The formate and redox mechanisms of watergas 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 Ag lt;sub gt;5 lt;/sub gt; cluster were investigated using DFT computations. All the elementary steps involved in both mechanisms were considered. It was observed that dissociation of H lt;sub gt;2 lt;/sub gt;O lt;sub gt;ads lt;/sub gt; and OH lt;sub gt;ads lt;/sub gt;, as well as formation of CO lt;sub gt;2(ads) lt;/sub gt;, 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 lt;sup gt;1 lt;/sup gt;) was related to the dissociation of OH lt;sub gt;ads lt;/sub gt; as the rate limiting step. The calculated activation energy of CO lt;sub gt;2(ads) lt;/sub gt; formation according to the redox mechanism (COads + O ads → CO2(ads)) was 9.32 kJ mol lt;sup gt;1 lt;/sup gt; indicating that this step was relatively fast on the surface of Ag lt;sub gt;5 lt;/sub gt; cluster. It was observed that, CO lt;sub gt;2(ads) lt;/sub gt; 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 lt;sup gt;1 lt;/sup gt;.