Article

# Comparative study for low temperature water-gas shift reaction on Pt/ceria catalysts: Role of different ceria supports

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## Abstract

Pt on ceria catalysts for water-gas shift (WGS) reaction were prepared by employing three ceria nanopowders synthesized with different processing techniques and having different surface area and porosities. Nano-Pt (∼0.5-2 nm) was deposited in the vapor phase onto each of the three ceria supports by Reactive Spray Deposition Technology (RSDT). The catalysts were performance tested for the WGS reaction in the temperature range of 150-450 °C at a gas hourly space velocity (GHSV) of 13,360 h-1. The structure-activity relationship for the ceria-based materials was studied. The most promising catalyst was Pt supported on mesoporous ceria with crystallite size of 5.8 nm and Brunauer-Emmett-Teller (BET) surface area of 187 m2/g. This configuration demonstrated complete CO conversion at 225 °C. The CO adsorption strength and the ability to dissociate H2O are the two main factors that determine the activity of a particular catalyst site for the water-gas shift (WGS) reaction. This study leads to the conclusion that the highest water-gas shift reaction activity was obtained on Pt supported on the mesoporous ceria with low crystallite size and high surface area, with well dispersed Pt, leading to enhanced Pt-ceria interaction.

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... On the one hand, H 2 is a very effective reducing agent, especially at very low DeNO x temperatures [20][21][22][23]; on the other hand, it plays an important role in the detection of a complete NO x regeneration because of the hydrogen sensitivity of the oxygen [30] sensor downstream of the LNT. Catalytic hydrogen formation over a LNT with a comparable formulation was frequently mentioned in the literature [22,[24][25][26][27][28][29] and takes place according to the water gas shift (WGS) mechanism (1): ...
... There are two theories discussed in literature how H 2 formation takes place on a LNT. In [27][28][29] a two-stage redox mechanism is supposed. First, CO adsorbs on platin out of the gas phase (2) and is further oxidized to CO 2 by oxygen from ceria oxide (CeO 2 ) (3). ...
... First, CO adsorbs on platin out of the gas phase (2) and is further oxidized to CO 2 by oxygen from ceria oxide (CeO 2 ) (3). Over reduced ceria (Ce ?3 ) an O atom is separated from a water molecule and hydrogen gets formed (4 [28,29]. Jain et al. [29] found that H 2 is formed via both described mechanisms. ...
Article
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In this paper, the aging impact of desulphation (DeSOx) procedures on lean NOx traps (LNT) was investigated. With accelerated aging procedures on an engine test bench and on a synthetic-gas test bench, LNTs were stressed with lean rich cycling under realistic desulphation conditions. Exhaust gas chassis dynamometer tests showed the impact on emissions of the lean rich treatment. High carbon monoxide (CO) slips were detected in NEDC tests during NOx regeneration (DeNOx). With light-off tests, the pattern of damage was further investigated. A pronounced deactivation in CO-rich gas conversion was found to be the main reason for the carbon monoxide emissions in the chassis dynamometer tests. A correlation between DeSOx duration (cumulated duration of rich pulses) and the inhibited CO conversion was observed. Determinations of oxygen storage capacities of aged catalysts indicated that the lean–rich cycling mainly damaged the ceria oxide of the LNT. Variations of the rich gas components indicated that hydrogen in the feed gas as well as in situ generated hydrogen out of feed gas components (steam reforming, water gas shift) is accountable for the degradation in carbon monoxide conversion in rich purges. Investigations for lower desulphation temperatures showed that the effect is negligible for temperature <350 °C. Therefore, catalyst deactivation throughout NOx regeneration events with much lower temperatures than at DeSOx was not observed. As reference to other aging treatments used in the literature, samples aged hydrothermally at 750 °C, a phosphorus poisoned and hydrothermally aged LNT as well as a LNT sample from a vehicle endurance run were compared to the DeSOx aged catalysts. All LNTs had the same conventional LNT coating.
... Furthermore, the effect of the fabrication process on polarization overpotentials is studied with a one-step direct deposition process, reactive spray deposition technology (RSDT). 21,[44][45][46][47][48] In the RSDT process, the Pt nanoparticles are synthesized in the flame via the decomposition of the Pt precursor followed by nucleation growth into particles. The Pt is deposited onto carbon coated with ionomer in the downstream of the process after the flame is cooled. ...
... The detailed process of CCM fabrication by RSDT can be found in our previous publications. 21,[44][45][46][47][48] Typically, Pt-2, 4-pentanedionate (Colonial Metals, Inc.) was used as Pt precursor and dissolved in a combination of 62.5 wt% xylene (Sigma Aldrich, ACS reagent, ≥ 98.5%), 21 wt% acetone (Sigma Aldrich, HPLC ≥ 99.9%) and 16.5 wt% liquid propane (Air gas, ≥ 90%) with 10 mM Pt concentration. High-surface-area Ketjen Black EC-600JD (KB, Akzo Nobel) was employed as catalyst support. ...
Article
... RSDT is a subset of flame spray pyrolysis which was developed by Maric et al. [39] for the synthesis of nanoparticles. This process can employ a broad selection of precursors [40][41][42][43][44][45][46] compared to conventional vapor-fed flame reactors. In RSDT, nanoparticles are generated in the flame, and then are either directly deposited on the substrate as a film or collected as a nanopowder. ...
... This eliminates the intermediate steps of filtration, drying, and calcination. By incorporating a secondary spray system, nanoparticles can also be deposited on various supports such as carbon [47], magneli phase titania [48] and ceria [41,42]. RSDT provides complete control of the nanoparticle size, crystallinity [49], porosity, film thickness, and support concentration [47]. ...
... In this work, the mesoporous ceria with a crystallite size of 5.8 nm and synthesized via sol-gel method was found to be the best catalyst with a complete conversion of CO at 175°C (Fig. 8a-c) [105]. Byun et al. prepared a series of Cu-ZnO-CeO 2 catalysts to investigate the influence of CeO 2 addition on lowtemperature WGS reaction performance [106]. These catalysts included a fixed amount of Cu (50 wt%) and different ceria content (from 0 to 40%), which affects the Cu dispersion and binding energy. ...
... These catalysts included a fixed amount of Cu (50 wt%) and different ceria content (from 0 to 40%), which affects the Cu dispersion and binding energy. The result showed that a 10 wt% cerium could promote catalyst reduction and CO conversion at low temperatures of 200-400°C [106]. Petallidou et al. also investigated the effect of three synthesis methods (sol-gel, pechini, and urea co-precipitation) on 0.5 wt% Pt/Ce 0 . ...
Article
Full-text available
The water-gas shift reaction (WGSR) is an intermediate reaction in hydrocarbon reforming processes, considered one of the most important reactions for hydrogen production. Here, water and carbon monoxide molecules react to generate hydrogen and carbon dioxide. From the thermodynamics aspect, pressure does not have an impact, whereas low-temperature conditions are suitable for high hydrogen selectivity because of the exothermic nature of the WGSR reaction. The performance of this reaction can be greatly enhanced in the presence of suitable catalysts. The WGSR has been widely studied due do the industrial significance resulting in a good volume of open literature on reactor design and catalyst development. A number of review articles are also available on the fundamental aspects of the reaction, including thermodynamic analysis, reaction condition optimization, catalyst design, and deactivation studies. Over the past few decades, there has been an exceptional development of the catalyst characterization techniques such as near-ambient x-ray photoelectron spectroscopy (NA-XPS) and in situ transmission electron microscopy (in situ TEM), providing atomic level information in presence of gases at elevated temperatures. These tools have been crucial in providing nanoscale structural details and the dynamic changes during reaction conditions, which were not available before. The present review is an attempt to gather the recent progress, particularly in the past decade, on the catalysts for low-temperature WGSR and their structural properties, leading to new insights that can be used in the future for effective catalyst design. For the ease of reading, the article is divided into subsections based on metals (noble and transition metal), oxide supports, and carbon-based supports. It also aims at providing a brief overview of the reaction conditions by including a table of catalysts with synthesis methods, reaction conditions, and key observations for a quick reference. Based on our study of literature on noble metal catalysts, atomic Pt substituted Mn3O4 shows almost full CO conversion at 260 °C itself with zero methane formation. In the case of transition metals group, the inclusion of Cu in catalytic system seems to influence the CO conversion significantly, and in some cases, with CO conversion improvement by 65% at 280 °C. Moreover, mesoporous ceria as a catalyst support shows great potential with reports of full CO conversion at a low temperature of 175 °C.
... Raman shift of m-ceria with its characteristic peaks is shown in Fig. 2a. The Raman shift with the strongest intensity centered at 465 cm −1 is the first order Raman F2g mode from Ce-8O stretching vibrations [49]. No second-order peak is observed on the as-prepared mceria, i.e., no obvious peak near 260 cm −1 appears, suggesting that ceria lattice with high defects was not found in as-prepared m-ceria. ...
... shows no feature in Raman shifts region of m-ceria. XRD spectrum of asprepared m-ceria from SBA-15 (Fig. 2b) shows the fluorite cubic structure [51] and exposes {111} nanofacets, and is indexed as: 28°-CeO 2 (111), 32°-(200), 47°-(220), 57°-(311), 59°-(222); 70°-(400), 77°-(331), 78.5°-(420), and 88°-(422) [49]. Well-ordered hexagonal arrays of 2-D mesoporous channels for channel direction (110) was observed for the SBA-15 template (Fig. 3a). ...
... Further, the peak at 260 cm −1 can be assigned to the transverse mode (second-order transverse acoustic or doubly degenerate transverse optic) which is present only in ceria lattices with high defect concentration. 37 Secondary peaks are seen at Raman shifts of 830 and 1055 cm −1 (Figure 3b), which are assigned to peroxides (O 2 2− ) 38 and superoxide (O 2 − ) species, respectively, caused by adsorbed surface oxygen. Ceria nanoparticles synthesized in highly oxidizing conditions (CeO 2 cold, two-nozzle hybrid) and the commercially available Ce(IV)O 2 particles show no measurable defects, which indicates high Ce 4+ contents. ...
Article
Despite its use as a highly efficient and reusable catalyst in research and industrial settings, cerium oxide nanoparticles or nanoceria have yet to gain a foothold in the biomedical field. A variety of beneficial effects of nanoceria have been demonstrated, including its use as an inorganic nanoenzyme to mimic antioxidant enzymes, to protect mammalian cells and to suppress microbial growths. While these properties are of high interest for wound-management applications, literature offers contradicting reports on toxicity and enzymatic activity of nanoceria. These discrepancies can be attributed to differences between synthesis methods and insufficient physicochemical characterization, leading to incomparable studies. The activity of nanoceria is mostly governed by its Ce3+/Ce4+ ratio which needs to be controlled in order to compare different nanoceria systems. In this work, we demonstrate that liquid-feed flame spray pyrolysis offers excellent control over oxidation state in a one-step synthesis of nanoceria. This control allows a comprehensive comparison of different types of ceria nanoparticles. We connect physicochemical characteristics to biomedically relevant properties such as superoxide dismutase and catalase mimicry, human monocyte and macrophage protection, and antimicrobial activity. Furthermore, we demonstrate how the synthesis method also allows tailoring the properties of ceria-bioglass hybrid nanoparticles, thus creating nanoparticles with manifold biomedical prospects.
