Reoxidation Of Surface Research Articles

Water-gas shift: comparative screening of metal promoters for metal/ceria systems and role of the metal

In situ diffuse reflectance mode (DRIFTS) measurements for adsorption of CO and under the water-gas shift (WGS) reaction revealed that formates emerge on the surface of reduced ceria after the reaction of CO with geminal OH groups. These groups are formed after reduction of the ceria surface shell. X-ray absorption near edge spectroscopy (XANES) results demonstrated that the process of surface shell reduction was strongly catalyzed by the presence of metal, while changing very little, if at all, the catalysis of bulk ceria reduction. For 1% Pt/ceria, under steady state WGS at a high H 2O/CO ratio, surface formate concentrations were strongly limited at high CO conversions, while Pt-CO was affected only slightly. Under high H 2O/CO ratios, CO exhibits a first order rate dependency, and therefore, the active site is expected to move to sparser coverages of CO, which suggested that the WGS mechanism likely proceeded via formates. Later, XANES work gave no evidence for the reoxidation of ceria surface by water under a hydrogen environment, which would be necessary to substantiate an alternate mechanism, referred to as the ceria-mediated redox process. Later, isotope switching from H 2O to D 2O was carried out to validate the possibility that decomposition of surface formates could be the rate limiting step for the mechanism, as was proposed and demonstrated earlier by Shido and Iwasawa. In agreement with their findings, we also observed a normal isotope effect, consistent with a link between the activation energy barrier of the rate limiting step to the breaking of the CH bond of the formate. In this study, a variety of metals were screened to try to gain further insight into the role played by the metal in the catalysis of metal/ceria systems for WGS. One group of metals was selected on the basis of reduction temperature, since spillover from reduced metal likely catalyzes the reduction of ceria surface. Therefore, we tested the following metals, moving from lowest to highest reduction temperature: Pt<Ni<Co=Fe. These were found to catalyze reduction of ceria surface by the same trend, as well as the WGS rate. In each case, DRIFTS showed that as reduction of ceria surface occurred, marked increases in geminal OH intensities occurred, which yielded formates upon adsorption of CO. The second group of metals was a comparative study between Pt and Group 11 metals that have been purported to catalyze surface reduction at the same or even lower temperatures than Pt. Indeed, the surface reduction occurred at a lower temperature than for Pt with Au and at a similar temperature with Cu. However, on an equivalent atomic basis, the depth of reduction of ceria was found to be higher when Pt was used over Au, and the WGS rate was about 20 times higher with Pt than by Group 11 metal promotion at 250 °C and higher. WGS feed conditions were carefully chosen to mimic conditions found in a fuel cell reformer.

The role of surface sulfur species in the inhibition of pyrrhotite dissolution in acid conditions

Pyrrhotite, in anoxic acidic conditions, exhibits an induction period before rapid dissolution occurs. The length of the induction period is controlled by the amount of surface oxidation products on the mineral surface, acid strength, and temperature. During the induction period there is slow release of iron but little or no production of H 2S. The induction period is best described as a period of inhibited dissolution, before the onset of H 2S production and increased rate of iron release of at least 2 orders of magnitude. X-ray photoelectron spectroscopic (XPS) analysis of the acid-reacted surface shows the progress of the dissolution. Four stages of dissolution have been identified. (1) The immediate dissolution of an outermost layer of oxidised iron hydroxide/oxyhydroxide species and oxy-sulfur species. (2) Inhibited, diffusion limited dissolution during an induction period due to iron diffusion through the metal-deficient layer and oxidative dissolution of the polysulfide species. (3) Rapid, acid-consuming reaction of mono-sulfide species under nonoxidative or reductive conditions with production of H 2S. (4) Inhibited dissolution due to reoxidation of the sulfide surface by oxidising solution species (i.e., Fe 3+, residual oxygen) to produce polysulfide, elemental sulfur, and oxy-sulfur species. Dissolving synthetic pyrrhotite in similar, but aerated, acidic conditions, results in inhibited dissolution characterised by a lower rate of Fe release, minimal release of SO 4 2− and no release of H 2S . The XPS sulfur (S2p) spectrum shows sulfate and a form of elemental sulfur on the reacted surface. Only the first two stages of dissolution occur. The second stage differs in this case in that there is a plentiful supply of oxidising species (O 2). Two reaction mechanisms are proposed for the dissolution of the iron sulfide lattice of pyrrhotite in acidic conditions. The mechanisms are oxidative and nonoxidative dissolution. Two distinct activation energies are associated with the two regimes. A lower activation energy corresponds to inhibited dissolution with no production of H 2S. A t 1/2 rate law describes dissolution in air saturated solutions and supports diffusion controlled dissolution under these conditions. A higher activation energy corresponds to rapid dissolution with H 2S production. The mechanism of dissolution is determined by the state of the surface, particularly the sulfur species.

