A. V. Kalinkin

Size effect in the oxidation of platinum nanoparticles on graphite with nitrogen dioxide: An XPS and STM study

The interaction of NO2 with model catalysts prepared by platinum evaporation onto the surface of highly oriented pyrolytic graphite has been investigated at room temperature and a pressure of 3 × 10−6 Torr by X-ray photoelectron spectroscopy and scanning tunneling microscopy. In the catalyst containing only small (<2.5 nm) platinum particles, these particles oxidize to PtO and PtO2. The action of NO2 on the graphite support and on the graphite-supported Pt catalyst causes graphite oxidation. The oxygen concentration in the model catalyst is higher than on the support. This is supposed to be due to the spillover of oxygen atoms from platinum particles to graphite.

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.

Comparative XPS Study of Al2O3 and CeO2Sulfation in Reactions with SO2, SO2 + O2, SO2 + H2O, and SO2 + O2 + H2O

The interactions of Al2O3, CeO2, Pt/Al2O3, and Pt/CeO2 films with SO2, SO2 + H2O, SO2 + O2, and SO2 + O2 + H2O in the temperature range 300–673 K at the partial pressures of SO2, O2, and H2O equal to 1.5 × 102, 1.5 × 102, and 3 × 102 Pa, respectively, were studied using X-ray photoelectron spectroscopy. The formation of surface sulfite at T ≥ 473 K (the S 2p3/2 binding energy (Eb) is 167.5 eV) and surface sulfate at T   ≥  573 K (Eb = 169.2 eV) was observed in the reactions of Al2O3 and CeO2 with SO2. The formation of sulfates on the surface of CeO2 occurred much more effectively than in the case of Al2O3, and it was accompanied by the reduction of Ce(IV) to Ce(III). The formation of aluminum and cerium sulfates and sulfites on model Pt/Al2O3 and Pt/CeO2 catalysts occurred simultaneously with the formation of surface platinum sulfides (Eb of S 2p3/2 is 162.2 eV). The effects of oxygen and water vapor on the nature and yield of sulfur-containing products were studied.

XPS study of gold oxidation with nitrogen dioxide in model Au/C samples

The interaction of NO2 with model samples obtained by the gold sputter deposition onto the surface of highly oriented pyrolytic graphite (HOPG) has been studied by X-ray photoelectron spectroscopy (XPS). It has been shown that 3D metal particles characterized by an Au4f7/2 binding energy (BE) of 84.0 eV form on the initial smooth graphite surface. During the sputter deposition onto the surface of HOPG preliminarily activated by means of ion etching, gold atoms chemically bound to the carbon atoms form in the area of surface defects. Such atoms are characterized by a positive shift of BE(Au4f7/2). It has been established that the 3D particles are resistant to the action of NO2 under the pressure XXXXXXXX mbar at room temperature. On the contrary, atomic gold oxidizes under these conditions into Au(III) complexes bound to the graphite surface. It is assumed that gold atoms on the support surface play the role of active sites of gold catalysts in oxidation reactions.

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.

Reaction of CO oxidation on platinum, rhodium, a platinum-rhodium alloy, and a heterophase bimetallic platinum/rhodium surface

The reaction of CO oxidation on thin metal films of platinum, rhodium, and their alloy and on a heterophase bimetallic Pt/Rh surface that consisted of platinum particles of size 10–20 nm on the surface of rhodium was studied in the region of low reactant pressures (lower than 2 × 10−5 mbar). At low temperatures (T < 200°C), the activity of samples increased in the order Rh > Pt/Rh > Pt-Rh alloy > Pt. Above 200°C, the rate of reaction on the heterophase Pt/Rh surface was almost twice as high as the sum of the rates of reaction on the individual metals; this fact is indicative of a synergistic effect. The nature of this effect is considered.

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.

Model sulfur-resistant NSR catalysts: An XPS study of the interaction of BaO/TiO2-ZrO2 and Pt-BaO/TiO2-ZrO2 with NO2

The interaction of NO2 with BaO/TiO2-ZrO2 and Pt-BaO/TiO2-ZrO2 model NOx storage-reduction (NSR) catalysts prepared on the surface of FeCrAl alloy substrates has been investigated by X-ray photoelectron spectroscopy. The action of nitrogen dioxide on these catalysts at room temperature causes the consecutive formation of surface barium nitrite and nitrate. Supposedly, NO2 reduction yielding barium nitrite at the early stages of the interaction is due to the oxidation of amorphous carbon impurities. The introduction of platinum into the catalyst accelerates nitrogen dioxide sorption. On being exposed to NO2 for a long time, platinum oxidizes to PtO2. This process is more pronounced in the sample containing freshly deposited platinum. Heat treatment of this sample in a vacuum, which coarsens the platinum particles, makes them more resistant to oxidation.

Formation of platinum sites on layered double hydroxide type basic supports: III. Effect of the mechanism of [PtCl6]2− complex binding to aluminum-magnesium layered double hydroxides on the properties of supported platinum in Pt/MgAlOx catalysts

While synthesizing platinum catalysts supported on aluminum-magnesium oxides (Pt/MgAlOx), we established that, in the binding of the Pt(IV) chloro complex to aluminum-magnesium layered double hydroxides (LDHs), the mechanism of the metal complex-support interaction depends on the nature of the interlayer anion of the LDH. The synthesis may yield chemically identical Pt/MgAlOx samples differing in the particle size and electronic structure of supported platinum. The higher dehydrogenating activity of the catalyst obtained by binding the [PtCl6]2− complex in the interlayer space of LDH via exchange with interlayer OH− anions is possibly due to the larger proportion of metallic platinum (Pt0) in this catalyst. In the catalyst prepared from hydrolyzed platinum complex species using LDH with CO32− interlayer anions, platinum is mainly in an oxidized state similar to Pt2+.