Single Cobalt Atoms with Precise N‐Coordination as Superior Oxygen Reduction Reaction Catalysts

A new strategy for achieving stable Co single atoms (SAs) on nitrogen‐doped porous carbon with high metal loading over 4 wt % is reported. The strategy is based on a pyrolysis process of predesigned bimetallic Zn/Co metal–organic frameworks, during which Co can be reduced by carbonization of the organic linker and Zn is selectively evaporated away at high temperatures above 800 °C. The spherical aberration correction electron microscopy and extended X‐ray absorption fine structure measurements both confirm the atomic dispersion of Co atoms stabilized by as‐generated N‐doped porous carbon. Surprisingly, the obtained Co‐N_x single sites exhibit superior ORR performance with a half‐wave potential (0.881 V) that is more positive than commercial Pt/C (0.811 V) and most reported non‐precious metal catalysts. Durability tests revealed that the Co single atoms exhibit outstanding chemical stability during electrocatalysis and thermal stability that resists sintering at 900 °C. Our findings open up a new routine for general and practical synthesis of a variety of materials bearing single atoms, which could facilitate new discoveries at the atomic scale in condensed materials.     

Maximum Noble‐Metal Efficiency in Catalytic Materials: Atomically Dispersed Surface Platinum

Platinum is the most versatile element in catalysis, but it is rare and its high price limits large‐scale applications, for example in fuel‐cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst’s surface. To maximize the noble‐metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria “nanopocket”, which binds Pt²⁺ so strongly that it withstands sintering and bulk diffusion. On model catalysts we experimentally confirm the theoretically predicted stability, and on real Pt‐CeO₂ nanocomposites showing high Pt efficiency in fuel‐cell catalysis we also identify these anchoring sites.     

Effect of the synthetic method and support porosity on the structure and performance of silica‐supported CuBr/pyridylmethanimine atom transfer radical polymerization catalysts. I. Catalyst preparation and characterization

Silica‐supported CuBr/pyridylmethanimine (PMI) complexes that facilitate the atom transfer radical polymerization of methyl methacrylate have been prepared and characterized. Four different synthetic routes, including multistep‐grafting (M1), two‐step‐grafting (M2), one‐pot (M3), and preassembled‐complex (M4) methods, have been evaluated on three different silica supports (mesoporous SBA15 with 48‐ and 100‐Å pores and nonporous Cab‐O‐Sil EH5). The resulting solids have been characterized by a battery of techniques, including thermogravimetric analysis/differential scanning calorimetry, FT‐Raman spectroscopy, ¹³C and ²⁹Si magic‐angle‐spinning and cross‐polarity/magic‐angle‐spinning spectroscopy, low‐temperature nitrogen physisorption, and elemental analysis. The combination of elemental analysis and spectroscopic results has indicated that a variety of different surface species likely exist for most catalysts, including copper species that are both monocoordinated and biscoordinated by PMI ligands, and PMI‐free copper bromide species interacting with the silica surface. M4 appears to give a material that has the smallest amount of the uncomplexed ligand (by FT‐Raman spectroscopy) and is, therefore, the most homogeneous. After M4, the metallation efficiency decreases in the order M2 ≥ M3 > M1, with M1 giving a material with a highly heterogeneous surface composition. The ligand loading on all the catalysts has been determined to be approximately 1 mmol/g of SiO₂, with Cab‐O‐Sil‐supported materials giving much higher ligand densities because of its lower surface area. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1367–1383, 2004     

Strong Metal–Support Interactions between Pt Single Atoms and TiO2

Strong metal–support interaction (SMSI) has gained great attention in the field of heterogeneous catalysis. However, whether single‐atom catalysts can exhibit SMSI remains unknown. Here, we demonstrate that SMSI can occur on TiO₂‐supported Pt single atoms but at a much higher reduction temperature than that for Pt nanoparticles (NPs). Pt single atoms involved in SMSI are not covered by the TiO₂ support nor do they sink into its subsurface. The suppression of CO adsorption on Pt single atoms stems from coordination saturation (18‐electron rule) rather than the physical coverage of Pt atoms by the support. Based on the new finding it is revealed that single atoms are the true active sites in the hydrogenation of 3‐nitrostyrene, while Pt NPs barely contribute to the activity since the NP sites are selectively encapsulated. The findings in this work provide a new approach to study the active sites by tuning SMSI.     

