Methanol adsorption on a CeO2(1 1 1)/Cu(1 1 1) thin film model catalyst

Adsorption and desorption of methanol on a CeO₂(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce⁴⁺ → Ce³⁺ transition and an increase of the Ce 4f electronic state occupancy.     

Alumina supported model Pd-Ag catalysts: A combined STM, XPS, TPD and IRAS study

The bimetallic Pd-Ag model catalysts were prepared by physical vapor deposition on thin alumina films. The morphology and structure of the Pd-Ag particles were studied by STM, XPS, and by TPD and IRAS of CO. The results showed the formation of true alloy particles with Ag segregated at the surface. The addition of Ag first suppresses the most strongly bonded CO on threefold hollow sites of Pd. With further increasing Ag coverage, only isolated Pd atoms surrounded by Ag atoms are likely present on the surface. The results on CO adsorption suggest that the model Pd-Ag system mimics the structure of the real Pd-Ag catalysts.     

Influence of the spacing between metal particles on the kinetics of reaction with spillover on the supported metal catalyst

The influence of the spacing between active metal particles placed on the supported metal catalyst on the kinetics of the catalytic reaction with spillover was investigated. The 2A+B₂→2AB reaction, modelling the CO oxidation on Pd/Al₂O₃ catalyst, was studied using Dynamic Monte Carlo simulations. It was shown that there exists an optimal spacing, that provides the maximum reaction rate. It was postulated that this optimum is a consequence of both competition and cooperative effects occurring between metal particles.     

Au/TiO2/Ru(0 0 0 1) model catalysts and their interaction with CO

Au/TiO₂/Ru(0 0 0 1) model catalysts and their interaction with CO were investigated by scanning tunneling microscopy and different surface spectroscopies. Thin titanium oxide films were prepared by Ti deposition on Ru(0 0 0 1) in an O₂ atmosphere and subsequent annealing in O₂. By optimizing the conditions for deposition and post-treatment, smooth films were obtained either as fully oxidized TiO₂ or as partly reduced TiO_x, depending on the preparation conditions. CO adsorbed molecularly on both oxidized and reduced TiO₂, with slightly stronger bonding on the reduced films. Model catalyst surfaces were prepared by depositing submonolayer quantities of Au on the films and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. From X-ray photoelectron spectroscopy, a weak interaction between the Au and the TiO₂ substrate was found. At 100 K CO adsorption occurred on both the TiO₂ film and on the Au nanoparticles. CO desorbed from the Au particles with activation energies between 53 and 65 kJ/mol, depending on the Au coverage. If the Au deposit was annealed to 770 K prior to CO exposure, the CO adsorption energy decreased significantly. STM measurements revealed that the Au particles grow upon annealing, but are not encapsulated by TiO_x suboxides. The higher CO adsorption energy observed for smaller Au coverages and before annealing is attributed to a significantly stronger interaction of CO with mono- and bilayer Au islands, while for higher particles, the adsorption energy becomes more bulk-like. The implications of these effects on the known particle size effects in CO oxidation over supported Au/TiO₂ catalysts are discussed.     

Quantitative XPS studies of a natural catalyst

The surface characterization by X-ray photoelectron spectroscopy (XPS) of a natural mineral found in the Venezuelan Andes is reported. We present XPS studies for the natural mineral (sample ULA2) and for the mineral after it has served as a catalyst (sample ULA1) in a Fischer-Tropsch reaction for three months. The effect of different treatments on these samples is also investigated. Both samples exhibit a differential charge effect which is present once their surfaces have interacted with hydrogen. The quantitative XPS results allow us to advance the composition of these samples. The presence of a negatively charged hydrogen species in our samples is discussed based on our XPS results and supported by work function measurements.     

Analysis of XPS spectra of Fe and Fe ions in oxide materials

Samples of the iron oxides Fe_0.94O, Fe₃O₄, Fe₂O₃, and Fe₂SiO₄ were prepared by high temperature equilibration in controlled gas atmospheres. The samples were fractured in vacuum and high resolution XPS spectra of the fractured surfaces were measured. The peak positions and peak shape parameters of Fe 3p for Fe²⁺ and Fe³⁺ were derived from the Fe 3p XPS spectra of the standard samples of 2FeO·SiO₂ and Fe₂O₃, respectively. Using these parameters, the Fe 3p peaks of Fe₃O₄ and Fe_1−yO are analysed. The results indicate that high resolution XPS techniques can be used to determine the Fe²⁺/Fe³⁺ ratios in metal oxides. The technique has the potential for application to other transition metal oxide systems.     

