NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO₂Adsorption Sites

Abstract

Kinetic and isotopic methods show that NO oxidation on supported Pt clusters involves kinetically relevant reaction of O 2 with vacancy sites on surfaces nearly saturated with oxygen adatoms (O*). The oxygen chemical potential at Pt surfaces that determines the O* coverage is rigorously described by an O 2 virtual pressure and determined by the thermodynamics of NO 2 -NO interconversion reactions. NO oxidation and oxygen isotopic exchange processes are described by the same rate constant, consistent with similar kinetically relevant O 2 dissociation steps for both reactions. NO oxidation, NO 2 decomposition, andO 2 exchange rates increased markedly with increasing Pt cluster size (1-8 nm); these clusters remain metallic at all O 2 virtual pressures prevalent during NO oxidation. These effects of cluster size reflect the higher vacancy concentrations and more facile oxygen desorption on larger Pt clusters, which bind oxygen adatoms weaker than more coordinatively unsaturated surface Pt atoms on smaller clusters. These trends are similar to those found for methane and dimethyl ether combustion on Pt and Pd catalysts, which also require vacancy sites on O*-saturated cluster surfaces in their respective kinetically relevant steps. Inhibition of NO oxidation by NO 2 persists to undetectable NO 2 concentrations; thus, NO oxidation turnover rates increase significantly when NO 2 adsorption sites present on BaCO 3 /Al 2 O 3 are placed within diffusion distances of Pt clusters. NO oxidation rates on intrapellet catalyst-adsorbent mixtures are described accurately by a simple reaction-adsorption model in which NO 2 adsorbs via displacement of CO 2 on BaCO 3 surfaces.

The Effect of Pt Particle Size on the Oxidation of CO, C3H6, and NO Over Pt/Al2O3 for Diesel Exhaust Aftertreatment

“…Figures 5b and 5c indicate that the TOFs for C3H6 and NO oxidation follow the relative amount of Pt terrace 345 atoms, and the highest C3H6 and NO TOFs are obtained for large Pt particles, as previously reported [5,[12][13][14][15][16][17][18][19][20]. 346…”

“…Pt particle size for both HC oxidation [13][14][15][16][17][18] and NO oxidation [12,19,20]. The observation that the optimum 326 particle diameter coincides with a maximum in TOF for CO oxidation but not for HC and NO oxidation indicates 327 that the reasons for the maxima in rates of reaction are different.…”

“…The higher activity of the terraces for the oxidation of NO has been ascribed to a 362 reaction of O2 with vacancies on terraces nearly saturated with O* adatoms [19]. As terraces bind the oxygen 363 18 / 41 atoms weaker, the vacancies are easier to create on terraces, thus favoring the NO oxidation.…”

First-Principles-Based Kinetic Monte Carlo Simulation of Nitric Oxide Reduction over Platinum Nanoparticles under Lean-Burn Conditions

“…As was shown experimentally, the reaction of NO* + O* f NO 2 * controls the surface coverage of oxygen. 50,51 The temperature dependent steady-state coverages of NO*, N*, and O* averaged over the entire Pt particle are shown in Figure 6. The results indicate that the particle is predominately covered by oxygen which is consistent with experimental results reported for reduction 52 and NO oxidation.…”

“…The results indicate that the particle is predominately covered by oxygen which is consistent with experimental results reported for reduction 52 and NO oxidation. 50,51 As the temperature increases from 500 to 900 K, the surface coverage of oxygen decreases by almost a factor of 2 from 0.62 down to 0.35 ML. The coverage of NO also decreases by about a factor of 2 from 0.24 ML at 500 K to 0.11 ML at 900 K. The atomic nitrogen coverage over the entire particle surface is quite low, ranging from just 0.03-0.07 ML over the temperatures studied.…”