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.

Application of the Transient Response Technique to the Study of Co + NO + O2 Interaction on CuxCo3−xO4 Catalysts

NO + CO interaction has been studied in the temperature range 60 to 240°C on the CuxCo3−xO4 spinels, where x = 0.3, 0.54, and 0.94. At temperatures up to the "light-off" temperature, where degrees of surface reduction below 50% of monolayer oxygen are realized, the formation of N2O is an important intermediate: step of the NO + CO reaction. At higher temperatures, degrees of surface reduction up to several monolayers are attained and a route of direct NO reduction to N2 becomes important. On oxidized surfaces large amounts of NO are adsorbed which, however, are inactive towards reduction. Under oxidizing conditions the NO + O2 + CO interaction proceeds with slow reoxidation of the catalyst surface. The activity towards NO reduction even in oxidizing conditions is found to increase with increasing copper content (x). This is attributed to the enhanced capability of catalysts with x ≥ 0.5 to accumulate reduced sites in the surface layer.

Antimony Passivation of InP

Antimony passivation of InP was investigated. Sb was evaporated on a HCl-etched InP substrate and annealed at 300°C for 10 min. I-V characteristics of the Au/Sb/InP diode are substantially improved and the Schottky barrier height becomes higher as compared with the conventional Au/InP diode. The reverse current decreases by about two orders of magnitude upon Sb passivation. X-ray photoelectron spectroscopic (XPS) measurements show that the Sb oxide, Sb2O3, is formed near both the surface and the interface, i.e., the Sb2O3/Sb/Sb2O3 layered structure exists on the InP substrate. On the other hand, In2O3, the dominant component species of the native oxide of InP, is not observed in the Sb-passivated sample, which indicates that Sb passivation effectively removes that native oxide (In2O3) and suppresses reoxidation of the InP surface. Sb is considered to reduce In2O3 and is oxidized itself to become Sb2O3. This is a probable mechanism of Sb passivation. After the Sb-passivated substrate is washed in deionized water, the amount of Sb decreases and In2O3 is again observed. This is explained by the balance in the chemical reaction between In2O3 and Sb2O3 (the mass-action law). The low Schottky barrier height and the poor electrical characteristics are thus well correlated with the existence of the native oxide of InP, dominantly, In2O3.

Surface structure of crystalline and amorphous chromia catalysts for the selective catalytic reduction of nitric oxide IV. Diffuse reflectance FTIR study of NO adsorption and reaction

Adsorption of NO onto the oxidized surfaces of both crystalline and amorphous chromia results in the formation of nitrate complexes, by the reaction of NO with surface oxygen. Up to six differently bound species exhibiting different thermal stabilities have been identified. As the catalyst surfaces are saturated with adsorbed oxygen, the bound NO molecules are not decomposed: Upon heating, NO is molecularly desorbed, and neither formation of N 2O nor of N 2 is observed. Reduced chromia surfaces, which are partially depleted in surface oxygen, are more reactive towards NO. Decomposition of NO, to form N 2O and N 2, is taking place already at room temperature and is accompanied by a partial reoxidation of the catalyst surface. Nitrato complexes are the most abundant surface species also on the reduced chromia; signals of adsorbed mono- and dinitrosyl complexes are comparatively weak. This observation shows that NO preferentially binds to the Lewis basic surface oxygen ions. As a consequence of the decreased surface oxygen concentration, the coverage of bridging surface nitrate species (requiring two adjacent surface oxygens) is smaller than on the oxidized catalysts. The reaction of NO with surface hydroxyl groups, producing N2 and NO 2 −, results in a minor concentration of Cr-NO 2 surface nitro complexes. On the reduced amorphous catalyst, extensive reoxidation of the surface accompanied by N 2O formation is observed between 311 and 411 K in temperature-programmed reaction and desorption (TPRD) experiments, while this process is observed to a much smaller extent on reduced crystalline chromia. Above 473 K, bridging and bidentate surface nitrates are the most abundant species on both morphologies. Further heating results in the decomposition of these nitrates yielding N 2, N 2O (on crystalline chromia only), and surface oxygen.

