Reduction Of Surface Oxides Research Articles

CO oxidation on partially oxidized Pd nanoparticles

Combining multi-molecular beam (MB) experiments and in situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we studied the relationship between the formation of Pd surface oxides and the reaction kinetics of CO oxidation on a well-defined Fe 3O 4-supported Pd model catalyst. The model catalyst was prepared in-situ under ultra-high-vacuum (UHV) conditions by Pd deposition onto a well-ordered Fe 3O 4 film on Pt(111). In previous studies, structure, morphology, and adsorption properties of the model system, as well as the formation of Pd oxide species, were characterized in detail. At low reaction temperatures ( T < 450 K ), the CO oxidation activity of partially oxidized Pd particles was found to be significantly lower than that of metallic Pd particles. We address this deactivation of the catalyst to a weak CO adsorption on Pd surface oxides, leading to a very low reaction probability. Even after extended exposure to CO-rich reactants at 400 K, no significant gain in catalytic activity due to reduction of surface oxides was observed. As a result, we conclude that the formation of Pd oxide species at higher temperatures causes long-term deactivation of the catalyst at lower temperatures. At higher reaction temperatures ( T ⩾ 500 K ), however, Pd oxides can be dynamically formed and decomposed, depending on the composition of the reactant environment. Although Pd oxides are formed on the surface under O-rich conditions, such oxide species are decomposed under CO-rich conditions at these temperatures. Using a simple model, we qualitatively analyzed the formation and decomposition process of Pd oxides under reaction conditions. We found that at higher reaction temperatures, partial oxidation of the Pd particles generally led to reduced CO oxidation activity and slow hysteresis effects that were strongly dependent on the pretreatment of the sample.

Formation of interface and surface oxides on supported Pd nanoparticles

We have quantitatively studied the interaction between oxygen and an Fe 3O 4-supported Pd model catalyst by molecular beam (MB) methods, time resolved IR reflection absorption spectroscopy (TR-IRAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The well-shaped Pd particles were prepared in situ by metal evaporation and growth under ultrahigh vacuum (UHV) conditions on a well-ordered Fe 3O 4 film on Pt(1 1 1). It is found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (∼10 −6 mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface, stabilized by the iron-oxide support. Their formation and reduction is fully reversible. Upon decomposition, oxygen is released which migrates back onto the metallic part of the Pd surface. In consequence, the Pd interface oxide layer acts as an oxygen reservoir, the capacity of which by far exceeds the amount of chemisorbed oxygen on the metallic surface. Additionally, Pd surface oxides can also be formed at temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature. This effect is addressed to different onset temperatures for oxidation of the particle facets and sites. It is shown that the presence of Pd surface oxides sensitively modifies the adsorption and reaction properties of the model catalyst, i.e. by lowering the CO adsorption energy and CO oxidation probability. Still, a complete reduction of the Pd surface oxides can be obtained by extended CO exposure, fully reestablishing the metallic Pd surface.

Electrochemical behavior of Pd–Rh alloys

Pd–Rh alloys were prepared by electrochemical codeposition. Bulk compositions of the alloys were determined by the energy dispersive X-ray analysis method, while surface compositions were determined from the potential of the surface oxide reduction peak. Cyclic voltammograms, recorded in 0.5 M H2SO4 for Pd–Rh alloys of different bulk and surface compositions, are intermediate between curves characteristic of Pd and Rh. The influence of potential cycling on electrochemical properties and surface morphologies of the alloys was studied. Due to electrochemical dissolution of metals, both alloy surface and bulk become enriched with Pd. Carbon oxides were adsorbed at a constant potential from the range of hydrogen adsorption. The presence of adsorbed CO2 causes remarkable diminution of hydrogen adsorption but it does not significantly influence hydrogen insertion into the alloy bulk. On the other hand, in the presence of adsorbed CO, both hydrogen absorption and adsorption are strongly suppressed. Oxidative removal of the adsorbates results in a characteristic voltammetric peak, whose potential increases with the decrease in Rh surface content. Electron per site (eps) values calculated for the oxidation of the adsorbates change with alloy surface composition, more for CO2 than CO adsorption, indicating the variation of the structure and composition of CO2 and CO adsorption products. The course of the dependence of eps values on surface composition suggests that the products of CO2 and CO adsorption on Pd–Rh alloys are similar but not totally identical.

