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

Tin, The Enabler-Hydrogen Diffusion into Ruthenium

Hydrogen interaction with ruthenium is of particular importance for the ruthenium-capped multilayer reflectors used in extreme ultraviolet (EUV) lithography. Hydrogen causes blistering, which leads to a loss of reflectivity. This problem is aggravated by tin. This study aims to uncover the mechanism via which tin affects the hydrogen uptake, with a view to mitigation. We report here the results of a study of hydrogen interaction with the ruthenium surface in the presence of tin using Density Functional Theory and charge density analyses. Our calculations show a significant drop in the energy barrier to hydrogen penetration when a tin atom or a tin hydride molecule (SnHx) is adsorbed on the ruthenium surface; the barrier has been found to drop in all tested cases with tin, from 1.06 eV to as low as 0.28 eV in the case of stannane (SnH₄). Analyses show that, due to charge transfer from the less electronegative tin to hydrogen and ruthenium, charge accumulates around the diffusing hydrogen atom and near the ruthenium surface atoms. The reduced atomic volume of hydrogen, together with the effect of electron⁻electron repulsion from the ruthenium surface charge, facilitates subsurface penetration. Understanding the nature of tin's influence on hydrogen penetration will guide efforts to mitigate blistering damage of EUV optics. It also holds great interest for applications where hydrogen penetration is desirable, such as hydrogen storage.

Tuning the oxygen evolution reaction on a nickel-iron alloy via active straining

We report that one can gain active control of the electrocatalytic oxygen evolution reaction (OER) on Ni3Fe thin films via externally applied strains. The combination of theory and experiments shows that an elastic strain on the surface can tune the OER activity in a predictable way that is consistent with the d-band model. The OER overpotential can be lowered by uniaxial tensions and increased by compressions in a linear manner.

Electrochemical tuning of Pd100−xAux bimetallics towards ethanol oxidation: effect of an induced d-band center shift and oxophilicity

Electronegative Au improves the oxidation kinetics of Pd by inducing a downshift of the d-band center and increasing the coverage of adsorbed hydroxyls.

Au@PdAg core–shell nanotubes as advanced electrocatalysts for methanol electrooxidation in alkaline media

Au@PdAg core–shell nanotubes using the galvanic displacement reaction were prepared and they exhibited excellent electrocatalytic performance towards methanol electrooxidation.

Graphene-covered transition metal halide molecules as efficient and durable electrocatalysts for oxygen reduction and evolution reactions

Graphene-covered halides are designed as durable and efficient electrocatalysts in acid media. A design principle has been established through the DFT calculations, from which the best catalysts could be predicted for fuel cells.

Natural clay-supported palladium catalysts for methane oxidation reaction: effect of alloying

Bimetallic Pd-supported halloysite nanotubes revealed outstanding catalytic activity towards catalytic methane oxidation especially PdNi.

Systematic exploration of N, C configurational effects on the ORR performance of Fe–N doped graphene catalysts based on DFT calculations

Metal single-atom catalysts (MSATs), such as Fe–N coordination doped sp²-carbon matrices, have emerged as a promising oxygen reduction reaction (ORR) catalyst to replace their costly platinum (Pt) based counterparts in fuel cells. In this work, we employ density functional theory (DFT) to systematically discuss the electronic-structure and surface-stress effects of N, C configurations on Fe–N doped graphene in single and double vacancy. The formation energy (E_f) of Fe–N-gra dropped off with the increase of N atoms incorporated for both single and double vacancy groups. The theoretical overpotentials on Fe–N–C sites were calculated and revealed that moderate N-doping levels and doping configuration could enhance the ORR activity of Fe–N coordination structures in the double vacancy and that doping N atoms is not effective for ORR activity in single vacancy. By exploring the d-band centers, we found that ligand effects and surface tension effects contribute to the modification of the d-band centers of metal Fe atoms. An optimum Fe–N–C ORR catalyst should exhibit moderate surface stress properties and an ideal N, C ligand configuration. This study provides new insight into the effects of N atom doping in Fe–N-gra catalysts and could help guide the rational design of high-performance carbon-based ORR electrocatalysts.

An optimum Fe–N–C ORR catalyst should exhibit a moderate surface stress property and an ideal N, C ligand configurations that results in a moderate interaction between the ORR intermediates and its surface sites.