... Supported noble metal catalysts have been extensively studied for the development of WGS units in fuel processors, due to their advantages such as high activity, fast response, long catalyst lifetime and non-pyrophoricity. 14, [16][17][18] The performance of these materials in the WGS reaction is the result of a synergic effect between the noble metal and the support. 12,19 Among the explored metallic phases, Pt is particularly interesting due to its superior performance compared to other noble metals. ...
Article
For Pt catalysts which have demonstrated great activity on the WGS reaction, the activation of water is described as the rate-limiting step. Such limitation could be overcome through the design of supports able to supply water. In this study, the hexagonal and monoclinic phases of CePO4 have been evaluated as supports for Pt WGS catalysts. The hexagonal structure presents channels containing water, absent in the monoclinic structure. The presence of these channels in the hexagonal phase increases the interaction with the water molecule, leading to an enhancement of the WGS catalytic performance. DRIFTS results showed that dissociation of water does not occur on these supports, whereas calculated apparent activation energies present values similar to those reported in literature for the dissociation of water in Pt (111). These results suggest that cerium phosphates are acting as water suppliers, increasing the number of available species to be dissociated on the Pt surface.
... The Raman spectral profiles of CeO 2 _A and CeO 2 _S samples showed a similar pattern, nevertheless some differences were identified. The F 2g mode was located at 463 cm À1 in CeO 2 _S sample and at 475 cm À1 in CeO 2 _A sample, moreover there was a relevant difference in the broadness of the peak, suggesting a higher crystallite size in the commercial sample [42], in agreement with the results of Sherrer calculation, previously reported. The peak of the defect-induced mode was almost absent in CeO 2 _A sample, on the contrary it was prominent in CeO 2 _S sample, suggesting a different reducibility of the support. ...
Article
In this paper, we present a study on the preparation of ceria nanopowder by Supercritical Antisolvent technique and its use as catalysts support for water gas shift reaction. The effect of the concentration of ceria precursor and the solution flow rate was evaluated on particle size and granulometric distribution. The increase of concentration led to an increase in the average particle size, whereas the solution flow rate had a negligible effect. The platinum/ceria catalyst was prepared by wet impregnation and fully characterized; SEM and TEM-EDX showed a mean particle size of around 50 nm and a good dispersion of the active component. The results of the activity tests highlighted a good performance of the SAS-derived catalyst, that showed higher CO conversion with respect to a catalyst obtained from commercial ceria nanopowder.
... The latter is involved in the activation of the water molecule, generally recognized as the rate-limiting step of the WGS reaction [12][13][14]. Both reducible and non-reducible oxide-type supports have been extensively studied for the WGS reaction [5,[15][16][17][18][19]. Among them, CeO 2 has been proposed as an interesting candidate since it greatly enhances the water activation rate through the influence of its electronic properties (the redox pair Ce 3+ /Ce 4+ ), directly related to the presence of oxygen vacancies in the solid. ...
Article
The importance of water availability during the WGS reaction has been extensively reported. Thus, the search of new supports able to interact with the water molecule is of great importance. In this work, a series of phosphate-type supports containing Ce, Ca and Ti have been studied, demonstrating that water interaction with the support is closely related to the textural properties, surface composition and crystal structure of the solids. Additionally, DRIFTS results showed that different interaction mechanisms with the water molecule occur depending on the support. The system containing Ca dissociates the water molecule and interacts with it via the phosphate and Ca2+ ions. However, the Ce systems retain water in its molecular form, which interacts with the solids via hydrogen bonding with the phosphate groups. On the other hand, the Ti system experiences a loss of phosphorous, presenting a low degree of interaction with the water molecule. Additionally, the behavior of the supports with water has been successfully related to the WGS catalytic activity of the corresponding phosphate-supported Pt catalysts.
... The relevance of the latter variable in the catalytic performance of the WGS reaction is observed in the literature. For example, higher surface area catalysts of Pt/CeO 2 (187 m 2 /g) presented much higher activity than those with lower surface area (78 m 2 /g) (Jain et al., 2015) due to the presence of more defect sites, thus attracting more molecules of CO and H 2 O to the surface and favouring the reaction. Therefore, the novelty in including the surface area in the ANN model for the WGS reaction was really decisive since it was possible to verify its great influence on the process. ...
Article
Hydrogen (H 2 ) is considered a clean valuable energy source and its worldwide demand has increased in recent years. The Water-Gas Shift (WGS) Reaction is one of the major routes for hydrogen production and uses different catalysts depending on the operating process conditions. A catalyst is usually composed of an active phase and a support for its dispersion. There are currently an increasing number of researches on catalytic field focusing on transition metals nanoparticles supported on different compounds. In order to predict optimal catalyst compositions for the WGS reaction, Artificial Neural Networks (ANNs) were used to build a model from the literature catalytic data. A three-layer feedforward neural network was employed with active phase composition and support type as some of the input variables, and Carbon Monoxide (CO) conversion as output variable. The insertion of properties such as surface area, calcination temperature and time allowed predicting the reaction performance based on intrinsic catalyst variables not commonly used in phenomenological kinetic models. Also, unlike previous studies, a detailed sensitivity analysis was carried out to observe useful trends. An important outcome of this work is the proposition of ceria-supported catalysts for the WGS reaction that present larger surface areas, with Ru, Ni or Cu as active phases conducted at moderate temperatures (≈300 °C) and with reasonable space velocities (2000–6000 h ⁻¹ ). In addition, it was possible to predict the most relevant variables for the process: the temperature and the surface area. Thus, the results show the power of ANNs for predicting better catalysts and conditions for this important process in the environmental field.
... The detailed process of CCM fabrication by RSDT can be found in our previous publications [8,9,49,[54][55][56]. Typically, Pt-2, 4pentanedionate (Colonial Metals, Inc.) was used as Pt precursor and dissolved in a combination of 62.5 wt. ...
... The relevance of the latter variable in the catalytic performance of the WGS reaction is observed in the literature. For example, higher surface area catalysts of Pt/CeO 2 (187 m 2 /g) presented much higher activity than those with lower surface area (78 m 2 /g) (Jain et al., 2015) due to the presence of more defect sites, thus attracting more molecules of CO and H 2 O to the surface and favouring the reaction. Therefore, the novelty in including the surface area in the ANN model for the WGS reaction was really decisive since it was possible to verify its great influence on the process. ...
Conference Paper
A crescente utilização de técnicas de Machine Learning (ML) na área de Ciência e Engenharia dos Materiais visa a avaliar e melhorar o desempenho dos materiais em suas aplicações, fornecendo insights não facilmente perceptíveis "a olho nu" e reduzindo o número de experimentos dispendiosos em laboratório por meio de uma varredura de potenciais e elucidativas condições experimentais. Neste trabalho, foram reportados os principais avanços desta subárea da Inteligência Artificial (IA) no campo da Catálise ao longo dos últimos anos, suas aplicações no estudo da cinética das reações, no desenvolvimento de novos catalisadores e na aceleração do cálculo de simulações moleculares, como DFT. Além disso, foram aplicados algoritmos de Redes Neurais Artificiais (RNAs) e Árvores de Decisão em um conjunto de dados coletado da literatura para a Reação Water-Gas Shift (WGS), um importante processo para a produção de hidrogênio, com o intuito de aprimorar o design dos seus catalisadores. Os resultados comprovam o grande potencial desses métodos para a extração de informação na forma de modelos e regras heurísticas, auxiliando a condução de futuras pesquisas.
... The scalable methods of preparation of CeO 2 supports vary from the cerium salts solution hydrolysis [15,16] to the flame spray pyrolysis. [7,17] Among these methods the simplest and most straightforward one is the thermal decomposition of cerium salts, such as Ce (NO 3 ) 3 · 6H 2 O, Ce 2 (C 2 O 4 ) 3 , (NH 4 ) 2 [Ce(NO 3 ) 6 ], and Ce 2 (CO 3 ) 2 O · H 2 O. [8,[18][19][20] Despite its apparent simplicity, a variety of parameters including composition of the precursor, temperature, ramping rate, gaseous atmosphere, and precursor pretreatment, can be used to control the key characteristics of the resulting ceria materials. ...
Article
A new approach for preparing a PtOx/CeO2 catalyst for low‐temperature CO oxidation has been developed. The approach includes; i) preparation of a CeO2 support through the controllable thermal decomposition of Ce(NO3)3·6H2O, and then ii) deposition of polynuclear platinum nitrato complexes ([H3O18‐crown‐6]2[Pt2(μ2‐OH)2(NO3)8][Pt4(μ3‐OH)2(μ2‐OH)4(NO3)10]) on the CeO2 support. The NO3‐‐rich ceria support produced at the onset temperature of Ce(NO3)3 ‐> CeO2 transformation (220°C) yields the “PtOx/CeO2_220” catalyst that shows intriguingly high activity in CO oxidation, at room temperature and below. To understand the nature of the low‐temperature catalytic activity, prepared catalysts have been studied using X‐ray photoelectron spectroscopy (XPS), Raman and operando X‐ray absorption fine structure spectroscopy (XAFS) and transmission electron microscopy (TEM).
... As described previously, catalyst-coated membranes (CCMs) were fabricated by reactive spray deposition technology (RSDT) with 25 cm 2 active area on Nafion 1 212 membrane [12][13][14][15][16][17], with Pt loading was 0.11 AE 0.02 mg cm À2 for the cathode and 0.05 AE 0.01 mg cm À2 for anode, confirmed by inductive-coupled plasma optical emission spectroscopy (ICP-OES) analysis. The fabrication process for the control cathode is described briefly as follows: the Pt precursor solution was prepared by dissolving Pt-2, 4pentanedionate (Colonial Metals, Inc.) in a combination of 62.5 wt% xylene (Sigma Aldrich, ACS reagent, !98.5%), 21 wt% acetone (Sigma Aldrich, HPLC !99.9%) and 16.5 wt% liquid propane (Air gas, !90%), resulting 10 mM Pt concentration. ...
... The catalytic oxidation of CO by Pt-group metals has enormous technological, industrial and environmental relevance. [1][2][3][4] Pt itself is of the upmost importance as a catalyst for car exhaust cleaning or for the water gas shift reaction, 5,6 whereas Pt crystal surfaces are model systems for investigating the catalytic CO oxidation at the atomic scale. [7][8][9][10][11][12] In the last four decades many researchers have studied the separate, sequential and simultaneous interaction of CO [13][14][15][16][17][18][19][20][21][22][23] and oxygen [24][25][26][27][28][29][30][31][32][33][34][35] with Pt crystal surfaces under High or Ultra-High Vacuum (HV or UHV) conditions. ...
Preprint
Full-text available
We combine PLIF and NAP-XPS to investigate the CO oxidation reaction at vicinal Pt(111) surfaces in the millibar regime, using a curved sample. We find that the catalytic activation of Pt occurs in all vicinal planes simultaneously, irrespectively of the reaction parameters. The systematic analysis of chemical species across the entire curved surface provides the clues for this surprising behavior. As the surface CO concentration decreases when approaching ignition, minor amounts of oxygen build up at both steps and (111) terraces. First-principles theory indicates that the latter is forming a CO-Pt-O complex that binds CO molecules to terraces strongly, leveling its adsorption energy to that of low-coordinated steps, and explaining why CO abruptly desorbs at the same temperature along the di erent crystal facets that make up the curved Pt surface.
