Photoexcited Surface Frustrated Lewis Pairs for Heterogeneous Photocatalytic CO2 Reduction

Semi-solid and solid frustrated Lewis pair catalysts

This review presents the strategies for the construction of heterogeneous frustrated-Lewis-pair catalysts, their catalytic applications and future challenges and opportunities.

Room-Temperature Activation of CO2 by Dual Defect-Stabilized Nanoscale Hematite (Fe2-δO3-v ): Concurrent Role of Fe and O Vacancies

We demonstrate that synthetically controlled concurrent stabilization of Fe and O vacancy defects on the surface of interbraided nanoscale hematite (Fe_2-δO_3-v ) renders an interesting surface chemistry which can reduce CO₂ to CO at room temperature (RT). Importantly, we realized a highly enhanced output of 410 μmol h^-1 g^-1 at RT, as compared to that of 10 μmol h^-1 g^-1 for bulk hematite. It is argued based on the activity degradation under cycling and first principles density functional theory calculations that the excess chemical energy embedded in the defect-stabilized surface is expended in this high-energy conversion process, which leads to progressive filling up of oxygen vacancies.

Halogenated triphenylgallium and -indium in frustrated Lewis pair activations and hydrogenation catalysis

The Lewis acids Ga(C₆F₅)₃, In(C₆F₅)₃ and Ga(C₆Cl₅)₃ are prepared and their Lewis acidity has been probed experimentally and computationally. The species Ga(C₆F₅)₃ and In(C₆F₅)₃ in conjunction with phosphine donors are shown to heterolytically split H₂ and catalyse the hydrogenation of an imine. In addition, frustrated Lewis pairs (FLPs) derived from Ga(C₆F₅)₃ and In(C₆F₅)₃ and phosphines react with diphenyldisulfide to phosphoniumgallates or indates of the form [tBu₃PSPh][PhSE(C₆F₅)₃] and [tBu₃PSPh][(μ-SPh)(E(C₆F₅)₃)₂] (E = Ga, In). The potential of the FLPs based on Ga(C₆F₅)₃, In(C₆F₅)₃ and Ga(C₆Cl₅)₃ and phosphines is also shown in reactions with phenylacetylene to give pure or mixtures of the products [tBu₃PH][PhCCE(C₆X₅)₃] and R₃P(Ph)C=C(H)E(C₆X₅)₃ A number of these species are crystallographically characterized. The implications for the use of these species in FLP chemistry are considered.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

The broadening reach of frustrated Lewis pair chemistry

The revelation that combinations of Lewis acids and bases for which dative bonding is impeded can activate dihydrogen led to the concept of "frustrated Lewis pairs" (FLPs). Over the past decade, a range of FLP systems and substrate molecules have precipitated a paradigm change in main-group chemistry and metal-free catalysis. The FLP motif has also found application in a growing body of chemical problems in organic synthesis, transition metal and free radical chemistry, materials, enzymatic models, and surface chemistry. The current state of FLP chemistry is assessed herein, and the outlook for the future considered.

Solid frustrated-Lewis-pair catalysts constructed by regulations on surface defects of porous nanorods of CeO2

Identification on catalytic sites of heterogeneous catalysts at atomic level is important to understand catalytic mechanism. Surface engineering on defects of metal oxides can construct new active sites and regulate catalytic activity and selectivity. Here we outline the strategy by controlling surface defects of nanoceria to create the solid frustrated Lewis pair (FLP) metal oxide for efficient hydrogenation of alkenes and alkynes. Porous nanorods of ceria (PN-CeO₂) with a high concentration of surface defects construct new Lewis acidic sites by two adjacent surface Ce³⁺. The neighbouring surface lattice oxygen as Lewis base and constructed Lewis acid create solid FLP site due to the rigid lattice of ceria, which can easily dissociate H–H bond with low activation energy of 0.17 eV.

Surface engineering of catalysts allows the tailoring of active sites. Here the authors produce a heterogeneous nanoceria catalyst with engineered defects producing active solid frustrated Lewis pair sites, and use these materials for the hydrogenation of alkynes and alkenes.

