The basic unsettled problem of CO poisoning of platinum catalyst is addressed with the introduction of single dopant atoms on small cationic platinum clusters. Here, 'Rh', 'Ru' and 'Sn' doped on Pt-n(+) (n = 3 and 4) clusters are investigated under three DFT functional (MN12-SX, M06-D3 and TPSSh) using def2TZVPP basis set. Our findings reveal a notable amount of reduction of adsorption energy equal to 0.3 eV for dopant Sn over dopant Ru and Rh in Pt(3)(+)cluster with MN12-SX functional. Thus, the promotional effect of Sn on N catalyst could be a suitable system for improved CO tolerance.