XPS results revealed the presence of residual sulfur at the surface of the TiO2 based catalysts synthesized via the algae route, regardless of whether Ni was incorporated before or after crystallization, and whatever the reduction temperature, with a S/Ti surface atomic ratio of ca. 0.023-0.026 (Figure ). The XPS spectra recorded on selected Ni-based catalysts are shown in Fig. . The Ni 2p3/2 orbital spectra revealed a complex feature with a broad multicontribution envelope containing the core level peaks and shake-up satellite peaks corresponding to a multi-electron excitation and being characteristic of Ni species in oxidized states, with main contribution at ca. 855.7 eV corresponding. Depending on the catalyst, a more or less visible contribution at lower binding energy was observed at ca. 852.5 eV, that corresponded to metallic Ni. Metallic Ni however undergoes natural surface re-oxidation to air. Further, Table shows to which extent the activity of the most active Ni/TiO2 catalysts in the HMF hydrodeoxygenation is impacted by the reduction temperature and the reaction conditions, in particular the reaction time and the catalyst amount. First, it was observed that extending the reaction time up to 2 h on the Ni(pre)/TiO2(ALG) catalyst allowed for a strong increase in the HMF conversion from 63% to 98%, while maintaining unchanged the selectivity to 5-MF at 86%, so that an important enhancement of the 5-MF yield from 54% to 84% was observed. Further, applying a higher reduction temperature of 600°C for a reaction time of 1 h was detrimental to the HMF conversion, that decreased to 54%, but also to the selectivity pattern, as selectivities to 5-MF and BHMF of 48% and 39% were obtained, respectively. As a result, low yields to 5-MF and BHMF of 26% and 21% were achieved, respectively. Increasing the reaction time up to 2 h increased the HMF conversion to 85%, but with no improvement in terms of selectivities, so that low yield to 5-MF and BHMF of 40% and 33% were achieved, respectively. Increasing the reduction temperature to 700°C was further detrimental to the HMF conversion, that was lowered to 33%, but with a switch of the selectivity in favor of BHMF (82%), only traces of 5-MF being formed. Interestingly extending the reaction duration to 2 h and tripling the amount of catalyst when reduced at 700°C allowed for a strong increase in the HMF conversion up to 97%, while keeping very high the BHMF selectivity at 86%. In that case, a high yield to BHMF of 83% at nearly full HMF conversion was observed.