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Catalytic reactivity of face centered cubic PdZn α for the steam reforming of methanol
... 12 Pd-based catalysts exhibit better thermal stability than Cu-based ones with a comparable CO 2 selectivity of ≥98%. 13 The CO 2 selectivity of Pd/ZnO catalysts is attributed to the intermetallic compound (IMC) ZnPd that forms upon reductive pretreatment at around 400°C. 7,8 A recent review addresses the role of the intermetallic compound ZnPd in MSR. 3 Studies on unsupported ZnPd reveal a strong dependence of the CO 2 selectivity on the chemical composition of the compound. ...
... The formation of ZnPd is typically identified by XRD reflections at 2θ of 41.2°and 44.2°in the case of Cu radiation. 13,40 Thus, the formation of ZnPd (also elemental Pd and Zn as well as ZnO) in substituted LaCo 1−x−y Pd x Zn y O 3±δ catalysts during TPR is carefully analyzed. However, the identification of ZnPd reflections is not straightforward from the XRD patterns of the materials due to reflections arising from other phases derived from the perovskite-type oxides such as oxygendeficient perovskite-type oxides, Co and La 2 O 3 that overlap at the same 2θ angles. ...
... The activity data reflect the literature, concluding that elemental Pd is active but not selective for the MSR reaction, 43 while ZnPd is more selective. 13,44 It is also suggested that complete alloy formation is not a prerequisite for achieving high CO 2 selectivity, but elimination of small monometallic Pd particles (<2 nm) formed during reductive pretreatment is essential. 45 In line with these observations, the CO 2 selectivity profile of the LCPZO-1-0.15 ...
Methanol steam reforming (MSR) catalysts are derived from perovskite-type oxides LaCo1-x-yPdxZnyO3±δ by reductive pretreatment. The unsubstituted LaCoO3±δ (LCO) and LaCo1-x-yPdxZnyO3±δ (Co substituted with Pd and/or Zn) are synthesized by a citrate method and characterized by different techniques. The perovskite-type oxides exhibit a rhombohedral crystal structure and a comparable surface area (≈8.5 (±2) m2 g-1). The temperature-programmed reduction (TPR) shows low (100°C < T < 450°C) and high (T > 450°C) temperature reduction events that correspond to partial and complete reduction of the non-rare-earth metal ions, respectively. At high temperatures, Pd-Zn alloy nanoparticles are formed exclusively on Pd-and Zn-containing LaCo1-x-yPdxZnyO3±δ, as evident from high angular annular dark-field scanning transmission electron microscopy (HAADF-STEM). The CO2-selective MSR performance of the catalysts strongly depends on the reductive pretreatment temperature, catalyst composition (i.e., the Pd:Zn molar ratio and the degree of Co substitution) and reaction temperature. Only LaCo1-x-yPdxZnyO3±δ catalysts show a low-temperature CO2 selectivity maximum between 225 and 250°C, while all catalysts present similar high-temperature selectivity maxima at T > 400°C. The former is missing on LCO, LaCo1-xPdxO3±δ or LaCo1-yZnyO3±δ. Pd-Zn nanoparticles facilitate Zn(OH)2 and Co(OH)2 formation exclusively on LaCo1-x-yPdxZnyO3±δ, as evident from in situ XRD under steam atmosphere. This indicates the important role of Pd-Zn nanoparticles in the low-temperature CO2 selectivity, which is improved from 0 to 76% at 225°C on LCO and LaCo0.75Pd0.125Zn0.125O3±δ, respectively. The high-temperature CO2 selectivity is governed by the bulk catalyst composition and the occurrence of reverse water gas shift reaction.
... Further details on the characterization of the a and b-PdZn phases have been described elsewhere. 20,21 Pure palladium metal was obtained from Sigma Aldrich and used as a standard benchmark for the CO oxidation test reactions. The reference Pd powder (Sigma Aldrich CAS #7440-05-3) was calcined in air at 350 1C for 3 hours, then reduced at 500 1C in flowing 5% H 2 for 4 hours. ...
... As shown in Table 1, the desorption temperature of CO on Pd metal powder was 530 K, on a-PdZn was 450 K and on b-PdZn was 430 K. Temperature programmed desorption (TPD) results for adsorbed CO on the three powder samples were consistent with previous studies. 21 The TPD results are also consistent with the DFT calculations (vide infra) and indicate a weakening of the CO binding with the addition of Zn to Pd. The total amount of adsorbed CO also diminishes significantly on the b sample, observed as the reduction of area under the CO-TPD curve. ...
... The total amount of adsorbed CO also diminishes significantly on the b sample, observed as the reduction of area under the CO-TPD curve. 21 In our previous work, it was found that when CO oxidation reaction was initiated on the PdZn samples, there was a transient period where the activity decayed with time. Therefore, we first performed experiments under isothermal conditions to investigate the deactivation behavior. ...
The effect of Zn on the CO adsorption and oxidation reaction is examined experimentally and theoretically on two PdZn catalysts with different compositions, namely the intermetallic 1 : 1 β-PdZn and α-PdZn as a solid solution of 9 at% Zn in Pd. These bimetallic catalysts, made using an aerosol derived method, are homogeneous in phase and composition so that the measured reactivity excludes support effects. Both specific reactivities for CO oxidation on these two PdZn catalysts were measured. It was found that the initial rates are high and different between these catalysts, presumably due to the weakening of the CO adsorption and easier binding of oxygen to Pd sites modified by Zn. However, the rates decrease with time and become comparable to that on Pd at the steady state. With the help of density functional theory, it was suggested that the transient kinetics are due to the oxidation of Zn during the catalysis, which yields pure Pd where the reaction takes place.
... Formate species were observed on the Pd/ZnO catalyst surface in methanol steam reforming [7] and play a role of reaction intermediates or spectators. Formic acid and formates could be obtained in this reaction by hydroxylation/hydrolysis of formaldehyde and methylformate intermediates [1,14]. Jeroro and Vohs [15] studied interaction of formic acid with Zn/Pd(111) surfaces by temperature programmed reduction (TPR) and high resolution electron energy loss spectroscopy (HREELS), while Yuan et al. [16] investigated the same system by a density functional theory (DFT) modeling. ...
... Pd is known as a good catalyst for the CeH bond scission. The range of E a values reported for methanol steam reforming over various Pd/ZnO catalysts is quite broad (48-120 kJ/Mol) [2,14] and covers our values measured for formic acid decomposition. The E a values should depend on composition of the catalyst and reaction conditions. ...
This is one of the first reports, which is related to hydrogen production through formic acid decomposition over Pd/ZnO catalysts widely used for methanol steam-reforming. These catalysts have been investigated in comparison with Pt/ZnO and Pd/Al2O3 catalysts as well as ZnO support. HAADF/STEM, XRD, XPS and DRIFTS in situ studies of the systems were performed. The measured catalyst activity corresponds to the following order: Pd/Al2O3≥Pd/ZnO > Pt/ZnO > ZnO. Among the studied catalysts, Pd/ZnO showed the highest selectivity to hydrogen (up to 99.3%). This was assigned to the formation of a PdZn alloy during the reductive pre-treatment of the catalyst. An increase of the pre-treatment temperature from 573 to 773 K led to a significant increase of the mean PdZn (PtZn) nanoparticle size. However, the catalyst activity did not change, but the selectivity to hydrogen increased. These features closely remind the behavior of Pd/ZnO catalysts in methanol steam reforming implying that the mechanism of formic acid decomposition involves the same key steps and active sites.
... ZnO was also found to affect the activity and selectivity of PdZnO catalysts in MSR reaction [32]. In line with this finding, a recent study about the influence of ZnO facets on the performances of Pd/ZnO catalysts for MSR also reached the same conclusion [41,42]. Authors reported that at comparable Pd/ZnO catalyst composition, the polar sample was more selective than the nonpolar one due to the preferential formation of the PdZn phase, which is selective toward CO 2 , on the polar ZnO [41,42]. ...
... In line with this finding, a recent study about the influence of ZnO facets on the performances of Pd/ZnO catalysts for MSR also reached the same conclusion [41,42]. Authors reported that at comparable Pd/ZnO catalyst composition, the polar sample was more selective than the nonpolar one due to the preferential formation of the PdZn phase, which is selective toward CO 2 , on the polar ZnO [41,42]. ...
... 225 Datye et al. proposed that the deactivation of PdZn is caused by coke formation. 226 Pérez-Hernández et al. studied the performance of Pd supported on TiO 2 , ZrO 2 , and ZrO 2 -TiO 2 mixed oxide and ascribed the deactivation to sintering of active phase and coke deposition. 227 Au-Based catalysts. ...
Hydrogen energy, often dubbed the “ultimate energy source”, boasts zero carbon emissions and no harmful by-products. Nevertheless, the storage and transportation of hydrogen remain significant hurdles for its commercialization and large-scale implementation. Liquid hydrogen carriers (LHC), such as cyclohexane, methylcyclohexane, N-heterocycles, methanol, and ammonia, have emerged as promising solutions in hydrogen energy conversion systems. The storage and release of hydrogen rely on molecular hydrogenation and dehydrogenation processes, which are heavily influenced by the presence of catalysts. As such, a thorough understanding of catalyst design and mechanism is essential to facilitate (de)hydrogenation reactions under milder conditions. In this review, we explore three prevalent LHC systems and the catalysts employed during (de)hydrogenation processes. While noble metal catalysts exhibit superior performance in catalytic hydrogen storage, non-noble metal catalysts have also made considerable advancements. Furthermore, some liquid organic molecules are close to commercialization, potentially providing new options for energy storage and transportation. This article aims to trigger interest in LHC research and inspire the development of innovative catalytic systems for the catalytic hydrogen storage process.’
