The reduction profile of Ni species occurs in one step, usually starting around 250-300°C, with shape and temperature of maximum reduction depending on the particle size and the strength of the interaction with the support. The profile recorded for the Ni(post)/TiO2(SG) catalyst suggested the presence of non-uniformly sized nanoparticles and potentially exhibiting different interactions with the TiO2 support. By contrast, the reduction occured at a lower temperature for the Ni/TiO2(PC500) catalyst with a single, broad peak with maximum temperature of 325 °C, what suggested the stabilization of smaller nanoparticles and/or in weaker interaction with the support. The very low intensity broad band observed for the Ni(pre)/TiO2(SG) sample revealed the absence of Ni nanoparticles in significant amount at the TiO2 surface, and supports the hypothesis of an incorporation of most of the Ni cations within the TiO2 network. Indeed, the doping of the TiO2 crystallographic anatase structure by transition metals (e.g. Ni) has been often reported when introducing the Ni precursor during the sol-gel process prior to the crystalisation of ther anatase phase. FTIR spectra of CO adsorbed on the catalysts were recorded to characterize to which extent the preparation method and the support nature influence the type and availability of the Ni centers available at the surface (Fig. ). Absorption bands within the 2000-2100 cm -1 range are usually assigned to linearly-adsorbed CO on metallic Ni crystallites, with CO adsorbed in a mono-and tetra-carbonyl Ni configuration, while the bands below 2000 cm -1 correspond generally to bridged CO adsorption. First, all studied catalysts showed absorption band centered at 2056 cm -1 characteristic of linearly adsorbed CO tetra-carbonyl Ni configuration, and no bridged CO adsorption was Table reports the ToF-SIMS and XPS semi-quantitative surface analyses performed on the catalysts in terms of surface ratio. The results showed that increasing the reduction temperature from 500°C to 700°C led to a gradual increase in the Ni + /Ti + ratio from 0.15 to 0.41 for the catalysts of the Ni(pre)/TiO2(ALG) series, which revealed a decrease in the TiO2 availability. The same behavior was observed for the TiO2 PC500 based series, with an increase in the Ni + /Ti + ratio from 0.29 to 0.57. Compared to the Ni(pre)/TiO2(ALG)-500 catalyst, the counterpart prepared by impregnation displayed a lower Ni + /Ti + ratio of 0.052, which resulted probably from far bigger Ni particles at the surface, in agreement with the TPR pattern.