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Insights on hydride formation over cerium-gallium mixed oxides: A mechanistic study for efficient H2 dissociation

Vecchietti, Julia, Baltanás, Miguel A., Gervais, Christel, Collins, Sebastián E., Blanco, Ginesa, Matz, Olivier, Calatayud, Monica, Bonivardi, Adrian
Journal of catalysis 2017 v.345 pp. 258-269
X-ray photoelectron spectroscopy, activation energy, alkanes, alkynes, catalytic activity, cations, ceric oxide, cerium, dehydrogenation, density functional theory, dissociation, gallium, heterolytic cleavage, hydrides, hydrogen, hydrogenation, infrared spectroscopy, mathematical models, moieties, nuclear magnetic resonance spectroscopy, reflectance
A four-step reaction mechanism is proposed for the H2 dissociation over pure ceria and gallium-promoted mixed oxide materials, in a combined experimental and computational investigation. Two samples of cerium-gallium mixed oxides with Ce/Ga atomic ratios equal to 90/10 and 80/20 were studied by time-resolved diffuse reflectance infrared spectroscopy under H2 (D2) flow at isothermal condition in the range of 523–623K. X-ray photoelectron spectrometry allowed to conclude that only Ce⁴⁺ is reduced to Ce³⁺ (Ga³⁺ is not reduced), in agreement with density functional theory (DFT) results. The time evolution profiles of gallium hydride (GaH) species, hydroxyl groups (OH) and Ce³⁺ infrared signals were analyzed and kinetic rate parameters for each step were obtained by mathematical modeling. The values for activation energies were in agreement with those calculated by DFT, for the different elementary pathways. A small activation energy (∼4kcal/mol) was found for H2 dissociation found on Ga⋯OCe sites assuming that the heterolytic cleavage of the HH bond is the rate determining step. On pure ceria, the experimental activation energy is ∼23kcal/mol, showing that the addition of Ga³⁺ cations boosts the splitting of H2. Interestingly, the reduction step of pure CeO2 surface domains seems to proceed via a CeH/OH pair intermediate, according to DFT calculations. Moreover, ⁷¹Ga NMR experiments indicate the possible presence of gallia nanodomains. It is proposed that the generation of Ga⋯OCe sites in the perimeter of such surface gallia nanodomains is responsible for the enhanced reactivity of the mixed materials. The key role of this new type of sites to improve the efficiency of relevant catalytic reactions such as selective alkyne hydrogenation and light alkane dehydrogenation is then analyzed.