... 77 This tendency is however reverse to the experimental findings, suggesting that formaldehyde is more strongly bound on PdZn. 78,79 This may arise from the structural complexity of actual catalyst surfaces, as well as the influence of reaction conditions. The addition of Zn changes the most favorable adsorption site from η Pd 1 (C)η Pd 1 (O) on Pd ( Figure S5) to η Pd 1 (C)η PdZn 2 3 (O) on PdZn ( Figure S4), in agreement with the results of Smith et al. 80 The experimental observations by Jeroro and Vohs, who reported that H 2 CO binds to Pd−Zn dimers, with the carbon end of the molecule bound to Pd and the oxygen end bound to Zn, 78 slightly differ from our findings. ...
The tunability offered by alloying different elements is useful to design catalysts with greater activity, selectivity, and stability than single metals. By comparing the Pd(111) and PdZn(111) model catalysts for CO2 hydrogenation to methanol, we show that intermetallic alloying is a possible strategy to control the reaction pathway from the tuning of adsorbate binding energies. In comparison to Pd, the strong electron-donor character of PdZn weakens the adsorption of carbon-bound species and strengthens the binding of oxygen-bound species. As a consequence, the first step of CO2 hydrogenation more likely leads to the formate intermediate on PdZn, while the carboxyl intermediate is preferentially formed on Pd. This results in the opening of a pathway from carbon dioxide to methanol on PdZn similar to that previously proposed on Cu. These findings rationalize the superiority of PdZn over Pd for CO2 conversion into methanol, and suggest guidance for designing more efficient catalysts by promoting the proper reaction intermediates.
... In addition, it was reported that CO 2 selectivity of the MSR reactioni s relatedt ot he ratio of Pd/Zn in the alloy.T he Zn-rich PdZn alloy catalysts show higher activity and higher CO 2 selectivity than that of Pd-richc atalysts. [123][124][125] Zhang et al. recently reported the Zn-rich PdZn b-phase (Pd/Zn = 1:1) was prone to form on polar (0 001)f acets of ZnO rather than on nonpolar (1 01 0) facets. [34] Thus, the catalytic ability can be further enhanced by the morphologyc ontrol of the support. ...
As one of the alternative metals to platinum, which supports a wide range of applications in chemistry and catalysis in industry, palladium increasingly receives attention because of its similar physicochemical properties. However, Pd is generally less expensive than Pt and has a richer natural reserve. Herein, some recently developed techniques for the preparation and characterization of Pd-based bimetallic catalysts are reviewed. The impact on catalytic reactions of interest, including hydrogenation, dehydrogenation, hydrogenolysis, reforming, the oxygen reduction reaction, and hydrodesulfurization are also discussed. It is shown that the catalytic performance of Pd-based bimetallic catalysts is strongly dependent on the geometric and electronic states of Pd, which can be significantly affected by blended foreign element(s). Rationalization of the structure–activity relationship can provide useful guidelines to the fine tuning of these important catalytic reactions.
... In the mTorr pressure range a very high coverage of adsorbates is still produced which, as previous experiments showed, can induce important changes in the NP structure. [ 18,[41][42][43][44][45][46] After introduction of CO 2 and H 2 the samples were heated to 250 °C while XPS were acquired ( Figure 4 ). The Ni and Co 2p spectra were collected using a photon energy of 1115 eV and the 3p spectra with a photon energy of 700 eV. ...
Bimetallic nanoparticle (NP) catalysts are interesting for the development of selective catalysts in reactions such as the reduction of CO2 by H2 to form hydrocarbons. Here the synthesis of Ni-Co NPs is studied, and the morphological and structural changes resulting from their activation (via oxidation/reduction cycles), and from their operation under reaction conditions, are presented. Using ambient-pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and transmission electron microscopy, it is found that the initial core-shell structure evolves to form a surface alloy due to nickel migration from the core. Interestingly, the core consists of a Ni-rich single crystal and a void with sharp interfaces. Residual phosphorous species, coming from the ligands used for synthesis, are found initially concentrated in the NP core, which later diffuse to the surface.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
... The surface segregation of one element in multicomponent material strongly depends on the chemical environments. Switching of surface composition of alloys has been extensively studied with (NAP-)XPS under different condi-tions for Ag-Pd [79][80][81], Pt-Sn [82][83][84], Pd-Au [85,86], Pd-Zn [87][88][89] and Pd-In [90] alloys. Among them, relationship between surface composition and catalytic activity has been particularly paid much attention; for example, the NAP-XPS studies revealed active phases of multicomponent catalysts for CO oxidation [84,86] and methanol steam reforming [87]. ...
Catalytic chemical reactions proceeding on solid surfaces are an important topic in fundamental science and industrial technologies such as energy conversion, pollution control and chemical synthesis. Complete understanding of the heterogeneous catalysis and improving its efficiency to an ultimate level are the eventual goals for many surface scientists. Soft x-ray is one of the prime probes to observe electronic and structural information of the target materials. Most studies in surface science using soft x-rays have been performed under ultra-high vacuum conditions due to the technical limitation, though the practical catalytic reactions proceed under ambient pressure conditions. However, recent developments of soft x-ray based techniques operating under ambient pressure conditions have opened a door to the in-situ observation of materials under realistic environments. The near-ambient-pressure x-ray photoelectron spectroscopy (NAP-XPS) using synchrotron radiation enables us to observe the chemical states of surfaces of condensed matters under the presence of gas(es) at elevated pressures, which has been hardly conducted with the conventional XPS technique. Furthermore, not only the NAP-XPS but also ambient-pressure compatible soft x-ray core-level spectroscopies, such as near-edge absorption fine structure (NEXAFS) and x-ray emission spectroscopy (XES), have been significantly contributing to the in-situ observations. In this review, first we introduce recent developments of in-situ observations using soft x-ray techniques and current status. Then we present recent new findings on catalytically active surfaces using soft x-ray techniques, particularly focusing on the NAP-XPS technique. Finally we give a perspective on the future direction of this emerging technique.
... The optimal promotional rates for POM and SRM correspond to the electrochemical supply of 1.38 Â 10 À6 and 5.14 Â 10 À7 mol K + , respectively, calculated from the integration of the current vs. time curves. In addition, it should be noted that in both POM (Fig. 6) and SRM (Fig. 7) under optimum electropromoted conditions, the calculated turnover frequencies of methanol consumption (TOF) (8.45 and 0.51 s À1 , respectively) were shown to be of the same order as those obtained with conventional catalysts such as Cu/ ZnO- [53], Cu/CeO 2 - [54], PdZn- [55], and VO-based catalysts [56] under similar reaction conditions. However, unlike the experiments under POM conditions, a single promotional effect was found (at V WR 6 0.5 V) for the SRM process. ...
This study reports for the first time in the literature that the electrochemical promotion (EPOC) of Pt nanoparticles (of around 3 nm) dispersed on a diamond-like carbon (DLC) matrix. A novel Pt-DLC catalyst film has been prepared by the technique of cathodic arc deposition onto a K-beta Al2O3 solid electrolyte support. The catalyst film has been characterized and tested under EPOC conditions in H-2 production from methanol via partial oxidation (POM) and steam reforming (SRM). After a temperature-programmed pretreatment, the Pt-DLC film achieved a suitable electrical conductivity due to the transition of the sp(3)-hybridized carbon form into a more graphitic structure (sp(2)-hybridized) as supported by different characterization measurements such as STEM and EELS. Under EPOC conditions, the H-2 production rates were increased 2.5 and 3.4 times under optimal promoter coverage for POM and SRM, respectively. In addition, under POM conditions, two different electropromotional states were achieved at two different potentials attributed to the formation of different kinds of promoter phases, as confirmed by cyclic voltammetry. Finally, a comparison of the Pt-DLC catalyst film vs. a pure dense Pt layer prepared by the same technique demonstrated the higher activity of the former, due to the lower size of the Pt nanoparticles supported on the DLC matrix.
... [60] On top of this result, even more recent studies on "isolated" bulk intermetallic tetragonal PdZn compounds revealed the presence of ZnO on the surface of a sufficiently Znrich PdZn phase. [52] Very recently, a corresponding PdZn phase with a face-centered cubic structure was reported to be stable under MSR conditions, that is, no oxidized Zn species were observed on the surface during the MSR reaction, but also zero CO 2 selectivity was observed; actually, this intermetallic compound showed 100 % selectivity for CO. [83] The high importance of the phase boundary has also been highlighted in a recent paper by Friedrich et al., who showed that this phase boundary was critically dependent on the exact Zn content in the bulk PdZn compound. Higher Zn content led to the segregation of Zn on the surface and, hence, to the formation of oxidized Zn species and high CO 2 selectivity. ...
