Explore the Catalytic Reaction Mechanism in the Reduction of NO by CO on the Rh₇⁺Cluster: A Quantum Chemical Study

Abstract

Rhodium has been proved to possess unique reactivity to convert NO x into N 2 with high conversion efficiency and selectivity. In this study, we have carried out DFT calculations on the reaction mechanism in the reduction of NO by CO on the surface of the Rh 7 + cluster. The calculated results suggest that the reaction proceeds via three steps. First, the NO and CO are adsorbed on the Rh 7 + cluster, then the adsorbed NO decomposes to N and O atoms. The O atom reacts with the adsorbed CO leading to the formation of CO 2 molecule. Second, another NO is adsorbed on the rhodium cluster and decomposes to N and O atoms, then the two N atoms couple with each other to yield N 2 molecule. Finally, the second CO can be adsorbed on the Rh 1 or Rh 7 atom of the Rh 7 + cluster and oxidized to CO 2 molecule. On the basis of present calculations from gas-phase Gibbs free energy profiles, the reaction path related to CO adsorption on the Rh 7 atom is energetically more favorable. The second adsorbed NO generating N and O atoms in the second step is the rate-limiting step of whole catalytic cycle. The high activation barrier (TS 67 ) of 56.6 kcal/mol can be driven by large exergonic reaction. Our work would provide some valuable fundamental insights into the reaction mechanism between NO and CO on the rhodium surface, which is vitally important to decrease NO emissions in automotive exhaust gas.

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

“…1 Zeinalipour-Yazdi et al performed a systematic study on the adsorption of CO on a Rh 4 cluster, 12 indicating the vertical adsorption of CO to the metal-metal bond. 13 These theoretical studies support the process of the NO molecule dissociating into N and O atoms, and the mechanism of CO 2 formation accords to the recombination of CO and O atoms, the latter originating from NO decomposition. 13 They suggested that the reaction proceeds via the following steps: the coadsorption of NO and CO on the Rh 7 + cluster, then the decomposition of adsorbed NO into N and O atoms, the reaction of the O atom with the adsorbed CO to form CO 2 molecules, the adsorption of the second NO on the Rh 7 + cluster, the decomposition of the adsorbed NO into N and O atoms, the coupling of two N atoms to yield the N 2 molecule, and the recombination of the second CO with an O atom to yield the CO 2 molecule, in which the second adsorbed NO that generated the N and O atoms is the rate-limiting step of whole catalytic cycle.…”

“…13 These theoretical studies support the process of the NO molecule dissociating into N and O atoms, and the mechanism of CO 2 formation accords to the recombination of CO and O atoms, the latter originating from NO decomposition. 1,[11][12][13] The mechanism of CO 2 formation is experimentally in debate, with one mechanism being proposed to occur through the reaction between CO and NO, 14 and another through recombination of the CO and O atom, the latter originating from NO decomposition. [15][16][17] We previously investigated the reaction mechanism of NO and CO catalyzed by the Rh atom, 18 which involves two main reaction stages -NO deoxygenation to generate N 2 O and then the deoxygenation of N 2 O with CO to form N 2 and CO 2 .…”

“…1 They proposed that the dissociation of the N-O bond occurs with greater ease on square facets than on triangular atomic environments, and that the energy barriers to break the N-O bond depend on the charge state of the systems. 13 They suggested that the reaction proceeds via the following steps: the coadsorption of NO and CO on the Rh 7 + cluster, then the decomposition of adsorbed NO into N and O atoms, the reaction of the O atom with the adsorbed CO to form CO 2 molecules, the adsorption of the second NO on the Rh 7 + cluster, the decomposition of the adsorbed NO into N and O atoms, the coupling of two N atoms to yield the N 2 molecule, and the recombination of the second CO with an O atom to yield the CO 2 molecule, in which the second adsorbed NO that generated the N and O atoms is the rate-limiting step of whole catalytic cycle. Later, Xie et al explored the catalytic reduction mechanism of NO by CO on Rh 7 + clusters.…”

“…Later, Xie et al explored the catalytic reduction mechanism of NO by CO on Rh 7 + clusters. 1,[11][12][13] The mechanism of CO 2 formation is experimentally in debate, with one mechanism being proposed to occur through the reaction between CO and NO, 14 and another through recombination of the CO and O atom, the latter originating from NO decomposition. 13 These theoretical studies support the process of the NO molecule dissociating into N and O atoms, and the mechanism of CO 2 formation accords to the recombination of CO and O atoms, the latter originating from NO decomposition.…”

“…13 They suggested that the reaction proceeds via the following steps: the coadsorption of NO and CO on the Rh 7 + cluster, then the decomposition of adsorbed NO into N and O atoms, the reaction of the O atom with the adsorbed CO to form CO 2 molecules, the adsorption of the second NO on the Rh 7 + cluster, the decomposition of the adsorbed NO into N and O atoms, the coupling of two N atoms to yield the N 2 molecule, and the recombination of the second CO with an O atom to yield the CO 2 molecule, in which the second adsorbed NO that generated the N and O atoms is the rate-limiting step of whole catalytic cycle. 13 These theoretical studies support the process of the NO molecule dissociating into N and O atoms, and the mechanism of CO 2 formation accords to the recombination of CO and O atoms, the latter originating from NO decomposition. 1,[11][12][13] The mechanism of CO 2 formation is experimentally in debate, with one mechanism being proposed to occur through the reaction between CO and NO, 14 and another through recombination of the CO and O atom, the latter originating from NO decomposition.…”