Oxygen-induced Restructuring of a Pd/Fe3O4 Model Catalyst

We have studied the influence of oxygen on the structure and morphology of a Pd/Fe3O4 model catalyst using molecular beam (MB) methods, IR reflection absorption spectroscopy (IRAS) and scanning tunneling microcopy (STM). The model catalyst was prepared under ultrahigh vacuum (UHV) conditions by physical vapor deposition (PVD) and growth of Pd nanoparticles on an ordered Fe3O4 thin film on Pt(111). It is found that surface oxides are formed on the Pd nanoparticles even under mild oxidation conditions (temperatures of 500 K and effective oxygen partial pressures of around 10−6 mbar). These surface oxides are initially generated at the Pd/Fe3O4 interface and, subsequently, are formed at the Pd/gas interface. The process of formation and reduction of surface and interface oxides on the Pd particles is fully reversible in that all oxides formed can be fully reduced. As a result, the oxide phase acts like a storage medium for oxygen during oxidation reactions, as probed via CO oxidation. The process of surface and interface oxidation is directly connected with the onset of a non-reversible sintering process of the Pd particles. It is suggested that this sintering process occurs via a mobile Pd oxide species, which is stabilized by interaction with the Fe3O4 support. The restructuring is monitored via STM and IRAS using CO as a probe molecule. In addition to a decrease in particle density and Pd surface area, a reshaping of the particles occurs, which is characterized by the formation of well-ordered crystallites and with a relatively large fraction of (100) facets. After a few oxidation/reduction cycles at 500 K, the sintering process becomes very slow and the system shows a stable behavior under conditions of CO oxidation.

Voltammetry of Metallic Powder Suspensions on Mercury Electrodes

AbstractUltrafine powder samples of Cu, Fe, Ni, Mo and W were examined voltammetrically in aqueous suspensions with hanging mercury drop electrode. According to chemical interaction of the metallic powder particles with the dispersing medium the voltammetric curves either consist of a set of individual current peaks given by particulate reduction of surface oxides on impingements of particles from their suspension upon the electrode, or correspond to electroactivity of dissolved products of chemical reaction between particles and the medium.

Oxygen Reduction at Platinum Monolayer Islands Deposited on Au(111)

Atomic monolayer islands of Pt, namely, two-dimensional Pt nanoparticles, on a Au(111) electrode have been studied for the first time, focusing on their electrocatalytic activities for oxygen reduction in acid solutions. The Pt islands' electrodes were prepared using the self-assembled technique of thiols together with the replacement of Pt with a Cu monolayer. The states of adsorbed OH and the catalytic activities of oxygen reduction were sensitive to the Pt island size. As island size decreased, a delay in the reduction of surface oxide was observed. However, negligible influence of adsorbed OH on activity for oxygen reduction was observed. Pt islands of sizes ranging from 5 to 10 nm showed higher specific catalytic activities for oxygen reduction. Specific catalytic activities decreased by a factor of 10 with a decrease in island sizes from 5.5 to 3.1 nm. Size effects observed in Pt monolayer islands were discussed in comparison with three-dimensional nanoparticles, to obtain information concerning the size effects of metal nanoparticles.

Surface oxide reduction and bilayer molecular assembly of a thiol-terminated organosilane on Cu

We demonstrate the use of thiol-terminated organosilane to reduce the surface oxide and form a protective layer on Cu surfaces. The thiol termini of mercapto-propyl-trimethoxy-silane molecules reduce the copper oxide, and release disulfide- and sulfonate-terminated silanes. Unreacted mercaptosilanes and disulfides then assemble on the clean Cu surface forming a monolayer via chemisorption. The outward pointing methoxy groups react with other methoxysilane termini of sulfonated- and unreacted organosilanes, forming a molecular bilayer with Si–O–Si linkages between the two layers. These findings open up new possibilities for surface cleaning and passivating Cu interconnects with molecular nanolayers, and minimize surface-scattering-induced conductivity decrease in nanometer-thick Cu lines, without destructively etching the surface Cu oxide.

