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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.

We present B3LYP calculations on a selection of small-size clusters with (TiO 2 ) N stoichiometry (N ) 1-10) built on previous works on TiO 2 and SiO 2 or derived by kinship with stable clusters of different sizes. Their reactivity is analyzed as a function of size and electronic structure. Gas-phase acidity is probed by H + interaction with the oxygen sites, while basicity is tested by interaction of molecular NH 3 with titanium sites. Correlation with size, topology, or electronic properties is observed for some systems. In general, the correlation with electronic levels (highest occupied and lowest unoccupied molecular orbitals, HOMO and LUMO) is good criteria of reactivity, although this is not always observed. The calculated values generally decrease with the size. The HOMO-LUMO gaps show oscillations and a general decrease with the size. Coordination of the active site influences both the levels of the frontier orbitals and their effect upon reactivity. The protonation testing the cluster basicity is found to be higher for clusters with N ) 3 and N ) 9 in accordance with the high HOMO values. A remarkable exception is found for N ) 4 for which the most stable protonated structure is different from the most stable naked cluster. We have not tested here the flexibility of the naked cluster; however, this case means that structure reorganization should be considered for reactivity. Regarding ammonia adsorption testing cluster acidity, clusters with N ) 3 and N ) 8 present the highest adsorption energy toward NH 3 in accordance with a low LUMO value for the former but because of the local topology of the adsorption site for the latter. Acidic and basic character decreases for N ) 8-10 probably because of the increase in cohesive energy. A structure with N ) 9 emerges as the strongest base with the largest protonation energy. A tetrahedral structure with N ) 10 is remarkably stable and presents the lowest adsorption energy values.

Bottom-up and top-down derived nanoparticle structures refined by accurate ab initio calculations are used to investigate the size dependent emergence of crystallinity in titania from the monomer upwards. Global optimisation and data mining are used to provide a series of (TiO) global minima candidates in the range N = 1-38, where our approach provides many new low energy structures for N > 10. A range of nanocrystal cuts from the anatase crystal structure are also considered up to a size of over 250 atoms. All nanocrystals considered are predicted to be metastable with respect to non-crystalline nanoclusters, which has implications with respect to the limitations of the cluster approach to modelling large titania nanosystems. Extrapolating both data sets using a generalised expansion of a top-down derived energy expression for nanoparticles, we obtain an estimate of the non-crystalline to crystalline crossover size for titania. Our results compare well with the available experimental results and imply that anatase-like crystallinity emerges in titania nanoparticles of approximately 2-3 nm diameter.

The geometry, energetic, and spectroscopic properties of molecular structures of silica-supported vanadium oxide catalysts are studied using periodic density functional calculations. Isolated vanadia units deposed on amorphous silica are modeled at low coverage, 0.44 atoms nm -2 . The models are built following the grafting process through the reaction of a vanadium precursor with surface silanols: OV(OHThe most stable grafted structures involve one vanadyl group together with n(V-O-Si) bonds. The predominance of the vanadate groups is analyzed as a function of hydration by means of atomistic thermodynamics. At dehydrated conditions, the trigrafted pyramidal OV(O-Si) 3 species are predominant, whereas partial hydration stabilizes digrafted OV(OH)(O-Si) 2 and monografted OV(OH) 2 (O-Si) species. The harmonic vibrational spectra for selected models are compared to recent experimental infrared and Raman data, for representative bands, and vibrational modes. Hydration effects are discussed in terms of thermodynamic stability and vibrational spectra. The results obtained in this study show that the pyramidal OV(O-Si) 3 , digrafted OV(OH)(O-Si) 2 , and monografted OV(OH) 2 (O-Si) models can exist at a support surface, a trend in agreement with recent experimental findings.

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