Ru Crystal Research Articles

Effect of Ru crystal phase on the catalytic activity of hydrolytic dehydrogenation of ammonia borane

Ruthenium (Ru) is one of the most attractive catalysts in the hydrolytic dehygrogenation of ammonia borane. So far, the employed Ru-based catalysts are mainly focusing on hexagonal close packed structured Ru. In this work, we study the crystal phase effect of Ru on the hydrolytic dehygrogenation of ammonia borane, using face-centered cubic structured and hexagonal close packed structured Ru nanoparticles supported on γ-Al2O3. The results show that the catalytic activity is different even these two kinds of Ru nanoparticles hold similar size. Impressively, with the Ru particle size change, the variation trend of activity is opposite for face-centered cubic and hexagonal close packed structured Ru/γ-Al2O3. Finally, some possible reasons for their difference are proposed based on density functional theory calculations.

Electron transfer in the structure of a photorefractive doped Bi12TiO20: Ru crystal

The ac conductivity of Bi12TiO20: Ru crystals has been studied in the frequency range 102–106 Hz and at temperatures 293–773 K. The experimental data have been analyzed in the framework of the model of correlated barrier hops. In this material, the potential barriers are due to the existence of a block structure, crystal lattice defects, and also the presence of a ruthenium impurity. The microparameters characterizing the charge transfer in doped bismuth titanate single crystals have been determined.

Extreme UV induced dissociation of amorphous solid water and crystalline water bilayers on Ru(0001)

The extreme ultraviolet (EUV, λ=13.5nm) induced dissociation of water layers on Ru(0001) was investigated. We irradiated amorphous and crystalline water layers on a Ru crystal with EUV light, and measured the surface coverage of remaining water and oxygen as a function of radiation dose by temperature programmed desorption (TPD). The main reaction products are OH and H with a fraction of oxygen from fully dissociated water. TPD spectra from a series of exposures reveal that EUV promotes formation of the partially dissociated water overlayer on Ru. Furthermore, loss of water due to desorption and dissociation is also observed. The water loss cross sections for amorphous and crystalline water are measured at 9±2×10−19cm2 and 5±1×10−19cm2, respectively. Comparison between the two cross sections suggests that crystalline water is more stable against EUV induced desorption/dissociation. The dissociation products can oxidize the Ru surface. For this early stage of oxidation, we measured a smaller (compared to water loss) cross section at 2×10−20cm2, which is 2 orders of magnitude smaller than the photon absorption cross section (at 92eV) of gas phase water. The secondary electron (SE) contributions to the cross sections are also estimated. From our estimation, SE only forms a small part (20–25%) of the observed photon cross section.

Cu Underpotential Deposition on Ru(0001)

