Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters.

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Full Selectivity Control over the Catalytic Hydrogenation of Nitroaromatics into Six Products.

A full selectivity control over the catalytic hydrogenation of nitroaromatics leads to the production of six possible products, i.e., nitroso, hydroxylamine, azoxy, azo, hydrazo or aniline compounds, which has however not been achieved in the field of heterogeneous catalysis. Currently, there is no sufficient evidence to support that the catalytic hydrogenation of nitroaromatics with the use of heterogeneous metal catalysts would follow the Haber's mechanistic scheme based on electrochemical reduction. We now demonstrate in this work that it is possible to fully control the catalytic hydrogenation of nitroaromatics into their all six products using a single catalytic system under various conditions. Employing SnO2-supported Pt nanoparticles facilitated by the surface coordination of ethylenediamine and vanadium species enabled this unprecedented selectivity control. Through systematic investigation into the controlled production of all products and their chemical reactivities, we have constructed a detailed reaction network for the catalytic hydrogenation of nitroaromatics. Crucially, the application of oxygen-isolated characterization techniques proved indispensable in identifying unstable compounds such as nitroso, hydroxylamine, hydrazo compounds. The insights gained from this research offer invaluable guidance for selectively transforming nitroaromatics into a wide array of functional N-containing compounds, both advancing fundamental understanding and fostering practical applications in various fields.

Hydrogen Bond Network Induced by Surface Ligands Shifts the Semi-hydrogenation Selectivity over Palladium Catalysts.

Tuning the metal-ligand interfaces of heterogeneous catalysts has emerged as an effective strategy to optimize their catalytic performance. However, improving the selectivity via organic modification remains a challenge so far. In this work, we demonstrate a simple ligand modification by preparing cysteamine-coated ultrathin palladium nanosheets. The as-prepared catalyst exhibits excellent selectivity with durability during catalytic hydrogenation of terminal alkynes, superior to most previously reported ligand-protected palladium catalysts. Further study reveals that a zwitterionic transformation occurs on the palladium interface under the H2 conditions, generating a rigid hydrogen bond network. Such an unexpected effect beyond the traditional steric effect derived from van der Waals interactions makes the catalytic surface favor the hydrogenation of alkynes over alkenes without significantly sacrificing the catalytic activity. These results not only provide a unique steric effect concept for surface coordination chemistry but also provide a practical application to improve the selectivity and activity comprehensively.

Heterogeneous Hydrogenation with Hydrogen Spillover Enabled by Nitrogen Vacancies on Boron Nitride-Supported Pd Nanoparticles.

Heterogeneous hydrogenation with hydrogen spillover has been demonstrated as an effective route to achieve high selectivity towards target products. More effort should be paid to understand the complicated correlation between the nature of supports and hydrogenation involving hydrogen spillover. Herein, we report the development of the hydrogenation system of hexagonal boron nitride (h-BN)-supported Pd nanoparticles for the hydrogenation of aldehydes/ketones to alcohols with hydrogen spillover. Nitrogen vacancies in h-BN determine the feasibility of hydrogen spillover from Pd to h-BN. The hydrogenation of aldehydes/ketones with hydrogen spillover from Pd proceeds on nitrogen vacancies on h-BN. The weak adsorption of alcohols to h-BN inhibits the deep hydrogenation of aldehydes/ketones, thus leading to high catalytic selectivity to alcohols. Moreover, the hydrogen spillover-based hydrogenation mechanism makes the catalyst system exhibit a high tolerance to CO poisoning.

N-Heterocyclic Carbene-Stabilized Gold Nanoclusters with Organometallic Motifs for Promoting Catalysis.

The complexity of heterogeneous metal catalysts makes it challenging to gain insights into their catalytic mechanisms. Thus, there exists a huge gap between heterogeneous catalysis and organometallic catalysis. With the success in the preparation of highly robust atomically precise metal nanocluster catalysts (i.e., [Au16(NHC-1)5(PA)3Br2]3+ and [Au17(NHC-1)4(PA)4Br4]+, where NHC-1 is a bidentate NHC ligand, and PA is phenylacetylide) with surface organometallic motifs anchored on the metallic core, we demonstrate in this work how the metallic core works synergistically with the surface organometallic motifs to enhance the catalysis. More importantly, the discovery allows the development of highly stable and recyclable heterogeneous metal catalysts to achieve efficient hydroamination of alkynes with an extremely low catalyst dosage (0.002 mol %), helping bridge the gap between heterogeneous and homogeneous metal catalysis. The surface modification of metal nanocatalysts with organometallic motifs provides a new design principle of metal catalysts with enhanced catalysis.

