[Show abstract][Hide abstract] ABSTRACT: Impregnation and flame spray pyrolysis (FSP) as complementary preparation routes were applied for the synthesis of Pd-doped SnO2 based sensors. The structure of the sensors, in particular the palladium constituent, was found to be significantly different as analyzed by X-ray diffraction, BET surface area, electron microscopy, X-ray absorption spectroscopy, temperature-programmed reduction (TPR), and their sensing behavior by DC-resistance measurements. XAS was additionally applied under in situ and sensing conditions to derive structure–function relationships. The impregnation of sol–gel derived and calcined SnO2 powder led mainly to Pd/SnO2 sensors where PdO particles were located on the SnO2 surface as evidenced by X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and TPR-XANES. Flame-spray pyrolysis led to a mostly homogeneously mixed Pd–SnO2 powder with Pd being mainly dispersed in the SnO2. In this case both a directly deposited and a screen printed Pd–SnO2 layer were prepared. Sensing of hydrogen in air showed that its sensing could be significantly improved by direct deposition of the FSP-derived Pd/SnO2. Comparison of the screen printed FSP and the samples synthesized by impregnation supports earlier reports that small PdO clusters decorating the surface of the SnO2 grains is beneficial for sensing.
No preview · Article · Nov 2015 · Sensors and Actuators B Chemical
[Show abstract][Hide abstract] ABSTRACT: Second generation biofuels are produced in the bioliq® process at the Karlsruhe Institute of Technology via gasification of pyrolysis oil and synthesis of gasoline from the emerging synthesis gas. An alternative strategy is the direct upgrading of the pyrolysis oil by hydrodeoxygenation (HDO). The present study reports on the HDO of guaiacol as one of the phenolic compounds strongly abundant in such mixtures. Special focus was laid on the solvent influence using Pt-based catalysts. Higher HDO ability was seen using nonpolar solvents and acidic supports. Characterization of the catalysts before and after the test showed that the solvent did not only influence the reactivity, but also the catalyst stability.
No preview · Article · Nov 2015 · Chemie Ingenieur Technik
[Show abstract][Hide abstract] ABSTRACT: A nanodispersed intermetallic GaPd2/SiO2 catalyst is prepared by simple impregnation of industrially relevant high-surface-area SiO2 with Pd and Ga nitrates, followed by drying, calcination, and reduction in hydrogen. The catalyst is tested for CO2 hydrogenation to methanol at ambient pressure, revealing that the intrinsic activity of the GaPd2/SiO2 is higher than that of the conventional Cu/ZnO/Al2O3, while the production of the undesired CO is lower. A combination of complementary in situ and ex situ techniques are used to investigate the GaPd2/SiO2 catalyst. In situ X-ray diffraction and in situ extended X-ray absorption fine structure spectroscopy show that the GaPd2 intermetallic phase is formed upon activation of the catalyst via reduction and remains stable during CO2 hydrogenation. Identical location-transmission electron microscopy images acquired ex situ (i.e., micrographs of exactly the same catalyst area recorded at the different steps of activation and reaction procedure) show that nanoparticle size and dispersion are defined upon calcination with no significant changes observed after reduction and methanol synthesis. Similar conclusions can be drawn from electron diffraction patterns and images acquired using environmental TEM (ETEM), indicating that ETEM results are representative for the catalyst treated at ambient pressure. The chemical composition and the crystalline structure of the nanoparticles are identified by scanning TEM energy dispersive X-ray spectroscopy, selected area electron diffraction, and atomically resolved TEM images.
[Show abstract][Hide abstract] ABSTRACT: Mo2C/ZrO2 was investigated as catalyst for hydrodeoxygenation (HDO) of phenol in 1-octanol as a simplified bio-oil model system in a fixed-bed setup at 100 bar. Mo2C/ZrO2 selectively converted phenol to benzene above 320 °C. During long-term testing, limited stability of the catalyst was observed, with the conversion of 1-octanol and phenol decreasing from 70% to 37% and from 37% to 19%, respectively, over 76 h of operation. Repeating the experiment but also co-feeding 30% water, the catalyst deactivated completely within 12 h of operation. Thermodynamic calculations and in situ XRD analysis showed that Mo2C is transformed to MoO2 in the presence of water at the given conditions, and this was probably the source of deactivation in the experiments. Thus, Mo2C-based catalyst for HDO seems interesting, but requires further stabilization or regeneration of the carbide phase as bio-oil contains high levels of water and water is a by-product during HDO.
