Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts

JULIA VECCHIETTI; ADRIAN BONIVARDI; WENQIAN XU; DARIO STACCHIOLA; JUAN J. DELGADO; MONICA CALATAYUD; SEBASTIÁN COLLINS

ASC Catalysis

American Chemical Society

Año: 2014 vol. 4 p. 2088 - 2096

2155-5435

Reducible oxides have been shown to greatly improve the activity of water gas shift (WGS) catalysts. The precise mechanism for this effect is a matter of intense debate, but the dissociation of water is generally considered to be the key step in the reaction. We present here a study of the water activation on oxygen vacancies at the support, as part of the mechanism of the WGS reaction on Pt supported on pure and gallium-doped ceria. Doping the ceria with gallium allows tuning the vacancies in the support, while maintaining constant the metal dispersion. An inverse relationship was found between the catalytic activity to WGS and the amount of oxygen vacancies. In situ time resolved X-ray diffraction, mass spectrometry and diffuse reflectance infrared spectroscopy (DRIFT) showed that the oxygen vacancy filling by water is always fast either in Pt/CeO2 or Pt/CeGa. DFT calculation provides molecular insights to understand the pathway of water reaction with vacancies at the of metal-oxide interface sites. Our results suggest that the activation of water molecule in the WGS mechanism is not the rate limiting step in these systems. Concentration-modulation spectroscopy in DRIFT mode under WGS reaction conditions allows the selective detection of key reaction intermediates, a monodentate formate (HCOO) and carboxylate (CO2-) species, which clearly indicate the prevalence of an associative mechanism activated at the oxide-metal interface of the catalyst.