Th. Kammler

Interaction of H atoms with Cu(111) surfaces: Adsorption, absorption, and abstraction

The interaction of H (D) atoms with clean and D (H) covered Cu(111) surfaces was studied with TDS and direct product detection methods. H (D) atoms exhibit an initial sticking coefficient of 0.22. Due to abstraction, the surface saturation coverage is achieved at Θ=0.34, significantly less than the half monolayer coverage obtained through exposure of energetic H2 molecules to Cu(111) surfaces. Adsorbed H (D) desorbs recombinatively between 250 and 400 K. Desorption of absorbed H (D) via gaseous H2(D2) around 200 (210) K was observed according to a zero-order rate law with an activation energy of 0.40 (0.35) eV. Abstraction of D (H) by H (D) at 80 K lead to gaseous HD and D2(H2) formation. About 1% of the adsorbed species occurred in homonuclear products. Throughout the abstraction reaction the HD rate was found strictly proportional to coverage and flux, in line with a purely quasifirst-order, exponentially decreasing Eley–Rideal-type product rate. However, this phenomenology as well as the occurrence of ...

A hot-atom reaction kinetic model for H abstraction from solid surfaces

Measurements of the abstraction reaction kinetics in the interaction of gaseous H atoms with D adsorbed on metal and semiconductor surfaces, H(g)+D(ad)/S→ products, have shown that the kinetics of the HD products are at variance with the expectations drawn from the operation of Eley–Rideal mechanisms. Furthermore, in addition to HD product molecules, D2 products were observed which are not expected in an Eley–Rideal scenario. Products and kinetics of abstraction reactions on Ni(100), Pt(111), and Cu(111) surfaces were recently explained by a random-walk model based solely on the operation of hot-atom mechanistic steps. Based on the same reaction scenario, the present work provides numerical solutions of the appropriate kinetic equations in the limit of the steady-state approximation for hot-atom species. It is shown that the HD and D2 product kinetics derived from global kinetic rate constants are the same as those obtained from local probabilities in the random walk model. The rate constants of the hot-atom kinetics provide a background for the interpretation of measured data, which was missing up to now. Assuming that reconstruction affects the competition between hot-atom sticking and hot-atom reaction, the application of the present model at D abstraction from Cu(100) surfaces reproduces the essential characteristics of the experimentally determined kinetics.

The role of sticking and reaction probabilities in hot-atom mediated abstraction reactions of D on metal surfaces by gaseous H atoms

Recent experiments on the abstraction of D adsorbed on metal surfaces with gaseous hydrogen atoms revealed a kinetics of HD formation which is not compatible with the operation of Eley–Rideal (ER) mechanisms. Furthermore, homonuclear products were observed during abstraction, which are not expected through an ER reaction scheme. It was therefore suggested that hot-atom (HA) mechanisms are more appropriate to explain the measured kinetics and products. Random walk calculations of the abstraction kinetics are presented based on a model which exclusively relies on elementary reaction steps which are HA mediated processes. Within this model, the ratio of two variables, the probabilities for hot-atom sticking at empty sites ps and hot-atom reaction with adsorbed species pr, was found to control the kinetics of HD and D2 formation. The essential features of measured kinetic data at Ni(100), Pt(111), and Cu(111) surfaces were reproduced by simple and reasonable assumptions on ps/pr.

Interaction of thermal H atoms with Ni(100) H surfaces: through surface penetration and adsorbed hydrogen abstraction

Abstract Considerable quantities, > 3 monolayers, of hydrogen (deuterium) were absorbed in subsurface sites at Ni(100) surfaces via impact of thermal H (D) atoms. Competitive to a direct transition (penetration) into the bulk with an initial probability of 4.5 × 10 −2 impinging H (D) atoms abstract adsorbed D (H) with an initial abstraction probability R = 0.25 and cross section of 1.6 A 2 . The filling of subsurface sites obeys a Langmuir-type rate law. An adsorbed H (D) monolayer does not affect the probability for penetration into the bulk. Above 150 K subsurface H (D) move to the surface and either recombine with adsorbed H (D) followed by recombinative desorption or get adsorbed at the surface if they encounter an empty site. The desorption kinetics from subsurface sites is in quantitative agreement with thermodynamic bulk Ni H data. Isotope effects were neither observed for bulk penetration nor for abstraction.

Interaction of H atoms with oxygen adsorbed on Ni(100) surfaces: Direct reactions towards OH and H2O

Abstract The reactions of oxygen adlayers on Ni(100) surfaces with gaseous thermal H and D atoms were studied with AES, TDS and reaction-product monitoring methods in the temperature range 120–400 K. Independent of temperature and coverage, adsorbed O atoms are hydrogenated to adsorbed OH (OD) by impinging H (D) atoms. A cross-section of σ = 0.3 A 2 suggests that this reaction proceeds via an Eley-Rideal (ER)-type direct mechanism. Independent of temperature, in a second ER step with a cross-section of σ = 4.5 A 2 , adsorbed OH (OD) groups are hydrogenated to water. Well above the water desorption temperature (180 K) the product molecules desorb either through a thermally activated or an energetically mediated step. Below the water desorption temperature (at 120 K), 90% of the product molecules remain adsorbed on the surface and 10% desorb through the energy set free at reaction. An isotope effect (H/D) on the cross-sections was not observed. The processes were observed independent of the oxygen coverage and also at oxidized Ni surfaces, which could be reduced completely at sufficiently high H or D atom fluences.

INTERACTION OF HYDROGEN ATOMS WITH COADSORBED D/CH3I ADLAYERS ON NI (100) SURFACES: EVIDENCE FOR HOT ATOM MEDIATED REACTIONS

Adlayers of coadsorbed hydrogen (deuterium) atoms and methyliodide, H/CH3I, D/CH3I, H/CD3I, D/CD3I, were prepared on Ni(100) surfaces by adsorption of methyliodide on hydrogen (deuterium) saturated surfaces at 120 K. Upon directing thermal D (H) atoms at these adlayers as gaseous products hydrogen and methane molecules were observed which constitute of surface species only, e.g., H2 and CH4 in the combination: D→H/CH3I. The rates of formation of these products give evidence for reaction mechanisms in which adsorbed atoms, e.g., H, activated by collisions with incoming atoms, e.g., D, react with surface species, either H or CH3I to form H2 and CH4. The reaction cross sections and isotope effects in H→D/CD(H)3I or D→H/CH(D)3I reactions support a collision model in which hot atoms act as mediators in the reaction pathways.

Methanation of carbon on Ni(100) surfaces at 120 K with gaseous H atoms

Abstract Upon directing a beam of thermal (2000 K) H atoms at carbon covered Ni(100) surfaces between 120 and 250 K methane was observed as the ohey only reaction product. The initiating step H gas + C ad → CH ad was identified as rate determining. Above 250 K thermal decomposition of CH 3,ad prevents C methanation. Only direct Eley-Rideal type reactions are involved in the stepwise hydrogenation of C to methane.