(681h) The Influence of Sulfur Promoters on the Morphology and Reactivity of Pt Nanoparticles for Methyl Cyclohexane Dehydrogenation

Central towards using liquid organic hydrogen carriers like the toluene(TOL)/methylcyclohexane(MCH) pair, is identifying dehydrogenation catalysts that selectively dissociate C-H bonds. Promoters like sulfur modify the active sites of Pt such that TOL desorption is favored over competitive C-C cleavage to benzene [1]. Using density functional theory (DFT) calculations, we determine how sulfur modifies the morphologies of Pt nanoparticles and how such structural modifications, alter the rate and selectivity of MCH dehydrogenation. We formulate a graph-based model that predicts the surface energies (errors < ~ 5 meV/Ų) of crystal planes decorated with S* having any arbitrary coverage and configuration. Leveraging this graph-based model, we screen > 10⁵ configurations on (111), (100), and (211) surfaces to identify the most thermodynamically stable S* coverages and configurations. Equilibrium morphologies of metal nanoparticles covered by S* are then determined using Wulff constructions, which reveal that S* restructures the nanoparticle (Figure 1b) such that the densities of terrace sites (selective sites) increase at the expense of step edges. Upon determining the nanoparticle structures, we then elucidate the MCH dehydrogenation pathway to TOL and to the undesired product, benzene. We consider sulfur-free Pt-55 cuboctahedral nanoparticles as a reference catalyst (Figure 1c) and compare reactivity and selectivity trends against S*- decorated Pt nanoparticles, (111), and (100) surfaces. Through this reaction pathway analysis, we discuss how S*: (a) blocks low-coordinated metal sites which promote benzene formation, (b) weakens adsorption of MCH dehydrogenation reaction intermediates, and (c) influences C-H dissociation pathways. Insights from the reaction pathway analysis are compared with experimentally measured rates and selectivity on both sulfur-free and sulfur-modified Pt nanoparticles. This work introduces an approach to quantify how promoters like sulfur alter nanoparticle morphology and their reactivity.

[1] Okada Y., Extended abstracts of the 9th Tokyo Conference on Advanced Catalytic Science and Technology, Fukuoka, KL14, (2022).