Origin of CO2 as the main carbon source in syngas-to-methanol process over Cu: theoretical evidence from a combined DFT and microkinetic modeling study

Abstract

The main carbon source for methanol synthesis from syngas (CO, CO₂ and H₂) has been studied experimentally for decades. In the current work, we attempt to understand the origin of such observations at a molecular level by combining density functional theory (DFT) and a microkinetic modeling approach. The energetics of elementary steps in the process of CO and CO₂ hydrogenation to methanol on Cu(211) were calculated by DFT. We found that the hydrogenation of CO to methanol over Cu(211) proceeds via the reaction pathway of CO → HCO → CH₂O → CH₃O → CH₃OH. Subsequently, microkinetic modeling was performed, taking a mixture of CO, CO₂ and H₂ as the reactant, with a variable ratio between CO and CO₂. The microkinetic results showed that the Cu(211) surface will have a certain coverage (around 0.083 monolayer) of spectating HCOO at the steady state. More importantly, the formation rate of methanol will increase dramatically with a small amount of CO₂ added to the mixture of CO and H₂ over the surface with spectating HCOO, which is in good agreement with previous experimental results. In addition, we found that the formation of methanol from CO₂ hydrogenation is always faster than that from CO hydrogenation, which further supports the conclusion that the main carbon source in the syngas-to-methanol process over Cu catalyst is CO₂, rather than CO.