Visualization of Atomic Processes on Ruthenium Dioxide using Scanning Tunneling Microscopy

Electrocatalysis: From Planar Surfaces to Nanostructured Interfaces

The reactions critical for the energy transition center on the chemistry of hydrogen, oxygen, carbon, and the heterogeneous catalyst surfaces that make up electrochemical energy conversion systems. Together, the surface-adsorbate interactions constitute the electrochemical interphase and define reaction kinetics of many clean energy technologies. Practical devices introduce high levels of complexity where surface roughness, structure, composition, and morphology combine with electrolyte, pH, diffusion, and system level limitations to challenge our ability to deconvolute underlying phenomena. To make significant strides in materials design, a structured approach based on well-defined surfaces is necessary to selectively control distinct parameters, while complexity is added sequentially through careful application of nanostructured surfaces. In this review, we cover advances made through this approach for key elements in the field, beginning with the simplest hydrogen oxidation and evolution reactions and concluding with more complex organic molecules. In each case, we offer a unique perspective on the contribution of well-defined systems to our understanding of electrochemical energy conversion technologies and how wider deployment can aid intelligent materials design.

Mechanistic insights into the contribution of Lewis acidity to brominated VOCs combustion over titanium oxide supported Ru catalyst

CH₃Br catalytic oxidation as the probe reaction was investigated over Ru supported on TiO₂ with different crystalline phases. 1% Ru/anatase TiO₂ (a-TiO₂) exhibited superior stability at 240 °C after a 180 h time-on-stream run. And there was an induced activation for 1% Ru/a-TiO₂ during the initial 60 h reaction. Then the activity sustained stable. To elucidate the intrinsic mechanism, a series of characterizations were performed such as XRD, CO-Pulse, H₂-TPR, XPS and NH₃-TPD etc. Results showed that the Ru particle size increased and the Ru⁰ content decreased as the reaction proceeded, which were not conductive to the reaction. It was assumed that the catalytic activity was strongly dependent on other factors. In combination with NH₃-TPD and Py-FTIR measurements, it was confirmed that the enhanced activity and stability was strongly associated with the surface acidity, especially moderate strong Lewis acid (L acid). The increase of the acid amount and acidity strength was led by the generation and adsorption of HBr, Br₂ and RuO_xBr_y during the reaction, among which HBr and Br₂ was easier to desorb at 250 °C. While moderate strong L acid was sourced from the formation of RuO_xBr_y. The addition of transition metal (Ce, Co, Mn, Nb and Ni) further validated that the moderate strong L acid played a decisive role in the CH₃Br catalytic oxidation.

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.

Stable reconstruction of the (110) surface and its role in pseudocapacitance of rutile-like RuO2

Surfaces of rutile-like RuO₂, especially the most stable (110) surface, are important for catalysis, sensing and charge storage applications. Structure, chemical composition, and properties of the surface depend on external conditions. Using the evolutionary prediction method USPEX, we found stable reconstructions of the (110) surface. Two stable reconstructions, RuO₄-(2 × 1) and RuO₂-(1 × 1), were found, and the surface phase diagram was determined. The new RuO₄-(2 × 1) reconstruction is stable in a wide range of environmental conditions, its simulated STM image perfectly matches experimental data, it is more thermodynamically stable than previously proposed reconstructions, and explains well pseudocapacitance of RuO₂ cathodes.

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.

Direct Visualization of Catalytically Active Sites at the FeO-Pt(111) Interface

Within the area of surface science, one of the "holy grails" is to directly visualize a chemical reaction at the atomic scale. Whereas this goal has been reached by high-resolution scanning tunneling microscopy (STM) in a number of cases for reactions occurring at flat surfaces, such a direct view is often inhibited for reaction occurring at steps and interfaces. Here we have studied the CO oxidation reaction at the interface between ultrathin FeO islands and a Pt(111) support by in situ STM and density functional theory (DFT) calculations. Time-lapsed STM imaging on this inverse model catalyst in O2 and CO environments revealed catalytic activity occurring at the FeO-Pt(111) interface and directly showed that the Fe-edges host the catalytically most active sites for the CO oxidation reaction. This is an important result since previous evidence for the catalytic activity of the FeO-Pt(111) interface is essentially based on averaging techniques in conjunction with DFT calculations. The presented STM results are in accord with DFT+U calculations, in which we compare possible CO oxidation pathways on oxidized Fe-edges and O-edges. We found that the CO oxidation reaction is more favorable on the oxidized Fe-edges, both thermodynamically and kinetically.