Surface Coordination Chemistry of Atomically Dispersed Metal Catalysts.

Atomically dispersed metal catalysts (ADCs), as an emerging class of heterogeneous catalysts, have been widely investigated during the past two decades. The atomic dispersion nature of the catalytic metal centers makes them an ideal system for bridging homogeneous and heterogeneous metal catalysts. The recent rapid development of new synthetic strategies has led to the explosive growth of ADCs with a wide spectrum of metal atoms dispersed on supports of different chemical compositions and natures. The availability of diverse ADCs creates a powerful materials platform for investigating mechanisms of complicated heterogeneous catalysis at the atomic levels. Considering most dispersed metal atoms on ADCs are coordinated by the donors from supports, this review will demonstrate how the surface coordination chemistry plays an important role in determining the catalytic performance of ADCs. This review will start from the link between coordination chemistry and heterogeneous catalysis. After the brief description on the advantages and limitations of common structure characterization methods in determining the coordination structure of ADCs, the surface coordination chemistry of ADCs on different types of supports will be discussed. We will mainly illustrate how the local and vicinal coordination species on different support systems act together with the dispersed catalytic metal center to determine the catalytic activity, selectivity, and stability of ADCs. The dynamic coordination structure change of ADCs in catalysis will be highlighted. At the end of the review, personal perspectives on the further development of the field of ADCs will be provided.

Heterogeneous Isomerization for Stereoselective Alkyne Hydrogenation to trans-Alkene Mediated by Frustrated Hydrogen Atoms.

Stereoselective production of alkenes from the alkyne hydrogenation plays a crucial role in the chemical industry. However, for heterogeneous metal catalysts, the olefins in cis-configuration are usually dominant in the products due to the most important and common Horiuti-Polanyi mechanism involved over the metal surface. In this work, through combined theoretical and experimental investigations, we demonstrate a novel isomerization mechanism mediated by the frustrated hydrogen atoms via the H2 dissociation at the defect on solid surface, which can lead to the switch in selectivity from the cis-configuration to trans-configuration without overhydrogenation. The defective Rh2S3 with exposing facet of (110) exhibits outstanding performance as a heterogeneous metal catalyst for stereoselective production of trans-olefins. With the frustrated hydrogen atoms at spatially separated high-valence Rh sites, the isolated hydrogen mediated cis-to-trans isomerization of olefins can be effectively conducted and the overhydrogenation can be completely inhibited. Furthermore, the bifunctional Rh-S/Pd nanosheets have been synthesized through the surface modification of Pd nanosheets with rhodium and sulfide. With the selective semihydrogenation of alkynes into cis-olefins catalyzed by the small surface PdSx ensembles, the bifunctional Rh-S/Pd nanosheets exhibit excellent activity and stereoselectivity in the one-pot alkyne hydrogenation into trans-olefin, which surpasses the most reported homogeneous and heterogeneous catalysts.

Insights into the Interfacial Effects in Heterogeneous Metal Nanocatalysts toward Selective Hydrogenation.

Heterogeneous metal catalysts are distinguished by their structure inhomogeneity and complexity. The chameleonic nature of heterogeneous metal catalysts have prevented us from deeply understanding their catalytic mechanisms at the molecular level and thus developing industrial catalysts with perfect catalytic selectivity toward desired products. This Perspective aims to summarize recent research advances in deciphering complicated interfacial effects in heterogeneous hydrogenation metal nanocatalysts toward the design of practical heterogeneous catalysts with clear catalytic mechanism and thus nearly perfect selectivity. The molecular insights on how the three key components (i.e., catalytic metal, support, and ligand modifier) of a heterogeneous metal nanocatalyst induce effective interfaces determining the hydrogenation activity and selectivity are provided. The interfaces influence not only the H2 activation pathway but also the interaction of substrates to be hydrogenated with catalytic metal surface and thus the hydrogen transfer process. As for alloy nanocatalysts, together with the electronic and geometric ensemble effects, spillover hydrogenation occurring on catalytically "inert" metal by utilizing hydrogen atom spillover from active metal is highlighted. The metal-support interface effects are then discussed with emphasis on the molecular involvement of ligands located at the metal-support interface as well as cationic species from the support in hydrogenation. The mechanisms of how organic modifiers, with the ability to induce both 3D steric and electronic effects, on metal nanocatalysts manipulate the hydrogenation pathways are demonstrated. A brief summary is finally provided together with a perspective on the development of enzyme-like heterogeneous hydrogenation metal catalysts.