The underpotential deposition (UPD) has attracted research attention for many years due to its fundamental and practical importance[i]. Development of scanning probe methods (STM and AFM), and X-ray scattering techniques has enabled mechanistic in situ studies for many substrate-adlayer systems with great insight in details of UPD adlayer formation and structure, alloying and anion co-adsorption[ii]. However, in some cases, the noble metal surfaces with complex reactivity and interaction with water based electrolytes have rendered UPD studies a very difficult endeavor. One example is Ru surface for which a just few UPD studies have been reported[iii]-,[iv],[v],[vi],[vii]. Our talk presents a study of Cu UPD on Ru(0001). The Ru(0001) surface was prepared by annealing at 1400 C for one hour in Ar+H2 atmosphere (96:4), Figure 1. After cooling the Ru surface was protected by CO-saturated 0.1 M HClO4 solution and transferred to the electrochemical cell or STM cell for further studies. The example of the cyclic voltammetry and insitu STM results are shown in Figure 1. The potential has been gradually changed towards the region where Cu UPD on Ru(0001) occurs and high resolution/quality STM images were recorded. The morphological change of Ru(0001) surface due to Cu UPD process were correlated with CV data and characteristic cathodic and anodic peaks in CV were associated with particular features/process occurring during Cu UPD deposition and/or stripping. The Cu UPD process and adlayer structure on Ru(0001) is compared with corresponding Cu UPD process on other closed pack surfaces such as Au(111) and Pt(111). The apparent similarities and differences were identified and discussed during the talk. The authors acknowledge the support from NSF Chemistry division under the contract # 0955922. Figure 1. Cyclic voltammetry for Cu UPD on Ru(0001), scan rate 10 mV/s. Solution: 0.1 M HClO4 + 10-3 M Cu2+. Inset shows Ru crystal during inductive annealing. [i] S. Trasatti, J. Electroanal. Chem. 33, 351 (1971), [ii] E. Budevski, G. Staikov and W. J. Lorenz in Electrochemical Phase Formation and Growth, ed. R.C. Alkire et al. VCH, Berlin (1996), see Ref. herein. [iii] M. Vukmirovic, in Modern Aspects of Electrochemistry, vol. 42, ed. C. Vayenas et al. Springer, NY (2008). [iv] N. Bogolowski, S. Huxter, A. –E. Abd-El-Latif,G.A. Attard, H. Baltruschat, Journal of Electroanalytical Chemistry, 646, 68 (2010). [v] D. K. Gebregziabiher,Y-G. Kim, C. Thambidurai, V. Ivanova, P-H. Haumesser, J.L. Stickney, Journal of Crystal Growth, 312 1271 (2010). [vi] Y. Zhang, L. Huang, T. N. Arunagiri, O. Ojeda, S. Flores, O. Chyan, R. M. Wallace, Electrochemical and Solid-State Letters, 7 C107 (2004). [vii] M. A. Quiroz, L. Salgado, Y. Meas, Electrochimica Acta, 33 435 (1988). Figure 1

RuSe Electrocatalysts and Single Wall Carbon Nanohorns Supports for the Oxygen Reduction Reaction

Selenium modified ruthenium electrocatalysts supported on carbon black were synthesized using NaBH4 reduction of the metal precursor. Prepared Ru/C electrocatalysts showed high dispersion and very small averaged particle size. These Ru/C electrocatalysts were subsequently modified with Se following two procedures: (a) preformed Ru/carbon catalyst was mixed with SeO2 in xylene and reduced in H2 and (b) Ru metal precursor was mixed with SeO2 followed by reduction with NaBH4. The XRD patterns indicate that a pyrite-type structure was obtained at higher annealing temperatures, regardless of the Ru:Se molar ratio used in the preparation step. A pyrite-type structure also emerged in samples that were not calcined; however, in this case, the pyrite-type structure was only prominent for samples with higher Ru:Se ratios. The characterization of the RuSe/C electrocatalysts suggested that the Se in noncalcined samples was present mainly as an amorphous skin. Preliminary study of activity toward oxygen reduction reaction (ORR) using electrocatalysts with a Ru:Se ratio of 1:0.7 indicated that annealing after modification with Se had a detrimental effect on their activity. This result could be related to the increased particle size of crystalline RuSe2 in heat-treated samples. Higher activity of not annealed RuSe/C catalysts could also be a result of the structure containing amorphous Se skin on the Ru crystal. The electrode obtained using not calcined RuSe showed a very promising performance with a slightly lower activity and higher overpotential in comparison with a commercial Pt/C electrode. Single wall carbon nanohorns (SWNH) were considered for application as ORR electrocatalysts' supports. The characterization of SWNH was carried out regarding their tolerance toward strong catalyzed corrosion conditions. Tests indicated that SWNH have a three times higher electrochemical surface area (ESA) loss than carbon black or Pt commercial electrodes.

Scanning Tunneling Microscopy Evidence for the Dissociation of Carbon Monoxide on Ruthenium Steps

In heterogeneous catalysis, identifying the active site for key reaction steps is an important contribution for the optimization of industrial synthesis. The structure sensitivity of CO dissociation on a metal catalyst, which is the rate-limiting step for the methanation and the Fischer–Tropsch processes under certain conditions, has been debated for years. Here, scanning tunneling microscopy (STM) and density functional theory (DFT) are used to clarify the role of monatomic steps in the splitting of CO on a stepped Ru(0 1 54) crystal, which displays alternating steps with either 4-fold or 3-fold symmetry. After CO doses at elevated temperatures, the STM images reveal step decorations characteristic of atomic oxygen resulting from CO dissociation on every second step. The comparison of the STM images with the results of DFT calculations shows that the step decoration occurs on the steps displaying the 4-fold symmetry. We conclude that the active sites for CO dissociation on ruthenium are located on the 4-fold symmetry monatomic steps.