Be12O12 Nano-cage as a Promising Catalyst for CO2 Hydrogenation

An efficient conversion of CO₂ into valuable fuels and chemicals has been hotly pursued recently. Here, for the first time, we have explored a series of M₁₂x₁₂ nano-cages (M = B, Al, Be, Mg; X = N, P, O) for catalysis of CO₂ to HCOOH. Two steps are identified in the hydrogenation process, namely, H₂ activation to 2H*, and then 2H* transfer to CO₂ forming HCOOH, where the barriers of two H* transfer are lower than that of the H₂ activation reaction. Among the studied cages, Be₁₂O₁₂ is found to have the lowest barrier in the whole reaction process, showing two kinds of reaction mechanisms for 2H* (simultaneous transfer and a step-wise transfer with a quite low barrier). Moreover, the H₂ activation energy barrier can be further reduced by introducing Al, Ga, Li, and Na to B₁₂N₁₂ cage. This study would provide some new ideas for the design of efficient cluster catalysts for CO₂ reduction.

Heterogeneous catalytic hydrogenation of CO2by metal oxides: defect engineering – perfecting imperfection

In this review, we discuss how metal oxides with designed defects can be synthesized and engineered, to enable heterogeneous catalytic hydrogenation of gaseous carbon dioxide to chemicals and fuels.

Heterogeneous Metal-Free Hydrogenation over Defect-Laden Hexagonal Boron Nitride

Catalytic hydrogenation is an important process used for the production of everything from foods to fuels. Current heterogeneous implementations of this process utilize metals as the active species. Until recently, catalytic heterogeneous hydrogenation over a metal-free solid was unknown; implementation of such a system would eliminate the health, environmental, and economic concerns associated with metal-based catalysts. Here, we report good hydrogenation rates and yields for a metal-free heterogeneous hydrogenation catalyst as well as its unique hydrogenation mechanism. Catalytic hydrogenation of olefins was achieved over defect-laden h-BN (dh-BN) in a reactor designed to maximize the defects in h-BN sheets. Good yields (>90%) and turnover frequencies (6 × 10^-5-4 × 10^-3) were obtained for the hydrogenation of propene, cyclohexene, 1,1-diphenylethene, (E)- and (Z)-1,2-diphenylethene, octadecene, and benzylideneacetophenone. Temperature-programmed desorption of ethene over processed h-BN indicates the formation of a highly defective structure. Solid-state NMR (SSNMR) measurements of dh-BN with high and low propene surface coverages show four different binding modes. The introduction of defects into h-BN creates regions of electronic deficiency and excess. Density functional theory calculations show that both the alkene and hydrogen-bond order are reduced over four specific defects: boron substitution for nitrogen (B_N), vacancies (V_B and V_N), and Stone-Wales defects. SSNMR and binding-energy calculations show that V_N are most likely the catalytically active sites. This work shows that catalytic sites can be introduced into a material previously thought to be catalytically inactive through the production of defects.

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO2 reduction

In₂O_3-x(OH)_y nanoparticles have been shown to function as an effective gas-phase photocatalyst for the reduction of CO₂ to CO via the reverse water-gas shift reaction. Their photocatalytic activity is strongly correlated to the number of oxygen vacancy and hydroxide defects present in the system. To better understand how such defects interact with photogenerated electrons and holes in these materials, we have studied the relaxation dynamics of In₂O_3-x(OH)_y nanoparticles with varying concentration of defects using two different excitation energies corresponding to above-band-gap (318-nm) and near-band-gap (405-nm) excitations. Our results demonstrate that defects play a significant role in the excited-state, charge relaxation pathways. Higher defect concentrations result in longer excited-state lifetimes, which are attributed to improved charge separation. This correlates well with the observed trends in the photocatalytic activity. These results are further supported by density-functional theory calculations, which confirm the positions of oxygen vacancy and hydroxide defect states within the optical band gap of indium oxide. This enhanced understanding of the role these defects play in determining the optoelectronic properties and charge carrier dynamics can provide valuable insight toward the rational development of more efficient photocatalytic materials for CO₂ reduction.

Spatial Separation of Charge Carriers in In2O3-x(OH)y Nanocrystal Superstructures for Enhanced Gas-Phase Photocatalytic Activity

The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst-support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water-gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts.