Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects

Sulfur stabilizing metal nanoclusters on carbon at high temperatures

Supported metal nanoclusters consisting of several dozen atoms are highly attractive for heterogeneous catalysis with unique catalytic properties. However, the metal nanocluster catalysts face the challenges of thermal sintering and consequent deactivation owing to the loss of metal surface areas particularly in the applications of high-temperature reactions. Here, we report that sulfur-a documented poison reagent for metal catalysts-when doped in a carbon matrix can stabilize ~1 nanometer metal nanoclusters (Pt, Ru, Rh, Os, and Ir) at high temperatures up to 700 °C. We find that the enhanced adhesion strength between metal nanoclusters and the sulfur-doped carbon support, which arises from the interfacial metal-sulfur bonding, greatly retards both metal atom diffusion and nanocluster migration. In catalyzing propane dehydrogenation at 550 °C, the sulfur-doped carbon supported Pt nanocluster catalyst with interfacial electronic effects exhibits higher selectivity to propene as well as more stable durability than sulfur-free carbon supported catalysts.

When Fluxionality Beats Size Selection: Acceleration of Ostwald Ripening of Sub-Nano Clusters

Size selection was demonstrated to suppress Ostwald ripening of supported catalytic nanoparticles. When the supported clusters are subnanometer in size and highly fluxional, such as Pt clusters on the rutile TiO₂ (110) surface, this paradigm breaks down, and the established theory of sintering needs a revision. At temperatures characteristic of catalysis (i.e. 700 K), sub-nano clusters thermally populate many low-energy metastable isomers. As these isomers all have different geometric and electronic structures, and thus, formation and dissociation energies (in lieu of surface energy), Ostwald ripening is not suppressed, despite the size-selection. However, some clusters arise as magic numbers in terms of sintering stability at the ensemble level. Acceleration of sintering by metastable species persists though weakens in polydisperse cluster systems. We propose a competing pathways theory for sintering, which at the atomistic level describes the found size-specific sintering behavior.

Size-Sensitive Dynamic Catalysis of Subnanometer Cu Clusters in CO2 Dissociation

Small cluster catalysts are highly size-dependent and exhibit complex structural dynamic effects during catalytic reactions. Understanding their structural dynamics is of great importance in tuning the catalytic performances of small clusters that widely exist in supported catalysts. However, very little is known about the size dependence of the dynamic effect of small clusters. In this work, we systematically study the free energies and barriers of catalytic dissociation of CO₂ at different temperatures on dynamical Cu clusters with different sizes by ab initio molecular dynamics. The reaction shows an abnormal entropic effect on Cu clusters, and more interestingly, it shows size sensitivity. On the Cu₇ cluster, the entropy curve shows a reverse peak shape with increasing temperature, and it is surprising to find that it has a complex pulse shape on the Cu₁₉ cluster. The detailed analysis shows that such temperature dependences can be attributable to the nontrivial behaviors of adsorption-induced phase transitions of the subnanometer Cu clusters during the dissociation of CO₂. Our work not only demonstrates the complexity of the temperature dependence of the surface reaction on cluster sizes but also provides useful insight into the phase transition catalysis of dynamic clusters.

Plasmonic Photocatalytic Enhancement of L-Cysteine Self-Assembled Gold Nanoparticle Clusters for Fenton Reaction Catalysis

Plasmon-enhanced photocatalysis has the potential to reduce activation energies and decrease temperature requirements, which increases catalyst stability and lowers process operating costs. The near-field enhancement that occurs at junctions between plasmonic nanoparticle clusters (i.e., hot spots) has been well-studied for sensing applications (e.g., Raman scattering). However, experimental insight into the effect of nanoparticle cluster hot spots on plasmon-enhanced photocatalysis is lacking. We demonstrate that catalytic activity is increased when clusters of gold nanoparticles (AuNPs) are formed relative to isolated particles using the same catalyst loading. Through experimental controls, we conclude that this catalytic enhancement is most likely due to the formation of plasmonic hot spots. Clusters of AuNPs were formed by adding L-cysteine to an AuNP dispersion, and a 20 ± 12% enhancement in the photocatalytic dye degradation rate was observed using a Fenton process. While this report may be a modest enhancement relative to the spectacular near-field electromagnetic field enhancements predicted by simulation at the nanoparticle junction, this finding supports the recent work of Srimanta et al. that plasmonic hot spots contribute to catalytic rate enchantments. It is anticipated that further self-assembly strategies to optimize interparticle orientations and cluster size distributions will improve the enhancement due to the formation of hot spots, and careful control will be required. For example, excess L-cysteine addition revealed extensive aggregation and subsequent rate reductions.

