Surface Analogues of Molecular Frustrated Lewis Pairs in Heterogeneous CO2 Hydrogenation Catalysis

Multivariate Bayesian Optimization of CoO Nanoparticles for CO2 Hydrogenation Catalysis

The hydrogenation of CO2 holds promise for transforming the production of renewable fuels and chemicals. However, the challenge lies in developing robust and selective catalysts for this process. Transition metal oxide catalysts, particularly cobalt oxide, have shown potential for CO2 hydrogenation, with performance heavily reliant on crystal phase and morphology. Achieving precise control over these catalyst attributes through colloidal nanoparticle synthesis could pave the way for catalyst and process advancement. Yet, navigating the complexities of colloidal nanoparticle syntheses, governed by numerous input variables, poses a significant challenge in systematically controlling resultant catalyst features. We present a multivariate Bayesian optimization, coupled with a data-driven classifier, to map the synthetic design space for colloidal CoO nanoparticles and simultaneously optimize them for multiple catalytically relevant features within a target crystalline phase. The optimized experimental conditions yielded small, phase-pure rock salt CoO nanoparticles of uniform size and shape. These optimized nanoparticles were then supported on SiO2 and assessed for thermocatalytic CO2 hydrogenation against larger, polydisperse CoO nanoparticles on SiO2 and a conventionally prepared catalyst. The optimized CoO/SiO2 catalyst consistently exhibited higher activity and CH4 selectivity (ca. 98%) across various pretreatment reduction temperatures as compared to the other catalysts. This remarkable performance was attributed to particle stability and consistent H* surface coverage, even after undergoing the highest temperature reduction, achieving a more stable catalytic species that resists sintering and carbon occlusion.

Advances in Catalysts for Urea Electrosynthesis Utilizing CO2 and Nitrogenous Materials: A Mechanistic Perspective

Electrocatalytic urea synthesis from CO2 and nitrogenous substances represents an essential advance for the chemical industry, enabling the efficient utilization of resources and promoting sustainable development. However, the development of electrocatalytic urea synthesis has been severely limited by weak chemisorption, poor activation and difficulties in C-N coupling reactions. In this review, catalysts and corresponding reaction mechanisms in the emerging fields of bimetallic catalysts, MXenes, frustrated Lewis acid-base pairs and heterostructures are summarized in terms of the two central mechanisms of molecule-catalyst interactions as well as chemical bond cleavage and directional coupling, which provide new perspectives for improving the efficiency of electrocatalytic synthesis of urea. This review provides valuable insights to elucidate potential electrocatalytic mechanisms.

Defect-Induced All-Solid-State Frustrated Lewis Pair on Metal-Organic Monolayer Accelerating Photocatalytic CO2 Reduction with H2O Vapor

Understanding the structure-performance relationships of a frustrated Lewis pair (FLP) at the atomic level is key to yielding high efficiency in activating chemically "inert" molecules into value-added products. A sound strategy was developed herein through incorporating oxygen defects into a Zr-based metal-organic layer (Zr-MOL-D) and employing Lewis basic proximal surface hydroxyls for the in situ formation of solid heterogeneous FLP (Zr4-δ-VO-Zr-OH). Zr-MOL-D exhibits a superior CO2 to CO conversion rate of 49.4 μmol g-1 h-1 in water vapor without any sacrificing agent or photosensitizer, which is about 12 times higher than that of pure MOL (Zr-MOL-P), with extreme stability even after being placed for half a year. Theoretical and experimental results reveal that the introduction of FLP converts the process of the crucial intermediate COOH* from an endothermic reaction to an exothermic spontaneous reaction. This work is expected to provide new prospects for developing efficient MOL-based photocatalysts in FLP chemistry through a sound defect-engineering strategy.