Engineering the Coordination Environment of Single‐Atom Platinum Anchored on Graphdiyne for Optimizing Electrocatalytic Hydrogen Evolution

Two Pt single‐atom catalysts (SACs) of Pt‐GDY1 and Pt‐GDY2 were prepared on graphdiyne (GDY)supports. The isolated Pt atoms are dispersed on GDY through the coordination interactions between Pt atoms and alkynyl C atoms in GDY, with the formation of five‐coordinated C₁‐Pt‐Cl₄ species in Pt‐GDY1 and four‐coordinated C₂‐Pt‐Cl₂ species in Pt‐GDY2. Pt‐GDY2 shows exceptionally high catalytic activity for the hydrogen evolution reaction (HER), with a mass activity up to 3.3 and 26.9 times more active than Pt‐GDY1 and the state‐of‐the‐art commercial Pt/C catalysts, respectively. Pt‐GDY2 possesses higher total unoccupied density of states of Pt 5d orbital and close to zero value of Gibbs free energy of the hydrogen adsorption (|Δ |) at the Pt active sites, which are responsible for its excellent catalytic performance. This work can help better understand the structure–catalytic activity relationship in Pt SACs.     

Catalytic Platinum Nanoparticles Decorated with Subnanometer Molybdenum Clusters for Biomass Processing

The development of improved technologies for biomass processing into transportation fuels and industrial chemicals is hindered due to a lack of efficient catalysts for selective oxygen removal. Here we report that platinum nanoparticles decorated with subnanometer molybdenum clusters can efficiently catalyze hydrodeoxygenation of acetic acid, which serves as a model biomass compound. In contrast with monometallic Mo catalysts that are inactive and monometallic Pt catalysts that have low activities and selectivities, bimetallic Pt–Mo catalysts exhibit synergistic effects with high activities and selectivities. The maximum activity occurs at a Pt to Mo molar ratio of three. Although Mo atoms themselves are catalytically inactive, they serve as preferential binding anchors for oxygen atoms while a catalytic transformation proceeds on neighboring surface Pt atoms. Beyond biomass processing, Pt–Mo nanoparticles are promising catalysts for a wide variety of reactions that require a transformation of molecules with an oxygen atom and, more broadly, in other fields of science and technology that require tuning of surface–oxygen interactions.     

Single‐Atom Iron Catalysts on Overhang‐Eave Carbon Cages for High‐Performance Oxygen Reduction Reaction

Single‐atom catalysts have drawn great attention, especially in electrocatalysis. However, most of previous works focus on the enhanced catalytic properties via improving metal loading. Engineering morphologies of catalysts to facilitate mass transport through catalyst layers, thus increasing the utilization of each active site, is regarded as an appealing way for enhanced performance. Herein, we design an overhang‐eave structure decorated with isolated single‐atom iron sites via a silica‐mediated MOF‐templated approach for oxygen reduction reaction (ORR) catalysis. This catalyst demonstrates superior ORR performance in both alkaline and acidic electrolytes, comparable to the state‐of‐the‐art Pt/C catalyst and superior to most precious‐metal‐free catalysts reported to date. This activity originates from its edge‐rich structure, having more three‐phase boundaries with enhanced mass transport of reactants to accessible single‐atom iron sites (increasing the utilization of active sites), which verifies the practicability of such a synthetic approach.     

Geometry‐Assisted Three‐Dimensional Superlocalization Imaging of Single‐Molecule Catalysis on Modular Multilayer Nanocatalysts

To establish the structure–catalytic property relationships of heterogeneous catalysts, a detailed characterization of the three‐dimensional (3D) distribution of active sites on a single catalyst is essential. Single‐particle catalysis of a modular multilayer catalytic platform that consists of a solid silica core, a mesoporous silica shell, and uniformly distributed Pt nanoparticles sandwiched in between these layers is presented. The first 3D high‐resolution super‐localization imaging of single fluorescent molecules produced at active sites on the core‐shell model nanocatalysts is demonstrated. The 3D mapping is aided by the well‐defined geometry and a correlation study in scanning electron microscopy and total internal reflection fluorescence and scattering microscopy. This approach can be generalized to study other nano‐ and mesoscale structures.     

