Metal Single-atom Catalysts Research Articles

Photoresponses of Supported Au Single Atoms on TiO2(110) through the Metal-Induced Gap States.

When a metal single-atom (SA) catalyst is supported on a semiconducting photocatalyst, the charge transfer of the photoexcited carriers to metal SAs can provide a synergetic activity for the co-catalysts. Here, we report the interfacial electronic coupling of the Au SAs on the TiO2(110) surface using scanning tunneling microscopy/spectroscopy, in combination with first-principles calculations. Distinct energy and spatial distributions of the metal-induced gap states (MIGSs) are experimentally revealed for the Au SAs adsorbed at the terminal Ti sites and the oxygen vacancies. The localized MIGS below the Fermi level provides a dedicated channel for the transfer of a photoexcited hole from the TiO2 substrate to the adsorbed Au SAs. The hole can weaken the Ti-Au bonding and activate the diffusion of Au SAs. Our results shed light on combining the advantages of photocatalysis and metal SA catalysis using a co-catalyst, which is promising to promote chemical reactions at low temperatures.

Atomically Dispersed Semimetallic Selenium on Porous Carbon Membrane as an Electrode for Hydrazine Fuel Cells.

Electrochemically functional porous membranes of low cost are appealing in various electrochemical devices used in modern environmental and energy technologies. Herein we describe a scalable strategy to construct electrochemically active, hierarchically porous carbon membranes containing atomically dispersed semi-metallic Se, denoted SeNCM. The isolated Se atoms were stabilized by carbon atoms in the form of a hexatomic ring structure, in which the Se atoms were located at the edges of graphitic domains in SeNCM. This configuration is different from that of previously reported transition/noble metal single atom catalysts. The positively charged Se, enlarged graphitic layers, robust electrochemical nature of SeNCM endow them with excellent catalytic activity that is superior to state-of-the-art commercial Pt/C catalyst. It also has long-term operational stability for hydrazine oxidation reaction in practical hydrazine fuel cell.

Atomically Dispersed Semimetallic Selenium on Porous Carbon Membrane as an Electrode for Hydrazine Fuel Cells

AbstractElectrochemically functional porous membranes of low cost are appealing in various electrochemical devices used in modern environmental and energy technologies. Herein we describe a scalable strategy to construct electrochemically active, hierarchically porous carbon membranes containing atomically dispersed semi‐metallic Se, denoted SeNCM. The isolated Se atoms were stabilized by carbon atoms in the form of a hexatomic ring structure, in which the Se atoms were located at the edges of graphitic domains in SeNCM. This configuration is different from that of previously reported transition/noble metal single atom catalysts. The positively charged Se, enlarged graphitic layers, robust electrochemical nature of SeNCM endow them with excellent catalytic activity that is superior to state‐of‐the‐art commercial Pt/C catalyst. It also has long‐term operational stability for hydrazine oxidation reaction in practical hydrazine fuel cell.

Metal-organic framework encapsulated single-atom Pt catalysts for efficient photocatalytic hydrogen evolution

Atomically dispersed metal catalysts have emerged as a new research frontier in the catalytic community due to the utmost atom efficiency and tunable functionality in a wide variety of catalytic reactions. In the present work, a general operable synthetic platform for stable and well-defined positioned single-atom metal catalysts encapsulated in metal-organic framework (MOF) architecture was developed. Using the MOF-808 based single ion trap method, single metal ions were effectively captured by ethylenediaminetetraacetic acid (EDTA) ligands, which were exchanged with the original formate ligands anchored at the Zr6 cluster metal nodes. Upon activated in hydrogen atmosphere at 200 °C, the stable MOF-808-EDTA encapsulated single-atom metal catalysts with well-defined atomic positions were synthesized. The synthesized single-atom metal catalysts were confirmed using various experimental characterization techniques including high-angle annular dark-field scanning transmission electron microscope, extended X-ray absorption fine structure spectroscopy analysis, and diffuse reflectance infrared Fourier transform, as well as theoretical density functional theory calculations. As a demonstration case, it has been found that the as-synthesized MOF-808-EDTA encapsulated single-atom Pt catalyst shows the excellent photocatalytic hydrogen evolution activity (68.33 mmol g−1 h−1) and high stability. The apparent quantum efficiency reaches up to 67.6% at 420 nm. This work not only provides a general and effective synthesis approach for preparing stable, uniformly distributed, MOF encapsulated single-atom metal catalysts such as Pd, Rh, Ru, Cu, Co, and Ni, but also sheds new insights into single-atom metal catalysts for photocatalytic water splitting for hydrogen production.

