Segregation Of Ru Research Articles

Formation and Segregation of the Ru- and Rh-MgO Solid Solutions.

Structural changes in Ru- and Rh-loaded magnesium oxide (MgO) subjected to thermal treatment were investigated by using X-ray absorption spectroscopy. The thermal treatment of the MgO-loaded Ru and Rh nanoparticles led to the formation of Ru-MgO and Rh-MgO solid solutions, respectively. The valences of Ru and Rh in the solutions were 4+ and 3+, respectively, as determined by the Ru and Rh K-edge X-ray absorption near-edge structure (XANES). The degree of solid solution formation was the highest at 873 and 1073 K in Ru-MgO and Rh-MgO, respectively, as observed in the extended X-ray absorption fine structure and XANES analyses. The nearest neighboring Ru-O and Rh-O bond distances were shorter than the Mg-O bond in MgO. The dispersion of Ru and Rh on the MgO surface, measured by CO adsorption, increased for samples thermally heated at 1273 K, suggesting that the segregation of Ru or Rh from the solid solutions occurred at this temperature. Consequently, a relatively high dispersion was realized in the sample thermally heated to 1273 K. A good correlation was found between the dispersion value of Ru in Ru/MgO and the NH3 decomposition activity.

Effect of Ru addition on γ/γ′ microstructural stability in a low-density CoNi based superalloy

The present work explores the effect of ruthenium (Ru) addition to the γ/γ′ strengthened Co-30Ni-10Al-5Mo-2Nb superalloy. A 2 at.% Ru increases the γ/γ′ microstructural stability at 900°C. XRD and APT revealed a reduction of γ/γ′ lattice misfit by ∼ 26% and segregation of Ru at γ/γ′ interface, respectively that contribute to the decrease of γ/γ′ interfacial energy. The temporal evolution of γ′ precipitates at 900°C shows the classical matrix diffusion controlled LSW mechanism with a coarsening rate constant (Kr) ∼ 6.03 nm3/s similar to the values for Co-Al-W based superalloys. Furthermore, Ru addition increases the degree of anomalous strengthening with a peak 0.2% proof stress (PS) value of ∼ 630 MPa at 770°C. The results show Ru as a promising alloying addition to increase the γ/γ′ microstructural stability at high temperatures for the low-density class of CoNi-based superalloys.

Segregation of alloying elements and their effects on the thermodynamic stability and fracture strength of γ-Ni/γ′-Ni3Al interface

The γ/γ’ phase interface, being the main plane defect in nickel-based single-crystal superalloys, can have a significant effect on the microstructural stability and mechanical properties of superalloys. To improve the thermodynamic stability and fracture strength of the γ-Ni/γ’-Ni3Al interface and gain insight into the underlying micro-mechanisms, the segregation tendency of 12 alloying elements X at the γ-Ni/γ’-Ni3Al interface and their effects on the interfacial formation and Griffith fracture works were investigated by using first-principles calculations. It is found that all alloying elements X can segregate to the corner-point site of the γ-Ni (001) layer except the element Y. The Re-, Ti-, Mo-, W-, Cr-, Nb-, Ta-, Hf- and Zr-segregated interfaces are more stable than the unalloyed interface due to the presence of pseudogap and lower values of densities of states at the Fermi level. The segregation of Ru, Co, Re, Mo, W, Cr, Nb and Ta strengthens the interfacial fracture strength, which can be mainly attributed to the enhanced bonding strengths of X–Ni bonds formed in these segregated interfaces. The interfacial segregation of Cr, Re, Mo, W, Nb and Ta can not only improve the thermodynamic stability but also enhance the fracture strength of the phase interface. The segregation of Ta and Re is able to improve the thermodynamic stability and fracture strength of the γ-Ni/γ’-Ni3Al interface to the maximum extent, respectively. The effect of interfacial segregation of alloying elements on the fracture strength and thermodynamic stability of the γ-Ni/γ’-Ni3Al phase interface and the underlying mechanisms are studied.

