(91d) Structure Sensitivity in the Reaction of Ethyl Groups On Model Chromia Surfaces: Ethyl Chloride On α-Cr2O3 (1012) and (0001)

The reactions of ethyl intermediates are studied on both the α-Cr₂O₃ (1012) and (0001) surfaces for insight into the chemical details of the catalytic dehydrogenation of ethane over chromia [1]. The α-Cr₂O₃ (1012) and (0001) surfaces were chosen because they exhibit the lowest free energies of the low-index surfaces [2], with (1012) being the predominantly exposed face on microcrystalline α-Cr₂O₃ powders [3]. The two surfaces differ primarily in terms of the local coordination of the surface Cr³⁺ cations: five coordinate for (1012), and three coordinate on (0001).

Ethyl (CH₃CH₂-) intermediates are formed from the dissociative adsorption of ethyl chloride, via C-Cl bond cleavage. In thermal desorption on the (1012) surface, the surface ethyl group reaction is initiated via a rate-limiting β-hydride elimination near 500 K in thermal desorption experiments forming CH₂=CH₂ and surface hydrogen. Subsequently, two parallel competing reactions form CH₃-CH₃, via α-hydrogen addition to remaining surface ethyl groups, and H₂, via the combination of two surface hydrogen atoms. On the (0001) surface, two seperate reaction channels are observed for surface ethyl species that appear to occur via the same elementary reaction steps seen on (1012), and produce similar products: CH₂=CH₂, CH₃-CH₃ and H₂. The lower-temperature reaction channel produces all three products near 500 K with approximately the same activation barrier to reaction observed on the (1012) surface. A higher-temperature reaction channel at 600 K produces CH₂=CH₂ and H₂, but no CH₃-CH₃. The lack of CH₃-CH₃ production from the higher energy channel suggests isolated reaction sites where remaining ethyl group coverage is limited, and the recombination of surface hydrogen from the rate-limiting production of CH₂=CH₂ is the preferred pathway.

The chlorine freed from the dissociative adsorption of ethyl chloride on both surfaces binds at the surface Cr sites and inhibits the surface chemistry by simple site blocking. No surface carbon deposition is observed from reactions of ethyl intermediates on either surface and indicates that ethyl intermediates are not principle coke formers in the dehydrogenation of ethane over (1012) or (0001) facets of α-Cr₂O₃.

References

1. B.M. Weckhuysen, R.A. Schoonheydt, Catalysis Today 51 (1999) 223.

2. P.J. Lawrence, S.C. Parker, P.W. Tasker, Journal of the American Ceramic Society 71 (1988) C389.

3. D. Scarano, G. Spoto, S. Bordiga, G. Ricchiardi, A. Zecchina, Journal of Electron Spectroscopy and Related Phenomena 64-5 (1993) 307.