Surface Chemistry of Ruthenium Dioxide in Heterogeneous Catalysis and Electrocatalysis: From Fundamental to Applied Research

Solvent and additive-free efficient aerobic oxidation of alcohols by a perovskite oxide-based heterogeneous catalyst

A perovskite oxide has been utilized for the solvent and additive-free heterogeneous oxidation of various alcohols.

RuO2 photodeposited on W-doped and Cr-doped TiO2 nanotubes with enhanced photoelectrochemical water splitting and capacitor properties

Two series of highly ordered Cr-doped TiO₂ (CT) and W-doped TiO₂ (WT) nanotubes were prepared by in situ anodizing and modified by photodeposition of RuO₂ for water splitting study.

Synergistic activity of binary metal sulphide WS2–RuS2 nanospheres for the electrochemical detection of the antipsychotic drug promazine

Schematic presentation for the synthesis of tungsten disulfide–ruthenium disulfide (WS₂–RuS₂) nanospheres and application for the electrochemical determination of antipsychotic drug promazine.

Au@Co2P core/shell nanoparticles as a nano-electrocatalyst for enhancing the oxygen evolution reaction

Core/shell nanoparticles (NPs) of Au@Co₂P, each comprising a Au core with a Co₂P shell, were prepared, and shown to efficiently catalyze the oxygen evolution reaction (OER). In particular, Au@Co₂P has a small overpotential of 321 mV at 10 mA cm^-2 in 1 M KOH aqueous solution at room temperature, which is about 95 mV less than pure Co₂P. More importantly, the Tafel slope of Au@Co₂P, at 57 mV dec^-1, is 44 mV dec^-1 lower than that of Co₂P. Hence, Au@Co₂P outperforms Co₂P drastically in practical production when a high current density is required.

Amorphous Oxide Nanostructures for Advanced Electrocatalysis

Amorphous oxides have attracted special attention as advanced electrocatalysts owing to their unique local structural flexibility and attractive electrocatalytic properties. With abundant randomly oriented bonds and surface-exposed defects (e.g., oxygen vacancies) as active catalytic sites, the adsorption/desorption of reactants can be optimized, leading to superior catalytic activities. Amorphous oxide materials have found wide electrocatalytic applications ranging from hydrogen evolution and oxygen evolution to oxygen reduction, CO₂ electroreduction and nitrogen electroreduction. The amorphous oxide electrocatalysts even outperform their crystalline counterparts in terms of electrocatalytic activity and stability. Despite of the merits and achievements for amorphous oxide electrocatalysts, there are still issues and challenges existing for amorphous oxide electrocatalysts. There are rarely reviews specifically focusing on amorphous oxide electrocatalysts and therefore it is imperative to have a comprehensive overview of the research progress and to better understand the achievements and issues with amorphous oxide electrocatalysts. In this minireview, several general preparation methods for amorphous oxides are first introduced. Then, the achievements in amorphous oxides for several important electrocatalytic reactions are summarized. Finally, the challenges and perspectives for the development of amorphous oxide electrocatalysts are outlined.

Ruthenium Nanoparticles for Catalytic Water Splitting

Both global warming and limited fossil resources make the transition from fossil to solar fuels an urgent matter. In this regard, the splitting of water activated by sunlight is a sustainable and carbon-free new energy conversion scheme able to produce efficient technological devices. The availability of appropriate catalysts is essential for the proper kinetics of the two key processes involved, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). During the last decade, ruthenium nanoparticle derivatives have emerged as true potential substitutes for the state-of-the-art platinum and iridium oxide species for the HER and OER, respectively. Thus, after a summary of the most common methods for catalyst benchmarking, this review covers the most significant developments of ruthenium-based nanoparticles used as catalysts for the water-splitting process. Furthermore, the key factors that govern the catalytic performance of these nanocatalysts are discussed in view of future research directions.

Recent Advancements Towards Closing the Gap between Electrocatalysis and Battery Science Communities: The Computational Lithium Electrode and Activity-Stability Volcano Plots

Despite of the fact that the underlying processes are of electrochemical nature, electrocatalysis and battery research are commonly perceived as two disjointed research fields. Herein, recent advancements towards closing this apparent community gap by discussing the concepts of the constrained ab initio thermodynamics approach and the volcano relationship, which were originally introduced for studying heterogeneously catalyzed reactions by first-principles methods at the beginning of the 21st century, are summarized. The translation of the computational hydrogen electrode (CHE) approach or activity-based volcano plots to a computational lithium electrode (CLiE) or activity-stability volcano plots, respectively, for the investigation of electrode surfaces in batteries may refine theoretical modeling with the aim that enhancements of the underlying concepts are transferred between the research communities. The presented strategy of developing novel approaches by interdisciplinary research activities may trigger further progress of improved theoretical concepts in the near future.

