Atomic-scale structure and catalytic reactivity of the RuO(2)(110) surface

In situ studies of NO reduction by H2 over Pt using surface X-ray diffraction and transmission electron microscopy

Exposure to H₂ induces faceting of the Pt nanoparticle, while exposure to NO induces rounding of the nanoparticle.

The Nanostructuring of Atomically Flat Ru(0001) upon Oxidation and Reduction

The O/Ru(0001) system is widely studied due to its rich phase variety of various stoichiometry and atomic arrangements, including the formation of a RuO₂/Ru(0001) oxide layer. Apart from homogeneous ruthenium surfaces in certain oxidation states, also strongly heterogeneous surfaces can exist due to oxidation state's variation at the nanoscale. We report on a scanning tunneling microscopy (STM) study of the nanostructuring of the oxidized Ru(0001) surface as a result of its interaction with molecular oxygen at elevated temperatures and subsequent reduction of a resulting RuO₂ film by CO or HCl molecules from the gas phase in high-vacuum environment.

Understanding the pseudocapacitance of RuO2 from joint density functional theory

Pseudocapacitors have been experimentally studied for many years in electric energy storage. However, first principles understanding of the pseudocapacitive behavior is still not satisfactory due to the complexity involved in modeling electrochemistry. In this paper, we applied joint density functional theory (JDFT) to simulate the pseudocapacitive behavior of RuO2, a prototypical material, in a model electrolyte. We obtained from JDFT a capacitive curve which showed a redox peak position comparable to that in the experimental cyclic voltammetry (CV) curve. We found that the experimental turning point from double-layer to pseudocapacitive charge storage at low scan rates could be explained by the hydrogen adsorption at low coverage. As the electrode voltage becomes more negative, H coverage increases and causes the surface-structure change, leading to bended -OH bonds at the on-top oxygen atoms and large capacitance. This H coverage-dependent capacitance can explain the high pseudocapacitance of hydrous RuO2. Our work here provides a first principles understanding of the pseudocapacitance for RuO2 in particular and for transition-metal oxides in general.

Building better lithium-sulfur batteries: from LiNO3 to solid oxide catalyst

Lithium nitrate (LiNO₃) is known as an important electrolyte additive in lithium-sulfur (Li-S) batteries. The prevailing understanding is that LiNO₃ reacts with metallic lithium anode to form a passivation layer which suppresses redox shuttles of lithium polysulfides, enabling good rechargeability of Li-S batteries. However, this view is seeing more challenges in the recent studies, and above all, the inability of inhibiting polysulfide reduction on Li anode. A closely related issue is the progressive reduction of LiNO₃ on Li anode which elevates internal resistance of the cell and compromises its cycling stability. Herein, we systematically investigated the function of LiNO₃ in redox-shuttle suppression, and propose the suppression as a result of catalyzed oxidation of polysulfides to sulfur by nitrate anions on or in the proximity of the electrode surface upon cell charging. This hypothesis is supported by both density functional theory calculations and the nitrate anions-suppressed self-discharge rate in Li-S cells. The catalytic mechanism is further validated by the use of ruthenium oxide (RuO₂, a good oxygen evolution catalyst) on cathode, which equips the LiNO₃-free cell with higher capacity and improved capacity retention over 400 cycles.

Size dependence of structural parameters in fcc and hcp Ru nanoparticles, revealed by Rietveld refinement analysis of high-energy X-ray diffraction data

To reveal the origin of the CO oxidation activity of Ruthenium nanoparticles (Ru NPs), we structurally characterized Ru NPs through Rietveld refinement analysis of high-energy X-ray diffraction data. For hexagonal close-packed (hcp) Ru NPs, the CO oxidation activity decreased with decreasing domain surface area. However, for face-centered cubic (fcc) Ru NPs, the CO oxidation activity became stronger with decreasing domain surface area. In comparing fcc Ru NPs with hcp Ru NPs, we found that the hcp Ru NPs of approximately 2 nm, which had a smaller domain surface area and smaller atomic displacement, showed a higher catalytic activity than that of fcc Ru NPs of the same size. In contrast, fcc Ru NPs larger than 3.5 nm, which had a larger domain surface area, lattice distortion, and larger atomic displacement, exhibited higher catalytic activity than that of hcp Ru NPs of the same size. In addition, the fcc Ru NPs had larger atomic displacements than hcp Ru NPs for diameters ranging from 2.2 to 5.4 nm. Enhancement of the CO oxidation activity in fcc Ru NPs may be caused by an increase in imperfections due to lattice distortions of close-packed planes and static atomic displacements.