Application of heterogeneous catalysts in olefin hydroformylation

Hydroformylation is a 100% atom-economical reaction using syngas and olefins to produce aldehydes, which find wide application in plasticizers, surfactants, pharmaceuticals, agrochemicals, flavorings, etc. The increasing demand of aldehydes and alcohols in the national economy, as well as the development of the petrochemical industry, boost the rapid development of olefins hydroformylation. Nowadays, hydroformylation has become one of the most important technologies using olefins to produce oxo chemicals. More than 10 million tons of oxo chemicals are produced via homogeneous hydroformylation around the world per year. Homogeneous hydroformylation catalysts show excellent catalytic activity and selectivity. But the difficulty in separation of catalysts from reaction mixtures cannot be ignored. Although heterogeneous catalysts are inferior in activity and/or selectivity, they are extremely more preeminent in separation than homogeneous catalysts. Therefore, how to combine the advantages of heterogeneous and homogeneous catalysts to realize efficient heterogeneous hydroformylation is the research hotspot currently. In the past several decades, a good deal of efforts has been devoted to developing heterogeneous catalysts for hydroformylation, including molecular catalysts, metal nanoparticles catalysts and metal single atom catalysts: (1) Heterogeneous molecular catalysts are prepared by anchoring organometallic complex on the surface/framework/pores of supports. The study focuses on keeping the activity and stability of the immobilized organometallic complex. Currently, porous organic copolymers supported Rh complex, which can steady run for 1000 h, show great potential in the industrial application. (2) The activity, selectivity, and stability of metal nanoparticles catalysts are relatively low. Both phosphorus ligands and inorganic promoters have been employed to improve the catalytic performance of metal nanoparticles catalysts. However, the unclear relationship of structure-activity limits the development of an efficient catalyst. Combining in situ characterization technique, illustrating the effect of promoters on catalytic performance of metal nanoparticles catalysts in hydroformylation will be one of the primary targets in the future. On the other hand, developing new metal nanoparticles catalysts, such as highly crystalline metal phosphides, may be a possible breakthrough in this field. (3) Although single atom catalysts show comparable activity with homogeneous catalysts, their regioselectivity and stability are still unclear. Further research should focus on the relationship of single atom catalysts with hydroformylation, and developing efficient and stable single atom catalysts.

Building Up a Picture of the Electrocatalytic Nitrogen Reduction Activity of Transition Metal Single-Atom Catalysts.

The lack of chemical understanding and efficient catalysts impedes the development of electrocatalytic nitrogen reduction reaction (eNRR) for ammonia production. In this work, we employed density functional theory calculations to build up a picture (activity trends, electronic origins, and design strategies) of single-atom catalysts (SACs) supported on nitrogen-doped carbons as eNRR electrocatalysts. To construct such a picture, this work presents systematic studies of the eNRR activity of SACs covering 20 different transition metal (TM) centers coordinated by nitrogen atoms contained in three types of nitrogen-doped carbon substrates, which gives 60 SACs. Our study shows that the intrinsic activity trends could be established on the basis of the nitrogen adatom adsorption energy (Δ EN*). Furthermore, the influence of metal and support (ligands) on Δ EN* proved to be related to the bonding/antibonding orbital population and regulating the scaling relations for adsorption of intermediates, respectively. Accordingly, a two-step strategy is proposed for improving the eNNR activity of TM-SACs, which involves the following: (i) selection of the most promising family of SACs (g-C3N4 supported SACs as predicted in this work) and (ii) further improvement of the activity of the best candidate in the aforementioned family via tuning the adsorption strength of the key intermediates. Also, the stability of N-doped carbon supports and their selectivity in comparison to the competing hydrogen evolution need to be taken into consideration for screening the durable and efficient candidates. Finally, an effective strategy for designing active, stable, and selective SACs based on the mechanistic insights is elaborated to guide future eNRR studies.

