First-Principles Microkinetic Analysis of NO + CO Reactions on Rh(111) Surface toward Understanding NO<i>_x</i> 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