A. Adnot

Ce 3d XPS study of composite CexMn1 xO2 y wet oxidation catalysts

Abstract X-ray photoelectron spectroscopy (XPS) was used to investigate the valence state of cerium in unsupported composite Ce x Mn 1− x O 2− y catalysts. Manganese–cerium composite oxides are being widely used in sub- and supercritical catalytic wet oxidation for the treatment of wastewater containing toxic organic pollutants. The performance of such catalysts depends, among others, on the redox reactions involving CeO 2− x suboxides and manganese oxides. In order to investigate the surface chemistry of such environmental catalysts, 10 samples with Mn/Ce ratios ranging from 0/10 to 9/1—molar basis—were synthesized by co-precipitation and characterized as-prepared and after gold coating. Principal component analysis and curve fitting of Ce 3d and O 1s core-level spectra were used to determine the types of valence states and their proportions on the catalysts’ surface. The study revealed valence state modifications in cerium and changes in the amount of lattice oxygen depending on the Mn/Ce ratio and on whether or not the as-prepared catalysts were subjected to gold deposition. The proportion of surface Ce 3+ species in the non-stoichiometric CeO 2− x oxides was successfully quantified through linear correlation between v 2 ′ peak area and integral of the Ce 4+ 3d spectrum.

Composition–activity effects of Mn–Ce–O composites on phenol catalytic wet oxidation

Abstract Mn–Ce–O composite catalysts have been widely used in sub- and supercritical catalytic wet oxidation of toxic organics contained in aqueous streams. In order to investigate their composition–activity relationship, 11 samples with Ce/(Mn+Ce) atomic bulk ratios ranging from 0 to 100% were prepared by co-precipitation. Phenol was selected as a model pollutant and the catalytic oxidation was carried out in a batch slurry reactor using oxygen as the oxidizing agent under mild reaction conditions. The results showed that the catalytic activity was greatly influenced by the catalyst composition. The catalyst with Mn/Ce ratio=6/4 was found to be the most active in reducing both phenol concentration and total organic carbon (TOC). All catalysts were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature programmed reduction (TPR) and nitrogen adsorption techniques. Systematic shifts in binding energy, diffraction angle, and reduction temperature were observed in the XPS, XRD and TPR spectra, respectively. XPS and XRD data revealed the occurrence of significant interactions between Mn and Ce oxides, resulting in the evolution of textural, structural and oxidation state with composition. TPR analysis showed that the interaction between Mn and Ce greatly improved the oxygen storage capacity of manganese and cerium oxides as well as oxygen mobility on the surface of catalyst. Catalytic active sites have been ascribed to manganese oxide species exhibiting higher oxidation state. Furthermore, XPS revealed that the most active catalyst, i.e. Mn/Ce 6/4, exhibits an electron-rich surface which may be very important in the activation of adsorbed oxygen.

Structurally stable composite PdAg alloy membranes: Introduction of a diffusion barrier

Abstract This work investigates the improvement of the structural stability of PdAg alloy membranes by introduction of a diffusion barrier. Asymmetric PdAg films were deposited on porous stainless steel (SS) substrate by electroless plating. The formation of these alloys was achieved by annealing the as-deposited membranes at temperatures higher than Tamman temperatures of the alloy components in a hydrogen atmosphere. The composite PdAg membranes were characterized using XRD and Auger electron depth profiling. The atomic interdiffusion of silver and palladium resulted in PdAg alloys in an fcc structure. To improve the structural stability of PdAg alloy/SS membranes, an ultrathin intermediate layer of titanium nitride being 0.1 μm thick was introduced as a diffusion barrier between PdAg and the SS substrate. The Auger electron depth profiling analysis indicated that the improved membranes were thermally stable at temperatures as high as 973 K, and practical for the catalytic membrane reactor use. An estimation of diffusion coefficients revealed that the presence of hydrogen in the annealing atmosphere favoured the PdAg interdiffusion and thus the formation of PdAg alloys.

Characterization of basicity in alkaline cation faujasite zeolites - An XPS study using pyrrole as a probe molecule

An X-ray photoelectron spectroscopic method is proposed for the characterization of Lewis basic sites in alkali-cation faujasite zeolites. The method consists of deconvoluting the N1s XPS line of chemisorbed pyrrole and measuring the relative intensities of the peak components. It was found that whenever the zeolite sample contains two kinds of alkali cation, two different deconvoluted N1s peaks corresponding to pyrrole chemisorbed on these two sites are obtained. Combined with previous infrared observations, the results offer further evidence for the basic sites being the framework oxygens adjacent to the alkali cations in faujasite zeolites. The stronger the basic site, the lower the N1s binding energy level of the corresponding peak component is. An N1s peak corresponding to polymerized pyrrole species was also detected in the cases of LiY and NaY zeolites.

An ESCA study of the interaction of oxygen with the surface of ruthenium

The interaction of oxygen with the surface of a Ru wafer in the range of room temperature to 600°C was investigated by ESCA. Ru3d,3p,MVV, valence band and 01s spectra were recorded. Three different states of oxygen were observed on the Ru wafer surface. Definitive identification of two of the three states was achieved according to the multiplet structure in the photoemission spectra of oxygen and the binding energy of various states of oxygen. The oxygen species identified are O2- and dissolved atomic oxygen. The third oxygen state which gives rise to the peak at a BE of 532.3 eV in the O1s spectrum and 11.7 eV in the photoemission spectrum is tentatively assigned to a chemisorbed hydroperoxy radical. A discussion of the Ru oxidation process is also given.

