G.A. Bootsma

A study of the kinetics of the interactions of O2 and N2O with a Cu(111) surface and of the reaction of CO with adsorbed oxygen using aes, LEED and ellipsometry

The interactions of O2 and N2O in the low pressure range with a Cu(111) surface and of CO with adsorbed oxygen have been studied with ellipsometry, Auger electron spectroscopy and LEED. The adsorption of O2 was investigated in the 10−6–10−4 Torr range and at crystal temperatures ranging from 23 to 400°C. O2 chemisorbs dissociatively with an initial reaction probability of about 10−3 and an apparent activation energy of 2–4 kcal/mol, which depends on the substrate temperature, up to a saturation coverage of 0.45. The probability of decomposition of N{ib2}O is 10−5 at 300°C, and the activation energy is 10.4 kcal/mol for 250 < T < 400°C. The oxygen coverage saturates at θ = 0.45 as well. For both oxidation reactions the kinetics can be described with a precursor state model. With LEED no superstructures were observed. The probability of the reaction of CO with adsorbed oxygen is 4 × 10−5 at 250°C and is initially independent of the oxygen coverage. The reaction is assumed to proceed via a Langmuir-Hinshelwood mechanism. The activation energy for the reaction COad + Oad → CO2 is 18–20 kcal/mol.

The kinetics of the interactions of O2 and N20 with a Cu(110) surface and of the reaction of CO with adsorbed oxygen studied by means of ellipsometry, AES and LEED

Ellipsometry, LEED and Auger electron spectroscopy have been used to study the interactions of O2 and N2O with a clean annealed Cu(110) surface and the reaction of CO with adsorbed oxygen in the monolayer range. Gas pressures were in the range 10−8–10−4 Torr and crystal temperatures varied between 23–400°C. The changes in the ellipsometric angles Δ and ψ per oxygen atom upon adsorption and removal of oxygen depend on the coverage θ, the temperature and on the azimuth of the plane of incidence of the light beam. The kinetics of the chemisorption of oxygen is independent of the crystal temperature, initial sticking probability ≈ 0.2. The LEED data and the adsorption kinetics indicate an attractive interaction in the adsorbed layer in the [001] direction. The initial decomposition probability of N2O at room temperature is 0.15 and decreases with increasing temperature; the maximum coverage is 0.5 monolayer. The LEED patterns observed were the same as those with O2. The reaction probability of CO with adsorbed oxygen increases with decreasing oxygen coverage (order of magnitude ∼10−5, apparent activation energy ∼6 kcal/mol). This increase has been attributed to the operation of the Langmuir-Hinshelwood mechanism.

AES-LEED-ellipsometry study of the kinetics of the interaction of methane with Ni(110)

The interaction of methane with Ni(110) was studied with AES, LEED and ellipsometry. Sticking coefficients were determined in the temperature range 298–600 K at methane pressures of 10−4–10−2 Torr. The carbon coverages were derived from Auger spectra by calibration with ellipsometry. At room temperature no detectable adsorption was observed without use of electron sources. In the temperature range 473–579 K the coverage versus exposure curves show an induction effect at low coverage followed by an almost linear increase up to a saturation coverage of about 13 monolayer of carbon. At these temperatures a Ni(110)-(2 × 3)-C structure was observed with streaks in the direction of constant h. The observed behaviour is explained with a nucleation and growth model in which mobile carbon species are captured at the edges of surface nickel carbide islands. At temperatures above 600 K carbon diffuses into the bulk and the Ni(110)-(4 × 5)-C superstructure is observed.

The adsorption and incorporation of oxygen on Cu(110) and its reaction with carbon monoxide

The initial interaction of oxygen with a Cu(110) surface occurs in two stages, characterised by (2 × 1) and c(6 × 2) overlayer structures and then a third stage where the c(6 × 2) structure remains but further oxygen uptake is registered only with ellipsometry. The first stage is nonactivated and is accompanied by a work function increase of 370–420 meV, depending on sample temperature. The subsequent appearance of the extra features associated with the c(6 × 2) structure in the second stage is accompanied by a decrease in work function of ∼100 meV and is characterised by an apparent activation energy of ∼18 kJ mol−1. The adsorbed oxygen can be removed at all coverages by gas-phase CO. The reaction appears to follow Langmuir-Hinshelwood kinetics with an apparent activation energy of ∼25 kJ mol−1.