Ru/CeO2 Catalyst with Optimized CeO2 Support Morphology and Surface Facets for Propane Combustion

Tailoring the interfaces between active metal centers and supporting materials is an efficient strategy to obtain a superior catalyst for a certain reaction. Herein, an active interface between Ru and CeO₂ was identified and constructed by adjusting the morphology of CeO₂ support, such as rods (R), cubes (C), and octahedra (O), to optimize both the activity and the stability of Ru/CeO₂ catalyst for propane combustion. We found that the morphology of CeO₂ support does not significantly affect the chemical states of Ru species but controls the interaction between the Ru and CeO₂, leading to the tuning of oxygen vacancy in the CeO₂ surface around the Ru-CeO₂ interface. The Ru/CeO₂ catalyst possesses more oxygen vacancy when CeO₂-R with predominantly exposed CeO₂{110} surface facets is used, providing a higher ability to adsorb and activate oxygen and propane. As a result, the Ru/CeO₂-R catalyst exhibits higher catalytic activity and stability for propane combustion compared with the Ru/CeO₂-C and Ru/CeO₂-O catalysts. This work highlights a new strategy for the design of efficient metal/CeO₂ catalysts by engineering morphology and associated surface facet of CeO₂ support for the elimination of light alkane pollutants and other volatile organic compounds.

Rational approach to guest confinement inside MOF cavities for low-temperature catalysis

Geometric or electronic confinement of guests inside nanoporous hosts promises to deliver unusual catalytic or opto-electronic functionality from existing materials but is challenging to obtain particularly using metastable hosts, such as metal–organic frameworks (MOFs). Reagents (e.g. precursor) may be too large for impregnation and synthesis conditions may also destroy the hosts. Here we use thermodynamic Pourbaix diagrams (favorable redox and pH conditions) to describe a general method for metal-compound guest synthesis by rationally selecting reaction agents and conditions. Specifically we demonstrate a MOF-confined RuO₂ catalyst (RuO₂@MOF-808-P) with exceptionally high catalytic CO oxidation below 150 °C as compared to the conventionally made SiO₂-supported RuO₂ (RuO₂/SiO₂). This can be caused by weaker interactions between CO/O and the MOF-encapsulated RuO₂ surface thus avoiding adsorption-induced catalytic surface passivation. We further describe applications of the Pourbaix-enabled guest synthesis (PEGS) strategy with tutorial examples for the general synthesis of arbitrary guests (e.g. metals, oxides, hydroxides, sulfides).

Loading guests inside the pre-existing pores of nanoporous hosts remains challenging. Here, the authors introduce a rational route for incorporation of guest compounds into an arbitrary nanoporous host, enabling the investigation of multiple host-guest systems with surprising functionalities.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

Shaping well-defined noble-metal-based nanostructures for fabricating high-performance electrocatalysts: advances and perspectives

This review highlights recent advances in shaping protocols and structure-activity relationships of noble-metal-based catalysts with well-defined nanostructures in electrochemical reactions.

Highly effective ruthenium-doped TiO2nanoparticles photocatalyst for visible-light-driven photocatalytic hydrogen production

This paper reports the synthesis of bare TiO₂and various molar concentrations of ruthenium (Ru)-doped TiO₂nanoparticles by the precipitation method.

Evidence for tetranuclear bis-μ-oxo cubane species in molecular iridium-based water oxidation catalysts from XAS analysis

Latest EXAFS results suggest a ‘dimer-of-dimers’ as the dominant resting state of Ir-pyalk WOCs in aqueous solution.

Applications of 2D MXenes in energy conversion and storage systems

This article provides a comprehensive review of MXene materials and their energy-related applications.