Conjugation-Driven "Reverse Mars-van Krevelen"-Type Radical Mechanism for Low-Temperature C-O Bond Activation

C-O bond activation on monofunctional catalysts (metals, carbides, and oxides) is challenging due to activity constraints imposed by energy scaling relationships. Yet, contrary to predictions, recently discovered multifunctional metal/metal oxide catalysts (e.g., Rh/ReOx, Rh/MoOx, Ir/VOx) demonstrate unusually high C-O scission activity at moderate temperatures. Herein, we use extensive density functional theory calculations, first-principles microkinetic modeling, and electronic structure analysis to elucidate the metal/metal oxide synergy in the Ru/RuO2 catalyst, which enables up to 76% yield of the C-O scission product (2-methyl furan) in catalytic transfer hydrogenolysis of furfural at low temperatures. Our key mechanistic finding is a facile radical-mediated C-O bond activation on RuO2 oxygen vacancies, which directly leads to a weakly bound final product. This is the first time the radical reduction mechanism is reported in heterogeneous catalysis at temperatures <200 °C. We attribute the unique catalytic properties to the formation of a conjugation-stabilized furfuryl radical upon C-O bond scission, the strong hydroxyl affinity of oxygen vacancies due to the metallic character of RuO2, and the acid-base heterogeneity of the oxide surface. The conjugation-driven radical-assisted C-O bond scission applies to any catalytic surface that preserves the π-electron system of the reactant and leads to C-O selectivity enhancement, with notable examples including Cu, H-covered Pd, self-assembled monolayers on Pd, and oxygen-covered Mo2C. Furthermore, we reveal the cooperativity of active sites in multifunctional catalysts. The mechanism is fully consistent with kinetic studies and isotopic labeling experiments, and the insights gained might prove useful more broadly in overcoming activity constraints induced by energy scaling relationships.

In search of non-conventional surface oxidic motifs of Cu on Au(111)

Growing ultrathin oxide layers on metal surfaces presents a new class of low-dimensional nanomaterials with exceptional chemical and physical properties. These "new oxides" can be used in many niche technologies and applications such as nanoscale electronics and heterogeneous nanocatalysis. In this work, we study the formation of surface oxidic structures and motifs of Cu, supported on the Au(111) substrate, using first-principles density-functional theory calculations in conjunction with an ab initio atomistic thermodynamics model. In particular, we systematically examine and analyze the detailed atomic structure and surface energetics of various oxidic motifs of Cu on Au(111), in particular, p2, p2s, p2(6q6) and the newly suggested metastable p2(6q6) + O3, in comparison to both the binary O/Cu(111) and O/Au(111) systems. Depending on the oxygen atmosphere and the type of surface defects introduced in the oxidic layer, various non-conventional, non-hexagonal surface oxidic motifs of Cu could be obtained. Our theoretical results agree with recent scanning tunneling microscopy (STM) experiments and we propose that metastable non-hexagonal surface motifs may pave a way to pursue further studies of these interesting complex surface oxidic layers on various metal supports.

Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes

Chlorine gas and sodium chlorate are two base chemicals produced through electrolysis of sodium chloride brine which find uses in many areas of industrial chemistry. Although the industrial production of these chemicals started over 100 years ago, there are still factors that limit the energy efficiencies of the processes. This review focuses on the unwanted production of oxygen gas, which decreases the charge yield by up to 5%. Understanding the factors that control the rate of oxygen production requires understanding of both chemical reactions occurring in the electrolyte, as well as surface reactions occurring on the anodes. The dominant anode material used in chlorate and chlor-alkali production is the dimensionally stable anode (DSA), Ti coated by a mixed oxide of RuO2 and TiO2. Although the selectivity for chlorine evolution on DSA is high, the fundamental reasons for this high selectivity are just now becoming elucidated. This review summarizes the research, since the early 1900s until today, concerning the selectivity between chlorine and oxygen evolution in chlorate and chlor-alkali production. It covers experimental as well as theoretical studies and highlights the relationships between process conditions, electrolyte composition, the material properties of the anode, and the selectivity for oxygen formation.