Reactivity of Cobalt Clusters Con±/0 with Dinitrogen: Superatom Co6+ and Superatomic Complex Co5N6. J Phys Chem A 2021;125:2130-2138. [PMID: 33689326 DOI: 10.1021/acs.jpca.1c00483]

We report a joint experimental and theoretical study on the reactions of cobalt clusters (Co_n^±/0) with nitrogen using the customized reflection time-of-flight mass spectrometer combined with a 177.3 nm deep-ultraviolet laser. Comparing to the behaviors of neutral Co_n (n = 2-30) and anionic Co_n^- clusters (n = 7-53) which are relatively inert in reacting with nitrogen in the fast-flow tube, Co_n⁺ clusters readily react with nitrogen resulting in adducts of one or multiple N₂ except Co₆⁺ which stands firm in the reaction with nitrogen. Detailed quantum chemistry calculations, including the energetics, electron occupancy, and orbital analysis, well-explained the reasonable reactivity of Co_n⁺ clusters with nitrogen and unveiled the open-shell superatomic stability of Co₆⁺ within a highly symmetric (D_3d) structure. The D_3d Co₆⁺ bears an electron configuration of a half-filled superatomic 1P orbital (i.e., 1S²1P³||1D⁰), a large α-highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, symmetric multicenter bonds, and reasonable electron delocalization pertaining to metallic aromaticity. Topology analysis by atom-in-molecule illustrates the interactions between Co_n⁺ and N₂ corresponding to covalent bonds, but the Co-N interactions in cationic Co₂⁺N₂ and Co₆⁺N₂ clusters are apparently weaker than those in the other systems. In addition, we identify a superatomic complex Co₅N₆⁺ which exhibits similar frontier orbitals as the naked Co₅⁺ cluster, but the alpha HOMO-LUMO gap is nearly double-magnified, which is consistent with the high-abundance peak of Co₅N₆⁺ in the experimental observation. The enhanced stability of such a ligand-coordinated superatomic complex Co₅N₆⁺, along with the superatom Co₆⁺ with aromaticity, sheds light on special and general superatoms.

Plasmon-assisted click chemistry at low temperature: an inverse temperature effect on the reaction rate

Plasmon assistance promotes a range of chemical transformations by decreasing their activation energies. In a common case, thermal and plasmon assistance work synergistically: higher temperature results in higher plasmon-enhanced catalysis efficiency. Herein, we report an unexpected tenfold increase in the reaction efficiency of surface plasmon-assisted Huisgen dipolar azide-alkyne cycloaddition (AAC) when the reaction mixture is cooled from room temperature to -35 °C. We attribute the observed increase in the reaction efficiency to complete plasmon-induced annihilation of the reaction barrier, prolongation of plasmon lifetime, and decreased relaxation of plasmon-excited-states under cooling. Furthermore, control quenching experiments supported by theoretical calculations indicate that plasmon-mediated substrate excitation to an electronic triplet state may play the key role in plasmon-assisted chemical transformation. Last but not least, we demonstrated the possible applicability of plasmon assistance to biological systems by AAC coupling of biotin to gold nanoparticles performed at -35 °C.