... Jain et al. [114] investigated the effect of ceria structural properties in a series of Pt/CeO2 catalysts where ceria was synthesized with three different methods. 5%wt nano-Pt (0.5-2 nm) was deposited by reacting spray deposition technology respectively on: C1, sol-gel method synthesized ceria; C2, combustion chemical vapor deposition prepared ceria; C3, commercial ceria provided by Sigma-Aldrich. ...
Article
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The water gas shift (WGS) is an equilibrium exothermic reaction, whose corresponding industrial process is normally carried out in two adiabatic stages, to overcome the thermodynamic and kinetic limitations. The high temperature stage makes use of iron/chromium-based catalysts, while the low temperature stage employs copper/zinc-based catalysts. Nevertheless, both these systems have several problems, mainly dealing with safety issues and process efficiency. Accordingly, in the last decade abundant researches have been focused on the study of alternative catalytic systems. The best performances have been obtained with noble metal-based catalysts, among which, platinum-based formulations showed a good compromise between performance and ease of preparation. These catalytic systems are extremely attractive, as they have numerous advantages, including the feasibility of intermediate temperature (250–400 °C) applications, the absence of pyrophoricity, and the high activity even at low loadings. The particle size plays a crucial role in determining their catalytic activity, enhancing the performance of the nanometric catalytic systems: the best activity and stability was reported for particle sizes < 1.7 nm. Moreover the optimal Pt loading seems to be located near 1 wt %, as well as the optimal Pt coverage was identified in 0.25 ML. Kinetics and mechanisms studies highlighted the low energy activation of Pt/Mo2C-based catalytic systems (Ea of 38 kJ·mol−1), the associative mechanism is the most encountered on the investigated studies. This review focuses on a selection of recent published articles, related to the preparation and use of unstructured platinum-based catalysts in water gas shift reaction, and is organized in five main sections: comparative studies, kinetics, reaction mechanisms, sour WGS and electrochemical promotion. Each section is divided in paragraphs, at the end of the section a summary and a summary table are provided.
... The average crystallite sizes were calculated with the Scherrer equation from the line broadening at half of the maximum intensity (FWHM) of selected CeO 2 and NiO diffraction peaks. No significant Pt peaks were observed in the XRD pattern, possibly due to the small size of Pt crystallites (0.5e2 nm) [31]. Conversely, the ceria crystallite dimensions in the NiePt/CeO 2 catalyst (23 nm) were slightly higher than that of the support alone (17 nm). ...
Article
The reaction rate of low-temperature steam reforming was studied over two Ni-based catalysts. Experimental tests were carried out for temperatures between 400 and 550 °C and pressures between 1 and 7 bar, using methane and simulated biogas (1:1 methane-carbon dioxide mixture) as carbonaceous feed. The results were analyzed with a simplified reaction rate expression. The parameters of such expression were optimized in order to provide a tool for the design of an industrial scale reactor using the catalysts considered.
... This implies that extensive research is needed to develop and improve catalysts with high activity, selectivity, and stability. In the development of WGS catalysts, innovative formulations based on ceria and titania-based platinum catalysts have been studied [16,17]. ...
Article
Full-text available
This paper reports the findings of a FP7 project (DEMCAMER) that developed materials (catalysts and membranes) and new processes for four industrially relevant reaction processes. In this project, active, stable, and selective catalysts were developed for the reaction systems of interest and their production scaled up to kg scale (TRL5 (TRL: Technology Readiness Level)). Simultaneously, new membranes for gas separation were developed; in particular, dense supported thin palladium-based membranes for hydrogen separation from reactive mixtures. These membranes were successfully scaled up to TRL4 and used in various lab-scale reactors for water gas shift (WGS), using both packed bed and fluidized bed reactors, and Fischer-Tropsch (FTS) using packed bed reactors and in prototype reactors for WGS and FTS. Mixed ionic-electronic conducting membranes in capillary form were also developed for high temperature oxygen separation from air. These membranes can be used for both Autothermal Reforming (ATR) and Oxidative Coupling of Methane (OCM) reaction systems to increase the efficiency and the yield of the processes. The production of these membranes was scaled up to TRL3–4. The project also developed adequate sealing techniques to be able to integrate the different membranes in lab-scale and prototype reactors.
... In situ DRIFT spectra collected in Xia et al.'s work [71,72] presented no peaks mentioned above, suggesting that the production of CO 2 went through the pathway where CO reacted with adsorbed O intermediates on the catalyst, which was from the dissociation of OH species after H 2 O activation. Nevertheless, there were a few studies which identified formate as an active reaction intermediate [51,[73][74][75][76] while others claimed it as a spectator species or decisive species [56,77,78]. The determination of formate to be active intermediate continues to be the focus on future WGS mechanistic studies. ...
Article
The use of in situ and operando spectroscopy in applied catalysis is driven by the recognition that it is a powerful tool to elucidate the nature and changes of the atomic arrangements and chemical species at a surface during heterogeneous catalytic reactions. The aim of this article is to provide an overview of recent processes in using in situ and operando spectroscopy to investigate the relationship between structure and activity/selectivity, as well as mechanisms of gas-surface reactions on oxide-supported metal catalysts and at metal-oxide interfaces. The selected reactions include water-gas shift reaction, CO oxidation, and selective CO2 hydrogenation. Special attention is given to oxide-supported single-atom catalysts, which are known to provide maximum atom utilization and high efficiency towards catalytic performance but is lack of detailed understanding under operando conditions. Recent research from 2018 to 2021 using in situ and operando spectroscopy with an emphasis on diffuse-reflectance infrared spectroscopy (DRIFTS), combined with X-ray spectroscopy and theoretical calculations, are reviewed for the three reactions on relevant catalysts for general audience in applied catalysis. The perspectives on how to improve catalytic performance are discussed.
Article
In this paper, a facile strategy by etching CeO2 spheres pretreated by different concentrations of H2SO4 or NaOH solution with an ionic liquid (IL) of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) under hydrothermal condition was developed to achieve CeO2 nanocubes. By depositing CuxO clusters on CeO2 nanocubes via a deposition−precipitation method, CuxO/CeO2 nanocube catalysts loading with different amounts of CuxO clusters were synthesized and the CuxO clusters were highly dispersed onto the surface of CeO2 nanocubes. The Raman spectroscopy, H2-TPR, XPS, and DRIFTS results reveal that there is a strong interaction between CuxO and CeO2. Among these CuxO/CeO2 catalysts, the 10CuCe-2OH-ILs, which was obtained via immersing CeO2 spheres using 2 M NaOH, and subsequently being etched by ILs, and then loading ∼10% of copper, exhibits the optimal catalytic performances and stability. The catalytic activity of different catalysts increases in the order of 10CuCe-ILs < 10CuCe-1H-ILs < 10CuCe-0.1OH-ILs < 10CuCe-5OH-ILs < 10CuCe-2OH-ILs. The result is in agreement with the order of the surface oxygen vacancy concentration, reducibility and Cu⁺ content, suggesting that the interaction between CuxO species and surface oxygen vacancies of these CuxO/CeO2 catalysts plays an important role for CO preferential oxidation in H2-rich gases.
Article
The physicochemical and catalytic properties of Cu-containing crystalline zirconia, obtained via sol–gel synthesis in the presence of Yb 3+ ions and polyvinylpyrrolidone, are studied. DTG/DSC, TEM, XRD and BET methods are used to analyze the crystallization, texture, phase uniformity, surface and poros-ity of ZrO 2 nanopowders. It is shown that increasing the copper content (1, 3, and 5 wt % from ZrO 2) raises the dehydrogenation activity in the temperature range of 100–400°C and lowers the activation energy of acet-aldehyde formation. It is found that the activity of all Cu/t-ZrO 2 catalysts grows under the effects of the reaction medium, due to the migration and redispersion of copper.
Article
Well-dispersed, random alloy, palladium-gold nanoparticles (2.66 ± 0.51 nm) were immobilized onto several reducible mesoporous transition metal oxide materials. The composites (palladium-gold nanoparticles immobilized onto mesoporous transition metal oxide (PdAu–MTMO)) were characterized through several analytical methods such as UV–vis spectroscopy, BET, XRD, FT-IR, ICP-OES, TEM and TPR analyses. Catalytic oxidation of morin (quercetin) was performed as a model reaction in the presence of hydrogen peroxide to investigate the synergic catalytic activity of the composite. Silica was used as inert support to isolate the catalytic activity of the metal nanoparticles (32.69 ± 9.93 kJ mol⁻²). Synergistic interaction of PdAu–MTMO was mechanically described according to Langmuir-Hinshelwood and Mars-van Krevelen approaches. The TOF of PdAu–Co3O4 (6073.23 ± 85.01 s⁻¹ mol⁻¹) was considerably larger than that of random alloy nanoparticles (PdAu–SiO2 (25.71 ± 2.35 s⁻¹ mol⁻¹). The Arrhenius-type plot was constructed to determine the synergistic activity of the composite, where PdAu–Co3O4 described the best synergistic interaction.
Article
This study explores the removal of an anti-inflammatory drug Mesalamine a hazardous pharmaceutical pollutants, from aqueous solution by Unsaturated Polyester Resin (UPR) a non-carbon adsorbent. The sorption of Mesalamine species has been carried out as a function of pH, contact time, adsorbate concentration (0.1–0.7 mg/mL) and temp. (30–50 °C). The optimum pH for maximum adsorption of Mesalamine was found to be at pH 10.2. The amounts adsorbed by UPR increased with the increasing concentration of hydroxide ions. The results indicate that the Langmuir adsorption isotherm model fits the data better than the Freundlich adsorption isotherm model. The calculated thermodynamic parameters (ΔGo, ΔHo and ΔSo) showed that the adsorption of Mesalamine onto UPR was feasible, spontaneous and endothermic. Mass transfer property of the sorption process was studied using Lagergren pseudo-first-order kinetic models. The values of kad for Mesalamine were calculated at different temperatures (303 − 323 K). The mechanism of the adsorption process was determined from the intraparticle diffusion model. The linear portions of the curves do not pass through the origin indicating that mechanism of Mesalamine removal onto UPR is complex and both the surface adsorption as well as intra-particle diffusion contributes to the rate determining step.
Article
Aqueous-phase reforming (APR) of methanol over nickel supported on zirconium, cerium and lanthanum oxides was performed in continuous laboratory scale reactor and discussed in this paper. The role of composition and physico-chemical properties of the supports were investigated and significant benefit of using mixed oxides CeO2-ZrO2 and La2O3-ZrO2 over the pure oxides, in term of methanol conversion and hydrogen production, was demonstrated. Methanol conversion of over 50% with hydrogen production efficiency of over 40% were achieved with the most active catalyst Ni/25%CeO2-ZrO2. Furthermore, catalyst stability, the most challenging issue within APR studies, was thoroughly investigated and discussed. Slight deactivation of the prepared catalysts during APR experiments or reduction was observed and addressed to the Ni particles sintering. On the other hand, other common reasons causing catalysts deactivation under APR conditions, such as leaching of Ni, changes in Ni oxidation state or changes in the supports lattice were not observed by wide range of characterization methods The most stable catalyst, Ni/10%La2O3-ZrO2, exhibited a slight decrease of MeOH conversion within two subsequent experiments (each per 6 h) from 46.3% to 42.7%.
