First-Principles Microkinetic Analysis of NO + CO Reactions on Rh(111) Surface toward Understanding NOx Reduction Pathways

In the NO + CO catalytic reaction on Rh(111), it is known from experiments that N2O and N2 are formed at low and high reaction temperatures, respectively, although the mechanism has not been fully understood. Here, we clarified its detailed mechanism using ab initio density functional theory (DFT) and microkinetic analysis. We considered that the catalytic cycle consists of following steps: NO dissociation, N2O formation, N2 formation (via N–N recombination or N2O decomposition), and CO2 formation. Their reaction energies and activation barriers were evaluated by DFT calculations and were then employed for the microkinetics and reactor simulation. We then demonstrated that N2O and N2 are mainly formed at low and high temperatures, respectively, in agreement with experiments. This is because (i) N2O formation has a lower activation barrier than that of N2 formation and thus has a faster rate at low temperature, whereas N2 formation is dominant at high temperature because of the large exothermicity, and (ii) at a higher temperature, NO dissociation occurs more and thus sufficient amount of surface N atom is provided, accelerating N + N → N2. This study demonstrated that to analyze the catalytic reactions in a wide temperature range the combination of the DFT calculation, surface microkinetics, and reactor simulation plays a crucial role.