Effect of Rh Loading on the Performance of Rh/CeO₂ in CH₄ Combustion: Important Role of Forming RhO Nanoparticles

In order to avoid the emission of CH₄ into the air, catalytic combustion of CH₄ is a practical solution, but it is challenging to develop efficient catalysts due to the inertness of CH₄. Herein, Rh/CeO₂ catalysts with different Rh loadings were synthesized and compared. The catalytic activities in CH₄ oxidation were found to increase with an increase of Rh loading. Thus, 0.3Rh/CeO₂ (with 0.3 wt % Rh) and 2.0Rh/CeO₂ (with 2.0 wt % Rh) were chosen as representatives to study the difference. It was found that the Rh species exist as Rh single atoms with a valence of +3 in 0.3Rh/CeO₂, and there are RhO nanoparticles showing the coexistence of Rh³⁺ and Rh^δ+ in 2.0Rh/CeO₂. Theoretical calculations show that, in the CeO₂-supported RhO nanoparticles catalyst, the band gap between the highest occupied band orbital and the lowest unoccupied band orbital of CeO₂ is filled with the Rh density of state, while there remains a gap of ∼0.6 eV for the single-atom catalyst. The smaller gap between the highest occupied band orbitals and the lowest unoccupied band orbitals makes the RhO nanoparticles more favorable for electron transfer than the single-atom catalyst, resulting in a lower energy barrier in C-H bond activation and higher catalytic activity. This work provides a rationale for developing high-activity catalysts for CH₄ oxidation.