E. I. Vovk

An XPS study of the oxidation of noble metal particles evaporated onto the surface of an oxide support in their reaction with NO x

The interaction of the model catalysts Rh/Al2O3, Pd/Al2O3, Pt/Al2O3, and Pt/SiO2 with NOx (mixture of 10 Torr of NO and 10 Torr of O2) was studied by X-ray photoelectron spectroscopy (XPS). Samples of the model catalysts were prepared under vacuum conditions as oxide films ≥100 Å in thickness on tantalum foil with evaporated platinum-group metal particles. According to transmission electron microscopic data, the platinum-group metal particle size was several nanometers. It was found by XPS that the oxidation of Rh and Pd nanoparticles in their interaction with NOx occurs already at room temperature. The particles of platinum were more stable: their oxidation under the action of NOx was observed at elevated temperatures of ∼300°C. At room temperature, the interaction of platinum nanoparticles with NOx hypothetically leads to the dissolution (insertion) of oxygen atoms in the bulk of the particles with the retention of their metallic nature. It was found that dissolved oxygen is much more readily reducible by hydrogen than the lattice oxygen of the platinum oxide particles.

Use of the differential charging effect in XPS to determine the nature of surface compounds resulting from the interaction of a Pt/BaCO3

A Pt/BaCO3/Al2O3 model NOx storage-reduction catalyst, which was prepared as a thin film (∼100 Å) on the surface of tantalum foil, was studied by X-ray photoelectron spectroscopy (XPS). It was found that the Pt/BaCO3 and Pt/Al2O3 catalyst constituents acquired different surface charges in the course of photoelectron emission; that is, differential charging occurred. An analysis of this effect allowed us to determine the nature of the products formed in the interaction of the catalyst with a mixture of NO (260 Pa) + O2 (2600 Pa) + H2O (525 Pa) at 250°C followed by reduction with a mixture of CO (2100 Pa) + H2O (525 Pa) at 450°C. It was found that barium carbonate was converted into barium nitrate as a result of reaction with NOx on the surface of BaCO3. Simultaneously, platinum supported on both BaCO3 and Al2O3 was oxidized. The reduction of the catalyst treated with a mixture containing NO resulted in nitrate decomposition and regeneration of a carbonate coating on the surface of BaCO3; this is accompanied by the reduction of oxidized platinum particles to platinum metal.

Use of the differential charging effect in XPS to determine the nature of surface compounds resulting from the interaction of a Pt/(BaCO3 + CeO2) model catalyst with SOx

Changes in the chemical composition of the surface of a Pt/(BaCO3 + CeO2) model NOx storage-reduction catalyst upon its interaction with SOx (SO2 (260 Pa) + O2 (2600 Pa) + H2O (525 Pa)) followed by regeneration in a mixture of CO (2100 Pa) with H2O (525 Pa) were studied by X-ray photoelectron spectroscopy (XPS). Model catalyst samples were prepared as a thin film (about several hundreds of angstrom units in thickness) on the surface of tantalum foil coated with a layer of aluminum oxide (∼100 Å). It was found that the Pt/BaCO3 and Pt/CeO2 catalyst constituents acquired different surface charges (differential charging) in the course of photoelectron emission; because of this, it was possible to determine the nature of surface compounds formed as a result of the interaction of the catalyst with a reaction atmosphere. It was found that barium carbonate was converted into barium sulfate as a result of reaction with SOx on the surface of BaCO3 at 150°C. As the treatment temperature in SOx was increased to 300°C, the formation of sulfate on the surface of CeO2 was observed. The sulfatization of CeO2 was accompanied by the reduction of Ce(IV) to Ce(III). The regeneration reaction of the catalyst treated in SOx at 300°C resulted in the consecutive decomposition of cerium(III) sulfate at ≤500°C and then barium sulfate at 600–700°C. Upon the decomposition of BaSO4, a portion of sulfur was converted into a sulfide state, probably, because of the formation of BaS.

