(47d) Microkinetic Analysis of Ethylene Hydrogenation on Pd-Based Catalysts: Effect of Subsurface Hydrogen on Mechanism and Rate Control

Ethylene, C₂H₄, hydrogenation to ethane, C₂H₆, has been widely studied to gain insight into more complex hydrogenation reactions. Yet, hydrogenation of C₂H₄ is a complex reaction itself, as it occurs through a series of elementary steps. Reaction kinetics depend sensitively on temperature, reactant partial pressures and the catalyst. Hence, it is difficult to analyze the reaction mechanism and elucidate the kinetic parameters of each elementary step, experimentally. Moreover, studies in recent years show that subsurface hydrogen plays a critical role in hydrogenation reactions. However, presence of subsurface hydrogen further complicates the reaction mechanism and kinetics.

In this study, activity of C₂H₄ hydrogenation on Pd, Pd₈₅Cu₁₅, and Pd₇₀Cu₃₀ foil catalysts were measured experimentally to elucidate a reaction mechanism that accounts for subsurface hydrogen. Experiments were performed over a temperature range of 300 – 450 K at 1 bar, with various H₂ and C₂H₄ inlet partial pressures. Experimental C₂H₆ conversion data was fit to models derived from two mechanisms; (i) classical Horiuti-Polayni (HP) mechanism, and (ii) mechanism that includes subsurface hydrogen in elementary steps. For each catalyst, suitable model for the reaction was selected through statistical analysis. Degree of rate control (DRC) analysis for each intermediate (IS) and transition (TS) state was also performed based on the selected model. The experimental data and the analysis results show that:

• The activity of catalysts for C₂H₄ hydrogenation increase in the order: Pd₇₀Cu₃₀ < Pd₈₅Cu₁₅ < Pd.
• Mechanism that includes the effect of subsurface hydrogen defines the reaction better compared to classical HP mechanism on all Pd-based catalysts used in this study.
• C₂H₅ - - H complex, with H supplied from surface for Pd, from subsurface for Pd₇₀Cu₃₀ and Pd₈₅Cu₁₅, is the significant TS in DRC.
• DRC analysis for IS showed that H occupying a surface site is the important species on Pd and Pd₈₅Cu₁₅, whereas it is the subsurface H on Pd₇₀Cu₃₀.

To our knowledge, this is the first experimental study on Pd-based foil catalysts that elucidates the detailed reaction mechanism, kinetics and DRC of species/states in ethylene hydrogenation, including the effect of subsurface hydrogen. Results of this study emphasize (i) the importance of subsurface hydrogen by proposing a new mechanism for ethylene hydrogenation on Pd-based foil catalysts, and (ii) the fact that only one state/species or elementary reaction cannot be considered as rate determining, as overall reaction is a combination off all these steps and states.