The formate and redox mechanisms of water-gas shift reaction on the surface of Ag: A nanocluster model based on DFT study

Two different possible mechanisms of water gas shift reaction including formate and redox mechanisms on the Ag5 cluster were investigated using DFT computations. All the elementary steps involved in both mechanisms were considered. It was observed that dissociation of H2Oads and OHads, as well as formation of CO2(ads), required activation energy. For these steps, transition state structures were determined and their corresponding activation energies were calculated. For both mechanisms, the highest activation energy (402.34 kJ mol-1) was related to the dissociation of OHads as the rate limiting step. The calculated activation energy of CO2(ads) formation according to the redox mechanism (COads + O ads → CO2(ads)) was 9.32 kJ mol-1 indicating that this step was relatively fast on the surface of Ag5 cluster. It was observed that, CO2(ads) formation according to the formate mechanism occurred through three consecutive steps where the dissociation of formate (HCOO(ads) → CO2(ads) + Hads) had the highest activation energy, 171.53 kJ mol-1.