Dehydrogenation mechanisms of methyl-cyclohexane on γ-Al2O3 supported Pt13: Impact of cluster ductility

• Published in: Journal of catalysis Vol. 370; pp. 118 - 129
• Main Authors: Zhao, Wei, Chizallet, Céline, Sautet, Philippe, Raybaud, Pascal
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.02.2019
• Subjects: active sites; adsorption; Alumina; aluminum oxide; cleavage (chemistry); Dehydrogenation; density functional theory; DFT; Gibbs free energy; hydrogen; kinetics; molecular dynamics; platinum; Platinum cluster; Reforming catalyst; toluene; Reforming catalyst; Platinum cluster; DFT; Dehydrogenation; Alumina
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2018.12.004

[Display omitted] •The mechanism of methyl-cyclohexane dehydrogenation is decoded on Pt13/Al2O3.•The Pt13 cluster’s reconstruction challenges the structure-sensitivity concept.•Several CH bond dissociation steps exhibit competing activation energies.•Dispersion corrections impact the free energy profile and BEP relationships.•The support influences the stability of some reaction intermediates. By using density functional theory (DFT) and ab initio molecular dynamics, we investigate the dehydrogenation reactivity of 13 atoms platinum cluster supported on the γ-alumina (1 0 0) surface. We provide a detailed free energy profile and structural analysis of the dehydrogenation mechanisms of methyl-cyclohexane (MCH) into toluene. We highlight the quantitative impact of dispersion corrections on the free energy profile and on the adsorption configurations of the intermediates exhibiting a dual interaction with the cluster and with the alumina surface. During the step by step dehydrogenation of MCH, several reconstructions of the Pt cluster and hydrogen migrations occur. Due to the cluster ductility, they are moderately activated and provide optimal active sites catalyzing the CH bond cleavages. According to a preliminary kinetic analysis based either on energetic spans or on activation free energies of elementary steps, we found that many states and/or steps may be considered as determining ones. This may explain some diverging interpretations brought by previous experimental kinetic studies. We finally discuss how the cluster ductility challenges the historical concept of structure sensitivity/insensitivity for a given reaction in the case of nanometer-size metallic clusters dispersed on a support.