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 AgxPd1–x Composition Spread Alloy Film (CSAF). The ethylene hydrogenation reaction order in H2 predicted by the 2H' mechanism, nH2, was consistent with nH2 = 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, keff, and the energy barriers for ethylene adsorption and desorption. Kinetic parameter estimation bounded the effective hydrogenation rate constant, keff, to between 1010 and 1014 mol/m2/sec and predicted that the ethylene adsorption energy, ΔEadsE, 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.