Studying the elementary reactions of single-noble-metal-atom-doped species can give theoretical guidance for the design of related single-atom catalysis. Using a combination of mass spectrometry and density functional theory calculations, the reaction of RhAl2O4- with the most stable alkane molecule CH4 under thermal conditions has been studied. The methane tends to be converted into syngas (free H-2 and adsorbed CO) with activation of four C-H bonds. In sharp contrast, formaldehyde was generated in the previously reported reaction of PtAl2O4- with CH4. Density functional theory calculations show that the difference in reactivity between RhAl2O4- and PtAl2O4- is found to be due to a higher energy barrier of the third C-H bond activation for the Pt analogue. This work provides the first comparative study on the reactivity of single noble-metal atoms (Rh, Pt) on the same cluster support (Al2O4-) and can be helpful for rational design of single-atom catalysts for selective methane conversion.