Copper-ceria interaction: A combined photoemission and DFT study

Stoichiometric and partially reduced ceria films were deposited on preoxidized Ru(0001) crystal by Ce evaporation in oxygen atmosphere of different pressures at 700 K. Copper-ceria interaction was investigated by deposition of metalic copper on both types of substrate. The samples were characterized by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) of core states and resonant photoelectron spectroscopy (RPES) of the valence bands. Copper adsorption on stoichiometric ceria caused reduction of CeO2, while on the oxygen-defficient ceria it partially reoxidized the substrate. This is in agreement with DFT+U calculations of copper adsorption on stoichiometric and defective ceria surfaces.

Carbon Nanofiber-Supported Ru Catalysts for Hydrogen Evolution by Ammonia Decomposition

Carbon nanofibers (CNFs) with fish-bone graphene alignment and carbon nanotubes (CNTs) were used to support ruthenium for ammonia decomposition. The Ru nanoparticles on the CNF supports are more active than those on CNT supports. The Ru particle size was adjusted by changing the Ru loading or by introducing oxygen containing groups onto the CNF surface. The site activity increases when the Ru crystal size increases. The oxygen groups on the CNFs have a remarkable effect on ammonia decomposition over the Ru nanoparticles. On identically sized Ru crystals, oxygen on the CNFs clearly enhances ammonia decomposition over the Ru/CNFs.

Formation of graphene on Ru(0001) surface

We report on the formation of a graphene monolayer on a Ru(0001) surface by annealing the Ru(0001) crystal. The samples are characterized by scanning tunnelling microscopy (STM) and Auger electron spectroscopy (AES). STM images show that the Moiré pattern is caused by the graphene layer mismatched with the underlying Ru(0001) surface and has an N × N superlattice. It is further found that the graphene monolayer on a Ru(0001) surface is very stable at high temperatures. Our results provide a simple and convenient method to produce a graphene monolayer on the Ru(0001) surface, which is used as a template for fabricating functional nanostructures needed in future nano devices and catalysis.

