No abstract

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 in either Pt/CeO2 or Pt/CeGa. DFT calculation provides molecular insights to understand the pathway of water reaction with vacancies at the metal–oxide interface sites. Our results suggest that the activation of the 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 δ−) species, which suggests the prevalence of a carboxyl (HOCO) mechanism activated at the oxide–metal interface of the catalyst.

The mechanisms of adsorption of hydrogen on the anatase TiO 2 (101) surface and of its diffusion in the bulk are investigated with DFT calculations and compared with similar results obtained for the diffusion of hydrogen on the rutile (110) surface. Because of the different oxygen environments in anatase and rutile surfaces, the H binding energy on the anatase surface is 0.2À0.3 eV smaller than in rutile. Various processes for H diffusion are investigated using the climbing nudged-elastic-band (cNEB) approach. We have identified three main diffusion mechanisms, leading to migration of H on the surface, diffusion into the bulk, and desorption of H 2 molecule. Our calculated activation barrier (E act ) shows that migration of H into the bulk is the kinetically most favorable process.