(544c) Tuning the Selectivity of Ethyl Disproportionation on Pt-Doped Cu(111) Surfaces: Do You Want Ethane or Ethylene?

The recent development (synthesis and characterisation) of single-atom alloys has, over the last five years, been a real breakthrough in the field of heterogeneous catalysis [1]. When atomically dispersed and embedded in the network of a coinage metal (Au, Ag, Cu), late transition metals show high activity and selectivity. This is related to their electronic properties, which were shown to be more similar to atoms than extended metallic surfaces [2]. In particular, Pt-doped Cu(111) single atom alloys (SAA), later referred to as Pt·Cu(111), are very promising as they activate C-H bonds (like pure Pt) while displaying a strong resistance to the formation of over-unsaturated hydrocarbons and ultimately coke formation (property inherited from Cu) [3].

Here we study the reactivity of Ethyl as a probe molecule on both Cu(111) and Pt·Cu(111) surfaces, performing periodic Density Functional Theory calculations (VASP – optB86b-vdW) and kinetic Monte-Carlo simulations (Zacros – KMC). We consider two pathways, namely the hydrogenation to ethane and the beta C-H activation to ethylene. On Cu(111) both hydrogenation and dehydrogenation pathways show similar activation energies (Ea = 0.55 eV). Because of the contribution of the pre-exponential factors to the kinetic parameters and the difference in the order of the elementary processes, dehydrogenation happens to be 10 times faster than hydrogenation. When Pt substitutes one of the surface Cu atoms, Ethyl is more stable on top of Pt than on Cu. Moreover, Pt selectively opens the route to hydrogenation (Ea = 0.54 eV) over dehydrogenation (Ea = 0.74 eV). Pt·Cu(111) therefore becomes an interdependent bifunctional catalyst, with Cu and Pt being the active sites of dehydrogenation (production of atomic hydrogen) and hydrogenation (consumption of atomic hydrogen) respectively. Moreover, and unlike pure Cu(111), Pt is able to activate H₂ on Pt·Cu(111). With similar activation energies for hydrogenation and dehydrogenation on Pt·Cu(111), we can easily control the selectivity to either ethane or ethylene tuning the partial pressure of the atmosphere of H₂ in contact with the metallic surface.

This theoretical study is compared with surface science experiments, namely Temperature Program Desorption (TPD) spectra. The theoretical model (DFT + KMC) agrees almost quantitatively, as it is able to predict TPD peak temperatures and trends in selectivities under ultra-high vacuum conditions. Further comparison with micro-reactor studies is on-going.

This study expands our knowledge on the reactivity of Pt·Cu(111) as a single-atom alloy catalyst. It further proves the promising properties of Pt·Cu(111) as being a versatile heterogeneous catalyst with tuneable selectivity.

References:

[1] Darby M. T., Stamatakis M., Michaelides A., Sykes E. C. H. J. Phys. Chem. Lett., 2018, 9, 5636-5646
[2] Thirumalai H., Kitchin J. R. Topics In Catal. 2018, 61, 462-474
[3] Marcinkowski M. D., Darby M. T., Liu J., Wimble J. W., Lucci F. R., Sungsik L., Michaelides A., Flytzani-Stephanopoulos M., Stamatakis M., Sykes, E. C. H. Nature Chem. 2018, 10, 325-332