Forming Aromatic Hemispheres on Transition‐Metal Surfaces

This study details a new method to create materials by integrating polycyclic aromatic hydrocarbons into reactive transition-metal surfaces. We describe the interaction of hexabenzocoronene (1, HBC, Figure 1a) with a clean ruthenium surface. Interest in 1 and its interaction with transition-metal surfaces is driven by its relationship to the end cap of a carbon nanotube, to polynuclear aromatic hydrocarbons, and to reactor carbonization. Even though 1 is a large and multifunctional molecule, its high symmetry (there are only five types of C atom) simplifies the interpretation of the fundamental surface-molecule chemistry. Ruthenium provides a catalytically active metal that is capable of p bonding to polycyclic aromatic hydrocarbons and is known to be active in many processes such as Fischer– Tropsch and olefin metathesis. Our experimental approach utilizes a combination of scanning tunneling microscopy (STM), reflectance absorbance infrared spectroscopy (RAIRS), and temperature-programmed desorption (TPD) to study the surface chemistry of 1. Before depositing 1, we cleaned the (0001) face of a Ru crystal by repeated cycles of Ar sputtering, annealing in oxygen at around 940 8C, and then annealing at 1100 8C. STM images of the crystal prior to chemisorption show large flat terraces (ca. 100 nm wide) in which the individual atoms could be resolved. One of these images is shown in Figure S1A in the Supporting Information. There are a few larger bright features that apparently resulted from underlying damage to the Ru crystal by sputtering and annealing (Figure S1B). Despite these sparse defects, the arrangement of surface atoms is essentially unaffected. These features had no measurable effect on the surface chemistry or spectroscopy described below. Furthermore, the low-energy electron diffraction (LEED) pattern at 59 eV showed a hexagonally close-packed 1! 1 arrangement, as expected for the (0001) surface. No significant amount of surface oxygen atoms was detected by Auger spectroscopy. It is difficult to quantify by Auger electron spectroscopy how much carbon is on the surface, but the amount must be relatively low in view of the high quality of the STM image (Figure S1). Moreover, minor impurities on the surface will not affect the experiments because the surfaces are covered with substantially less than a monolayer of 1. Figure 1 shows the STM measurements of 1 on the Ru single crystal at a base pressure of 3! 10!10 Torr and at room temperature. 17] The features on the surface are hexagonally shaped. We measured the width of these features from the full width at half maximum in the topographic images to counteract tip effects, which tend to inflate the lateral size of molecules on the surface of metals. The diameter of 1 measured in Figure 1d (1.5 0.1 nm) is slightly larger than the diameter of 1 (ca. 1.4 nm) measured from its crystal structure. The important conclusion is that 1 bonds “face-on” to the Ru(0001) surface. We observed no mobility of the molecules on the surface of ruthenium over duration of the experiment (typically several hours). By contrast, we observed high mobility of [*] K. T. Rim, M. Siaj, S. Xiao, M. Myers, L. Liu, M. L. Steigerwald, Prof. G. W. Flynn, Prof. C. Nuckolls Department of Chemistry and The Center for Electron Transport in Molecular Nanostructures Columbia University New York, NY 10027 (USA) Fax: (+1)212-932-1289 (C.N.) or (+1)212-854-8336 (G.W.F.) E-mail: gwf1@columbia.edu cn37@columbia.edu Homepage: nuckolls.chem.columbia.edu V. D. Carpentier, P. H. McBreen Department de Chemie Universite Laval Quebec, QC, G1K 7P4 (Canada) C. Su Department of Molecular Science and Engineering National Taipei University of Technology Taipei 106 (Taiwan) M. S. Hybertsen Center for Functional Nanomaterials Brookhaven National Laboratory Upton, NY 11973-5000 (USA) [**] This work was funded by the Department of Energy under Grant No. DE-FG02-88ER13937 (G.W.F.). Equipment support provided by the National Science Foundation under grant CHE-03-52582 (G.W.F.). We acknowledge financial support from the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number CHE-0117752 and by the New York State Office of Science, Technology, and Academic Research (NYSTAR). We acknowledge support from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, US D.O.E. (DE-FG02-01ER15264). C.N. thanks the Alfred P. Sloan Fellowship Program (2004). We thank the MRSEC Program of the National Science Foundation under Award Number DMR-0213574 and by the New York State Office of Science, Technology and Academic Research (NYSTAR) for financial support for MLS and the shared instrument facility. We acknowledge research support from the Natural Sciences and Engineering Research Council (NSERC) of Canada. M.S. acknowledges the receipt of an NSERC postdoctoral scholarship. Supporting information for this article (experimental and theoretical methods, STM images, current–voltage characteristics, RAIRS spectrum, and computed structures/frontier orbitals for 3) is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie

Characterization and study on performance of the Ru–La–B/ZrO2 amorphous alloy catalysts for benzene selective hydrogenation to cyclohexene under pilot conditions

Various characterizations of Ru–La–B/ZrO2 catalysts were carried out by means of XRD, TEM, SEAD, XPS and N2-physisorption, and the activity and selectivity of the catalysts were measured. XRD results showed that Ru–La–B/ZrO2 catalysts were considerable stable and no significant peaks corresponding to Ru crystal were observed for the catalysts treated below 673 K and ran for 500 h under pilot conditions. The study on N2-physisorption showed that with extending of the reaction time the most probable pore diameters, specific surface areas and pore volumes became smaller supposed to be relative to the adsorption of substances in the reaction mixture and to be one of the reasons for decreasing of the activity and selectivity of the catalysts. XPS showed that ruthenium exists in metallic state; lanthanum exists in La3+ or in metallic state. One of the most remarkable features of Ru–La–B/ZrO2 catalysts is the high activity and good selectivity for benzene selective hydrogenation to cyclohexene. On the other hand, the catalysts can be regenerated and have a long life under pilot conditions.

