Single crystal RuO2 (110): Surface structure

The surface structure of RuO₂ (110) has been studied with LEED, AES and XPS. The “as-grown” surface shows no LEED patterns and both AES and XPS indicate that the surface is depleted in oxygen in high vacuum. After extensive annealing in an O₂ atmosphere reproducible LEED patterns characteristic of the (110) surface were obtained. For the well-ordered surface the oxygen XPS results revealed oxygen associated with the bulk RuO₂, the presence of RuO₃ and oxygen bound to surface atoms.     

Adsorption and interaction of ethylene on RuO2(110) surfaces

Ethylene (C2H4) adsorbed on the stoichiometric and oxygen-rich RuO2(110) surfaces, exposing coordinatively unsaturated Ru-cus and O-cus atoms, is investigated by applying high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy in combination with isotope labeling experiments. On the stoichiometric RuO2(110) surface C2H4 adsorbs and desorbs molecularly. In contrast, on the oxygen-rich RuO2(110) surface ethylene adsorbs molecularly at 85 K and is completely oxidized through interaction with O-cus and O-bridge upon annealing to 500 K. The first couple of reactions are observed at 200 K taking place on Ru-cus: A change from pi- to sigma-bonding, formation of -C=O and -C-O groups, and dehydrogenation giving rise to H2O adsorbed at Ru-cus. Maximum reaction rate is reached for C2H4 chemisorbed at Ru-cus with O-cus neighbors on each side. A model for the first couple of reactions is sketched. For the final combustion, C2H4 reacts both with O-cus and O-bridge. Ethylene oxide is not detected under any circumstance.     

Adsorption of platinum on the stoichiometric RuO2(110) surface

Density functional theory was used to calculate the geometries and electronic structures of Pt adsorption on the stoichiometric RuO(2)(110) surface at different coverages. The calculated results revealed that the Pt atoms strongly adsorb on RuO(2), and two-dimensional growth up to 1.25 ML deposition is energetically favorable. At low coverage, the binding between Pt and RuO(2) is very strong, accompanied by a significant transfer of electron density from Pt to the support and a large downshift of the d-band compared to that of the unsupported Pt. At high coverage, a weak interaction of RuO(2) with the Pt cluster is observed, and the electronic structure of Pt is only slightly modified with respect to that of the unsupported material. Our results suggest that among the systems investigated, the RuO(2)-supported Pt at a coverage of 1 ML may become one of the best alternatives to pure Pt as a catalyst because it combines a high stability and a moderate activity similar to Pt.     

Electrooxidation of CO on Ru（0001） and RuO2（ 100） Electrode Surfaces

The electrooxidation of CO on Ru(0001) and RuO2(100) electrode surfaces were characterized by cyclic voltammetry,AES and RHEED,The CO adlayer was first partially oxidized at 0.8 V, which is controlled by the attack of oxygen species toward the Ru(0001) surface. The remaining CO aldayer oxidation at 0.55 V is related to the combination of CO molecules with oxygen species already located on the surface,In contrast,successive peaks on RuO2(100) at 0.4 V and 0.72 V are observed ,which shows that CO molecules can directly react with two different lattice-oxygen on the surface to carbon dioxide.     

Inhibition of CO oxidation on RuO2(110) by adsorbed H2O molecules

Catalytic CO oxidation on the RuO(2)(110) surface was studied at 300 K by scanning tunneling microscopy (STM), high-resolution electron-energy-loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). Upon repeatedly exposing the surface to several 10 L of CO and O(2) at 300 K, STM shows that unreactive features accumulate with each CO and O(2) titration run. HREELS and TDS show formation of increasing amounts of H(2)O, retarded formation of O-cus atoms and incomplete removal of CO-bridge molecules during O(2) dosing, and a changing ratio of single- and double-bonded CO-bridge molecules. It is concluded that H(2)O (presumably from the residual gas) is accumulating at the Ru-cus sites thus blocking them, so that the dissociative adsorption of oxygen is prevented and the CO oxidation reaction is suppressed. Some 10% CO- bridge remains on the surface even during oxygen exposure. Consistent with this interpretation, deactivation of the surface is suppressed at 350 K, at the onset of H(2)O desorption.     

