**4. Conclusions**

This paper describes the synthesis, characterization and catalytic behavior of Rh-based catalysts, obtained by using the Rh4(CO)12 neutral cluster as the active-phase precursor. In particular, the preparation method allowed the deposition of the cluster on the surface of Ce0.5Zr0.5O2 and ZrO2 supports, which were synthetized by w/o microemulsion technique. The catalysts were found to be active in the low-temperature steam reforming process for syngas production. At high Rh loadings (0.6%) the CZOm-supported catalyst was active at 350 ◦C and was able to reach the equilibrium conversion, especially at low S/C ratio or at high pressures at 450 and 500 ◦C. It showed good stability, and this opens the possibility of employing such catalyst in membrane reactors, enhanced reformers or chemical loop based on reforming. At lower concentrations (0.05%) and high temperature, the CZOm-supported cluster sample showed better results with respect to the analogous ZOm-supported one and to a classical Rh-impregnated CeZr catalyst. At low temperature, a deactivation effect was observed for the CZOm-supported catalyst, which could be overcome by employing a ZOm support. A detailed analysis provided evidences that the oxidation of the Rh promoted by Ce and high oxygen mobility was responsible for the fast deactivation. In these conditions, it was also observed that the cluster-based catalyst which had not been treated with hydrogen at 500 ◦C was more active than the treated one, due to the sintering of the Rh particles. Finally, the unreduced 0.05% Rh cluster deposited on the ZrO2 support showed significant activity at 500 ◦C.

**Author Contributions:** A.F. investigated the catalytic activity and the support characterization and wrote the first draft; S.R. investigated the cluster preparation and deposition, and collaborated to write the first draft; C.F. supervised the catalyst preparation, investigated the IR and cluster analysis, and collaborated in writing, reviewing, and editing; F.B. defined the methodology for support preparation, supervised the project, validated the catalytic data, and took care of the conceptualization and of writing, reviewing, and editing of the paper.

**Funding:** This research received no external funding.

**Acknowledgments:** The contribution of Francesca Ospitali of the Department of Industrial Chemistry "Toso Montanari" is acknowledged for the TEM analysis.

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