This Minireview summarizes the fundamental results of a comparative inverse‐model versus real‐model catalyst approach toward methanol steam reforming (MSR) on the highly CO2‐selective H2‐reduced states of supported Pd/ZnO, Pd/Ga2O3, and Pd/In2O3 catalysts. Our model approach was extended to the related Pd/GeO2 and Pd/SnO2 systems, which showed previously unknown MSR performance. This approach allowed us to determine salient CO2‐selectivity‐guiding structural and electronic effects on the molecular level, to establish a knowledge‐based approach for the optimization of CO2 selectivity. Regarding the inverse‐model catalysts, in situ X‐ray photoelectron spectroscopy (in situ XPS) studies on near‐surface intermetallic PdZn, PdGa, and PdIn phases (NSIP), as well as bulk Pd2Ga, under realistic MSR conditions were performed alongside catalytic testing. To highlight the importance of a specifically prepared bulk intermetallicoxide interface, unsupported bulk intermetallic compounds of Pdx Gay were chosen as additional MSR model compounds, which allowed us to clearly deduce, for example, the water‐activating role of the special Pd2Ga‐β‐Ga2O3 intermetallicoxide interaction. The inverse‐model studies were complemented by their related “real‐model” experiments. Structure–activity and structure–selectivity correlations were performed on epitaxially ordered PdZn, Pd5Ga2, PdIn, Pd3Sn2, and Pd2Ge nanoparticles that were embedded in thin crystalline films of their respective oxides. The reductively activated “thin‐film model catalysts” that were prepared by sequential Pd and oxide deposition onto NaCl(001) exhibited the required large bimetaloxide interface and the highly epitaxial ordering that was required for (HR)TEM studies and for identification of the structural and catalytic (bi)metalsupport interactions. To fully understand the bimetalsupport interactions in the supported systems, our studies were extended to the MeOH‐ and formaldehyde‐reforming properties of the clean supporting oxides. From a direct comparison of the “isolated” MSR performance of the purely bimetallic surfaces to that of the “isolated” oxide surfaces and of the “bimetaloxide contact” systems, a pronounced “bimetaloxide synergy” toward optimum CO2 activity/selectivity was most evident. Moreover, the system‐specific mechanisms that led to undesired CO formation and to spoiling of the CO2 selectivity could be extracted.
La0.6Sr0.4CoO3−δ (LSC) perovskite, as a potential catalyst precursor for hydrogen (H2)-rich production by steam reforming of methanol (SRM) and oxidative steam reforming of methanol (OSRM), was investigated. For this purpose, LSC was synthesized by the citrate sol–gel method and characterized by complementary analytical techniques. The catalytic activity was studied for the as-prepared and prereduced LSC and compared with the undoped LaCoO3−δ (LCO) at several feed gas compositions. Furthermore, the degradation and regeneration of LSC under repeated redox cycles were studied. The results evidenced that the increase in the water/methanol ratio under SRM, and the O2 addition under OSRM, increased the CO2 formation and decreased both the H2 selectivity and catalyst deactivation caused by carbon deposition. Methanol conversion of the prereduced LSC was significantly enhanced at a lower temperature than that of as-prepared LSC and undoped LCO. This was attributed to the performance of metallic cobalt nanoparticles highly dispersed under reducing atmospheres. The reoxidation program in repetitive redox cycles played a crucial role in the regeneration of catalysts, which could be regenerated to the initial perovskite structure under a specific thermal treatment, minimizing the degradation of the catalytic activity and surface.
A catalytic membrane reactor equipped with Pd–Ag metallic membranes and loaded with PdZn/ZnAl2O4/Al2O3 catalytic pellets was tested for the methanol steam reforming reaction (S/C = 1) aimed at producing a pure hydrogen stream for PEM fuel cell feeding. The catalyst was prepared in two steps. First, commercial γ-Al2O3 pellets were impregnated with ZnCl2 and calcined at 700 °C to obtain a ZnAl2O4 shell, and subsequently impregnated with PdCl2 and reduced at 600 °C to obtain PdZn alloy nanoparticles. The catalyst was tested both in a conventional packed bed reactor and in a catalytic membrane reactor. A 3D CFD non-isothermal model with mass transfer limitations was developed and validated with experimental data. The reactions of methanol steam reforming, reverse water-gas shift and methanation were modeled under different pressure, temperature and feed load values. The model was used to study and simulate the CMR under different operation conditions.
Catalytic hydrogenation of sulfur-containing substrates is an important and challenging reaction in the chemical industry. In this work, active carbon supported PdZn alloy catalyst was prepared by self-reduction method using zinc acetate as precursor without H2 atmosphere. During the process of self-reduction, Zn²⁺ was firstly reduced to Zn⁰ at 300 οC by active carbon and reducing gas from the decompose of acetate under the promotion of metal Pd, and Zn⁰ further reacted with metal Pd to form PdZn alloy phase at 500 οC. These PdZn/AC-X catalysts showed the higher conversion and stability for the hydrogenation of 4-nitrothioanisole than the Pd/AC-600 catalyst. The excellent catalytic performance of PdZn/AC-600 catalyst can be attributed to formation of PdZn alloy, in which electron-rich Pd atoms weaken the binding ability between Pd and S and enhance the sulfur-resistance of catalyst. On the other hand, H2-TPR and DFT theory calculation further indicated that the PdZn alloy phase weakens the adsorption capacity of S. Compared with the Pd/AC-600 catalyst, the PdZn alloy phase in PdZn/AC-600 catalyst has not changed and only a small amount of sulfur-containing substrates deposited on the catalyst surface after three cycles. PdZn/AC-600 catalyst exhibited improved stability in the hydrogenation of 4-nitrothioanisole and can be used three cycles with little decrease in activity.
Fuel cells (FC) produce electricity in a continuous mode through a catalytic reaction and have many possible applications, as for example, in the transportation sector substituting the combustion engines. These devices can be regarded as a free emission technology if the fuel used in them is obtained in a renewable mode, such as hydrogen from the reforming of light alcohols obtained from biomass fermentation or gasification. In fact, proton exchange membrane fuel cells (PEMFC) use hydrogen as fuel that, in turn, has to be free of carbon monoxide (CO) since the later chemical species poisons the platinum based catalyst applied in the electrochemical process. This review aims at clarifying how multicomponent catalysts can be used in the hydrogen production from light alcohols reforming to overcome the limitations of current catalysts. Specifically, their low thermal stability, the CO formation that is not suitable for FC use, the carbon (coke) production that poisons the reforming catalyst, or byproducts (i.e. CH4) generation that reduces the hydrogen amount produced. Special emphasis is paid to the applicability of theoretical methods for the study and development of improved multicomponent catalysts for light alcohols reforming.
Supported Pd catalysts with varied Pd loadings (x = 0.5 wt%, 2.0 wt%, 5.0 wt%, 7.5 wt%, 15.0 wt%) were prepared by the incipient wetness impregnation method using a ZnAl2O4 spinel support. We found that ZnAl2O4 supported Pd catalysts with low Pd loadings (e.g., 0.5 wt%) are very selective in syngas conversion to methanol and dimethyl-ether (DME). XRD and TEM characterization shows that, after reduction at 350 °C, PdZnβ phase with Pd:Zn molar ratio of 1:1 is favored to form predominantly on the spinel support at relatively low Pd loadings, i.e. less than 5.0 wt%, while Pd-rich PdZnα alloy phase exists at Pd loadings above 5.0 wt%. A higher reduction temperature such as 500 °C can facilitate the transformation from PdZnα to PdZnβ phase in those catalysts with high Pd loading. We further found that catalysts with predominant PdZnβ phase are selective in the methanol and DME production from syngas, while the presence of PdZnα phase leads to the notable formation of alkanes byproducts, resulting in reduced methanol and DME selectivity. DME formation from dehydration of methanol depends on the acidity of catalysts, which was found to increase with Pd loading, probably due to the formation of isolated Al2O3 as a result of Zn migrating from ZnAl2O4 spinel phase to form the PdZn phases with Pd.
In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy publications that have appeared between the 1970s and the end of 2018 according to their scientific field. We find that catalysis, surface science and materials science are predominant, while, for example, electrocatalysis and thin film growth are emerging. All catalysis publications that we could identify are cited, and selected case stories with increasing complexity in terms of surface structure or chemical reaction are discussed. For thin film growth we discuss recent examples from chemical vapour deposition and atomic layer deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron spectroscopy research, indicating some directions of future development of the field.
Cobalt germanides have been widely studied as semiconductor contact materials, but recent theoretical studies suggest that they may also be excellent catalysts for methane steam reforming with stabilities and activities comparable to more expensive noble metal catalysts. We have sputter deposited CoGe alloy films and characterized their structure and morphology after post-deposition annealing in high vacuum up to 1000 °C. We used X-ray photoelectron spectroscopy to study the initial oxidation of amorphous and crystalline CoGe alloy surfaces under low pressures of O2 and H2O. The oxidation rate in O2 was found to be faster for an amorphous CoGe surface compared to a crystalline surface. We also found that there was little difference in the oxidation rate in H2O for either amorphous or crystalline surfaces. During O2 oxidation, the crystalline surface preferentially forms GeO and the amorphous surface preferentially forms GeO2. We have also observed preferential oxidation of Ge in the CoGe thin films. During temperature programmed desorption studies, we found that GeO desorption begins near 350 °C and that GeO2 decomposes to GeO and desorbs near 700 °C. More studies of CoGe catalysts are warranted, however GeO desorption may be a concern under reaction conditions when the film is subjected to an oxidizing environment.
