Syn-gas generation in the absence of oxygen and isotopic exchange reactions over Rh & Pt/doped-ceria catalysts

Syn-gas generation in the absence of oxygen by methane decomposition offers an interesting route to decrease reactor size and cost because methane is the only reactant in the gas phase. In this work, several catalysts were studied, Rh/CeO2, Pt/CeO2, Rh/(Ce0.91Gd0.09)O2−x, Pt/(Ce0.91Gd0.09)O2−x, Rh/γ-Al2O3 and Pt/γ-Al2O3 for methane reforming in the absence of gaseous oxygen. Rhodium showed a superior catalytic activity and selectivity with respect to Pt. This catalytic behavior may be due to the strong metal-support interaction, associated with the formation of mixed metal–oxide species at the interface. The addition of Gd3+ to ceria lowered the required temperatures for catalyst activation with respect to the un-doped material. Conversely to oxygen ion conducting materials, which showed a high selectivity for syn-gas generation, the non-oxygen conducting catalysts did not generated carbon monoxide. These results may be correlated to their oxygen storage capacity and ionic conductivity. Since gaseous oxygen was not delivered to the reactor, it is clear that the only source of oxygen was the catalyst. During the isothermal isotopic oxygen exchange experiments over Pt/(Ce0.91Gd0.09)O2−x and Pt/γ-Al2O3, results illustrated that oxygen in the gas phase was exchanged with the oxygen from the catalyst. Three different molecules were detected 16O–18O, 16O–16O and 18O18O. A higher amount of oxygen was exchanged over Pt/(Ce0.91Gd0.09)O2−x with respect to Pt/γ-Al2O3. It is proposed that mainly lattice and surface oxygen were exchanged over Pt/(Ce0.91Gd0.09)O2–x and Pt/γ-Al2O3, respectively. It is also suggested that two types of reaction mechanisms take place, the simple and multiple hetero-exchange with the participation of one and two catalyst oxygen atoms, respectively. Similarly to methane reforming, lower temperatures were required for the oxygen exchange experiments over Rh than over Pt, as illustrated by results of the temperature-programmed exchange reactions. In summary, the properties of doped ceria may open new catalytic routes for oxidation reactions without gaseous oxygen because post-oxidation can restore its oxygen storage capacity.

► Rh showed higher syn-gas production with respect to Pt.
► The addition of Gd3+ to ceria enhanced its catalytic activity with respect to the un-doped material.
► Rh/Alumina did not generate CO in the absence of gaseous O2.
► Higher oxygen exchange rates were shown by Rh than Pt.