Surface electronic structure and reactivity of transition and noble metals1Communication presented at the First Francqui Colloquium, Brussels, 19–20 February 1996.1

Adsorption Energy in Oxygen Electrocatalysis

Adsorption energy (AE) of reactive intermediate is currently the most important descriptor for electrochemical reactions (e.g., water electrolysis, hydrogen fuel cell, electrochemical nitrogen fixation, electrochemical carbon dioxide reduction, etc.), which can bridge the gap between catalyst's structure and activity. Tracing the history and evolution of AE can help to understand electrocatalysis and design optimal electrocatalysts. Focusing on oxygen electrocatalysis, this review aims to provide a comprehensive introduction on how AE is selected as the activity descriptor, the intrinsic and empirical relationships related to AE, how AE links the structure and electrocatalytic performance, the approaches to obtain AE, the strategies to improve catalytic activity by modulating AE, the extrinsic influences on AE from the environment, and the methods in circumventing linear scaling relations of AE. An outlook is provided at the end with emphasis on possible future investigation related to the obstacles existing between adsorption energy and electrocatalytic performance.

Electronic structure and catalytic activity of exsolved Ni on Pd core-shell nanoparticles

This study reports first principles calculations performed to study the electronic structure and catalytic activity of exsolved Ni on Pd core-shell catalysts reported in recent experimental literature. The modification in the electronic and geometric properties of the Ni/Pd bimetallic system as successive layers of Ni are added on top of Pd is systematically investigated using the d-band model as well as the adsorption of O and CO on the surface of these core-shell structures. The results show that the adsorption of O and CO is more favourable on Ni/Pd core-shell catalysts compared to the pure Ni surface. As the dissociation of the O₂ molecule into atomic oxygen and CO oxidation are key steps in metal-catalysed oxidation reactions, we have examined the energetics of O₂ dissociation and CO oxidation reaction over the (111) faces of Ni as well as Ni/Pd structures. Our results suggest that both adsorption and dissociation are easier on Ni/Pd surfaces compared to a simple Ni surface. Unlike O₂ dissociation, we find that CO oxidation is unfavourable on Ni/Pd in comparison to Ni. The energetics of both reactions follow Brønsted-Evans-Polanyi relationships where the activation energy is linearly related to the reaction energy for all surfaces studied here. We found that a single monolayer of Ni on Pd, due to the synergistic effect of geometric and electronic factors, is the most active among the surfaces studied here towards the adsorption and dissociation of O₂. Both adsorption and dissociation become less favourable with an increase in the thickness of the Ni shell in these core-shell catalysts. A close analysis of the results indicates that both strain and ligand effects are active in the improved catalytic activity seen in Ni on Pd catalysts. Quite understandably, the ligand effect is only seen for the single monolayer of Ni on Pd and fades off as we go to two monolayers of Ni. The results reported here help us understand the connections between the electronic structure and catalytic activity of Ni/Pd core-shell nanoparticles, and these insights are expected to be useful in the development of core-shell catalysts.

Strain-mediated oxygen evolution reaction on magnetic two-dimensional monolayers

By screening 56 magnetic 2D monolayers through first-principles calculations, it was found that 8 magnetic 2D monolayers (CoO₂, FeO₂, FeSe, FeTe, VS₂, VSe₂, VTe₂ and CrSe₂) can bind O*, OH* and OOH* intermediates of the oxygen evolution reaction (OER), in which the overpotentials of CoO₂, FeO₂, VSe₂, and VTe₂ monolayers are 0.684, 1.107, 0.863 and 0.837 V, respectively. After applying suitable biaxial tensile strains, the overpotentials of CoO₂, FeO₂ and VTe₂ monolayers are reduced over 40%. In particular, the overpotentials of CoO₂ and VTe₂ monolayers decrease to 0.372 V and 0.491 V under the biaxial tensile strains of 4.0% and 3.0%, respectively, which are comparable to the reported overpotentials of noble metal and low-dimensional materials. Tensile strains modify the potential determining step for the OER and enhance the catalytic activity of metal atoms of magnetic 2D monolayers. Magnetic 2D monolayers could be activated by strain engineering as catalysts for the OER.