Stable single atomic silver wires assembling into a circuitry-connectable nanoarray

Atomic metal wires have great promise for practical applications in devices due to their unique electronic properties. Unfortunately, such atomic wires are extremely unstable. Here we fabricate stable atomic silver wires (ASWs) with appreciably unoccupied states inside the parallel tunnels of α-MnO₂ nanorods. These unoccupied Ag 4d orbitals strengthen the Ag–Ag bonds, greatly enhancing the stability of ASWs while the presence of delocalized 5s electrons makes the ASWs conducting. These stable ASWs form a coherently oriented three-dimensional wire array of over 10 nm in width and up to 1 μm in length allowing us to connect it to nano-electrodes. Current-voltage characteristics of ASWs show a temperature-dependent insulator-to-metal transition, suggesting that the atomic wires could be used as thermal electrical devices.

One-dimensional atomic metal wires can exhibit useful properties distinct from their bulk equivalents; however they typically suffer from limited stability. Here, Chen et al create atomic silver wires which are stable and exhibit a temperature dependent metal to insulator transition.

Tuning the coalescence degree in the growth of Pt-Pd nanoalloys

Coalescence is a phenomenon in which two or more nanoparticles merge to form a single larger aggregate. By means of gas-phase magnetron-sputtering aggregation experiments on Pt-Pd nanoalloys, it is shown that the degree of coalescence can be tuned from a growth regime in which coalescence is negligible to a regime where the growth outcome is dominated by coalescence events. This transition is achieved by varying both the length of the aggregation zone and the pressure difference between the aggregation and the deposition chamber. In the coalescence-dominated regime, a wide variety of coalescing aggregates is produced and analyzed by TEM. The experimental results are interpreted with the aid of molecular-dynamics simulations. This allows to distinguish four different steps through which coalescence proceeds towards equilibrium. These steps, occurring on a hierarchy of well-separated time scales, consist in: (i) alignment of atomic columns; (ii) alignment of close-packed atomic planes; (iii) equilibration of shape; (iv) equilibration of chemical ordering.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

Critical Aspects of Metal-Organic Framework-Based Materials for Solar-Driven CO2 Reduction into Valuable Fuels

Photoreduction of CO2 into value-added fuels is one of the most promising strategies for tackling the energy crisis and mitigating the "greenhouse effect." Recently, metal-organic frameworks (MOFs) have been widely investigated in the field of CO2 photoreduction owing to their high CO2 uptake and adjustable functional groups. The fundamental factors and state-of-the-art advancements in MOFs for photocatalytic CO2 reduction are summarized from the critical perspectives of light absorption, carrier dynamics, adsorption/activation, and reaction on the surface of photocatalysts, which are the three main critical aspects for CO2 photoreduction and determine the overall photocatalytic efficiency. In view of the merits of porous materials, recent progress of three other types of porous materials are also briefly summarized, namely zeolite-based, covalent-organic frameworks based (COFs-based), and porous semiconductor or organic polymer based photocatalysts. The remarkable performance of these porous materials for solar-driven CO2 reduction systems is highlighted. Finally, challenges and opportunities of porous materials for photocatalytic CO2 reduction are presented, aiming to provide a new viewpoint for improving the overall photocatalytic CO2 reduction efficiency with porous materials.

Crystal-plane-controlled selectivity and activity of copper catalysts in propylene oxidation with O2

The selectivity of PO is inversely proportional to its activity on copper catalysts.

A review of the hot spot analysis and the research status of single-atom catalysis based on the bibliometric analysis

The bibliometric method was used to analyze the development trend and research hotspots in past 10 years since the concept of single-atom catalysis was proposed in 2011. This article can provide some guidance for future research of SACs.

Control of the single atom/nanoparticle ratio in Pd/C catalysts to optimize the cooperative hydrogenation of alkenes

Control of the single atom/nanoparticle ratio allows preparation of highly active Pd/C hydrogenation catalysts integrating the ultra-rational use of Pd.

Catalysis with Silver: From Complexes and Nanoparticles to MORALs and Single-Atom Catalysts

Silver catalysis has a rich and versatile chemistry now expanding from processes mediated by silver complexes and silver nanoparticles to transformations catalyzed by silver metal organic alloys and single-atom catalysts. Focusing on selected recent advances, we identify the key advantages offered by these highly selective heterogeneous catalysts. We conclude by offering seven research and educational guidelines aimed at further progressing the field of new generation silver-based catalytic materials.