Tailoring Electronic Structure of Atomically Dispersed Metal–N3S1 Active Sites for Highly Efficient Oxygen Reduction Catalysis

Rational design of oxygen electrocatalysts with high catalytic activity and long-term durability is currently a severe challenge for rechargeable Zn–air batteries. Here, we highlighted an electronic structure engineering on Cu-based catalyst with atomically dispersed Cu–N3S1 active sites, representing as highly active electrode material for oxygen reduction. The structurally-new Cu–N3S1 species shows unique electron interactions with both phosphorus atoms/carbon support, which not only activate the electron transfer around the Cu–N3S1 sites but also enhance the interaction with oxygenated species, resulting in a promoting reaction kinetics process. As expected, this catalyst exhibits superior catalytic activity for oxygen reduction and evolution. A rechargeable flexible solid Zn–air battery based on this catalyst behaves superior performance of a high open-circuit voltage (1.41 V), a large power density (138.2 mW/cm2), and a small charge/discharge voltage gap (0.72 V at 2.0 mA cm–2) even under different bending angles. Specifically, this strategy is general and can be extended to other metal single atom catalyst systems (such as Fe–N3S1 species). This work will open a new avenue to design structurally-new single atom catalysts for energy-related field.

Bridging the Homogeneous-Heterogeneous Divide: Modeling Spin for Reactivity in Single Atom Catalysis.

Single atom catalysts (SACs) are emergent catalytic materials that have the promise of merging the scalability of heterogeneous catalysts with the high activity and atom economy of homogeneous catalysts. Computational, first-principles modeling can provide essential insight into SAC mechanism and active site configuration, where the sub-nm-scale environment can challenge even the highest-resolution experimental spectroscopic techniques. Nevertheless, the very properties that make SACs attractive in catalysis, such as localized d electrons of the isolated transition metal center, make them challenging to study with conventional computational modeling using density functional theory (DFT). For example, Fe/N-doped graphitic SACs have exhibited spin-state dependent reactivity that remains poorly understood. However, spin-state ordering in DFT is very sensitive to the nature of the functional approximation chosen. In this work, we develop accurate benchmarks from correlated wavefunction theory (WFT) for relevant octahedral complexes. We use those benchmarks to evaluate optimal DFT functional choice for predicting spin state ordering in small octahedral complexes as well as models of pyridinic and pyrrolic nitrogen environments expected in larger SACs. Using these guidelines, we determine Fe/N-doped graphene SAC model properties and reactivity as well as their sensitivities to DFT functional choice. Finally, we conclude with broad recommendations for computational modeling of open-shell transition metal single-atom catalysts.

H2Dissociation on Noble Metal Single Atom Catalysts Adsorbed on and Doped into CeO2(111)

We used density functional theory (DFT) calculations to investigate the dissociation of H2 on an Ag single atom catalyst adsorbed on the pristine CeO2 (111) surface (Ag/CeO2), or substituting a surface Ce atom on the reduced (Ag:CeO2-x) and partially hydrogenated (Ag:H–CeO2) surfaces. The initial state of the H2 dissociation reaction in the different investigated models corresponds to distinct oxidation states, +1, +2, or +3, of the Ag atom, thus allowing us to examine the influence of the charge transfers between the noble metal, the oxide, and the hydrogen atoms on the reaction pathway and activation energy. In all investigated models, the computed barrier of H2 dissociation is lowered by about 0.6 eV in comparison to that on metal-free CeO2. On Ag/CeO2 and Ag:CeO2-x, also the energy of H2 dissociative adsorption is smaller than that on metal-free ceria. These results suggest that CeO2 modified with dispersed Ag atoms is a promising anode material for proton exchange membrane fuel cells. Further comparison of our results for Ag to analogous calculations for Cu and Au single atom catalysts reveals trends in the computed barriers that can be related to the change of the metal oxidation state in the reaction.

