Experimental and Computational Investigation of the Role of P in Moderating Ethane Dehydrogenation Performance over Ni-Based Catalysts

Abstract

We investigated the influence of P incorporation into a Ni catalyst on ethane dehydrogenation (EDH). Density functional theory calculations on model Ni(111) and Ni2P­(001) surfaces reveal that surface P generally decreases adsorption energies of fragments relevant to EDH at surface Ni sites but that P itself participates in binding some of these intermediates. These nonlinear influences of P cause CH3CH2–H activation to occur with similar facility on metal and phosphide surfaces, while CH2CH–H activation, an indicator of coking tendency, has much greater barriers on the phosphide. We prepared Ni and Ni–P catalysts on an SBA-15 support to test these predictions. A Ni–P catalyst with a 2:1 ratio (Ni2P­(2)/SBA-15), corresponding to the Ni2P phase, showed >80% ethylene selectivity during EDH at 873 K, compared to <1% ethylene selectivity on Ni/SBA-15, and maintained this selectivity up to 4 h time-on-stream. Diffuse reflectance infrared Fourier transform spectroscopy observations following ethylene exposure and heating under an inert flow indicate the appearance of carbon deposits on Ni/SBA-15 compared to ethylene desorption from Ni2P­(2)/SBA-15, consistent with predicted adsorption energy trends. Thermogravimetric analysis of spent EDH catalysts indicates significantly less carbon deposition on Ni2P­(2)/SBA-15 relative to Ni/SBA-15. The results highlight the potential of metal phosphides as selective and robust alkane dehydrogenation catalysts.

Ethane dehydrogenation performance and high temperature stability of silica supported cobalt phosphide nanoparticles

“…These results demonstrate that P incorporation significantly enhances the EDH performance of Co, which is consistent with the trends observed for light alkane dehydrogenation with other P promoted metals such as Ni, Ru, and Pt. [16][17][18][19][20]36 Co 2 P-E/SBA-15 was compared to reported catalytic results of EDH in literature and displayed comparable TOF values to Pt based alloys and other materials tested for EDH (Table S2 †). The H 2 to C 2 H 6 ratio for Co 2 P-E/ SBA-15 was also varied from 0 : 1 to 1 : 1 (Fig.…”

“…14,15 For light alkane dehydrogenation reactions, P promotion of Ni results in improved selectivity to the desired alkene as well as improved stability over monometallic Ni nanoparticles. [16][17][18][19] These enhancements in dehydrogenation performance were attributed to geometric and electronic effects of P incorporation. Specifically, geometric isolation of the metal active sites through the presence of P in the crystal structure helps to limit structure sensitive side reactions, such as hydrogenolysis.…”

“…Specifically, geometric isolation of the metal active sites through the presence of P in the crystal structure helps to limit structure sensitive side reactions, such as hydrogenolysis. 16,17,20,21 Electronic effects from P incorporation lead to a partial positive charge on Ni. This positive charge has been shown to weaken hydrocarbon-metal bonds, resulting in improved selectivity.…”

“…This positive charge has been shown to weaken hydrocarbon-metal bonds, resulting in improved selectivity. 16,17,20,21 In our previous work, we synthesized and evaluated Ni 2 P for ethane dehydrogenation (EDH) and found that P not only promotes the active metal site but also participates in the reaction pathway. In particular, density functional theory calculations predicted the pathway for deep dehydrogenation of ethylene involves the migration of surface bound ethylene to a Ni-P bridge site, resulting in a higher barrier for deep dehydrogenation of ethylene on Ni 2 P and suggesting ethylene desorption as the preferred pathway.…”

“…By preventing deep dehydrogenation, carbon formation during reaction is reduced, which results in improved stability. 16 In this work, we probe the effect of P promotion on Co for EDH. Co was chosen because it is a non-noble metal that has shown promising light alkane dehydrogenation performance when isolated, in oxide form, or promoted with elements such as Zn.…”