Surface Coordination of Multiple Ligands Endows N-Heterocyclic Carbene-Stabilized Gold Nanoclusters with High Robustness and Surface Reactivity.

Deciphering the molecular pictures of the multi-component and non-periodic organic-inorganic interlayer is a grand technical challenge. Here we show that the atomic arrangement of hybrid surface ligands on metal nanoparticles can be precisely quantified through comprehensive characterization of a novel gold cluster, Au44 (i Pr2 -bimy)9 (PA)6 Br8 (1), which features three types of ligands, namely, carbene (1,3-diisopropylbenzimidazolin-2-ylidene, i Pr2 -bimy), alkynyl (phenylacetylide, PA), and halide (Br), respectively. The delicately balanced stereochemical effects and bonding capabilities of the three ligands give rise to peculiar geometrical and electronic structures. Remarkably, despite its complex and highly distorted surface structure, cluster 1 exhibits unusual catalytic properties and yet it is highly stable, both chemically and thermally. Moreover, rich reactive sites on the cluster surface raise the prospect of bio-compatibility (as it can be functionalized to yield water-soluble derivatives) and bio-applications.

Chemoselective Hydrogenation of Nitroaromatics at the Nanoscale Iron(III)-OH-Platinum Interface.

Catalytic hydrogenation of nitroaromatics is an environment-benign strategy to produce industrially important aniline intermediates. Herein, we report that Fe(OH)x deposition on Pt nanocrystals to give Fe(OH)x /Pt, enables the selective hydrogenation of nitro groups into amino groups without hydrogenating other functional groups on the aromatic ring. The unique catalytic behavior is identified to be associated with the FeIII -OH-Pt interfaces. While H2 activation occurs on exposed Pt atoms to ensure the high activity, the high selectivity towards the production of substituted aniline originates from the FeIII -OH-Pt interfaces. In situ IR, X-ray photoelectron spectroscopy (XPS), and isotope effect studies reveal that the Fe3+ /Fe2+ redox couple facilitates the hydrodeoxygenation of the -NO2 group during hydrogenation catalysis. Benefitting from FeIII -OH-Pt interfaces, the Fe(OH)x /Pt catalysts exhibit high catalytic performance towards a broad range of substituted nitroarenes.

Economizing Production of Diverse 2D Layered Metal Hydroxides for Efficient Overall Water Splitting.

2D layered metal hydroxides (LMH) are promising materials for electrochemical energy conversion and storage. Compared with exfoliation of bulk layered materials, wet chemistry synthesis of 2D LMH materials under mild conditions still remains a big challenge. Here, an "MgO-mediated strategy" for mass production of various 2D LMH nanosheets is presented by hydrolyzing MgO in metal salt aqueous solutions at room temperature. Benefiting from this economical and scalable strategy, ultrathin LMH nanosheets (M = Ni, Fe, Co, NiFe, and NiCo) and their derivatives (e.g., metal oxides and sulfides) can be synthesized in high yields. More importantly, this strategy opens up opportunities to fabricate hierarchically structured LMH nanosheets, resulting in high-performance electrocatalysts for the oxygen- and hydrogen-evolution reactions to realize stable overall water splitting with a low cell voltage of 1.55 V at 10 mA cm-2 . This work provides a powerful platform for the synthesis and applications of 2D materials.

Electrochemical Reduction of Carbon Dioxide to Methanol on Hierarchical Pd/SnO2 Nanosheets with Abundant Pd-O-Sn Interfaces.

Electrochemical conversion of CO2 into fuels using electricity generated from renewable sources helps to create an artificial carbon cycle. However, the low efficiency and poor stability hinder the practical use of most conventional electrocatalysts. In this work, a 2D hierarchical Pd/SnO2 structure, ultrathin Pd nanosheets partially capped by SnO2 nanoparticles, is designed to enable multi-electron transfer for selective electroreduction of CO2 into CH3 OH. Such a structure design not only enhances the adsorption of CO2 on SnO2 , but also weakens the binding strength of CO on Pd due to the as-built Pd-O-Sn interfaces, which is demonstrated to be critical to improve the electrocatalytic selectivity and stability of Pd catalysts. This work provides a new strategy to improve electrochemical performance of metal-based catalysts by creating metal oxide interfaces for selective electroreduction of CO2 .