Article
We herein report the preparation of ceria (CeO2) via a simple precipitation method for use as a catalyst support in the water-gas shift (WGS) reaction. More specifically, we optimized the titration time required to obtain highly active CeO2-supported catalysts for the WGS reaction. As such, Cu was employed as the active metal coupled with the CeO2 support. Notably, the CeO2–0 supported Cu catalyst (where the precipitant was immediately injected into a cerium nitrate solution) exhibited the highest CO conversion at a gas hourly space velocity of 36,050 h⁻¹. This high catalytic activity of the Cu/CeO2–0 catalyst was mainly due to its high Brunauer-Emmett-Teller (BET) surface area, enhanced Cu dispersion, high number of oxygen vacancies, and enhanced reducibility.
Article
At present, lignocellulosic biomass is the only renewable and available resource capable of supplanting fossil fuel resources. Fast pyrolysis is the conversion of biomass at elevated temperature, high heating rate and under inert atmosphere to produce bio-oil. This bio-oil is a blend of oxygenated hydrocarbons, which must be upgraded prior to use as a transportation fuel. The focus of this thesis is the investigation of bio-oil upgrading reaction pathways and catalysts for the production of biofuels. There are two primary pathways for pyrolysis vapor upgrading: catalytic cracking and hydrodeoxygenation (HDO). During catalytic cracking, pyrolysis vapors react over the acid sites of ZSM-5 zeolite, which crack C-C bonds to release oxygen in the form of COx. Biofuel compound yield is greater when cracking occurs in-situ the pyrolysis reactor, but selectivity is greater when cracking occurs ex-situ. Because of its microporous structure, the ZSM-5 catalyst may exclude some of the bulky oxygenates formed during pyrolysis. This accessibility limitation can be alleviated through the introduction of mesoporosity in the ZSM-5 zeolite. Zeolite mesoporosity is beneficial for increasing aromatic yield and reducing coke on catalyst. While catalytic cracking is effective for removing oxygen from the pyrolysis vapors, this oxygen is removed as COx, reducing carbon return in the bio-oil. Moreover, the upgraded bio-oil is aromatic in nature, and aromatics in gasoline are limited. Incorporation of hydrogen into the pyrolysis reactor in the presence of Ni-ZSM-5 catalyst reduces char formation and substantially increases CH4 yield, but the bio-oil does not contain many alkanes. The high reaction temperature demanded by biomass volatilization thermodynamically limits hydrogenation reactions. Catalytic hydropyrolysis followed by secondary hydroprocessing produces a biofuel with heating value and aromaticity similar to gasoline. Liquid phase HDO is another pathway for removal of bio-oil oxygen. Catalytic HDO of anisole, 4-ethylphenol and benzofuran was performed with Ni, Ru and Pd supported on USY zeolite. Kinetic rate measurements show Pd is more effective than Ni and Ru in terms of reaction rate, deoxygenation activity and C-C coupling. Finally, controlled mesoporosity of the USY support is beneficial for enhancing access of oxygenates to the impregnated metal species for efficient hydrogenation.
Article
Five different rare‐earth‐based nanocrystalline high entropy oxides (HEOs) with fluorite structure and average crystallite sizes between 6 and 8 nm are prepared and their photocatalytic behavior toward azo dye degradation and photoelectrochemical water splitting for hydrogen generation is examined. The cationic site in the fluorite lattice consists of five equimolar elements selected from the group of rare‐earth elements including La, Ce, Pr, Eu, and Gd and second‐row transition metals, Y and Zr. The studied HEOs exhibit bandgaps in the range from 1.91 to 3.0 eV and appropriate valence and conduction bands for water splitting. They reveal high photocatalytic activity that is mostly attributed to the accessibility of more photocatalytic active sites, which provide radicals responsible for the azo dye degradation. The materials successfully produce hydrogen by photocatalytic water splitting, suggesting the potential of HEOs as new photocatalysts. The photocatalytic performances of all studied HEOs outperform the single fluorite oxides or equivalent mixed oxides. The Ce0.2Zr0.2La0.2Pr0.2Y0.2O2 (CZLPY) engender hydrogen in 9.2 µmol mg−1 per hour that is much higher content than for pristine CeO2 material which amounts to 0.8 µmol mg−1 per hour. Five different nanocrystalline rare‐earth‐based high entropy oxides (HEOs) with fluorite crystal structure are synthesized. The studied HEO photocatalysts for azo dye degradation and photochemical water splitting for hydrogen generation outperform the single fluorite oxides or equivalent mixed oxides. The photocatalyst with the best performance (Ce0.2Zr0.2La0.2Pr0.2Y0.2O2) is additionally investigated by theoretical modelling using density functional theory.
Article
PEM fuel cells are of great interest for vehicular applications. A highly pure, sustainably obtained H2 stream is desirable to use as the cell feed. The Water Gas Shift reaction (WGS) is one of the stages in which large amounts of CO, a poison for the cell anode, are removed from a bio‐alcohol derived H2 stream, before entering the cell. In this study, Cu‐Ni catalysts supported on a combination of La‐doped ceria solids are analyzed and proposed for the WGS reaction. The solids were prepared via the urea thermal decomposition method, with different percentages of La as a promoter of the ceria support. The metal phase is incorporated via incipient wet impregnation. Many characterization techniques have been employed in this work (BET, XRD, SEM, ICP, TPR, OSC), and a relation between the properties obtained from these studies and the solid's catalytic performance is attempted in order to determine the effect of La presence. A mixture of both Cu and Ni was found to be the most effective active phase out of all the studied samples, having considerable activity and selectivity towards the desired reaction. Low contents of La doping on ceria enhance oxygen mobility in the lattice. From the results presented in this work it can be concluded that a commercial ceria salt containing La as its main impurity (ca. 2%) is quite convenient, given its good performance and its low market value compared to a high purity salt.
Article
Increasing CO2 emissions into the environment has triggered intensive research on CO2 capture and utilization. Downsizing catalyst nanoparticles (NPs) to an atomic dispersion, exposing all atoms as active sites on the surface, is highly desirable to reduce noble metal usage and see improved activity on many catalytic reactions such as CO oxidation and CO2 reduction. Yet, current studies on atomic-level understanding of the catalytic CO2 reduction mechanism are poorly understood. Here, we report the synthesis of CeO2 NPs decorated with atomically dispersed Pt atoms and scrutinize the reaction mechanism of CO2 reduction catalyzed by single-atom (0.05wt%) Pt/CeO2 and nano-clustered (2wt%) Pt/CeO2 using in-situ DRIFTS. The activity results indicate that the single atom Pt/CeO2 exhibited a 7.2 times higher reaction rate, despite a 40 times lower Pt loading than for the nano-clustered Pt/CeO2 catalyst, and possessed good thermal stability at 500 °C. In-situ spectroscopy demonstrated that CO2 activation occurs on the oxide support while H2 dissociation occurs on the Pt metal. The single atom or nano-clustered nature of the Pt catalyst impacts on the selectivity of the reaction products towards CO or CH4, whereby different mechanistic pathways for CO2 reduction are suggested based on the geometric Pt arrangement. The isolated Pt atom geometry, unlike nano-clustered Pt with continuous Pt-Pt bond, weakly binds CO which restricts further hydrogenation and prevents CO poisoning. The findings illustrate the unique opportunities available for tuning catalyst activity and chemoselectivity by the rational design of atomically dispersed catalysts.
Article
Synthesis of functional nanomaterial thin films using a scalable flame combustion technique called Reactive Spray Deposition Technology (RSDT) was explored. Nanomaterials were used as sensing film for local gas monitoring and human breath analysis for medical diagnosis (different phases of WO3) and catalysts for water-gas shift (WGS) reaction (Pt supported on ceria). Areas of application include: handheld portable devices for immediate breath composition monitoring, medical diagnosis, and environment monitoring (workplace, residence and automobile). Two case studies will be explained in detail: (1) acetone sensing in human breath for blood glucose monitoring and (2) NO2 sensing for air quality monitoring. A study of the RSDT synthesis technique and control of crystal structure, porosity, and nanoparticle size will be demonstrated. The detailed study of acetone and NO2 sensing mechanism will be explained in detail, including sensor performance and stability testing.
Article
A comparative study of three different ceria synthesis procedures (template- and MW- assisted hydrothermal synthesis and urea homogeneous precipitation) is reported in this paper. The obtained materials were employed as supports for Pt nanoparticles, and the Pt/CeO2 catalysts were evaluated in the WGS reaction under model and realistic conditions. The influence of the support, e.g., its morphology and electronic properties, has been studied in detail by means of XRD, H2-TPR, XPS, UV–Vis spectroscopy and toluene hydrogenation (for metal dispersion assessment). The catalytic performance of the samples is directly correlated with the modification of the electronic properties, as a result of the preparation method used. The conventional homogeneous precipitation method with urea resulted to be the best option, leading to enhanced ceria reducibility and adequate Pt dispersion, which in turns resulted in a very efficient WGS catalyst.
Article
In present study, the preparation of solar active photocatalyst and its application for the detoxification and degradation of dye molecules in aqueous medium is demonstrated. The reduced graphene oxide (rGO) is prepared from graphene oxide (GO) using Carica papaya leaf extract. Nanosize CeO2 was coated over the rGO surface to make a nanocomposite photocatalyst. Prepared composite catalyst was characterized using SEM, HRTEM, XRD, EDX, FTIR, FT-Raman and UV-DRS techniques. The prepared composite catalyst was used for the degradation of auramine-O dye in its aqueous solution using UV and solar irradiations. The degradation kinetics is also evaluated using Langmuir-Hinshelwood kinetic model. The kinetic curves are analyzed using basic algorithm. A deviation from the experimental values and the reaction order is enumerated.
Article
Curved crystal surfaces enable the systematic and accurate comparison of physical and chemical processes for a full set of vicinal crystal planes, which are probed in the very same environment. Here we examine the early stages of the CO chemisorption on vicinal Rh(111) surfaces using a curved Rh crystal that exposes a smoothly variable density of $\{100\}$ (A-type) and $\{111\}$ (B-type) steps. We readily identify and quantify step and terrace species by resolving their respective core-level lines in X-ray Photoelectron Spectroscopy at different locations on the curved surface. Uptake experiments show similar sticking probabilities at all surface planes, subtle asymmetries between A- and B-type steps, and significantly lower saturation coverage at densely stepped surfaces as compared to the (111) plane. The analysis of the C 1\emph{s} intensity variation across the curved sample allows us to discuss the adsorption geometry around the step edge.
Article
Pt/CeO2 catalysts with unitaryPt species,nanoparticles,clustersorsingle atoms, often exhibit excellent activity and unique selectivity in many catalytic reactions benefiting from their small size, abundant unsaturated active sites, and unique electronic structure. In recent years, a tremendous number of related articles have providedgreat inspiration to future research and development of Pt/CeO2 catalysts. In this review, the state-of-the-art evolution of Pt nanoparticles to Pt single atomson CeO2 is reviewed with the emphasis on syntheticstrategies, advanced characterization techniques (allowing one to clarify the single atoms from clusters), the catalytic applications and mechanisms from the viewpoint of theoretical calculation. Finally, the critical outlooksand the challenges faced in developing the single-atom Pt/CeO2 catalysts are highlighted.