Manipulating the Water Dissociation Electrocatalytic Sites of Bimetallic Nickel-Based Alloys for Highly Efficient Alkaline Hydrogen Evolution

Transition-metal alloys are currently drawing increasing attention as promising electrocatalysts for the alkaline hydrogen evolution reaction (HER). However, traditional density-functional-theory-derived d-band theory fails to describe the hydrogen adsorption energy (ΔG_H ) on hollow sites. Herein, by studying the ΔG_H for a series of Ni-M (M=Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mo, W) bimetallic alloys, an improved d-band center was provided and a potential NiCu electrocatalyst with a near-optimal ΔG_H was discovered. Moreover, oxygen atoms were introduced into Ni-M (O-NiM) to balance the adsorption/desorption of hydroxyl species. The tailored electrocatalytic sites for water dissociation can synergistically accelerate the multi-step alkaline HER. The prepared O-NiCu shows the optimum HER activity with a low overpotential of 23 mV at 10 mA cm^-2 . This work not only broadens the applicability of d-band theory, but also provides crucial understanding for designing efficient HER electrocatalysts.

Yttrium-based Double Perovskite Nanorods for Electrocatalysis

Herein, we investigate the effect of the chemical composition of double perovskite nanorods on their versatile electrocatalytic activity not only as supports for the oxidation of small organic molecules but also as catalysts for the oxygen evolution reaction. Specifically, Y₂CoMnO₆ and Y₂NiMnO₆ nanorods with average diameters of 300 nm were prepared by a two-step hydrothermal method, in which the individual effects of synthetic parameters, such as the pH, annealing temperature, and precursor ratios on both the composition and morphology, were systematically investigated. When used as supports for Pt nanoparticles, Y₂CoMnO₆/Pt catalysts exhibited an electrocatalytic activity for the methanol oxidation reaction, which is 2.1 and 1.3 times higher than that measured for commercial Pt/C and Y₂NiMnO₆/Pt, respectively. Similarly, the Co-based catalyst support material displayed an ethanol oxidation activity, which is 2.3 times higher than both Pt/C and Y₂NiMnO₆/Pt. This clear enhancement in the activity for Y₂CoMnO₆ can largely be attributed to strong metal-support interactions, as evidenced by a downshift in the binding energy of the Pt 4f bands, measured by X-ray photoelectron spectroscopy (XPS), which is often correlated not only with a downshift in the d-band center but also to a decreased adsorption of poisoning adsorbates. Moreover, when used as catalysts for the oxygen evolution reaction, Y₂CoMnO₆ displayed a much greater activity as compared with Y₂NiMnO₆. This behavior can largely be attributed not only to a preponderance of comparatively more favorable oxidation states and electronic configurations but also to the formation of an active layer on the surface of the Y₂CoMnO₆ catalyst, which collectively gives rise to improved performance metrics and greater stability as compared with both IrO₂ and Y₂NiMnO₆. Overall, these results highlight the importance of both the chemical composition and the electronic structure of double perovskites, especially when utilized in multifunctional roles as either supports or catalysts.

Strain Engineering: A Boosting Strategy for Photocatalysis

Whilst the photocatalytic technique is considered to be one of the most significant routes to address the energy crisis and global environmental challenges, the solar-to-chemical conversion efficiency is still far from satisfying practical industrial requirements, which can be traced to the suboptimal bandgap and electronic structure of photocatalysts. Strain engineering is a universal scheme that can finely tailor the bandgap and electronic structure of materials, hence supplying a novel avenue to boost their photocatalytic performance. Accordingly, to explore promising directions for certain breakthroughs in strained photocatalysts, an overview on the recent advances of strain engineering from the basics of strain effect, creations of strained materials, as well as characterizations and simulations of strain level is provided. Besides, the potential applications of strain engineering in photocatalysis are summarized, and a vision for the future controllable-electronic-structure photocatalysts by strain engineering is also given. Finally, perspectives on the challenges for future strain-promoted photocatalysis are discussed, placing emphasis on the creation and decoupling of strain effect, and the modification of theoretical frameworks.