Argon scattering fromRu(0001): Calculations and comparison with experiment

Recently reported measurements of Ar atom scattering from $\mathrm{Ru}(0001)$ at thermal and hyperthermal energies exhibited a number of characteristics that are unusual in comparison with other systems for which molecular beam experiments have been carried out under similar conditions: the measured energy losses are unusually small, some of the angular distributions exhibited an anomalous shoulder feature in addition to a broad peak near specular, and quantum mechanical diffraction was observed under conditions for which it was not expected. The data are analyzed and compared to calculations with classical scattering theory. Many of the unusual features observed in the measurements are explained but only upon using an effective surface mass of 2.3 Ru atomic masses, which implies collective effects in the Ru crystal. The bigger effective mass leads to substantially larger Debye-Waller factors and explains and confirms the observations of diffraction features. It also leads to the interesting conclusion that Ru is a metal for which molecular beam scattering measurements in the purely quantum mechanical regime can be performed with a range of atomic and molecular projectiles with masses substantially heavier than He.

Role of Vibrationally Excited NO in Promoting Electron Emission When Colliding with a Metal Surface: A Nonadiabatic Dynamic Model

A nonadiabatic quantum dynamic model has been developed to study the process of electron emission from a low-work-function metal surface. The process is initiated by scattering a highly vibrationally excited NO molecule from a surface composed of a Cs layer covering a Ru crystal. The model addresses the increasing quantum yield of the electron emission as a function of the molecular vibrational excitation and incident kinetic energy. The reaction mechanism is identified as a long-range harpooning electron transfer to a molecular ion which is then accelerated toward the surface. Upon impact, the molecular ion emits its excess electron.

New methods of controlled monolayer-to-multilayer deposition of Pt for designing electrocatalysts at an atomic level

Two new methods for monolayer-to-multileyer Pt deposition are presented. One involves Pt deposition by the replacement of an UPD metal monolayer on an electrode surface and the other the spontaneous deposition of Pt on Ru. The first method, exemplified by the replacement of a Cu monolayer on a Au(111) surface, occurs as a spontaneous irreversible redox reaction in which the Cu monolayer is oxidized by Pt cations, which are reduced and simultaneously deposited. The second method is illustrated by the deposition of Pt on a Ru(0001) surface and on carbon-supported Ru nanoparticles. This deposition takes place upon immersion of a UHV-prepared Ru(0001) crystal or Ru nanoparticles, reduced in H2, in a solution containing PtCl6 2- ions. The oxidation of Ru to RuOH by a local cell mechanism appears to be coupled with Pt deposition. This method facilitates the design of active Pt-Ru catalysts with ultimately low Pt loadings. Only a quarter of a monolayer of Pt on Ru nanoparticles yields an electrocatalyst with higher activity and CO tolerance for H2/CO oxidation than commercial Pt-Ru alloy electrocatalysts with considerably higher Pt loadings.

Structure sensitivity of supported ruthenium catalysts for ammonia synthesis

The catalytic ammonia synthesis activities of four supported ruthenium catalysts are reported. It is seen that Ru/MgAl 2O 4 is more active than two similar Ru/C catalysts, which are significantly more active than Ru/Si 3N 4. The activity differences cannot be satisfactorily explained solely by the differences in dispersion. Recent results from single crystal studies and DFT calculations have shown that ammonia synthesis over ruthenium catalysts is a very structure sensitive reaction, more so than on iron catalysts. It is suggested that special B 5-type sites are primarily responsible for the catalytic activity of the present supported Ru catalysts. It is shown how the number of such B 5-type sites depends on the Ru crystal size for a given crystal morphology. We have found that the activity of the Ru/MgAl 2O 4 catalyst increases significantly during the initial part of a test run. This activity increase is paralleled by the disappearance of crystals smaller than ca. 1.0 nm due to sintering and a resulting formation of larger crystals. We conclude that there exists a lower limit to the desired crystal size of Ru in supported ammonia synthesis catalysts. This is in agreement with a low number of B 5-type sites expected for such crystal sizes. Furthermore, we suggest that the support plays a decisive role in controlling the morphology of the Ru crystals and the resulting change in abundance of B 5-type sites is the main cause for the significant activity variations observed for Ru catalysts with different supports. Finally, the support may also influence the electronic and catalytic properties of neighboring B 5-type sites.