Full-text · Article · Aug 2015 · Journal of Catalysis
[Show abstract][Hide abstract] ABSTRACT: The methanation of CO2 as a relevant strategy for energy storage has been studied by operando X-ray absorption spectroscopy under dynamic H2/CO2 and CO2 reaction atmospheres. A typical CO2 conversion of 81% was reached at 400 °C with a 23 wt.-% Ni/CaO–Al2O3 catalyst, yielding 80% of CH4. The operando XAS experiment under working conditions revealed pronounced structural changes, e.g., a fast bulk oxidation of the Ni particles after removal of H2 from the H2/CO2 (4:1) gas stream. A lower performance of the catalyst was observed in the subsequent methanation cycle due to the presence of a residual oxidized fraction of Ni.
No preview · Article · Jul 2015 · Journal of Catalysis
[Show abstract][Hide abstract] ABSTRACT: Ni/Al2O3 catalysts were tested for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) as an important bio-based platform molecule for chemical products based on renewable feedstocks. The catalysts were prepared by wet impregnation, incipient wetness impregnation, precipitation, and flame spray pyrolysis; both the influence of different solvents (monovalent alcohols and water) as well as solvent free reaction conditions were screened in batch autoclaves. Whereas alcohols led to a number of side reactions that could only be suppressed by high hydrogen pressures (>20 bar), water as solvent resulted in a GVL selectivity of 100 %. The GVL yields reached 57%. Further improvement was achieved without any solvent, whereby the GVL yield increased to 92% at 100% LA conversion. Reuse of the Ni catalysts resulted in a significant drop in activity. The catalysts were thoroughly characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), linear combination analysis of X-ray absorption near edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS). The results indicated that incorporated Ni2+ as present in flame-derived catalysts was less active for GVL synthesis compared to surface Ni particles as present in the wet impregnated catalyst.
No preview · Article · May 2015 · Applied Catalysis A General
[Show abstract][Hide abstract] ABSTRACT: The structure of copper sites in Cu-SSZ-13 during NH3-SCR was unravelled by a combination of novel operando X-ray spectroscopic techniques. Strong adsorption of NH3 on Cu, its reaction with weakly adsorbed NO from the gas phase, and slow re-oxidation of Cu(I) were proven. Thereby the SCR reaction mechanism is significantly different to Fe-ZSM-5.
Full-text · Article · Apr 2015 · Chemical Communications
[Show abstract][Hide abstract] ABSTRACT: The total oxidation of methane was studied over a Pd-Pt/Al2O3 catalyst with selected variations of typical gaseous emission components of natural gas engines being oxygen, methane, water, carbon dioxide, higher hydrocarbons, CO, NOx, and SO2. The light-off, durability and reactivation of deactivated samples were studied. A continuous deactivation of the catalyst was observed in methane/air. In situ XAS revealed a Pd oxidation state +2 under these conditions. No pronounced changes in BET surface area, noble metal dispersion, and oxidation state were observed for the deactivated samples. However, the deactivation is accompanied by segregation of Pt and Pd in core-shell bimetallic particles. This deactivation did not occur in the presence of NOx. The catalyst could furthermore be reactivated in the presence of NOx as well as by the reduction in hydrogen. Even a small addition of SO2 was observed to have a pronounced negative impact on the catalyst activity and durability. This deactivation is attributed to the blocking of active noble metal sites by sulfur compounds, because the number of active sites is drastically reduced as observed by CO-chemisorption measurements.
[Show abstract][Hide abstract] ABSTRACT: A series of ceria-supported Rh and Pt catalysts was synthesized by flame spray pyrolysis for high temperature water gas shift (WGS) reactions in a membrane reactor. The structural and catalytic properties were compared with a Pt/CeO2 catalyst prepared conventionally by incipient wetness impregnation. BET results show that compared to conventionally prepared catalysts the specific surface area of the flame made catalysts is larger and decreases with increasing metal content. This is in line with X-ray diffraction (XRD) results uncovering a high nanocrystallinity of the ceria, whose crystallite size slightly increases with higher noble metal loading. Both Pt and Rh particles are not detected by XRD due to the small crystallite size. In fact, transmission electron microscopy (TEM) images of the flame made catalysts with high loading showed Pt and Rh particles with diameters of ca. 3 nm which remained stable after catalytic testing. X-ray absorption near edge spectra (XANES) at the Rh K- and Pt L3-edge showed that both Pt and Rh were in an oxidized state after preparation. In situ XANES during temperature programmed reduction in 5% H2/He indicated a lower reduction temperature than for the conventional Pt/CeO2 catalyst. Hence, Rh and Pt species are located on the surface of the CeO2 particles. Extended X-ray absorption fine structure (EXAFS) spectra provide further evidence for oxidized Rh, Pt before reduction and full reduction after activation in hydrogen, and small particle sizes in agreement with the TEM and XRD results. The catalytic activity measurements were performed at atmospheric pressure in two different gas mixtures by measuring CO conversion as a function of temperature. In all cases the Pt/CeO2 catalysts prepared by flame spray pyrolysis showed high catalytic activity and much higher selectivity than the Rh/CeO2 catalyst, where methane formation is observed above 300 °C. Furthermore, the reaction rate in terms of turnover frequencies is much higher on the Pt-based catalysts, which makes them promising candidates for high temperature WGS catalysts in membrane and microreactors.