Biaxially Compressive Strain in Ni/Ru Core/Shell Nanoplates Boosts Li-CO2 Batteries

Regulating surface strain of nanomaterials is an effective strategy to manipulate the activity of catalysts, yet not well recognized in rechargeable Li-CO₂ batteries. Herein, biaxially compressive strained nickel/ruthenium core/shell hexagonal nanoplates (Ni/Ru HNPs) with lattice compression of ≈5.1% and ≈3.2% in the Ru {10-10} and (0002) facets are developed as advanced catalysts for Li-CO₂ batteries. It is demonstrated that tuning the electronic structure of Ru shell through biaxially compressive strain engineering can boost the kinetically sluggish CO₂ reduction and evolution reactions, thus achieving a high-performance Li-CO₂ battery with low charge platform/overpotential (3.75 V/0.88 V) and ultralong cycling life (120 cycles at 200 mA g^-1 with a fixed capacity of 1000 mAh g^-1 ). Density functional theory calculations reveal that the biaxially compressive strain can downshift the d-band center of surface Ru atoms and thus weaken the binding of CO₂ molecules, which is energetically beneficial for the nucleation and decomposition of Li₂ CO₃ crystals during the discharge and charge processes. This study confirms that strain engineering, though constructing a well-defined core/shell structure, is a promising strategy to improve the inherent catalytic activity of Ru-based materials in Li-CO₂ batteries.

Limits to scaling relations between adsorption energies?

Linear scaling relations have led to an understanding of trends in catalytic activity and selectivity of many reactions in heterogeneous and electro-catalysis. However, linear scaling between the chemisorption energies of any two small molecule adsorbates is not guaranteed. A prominent example is the lack of scaling between the chemisorption energies of carbon and oxygen on transition metal surfaces. In this work, we show that this lack of scaling originates from different re-normalized adsorbate valence energies of lower-lying oxygen vs higher-lying carbon. We develop a model for chemisorption of small molecule adsorbates within the d-band model by combining a modified form of the Newns-Anderson hybridization energy with an effective orthogonalization term. We develop a general descriptor to a priori determine if two adsorbates are likely to scale with each other.

Insights into the Adsorption, Alloy Formation, and Poisoning Effects of Hg on Monometallic and Bimetallic Adsorbents

The removal of elemental mercury (Hg⁰) from coal-derived syngas at high temperatures is desired to improve the thermal efficiency of the coal-to-chemical processes. First-principles density functional theory (DFT) calculations for Hg⁰ adsorption are performed using different exchange correlation functionals (PBE, optPBE-vdW, and optB88-vdW). Gibbs free energy (ΔG) calculations are further performed to evaluate the feasibility of Hg⁰ adsorption on various exposed planes of metal nanoparticles and to obtain bimetallic compositions for Hg⁰ removal at various temperatures. Pd and Pt are shown to be suitable for Hg⁰ adsorption at high temperatures (473 K), whereas Rh and Ru are effective only until 373 K. The bimetallic adsorbents comprising Ag or Au along with Rh, Ru, Pd, or Pt are identified for Hg⁰ removal at high temperatures (473 K). The increase in Hg⁰ adsorption strength on various bimetallic surfaces is correlated to the upward shift in the d-band center. Further, calculations predict the tendency of Hg to segregate toward the surface of amalgams and disturb the perfect planar geometry of the Pd, Pt, Rh, Ru, Ir, Cu, Ag, and Au surfaces to form a noncrystalline Hg-rich amalgam surface. An analysis of the binding of various adsorbates (H, O, N, and S) shows that the adsorption becomes significantly weaker on various sites in close proximity to pre-adsorbed Hg. Moreover, for specific combinations of the adsorbate, surface composition, and the site location, the adsorption does not take place on the proximal sites. These results are complemented by the partial density of states calculations, which show changes in the electronic properties of the amalgam surface, thus explaining the poisoning effect of Hg on metallic catalysts.

Strategic Modulation of Target-Specific Isolated Fe,Co Single-Atom Active Sites for Oxygen Electrocatalysis Impacting High Power Zn-Air Battery

An effective modulation of the active sites in a bifunctional electrocatalyst is essentially desired, and it is a challenge to outperform the state-of-the-art catalysts toward oxygen electrocatalysis. Herein, we report the development of a bifunctional electrocatalyst having target-specific Fe-N₄/C and Co-N₄/C isolated active sites, exhibiting a symbiotic effect on overall oxygen electrocatalysis performances. The dualism of N-dopants and binary metals lower the d-band centers of both Fe and Co in the Fe,Co,N-C catalyst, improving the overpotential of the overall electrocatalytic processes (ΔE_ORR-OER = 0.74 ± 0.02 V vs RHE). Finally, the Fe,Co,N-C showed a high areal power density of 198.4 mW cm^-2 and 158 mW cm^-2 in the respective liquid and solid-state Zn-air batteries (ZABs), demonstrating suitable candidature of the active material as air cathode material in ZABs.