Methanol steaming reforming (MSR), catalyzed by Pd/ZnO, is a promising process to produce onboard hydrogen for fuel cell. The reactivity of Pd/ZnO, especially selectivity to CO2 and H2, changes with the formation of ZnPd intermetallic compound and ZnPd-ZnO interface. In this work, we measured the adsorption energetics and natures of adsorbed species on ZnO, Pd/ZnO, and ZnPd/ZnO catalysts by combining adsorption microcalorimetry and infrared spectroscopy with the reactants (methanol and water) and intermediate (formaldehyde) of MSR as probe molecules, and correlated the adsorption energetics to the reactivities of the samples. ZnO exhibits weakly molecular adsorption for methanol while strongly interacting with water. In contrast, the adsorption energetic gap between methanol and water decreases on Pd/ZnO and disappears on ZnPd/ZnO. This might be responsible for the highest activity of MSR on ZnPd/ZnO since methanol could competitively adsorb and react with water. Though the introduction of Pd onto ZnO lowers the thermodynamic stability of adsorbed formaldehyde, the formed ZnPd intermetallic compound strengthens the bonding of adsorbed formaldehyde, allowing the further reaction with water that results in the progress of the reaction pathway to CO2 and H2. This might be an important factor for the high selectivity to CO2 and H2 on ZnPd/ZnO.
ZnPd bimetallic nanoparticles supported on the Al-doped ZnO support showed efficient catalytic performance for glycerol hydrogenolysis to 1,2-propanediol (1,2-PDO). The Al species in the ZnPd/ZnO-3Al catalyst were highly related to the reaction activity of the catalysts. It was confirmed that the AlA(IV) species in the ZnPd/ZnO-3Al catalyst determined the reduction of ZnO, the size of ZnPd, oxygen vacancy density, and the reaction activity. Calcination temperature had significant influences on the distribution and migration of Al(IV) in ZnO lattice, and hence affected the reaction activity.
This Review describes recent advances in the design, synthesis, reactivity, selectivity, structural, and electronic properties of the catalysts for reforming of a variety of oxygenates (e.g., from simple monoalcohols to higher polyols, then to sugars, phenols, and finally complicated mixtures like bio-oil). A comprehensive exploration of the structure-activity relationship in catalytic reforming of oxygenates is carried out, assisted by state-of-the-art characterization techniques and computational tools. Critical emphasis has been given on the mechanisms of these heterogeneous-catalyzed reactions and especially on the nature of the active catalytic sites and reaction pathways. Similarities and differences (reaction mechanisms, design and synthesis of catalysts, as well as catalytic systems) in the reforming process of these oxygenates will also be discussed. A critical overview is then provided regarding the challenges and opportunities for research in this area with a focus on the roles that systems of heterogeneous catalysis, reaction engineering, and materials science can play in the near future. This Review aims to present insights into the intrinsic mechanism involved in catalytic reforming and provides guidance to the development of novel catalysts and processes for the efficient utilization of oxygenates for energy and environmental purposes.
ZnO nanowires (NWs) supported Pd catalysts were prepared by modified deposition-precipitation method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), transmission electron microscopy (TEM) and high angle annular dark field scanning transmission electron microscope (HAADF-STEM). The effects of Pd loading and reduction temperature on the formation of PdZn were investigated. The results showed that the high CO selectivity of 2.1% Pd/ZnO NW catalyst after reduced at 400℃ is due to the formation of PdxZny(x>y) alloy. Increasing of Pd loading or reduction temperature is favorable for the transformation of PdxZny(x>y) to PdZn alloy which can inhibit CO formation. By a modified wet chemistry method and apt post-synthesis treatment, we have been able to grow PdZn alloy nanoparticles epitaxially onto the {1010} nanofacets of the ZnO NWs. It showed better stability than PdZn supported on commercial ZnO powders in long term reaction. The methanol conversion at 300℃ over Pd/ZnO NW remained constant for about 40 h without obviously deactivation. On the contrary, the methanol conversion over Pd/ZnO powders catalyst gradually decreased with time on stream by more than 30%. The better stability of Pd/ZnO NW catalysts is due to the highly stable ZnO NWs primarily enclosed by the low-energy {1010} surfaces and the epitaxial relationships between PdZn nanoparticles and ZnO NWs support.
Stability and reusability are crucial properties for heterogeneous catalysts in view of their industrial applications. Here, we report a very stable catalyst, ZnPd/ZnO@Al2O3, for the aqueous phase hydrogenolysis of glycerol, which was prepared by a facile strategy via pretreatment of ZnO with Al(NO3)3 and conventional impregnation and annealing processes. The as-prepared ZnPd/ZnO@Al2O3 showed excellent stability, and it could be recycled 5 times, retaining high activity and selectivity (92%) to 1,2-propanediol. It was confirmed that the surface porous Al2O3 skin layer could suppress the aggregation and growth of ZnPd alloy particles and protect the imbedded ZnO from etching by water. This work provides a facile method for preparing stable catalysts with metal particles imbedded into a surface Al2O3 layer, and the present ZnPd/ZnO@Al2O3 is expected to have a potential application in other aqueous catalysis reactions.
To understand steam reforming of methanol (SRM) reaction pathways over the surfaces of various metal-modified molybdenum carbides, the reaction intermediate species generated during the SRM reaction are investigated by using a temperature programmed surface reaction (TPSR) approach, in which a mass spectroscopy is applied to detect the produced species on-line. It is found that the reaction temperature affects the formation of different intermediate species over different catalysts. At the temperature lower than 180 °C, SRM reaction proceeds over Cu modified molybdenum carbide surface through formic acid (HCOOH) intermediate pathway, but at the temperature over 180 °C, the reactions over pure β-Mo2C, Ni, Cu and Pt modified molybdenum carbide surfaces proceed through methyl-formate (HCOOCH3) intermediate pathway. It is expected that these findings can help us to understand the mechanism on SRM reaction over molybdenum carbide based catalysts.
Density functional theory (DFT) calculations were employed to study the dissociation of the O─H bond in methanol on several planar and stepped bimetallic transition metal surfaces, composed of elements showing high or moderate activity towards this reaction, namely, Ni, Rh, Ru, Ir, Pd, Au, Zn and Cu. The activation energies for the O─H bond cleavage were compared with those estimated using a Brønsted-Evans-Polanyi (BEP) relationship for the RO─H bond breakage on pure metal transition surfaces, relating the activation energy for the reaction with the adsorption energies of the reaction products, RO• and H• adsorbed on the surface of the catalyst. Furthermore, the average differences between the values of the activation energies calculated with the exhaustive determination of the full reaction path and location of the transition state on each surface model and the activation energies obtained from the BEP relationship with the simple calculation of the adsorption energies of the RO• and H• species are ~ 0.14 eV. This suggests that the BEP relationship developed upon the consideration of data for dissociation of the O─H bond in alcohols and water on pure metal surfaces is also valid for a qualitative prediction of the methanol activation energy on bimetallic surfaces.
Over 7000 papers are published in the field of catalysis each year. While the majority appear within a handful publications, keeping up with the literature can be difficult. Now in its 25th volume, the Specialist Periodical Report on Catalysis presents critical and comprehensive reviews of the hottest literature published over the last twelve months.
Industrial and academic scientists face increasing challenges to find cost-effective and environmentally sound methods for converting natural resources into fuels, chemicals and energy. This series is edited by two leading researchers in the field and provides a balanced and in-depth review of the modern approaches to these challenges, covering major areas of heterogeneous and homogenous catalysis, as well as specific applications of catalysis, such as NOx control, kinetics and experimental techniques, such as microcalorimetry.
With chapters detailing specific areas within the field, this series is a comprehensive reference for anyone working in Catalysis and an essential resource for any Chemistry Library.
Pd/ZnO catalysts with different Pd content have been synthesized, thoroughly characterized and investigated with regard to their reduction behavior in hydrogen or carbon monoxide containing atmospheres, by applying CO-chemisorption, photoelectron spectroscopy, X-ray diffraction, electron microscopy, TPR and DRIFTS techniques. As a catalytic test reaction, CO-oxidation has been applied. The interaction of the noble metal with the support has been revealed in a way that can distinguish between alloying and other surface spreading/wetting phenomena, induced by strong metal-support interaction (SMSI). It was found that while alloy formation promoted CO-oxidation activity additional ZnOx formation by SMSI had the opposite effect. Zinc enrichment at the surface was detected during reduction of the catalysts, depending on the reducing agent and the Pd particle size.
Two-monolayer-Zn covered Pd(1 1 1) annealed at (low) 500 K exhibits nice CO2 selectivity for CH3OH+H2O to CO2+H2, whereas CO is yielded exclusively when annealed at (high) 650 K. To unravel the reason behind the phenomenon, kinetic Monte Carlo (KMC) simulations were used to study the alloying process. It shows the low temperature annealing produces a multilayer 1:1 PdZn alloy, whereas the high temperature operation results in a Zn-lean multilayer alloy, not the previously assumed monolayer PdZn alloy on Pd. The geometry and electronic structures of the models derived from KMC simulation agrees with the relevant experiments. The low temperature sample is more active than the high temperature one for H2O dissociation, in line with the assumption that H2O dissociation controls CO2 selectivity. It is revealed that triple Zn ensembles which form three-fold hollow sites account for the activity for H2O dissociation.
