On the mechanism of alkyne hydrogenation catalyzed by Ga-doped ceria

Heterogeneous Frustrated Lewis Pair Catalysts: Rational Structure Design and Mechanistic Elucidation Based on Intrinsic Properties of Supports

ConspectusThe discovery of reversible hydrogenation using metal-free phosphoborate species in 2006 marked the official advent of frustrated Lewis pair (FLP) chemistry. This breakthrough revolutionized homogeneous catalysis approaches and paved the way for innovative catalytic strategies. The unique reactivity of FLPs is attributed to the Lewis base (LB) and Lewis acid (LA) sites either in spatial separation or in equilibrium, which actively react with molecules. Since 2010, heterogeneous FLP catalysts have gained increasing attention for their ability to enhance catalytic performance through tailored surface designs and improved recyclability, making them promising for industrial applications. Over the past 5 years, our group has focused on investigating and strategically modifying various types of solid catalysts with FLPs that are unique from classic solid FLPs. We have explored systematic characterization techniques to unravel the underlying mechanisms between the active sites and reactants. Additionally, we have demonstrated the critical role of catalysts' intrinsic electronic and geometric properties in promoting FLP formation and stimulating synergistic effects. The characterization of FLP catalysts has been greatly enhanced by the use of advanced techniques such as synchrotron X-ray diffraction, neutron powder diffraction, X-ray photoelectron spectroscopy, extended X-ray absorption fine structure, elemental mapping in scanning transmission electron microscopy, electron paramagnetic resonance spectroscopy, diffuse-reflectance infrared Fourier transform spectroscopy, and solid-state nuclear magnetic resonance spectroscopy. These techniques have provided deeper insights into the structural and electronic properties of FLP systems for the future design of catalysts.Understanding electron distribution in the overlapping orbitals of LA and LB pairs is essential for inducing FLPs in operando in heterogeneous catalysts through target electron reallocation by external stimuli. For instance, in silicoaluminophosphate-type zeolites with weak orbital overlap, the adsorption of polar gas molecules leads to heterolytic cleavage of the Alδ+-Oδ- bond, creating unquenched LA-LB pairs. In a Ru-doped metal-organic framework, the Ru-N bond can be polarized through metal-ligand charge transfer under light, forming Ru+-N- pairs. This activation of FLP sites from the framework represents a groundbreaking innovation that expands the catalytic potential of existing materials. For catalysts already employing FLP chemistry to dynamically generate products from substrates, a complete mechanistic interpretation requires a thorough examination of the surface electronic properties and the surrounding environment. The hydrogen spillover ability on the Ru-doped FLP surfaces improves conversion efficiency by suppressing hydrogen poisoning at metal sites. In situ H2-H2O conditions enable the production of organic chemicals with excellent activity and selectivity by creating new bifunctional sites via FLP chemistry. By highlighting the novel FLP systems featuring FLP induction and synergistic effects and the selection of advanced characterization techniques to elucidate reaction mechanisms, we hope that this Account will offer innovative strategies for designing and characterizing FLP chemistry in heterogeneous catalysts to the research community.

Single-Atom Metal Species as A Promoter to Enhance Heterogeneous Catalysis

Single-atom catalysts (SACs) have emerged as a focal point of research in the field of heterogeneous catalysis. This paper reviews the progress in the studies of single atoms as promoters in various catalytic reactions, elucidating their distinctive role in comparison to the dominant active sites. We provide a discussion on the application of single-atom promoters (SAP) within host-guest systems in various catalysts, including metal oxide supported catalysts, molybdenum carbide-based catalysts, bimetallic catalysts, and others. The behavior of SAP is diverse. They often promote the formation of oxygen vacancies for oxide support, leading to local site reconstruction that creates specific reaction route. Moreover, they can also precisely modify the electronic structure of hetero-metal atomic or nanoparticle sites, then regulating the adsorption of reactants or intermediates and catalytic performance. Finally, the potential for the development of SAP is outlined, proposing novel approach for the design of SACs with enhanced activity and stability.

CeO2-Based Frustrated Lewis Pairs via Defective Engineering: Formation Theory, Site Characterization, and Small Molecule Activation

Activation of small molecules is considered to be a central concern in the theoretical investigation of environment- and energy-related catalytic conversions. Sub-nanostructured frustrated Lewis pairs (FLPs) have been an emerging research hotspot in recent years due to their advantages in small molecule activation. Although the progress of catalytic applications of FLPs is increasingly reported, the fundamental theories related to the structural formation, site regulation, and catalytic mechanism of FLPs have not yet been fully developed. Given this, it is attempted to demonstrate the underlying theory of FLPs formation, corresponding regulation methods, and its activation mechanism on small molecules using CeO2 as the representative metal oxide. Specifically, this paper presents three fundamental principles for constructing FLPs on CeO2 surfaces, and feasible engineering methods for the regulation of FLPs sites are presented. Furthermore, cases where typical small molecules (e.g., hydrogen, carbon dioxide, methane oxygen, etc.) are activated over FLPs are analyzed. Meanwhile, corresponding future challenges for the development of FLPs-centered theory are presented. The insights presented in this paper may contribute to the theories of FLPs, which can potentially provide inspiration for the development of broader environment- and energy-related catalysis involving small molecule activation.

