Formation and Reactivity of Silicon Oxyhydrides in Nickel Hydrogenation Catalysts Supported on Silica

Silica is generally regarded as an inert support in the field of heterogeneous catalysis, including hydrogenation-dehydrogenation catalysis, on which hydrogen spillover is debated. Since supported hydrogenation and dehydrogenation catalysts are generally activated in H2 as reducing gas at medium-high temperatures (350–500 °C), we suspected that hydride formation could occur during such catalyst pretreatments. In this work, we investigated silicon oxyhydride (SiHxOy, SiHx for brevity) formation during the reduction pretreatment of a set of Ni/SiO2 catalyst materials using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We observed two distinct vibrational bands located at ~ 2275 and ~ 2257 cm−1, which we assigned to SiH and SiH2 species, as supported by density functional theory (DFT) calculations. Since these vibrational bands are absent after hydrogenation of pure silica, we propose that the formation of hydrides was caused by hydrogen spillover from Ni metal nanoparticles supported on silica. The decrease in the band areas with rising temperature was used in a van t ‘Hoff plot to yield enthalpy values of ~ 26 and ~ 42 kJ/mol for the apparent reaction of hydride species, suggesting an equilibrium with IR-inactive or very mobile species. The reactivity of the SiHx species was also investigated, by dosing methanol, water and CO2 as probe molecules on the pre-reduced Ni/SiO2 catalysts. Methanol reacted the fastest with the hydride species, followed by water, while CO2 did not show any interaction with the hydrides. We anticipate this work will contribute to changing the view of silica as an inert support material, and to the understanding of H spillover in more general.