Polypropylene natural aging studied by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) has been used to investigate the effect of natural aging on polypropylene (PP). The analysis showed a marked degradation of the surface, which is mainly caused by the contamination of the superficial structure by oxygen. This study was conducted with the aid of three samples of PP: the first was new, the second was aged under ambient atmosphere and protected from ambient light, and the third was exposed to both ambient atmosphere and ambient light. Spectral analysis shows that oxygen is the predominant element of contamination. The aging process induces a chemical modification of the surface, creating double bonds and bonding to more than one atom causing the polymer chain to break. The presence of ambient light through time seems to favour double bonding. According to the spectra obtained at different analysis angles, this type of bonding spreads and remains homogeneous to a depth of a few monolayers (8 nm). The comparison of this study with that of the literature has allowed us to propose a reaction mechanism for the polymer oxidation.

Surface segregation of PdAg membranes upon hydrogen permeation

Abstract PdAg membranes are permeable to hydrogen. Hydrogen treatment results in a small chemical shift (±0.1–0.2 eV) of Pd 3d core level but no change in the Ag3d level. A new valence band in the binding energy region of 7–9 eV corresponding to the interaction between H 1s and Pd4d appears on a hydrogen permeated membrane surface. Quantitative XPS analysis reveals that Pd segregates at the membrane surface toward the high hydrogen pressure side while Ag segregation occurs at the surface on the low hydrogen pressure side after hydrogen permeation. Both surface segregations are explained based on an MTCIP-1A (modern thermodynamic calculation of interface properties — first approximation) approach. It is concluded that hydrogen chemisorption induces palladium segregation on the PdAg membrane surface.

Spectroscopic and catalytic study of P-modified ZSM-5

Abstract Postsynthesis modification of ZSM-5 with phosphorus was performed by gas-phase adsorption of triphenylphosphine. IR spectra of adsorbed pyridine indicated an interaction of phosphorus species with Bronsted acid sites. ESCA analysis of noncalcined catalysts suggests a model for this interaction. A quantitative treatment of ESCA intensity ratios for the calcined catalysis before and after grinding allows one to calculate the size and loading of both extraparticle and intra-pore-lattice (IPL) phosphorus oxide particles. The IPL loading was found to be very close to the loading of exchanged phosphorus calculated from the IR of adsorbed pyridine. The product distribution of MTG conversion was found to be correlated with the extent of Bronsted acid site poisoning following exchange with phosphorus species.

Acid sites in Al-ZSM-22 and Fe-ZSM-22

A crystalline ferrisilicate zeolite having the framework structure of ZSM-22 is reported. The ferrisilicate was characterized by XRD, SEM t.g.a. t.p.d. of NH 3 , Ft i.r., XPS, and hydrocarbon sorption measurements. XRD patterns of as-synthesized as well as calcined ferrisilicate zeolite matched well with that of ZSM-22 zeolite except for some minor changes in the peak intensities. The Fe 3+ ions in the ferrisilicate zeolite were in the trivalent oxidation state as confirmed by XPS study. The white color of the as-synthesized as well as that of calcined ferrisilicate sample, ion-exchange capacity, and hydrocarbon sorption data have indicated that extraframework material was not present in major amount. The O 1s XPS spectra, however, showed the presence of small amounts of extraframework iron. The acidic properties of the ferrisilicate zeolite were studied by t.p.d. of NH 3 , Ft i.r. spectra in the OH as well in the pyridine regions, and the N 1s XPS spectra of chemisorbed pyridine. I.r. spectroscopy revealed a distinct absorption band at 3628 cm −1 assigned to structural OH groups of Fe-ZSM-22 associated with lattice iron. T.p.d of NH 3 and N 1s XPS spectra of chemisorbed pyridine showed that Bronsted acid sites present in ferrisilicate zeolite are weaker than those present in the Al-ZSM-22 zeolite. From the comparison of these results with those obtained with Al-ZSM-5 and Al-ZSM-22 zeolites, the following order for the acidic strength of the framework hydroxyls is derived: Al-ZSM-22 > Fe-ZSM-22 ≥ Al-ZSM-5

IR spectroscopy study of polypropylene natural aging

Structural modifications of a commercial polypropylene (PP) sample are studied by IR spectroscopy. Aging this sample under an ambient atmosphere with or without the presence of ambient light shows the oxidation process to be the most predominant. Spectral analysis reveals that the commercial sample is isotactic and also indicates that, for the new sample, oxygen establishes single bonds with carbon. However, through the aging process, spectral changes essentially occur in the regions of ∼3400, ∼1712, and 1170 cm−1 which correspond to hydroxyl and/or hydroperoxide groups, a carbonyl group, and CO, respectively. The deconvolution of the bands corresponding to a carbonyl group reveals the presence of a complicated mixture of oxidation products: aldehydes, ketones, esters, acids, peresters, and peracids. Spectral analysis also shows that the most favorable site for oxidation is at the methylene group.