Aspects of using the factor analysis for XPS data interpretation

In this work, the use of factor analysis to chemical state quantification of XPS data is studied. First, the theory of the method is reviewed with a special emphasis on the issues related to XPS data analysis. In particular, we concentrate on the transformation of the abstract components into physically meaningful ones in the case where reliable reference spectra are not available. We have observed that in the commonly used iterative target transformation factor analysis (ITTFA), in which a delta peak serves as the initial guess, the shape of the obtained component depends strongly on the position of the delta peak and on the minimum allowed intensity level. We propose an approach in which these parameters are varied in order to generate different representations for each component of the data. With simulated model data we show that if the variation is done with a sufficiently small step size, the correct representation will be generated. We also show that in the case of two-component data the iteration of the components is not necessary because a position can be found where a delta peak directly transforms into the correct component without unphysical features. Besides the model data, the proposed method is applied to experimental 2p photoelectron spectra of iron and chromium oxides.     

Magnetic composites based on hybrid spheres of aluminum oxide and superparamagnetic nanoparticles of iron oxides

Materials containing hybrid spheres of aluminum oxide and superparamagnetic nanoparticles of iron oxides were obtained from a chemical precursor prepared by admixing chitosan and iron and aluminum hydroxides. The oxides were first characterized with scanning electron microscopy, X-ray diffraction, and Mössbauer spectroscopy. Scanning electron microscopy micrographs showed the size distribution of the resulting spheres to be highly homogeneous. The occurrence of nano-composites containing aluminum oxides and iron oxides was confirmed from powder X-ray diffraction patterns; except for the sample with no aluminum, the superparamagnetic relaxation due to iron oxide particles were observed from Mössbauer spectra obtained at 298 and 110 K; the onset six line-spectrum collected at 20 K indicates a magnetic ordering related to the blocking relaxation effect for significant portion of small spheres in the sample with a molar ratio Al:Fe of 2:1.     

Characterization of iron surface modified by 2-mercaptobenzothiazole self-assembled monolayers

A self-assembled monolayer of 2-mercaptobenzothiazole (MBT) adsorbed on the iron surface was prepared. The films were characterized by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared reflection spectroscopy (FT-IR) and scanning electron microscopy (SEM). Besides, the microcalorimetry method was utilized to study the self-assembled process on iron surface and the adsorption mechanism was discussed from the power-time curve. The results indicated that MBT was able to form a film spontaneously on iron surface and the presence of it could protect iron from corrosion effectively. However, the assembling time and the concentration influence the protection efficiency. Quantum chemical calculations, according to which adsorption mechanism was discussed, could explain the experimental results to some extent.     

A resonant photoemission study of the Ce and Ce-oxide/Pd(1 1 1) interfaces

We have investigated the Ce 4f electronic states in the Ce/Pd(1 1 1) and Ce-oxide/Pd(1 1 1) systems, using resonant photoemission (Ce 4d → 4f transitions), and XPS to understand Pd-Ce interactions in ultra thin layers of cerium and ceria deposited on Pd(1 1 1). Cerium deposited on Pd(1 1 1) at room temperature forms surface Ce-Pd alloys with Ce rich character, while a Pd rich Ce-Pd alloy is formed by heating to 700 °C. A modification of the chemical state of Ce can also be seen after oxygen exposure. RPES provides evidence that Ce-oxide layers deposited on Pd(1 1 1) have a CeO₂ (Ce⁴⁺) character, however a net contribution of the Ce³⁺ states is also revealed. The Ce³⁺ states have surface character and are accompanied by oxygen vacancies. Heating to 600 °C causes Ce-oxide reduction. A significant shift of Pd 4d-derived states, induced by Pd 4d and Ce 4f hybridization, was observed. The resonant features in the valence band corresponding to Ce⁴⁺, Ce³⁺ and Ce⁰ states have been investigated for various Pd−Ce(CeO_x) coverages.     

