Rational Design of Fe Single Sites Supported on Hierarchical Zeolites via Atomic Layer Deposition for Few-Walled Carbon Nanotube Production.

Metal single-site catalysts have recently played an essential role in catalysis due to their enhanced activity, selectivity, and precise reaction control compared to those of conventional metal cluster catalysts. However, the rational design and catalytic application of metal single-site catalysts are still in the early stages of development. In this contribution, we report the rational design of Fe single sites incorporated in a hierarchical ZSM-5 via atomic layer deposition (ALD). The designer catalysts demonstrated highly dispersed Fe species, predominantly stabilized by oxygen atoms in the zeolite framework at terminal, isolated, and vicinal silanol groups within the micropores and external surfaces of the zeolite. The successful incorporation of highly thermally stable and uniform Fe single sites into hierarchical zeolite through ALD represents a significant advancement in few-walled carbon nanotube production. The inner and outer diameters of produced CNTs are approximately 4.4 ± 2.4 and 8.6 ± 1.8 nm, respectively, notably smaller than those produced via traditional impregnated catalysts. This example emphasizes the concept of rational design of a single Fe site dispersed on a hierarchical ZSM-5 surface, which is anticipated to be a promising catalyst for advancing catalytic applications.

Correlation between Heteroatom Coordination and Hydrogen Evolution for Single-site Pt on Carbon-based Nanocages.

The electrocatalytic performance of single-site catalysts (SSCs) is closely correlated with the electronic structure of metal atoms. Herein we construct a series of Pt SSCs on heteroatom-doped hierarchical carbon nanocages, which exhibit increasing hydrogen evolution reaction (HER) activities along S-doped, P-doped, undoped and N-doped supports. Theoretical simulation indicates a multi-H-atom adsorption process on Pt SSCs due to the low coordination, and a reasonable descriptor is figured out to evaluate the HER activities. Relative to C-coordinated Pt, N-coordinated Pt has higher reactivity due to the electron transfer of N-to-Pt, which enriches the density of states of Pt 5d orbital near the Fermi level and facilitates the capturing of protons, just the opposite to the situations for P- and S-coordinated ones. The stable N-coordinated Pt originates from the kinetic stability throughout the multi-H-atom adsorption process. This finding provides a significant guidance for rational design of advanced Pt SSCs on carbon-based supports.

XPS and HR TEM Elucidation of the Diversity of Titania-Supported Single-Site Ir Catalyst Performance in Spin-Selective Propene Hydrogenation.

Immobilized [Ir(COD)Cl]2-Linker/TiO2 catalysts with linkers containing Py, P(Ph)2 and N(CH3)2 functional groups were prepared. The catalysts were tested via propene hydrogenation with parahydrogen in a temperature range from 40 °C to 120 °C which was monitored via NMR. The catalytic behavior of [Ir(COD)Cl]2-Linker/TiO2 is explained on the basis of quantitative and qualitative XPS data analysis performed for the catalysts before and after the reaction at 120 °C. It is shown that the temperature dependence of propene conversion and the enhancement of the NMR signal are explained via a combination of the stabilities of both the linker and immobilized [Ir(COD)Cl]2 complex. It is demonstrated that the N(CH3)2-linker is the most stable at the surface of TiO2 under used reaction conditions. As a result, only this sample shows a rise in the enhancement of the NMR signal in the 100-120 °C temperature range.

Reaction Chemistry at Discrete Organometallic Fragments on Black Phosphorus.

Black phosphorus (bP) is a two-dimensional van der Waals material unique in its potential to serve as a support for single-site catalysts due to its similarity to molecular phosphines, ligands quintessential in homogeneous catalysis. However, there is a scarcity of synthetic methods to install single metal centers on the bP lattice. Here, we demonstrate the functionalization of bP nanosheets with molecular Re and Mo complexes. A suite of characterization techniques, including infrared, X-ray photoelectron and X-ray absorption spectroscopy as well as scanning transmission electron microscopy corroborate that the functionalized nanosheets contain a high density of discrete metal centers directly bound to the bP surface. Moreover, the supported metal centers are chemically accessible and can undergo ligand exchange transformations without detaching from the surface. The steric and electronic properties of bP as a ligand are estimated with respect to molecular phosphines. Sterically, bP resembles tri(tolyl)phosphine when monodentate to a metal center, and bis(diphenylphosphino)propane when bidentate, whereas electronically bP is a σ-donor as strong as a trialkyl phosphine. This work is foundational in elucidating the nature of black phosphorus as a ligand and underscores the viability of using bP as a basis for single-site catalysts.