Article
The hexagonal and monoclinic phases of CePO4 have demonstrated to be excellent catalytic supports for Pt-based WGS catalysts. Consequently, the elucidation of the WGS reaction mechanism in these materials constitutes a fundamental aspect in order to explain their catalytic behavior. Since the observed WGS reaction path is closely related to the absence or presence of oxygen vacancies in the support, the study of the oxygen mobility in these solids constitutes a key factor for the understanding of the materials structure and its influence on the reaction mechanism. In this study, the oxygen mobility in the CePO4 supports and the corresponding Pt catalysts has been evaluated by means of isotopic exchange experiments using ¹⁸O2 and C¹⁸O2 as probe molecules. Results demonstrate that the evaluated solids present a low exchange activity when ¹⁸O2 is used, indicating the absence of oxygen vacancies in these solids and thus, suggesting a poor influence of the WGS redox mechanism. On the contrary, a high oxygen exchange activity is observed using C¹⁸O2, demonstrating that the exchange in these materials takes place through the formation of carbonate-like intermediates, and thus suggesting the associative mechanism of the WGS reaction as the preferred path in these solids. Operando DRIFTS experiments under WGS reaction conditions confirm these results, proving that the WGS reaction in the studied materials takes place through a formate-mediated associative mechanism.
Article
The catalytic oxidation of carbon monoxide (CO) on transition metals, such as platinum (Pt), is commonly viewed as a sharp transition from the CO‐inhibited surface to the active metal, covered with oxygen (O). However, we find that minor amounts of O are present in the CO‐poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near‐ambient pressure X‐ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. The systematic analysis of carbon and oxygen core levels across the curved crystal reveals that, right before light‐off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces, as indicated by ab‐initio theory: a CO‐Pt‐O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.
Article
The light‐off of the CO oxidation is simultaneous on all Pt crystal surfaces vicinal to the (111) plane, regardless of the reaction conditions, and in contrast with the structural dependence of Pd. Using ambient‐pressure XPS we find that, immediately prior to ignition, atomic oxygen incorporates to the subsurface plane, leading to buckling of the topmost CO‐Pt layer, and effectively equaling the CO desorption temperature at terraces and steps. Abstract The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO‐inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO‐poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near‐ambient pressure X‐ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light‐off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO‐Pt‐O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.
Article
The Intermittent Microwave Heating (IMH) assisted polyol method was used to disperse Pt nanoparticles on Vulcan XC-72 (C) and reduced Graphene Oxide (rGO) functionalized with [(η 6-C 6 H 5 OCH 2 CH 2 OH)RuCl 2 ] 2 (Ru-dim) and [(η 6-C 6 H 4 (CHMe 2)Me)RuCl 2 ] 2 (Ru-cym). The nanocatalysts were labeled as Pt/C Ru-dim , Pt/C Ru-cym , Pt/rGO Ru-dim and Pt/rGO Ru-cym. Their catalytic activity was evaluated for the Methanol (MOR) and Ethanol Oxidation Reactions (EOR). 1 H and APT 13 C NMR characterization showed the coordination of arene ligands with ruthenium atoms, supporting the formation of Ru-dim and Ru-cym. Raman spectroscopy indicated that C and rGO preserve their graphitic band structure after functionalization. Functionalization of the supports resulted in the development of several surface chemical groups. The electrochemical characterization showed that: i) Pt/C Ru-dim was the nanocatalyst with the highest catalytic activity for the MOR, demonstrating also a high performance for the EOR; ii) Pt/rGO Ru-cym showed a good electrocatalytic behavior for both reactions at more negative potentials, nevertheless delivering lower current densities (j). In terms of the organic molecule, higher j values have been obtained from the MOR, compared to the EOR. The results showed that these nanocatalysts can be considered as anode materials in Direct Alcohol Fuel Cells applications.
Article
We synthesized CeO 2 nanowires and nanocubes showing {110} + {100} and {100} surfaces predominantly, respectively, once the different surface energies play a crucial role in the behavior of Ce 4+ /Ce 3+ reversibility. The differences in the Pt‐Ce and Pt‐CO interactions were studied in detail by several techniques. We revealed that Pt/CeO 2 nanowires presented more Ce 3+ content, oxygen vacancies, and atomically dispersed Pt, indicating a stronger Pt‐Ce interaction, as attested by H 2 ‐TPR, HR‐STEM, CO‐DRIFT, XPS, and Raman. In contrast, the Pt/CeO 2 nanocubes presented a higher contribution of Pt δ+ species, suggesting a well‐controlled Pt particle size (~1nm) and significant interaction with ceria, with oxygen species more available at the surface. Thus, we suggest that the ceria's different surface energies may lead to different Pt distributions of species over the supports. H 2 ‐reduction treatment led to changes in Pt structure that showed a better catalysis performance for the Pt/CeO 2 nanowires essentially, supported by XPS and CO‐DRIFTS. Nevertheless, this step did not cause improved activity to the point of overcoming the nanocubes‐based catalyst, and the reasons were fully discussed. Herein, we propose that catalysts' performance depends on a complex combination of several materials' characteristics. These features may lead to different reaction pathways depending on the pre‐treatment of the samples.
Article
Magnetic, electrical, and photocatalytic characteristics of a series of nanoparticles (NPs) of Gd-doped CeO2 (GdxCe1-xO2 (100×% GDC); x = 0.03, 0.05, 0.10, and 0.20) were analyzed in correlation with their surface structure and defect behavior. The GDC NPs were synthesized by a simple chemical process through a polymer-based precursor. X-ray diffraction (XRD) studies confirmed the effective substitution of Gd³⁺ in the face centered cubic (fcc) lattice of the host matrix of CeO2. The optical band gap (Eg) and oxygen vacancies (VO) increased with Gd-doping as validated by UV–visible reflectance and Raman spectroscopy. Further analysis with X-ray photoelectron spectroscopy (XPS) substantiated the formation of non-stoichiometric CeO2 structure through Ce⁴⁺ → Ce³⁺ reduction to maintain charge neutrality after Gd-doping. Samples of 20% GDC processed at 400 °C, showed the highest saturation magnetization (MS) of 17.58 memu/g at room temperature. A maximum conductivity of 2.11 × 10⁻³ S/cm was recorded at 650 °C in air in the 20% GDC samples processed at 1100 °C. Using 20% GDC NPs processed at 400 °C, a typical value of apparent degradation rate constant (k) of 2.13 × 10⁻² min⁻¹ was obtained for methyl orange (MO) dye under UV–visible irradiation for 10 min.
Thesis
Synthesis of functional nanomaterial thin films using a scalable flame combustion technique called Reactive Spray Deposition Technology (RSDT) was explored. Nanomaterials were used as sensing film for local gas monitoring and human breath analysis for medical diagnosis (different phases of WO3) and catalysts for water-gas shift (WGS) reaction (Pt supported on ceria). Areas of application include: handheld portable devices for immediate breath composition monitoring, medical diagnosis, and environment monitoring (workplace, residence and automobile). Two case studies will be explained in detail: (1) acetone sensing in human breath for blood glucose monitoring and (2) NO2 sensing for air quality monitoring. A study of the RSDT synthesis technique and control of crystal structure, porosity, and nanoparticle size will be demonstrated. The detailed study of acetone and NO2 sensing mechanism will be explained in detail, including sensor performance and stability testing.
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Thin films of RuxIr1-xO2-y have been deposited by a dry combustion process directly onto polypropylene and quartz coupons for characterization and on a gold disk to evaluate the electrochemical suitability as an oxygen evolution electrode. Compositional analysis on the oxygen evolution catalyst suggests a chemical formula of Ru0.41Ir0.59O2-y. Bulk diffraction of the thin film Ru0.41Ir0.59O2-y suggests phases of IrO2 and RuO2 and to a lesser extent Ru metal with no Ir metal. The photoelectron emission spectrum of the film suggests that Ru exists primarily in the oxidized state. The integrated area of the Ir metal peaks is an order of magnitude larger than the RuO2 peaks and 6 times larger than the corresponding oxide suggesting a surface enriched in Ir metal in contrast to the diffraction findings. An exceptional oxygen evolution current of 25-40 mA/cm2 was observed for a Ru 0.41Ir0.59O2-y thin film supported on a gold rotating disk electrode in a 0.5 M H2SO4 electrolyte measured at a potential of 1.6 V. The corresponding mass activity of the electrode is exceptionally high at 400 mA/mg of Ru0.5Ir 0.5O2 compared to 50-75 mA/mg of Ru0.5Ir 0.5O2 from the literature.
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Gestational diabetes mellitus (GDM) is a glucose tolerance disorder that occurs or is diagnosed for the first time during pregnancy. Perinatal morbidity is more and women with GDM have more risk of developing diabetes. Uttar Pradesh is a state of India with one of the highest rate of infant as well as maternal mortality which might be, at least partially, due to GDM. Thus, appropriate assessment and management of GDM can improve the outcomes. Aims & Objectives: Primary objective of this study was to determine the prevalence of GDM and evaluate the maternal and fetal outcome in and around Kanpur. Thus, this study was undertaken to know the extent of burden on the healthcare, before scope of intervention could be defined. Materials and Methods: A prospective study (September, 2012 - October, 2014) was done at 198 healthcare facilities. 24,656 mothers were screened (24th- 28th weeks of pregnancy) as per guidelines of Diabetes in Pregnancy Study Group India (DIPSI) and Federation of Obstetric and Gynecological Societies of India (FOGSI). Results: > 94% pregnant women did not know about GDM. Prevalence of GDM was 14.42%. Stillbirth, Perinatal & neonatal mortality were respectively 2, 3.3 & 6 times higher in GDM. Most of the GDM were diagnosed in primigravida (62%). Congenital Malformation was 8 times higher. Low Birth Weight (LBW) was 35% in GDM (16% in Non GDM). GDM positive cases had 20.6% positive family history of diabetes (compared to 6.5% in non-GDM). Relative risks for PBU (post birth unit), LGA (large for gestational age), LBW (low birth weight), pre-eclampsia and jaundice were also higher. Conclusion: A well predictive screening criteria is needed. As the ignorance about GDM among pregnant ladies is high, to reduce the risk, awareness can be an area of thrust. Key Words: Pregnancy; Gestational Diabetes; Perinatal Complication; Maternal Complication; 24-28 Week
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We propose a method to enhance the fuel cell efficiency with the simultaneous removal of toxic heavy metal ions. Carbon monoxide (CO), an intermediate of methanol oxidation that is primarily responsible for Pt catalyst deactivation, can be used as an in-situ reducing agent for hexavalent chromium (Cr (VI)) with reactivating the CO-poisoned Pt catalyst. Using electro-oxidation measurements, the oxidation of adsorbed CO molecules coupled with the concurrent conversion of Cr (VI) to Cr (III) was confirmed. This concept was also successfully applied to a methanol fuel cell to enhance its performance efficiency and to remove toxic Cr (VI) at the same time.
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Mesoporous oxides attract a great deal of interest in many fields, including energy, catalysis and separation, because of their tunable structural properties such as surface area, pore volume and size, and nanocrystalline walls. Here we report thermally stable, crystalline, thermally controlled monomodal pore size mesoporous materials. Generation of such materials involves the use of inverse micelles, elimination of solvent effects, minimizing the effect of water content and controlling the condensation of inorganic frameworks by NOx decomposition. Nanosize particles are formed in inverse micelles and are randomly packed to a mesoporous structure. The mesopores are created by interconnected intraparticle voids and can be tuned from 1.2 to 25 nm by controlling the nanoparticle size. Such phenomena allow the preparation of multiple phases of the same metal oxide and syntheses of materials having compositions throughout much of the periodic table, with different structures and thermal stabilities as high as 800 °C.
