Spectroscopic Characterization of Gold-Catalysts for H2 Purification and Production

Gold 2003
Anna. Chiorino, Maela Manzoli,
Abstract F. Boccuzzi, M. Manzoli and A. Chiorino
Dipartimento di Chimica I. F. M. Università di Torino, via P. Giuria 7, 10125 Torino, Italy

The development of clean H2 fuel for feeding PEM fuel cells is highly desired for “zero emission vehicles”, where hydrogen can be produced on-board by conventional fuels decomposition, as gasoline or methanol. In both cases the resulting gas mixture contains significant amounts of CO that are not tolerated by the fuel cell electrodes. Therefore highly efficient catalysts for CO selective oxidation and hydrogen production are needed. Moreover, for both reactions, catalysts active at low temperature, compatible with PEM fuel cells working conditions, are requested .
Supported nanosized gold catalysts are good candidates: they show high catalytic activity at low temperature in a number of important reactions as CO oxidation and water gas shift reaction [ , ] and they are also active in CO selective oxidation [ ]. The aim of the work is to elucidate the nature and the abundance of the surface intermediates during the CO oxidation, in absence or in presence of hydrogen from 90 K up to 500 K and to get a full understanding of the mechanism of methanol decomposition, pure or with steam and oxygen, on two differently supported gold catalysts. We have previously discussed the methanol decomposition on Au/TiO2[ ].
A first section of the work, devoted to the purification of hydrogen from CO, discusses the formation and the evolution with the temperature of the surface intermediates appearing during the CO oxidation in pure CO or in presence of an excess of hydrogen. A detailed analysis of the nature and concentration of the reaction intermediates in different experimental conditions is presented, in order to clarify the origin of the regenerative effect produced by the presence of hydrogen on the catalytic activity of gold catalysts. The FTIR and quadrupole mass study of CO oxidation in absence and in presence of hydrogen on the two catalysts provide an insight on the origin of the regenerative effect of the hydrogen and on the role of hydroxyls groups possibly bonded to gold cations at the interface with the supports. There are evidences that the presence of hydrogen in the mixture inhibits the formation of transient and stable species at the interface between the metal and the oxides. Moreover, a significant effect of the different acidity/ basicity of the two supports has been evidenced.
The second section is devoted to the study of methanol decomposition in different experimental conditions. The role of oxygen adsorbed on gold particles near oxygen vacancies of the supports in the oxidative dehydrogenation of methanol in the methanol-water-oxygen mixture is discussed. FTIR spectra run at room temperature on the two catalysts in mixtures containing methanol show that differently coordinated methoxy species are produced: on the Au/TiO2 sample two different on top methoxy and a doubly bridged one on the support cations are detected, on the Au/ZnO sample a doubly bridged methoxy species on the support cations is detected. Formate species on the supports are produced by increasing the temperature on both catalysts in methanol and in methanol-water-oxygen. Some carbonaceous species are observed. At 473 K, all the adsorbed species are almost completely depleted and hydrogen and CO2 are produced, as revealed by quadrupole mass and by FTIR. Also in this case a role of the support surface properties on the reaction is evident: some methane is produced on the Au/TiO2 catalysts, not detected at all on Au/ZnO.

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