Coordinative Stabilization of Single Bismuth Sites in a Carbon-Nitrogen Matrix to Generate Atom-Efficient Catalysts for Electrochemical Nitrate Reduction to Ammonia.

Electrochemical nitrate reduction to ammonia powered by renewable electricity is not only a promising alternative to the established energy-intense and non-ecofriendly Haber-Bosch reaction for ammonia generation but also a future contributor to the ever-more important denitrification schemes. Nevertheless, this reaction is still impeded by the lack of understanding for the underlying reaction mechanism on the molecular scale which is necessary for the rational design of active, selective, and stable electrocatalysts. Herein, a novel single-site bismuth catalyst (Bi-N-C) for nitrate electroreduction is reported to produce ammonia with maximum Faradaic efficiency of 88.7% and at a high rate of 1.38 mg h-1 mgcat -1 at -0.35 V versus reversible hydrogen electrode (RHE). The active center (described as BiN2 C2 ) is uncovered by detailed structural analysis. Coupled density functional theory calculations are applied to analyze the reaction mechanism and potential rate-limiting steps for nitrate reduction based on the BiN2 C2 model. The findings highlight the importance of model catalysts to utilize the potential of nitrate reduction as a new-generation nitrogen-management technology based on the construction of efficient active sites.

Single-Site Carbon-Supported Metal-Oxo Complexes in Heterogeneous Catalysis: Structure, Reactivity, and Mechanism.

When early transition metal complexes are molecularly grafted onto catalyst supports, well-defined, surface-bound species are created, which are highly active and selective single-site heterogeneous catalysts (SSHCs) for diverse chemical transformations. In this minireview, we analyze and summarize a less conventional type of SSHC in which molybdenum dioxo species are grafted onto unusual carbon-unsaturated scaffolds, such as activated carbon, reduced graphene oxide, and carbon nanohorns. The choice of earth-abundant, low-toxicity, versatile metal constituents, and various carbon supports illustrates "catalyst by design" principles and yields insights into new catalytic systems of both academic and technological interest. Here, we summarize experimental and computational investigations of the bonding, electronic structure, reaction scope, and mechanistic pathways of these unusual catalysts.

Sulfur Poisoning and Self-Recovery of Single-Site Rh1 /Porous Organic Polymer Catalysts for Olefin Hydroformylation.

Sulfur poisoning and regeneration are global challenges for metal catalysts even at the ppm level. The sulfur poisoning of single-metal-site catalysts and their regeneration is worthy of further study. Herein, sulfur poisoning and self-recovery are first presented on an industrialized single-Rh-site catalyst (Rh1 /POPs). A decreased turnover frequency of Rh1 /POPs from 4317 h-1 to 318 h-1 was observed in a 1000 ppm H2 S co-feed for ethylene hydroformylation, but it self-recovered to 4527 h-1 after withdrawal of H2 S, whereas the rhodium nanoparticles demonstrated poor activity and self-recovery ability. H2 S reduced the charge density of the single Rh atom and lowered its Gibbs free energy with the formation of inactive (SH)Rh(CO)(PPh3 -frame)2 , which could be regenerated to active HRh(CO)(PPh3 -frame)2 after withdrawing H2 S. The mechanism and the sulfur-related structure-activity relationship were highlighted. This work provides an understanding of heterogeneous ethylene hydroformylation and sulfur-poisoned regeneration in the science of single-atom catalysts.

Transition Metal-(µ-Cl)-Aluminum Bonding in α-Olefin and Diene Chemistry.

Olefin and diene transformations, catalyzed by organoaluminum-activated metal complexes, are widely used in synthetic organic chemistry and form the basis of major petrochemical processes. However, the role of M−(µ-Cl)−Al bonding, being proven for certain >C=C< functionalization reactions, remains unclear and debated for essentially more important industrial processes such as oligomerization and polymerization of α-olefins and conjugated dienes. Numerous publications indirectly point at the significance of M−(µ-Cl)−Al bonding in Ziegler−Natta and related transformations, but only a few studies contain experimental or at least theoretical evidence of the involvement of M−(µ-Cl)−Al species into catalytic cycles. In the present review, we have compiled data on the formation of M−(µ-Cl)−Al complexes (M = Ti, Zr, V, Cr, Ni), their molecular structure, and reactivity towards olefins and dienes. The possible role of similar complexes in the functionalization, oligomerization and polymerization of α-olefins and dienes is discussed in the present review through the prism of the further development of Ziegler−Natta processes and beyond.

