(118b) Role of Support Vacancies on the Reactivity of Gold Catalysts In the CO-PROX Reaction

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The search for renewable energy sources that can replace fossil fuels has driven the interest in CO abatement reactions. Hydrogen production through either reforming fossil or renewable fuels results in mixtures of H₂ and carbon oxides as main products. For fueling PEM fuel cells hydrogen must contain ultralow levels of carbon monoxide (typically bellow 10 ppm) to prevent poisoning of the Pt-based PEMFC catalyst. The preferential oxidation with air of the pre-cleaned reformate (PROX) is a cheap and effective solution, since the range of working temperature matches the one at which PEM fuel cells operate [1].

Although there is no agreement in the literature on which is the active gold species in the CO oxidation reaction, it is well established that small gold NPs (2-3 nm) deposited on reducible supports like TiO2 or CeO2 are more active than when supported on non-reducible supports [2]. We have previously shown that the creation of oxygen vacancies in CeO2, results in a decreasing of the average size of the gold particles deposed on it. [3]  We have also shown that the support structural defects play a key role on the activity of gold catalysts in the oxidation of CO, being the catalytic activity a function of defect concentration [3-5]. Therefore, the modification of the reducible oxides network with different cations will result in modified defect structures that will depend on the nature of the modifier. The study of the effect of the interaction of these modified supports with metallic gold particles and their influence in the catalytic activity in the PROX reaction will allow understanding the synergy in these systems.

The incorporation of yttrium into the titania lattice, that is accompanied by creation of oxygen vacancies, favors the conversion of CO at low temperatures. Our results show that the amount of dopant cations introduced into the titania controls the number of surface oxygen vacancies created and, as a result, the gold particle size, directly influencing the catalytic activity; and the band gap of the resulting solid, existing a linear relationship between the catalytic activity and the band gap values found for the studied solids. These results agrees fairly well with theoretical data on these Systems [6].

On the other hand, the modification of CeO₂ with three doping metallic elements Zr, Zn and Fe, resulted in materials whose structure depends on the nature of the modifier. Solid solutions are formed on doping with Zr and Fe cations, whereas ZnO segregation is observed on doping with Zn. Whatever the case an enhancement of the reducibility or the oxygen exchange ability is observed. However, oxygen vacancies depends on the nature of the modifier, being enhanced or decreased the Lumber of vacancias on the nature of the dopant cation. Nevertheless, a high dispersion of gold nanoparticles is achieved in all cases. These results allow stating the formation of oxygen vacancies is not the only way to increase the dispersion of gold nanoparticles. In general, surfaces sites with increased electronic density act in a similar way that oxygen vacancies, being sites for the preferential deposition of the gold nanoparticles.

References

[1] F. Mariño, G. Baronetti, M. Laborde, N. Bion, A. Le Valant, F. Epron, D. Duprez, Int. J. Hydrogen Energy 33, 2008, 1345.

[2] M. Schubert, S. Hackenberg, A.C. van Veen, M. Muhler, V. Plzak, R.J. Behm, J. Catal. 197, 2001, 113.

[3] F. Romero-Sarria, L.M. Martìnez, M.A. Centeno, J.A. Odriozola, J. Phys. Chem. C 111, 2007, 14469.

[4] W.Y. Hernández, F. Romero-Sarria, M.A. Centeno, J.A. Odriozola, J. Phys. Chem. C 114 (2010) 10857–10865. 870

[5] M.I. Domínguez, F. Romero-Sarria, M.A. Centeno, J.A. Odriozola, Appl. Catal. B 87, 2009, 245.

[6] J.J. Plata, A.M. Márquez, J. Fdez. Sanz, R. Sánchez Avellaneda , F. Romero-Sarria, M.I. Domínguez, M.A. Centeno, J.A. Odriozola, Topics Catal. In the press