Electron spectroscopy (ESCA) studies of RuCu catalysts

Electron spectroscopy (ESCA) studies were conducted on a series of unsupported ruthenium-copper catalysts with copper contents ranging from 1.5 to 5 at%. Previous chemisorption and catalytic studies had indicated that the copper in these catalysts tends to cover the surface of the ruthenium. The ESCA data, as represented by the ratio of the intensity of the Cu $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ peak to that of the Ru $\text{3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ peak, are consistent with this view. The ESCA data were obtained with and without in situ hydrogen pretreatment of the catalysts at elevated temperature. When data on a number of ruthenium-copper catalysts are considered, there is an inverse correlation between hydrogen chemisorption capacity and the Cu/Ru intensity ratio obtained in the ESCA measurements. Moreover, when catalytic activity for ethane hydrogenolysis is considered as a function of the Cu/Ru intensity ratio, a very pronounced inverse correlation is obtained. With regard to the electronic structure of ruthenium-copper catalysts, the ESCA data showed no evidence of shifts in core level binding energies for either copper or ruthenium.     

Esca investigations of some NiO/SiO2 and NiO—Al2 O3 /SiO2 catalysts

The ESCA spectra of a series of NiO/SiO₂ and NiO—Al₂ O₃/SiO₂ catalysts are reported, together with those of some reference compounds. The positions and shapes of the lines, in conjunction with a quantitative surface analysis from relative intensities, allow the identification of different surface phases, e.g. an NiO-like phase in the impregnated catalysts with very low catalytic activity and an Ni talc-like phase in the precipitated catalysts which have higher activity. The addition of Al₂O₃ has a great influence on the surface structure (formation of alumosilicate).     

A method for measuring the proportion of different plane orientations in metal supported catalysts by gas chemisorption

A method is presented, which allows us to calculate the percentage of metal surface atoms with different coordination numbers, on metal supported catalysts. This method is based on gas-metal stoichiometry values reported for the chemisorption of H₂, O₂ and CO on well defined monocrystals, and on gas chemisorption measurements on metal supported catalysts. It has here been applied to a series of Pd/SiO₂ catalysts.     

ESCA studies of Cr-Co alloys

ESCA examination of films formed on Cr-Co alloys after immersion in 0.1M NaCl for 24 h has shown that the thickness of passive films decreased with an increase in chromium content. Surface films consisted of chromium and cobalt oxides as Cr₂O₃ and CoO. The amount of CoO in the surface film of the alloy was decreased with an increase in chromium but Cr₂O₃ was found at a greater depth in the passive film at any composition. Cr₂O₃ was a major component of the surface film when the chromium content in the alloy was 10% or higher. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate. Both Co-10 wt.% Cr and Co-30 wt.% Cr alloys investigated showed a lower corrosion rate than the Co-5 wt.% Cr alloy. Corrosion rate measured could be correlated to the surface film composition and structure as determined by ESCA.     

Use of X-ray photoelectron and Mössbauer spectroscopies in the study of iron pentacyanide complexes

A number of iron pentacyanide complexes of the Na_itn [Fe(CN)₅X] type have been synthesized. The compounds have been examined by X-ray photoelectron (ESCA) and Mössbauer spectroscopies. The influence of the nature of ligand X upon the properties of iron- (II) pentacyanide complexes has been studied. The donor properties of dinitrogen as a ligand have been compared with those of other nitrogen-containing ligands. A correlation has been established between the ESCA and Mössbauer data.     

Efficiency of Pd impregnated sol-gel derived γ-alumina porous spheres as catalyst

Porous γ-alumina spheres having surface area as large as 300 m ² /gm are obtained by using sol-gel process. γ-alumina supported Pd catalyst is synthesized by soaking it in PdCl ₂ solution followed by drying and heating at 300 ^° C/1 hr in hydrogen atmosphere. Specific Surface Area of Pd coated alumina remained unaltered. The scanning electron microscopy revealed uniform coating of Pd metal on the surface. The equivalent spherical diameter (ESD) of the coated Pd metal particles is estimated to be 50. The catalytic efficiency is evaluated by H ₂ -O ₂ combination reaction. The performance of this supported catalyst is found to be much superior in comparison with Pd catalyst realized on commercially available alumina having surface area of 80 m ² /gm. The improved efficiency of the catalyst could be co-related to enhanced surface of nano-structured Pd available for catalytic action. The added advantage of spherical shape of the support material appeared in uniform and homogeneous loading of the catalyst bed. The results highlight the important role played by support material in these type of catalysts. Received 24 November 2000     

Diffraction of picosecond bulk longitudinal and shear waves in micron thick films

Investigation of thin metallic film properties by means of picosecond ultrasonics [C. Thomsen et al., Phys. Rev. Lett. 53, 989 (1984)] has been under the scope of several studies. Generation of longitudinal and shear waves [T. Pézeril et al., Phys. Rev. B 73, 132301 (2006); O. Matsuda et al., Phys. Rev. Lett. 93, 095501 (2004)] with a wave vector normal to the film free surface has been demonstrated. Such measurements cannot provide complete information about properties of anisotropic films. Extreme focusing of a laser pump beam (≈0.5 μm) on the sample surface has recently allowed us to provide evidence of picosecond acoustic diffraction in thin metallic films (≈1 μm) [C. Rossignol et al., Phys. Rev. Lett. 94, 166106 (2005)]. The resulting longitudinal and shear wavefronts propagate at group velocity through the bulk of the film. To interpret the received signals, source directivity diagrams are calculated taking into account material anisotropy, optical penetration, and laser beam width on the sample surface. It is shown that acoustic diffraction increases with optical penetration, so competing with the increasing of directivity caused by beam width. Reflection with mode conversion at the film-substrate interface is discussed.     

