The state of nickel in Ni/SiO2 and Ni/TiO2-calcined catalysts

Ni/TiO2- and Ni/SiO2-calcined catalysts prepared by an ion exchange method have been studied by TPR, UV visible spectroscopy, XPS, and EXAFS. In Ni/SiO2 it is shown that Ni2+ cations exist in an octahedral coordination forming very stable nickel silicate structures. On the contrary, in Ni/TiO2, the existence of Ni3+ is revealed by EXAFS (Ni-O distance at 1.841Å) and UV visible (bands at 29400 and 24400 cm−1). In this case Ni2+ together with Ni3+ cations exists in small patches of nickel oxide on the surface of the TiO2. The formation of a Ni3+-O2n- complex at the surface is also likely.