Two series of supported palladium catalysts (2 wt%) were prepared on ZnO–CeO2 nanocomposites (Zn-to-Ce atomic ratio between 0.5 and 2) obtained by oxalate or carbonate coprecipitation (OC and CC series, respectively). Methanol steam reforming (MSR) reaction was tested in a wide range of temperature (398–623 K) for CH3OH/H2O = 1/1 gas mixtures. Pd on pure CeO2 was only able to decompose methanol to CO, under 523 K, but the reverse water gas shift reaction took place at higher temperatures. The MSR reaction only occurred in the presence of zinc oxide and the selectivity to CO2 was higher for the CC series, due to the better dispersion of the ZnO phase over these carbonate-derived nanocomposites. Although the CO2 selectivity seems to be modulated by the reverse water gas shift reaction, the palladium supported on the mixed oxides was more stable than Pd/ZnO, which continuously deactivated. A detailed characterization by high resolution atomic microscopy, X-ray photoelectron spectroscopy and a novel carbon monoxide step chemisorption technique, proved the formation of bulk and surface PdZn alloying in the ternary catalyst, Pd supported on the nanosized ceria and zinc oxide supports. It is concluded that although a better catalytic stability was observed on the ZnO–CeO2 nanocomposites, the employment of temperatures higher than 450 K would impose an insurmountable limitation in terms of CO2 selectivity.
We report the structural evolution of Pd-Zn alloys in a 3.6% Pd-12% Zn/Al2O3 catalyst which is selective for propane dehydrogenation. High signal-to-noise, in situ synchrotron X-ray diffraction (XRD) was used quantitatively, in addition to in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) and extended X-ray absorption fine structure (EXAFS) to follow the structural changes in the catalyst as a function of reduction temperature. XRD in conjunction with DRIFTS of adsorbed CO indicated that the β1-PdZn intermetallic alloy structure formed at reduction temperatures as low as 230 °C, likely first at the surface, but did not form extensively throughout the bulk until 500 °C which was supported by in situ EXAFS. DRIFTS results suggested there was little change in the surfaces of the nanoparticles above 325 °C. The intermetallic alloy which formed was Pd-rich at all temperatures but became less Pd-rich with increasing reduction temperature as more Zn incorporated into the structure. In addition to the β1-PdZn alloy, a solid solution phase with face-center cubic structure (α-PdZn) was present in the catalyst, also becoming more Zn-rich with increasing reduction temperature.
Ein heterogener Katalysator ist ein Funktionsmaterial, das mit seinen Reaktanten unter Reaktionsbedingungen kontinuierlich aktive Zentren erzeugt. Diese Zentren verändern die Geschwindigkeit chemischer Reaktionen der an sie gebundenen Reaktanten, ohne die Lage des thermodynamischen Gleichgewichtes dieser Stoffe zu verändern.
A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials. Filling the gaps: The understanding of heterogeneous catalysis is built on a standard model of interface catalysis that was developed from surface physics and theory. This model has significant gaps with regards to transferring knowledge yielded to high-performance catalysts, and approaches to fill these gaps are proposed in this Review.
PdZn mixed oxides are precursors for the formation of intermetallic PdZn phases, which show improved catalytic performance for methanol steam reforming. In this work we have prepared mixed oxides (PdxZn1-xO) that span a range of compositions around the tetragonal PdZn 1:1 L10 phase (x = 0.25, 0.5 and 0.75). We find that Pd+2 can be isomorphously substituted into hexagonal ZnO and likewise Zn+2 can also be substituted within tetragonal PdO. Our results show that the mixed oxide has a composition Pd0.75Zn0.25O within the tetragonal PdO lattice with a slight contraction in unit cell volume. The results are relevant for understanding the enhanced sensing properties of ZnO and the nature of the oxide precursors for the synthesis of intermetallic PdZn nanoparticles for heterogeneous catalysis.
We present a study of the structure and reactivity of Ru nanopartides of different sizes (1.3, 1.9, and 3.1 nm) for CO hydrogenation using gas-phase nuclear magnetic resonance and mass spectroscopy. In addition, the nanopartides were characterized under reaction mixtures in situ by ambient-pressure X-ray photoelectron spectroscopy. We found that during reaction the Ru is in the metallic state and that the diphosphine ligands [bis(diphenylphosphino)butane (dppb)] on the surface of 1.9 and 3.1 nm nanopartides not only act as capping and protecting agents but also stay on the surface during reaction and improve their activity and selectivity toward C-2 C-4 hydrocarbons. KEYWORDS: ruthenium nanoparticles, model Fischer Tropsch synthesis, surface chemistry, ligand effect, ambient-pressure XPS, NMR, FTIR, mass spectrometry
The review is concerned with correlations between the synergistic effects and structural organization of the surface of bimetallic alloys that are used as active components of catalysts for selective hydrogenation of organic compounds and for CO oxidation in hydrogen-rich mixtures. Studies on the preparation of novel highly efficient catalysts using modern theoretical approaches, computer-assisted molecular design and original synthetic procedures are considered. It is shown that introduction of the second metal into the monometallic catalyst and subsequent formation of alloy particles with modified structure of the surface and near-surface layers leads to nonadditive enhancement of catalytic activity and/or selectivity. The bibliography includes 203 references.
Bimetallic PdZn catalysts supported on carbon black (CB) and carbon nanotubes (CNTs) were found to be selective for CO-free H2 production from ethanol at low temperature (250 oC). On Pd, the H2 selectivity was low (~ 0.3 mol H2/ethanol reacted) and the CH4/CO2 ratio was high (~1.7). Addition of Zn to Pd formed the intermetallic PdZnβ phase (atomic ratio of Zn to Pd is 1) with increased H2 selectivity (~1.9) and CH4/CO2 ratio of < 1. The higher H2 selectivity and low CH4 formation was related to the improved dehydrogenation activity of the L10 PdZnβ phase. The TOF increased with particle size and the CNTs provided the most active and selective catalysts, which may be ascribed to pore-confinement effects. Furthermore, no significant changes in either the supports or the PdZnβ particles was found after aqueous-phase reforming (APR) indicating that the metal nanoparticles and the carbon support are hydrothermally stable in the aqueous phase at elevated temperatures and pressures (> 200 oC, 65 bar). The main reaction products from APR of ethanol are H2, CO2, CH4, CH3CHO, CH3COOH and CH3COCH3. No CO was detected for all the catalysts performed in aqueous-phase reaction, indicating that both monometallic Pd and bimetallic PdZn catalysts have high water-gas shift activity during APR. However, the selectivity towards H2 is considerable lower than the theoretical value of 6 H2 per mole ethanol which is achieved during high temperature vapor phase steam reforming.
In order to provide insight into how alloying Pt with Zn affects its reactivity for the steam reforming of methanol (SRM), the adsorption and reaction of CO, CH2O, and CH3OH on Zn-modified Pt(111) surfaces was investigated using a combination of X-ray photoelectron spectroscopy (XPS), high resolution electron energy loss spectroscopy (HREELS), and temperature-programmed desorption (TPD). The reactivity of PtZn near surface alloys were found to be similar to that of Pt(111) and have relatively low activity for methanol decomposition. In contrast, on Zn-modified Pt(111) surfaces where the Zn was present only as adatoms, methanol adsorbs dissociatively to form methoxide groups bound to both Pt and Zn sites. The Pt-bound methoxides are stable up to 450 K at which point they undergo dehydrogenation to produce H2 and CO, while those adsorbed on Zn sites undergo partial dehydrogenation at 300 K to produce formaldehyde which desorbs intact. These differences in the reactivity of the two types of surfaces studied suggest that Zn adatoms may play a role as active sites for the SRM reaction which is in agreement with studies of high surface area PtZn catalysts that indicate that excess surface Zn is required to obtain high selectivity to CO2 and H2.
A series of Pd/ZnO catalysts with different Pd loadings were prepared using needlelike ZnO crystallites (ZnO-N) with predominant (10–10) nonpolar facets exposed and commercial ZnO (ZnO-P) without any dominant facets. The Pd/ZnO catalysts were characterized using complementary techniques, such as nitrogen physisorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Fourier transform infrared spectroscopy analysis of adsorbed CO (CO-FTIR). The effect of ZnO crystallite faceting on the activity and selectivity of the Pd/ZnO catalysts was studied in methanol steam reforming (MSR). It was found that the Pd-rich phases (PdxZny, x > y) are predominantly formed at low Pd loadings on both ZnO supports (ZnO-N and ZnO-P), resulting in high CO selectivity. As Pd loading increases, the x/y ratio in the PdxZny phases decreases, leading to decreased CO selectivity. At similar Pd loadings, Pd/ZnO-P catalysts are more selective in MSR than Pd/ZnO-N, which is due to more facile formation of the stable PdZnβ phase on polar ZnO (0001) facets than on nonpolar ZnO (10–10) facets. The current study provides insight into the structure–performance relationships in Pd/ZnO catalysts for MSR, helping shed light on the rational design of selective MSR catalysts to minimize CO formation.
Despite extensive studies on hydrogen production via steam reforming of alcohols and sugar alcohols, catalysts typically suffer a variety of issues from poor hydrogen selectivity to rapid deactivation. Here, we summarize recent advances in fundamental understanding of functionality and structure of catalysts for alcohol/sugar alcohol steam reforming, and provide perspectives on further development required to design highly efficient steam reforming catalysts.
Heterogeneous chemical reactions at vapor/solid interfaces play an important role in many processes in the environment and technology. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a valuable tool to investigate the elemental composition and chemical specificity of surfaces and adsorbates on the molecular scale at pressures of up to 130 mbar. In this review we summarize the historical development of APXPS since its introduction over forty years ago, discuss different approaches to minimize scattering of electrons by gas molecules, and give a comprehensive overview about the experimental systems (vapor/solid interfaces) that have been studied so far. We also present several examples for the application of APXPS to environmental science, heterogeneous catalysis, and electrochemistry.