An S/XPS study of hydrogen terminated, ordered silicon (100) and (111) surfaces prepared by chemical etching

There is considerable current interest in the passivation of single crystal silicon surfaces, particularly with reference to the fabrication of epitaxially grown solid state device structures.In this paper, we discuss ordered Si(100) and (111) surfaces prepared by chemical etching in solutions of hydrofluoric acid and studied by use of Soft XPS, XPS and LEED. Despite transfer through the atmosphere before being loaded into the UHV of our analysis instruments, the hydrogen terminated surfaces observed are shown to be among the most perfect yet produced by such chemical means. Under optimum conditions, the etch can yield levels of carbon and oxygen impurities of less than 2% of a monolayer and lead to surfaces where the Fermi level is not pinned.The etched surfaces are modified by the hydrogen termination such that they are electrically passivated and ordered into a (1 × 1) periodicity. Upon annealing at (520 ± 10)°C in UHV, the hydrogen is removed, the surface changes to a (2 × 1) reconstruction and also exhibits band bending due to Fermi level pinning by surface strates.The relationship between residual contamination and etching parameters such as time and etchant concentration has been examined, together with the stability of the passivated surface under exposure to wet and dry environments. This has shown that the surface condition is sensitive to etching procedure and that the optimum surface may be obtained by using a 5% non-aqueous solution, or by pre-cleaning a surface using a UV generated ozone exposure prior to an aqueous etch. It has also been shown that a controlled reoxidation of an etched surface using UV-ozone can produce a non-abrupt Si-SiO2 interface. This may form a useful part of a reduced temperature surface preparation, compatible with any subsequent epitaxial overlayer growth.

AES and TDS study of the adsorption of NH 3 and NO on V 2O 5 and TiO 2 surfaces: Mechanistic implications

The adsorption of NH 3 and NO on pure V 2O 5 and on TiO 2 samples at pressures (ca. 25 Torr) and temperatures (room temperature to 675 K) close to those used in industry with V 2O 5 TiO 2 catalysts to carry out the NO reduction with NH 3 has been studied by Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS), using a UHV chamber equipped with a built-in high pressure cell (up to 1 atm). NH 3 adsorption takes place over the whole range of temperatures studied on TiO 2 and on V 2O 5 as well as on reduced V 2O 5 surfaces. NO adsorption occurs only on TiO 2 and on reduced V 2O 5 but not on oxidized V 2O 5 surfaces. It is shown that NH 3 reduces the vanadium pentoxide surface resulting in the production of N 2O. The presence of N 2O, NO, and O 2 results in the reoxidation of the vanadium surface with the nitrogen oxides being converted into N 2. No evidence for the NO + NH 3 reaction has been obtained on the TiO 2 surface. However, NO adsorption on TiO 2 appears to be a main factor for the catalyst effectiveness in V 2O 5 TiO 2 samples since it may provide a source of NO at the interface of vanadia islands on TiO 2 in the actual catalyst. Finally, from TDS and AES data a reaction mechanism is suggested that may explain the reduction of NO to N 2 as well as the side reaction responsible for the production of N 2O.

On the ecology of meiofauna in an organically polluted estuarine mudflat

The structure, distribution and seasonal changes of the benthic meiofauna in an organically polluted, tidal, brackish-water mudflat in the Ems-Dollart estuary were analysed. Towards the outfall of polluted fresh water, macrofauna disappeared, numbers of meiofauna increased but the diversity of the meiofauna decreased. In the area surrounding the outfall the numbers and biomass of nematodes and oligochaetes increased rapidly in spring and remained high until autumn ( c. 13 × 10 6 individuals m −2; c. 2gCm −2). The benthic fauna comprised small numbers of species, dominated by a few fast-growing diatom-feeding nematodes ( Eudiplogaster pararmatus and Dichromadora geophila) and oligochaetes ( Amphichaeta sannio and Paranais litoralis). Eudiplogaster pararmatus exhibits brood care and it tolerates low salinities. Dichromadora geophila which is oviparous, behaved similarly regarding tolerances, life cycle and feeding but this species was less successful than Eudiplogaster in colonizing the mudflats near the outfall. The success of the two naidid oligochaete species results from their method of reproduction by means of binary fission. Most organisms fed on benthic diatoms. In spite of intensive mineralization in the mudflat, only one bacteria-feeding organism was found in abundance (the nematode Leptolaimus papilliger). The absence of macrofaunal organisms, e.g. Nereis diversicolor, is probably also responsible for the development of high densities of meiofauna in the upper sediment layers of the mudflats in the vicinity of the outfall. The heavy load of organic waste entering the Dollart reduced the diversity within the benthic ecosystem, but nevertheless a simple system remained, based on the recurrent reoxidation of the sediment surface.

