Autothermal reforming of methane over Rh/Ce0.5Zr0.5O2 catalyst: Effects of the crystal structure of the supports

Autothermal reforming of methane (ATR) was studied over Rh catalysts supported on Ce_0.5Zr_0.5O₂ solid solution, which were synthesized by four different routes, including reverse micro-emulsion (ME), co-precipitation (CP), urea-combustion (UC) and sol-gel (SG) method. The textural and structural properties of the as-prepared solid solutions were carefully examined by means of BET, TEM, XRD and Raman techniques. Results showed that the ME sample exhibited a single cubic phase, whereas tetragonal or mixed phases such as cubic CeO₂-rich and tetragonal ZrO₂-rich phases, were found in the case of CP, UC and SG. Vegard's rule revealed that the homogeneity of these as-prepared solid solutions followed the order of ME > CP > UC > SG. TPR and CO-pulse experiments were adopted to evaluate the reducibility and the oxygen storage capacity (OSC) of the catalysts. It was found that the more homogenous the solid solution is, the more reducibility it is, i.e. both the reducibility and OSC followed the same order as that of homogeneity.Rh/ME showed the highest activity and H₂/CO ratio and such performance was maintained without significant loss during 10 h experiment. On the contrary, the other three catalysts having mixed phases showed remarkably deactivation in terms of H₂/CO due to the loss of BET area. To elucidate the resistance toward carbon formation of these catalysts, methane decomposition experiments and following temperature-programmed-oxidation (TPO) were studied. As expected, the resistance toward carbon formation could be enhanced by the improved OSC of the catalyst.