Strain engineering of metal-based nanomaterials for energy electrocatalysis

The strain effect, along with the ligand effect and synergistic effect, contributes primarily to the optimization of electrocatalytic activity and stability. The strain effect leads to a shift in the d-band center and alters binding energies toward adsorbates. Under electrocatalytic circumstances, the strain effect and ligand effect by and large function in combination; however, the decay and vanishing of the ligand effect precede the strain effect as the thickness of the shell in the core/shell structure or metallic overlayers on substrates increases. The strain effect on electrocatalytic activity can be well engineered by tuning the thickness of shells or atomic composition. Microstrain, or localized lattice strain, is another type of strain associated with structural defects such as grain boundaries and multi-twinning. In this review, we discuss the origin of the strain effect and how it affects electrocatalytic activity based on the d-band model. We present the structural characterization and quantitative determination of strain. Metal-based nanocrystals are basically grouped into two types of structures to which the strain engineering applies, i.e. lattice strain-associated structures (which include the general core/shell structure and solid solution alloy) and multiple defects-induced structures. Then analysis is performed on the correlation of strain and ligand effects and on the tuning strategies of the strain effect for electrocatalysis. After that, we use representative examples to demonstrate how strain engineering assists in typical electrocatalytic reactions on anodes and cathodes. Finally, we summarize and propose potential research areas in terms of enhancing electrocatalytic activities by strain engineering in the future.

Application of silica-supported Ir and Ir-M (M = Pt, Pd, Au) catalysts for low-temperature hydrodechlorination of tetrachloromethane

Herein, it is presented a catalytic system for gas-phase hydrodechlorination of tetrachloromethane at low temperature and atmospheric pressure, using iridium supported on silica as parent catalyst. Iridium electronic configuration is suitable to catalyse the hydrodechlorination reactions, however, it has been rarely used in this reaction to date. The catalytic abilities were significantly improved when a second transition metal was added. Catalysts' stability and selectivity to the desired products (i.e. C₁-C₄ hydrocarbons) improved compared to conventional activation in hydrogen when catalysts were activated shortly with microwave irradiation. Microwave irradiation of catalysts favourably influences the homogeneity of the metallic active phase, both in terms of the size of metal crystals and the homogeneity of bimetallic systems. Addition of platinum to the 'parent' iridium catalyst improved its catalytic properties and decreased deactivation. Fresh and spent catalysts were comprehensively characterized using several techniques (BET, CO-chemisorption, XRD, XPS, electron microscopy and mass spectrometry) to determine structure-activity relationships and potential causes for catalyst deactivation. No significant changes in crystalline size or bimetallic phase composition were observed for spent catalysts (with the exception of Ir-Pd catalysts which underwent bulk carbide during the reaction).

Nanostructure Optimization of Platinum-Based Nanomaterials for Catalytic Applications

Platinum-based nanomaterials have attracted much interest for their promising potentials in fields of energy-related and environmental catalysis. Designing and controlling the surface/interface structure of platinum-based nanomaterials at the atomic scale and understanding the structure-property relationship have great significance for optimizing the performances in practical catalytic applications. In this review, the strategies to obtain platinum-based catalysts with fantastic activity and great stability by composition regulation, shape control, three-dimension structure construction, and anchoring onto supports, are presented in detail. Moreover, the structure-property relationship of platinum-based nanomaterials are also exhibited, and a brief outlook are given on the challenges and possible solutions in future development of platinum-based nanomaterials towards catalytic reactions.

Boosting Tunable Syngas Formation via Electrochemical CO2 Reduction on Cu/In2O3 Core/Shell Nanoparticles

