D.Yu. Zemlyanov

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 unreconstructed and reconstructed Pt(100) surface at 300 K: HREELS studies

Dissociative adsorption at 300 K is shown to proceed only on unreconstructed Pt(100)−(1×1) surface. When heated, NO (ad) layers behave in a similar way on both surfaces. Dissociation products are N2 and O(ad). Adsorption on the (hex) surface is supported to occur in islands. The island size in the saturation layer appears to be ∼ 40 Å.

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.

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.

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).