WGS reaction on ceria-supported Pt catalysts: The role of oxygen vacancies

J. VECCHIETTI; A. BONIVARDI; W. XU; D. STACCHIOLA; J.J. DELGADO; M. CALATAYUD; S. COLLINS

Barcelona

Workshop; 3rd General Meeting COST Actin CM1104, Reducible oxide chemistry, structure and functions; 2014

The precise mechanism of the water gas shift WGS reaction is a matter of intense debate, butthe dissociation of water is generally considered a key step in the reaction. In this work, wereport a study of the relationship between the catalytic activity and the reducibility of thesupport (oxygen vacancies) to activate water on well characterized platinum catalystssupported on pure and gallium-doped ceria [1-4]. Pt/CeO2 and Pt/CeGaOx catalystsinvestigated here presented a remarkable reducibility, enhanced by the incorporation of Ga3+cations [1,3] and a very stable metal dispersion. An inverse correlation was found between thecatalytic activity to WGS and the amount of oxygen vacancies [4]. In situ time resolved X-raydiffraction, mass spectrometry and diffuse reflectance infrared spectroscopy (DRIFT) showedthat the replenishment of oxygen vacancies by water is always fast either in Pt/CeO2 orPt/CeGaOx. DFT calculation provides molecular insights to understand the pathway of waterreaction with vacancies at the of metal?oxide interface sites. Concentration-modulationspectroscopy (c-MES) in DRIFT mode under WGS reaction conditions allows the selectivedetection of key reaction intermediates. Altogether, the experimental and DFT results clearlysuggest that although the oxygen vacancies can react with water, it is not the rate-determiningstep in the WGS reaction mechanism on these catalysts. The c-MES results suggest thatmonodentate formate (m-HCOO) and carboxylate (CO2
−) are reaction intermediates, whichclearly indicate the prevalence of an associative mechanism activated at the oxide-metalinterface of the catalyst.