(694a) Selective C?H Bond Activation of Light Alkane Using Metal Phosphide Catalysts

Conversion of light alkanes derived from shale gas to light alkenes, which are potential building blocks for transportation fuels and chemicals, is of great practical importance. The challenges of alkane dehydrogenation are (1) to suppress coke formation by giving a balance between the ability to activate C−H bonds while avoiding over-activation and (2) to replace current expensive Pt-based alloy catalysts. In this regard, we have chosen to focus on metal-rich phoshpides as an alternative because they are commonly metallic with reactivities similar to metals, and potentially offer site isolation and electronic modifications similar to alloys.

Here we employed density functional theory (DFT) calculations to figure out different dehydrogenation reactivities on Ni and Ni₂P, which is known to be active for hydrogenations[1] and dehydrogenations[2]. To begin with, we compared adsorption energies and site preferences of C1 and C2 hydrocarbon species on Ni(111) and Ni₂P(001). While CH-containing species generally similar adsorption energies on both surfaces, sp³ and sp² C tend to favor binding at P sites over Ni sites. This preference has consequences for predicted dehydrogenation performance. Further, we found that Ni₂P has a similar initial dehydrogenation (C₂H₆ * + * → C₂H₅* + H*) activity to Ni even though their reaction pathways differ because of the influence of the P sites. On the other hand, we predicted ethylene selectivity by comparison with activation barrier for ethylene dehydrogenation (C₂H₄* + * → C₂H₃* + H, a potential pathway towards coke) and ethylene desorption energy on both surfaces. While both surfaces bind ethylene nearly isoenergetically, further dehydrogenation of ethylene to vinyl is considerably more activated on Ni than on Ni₂P due to the fact that site isolation by P atoms inhibits its reaction pathway. These results suggest a higher selectivity towards ethylene and against other products on Ni₂P than on Ni. Such a selectivity ordering has been reported for isobutane dehydrogenation[3]. Our results provide evidence that metal phosphides can be both selective and robust dehydrogenation catalysts.

REFERENCES

[1] S.T. Oyama, T. Gott, H. Zhao, Y.-K. Lee, Transition metal phosphide hydroprocessing catalysts: A review, Catal. Today. 143 (2009) 94–107.

[2] J. Kibsgaard, C. Tsai, K. Chan, J.D. Benck, J.K. Norskov, F. Abild-Pedersen, T.F. Jaramillo, Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends, Energy Environ. Sci. 8 (2015) 3022–3029.

[3] Y. Xu, H. Sang, K. Wang, X. Wang, Catalytic dehydrogenation of isobutane in the presence of hydrogen over Cs-modified Ni2P supported on active carbon, Appl. Surf. Sci. 316 (2014) 163–170.