2.1. Catalyst preparation
A TiO2 material supplied by Catalysis Society of Japan was used, which was a mixture of anatase and rutile TiO2. The structural properties will be presented later (Table 1). The doping of Ni species was made by conventional impregnation with nickel nitrate (Wako) and the loading of Ni species was 10% by weight assuming Ni species was in the form of NiO. A weighed TiO2 (1 g) was dispersed in 10 cm3 Ni(NO3)2 aqueous solution (0.15 M) and the slurry formed was continuously stirred overnight for sufficient impregnation. The mixture was kept in an oven at 80 °C for 12 h to remove the solvent (water). Then, the solid sample was ground and calcined in a muffle furnace while passing air at 50 cm3 min−1 at 250, 450 and 650 °C for 3 h. The catalysts so prepared are denoted by NiOx/TiO2 hereinafte