Research interest in bimetallic catalysts is mainly due to their tunable chemical/physical properties by a number of parameters like composition and morphostructure. In catalysis, numerous bimetallic catalysts have been shown to exhibit unique properties which are distinct from those of their monometallic counterparts. To meet the growing energy demand while mitigating the environmental concerns, numerous endeavors have been made to seek green and sustainable energy resources, among which hydrogen has been identified as the most promising one with bimetallic catalysts playing important roles. This tutorial review intends to summarize recent progress in bimetallic catalysts for hydrogen production, specifically focusing on that of reforming technologies as well as the relevant processes like water-gas shift (WGS) and CO preferential oxidation (PROX), and emphasizing on the fundamental understanding of the nature of catalytic sites responsible for generating high purity hydrogen and minimizing carbon monoxide formation. Meanwhile, some important synthesis and characterization methods of bimetallic catalysts developed so far are also summarized.
An intermetallic compound, PdZn, exhibits a similar valence electron density of states as pure Cu, which has been confirmed by energy band calculation and X-ray photoelectron spectroscopy. The catalytic function was verified to be identical for PdZn and Cu. This explains the origin of the high selectivity of PdZn for the steam reforming of methanol (SRM), CH3OH+H2O-->3H2+CO2, a chemical reaction for which Cu is known to be one of the best catalyst; namely, PdZn is catalytically equivalent to Cu. Compared with PdZn, PtZn and NiZn, which have the same crystal structure but different valence band structures, exhibit a poorer selectivity of CO2. This suggests that the catalytic function, at least for SRM, is solely governed by the valence band structure of the catalyst. A simple but very important principle has been derived that an intermetallic compound may be logically designed by band structure calculation, aiming at replacing a selected metallic element without changing the catalytic function. Using this principle, we designed a compound, PdCd, which exhibits a similar valence electron density of state and selectivity for SRM with Cu.
Low-energy ion scattering with monolayer sensitivity was applied to investigate ultrathin films of zinc on Pd(1 1 1). Uptake curves taken at 150 K indicate the simultaneous growth of multilayers with negligible interlayer transport. Annealing experiments for two-monolayer films reveal a rapid decrease in the zinc content on the surface layer at temperatures above 300 K, forming a metastable state with a Pd:Zn surface ratio of approx. 1:1 in the temperature region between 400 and 550 K. This state is most easily explained as a slightly buckled p(2 × 1)-PdZn surface alloy, with Zn atoms located approx. 0.25 Å above their Pd counterparts.
The growth and structure of Pd films on ZnO(0001) were investigated using high resolution electron energy loss spectroscopy, x-ray photoelectron spectroscopy, and low energy electron diffraction. Vapor deposited Pd films at 300 K were found to follow a two-dimensional (2D) island growth mode, in which 2D metal islands are formed up to a critical coverage at which point growth occurs primarily in a layer-by-layer fashion on top of the islands. Heating to only 350 K was found to be sufficient to induce partial agglomeration of Pd films into three-dimensional particles. In addition to causing further agglomeration into particles, heating to 700 K resulted in partial reduction of the ZnO surface and the formation of a PdZn alloy.
The thermal oxidation process of metallic zinc on 6H-SiC(0 0 0 1) surface has been investigated by using atomic force microscopy (AFM), synchrotron radiation photoelectron spectroscopy (SRPES) and XPS methods. The AFM images characterize the surface morphology of ZnO film formed during the thermal oxidation and SRPES record the valence band, Si 2p and Zn 3d spectra at different stages. The O 1s peak is recorded by XPS because of the energy limit of the synchrotron radiation. Our results reveal that the silicon oxides layer of SiC substrate can be reduce by hot metallic zinc atom deposition. The oxygen atoms in the silicon oxides are captured by the zinc atoms to form ZnOx at the initial stage and as a result, the oxidized SiC surface are deoxidized. After the zinc deposition with the final thickness of 2.5 nm, the sample is exposed in oxygen atmosphere and annealed at different temperatures. According to the evolution of peaks integrated intensities, it is considered that the Zn/SiC system will lose zinc atoms during the annealing in oxygen flux at high temperature due to the low evaporation temperature of pure zinc. After further annealing in oxygen flux at higher temperature, the substrate is also oxidized and finally the interface becomes a stable SiC SiOx ZnO sandwich structure.
Methyl formate is selectively produced in the dehydrogenation of methanol over Pd/ZnO; the reaction is suggested to proceed through the same steps as over copper-based catalysts.
The CO dissociation probability on transition metals is often invoked to explain the product distribution (selectivity) of catalytic CO hydrogenation. Along these lines, we have investigated CO adsorption and dissociation on smooth and ion-bombarded Pd(111) at pressures up to 1mbar using vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS). Under high pressure, CO adsorbate structures were observed that were identical to high-coverage structures in UHV. On ion-bombarded surfaces an additional species was detected which was attributed to CO bridge bonded to defect (low-coordinated) sites. On both surfaces, no indications of CO dissociation were found even after hours of 0.1mbar CO exposure. However, exposing CO/H2 mixtures to ion-bombarded Pd(111) produced carbonaceous deposits suggesting CHxO species as precursors for CO bond cleavage and that the formation of CHxO is facilitated by surface defects. The relevance of the observations for CO hydrogenation on Pd catalysts is discussed.
Activation energies, kinetic orders, and relative activities have been determined for the oxidation of CO by O2 over five supported noble metals. With ruthenium drifts in reaction rate occurred for many hours following pressure and temperature changes but the initial responses were qualitatively similar to the simple behavior found for iridium, rhodium, and palladium (above 390 K), namely, kinetic orders near −1 and +1 in CO and O2, respectively, and an activation energy near 100 kJ mol−1. The platinum-catalyzed reaction had a much lower activation energy (56 kJ mol−1), and the kinetic order was only slightly negative in CO with a possible tendency for these findings to change toward those found for the other metals at temperatures above 480 K. These results were compared with the predictions of a model which used parameters derived from measurements made under uhv conditions and/or with only one reactant present in the gas phase. The two reactions indicated were those oxygen molecularly adsorbed on a near complete carbon monoxide layer and between adsorbed oxygen atoms and gas-phase carbon monoxide molecules. The kinetic results which were common to each metal conformed to that expected for this latter reaction with the rate limited by CO desorption. Under these conditions the order of metal activity should be opposite to the order of heats of adsorption of carbon monoxide, and from the measurements available such a relationship did seem to hold.
The adsorption of CO on Pd(111) has been studied with scanning tunneling microscopy in the very low coverage regime. Isolated CO molecules were imaged with two different corrugations of ∼0.25Å and ∼0.15Å, which are attributed to adsorption of the hcp and fcc hollow sites. Total energy calculations reveal that these two sites are the most favorable ones. Calculated image profiles using the ESQC method agree well with the experimental corrugations.
Infrared reflection-absorption spectroscopy (IRAS) has been used to study the adsorption of carbon monoxide on a Pd(111) surface. IRAS spectra were collected at temperatures from 100 to 1000 K and at pressures from 1.0 × 10−7 to 10.0 Torr. The IRAS data at temperatures > 250 K showed the coverage versus temperature behavior anticipated from the data acquired under UHV conditions. However, at temperatures < 250 K, multiple CO adsorption structures were observed which were dependent upon the adsorption temperature and pressure. The low temperature/low pressure adsorption data extrapolate to the high temperature/high pressure regime only if appropriate adsorption conditions are employed.
The catalytic performance of Pd/ZnO for steam reforming of methanol (CH3OH + H2O → CO2 + 3H2) was greatly improved by previously reducing the catalysts at higher temperatures. The original catalytic functions of metallic palladium were greatly modified as a result of the formation of PdZn alloys. Over the catalysts containing alloys, formaldehyde species formed in the reaction were suggested to be effectively attacked by water, being transformed into carbon dioxide and hydrogen. By contrast, the formaldehyde species decomposed selectively to carbon monoxide and hydrogen over catalysts containing metallic palladium.
The O-terminated ZnO(000-1) surface and Mn/ZnO(000-1) interface have been investigated by synchrotron radiation photoemission spectroscopy (SRPES), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) systematically. Our results show that ordered O-polar ZnO(000-1) surface can be prepared by annealing in an oxygen ambience and this polar surface expresses good chemical stability. At room temperature, metallic Mn film is deposited onto the cleaned ZnO(000-1)surface and grows in a layer-by-layer mode. During the process of Mn film deposition a downward Fermi level movement is observed, and the final resultant Schottky barrier height is 1.07 ± 0.05 eV. High temperature annealing is performed and the interfacial reaction happens evidently. The interfacial chemical reaction and the effect of interfacial dipole layer have been briefly discussed. Copyright © 2007 John Wiley & Sons, Ltd.
Recent experiments suggested that PdZn alloy on ZnO support is a very active and selective catalyst for methanol steam reforming (MSR). To gain insight into MSR mechanism on this catalyst, plane-wave density functional theory calculations were carried out on the initial steps of MSR on both PdZn and ZnO surfaces. Our calculations indicate that the dissociation of both methanol and water is highly activated on flat surfaces of PdZn such as (111) and (100), while the dissociation barriers can be lowered significantly by surface defects, represented here by the (221). (110), and (321) faces of PdZn. The corresponding processes on the polar Zn-terminated ZnO(0001) surfaces are found to have low or null barriers. Implications of these results for both MSR and low temperature mechanisms are discussed. (C) 2011 Elsevier B.V. All rights reserved.