ORDERED STRUCTURES OF HEXAGONAL HONEYCOMB LATTICE

In this paper we determined 6 ordered structure types and 7 perfect ordered structures of hexagonal honeycomb lattice by using concentration-wave theory.And we proposed an explanation for the ordered structures of O atoms absorbed by (0001) surfaces of Ru crystal and Na ions in intercalation compound Nax TiS 2.

Influence of a magnetic field on the magnetoelasticity of commensurate spin-density-wave Cr–Ru and Cr–Si alloy single crystals

Previously reported attempts to produce a single-Q commensurate (C) spin-density-wave (SDW) state in a dilute Cr alloy by magnetic field cooling through the Néel temperature (TN), were unsuccessful. In the present study the influence of a cooling field of 4.5 T on the magnetoelasticity of Cr+0.3 at.% Ru and Cr+1.6 at.% Si alloy single crystals, both having CSDW phases below TN, are reported. The results show that cooling the Cr+0.3 at.% Ru crystal through TN in a field of 4.5 T has no influence on the first-order commensurate–incommensurate SDW magnetic phase transition. On the other hand, cooling the Cr+1.6 at.% Si crystal in a field of 4.5 T through TN noticeably affects the elasticity in the CSDW state, suggesting a condensation of part of this crystal into a single-Q CSDW phase.

Isotopic equilibration reaction of dinitrogen over raney ruthenium: Importance of the structural factor

The isotopic equilibration reaction of dinitrogen ( 28N 2 + 30N 2 = 2 29N 2) was studied over three Ru catalysts: stabilized Raney Ru, AlRu alloy, and Ru powder. The turnover frequency (TOF) at 588 K ranges from 10 −6 to 10 −6 and depends on the Al/Ru ratio and the microcrystal size of Ru. The highest TOFs are obtained over samples leached at 373 K from alloys having an atomic percent of Ru from 6 to 33. These samples have a smaller average Ru crystal size and a lower Al content, ranging from 8 to 18 mol% as measured by X-ray photoelectron spectroscopy (XPS). We also suggest that the high activity is related to strongly adsorbed nitrogen, the uptake of which exceeds hydrogen adsorption. Almost no XPS chemical shift is observed for any leached Raney Ru, whereas the Ru 3 d binding energy in AlRu alloys is about 1 eV lower than that in Ru metal powder. In spite of the pronounced chemical shift, the TOF on AlRu alloy is quite low. Results suggest that structural factors are most important in this system, rather than the electronic factor that has been emphasized for supported Ru catalysts.

Coadsorption of water and lithium on the Ru(001) surface

The interactions between water and lithium have been studied on the surface of a Ru(001) crystal using thermal desorption spectroscopy, electron stimulated desorption ion angular distributions (ESDIAD), Auger spectroscopy and LEED. The presence of Li was found to influence strongly the H + ESD yield and the ESDIAD patterns from adsorbed water even at Li coverages of 0.02 monolayer; changes in the thermal desorption states for water were also observed at low Li coverages. For coadsorbed Li coverages above 0.05, ESDIAD measurements provided clear evidence of water decomposition, even at surface temperatures near 80 K; evidence for dissociation was also obtained from thermal desorption and Auger measurements. The present results are compared and contrasted to those reported previously for the H 2O/Na/Ru(001) system.

Effects of an electron beam on adsorption and desorption of ammonia on ruthenium (0001)

The effects of an electron beam on ammonia adsorption and desorption on Ru(0001) have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. Appreciable adsorption at room temperature occurred only on the area of the Ru crystal which had been bombarded by an electron beam during dosing. The adsorption rate was a function of beam current density and ammonia pressure, and an apparent (2 × 2) diffraction pattern appeared in the area bombarded by the electron beam. Electron bombardment of the molecular γ states of ammonia followed by flash desorption showed that less ammonia and more hydrogen and nitrogen were desorbed as the bombardment time increased. An analysis of this process based on electron-induced dissociation of the ammonia molecule yielded an effective initial dissociation cross section of 3 × 10 −16 cm 2. Hydrogen flash desorption spectra after bombardment of the γ states showed two states obeying first order kinetics with desorption energies of 0.78 and 1.0 eV. Electron bombardment of the γ states for short times produced the same effects on the ammonia flash desorption spectra as preadsorption of hydrogen.