Fabricating Single-Atom Catalysts from Chelating Metal in Open Frameworks.

In the present study, a highly efficient strategy is reported using open framework platforms with abundant chelating ligands to fabricate a series of stable metal single-atom catalysts (SACs). Here, the metal ions are initially anchored onto the active bipyridine sites through postsynthetic modification, followed by pyrolysis and acid leaching. The resulting single metal atoms are uniformly distributed on a nitrogen-doped carbon (N-C) matrix. Interestingly, each metal atom is found to be coordinated with five N atoms, in contrast to the average coordination number of four as previously reported. The as-prepared Fe SAC/N-C catalyst exhibits excellent oxygen reduction reaction (ORR) activity (with a half-wave potential of 0.89 V), outstanding stability, and good methanol tolerance. The density functional calculations reveal that the coordinated pyridine can favorably modulate the interaction strength of oxygen on the Fe ion and thus improve the ORR activity. More importantly, it is demonstrated that this strategy can be successfully extended to the preparation of other transition metal SACs, simply by altering the metal precursors used in the metalation step.

Boosting Oxygen Reduction Catalysis with Fe–N4 Sites Decorated Porous Carbons toward Fuel Cells

It is highly desired but remains a great challenge to develop nonprecious metal single-atom catalysts to supersede the Pt-based material for oxygen reduction reaction (ORR). Herein, we report a porous nitrogen-doped carbon matrix catalyst with 3.5 wt % single Fe atoms (Fe SAs/N–C) through a versatile molecules-confined pyrolysis strategy. In 0.1 M KOH condition, the Fe SAs/N–C catalyst possesses a half-wave potential of 0.91 V vs RHE. In a more challenging acidic solution, Fe SAs/N–C catalyst also offers good ORR activity, comparable with the commercial Pt/C. Impressively, Fe SAs/N–C shows extremely high stability both in alkaline and acidic media. In addition, this Fe SAs/N–C-derived Zn–air battery and proton exchange membrane fuel cells (PEMFCs) exhibit high performance. This work opens an avenue for designing and preparing single-atom catalysts.

Dispersive Single-Atom Metals Anchored on Functionalized Nanocarbons for Electrochemical Reactions.

The use of dispersive single-atom metals anchored on functionalized carbon nanomaterials as electrocatalysts for electrochemical energy conversion reactions represents a burgeoning area of research, due to their unique characteristics of low coordination number, uniform coordination environment, and maximum atomic utilization. Here we highlight the advanced synthetic methods, characterization techniques, and electrochemical applications for carbon-based single-atom metal catalysts, and provide illustrative correlations between molecular/electronic structures and specific catalytic activity for O2 reduction, water splitting, and other emerging reactions including CO2 reduction, H2O2 production, and N2 reduction. We also discuss fundamental principles for the future design of carbon-based single-atom metal catalysts for specific electrochemical reactions. In addition, we explore the challenges and opportunities that lie ahead in further work with carbon-based single-atom metal electrocatalysts.

Toward Understanding of the Support Effect on Pd1 Single-Atom-Catalyzed Hydrogenation Reactions

Metal single-atom catalysts (SACs) have received significant attention in the catalysis field due to maximized atom efficiency and their unique catalytic properties. The support could have prominen...

Recent Advances in Electrochemical CO2 -to-CO Conversion on Heterogeneous Catalysts.

