The Dual Subsurface Hydrogen (2H') Mechanism for Ethylene Hydrogenation on Pd

A microkinetic model for ethylene hydrogenation on Pd that includes the presence of subsurface hydrogen (H') was developed by adapting the existing Horiuti-Polanyi framework. This reaction mechanism, known as the Dual Subsurface Hydrogen (2H') mechanism, is an extension of a reaction model that was initially proposed to resolve inconsistencies in the Langmuir-Hinshelwood mechanism for the H₂-D₂ exchange reaction. The 2H' mechanism accurately characterizes surface reactions on Pd-based alloy surfaces by accounting for the presence of H' in the subsurface, which activates the adsorbed H atoms on the top surface, causing them to react. In this work, we derive a 2H' mechanism for the hydrogenation of ethylene to ethane and compare the implications of the model to experimental results obtained on a Ag _x Pd_1-x Composition Spread Alloy Film (CSAF). The ethylene hydrogenation reaction order in H₂ predicted by the 2H' mechanism, n _H2, was consistent with n _H2 = 0.69 ± 0.18 measured on Pd within the temperature range 345-405 K. In addition, the 2H' rate law for ethane production was fit to experimental measurements of ethane production on Pd to estimate the effective hydrogenation rate constant, k _eff, and the energy barriers for ethylene adsorption and desorption. Kinetic parameter estimation bounded the effective hydrogenation rate constant, k _eff, to between 10¹⁰ and 10¹⁴ mol/m²/sec and predicted that the ethylene adsorption energy, $\Delta E_{ads}^E$ , is on the order of ∼10 kJ/mol. Development of the 2H' mechanism for more complex reactions, like ethylene hydrogenation, shows the necessity for considering the presence of subsurface hydrogen in properly modeling surface reactions on Pd.