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

Authors: 
Gumuslu-Gur, G., Carnegie Mellon University
Gellman, A. J., Carnegie Mellon University
Miller, J. B., Carnegie Mellon University
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.