Regulation of frustrated Lewis pairs on CeO2 facilitates tandem transformation of styrene and CO2

The frustrated Lewis pair (FLP) site of (Ce, Ce)-O on the CeO2(110) surface undergoes reconstruction to form (La, Ce)-O upon La-doping. The FLP site of (La, Ce)-O with the tailored local Lewis acid-base property and increased spatial distance between the Lewis acid and base facilitates the tandem transformation of styrene and CO2 through the weakened adsorption of CO2 while maintaining activation.

Characteristics of the Frustrated Lewis Pairs (FLPs) on the Surface of Albite and the Corresponding Mechanism of H2 Activation

The characteristics of frustrated Lewis pairs (FLPs) on albite surfaces were analyzed with density functional theory, and the reaction mechanism for H2 activation by the FLPs was studied. The results show that albite is an ideal substrate material with FLPs, and its (001) and (010) surfaces have the typical characteristics of FLPs. In the case of H2 activation, the interaction between the HOMO of H2 and the SOMO of the Lewis base and the electron acceptance characteristics of the Lewis acid are the key factors. In fact, the activation energy of H2 is the required activation energy from the ground state to the excited state, and once the excited state is produced, the dissociative adsorption of H2 will occur directly. This study provides a new ideas and a reference for research on the construction of novel solid FLPs catalysts using ultramicro channel materials.

Coordination Defect-Induced Frustrated Lewis Pairs in Polyoxo-metalate-Based Metal-Organic Frameworks for Efficient Catalytic Hydrogenation

Precise control of the structure and spatial distance of Lewis acid (LA) and Lewis base (LB) sites in a porous system to construct efficient solid frustrated Lewis pair (FLP) catalyst is vital for industrial application but remains challenging. Herein, we constructed FLP sites in a polyoxometalate (POM)-based metal-organic framework (MOF) by introducing coordination-defect metal nodes (LA) and surface-basic POM with abundant oxygen (LB). The well-defined and unique spatial conformation of the defective POM-based MOF ensure that the distance between LA and LB is at ~4.3 Å, a suitable distance to activate H2 . This FLP catalyst can heterolytically dissociate H2 into active Hδ- , thus exhibiting high activity in hydrogenation, which is 55 and 2.7 times as high as that of defect-free POM-based MOF and defective MOF without POM, respectively. This work provides a new avenue toward precise design multi-site catalyst to achieve specific activation of target substrate for synergistic catalysis.

Engineering Nanostructured Interfaces of Hexagonal Boron Nitride-Based Materials for Enhanced Catalysis

ConspectusHexagonal boron nitrides (h-BNs) are attractive two-dimensional (2D) nanomaterials that consist of alternating B and N atoms and layered honeycomb-like structures similar to graphene. They have exhibited unique properties and promising application potentials in the field of energy storage and transformation. Recent advances in utilizing h-BN as a metal-free catalyst in the oxidative dehydrogenation of propane have triggered broad interests in exploring h-BN in catalysis. However, h-BN-based materials as robust nanocatalysts in heterogeneous catalysis are still underexplored because of the limited methodologies capable of affording h-BN with controllable crystallinity, abundant porosity, high purity, and defect engineering, which played important roles in tuning their catalytic performance. In this Account, our recent progress in addressing the above issues will be highlighted, including the synthesis of high-quality h-BN-based nanomaterials via both bottom-up and top-down pathways and their catalytic utilization as metal-free catalysts or as supports to tune the interfacial electronic properties on the metal nanoparticles (NPs). First, we will focus on the large-scale fabrication of h-BN nanosheets (h-BNNSs) with high crystallinity, improved surface area, satisfactory purity, and tunable defects. h-BN derived from the traditional approaches using boron trioxide and urea as the starting materials generally contains carbon/oxygen impurities and has low crystallinity. Several new strategies were developed to address the issues. Using bulk h-BN as the precursor via gas exfoliation in liquid nitrogen, single- or few-layered h-BNNS with abundant defects could be generated. Amorphous h-BN precursors could be converted to h-BN nanosheets with high crystallinity assisted by a magnesium metallic flux via a successive dissolution/precipitation/crystallization procedure. The as-fabricated h-BNNS featured high crystallinity and purity as well as abundant porosity. An ionothermal metathesis procedure was developed using inorganic molten salts (NaNH₂ and NaBH₄) as the precursors. The h-BN scaffolds could be produced on a large scale with high yield, and the as-afforded materials possessed high purity and crystallinity. Second, utilization of the as-prepared h-BN library as metal-free catalysts in dehydrogenation and hydrogenation reactions will be summarized, in which they exhibited enhanced catalytic activity over the counterparts from the previous synthesis method. Third, the interface modulation between metal NPs with the as-prepared defects' abundant h-BN support will be highlighted. The h-BN-based strong metal-support interaction (SMSI) nanocatalysts were constructed without involving reducible metal oxides via the ionothermal procedure we developed by deploying specific inorganic metal salts, acting as robust nanocatalysts in CO oxidation. Under conditions simulated for practical exhaust systems, promising catalytic efficiency together with high thermal stability and sintering resistance was achieved. Across all of these examples, unique insights into structures, defects, and interfaces that emerge from in-depth characterization through microscopy, spectroscopy, and diffraction will be highlighted.