The plasmon structure accompanying core-level x-ray photoemission in Na,
Mg, and Al is reported and analyzed using a model of random spatial
emission of extrinsic plasmons. The Mg and Al spectra are well fit by
this model without intrinsic plasmon production. The Na data are not
explained by purely extrinsic plasmon emission.
The adsorption and reaction of CH3OH on Pd and PdZn films and particles supported on the (0001) surface of a ZnO single crystal were studied using temperature-programmed desorption (TPD). At 300 K, vapor-deposited Pd films followed a two-dimensional island growth mode and preferentially blocked the active sites for dissociative adsorption of methanol on the ZnO(0001) surface. Upon heating, the Pd films agglomerated into particles and reacted with the ZnO substrate to form a PdZn alloy. Methanol was found to undergo complete dehydrogenation on the supported Pd and PdZn films and particles producing CO and H2. Consistent with previous studies, alloying of Pd with Zn decreased the strength of the interaction of CO with the metal surface, as evidenced by a decrease in the CO desorption temperature. A low-temperature pathway for the oxidation of CO to CO2 on Pd/ZnO(0001) and PdZn/ZnO(0001), in which oxygen was supplied by the ZnO(0001) support, was also identified.
Cu is an intracrystalline promoter for methanol synthesis catalyzed by ZnO. High-affinity CO bonding to Cu/ZnO has been shown (Didziulis, S. V.; Butcher, K. D.; Cohen, S. L.; Solomon, E. I. J. Am. Chem. Soc. 1989, 111, 7110) to involve a coordinatively unsaturated tetrahedral Cu(I) site which is also present on the CuCl(111) model surface. CO is found to bind to the CuCl(111) surface nondissociatively with its molecular axis normal to the surface and with a high heat of adsorption of 23 +/- 2 kcal/mol, similar to that of CO on Cu/ZnO. Variable photon energy photoelectron spectroscopy (PES) has been used to investigate the nature of this bonding and to compare this to CO bonding to the coordinatively unsaturated Zn(II) site on ZnO(1010BAR). CO binding to CuCl is found to induce a shift of Cu 3d-pi levels to deeper binding energy and to result in the presence of intense CO core level satellites, both results indicating the presence of pi back-bonding of intermediate strength, which is not found to be present for CO bonding to ZnO. Differences in the sigma bonding interaction are indicated by the 2.2 vs 1.3 eV shift of the CO 5-sigma peak to deeper binding energy upon adsorption on CuCl vs ZnO. This however is complicated by the CO 5-sigma interaction with the fully occupied Cu 3d band, which is found to be present from the 5-sigma photoemission cross section and shifts the 5-sigma peak energy but does not contribute to the net bonding interaction. A more direct probe of sigma bonding is given by constant initial state (CIS) resonance PES studies at the metal 3p --> 4s transition edge which gives a 2.0 vs 0.8 eV shift in the M4s energy for CO adsorption on CuCl vs ZnO, demonstrating that sigma bonding is stronger for CO/CuCl than CO/ZnO. This decrease in the sigma and pi bonding of CO to Zn(II) as compared to Cu(I) sites is ascribed to orbital contraction and d band energy stabilization associated with the higher effective nuclear charge on Zn(II). From He I PES work function studies the surface dipole moment is found to decrease and from NEXAFS studies the C-O bond length shortens upon CO adsorption to both d10 metal ion surfaces, demonstrating that in contrast to CO adsorption on transition-metal surfaces sigma bonding plays a dominate role over pi back-bonding for these two d10 ions. Analysis of XPS and Auger results shows the presence of positive effective atomic charge on the carbon end of the adsorbed CO molecule on both CuCl and ZnO. This positive charge on the carbon results from the dominance of sigma donation and would activate the CO for nucleophilic attack by hydride. However the positive charge on carbon is found to be larger for Zn(II) than Cu(I), demonstrating that the additional promotional effect of Cu(I) is not electrostatic activation of CO but would appear to relate to the ability of Cu(I) to pi back-bond which would stabilize later reaction intermediates in the catalytic mechanism of methanol synthesis.
The properties of PdZn surfaces have been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, and CO chemisorption. The formation of Pd-Zn bonds increases the binding energy of the core and valence levels of Pd and reduces the binding energy of the core and valence levels of Zn. An equiatomic PdZn alloy exhibits a large depletion of Pd(4d) states near the Fermi level and a shift of similar to+0.7 eV in the binding energy of the Pd(3d) levels. Ab initio SCF calculations for the PdZn molecule and PdnZnn (n = 2-4) clusters show a decrease in the electron population of the Pd(4d) orbitals as a consequence of Pd(4d) --> Zn(4p) charge transfer and Pd(4d) --> Pd(5s,5p) rehybridization. These electron transfers reduce electron-electron repulsion within a Pd atom, shifting its core and valence levels toward higher binding energy. The electronic perturbations induced by Zn on Pd reduce the CO-chemisorption ability of Pd by weakening the Pd(4d)-CO(2 pi) bonding interactions. PdZn surfaces that are rich in Zn show Pd-CO bonds that are 12-16 kcal/mol weaker than those seen on pure Pd surfaces. The electronic and chemical perturbations observed after bonding Pd to Zn are as large as those found for Pd bonded to early-transition metals and much bigger than those found when Pd is bonded to late-transition metals.
Exposure of a clean Zn metal to oxygen in ultra high vacuum provides a mean to gradually form ZnO. With in situ synchrotron photoelectron measurement, the progressive change in the spectra with the oxygen exposure time is observed. The analysis of the spectra allows the determination of ZnO formation. It was found that the oxidation process takes place until reaching the critical thickness, at which the oxidation rate reduces greatly to nearly zero. The critical thickness was determined to be about 2 monolayers.
The growth and alloying of thin Zn layers on Pd(1 1 1) was investigated using X-ray and ultraviolet photoelectron spectroscopy as well as low energy electron diffraction and correlated with density functional calculations. At 105 K, the formation of a pseudomorphic Zn monolayer is observed. Upon heating this layer to 550 K or upon deposition of 1 ML at 550 K, an ordered p(2 × 1) PdZn surface alloy with a Pd:Zn ratio of ∼1:1 is formed, with a characteristic Pd 3d5/2 peak at a binding energy of ∼335.6 eV. For deposition of 3 ML Zn at 550 K or by heating 3 ML, deposited at low temperature, to 500 or 600 K, a PdZn alloy with a Pd:Zn ratio of again ∼1:1 is found in the surface region, with a Pd 3d5/2 peak at ∼335.9 eV; the direct preparation at 550 K leads to a more homogeneous and better ordered alloy. The valence band spectrum of this alloy with a low density of states at the Fermi level and pronounced maxima due to the “Pd 4d” band at ∼2.4 and 3.9 eV closely resembles the spectrum of Cu(1 1 1), in good agreement with the calculated density of states for a PdZn alloy of 1:1 stoichiometry. The shift of the “Pd 4d” band to higher binding energies as compared to Pd(1 1 1) indicates a charge transfer from Zn to the Pd 4d levels. Overall, the similarity between the ultraviolet photoelectron spectra for the PdZn alloy and for Cu(1 1 1) is taken as explanation for the similar chemical activity of both systems in methanol steam reforming.
The β1 PdZn intermetallic of nominal 50:50 Pd:Zn at% was synthesized using an aerosol method. The aerosol method provided atomically mixed precursor oxy-nitrate powder that was then reduced to form β1 PdZn, having a surface area amenable to catalytic measurements. Formation of the β1 PdZn during reduction was found to occur rapidly (4 h) and at moderate temperature (500 °C), serving to minimize the loss of volatile Zn. Chemical and structural characterization confirms that β1 PdZn (95–99 wt% phase pure) of the same composition as the nitrate feedstock solution can be prepared using this method. Detailed structural analysis shows that this material contains little or no vacancies and minimal Pd/Zn disorder.Research highlights▶ Zn-volatility can lead to poor compositional control during PdZn intermetallic phase synthesis. ▶ Aerosol synthesis methods provide rapid intermetallic phase formation at reduced temperatures, reducing Zn-volatility effects and providing tighter control of composition. ▶ Combined X-ray/Neutron powder diffraction data analysis in conjunction with electron microprobe analysis leads to a precise description of the PdZn L10 structure.
The adsorption of methanol, formaldehyde, methoxy, carbon monoxide and water on a (2 × 1) PdZn surface alloy on Pd(1 1 1) has been studied using DFT calculations. The most stable adsorption structures of all species have been investigated with respect to the structure and the electronic properties. It was found that methanol is only weakly bound to the surface. The adsorption energy only increases with higher methanol coverage, where chain structures with hydrogen bonds between the methanol molecules are formed. The highest adsorption energy was found for the formate species followed by the methoxy species. The formaldehyde species shows quite some electronic interaction with the surface, however the stable η2 formaldehyde has only an adsorption energy of about 0.49 eV. The calculated IR spectra of the different species fit quite well to the experimental values available in the literature.
