Role of redox couples of Rh0/Rhδ+ and Ce4+/Ce3+ in CH4/CO2 reforming over Rh–CeO2/Al2O3 catalyst

The interaction between Rh and CeO2 over the Rh–CeO2/Al2O3 catalyst was investigated for the reaction of methane reforming with CO2. It was shown that the activity and coke resistance of Rh/Al2O3 were enhanced by the addition of CeO2, which greatly depend on the interaction between rhodium and ceria under the reaction atmosphere. In situ electrical conductivity results showed that the reducing agents (CH4, H2) in the reaction system could be activated and dissociated on Rh0, releasing electrons to CeO2 in close contact with Rh0 and generating the Ce4+/Ce3+ redox couple. Meanwhile, it was found from XPS measurements that the electron transfer could also happen from Rh0 to CeO2, creating the Rh0/Rhδ+ couple. Thus, the very coexistence of Ce4+/Ce3+ and Rh0/Rhδ+ redox couples facilitated the activation of CH4 and CO2 and further enhanced the catalytic activity and coke resistance of Rh/Al2O3. For CH4 activation, the electron-deficient state of Rhδ+ had higher ability to accept σ electrons of CH4 to promote CH4 adsorption and C–H bond cleavage, while CO2 activation was mainly facilitated by accepting free electrons of Ce3+ species, which resulted in the enhancement of carbon elimination to yield CO. Finally, a cycle mechanism of redox couples over Rh–CeO2/Al2O3 in CH4/CO2 reforming was proposed.