Pt–Pd Bimetallic Aerogel as High-Performance Electrocatalyst for Nonenzymatic Detection of Hydrogen Peroxide

Hydrogen peroxide (H2O2) plays an indispensable role in the biological, medical, and chemical fields. The development of an effective H2O2 detecting method is of great importance. In the present work, a series of PtxPdy bimetallic aerogels and Pt, Pd monometallic aerogels were controllably synthesized by one-step gelation method. Their morphologies and compositions were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, and so forth. These aerogels were used as nonenzyme electrocatalysts for the detection of H2O2. The cyclic voltammetric and amperometric results demonstrated that the performance of the metal aerogels showed volcano-type behavior, with the Pt50Pd50 aerogel sitting on top. The Pt50Pd50 aerogel-based electrochemical sensor exhibited excellent comprehensive performance, with a low overpotential of −0.023 V vs. Ag/AgCl, a broad linear range from 5.1 to 3190 μM (R2 = 0.9980), and a high sensitivity of 0.19 mA mM−1 cm−2, in combination with good anti-interference ability and stability. A comprehensive study indicated that the superior sensing performance of the Pt50Pd50 aerogel is closely related to its optimized d-band center and larger cumulative pore volume. This work first applied Pt–Pd bimetallic aerogels into the detection of H2O2 and shows the promising application of noble metal aerogels in the electrochemical sensing area.

Ni2+ -Directed Anisotropic Growth of PtCu Nested Skeleton Cubes Boosting Electroreduction of Oxygen

Structure-controlled Pt-based nanocrystals have the great potential to provide a flexible strategy for improving the catalysis of the oxygen reduction reaction (ORR). Here, a new synthetic approach is developed to tune the 3D structure of Pt-based alloys, and switch a synthetic reaction which produces solid PtCu octahedral stars (OSs) to produce PtCu nested skeleton cubes (NSCs) by simple addition of Ni(acac)₂ . In particular, Ni²⁺ -guided anisotropic growth is observed to generate the nested skeleton structure in PtCu NSCs. Ni²⁺ , though absent from the nanoalloys, not only endows faster Cu reduction kinetics but also acts as a structure-directing agent. Moreover, it is shown that acetic acid treatment of PtCu NSCs/C exposes Pt-rich surface with a fine-tuned Pt d-band center energy and the reduced Cu leaching, resulting in strikingly high activity and stability. Acid-treated PtCu NSCs/C shows a remarkable ORR mass activity of 5.13 A mg_Pt ^-1 , about 26 times higher than commercial Pt/C catalyst. This catalyst also exhibits excellent stability with a lower activity decay of 11.5% and the negligible variation in structure after 10 000 cycles.

Magnetism versus band-gap relationship in diluted magnetic semiconductors: megatom impurity behavior of the magnetic dopant complexes

An analysis ofab initionumerical results obtained for the total energy of diluted magnetic semiconductors (DMSs) doped with dopant formations of various structural and spin conformations consisting of 2-4 3D transition metal (TM atoms) has revealed that a dopant formation acts as large impurity atom i.e., as amegatom, in a reverse analogy to the process of the adsorption ofsp-atoms onto metallic surfaces. As a result, thed-orbitals of the magnetic dopant formation (themegatom) become hybridized with thesp-bands of the host anions thus creating a number of impurity states which are reflected in the changes of the band gap of the DMS establishing an implicit relationship between the band gap and magnetism. Additional findings also indicate that: (i) the total magnetic momentMtot(α)and the band gapegap(α)which characterize a DMS with a dopant formation in spin conformation (α) do not vary independently from each other but instead form one composite system parameter. (ii) The per dopant-pair magnetic interactions in dopant formations consisting of more than two dopants are smaller than those obtained for an isolated dopant-pair. These are demonstrated with results obtained for GaN doped with 3D-TM dopant formations.

