Cerium-niobium mixed oxides for the water gas shift reaction

SANDRA L. HERNÁNDEZ,; SEBASTIÁN COLLINS; JAVIER CARRASCO; MARÍA V. GANDUGLIA-PIROVANO; JUAN J. DELGADO; ADRIAN BONIVARDI

Osnabrück

Workshop; Reducible oxide chemistry, structure and functions. COST CM1104 Final Meeting; 2016

CeO2, as a support, has proved to enhance the global rate for the water gas shift (WGS) reaction. It has been claimed that this behavior relies on the redox property of ceria that results in an increase of the rate of re-oxidation by water in the catalytic cycle, as compare to others non-redox supports (e.g., alumina) [1]. However, some of us have previously shown that although oxygen vacancies can react faster with water on gallium-promoted ceria catalysts, this is not the rate-determining step in the WGS reaction mechanism since an inverse correlation was found between the catalytic activity and the Ce3+ concentration [2]. Recent results for Pt/Nb2O5-CeO2 catalysts show that niobia clearly improved the activity of Pt/ceria based catalyst for the WGS reaction. Moreover, the turnover rate per Ce3+ (sub)surface sites (measured by c-MES-DRIFTS under operando conditions) suggests that those sites should be part of the catalytic site. We report here, the results of the characterization of the ceria-niobia supports by XEDS (STEM-HAADF mode, Fig. 1), XPS, TPR-H2 (IR, MS) and TPR-CO (IR), which show that well-dispersed niobia on ceria increased the reducibility of CeO2, but not of niobia. Theoretical calculations by DFT also show that ceria always stabilizes Nb5+ ions, even for the supported NbO2, by removing an additional electron of the Nb 3d states which then occupies Ce 4f states. Low energy is demanded for the additional reduction by oxygen vacancy formation, which in turns increases the Ce3+ amount (Fig. 2). The combined approach of theoretical and experimental results proved that the ability of ceria for the stabilization of reduced states is favored by niobium oxide species in 5+ oxidation state.