Measurements of Oxygen Electroadsorption Energies and Oxygen Evolution Reaction on RuO2(110): A Discussion of the Sabatier Principle and Its Role in Electrocatalysis

We report the hydroxide (OH_ad) and oxide (O_ad) experimental electroadsorption free energies, their dependences on pH, and their correlations to the oxygen evolution reaction (OER) electrocatalysis on RuO₂(110) surface. The Sabatier principle predicts that catalyst is most active when the intermediate stabilization is moderate, not too strong such that the bound intermediate disrupts the subsequent catalytic cycle, nor too weak such that the surface is ineffective. For decades, researchers have used this concept to rationalize the activity trend of many OER electrocatalysts including RuO₂, which is among the state-of-the-art OER catalysts. In this article, we report an experimental assessment of the Sabatier principle by comparing the oxygen electroadsorption energy to the OER electrocatalysis for the first time on RuO₂. We find that the OH_ad and O_ad electroadsorption energies on RuO₂(110) depend on pH and obey the scaling relation. However, we did not observe a direct correlation between the OH_ad and O_ad electroadsorption energies and the OER activity in the comparative analysis that includes both RuO₂(110) and IrO₂(110). Our result raises a question of whether the Sabatier principle can describe highly active electrocatalysts, where the kinetic aspects may influence the electrocatalysis more strongly than the electroadsorption energy, which captures only the thermodynamics of the intermediates and not yet kinetics.

Oxidized Laser-Induced Graphene for Efficient Oxygen Electrocatalysis

An efficient metal-free catalyst is presented for oxygen evolution and reduction based on oxidized laser-induced graphene (LIG-O). The oxidation of LIG by O₂ plasma to form LIG-O boosts its performance in the oxygen evolution reaction (OER), exhibiting a low onset potential of 260 mV with a low Tafel slope of 49 mV dec^-1 , as well as an increased activity for the oxygen reduction reaction. Additionally, LIG-O shows unexpectedly high activity in catalyzing Li₂ O₂ decomposition in Li-O₂ batteries. The overpotential upon charging is decreased from 1.01 V in LIG to 0.63 V in LIG-O. The oxygen-containing groups make essential contributions, not only by providing the active sites, but also by facilitating the adsorption of OER intermediates and lowering the activation energy.

Methanol oxidation on stoichiometric and oxygen-rich RuO2(110)

We used temperature-programmed reaction spectroscopy (TPRS) to investigate the adsorption and oxidation of methanol on stoichiometric and O-rich RuO₂(110) surfaces. We find that the complete oxidation of CH₃OH is strongly preferred on stoichiometric RuO₂(110) during TPRS for initial CH₃OH coverages below ∼0.33 ML (monolayer), and that partial oxidation to mainly CH₂O becomes increasingly favored with increasing CH₃OH coverage from 0.33 to 1.0 ML. We present evidence that an adsorbed CH₂O₂ species serves as the key intermediate to complete oxidation and that CH₂O₂ formation is intrinsically facile but becomes limited by the availability of bridging O-atoms on stoichiometric RuO₂(110) at initial CH₃OH coverages above 0.33 ML. We show that methanol molecules adsorbed in excess of 0.33 ML dehydrogenate to mainly CH₂O and desorb during TPRS, with adsorbed CH₃O groups mediating the evolution of both CH₂O and CH₃OH. We find that O-rich RuO₂(110) surfaces are also highly active toward methanol oxidation and that selectivity toward the complete oxidation of methanol increases markedly with increasing coverage of on-top O-atoms (O_ot) on RuO₂(110). Our results demonstrate that CH₃OH species adsorbed within O_ot-rich domains react efficiently during TPRS, in parallel with reaction of CH₃OH adsorbed initially on cus-Ru sites. The data suggests that the facile hydrogenation of O_ot atoms and the resulting desorption of H₂O at low-temperature (<∼400 K) provides an efficient pathway for restoring reactive O-atoms and thereby promoting complete oxidation of methanol on the O-rich RuO₂(110) surface.