Structural and electronic properties of Pt modified Au(100) surface

Investigations on electronic and geometric structures of platinum adsorbed on monocrystalline gold surfaces are important for understanding the remarkable catalytic properties of bimetallic Pt-Au systems. Herein, the morphology of quasi-hexagonal (hex) Au(100) surface after deposition of platinum for coverage up to 0.5 monolayer (ML) has been investigated by scanning tunneling microscopy (STM). For coverage range 0.2-0.4 ML the creation of elongated islands with mono-atomic height is observed. The islands consist of flat phase of disordered Pt-Au alloy which coexists with nanowire-like features with a hex atom arrangement and quantized width. Annealing the Pt/Au(100) system at 100-150 °C changes the surface morphology. The islands disappear and the topmost layer of the surface consists of flat phase of Pt-Au alloy which coexists with the hex-stripes. Small domains of ordered c(2 × 2) structure of Pt-Au alloy are found. The electronic properties of this structure have been investigated by ab-initio calculations. The obtained results allow to distinguish the Pt from Au atoms by their appearance in the STM images. The calculated electronic structures indicate a bonding creation between Pt and Au atoms and an electron d-states redistribution of Pt in comparison to the bare Pt(100)-(1 × 1) surface.

Thermal activation of Ti(1-x)Au(x) thin films with enhanced hardness and biocompatibility

The lifetime of orthopaedic implants can be extended by coating the softer Ti₆Al₄V alloy with harder biocompatible thin films. In this work, thin films of Ti_(1-x)Au_(x) are grown on Ti₆Al₄V and glass substrates by magnetron sputtering in the entire x = 0–1 range, before their key biomechanical properties are performance tuned by thermal activation. For the first time, we explore the effect of in-situ substrate heating versus ex-situ post-deposition heat-treatment, on development of mechanical and biocompatibility performance in Ti–Au films. A ∼250% increase in hardness is achieved for Ti–Au films compared to bulk Ti₆Al₄V and a ∼40% improvement from 8.8 GPa as-grown to 11.9 and 12.3 GPa with in-situ and ex-situ heat-treatment respectively, is corelated to changes in structural, morphological and chemical properties, providing insights into the origins of super-hardness in the Ti rich regions of these materials. X-ray diffraction reveals that as-grown films are in nanocrystalline states of Ti–Au intermetallic phases and thermal activation leads to emergence of mechanically hard Ti–Au intermetallics, with films prepared by in-situ substrate heating having enhanced crystalline quality. Surface morphology images show clear changes in grain size, shape and surface roughness following thermal activation, while elemental analysis reveals that in-situ substrate heating is better for development of oxide free Ti₃Au β-phases. All tested Ti–Au films are non-cytotoxic against L929 mouse fibroblast cells, while extremely low leached ion concentrations confirm their biocompatibility. With peak hardness performance tuned to >12 GPa and excellent biocompatibility, Ti–Au films have potential as a future coating technology for load bearing medical implants.

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Combined study on biocompatibility and mechanical performance of Ti–Au films.

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Reports on effect of varying of thermal activation on quality of Ti–Au film structure.

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Clear development of super-hard β-Ti₃Au phase with in-situ thermal activation.

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Peak hardness value > 12 GPa attained for Ti rich films with ex-situ thermal activation.

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All Ti–Au films highly biocompatible with safe cytotoxic profile against L929 cells.

New insights into the key bifunctional role of sulfur in Fe-N-C single-atom catalysts for ORR/OER

Sulfur-doping of non-noble metal Fe-N-C single-atom catalysts (SACs) shows a key bifunctional role in promoting ORR and OER activity. The controversial claims about the enhanced ORR activity and the ambiguity of the OER activity brought about by S-doping demand in-depth investigation. Here, systematic theoretical investigation was carried out. Unlike previously believed, coordinative S-doping gives rise to a precisely regulated OOH* stabilization effect, which is revealed to be the origin of the bifunctional ORR/OER activity. The fine regulation is reflected in two aspects: (1) Compared with other intermediates, the regulation of OOH* adsorption is more obvious. (2) More sulfur-doping leads to excessive strong or weak stabilization, which is not conducive to ORR/OER. The single S doping elevates the charge density and opens the metallic spin channels of Fe-N₃|S, moves the d-band center towards the Fermi level, all contributing to moderate OOH* stabilization. It is hoped that these results will promote the development of heteroatom-doped bifunctional SACs.

