Role of Surface Species Interactions in Identifying the Reaction Mechanism of Methanol Synthesis from CO2 Hydrogenation over Intermetallic PdIn(310) Steps

Considerable attention has been paid to the development of new catalysts for methanol synthesis from CO2 hydrogenation for a long period of time. The PdIn intermetallic catalyst was found recently to exhibit good stability and activity for methanol formation. We thus performed a theoretical study to understand the reaction mechanism of methanol synthesis on stepped PdIn(310), combining density functional theory (DFT) calculations and microkinetic analysis. On the basis of energetics obtained on clean PdIn(310), we found that the preferred reaction pathway for CH3OH generation proceeds through the COOH intermediate and further CO hydrogenation. However, microkinetic results suggested that the coverage of formate and carbon monoxide at the steady state is nearly one monolayer at the corresponding preferred adsorption site, that is, the In site for HCOO and the Pd site for CO, within the whole temperature range studied. Therefore, further studies were carried out to reveal the influence of coverage of preadsorbed formate and carbon monoxide on adsorption energies. It turned out that the differential adsorption energy of formate at the In site is comparable to that at the Pd site when the surface is covered by two formate at the In step-bridge site, indicating that it is possible for an additional formate to adsorb at either the Pd or In site under such condition. On this basis, we found with further DFT calculations and microkinetic analysis that the preferred reaction mechanism of methanol formation would change to the one including the HCOOH intermediate and the reaction prefers to happen at the Pd site, with two formate preadsorbing at the In step-bridge site at the same time. It was found that such changes can be attributed to the reduced barriers of elementary steps in this path introduced by the formate coverage effect. Therefore, it is imperative to carry out a theoretical study for surface reactions by combining DFT calculations and microkinetic analysis and to take the interactions between dominant surface species into account when identifying the mechanism under reaction conditions.