**4. Conclusions**

Hydrogen production through the glycerol photo-reforming of titania-based systems was significantly improved on the incorporation of nickel (0.5% by weight) through deposition–precipitation (DP) or impregnation (IMP) methods. This improvement was more pronounced when solar light was used as the irradiation source. The absorption shift to the visible range in the presence of Ni (evidenced by UV-Vis) could account for this effect. The pre-reduction of the systems prior to the catalytic essays led to a substantial improvement in catalytic performance, despite the fact that XPS studies showed that i) nickel species were "in-situ" reduced under working conditions, and ii) surface chloride species (arising from the used precursor, NiCl2) are removed as the reaction proceeds. It is possible that pre-reduction treatment at 400 ◦C induced a strong metal–support interaction (SMSI) which could be positive to the catalytic performance, although this requires further studies. This SMSI effect would be favored for systems with smaller, more uniformly-distributed Ni particle sizes synthesized by the DP

method as compared to the IMP method. This would be consistent with the observed higher activities of Ni-0.5-DP Red as compared to Ni-0.5-IMP Red. In summary, the addition of a small percentage (0.5% by weight) of a transition metal such as Ni (ca. 2000 times cheaper than Pt) resulted in a 15.5-fold increase in the catalytic activity of Evonik P25, producing 2.6 mmol H2·g<sup>−</sup><sup>1</sup> after 6 h of UV irradiation. Thus, Ni proved to be a promising metal for use in photo-reforming processes of biomass-derived oxygenated compounds.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/12/17/3351/s1. Figure S1. Hydrogen calibration plot used in this work. Figure S2. A typical chromatogram obtained in the hydrogen quantification process through GC. Figure S3. X-ray diffractograms of Evonik P25, Ni-0.5-DP. Ni-0.5-IMP, Ni-0.5-DP-Red and Ni-0.5-IMP-Red. Figure S4. Band-gap energy (Eg) calculation for the Ni-0.5-IMP-Red catalysts. Figure S5. Particle size distribution of each catalyst. Figure S6. XPS profiles for the Ni 2p3/2 component of each catalyst.

**Author Contributions:** Conceptualization, A.M. and F.J.U.; methodology, J.H-C. and A.M.; validation, F.J.U., A.M. and J.H.-C.; formal analysis, A.M. and J.H-C; investigation, J.M-G., J.M. and J.C.E.; data curation, A.M., J.H.-C.; writing—original draft preparation, J.M.-G. and J.H.C.; writing—review and editing, F.J.U. and A.M.; supervision, J.H-C., F.J.U. and A.M.

**Funding:** The authors are thankful to MINECO-ENE2016-81013-R (AEI/FEDER, EU).

**Acknowledgments:** The scientific support from the Central Service for Research Support (SCAI) at the University of Cordoba is acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
