Photochemically Engineering the Metal-Semiconductor Interface for Room-Temperature Transfer Hydrogenation of Nitroarenes with Formic Acid

Design of the Synergistic Rectifying Interfaces in Mott-Schottky Catalysts

The functions of interfacial synergy in heterojunction catalysts are diverse and powerful, providing a route to solve many difficulties in energy conversion and organic synthesis. Among heterojunction-based catalysts, the Mott-Schottky catalysts composed of a metal-semiconductor heterojunction with predictable and designable interfacial synergy are rising stars of next-generation catalysts. We review the concept of Mott-Schottky catalysts and discuss their applications in various realms of catalysis. In particular, the design of a Mott-Schottky catalyst provides a feasible strategy to boost energy conversion and chemical synthesis processes, even allowing realization of novel catalytic functions such as enhanced redox activity, Lewis acid-base pairs, and electron donor-acceptor couples for dealing with the current problems in catalysis for energy conversion and storage. This review focuses on the synthesis, assembly, and characterization of Schottky heterojunctions for photocatalysis, electrocatalysis, and organic synthesis. The proposed design principles, including the importance of constructing stable and clean interfaces, tuning work function differences, and preparing exposable interfacial structures for designing electronic interfaces, will provide a reference for the development of all heterojunction-type catalysts, electrodes, energy conversion/storage devices, and even super absorbers, which are currently topics of interest in fields such as electrocatalysis, fuel cells, CO₂ reduction, and wastewater treatment.

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.

Metal-organic frameworks derived carbon-incorporated cobalt/dicobalt phosphide microspheres as Mott-Schottky electrocatalyst for efficient and stable hydrogen evolution reaction in wide-pH environment

Cobalt phosphides, as low cost and abundant non-noble materials for hydrogen evolution reaction (HER), are always constrained by their inferior charge transfer and sluggish intrinsic electrocatalytic kinetics. In this work, carbon-incorporated Co/Co₂P microspheres (Co/Co₂P@C) as a novel Mott-Schottky catalyst were synthesized successfully via carbonization and gradual phosphorization of Co based metal-organic frameworks. The unique merits, including Mott-Schottky effect at the interface formed between metal Co and semiconductor Co₂P, the incorporated carbon-layer on the surface and the spherical structure endow Co/Co₂P@C with favorable electrical conductivity, preferable kinetics and long-term stability when it was evaluated as electrocatalyst for HER in wide-pH range. As a result, the Co/Co₂P@C with the optimized phosphorization degree delivers a benchmark current density of 10 mA cm^-2 at the low overpotential of 192 and 158 mV in acidic and alkaline electrolytes, respectively, with a remarkable stability (CV cycling for 3000 cycles and continuous electrolysis at the overpotential of 200 mV for 48 h). Therefore, the as-designed Co/Co₂P@C should be one of the most promising catalysts for HER application.

On the catalytic transfer hydrogenation of nitroarenes by a cubane-type Mo3S4 cluster hydride: disentangling the nature of the reaction mechanism

Cubane-type Mo₃S₄ cluster hydrides decorated with phosphine ligands are active catalysts for the transfer hydrogenation of nitroarenes to aniline derivatives in the presence of formic acid (HCOOH) and triethylamine (Et₃N). The process is highly selective and most of the cluster species involved in the catalytic cycle have been identified through reaction monitoring. Formation of a dihydrogen cluster intermediate has also been postulated based on previous kinetic and theoretical studies. However, the different steps involved in the transfer hydrogenation from the cluster to the nitroarene to finally produce aniline remain unclear. Herein, we report an in-depth computational investigation into this mechanism. Et₃N reduces the activation barrier associated with the formation of Mo-HHOOCH dihydrogen species. The global catalytic process is highly exergonic and occurs in three consecutive steps with nitrosobenzene and N-phenylhydroxylamine as reaction intermediates. Our computational findings explain how hydrogen is transferred from these Mo-HHOOCH dihydrogen adducts to nitrobenzene with the concomitant formation of nitrosobenzene and the formate substituted cluster. Then, a β-hydride elimination reaction accompanied by CO₂ release regenerates the cluster hydride. Two additional steps are needed for hydrogen transfer from the dihydrogen cluster to nitrosobenzene and N-phenylhydroxylamine to finally produce aniline. Our results show that the three metal centres in the Mo₃S₄ unit act independently, so the cluster can exist in up to ten different forms that are capable of opening a wide range of reaction paths. This behaviour reveals the outstanding catalytic possibilities of this kind of cluster complexes, which work as highly efficient catalytic machines.

