XPS study of the nickel oxidation state in the reduced (Ag)NiMg/SiO2 vegetable oil hydrogenation catalysts

Two series of (Ag)NiMg/SiO2 catalysts are synthesized on SiO2 support derived from two silica sources: diatomite activated at 800oC (Series a; Mg/Ni=0.1 and SiO2/Ni=1.07) and synthetic water glass (Series b; Mg/Ni=0.1 and SiO2/Ni=1.15). The modification with silver is made at three molar Ag/Ni ratios, namely 0.0025, 0.025 and 0.1. The effect of the source of SiO2 support as well as of the Ag presence and content on the nickel oxidation state in the co-precipitated (Ag)NiMg/SiO2 catalysts after reduction-passivation procedure are studied by XPS. The peak-fitting of the composite Ni2p level confirms that nickel on the surface is presented in Ni0 and Ni2+ oxidation states. The higher content of Ni0 on the surface of the diatomite supported catalysts shows that the metallic nickel is more stable to oxidation in comparison of the water glass supported ones