The results are shown in Fig. and Fig. and S4. The images recorded on the Ni(pre)/TiO2(ALG) catalyst reduced at 500°C reveal that the TiO2 crystallites are uniform in size, and no Ni nanoparticles were evidenced at the TiO2 surface (Fig. ). This agrees with the hypothesis of an incorporation of the Ni atoms within the anatase crystallographic cell suggested by the structural XRD analysis. By contrast, it is worth noting that reducing the catalyst at 700°C led to the appearance of well-dispersed and uniform nanoparticles anchored at the TiO2 surface with ca. 2 nm mean size determined statistically over more than 300 nanoparticles (Fig. ). Hydrodeoxygenation is occurring easily in the presence of surface acid sites, therefore high acidity values stimulate the formation of the 5-MF product. As previously shown, this reaction can proceed thanks to the presence of Lewis acid sites at the titania surface, via intermolecular hydride transfer through the disproportionation reaction. This mechanism is supported by the study of Dong et al., that reported the high activity of the anatase titania polymorph and demonstrated that the activity was driven by the favored and easy HMF adsorption on anatase titania with the help of DFT calculations. So in our case we believe that anatase titania is playing a significant role in the 5-MF generation for catalysts exhibiting a high surface acidity, namely for acidity values within in the range 430-817 µmol•g -1 . Acid sites in the series of the tested Ni-based TiO2 catalysts have different origins, so that direct comparison between the catalysts is not clearly relevant. First, they can be generated during the synthesis process at the interface of the anatase support and the metal Ni centers as a result of the metal/support interaction. Second, residual sulphur at the surface of the algae-derived catalysts can also directly or indirectly participate to the surface acidity. Therefore, direct quantitative comparison of the acidity of the catalysts is not straightforward and should better be considered in a qualitative way. Increasing the reduction temperature up to 700°C led to a reduction of the surface acidity and of the titania surface exposed, as evidenced by XPS, ToF-SIMs and surface area measurements, while the formation of Ni nanoparticles was observed. Additionally, the temperature increase caused a strong increase in the rutile phase content in the TiO2 catalyst, which might influence the HMF adsorption mode as well as the protonation by surface hydroxyl groups, as both phenomenon were reported to be less favored on rutile TiO2 compared to on its anatase counterpart. Those factors together with the lower acidity are pushing the reaction towards the carbonyl group hydrogenation, as a not high-enough level of acidity does not allow for the hydrodeoxygenation pathway to be significant, so that high selectivity to BHMF (82%) was obtained. It must be said that the increase in the reduction temperature was accompanied The presence of high acidity might, however, not be sufficient when Ni is introduced via the classical impregnation method to a TiO2 material with a relatively moderate surface area (as when prepared via the alginate route), e.g. Ni(post)/TiO2(ALG). Indeed, in such case the selectivity towards 5-MF is not as high as for the other samples and we can also observe BHMF formation. This might be related to bigger size of Ni (FTIR analysis) or stronger interaction with the support as observed by TPR analysis.