The reduction of SO2 by CO at various sites of CnAun (n=4, 8 and 16) clusters is studied using the dispersion corrected density functional theory. The energy required for the desorption of the final product COS from the respective clusters is lower than the maximum activation energy barrier of the reaction indicating that the clusters considered are free from sulphur poisoning. The rate determining step of the reaction in presence of the above clusters is also determined. The competing pathways which arise due to the presence of other available possible reaction sites are discussed.