Geometric Requirements for Hydrocarbon Catalytic Sites on Platinum Surfaces

Vibrational spectroscopic measurements and density functional calculations were used to identify a preferential catalytic mechanism for the transformation of acetylene, HC? CH, to vinylidene, C? CH₂, on surfaces of Pt‐Sn ordered alloys. In this mechanism, two adjacent Pt atoms adsorb an acetylene molecule and a third neighboring Pt atom is required for stabilizing the reacting H atom during the transformation. Therefore, unlike a direct H shift along the C? C bond in organometallic compounds with a single transition‐metal atom, this mechanism has a geometric site requirement of three adjacent Pt atoms in the form of a three‐fold site. The same geometric site requirement is identified for preferential C? H bond cleavage of acetylene with the formation of adsorbed C? CH and H species. In the absence of three‐fold Pt sites, the reaction mechanism changes, and reactions of H transfer and C? H bond cleavage are suppressed.     

Mesoporous Pt-SiO2 and Pt-SiO2-Ta2O5 catalysts prepared using Pt colloids as templates.

Sol‐gel synthesis of silica and silica–tantalum oxide embedded platinum nanoparticles is carried out using Pt colloids as templates. These colloids are prepared by reduction with Na[AlEt₃H] and stabilized with different ligands (ammonium halide derivatives, non‐ionic surfactants with polyether chains, and 2‐hydroxy‐propionic acid). The aim of the present study is to prepare mesoporous silica embedded Pt colloids combining the “precursor concept” with the model of catalyst preparation using preformed spheres. Nanoparticles of Pt incorporated in high surface area mesoporous materials are formed after calcination. Further, it is observed that calcination of these catalysts causes partial aggregation and oxidation of the parent colloids, a process that is largely dependent on the nature of the stabilizing ligands. Several methods have been used for characterization of these materials: adsorption‐desorption isotherms at 77 K, H₂ chemisorption, X‐ray diffraction(XRD), ²⁹Si and ¹³C magic angle spinning (MAS) NMR, ammonia diffuse reflectance Fourier transform infrared spectroscopy (NH₃‐DRIFT), transmission electron microscopy (TEM), and X‐ray photoelectron spectroscopy (XPS). It is found that both metal oxide systems exhibit Brønsted acidity (weaker for silica and quite strong for silica–tantalum oxide). In addition, NH₃‐DRIFT experiments demonstrate the oxidative properties of the surface. Part of the adsorbed NH₄⁺ species is oxidized to N₂O. Testing these catalysts in the reduction of NO and NO₂ with isopentane under lean conditions indicate that the activity of these catalysts is indeed dependent on the size of the platinum particles, with those of size 8–10 nm demonstrating the best results. The support likely contributes to this effect, particularly after Ta incorporation into silica.     

Generation of Nanoparticle,Atomic‐Cluster,and Single‐Atom Cobalt Catalysts from Zeolitic Imidazole Frameworks by Spatial Isolation and Their Use in Zinc–Air Batteries

The size effect of transition‐metal nanoparticles on electrocatalytic performance remains ambiguous especially when decreasing the size to the atomic level. Herein, we report the spatial isolation of cobalt species on the atomic scale, which was achieved by tuning the zinc dopant content in predesigned bimetallic Zn/Co zeolitic imidazole frameworks (ZnCo‐ZIFs), and led to the synthesis of nanoparticles, atomic clusters, and single atoms of Co catalysts on N‐doped porous carbon. This synthetic strategy allowed an investigation of the size effect on electrochemical behavior from nanometer to Ångström dimensions. Single‐atom Co catalysts showed superior bifunctional ORR/OER activity, durability, and reversibility in Zn–air batteries compared with the other derivatives and noble‐metal Pt/C+RuO₂, which was attributed to the high reactivity and stability of isolated single Co atoms. Our findings open up a new avenue to regulate the metal particle size and catalytic performance of MOF derivatives.     