In this work, monodisperse core/shell Cu/In₂O₃ nanoparticles (NPs) were developed to boost efficient and tunable syngas formation via electrochemical CO₂ reduction for the first time. The efficiency and composition of syngas production on the developed carbon-supported Cu/In₂O₃ catalysts are highly dependent on the In₂O₃ shell thickness (0.4-1.5 nm). As a result, a wide H₂/CO ratio (4/1 to 0.4/1) was achieved on the Cu/In₂O₃ catalysts by controlling the shell thickness and the applied potential (from -0.4 to -0.9 V vs reversible hydrogen electrode), with Faraday efficiency of syngas formation larger than 90%. Specifically, the best-performing Cu/In₂O₃ catalyst demonstrates remarkably large current densities under low overpotentials (4.6 and 12.7 mA/cm² at -0.6 and -0.9 V, respectively), which are competitive with most of the reported systems for syngas formation. Mechanistic discussion implicates that the synergistic effect between lattice compression and Cu doping in the In₂O₃ shell may enhance the binding of *COOH on the Cu/In₂O₃ NP surface, leading to the enhanced CO generation relative to Cu and In₂O₃ catalysts. This report demonstrates a new strategy to realize efficient and tunable syngas formation via rationally designed core/shell catalyst configuration.

Epitaxial growth of Ni(OH)2 nanoclusters on MoS2 nanosheets for enhanced alkaline hydrogen evolution reaction

Constructing heterostructures is an effective strategy for designing efficient electrocatalysts. MoS2 is a star catalyst for hydrogen evolution reaction (HER) in acidic media; however, the alkaline HER activity is deficient due to the sluggish water dissociation process. Herein, Ni(OH)2/MoS2 heterostructures with Ni(OH)2 nanoclusters epitaxially decorated on the surface of MoS2 are synthesized towards the alkaline HER. As compared with MoS2, the epitaxial Ni(OH)2/MoS2 heterostructures show significantly enhanced HER activity in 1 M KOH, and the overpotential is decreased by nearly 150 mV to reach a current density of 10 mA cm-2. The substantial increase in turnover frequency (TOF) demonstrates that the intrinsic activity is greatly improved after the incorporation of Ni(OH)2 nanoclusters. The presence of Ni(OH)2 nanoclusters would provide additional water dissociation sites while MoS2 is ready for the adsorption and combination of the generated H*, and this so-called synergistic effect eventually induces significantly enhanced alkaline HER kinetics. Besides, the electron transfer from Ni(OH)2 to MoS2 increases the proton affinity of MoS2. The present results describe an interesting case of an atomic-scale electrochemically inert material promoted HER process, and would open a new avenue into designing efficient hetero-nanostructures towards electrocatalytic applications.

Electrochemical Synthesis of Mesoporous Au-Cu Alloy Films with Vertically Oriented Mesochannels Using Block Copolymer Micelles

We synthesized Au-Cu bimetallic alloy films with a controlled mesoporous architecture through electrochemical deposition using an electrolyte solution containing spherical polymeric micelles. The composition of the alloy films can be easily controlled by tuning the ratio between the Au and Cu species present in the electrolyte solution. At low Cu content, cage-type mesopores are formed, reflecting the parent micellar template. Surprisingly, upon increasing the Cu content, the cage-type mesopores fuse to form vertically aligned one-dimensional mesochannels. The vertical alignment of these mesopores is favorable for enhanced mass and ion transfer within the channels due to low diffusion resistance. The atomic distribution of Au and Cu is uniform over the entire film and free of any phase segregation. The as-synthesized mesoporous Au-Cu films exhibit excellent performance as a nonenzymatic glucose sensor with high sensitivity and selectivity, and the current response is linear over a wide range of concentrations. This work identifies the properties responsible for the promising performance of such mesoporous alloy films for the clinical diagnosis of diabetes. This micelle-assisted electrodeposition approach has a high degree of flexibility and can be simply extended from monometallic compounds to a multimetallic system, enabling the fabrication of various mesoporous alloy films suitable for different applications.

Mechanistic investigations of the Au catalysed C-H bond activations in on-surface synthesis

Recently, Au-based nanostructures have attracted extensive interest due to their excellent activities in heterogeneous catalysis. The reaction mechanisms have been interpreted qualitatively by the quantum confinement effect due to the low-coordination of Au atoms in nanostructures. In this work, systematic first-principles calculations were carried out to obtain an in-depth understanding of the origin of C-H bond activations with Au-based catalysts in on-surface synthesis. Combining density functional theory (DFT) calculations and scanning tunneling microscopy (STM) studies, we reveal that the d-band centre and the d-band width of the Au-5dz2 orbital in an energy window of -6.80 to 0.00 eV may serve as theoretical descriptors for the prediction of the activity of Au catalysts in C-H bond activations. This work may therefore inspire further investigations on the design of new catalysts.

