Mechanistic Understanding of Nitric Oxide Reactions on Supported Platinum Nanoparticles via Experiments and Computational Modeling 14 views

Supported nanoparticles exhibit heterogeneity in size and local coordination environments and this heterogeneity can have kinetic implications. Quantification of sites with specific coordination environments is therefore essential for determining the structure-activity relationships. Here, we quantify step sites on supported Pt nanoparticles using low-temperature nitric oxide (NO) reduction as a titrimetric probe reaction. We combined transient kinetic experiments, density functional theory calculations and microkinetic modeling to investigate NO reduction mechanism on Pt. Our transient kinetic experiments reveal that NO reduction proceeds stoichiometrically at 423 K, forming N2O and N2 until surface oxygen accumulation inhibits further reaction. Density functional theory calculations and microkinetic modeling show that NO reduction occurs via an associative mechanism that forms surface (NO)2 dimers on step sites, while N2 forms only by subsequent N2O decomposition. This site-specific reactivity is driven by NO coverage at 423 K, which prohibits NO reduction routes on terrace sites, and enables step sites to facilitate N2O formation. The total N2O and N2 produced per surface Pt atom remains constant across a range of reaction conditions and residence times for a given catalyst and correlates with the predicted abundance of step sites across catalysts with variable particle sizes. These findings establish NO reduction as a robust and mechanistically supported method for quantifying step sites on Pt catalysts, offering a technique for quantifying active site heterogeneity for supported metal catalysts. In addition to establishing NO reduction as a reliable probe for quantifying step sites on Pt nanoparticles, this thesis extends the mechanistic understanding of NO-metal chemistry through complementary studies on NO oxidation on Pt, and coverage-dependent NO reduction on Pd and Rh. A comprehensive literature review on NO reactions on Pt/Al2O3 examines how operating conditions, oxygen coverage, metal coordination, Pt oxide formation, and dynamic Pt oxidation behavior influence mechanistic paths and product distribution. Building on the mechanistic insights developed for NO reduction on Pt, density functional theory calculations are further extended to Pd and Rh model surfaces to investigate how NO dissociation evolve with metal identity, coordination environment, and NO* coverage. Taken together, these complementary studies provide an integrated perspective on how surface structure, metal oxidation state, and adsorbate coverage collectively govern NO reaction chemistry across supported metal catalysts. Finally, motivated by the industrial relevance of Pt mobility, this thesis includes a technical review of Pt contamination in selective catalytic reduction systems, highlighting how volatilized Pt can alter NOx reduction performance in practical aftertreatment systems.

PHD (Doctor of Philosophy)

Catalysis, Kinetics, emissions, reaction engineering

English

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2026-04-15