Size effect in the oxidation–reduction processes of platinum particles supported onto silicon dioxide

The interaction of the Pt/SiO2 model catalysts as thin films on the surface of tantalum supports with a mixture of NO + O2 (1: 1) was studied by X-ray photoelectron spectroscopy. The pressure of the reaction mixture was varied from 6 to 64 mbar, and the temperature was varied from room temperature to 500°C. Two types of the catalysts, in which the Pt/Si atomic ratios were ~0.1 and ~0.3 (0.1-Pt/SiO2 and 0.3-Pt/SiO2, respectively) according to the XPS data, were studied. In 0.1-Pt/SiO2, the particles of platinum predominantly had a size from 1 to 2.5 nm; a wide Pt particle size distribution in a range from 1 to 15 nm with a maximum at ~4 nm was characteristic of 0.3-Pt/SiO2. The interaction of all of the samples with NO + O2 at room temperature led to the dissolution of oxygen atoms in the bulk of platinum metal particles. As the reaction temperature was increased, PtOx platinum oxide particles were formed: from small Pt particles in 0.1-Pt/SiO2 at 300°C and from larger particles in 0.3-Pt/SiO2 at 400–500°C. It was established that the reactivity of platinum oxide particles toward hydrogen also depended on the particle size. The small particles of platinum oxide were converted into platinum metal under the action of hydrogen (16 mbar) at 300°C. The coarse particles of PtOx in the samples of 0.3-Pt/SiO2 were reduced much more easily starting with room temperature.

Analysis of the oxidation state of platinum particles in supported catalysts by double differentiation of XPS lines

In the work the double differentiation of functions describing the Pt4f7/2 band in the XPS spectra of model supported Pt/SiO2 catalysts is performed in order to determine the number of different chemical states of platinum particles. The functions for the differentiation are obtained by the deconvolution of the experimental spectral contour into two spin-orbit components. As a result of the performed analysis of the number and position of the minima of the second derivative of the function of Pt437/2 the conditions of the oxidation of platinum particles in the Pt/SiO2 sample on treating in a NO + O2 mixture and the reduction of platinum oxide particles on interacting of the PtOx/SiO2 sample with hydrogen are determined.

AN XPS STUDY OF THE INTERACTION OF MODEL Ba/TiO2 AND Ba/ZrO2 NSR CATALYSTS WITH NO2

X-ray photoelectron spectroscopy is used to study the interaction of model NO2 storage-reduction catalysts (NSR catalysts) Ba/TiO2 and Ba/ZrO2 with NO2. The catalysts are prepared on the surface of ultrathin Al2O3 film substrates obtained by the FeCrAl alloy oxidation. It is shown that at room temperature the model catalysts react with NO2 with the successive formation of surface barium nitrite and nitrate. The NO2 reduction with the formation of barium nitrite at the initial step of the interaction is assumed to be accompanied by the oxidation of residual metallic barium and amorphous carbon impurity. It is found that the formation of barium nitrate proceeds more efficiently on Ba/ZrO2 rather than on Ba/TiO2.

Mechanism of the Reaction NO + H 2 on the Pt(100)-hex Surface under Conditions of the Spatially Nonuniform Distribution of Reacting Species