Study of oxygen evolution reaction on amorphous Au13@Ni120P50 nanocluster

The pursuit of catalysts to promote effective water oxidization to produce oxygen has become a research subject of high priority for water splitting. Here, first-principles calculations are employed to study the water-splitting oxygen evolution reaction (OER) on ∼1.5 nm diameter Au13@Ni120P50 core-shell nanoclusters. Water splitting to produce oxygen proceeds in four intermediate reaction steps (OH*, O*, OOH* and O2). Adsorption configurations and adsorption energies for the species involved in OER on both Au13@Ni120P50 cluster and Ni12P5(001) supported by Au are presented. In addition, thermodynamic free energy diagrams and kinetic potential energy changes are systematically discussed. We show that the third intermediate reaction (O* reacting with H2O to produce OOH*) of the four elementary steps is the reaction-determining step, which accords with previous results. Also, the catalytic performance of OER for Au13@Ni120P50 is better than that for Ni12P5(001) supported by Au in terms of reactive overpotential (0.74 vs. 1.58 V) and kinetic energy barrier (2.18 vs. 3.17 eV). The optimal kinetic pathway for OER is further explored carefully for the Au13@Ni120P50 cluster. The low thermodynamic overpotential and kinetic energy barrier make Au13@Ni120P50 promising for industrial applications as a good OER electrocatalyst candidate.

CO oxidative coupling to dimethyl oxalate over Pd-Me (Me = Cu, Al) catalysts: a combined DFT and kinetic study

CO oxidative coupling to dimethyl oxalate (DMO) on Pd(111), Pd-Cu(111) and Pd-Al(111) surfaces was systematically investigated by means of density functional theory (DFT) together with periodic slab models and micro-kinetic modeling. The binding energy results show that Cu and Al can be fine substrates to stably support Pd. The favorable pathway for DMO synthesis on these catalysts starts from the formation of two COOCH₃ intermediates, followed by the coupling to each other, and the catalytic activity follows the trend of Pd-Al(111) > Pd(111) > Pd-Cu(111). Additionally, the formation of DMO is far favorable than that of dimethyl carbonate (DMC) on these catalysts. The results were further demonstrated by micro-kinetic modeling. Therefore, Pd-Al bimetallic catalysts can be applied in practice to effectively enhance the catalytic performance and greatly reduce the cost. This study can help with fine-tuning and designing of high-efficient and low-cost Pd-based bimetallic catalysts.

Electrical Matching at Metal/Molecule Contacts for Efficient Heterogeneous Charge Transfer

In a metal/molecule hybrid system, unavoidable electrical mismatch exists between metal continuum states and frontier molecular orbitals. This causes energy loss in the electron conduction across the metal/molecule interface. For efficient use of energy in a metal/molecule hybrid system, it is necessary to control interfacial electronic structures. Here we demonstrate that electrical matching between a gold substrate and π-conjugated molecular wires can be obtained by using monatomic foreign metal interlayers, which can change the degree of d-π* back-donation at metal/anchor contacts. This interfacial control leads to energy level alignment between the Fermi level of the metal electrode and conduction molecular orbitals, resulting in resonant electron conduction in the metal/molecule hybrid system. When this method is applied to molecule-modified electrocatalysts, the heterogeneous electrochemical reaction rate is considerably improved with significant suppression of energy loss at the internal electron conduction.

Ruthenium-Tungsten Composite Catalyst for the Efficient and Contamination-Resistant Electrochemical Evolution of Hydrogen

A new catalyst, prepared by a simple physical mixing of ruthenium (Ru) and tungsten (W) powders, has been discovered to interact synergistically to enhance the electrochemical hydrogen evolution reaction (HER). In an aqueous 0.5 M H₂SO₄ electrolyte, this catalyst, which contained a miniscule loading of 2-5 nm sized Ru nanoparticles (5.6 μg Ru per cm² of geometric surface area of the working electrode), required an overpotential of only 85 mV to drive 10 mA/cm² of H₂ evolution. Interestingly, our catalyst also exhibited good immunity against deactivation during HER from ionic contaminants, such as Cu²⁺ (over 24 h). We unravel the mechanism of synergy between W and Ru for catalyzing H₂ evolution using Cu underpotential deposition, photoelectron spectroscopy, and density functional theory (DFT) calculations. We found a decrease in the d-band and an increase in the electron work function of Ru in the mixed composite, which made it bind to H more weakly (more Pt-like). The H-adsorption energy on Ru deposited on W was found, by DFT, to be very close to that of Pt(111), explaining the improved HER activity.