Atomic-Scale View into the Degradation of Ir-Ru Alloys during Anodic Oxygen Evolution

Hydrogen is likely to play a key role as central energy carrier in the world’s transition towards a more sustainable future. Electricity from renewable sources can be used to power water electrolyzers to generate carbon-neutral hydrogen [1]. ‘Green’ hydrogen can then, for example, be used for the synthesis of carbon-neutral fuels, in the transportation sector to power H2 fuel-cell fitted vehicles, as a carbon-neutral agent for the direct reduction of metals from ores, or to re-generate electricity in time of high demand. However, the technological upscaling of proton exchange membrane water electrolysis (PEM) devices is currently hindered by the low reactivity and instability of most catalyzing materials for the anodic oxygen evolution reaction (OER). The development of electrocatalysts with superior performance requires a detailed understanding of how their surfaces evolve during OER, ideally at the atomic scale [2,3]. The identification of the role of individual atoms in different chemical and structural environments in the near-surface region of the catalyst is crucial to understand the electrochemical performance and degradation processes.Here, we provide atomic scale insights into the degradation mechanism of an Ir-Ru model alloy during electrochemical oxygen evolution. Ir-Ru alloys are promising candidates for PEM electrolysis combining the stability of Ir and high activity of Ru towards OER. The changes in electronic state of the catalysts induced by OER are studied by X-ray photoelectron spectroscopy (XPS), while an in-situ activity-stability analysis was performed online using an inductively coupled plasma mass spectrometer. Atom probe tomography (APT) was used to reveal the structure and composition of the first few atomic layers of the catalyst in three dimensions.APT reveals strong compositional differences between intragranular and grain boundary regions in the pristine alloy, resulting from significant Ru segregation to crystalline defects during the deposition process [4,5]. Significant Ru dissolution occurs during the OER, as revealed by online dissolution measurements, resulting in a few surface layers enriched in Ir, consistent with XPS results. APT further shows that the entire near-surface region affected by the oxidation exhibits signs of Ru dissolution. In addition, oxides formed in Ru-rich regions across the catalyst´s surface, i.e. in the proximity of grain boundaries, generally exhibit a higher oxygen content, compared to oxides on top of intergranular regions. Our data suggests that the defect structure of the material governs the enrichment of Ru in grain boundaries with consecutive preferential oxidation of these regions. This preferential oxidation potentially contributes to an accelerated, localized formation of volatile RuO4 species during the OER, leading to an enhanced Ru leaching along defect regions, eventually resulting in the deterioration of the overall reactivity of the material.Finally, using the new insights gained by this study, we discuss potential design strategy improvements of Ir-Ru alloys, to engineers an OER catalyst with a higher performance and longer durability. With this work we stress the need to investigate processes in catalytic materials with a combination of independent methods and at the near-atomic scale. Only the combination of insights enables the establishing of reliable structure-function relationships of electrocatalytic materials.

A single source method to generate Ru-Ni-MgO catalysts for methane dry reforming and the kinetic effect of Ru on carbon deposition and gasification

A single precursor RuxNiyMg1-x-y(OH)(OCH3) derived from solvothermal synthesis was used to generate Ru-Ni-MgO catalysts for methane reforming with CO2. Calcination-reduction pretreatment of precursors could easily cause segregation of Ru and formation of both large and small metallic particles as RuO2 has limited solubility in NiO-MgO solid solution. Uniform small Ru-Ni alloy particles could only be produced within their limited alloying composition range through direct reduction pretreatment of the precursor. Catalysts derived from calcination-reduction exhibited low initial activity that increased with time-on-stream, whereas catalysts derived from direct reduction demonstrated high and steady activity. Over spent catalysts, Ru was found to have changed the type of deposited carbon from a recalcitrant graphitic one that could only be gasified by O2 to a soft type that can be facilely gasified by CO2. Kinetic studies showed that Ru increased the activation barrier for the rate determining CH4 dissociation step and thereby slows down the carbon deposition rate. A first order reaction dependence for CH4 pressure variation and zeroth for CO2 pressure change for pristine Ni- and Ru-catalysts was identified, while a first order and a deviation from zeroth order for CH4 and CO2 pressure variation were observed on bimetallic Ru-Ni catalyst. Such a deviation is associated with the oxyphilic nature of Ru that is enriched in the alloy surface under reforming conditions. The effects of Ru on carbon gasification over spent catalysts were investigated using a modified CO2-TPO measurement based on an extrapolated Wigner–Polanyi equation for carbon gasification. Ru was found to accelerate carbon gasification by increasing the pre-exponential factor for CO2 oxidation of carbon, albeit a disfavored elevated activation barrier was obtained, thus showing a strong compensation effect. Carbon gasification is favored in high concentration of CO2 and at high temperatures for Ru-Ni catalyst.

