Catalytic reduction of NO by CO on Rh4+ clusters: a density functional theory study

Abstract

An extensive study was conducted to explore the catalytic reduction of NO by CO on Rh₄⁺ clusters at the ground and first excited states at the B3LYP/6-311+G(2d), SDD level. The main reaction pathway includes the following elementary steps: (1) the coadsorption of NO and CO; (2) the recombination of NO and CO molecules to form CO₂ molecules and N atoms, or the decomposition of NO to N and O atoms; (3) the reaction of the N atom with the second adsorbed NO to form N₂O; (4) the decomposition of N₂O to N₂ molecules and O atoms; and (5) the recombination of O atoms and CO to form CO₂. At low temperatures (300–760 K), the turnover frequency (TOF)-determining transition state (TDTS) is the simultaneous C–O bond formation and N–O bond cleavage, with a rate constant (s⁻¹) of k_Ps = 4.913 × 10¹²  exp(−272 724/RT). The formation of CO₂ should originate in half from the reaction between the adsorbed CO and NO. The presence of CO in some degree decreases the catalytic reduction temperature of NO on the Rh₄⁺ clusters. At high temperatures (760–900 K), the TDTS is applied to the N–O bond cleavage, with a rate constant (s⁻¹) of k_Pa = 6.721 × 10¹⁵  exp(−318 376/RT). The formation of CO₂ should stem solely from the surface reaction between the adsorbed CO and the O atom, the latter originating from NO decomposition. The bridge N_bRh₄⁺ is thermodynamically preferred. Once the bridge N_bRh₄⁺ is formed, N₂O- and NCO-contained species are predicted to exist, which is in good agreement with the experimental results.