Dehydrogenation mechanisms of liquid organic hydrogen carriers over Pt, Pd, Rh, and Ni surfaces: Cyclohexane as a model compound

Highlights: • Cyclohexane dehydrogenation over Pt, Pd, Rh, and Ni metals are calculated. • Adsorption and activation energies of elementary reactions are obtained. • Initial dehydrogenation of C₆H₁₂ is the rate-determining step. • Rh performs the best reactivity in the cyclohexane dehydrogenation. In order to make a better understanding of dehydrogenation mechanisms of liquid organic hydrogen carriers, cyclohexane was selected as a model compound to carry out its decomposition studies over Pt(1 1 1), Pd(1 1 1), Rh(1 1 1), and Ni(1 1 1) surfaces via periodic density functional theory calculations. The adsorption geometries and adsorption energies of reaction intermediates were presented. Similar linear relationships of the adsorption energies with respect to the number of hydrogen removal over these four surfaces were revealed for C₆Hx* (x = 8–12). Seven elementary reactions for cyclohexane successive dehydrogenation to C₆H₅* were considered. The initial dehydrogenation of C₆H₁₂* to generate C₆H₁₁* were identified as the rate-determining steps over all surfaces, association with activation energies of 1.04, 1.06, 0.96, and 1.14 eV for Pt(1 1 1), Pd(1 1 1), Rh(1 1 1), and Ni(1 1 1), respectively. The reactivity of dehydrogenation was in the order of Rh(1 1 1) > Pt(1 1 1) > Pd(1 1 1) > Ni(1 1 1).