Hetero-Interfaces on Cu Electrode for Enhanced Electrochemical Conversion of CO2 to Multi-Carbon Products

Electrochemical CO₂ reduction reaction (CO₂RR) to multi-carbon products would simultaneously reduce CO₂ emission and produce high-value chemicals. Herein, we report Cu electrodes modified by metal-organic framework (MOF) exhibiting enhanced electrocatalytic performance to convert CO₂ into ethylene and ethanol. The Zr-based MOF, UiO-66 would in situ transform into amorphous ZrO_x nanoparticles (a-ZrO_x), constructing a-ZrO_x/Cu hetero-interface as a dual-site catalyst. The Faradaic efficiency of multi-carbon (C2+) products for optimal UiO-66-coated Cu (0.5-UiO/Cu) electrode reaches a high value of 74% at - 1.05 V versus RHE. The intrinsic activity for C2+ products on 0.5-UiO/Cu electrode is about two times higher than that of Cu foil. In situ surface-enhanced Raman spectra demonstrate that UiO-66-derived a-ZrO_x coating can promote the stabilization of atop-bound CO* intermediates on Cu surface during CO₂ electrolysis, leading to increased CO* coverage and facilitating the C-C coupling process. The present study gives new insights into tailoring the adsorption configurations of CO₂RR intermediate by designing dual-site electrocatalysts with hetero-interfaces.

Defect-Regulated Frustrated-Lewis-Pair Behavior of Boron Nitride in Ambient Pressure Hydrogen Activation

The construction of heterogeneous frustrated Lewis pairs (FLPs) with performance comparable to or surpassing the homogeneous counterparts in H₂ activation is a long-standing challenge. Herein, sterically hindered Lewis acid ("B" center) and Lewis base ("N" center) sites were anchored within the rigid lattice of highly crystalline hexagonal boron nitride (h-BN) scaffolds. The active sites were created via precision defect regulation during the molten-salt-involved (NaNH₂ and NaBH₄) h-BN construction procedure. The as-afforded h-BN scaffolds achieved highly efficient H₂/D₂ activation and dissociation under ambient pressure via FLP-like behavior, and attractive catalytic efficiency in hydrogenation reactions surpassing the current heterogeneous analogues.

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.

