A microkinetic study of CO₂ hydrogenation to methanol on Pd₁-Cu(111) and Pd₁-Ag(111) catalysts: a DFT analysis

The thermochemical conversion of CO₂ into methanol, a process known for its selectivity, often encounters a significant obstacle: the reverse water gas reaction. This problem emerges due to the demanding high temperatures and pressures, causing instability in catalytic performance. Recent endeavours have focused on innovatively designing catalysts capable of withstanding such conditions. Given the costliness of experimental approaches, a theoretical framework has emerged as a promising avenue for addressing the challenges in methanol production. It has been reported that transition metals, especially Pd, provide ideal binding sites for CO₂ molecules and hydrogen atoms, facilitating their interactions and subsequent conversion to methanol. In the geometric single-atom form, their surface enables precise control over the reaction pathways and enhances the selectivity towards methanol. In our study, we employed density functional theory (DFT) to explore the conversion of CO₂ to CH₃OH on Pd₁-Cu(111) and Pd₁-Ag(111) single-atom alloy (SAA) catalysts. Our investigation involved mapping out the complex reaction pathways of CO₂ hydrogenation to CH₃OH using microkinetic reaction modelling and mechanisms. We examined three distinct pathways: the COOH* formation pathway, the HCOO* formation pathway, and the dissociation of CO₂* to CO* pathway. This comprehensive analysis encompassed the determination of adsorption energies for all reactants, transition states, and resultant products. Additionally, we investigated the thermodynamic and kinetic profiles of individual reaction steps. Our findings emphasised the essential role of the Pd single atom in enhancing the activation of CO₂, highlighting the key mechanism underlying this catalytic process. The favoured route for methanol generation on the Pd₁-Ag(111) single-atom alloy (SAA) surface unfolds as follows: CO₂* progresses through a series of transformations, transitioning successively into HCOO*, HCOOH*, H₂COOH*, CH₂O*, and CH₂OH*, terminating in the formation of CH₃OH*, due to lower activation energies and higher rate constants.