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.     

Visualization of atomic processes on ruthenium dioxide using scanning tunneling microscopy.

The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy (STM), we investigated the CO oxidation reaction over the RuO2(110) and RuO2(100) surfaces. For both surfaces the protruding bridging O atoms are imaged in STM as bright features. The reaction mechanism is identical on both orientations of RuO2. CO molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated O atoms to form CO2 at the oxide surface. In contrast to the RuO2(110) surface, the RuO2(100) surface stabilizes also a catalytically inactive c(2 x 2) surface phase onto which CO is not able to adsorb above 100 K. We argue that this inactive RuO2(100)-c(2 x 2) phase may play an important role in the deactivation of RuO2 catalysts in the electrochemical Cl2 evolution and other heterogeneous reactions.     

The role of weakly bound on-top oxygen in the catalytic CO oxidation reaction over RuO2(110)

RuO(2)-based catalysts are much more active in the oxidation of CO than related metallic Ru catalysts. This high catalytic activity (or low activation barrier) is attributed to the weak oxygen surface bonding of bridging O atoms on RuO(2)(110) in comparison with the strongly chemisorbed oxygen on Ru(0001). Since the RuO(2)(110) surface is able to stabilize an even more weakly bound on-top oxygen species, one would anticipate that the catalytic activity will increase further under oxidizing conditions. We will show that this view is far too simple to explain our temperature-programmed reaction experiments, employing isotope labeling of the potentially active surface oxygen species on RuO(2)(110). Rather, both surface O species on RuO(2)(110) reveal similar activities in oxidizing CO.     

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.     

Protonation of phosphovanadomolybdates H(3+x)PV(x)Mo(12-x)O40: computational insight into reactivity

Protonated phosphovanadomolybdates of the Keggin structure, H(3+x)PV(x)Mo(12-x)O(40) where x = 0, 1, 2, and derivatives with surface defects formed by loss of constitutional water were studied using high-level DFT calculations toward determination of the most stable species and possible active forms in oxidation catalysis in both the gas phase and in polar solutions. The calculations demonstrate that protonation at bridging positions is energetically much more favorable than protonation of terminal oxygen atoms. The preferential protonation site is determined by the stability of the metal-oxygen bond rather than the negative charge on the oxygen atom. In H(3)PMo(12)O(40), maximum distances between protons at bridging oxygen atoms are energetically favored. In contrast, for H(4)PVMo(11)O(40) and H(5)PV(2)Mo(10)O(40) protons prefer nucleophilic sites adjacent to vanadium atoms. Up to three protons are bound to the nucleophilic sites around the same vanadium atom in the stable isomeric forms of H(5)PV(2)Mo(10)O(40) that result in strong destabilization of oxo-vanadium(V) bonding to the Keggin unit. Such behavior arises from the different nature of the Mo-O and V-O bonds that can be traced to the different sizes of the valence d orbitals of the metals. Coordination of two protons at the same site yields water and an oxygen defect as a result of its dissociation. The energetic cost for the formation of surface defects decreases in the order: O(t) ? O(c) ? O(e) and is lower for the sites adjacent to vanadium atoms. Vanadium atoms near defects also have a significant contribution to the LUMO. Thus, vanadium-substituted polyoxometalates with defects near and, especially, between vanadium atoms present a plausible active form of polyoxometalates in oxidation reactions.     

Spectroscopic study of the properties of surface compounds in methanol conversions on Cu/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The reactions of methanol on the (10% Cu)/γ-Al₂O₃ surface were studied by the spectrokinetic method (simultaneous measurements of the conversion rates of surface compounds and the product formation rates). Bridging and linear methoxy groups result from the interaction of methanol with surface hydroxyl groups. Formate and aldehyde-like complexes form by the oxidative conversion of the linear methoxy groups. Hydrogen forms via the recombination of hydrogen atoms on copper clusters, and the hydrogen atoms result from interconversions of surface compounds. The source of CO₂ in the gas phase is the formate complex, and the source of CO is the aldehyde complex. In the absence of methanol in the gas phase, dimethyl ether forms by the interaction between two bridging methoxy groups. When present in the gas phase, methanol reacts with methoxy groups on the surface. The roles of oxygen and water vapor in the conversions of surface compounds are discussed.     

