(360bj) Theoretical Investigation of The Coverage Effect on Ni-In Intermetallic Catalysts for Selective Hydrogenation of Acetylene to Ethylene

Ethylene streams produced from naphtha steam crackers are the building blocks for polyethylene products and other key polymers. Acetylene impurities in ethylene streams impact the life span of the polymerization catalyst, and the quality of the polymeric products [1]. Catalysts are utilized to selectively hydrogenate acetylene to ethylene. The side reactions for the semi-hydrogenation of acetylene are the over hydrogenation of ethylene to ethane and the oligomerization of acetylene through C-C bond formation. Oligomeric species poison the active sites of the catalyst causing irreversible deactivation [1,2]. The deteriorating effects of these oligomers during the catalytic process intensify the demand for innovative selective solutions.

Palladium-based catalysts have been used for decades for hydrogenation reactions. The cost of Pd as well as the limited selectivity, introduce the need to find noble metal-free alternatives. Ni-based catalysts are cost-effective promising candidates, particularly the intermetallic compounds (IMC). IMC offer more control over the ensemble of active sites, improved surface dispersion, and less risk of surface segregation in comparison to disordered alloys. The Ni-In phase diagram shows that Ni3In, Ni2In, NiIn, and Ni2In3 all exist in the intermetallic phase. The promotional effect of introducing In to metallic Ni was evaluated against monometallic Ni surface. Exploring IMC with varied Ni surface content provides a better understanding of the optimum ratio of active transition metal to a second metal.

In this work, DFT-based modelling of Ni3In, Ni2In, Ni-In, and Ni2In3 was used to describe the coverage effect on the activity and the selectivity of semi-hydrogenation of acetylene. When constructing traditional coverage-independent DFT-based energy diagrams, adsorption energy and activation barriers were assumed to be coverage independent or at low coverage. This means that the adsorbate-adsorbate interactions on the surface were not considered. This assumption may cause disagreement between experimental and theoretical studies and mis-predict the selectivity of catalysts [2,3]. Therefore, coverage-independent and coverage-dependent reaction profiles were constructed based on the elementary steps of the hydrogenation and oligomerization reactions. Literature showed that the difference between the cross-interactions of C2 intermediates and the self-interaction was insignificant [3]. Thus, the coverage effect was approximated using a single type of species to simulate the adsorbate-adsorbate interactions on the surface and the binding energy of acetylene was evaluated at different surface coverages. Results show that the coverage effect on the energy barriers and rate-determining steps was not similar on all Ni-In surfaces. It was also shown that energy barriers were generally more processable at high coverage due to the weakened binding energies. Considering the coverage effect helps in accurately predicting the selectivity of the catalysts and provides insights to tune the reaction toward the desired product.

1. Vignola, E., Steinmann, S.N., Al Farra, A., Vandegehuchte, B.D., Curulla, D., and Sautet, P. ACS Catal. 8(3), 1662–1671 (2018).
2. Spanjers, C.S., Held, J.T., Jones, M.J., Stanley, D.D., Sim, R.S., Janik, M.J., and Rioux, R.M. J. Catal. 316, 164–173 (2014).
3. Zhao, J., Zha, S., Mu, R., Zhao, Z., and Gong, J. J. Phys. Chem. C 122,6005-6013 (2018).