V.V. Gorodetskii

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.

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.

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.

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.

HREELS/TDS study of NO reaction with hydrogen on Pt(100) surface

NO adsorption on a Pt(100)-(hex) surface and NOads reaction with hydrogen at 300 K have been studied by HREELS, LEED, TDS and isothermal desorption. NO adsorbs in molecular form, its molecules gathering in islands with a high local coverage. Surface reconstruction into a (1 × 1) phase proceeds within the boundaries of islands. Reaction NO + H2 is performed via NOads previous heating in vacuum atTh = 375–425 K. Kinetics of NOads titration appears to be autocatalytic. Nitrogen is the major reaction product.

NO ADSORPTION ON RECONSTRUCTED AND UNRECONSTRUCTED Pt(100) SURFACE AT 300 K TDS STUDIES

The consequence of filling NO and N2 thermodesorption states and the adsorption rate are found to depend on the initial surface structure. The initial sticking coefficients for (1×1) and (hex) structures are 1 and 0.35, respectively. If ΘNO < 0.3 ML, the dissociation probability of NO is shown to be higher when adsorption occurs on the unreconstructed surface.

HREELS study: low-temperature reaction of NO with isolated carbon atoms adsorbed on Pt(111) surface

The reaction between isolated carbon atoms and nitrogen oxide molecules in the adlayer on Pt(111) surface has been studied. Carbon atoms have been deposited on the surface from the special source. The reaction was found to proceed atT ≈ 100 K and to provide, at least, two intermediate surface species, which have been assigned to adsorbed isocyanate NCOads and fulminate CNOads particles. Both intermediates dissociated into on-top state of COads and Nads under heating toT ⩽300 K.

Kinetic isotope effect in the reaction of NOads and COads on the Pt(100) surface

The reaction of CO with 15NO and 14NO mixtures in a co-adsorption layer on the Pt(100)-(hex) surface was studied by TPR. The kinetic isotope effect (KIE) manifests itself in the variation of the temperature of the maximum of the N2 desorption peak depending on the isotopic composition: Tmax(14N2)<T max(14N15N)≈ Tmax(15N2). The KIE observed is consistent with the assumption that the NOads dissociation is the rate-determining step of the reaction.

Hreels/Tds Identification of Intermediates in The Low-Temperature H2+O2, NO+H2, NH3+O2 Reactions on Pt(111) Surface

Publisher Summary There is a possibility of the formation of different products during NO reduction with hydrogen and NH 3 oxidation depending on the conditions and the metals used. The character of the NO adsorbed state (molecular or dissociative) determines the formation of the key product (NH 3 or N 2 ). The activity of Pt toward breaking the N–O bond is very sensitive to the surface structure. The study of low-temperature oxidative catalysis described in the chapter establishes the mechanisms of the reactions proceeding with the participation of the reactants in the molecularly adsorbed states. In the study described in the chapter, high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS) were used for the study of the low-temperature adsorption of H 2 , O 2 , NO, and NH 3 and the H 2 + O 2 , NO + H 2 , and NH 3 + O 2 reactions on the Pt(III) surface. Experiments were carried out by VG ADES-400 spectrometer, and the monochromatic electron beam with a kinetic energy of 2.5 eV and resolution of 10 meV (80 cm −1 ) was used.

Hydrogenation of the atomic carbon deposited on the Pt(111) surface studied by low electron energy loss spectroscopy and thermal desorption mass spectrometry

Abstract The properties of carbon layer on the Pt(111) surface with submonolayer coverage and its activity towards hydrogen have been studied by low electron energy loss spectroscopy and thermal desorption mass spectrometry. The layer of C(ads) atoms was prepared from a special source which produced a carbon atom flow free of admixtures and clusters. It was shown that after deposition of carbon on Pt(111) at 100 K and with n C ≈ 2 × 10 14 cm -2 the layer was obtained chiefly containing the isolated C(ads) atoms. The high activity of the adsorbed atoms was established; the reaction C(ads) + H(ads) was initiated at 170 K and in the course of reaction CH(ads) was produced. These particles are stable towards dissociation up to 500 K. Interaction of the carbon layer ( n C ≈ 10 15 cm -2 with hydrogen gives rise to the formation of more complex particles and, in particular, ethylidyne ( (ads) identified according to the following vibrational bands: v (CC) at 1130 cm -1 and δ s (CH 3 ) at 1370 cm -1 .