Solid surface frustrated Lewis pair constructed on layered AlOOH for hydrogenation reaction

Designing heterogeneous solid surface frustrated Lewis pair (ssFLP) catalyst for hydrogenation is a new challenge in catalysis and no research has been reported on the construction of ssFLP on boehmite (AlOOH) surfaces up to now as far as we know. Herein, AlOOH with a layer structure is prepared and it is found that the Lewis basic O_Hv site (one H removed from OH) and an adjacent Lewis acidic unsaturated Al site (Al³⁺_unsatur.) proximal to a surface OH_v (OH vacancy) on AlOOH layers could form the ssFLP. The layered structure of AlOOH and its abundant OH defects over the surface result in a high concentration of O_Hv/Al³⁺_unsatur. FLPs, which are conducive to highly efficient hydrogen activation for hydrogenation of olefins and alkynes with low H-H bond dissociates activation energy of 0.16 eV under mild conditions (T = 80°C and P(H₂) = 2.0 MPa). This work develops a new kind of hydrogenation catalyst and provides a new perspective for creating solid surface FLP.

Designing heterogeneous solid surface frustrated Lewis pair (ssFLP) catalyst for hydrogenation is a new challenge in catalysis. Here, the authors show the ssFLP can be constructed on layered AlOOH for hydrogenation reactions under mild conditions.

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.

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides can catalyze the CO2 transformation reactions. On metal oxide-based catalysts, CO2 transformations are promoted significantly in the presence of surface oxygen vacancies or surface defect sites. Electrons transferable to the neutral CO2 molecule can be enriched on oxygen vacancies, which can also act as CO2 adsorption sites. CO2 activation is also possible without necessarily transferring electrons by tailoring catalytic sites that promote interactions at an appropriate energy level alignment of the catalyst and CO2 molecule. This review discusses CO2 activation on various catalysts, particularly the impacts of various structural factors, such as oxygen vacancies, on CO2 activation.

Lewis Acid Site-Promoted Single-Atomic Cu Catalyzes Electrochemical CO2 Methanation

Developing an efficient catalyst for the electrocatalytic CO₂ reduction reaction (CO₂RR) is highly desired because of environmental and energy issues. Herein, we report a single-atomic-site Cu catalyst supported by a Lewis acid for electrocatalytic CO₂ reduction to CH₄. Theoretical calculations suggested that Lewis acid sites in metal oxides (e.g., Al₂O₃, Cr₂O₃) can regulate the electronic structure of Cu atoms by optimizing intermediate absorption to promote CO₂ methanation. Based on these theoretical results, ultrathin porous Al₂O₃ with enriched Lewis acid sites was explored as an anchor for Cu single atoms; this modification achieved a faradaic efficiency (FE) of 62% at -1.2 V (vs RHE) with a corresponding current density of 153.0 mA cm^-2 for CH₄ formation. This work demonstrates an effective strategy for tailoring the electronic structure of Cu single atoms for the highly efficient reduction of CO₂ into CH₄.

Symmetry-Breaking Sites for Activating Linear Carbon Dioxide Molecules

ConspectusCO₂ is not only a greenhouse gas but also a pivotal carbon source as a promising supplement to fossil fuels. Both the environment and energy crises have compelled the researchers to explore how to efficiently transform CO₂ into liquid fuels and value-added chemicals. As the industrialized approach nowadays, heterogeneous CO₂ hydrogenation driven by thermal energy represents a potential strategy to help mitigate the greenhouse effect and reduce the reliance on fossil fuels. However, as the prerequisite for CO₂ hydrogenation, CO₂ activation is difficult due to the thermodynamic stability and chemical inertness of CO₂ molecules. It is not proper to activate CO₂ by directly increasing the reaction temperature, because CO₂ hydrogenation into liquid products is an exothermic process where elevating the temperature decreases both the balanced conversion of CO₂ and the balanced selectivity for target products. Therefore, the key scientific issue for CO₂ hydrogenation lies in how to design catalysts which enable efficient activation of CO₂. Up to date, a vast variety of active sites have been constructed for effective activation of CO₂. These active sites including step sites, alloys, interface, substitution, vacancies, etc. are generally symmetry-breaking rather than perfect flat surfaces.Herein, we propose a catalyst design principle of constructing symmetry-breaking sites to activate nonpolar CO₂ molecules. From the perspective of electronic properties, there is a prominent charge density gradient in a symmetry-breaking center, resulting in perturbing electronic structures of nonpolar CO₂ and polarizing the adsorbed species. From the perspective of adsorption configuration, a symmetry-breaking site gives a local torque which enables more effective overlapping of atomic orbitals and thus more facilely bending of linear CO₂ molecules, compared with symmetric sites. In this Account, we categorize the modes of CO₂ activation and put forward the design principle of constructing symmetry-breaking sites. Moreover, we illustrate how to construct symmetry-breaking sites from the perspectives of local and global structures. Strategies to break the symmetry of local structures include surface substitution, surface adatom, and surface vacancy. Strategies to break the symmetry of global structures comprise surface modification with ligands, high-index surface, and phase reconstruction. In the future, further improvements, such as quantified descriptors, function for C-C coupling, and applicability to other nonpolar molecules, are necessary.