A Route for Phase Control in Metal Nanoparticles: A Potential Strategy to Create Advanced Materials

There is untapped potential for materials whose crystal structures are unobtainable in the bulk state. Several examples of such structures have been found in nanomaterials, and these materials exhibit unique properties that arise from their unique electronic states and surface structures. Here, recent developments in the syntheses of these nanomaterials and their unique properties, such as hydrogen-storage ability and catalytic activity, are summarized. Firstly, the syntheses and properties of novel solid-solution alloy nanoparticles in immiscible alloy systems such as Ag-Rh and Pd-Ru are introduced. Following this, the crystal structure control of nanoscale Ru is discussed. These unique alloy materials show enhanced properties and highlight the potential of phase control to be a new strategy for nanomaterial development.

Influences of the Pb 6s2 lone pair effect and quantum size effect on the diffusion of oxygen atoms on Pb(111) films

Based on our previous studies revealing quantum oscillations in the adsorption energetics of atomic oxygen on Pb(111) films, here we study all the possible on-surface and subsurface adsorption sites of oxygen atoms on Pb(111) films at different coverages.

Role of RuO2(100) in surface oxidation and CO oxidation catalysis on Ru(0001)

Oxidation of Ru(0001) induces the simultaneous formation of RuO₂(100) and RuO₂(110) and a structure-sensitive oxygen spillover during CO oxidation.

Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution

Water splitting catalysed by earth-abundant materials is pivotal for global-scale production of non-fossil fuels, yet our understanding of the active catalyst structure and reactivity is still insufficient. Here we report on the structurally reversible evolution of crystalline Co3O4 electrocatalysts during oxygen evolution reaction identified using advanced in situ X-ray techniques. At electrode potentials facilitating oxygen evolution, a sub-nanometre shell of the Co3O4 is transformed into an X-ray amorphous CoOx(OH)y which comprises di-μ-oxo-bridged Co(3+/4+) ions. Unlike irreversible amorphizations, here, the formation of the catalytically-active layer is reversed by re-crystallization upon return to non-catalytic electrode conditions. The Co3O4 material thus combines the stability advantages of a controlled, stable crystalline material with high catalytic activity, thanks to the structural flexibility of its active amorphous oxides. We propose that crystalline oxides may be tailored for generating reactive amorphous surface layers at catalytic potentials, just to return to their stable crystalline state under rest conditions.

The Hubbard dimer: a density functional case study of a many-body problem

This review explains the relationship between density functional theory and strongly correlated models using the simplest possible example, the two-site Hubbard model. The relationship to traditional quantum chemistry is included. Even in this elementary example, where the exact ground-state energy and site occupations can be found analytically, there is much to be explained in terms of the underlying logic and aims of density functional theory. Although the usual solution is analytic, the density functional is given only implicitly. We overcome this difficulty using the Levy-Lieb construction to create a parametrization of the exact function with negligible errors. The symmetric case is most commonly studied, but we find a rich variation in behavior by including asymmetry, as strong correlation physics vies with charge-transfer effects. We explore the behavior of the gap and the many-body Green's function, demonstrating the 'failure' of the Kohn-Sham (KS) method to reproduce the fundamental gap. We perform benchmark calculations of the occupation and components of the KS potentials, the correlation kinetic energies, and the adiabatic connection. We test several approximate functionals (restricted and unrestricted Hartree-Fock and Bethe ansatz local density approximation) to show their successes and limitations. We also discuss and illustrate the concept of the derivative discontinuity. Useful appendices include analytic expressions for density functional energy components, several limits of the exact functional (weak- and strong-coupling, symmetric and asymmetric), various adiabatic connection results, proofs of exact conditions for this model, and the origin of the Hubbard model from a minimal basis model for stretched H2.

Nanoscale Origin of Mesoscale Roughening: Real-Time Tracking and Identification of Three Distinct Ruthenium Oxide Phases in Ruthenium Oxidation

The structural modification of the Ru(0001) surface is followed in real-time using low-energy electron microscopy at elevated temperatures during exposure to molecular oxygen. We observe the nucleation and growth of three different RuO2 facets, which are unambiguously identified by single-domain microspot low-energy electron diffraction (μLEED) analysis from regions of 250 nm in diameter. Structural identification is then pushed to the true nanoscale by employing very-low-energy electron reflectivity spectra R(E) from regions down to 10 nm for structural fingerprinting of complex reactions such as the oxidation of metal surfaces. Calculations of R(E) with an ab initio scattering theory confirm the growth of (110), (100), and (101) orientations of RuO2 and explain the shape of the R(E) spectra in terms of the conducting band structure. This methodology is ideally suited to identify the structure of supported ultrathin films and dynamic transformations at multicomponent interfaces down to few nanometer lateral resolution at elevated temperature and in reactive environments.