The surface oxide as a source of oxygen on Rh(1 1 1)

The reduction of a thin surface oxide on the Rh(1 1 1) surface by CO is studied in situ by photoemission spectroscopy, scanning tunneling microscopy, and density functional theory. CO molecules are found not to adsorb on the surface oxide at a sample temperature of 100 K, in contrast to on the clean and chemisorbed oxygen covered surface. Despite this behavior, the surface oxide may still be reduced by CO, albeit in a significantly different fashion as compared to the reduction of a phase containing only chemisorbed on surface oxygen. The experimental observations combined with theoretical considerations concerning the stability of the surface oxide, result in a model of the reduction process at these pressures suggesting that the surface oxide behaves as a source of oxygen for the CO-oxidation reaction.

Surface segregation and oxidation of Ti in a V–Ti alloy

The oxidation and reduction of surface oxides of the V–4%Ti alloy were examined by X-ray photoelectron spectroscopy. A specimen sheet of the alloy covered by oxide films was heated between 513 and 1223 K for 10 min in vacuum. Vanadium oxides started to be reduced to the metallic state at 673 K owing to oxygen dissolution in the bulk, while titanium remained in oxidized states up to 983 K. Titanium segregated to the surface above this temperature, and the surface concentration reached up to 40% at 1223 K. The specimen surface enriched in Ti was thus prepared and oxidized at 573 and 773 K under oxygen pressures between 10 −5 and 10 −3 Pa. At 573 K, titanium was selectively oxidized at 10 −5 Pa, while vanadium remained in a metallic state. Such selective oxidation of Ti took place also at 775 K under the pressure of 10 −4 Pa. Titanium segregating to the surface appeared to provide the barrier effect against tritium inventory and permeation under high temperature, low oxygen partial pressure conditions through selective oxidation.

Plasma Treatment of Artefacts

Corrosion products on ferrous artefacts generally consist of iron oxides and hydroxides. After excavation of the artefact, chlorinated products included between the object and its surface become very active owing to the oxygen and water vapour in ambient air. Therefore, the elimination of these chlorinated products is very important in order to preserve the excavated object. Chemical treatments are usually applied to these objects, such as alkaline sulphite baths, but this method takes a long time (a few months), and is also quite high maintenance. In the 1980s, it was shown by Veprek et al, at Zürich University, that plasma treatment of these artefacts could be of interest, mainly by using hydrogen plasma, which allows reduction of chlorinated products as well as oxides. Another reason for interest in this new method was that the treatment is much shorter: a few tens of hours compared to a few months for conventional chemical treatments. In this paper, a new type of large plasma reactor (approximately 200 L) for treatment of artefacts is presented, which is used at the Archaeological Museum of Val d’Oise. The reactor is described in terms of the high frequency power and plasma creation. The plasma itself is also analysed using optical analysis of the active species with and without an artefact inside the vacuum chamber. The operating conditions are also described. Results of plasma treatments on artefacts are presented with details of the treatment time and the restoration of the object after the plasma treatment. The potential extension of the use of this kind of plasma reactor is investigated for industrial applications such as fine cleaning or surface oxide reduction on metallic objects.

Surface Passivation Using P2S5/(NH4)2Sx and Hydrogen Fluoride Solutions on Ag/n-InAs and Ag/n-InSb Schottky Diodes

A method for surface passivation using both phosphorus sulphide/ammonia sulphide [P2S5/(NH4)2Sx] and hydrogen fluoride (HF) solutions has shown great effectiveness for the reduction of InAs and InSb substrate surface oxides. In comparison with samples treated by P2S5/(NH4)2Sx or HF solutions alone, the surface layer produced by the new passivation treatment has a high stability against oxidation even after an 18 h baking treatment at a temperature of 300°C in N2 ambient. In addition, the barrier heights of Ag/n-InAs and Ag/n-InSb Schottky diodes can still reach as high as 0.37 and 0.19 eV, respectively, even though the Ag/n-InAs and Ag/n-InSb diodes were baked for 18 h at 300°C.

Processes in PM steel compacts during the initial stages of sintering

Abstract In classical sintering procedures of ferrous precision part, important processes occur during the heating period and/or during presintering stages. Apart from the de-waxing process, which is not discussed here, changes in surface chemistry, formation of metallic bridges, and dissolution of alloy elements added as powders take place. In this work, desorption of gases, reduction of surface oxides, and neck formation have been studied as well as the dissolution of carbon and of metallic alloy elements. It showed that surface chemistry is changed within fairly narrow temperature intervals the position of which depends on the composition of the powder particles and in part on pretreatment. Dissolution of carbon in iron and steel matrices is a slower process than might be anticipated from the thermodynamic viewpoint, in plain Fe complete dissolution occurring only at T >900°C and the atmosphere also playing a major role. Formation of stable necks in steel–graphite compacts is retarded until dissolution of graphite has occurred to a significant extent. Finally, dissolution of metallic alloying elements, also if promoted by transient liquid phase, starts at still higher temperatures, in the case of Mo even T >1200°C being necessary, and full homogenization is complete only after extended isothermal sintering.