Understanding the effect of mechanical strains on the catalytic activity of transition metals

The effect of elastic strains on the catalytic activity for the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) was analyzed on thirteen late transition metals: eight (111) surfaces of fcc metals (Ni, Cu, Pd, Ag, Pt, Au, Rh, Ir) and five (0001) surfaces of hcp metals (Co, Zn, Cd, Ru, and Os). The corresponding adsorption energies for the different intermediate reactions up to strains dictated by the mechanical stability limits were previously obtained by means of density functional theory calculations. It was found that the elastic strains can be used to tune the catalytic activity of different metals by reducing the energy barrier of the rate limiting step and even to reach the cusp of the volcano plot. The largest changes in catalytic activity with strain for the HER were found in Pt, Au, and Ir while Co and Ni were very insensitive to this strategy. In the case of the ORR, the catalytic activity of Au could be enhanced by the application of tensile strains while that of Cu, Ni, Pt, Pd, Rh, Co, Ru, and Os was improved by the application of compressive strains. However, the catalytic activity of Ir was rather insensitive to mechanical deformations. Elastic strains were able to modify the rate limiting reaction in Au, Pt, Ag, and Os and it was possible to achieve the cusp of the volcano plot in these metals. Final, mechanical instabilities were attained at small strains in Zn and Cd, which did not lead to significant changes in the catalytic activity for the HER and the ORR. These results provide a framework to systematically investigate the application of elastic strains in the design of new catalysts.

Clean Electrochemical Synthesis of Pd–Pt Bimetallic Dendrites with High Electrocatalytic Performance for the Oxidation of Formic Acid

Pd–Pt bimetallic catalysts with a dendritic morphology were in situ synthesized on the surface of a carbon paper via the facile and surfactant-free two step electrochemical method. The effects of the frequency and modification time of the periodic square-wave potential (PSWP) on the morphology of the Pd–Pt bimetallic catalysts were investigated. The obtained Pd–Pt bimetallic catalysts with a dendritic morphology displayed an enhanced catalytic activity of 0.77 A mg⁻¹, almost 2.5 times that of the commercial Pd/C catalyst reported in the literature (0.31 A mg⁻¹) in acidic media. The enhanced catalytic activity of the Pd–Pt bimetallic catalysts with a dendritic morphology towards formic acid oxidation reaction (FAOR) was not only attributed to the large number of atomic defects at the edges of dendrites, but also ascribed to the high utilization of active sites resulting from the “clean” electrochemical preparation method. Besides, during chronoamperometric testing, the current density of the dendritic Pd–Pt bimetallic catalysts for a period of 3000 s was 0.08 A mg⁻¹, even four times that of the commercial Pd/C catalyst reported in the literature (about 0.02 A mg⁻¹).

ZnSn nanocatalyst: Ultra-high formate selectivity from CO2 electrochemical reduction and the structure evolution effect

The introduction of tin (Sn) into Zn-based catalyst can change its intrinsic properties of the electrochemically reduction of CO₂ to CO, obtaining a high formate yield. The electron transfer from Zn to Sn lowers down the d-band center of Sn, leading to a more reliable surface adsorption of the *OCHO intermediate and high formate selectivity. The obtained ZnSn catalyst enables formate formation with a drastically boosted Faradaic efficiency (FE) up to 94%, which is 2.04 and 1.34 times of pure Zn and Sn foils, respectively, indicating a synergistic effect between Zn and Sn. During the electrochemical CO₂ reduction reaction (eCO₂RR) process, the morphology of the ZnSn catalyst evolved from nanoparticles to nanosheets, nanoneedles and collapsed structures, corresponding to the activation, stabilization and decay stages, respectively. This study provides a facile and controllable approach for the construction of novel bimetallic catalyst favoring formate selectivity based on the synergistic effect.

Synergistic Effects of Crystal Phase and Strain for N2 Dissociation on Ru(0001) Surfaces with Multilayered Hexagonal Close-Packed Structures

The synergistic effects of strain and crystal phase on the reaction activity of nitrogen molecule dissociation have been studied using density functional theory calculations on Ru(0001) surfaces with multilayered hexagonal close-packed structures. The phase transformation from hexagonal close-packed phase (2H) to face-centered cubic (3C) phase or unconventional phases (4H, DHCP, 6H1, and 6H2) would occur under the uniaxial tensile strain loaded along the c axis. The close-packed surfaces of unconventional crystal phases show an enhanced chemical reactivity for N adsorption due to the upshifted d-band center of Ru. However, the N2 adsorption energy is almost independent of the applied strain and crystal phase. The optimized catalytic activity of Ru(0001) surfaces with the unconventional phases is found for the N2 dissociation through breaking the scaling relationships between the reaction barrier and reaction energy. Our results indicate that the strain-induced phase transformation is an effective method to improve the catalytic activity of noble metal catalysts toward the N2 dissociation reaction.