Theoretical investigation on RuO2 nanoclusters adsorbed on TiO2 rutile (110) and anatase (101) surfaces

Minimizing the energy of an $N$ -electron system as a functional of a two-electron reduced density matrix (2-RDM), constrained by necessary $N$ -representability conditions (conditions for the 2-RDM to represent an ensemble $N$ -electron quantum system), yields a rigorous lower bound to the ground-state energy in contrast to variational wave function methods. We characterize the performance of two sets of approximate constraints, (2,2)-positivity (DQG) and approximate (2,3)-positivity (DQGT) conditions, at capturing correlation in one-dimensional and quasi-one-dimensional (ladder) Hubbard models. We find that, while both the DQG and DQGT conditions capture both the weak and strong correlation limits, the more stringent DQGT conditions improve the ground-state energies, the natural occupation numbers, the pair correlation function, the effective hopping, and the connected (cumulant) part of the 2-RDM. We observe that the DQGT conditions are effective at capturing strong electron correlation effects in both one- and quasi-one-dimensional lattices for both half filling and less-than-half filling.     

RuO2-wired high-rate nanoparticulate TiO2 (anatase): Suppression of particle growth using silica

To enhance the high-rate capability (up to 120 C, 20 A/g) of nanoparticulate TiO₂ (anatase) formed by thermal treatment of protonated TiO₂ nanotubes, we used two types of additives: RuO₂ as an electron-conductive material [Y.-G. Guo, Y.-S. Hu, W. Sigle, J. Maier, Adv. Mater. 19 (2007) 2087] and silica as a suppressant of particle growth during heat treatment. We show systematically that both additives, when used separately, improve the high-rate performance of anatase by 25–55 mA h/g at 60 C. The combined use of both additives in a total amount of merely 2.5 wt.% leads to an improvement of more than 70 mA h/g at 60 C. The underlying mechanisms for these significant effects are briefly discussed.     

Catalytic oxidation of ammonia on RuO2(110) surfaces: mechanism and selectivity

The selective oxidation of ammonia to either N2 or NO on RuO2(110) single-crystal surfaces was investigated by a combination of vibrational spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and steady-state rate measurements under continuous flow conditions. The stoichiometric RuO2(110) surface exposes coordinatively unsaturated (cus) Ru atoms onto which adsorption of NH3 (NH3-cus) or dissociative adsorption of oxygen (O-cus) may occur. In the absence of O-cus, ammonia desorbs completely thermally without any reaction. However, interaction between NH3-cus and O-cus starts already at 90 K by hydrogen abstraction and hydrogenation to OH-cus, leading eventually to N-cus and H2O. The N-cus species recombine either with each other to N2 or with neighboring O-cus leading to strongly held NO-cus which desorbs around 500 K. The latter reaction is favored by higher concentrations of O-cus. Under steady-state flow condition with constant NH3 partial pressure and varying O2 pressure, the rate for N2 formation takes off first, passes through a maximum and then decreases again, whereas that for NO production exhibits an S-shape and rises continuously. In this way at 530 K almost 100% selectivity for NO formation (with fairly high reaction probability for NH3) is reached.     

Mechanism and Tafel lines of electro-oxidation of water to oxygen on RuO2(110)

How to efficiently oxidize H(2)O to O(2) (H(2)O → 1/2O(2) + 2H(+) + 2e(-)) is a great challenge for electrochemical/photo water splitting owing to the high overpotential and catalyst corrosion. Here extensive periodic first-principles calculations integrated with modified-Poisson-Boltzmann electrostatics are utilized to reveal the physical origin of the high overpotential of the electrocatalytic oxygen evolution reaction (OER) on RuO(2)(110). By determining the surface phase diagram, exploring the possible reaction channels, and computing the Tafel lines, we are able to elucidate some long-standing puzzles on the OER kinetics from the atomic level. We show that OER occurs directly on an O-terminated surface phase above 1.58 V vs NHE, but indirectly on a OH/O mixed phase below 1.58 V by converting first the OH/O mixed phase to the O-terminated phase locally. The rate-determining step of OER involves an unusual water oxidation reaction following a Eley-Rideal-like mechanism, where a water molecule from solution breaks its OH bond over surface Os with concurrent new O-OH bond formation. The free energy barrier is 0.74 eV at 1.58 V, and it decreases linearly with the increase of potential above 1.58 V (a slope of 0.56). In contrast, the traditionally regarded surface oxygen coupling reaction with a Langmuir-Hinshelwood mechanism is energetically less favored and its barrier is weakly affected by the potential. Fundamentally, we show that the empirical linear barrier~potential relation is caused by the linear structural response of the solvated transition state to the change of potential. Finally, the general strategy for finding better OER anode is also presented.     