Chemically modified carbon electrodes: Part XVII. Metallation of immobilized tetra(aminophenyl)porphyrin with manganese, iron, cobalt, nickel, copper and zinc, and electrochemistry of diprotonated tetraphenylporphyrin

Tetra(aminophenyl)porphyrin can be amide-bonded to oxidized glassy carbon surfaces following activation with acetyl chloride or thionyl chloride, with the latter affording higher coverage. The immobilized porphyrin can be metallated with manganese, iron, cobalt, nickel, copper and zinc. The metalloporphyrins undergo electrochemical reactions in DMSO solvent at metal and ring-centered reduction potentials expected on the basis of solution molecular analogs. In the case of iron-metallated porphyrin, electrochemistry can be done in aqueous acid, where a clear but porphyrin-debilitating dioxygen catalysis is observable. A small reverse wave at ca. −0.4 V vs. NaSCE in CH 3CN and DMSO solvent is interpreted as reoxidation of diprotonated surface bound porphyrin.

Correlations between catalytic and thermodynamic parameters of transition metal Oxides

AbstractThe oxidation of benzaldehyde to benzoic acid with V2O5, SnO2‐promoted V2O5 and MnO2 as catalysts has been investigated. The reaction mechanism includes the following steps: dissociative chemisorption of benzaldehyde, yielding an asymmetric covalent benzoate complex with one oxygen atom of the sufrace, atomic rearrangement to the symmetric benzoate ion, desorption of benzoic acid and reoxidation of the catalyst surface by adsorption of oxygen. The activity of the oxides is found to be related to their ability to release oxygen. An antibatic relation between the Gibbs free energy of oxygen release and the catalytic activity is demonstrated. The action of the tin promoter in the mixed oxide catalyst is due to the loosening of the metal‐oxygen bond. A parallelism is suggested between the catalytic selectivity and the differential increase of the enthalpy of oxygen release with increasing degree of reduction.

Reduction of Bi 2O 3MoO 3 catalyst during the ammoxidation of propylene in the absence of gaseous oxygen

The reduction of a P 2O 5Bi 2O 3z.sbnd;MoO 3 SiO 2 catalyst (a catalyst for the acrylonitrile process), consisting of Bi 2O 3z.sbnd;MoO 3 + BiPO 4 + amorphous SiO 2, during the ammoxidation of propylene to acrylonitrile in the absence of gaseous oxygen was studied quantitatively by X-ray diffraction techniques and by catalytic activity measurements. Amorphous SiO 2 and BiPO 4 are not affected in synthesis. The overall reduction of catalytically active Bi 2O 3 · 3MoO 3 to metallic Bi and MoO 2 proceeds according to the following two consecutive reactions, where Bi2O 2 · MoO 3 (koechlinite) is an intermediate: Bi 2 O 3·3 Mo O → ( C 3 H 6, NH 3) Bi 2 O 3· Mo O 3+2 Mo O 2+ O 2 Bi 2 O 3·3 Mo O → ( C 3 H 6, NH 3)2 Bi+ Mo O 2+2 O 2 The catalytic activity decreases rapidly with the increasing reduction of the catalyst. The phase changes of the catalyst and the product formation in synthesis correspond quantitatively to the depletion of lattice oxygen from the catalyst. In the absence of gaseous oxygen, the optimum activity for acrylonitrile of silica supported Bi 2O 3z.sbnd;MoO 3 catalysts is at a Bi:Mo atomic ratio of about 1. For the catalytic ammoxidation of propylene to acrylonitrile on Bi 2O 3z.sbnd;MoO 3 based catalysts, a steady state mechanism involving simultaneous reduction and reoxidation of the catalyst surface with gaseous oxygen is suggested.