Mechanism and Effects of Coverage and Particle Morphology on Rh-Catalyzed NO–H₂ Reactions

Abstract

Three-way catalysts, which typically include Rh, are used to treat automotive exhaust and reduce nitric oxide (NO) with a combination of CO and H2, although few kinetic and theoretical investigations have studied NO–H2 reactions on Rh. Here, we examine NO activation, which is believed to control the rate of NO reduction, through direct, NO-assisted, and H2-assisted dissociation pathways on NO*-covered Rh(111) surfaces and Rh nanoparticle models using density functional theory (DFT) and contrast these results with previously reported data on Pt(111) surfaces. Saturation coveragesdetermined by incrementally adsorbing NO*were determined to be 5/9 ML NO* on Pt(111), 6/9 ML on Rh(111), and 1.38 ML on a 201-atom Rh nanoparticle (∼2 nm). Free energies of activation and reaction were calculated by DFT for the pathways at these coverages and interpreted through maximum rate analyses over a wide range of NO and H2 pressures to predict NO activation mechanisms and kinetics. Rates are inhibited by NO at all relevant NO pressures and to similar extents on all catalyst models. On Pt(111) surfaces, NO is activated through NOH* formation and dissociation (to N* and OH*) at low H2 pressures (<0.5 bar) and through HNOH* (to HN* and OH*) at high H2 pressures (>0.5 bar), resulting in a shift in the H2 dependency from half order to first order. NO is activated through NOH* formation and dissociation on Rh(111) at all relevant H2 pressures, with all other pathways being >1000 times slower. NO activation occurs with similar rates through either NOH* or HNO* on Rh particles at 1.38 ML NO*, indicating that these high coverages can shift mechanistic preferences. Predicted NO consumption rates are half order in H2 on Rh particles and surfaces and are similar in magnitude to one another, despite shifts in the mechanism; these rates on Rh are 106 times slower than Pt, consistent with the prior reports that demonstrate that equal turnover rates for Pt at 60 °C occur for Rh at 200 °C. This work demonstrates that strong NO bonds activate through bimolecular (assisted) pathways and that particle models of catalysts enable high coverages of strongly bound species, which can then influence relative rates and mechanistic predictions.

Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al2O3-Supported Rh Nanoparticles

“…NO adsorption energies on Rh(111) surfaces and Rh 201 particles have been reported in our recent work . In brief, NO* prefers to bind in threefold hcp sites at low coverages in all functionals tested, although threefold fcc sites are only 1–2 kJ mol –1 less stable (Figure ).…”

“…Our previous work showed that supramonolayer coverages are possible on metal nanoparticle models because interactions between adsorbates across periodic boundaries are absent, particle curvature permits the adlayer to laterally relax and reduce strain, and surface M–M bonds can expand on nanoparticle models. ,− , Here, we show the same is true for CO* and NO* on 201-atom Rh nanoparticles and that the curvature of the particle is the predominant cause of the more exothermic binding energies of these adsorbates, which leads to higher coverages than those achievable on flat Rh(111) surfaces. CO* prefers to bind in atop modes on Rh 201 particles, with some CO* binding in bridge binding modes and saturating near 1 ML, while NO* occupies a mixture of threefold, bridge, and atop sites on Rh 201 with a saturation coverage of 1.38 ML.…”

“…Similarly, DFT calculations indicate that CO* saturates Ru 201 particles at > 1 ML but Ru(0001) surfaces at 0.75 ML ,, and that S* saturates Ru, Re, and Pt nanoparticles at higher coverages than their corresponding (111) or (0001) surfaces . Most relevantly, our prior work shows that Rh 201 nanoparticles saturate at 1.38 ML NO*, while Rh(111) surfaces saturate at ∼0.7 ML . These coverages affect relative reaction rates directly by altering reactant surface concentrations and indirectly by altering binding energies and barriers through coadsorbate interactions.…”

“…A differential binding energy (Δ E diff, i ) was calculated for each CO added to the surface where E CO*, i is the energy of the most stable configuration with i CO* and E CO(g) is the energy of gas-phase CO. Rh 201 particles have 122 surface Rh atoms, and thus, it is impractical to consider singular sequential CO* adsorption events and to consider all arrangements of submonolayer CO* adlayers. As such, CO* were added to fill all locations of one unique binding mode on the Rh particle, consistent with prior work. , For example, CO* can be added to all atop positions on corner atoms of the Rh particle or to all bridge positions along the edge between two adjacent (111) terraces. These unique binding positions are shown in Figure .…”

“…These characterization data indicate that single-crystal extended surfaces, large nanoparticles, and atomically dispersed Rh each have distinct adsorbate coverages and binding modes, but these complexities have seldom been considered in the development and use of DFT models of transition-metal catalysts. Recently, adsorptions on nanoparticle models have been studied with DFT and contrasted to single-crystal surfaces and have shown that nanoparticles can have higher coverages than surfaces, including supramonolayer coverages. − For example, our prior work shows that H*-covered 201-atom Ir and Pt particles saturate at 1.59 and 1.30 ML, respectively, during H 2 chemisorption, which is typically used to probe the size of metal nanoparticles. Similarly, DFT calculations indicate that CO* saturates Ru 201 particles at > 1 ML but Ru(0001) surfaces at 0.75 ML ,, and that S* saturates Ru, Re, and Pt nanoparticles at higher coverages than their corresponding (111) or (0001) surfaces .…”