X-ray photoelectron spectroscopy (XPS) for catalysts characterization

The general principles of X-ray photoelectron spectroscopy (XPS) as applied in the field of heterogeneous catalysis are reviewed. In particular, the use of this technique in the determination of chemical and physical changes of catalysts upon exposure to gaseous molecules and upon different thermal treatments is examined. Furthermore, examples of methods useful in obtaining the dispersion of supported catalysts are described and, for this purpose, theoretical models of the particle-support distribution are also discussed. The XPS characterization of supported Pd–Ag catalysts is reported, emphasising the advantages of using XPS to investigate surface segregation processes. In the case of supported Pd/Pt bimetallic catalysts it is shown how both, Auger and photoelectron peaks, characterized by different kinetic energies, allow to get depth profile non-destructive analysis. Finally the surface behaviour of CoMo catalysts, used for hydrodesulfurization reactions, is investigated on different supports and under different pre-treatment and reaction conditions.     

The ultrastructure of spruce kraft pulps studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS)

Surface properties of spruce (Picea abies) kraft pulps cooked for different times and further OD₀E₁D₁E₂D₂-bleached were investigated with atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). A rough correlation between the increasing relative amount of the fibrillar surface structure in AFM images and increasing O/C atomic ratio in XPS-spectra was found with proceeding delignification. At the end of cooking (120 min) only about 1/3 of the fibre surface consisted of cellulose. The detailed analysis of the relative peak areas of the different C1s components in the XPS-spectra indicated that the granules at the beginning of cooking at 170 °C consisted mainly of lignin and extractives. The analysis also showed that different steps of the bleaching sequence were quite specific in removing structural components. Furthermore, the lignin removal was shown not to result automatically in increased fraction of exposed cellulose surface, but could also lead in increased relative amount of surface extractives. Evidence for the high surface content of hemicelluloses for the D2-stage sample was observed. Hemicelluloses with both fibrillar and amorphous morphology were found to be present.     

XPS study of reductively and non-reductively modified coals

Two demineralised coals: Mequinenza and Illinois No. 6 have been subjected to reduction and reductive methylation in the potassium/liquid ammonia system and to non-reductive methylation by the Liotta method. The initial coals and the products obtained have been analysed by classical chemical methods and by the X-ray photoelectron spectroscopy (XPS) method. The coal surface has been found susceptible to oxidation by atmospheric oxygen, but the differences in the elemental composition between the surface and the bulk sample proved small. Deconvolution of the XPS spectrum has shown that the dominant form of oxygen in all samples is ether and hydroxyl oxygen and the dominant form of nitrogen is pyrrolic nitrogen. It has also been shown that the main sulphur species in the initial Mequinenza coal is sulphidic sulphur, while Illinois No. 6 is richer in thiophenic systems. The results have confirmed an earlier finding that modification of coals in the potassium/liquid ammonia system leads to elimination of sulphur from some thiophene compounds.     

X-ray absorption near-edge structure and photoelectron spectroscopy of single-walled carbon nanotubes modified by a HBr solution

X-ray absorption near-edge structure (XANES) spectroscopy and photoelectron spectroscopy (PES) have been used to investigate single-walled carbon nanotubes (SWNTs) modified by immersion in a HBr solution at room temperature. After treatment XANES spectra of SWNTs show a new pronounced feature, which has been assigned to new bonds between the sidewall of the SWNTs and Br atoms. This investigation demonstrates the unique capabilities of the XANES spectroscopy as a tool to achieve structural and bonding information of carbon nanotubes induced by chemical processes.     

X-ray photoelectron spectroscopy of polybithiophene–polystyrene composites

An investigation of the structure, stability, and charge distributions of conducting polybithiophene–polystyrene composite chemically synthetized using X-ray photoelectron spectroscopy (XPS) and electron microprobe analysis is described. XPS results confirm the reduction of the oxidant (Fe³⁺ is reduced to Fe²⁺ and Cu²⁺ to Cu⁺) during the bithiophene polymerization and indicate that the positive charges of doped polybithiophene are preferentially localized on the carbon atoms. Measurements versus ambient atmosphere exposure support a decreasing of the atomic ratio Cl/Fe or Cl/Cu and an increasing of the atomic ratio O/C, which could be responsible for the observed electrical conductivity instability. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1839–1845, 1997     