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The combined effects of strain and phonon confinement are seen to explain why the Raman peak near 464 cm-1 in CeO2-y nanoparticles shifts to progressively lower energies and the lineshape of this feature gets progressively broader and asymmetric (on the low-energy side) as the particle size gets smaller. The increasing lattice constant measured for decreasing particle size explains this Raman shift well. The linewidth change is fairly well explained by the inhomogenous strain broadening associated with the small dispersion in particle size and by phonon confinement. The spectra are also likely to be directly affected by the presence of oxygen vacancies. Comparison of the temperature dependence of the Raman lineshape in the nanoparticles and the bulk shows that phonon coupling is no faster in the nanoparticles, so size-dependent phonon coupling does not contribute to the large nanoparticle peak red shifts and broadening at room temperature. Irreversible thermally induced changes are observed in the Raman peak position of the nanoparticles.
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Cu-Pt bimetal catalysts supported on nanocrystalline CeO2 (nano-ceria) are synthesized via the low-cost sol-gel approach followed by impregnation processing. The average particle size of the catalytic composites is 63 nm. Ceria nanopowders sequentially impregnated in copper solution and then in Pt solution transformed into Pt-skin-structured Cu-Pt/ceria nanocomposite, based on the surface elemental and bulk compositional analyses. The ceria supporter has a fluorite structure, but the structure of Cu and Pt catalytic contents, not detected by X-ray diffraction spectroscopy due to the low loading level, is yet conclusive. The bimetallic catalytic nanocomposites may potentially serve as sulfur-tolerant anode in solid oxide fuel cells.
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Mesoporous ceria nanotubes are fabricated by the following process: ceria was deposited on the surface of ZnO nanorods by the ultrasonic assisted successive ionic layer adsorption and reaction (SILAR) method to obtain ceria/ZnO composite nanostructures, then, the ZnO core was removed by HNO3 solution, resulting in the formation of ceria nanotubes, which have the large surface area of 109 m2 g−1. Such ceria nanotubes have excellent catalytic performance in the CO → CO2 conversion, and the reaction start temperature is about 100 °C lower than that of commercial ceria (CeO2) particles. The specific catalytic activity of the ceria nanotubes is 9 times higher than that of commercial ceria particles at 320 °C. The XPS and Raman investigations have proved that the Ce3+ ion number in the surfaces is about 23% of total cerium ion number, which may be responsible for the high reduction performance to oxygen. The positron annihilation experiments prove that a large amount of vacancies and vacancy groups exist in these samples. A large amount Ce3+ formation is closely associated with these vacancies and vacancy groups. Temperature programmed reduction (H2-TPR) experiments further confirm that the enhanced activity of mesoporous ceria is in contrast to commercial ceria particles. Moreover, for ceria nanotubes the enhanced activity is attributed to enhancement of the surface activity caused by Ce3+ ions. The catalysis activity stability arises from contribution of the cycle transition between Ce3+ and Ce4+ ions.
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The effect of preparation method and pretreatment on the characteristic properties and selective CO oxidation activities of 50/50 CuO/CeO2, 25/25/50 CuO/Co3O4/CeO2 and 5/95 CuO/CeO2 catalysts was investigated. Catalysts were characterized by BET surface area measurement, N2 physisorption, SEM micrographs and X-ray diffraction. The selective oxidation reaction in H2 rich stream was carried out between the 110°C and 210°C temperatures. The 5/95 CuO/CeO2 catalysts gave the best CO conversion and selectivity result. The 5/95 CuO/CeO2 catalyst calcined at 700°C protected its activity and selectivity at 200°C during 24h.
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We have investigated the structure sensitivity of the water-gas shift (WGS) reaction on Cu–CeO2 catalysts prepared at the nanoscale by different techniques. On the surface of ceria, different CuOx structures exist. We show here that only the strongly bound Cu–[Ox]–Ce species, probably associated with the surface oxygen vacancies of ceria, are active for catalyzing the low-temperature WGS reaction. Weakly bound CuOx clusters and CuO nanoparticles are spectator species in the reaction. Isolated Cu2+ ions doping the ceria surface are not active themselves, but they are important in that they create oxygen vacancies and can be used as a reservoir of copper to replenish surface Cu removed by leaching or sintering. Accordingly, synthesis techniques such as coprecipitation that allow for extensive solubility of Cu in ceria should be preferred over impregnation, deposition–precipitation, ion exchange or another two-step method whereby the copper precursor is added to already made ceria nanocrystals. For the synthesis of different structures, we have used two methods: a homogeneous coprecipitation (CP), involving hexamethylenetetramine as the precipitating agent and the pH buffer; and a deposition–precipitation (DP) technique. In the latter case, the ceria supports were first synthesized at the nanoscale with different shapes (rods, cubes) to investigate any potential shape effect on the reaction. Cu–CeO2 catalysts with different copper contents up to ca. 20at.% were prepared. An indirect shape effect of CeO2, manifested by the propensity to form oxygen vacancies and strongly bind copper in the active form, was established; i.e. the water-gas shift reaction is not structure-sensitive. The apparent activation energy of the reaction on all samples was similar, 50±10kJ/mol, in a product-free (2% CO–10% H2O) gas mixture.
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Abstract Single stage water–gas shift reaction has been carried out at a gas hourly space velocity of 45,515 h−1 over supported Pt catalysts prepared by an incipient wetness impregnation method. CeO2, ZrO2, MgO, MgO–Al2O3 (MgO = 30 wt%) and Al2O3 were employed as supports for the target reaction in this study. 1 wt% Pt/CeO2 exhibited the highest CO conversion as well as very high CO2 selectivity due to high oxygen storage capacity of CeO2. Graphical Abstract
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A study was conducted to analysis the crystallite size of supported platinum catalysts by X-Ray diffraction (XRD) with transmission electron microscopy (TEM) using fuel cell electrocatalysts and cyclic voltammetry. Fresh or aged samples with catalytic activity was used to obtained the crystal/particle size information. The study calculated the average Pt crystallite size from XRD peak broadening using the Scherrer equation. The study used one non-Fourier transform method to determination of metric and microstructural parameters and Rietveld method was used to describe the measured X-ray pattern. The line of pattern was described by a split pseudo-Voigt function in terms of line width and Lorentzian fraction. The study analyzed the effects of Pt particles shape and microstructure, and found that the XRD is suitable for particles over TEM digital processing.
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Ceria is a crucial component of automotive catalysts, where its ability to be reduced and re-oxidized provides oxygen storage capacity. Because of these redox properties, ceria can greatly enhance catalytic activities for a number of important reactions when it is used as a support for transition metals. For reactions that use steam as an oxidant (e.g., the water-gas-shift reaction and steam reforming of hydrocarbons), rates for ceria-supported metals can be several orders of magnitude higher than that for ceria or the transition metal alone. Because the redox properties of ceria are strongly dependent on treatment history and the presence of additives, there are significant opportunities for modifying catalysts based on ceria to further improve their performance. This article will review some of the contributions from my laboratory on understanding and using ceria in these applications.
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Ceria–zirconias are popular catalysts and supports for metal particles. Even without supported metal particles, these materials are active for hydrogenation and hydrodeoxygenation reactions, where oxygen vacancies serve as the active site. To gain a detailed understanding of the ability of ceria-based catalysts to dissociatively adsorb hydrogen, H2–D2 exchange is studied as a test reaction. The density of exchangeable hydrogen per surface area approaches the values for typical metal surfaces. Below 250 °C, H2–D2 exchange occurs at oxygen vacancies with an activation energy of ca. 24 kJ mol−1. At higher temperatures, additional sites at the edges of ceria–zirconia crystallites contribute to the reaction. The kinetics and the density of active sites for dissociative adsorption of hydrogen are correlated with physicochemical properties of the catalysts. Specifically, the crystallite size has a strong influence on the reactivity of different samples.
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Judicious selection of mixed ionic–electronic conducting (MIEC) perovskite oxide as oxygen transport membrane (OTM) offers the potential to enhance overall process economics and systems performance for a wide variety of industrial applications ranging from clean and efficient energy conversion (oxy-combustion) to selective gas separation (high purity oxygen production) and value added chemicals (syngas and liquid fuel) production with near-zero greenhouse gas emissions. Doped lanthanum chromite perovskites have been considered as promising material of choice for oxygen transport membrane (OTM) due to their superior thermo-chemical stability in aggressive environment (800–1000 °C, 0.21–10−20 ) than the other mixed ionic–electronic conducting (MIEC) perovskites such as ferrites and cobaltite's. Thermo-physical properties of the lanthanum chromite, required for optimum oxygen transport can be tuned by modifying the crystal structure, chemical bonding, and ionic and electronic transport properties through selection of dopant's type and level. A perspective on the development of lanthanum chromite-based oxygen transport membranes is presented with an insight based on the pertinent literature and data analysis. The role of various A- and B-site dopants on the crystal structure, densification, thermal expansion, electrical transport, oxygen permeation, mechanical properties, and thermochemical stability of lanthanum chromite is discussed to enlighten ‘composition–structure–property’ correlations. It has been found that: the preferred dopants are strontium at A-site and manganese, nickel, iron, and titanium at B-site to obtain the desired thermo–chemo–electro–mechano properties. Challenges for long term performance and structural stability of doped lanthanum chromite as an oxygen transport membrane are outlined for the applications under ‘real system’ exposure conditions.
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The redox and associative carboxyl pathways of the water–gas shift reaction have been investigated at a corner Pt site of the Pt/TiO2 (110) interface using density functional theory and microkinetic modeling techniques. Overall rates calculated from the microkinetic model suggest that the redox pathway dominates in the temperature range of 473–673 K and that the oxygen vacancy structure plays a critical role in dissociating H2O. Because of the strong adsorption of CO at the corner Pt atoms, these sites are less active than the edge Pt sites at low temperatures; however, the activity of corner atoms becomes higher above 573 K. The CO adsorption strength and the ability to dissociate H2O are the two main factors that determine the activity of a particular site or catalyst for the water–gas shift reaction.
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Ceria doped with divalent or trivalent cations is a mixed conductor; conduction occurs predominantly by the motion of oxygen vacancies or by electrons, depending on the departure from stoichiometry. In order to establish the electrolytic domain at which behaves primarily as an ionic conductor with transference number , a careful study was made of the conductivity, σ, as a function of temperature and oxygen partial pressure, covering the range of 10⁰;–10⁻²² atm in small steps. From these data the electrolytic domain was determined; it extends to about 10⁻¹³ atm at 600°C. When compared to calcia‐stabilized zirconia (CSZ), doped ceria shows a higher conductivity, lower activation energy (0.76 eV) for anion vacancy migration, and absence of polarization effects to lower temperatures. These results indicate that doped ceria may be an attractive candidate for fuel cells and other applications at temperatures below those at which CSZ is useful.
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The importance of the exposed interface between Au and the support metal oxide has been suggested in the literature. Here we summarize evidence that provide strong support as well as conflicting evidence for this type of active sites and the potential reaction mechanism that takes place. Evidence from three reactions are summarized: CO oxidation, water gas shift (WGS) reaction, and selective oxidation of propane. Implications for potential future research and ability to truly design selective catalysts are discussed.
