1 / 1

Adsorption and recombination via abstraction of H(D) on graphite (0001) surfaces

Adsorption and recombination via abstraction of H(D) on graphite (0001) surfaces Thomas Zecho 2 and Jürgen Küppers 1,2 1 Experimentalphysik III, Universität Bayreuth, D-95440 Bayreuth, 2 Institut für Plasmaphysik, D-85748 Garching, EURATOM Association. n. d. sp 2. H. HD.

abra-weber
Download Presentation

Adsorption and recombination via abstraction of H(D) on graphite (0001) surfaces

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Adsorption and recombination via abstraction of H(D) on graphite (0001) surfaces Thomas Zecho2 and Jürgen Küppers 1,2 1 Experimentalphysik III, Universität Bayreuth, D-95440 Bayreuth, 2 Institut für Plasmaphysik, D-85748 Garching, EURATOM Association n d sp2 H HD s abs calculations (1/8 ML) by Jackson et al.: d C-H(D) n C-H(D) D D D 1100(760) 2600(1900) Eley-Rideal + steering effect The study was carried out in a UHV system with a base pressure of < 1*10-10 torr, equipped with: H/D source • QMS • (direct product detection) • TDS • (thermal desorption spectroscopy) • AES, EELS, HREELS • (electron spectrosopies) • H/D source The H(D) atoms were produced by dissociation of H2(D2) in a hot tungsten capillary. graphite flake The graphite crystals were clamped via tungsten wires to a precision manipulator. Resistive heating and cryocooling did allow sample temperatures between 80 K and 1600 K. suggested reconstruction: ZYA/H oxidiced in air monomer mixed dimer temperature [K] Workshop: Solid State Astrochemistry of Star Forming Regions 14 – 17 April 2003, Leiden University Experimental setup Surface characterisation (SEM) natural graphite flake Based on recent theoretical and experimental work [1,2,3] it is now established that H chemisorbs on top of a C of the graphite basal plane. The C atom moves out of the plane by about 0.4 Å, causing an activation barrier of about 0.2 eV. The subsequent recombination reaction higly oriented pyrolytic graphites (HOPGs) abstraction (1800 – 2200) K thermal H(D) ZYA quality hydrogen and traces of hydrocarbon desorption at elevated temperatures terrace reconstruction crystalline quality adsorption + kT thermal desorption edge ZYH quality 80 K – 1600 K via abstraction of H(D) is dominated by a strong attractive interaction between the impinging and chemisorbed hydrogen atom. This leads to a steering effect and to a high recombination cross section at low H(D) precoverages [4,5]. oxidiced in air x 10 10 mm Adsorption on terraces Thermal desorption from terraces Abstraction from terraces HOPG-ZYH HOPG-ZYH HOPG-ZYH H(D) uptake kinetics HREELS analysed with first order kinetics: Thermal desorption from edges adsorption: d[CD](t)/dt = σads Φ [C](t) oxidiced in air abstraction: d[D2](t)/dt = σabsΦ [CD](t) only surface with detectable edge desorption solution: [CD](t) = [CD]¥ (1 – exp[ – (σads + σabs ) Φ t ] ) with saturation coverage: [CD]¥ = [C]0 σads /(σads + σabs ) x 10 Activation barrieradsorption Analysis of saturation coverages (TDS) after dosing with variable D temperatures EELS HOPG-ZYA Ep variation Eact~ 0.18 eV surface effect Arrhenius description: σads = σ0 exp[ -Eact / (kTD) ] temperature [K] direct evidence of H(D) adsorption on the graphite basal plane • 3 desorption states ~ every D with E > Eact sticks hydrogen and hydrocarbon desorption from terrace edges • Qsat = 0.2...0.4 ML • strong isotope effect Analysis of thermal desorption from terraces Activation barrier monomer desorption shape of desorption peak does not vary with surface quality graphite flake Summary • first experimental evidence of H(D) adsorption and • abstraction reactions on the graphite basal plane • measurements are in excellent agreement with theory • additional dimer reconstruction structure is suggested increasing D exposure [1] L. Jeloaica, V. Sidis, Chem. Phys. Lett. 300, 157 (1999) [2] X. Sha, B. Jackson, Surf. Sci. 496, 318 (2002) [3] T. Zecho, A. Güttler, X. Sha, B. Jackson, J. Küppers, J. Chem. Phys. 117, 18 (2002) [4] X. Sha, B. Jackson, J. Chem. Phys. 116 (16), 7158 (2002) [5] T. Zecho, A. Güttler, X. Sha, D. Lemoine, B. Jackson, J. Küppers, Chem. Phys. Lett. 366, 188-195 (2002) monomer dimer • coverage dependence • isotope effect

More Related