Forming defect sites on catalyst supports and immobilizing precious metal atoms at these sites offers an efficient approach for preparing single-atom catalysts. In this study, we employed an Fe-Ce oxide solid solution (FC), which has surface oxygen that reduces more readily than that of ceria, to anchor Rh single atoms (Rh₁). When utilized in the selective catalytic reduction of NO with CO (CO-SCR), Rh₁/FC reduced at 500 °C-characterized by less oxidic Rh state induced by an oxygen-deficient coordination-exhibited superior activity and durability compared to Rh₁/ceria and Rh₁/FC reduced at 300 °C. This Rh single-atom structure was sustained after 100 hours of CO-SCR at 400 °C. Reaction intermediates formed on the catalyst surface were analyzed using in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) under NO and CO flow conditions. Additionally, the catalyst structure and the CO-SCR reaction mechanism were investigated using density functional theory (DFT). While Rh atoms located near surface Fe sites were found to be thermodynamically most stable, both NO and CO preferentially adsorbed on Rh sites. Fe plays a role in stabilizing Rh sites and facilitating oxygen transfer. This work provides valuable insights into the design of highly active and durable single-atom catalysts.