Reaction pathways and excited states in H(2)O(2)+OH-->HO(2)+H(2)O: a new ab initio investigation

The mechanism of the hydrogen abstraction reaction H(2)O(2)+OH-->HO(2)+H(2)O in gas phase was revisited using density functional theory and other highly correlated wave function theories. We located two pathways for the reaction, both going through the same intermediate complex OH-H(2)O(2), but via two distinct transition state structures that differ by the orientation of the hydroxyl hydrogen relative to the incipient hydroperoxy hydrogen. The first two excited states were calculated for selected points on the pathways. An avoided crossing between the two excited states was found on the product side of the barrier to H transfer on the ground state surface, near the transition states. We report on the calculation of the rate of the reaction in the gas phase for temperatures in the range of 250-500 K. The findings suggest that the strong temperature dependence of the rate at high temperatures is due to reaction on the low-lying excited state surface over a barrier that is much larger than on the ground state surface.     

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.     

On the Mechanisms of Hydrogen‐Atom Transfer from Water to the Heteronuclear Oxide Cluster [Ga2Mg2O5].+: Remarkable Electronic Structure Effects

Mechanistic insight into the homolytic cleavage of the O? H bond of water by the heteronuclear oxide cluster [Ga₂Mg₂O₅]^.+ has been derived from state‐of‐the‐art gas‐phase experiments in conjunction with quantum chemical calculations. Three pathways have been identified computationally. In addition to the conventional hydrogen‐atom transfer (HAT) to the radical center of a bridging oxygen atom, two mechanistically distinct proton‐coupled electron‐transfer (PCET) processes have been identified. The energetically most favored path involves initial coordination of the incoming water ligand to a magnesium atom followed by an intramolecular proton transfer to the lone‐pair of the bridging oxygen atom. This step, which is accomplished by an electronic reorganization, generates two structurally equivalent OH groups either of which can be liberated, in agreement with labeling experiments.     

Mechanism of the hydrogen transfer from the OH group to oxygen-centered radicals: proton-coupled electron-transfer versus radical hydrogen abstraction

High-level ab initio electronic structure calculations have been carried out with respect to the intermolecular hydrogen-transfer reaction HCOOH+.OH-->HCOO.+H(2)O and the intramolecular hydrogen-transfer reaction .OOCH2OH-->HOOCH(2)O.. In both cases we found that the hydrogen atom transfer can take place via two different transition structures. The lowest energy transition structure involves a proton transfer coupled to an electron transfer from the ROH species to the radical, whereas the higher energy transition structure corresponds to the conventional radical hydrogen atom abstraction. An analysis of the atomic spin population, computed within the framework of the topological theory of atoms in molecules, suggests that the triplet repulsion between the unpaired electrons located on the oxygen atoms that undergo hydrogen exchange must be much higher in the transition structure for the radical hydrogen abstraction than that for the proton-coupled electron-transfer mechanism. It is suggested that, in the gas phase, hydrogen atom transfer from the OH group to oxygen-centered radicals occurs by the proton-coupled electron-transfer mechanism when this pathway is accessible.     