Adsorption of alkanes on stoichiometric and oxygen-rich RuO2(110)

We investigated the molecular adsorption of methane, ethane, propane and n-butane on stoichiometric and oxygen-rich RuO2(110) surfaces using temperature-programmed desorption (TPD) and dispersion-corrected density functional theory (DFT-D3) calculations. We find that each alkane adsorbs strongly on the coordinatively-unsaturated Ru (Rucus) atoms of s-RuO2(110), with desorption from this state producing a well-defined TPD peak at low alkane coverage. As the coverage increases, we find that alkanes first form a compressed layer on the Rucus atoms and subsequently adsorb on the bridging O atoms of the surface until the monolayer saturates. DFT-D3 calculations predict that methane preferentially adsorbs on top of a Rucus atom and that the C2 to C4 alkanes preferentially adopt bidentate configurations in which each molecule aligns parallel to the Rucus atom row and datively bonds to neighboring Rucus atoms. DFT-D3 predicts binding energies that agree quantitatively with our experimental estimates for alkane σ-complexes on RuO2(110). We find that oxygen atoms adsorbed on top of Rucus atoms (Oot atoms) stabilize the adsorbed alkane complexes that bind in a given configuration, while also blocking the sites needed for σ-complex formation. This site blocking causes the coverage of the most stable, bidentate alkane complexes to decrease sharply with increasing Oot coverage. Concurrently, we find that a new peak develops in the C2 to C4 alkane TPD spectra with increasing Oot coverage, and that the desorption yield in this TPD feature passes through a maximum at Oot coverages between ∼50% and 60%. We present evidence that the new TPD peak arises from C2 to C4 alkanes that adsorb in upright, monodentate configurations on stranded Rucus sites located within the Oot layer.

The Redox Chemistry of Ruthenium Dioxide: A Cyclic Voltammetry Study—Review and Revision

By cyclic voltammetry at high scan rates, the electrochemical properties of RuO2 in acidic and alkaline solutions were investigated in detail. Thirteen current peaks can be distinguished in sulfuric acid and sodium hydroxide. With respect to the pH sensitivity of RuO2 electrodes, we considered charge calculations, peak currents, and apparent diffusion coefficients. The nature of the Ru(II) oxidation was clarified by Ru(I)−Ru(III) species.

Elucidating ultrafast electron dynamics at surfaces using extreme ultraviolet (XUV) reflection–absorption spectroscopy

Time-resolved XUV reflection–absorption spectroscopy probes core-to-valence transitions to reveal state-specific electron dynamics at surfaces.

A review of anion-regulated multi-anion transition metal compounds for oxygen evolution electrocatalysis

Recent advances in the anion regulation on multi-anion transition metal compounds as electrocatalysts for oxygen evolution reaction are reviewed.

Impact of Various Acids and Bases on the Voltammetric Response of Platinum Group Metal Oxides

The voltammetric response of platinum metal oxides is discussed with respect to novel pH sensors combining both miniaturization and stability. For practical applications in solutions of any kind, for example, in tap water and in domestic sewage, various interferences must be considered, such as chloride and reducing agents. This work clarifies the voltammetric behavior of RuO2 electrodes in solutions of different pH values and ionic strengths.

Oxygen evolution on Fe-doped NiO electrocatalysts deposited via microplasma

The oxygen evolution reaction (OER) in alkaline media was investigated on nanostructured Fe₂O₃, NiO, and Ni_1-xFe_xO (Fe-doped, rocksalt NiO, x = 0.05-0.19) electrocatalysts deposited via microplasma on indium tin oxide. A detailed investigation of film morphology, structure, and chemical surface state using SEM, XRD, and XPS, respectively, was carried out to understand catalytic activity, which was assessed using cyclic voltammetry and chronopotentiometry. Iron was seen to be fully incorporated into the parent rocksalt NiO lattice during microplasma deposition, and overpotentials (η) decreased from 360 mV for NiO to 310 mV for Ni_1-xFe_xO at 10 mA cm^-2. Interestingly, overpotential did not change significantly for Fe compositions from 5-19%. The Ni_1-xFe_xO films displayed relatively low Tafel slopes of 20-30 mV dec^-1 at 0.01-1 mA cm^-2, demonstrating their high activity for (OER). Turn-over-frequency (TOF, i.e., O₂ molecules per Ni atom per s) at η = 350 mV revealed a continuous improvement in activity of the NiO surface with increasing Fe content, where values of 0.07 and 0.48 s^-1 were measured for undoped NiO and Ni_0.81Fe_0.19O films, respectively. Chronopotentiometry measurements followed by SEM and XPS verified that the as-deposited Ni_1-xFe_xO catalysts were mechanically and chemically stable for OER under alkaline conditions. This work highlights that microplasma-based deposition is a general approach to realize conformal coatings of nanostructured, doped oxides with high activity for OER.