Pt- and Ru-Doped SnO2–Sb Anodes with High Stability in Alkaline Medium

Different Pt- and Ru-doped Ti/SnO2-Sb electrodes were synthesized by thermal decomposition. The effect of the gradual substitution of Sb by Ru in the nominal composition on the physicochemical and electrochemical properties were evaluated. The electrochemical stability of the electrodes was estimated from accelerated tests at 0.5 A cm(-2) in 1 M NaOH. Both as-synthesized and deactivated electrodes were thoroughly characterized by scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction analysis (XRD). The incorporation of a small amount (about 3 at. %) of both Pt and Ru into the SnO2-Sb electrodes produced a 400-times increase in their service life in alkaline medium, with no remarkable change in the electrocatalysis of the oxygen evolution reaction (OER). It is concluded that the deactivation of the electrodes is promoted by alkaline dissolution of metal species and coating detachment at high potentials. The introduction of Pt has a coating compacting effect, and Ru(IV), at low amounts until 9.75 at. %, replaces the Sn(IV) cations in the rutile-like SnO2 structure to form a solid solution that strongly increases the stability of the electrodes. The observed Ru segregation and decreased stability for larger Ru contents (x > 9.75 at. %), together with the selective dissolution of Ru after deactivation, suggest that the formation of a homogeneous (RuδSn1-δ)O2 single-phase is crucial for the stabilization of these electrodes.

Enhancing catalytic activity and stability for CO2 methanation on Ni–Ru/γ-Al2O3via modulating impregnation sequence and controlling surface active species

Co-impregnation and sequential impregnation were used to prepare a series of Ni–Ru/γ-Al2O3 catalysts for CO2 methanation. By comparing the structure, surface species and catalytic performances, we uncovered the surface species segregation of Ru in Ni–Ru catalysts, and its effect on catalytic properties.

Carbon monoxide and methanol oxidation on potential-modified Pt–Ru/C electrocatalyst for polymer electrolyte fuel cells

In this paper, the effect of the anodic treatment of carbon-supported Pt−Ru nanoparticles using a HP 20% 1:1 Pt−Ru alloy on Vulcan XC-72 carbon black on the CO stripping and methanol oxidation reaction (MOR) has been analyzed. Thin-layer electrodes were prepared by depositing catalyst inks of Pt−Ru/C and Nafion on glassy carbon (GC) disks. Steady-state polarization at potentials >1.00V vs. RHE in 0.5M H2SO4 was used for the electrochemical activation of the specimen. Cyclic voltammetry was applied for electrochemical testing. The morphological analysis was performed by transmission electron microscopy (TEM), energy dispersive X-ray (EDX), selected-area electron diffraction (SAED), and Fast Fourier Transform (FFT). The present results showed that the anodic potential treatment causes Ru segregation with some carbon oxidation and agglomeration of nanoparticles. Activation of CO and methanol oxidation was found, indicating the production of the beneficial hydrous Ru oxide, RuOxHy, the promoting species for both reactions. For the MOR, the optimum potential of anodic treatment was located at 1.40–1.60V. An apparent double Tafel slope behavior with values of 120 and 200mVdec−1 was obtained after this optimum potential treatment, which was tentatively attributed to the existence of two different active sites for the MOR.

