Construction of frustrated Lewis pairs on carbon nitride nanosheets for catalytic hydrogenation of acetylene

Finding Natural, Dense, and Stable Frustrated Lewis Pairs on Wurtzite Crystal Surfaces for Small-Molecule Activation

The surface frustrated Lewis pairs (SFLPs) open up new opportunities for substituting noble metals in the activation and conversion of stable molecules. However, the applications of SFLPs on a larger scale are impeded by the complex construction process, low surface density, and sensitivity to the reaction environment. Herein, wurtzite-structured crystals such as GaN, ZnO, and AlP are found for developing natural, dense, and stable SFLPs. It is revealed that the SFLPs can naturally exist on the (100) and (110) surfaces of wurtzite-structured crystals. All the surface cations and anions serve as the Lewis acid and Lewis base in SFLPs, respectively, contributing to the surface density of SFLPs as high as 7.26×1014 cm-2. Ab initio molecular dynamics simulations indicate that the SFLPs can keep stable under high temperatures and the reaction atmospheres of CO and H2O. Moreover, outstanding performance for activating the given small molecules is achieved on these natural SFLPs, which originates from the optimal orbital overlap between SFLPs and small molecules. Overall, these findings not only provide a simple method to obtain dense and stable SFLPs but also unfold the nature of SFLPs toward the facile activation of small molecules.

Frustrated Lewis Pairs: Bonding, Reactivity, and Applications

The outstanding capability of Frustrated Lewis Pair (FLP) catalysts to activate small molecules has gained significant attention in recent times. Reactivity of FLP is further extended toward the hydrogenation of various unsaturated species. Over the past decade, this unique catalysis concept has been successfully expanded to heterogeneous catalysis as well. The present review article gives a brief survey on several studies on this field. A thorough discussion on quantum chemical studies concerning the activation of H₂ is provided. The role of aromaticity and boron-ligand cooperation on the reactivity of FLP is discussed in the Review. How FLP can activate other small molecules by cooperative action of its Lewis centers is also discussed. Further, the discussion is shifted to the hydrogenation of various unsaturated species and the mechanism regarding this process. It also discusses the latest theoretical advancements in the application of FLP in heterogeneous catalysis across various domains, such as two-dimensional materials, functionalized surfaces, and metal oxides. A deeper understanding of the catalytic process may assist in devising new heterogeneous FLP catalysts through experimental design.

Theoretical understanding on all-solid frustrated Lewis pair sites of C2N anchored by single metal atom

Designing all-solid heterogeneous catalysts with frustrated Lewis pairs (FLPs) has aroused great attentions recently because of its appealing low dissociation energy for H2 molecule and thus a promotion of hydrogenation reaction is expected. The sterically encumbered Lewis acid (metal site) and base (nitrogen site) in the cavity of single transition metal atom doped M/C2N sheet makes it potential candidate with FLP, while a comprehensive understanding of its intrinsic property and reactivity is still required. Calculations show that the complete dissociation of H2 molecule into two H* at the N sites requires two steps, i.e., heterolytic cleavage of H2 molecule and the transfer of H* from metal site to N site, which are highly related to the acidity of the metal site. The Ni/C2N and Pd/C2N, which outperform over the other 8 transition metal atom (M) anchored M/C2N candidates, possess low energy barriers for the complete dissociation of H2 molecule, with values of only 0.30 and 0.20 eV, respectively. Furthermore, both Ni/C2N and Pd/C2N catalysts can achieve semi-hydrogenation of C2H2 into C2H4, with overall barriers of 0.81 and 0.75 eV, respectively, lower than many reported catalysts. It is speculated that M/C2N catalysts with intrinsic FLPs may also find applications in other important hydrogenation reaction.