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Ceria has been prepared by flame spray pyrolysis and tested for activity in catalytic soot oxidation. In tight contact with soot the oxidation activity (measured in terms of the temperature of maximal oxidation rate, Tmax) of the flame made ceria is among the highest reported for CeO2. This can to a significant degree be ascribed to the large surface area achieved with the flame spray pyrolysis method. The importance of the inherent soot reactivity for the catalytic oxidation was studied using various soot samples, and the reactivity of the soot was found to have a significant impact, as the Tmax-value for oxidation in tight contact with a catalyst scaled linearly with the Tmax-value in non-catalytic soot oxidation. The Tmax-value in non-catalytic soot oxidation was in turn observed to scale linearly with the H/C ratio of the carbonaceous materials. Graphical Abstract
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A versatile synthetic method, which is based on a low-temperature hydrothermal technique, is developed for the fabrication of a microporous ZnO/TiO2 composite catalyst with different structures (e.g., amorphous, amorphous/crystalline and crystalline). In particular, a novel microporous ZnO/TiO2 composite with amorphous/crystalline structure is obtained with a 3/1 M ratio of Ti/Zn. This novel ZnO/TiO2 composite heterostructure not only has a large specific surface area (311.9 m2 g-1) but also exhibits outstanding performance during solar water splitting reactions to generate hydrogen without a noble metal co-catalyst. Based on our in-depth mechanistic analysis, the synergistic effect between the amorphous ZnO and crystalline TiO2 is responsible for the enhanced performance of this material.
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Reactive Spray Deposition Technology (RSDT) was employed to synthesize 1 wt% Pt of 0.5–2 nm onto ceria of 8–30 nm. The catalyst was evaluated for water–gas shift (WGS) reaction with 1 vol% CO & 3 vol% H2O, atmospheric pressure, temperature range (100–350 °C) and gas hourly space velocity (GHSV) of 8622 h−1. CO conversion of 15% (150 °C), 18% (200 °C), 37% (225 °C) and 100% (250 °C) was observed. Comparison with conventionally prepared catalysts (sol–gel, co–precipitation, and incipient wetness impregnation) from literature revealed superior activity with RSDT synthesized catalysts. Catalyst morphology was investigated with TGA, ICP–OES, XRD, TPR, HRTEM, and SEM with XEDS. No evidence of sintering or agglomeration of Pt nanoparticles was observed in HRTEM which could account for the dramatic improvement in activity.
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The substitution of fossil fuels by renewable energy sources is needed to decrease greenhouse gas emissions, especially CO 2 . Wind and solar power are today considered as attractive alternatives for electric power generation, but are not suitable for providing base load. Thus, efficient storage of electrical energy is inevitable. Liquid hydrocarbons (HCs) exhibit an excellent volumetric energy density and offer various opportunities for storing electric energy. They can be produced by CO 2 and renewable H 2 (generated by water electrolysis) in a two step process. The first step is generation of syngas by reverse water-gas shift (RWGS) at elevated temperatures; the second step comprises the production of liquid hydrocarbons by Fischer-Tropsch (FT) synthesis. The experiments on RWGS with a commercial Ni-catalyst show that a CO 2 conversion of around 80 % can be reached at 800 °C within a very short residence time of less than < 0.1 s. The experiments on FTS with Fe as catalyst and syngas containing different amounts of CO 2 indicate that the influence of CO 2 on CO conversion and product selectivities (including net CO 2 production by water-gas shift) is insignificant if the inlet partial pressures of H 2 and CO are kept constant. If CO is substituted by CO 2 , less HCs are formed, the water-gas shift is repressed, and methane selectivity increases.
Article
Recent advancements in the field of electron microscopy, such as aberration correctors, have now been integrated into Environmental Transmission Electron Microscopes (TEMs), making it possible to study the behavior of supported metal catalysts under operating conditions at atomic resolution. Here, we focus on in situ electron microscopy studies of catalysts that shed light on the mechanistic aspects of catalyst sintering. Catalyst sintering is an important mechanism for activity loss, especially for catalysts that operate at elevated temperatures. Literature from the past decade is reviewed along with our recent in situ TEM studies on the sintering of Ni/MgAl2O4 catalysts. These results suggest that the rapid loss of catalyst activity in the earliest stages of catalyst sintering could result from Ostwald ripening rather than through particle migration and coalescence. The smallest particles are found to disappear in a few seconds as soon as the catalyst reaches the operating temperature. While particle migration and coalescence is evident in some of these in situ studies, it does not follow the classical model where the smallest particles are most mobile. Deterministic models of Ostwald ripening as well as atomistic Monte Carlo simulations are both in good agreement with these experimental observations, predicting a steep loss in catalyst activity at short times on stream. The in situ studies show the importance of direct observations to deduce mechanisms and show the important role played by the support and the gas atmosphere (especially the presence of H2O) on the rates of catalyst sintering.
Article
The mechanism of water–gas shift reaction at the three-phase boundary of Pt/CeO2 catalysts has been investigated using density functional theory and microkinetic modeling to better understand the importance of metal–oxide interface sites in heterogeneous catalysis. Analysis of a microkinetic model based on parameters obtained from first principles suggests that both the “Redox pathway” and the “Associative carboxyl pathway with redox regeneration” could operate on Pt/CeO2 catalysts. Although (1) only few interfacial Pt atoms are found to be catalytically active at low temperatures due to strong adsorption of CO and (2) interfacial O–H bond breakage is difficult due to the high reducibility of ceria, interface sites are 2–3 orders of magnitude more active than Pt (1 1 1) and stepped Pt surface sites and therefore effectively determine the overall activity of Pt/CeO2. The high activity of Pt/CeO2 interface sites originates from a significantly enhanced water activation and dissociation at interfacial oxygen vacancies.
Article
We synthesized dense CeO ${2 - x}$ polycrystals of ∼10 nm grain size and characterized their electrical conductivity, in order to determine whether the defect properties of nanocrystalline solids fundamentally differ from those of conventional materials. The nanocrystals exhibit enhanced electronic conductivity, greatly reduced grain boundary impedance, and a heat of reduction more than 2.4 eV lower per oxygen vacancy compared to their coarse-grained counterparts. We propose that defect formation at low energy grain boundary sites is responsible for these properties, and that nanocrystalline oxides represent bulk materials possessing the defect thermodynamics of interfaces.
Article
Fischer–Tropsch synthesis (FTS) process aims at converting synthesis gas to liquid fuels. Due to high activity and long catalyst life, cobalt-based catalyst is currently the catalyst of choice for gas to liquid (GTL) technology. Water is most undesirable byproduct of FTS process. Due to low water–gas-shift (WGS) activity of cobalt-based catalyst, the water concentration rises with time-on-stream (TOS) in FTS. This paper reviews the effects of water on the performances of various cobalt catalysts for FTS. The effects of water on FTS is quite complex and depends on the support and its nature, Co metal loading, its promotion with noble metals, and preparation procedure. Added water up to certain concentrations has positive effects (in terms of higher CO conversion, C5+ selectivity, olefin selectivity and lower methane and CO2 selectivity) on unsupported cobalt oxide catalysts. If the effects of support are taken into account, water has positive effect for silica-supported catalysts. The effects are negative for alumina where as for titania support, water has little positive effect. However in general, oxidation of cobalt active site depending on the cluster size and water partial pressure, the removal of transport restrictions via the formation of water-rich intra-pellet liquids, and kinetic effects have been considered as the main responsible factors. The effects are strongly influenced by the cobalt cluster size as well as on pore size of the support. Addition of noble metals at low cobalt loading increases the dispersion of cobalt on the support and hence improves its activity. Higher cobalt dispersion enhances the negative impact of water especially at higher water partial pressures under FTS conditions.
Article
The interaction between CO and Pt/ceria catalysts was investigated by oxygen storage capacity measurements, CO and CO2 isotope exchange and FT-IR measurements. A Pt/alumina sample was also investigated for comparison. The capacity to store and release oxygen as a function of ceria surface-area and Pt–ceria contact was estimated. The presence of chlorine from the Pt pre-cursor was found not to decrease the oxygen storage capacity. However, FT-IR measurements showed that chlorine hindered the carbonate formation during CO exposure, which was found to be the main form of carbon storage on the pre-oxidized sample. On a strongly reduced sample, CO disproportionation was found to occur to an increasing extent with increasing degree of reduction. CO and CO2 isotope exchange showed that these molecules exchange their oxygen with Pt/ceria and ceria quite easily at 400°C. CO chemisorption at −78°C as a method to determine the metal dispersion was also investigated. At this temperature, the CO uptake on ceria was strongly suppressed, especially on the Cl-free sample, but not completely hindered.
Article
Adsorption and reaction of formic acid on stoichiometric CeO2(111), partially reduced CeO2-x, and Pt/CeO2(111) films prepared on Cu(111) were studied by means of synchrotron radiation photoelectron spectroscopy (SRPES), resonant photoemission spectroscopy (APES), infrared reflection absorption spectroscopy (IRAS), and temperature-programmed desorption (TPD). On all studied samples, the principal species formed during formic acid adsorption below 160 K were formate and molecular formic acid. In the presence of Pt particles, formate species were predominantly localized on Pt at 100 K, and on ceria at or above 300 K. Below 400 K, molecular formic acid decomposes to formate with partial release of CO2, CO, hydrogen, and water. Analysis of the TPD fragmentation suggests additional evolution of methane. Above 400 K, desorption of CO2, CO, hydrogen, and water is observed. This process is controlled by the stoichiometry of ceria and the presence of Pt particles. In particular, desorption of CO2 is suppressed on CeO2-x but is enhanced on Pt/CeO2. APES suggests that the reaction of formic acid does not alter the oxidation state of cerium cations on CeO2(111). By contrast, we observed significant reoxidation on partially reduced CeO2-x, between 250 and 400 K, followed by reduction between 400 and 500 K.
Article
The platinum-encapsulated zeolitically microcapsular catalyst, associated with the immobilized Candida antartica lipase B (Novozyme®435), is successfully employed in the dynamic kinetic resolution of phenylethylamine. A conversion of 80% and a selectivity of 95% are achieved, and negligible loss of activity is detected even after reaction of 5 runs. It is found that the existence of the silicalite-1 shell not only effectively prevents the deactivation of both enzyme and Pt by isolating them in different regions of reaction system, but also significantly reduces the formation of by-products on the Pt nanoparticles within the protected space of zeolitic microcapsule. Such features of zeolitic shell should further promote the designing of various catalysts for multistep reaction network.
Article
The photocatalytic reaction of nitric oxide (NO) on TiO 2 and transition metal-loaded M (Cu, V, and Cr)/TiO 2 catalysts was studied using in situ FTIR spectroscopy under UV irradiation. TiO 2 and M/TiO 2 catalysts were prepared by the sol–gel method via controlled hydrolysis of titanium (IV) butoxide. Copper, vanadium, or chromium was loaded onto TiO 2 during the sol–gel procedure. After treatment at 500 • C under air flow, a large amount of surface peroxo species and OH groups were detected on the TiO 2 and M/TiO 2 catalysts. Nitric oxide was adsorbed on TiO 2 and M/TiO 2 in the form of bidentate nitrites and nitrates by reacting with OH groups, peroxo, or M=O species. In addition, NO can also be adsorbed on M n+ in the form of nitrosyls. Under UV irradiation, bidentate nitrite was oxidized to either monodentate or bidentate nitrate. Such oxidation was suggested to be induced by superoxo species generated by oxidizing peroxo species via photogenerated holes. The existence of nitrosyls deferred the oxidation of nitrites to nitrates due to the prior oxidation of nitrosyls by superoxo. The XRD and UV–vis spectra showed that the structures and the abilities of absorbing UV light of all catalysts were not influenced by the photocatalytic NO reaction. Possible mechanisms were proposed for the photocatalytic NO oxidation on TiO 2 and M/TiO 2 based on the intermediates found from the in situ FTIR study.