Surface Coordination Chemistry of Metal Nanomaterials.

Surface coordination chemistry of nanomaterials deals with the chemistry on how ligands are coordinated on their surface metal atoms and influence their properties at the molecular level. This Perspective demonstrates that there is a strong link between surface coordination chemistry and the shape-controlled synthesis, and many intriguing surface properties of metal nanomaterials. While small adsorbates introduced in the synthesis can control the shapes of metal nanocrystals by minimizing their surface energy via preferential coordination on specific facets, surface ligands properly coordinated on metal nanoparticles readily promote their catalysis via steric interactions and electronic modifications. The difficulty in the research of surface coordination chemistry of nanomaterials mainly lies in the lack of effective tools to characterize their molecular surface coordination structures. Also highlighted are several model material systems that facilitate the characterizations of surface coordination structures, including ultrathin nanostructures, atomically precise metal nanoclusters, and atomically dispersed metal catalysts. With the understanding of surface coordination chemistry, the molecular mechanisms behind various important effects (e.g., promotional effect of surface ligands on catalysis, support effect in supported metal nanocatalysts) of metal nanomaterials are disclosed.

Frustrated Lewis pairs on pentacoordinated Al3+-enriched Al2O3 promote heterolytic hydrogen activation and hydrogenation.

As an emerging class of metal-free catalysts, frustrated Lewis pairs (FLPs) catalysts have been greatly constructed and applied in many fields. Homogeneous FLPs have witnessed significant development, while limited heterogeneous FLPs catalysts are available. Herein, we report that heterogeneous FLPs on pentacoordinated Al3+-enriched Al2O3 readily promote the heterolytic activation of H2 and thus hydrogenation catalysis. The defect-rich Al2O3 was prepared by simple calcination of a carboxylate-containing Al precursor. Combinatorial studies confirmed the presence of rich FLPs on the surface of the defective Al2O3. In contrast to conventional alumina (γ-Al2O3), the FLP-containing Al2O3 can activate H2 in the absence of any transition metal species. More importantly, H2 was activated by surface FLPs in a heterolytic pathway, leading to the hydrogenation of styrene in a stepwise process. This work paves the way for the exploration of more underlying heterogeneous FLPs catalysts and further understanding of accurate active sites and catalytic mechanisms of heterogeneous FLPs at the molecular level.

Pectin extracted from Premna Microphylla Turcz for preparation of a "sandwich" multi-property sensor film involved with deep eutectic solvent.

An acidic deep eutectic solvent (DES, choline chloride/citric acid) was used to efficiently extract edible pectin from Premna microphylla Turcz (PMTP) and further prepare the film sensor with the purpose of "four birds with one stone" with the roles of extractant, coalescent, conductivity promoter and bacteriostatic agent. The optimized extraction process accorded with pseudo second-order kinetics, which was carried out at 78.2 °C for 1.29 h with the solid-liquid ratio of 1:34.66 g/mL with the yield up to 0.8210 g/g. After comprehensive characterizations of pectin product, a simple casting method was used to prepare the PMTP-DES based composite film. It showed that the composite film has promising compatibility, smooth surface, good breathability and ideal homogeneity. After 30 power on/power off cycles at 10 V, it exhibited satisfied conductivity stability. Moreover, the PMTP-DES film could be simply assembled as the flexible visual temperature sensor, with sensitive response at breathing or finger touch; it exhibited the highest sensitivity of 134 %/°C when the external temperature changed from 15 to 55 °C. Besides, the composite film also has preferable antimicrobial activity. The whole results and findings were aimed to contribute for the raw material, composition, preparation, and functions of the existing flexible functional materials.

Upgrading heterogeneous Ni catalysts with thiol modification.

Precious metal catalysts are the cornerstone of many industrial processes. Replacing precious metal catalysts with earth-abundant metals is one of key challenges for the green and sustainable development of chemical industry. We report in this work a surprisingly effective strategy toward the development of cost-effective, air-stable, and efficient Ni catalysts by simple surface modification with thiols. The as-prepared catalysts exhibit unprecedentedly high activity and selectivity in the reductive amination of aldehydes/ketones. The thiol modification can not only prevent the deep oxidation of Ni surface to endow the catalyst with long shelf life in air but can also allow the reductive amination to proceed via a non-contact mechanism to selectively produce primary amines. The catalytic performance is far superior to that of precious and non-precious metal catalysts reported in the literature. The wide application scope and high catalytic performance of the developed Ni catalysts make them highly promising for the low-cost, green production of high-value amines in chemical industry.