Carbon Monoxide Poisoning Resistance and Structural Stability of Single Atom Alloys

Platinum group metals (PGMs) serve as highly active catalysts in a variety of heterogeneous chemical processes. Unfortunately, their high activity is accompanied by a high affinity for CO and thus, PGMs are susceptible to poisoning. Alloying PGMs with metals exhibiting lower affinity to CO could be an effective strategy toward preventing such poisoning. In this work, we use density functional theory to demonstrate this strategy, focusing on highly dilute alloys of PGMs (Pd, Pt, Rh, Ir and Ni) with poison resistant coinage metal hosts (Cu, Ag, Au), such that individual PGM atoms are dispersed at the atomic limit forming single atom alloys (SAAs). We show that compared to the pure metals, CO exhibits lower binding strength on the majority of SAAs studied, and we use kinetic Monte Carlo simulation to obtain relevant temperature programed desorption spectra, which are found to be in good agreement with experiments. Additionally, we consider the effects of CO adsorption on the structure of SAAs. We calculate segregation energies which are indicative of the stability of dopant atoms in the bulk compared to the surface layer, as well as aggregation energies to determine the stability of isolated surface dopant atoms compared to dimer and trimer configurations. Our calculations reveal that CO adsorption induces dopant atom segregation into the surface layer for all SAAs considered here, whereas aggregation and island formation may be promoted or inhibited depending on alloy constitution and CO coverage. This observation suggests the possibility of controlling ensemble effects in novel catalyst architectures through CO-induced aggregation and kinetic trapping.

The surface and catalytic chemistry of the first row transition metal phosphides in deoxygenation

The proven utility of transition metal (TM) phosphides in catalytic deoxygenation reactions and their ability to preserve unsaturation or aromaticity in products has suggested the materials exhibit unique surface chemistry towards C, O, and H.

The adsorption of alcohols on strained Pt3Ni(111) substrates: a density functional investigation within the D3 van der Waals correction

In this theoretical study, we address the effect of strain and alloying on the adsorption of methanol, ethanol and glycerol on Pt₃Ni(111) surfaces.

Lattice mismatch as the descriptor of segregation, stability and reactivity of supported thin catalyst films

Surface properties of supported bimetallic films can be predicted from the lattice mismatch between the overlayer and the support already there for trilayers.

Formation of Surface and Quantum-Well States in Ultra Thin Pt Films on the Au(111) Surface

The electronic structure of the Pt/Au(111) heterostructures with a number of Pt monolayers n ranging from one to three is studied in the density-functional-theory framework. The calculations demonstrate that the deposition of the Pt atomic thin films on gold substrate results in strong modifications of the electronic structure at the surface. In particular, the Au(111) s-p-type Shockley surface state becomes completely unoccupied at deposition of any number of Pt monolayers. The Pt adlayer generates numerous quantum-well states in various energy gaps of Au(111) with strong spatial confinement at the surface. As a result, strong enhancement in the local density of state at the surface Pt atomic layer in comparison with clean Pt surface is obtained. The excess in the density of states has maximal magnitude in the case of one monolayer Pt adlayer and gradually reduces with increasing number of Pt atomic layers. The spin-orbit coupling produces strong modification of the energy dispersion of the electronic states generated by the Pt adlayer and gives rise to certain quantum states with a characteristic Dirac-cone shape.

Finite Size Effects in Submonolayer Catalysts Investigated by CO Electrosorption on PtsML/Pd(100)

A combination of scanning tunneling microscopy, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS), and density functional theory (DFT) is used to quantify the local strain in 2D Pt clusters on the 100 facet of Pd and its effect on CO chemisorption. Good agreement between SNIFTIRS experiments and DFT simulations provide strong evidence that, in the absence of coherent strain between Pt and Pd, finite size effects introduce local compressive strain, which alters the chemisorption properties of the surface. Though this effect has been widely neglected in prior studies, our results suggest that accurate control over cluster sizes in submonolayer catalyst systems can be an effective approach to fine-tune their catalytic properties.