Article
A facile hydrothermal synthesis of tin doped ceria-zirconia (Ce0.5Zr0.43Sn0.07O2) solid solutions was carried out using Ce(NH4)2(NO3)6, Zr(NO3)3·2H2O and SnCl4·5H2O as the starting materials. The synthesized Ce0.5Zr0.43Sn0.07O2 particles were characterized for their oxygen storage capacity (OSC) for automotive catalysis applications. For the characterization, X-ray diffraction, transmission electron microscopy and the Brunauer-Emmet-Teller (BET) technique were employed. The OSC values of all samples were measured using thermogravimetric-differential thermal analysis. Ce0.5Zr0.43Sn0.07O2 solid solutions with a BET surface area of 246 m2 g−1 exhibited a considerably high OSC of 1425 μmol-O g−1. The incorporation of tin ions in the lattice of the ceria based catalyst greatly enhanced the thermal stability and OSC. The influence of cation radius on the thermal stability and OSC was also discussed.
Article
A series of Ce–Fe mixed oxides as well as the pure oxides were synthesized by the microemulsions method. The solid solution formation was established for all the Fe-doped systems and only a hardly noticeable segregation of α-Fe2O3 was appreciated for the solid with the maximum iron content (50at.% Fe). The oxygen exchange is improved for all the Fe-doped systems; however the 10at.% Fe appears as the optimal iron content for achieving the maximum oxygen vacancies concentration and the higher reducibility efficiency. The CO oxidation (TOX, PROX) is especially achieved for the solids with the lower iron contents but with a superior oxygen vacancies proportion. These Ce–Fe systems prepared from microemulsions are very attractive to be considered as supports for depositing active phases capable of enhancing oxygen exchange ability of the whole system, allowing higher CO oxidation abilities.
Article
Interactions between ceria (CeO2) and supported metals greatly enhance rates for a number of important reactions. However, direct relationships between structure and function in these catalysts have been difficult to extract because the samples studied were either heterogeneous or were model systems dissimilar to working catalysts. Here, we report rate measurements on samples in which the length of the ceria-metal interface is tailored by the use of monodisperse Ni, Pd, and Pt nanocrystals (NCs). We demonstrate unambiguously that CO oxidation in ceria-based catalysts is greatly enhanced at the ceria-metal interface sites for a range of group VIII metal catalysts, clarifying the pivotal role played by the support.
Article
Reduced sintering temperature of doped ceria can greatly simplify the fabrication process of solid oxide fuel cells (SOFCs) by utilizing the co-firing of all cell components with a single step. In the present study, nano-crystalline gadolinium doped ceria (GDC) powders of high sinterability at lower sintering temperature has been synthesized by co-precipitation at room temperature. The successful synthesis of nano-crystalline GDC was confirmed by XRD, TEM and Raman spectroscopy analysis. Dilatometry studies showed that GDC prepared by this method can be fully densified (97% relative density) at a sintering temperature of 950°C which is fairly lower than ever before. It has also been found that the sintered samples have a higher ionic conductivity of 1.64×10−2Scm−1 at 600°C which is suitable for the intermediate temperature SOFC application.
Article
Ceria and doped ceria nanopowders synthesized in flames from submicrometer-sized droplets of precursor solutions demonstrate several characteristics that promise superior performance when used to fabricate electrolytes for solid oxide fuel cells relative to powders synthesized using other technologies. Combustion chemical vapor condensation has been used to produce CeO2, Ce0.8Sm0.2O2-x (SDC), and Ce0.8Gd0.2O2-x (GDC) nanopowders, with an average particle size below 20 nm. These nanopowders have been used to fabricate electrolyte test specimens. The effect of particle size, dopant, and sintering temperature on electrolyte morphology, density, and ac conductivity have been measured. Sintering of pressed nanopowder specimens at temperatures below 1150°C resulted in electrolytes with densities higher than 99% of theoretical density. The resulting sintered grain sizes range from 150 to 500 nm for CeO2 and GDC, and from 150 to 300 nm for SDC. AC impedance measurements show conductivities of 8 and 10 S/m at 800°C for SDC and GDC electrolytes, respectively. Conductivities at 500°C approached 2 S/m for both SDC and GDC.
Article
The mechanism responsible for the irreversible deactivation of ceria-supported precious metals for the water–gas-shift reaction has been investigated through accelerated aging tests. It is shown that deactivation of Pd/ceria occurs more rapidly at 673 than 523 K when operating with an integral reactor in 25 Torr each of CO and H2O. By heating a fresh catalyst in H2, H2O, CO, or CO2, it was shown that deactivation occurs due to the presence of CO. Measurements of metal dispersion by CO adsorption and by X-ray diffraction show that deactivation on Pt/ceria and Pd/ceria catalysts in our studies was due to a loss of metal surface area. Finally, water–gas-shift rates on a series Pd/ceria catalysts with ceria crystallite sizes ranging from 7.2 to 40 nm and Pd loadings of either 1 or 6 wt% demonstrated that rates were strictly proportional to the Pd surface area. Based on these observations, ceria–supported precious metals could be active and stable for the water–gas-shift reaction if steps are taken to avoid metal particle size growth.
Article
A series of Rh/SiO2 samples promoted with CeO2 have been prepared and used as catalyst in CO2 methanation. Their behaviour has been compared to that of unpromoted Rh/SiO2 and Rh/CeO2 catalysts. The activity has been monitored under transient and steady-state conditions and the catalysts have been characterized using quantitative temperature programmed reduction, chemisorption, X-ray diffraction, and transmission electron microscopy. By impregnation of amorphous silica with Ce(NO3)3. 6H2O followed by calcination at 923 K, aggregates of CeO2 particles with size ranging from 35 to 150 nm (depending on initial cerium loading) form on the surface. These large agglomerates are constituted by individual, smaller (5-15 nm), crystalline CeO2 particles. Redispersion of CeO2 in the presence of H2 at 773 K is observed in all samples. The presence of Rh, deposited by impregnation from RhCl3.3H2O solutions, accelerates the process. The reducibility of ceria is strongly enhanced by deposition on silica: complete reduction to Ce2O3 is observed for CeO2-supported samples at temperatures lower than 1100 K, while a maximum of 50% reduction (corresponding to CeO1.75) is observed for unsupported CeO2 in the range of temperature 295-1400 K. The activity of the virgin catalyst, as tested under unsteady-state conditions, is positively influenced by the reduction temperature and CeO2 crystallite size. We suggest that the observed enhancement of catalytic activity is linked to the presence of bulk vacancies created on ceria after reduction at high temperatures. Annihilation of these oxygen vacancies by oxygen from CO2, under reaction conditions, restores the normal catalytic behaviour.
Article
The addition of ceria, CeO{sub 2}, to automotive exhaust-gas catalysts has been linked to greater thermal stability of the alumina support, higher precious-metal dispersion, and wider range of parameters for three-way operation. Some, if not all, of these beneficial effects are undoubtedly influenced by the dispersion of the ceria, which is typically added at the 5-10 wt% (including the monolith) level. Methods of measuring its dispersion, or particle size, are thus important tools for studying how ceria functions. Traditionally, either transmission electron microscopy (TEM) or X-ray diffraction (XRD) have been applied to the problem of particle size determination in catalysts. In this report, the authors show how Raman spectroscopy may be used to determine the size of CeO{sub 2} particles in a way that is easier than TEM and faster than XRD.
Article
Reactant-promoted reaction mechanism for catalytic water-gas shift reaction (WGSR) on Rh-doped CeO[sub 2], including the activation of intermediate by a reactant, has been investigated by FT-IR. The intermediate of WGSR on Rh/CeO[sub 2] is bidentate formate produced from CO and terminal (on-top) surface OH groups on Ce ions: 65% of the surface formates decomposed to -OH + CO (backward decomposition) and only 35% of them decomposed to H[sub 2] + CO[sub 2] (forward decomposition) in vacuum. In the coexistence of second-adsorbed water molecules, almost 100% of the formate decomposed to H[sub 2] + CO[sub 2]. Besides the direct change of decomposition selectivity, the rate constant of the forward decomposition and the activation energy were surprisingly affected by water coadsorption. The rate constant increased one hundred times and the activation energy decreased from 55.6 to 33.3 kJ mol[sup [minus]1]. The rate-determining step for catalytic WGSR on Rh/CeO[sub 2] is the decomposition of the bidentate formate to H[sub 2] and unidentate carbonate. The desorption of CO[sub 2] by the decomposition of unidentate carbonate was also promoted by H[sub 2]O coexistence. The phonomena observed on Rh/CeO[sub 2] are entirely different from those on pure CeO[sub 2] and similar to the case of ZnO surface. The reactant-promoted reaction mechanism for WGSR is illustrated. 10 refs., 9 figs., 4 tabs.
Article
Mesoporous solid superbases have high potentials for applications as environmentally benign catalysts in diverse reactions, whereas their preparation remains a challenge. In the present study, an in situ functionalization strategy was designed to fabricate mesoporous solid superbases based on the hard-templating synthetic system of mesoporous ceria. Sodium and potassium modified mesoporous ceria (NaMC and KMC) were successfully prepared, and structural and basic properties were characterized by various methods. The obtained materials exhibit well-expressed mesostructure and superbasicity with a high strength of 27.0. The basic solutions (namely NaOH and KOH aqueous solutions) play a double role by removing the silica template SBA-15 and functioning as the guests. Therefore, sodium and potassium can be coated onto mesoporous ceria formed in situ. This strategy allows the fabrication and functionalization of mesoporous ceria in one step, which avoids the destruction of mesoporous ceria in post-modification. The obtained mesoporous solid superbases also exhibit excellent catalytic performance in dimethyl carbonate (DMC) synthesis. The yield of DMC can reach 64.6% in 4h over NaMC, which is apparently higher than those over the samples derived from post-modification (28.9%), nonmesoporous ceria (10.8%), as well as the conventional solid base magnesia (7.6%).
Article
Fuel cells and its fuel processor will be frequently shutdown and re-started depending on the power demand. Deactivation of Pt/CeO2 water-gas shift catalysts under conditions simulating shutdown was observed and studied using a combination of techniques such as Fourier-transform infrared (FT-IR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), CO chemisorption and catalytic activity test. Results show that formation of carbonates occurs on the catalyst surface as a consequence of shutdown in reformate, leading to deactivation. The carbonates originating from both CO and CO2 cover not only the surface of CeO2 but also the Pt metal surface and exert an effect on its electronic properties. It is found that a correlation exists between the content of carbonates and the degree of deactivation. Regeneration by air treatment removes the carbonates and restores activity. Pt metal sintering is not found to be a factor contributing to the catalyst deactivation under the shutdown and re-startup conditions.
Article
of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007