Room-temperature cataluminescence from CO oxidation in a non-thermal plasma-assisted catalysis system

Cataluminescence (CTL) is a kind of chemiluminescence during catalytic reaction on surface of catalysts under a heated condition. Due to the low catalytic reactivity of CO, normally low intensity of CTL is obtained during heterogeneously catalytic oxidation of CO under heated conditions (normally higher than 150°C), even catalyzed by precious-metal-based catalysts. Therefore, seeking enhanced CTL of CO at room temperature and using low-cost catalysts becomes significant. Here, CTL generated from CO oxidation was firstly reported at room temperature, which was carried out in a non-thermal plasma-assisted (NTPA) catalysis system. With air acting as discharge gas, carrier gas as well as oxidant, a Mn/SiO2 nanomaterials-based NTPA catalysis system was fabricated for CO catalytic oxidation at room temperature, whose temperature was much lower than previous CTL methods. Relatively high and selective CTL responses were acquired during CO oxidation on surface of Mn/SiO2 nanomaterials, whereas no significant CTL signal was recorded without plasma assistance or on other metals-doped SiO2 catalysts. Without any excitation light source or heating element, a low cost and simple CO sensor was fabricated by using common and easily synthesized catalysts. The present work has greatly simplified the constructions, and enlarged CTL applications.

Ab initio atomistic thermodynamics study on the oxidation mechanism of binary and ternary alloy surfaces

Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.

Real-Time Gas-Phase Imaging over a Pd(110) Catalyst during CO Oxidation by Means of Planar Laser-Induced Fluorescence

The gas composition surrounding a catalytic sample has direct impact on its surface structure, which is essential when in situ investigations of model catalysts are performed. Herein a study of the gas phase close to a Pd(110) surface during CO oxidation under semirealistic conditions is presented. Images of the gas phase, provided by planar laser-induced fluorescence, clearly visualize the formation of a boundary layer with a significantly lower CO partial pressure close to the catalytically active surface, in comparison to the overall concentration as detected by mass spectrometry. The CO partial pressure variation within the boundary layer will have a profound effect on the catalysts’ surface structure and function and needs to be taken into consideration for in situ model catalysis studies.

In-situ observations of catalytic surface reactions with soft x-rays under working conditions

Catalytic chemical reactions proceeding on solid surfaces are an important topic in fundamental science and industrial technologies such as energy conversion, pollution control and chemical synthesis. Complete understanding of the heterogeneous catalysis and improving its efficiency to an ultimate level are the eventual goals for many surface scientists. Soft x-ray is one of the prime probes to observe electronic and structural information of the target materials. Most studies in surface science using soft x-rays have been performed under ultra-high vacuum conditions due to the technical limitation, though the practical catalytic reactions proceed under ambient pressure conditions. However, recent developments of soft x-ray based techniques operating under ambient pressure conditions have opened a door to the in-situ observation of materials under realistic environments. The near-ambient-pressure x-ray photoelectron spectroscopy (NAP-XPS) using synchrotron radiation enables us to observe the chemical states of surfaces of condensed matters under the presence of gas(es) at elevated pressures, which has been hardly conducted with the conventional XPS technique. Furthermore, not only the NAP-XPS but also ambient-pressure compatible soft x-ray core-level spectroscopies, such as near-edge absorption fine structure (NEXAFS) and x-ray emission spectroscopy (XES), have been significantly contributing to the in-situ observations. In this review, first we introduce recent developments of in-situ observations using soft x-ray techniques and current status. Then we present recent new findings on catalytically active surfaces using soft x-ray techniques, particularly focusing on the NAP-XPS technique. Finally we give a perspective on the future direction of this emerging technique.

Heterogeneous catalysis

A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.

Surface resonances in electron reflection from overlayers

Electron scattering by oxygen monolayers on the Ru(0 0 0 1) surface is studied both experimentally and theoretically. Sharp transmission resonances at low energies are revealed and established to originate from critical points of a special kind in the complex band structure of the substrate. Electron reflection from the clean and oxidized Ru(0 0 0 1) is measured for kinetic energies up to 40 eV at normal incidence for oxygen coverages of 1/4, 1/2, 3/4, and one monolayer. The reflection spectra R(E) are analyzed using a Bloch-waves based ab initio scattering theory. In addition to the substrate-induced resonances the reconstructed (2 × 1) and (2 × 2) surfaces show surface resonances due to pre-emergent secondary diffraction beams. The R(E) spectra are shown to give unambiguous evidence of the hcp stacking of the oxygen layer.