Anisotropic line-shape broadening (peak width which is not a smooth function of d-spacing) is frequently observed in powder diffraction patterns, and can be a source of considerable dif®culty for whole-pattern ®tting or Rietveld analysis. A model of the multi-dimensional distribution of lattice metrics within a powder sample is developed, leading naturally to a few parameters which can be varied to achieve optimal line-shape ®ts. Conditions on these parameters are derived for all crystal systems, and the method is illustrated with two examples: sodium p-hydroxybenzoate and rubidium fulleride.
Steam reforming of methanol, CH3OH + H2O 3H2 + CO2, was carried out over various Pd catalysts (Pd/SiO2, Pd/Al2O3, Pd/La2O3, Pd/Nb2O5, Pd/Nd2O3, Pd/ZrO2, Pd/ZnO and unsupported Pd). The reaction was greatly affected by the kind of support. The selectivity for the steam reforming was anomalously high over Pd/ZnO catalysts.
The goal of work described in this paper was to better understand the methanol steam reforming (MSR) activity and selectivity patterns of ZnO and CeO2 supported Pd catalysts. This reaction is being used to produce H2-rich gas for a number of applications including hydrogen fuel cells. The Pd/ZnO catalysts had lower MSR rates but were more selective for the production of CO2 than the Pd/CeO2 catalysts. The CH3OH conversion rates were proportional to the H2 chemisorption uptake suggesting that the rate determining step was catalyzed by Pd. The corresponding turnover frequencies averaged 0.8 ± 0.3 s−1 and 0.4 ± 0.2 s−1 at 230 °C for the Pd/ZnO and Pd/CeO2 catalysts, respectively. The selectivities are explained based on the reaction pathways, and characteristics of the support. The key surface intermediate appeared to be a formate. The ZnO supported catalysts had a higher density of acidic sites and favored pathways where the intermediate was converted to CO2 while the CeO2 supported catalysts had a higher density of basic sites and favored the production of CO.
We present aerosol-derived alloy powders as a uniquely useful platform for studying the contribution of the metal phase to multifunctional supported catalysts. Multimetallic heterogeneous catalysts made by traditional methods are usually nonhomogenous while UHV-based methods, such as mass selected clusters or metal vapor deposited on single crystals, lead to considerably more homogeneous, well-defined samples. However, these well-defined samples have low surface areas and do not lend themselves to catalytic activity tests in flow reactors under industrially relevant conditions. Bimetallic alloy powders derived by aerosol synthesis are homogeneous and single phase and can have surface areas ranging 1-10 m2/g, making them suitable for use in conventional flow reactors. The utility of aerosol-derived alloy powders as model catalysts is illustrated through the synthesis of single phase PdZn which was used to derive the specific reactivity of the L10 tetragonal alloy phase for methanol steam reforming. Turnover frequencies on unsupported PdZn were determined from the experimentally determined metal surface area to be 0.21 molecules of methanol reacted per surface Pd at 250 °C and 0.06 molecules of CO oxidized to CO2 per surface Pd at 185 °C. The experimentally measured activation energies for MSR and CO-oxidation on PdZn are 48 and 87 kJ/mol, respectively.
Ultrathin PdZn surface alloys on Pd(111) are model systems well-suited for obtaining a microscopic understanding of the mechanisms of Pd/Zn-based catalysis for methanol steam reforming. The temperature-induced compositional and structural changes of these alloy films are investigated in the catalytically relevant temperature range. Heating of multilayer Zn films to 500 K results in the formation of multilayer PdZn alloy films with surface and near-surface composition close to 1:1. In the temperature regime above 550 K the subsurface layers deplete quickly in Zn due to diffusion of Zn atoms into the Pd bulk. In contrast, the composition of the surface layer changes only slightly, indicating formation of a PdZn film with strong monolayer character. This change in subsurface composition triggers a change of the original Zn-out/Pd-in surface corrugation, leading ultimately to a Pcl-out/Zn-in situation for annealing temperatures beyond 700 K. The altered corrugation pattern is also obtained when submonolayer amounts of Zn are heated to similar to 500 K. The observed structural changes are in qualitative agreement with predictions by DFT calculations.
The adsorption and bonding configuration of CO on clean and Zn-covered Pd(111) surfaces was studied using Low Energy Electron Diffraction (LEED), Temperature Programmed Desorption (TPD) and High Resolution Electron Energy Loss Spectroscopy (HREELS). LEED and TPD results indicate that annealing at 550 K is sufficient to induce reaction between adsorbed Zn atoms and the Pd(111) surface resulting in the formation of an ordered surface PdZn alloy. Carbon monoxide was found to bond more weakly to the Zn/Pd(111) alloy surfaces compared to clean Pd(111). Zn addition was also found to alter the preferred adsorption sites for CO from threefold hollow to atop sites. Similar behavior was observed for supported Pd-Zn/Al2O3 catalysts. The results of this study show that both ensemble and electronic effects play a role in how Zn alters the interactions of CO with the surface.
ZnO-supported palladium-based catalysts have been shown in recent years to be both active and selective towards the steam reforming of methanol, although they are still considered to be less active than traditional copper-based catalysts. The activity of PdZn catalysts can be significantly improved by supporting them on alumina. Here we show that the Pd/ZnO/Al2O3 catalysts have better long-term stability when compared with commercial Cu/ZnO/Al2O3 catalysts, and that they are also stable under redox cycling. The Pd/ZnO/Al2O3 catalysts can be easily regenerated by oxidation in air at 420 °C followed by re-exposure to reaction conditions at 250 °C, while the Cu/ZnO based catalysts do not recover their activity after oxidation. Reduction at high temperatures (>420 °C) leads to Zn loss from the alloy nanoparticle surface resulting in a reduced catalyst activity. However, even after such extreme treatment, the catalyst activity is regained with time on stream under reaction conditions alone, leading to highly stable catalysts. These findings illustrate that the nanoparticle surface is dynamic and changes drastically depending on the environment, and that elevated reduction temperatures are not necessary to achieve high CO2 selectivity.
More than skin deep: In spite of their identical 1:1 surface composition, the geometric and electronic structures of a multilayer and monolayer PdZn surface alloy are different, as are their catalytic selectivities. The CO2 selective multilayer alloy features surface ensembles of PdZn exhibiting a Zn-up/Pd-down corrugation (see picture). These act as bifunctional active sites both for water activation and for the conversion of methanol into CO2. On the monolayer alloy CO and not CO2 is produced.
The technique of high pressure photoemission or ambient pressure photoelectron spectroscopy is highlighted (APPES). APPES was recently used to investigate the surface composition of aqueous salt solutions, with the aim of determining the possible segregation of ions to the surface, an important problem in atmospheric sciences, where it has been predicted that reactions of sea-salt aerosols with gas phase oxidants such as OH and ozone provide a mechanism for the production of substantial amounts of gas-phase halogen compounds in the troposphere. It has its usefulness in the catalysis area for analysis of surface species, reactants and products during the reaction conditions, and throwing experimental light on the transition from surface-dominated reactions to sub-surface induced chemistry. It is expected that other areas of material and nanoscience will benefit from these developments with the advent of more instruments at various synchrotrons.
Methanol steam re-forming, catalyzed by Pd/ZnO, is a potential hydrogen source for fuel cells, in particular in pollution-free vehicles. To contribute to the understanding of pertinent reaction mechanisms, density functional slab model studies on two competing decomposition pathways of adsorbed methoxide (CH(3)O) have been carried out, namely, dehydrogenation to formaldehyde and C-O bond breaking to methyl. For the (111) surfaces of Pd, Cu, and 1:1 Pd-Zn alloy, adsorption complexes of various reactants, intermediates, transition states, and products relevant for the decomposition processes were computationally characterized. On the surface of Pd-Zn alloy, H and all studied C-bound species were found to prefer sites with a majority of Pd atoms, whereas O-bound congeners tend to be located on sites with a majority of Zn atoms. Compared to Pd(111), the adsorption energy of O-bound species was calculated to be larger on PdZn(111), whereas C-bound moieties were less strongly adsorbed. C-H scission of CH(3)O on various substrates under study was demonstrated to proceed easier than C-O bond breaking. The energy barrier for the dehydrogenation of CH(3)O on PdZn(111) (113 kJ mol(-)(1)) and Cu(111) (112 kJ mol(-)(1)) is about 4 times as high as that on Pd(111), due to the fact that CH(3)O interacts more weakly with Pd than with PdZn and Cu surfaces. Calculated results showed that the decomposition of methoxide to formaldehyde is thermodynamically favored on Pd(111), but it is an endothermic process on PdZn(111) and Cu(111) surfaces.
Using a photoemission spectroscometer that operates close to ambient conditions of pressure and temperature we have determined the Pd-O phase diagram and the kinetic parameters of phase transformations. We found that on the (111) surface oxidation proceeds by formation of stable and metastable structures. As the chemical potential of O2 increases chemisorbed oxygen forms followed by a thin surface oxide. Bulk oxidation is a two-step process that starts with the metastable growth of the surface oxide into the bulk, followed by a first-order transformation to PdO.
We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.
The adsorption and reaction of methanol and formaldehyde on two-dimensional PdZn alloys on a Pd(111) surface were studied as a function of the Zn content in the alloy in order to understand the role of Zn in Pd/ZnO catalysts for the steam reforming of methanol (SRM). Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) data show that Zn atoms incorporated into the Pd(111) surface dramatically decrease the dehydrogenation activity and alter the preferred bonding sites for adsorbed CO, CH3O, and CH2O intermediates. The experimental results obtained in this study are consistent with previous theoretical studies of this system and provide new insight into how Zn alters the reactivity of Pd.
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