Promoting propane dehydrogenation via strain engineering on iridium single-atom catalyst

Abstract

The rational design of active and selective catalysts is a challenge for heterogeneous catalysis, also true for propane dehydrogenation (PDH). Owing to the separated active sites, Single-atom catalysts (SACs) exhibit promising PDH applications with high propylene selectivity. Here, by means of density functional theory and the energetic span model, we systemically investigate the effect of strain on PDH over the nitrogen-coordinated iridium SAC (IrN4). We find that the thermodynamics and kinetics of each elementary step in PDH process exhibit linear relationship with applied strain (from −4.0% to 4.0%), as well as the geometry and electronic structure of the intermediates. We show that the compression strain can promote the first dehydrogenation while tension strain hinders it, which originates from the enhanced electron-donating of IrN4 site under compression strain and more bonding orbitals filling between Ir and carbon adatom (Ir-C). The first dehydrogenation and the second dehydrogenation are presented opposite energy trends under applied strain. This is attributed to strain-induced opposite changes in the energy levels of the d-band center of the IrN4 site relative to the p-band and s-band center of the intermediates. Further, using the energetic span model, we show that the first dehydrogenation dominates the activity of the PDH process, where the overall turnover frequency (TOF) of PDH can be greatly improved by stabilizing the first dehydrogenation intermediate (C3H7 + H)*. Thereby, compressive strain is suggested to improve PDH performance. Moreover, the selectivity of PDH over IrN4 SAC to propylene can be not affected by the applied strain qualitatively, which is due to the propylene-π adsorption mode at the single-atom site.