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There is a growing need to find new energy sources for automotive applications. One promising method is to use fuel cells as engines for which hydrogen is produced on-board. The raw material for hydrogen production can be bioethanol originated from biomass that is readily available in large quantities in every modern economies.
A number of catalysts are known to convert bioethanol by reforming into among others hydrogen. One of the most active and selective catalysts in this reaction is Rh/CeO2. The catalyst surface and the surface species formed were determined and characterized by X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared spectroscopy (DRIFTS) and temperature programmed desorption (TPD).
The exact characterization of Ce compounds by X-ray photoelectron spectroscopy is a great challenge because of the complex nature of the Ce 3d spectrum. The 3d spin-orbit doublet can split up into maximum ten individual peaks when both Ce3+ and Ce4+ species are present on the surface. The shake-up and possibly shake-down features reflect the interaction between the ligand's valence band and the Ce 4f orbital.
After a mild pretreatment consisting of oxidation at 473 K and reduction at 673 K ethanol was adsorbed onto the catalyst surface at room temperature.
XP spectra revealed that following the pretreatment both Ce3+ and Ce4+ species are present on the surface. After ethanol adsorption the intensity of the peaks characteristic of Ce3+ decreased and the binding energies of the Rh 3d orbitals slightly increased showing the partial re-oxidation of the surface. Upon heating this sample in vacuo to different temperatures, the Ce3+ peaks began to grow again marking the reduction of CeOx while the Rh 3d binding energies remained constant.
IR spectra were taken after the pretreatment described above. It was found that at low temperature not only adsorbed ethanol, different types of ethoxi species and adsorbed CO but also acetaldehyde and a significant amount of acetate groups were detectable on the surface. On the TPD spectra of adsorbed ethanol on Rh/CeO2, there is a high temperature desorption stage which was explained by the formation and decomposition of surface acetate species. In the presence of water the changes mentioned above were more pronounced.
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