Synthetic strategies of supported atomic clusters for heterogeneous catalysis

Supported atomic clusters with uniform metal sites and definite low-nuclearity are intermediate states between single-atom catalysts (SACs) and nanoparticles in size. Benefiting from the presence of metal–metal bonds, supported atomic clusters can trigger synergistic effects among every metal atom, which contributes to achieving unique catalytic properties different from SACs and nanoparticles. However, the scalable and precise synthesis and atomic-level insights into the structure–properties relationship of supported atomic clusters is a great challenge. This perspective presents the latest progress of the synthesis of supported atomic clusters, highlights how the structure affects catalytic properties, and discusses the limitations as well as prospects.

Supported atomic clusters with precise nuclearity are intermediate states between single-atom catalysts and nanoparticles in size. Here the authors summarize and discuss synthetic strategies of supported atomic clusters with unique catalytic properties for heterogeneous reactions.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Ultra-Fine CeO2 Particles Triggered Strong Interaction with LaFeO3 Framework for Total and Preferential CO Oxidation

Interactions between the active components with the support are one of the fundamentally factors in determining the catalytic performance of a catalyst. In contrast to the comprehensive understanding on the strong metal-support interactions (SMSI) in metal-based catalysts, it remains unclear for the interactions among different oxides in mixed oxide catalysts due to its complexity. In this study, we investigated the interaction between CeO₂ and LaFeO₃, the two important oxygen storage materials in catalysis area, by tuning the sizes of CeO₂ particles and highlight a two-fold effect of the strong oxide-oxide interaction in determining the catalytic activity and selectivity for preferential CO oxidation in hydrogen feeds. It is found that the anchoring of ultra-fine CeO₂ particles (<2 nm) at the framework of three-dimensional-ordered macroporous LaFeO₃ surface results in a strong interaction between the two oxides that induces the formation of abundant uncoordinated cations and oxygen vacancy at the interface, contributing to the improved oxygen mobility and catalytic activity for CO oxidation. Hydrogen spillover, which is an important evidence of the strong metal-support interactions in precious metal catalysts supported by reducible oxides, is also observed in the H₂ reduction process of CeO₂/LaFeO₃ catalyst due to the presence of ultra-fine CeO₂ particles (<2 nm). However, the strong interaction also results in the formation of surface hydroxyl groups, which when combined with the hydrogen spillover reduces the selectivity for preferential CO oxidation. This discovery demonstrates that in hybrid oxide-based catalysts, tuning the interaction among different components is essential for balancing the catalytic activity and selectivity.

Efficient synthesis of sub-5 nm Ag nanoparticles by the desorption effect of supercritical CO2 in SBA-15

Herein, the desorption effect of supercritical CO₂ (scCO₂) was utilized to obtain sub-5 nm Ag nanoparticles (NPs) with a high Ag loading in SBA-15. The size of the Ag NPs decreased from 3.54 ± 0.79 nm (Ag loading of 25.3 wt.% wt.%) to 2.38 ± 0.68 (Ag loading of 10.5 wt%) nm by changing the depressurization curve from 0.1 MPa min^-1 (20-14 MPa) to 3 MPa min^-1 (20-12 MPa). Meanwhile, the intensity of the crystalline Ag characteristic peaks was obviously higher than the latter sample from the x-ray diffraction (XRD) patterns. However, compared with the adsorption kinetics of the two precursors of AgNO₃ and Cu(NO₃)₂ on SBA-15, under the same deposition and depressurization conditions, when the two depositional systems used water as a co-solvent, the time taken to reach the adsorption equilibrium of Ag⁺ on the supports was longer than the time taken by Cu²⁺, which existed in the water as [Cu(H₂O)₄]²⁺. The surface of SBA-15 was hydrophilic, and then the interaction of Ag⁺ and the surface was weaker compared with Cu²⁺, making Ag⁺ highly dispersed on the surface under the scCO₂ desorption effect. After calcination, the size of the Ag NPs decreased, but the morphology of CuO was mainly characterized by nanorods (NRs). Moreover, by comparing the experiments of wetness impregnation, the dispersion ability of the bulk scCO₂ of the reactor was inefficient for Ag⁺ adsorbed on the channels in the depressurization process.