Crystal growth, observation, and characterization of the low-temperature structure of the fluorite-related ruthenates: Sm(3)RuO(7) and Eu(3)RuO(7)

The compounds Sm(3)RuO(7) and Eu(3)RuO(7) were grown as single crystals from molten hydroxide fluxes. They crystallize in the orthorhombic space group Cmcm and are part of a well-known family of fluorite-related oxides of stoichiometry Ln(3)MO(7). This structure contains rare earth cations in two different coordination environments, 8-fold pseudocubic and 7-fold pentagonal bipyramidal, and contains Ru(V) cations that are octahedrally coordinated. The RuO(6) octahedra are trans vertex-sharing to yield chains oriented along the c-axis. Upon cooling, single crystals of Sm(3)RuO(7) and Eu(3)RuO(7) undergo a structural transition at 190 and 280 K, respectively, from space group Cmcm to P2(1)nb. The structure transition results in a loss of lattice centering, a doubling of the b-axis, a distortion of the vertex-shared Ru-O chains, and a reduction in the coordination of one of the rare earth cations from 8-fold to 7-fold. Accompanying this structural transition are anomalies in the magnetic susceptibility at about 190 and 280 K for Sm(3)RuO(7) and Eu(3)RuO(7), respectively. The structures of these low-temperature phases of Ln(3)RuO(7) have been determined for the first time and are described.     

Synthesis and characterization of a magnetic semiconductor Na(2)RuO(4) containing one-dimensional chains of Ru(6+)

A new ternary ruthenium oxide Na(2)RuO(4) was prepared and shown to crystallize with a new structure type. Single crystal X-ray diffraction measurements reveal that Na(2)RuO(4) consists of RuO(4) chains made up of RuO(5) trigonal bipyramids by sharing axial corners. Na(2)RuO(4) is a magnetic semiconductor with a variable range hopping behavior, and its molar magnetic susceptibility chi(mol) has a broad maximum at approximately 74 K. The derivative d(chi(mol).T)/dT exhibits a peak at 37.7 K which has been confirmed by heat capacity measurement to be due to long-range antiferromagnetic ordering.     

Dynamic response of chlorine atoms on a RuO(2)(110) model catalyst surface

The dynamic behavior of surface accommodated chlorine atoms on RuO(2)(110) was studied by a variety of experimental methods including high resolution core level shift, thermal desorption-, and in situ infrared spectroscopy as well as in situ surface X-ray diffraction in combination with state-of-the-art density functional theory calculations. On the chlorinated RuO(2)(110) surface the undercoordinated oxygen atoms have been selectively replaced by chlorine. These strongly bound surface chlorine atoms shift from bridging to on-top sites when the sample is annealed in oxygen, while the reverse shift of Cl from on-top into bridge positions is observed during CO exposure; the vacant bridge position is then occupied by either chlorine or CO. For the CO oxidation reaction over chlorinated RuO(2)(110), the reactant induced site switching of chlorine causes a site-blocking of the catalytically active one-fold coordinatively unsaturated (1f-cus) Ru sites. This site blocking reduces the number of active sites and, even more important, on-top Cl blocks the free migration of the adsorbed reactants along the one-dimensional 1f-cus Ru rows, thus leading to a loss of catalytic activity.     

不同RuO2含量对RuO2-Fe2O3催化剂氨选择性催化氧化性能的影响

采用溶胶-凝胶法（sol-gel）制备了一系列具有不同RuO₂含量的RuO₂-Fe₂O₃催化剂,并将其应用于氨选择性催化氧化（NH₃-SCO）研究中。结果表明,所有RuO₂-Fe₂O₃催化剂都表现出较好的低温活性,且RuO₂含量对催化剂的NH₃催化氧化活性影响显著。此外,利用BET、XRD、H₂-TPR和DRIFTS等表征手段研究了催化剂的物理化学性质和催化活性之间的关系。结果表明,RuO₂的加入增大了催化剂的比表面积。RuO₂与Fe₂O₃之间存在的协同效应提高了催化剂的氧化还原能力,从而提高了催化剂的氨氧化活性。同时,RuO₂含量对催化剂表面酸性影响很大,且催化剂表面主要存在Lewis酸性位点。     

Interaction of hydrogen with RuO2(110) surfaces: activity differences between various oxygen species