MOLECULAR RIBBONS COMPOSED OF CALCIUM(II) IONS BRIDGED BY CARBOXYLATE AND WATER OXYGEN ATOMS IN THE CRYSTALS OF Ca(II) COMPLEX WITH 5-METHYLPYRAZINE-2-CARBOXYLATE LIGANDS

Abstract

Crystals of monoaquo(μ-5-methylpyrazine-2-carboxylato-N,O,O′), (5-methylpyrazine-2-carboxyato-N,O)di(μ-aquo-O,O)calcium(II) contain molecular ribbons in which two adjacent calcium(II) ions are bridged by two bidentate oxygen atoms donated by two ligand molecules on one side and bidentate oxygen atoms of two water molecules on the other. The coordination polyhedron around the Ca(II) ion is a pentagonal bipyramid. The vertices of its pentagonal base are composed of two bridging water oxygen atoms, two carboxylate oxygen atoms of two ligand molecules and a nitrogen atom belonging to one of the bridging ligands. A coordinated water molecule constitutes the apex of the pyramid on one side of the base, while the N, O bonding moiety of a second ligand molecule makes two apices on the other side of the base. The ribbons are held together by a system of hydrogen bonds.     

High physisorption affinity of water molecules to the hydroxylated aluminum oxide (001) surface

The adsorption mechanism of water on the hydroxylated (001) plane of α-Al(2)O(3) was studied by measuring adsorption isotherms and GCMC simulations. The experimental adsorption isotherms for three α-Al(2)O(3) samples from different sources are typical type II, in which adsorption starts sharply at low pressures, suggesting a high affinity of water to the Al(2)O(3) surface. Water molecules are adsorbed in two registered forms (bilayer structure). In the first form, water is registered at the center of three surface hydroxyl groups by directing a proton of the water. In the second form, a water molecule is adsorbed by bridging two of the first-layer water molecules through hydrogen bonding, by which a hexagonal ring network is constructed over the hydroxylated surface. The network domains are spread over the surface, and their size decreases as the temperature increases. The simulated adsorption isotherms present a characteristic two-dimensional (2D) phase diagram including a 2D critical point at 365K, which is higher than that on the hydroxylated Cr(2)O(3) surface (319 K). This fact substantiates the high affinity of water molecules to the α-Al(2)O(3) surfaces, which enhances the adsorbability originating from higher heat of adsorption. The higher affinity of water molecules to the α-Al(2)O(3) (001) plane is ascribed to the high compatibility of the crystal plane to form a hexagonal ring network of (001) plane of ice Ih.     

析氯钛基RuO_2－Co_3O_4－TiO_2（60）涂层的研究

用稳定极化、Ｘ射线衍射、透射电镜和扫描电镜的方法研究了钛基ＲｕＯ_２－Ｃｏ_３Ｏ_４－ＴｉＯ_２（６０）涂层的析氯活性、导电性、使用寿命、微观结构和表面形貌。并对其活性表面积进行了评价。详细讨论了涂层成份、微观结构和表面形貌对涂层析氯活性和活性表面积的影响。发现在０～１０ｍ／ｏＣｏ_３Ｏ_４和６０ｍ／ｏＴｉＯ_２成份范围内ＲｕＯ_２、Ｃｏ_３Ｏ_４和ＴｉＯ_２可形成单一金红石型固溶体,且有优异的析氯活性、电导率和使用寿命。以此研究为基础可改进氯碱工业广泛使用的传统ＲｕＯ_２－ＴｉＯ_２阳极,降低阳极涂层中贵金属含量,提高阳极的电化学性能．     

Ab initio QM/MM molecular dynamics study on the excited-state hydrogen transfer of 7-azaindole in water solution

Ab initio molecular dynamics (AIMD) simulations for the excited-state hydrogen transfer (ESHT) reaction of 7-azaindole (7AI-(H2O)n; n = 1, 2) clusters in the gas phase and in water are presented. The effective fragment potential (EFP) is employed to model the surrounding water molecules. The AIMD simulations for 7AI-H2O and 7AI-(H2O)2 clusters show an asynchronous hydrogen transfer at t approximately 50 fs after the photoexcitation. While the ESHT mechanism for 7AI-H2O in water does not change appreciably compared with that in the gas phase, the AIMD simulations on 7AI-(H2O)2 in water solution exhibit two different mechanisms. Since the tautomer form is lower in energy compared to the normal form in the S1 state, 7AI and (H2O) n fragments separate from each other after the ESHT. With the use of the results of the AIMD trajectories, the minimum energy conical intersection point in the tautomer region has also been located.     