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).

High chemical activity of a perovskite surface: reaction of CO with Sr(3)Ru(2)O(7)

Adsorption of CO at the Sr(3)Ru(2)O(7)(001) surface was studied with low-temperature scanning tunneling microscopy (STM) and density functional theory. In situ cleaved single crystals terminate in an almost perfect SrO surface. At 78 K, CO first populates impurities and then adsorbs above the apical surface O with a binding energy E(ads)=-0.7  eV. Above 100 K, this physisorbed CO replaces the surface O, forming a bent CO(2) with the C end bound to the Ru underneath. The resulting metal carboxylate (Ru-COO) can be desorbed by STM manipulation. A low activation (0.2 eV) and high binding (-2.2  eV) energy confirm a strong reaction between CO and regular surface sites of Sr(3)Ru(2)O(7); likely, this reaction causes the "UHV aging effect" reported for this and other perovskite oxides.

Capacitive performance enhancements of RuO2 nanocrystals through manipulation of preferential orientation growth originated from the synergy of Pluronic F127 trapping and annealing

The capacitive performances of RuO2 prepared by oxidation precipitation of Ru precursors (RuCl3·xH2O) surrounded with tri-block co-polymer, Pluronic F127, in aqueous media can be enhanced through manipulating its preferential orientation growth of nanocrystals. From the heterogeneous surface chemistry viewpoints with the support of structure characterizations, such enhancement originates from the preferential orientation growth of the {101} facet due to the adsorption of the highly polarisable, non-ionic ligands of Pluronic F127 on the high surface energy facets on RuO2 nanocrystallites. In this case, the F127-trapped sample with annealing at 300 °C enhances the specific capacitance 1.6-fold in comparison to its counterpart without F127. With the mechanistic insight into the heterogeneous surface crystal growth pathways, our results materialize the development of RuO2 with tuneable capacitive performances. Furthermore, due to the different propagation models of RuO2 with and without F127 trapping, a schematic diagram is proposed to interpret such a unique crystal growth evolution phenomenon.

Ruthenia-based electrochemical supercapacitors: insights from first-principles calculations

Electrochemical supercapacitors (ECs) have important applications in areas wherethe need for fast charging rates and high energy density intersect, including in hybrid and electric vehicles, consumer electronics, solar cell based devices, and other technologies. In contrast to carbon-based supercapacitors, where energy is stored in the electrochemical double-layer at the electrode/electrolyte interface, ECs involve reversible faradaic ion intercalation into the electrode material. However, this intercalation does not lead to phase change. As a result, ECs can be charged and discharged for thousands of cycles without loss of capacity. ECs based on hydrous ruthenia, RuO2·xH2O, exhibit some of the highest specific capacitances attained in real devices. Although RuO2 is too expensive for widespread practical use, chemists have long used it as a model material for investigating the fundamental mechanisms of electrochemical supercapacitance and heterogeneous catalysis. In this Account, we discuss progress in first-principles density-functional theory (DFT) based studies of the electronic structure, thermodynamics, and kinetics of hydrous and anhydrous RuO2. We find that DFT correctly reproduces the metallic character of the RuO2 band structure. In addition, electron-proton double-insertion into bulk RuO2 leads to the formation of a polar covalent O-H bond with a fractional increase of the Ru charge in delocalized d-band states by only 0.3 electrons. This is in slight conflict with the common assumption of a Ru valence change from Ru(4+) to Ru(3+). Using the prototype electrostatic ground state (PEGS) search method, we predict a crystalline RuOOH compound with a formation energy of only 0.15 eV per proton. The calculated voltage for the onset of bulk proton insertion in the dilute limit is only 0.1 V with respect to the reversible hydrogen electrode (RHE), in reasonable agreement with the 0.4 V threshold for a large diffusion-limited contribution measured experimentally. DFT calculations also predict that proton diffusion in RuO2 is hindered by a migration barrier of 0.8 eV, qualitatively explaining the observed strong charging rate-dependence of the diffusion-limited contribution. We found that reversible adsorption of up to 1.5 protons per Ru on the (110) surface contributes to the measured capacitive current at higher voltages. PEGS-derived models of the crystal structure of hydrated ruthenia show that incorporation of water in Ru vacancies or in bulk crystals is energetically much more costly than segregation of water molecules between slabs of crystalline RuO2. These results lend support to the so-called "water at grain boundaries" model for the structure of hydrous RuO2·xH2O. This occurs where metallic nanocrystals of RuO2 are separated by grain boundary regions filled with water molecules. Chemists have attributed the superior charge storage properties of hydrous ruthenia to the resulting composite structure. This facilitates fast electronic transport through the metallic RuO2 nanocrystals and fast protonic transport through the regions of structural water at grain boundaries.

