Understanding the Facile Heterolytic Dissociation of Hydrogen on Natural Surface Frustrated Lewis Pairs

Abstract

The discovery of naturally frustrated Lewis pairs (FLPs) on wurtzite-structured surfaces provides a shortcut to obtain dense and stable surface FLPs without complex surface engineering. However, the catalytic performance and potential applications in the catalysis of natural FLPs have not been thoroughly investigated. Herein, the heterolytic dissociation of hydrogen is studied at the natural FLPs of wurtzite-structured GaN, ZnO, and SiC surfaces by using theoretical methods. Compared with classical Lewis pairs (CLPs), the FLPs show lower activation barriers ranging from 0.07 to 0.16 eV but higher reaction energies in the heterolytic dissociation of hydrogen. By splitting the energy into different items via a chemical–physical model, the optimal substrate–surface interaction and the decreasing stability of lone pair electrons on N atoms are recognized as the main reason for the lower activation energy and the higher reaction energy at FLPs, respectively. Because of the relatively favorable kinetics and unfavorable thermodynamics in hydrogen dissociation, FLPs present superior performance in the selective hydrogenation of acetylene to ethylene, reflected by the lower activation energy by 0.89 eV in the rate-determining step than CLPs. Overall, this study not only provides mechanistic insights into the heterolytic dissociation of hydrogen at FLPs but also unfolds the advantages of FLPs in hydrogenation reactions.

Theoretical Perspective on the Design of Surface Frustrated Lewis Pairs for Small-Molecule Activation

“…The activation energies of methane dissociation are 0.62 eV at FLP sites on GaN(100) and 0.82 eV at FLP sites on ZnO(100), which are close to the activation energy (0.69 eV) on Pt(111). The optimal substrate–surface interactions in the transition states are the main reason for the low activation energy of small-molecule activation at the natural surface FLP . As shown in Figure a, the energies of the lone pair electrons of the nitrogen atom (N I 2 p z ) not participating in the H 2 dissociation at CLP and FLP sites are close (−1.09 vs −0.97 eV), whereas the lone pair electrons of the nitrogen atom (N II 2 p z ) directly involved in the H 2 dissociation at FLP sites have lower energy level (−1.51 eV) in comparison with the nitrogen atom (N II 2 p z , –1.06 eV) directly involved in the H 2 dissociation at CLP sites.…”

“…As shown in Figure a, the energies of the lone pair electrons of the nitrogen atom (N I 2 p z ) not participating in the H 2 dissociation at CLP and FLP sites are close (−1.09 vs −0.97 eV), whereas the lone pair electrons of the nitrogen atom (N II 2 p z ) directly involved in the H 2 dissociation at FLP sites have lower energy level (−1.51 eV) in comparison with the nitrogen atom (N II 2 p z , –1.06 eV) directly involved in the H 2 dissociation at CLP sites. The partial charge density maps in Figure b,c reflect that the 2 p z orbitals of N atom in CLP sites are unfavorable for the H 2 dissociation owing to the larger repulsion between the lone pair electrons of N II and the electrons in the Ga II −N II bond, whereas the 2 p z orbitals in FLP sites can have a larger overlap with the H 2 antibonding orbital without increasing the repulsion between the lone pair electrons of N II and surface electrons in the Ga II –N II bond …”

“…Partial charge density maps of hydrogen dissociation at (b) CLP (energy ranging from −1.2 eV to −1.7 eV) and (c) FLP sites (energy ranging from −1.7 eV to −0.4 eV), respectively. Reproduced from ref . Copyright 2023 American Chemical Society.…”