Cobalt Entrapped in N,S-Codoped Porous Carbon: Catalysts for Transfer Hydrogenation with Formic Acid

Catalysts with Co nanoparticles (NPs) entrapped in N,S-codoped carbon shells were successfully fabricated by pyrolysis of porous organic polymers (POPs) with cobalt salts. The encapsulated structure consisting of Co NPs and N,S-codoped carbon layers was verified by TEM, XRD, and X-ray photoelectron spectroscopy. The catalysts displayed excellent activity and stability for the catalytic transfer hydrogenation (CTH) of nitrobenzene with formic acid under base-free conditions. Furthermore, the resultant catalysts allowed for highly efficient and selective transfer hydrogenation of various functionalized nitroarenes to the corresponding anilines. Through control experiments, the covered Co NPs were identified as active sites for CTH. The incorporation of S into the N-doped carbon lattice promoted the electron transfer from metallic cobalt NPs to their shells, which played a significant role in the acceleration of CTH. Moreover, the Co-NSPC-850 catalyst pyrolyzed at 850 °C showed excellent stability in the recycling experiments.

Ultrafine Pd Nanoparticles Anchored on Nitrogen-Doping Carbon for Boosting Catalytic Transfer Hydrogenation of Nitroarenes

The catalytic performance of metal particles is closely related to the particle size. In this article, ultrafine palladium nanoparticles anchored on nitrogen-doping carbon support (Pd/N-XC72R) were fabricated, wherein the N-XC72R was prepared through low-temperature annealing of Vulcan XC72R carbon with urea at 300 °C. Nitrogen dopant on the surface of carbon support can remarkably strengthen the affinity of the metal nanoparticles onto the support. Compared with the Vulcan XC-72R-supported Pd catalyst, the prepared Pd/N-XC72R delivered superior catalytic activity for the transfer hydrogenation of nitroarenes with formic acid as the hydrogen donor at ambient temperature. Our strategy may provide an effective and feasible approach to fabricate N-functionalized carbon materials and construct high-performance ultrasmall metal nanoparticle heterogeneous catalysts.

Visible-Light-Driven Chemoselective Hydrogenation of Nitroarenes to Anilines in Water through Graphitic Carbon Nitride Metal-Free Photocatalysis

Green and efficient procedures are essential for the chemoselective hydrogenation of functionalized nitroarenes to form industrially important anilines. Herein, it is shown that visible-light-driven, chemoselective hydrogenation of functionalized nitroarenes with groups sensitive to forming anilines can be achieved in good to excellent yields (82-100 %) in water under relatively mild conditions and catalyzed by low-cost and recyclable graphitic carbon nitride. The process is also applicable to gram-scale reaction, with a yield of aniline of 86 %. A study of the mechanism reveals that visible-light-induced electrons are responsible for the hydrogenation reactions, and thermal energy can also promote the photocatalytic activity. A study of the kinetics shows that this reaction possibly occurs through one-step hydrogenation or stepwise condensation routes. A wide range of applications can be expected for this green, efficient, and highly selective photocatalysis system in reduction reactions for the synthesis of fine chemicals.

An air and moisture tolerant iminotrihydroquinoline-ruthenium(ii) catalyst for the transfer hydrogenation of ketones

Both amine- and imine-containing E and F are highly effective in the transfer hydrogenation of a wide range of ketones with F significantly operating in bench quality 2-propanol and in vessels open to the air.

Pd supported on g-C3N4 nanosheets: Mott–Schottky heterojunction catalyst for transfer hydrogenation of nitroarenes using formic acid as hydrogen source

Well-dispersed Pd nanoparticles anchored on g-C₃N₄ nanosheets displayed excellent catalytic activity for the transfer hydrogenation of nitroarenes.

