Heterogeneous single-metal-site catalysts (HSMSCs) have been attracting increasing interest because of their unique traits. However, deactivation induced by metal agglomeration restrains the application especially when they have high metal loading. Herein, we present a method of CO/CH3I thermal treatment to prepare and regenerate Ir HSMSCs on activated carbon (AC) via atomically dispersive strategy of nanoparticles (NPs). The intermediate key species during this process, radicals, have been detected in situ by photoelectron photoion coincidence spectroscopy with vacuum ultraviolet synchrotron radiation (PEPICO-VUV). Our findings imply a strong relationship between I radicals' as well as CO molecule's roles in the breakage of NPs metal-metal bond. NPs with size larger than about 5 nm are difficult to be dispersed due to strong Ir-Ir bond together with the kinetic stability. Besides, 5 wt% Ir HSMSCs deactivated by metal agglomeration can be regenerated repeatedly and regain their initial activity for heterogeneous methanol carbonylation. (C) 2020 Elsevier Inc. All rights reserved.