Single-Site Fe-N-C Atom Based Carbon Nanotubes for Mutually Promoted and Synergistic Oncotherapy.

A carbon nanotube (CNT) supported single-site Fe-N-C catalyst (CNTs/Fe-N-C) exhibited attractive properties in peroxidase (POD)-like activity and photothermal effect. Herein, we designed a therapeutic platform by wrapping doxorubicin (DOX) in mesoporous CNTs/Fe-N-C with the cell membrane (CM) of breast cancer. The ultimate nanoagent (CNTs/Fe-N-C/DOX/CM) exhibited high antitumor activity on account of its efficient catalytic ability, increased drug release rates, and significant photothermal effect. Due to the POD-like activity, CNTs/Fe-N-C could effectively catalyze hydrogen peroxide (H2O2) into cytotoxic hydroxyl radicals (•OH) for chemodynamic therapy (CDT) of the tumor. Besides, the released DOX not only merely induced the diagnosis of the tumor cells for chemotherapy (CT) but also generated H2O2 to promote CDT. Moreover, the photothermal effect of the nanoagent could use for photothermal therapy (PTT). The increasing temperature was conducive to the release of DOX from micropore into the cell, which indirectly enhanced CT and CDT effects. As an intelligent and multifunctional drug delivery platform, the present CNTs/Fe-N-C/DOX/CM nanoagent could be engineered with synergistic treatments and favorable biosafety, which provides a promising paradigm in site-specific antitumor treatment and biomedicine.

Doping Effect on Mesoporous Carbon-Supported Single-Site Bifunctional Catalyst for Zinc-Air Batteries.

Rechargeable zinc-air batteries (ZABs) require bifunctional electrocatalysts presenting high activity in oxygen reduction/evolution reactions (ORR/OER), but the single-site metal-N-C catalysts suffer from their low OER activity. Herein, we designed a series of single-site Fe-N-C catalysts, which present high surface area and good conductivity by incorporating into mesoporous carbon supported on carbon nanotubes, to study the doping effect of N and P on the bifunctional activity. The additional P-doping dramatically increased the content of active pyridine-N and introduced P-N/C/O sites, which not only act as extra active sites but also regulate the electron density of Fe centers to optimize the absorption of oxygenated intermediates, thereby ultimately improving the bifunctional activity of Fe-N-C sites. The optimized catalyst displayed a half-wave potential of 0.882 V for ORR and a low overpotential of 365 mV at 10 mA cm-2 for OER, which significantly outperforms the counterpart without P, as well as noble-metal-based catalysts. The ZABs with air cathodes containing the N,P-co-doped catalysts exhibited a high peak power density of 201 mW cm-2 and a long cycling stability beyond 600 h. Doping has shown to be an effective way to optimize the performance of single-site catalysts in bifunctional oxygen electrocatalysis, which can be extended to other catalyst systems.

The Control of the Coordination Chemistry for the Genesis of Heterogeneous Catalytically Active Sites in Oxidation Reactions.

This Review provides a summary of the use and the role of coordination complexes as precursors for the generation of heterogeneous catalysts for oxidation reactions of interest for fine chemical synthesis. Specific attention is paid to the comprehension of phenomena explaining the formation of active sites in heterogeneous catalysts. Different families of coordination precursors are considered. For each example, a brief critical description of the synthesis, characterization, and catalytic performance is reported. Special attention is paid to the chemical environment of the first coordination sphere of the active metal centre. The catalysts obtained by heterogenization, grafting, or anchoring of homogeneous catalysts can therefore exhibit enhanced catalytic performance by merging advantages of both homogeneous and heterogeneous systems. The deposition of coordination complexes over a preformed support is indeed a conceptually versatile strategy to design novel catalysts with tuned and controlled properties.

Atomic-Level Pt Electrocatalyst Synthesized via Iced Photochemical Method for Hydrogen Evolution Reaction with High Efficiency.

