(688c) The Dehydrogenation of Methyl Cyclohexane on Pt Nanoclusters: Insights from a First Principles Microkinetic Model.

Methyl cyclohexane has emerged as a promising liquid organic hydrogen carrier [1]. Methyl cyclohexane dehydrogenation to toluene is catalysed using Pt-based nanoparticles. Considering a cubo-octahedral Pt₅₅ nanoparticle as a model, we present a density functional theory (DFT) based microkinetic study. We determined the intrinsic activation barriers and thermodynamic barriers of elementary steps (Figure a). Considered pathways included methyl cyclohexane dehydrogenation to toluene, and one unselective route; toluene dissociation to benzene and methane. The degree of rate control (DRC) and the degree of thermodynamic control (DTC) were calculated to identify the rate-determining transition states and reaction intermediates, respectively. When adsorbate-adsorbate interactions were neglected, the desorption energy of toluene emerged as the sole rate-determining step. Including adsorbate-adsorbate interactions however modified the values of DRCs and DTCs (Figure b), such that both toluene desorption and the first C-H scission of methyl cyclohexane contribute to the overall rate. We constructed a linear model to determine coverage-dependent reaction energies. To reduce the computation time for transition-state calculations, another surrogate model was developed to estimate the intrinsic activation energy (ΔE_act) on different active sites for all the dehydrogenation elementary steps (Figure c). This surrogate model used both thermodynamic barriers and metal-adsorbate bond lengths as inputs. The transition-state energy predicted by the surrogate model showed improved accuracy over Brønsted-Evans-Polanyi (BEP) relations; predicting ΔE_act with errors below 0.1 eV. Coverage-dependent reaction energies and the surrogate model for determining ΔE_act were integrated within the microkinetic model. Based on the numerical solutions, an analytical expression for the reaction rate and selectivity is derived. This expression establishes the key rate determining steps that catalyst screening studies should focus on. The experimentally measured product distributions favourably compare with outputs from the microkinetic model.

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