The interaction of hydrogen with NOads/1 × 1 islands produced by NO adsorption on the reconstructed surface Pt(100)-hex was studied by high-resolution electron energy loss spectroscopy (HREELS) and the temperature-programmed reaction (TPR) method. The islands are areas of the unreconstructed surface Pt(100)-1 × 1 saturated with NOads molecules. The hexagonal phase around these islands adsorbs much more hydrogen near room temperature than does the clean Pt(100)-hex surface. It is assumed that hydrogen is adsorbed on the hexagonal surface areas that are adjacent to, and are modified by, the NOads/1 × 1 islands. The reaction of adsorbed hydrogen atoms with NOads takes place upon heating and has the character of so-called surface explosion. The TPR peaks of the products of this reaction—nitrogen and water—occur at Tdes ∼ 365–370 K, their full width at half-maximum being ∼5–10 K. In the case of the NOads/1 × 1 islands preactivated by heating in vacuo above the NO desorption onset temperature (375–425 K), after the admission of hydrogen at 300 K, the reaction proceeds in an autocatalytic regime and the product formation rate increases monotonically at its initial stage. In the case of activation at 375 K, during the initial, slow stage of the reaction (induction period), hydrogen reacts with nitric oxide molecules bound to structure defects (NOdef). After activation at 425 K, the induction period is characterized by the formation and consumption of imido species (NHads). It is assumed that NHads formation involves Nads atoms that have resulted from NOads dissociation on defects upon thermal activation. The induction period is followed by a rapid stage of the reaction, during which hydrogen reacts with NO1 × 1 molecules adsorbed on 1 × 1 areas, irrespective of the activation temperature. After the completion of the reaction, the areas of the unreconstructed phase 1 × 1 are saturated with adsorbed hydrogen. The formation of Hads is accompanied by the formation of a small amount of amino species (NH2ads).

Catalytic Activity of the Oxide Catalysts Based on Ni 0.75 Co 2.25 O 4 Modified with Cesium Cations in a Reaction of N 2 O Decomposition

The Ni0.75Co2.25O4 catalysts were prepared by a coprecipitation method and modified with cesium cations by impregnation with a solution of cesium nitrate or cesium nitrate with citric acid and ethylene glycol additives (the Pechini method). The catalysts obtained were investigated by X-ray diffraction analysis, the BET method, X-ray photoelectron spectroscopy, temperature-programmed reduction, and the temperatureprogrammed desorption of oxygen. The activity of the samples in a reaction of nitrous oxide decomposition was determined at temperatures of 200–300°C, in particular, in the presence of oxygen and water in the reaction mixture. It was found that the use of the Pechini method for supporting Cs makes it possible to obtain a more active catalyst, as compared with that prepared by impregnation with cesium nitrate, at the same cesium content (~2%) of the samples.

Comparison of thermal stability of gold nanoparticles deposited on Al2O3 and Fe2O3 in the CO + O2 reaction medium

A method for the synthesis of catalytic systems consisting of gold particles deposited on thin Al2O3 and Fe2O3 films supported by perforated carbon films was developed. The prepared samples were treated in a chamber of the photoelectron spectrometer under conditions of CO oxidation (5 mbar of CO + 5 mbar of O2, 200 ℃) and then examined under a microscope. A comparative study using X-ray photoelectron spectroscopy and transmission electron microscopy showed that the size of gold nanoparticles deposited on the Fe2O3 surface increased upon heating of the reaction medium, whereas the size of those deposited on the Al2O3 surface remained unchanged.

Effect of the composition of the reaction atmosphere on the thermal stability of highly dispersed gold particles on an oxide support (Au/Al2O3 system)

Metal gold particles were supported onto the surface of aluminum oxide by physical vapor deposition. The effects of thermal treatments at 30−800°C both in a vacuum and in an atmosphere of O2 (5 mbar), CO (5 mbar), or a mixture of CO + O2 (5 mbar of each) on the samples of Au/Al2O3 were studied by X-ray photoelectron spectroscopy. An increase in the Au4f line intensity in the course of gold deposition was accompanied by a shift of this line toward smaller binding energy. Upon the supporting of a maximum quantity of gold, the binding energy Eb(Au4f7/2) became smaller than the value characteristic of the bulk metal. It was hypothesized that this can be explained by the formation of negatively charged Auδ− particles due to electron density transfer from the support to the particles of gold. In the course of the heating of Au/Al2O3 in a vacuum or in a reaction atmosphere, the agglomeration of small gold particles occurred; this fact manifested itself in a decrease in the atomic ratio [Au]/[Al]. In all of the atmospheres, the Au particles supported on Al2O3 exhibited high thermal stability; considerable changes in the ratio [Au]/[Al] were observed only at temperatures higher than 600°C.