Promotion of Pt–Ru/C catalysts driven by heat treated induced surface segregation for methanol oxidation reaction

Carbon supported Pt–Ru/C (1:1) alloy catalysts supplied by E-TEK are widely used for fuel cell research. Heat treatments in various atmospheres are conducted for the promotion of the methanol oxidation reaction (MOR) and the investigation of the structure–activity relationship (SAR) of the catalysts. The alloy structures, surface compositions, surface species, and electro-catalytic activities of the alloy catalysts are characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV), respectively. The as-received Pt–Ru/C catalysts have a Ru rich in the inner core and Pt rich on the outer shell structure. Thermal treatments on the catalysts induce Ru surface segregation in different extents and thereby lead to their alteration of the alloying degrees. O 2 treatment results in obvious Ru segregation and formation of RuO 2. Catalysts treated in H 2 have the highest I f/ I b value in the CV scans among all samples, indicating the catalysts have the excellent CO de-poisoning ability as evidenced by anodic CO stripping experiments. N 2 treatment may serve as an adjustment process for the surface composition and structure of the catalysts, which can suppress the surface Pt depletion (∼60% Pt on the surface), make the components stable and hence promote the MOR significantly.

Antiferromagnetic iridium-manganese intermediate layers for perpendicular recording media (invited)

Current generation of cobalt-oxide-based perpendicular magnetic recording media uses single or dual ruthenium intermediate layers in order to grow crystallographically textured, and magnetically isolated granular media. In this work, the potential advantages of an antiferromagnetic iridium-manganese intermediate layer directly under the recording layer are highlighted. Owing to its close lattice matching with hexagonal cobalt, iridium-manganese which has the L12, or AuCu3-type crystal structure, can support the heteroepitaxial growth of the cobalt-based recording layer. In one of the media schemes described here, (111) textured iridium-manganese thin film was grown on 7.5 nm thick ruthenium layer. On the iridium-manganese as segregation layer, the Co-oxide-based magnetic recording layer showed perpendicular texture with Δθ50 below 4°, coercivity of over 4000 Oe alongside magnetic exchange decoupling, average grain sizes of 6 nm with distributions under 14%, and improved thermal stability. Measurements of the anisotropy constant did not show any significant change and even an IrMn capping layer was observed to improve the thermal stability. The possible mechanisms through which the IrMn layer could affect the thermal stability are hypothesized. The initial layers of the magnetic recording layer on IrMn segregation layers also showed exchange-decoupled and segregated grains, which is unlike that observed on Ru segregation layers. In a second media scheme, (111) textured iridium-manganese thin film was grown on a crystalline soft magnetic underlayer belonging on top of amorphous soft underlayers. In this scheme, partial pinning of the soft underlayer due to exchange-bias interaction with the IrMn layer was observed. This scheme offers the possibility to reduce the intermediate layer thickness, thus improve media writability, and with further optimization, could potentially facilitate the approach toward 1 Tbits/in.2.

Influence of Ru segregation on the activity of Ru–Co/γ-Al2O3 during FT synthesis: A comparison with that of Ru–Co/SiO2 catalysts

γ-Al 2 O 3 and SiO 2 supported Co catalysts, with varying amounts of Ru, were prepared and evaluated for Fischer–Tropsch synthesis (FTS). The composition of Ru for optimum activity was found to be support-dependent. The reducible Co 3 O 4 was high in the region of 0–1.64 wt.% of Ru in Co/SiO 2 catalysts. Co/γ-Al 2 O 3 displayed a maximum for reducible Co species at 0.42 wt.% Ru. Segregation of Ru occurred beyond this composition decreasing the extent of reduction. Co/γ-Al 2 O 3 catalysts showed lower activity and olefin selectivity, in spite of higher Co dispersion, than Co/SiO 2 catalysts. The catalytic performance depends on the amount of reducible Co species, which again depends upon the optimum content of Ru.

