Hydride formation and stability on a Pd–SiO2 thin-film model catalyst studied by TEM and SAED

Hydride formation was studied on Pd particles supported on SiO2, and the results were evaluated with reference to a corresponding ZnO-promoted Pd/SiO2 catalyst reported recently. Pd particles (mean size, ∼5 nm) were epitaxially grown on NaCl(001) cleavage faces and subsequently covered by a layer of amorphous SiO2, thereby maintaining their epitaxial orientation. The films were subjected to various hydrogen and annealing treatments in the temperature regime of 373–873 K, and their stability in an O2 environment (373–573 K) was also tested. The structural and morphological changes were followed by transmission electron microscopy and selected area electron diffraction. Reduction at 523 K increased the mean particle size considerably and converted the Pd particles into an amorphous hydride phase that decomposed at and above 673 K either by reduction in hydrogen or by annealing in He environment, forming a crystalline Pd2Si phase. Oxidative treatments of the PdH phase at temperatures above 523 K induce transformation into partially oriented Pd particles. The Pd2Si phase was stable under reductive conditions up to 873 K and decomposed into disoriented Pd particles on oxidation at temperatures at and above 573 K. Although the Pd/SiO2 catalyst can be readily converted into an amorphous hydride phase, it loses its hydrogen storage capability on entering the silicide state at 673 K, and hence no hydride formation was observed. These results are in contrast to previous observations on a corresponding Pd/ZnO/SiO2 catalyst, where the formation of a well-ordered PdZn alloy prevented the formation of the hydride phase and significantly enhanced the structural stability of the catalyst and its resistance against sintering. In contrast, the Pd/SiO2 system was converted into the amorphous hydride state under “real” catalytic conditions, for example, during CO2 hydrogenation at around 523 K.