Gallium-hydrogen bond formation on gallium and gallium-palladium silica-supported catalysts

The interaction of H-2 with Ga/SiO2, Pd/SiO2, and Ga-Pd/SiO2 catalysts has been investigated by in situ X-ray photoelectron and transmission infrared spectroscopies (XPS and FTIR). The precursors, calcined in dry air at 673 K, yield PdO and Ga2O3. Upon exposure to hydrogen reduction at 423 K, the fraction of reduced gallium ions (Gadelta+ cations, delta < 2) was 16% on Ga-promoted Pd/SiO2, whereas no Ga3+ was reduced over Ga/SiO2 materials. It is suggested that this promotional reduction effect of the noble metal over gallium(III) is accomplished through hydrogen spillover from metallic palladium, early reduced at 423 K. Higher reduction temperatures (723 K) lead to highly dispersed metallic palladium crystallites on the catalysts surface, and to the reduction of about 23-28% of the total gallium content in both the Ga/SiO2 and the GaPd/SiO2 materials. Above 473 K and under hydrogen flow, a band at 2020 cm(-1) developed over all these reduced gallium-containing catalysts, which was assigned to Gadelta+-H bond stretching. A linear relationship was found between the intensity of this infrared signal and the total gallium or Gadelta+ loading over the catalysts, at 723 K and 760 Torr of flowing H-2. We propose that gallium-hydrogen bond formation can be achieved on Ga/SiO2 by heterolytic hydrogen dissociation on the Gadelta+ cations, which are stabilized on the silica surface, to yield additional GaO-H bonds. In the Ga-Pd/SiO2 catalysts the process is further aided by the metal particles. The Gadelta+-H species were unstable below 450 K and/or decomposed under evacuation but could be immediately regenerated after restoring H-2 pressure. The impact of the formation of this gallium-hydrogen bond over the hydrogenation of carbon dioxide to oxygenated compounds is also discussed. (C) 2002 Elsevier Science (USA).