These changes mean, that surface of catalyst undergoes notable structural transformation upon thermal treatment (Table ). At 600°C specific surface area slightly decreases, which goes along with increase in average size of crystallites. Increasing the reduction temperature for the Ni(pre)/TiO2(ALG) catalysts led to a strong reduction of the surface area down to 53 m 2 g -1 and 9 m 2 g -1 for reduction temperature of 600°C and 700°C, respectively, together with the increase of the mean crystallite size, and a decrease of the acidity down to 311 µmol•g -1 and 136 µmol•g - [a] All the Ni/TiO2 were reduced at 500°C, except if specifically mentioned A-anatase, R-rutile derived from XRD measurements and defined as the average size of the coherently diffracting domains, derived by applying the Scherrer equation to the (101) peak of anatase at 25.7° and to the (110) peak of rutile at 27.3°, with the usual assumption of spherical crystallites taking into account the intrinsic broadening of the peaks due to the instrumentation. quantitative calculations of the crystallized phase composition performed by RIR (relative intensity ratio) method, reduced at 600 °C To understand the nature of the supported Ni nanoparticles and the interactions between the metal and the support, the reducibility of the catalysts was studied on the calcined samples and the TPR profiles are shown in Figure . Both catalysts supported on titania syntesized by the alginate route exhibited very intense hydrogen consumption peaks at relatively high temperature -starting above 400°C -in comparison to the sol-gel TiO2 and TiO2 PC500 based catalysts. This can be ascribed to the reduction of residual sulfur-containing compounds such as titanium oxysulfate, that may mask the reduction of the Ni-based species. The decrease of the sulfur content during a thermal treatment under hydrogen was confirmed by ToF-SIMS analysis (not shown). While the low-temperature shoulder might correspond to the reduction of the surface Ni-based species in the case of the Ni(post)/TiO2(ALG) catalyst, the incorporation of Ni within the TiO2 network might explain its absence in the case of the Ni(pre)/TiO2(ALG) catalyst.