In heterogeneous catalysis, metal particle morphology and size can influence markedly the activity. It is of great significance to rationally design and control the synthesis of Pt at the atomic level to demonstrate the structure-activity relationship toward electrocatalysis. Herein, a powerful strategy is reported to synthesize graphene-supported platinum-based electrocatalyst, that is, nanocatalysts with controllable size can be prepared by iced photochemical method, including single atoms (Pt-SA@HG), nanoclusters (Pt-Clu@HG), and nanocrystalline (Pt-Nc@HG). The Pt-SA@HG exhibits unexpected electrocatalytic hydrogen evolution reaction (HER) performances with 13 mV overpotential at 10 mA cm-2 current densities which surpass Pt-Clu@HG and Pt-Nc@HG. The in situ X-ray absorption fine structure spectroscopy (XAFS) and density functional theory (DFT) calculations determine the Pt-C3 active site is linchpin to the excellent HER performance of Pt-SA@HG. Compared with the traditional Pt-Nx coordination structure, the pure carbon coordinated Pt-C3 site is more favorable for HER. This work opens up a new way to adjust the metal particle size and catalytic performance of graphene at a multiscale level.

Solid Single-Atom Catalysts in Tandem Catalysis: Lookout, Opportunities and Challenges.

Tandem catalysis stands out as a major instrument towards the intensification of existing and future chemical processes. Initially formulated in the field of homogeneous catalysis, the concept relies on the single-pot integration of two (or more) catalysts showing high specificity for mechanistically decoupled reactions, while being operational and compatible under a single set of operation conditions. Isolated metal atoms stabilized on solid carriers in single-atom catalysts (SACs) hold the potential to reconcile the high reaction specificities of mononuclear sites in molecular catalysts with an intrinsic catalyst compartmentalization on inorganic matrices. Understandably, SACs have started to be considered as platforms in tandem catalysis. Tandem (electro)catalytic processes based on SACs have been showcased recently. While this sets excellent prospects for the expansion of this research subarea, challenges are faced, particularly as to the verification of the tandem nature of the processes.

Heterogeneous Catalysis and Parahydrogen-Induced Polarization.

Parahydrogen-induced polarization with heterogeneous catalysts (HET-PHIP) has been a subject of extensive research in the last decade since its first observation in 2007. While NMR signal enhancements obtained with such catalysts are currently below those achieved with transition metal complexes in homogeneous hydrogenations in solution, this relatively new field demonstrates major prospects for a broad range of advanced fundamental and practical applications, from providing catalyst-free hyperpolarized fluids for biomedical magnetic resonance imaging (MRI) to exploring mechanisms of industrially important heterogeneous catalytic processes. This review covers the evolution of the heterogeneous catalysts used for PHIP observation, from metal complexes immobilized on solid supports to bulk metals and single-atom catalysts and discusses the general visions for maximizing the obtained NMR signal enhancements using HET-PHIP. Various practical applications of HET-PHIP, both for catalytic studies and for potential production of hyperpolarized contrast agents for MRI, are described.

Fair Look at Coordination Oligomerization of Higher α-Olefins.

Coordination catalysis is a highly efficient alternative to more traditional acid catalysis in the oligomerization of α-olefins. The distinct advantage of transition metal-based catalysts is the structural homogeneity of the oligomers. Given the great diversity of the catalysts and option of varying the reaction conditions, a wide spectrum of processes can be implemented. In recent years, both methylenealkanes (vinylidene dimers of α-olefins) and structurally uniform oligomers with the desired degrees of polymerization have become available for later use in the synthesis of amphiphilic organic compounds and polymers, high-quality oils or lubricants, and other prospective materials. In the present review, we discussed the selective dimerization and oligomerization of α-olefins, catalyzed by metallocene and post-metallocene complexes, and explored the prospects for the further applications of the coordination α-olefin dimers and oligomers.

Atomic Layer Deposition for Preparing Isolated Co Sites on SiO2 for Ethane Dehydrogenation Catalysis.