Electrochemical reduction of carbon dioxide (CO2 ) to fuels and chemicals provides a promising solution for renewable energy storage and utilization. Among the many possible reaction pathways, CO2 conversion to carbon monoxide (CO) is the first step in the synthesis of more complex carbon-based fuels and feedstocks, and holds great significance for the chemical industry. Herein, recent advances in heterogeneous catalysts for selective CO evolution from electrochemical reduction of CO2 are described. With Au catalysts as a paradigm, principles for catalyst design including size, morphology, and grain boundary densities tuning, surface modifications, as well as metal-support interaction are comprehensively summarized, which shed light on the development of other transition metal catalysts targeting efficient CO2 -to-CO conversion. In addition, recently emerged novel materials including transition metal single-atom catalysts, which present significantly different catalytic behaviors compared to their bulk counterparts and thus open up many unexpected opportunities, are summarized. Furthermore, the technical aspects with respect to large-scale production of CO are presented, focusing on the full-cell design and implementation. Finally, short comments related to the future direction of real-word CO2 electrolysis for CO supply are provided in terms of catalyst optimization and technical breakthrough.

Understanding the underlying mechanism of improved selectivity in pd1 single-atom catalyzed hydrogenation reaction

Supported metal single-atom catalysts (SACs) with maximized metal atom efficiency and unique catalytic properties have drawn tremendous attention in the catalysis field. In this work, we reported that in selective hydrogenation of 1,3-butadiene, graphene supported Pd1 single atoms, synthesized by atomic layer deposition, exhibited expressively 100% butenes selectivity at 100% conversion at near ambient temperature, regardless of hydrogen partial pressures. The hydrogen reaction order was found to be about 1.2, indicating that hydrogenation dissociation is the rate-determining step. Combining with other structural characterization techniques, in situ X-ray absorption fine structure spectroscopy suggests that the Pd1 single atom likely bonds to the graphene support through three PdC and one PdOC coordinations. Density functional theory calculations show that 1,3-butadiene adsorbs on Pd1 via a mono-π adsorption mode (−0.98 eV), much stronger than that of H2 (−0.30 eV), which makes Pd1 to be predominantly covered with a 1,3-butadiene molecule during reaction, consistent very well with the results of H-D exchange reaction. Facilitated by the bridging O atom in the PdOC coordination, H2 dissociates at the 1,3-butadiene covered Pd1 atom in the heterolytic way, then hydrogenates the adsorbed 1,3-butadiene molecule by following the pseudo Horiuti-Polanyi mechanism. Distinct from its adsorption on extended Pd surfaces, such mono-π adsorption of 1,3-butadiene on Pd1 favors 1-butene formation, but impedes the secondary hydrogenation to butane owing to intensified steric hindrance, thus yielding the expressively high butenes selectivity over Pd1 SAC. These findings pave a new way to rational design SACs catalyst in selective hydrogenation reactions.

Carbon-supported metal single atom catalysts

Carbon-supported metal single atom catalysts

RETRACTED ARTICLE: A universal principle for a rational design of single-atom electrocatalysts

Developing highly active single-atom catalysts for electrochemical reactions is a key to future renewable energy technology. Here we present a universal design principle to evaluate the activity of graphene-based single-atom catalysts towards the oxygen reduction, oxygen evolution and hydrogen evolution reactions. Our results indicate that the catalytic activity of single-atom catalysts is highly correlated with the local environment of the metal centre, namely its coordination number and electronegativity and the electronegativity of the nearest neighbour atoms, validated by available experimental data. More importantly, we reveal that this design principle can be extended to metal–macrocycle complexes. The principle not only offers a strategy to design highly active nonprecious metal single-atom catalysts with specific active centres, for example, Fe-pyridine/pyrrole-N4 for the oxygen reduction reaction; Co-pyrrole-N4 for the oxygen evolution reaction; and Mn-pyrrole-N4 for the hydrogen evolution reaction to replace precious Pt/Ir/Ru-based catalysts, but also suggests that macrocyclic metal complexes could be used as an alternative to graphene-based single-atom catalysts.