Crystal-Facet Modulated CrOx/γ-Al2O3: Quasi-Liquid Surface Modification by Bonded Polydimethylsiloxane for Catalytic Oxidation of Propene

The crystal-facet effect of catalytic supports plays a crucial role in tailoring the physicochemical properties of active sites and the surface chemically bonded polymer can also regulate the local environment around active sites for optimizing catalytic performance. Herein, we report the effect of exposed facets of γ-Al₂O₃ supports and further modification by surface bonded long-chain polydimethylsiloxane (PDMS) on the properties of CrO_x/γ-Al₂O₃ catalysts for selective oxidation of propene. The {111} facets of γ-Al₂O₃ stabilize "non-redox Cr³⁺" and promote the overall oxidation rates compared with catalysts on {110} facets of γ-Al₂O₃. The surface bonded PDMS, with grafting density being about 0.13 chains/nm², endows a hydrophobic environment to facilitate the enrichment of the hydrophobic substrate and the desorption of hydrophilic products and occupies some acid sites on catalysts to limit acid-catalyzed side reactions. The inherent liquidlike nature of bonded PDMS also forms a setting that can regulate the redox ability of surface Cr species, that lead to modified activation of oxygen toward more surface adsorbed species. As a result, the modified catalysts enhance the whole oxidation process with favorable formation of epoxide product at low reaction temperatures (<225 °C). Our findings highlight the impact of surface chemically bound polydimethylsiloxane (PDMS) upon tailoring the surroundings of the catalyst surface, and that combined with facet-effect of supports can tune the reaction process toward selective ones.

Atomically-precise dopant-controlled single cluster catalysis for electrochemical nitrogen reduction

The ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped Au₄Pt₂ clusters supported on defective graphene. This creates a bimetal single cluster catalyst (Au₄Pt₂/G) with exceptional activity for electrochemical nitrogen (N₂) reduction. Our mechanistic study reveals that each N₂ molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N₂ via an enhanced back donation of electrons to the N₂ LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant.

The fabrication of singly dispersed metal cluster catalysts with atomic-level control of dopants is a long-standing challenge. Here, the authors report a strategy for the synthesis of a precisely doped single cluster catalyst which shows exceptional activity for electrochemical dinitrogen reduction.

Adsorption and activation of molecular oxygen over atomic copper(I/II) site on ceria

Supported atomic metal sites have discrete molecular orbitals. Precise control over the energies of these sites is key to achieving novel reaction pathways with superior selectivity. Here, we achieve selective oxygen (O₂) activation by utilising a framework of cerium (Ce) cations to reduce the energy of 3d orbitals of isolated copper (Cu) sites. Operando X-ray absorption spectroscopy, electron paramagnetic resonance and density-functional theory simulations are used to demonstrate that a [Cu(I)O₂]^3- site selectively adsorbs molecular O₂, forming a rarely reported electrophilic η²-O₂ species at 298 K. Assisted by neighbouring Ce(III) cations, η²-O₂ is finally reduced to two O^2-, that create two Cu-O-Ce oxo-bridges at 453 K. The isolated Cu(I)/(II) sites are ten times more active in CO oxidation than CuO clusters, showing a turnover frequency of 0.028 ± 0.003 s^-1 at 373 K and 0.01 bar P_CO. The unique electronic structure of [Cu(I)O₂]^3- site suggests its potential in selective oxidation.