Characterization of the synthesis and reactivity behavior of nanostructured vanadia model catalysts using XPS and vibrational spectroscopy

Nanostructured vanadia model catalysts, i.e., highly dispersed vanadium oxide supported on mesoporous silica SBA-15 (VO_x/SBA-15), were prepared. The mechanism for the synthesis of VO_x/SBA-15 was elucidated by detailed characterization of the individual synthesis steps using XPS and vibrational spectroscopy. The resulting surface vanadium oxide species (0-2.3 V/nm²), grafted on the inner pores of the SBA-15 silica matrix, consists of tetrahedrally coordinated vanadia as inferred from UV-VIS- and Raman spectroscopy. The prepared vanadia model catalysts were tested in the partial oxidation of methanol to formaldehyde yielding high formaldehyde selectivities of 94% at 350 °C. XPS and Raman analysis of the catalyst after reaction reveal the presence of methoxy as well as a significant amount of carbonaceous species on the surface. Our results demonstrate that a detailed understanding of partial oxidation reactions requires the combination of complementary spectroscopic techniques ultimately within one experimental set-up.     

Heteroepitaxial praseodymium sesquioxide films on Si(1 1 1): A new model catalyst system for praseodymium oxide based catalysts

The structure, growth and stoichiometry of heteroepitaxial Pr₂O₃ films on Si(1 1 1) were characterized by a combined RHEED, XRD, XPS and UPS study in view of future applications as a surface science model catalyst system. RHEED and XRD confirm the growth of a (0 0 0 1) oriented hexagonal Pr₂O₃ phase on Si(1 1 1), matching the surface symmetry by aligning the oxide in-plane direction along the Si azimuth. After an initial nucleation stage RHEED growth oscillation studies point to a Frank-van der Merwe growth mode up to a thickness of approximately 12 nm. XPS and UPS prove that the initial growth of the Pr₂O₃ layer on Si up to ∼1 nm thickness is characterized by an interface reaction with Si. Nevertheless stoichiometric Pr₂O₃ films of high crystalline quality form on top of these Pr-silicate containing interlayers.     

Characterization of the “native” surface thin film on pure polycrystalline iron: A high resolution XPS and TEM study

The characterization of the “native” surface thin film on pure polycrystalline iron has been studied by high resolution X-ray photoelectron (XP) spectroscopy of Fe 2p and O 1s regions. The film was allowed to form by exposing the sample to atmosphere at ambient conditions for a period of 1 h. The systematic approach used here includes the determination of curve fitting parameters from external standards and their use in fitting the raw data for the surface thin film. The quantitative high resolution XPS analysis involved an angle resolved study of the surface to determine the chemical composition and thickness of this native film. The film was found to be a mixture of Fe₃O₄ and Fe(OH)₂ with a thickness of 1.2 ± 0.3 nm. This conclusion is consistent with thermodynamics as indicated by the Pourbaix diagram for the Fe-H₂O system and the phase diagram for the Fe-oxygen system. A detailed TEM study of the native surface film also supports this conclusion.     

Synthesis of carbon supported palladium nanoparticles catalyst using a facile homogeneous precipitation-reduction reaction method for formic acid electrooxidation

A highly dispersed and ultrafine carbon supported Pd nanoparticles (Pd/C) catalyst is synthesized by a facile homogeneous precipitation-reduction reaction method. Under the appropriate pH conditions, [PdCl₄]²⁻ species in PdCl₂ solution are slowly transformed into the insoluble palladium oxide hydrate (PdO·H₂O) precipitation by heat treatment due to a slow hydrolysis reaction, which results in the generation of carbon supported PdO·H₂O nanoparticles (PdO·H₂O/C) sample with the high dispersion and small particle size. Consequently, a highly dispersed and ultrafine Pd/C catalyst can be synthesized by PdO·H₂O → Pd⁰ in situ reduction reaction path in the presence of NaBH₄. As a result, the resulting Pd/C catalyst possesses a significantly electrocatalytic performance for formic acid electrooxidation, which is attributed to the uniformly sized and highly dispersed nanostructure.     