Electroless deposition of Pt at a Pd electrode by reaction with sorbed H in Pd/H

Abstract In the course of cycling a Pt electrode into the platinum surface oxide formation and reduction region in an aqueous Cl − ion solution, using a Pd ∣ H reference electrode, it was found that the Pd surface becomes coated with Pt. It is shown that this phenomenon arises from electroless deposition of Pt from Pt anodically dissolved from the Pt working electrode; the adsorbed H at Pd ∣ H is the reducing agent. These observations provide a warning that care must be exercised in the use of Pd ∣ H as a reference electrode.

The cyclohexanol dehydrogenation on Rh [sbnd]Cu/Al 2O 3 catalysts: 2. Chemisorption and reaction

Rh Cu supported on alumina were selected for the dehydrogenation of cyclohexanol. Highly dispersed Rh Cu supported on alumina were obtained and characterized after reduction. The metallic surface area was determined by frontal H 2 chemisorption and by N 2O decomposition followed by TPR. The TPR method for surface oxide reduction after exposing the catalyst to N 2O decomposition for the determination of the active phase is in good agreement with the frontal H 2 chemisorption method on rhodium, copper and bimetallic catalysts. The temperature and time exposition are important parameters for a complete oxidation of the monolayer, in particular for lower concentration of copper. An exposition time of 20 h is needed to complete a monolayer oxidation. Metallic copper is oxidized to Cu 1+ and Cu 2+ although not all Cu 2+ is reduced to Cu 1+ without oxidation of a sublayer. For the bimetallic catalysts Rh Cu a complete monolayer was attained. The TOF values were calculated based on the total active sites which give correct interpretation of the activity. The activity decreases when copper is added to rhodium which was attributed to an ensemble effect. For lower contents the selectivity of ketone was enhanced, which could be explained by simultaneous geometric and electronic effects. The selectivity on the monometallic catalysts prevails toward cyclohexene formation, indicating strong influence of the acidic sites. However, for the bimetallic catalysts it changed drastically toward dehydrogenation. The second metal affects the surface sites enhancing the metallic or bimetallic formation for dehydrogenation. This phenomena explains the electronic effect on bimetallic Rh Cu systems.

Comprehensive studies of formation and reduction of surface oxides at rhodium electrodes at 298 K

Surface oxides on Rh were formed by anodic polarization at various potentials, Ep, from 0.70 to 1.40 V, RHE, for polarization times, tp, up to 104, at 298 K; their thickness expressed as a charge density, qOX, was evaluated from cyclic voltammetry, CV, profiles. The oxide growth proceeds in two steps whereas its reduction occurs in one. Upon extension of Ep and (or) tp to higher values, qOX increases. Under the above experimental conditions, qOX varied from 110 ± 5 to 985 ± 10 μC cm−2; after conversion, these charge densities correspond to 0.5 monolayer, ML, of RhOH and 1.56 ML of Rh(OH)3, respectively. Augmentation of Ep and tp results in thermodynamically more stable oxides. Plots of qOX versus log tp are linear over the whole range of qOX whereas plots of 1/qOX versus log tp are linear only when qOX &gt; 210 μC cm−2. Thus, the oxide growth is either logarithmic in time over the whole region of qOX or it is logarithmic when qOX &lt; 210 μC cm−2 and inverse-logarithmic when qOX &gt; 210 μC cm−2. It was observed that the oxide growth continues when the positive-going scan is stopped in the oxide-formation potential region. Similarly, the oxide reduction proceeds when the negative-going scan is stopped in the oxide-reduction potential range. Finally, the authors present the first-ever studies of the Rh oxide reduction. The results indicate that the reduction takes place at E ≤ 0.70 V, RHE. The oxide reduction kinetics are not understood but, in general, the lower the reduction potential and the longer the reduction time, the greater the amount of the reduced oxide. Keywords: rhodium oxides, oxide films, growth kinetics, reduction kinetics.

