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To reduce the loading of noble metals on fuel cell catalysts a synthesis method providing evenly distributed nanoparticles on the support surface is needed. Narrow size distribution palladium nanoparticles were prepared on a porous carbon support by atomic layer deposition (ALD), and their activity for ethanol and isopropanol oxidation was studied electrochemically in alkaline media. Palladium particles had smaller average particle sizes on the support material resulting in ∼50 mV lower onset potential and 2.5 times higher mass activity for alcohol oxidation compared with a commercial catalyst. The results indicate that the use of ALD allows the preparation of a noble metal nanoparticle catalyst, and this catalyst can provide similar mass activity with lower catalyst loading than current commercial fuel cell catalysts. This would significantly reduce the cost of the cell and provide a competitive advantage compared with other power sources.

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Abstract:The potential of atomic XAFS (AXAFS) to directly probe the catalytic performances of a set of supported metal oxide catalysts has been explored for the first time. For this purpose, a series of 1 wt % supported vanadium oxide catalysts have been prepared differing in their oxidic support material (SiO2, Al2O3, Nb2O5, and ZrO2). Previous characterization results have shown that these catalysts contain the same molecular structure on all supports, i.e., a monomeric VO4 species. It was found that the catalytic activity for the selective oxidation of methanol to formaldehyde and the oxidative dehydrogenation of propane to propene increases in the order SiO 2 < Al2O3 < Nb2O5 < ZrO2. The opposite trend was observed for the dehydrogenation of propane to propene in the absence of oxygen. Interestingly, the intensity of the Fourier transform AXAFS peak decreases in the same order. This can be interpreted by an increase in the binding energy of the vanadium valence orbitals when the ionicity of the support (increasing electron charge on the support oxygen atoms) increases. Moreover, detailed EXAFS analysis shows a systematic decrease of the V-O b (-Msupport) and an increase of a the V-O(H) bond length, when going from SiO2 to ZrO2. This implies a more reactive OH group for ZrO2, in line with the catalytic data. These results show that the electronic structure and consequently the catalytic behavior of the VO4 cluster depend on the ionicity of the support oxide. These results demonstrate that AXAFS spectroscopy can be used to understand and predict the catalytic performances of supported metal oxide catalysts. Furthermore, it enables the user to gather quantitative insight in metal oxide support interactions.

The kinetics of isobutane dehydrogenation was studied on a chromia/alumina catalyst developed for fluidized-bed operation. The dehydrogenation activity measurements were carried out in a laboratory-scale plug-flow reactor at 520−580 °C under atmospheric pressure. Several kinetic models were derived from different mechanisms, and the suitability of the models in describing the rate of the dehydrogenation reaction was tested. The results of the kinetic modeling suggested that the rate-determining step is isobutane adsorption, possibly on a pair of chromium and oxygen ions. A satisfactory description of the reaction rate depended on inclusion of the isobutene and hydrogen adsorption parameters in the mathematical model. The activation energy of the dehydrogenation reaction was estimated to be 133−142 kJ/mol.