Waveguide photoreactor enhances solar fuels photon utilization towards maximal optoelectronic - photocatalytic synergy

A conventional light management approach on a photo-catalyst is to concentrate photo-intensity to enhance the catalytic rate. We present a counter-intuitive approach where light intensity is distributed below the electronic photo-saturation limit under the principle of light maximization. By operating below the saturation point of the photo-intensity induced hydroxide growth under reactant gaseous H₂+CO₂ atmosphere, a coating of defect engineered In₂O_3-x(OH)_y nanorod Reverse Water Gas Shift solar-fuel catalyst on an optical waveguide outperforms a coated plane by a factor of 2.2. Further, light distribution along the length of the waveguide increases optical pathlengths of the weakly absorptive green and yellow wavelengths, which increases CO product rate by a factor of 8.1-8.7 in the visible. Synergistically pairing with thinly doped silicon on the waveguide enhances the CO production rate by 27% over the visible. In addition, the persistent photoconductivity behavior of the In₂O_3-x(OH)_y system enables CO production at a comparable rate for 2 h after turning off photo-illumination, enhancing yield with 44-62% over thermal only yield. The practical utility of persistent photocatalysis was demonstrated through outdoor solar concentrator tests, which after a day-and-night cycle showed CO yield increase of 19% over a day-light only period.

Heterolytic cleavage of dihydrogen (HCD) in metal nanoparticle catalysis

Supports, ligands and additives can promote heterolytic H₂ splitting by a cooperative mechanism with metal nanoparticles.

HI-Light: A Glass-Waveguide-Based "Shell-and-Tube" Photothermal Reactor Platform for Converting CO2 to Fuels

In this work, we introduce HI-Light, a surface-engineered glass-waveguide-based “shell-and-tube” type photothermal reactor which is both scalable in diameter and length. We examine the effect of temperature, light irradiation, and residence time on its photo-thermocatalytic performance for CO₂ hydrogenation to form CO, with a cubic phase defect-laden indium oxide, In2O3-x(OH)y, catalyst. We demonstrate the light enhancement effect under a variety of reaction conditions. Notably, the light-on performance for the cubic nanocrystal photocatalyst exhibits a CO evolution rate at 15.40 mmol g_cat⁻¹ hr⁻¹ at 300°C and atmospheric pressure. This is 20 times higher conversion rate per unit catalyst mass per unit time beyond previously reported In2O3-x(OH)y catalyst in the cubic form under comparable operation conditions and more than 5 times higher than that of its rhombohedral polymorph. This result underscores that improvement in photo-thermocatalytic reactor design enables uniform light distribution and better reactant/catalyst mixing, thus significantly improving catalyst utilization.