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

The excellent reactivity of frustrated Lewis pairs (FLP) to activate small molecules has gained increasing attention in recent decades. Though the development of surface FLP (SFLP) is prompting the application of FLP in the chemical industry, the design of SFLP with superior activity, high density, and excellent stability for small-molecule activation is still challenging. Herein, we review the progress of designing SFLP by surface engineering, screening natural SFLP, and the dynamic formation of SFLP from theoretical perspectives. We highlight the breakthrough in fine-tuning the activity, density, and stability of the designed SFLP studied by using computational methods. We also discuss future challenges and directions in designing SFLP with outstanding capabilities for small-molecule activation.

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.

Substitution of B-site in BaSb2O6 perovskite for surface lattice oxygen activation and boosted photocatalytic toluene mineralization

Perovskite oxides possess significant prospects in environment application because of their compositional versatility and controllable band structure for redox reactions. Nevertheless, low charge separation and limited reactants activation restrict their performance for practical applications. In this work, we reveal that the electronic structure of BaSb₂O₆ can be modulated effectively by substituting B-site cations, leading to broadened light response range and promoted carrier separation. The Ga atoms substitute the Sb atoms to form GaO bonds and enable octahedral distortion, resulting in the electron transfer from Ga atom to O atoms and realizing lattice oxygen activation. The unique electronic localization in the BaSb₂O₆ surface facilitates the adsorption and activation of O₂, H₂O, toluene and reaction intermediates, thus enhancing ROS generation for toluene mineralization. Compared with the performance of pure BaSb₂O₆, the photocatalytic toluene degradation and mineralization of 5 wt% Ga-BaSb₂O₆ are increased by 4.5 times and 4.8 times without obvious deactivation. The reported facile and valid strategy for in situ controlling of B-site in perovskite and their unique effects on the electronic structure would benefit the development of high-performance perovskites for environmental applications.

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.

Elucidating the Roles of Amorphous Alumina Overcoat in Palladium-Catalyzed Selective Hydrogenation

Amorphous alumina overcoats generated by atomic layer deposition (ALD) have been shown to improve the selectivity and durability of supported metal catalysts in many reactions. Several mechanisms have been proposed to explain the enhanced catalytic performance, but the accessibilities of reactants through the amorphous overcoats remain elusive, which is crucial for understanding reaction mechanisms. Here, we show that an AlOx ALD overcoat is able to improve the alkene product selectivity of a supported Pd catalyst in acetylene (C2H2) hydrogenation. We further demonstrate that the AlOx ALD overcoat blocks the access of C2H2 (kinetic diameter of 0.33 nm), O2 (0.35 nm), and CO (0.38 nm) but allows H2 (0.29 nm) to access Pd surfaces. A H-D exchange experiment suggests that H2 might dissociate heterolytically at the Pd-AlOx interface. These findings are in favor of a hydrogen spillover mechanism.

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.

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen

The articulation of the notion of "frustrated Lewis pairs" (FLPs) emerged from the discovery that H₂ can be reversibly activated by combinations of sterically encumbered main group Lewis acids and bases. This has prompted numerous studies focused on various perturbations of the Lewis acid/base combinations and the applications to organic reductions. This Perspective focuses on the new directions and developments that are emerging from this FLP chemistry involving hydrogen. Three areas are discussed including new applications and approaches to FLP reductions, the reductions of small molecules, and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chemistry, this simple concept of FLPs is being applied across the discipline.

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

Here, we studied Al or B atom-doped carbon nitride (g-C₃N₄ and C₂N) as catalysts for H₂ activation and acetylene hydrogenation using density functional theory calculations. The Al or B could be assembled with the surface N atoms of carbon nitride to form diverse frustrated Lewis pairs (FLPs). The results show that Al-N FLPs had lower barriers of H₂ activation in comparison with B-N FLPs. The heterolytic H₂ dissociation catalyzed by Al-N FLPs led to the formation of Al-H and N-H species. The Al-H species were highly active in the first hydrogenation of acetylene to C₂H₃*, yielding a mild barrier, while in the second hydrogenation step, the reaction between C₂H₃ and the H of N-H species caused a relatively high barrier. Electronic structure analysis demonstrated the electron transfer in the heterolytic H₂ cleavage and explained the activity differences in various FLPs. The results suggest that Al with the surface N of carbon nitride can act as an FLP to catalyze the H₂ activation and acetylene hydrogenation, thus providing a new strategy for the future development of noble metal-free hydrogenation catalysts.

Selective Hydrogenation on a Highly Active Single-Atom Catalyst of Palladium Dispersed on Ceria Nanorods by Defect Engineering

Single-atom catalysis represents a new frontier that integrates the merits of homogeneous and heterogeneous catalysis to afford exceptional atom efficiency, activity, and selectivity for a range of catalytic systems. Herein we describe a simple defect engineering strategy to construct an atomically dispersed palladium catalyst (Pd^δ+, 0 < δ < 2) by anchoring the palladium atoms on oxygen vacancies created in CeO₂ nanorods. This was confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurement. The as-prepared catalyst showed exceptional catalytic performance in the hydrogenation of styrene (99% conversion, TOF of 2410 h^-1), cinnamaldehyde (99% conversion, 99% selectivity, TOF of 968 h^-1), as well as oxidation of triethoxysilane (99% conversion, 79 selectivity, TOF of 10 000 h^-1). This single-atom palladium catalyst can be reused at least five times with negligible activity decay. The palladium atoms retained their dispersion on the support at the atomic level after thermal stability testing in Ar at 773 K. Most importantly, this synthetic method can be scaled up while maintaining catalytic performance. We anticipate that this method will expedite access to single-atom catalysts with high activity and excellent resistance to sintering, significantly impacting the performance of this class of catalysts.