Insights into the gas phase oxidation of Ru(0001) on the mesoscopic scale using molecular oxygen

We present an extensive mesoscale study of the initial gas phase oxidation of Ru(0001), employing in situ low-energy electron microscopy (LEEM), micro low-energy electron diffraction (μ-LEED) and scanning tunneling microscopy (STM).

When ruthenia met titania: achieving extraordinary catalytic activity at low temperature by nanostructuring of oxides

Nanostructured ruthenia shows strongly modified properties compared to the pure oxide thereby becoming a low-temperature high-activity catalyst.

High electrochemical selectivity of edge versus terrace sites in two-dimensional layered MoS2 materials

Exploring the chemical reactivity of different atomic sites on crystal surface and controlling their exposures are important for catalysis and renewable energy storage. Here, we use two-dimensional layered molybdenum disulfide (MoS2) to demonstrate the electrochemical selectivity of edge versus terrace sites for Li-S batteries and hydrogen evolution reaction (HER). Lithium sulfide (Li2S) nanoparticles decorates along the edges of the MoS2 nanosheet versus terrace, confirming the strong binding energies between Li2S and the edge sites and guiding the improved electrode design for Li-S batteries. We also provided clear comparison of HER activity between edge and terrace sites of MoS2 beyond the previous theoretical prediction and experimental proof.

A More Accurate Kinetic Monte Carlo Approach to a Monodimensional Surface Reaction: The Interaction of Oxygen with the RuO2(110) Surface

The theoretical study of catalysis would substantialy benefit from the use of atomistic simulations that can provide information beyond mean-field approaches. To date, the nanoscale understanding of surface reactions has been only qualitatively achieved by means of kinetic Monte Carlo coupled to density functional theory, KMC-DFT. Here, we examine a widely employed model for oxygen interaction with the RuO₂(110) surface, a highly anisotropic system. Our analysis reveals several covert problems that render as questionable the model’s predictions. We suggest an advanced approach that considers all the relevant elementary steps and configurations while smoothing the intrinsic errors in the DFT description of oxygen. Under these conditions, KMC provides quantitative agreement to temperature-programmed desorption experiments. These results illustrate how KMC-based simulations can be pushed forward so that they evolve toward being the standard methodology to study complex chemistry at the nanoscale.

Highly electrocatalytic activity of RuO₂ nanocrystals for triiodide reduction in dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) are promising alternatives to conventional silicon devices because of their simple fabrication procedure, low cost, and high efficiency. Platinum is generally used as a superior counter electrode (CE) material, but the disadvantages such as high cost and low abundance greatly restrict the large-scale application of DSCs. An efficient and sustainable way to overcome the limited supply of Pt is the development of high-efficiency Pt-free CE materials, which should possess both high electrical conductivity and superior electrocatalytic activity simultaneously. Herein, for the first time, a two-step strategy to synthesize ruthenium dioxide (RuO₂) nanocrystals is reported, and it is shown that RuO₂ catalysts exhibit promising electrocatalytic activity towards triiodide reduction, which results in comparable energy conversion efficiency to that of conventional Pt CEs. More importantly, by virtue of first-principles calculations, the catalytic mechanism of electrocatalysis for triiodide reduction on various CEs is investigated systematically and it is found that the electrochemical triiodide reduction reaction on RuO₂ catalyst surfaces can be enhanced significantly, owing to the ideal combination of good electrocatalytic activity and high electrical conductivity.

CO oxidation on single and multilayer Pd oxides on Pd(111): mechanistic insights from RAIRS

Strong binding on oxygen vacancies and metallic domains promotes CO oxidation on partially-reduced PdO(101), while adsorption only on metallic sites promotes CO oxidation when 2D oxide coexists with Pd(111).

Development of a ReaxFF potential for Pt–O systems describing the energetics and dynamics of Pt-oxide formation

A ReaxFF force field description of Pt–O systems has been developed, validated and applied to oxygen diffusion on Pt(111).

Gas-phase perspective on the thermodynamics and kinetics of heterogeneous catalysis

Gas-phase studies of small transition metal cluster cations provide thermochemistry of utility to surface science and heterogeneous catalysis.