M. Yu. Smirnov

An XPS study of the composition of iridium films obtained by MO CVD

Abstract Iridium films were deposited on flat quartz substrates by MO CVD from iridium tris-acetilacetonate; these processes were performed at atmospheric pressure in the presence of hydrogen, and substrate temperatures were varied in the range 350–550°C. X-ray photoelectron spectroscopy was used to study film composition: carbon- and oxygen-containing impurities (up to one monolayer) are present on the surface of all the films. During etching by argon ions up to the film-substrate boundary only iridium lines were observed in the spectra. Subsequent etching results in the decrease of the intensity of the iridium lines and the appearance of lines of the substrate material — Si2p, Si2s and O 1s. In the region of the IrSiO 2 boundary there is a transition layer where a compound of the type IrSi x O y is formed. The thickness of this layer increases with increasing deposition temperature. For films deposited at T > 500°C differential charging effects were found, i.e. the samples contain two phases - conductive and nonconductive ones. This might be due to the crystallization of two phases - metallic and “silicate”. The existence of differential charging can also be explained by grain size effects which depend on the deposition temperature.

HREELS characterization of hydrogen adsorption states on the Pt(100)-(hex) and (1×1) surfaces

The adsorption states of hydrogen on Pt(100)-(hex) and (1×1) surfaces have been studied using TDS and HREELS in the temperature range of 170-330 K. Hydrogen atoms are shown to adsorb on the (1×1) surface in bridge and 4-fold hollow sites. On the (hex) surface hydrogen adsorption induces a partial lifting of the (hex) reconstruction with the consequent appearance of the bridge and 4-fold hollow states on (1×1) patches. Additional adsorption states appear due to the population of structural defects and patches of the (hex) phase.

Non-linear processes on Pt, Rh, Pd, Ir and Ru surfaces during the NOhydrogen reactions

Abstract The present paper reviews our research in the field of NO+H 2 reactions over Pt, Rh, Pd, Ir and Ru surfaces. Emphasis will be placed on the ability of these platinum group metals to produce oscillations in both reaction rate and selectivity, in addition to other non-linear processes such as hysteresis phenomena. In the case of Rh, non-linearity is related to periodic transitions between N- and O-rich surfaces, with O destabilising the N-adlayer and causing an acceleration in the N 2 production. However, in the case of Pt, the important step involves the creation of vacant sites required for NO dissociation, whereby products leaving the surface facilitate an autocatalytic rise in the concentration of such vacancies and, hence, reaction rate. Ir positions itself between the former two, because oscillations have been observed in two different regimes. Both a lower-temperature Pt-like and higher-temperature Rh-like behaviours have been observed on different Ir surfaces. New results gained from field emission microscopy (FEM) and field ion microscopy (FIM) measurements are presented to consolidate the lower-temperature Pt-like behaviour, and confirm the dualistic nature of Ir. No oscillations in rate have been observed over Pd(111) or Ru(0001). The Ru surface exhibits selectivity to N 2 of almost 100% even in large excess of hydrogen.

Hydrogenation of isolated atoms and small clusters of carbon on Pt(111) surface: HREELS/TDS studies

Abstract The reaction of hydrogen with isolated atoms and small clusters of carbon adsorbed on Pt(111) surface was investigated by HREELS and TDS. The carbon adsorption layers were prepared by evaporation of carbon atoms onto the metal surface cooled down to 100 K. The surface carbon produced by this method reveals a high activity towards hydrogen: the reaction occurs at T ⩾ 170 K. The initial carbon concentration is found to determine the chemical content of products in the adlayer. At n c 14 at / cm 2 , when isolated atoms C ads prevail in the initial adlayer, methine CH ads formation is chiefly observed revealing two bands in HREELS: δ(CH) at 800 and v ( CH ) at 2960 cm −1 . The CH ads particles dissociate at 510 K leading to hydrogen evolution and formation of carbon islands with a graphite-like structure. At higher concentration, the carbon adlayer contains small clusters C x ads in addition to the isolated atoms leading to more complex reaction products. An ethylidyne species, , is detected among the products with characteristic bands δ s (CH 3 ) at 1360 and v ( CC ) at 1130 cm −1 . It is assumed that ethylidyne molecules are produced in the course of a consecutive hydrogenation of the C 2 ads cluster. Dehydrogenation of hydrocarbon surface species causes hydrogen evolution at T ≈350, 410, 450, 510 and 600–700 K . Ethylidyne dissociation is associated with the desorption peak at T ≈ 450 K ; an ethynyl CCH ads being the product. It is shown that the highest temperature stage of the dehydrogenation in the adlayer occurring at T > 600 K is accompanied by an increase of the carbon content in the C x CH ads molecules, finally resulting in the formation of islands with a graphite-like structure.

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.

NH2 formation in the reaction of Hads with NO on the Pt(100)-(1 × 1) surface

The reaction of a saturated layer of Hads with NO on the unreconstructed Pt(100)-(1 × 1) surface at 300 K has been studied by high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). The NH2ads species is produced as an intermediate of the reaction. The vibrational losses of NH2ads at 480 cm−1 (ν(PtNH2) stretching), 825 cm−1 (ω(NH2) wagging), 1450 cm−1 (δ(NH2) scissors), 3298 cm−1 (νs(NH2) symmetric stretching) and 3388 cm−1 (νas(NH2) asymmetric stretching) are observed. The hypothesis of the formation of NH2ads is supported by modeling by means of the Wilson GF matrix method and the simplest valence force approximation. The reaction of an adlayer consisting of a 1:1 mixture of HadsDads with NO produces an isotopic mixture of NH2ads, ND2ads and NHDads. The vibrational spectra of NH2ads and its isotopic counterparts ND2ads and NHDads have been discussed in detail. Based on the detailed analysis of the vibrational modes, the NH2ads species is suggested to occupy the bridge adsorption place with C2v symmetry.

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.

Hreels study and catalytic significance of low-temperature interaction of isolated carbon atoms with hydrogen on Pt(111)

Isolated atoms of carbon evaporated on to Pt(111) react with hydrogen atT⩾170 K to form methine species, characterized with vibrational modesv(CH) at 2960 and δ(CH) at 800 cm−1. The high reactivity ofCads is in line with their ability to take part as intermediates in the metanation reaction. CHads species are stable up toT ≈ 500 K; further heating leads to their dissociation accompanied by H2 desorption and formation of unreactive graphite-like islands.