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- A glass-waveguide-based “shell-and-tube” type photothermal reactor was developed

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The reactor exhibited a high photothermal catalytic performance for CO₂ reduction

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- The modular reactor has potential for scale-up, both in diameter and length

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The reactor design improves light distribution and reactant/catalyst mixing

Plasmonic Titanium Nitride Facilitates Indium Oxide CO2 Photocatalysis

Nanoscale titanium nitride TiN is a metallic material that can effectively harvest sunlight over a broad spectral range and produce high local temperatures via the photothermal effect. Nanoscale indium oxide-hydroxide, In₂ O_{3- x} (OH)_y , is a semiconducting material capable of photocatalyzing the hydrogenation of gaseous CO₂ ; however, its wide electronic bandgap limits its absorption of photons to the ultraviolet region of the solar spectrum. Herein, the benefits of both nanomaterials in a ternary heterostructure: TiN@TiO₂ @In₂ O_{3- x} (OH)_y are combined. This heterostructured material synergistically couples the metallic TiN and semiconducting In₂ O_{3- x} (OH)_y phases via an interfacial semiconducting TiO₂ layer, allowing it to drive the light-assisted reverse water gas shift reaction at a conversion rate greatly surpassing that of its individual components or any binary combinations thereof.

Relating surface defect energetics with reactant gas adsorption during the photo-catalytic reduction of CO2 by partially hydrolyzed In2O3 nanorods

Photo-Induced Transient Current Spectroscopy (PICTS) is a versatile technique for measurements of defect state energies and densities in photo-active materials. It is suitable for investigating the surface-gas adsorbate behavior and the defect characteristics of defect laden In2O3-x(OH)y nanorods, having oxygen vacancies and hydroxide surface groups, under in situ reactor conditions of dark ambient temperature, dark 150 °C and photo-illuminated 150 °C, for the photo-assisted Reverse Water Gas Shift reaction. From glovebox-protected X-ray Photoelectron Spectroscopy and in situ PICTS measurements we determined that the reduction of CO2 is associated with heterolytic dissociation of H2 into In-H§- and HO-H§+ centres accompanied by an increase in average carrier trap energies; increased carbonate formation in a photo/thermal reactor state of H2 + CO2, and an average trap energy decrease of 0.11 eV from H2 to a CO2 + H2 mixture, which correlates with binding energy shifts of the OH shoulder of the O1s spectra. These results show the reactivity link between the various OH groups, oxygen vacancies and trap energies of In2O3-x(OH)y in the reactant gas atmosphere components.

Heterolytic Scission of Hydrogen Within a Crystalline Frustrated Lewis Pair

We report the heterolysis of molecular hydrogen under ambient conditions by the crystalline frustrated Lewis pair (FLP) 1-{2-[bis(pentafluorophenyl)boryl]phenyl}-2,2,6,6-tetramethylpiperidine (KCAT). The gas-solid reaction provides an approach to prepare the solvent-free, polycrystalline ion pair KCATH2 through a single crystal to single crystal transformation. The crystal lattice of KCATH2 increases in size relative to the parent KCAT by approximately 2%. Microscopy was used to follow the transformation of the highly colored red/orange KCAT to the colorless KCATH2 over a period of 2 h at 300 K under a flow of H₂ gas. There is no evidence of crystal decrepitation during hydrogen uptake. Inelastic neutron scattering employed over a temperature range from 4-200 K did not provide evidence for the formation of polarized H₂ in a precursor complex within the crystal at low temperatures and high pressures. However, at 300 K, the INS spectrum of KCAT transformed to the INS spectrum of KCATH2. Calculations suggest that the driving force is more favorable in the solid state compared to the solution or gas phase, but the addition of H₂ into the KCAT crystal is unfavorable. Ab Initio methods were used to calculate the INS spectra of KCAT, KCATH2, and a possible precursor complex of H₂ in the pocket between the B and N of crystalline KCAT. Ex-situ NMR showed that the transformation from KCAT to KCATH2 is quantitative and our results suggest that the hydrogen heterolysis process occurs via H₂ diffusion into the FLP crystal with a rate-limiting movement of H₂ from inactive positions to reactive sites.

