First-principles based kinetic model for the hydrogenation of toluene

A fundamental kinetic model for the hydrogenation of toluene over platinum has been constructed, based on detailed first-principles density functional theory calculations for the hydrogenation of benzene over Pt(111). A Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model was derived based on an ab initio reaction path analysis [M. Saeys, M.-F. Reyniers, M. Neurock, G.B. Marin, J. Phys. Chem. B 109 (2005) 2064], enabling easy interpretation and evaluation of the parameters appearing in the rate equation. The activation energy, adsorption, and reaction enthalpies in the model were obtained from the first-principles calculations. The pre-exponential factors were calculated from statistical mechanics, using assumptions derived from the first-principles results. The coverage-dependent hydrogen chemisorption enthalpy was optimized to accurately model lab-scale experimental data for the gas phase hydrogenation of toluene over a 0.5 wt% Pt/ZSM-22 catalyst. The resulting hydrogen chemisorption enthalpy of −54.0±1.0 kJ/mol falls between the high and low coverage values, consistent with a simulated average hydrogen surface coverage of 61%. The LHHW model based on the ab initio calculations captures the main trends in the reaction rates. The reaction model predicts reaction orders for the inlet partial pressure of hydrogen and toluene ranging from 1.6 to 2.2 and from −0.4 to 0.7, respectively. This is in reasonable agreement with experimental values, which range from 1.3 to 1.8 and from −0.3 to 0.3, respectively.