Generation of Nanoparticle,Atomic‐Cluster,and Single‐Atom Cobalt Catalysts from Zeolitic Imidazole Frameworks by Spatial Isolation and Their Use in Zinc–Air Batteries

The size effect of transition‐metal nanoparticles on electrocatalytic performance remains ambiguous especially when decreasing the size to the atomic level. Herein, we report the spatial isolation of cobalt species on the atomic scale, which was achieved by tuning the zinc dopant content in predesigned bimetallic Zn/Co zeolitic imidazole frameworks (ZnCo‐ZIFs), and led to the synthesis of nanoparticles, atomic clusters, and single atoms of Co catalysts on N‐doped porous carbon. This synthetic strategy allowed an investigation of the size effect on electrochemical behavior from nanometer to Ångström dimensions. Single‐atom Co catalysts showed superior bifunctional ORR/OER activity, durability, and reversibility in Zn–air batteries compared with the other derivatives and noble‐metal Pt/C+RuO₂, which was attributed to the high reactivity and stability of isolated single Co atoms. Our findings open up a new avenue to regulate the metal particle size and catalytic performance of MOF derivatives.     

Isolated Single Iron Atoms Anchored on N‐Doped Porous Carbon as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

The development of low‐cost, efficient, and stable electrocatalysts for the oxygen reduction reaction (ORR) is desirable but remains a great challenge. Herein, we made a highly reactive and stable isolated single‐atom Fe/N‐doped porous carbon (ISA Fe/CN) catalyst with Fe loading up to 2.16 wt %. The catalyst showed excellent ORR performance with a half‐wave potential (E _1/2) of 0.900 V, which outperformed commercial Pt/C and most non‐precious‐metal catalysts reported to date. Besides exceptionally high kinetic current density (J _k) of 37.83 mV cm⁻² at 0.85 V, it also had a good methanol tolerance and outstanding stability. Experiments demonstrated that maintaining the Fe as isolated atoms and incorporating nitrogen was essential to deliver the high performance. First principle calculations further attributed the high reactivity to the high efficiency of the single Fe atoms in transporting electrons to the adsorbed OH species.     

Thermally Stable and Regenerable Platinum–Tin Clusters for Propane Dehydrogenation Prepared by Atom Trapping on Ceria

Ceria (CeO₂) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single‐atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single‐atom Pt/CeO₂ catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO₂, the single‐atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO₂‐supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO₂, making Pt–Sn/CeO₂ a fully regenerable catalyst.     

Pt在MgAl2O4载体上的分散程度对催化苯甲醛加氢反应影响

构建催化剂特别是在亚纳米尺度下分散的贵金属催化剂的构效关系是多相催化研究领域中的主要任务之一.我们采用与金属Pt具有强相互作用的MgAl2O4尖晶石作为载体,通过简单浸渍法制备了在纳米、亚纳米和单原子尺度上分散的Pt催化剂.首先利用X射线衍射和原子分辨的球差校正电镜,确定了Pt在MgAl2O4尖晶石载体表面上随负载量增大逐渐形成孤立的和相邻的单原子Pt,然后逐渐形成无定形Pt聚集体和小晶粒;然后利用电感耦合等离子体光谱和CO化学吸附测定了催化剂中Pt的含量和分散度;进一步通过测定CO在Pt表面吸附的红外光谱,区分了载体表面单原子和金属颗粒表面原子的CO吸附特征结构,并据此对不同结构的Pt原子进行了半定量估算.考察了具有不同Pt分散结构的Pt/MgAl2O4催化剂的催化苯甲醛选择性加氢能力,发现以载体表面Pt单原子物种为主的催化剂,可在较宽的温度区间内保持较高的部分加氢产物苯甲醇的选择性(60–150oC,苯甲醇选择性99.4–97.9％,甲苯选择性～0.4％),而以Pt纳米颗粒为主的催化剂上苯甲醇选择性降低显著,同时生成较多深度加氢产物甲苯(60–150oC,苯甲醇选择性99.0–93.1％,甲苯选择性0.7–5.0％).此外,我们测定了各催化剂在不同转化率(～20–90％)时催化剂加氢反应的质量比活性和转化频率(TOF),并在较低苯甲醛转化率(～20％)时,估算了不同结构Pt物种对苯甲醛加氢反应的本征活性,发现Pt纳米颗粒表面原子比MgAl2O4载体表面Pt单原子本征活性更高(4807 h–1 versus 3277 h–1).综上,Pt单原子催化剂具有贵金属原子利用率高,本征活性和加氢选择性高等优点;Pt纳米催化剂表面原子深度加氢能力强,加氢选择性较差,虽本征活性更高,但不足以补偿贵金属原子利用率降低带来的活性损失,Pt质量比活性显著低于单原子催化剂.此外,MgAl2O4尖晶石负载的单原子Pt催化剂也具有良好的催化反应循环稳定性,是一种较为理想的催化苯甲醛选择性加氢制苯甲醇催化剂.     