Heavy metal-adsorption on micas and clay minerals studied by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to study the adsorption of Cs-, Ba-, Cu-, Zn-, and Pb-ions on the external surfaces of various, well characterized 2:1 layer silicates (micas and illites).Before studying metal adsorption, it was necessary to determine the charge magnitude of the adsorption surface. This was done for chemically well-characterized micas (margarite, muscovite, sericite). The XPS analyses showed that the depth of analysis is about 15 Å. As a result it was possible to measure the surface- and interlayer ions on both sides of the outermost 2:1 layer. In determining the layer charges, the following strategy was used. The outer surface cations were replaced by Ba²⁺, giving, for ideal margarite an interlayer cation (Ca²⁺)/surface cation (Ba²⁺) ratio of 2:1 and in the case of muscovite a K⁺/Ba²⁺ ratio of 4:1. Deviations from these ratios indicate an asymmetry of layer charge in the outer sheet. Using the margarite, muscovite and sericite as standards, surface charge determination of a number of micas, illites, and I/S clays could be carried out by XPS.The properties of the metal-ions (charge, ionic radius, ionic potential), as well as layer charge characteristics of the clay, including surface charge magnitude and point of origin from tetrahedral or octahedral substitution, are factors which influence adsorption selectivity [Sposito, G., 1989. Surface reactions in natural aqueous colloidal solutions, G. Chimia, 43, 169–176]. The selection of previously well-characterized minerals, margarite, muscovite, celadonite, illite, montmorillonite, and beidellite for XPS study made it possible to relate these factors to heavy metal adsorption by the clay minerals.The results show that Cu²⁺ and Zn²⁺ are adsorbed as monovalent ions, presumably as (CuOH)¹⁺ and (ZnOH)¹⁺ hydroxy surface-complexes, due to their high ionic potential. Saturating the mica series with equimolar pairs of Cu–Zn and Cu–Pb, the ratios of Cu/Zn and Cu/Pb increase systematically with external surface charge. The higher the surface charge, the more selective is the exchange process for Cu with respect to Zn or Pb. Increasing external surface charge parallels increasing tetrahedral charge, which indicates that selectivity takes place at points of tetrahedral negativity on the crystallite surface, whereas octrahedral charge plays little role in the selective adsorption process.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.     

X-ray photoelectron spectroscopy study of anodically oxidized SIMFUEL surfaces

The surface composition of anodically oxidized SIMFUEL (doped uranium dioxide) has been determined as a function of applied potential over the range −500 to +500 mV (versus SCE). Cathodically cleaned UO₂ specimens were anodically oxidized for 1 h and subsequently analyzed by XPS. Using published binding energies, the U (4f_7/2) and O (1s) peaks were resolved into contributions from U^IV, U^V, U^VI, O^II, OH⁻ and H₂O. It was shown that over the potential range −500 to approximately +50 mV a thin surface layer of U^IV/U^V oxide (UO_2+x) formed. At more positive potentials, a U^VI hydrated oxide (UO₃·yH₂O) was deposited on the electrode surface. At very positive potentials (≥400 mV) the rapid anodic formation and hydrolysis of UO₂²⁺ led to local acidification in pores in the deposited UO₃·yH₂O layer and its subsequent re-dissolution.     

X-ray absorption near edge structure and X-ray photoelectron spectroscopy studies of chloride in passive oxide films

X-ray absorption near edge structure (XANES), utilizing both electron yield and X-ray fluorescence detectors, and X-ray photoelectron spectroscopy (XPS) were used to follow chloride uptake by oxide-covered aluminum in 0.1 M NaCl solutions. The aluminum samples were polarized at selected potentials below (less positive than) the pitting potential. The electron yield XANES and XPS showed multiple peaks. The XPS chloride spectra showed two distinct sets of doublets. One doublet is related to chloride on the surface and the second is related to chloride incorporated in the oxide film. The XANES results also showed two peaks which are attributed to chloride on the surface and in the bulk of the oxide.     

Surface analysis of emulsion adhesives by fast atom bombardment mass spectrometry and X-ray photoelectron spectroscopy