Electronic Structure-Based Descriptors for Oxide Properties and Functions

ConspectusThe transition from fossil fuels to renewable energy requires the development of efficient and cost-effective energy storage technologies. A promising way forward is to harness the energy of intermittent renewable sources, such as solar and wind, to perform (electro)catalytic reactions to generate fuels, thus storing energy in the form of chemical bonds. However, current catalysts rely on the use of expensive, rare, or geographically localized elements, such as platinum. Widespread adoption of new (electro)catalytic technologies hinges on the discovery and development of materials containing earth-abundant elements, which can efficiently catalyze an array of (electro)chemical reactions.In the context of catalysis, descriptors provide correlations between fundamental physical properties, such as the electronic structure, and the resulting catalytic activity. The use of easily accessible descriptors has proven to be a powerful method to advance and accelerate discovery and design of new catalyst materials. The position of the oxygen electronic 2p band center has been proposed to capture the basic physical properties of oxides, including oxygen vacancy formation energy, diffusion barrier of oxygen ions, and work function. Moreover, the adsorption strength of relevant reaction intermediates at the surface of oxides can be strongly correlated with the energy of the oxygen 2p states, which affects the catalytic activity of reactions, such as oxygen electrocatalysis, and oxidative dehydrogenation of organic molecules. Such descriptors for catalytic activity can be used to predict the activity of new catalysts and understand trends and behavior among different catalysts.In this Account, we discuss how the energy of the oxygen 2p states can be used as a descriptor for oxide bulk and surface chemical properties. We show how the oxide redox properties vary linearly with the position of the oxygen 2p band center with respect to the Fermi level, and we discuss how this descriptor can be expanded across different materials and structural families, including possible generalizations to compounds outside oxides. We highlight the power of the oxygen 2p band center to predict the catalytic activity of oxides. We conclude with an outlook examining under which conditions this descriptor can be applied to predict oxide properties and possible opportunities for further refining and accelerating property predictions of oxides by leveraging material databases and machine learning.

Single-Molecule Nanocatalysis Reveals the Kinetics of the Synergistic Effect Based on Single-AuAg Bimetal Nanocatalysts

Decades of extensive research efforts by scientists in the field of catalysis and nanomaterials have led to a large number of excellent bimetallic nanocatalysts. However, in many cases, the mechanism of the synergistic effect in bimetal catalyst-catalyzed reactions has been systematically neglected due to technical limitations. Herein, we use single-molecule fluorescence microscopy (SMFM) to reveal the mechanism of the synergy of the Au and Ag bimetal catalyst. Compared with that of the Ag nanocatalyst, the incorporation of Au changes the reaction pathway of Amplex Red and H₂O₂ from a noncompetitive to a competitive reaction mechanism, showing much higher catalytic efficiency. Additionally, the incorporation also inhibits the spontaneous surface reconstruction and facilitates the reaction-induced surface restructuring of the nanocatalyst, resulting in the enhancement of stability and reactivity. These findings provide useful insights into tailoring the reactivity of metal catalysts. This work also confirms the power of SMFM in revealing the origin of the catalytic activity of composite catalysts.

An efficient factor for fast screening of high-performance two-dimensional metal-organic frameworks towards catalyzing oxygen evolution reaction

Two-dimensional (2D) Metal-Organic frameworks (MOFs) are promising materials for catalyzing oxygen evolution reaction (OER) due to abundant exposed active sites and high specific surface area. However, how to fast screen...

Extreme Environmental Thermal Shock Induced Dislocation-Rich Pt Nanoparticles Boosting Hydrogen Evolution Reaction

Crystal structure engineering of nanomaterials is crucial for the design of electrocatalysts. Inducing dislocations is an efficient approach to generate strain effects in nanomaterials to optimize the crystal and electronic structures and improve the catalytic properties. However, it is almost impossible to produce and retain dislocations in commercial mainstream catalysts, such as single metal platinum (Pt) catalysts. In this work, a non-equilibrium high-temperature (>1400 K) thermal-shock method is reported to induce rich dislocations in Pt nanocrystals (Dr-Pt). The method is performed in an extreme environment (≈77 K) created by liquid nitrogen. The dislocations induced within milliseconds by thermal and structural stress during the crystallization process are kinetically frozen at an ultrafast cooling rate. The high-energy surface structures with dislocation-induced strain effects can prevent surface restructuring during catalysis. The findings indicate that a novel extreme environmental high-temperature thermal-shock method can successfully introduce rich dislocations in Pt nanoparticles and significantly boost its hydrogen evolution reaction performance.