Mild and selective hydrogenation of CO2 into formic acid over electron-rich MoC nanocatalysts

The direct hydrogenation of CO₂ using H₂ gas is a one-stone-two-birds route to produce highly value-added hydrocarbon compounds and to lower the CO₂ level in the atmosphere. However, the transformation of CO₂ and H₂ into hydrocarbons has always been a great challenge while ensuring both the activity and selectivity over abundant-element-based nanocatalysts. In this work, we designed a Schottky heterojunction composed of electron-rich MoC nanoparticles embedded inside an optimized nitrogen-doped carbon support (MoC@NC) as the first example of noble-metal-free heterogeneous catalysts to boost the activity of and specific selectivity for CO₂ hydrogenation to formic acid (FA) in liquid phase under mild conditions (2 MPa pressure and 70 °C). The MoC@NC catalyst with a high turnover frequency (TOF) of 8.20 mol_FA mol_MoC^-1 h^-1 at 140 °C and an excellent reusability are more favorable for real applications.

Shining light on CO2: from materials discovery to photocatalyst, photoreactor and process engineering

Materials engineering, theoretical modelling, reactor engineering and process development of gas-phase photocatalytic CO₂ reduction exemplified by indium oxide systems.

Piperazine-promoted gold-catalyzed hydrogenation: the influence of capping ligands

The presence of capping ligands can block the adsorption of the amine ligand on gold NPs, preventing the formation of a ligand–metal interface able to activate H₂ for selective hydrogenation reactions.

X-ray Photospectroscopy and Electronic Studies of Reactor Parameters on Photocatalytic Hydrogenation of Carbon Dioxide by Defect-Laden Indium Oxide Hydroxide Nanorods

In the study reported herein, glovebox-protected X-ray photoelectron spectroscopy (XPS) and in situ Hall charge carrier measurements provide new insights into the surface physical chemistry of gaseous H₂, CO₂, and H₂+CO₂ combined with nanostructured In₂O_(3−x)(OH)_y nanorods, which ensue under photochemical and thermochemical operating conditions. Heterolytic dissociation of H₂ in H₂-only atmosphere appears to occur mainly under dark and ambient temperature conditions, while the greatest amount of OH shoulder expansion in H₂+CO₂ atmosphere appears to mainly occur under photoilluminated conditions. These results correlate with those of the Hall measurements, which show that the prevalence of homolytic over heterolytic dissociation at increasing temperatures leads to a steeper rate of increase in carrier concentrations; and that H₂ adsorption is more prevalent than CO₂ in H₂+CO₂ photoillumination conditions.

Polymorph selection towards photocatalytic gaseous CO2 hydrogenation

Titanium dioxide is the only known material that can enable gas-phase CO₂ photocatalysis in its anatase and rutile polymorphic forms. Materials engineering of polymorphism provides a useful strategy for optimizing the performance metrics of a photocatalyst. In this paper, it is shown that the less well known rhombohedral polymorph of indium sesquioxide, like its well-documented cubic polymorph, is a CO₂ hydrogenation photocatalyst for the production of CH₃OH and CO. Significantly, the rhombohedral polymorph exhibits higher activity, superior stability and improved selectivity towards CH₃OH over CO. These gains in catalyst performance originate in the enhanced acidity and basicity of surface frustrated Lewis pairs in the rhombohedral form.

Polymorphs, compounds with identical chemical stoichiometries yet different atomic configurations, expand the range of potential chemical properties and new applications. Here, authors show rhombohedral indium oxides to be highly active and selective for photocatalytic CO₂ hydrogenation.