Atomic overlayer of permeable microporous cuprous oxide on palladium promotes hydrogenation catalysis.

The interfacial sites of metal-support interface have been considered to be limited to the atomic region of metal/support perimeter, despite their high importance in catalysis. By using single-crystal surface and nanocrystal as model catalysts, we now demonstrate that the overgrowth of atomic-thick Cu2O on metal readily creates a two-dimensional (2D) microporous interface with Pd to enhance the hydrogenation catalysis. With the hydrogenation confined within the 2D Cu2O/Pd interface, the catalyst exhibits outstanding activity and selectivity in the semi-hydrogenation of alkynes. Alloying Cu(0) with Pd under the overlayer is the major contributor to the enhanced activity due to the electronic modulation to weaken the H adsorption. Moreover, the boundary or defective sites on the Cu2O overlayer can be passivated by terminal alkynes, reinforcing the chemical stability of Cu2O and thus the catalytic stability toward hydrogenation. The deep understanding allows us to extend the interfacial sites far beyond the metal/support perimeter and provide new vectors for catalyst optimization through 2D interface interaction.

Nanoscale engineering of catalytic materials for sustainable technologies.

Nanostructured materials of diverse architecture are ubiquitous in industrial catalysis. They offer exciting prospects to tackle various sustainability challenges faced by society. Since the introduction of the concept a century ago, researchers aspire to control the chemical identity, local environment and electronic properties of active sites on catalytic surfaces to optimize their reactivity in given applications. Nowadays, numerous strategies exist to tailor these characteristics with varying levels of atomic precision. Making headway relies upon the existence of analytical approaches able to resolve relevant structural features and remains challenging due to the inherent complexity even of the simplest heterogeneous catalysts, and to dynamic effects often occurring under reaction conditions. Computational methods play a complementary and ever-increasing role in pushing forward the design. Here, we examine how nanoscale engineering can enhance the selectivity and stability of catalysts. We highlight breakthroughs towards their commercialization and identify directions to guide future research and innovation.

Carbon Deposition on Heterogeneous Pt Catalysts Promotes the Selective Hydrogenation of Halogenated Nitroaromatics.

Pt-based heterogeneous metal catalysts are commonly used in the hydrogenation of nitro groups. However, the continuous large metal domain size and the complex interaction between the substrates and metal surface usually inevitably result in side reactions, such as dehalogenation of halogenated nitroaromatics. In this work, by simply treating the Pt-based catalysts with CO at elevated temperature, the Boudouard reaction taking place on Pt surface results in the deposition of carbon species thereon. The carbon deposition not only inhibits the dehalogenation by restricting the planar interaction of the aromatic group with the Pt surface that is segmented by C, the hydrogenation activity of the nitro group is also enhanced on the partially positively charged surface Pt sites created by the electron-withdrawing interstitial C. Such a simple treatment strategy is effective for fabricating Pt-based catalysts on different supports toward the selective hydrogenation of a wide range of halogenated nitroaromatics.

Carbon Monoxide Promotes the Catalytic Hydrogenation on Metal Cluster Catalysts.

Size effect plays a crucial role in catalytic hydrogenation. The highly dispersed ultrasmall clusters with a limited number of metal atoms are one candidate of the next generation catalysts that bridge the single-atom metal catalysts and metal nanoparticles. However, for the unfavorable electronic property and their interaction with the substrates, they usually exhibit sluggish activity. Taking advantage of the small size, their catalytic property would be mediated by surface binding species. The combination of metal cluster coordination chemistry brings new opportunity. CO poisoning is notorious for Pt group metal catalysts as the strong adsorption of CO would block the active centers. In this work, we will demonstrate that CO could serve as a promoter for the catalytic hydrogenation when ultrasmall Pd clusters are employed. By means of DFT calculations, we show that Pd n (n = 2-147) clusters display sluggish activity for hydrogenation due to the too strong binding of hydrogen atom and reaction intermediates thereon, whereas introducing CO would reduce the binding energies of vicinal sites, thus enhancing the hydrogenation reaction. Experimentally, supported Pd2CO catalysts are fabricated by depositing preestablished [Pd2(µ-CO)2Cl4]2- clusters on oxides and demonstrated as an outstanding catalyst for the hydrogenation of styrene. The promoting effect of CO is further verified experimentally by removing and reintroducing a proper amount of CO on the Pd cluster catalysts.