Oxidation of NO with O₂ on Pt(111) and Pt(321) Large Single Crystals

Abstract

The kinetics of the oxidation of NO by O(2) was studied on 1 cm diameter single crystals, Pt(111) and Pt(321), at atmospheric pressure. The surface of the (321) crystal is composed of 20% kink, 20% step, and 60% terrace atoms and simulates small 1-3 nm size Pt particles on supported catalysts, while the (111) surface simulates the most stable plane found on large, >5 nm, particles. The turnover rates (TORs), that is, rate normalized by the exposed platinum, on the two single crystals differ by less than a factor of 2 over the range of conditions studied and are also similar to the TOR on a supported catalyst with an average particle size of 9 nm. Both surfaces show a dynamic kinetic behavior as evidenced by a change in the apparent activation energy and reaction orders as a function of reaction conditions. The oxygen coverage after initial rate experiments on Pt(111) was 0.6 monolayer (ML) on average which is similar to that measured previously by in situ X-ray photoelectron spectroscopy (XPS) under similar conditions. This oxygen overlayer, which is likely controlled by the relative presence of NO and NO(2), inhibits O(2) dissociation but lowers the binding energy of reactants enough to allow the catalysis. Long-term stability studies on Pt(111) correlate catalyst deactivation with irreversibly bound oxygen on the surface at coverages over 1 ML, as measured after reaction. Ex situ Auger electron spectroscopy (AES) and XPS results suggest that the surface defect sites on Pt(321) begin to oxidize relative to atoms on the (111) plane at lower NO(2) to NO ratios.

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.…”

Beyond mean-field approximations for accurate and computationally efficient models of on-lattice chemical kinetics

“…27,28 NO oxidation rates have been measured on single-crystal Pt surfaces as well as on supported Pt nanoparticles. 29,30 However, the NO oxidation mechanism and its transferability from surfaces to nanoparticles remain unresolved. 31 It is known that the greatest TOFs of this reaction occur over the close-packed Pt(111) facets of supported particles.…”

“…31 It is known that the greatest TOFs of this reaction occur over the close-packed Pt(111) facets of supported particles. 29,30 Moreover, it has been recognized that, during reaction conditions, oxygen (denoted as O) dominates the Pt surface, and high O coverages have been observed to account for NO oxidation activity. 32 Due to short-range repulsive interactions, the adsorbed oxygen atoms exhibit superlattice ordering, with their primary location given by threefold face-centered-cubic (fcc) sites of the Pt(111) hexagonal surface.…”

Implications of sterically constrained n-butane oxidation reactions on the reaction mechanism and selectivity to 1-butanol

“…Based on the typical nanoparticle sizes encapsulated in MOFs by the Liu et al [13] NP@MOF synthesis method, we chose the (111) facet for our model, which is representative of large nanoparticles (N5 nm) [28]. Although both metal and metal oxide catalysts could be encapsulated in NP@MOF systems [13], we opted for a transition metal catalyst motivated by the reported examples of catalytically active MOF-encapsulated catalysts [9,13].…”