Unlike Co clusters, isolated Co atoms have been shown to be selective for catalytic dehydrogenation of ethane to ethylene; however, preparation of isolated Co sites requires special preparation procedures. Here, we demonstrate that Atomic Layer Deposition (ALD) of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)cobalt(III) (Co(TMHD)3) on silica and other supports is effective in producing these isolated species. Silica-supported catalysts prepared with one ALD cycle showed ethylene selectivities greater than 96% at 923 K and were stable when CO2 was co-fed with the ethane. Co catalysts prepared by impregnation formed clusters that were significantly less active, selective, and stable. Rates and selectivities also decreased for catalysts with multiple ALD cycles. Isolated Co catalysts prepared on Al2O3 and MgAl2O4 showed reasonable selectivity for ethane dehydrogenation but were not as effective as their silica counterpart.

Hierarchical Porous Organometallic Polymers Fabricated by Direct Knitting: Recyclable Single-Site Catalysts with Enhanced Activity.

Porous organometallic polymers (POMPs) with hierarchical pore structures, high specific surface areas, and atomically dispersed metal (Ir, Pd, Ru) centers are successfully fabricated by a facile one-pot method through direct knitting of diverse N-heterocyclic carbene metal (NHC-M) complexes. These polymers can function as recyclable solid single-site catalysts and exhibit excellent catalytic activity and selectivity in both dehydrogenation and hydrogenation reactions even at ppm-level catalyst loadings. Remarkably, a record turnover number (TON) of 1.01 × 106 is achieved in the hydrogenation of levulinic acid to γ-valerolactone, which is 750 times higher than that attained with corresponding bis-NHC-Ir complex.

Toward Efficient Carbon and Water Cycles: Emerging Opportunities with Single-Site Catalysts Made of 3d Transition Metals.

Advances in the chemical and electrochemical transformation of carbon and water are vital for delivering affordable and environmentally friendly energy sources and chemicals. Central to this challenge is the performance of materials. Traditionally, noble metal particles or metal complexes have been used as catalysts for many reactions. Recently, 3d transition-metal single-site catalysts (3dTM-SSCs) have emerged as potentially transformational candidates for the next-generation high-performance noble-metal-free catalysts. Designing catalysts at the molecular level can lead to a more efficient utilization of metal atoms and at the same time enhance catalytic performance under harsh reaction conditions. Despite this promise, several fundamental issues remain, in particular the structural evolution of 3dTM-SSCs during the synthesis, the molecular-level insights into the structure of the active sites, catalytic mechanisms, and the long-term cycling stability. Here, the material chemistries that facilitate the 3dTM-SSCs generation through a controlled pyrolytic synthesis are discussed, with focus on elucidating the underlying performance descriptors that can tune the catalytic properties in various critical reactions in carbon and water cycles. The current challenges and possible solutions for improving these novel catalytic materials are also highlighted.

Zirconocene-Catalyzed Dimerization of α-Olefins: DFT Modeling of the Zr-Al Binuclear Reaction Mechanism.

Zirconocene-mediated selective dimerization of α-olefins usually occurs when precatalyst (η5-C5H5)2ZrCl2 is activated by minimal excess of methylalumoxane (MAO). In this paper, we present the results of density functional theory (DFT) simulation of the initiation, propagation, and termination stages of dimerization and oligomerization of propylene within the framework of Zr-Al binuclear mechanism at M-06x/DGDZVP level of theory. The results of the analysis of the reaction profiles allow to explain experimental facts such as oligomerization of α-olefins at high MAO/(η5-C5H5)2ZrCl2 ratios and increase of the selectivity of dimerization in the presence of R2AlCl. The results of DFT simulations confirm the crucial role of the presence of chloride in the selectivity of dimerization. The molecular hydrogen was found in silico and proven experimentally as an effective agent that increases the rate and selectivity of dimerization.

Reordering d Orbital Energies of Single-Site Catalysts for CO2 Electroreduction.

The single-site catalyst (SSC) characteristic of atomically dispersed active centers will not only maximize the catalytic activity, but also provide a promising platform for establishing the structure-activity relationship. However, arbitrary arrangements of active sites in the existed SSCs make it difficult for mechanism understanding and performance optimization. Now, a well-defined ultrathin SSC is fabricated by assembly of metal-porphyrin molecules, which enables the precise identification of the active sites for d-orbital energy engineering. The activity of as-assembled products for electrocatalytic CO2 reduction is significantly promoted via lifting up the energy level of metal d z 2 orbitals, exhibiting a remarkable Faradaic efficiency of 96 % at the overpotential of 500 mV. Furthermore, a turnover frequency of 4.21 s-1 is achieved with negligible decay over 48 h.