The synthesis and synergistic catalysis of iron phthalocyanine and its graphene-based axial complex for enhanced oxygen reduction

Exploring an efficient route to prepare novel graphene-based nanostructure of organic small molecule as metal single-atom catalyst still faces a great challenge due to the difficult synergy of complicated nanostructure for multiple catalytic steps. Herein, the iron phthalocyanine (FePc)-based nanostructures are synthesized via the graphene-assisted colloid chemical reaction for the first time. Compared to the commercial Pt/C for oxygen reduction reaction (ORR), the optimal FePc/graphene nanostructure exhibits not only more positive onset potential (Eonset, 1.000 vs 0.973 V) and half-wave potential (△E1/2 = 100 mV), but also better stability and durability in alkaline solution. The corresponding zinc-air battery also shows better the maximum power density. The systematical characterizations prove the axial coordination between FePc and graphene via Fe-O-C bond and the formation of Fe(II)Pc/Fe(III)Pc/graphene nanostructures. The comprehensive analysis and simulated calculation reveal the synergistic ORR catalysis of four- and five- coordinated Fe atoms in these samples. This work provides a new insight into the synthesis and application of metal-organic complex for metal single-atom catalysis.

Ethanol synthesis catalyzed by single Ni atom supported on Mo6S8 support

Conversion of carbon monoxide (CO) into ethanol has attracted significant interest due to ethanol can be used as alternative energy sources to satisfy the rising global energy demands and to solve the serious environmental problems, although it suffers from slow kinetics and low selectivity. Metal single-atom catalyst of Ni single atom supported on Mo6S8 (Ni-SA/Mo6S8) catalyst was developed for the exclusive conversion of CO into ethanol. Herein, ethanol synthesis on Ni-SA/Mo6S8 was systematically investigated using density functional theory (DFT) and microkinetic modeling. The adsorption situations and the reaction routes for ethanol reactions on Ni-SA/Mo6S8 were clarified. Interestingly, the selectivity to ethanol is controlled by CH3 formation and CC bond formation between CH3 species and CHO. Our results indicated that Ni-SA/Mo6S8 increases the stability of the reaction intermediates, and CH3 is the most favorable formed by CH3OH dissociation. Then CHO insertion into CH3 to form CH3CHO* is spontaneous, which suggested that the formation of ethanol is the most favorable on the Ni-SA/Mo6S8 catalyst. The BEP relationships of five different types of bonds in ethanol synthesis identified CO bond scission does not readily occur compared to the other four types of bonds, the formation of CH, OH, and CC bonds are easy to occur. Microkinetic modeling also confirmed Ni-SA/Mo6S8 shows extremely activity and selectivity for ethanol formation. We hope our study can provide the basis to understand and develop single-atom catalysis for ethanol synthesis.

Bottom-up precise synthesis of stable platinum dimers on graphene

Supported metal clusters containing only a few atoms are of great interest. Progress has been made in synthesis of metal single-atom catalysts. However, precise synthesis of metal dimers on high-surface area support remains a grand challenge. Here, we show that Pt2 dimers can be fabricated with a bottom–up approach on graphene using atomic layer deposition, through proper nucleation sites creation, Pt1 single-atom deposition and attaching a secondary Pt atom selectively on the preliminary one. Scanning transmission electron microscopy, x-ray absorption spectroscopy, and theoretical calculations suggest that the Pt2 dimers are likely in the oxidized form of Pt2Ox. In hydrolytic dehydrogenation of ammonia borane, Pt2 dimers exhibit a high specific rate of 2800 molH2 molPt−1 min−1 at room temperature, ~17- and 45-fold higher than graphene supported Pt single atoms and nanoparticles, respectively. These findings open an avenue to bottom–up fabrication of supported atomically precise ultrafine metal clusters for practical applications.