This paper reports an investigation by fast atom bombardment mass spectrometry (fabms) and X-ray photoelectron spectroscopy (xps) of the nature of surfaces of films cast from various emulsion adhesives against air and against polystyrene or polytetrafluoroethylene (ptfe) surfaces. An objective was to ascertain the extent to which the nature of the surfaces of films derived from emulsion adhesives depends upon the nature of the substrate against which the film is cast. The emulsion adhesives contained copolymers of vinyl acetate with vinyl neodecanoate (vaw), with ethylene (vae1, vae2, vae3) and with n-butyl acrylate (vaba). FABMS spectra show that the compositions of the air-dried surfaces of the films from the vavv, vae1, and vae2 emulsions are essentially those of the respective copolymers, that the surfaces of films from the vaba adhesives comprise mainly copolymer, but there is some dodecylbenzenesulphonate surfactant also present, and that the composition of the surface of films from the vae3 emulsion is dominated by potassium ions. The fabms spectra indicate no differences between the surfaces of vae3 adhesive films cast against polystyrene and ptfe on the one hand, and the surface of vae3 adhesive dried in air on the other. The fabms spectra clearly demonstrate that interfacial failure occurs between vae3 adhesive films and polystyrene and ptfe substrates when the films are separated from the substrates. XPS spectra show that the air- and polymer-cast surfaces of films from vae3 and vaba emulsions are very similar, and that the surfaces of films from the vae3 emulsions contain 5–6% of potassium together with significant levels of carboxylate moieties.     

sp/sp characterization of carbon materials from first-principles calculations: X-ray photoelectron versus high energy electron energy-loss spectroscopy techniques

We have performed X-ray photoelectron spectroscopy (XPS) and high energy electron energy-loss spectroscopy (HEELS) calculations from first principles on a series of Monte Carlo generated amorphous carbon materials and have used a technique which separates the π* and σ* components of the energy-loss near-edge structure spectra of carbon materials on the basis of the ab initio electronic structure calculations of graphite to determine the sp³ fraction of the carbon systems. While the XPS technique is found to probe the local coordination geometry, the sp³ fractions resulting from the HEELS technique are found to be in very good agreement with those based on the π-orbital axis vector analysis which accounts for the effects of non-planarity in 3-coordinated systems.     

Nano-particles seeding and its characterization by X-ray photoelectron spectroscopy (XPS)

We present a new procedure for pretreatment seeding by ultrasonic agitation of silicon substrates in diamond nano-powder suspensions to which HF and KOH were added X-ray photoelectron spectroscopy (XPS) was used to measure the surface coverage by diamond nuclei immediately after the pretreatment. Coverage percentages of 70, 40 and 55% were obtained for the HF, KOH and the original diamond slurry, respectively. The seeding density (S_D) was calculated from the known nano-particles size, determined independently from X-ray diffraction of the powder. For nano-particle size of ∼6 nm, we obtain nominal seeding densities of the order ∼10¹² cm⁻². The advantage of the high coverage was most evident for films deposited at low substrate temperature (570 °C). The potential of the new seeding procedure and the XPS characterization method are discussed.     

X-ray photoelectron spectroscopy study of sodium reactions in carbon cathode blocks of aluminium oxide reduction cells

Aluminium oxide reduction was performed in a laboratory electrolysis cell with different industrial carbon cathode blocks (semi-graphitic, graphitic, and graphitized blocks). During electrolysis, sodium species migrate from the bath into the carbon cathode. Consequences of this migration include expansion of the blocks—the so-called sodium swelling—that may lead to failure of the cell. Characterisation of the blocks by XPS indicated that in addition to ionic sodium species (e.g. NaF and NaHCO₃), two different types of metallic sodium were present in the cathodes. One type of metallic sodium is associated with a degradation of the graphitic structure, suggesting that this sodium is intercalated between the graphene layers, whereas the other type of metallic sodium was most probably present in micropores. Both types of metallic sodium were detected in semi-graphitic blocks while only the “micropore” sodium was found in graphitic and graphitized blocks. The metallic sodium was remarkably stable in the laboratory atmosphere, probably due to the fact that, after electrolysis, the entire porosity of the carbon cathode is filled with penetrated bath. This limits the access of oxygen and humidity to the metallic sodium.     

Surface carbonation of synthetic C-S-H samples: A comparison between fresh and aged C-S-H using X-ray photoelectron spectroscopy

This paper presents a continuation of studies into silicate anion structure using X-ray photoelectron spectroscopy (XPS). A series of C-S-H samples have been prepared mechanochemically, and then stored under ambient conditions for six months. Storage led to surface carbonation, the extent of which was dependent upon the calcium/silicon ratio of the fresh sample. Carbonation arose through decalcification of the C-S-H, leading to increased silicate polymerisation. The surfaces of the most calcium-rich phases (C/S = 1.33 and 1.50) underwent complete decalcification to yield silica (possibly containing some silanol groups) and calcium carbonate. Carbonation, and hence changes in silicate anion structure, was minimal for the C-S-H phases with C/S = 0.67 and 0.75.     