Interfacial Engineering of Metal/Metal Oxide Heterojunctions toward Oxygen Reduction and Evolution Reactions

Oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) are two very important electrochemical processes for renewable energy conversion and storage devices. Electrocatalysts are needed to accelerate their sluggish kinetics to improve energy conversion efficiencies. Hence, extensive efforts have been devoted to the development of OER and ORR electrocatalysts with high activity and stability as well as low cost. Among these developed electrocatalysts, metal/metal oxide heterostructures attract a great deal of research interest because their catalytic performances can be tuned by interface engineering. In this Review, the latest achievements in interface engineering of metal/metal oxides heterostructures toward ORR and OER are described. The effects of the metal/metal oxide interface on catalysis are first discussed. Then, the approaches for interface engineering are illustrated. The developments of interface engineering in OER and ORR catalysis as well as bifunctional electrocatalysis are further introduced. Lastly, a perspective for future development of interface engineering in metal/metal oxide for OER and ORR is discussed.

Local Environment Determined Reactant Adsorption Configuration for Enhanced Electrocatalytic Acetone Hydrogenation to Propane

We demonstrate a widely applicable method to alter the adsorption configuration of multi-carbon containing reactants by no catalyst engineering but simply adjusting the local reaction environment of the catalyst surface. Using electrocatalytic acetone to propane hydrogenation (APH) as a model reaction and common commercial Pt/Pt-based materials as catalysts, we found local H⁺ concentration can significantly influence the adsorption mode of acetone reactant, for example, in vertical or flat mode, and target product selectivity. Electrocatalytic measurement combined with in situ spectroscopic characterizations reveals that the vertically adsorbed acetone is favorable for propane production while the flatly adsorbed mode suppresses the reaction. DFT calculations indicate that the H coverage on catalyst surface plays a decisive role in the adsorption configuration of acetone. The increased local acidity can facilitate the adsorption configuration of acetone from flat to vertical mode and suppress the competing hydrogen evaluation reaction, which consequently enhances the APH selectivity.

Boron Coordination Effect in Ni-Nx Doped Graphene Catalysts on the ORR Performance Based on DFT Calculations

The coordination atoms of metal active site in transition metal N-doped carbon single atom electrocatalysts play a vital role in dominating the catalytic performance of oxygen reduction reaction (ORR) at the cathode of fuel cells or metal-air cells. In view of weak adsorption ability of Ni active site in NiN₄ -C catalysts to oxygen intermediate states, herein we introduce boron atoms with smaller electronegativity than N and C atoms to modulate the local coordination environment and electronic structures of Ni site. First-principles density functional calculations reveal that both B substitution for N atoms (NiN₂ B₂ -C) and B coordinating with N and C (NiN₄ B₈ -C) can effectively optimize the Gibbs free energy of oxygen intermediate states and hence improve the catalytic activity of the materials. In addition, we propose that the trend change in catalytic activity is mainly governed by the filling of antibonding orbitals between Ni-3d and O-2p states near the Fermi level.

Periodic Trends in Adsorption Energies around Single-Atom Alloy Active Sites

Single-atom alloys (SAAs) make up a special class of alloy surface catalysts that offer well-defined, isolated active sites in a more inert metal host. The dopant sites are generally assumed to have little or no influence on the properties of the host metal, and transport of chemical reactants and products to and from the dopant sites is generally assumed to be facile. Here, by performing density functional theory calculations and surface science experiments, we identify a new physical effect on SAA surfaces, whereby adsorption is destabilized by ≤300 meV on host sites within the perimeter of the reactive dopant site. We identify periodic trends for this behavior and demonstrate a zone of exclusion around the reactive sites for a range of adsorbates and combinations of host and dopant metals. Experiments confirm an increased barrier for diffusion of CO toward the dopant on a RhCu SAA. This effect offers new possibilities for understanding and designing active sites with tunable energetic landscapes surrounding them.