Gasification reaction of organic compounds catalyzed by RuO2 in supercritical water

Nearly complete gasification of organic compounds has been achieved by stoichiometrically insufficient amounts of RuO2 in supercritical water (SCW) to provide CH4, CO2 and H2, all the hydrogen atoms of which originate from water, and the catalytic effect of RuO2 results from a redox couple of Ru(IV)/Ru(II) induced by SCW.     

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.     

X-Ray Diffraction Study on Manganese(II) Complexes with Thiophene-2-Carboxylate and Furan-3-Carboxylate Ligands

Abstract

Mn(II) cations in the crystals of trisaquobis(μ-thiophen-2-carboxylato-O,O′)(thiophen-2-carboxylato-O)manganese(II) monohydrate are bridged by oxygen atoms donated by bidentate carboxylic groups of two thiophen-2-carboxylate ligands. In addition, each Mn(II) ion is coordinated by an oxygen atom of a monodentate carboxylic group of this ligand and three oxygen atoms of water molecules. The coordination around the Mn(II) cation is octahedral. The bridging of the ligands results in molecular ribbons propagating in the c-direction of the crystal held together by C?H…O hydrogen bonds. The crystal structure of diaquobis(μ-furan-3-carboxylato-O,O′)di(μ-furan-3-carboxylato-O,O)(μ-aqua-O)manganese(II) consists of dinuclear structural units. In each molecule Mn(II) cations are O,O′ bridged by oxygen atoms of bidentate carboxylic groups of two furan-3-carboxylate ligands and have a water located between the Mn cations. The units are O,O′ bridged to Mn(II) ions located in adjacent units by bidentate oxygen atoms, forming molecular ribbons extending in the c-direction. Octahedral coordination around each Mn(II) ion is completed by two water molecules. The octahedra around two adjacent metal ions in the unit share a common apex - the bridging oxygen atom of the water molecule. The ribbons are held together by C?H…O hydrogen bonds between furan ring oxygen atoms and the carbon atoms of adjacent furan rings.     

Metal-assisted reactions. Part 16.: Investigation of mechanisms of heterogeneous liquid phase catalytic transfer hydrogenolysis through deuterium-labelling

Hydrogen isotope studies have shown that heterogeneous catalytic transfer hydrogenolysis of C-O bonds in tetrazolyl ethers of phenols (1) in the liquid phase proceeds via direct transfer of hydrogen from an active hydrogen donor centre to the ether on the catalyst surface and not through transfer of hydrogen atoms from the catalyst surface.     

Properties of surface compounds in methanol conversion on copper-containing catalysts based on CeO2 according to in situ IR-spectroscopic data

Formate and carbonate complexes and bridging and linear methoxy groups were detected on the surfaces of CeO₂ and 5.0% Cu/CeO₂ under the reaction conditions of methanol conversion using IR spectroscopy. The reaction products were H₂, methyl formate, CO, CO₂, and H₂O. The bridging and linear methoxy groups were the sources of formation of bi- and monodentate formate complexes, respectively. Methyl formate was formed as a result of the interaction of the linear methoxy group and the formate complex. The study demonstrated that the recombination of hydrogen atoms on copper clusters and the decomposition of methyl formate were the main reactions of hydrogen formation. Formate and carbonate complexes were the source of CO₂ formation in the gas phase, and the decomposition of methyl formate was the source of CO. It was found that the addition of water vapor to the reaction flow considerably decreased the rate of CO formation at a constant yield of hydrogen. The effects of water vapor and oxygen on the course of surface reactions and the formation of products are discussed. To explain the mechanism of methanol conversion, a scheme of surface reactions is proposed.