Frustrated Lewis Pairs in Heterogeneous Catalysis: Theoretical Insights

Frustrated Lewis pair (FLP) catalysts have attracted much recent interest because of their exceptional ability to activate small molecules in homogeneous catalysis. In the past ten years, this unique catalysis concept has been extended to heterogeneous catalysis, with much success. Herein, we review the recent theoretical advances in understanding FLP-based heterogeneous catalysis in several applications, including metal oxides, functionalized surfaces, and two-dimensional materials. A better understanding of the details of the catalytic mechanism can help in the experimental design of novel heterogeneous FLP catalysts.

Acetylene hydrogenation catalyzed by bare and Ni doped CeO2(110): the role of frustrated Lewis pairs

Ceria (CeO₂) has recently been found to catalyze the selective hydrogenation of alkynes, which has stimulated much discussion on the catalytic mechanism on various facets of reducible oxides. In this work, H₂ dissociation and acetylene hydrogenation on bare and Ni doped CeO₂(110) surfaces are investigated using density functional theory (DFT). Similar to that on the CeO₂(111) surface, our results suggest that catalysis is facilitated by frustrated Lewis pairs (FLPs) formed by oxygen vacancies (O_vs) on the oxide surfaces. On bare CeO₂(110) with a single O_v (CeO₂(110)-O_v), two surface Ce cations with one non-adjacent O anion are shown to form (Ce³⁺-Ce⁴⁺)/O quasi-FLPs, while for the Ni doped CeO₂(110) surface with one (Ni-CeO₂(110)-O_v) or two (Ni-CeO₂(110)-2O_v) O_vs, one Ce and a non-adjacent O counterions are found to form a mono-Ce/O FLP. DFT calculations indicate that Ce/O FLPs facilitate the H₂ dissociation via a heterolytic mechanism, while the resulting surface O-H and Ce-H species catalyze the subsequent acetylene hydrogenation. With CeO₂(110)-O_v and Ni-CeO₂(110)-2O_v, our DFT calculations suggest that the first hydrogenation step is the rate-determining step with a barrier of 0.43 and 0.40 eV, respectively. For Ni-CeO₂(110)-O_v, the reaction is shown to be controlled by the H₂ dissociation with a barrier of 0.41 eV. These barriers are significantly lower than that (about 0.7 eV) on CeO₂(111), explaining the experimentally observed higher catalytic efficiency of the (110) facet of ceria. The change of the rate-determining step is attributed to the different electronic properties of Ce in the Ce/O FLPs - the Ce f states closer to the Fermi level not only facilitate the heterolytic dissociation of H₂ but also lead to a higher barrier of acetylene hydrogenation.

Cu/O Frustrated Lewis Pairs on Cu Doped CeO2(111) for Acetylene Hydrogenation: A First-Principles Study

In this work, the H2 dissociation and acetylene hydrogenation on Cu doped CeO2(111) were studied using density functional theory calculations. The results indicated that Cu doping promotes the formation of oxygen vacancy (Ov) which creates Cu/O and Ce/O frustrated Lewis pairs (FLPs). With the help of Cu/O FLP, H2 dissociation can firstly proceed via a heterolytic mechanism to produce Cu-H and O-H by overcoming a barrier of 0.40 eV. The H on Cu can facilely migrate to a nearby oxygen to form another O-H species with a barrier of 0.43 eV. The rate-determining barrier is lower than that for homolytic dissociation of H2 which produces two O-H species. C2H2 hydrogenation can proceed with a rate-determining barrier of 1.00 eV at the presence of Cu-H and O-H species., While C2H2 can be catalyzed by two O-H groups with a rate-determining barrier of 1.06 eV, which is significantly lower than that (2.86 eV) of C2H2 hydrogenated by O-H groups on the bare CeO2(111), showing the high activity of Cu doped CeO2(111) for acetylene hydrogenation. In addition, the rate-determining barrier of C2H4 further hydrogenated by two O-H groups is 1.53 eV, much higher than its desorption energy (0.72 eV), suggesting the high selectivity of Cu doped CeO2(111) for C2H2 partial hydrogenation. This provides new insights to develop effective hydrogenation catalysts based on metal oxide.