Promotion of the Electrochemical Activity of a Bimetallic Platinum−Ruthenium Catalyst by Oxidation-Induced Segregation

An alloy catalyst of 15 wt % Pt(50)Ru(50)/C was prepared by the method of incipient wetness impregnation and activated by hydrogen reduction at 620 K. Physical characterization of the freshly reduced catalyst indicated that bimetallic crystallites, Pt rich in the shell and Ru rich in the core, were finely dispersed in a diameter of dPtRu approximately 2 nm on carbon support. The reduced catalyst was subsequently modified by oxidization in air. On increasing the temperature of oxidation (T(o)), atoms of Ru in the core were found segregated to the surface of bimetallic crystallites and oxidized to amorphous RuO(2). Crystalline RuO(2) (RucO(2)) was formed on extensive segregation at To > 520 K. Catalytic activity of the alloy catalyst for electro-oxidation of methanol was examined by cyclic voltammetry. Electrochemical activity of the Pt-Ru/C catalyst was found to be significantly enhanced by oxidation treatments. The enhancement was, therefore, attributed to the segregation of Ru and the formation of RucO(2). Extensive oxidation treatment at elevated temperatures of To > 600 K, however, caused the deactivation of the electroactivity. The deactivation should be the result of excessive oxidation of the carbon support.

Soldification segregation in ruthenium-containing nickel-base superalloys

Ruthenium-containing multicomponent Ni-base superalloys with large variations in refractory alloying elements (Re, Ru, Ta, and W) have been investigated with respect to solidification, segregation characteristics, and the tendency to develop grain defects during directional solidification. Phase transformation temperatures and the effects of alloy composition on the liquidus temperature were determined by differential thermal analysis (DTA). The liquidus temperatures for most Ru-containing superalloys are generally higher than those of current commercial single-crystal superalloys. The partitioning behavior of individual constituents under the influence of alloy chemistry was characterized using a quantitative segregation mapping technique combined with a Scheil-type analysis. Whereas ruthenium partitioned preferentially to the dendrite cores during soldification, segregation of Ru is much less pronounced than Re and W. A higher degree of rhenium segregation was observed in Ru-containing superalloys. For the fixed processing conditions and moderate levels of Ru+Re, single-crystal solidification occurred without freckle formation or convection-induced breakdown of the solidification front. However, with high levels of Ru (9.6 ∼ 14.1 wt pct) and Re (7.2 wt pct), grain defects or the complete breakdown of single-crystal solidification was observed. Results from segregation and DTA analyses were used to estimate the corresponding Rayleigh numbers present during solidification of the experimental alloys. The Rayleigh criterion is effective for predicting the conditions under which the grain defect formation occurs during directional solidification of Ru-containing superalloys.

Characterization of Surface Composition of Platinum and Ruthenium Nanoalloys Dispersed on Active Carbon

Supported samples of 8 wt % monometallic Pt/C and Ru/C, as well as 12 wt % bimetallic Pt50Ru50/C, were prepared by the method of incipient wetness impregnation. Impregnated samples were subsequently reduced by hydrogen and then oxidized in air at different To temperatures. TEM and XRD examinations indicated that metal crystallites were finely dispersed with a diameter of dM < or = 3 nm on the reduced samples. Reductive behavior of the oxidized samples by hydrogen was pursued with the technique of temperature programmed reduction (TPR). The temperature of the reduction peaks (Tr) noticed in the TPR profiles varied with the metal composition of catalysts and To temperature of oxidation. At To = 300 K, oxidation was confined to the surface layer of metallic crystallites. As a result, Pts O (with a peak at Tr = 230 K) or PtsO2 (Tr = 250 K) was formed on monometallic Pt/C while RusO2 (Tr approximately 380 K) was formed on Ru/C. A reductive peak with Tr = 250 K was found from the bimetallic sample from Pt50Ru50/C oxidized at To = 300 K. The reductive peak suggests bimetallic crystallites were dispersed with cherry type structure, with Pt exposed at the surface and Ru in the core. On increasing the To temperature of oxidation treatment to 370 K and higher, Tr peaks between 270 and 350 K were gradually noticed on the oxidized bimetallic sample. Peaks in this Tr region are assigned to reduction of the oxidized alloy surface (AsO). Evidently, a segregation of Ru to the surface of the bimetallic crystallites is indicated upon oxidation at To > 380 K.