No preview · Article · Feb 2015 · Applied Catalysis A General
[Show abstract][Hide abstract] ABSTRACT: In this work, we present a detailed study of the formation of supported intermetallic Ni-Ga catalysts for CO2 hydrogenation to methanol. The bimetallic phase is formed during a temperature-programmed reduction of the metal nitrates. By utilizing a combination of characterization techniques, in particular in situ and ex situ X-ray diffraction, in situ X-ray absorption spectroscopy, transmission electron microscopy combined with electron energy loss spectroscopy and X-ray fluorescence, we have studied the formation of intermetallic Ni-Ga catalysts of two compositions: NiGa and Ni5Ga3. These methods demonstrate that the catalysts with the desired intermetallic phase and composition are formed upon reduction in hydrogen and enable us to propose a mechanism of the Ni-Ga nanoparticles formation. By studying the effect of calcination prior to catalyst reduction, we show that the reactivity depends on particle size, which suggests that the reaction is structure sensitive.
No preview · Article · Dec 2014 · Journal of Catalysis
[Show abstract][Hide abstract] ABSTRACT: The effect of Ni catalysts promoted with Mg on catalytic activity and carbon deposition was investigated in acetic acid steam reforming. Acetic acid was chosen as representative compound for the steam reforming of bio-oil derived from biomass pyrolysis. In this study, nickel catalysts modified with Mg have been prepared by sequential impregnation method with 15 wt.% Ni and variable loadings of Mg (1-10 wt.%). The samples were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), high resolution transmission electron microscopy (HRTEM), specific surface area (BET) and temperature programmed reduction (TPR-H-2). Steam reforming of acetic acid was conducted in a fixed bed reactor at a temperature of 500 degrees C and 600 degrees C. Under reactive conditions, the 15Ni5Mg/Al catalyst proved to be superior to the other catalysts at 600 degrees C at which it presented an effluent gaseous mixture with the highest H-2 selectivity and reasonable low coke formation.
No preview · Article · Nov 2014 · Applied Catalysis B Environmental
[Show abstract][Hide abstract] ABSTRACT: Hydrodeoxygenation of guaiacol over Pt-based catalysts was studied as a representative for phenolic compounds in pyrolysis oil. Screening of various Pt-based catalysts supported on different oxides and using different preparation methods resulted in 1% Pt/SiO2 and platinum supported on zeolites, such as 1% Pt/H-MFI-90, as the most promising catalysts in a temperature range up to 200 °C. Thereby conversions of 86 % and 100 % were received, respectively. Particularly, selectivities to cyclohexane above 90 % were achieved for 1% Pt/H-MFI-90.
No preview · Article · Oct 2014 · Applied Catalysis A General
[Show abstract][Hide abstract] ABSTRACT: Die Effizienz von Polymerelektrolytmembran-Brennstoffzellen hängt stark von der Funktionalität der Elektrokatalysatoren ab, d. h. deren Aktivität und Stabilität. Wir beschreiben hier die Entwicklung eines Katalysators, der die typische Aktivität nanoskaliger Pt-Legierungen in der kathodischen Sauerstoffreduktion mit der exzellenten Stabilität eines nanostrukturierten Trägermaterials vereint. Aufgrund seiner hohen spezifischen Aktivität und großen aktiven Oberfläche zeigt der Katalysator eine ungewöhnliche massenbezogene Aktivität von 10 A mgPt−1. In beschleunigten Alterungstests behielt der Katalysator seine ursprüngliche Oberfläche und intrinsische Aktivität unter üblichen Betriebsbedingungen bei. Dieses exzellente Verhalten wird durch die raumbegrenzte Legierung der Nanopartikel in den Poren des Trägermaterials erreicht.