The interaction of hydrogen with RuO(2)(110) surfaces was studied by means of thermal desorption and vibration spectroscopies. The stoichiometric surface exposes two types of coordinatively unsaturated atoms: double-bonded O-bridge and five-fold-bonded Ru-cus, while at the O-rich surface the Ru-cus atoms are covered with single-bonded O-cus. On the stoichiometric RuO(2)(110) surface at 90 K, H(2) either adsorbs molecularly on Ru-cus sites or dissociates and forms with O-bridge an H(2)O-like surface group. If, in addition, also O-cus is present at the surface, hydrogen interacts exclusively with this species forming H(2)O-cus. This demonstrates that hydrogen reacts much more readily with O-cus than with O-bridge as expected from the reduced bond order and smaller binding energy of O-cus. It is furthermore shown that at surface temperatures below 90 K free coordinatively unsaturated Ru-cus sites are needed to activate the incoming H(2) molecules prior to any reaction with O-cus or O-bridge. Generally, Ru-cus sites play a key role for reactions of a number of molecules at the RuO(2)(110) surface. These findings are supported by recent DFT-based calculations but are at variance with other reports.     

The oxidation of water by cerium(IV) catalysed by nanoparticulate RuO2 on mesoporous silica

Mesoporous silicates are prepared by templating on the hexagonal (H1) mesophase of surfactant bipyridine complexes of ruthenium(II) using a true liquid-crystal templating approach. On calcination, the surfactant template is removed except for the central metal ion that is oxidised, forming nanoparticles of RuO2 that deposit within the pores. RuO2 is a known oxidation catalyst and, despite its anhydrous nature in these silicates, is found to be very active in catalyzing the oxidation of water by acidic CeIV.     

A general route for RuO2 deposition on metal oxides from RuO4

A novel method for the deposition of RuO(2) from RuO(4)(g) on diverse metal oxides has been developed by grafting dopamine onto the otherwise un-reactive metal oxide surface. Oxygen evolution reaction on TiO(2) and the photoelectrochemical improvement of WO(3) by deposition of RuO(2) are just a few examples where this novel deposition method can be used.     

Effects of Composition on the Structure and Capacitive Properties of Ti/(ZrO2)x(RuO2)1-x Electrodes

<正>A series of nominal composition Ti/(ZrO_2)_x(RuO_2)_(1-x) (0.1≤x≤0.9) coatings chan- ged in 10% steps was deposited on titanium substrate from RuCl_3·nH_2O and ZrCl_4 containing ethanol solution by thermal decomposition method. The X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and electrochemical tests were performed to clarify the effects of ZrO_2 content on the structure and capacitive property of Ti/(ZrO_2)_x(RuO_2)_(1-x). The results show that by adding ZrO_2 into the coatings the degree of crystallization of RuO_2 decreases. The specific capacitance firstly increases and then deceases with the increase of ZrO_2 content in the mixed oxide coatings. The film of Ti/(ZrO_2)_(0.6)(RuO_2)_(0.4) consisting of amorphous matrix and fine nano-crystalline RuO_2 (about 4 nm) has the maximum specific capacitance of 713.27 F/g(RuO_2).     

Syntheses and Crystal Structures of Two Sodium Ruthenates: Na2RuO4 and Na2RuO3

Na₂RuO₄, prepared from Na₂O₂ and RuO₂ via high oxygen pressure synthesis, crystallises monoclinic in space group P2₁/c (a = 10.721(6), b = 7.033(4), c = 10.871(6) Å, β = 119.10(4)°, Z = 8, 2503 unique reflections, R₁ = 0.049). Structure determination from single crystal data shows that the compound consists of infinite chains of RuO₅ trigonal bipyramids connected through their axial vertices. The Na cations connect the pseudohexagonally packed equation/tex2gif-stack-1.gif[RuO₃O_2/2] chains and are coordinated by six or seven oxygen atoms, respectively. The compound exhibits an one‐dimensional spin system with μ = 2.80 μ_B and Θ = —222 K and a three‐dimensional antiferromagnetic ordering below 50 K. Na₂RuO₃ was obtained from Na₂RuO₄ at 850 °C under a flow of argon. The structure was determined from X‐ray powder diffraction. It is closely related to the α‐NaFeO₂ and the Li₂SnO₃ structure types, layered variants of the NaCl type. In Na₂RuO₃ the Na and Ru atoms are partially disordered. This partially disordered state was approximated by a Rietveld refinement of two superimposed structural models (model I: R 3¯ m, a = 3.12360(5), c = 16.0370(4) Å, Z = 2; model II: C2/c, a = 5.4141(4), b = 9.3663(6), c = 10.8481(4) Å, β = 99.636(9)°, Z = 8).     

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.