NMR studies of magnetic superconductor RuSr 2RECu 2O 8 (RE=Gd, Eu and Y)

We have carried out 99/101Ru and 63/65Cu nuclear magnetic resonance experiments in order to investigate magnetic and electronic properties of the magnetic superconductor RuSr 2RECu 2O 8 (RE=Gd, Eu and Y). The two kinds of 99/101Ru signals were observed in the magnetically ordered state for each system, suggesting a charge segregation of Ru 5+ (S=3/2) and Ru 4+ ( S=1) ions in the RuO 2 layers. The internal field at the Cu sites is revealed to be of the order of kilo Oe, indicating weak magnetic interactions between the CuO 2 and RuO 2 planes. The temperature dependence of nuclear spin-lattice relaxation time T 1 of 63Cu in RE=Y shows a ‘spin gap’ like behavior, suggesting the system is under-doped.

Long-range chemical order and induced lattice deformation along the growth direction in epitaxial [0001] Co 1− xRu x alloys

[0001] Co 1− x Ru x epitaxial films of about 50 nm thickness have been grown on a Ru buffer at temperatures ( T G) ranging from 450 K to 820 K. Most of these alloy thin films display (i) a composition modulation along the growth direction with a periodicity double that of a disordered hexagonal alloy and (ii) an induced decrease of the interplanar distance correlated to an increase of the in-plane parameter. The so-observed long-range ordering, that was not yet observed in bulk alloys, increases with T G between 450 and 600 K, passes through a maximum at 600 K and disappears above 700 K. This behavior is explained as resulting from the competition between two phenomena occurring simultaneously during the growth process: a surface effect driven by surface interactions and surface diffusion that tends to enrich the surface layer in the heavier element (Ru segregation) and a bulk effect driven by bulk interactions and bulk diffusion that tends to restore the equilibrium disordered state when the bulk diffusion becomes efficient during the growth time. A model that takes into account both effects and different diffusion rates in surface and in volume is proposed. It reproduces quite satisfactorily the T G dependence of the long-range order in these Co 3Ru films as well as similar results previously obtained in Co 3Pt films.

Surface segragation in the alloys Fe40Cr3Ru, Fe40Cr3Pt and Fe40Cr with a 1000 Å Pt Overlayer

No surface segregation of Ru or Pt could be detected in these Fe40Cr alloys at temperatures below 600°C in contrast with the behaviour of Pd. Strong preferred segregation of Cr and N with respect to Fe was observed in the Ru alloy between 400 and 500°C. Above 600°C, S segregated to the surface very strongly in both the Ru and Pt alloys but had a lower activation energy (97 ± 5 kJ/mol) for segregation in the Pt alloy compared to the 139 ± 6 kJ/mol for the Ru alloy. As was the case 0with Pd, the Pt overlayer of 950 ± 50 Å offered an easy diffusion path to the Cr and C atoms but inhibited the passage of Fe through this layer.

XPS, UPS and AES study of surface segregation and oxidation of Ni-Ru alloy: Evidence for decrease in dissociation temperature of NiO in the presence of Ru

Our AES study on the Ni-4.9at%Ru alloy shows Ru segregation to the surface with an enthalpy of segregation of −4 kJ/mol. On exposing the alloy to oxygen at 300 K, XPS and UPS studies show two types of oxygen species, O 2− (529.4 eV) and O 1− (531.4 eV); formation of NiO is also clearly observed. On heating the oxidized alloy surface in vacuum at 700 K, desorption of oxygen occurs as seen from the decrease in the O(1s) and O(KLL) peaks as well as the O(2p) band which was further confirmed by a thermal desorption study. Such a behaviour is attributed to the decrease in the dissociation temperature of NiO in the presence of Ru via O 2− →O 1− conversion.

Synthesis of some ring-substituted ruthenocenes and their use in the preparation of Ru/ZSM-5 catalysts

A method is proposed for the preparation of Ru/ZSM-5 catalysts using substituted and non-substituted ruthenocenes. To this end, the following complexes have been synthesized and characterized: dimethyl-1,1′; diphenyl-1,1′; dibenzoyl-1,1′, and monobenzoyl ruthenocene. Results of ESC A intensity ratio and ir of adsorbed pyridine show differences in the surface segregation of Ru as well as in the cationic exchange of Ru with Brønsted acid sites of the zeolite, when the catalyst is prepared using ring-substituted ruthenocene instead of ruthenocene itself. The binding energies of Ru 3d5/2 measured by ESCA are discussed.