No preview · Article · Oct 2014 · Angewandte Chemie
[Show abstract][Hide abstract] ABSTRACT: The efficiency of polymer electrolyte membrane fuel cells is strongly depending on the electrocatalyst performance, that is, its activity and stability. We have designed a catalyst material that combines both, the high activity for the decisive cathodic oxygen reduction reaction associated with nanoscale Pt alloys, and the excellent durability of an advanced nanostructured support. Owing to the high specific activity and large active surface area, the catalyst shows extraordinary mass activity values of 1.0 A mgPt−1. Moreover, the material retains its initial active surface area and intrinsic activity during an extended accelerated aging test within the typical operation range. This excellent performance is achieved by confined-space alloying of the nanoparticles in a controlled manner in the pores of the support.
No preview · Article · Oct 2014 · Angewandte Chemie International Edition
[Show abstract][Hide abstract] ABSTRACT: An in-depth understanding of the active site requires advanced operando techniques and the preparation of defined catalysts. We elucidate here the mechanism of the selective catalytic reduction of NO by NH3 (NH3-SCR) over a Fe-ZSM-5 zeolite catalyst. 1.3 wt% Fe-ZSM-5 with low nuclearity Fe sites was synthesized, tested in the SCR reaction and characterized by UV-Vis, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy. Next, this defined Fe-zeolite catalyst was studied by complementary high-energy-resolution fluorescence-detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (V2C XES) under different model in situ and realistic working (operando) conditions identical to the catalyst test bench including the presence of water vapor. In particular, this included the presence of water vapor which strongly influences the catalytic performance and the structure of the active centre. HERFD-XANES uncovered that the coordination (between 4 and 5), geometry (tetrahedral, partly 5-fold), and oxidation state of the Fe centres (reduced in NH3, partly in SCR mixture, slight reduction in NO) strongly changed. V2C XES supported by DFT calculations provided important insight into the chemical nature of the species adsorbed on Fe sites. The unique combination of techniques applied under realistic reaction conditions and the corresponding catalytic data unraveled the adsorption of ammonia via oxygen on the iron site. The derived reaction model supports a mechanism where adsorbed NOx reacts with ammonia coordinated to the Fe3+ site yielding Fe(2+) which reoxidation is slow.
No preview · Article · Aug 2014 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: The long term stability and resistance toward carbon deposition, sulfur, chlorine, and potassium of Ni/ZrO2 as a catalyst for the hydrodeoxygenation (HDO) of guaiacol in 1-octanol (as a model compound system for bio-oil) has been investigated at 250 [degree]C and 100 bar in a trickle bed reactor setup. Without impurities in the feed good stability of the Ni/ZrO2 catalyst could be achieved over more than 100 h of operation, particularly for a sample prepared with small Ni particles, which minimized carbon deposition. Exposing the catalyst to 0.05 wt% sulfur in the feed resulted in rapid deactivation with complete loss of activity due to the formation of nickel sulfide. Exposing Ni/ZrO2 to chlorine-containing compounds (at a concentration of 0.05 wt% Cl) on-stream led to a steady decrease in activity over 40 h of exposure. Removal of the chlorine species from the feed led to the regaining of activity. Analysis of the spent catalyst revealed that the adsorption of chlorine on the catalyst was completely r
[Show abstract][Hide abstract] ABSTRACT: PMMA-polysilsesquioxane (PMMA-Sil) class II organic-inorganic hybrids were prepared by the sol-gel method from a PMMA-based polymer precursor containing trimethoxysilane groups. An analysis was made of the effect of siloxane content, adjusted by addition of tetraethyl orthosilicate (TEOS), on the structure and thermal stability of the dried gels. 13C nuclear magnetic resonance measurements confirmed PMMA as the organic phase, while 29Si measurements revealed the presence of both T and Q silicon species, the most abundant being T2 and Q3. X-ray diffraction results showed that the inorganic SiO2 phase was amorphous, while small angle X-ray scattering analyses indicated that the average gyration radius size of the silicate particles and the correlation distance between the particles increased with greater addition of TEOS. Thermal stability was improved by increasing the amount of the inorganic phase. This effect was more evident under an air atmosphere than under N2, indicating that the silicate phase acted to limit oxygen diffusion.
No preview · Article · Jun 2014 · Polymer Degradation and Stability