Characterization of Supported Gold Catalysts with 197Au Mössbauer Effect Spectroscopy

We report on a ¹⁹⁷Au Mössbauer study of several types of supported gold catalysts. Differences in particle size show up in the Mössbauer spectra by a change in the relative weight of the spectral contribution of the surface atoms. The presence of ionic gold in active gold catalysts is not observed. The spectra can be interpreted in terms of bulk-like contributions from the inner-core atoms plus contributions from the outermost atoms at the surface of the particles.     

In-situ X-ray photoelectron spectroscopy study of supported gallium oxide

In-situ XPS study of supported gallium oxide catalysts prepared by impregnation of different metal oxides supports (SiO₂, γ-Al₂O₃, TiO₂, and MgO) has been performed. It has been found that the formation of metallic gallium takes place upon high-temperature reduction of the catalysts in flowing hydrogen. Relative concentrations of metallic gallium were most high in the cases of alumina- and titania-supported catalysts. It was suggested that interaction of gallium oxide with a support surface favors the formation of metallic gallium upon high-temperature reduction. A possible mechanism of metallic gallium formation upon the reduction is discussed.     

An ESCA study of iron sulfidation in H2S

The surface reaction of polycrystalline iron with H₂S has been studied in situ by means of ESCA in the temperature range 100–773 K for H₂S at     

X-ray photoionization cross sections for quantitative analysis

The relative photoemission intensities from subshells of 51 elements and simple compounds have been measured using X-ray photoelectron spectroscopy. The chemical composition of each specimen surface was monitored by Auger electron spectroscopy and contamination was minimized by Ar⁺ ion bombardment. Experimental photoelectron cross sections derived for MgKα (1254 eV) radiation were compared with theoretical Hartree-Slater subshell photoionization cross sections calculated by Scofield. Good correlation (r = 0.96) between theory and experiment was observed over two orders of magnitude for most of the elements studied. Relative cross sections for subshells of a given element are also consistent with theoretical values, with several notable exceptions. The results indicate that quantitative analysis of ESCA spectra is possible using the Scofield cross sections.     

XPS characterization of supported Ziegler-Natta catalysts

Surface analytical technique ESCA (electron spectrometer for chemical analysis) has been used for analysis of catalysts used in propylene polymerization. As a result of this analysis it has been shown that productivity of a catalyst can be correlated to Ti/Mg atomic ratio that indicates dispersion of titanium atoms on magnesium support. A quantitative indicator of productivity, i.e. “titanium index” has also been evaluated for studied catalysts.     

ESCA studies of Ni-Cr alloys

ESCA examination on Ni-Cr alloys has shown that a thin passive film was formed after 24 h immersion in 0.1 M NaCl. The film contained only chromium oxide in the form of Cr₂O₃. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate of the alloys. Both Ni-10 wt. % Cr and Ni-20 wt. % Cr alloys showed a slightly higher corrosion rate than the Ni-40 wt. % Cr alloy.The present ESCA study of the Ni-Cr system is part of our programme which involves an examination of the four binary alloy systems Fe-Si, Cr-Co, Ni-Cr, and Mo-Ni [1]. The aim is to correlate the structure and composition of the passive films formed in 0.1 M NaCl to the corrosion behaviour in the same solution.     

The state of the components in Cu-CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> catalysts for selective oxidation of CO

Cu-CeO₂ catalysts deposited on a commercial ZrO₂ support have been investigated. The catalyst composition has been optimized: the optimum copper content was found to be 5–10 wt % at a cerium oxide content of 23 wt %. The catalysts were investigated by X-ray diffraction analysis and X-ray photoelectron spectroscopy. According to the X-ray diffraction data, the support consists of the monoclinic ZrO₂ phase and cubic CeO₂ phase with an enlarged lattice parameter, while the catalysts contain CuO. The X-ray photoelectron data indicate the presence of a highly dispersed CuO₂ phase interacting with cerium oxide and zirconia on the surface of the catalyst having the optimum copper content.     