Structure-Activity Correlations for the Oxidation of CO over Polycrystalline RuO2 Powder Derived from Steady-State and Transient Kinetic Experiments

The oxidation of carbon monoxide was studied at atmospheric pressure in a plug-flow reactor over polycrystalline ruthenium dioxide powder in the temperature range from 363 to 453 K as a function of the pretreatment. Calcining RuO2 in flowing oxygen resulted in purified bulk RuO2, whereas reduction in hydrogen led to bulk Ru metal, which was partially oxidized again in flowing oxygen at increasing temperatures (T ox) up to 573 K to obtain RuO2/Ru shell-core particles with increasing RuO2 shell thickness. Using the TPR technique subsequent to steady-state CO oxidation to monitor the degree of oxidation, the most active and stable state of the unsupported ruthenium catalysts was identified as an ultra-thin RuO2 layer covering a metallic Ru core in agreement with the shell-core model established for supported Ru catalysts. Steady-state turnover frequencies (TOFs) obtained with the ultra-thin RuO2 films are in good agreement with TOFs reported for studies on Ru single crystal surfaces and with supported Ru catalysts. Only for RuO2 films thicker than 1 nm (T ox ≥ 473 K) and for fully oxidized RuO2 deactivation was observed, presumably due to the formation of inactive RuO2 surfaces such as the RuO2(100)-c(2×2) facet. Moreover, it was demonstrated that the presence of moisture in the reactant feed inhibits the oxidation of CO completely.

Effects of coverage on the structures, energetics, and electronics of oxygen adsorption on RuO2(110)

Plane-wave supercell DFT calculations within the PW91 generalized gradient approximation are used to examine the influence of oxygen coverage on the structure, energetics, and electronics of the RuO2(110) surface. Filling of O(br) and O(cus) sites is exothermic with respect to molecular O2 at all coverages and causes changes in local Ru electronic structure consistent with the changing metal coordination. By fitting the surface energies of a large number of surface configurations to a two-body interaction model, an O atom is calculated to be bound by 2.55 eV within a filled O(br) row and by 0.98 eV along an otherwise vacant O(cus) row. Lateral interactions modify these binding energies by up to 20%. O(cus)-O(cus) interactions are repulsive and diminish binding energy with increasing O(cus) filling. Due to the favorable relief of local strain, O(br)-O(br) interactions are attractive and favor filling of neighbor br sites. These interaction effects are relatively modest in absolute magnitude but are large enough to influence the ability of the RuO2(110) surface to promote oxidation of relatively weak reductants, such as NO and C2H4.

Unusual Process of Water Formation on RuO2(110) by Hydrogen Exposure at Room Temperature

The reduction mechanism of the RuO(2)(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO(2)(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (O(br)-H) species and subsequently part of these O(br)-H groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.