Decomposition and reduction of NO on transition metal surfaces: bond order conservation Morse potential analysis

Abstract

Periodic trends in the decomposition of NO and its reduction to N 2 and NH 3 by CO and H 2 on transition metal surfaces have been analyzed theoretically using the bond order conservation Morse potential (BOCMP) method. The analysis is based on calculations of the energetics, the reaction enthalpies ΔH and activation barriers ΔH ∗ , of elementary steps thought to comprise the mechanisms of the NO transformations. As the periodic series, the close-packed surfaces Pt(111), Rh(111), Ru(001), and Re(001) were chosen. The calculated heats of chemisorption Q of NH 3, NH 2, NH, NO, H 2O and OH are in good agreement with experiment. The activation barriers for dissociation of NO from a chemisorbed state, ΔE NO s ∗ were found to decrease in the order Pt > Rh > Ru > Re. For reasonable values of Q N and Q O, in the zero-coverage extreme these activation barriers were calculated to be much smaller than the relevant heats of chemisorption Q NO, so that dissociation of No upon heating is projected for all the surfaces studied with the possible exception of Pt(111). The presence of adsorbed N S and O S atoms may dramatically increase the values of ΔE NO S ∗ , for example, from 7–9 to 24–27 kcal/mol for Rh(100) and Rh(100)c(2 × 2)O,N, respectively. This sensitiv the values of ΔE NO s ∗ to NO S coverage may explain the diversity of experimental results obtained for different coverages (exposu and temperatures even for the same single crystal face. The anisotropy of the values of Q X(X = NO, N, O) for different surfaces and possible reconstructions of these surfaces also contribute to the balance between dissociation and desorption of NO. Of the two channels for recombinative desorption of N 2, 2N S→ N 2,g and N S + NO S→ N 2,g + O S, the latter has the smaller activati barrier. Because the N 2 formation barriers rapidly increase in the order Pt ∼ Rh ⪡ Ru ⪡ Re, Rh or Rh-Pt surfaces are projected to be the most efficient catalysts for NO reduction by CO (to N 2 and CO 2). Similarly, Pt surfaces are projected to be the most efficient catalysts for NO reduction by H 2 (to NH 3 and H 2O). The general mechanistic picture emerging from the BOCMP analysis concurs with experimental data. Some apparent inconsistencies, particularly concerning the dissociation of NO, are discussed and some model conclusions to be verified by future experiments are noted.