Ceria-Based Materials in Hydrogenation and Reforming Reactions for CO2 Valorization

Reducing greenhouse emissions is of vital importance to tackle the climate changes and to decrease the carbon footprint of modern societies. Today there are several technologies that can be applied for this goal and especially there is a growing interest in all the processes dedicated to manage CO2 emissions. CO2 can be captured, stored or reused as carbon source to produce chemicals and fuels through catalytic technologies. This study reviews the use of ceria based catalysts in some important CO2 valorization processes such as the methanation reaction and methane dry-reforming. We analyzed the state of the art with the aim of highlighting the distinctive role of ceria in these reactions. The presence of cerium based oxides generally allows to obtain a strong metal-support interaction with beneficial effects on the dispersion of active metal phases, on the selectivity and durability of the catalysts. Moreover, it introduces different functionalities such as redox and acid-base centers offering versatility of approaches in designing and engineering more powerful formulations for the catalytic valorization of CO2 to fuels.

Alkylammoniotrifluoroborate functionalized polystyrenes: polymeric pre-catalysts for the metal-free borylation of heteroarenes

Three polymeric versions of ansa-N,N-dialkylammoniumtrifluoroborate ambiphilic molecules based on the styrene motif (poly(1-NMe2H+-2-BF3--4-styrene) (P-Me), poly(1-NEt2H+-2-BF3--4-styrene) (P-Et) and poly(1-piperidinyl-H+-2-BF3--4-styrene) (P-Pip)) were synthesized, characterized and tested as heterogeneous pre-catalysts for the borylation of electron-rich heteroarenes. These heterogeneous versions of previously reported pre-catalysts show similar reactivity patterns and represent the first examples of solid-supported FLP metal-free catalysts for the C-H borylation of heteroarenes.

Tailoring Surface Frustrated Lewis Pairs of In2O3-x (OH)y for Gas-Phase Heterogeneous Photocatalytic Reduction of CO2 by Isomorphous Substitution of In3+ with Bi3

Frustrated Lewis pairs (FLPs) created by sterically hindered Lewis acids and Lewis bases have shown their capacity for capturing and reacting with a variety of small molecules, including H2 and CO2, and thereby creating a new strategy for CO2 reduction. Here, the photocatalytic CO2 reduction behavior of defect-laden indium oxide (In2O3-x (OH) y ) is greatly enhanced through isomorphous substitution of In3+ with Bi3+, providing fundamental insights into the catalytically active surface FLPs (i.e., In-OH···In) and the experimentally observed "volcano" relationship between the CO production rate and Bi3+ substitution level. According to density functional theory calculations at the optimal Bi3+ substitution level, the 6s2 electron pair of Bi3+ hybridizes with the oxygen in the neighboring In-OH Lewis base site, leading to mildly increased Lewis basicity without influencing the Lewis acidity of the nearby In Lewis acid site. Meanwhile, Bi3+ can act as an extra acid site, serving to maximize the heterolytic splitting of reactant H2, and results in a more hydridic hydride for more efficient CO2 reduction. This study demonstrates that isomorphous substitution can effectively optimize the reactivity of surface catalytic active sites in addition to influencing optoelectronic properties, affording a better understanding of the photocatalytic CO2 reduction mechanism.

Designing effective 'frustrated Lewis pair' hydrogenation catalysts

The past decade has seen the subject of transition metal-free catalytic hydrogenation develop incredibly rapidly, transforming from a largely hypothetical possibility to a well-established field that can be applied to the reduction of a diverse variety of functional groups under mild conditions. This remarkable change is principally attributable to the development of so-called 'frustrated Lewis pairs': unquenched combinations of bulky Lewis acids and bases whose dual reactivity can be exploited for the facile activation of otherwise inert chemical bonds. While a number of comprehensive reviews into frustrated Lewis pair chemistry have been published in recent years, this tutorial review aims to provide a focused guide to the development of efficient FLP hydrogenation catalysts, through identification and consideration of the key factors that govern their effectiveness. Following discussion of these factors, their importance will be illustrated using a case study from our own research, namely the development of FLP protocols for successful hydrogenation of aldehydes and ketones, and for related moisture-tolerant hydrogenation.

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.

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.

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.