Copper-Based Intermetallic Electride Catalyst for Chemoselective Hydrogenation Reactions

The development of transition metal intermetallic compounds, in which active sites are incorporated in lattice frameworks, has great potential for modulating the local structure and the electronic properties of active sites, and enhancing the catalytic activity and stability. Here we report that a new copper-based intermetallic electride catalyst, LaCu_0.67Si_1.33, in which Cu sites activated by anionic electrons with low work function are atomically dispersed in the lattice framework and affords selective hydrogenation of nitroarenes with above 40-times higher turnover frequencies (TOFs up to 5084 h^-1) than well-studied metal-loaded catalysts. Kinetic analysis utilizing isotope effect reveals that the cleavage of the H-H bond is the rate-determining step. Surprisingly, the high carrier density and low work function (LWF) properties of LaCu_0.67Si_1.33 enable the activation of hydrogen molecules with extreme low activation energy (E_a = 14.8 kJ·mol^-1). Furthermore, preferential adsorption of nitroarenes via a nitro group is achieved by high oxygen affinity of LaCu_0.67Si_1.33 surface, resulting in high chemoselectivity. The present efficient catalyst can further trigger the hydrogenation of other oxygen-containing functional groups such as aldehydes and ketones with high activities. These findings demonstrate that the transition metals incorporated in the specific lattice site function as catalytically active centers and surpass the conventional metal-loaded catalysts in activity and stability.

Oxygen Vacancy Engineering of Co3 O4 Nanocrystals through Coupling with Metal Support for Water Oxidation

Oxygen vacancies can help to capture oxygen-containing species and act as active centers for oxygen evolution reaction (OER). Unfortunately, effective methods for generating a high amount of oxygen vacancies on the surface of various nanocatalysts are rather limited. Here, we described an effective way to generate oxygen-vacancy-rich surface of transition metal oxides, exemplified with Co₃ O₄ , simply by constructing highly coupled interface of ultrafine Co₃ O₄ nanocrystals and metallic Ti. Impressively, the amounts of oxygen vacancy on the surface of Co₃ O₄ /Ti surpassed the reported values of the Co₃ O₄ modified even under highly critical conditions. The Co₃ O₄ /Ti electrode could provide a current density of 23 mA cm^-2 at an OER overpotential of 570 mV, low Tafel slope, and excellent durability in neutral medium. Because of the formation of a large amount of oxygen vacancies as the active centers for OER on the surface, the TOF value of the Co₃ O₄ @Ti electrode was optimized to be 3238 h^-1 at an OER overpotential of 570 mV, which is 380 times that of the state-of-the-art non-noble nanocatalysts in the literature.

Encapsulating Palladium Nanoparticles Inside Mesoporous MFI Zeolite Nanocrystals for Shape-Selective Catalysis

Pd nanoparticles were successfully encapsulated inside mesoporous silicalite-1 nanocrystals (Pd@mnc-S1) by a one-pot method. The as-synthesized Pd@mnc-S1 with excellent stability functioned as an active and reusable heterogeneous catalyst. The unique porosity and nanostructure of silicalite-1 crystals endowed the Pd@mnc-S1 material general shape-selectivity for various catalytic reactions, including selective hydrogenation, oxidation, and carbon-carbon coupling reactions.

A simple and efficient in situ generated ruthenium catalyst for chemoselective transfer hydrogenation of nitroarenes: kinetic and mechanistic studies and comparison with iridium systems

A convenient and highly efficient in situ generated Ru(ii) system for synthesizing functionalized amines and mechanistic studies and comparison with iridium systems is presented.

Efficient and highly selective boron-doped carbon materials-catalyzed reduction of nitroarenes

Boron-doped carbon materials are demonstrated to be excellent catalysts in nitroarene reduction reactions.

General transfer hydrogenation by activating ammonia-borane over cobalt nanoparticles

Cobalt nanoparticles containing both Co²⁺ and Co⁰ species supported on carbon nitride can function as heterogeneous nanocatalysts for a general transfer hydrogenation reaction in aqueous ammonia-borane solution at room temperature.