Ultrasonic-assisted hydrothermal synthesis of cobalt oxide/nitrogen-doped graphene oxide hybrid as oxygen reduction reaction catalyst for Al-air battery

A persistent ultrasound-assisted hydrothermal method has been developed to prepare cobalt oxide incorporated nitrogen-doped graphene (Co₃O₄/N-GO) hybrids. The electrochemical behaviors and catalytic activity of the prepared hybrids have been systematically investigated as cathode materials for Al-air battery. The results show that ultrasonication can promote the yield ratio of Co₃O₄ from 63.1% to 70.6%. The prepared Co₃O₄/N-GO hybrid with ultrasonication exhibits better ORR activity over that without ultrasonication. The assembled Al-air battery using the ultrasonicated Co₃O₄/N-GO hybrid exhibited an average working voltage of 1.02 V in 4 M KOH electrolyte at 60 mA∙cm⁻², approximately 60 mV higher than that using hybrid without ultrasonication. This should be attributed to large number density of fine Co₃O₄ particles growing on the dispersed GO sheets under the persistent ultrasonication. The related ultrasonic mechanism has been discussed in details.     

Formation of interface and surface oxides on supported Pd nanoparticles

We have quantitatively studied the interaction between oxygen and an Fe₃O₄-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 Fe₃O₄ 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.     

A Mössbauer spectroscopic study of an industrial catalyst for dehydrogenation of etylbenzene to styrene

Iron oxide catalyst with spinel structure used for dehydrogenation of ethylbenzene is one kind of important catalyst in petrochemical industry. In this work several series of industrial catalyst were prepared with different components and different manufacturing processes. Mössbauer Spectroscopy has been used to determine the optimal components and the better manufacturing process for spinel structure formation. The results may prove useful for producing the industrial dehydrogenation catalyst with better catalytic property.     

Fractional distribution of graphene oxide and its potential as an efficient and reusable solid catalyst for esterification reactions

Graphene oxide (GrO) prepared by the Hummers method was separated into three different fractions (GrO₅₀₀₀, GrO₂₀₀₀, and GrO_res) on the basis of their dispersion stability in the water. Infrared, nuclear magnetic resonance, X‐ray photoelectron spectroscopy, and elemental analyses revealed that GrO₅₀₀₀ possesses a high degree of oxygen functionalities including phenolic, carboxylic, and ?OSO₂H groups, compared with the other fractions. The GrO₅₀₀₀ was found to be a highly efficient and reusable solid catalyst for the esterification of various carboxylic acids with a variety of alcohols to furnish corresponding esters in high to excellent yields. The catalytic activity of the GrO₅₀₀₀ was attributed to the ability of highly polar GrO₅₀₀₀ scaffold to adsorb/attract reactants, where the acid functionalities of GrO₅₀₀₀ facilitated the esterification process efficiently. The chemical and structural features of GrO₅₀₀₀ were discussed to understand the improved catalytic activity compared with GrO₂₀₀₀ and conventional solid acid catalysts. Copyright © 2014 John Wiley & Sons, Ltd.     

XPS depth profiling study on the passive oxide film of carbon steel in saturated calcium hydroxide solution and the effect of chloride on the film properties

X-ray photoelectron spectroscopy (XPS) was used to study the properties of passive oxide film that form on carbon steel in saturated calcium hydroxide solution and the effect of chloride on the film properties. The thickness of the oxide films was determined to be approximately 4 nm and was not affected by the exposure time. Near the film/substrate interface the concentration of the Fe²⁺ oxides was higher than the concentration of the Fe³⁺ oxides; the layers near the free surface of the film mostly contained Fe³⁺ oxides. Chloride exposure decreased the thickness of the oxide films and changed their stoichiometry such that near the film/substrate interface Fe³⁺/Fe²⁺ ratio increased.     

The characterization of oxygen surface arrangements on a heated model Pt3Pb bimetallic catalyst: A case study by SIMS

SIMS in the low damage mode is applied to characterize different surface arrangements of O on a heated polycrystalline Pt₃Pb sample which is exposed to several oxygen doses. Small oxygen doses at 600 K lead to a surface characterized by O in on-top positions principally. At higher doses oxygen saturation occurs which is probably accompanied by a transition to a reconstructed state of the treated surface. At 790 K immediately place-exchange between O and the substrate metal atoms happens after all doses supplied. The SIMS results are elucidated using recent presentations of the interaction of electronegative species with metallic surfaces. The connection between work function and oxygen surface arrangement is discussed in detail.