Effect of Rh Loading on the Performance of Rh/CeO in CH Combustion: Important Role of Forming RhO Nanoparticles.

In order to avoid the emission of CH into the air, catalytic combustion of CH is a practical solution, but it is challenging to develop efficient catalysts due to the inertness of CH. Herein, Rh/CeO catalysts with different Rh loadings were synthesized and compared. The catalytic activities in CH oxidation were found to increase with an increase of Rh loading. Thus, 0.3Rh/CeO (with 0.3 wt % Rh) and 2.0Rh/CeO (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/CeO, and there are RhO nanoparticles showing the coexistence of Rh and Rh in 2.0Rh/CeO. Theoretical calculations show that, in the CeO-supported RhO nanoparticles catalyst, the band gap between the highest occupied band orbital and the lowest unoccupied band orbital of CeO 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 CH oxidation.