Anionic Copper Clusters Reacting with NO: An Open-Shell Superatom Cu18. J Phys Chem Lett 2020;11:5807-5814. [PMID: 32597656 DOI: 10.1021/acs.jpclett.0c01643]

Gas-phase metal clusters have been a subject of research interest for allowing reliable strategies to explore the stability and reactivity of materials at reduced sizes with atomic precision. Here we have prepared well-resolved copper cluster anions Cu_n^- (n = 7-37) and systematically studied their reactivity with O₂, NO, and CO. We found remarkable stability of an open-shell cluster Cu₁₈^-, which is comparable with the closed-shell clusters Cu₁₇^- and Cu₁₉^- within the picture of an electronic shell model. Even without having a magic number of valence electrons, intriguingly, the unpaired electron on the singly occupied molecular orbital of Cu₁₈^- is mainly contributed by the central copper atom, while the other 18 delocalized valence electrons occupy the lower-energy superatomic orbitals of the cluster. The finding of such an open-shell superatom Cu₁₈^-, with an electron configuration of 1S²1P⁶1D¹⁰2S¹||1F⁰, is interesting in the sense that an elementary cluster of coinage metal atoms could still behave as a superatom mimicking coinage metals like silver or gold atoms with an empty f orbital. The superatomic stability of this Cu₁₈^- cluster is reinforced by the unique electrostatic interaction between the Cu^- core and Cu₁₇ shell, which provides new insights into the chemistry of metal clusters.

Present and new frontiers in materials research by ambient pressure x-ray photoelectron spectroscopy

In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy publications that have appeared between the 1970s and the end of 2018 according to their scientific field. We find that catalysis, surface science and materials science are predominant, while, for example, electrocatalysis and thin film growth are emerging. All catalysis publications that we could identify are cited, and selected case stories with increasing complexity in terms of surface structure or chemical reaction are discussed. For thin film growth we discuss recent examples from chemical vapour deposition and atomic layer deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron spectroscopy research, indicating some directions of future development of the field.

A nanostructured MoO2/MoS2/MoP heterojunction electrocatalyst for the hydrogen evolution reaction

Electrocatalytic production of hydrogen from water is considered to be a promising and sustainable strategy. In this work, the low-cost nanostructured MoO₂/MoS₂/MoP heterojunction is successfully synthesized by phosphorization of the pre-prepared urchin-like MoO₂/MoS₂ nanospheres as the stable, highly efficient electrocatalysis for the hydrogen evolution reaction (HER). The MoO₂/MoS₂/MoP-800 (MoO₂/MoS₂ nanospheres are phosphated at 800 °C) displays a catalytic ability for the HER with an overpotential of 135 mV to achieve 10 mA cm^-2 and a Tafel slope of 67 mV dec^-1 in 0.5 M H₂SO₄, which is superior to MoO₂/MoS₂ nanospheres (200 °C; 24 h), MoO₂/MoS₂/MoP-700 (MoO₂/MoS₂ nanospheres are phosphated at 700 °C) and MoO₂/MoS₂/MoP-900 (MoO₂/MoS₂ nanospheres are phosphated at 900 °C). Meanwhile, the catalyst exhibits superior properties for HER with an overpotential of 145 mV to achieve 10 mA cm^-2 and a Tafel slope of 71 mV dec^-1 in 1 M KOH solution. Detailed characterizations reveal that the improved HER performances are significantly related to P-doping and the spherical nanostructure. This work not only provides a low-cost selective for electrocatalytic production of hydrogen, but also serves as a guide to optimize the composition and structure of nanocomposites.

Metal Clusters on Semiconductor Surfaces and Application in Catalysis with a Focus on Au and Ru

Metal clusters typically consist of two to a few hundred atoms and have unique properties that change with the type and number of atoms that form the cluster. Metal clusters can be generated with a precise number of atoms, and therefore have specific size, shape, and electronic structures. When metal clusters are deposited onto a substrate, their shape and electronic structure depend on the interaction with the substrate surface and thus depend on the properties of both the clusters and those of the substrate. Deposited metal clusters have discrete, individual electron energy levels that differ from the electron energy levels in the constituting individual atoms, isolated clusters, and the respective bulk material. The properties of clusters with a focus on Au and Ru, the methods to generate metal clusters, and the methods of deposition of clusters onto substrate surfaces are covered. The properties of cluster-modified surfaces are important for their application. The main application covered here is catalysis, and the methods for characterization of the cluster-modified surfaces are described.