Stabilizing a Platinum1 Single‐Atom Catalyst on Supported Phosphomolybdic Acid without Compromising Hydrogenation Activity

In coordination chemistry, catalytically active metal complexes in a zero‐ or low‐valent state often adopt four‐coordinate square‐planar or tetrahedral geometry. By applying this principle, we have developed a stable Pt₁ single‐atom catalyst with a high Pt loading (close to 1 wt %) on phosphomolybdic acid(PMA)‐modified active carbon. This was achieved by anchoring Pt on the four‐fold hollow sites on PMA. Each Pt atom is stabilized by four oxygen atoms in a distorted square‐planar geometry, with Pt slightly protruding from the oxygen planar surface. Pt is positively charged, absorbs hydrogen easily, and exhibits excellent performance in the hydrogenation of nitrobenzene and cyclohexanone. It is likely that the system described here can be extended to a number of stable SACs with superior catalytic activities.     

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 Fe^III‐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 Fe^III‐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 Fe^III‐OH‐Pt interfaces, the Fe(OH)_x/Pt catalysts exhibit high catalytic performance towards a broad range of substituted nitroarenes.     

Bidentate Disilicate Framework for Bis-Grafted Surface Species

Recent advances in surface organometallic chemistry have enabled the detailed characterization of the surface species in single-site heterogeneous catalysts. However, the selective formation of bis-grafted surface species remains challenging because of the heterogeneity of the supporting surface. Herein, we introduce a metal complex bearing bidentate disilicate ligands, −OSi(O^tBu)₂OSi(O^tBu)₂O−, as a molecular precursor, which has a silicate framework adjacent to the metal (Pt) center. The grafting of the precursors on silica supports (MCM-41 and CARiACT Q10) proceeded through a substitution reaction on the silicon atoms of the disilicate ligand, which was verified by the detection of isobutene and ^tBuOH as the elimination products, to selectively yield bis-grafted surface species. The chemical structure of the surface species was characterized by solid-state NMR, and the chemical shift values of the ancillary ligands and ¹⁹⁵Pt nuclei suggested that the bidentate coordination sphere was maintained following grafting.     

Silica‐Supported Pentamethylcyclopentadienyl Ytterbium(II) and Samarium(II) Sites: Ultrahigh Molecular Weight Polyethylene without Co‐Catalyst

Designing highly active supported ethylene polymerization catalysts that do not require a co‐catalyst to generate electrophilic metal alkyl species is still a challenge despite its industrial relevance. Described herein is the synthesis and characterization of well‐defined silica‐supported cyclopentadienyl Ln^II sites (Ln=Yb and Sm) of general formula [(≡SiO)LnCp*]. These well‐defined surface species are highly activite towards ethylene polymerization in the absence of added co‐catalyst. Initiation is proposed to occur by single electron transfer.     

Strong Metal–Support Interactions between Pt Single Atoms and TiO2

Strong metal–support interaction (SMSI) has gained great attention in the field of heterogeneous catalysis. However, whether single-atom catalysts can exhibit SMSI remains unknown. Here, we demonstrate that SMSI can occur on TiO₂-supported Pt single atoms but at a much higher reduction temperature than that for Pt nanoparticles (NPs). Pt single atoms involved in SMSI are not covered by the TiO₂ support nor do they sink into its subsurface. The suppression of CO adsorption on Pt single atoms stems from coordination saturation (18-electron rule) rather than the physical coverage of Pt atoms by the support. Based on the new finding it is revealed that single atoms are the true active sites in the hydrogenation of 3-nitrostyrene, while Pt NPs barely contribute to the activity since the NP sites are selectively encapsulated. The findings in this work provide a new approach to study the active sites by tuning SMSI.