Surface investigation of solids by the statical method of secondary ion mass spectroscopy (SIMS)

The physical and chemical properties of a solid surface are determined by its uppermost monolayers. Besides other methods like Auger electron spectroscopy (AES) and X-ray electron spectroscopy (ESCA) for example, the chemical composition of these uppermost monolayers can be investigated by the statical method of secondary ion mass spectroscopy (SIMS). In this method a relatively large target area (0.1 cm²) is bombarded with a small primary ion current density (10^?9 A cm^?2). Thus a sputtering time of several hours is achieved for an individual monolayer. A mass analysis of the emitted positive and negative secondary ions gives information about the chemical composition of the uppermost monomolecular layer of the bombarded surface. Important features of SIMS are: detection of chemical compounds; isotope sensitivity; detection of hydrogen and its compounds ; depth resolution in the range of a single monolayer ; low detection limits (< 10^?6 monolayer or < 10^?14 g) for many elements and compounds.The capacity of this method is demonstrated using as examples the initial surface oxidation of metals and semiconductors and adsorption phenomena on clean metal surfaces. In some special cases additional information on the chemical composition of the uppermost monolayer can be obtained by the electron induced ion emission from the surface.     

High resolution surface imaging of Co/ZrO2 catalyst by TOF-SIMS

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a useful tool for the characterization of supported catalyst. Especially, the possibility of surface imaging appears very interesting. The images of not only elements distribution but also molecules or their fragments may be performed. Owing to the fast improvement of time-of-flight secondary ions mass spectrometers, obtaining the surface images with lateral resolution even below 100 nm is possible. The application of TOF-SIMS to the high resolution surface imaging of catalysts surfaces was shown. The influence of the preparation conditions of Co/ZrO₂ catalyst on the dispersion and composition of the active phase was investigated. The Co⁺/Zr⁺ and CoOH⁺/Zr⁺ intensity ratio values were calculated.     

Deposition of sputtered iridium oxide—Influence of oxygen flow in the reactor on the film properties

Thin films of iridium oxide have been deposited by reactive magnetron sputtering. The influence of oxygen partial pressure in the sputtering plasma on the composition, surface structure and morphology of the films has been studied by XRD, SEM and AFM analysis. An optimal combination of sputtering parameters yields stable microporous amorphous films with highly extended fractal surface. The electrochemical properties of these films have been investigated in view of their application as catalysts for water splitting, using the electrochemical techniques of cyclovoltammetry, electrochemical impedance spectroscopy, and steady state polarization. The SIROFs have shown an excellent electrochemical reversibility and a high catalytic activity toward the oxygen evolution reaction in 0.5 M H₂SO₄. A current density of 150 mA cm⁻² at potential of 1.7 V (versus Ag/AgCl) has been obtained at catalyst load of only 100 μg cm⁻². These results combined with the established long-term mechanical stability of the sputtered iridium oxide films (SIROFs) proved the advantages of the reactive magnetron sputtering as simple and reliable method for preparation of catalysts with precisely controlled composition, loading, and surface characteristics.     

Laser Raman Spectra of Transition Metal Oxides and Catalysts

Abstract

During recent years the study of the vibrational structure of catalysts by laser Raman spectroscopy (LRS) and of the interfacial properties of adsorbed species on solid surfaces by resonance Raman spectroscopy (RRS) and surface-enhanced Raman spectroscopy (SERS) have comprised one of the major research activities in the area of Raman spectroscopy [1–10] as applied to catalysts [11, 12].     

An x-ray photoelectron spectroscopy study of the activated cobalt-molybdenum-alumina hydrodesulfurization catalysts

ESCA was used to monitor the chemical modification of a series of commercial cobalt-molybdenum-alumina hydrodesulfurization catalysts subjected to different reaction conditions. In the oxide form, the surface phase $\text{resembles}$ MoO₃ and CoAl₂O₄. Reduction and sulfiding using a 15:85 H₂S/H₂ mixture shifts the molybdenum peaks to the region of MoS₂. Part of the cobalt is apparently reduced and sulfided, but this species is susceptible to re-oxidation by moist air. The supported cobalt which is not changed by this treatment is interpreted as actually being CoAl₂O₄ formed by interaction of the dispersed material with the support during calcination. The presence of this aluminate phase seems to be important for the stability of the catalyst under the harsh conditions of industrial usage. Reduction of the oxide form by pure hydrogen leaves the molybdenum binding energy unchanged and shifts the cobalt peak to higher binding energy. This is interpreted as arising from a shift in reference level and it is concluded that the actual transformation     

Study to improve the quality of a Mexican straight run gasoil over NiMo/γ-Al2O3 catalysts

Four NiMo catalyst supported on Al₂O₃ with different textural properties have been studied in the hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrodearomatization (HDA) of a Mexican straight run gasoil (SRGO). All reactions were carried out at three different temperatures 613, 633, and 653 K. Alumina supports were analysed by pyridine FTIR-TPD and nitrogen physisorption in order to determine their surface acidity and textural properties, respectively. TPR studies of the NiMo catalysts were analysed to correlate their hydrogenating properties. Metallic particles were characterized (after sulfidation) using transmission electron microscopy (TEM). Catalytic activities are discussed in relation to the physicochemical properties of NiMo catalysts. The importance of textural properties on coke deposition has been emphasized. The results of catalytic activity of these materials varied depending on dispersed MoS particles and pore distribution in final catalysts. The optimum pore diameter was found around 80 Å for HDS and HDN.