Removal of Oxygen from the Si(100) Surface in a DC Hydrogen Plasma

This paper focuses on the removal of oxygen from a Si(100) surface by hydrogen plasma etching under high-vacuum conditions, i.e., the reaction-chamber base pressure is ≈10-7 mbar. The efficiency of oxygen removal from the Si(100) surface in the employed DC hydrogen glow discharge, with the sample at room temperature, is found to depend on the ratio of H2- and H2O partial pressures, the latter of which depends on the reaction-chamber base pressure and the amount of H2O liberated from the reaction-chamber walls by the plasma. This ratio reflects the competition between reduction of the Si surface by atomic hydrogen and reoxidation of the Si surface by residual H2O. A low-energy hydrogen ion bombardment is found to facilitate the reduction of the Si surface oxides. This process is most efficient when the sample potential is kept at 25 V below the plasma potential, i.e., when the surface is struck by 25-V protons.

Effect of CO on surface oxidation of uranium metal

The surface reactions of uranium metal with carbon monoxide at 25 and 200 °C have been studied by X-ray photoelectron spectroscopy (XPS); respectively. Adsorption of carbon monoxide on the surface layer of uranium metal leads to partial reduction of surface oxide and results in U4f photoelectron peak shifting to the lower binding energy. The content of oxygen in the surface oxide is decreased and O1s/O4f ratio decreases with increasing the exposure of carbon monoxide. The investigation indicates the surface layer of uranium metal has resistance to further oxidation in the atmosphere of carbon monoxide.

Changes in cathode catalyst structure and activity in phosphoric acid fuel cell operation

Changes in the cathode catalyst structure and activity obtained from a small size phosphoric acid fuel cell (PAFC) operated for various times up to 12 000 h, were examined. It was confirmed that the platinum surface oxide reduction potential in cyclic voltammograms (CV) shifted in the positive direction with cell operation. This may be one of the manifestations of the activity enhancement for the oxygen reduction reaction (ORR). It was assumed that this activity increase for the ORR was caused by an increase in the surface roughness, due to the dissolution of the alloyed base metals. Changes in the platinum chemical state of the alloy surface, from PtO to Pt, and growth of the Pt (1 1 0) plane would also contribute to this effect.

Cadmium sulfide surface stabilization for InP-based optoelectronic devices

Thin layers of chemical bath deposited cadmium sulfide were used to improve the surface and interface properties of InP and its latticed-matched III-V compounds. X-ray photoelectron spectroscopy indicates chemical reduction of surface oxides and the prevention of subsequent group III or V oxide formation. Photoluminescence spectra, measured between 1.0 and 1.3 μm, indicate a dramatic reduction in phosphorus vacancies following CdS treatment. Metalinsulator-semiconductor capacitors fabricated onn-type InP substrates with CdS interlayers display near-ideal quasi-static response and interface-state densities in the low 1011/eVcm2 range. Thin CdS layers were used to passivate the surface of InAlAs/InGaAs high electron mobility transistors (HEMTs) and metal-semiconductor-metal (MSM)photodetectors.AfterCdS treatment, Schottky diode barrier heights of 0.6 eV were regularly obtained. For HEMTs, drain-togate current ratios of 8 × 104 were observed after CdS treatment. For a new backside illuminated MSM design, the dark current of CdS-treated samples was reduced three orders of magnitude to below 1 nA.

In situ ftirs investigations of surface processes of Rh electrode—novel observation of geminal adsorbates of carbon monoxide on Rh electrode in acid solution

With the help of in situ multi-step FTIR Spectroscopy, two types of adsorbed geminal CO have been observed for the first time at an electrochemically modified Rh electrode. A doublet band of two broad peaks at 2166 and 2112 cm −1 is assigned to geminal CO on Rh surface oxide (or hydroxide) produced by the electrochemical modification process, and a doublet band of two peaks near 2103 and 2033 cm −1 is ascribed to geminal CO on surface clusters of Rh formed by reduction of Rh surface oxide. Based on the evolution of FTIR spectra with the electrode potential, the surface processes of a Rh electrode, subjected to a potential cycling treatment at 1.5 Vs −1 between −0.275 and 2.4V for 2min, have been elucidated. The present results at the solid/liquid electrochemical interface were compared with those obtained at the solid/gas interface, and consistent conclusions were achieved.