Examination of de-coking of promoted (Co, Ni) Mo/γ-Al2O3 catalysts by X-ray photoelectron spectroscopy

Promoted (Co, Ni)-Mo/γ-Al₂O₃ hydrodesulphurization catalysts have been decoked using hydrogen gasification. The catalysts were exposed to a benzene, toluene and xylene (BTX) feed for one week until a significant level of coke had been built up on the surface. The catalysts were then decoked by gasifi- cation at 400°C in hydrogen. The decoking was monitored using the X-ray photoelectron spectroscopy C/Al ratio. After 6 h gasification there was a tenfold decrease in the C/A1 ratio indicating a significant amount of decoking. Furthermore, the initial activity was regained as determined by the conversion of a standard BTX feed.     

Atomic force microscopy,X-ray diffraction,X-ray photoelectron spectroscopy and thermal studies of the new melamine fiber

This paper reports the characterization of unaged and aged melamine fibers using various characterization techniques including atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis. Since melamine fiber is a relatively new fiber, very few studies on its characterization have been made. Morphological studies of the fiber surface using SEM display die lines running along the filament surface, which are characteristics of synthetic fibers and generally occur during spinning of the molten prepolymer through the spinnerets. AFM studies show that the surface of a melamine fiber filament contains a large number of hills and valleys, which are triangular in shape. AFM roughness analysis shows that melamine fiber surface is considerably rough which may aid in adhesion of the fiber with polymeric matrices. Ageing causes an increase in the surface roughness with simultaneous increase in the crystallinity of the fiber from 19.4% to 22.6%. In XPS studies, high concentrations of carbonyl and hydroxyl groups on the filament surface have been detected. Ageing causes a reduction in the hydroxyl group concentration and an increase in the carbonyl group concentration due to surface oxidation. The reduction in the surface hydroxyl groups due to ageing has also been detected in the Fourier-Transform infrared (FT-IR) spectra of the aged fibers. Thermogravimetric (TG) studies reveal a high thermal stability of the melamine fiber even in an oxidative environment such as air.     

Impact of Mn2+-Si4+ co-substitution on the electronic structure of Zn0.3Mn0.7Fe2O4 ferrites studied by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) has been employed to explore the electronic structure of Zn_0.3Mn_0.7+xSi_xFe_2-2xO₄ (x = 0.0–0.3) ferrite series. The Si2p XPS spectra insinuated the presence of Si ions in the +4 valence state. The elemental Si⁰ and suboxide SiO_x are present in the system, the former showing an increase and the latter a decrease in atomic percentage upon Mn–Si substitution. It is also inferred that a fraction of Si⁰ might be residing at the grain boundaries; however, more studies are required to substantiate this. The Fe2p XPS spectra stipulate that ferrous and ferric ions co-occur in the system. The ferrous ions occupy the octahedral sites while the ferric ions dwell on both the octahedral and the tetrahedral sites. The O1s spectra indicate a remarkable increase in the oxygen defects with increasing Mn–Si substitution (x). The Mn2p XPS data indicate that the Mn⁺² states show an overall increasing tendency with increasing Mn–Si concentration. Also, the Mn⁺⁴/Mn⁺³ ratio shows an increment with an increase in Mn–Si substitution.     

XPS Analysis of Submicrometer Barium Titanate Powder

A commercial submicrometer BaTiO₃ powder was analyzed using X-ray photoelectron spectroscopy. The analysis revealed the powder surfaces to be covered with a layer of physisorbed H₂O and chemisorbed –OH ions. A BaCO₃ residual not detected with XPS was shown to be present in the powder using X-ray diffraction, suggesting that the carbonate takes the form of discrete particles rather than of a continuous surface layer. A relaxed surface phase detected in previous XPS analyses of bulk BaTiO₃ was also shown to be present. Depth profiling revealed the powder surfaces to be Ti-rich, confirming the presence of a phase, or phases, to stoichiometrically balance the barium carbonate.     

Study of sulphur in Assam coals by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) has been used to study the nature and content of sulphur contained in coal samples from Assam. In each case two sulphur 2p (S 2p) peaks were observed. The 169–171 eV 2p peak corresponds to sulphate, whilst the 164–165 eV peak is assigned to S in iron sulphide compounds such as pyrite, marcasite, etc. and also organic sulphur. The complete absence of a 169–171 eV S peak from a coal sample from which both pyritic and sulphate sulphur were removed by hydrodesulphurization strongly indicates that the remaining 164.7 eV S 2p peak corresponds to the total organic sulphur present in the coal.