THE ADSORPTION AND INCORPORATION OF OXYGEN ON Cu( 100) AND ITS REACTION WITH CARBON MONOXIDE; COMPARISON WITH Cu( 111) AND Cu( 110)

Abstract Ellipsometry, LEED, Auger electron spectroscopy and monitoring of work function changes have been used to study the interactions of O2 and N2O with a clean annealed Cu(100) surface and of the reaction of CO with sorbed oxygen. Gas pressures were in the range 10−7−10−4 Torr and crystal temperatures varied between 25–400°C. The initial interaction of oxygen with Cu(100) occurs in three stages. Oxygen chemisorbs with an initial sticking coefficient of ~10−7 at room temperature and an apparent activation energy of 1.3−3.5 kcal/mol, depending on the substrate temperature. The first stage is the formation of a (√2 × √2)R45° LEED pattern up to a coverage of 0.5, which is converted with an apparent activation energy of 3.2 kcal/mol to a (√2 × 2√2)R45° structure at a coverage of 0.75 in the second stage. The work function increases inthe first stage in an amount of ~300 meV, but decreases in the second stage to the value of the clean surface. In a third stage after an induction period further oxygen uptake could be registered only with ellipsometry. The apparent activation energy is 4.5 kcal/mol. The initial decomposition probability of N2O at room temperature is 5 × 10−5, its apparent activation energy 3.2 kcal/mol. The LEED patterns observed were the same as with O2. The sorbed oxygen can be removed at all coverages with CO. The reaction appears to follow Langmuir-Hinshelwood kinetics with an activation energy for the reaction COad + Oad → CO2 of 19–20 kcal/mol. A comparison is made with the data obtained for Cu(111) and Cu(110).

Interaction of methane with Ni(111) and Ni(100); diffusion of carbon into nickel through the (100) surface; An aes-leed study

The interaction of methane with Ni(100) results in the deposition of carbon in a carbidic form onto the surface. The sticking coefficient is initially 5 × 10−9 and does not depend on temperature in the range 474–563 K. This surface carbide consists of two different types with respect to their stability. A first kind may be made to diffuse into the bulk between 626–670 K, similar to experiments on Ni(110). A second kind is more tightly bound and leads to a segregation type of behaviour. Methane does not interact with Ni(111).

Ellipsometry-LEED study of oxygen adsorption and the carbon monoxide- oxygen interaction on Ag(110)

Abstract The adsorption of oxygen on Ag(110) has been studied by ellipsometry and LEED. Oxygen pressures varied between 10 −6 and 5 × 10 −5 Torr and the crystal temperature between room temperature and 200° C. The change in the ellipsometric parameter Δ was found to be proportional to the oxygen coverage derived from ( n × 1) superstructures in LEED. Initial sticking coefficients are about 5 × 10 −4 and the maximum coverage is 0.5 O atom Ag atom. The reaction of CO with adsorbed oxygen was studied between room temperature and 200°C at CO pressures between 10 −7 and 4 × 10 −6 Torr. The decrease in oxygen coverage is slow at high oxygen coverages and accelerates at lower coverages. Two distinct rate constants may be derived from a simple model, each proportional to the CO pressure. LEED evidence points to considerable freedom of motion for O-atoms along the troughs of the Ag(110) face. Above 200°C oxygen desorption takes place. Ellipsometric measurements indicate that at these temperatures either a surface rearrangement takes place, or that the bonding character of oxygen to the substrate changes. Isosteric heats of adsorption were estimated for crystal temperatures between 240 and 340°C.

Ellipsometric study of oxygen adsorption and the carbon monoxide-oxygen interaction on ordered and damaged Ag(111)

Abstract The adsorption of oxygen on Ag(111) has been studied by ellipsometry in conjunction with AES and LEED. The oxygen pressure varied between 10 −5 and 10 −3 Torr and the crystal temperature between room temperature and 250° C. Changes in the Auger spectrum and the LEED pattern upon oxygen adsorption are very small. Oxygen coverages were derived from the changes in the ellipsometric parameter Δ. At room temperature a maximum coverage is reached within a few minutes. Its value increases with the damage produced by the preceding argon ion bombardment. The sticking coefficient derived from the initial rate of Δ-change amounts to 3 × 10 −5 for well-annealed surfaces and 2.5 − 5 × 10 −4 for damaged surfaces. After evacuation no desorption takes place. Other types of adsorption, associated with much larger changes in Δ, were observed upon bombardment with oxygen ions and with oxygen activated by a hot filament. The reaction of CO with adsorbed oxygen was studied ellipsometrically at room temperature in the CO pressure range 10 −7 –10 −6 Torr. The initial reaction rate is proportional to the CO pressure. The reaction probability (number of oxygen atoms removed per incident CO molecule) is 0.36.

The adsorption and decomposition of carbon monoxide on Ni(100) and the oxidation of the surface carbide by oxygen

The interaction of carbon monoxide with Ni(100) has been studied by ellipsometry and Auger electron spectroscopy. Bombardment by electrons of a relatively high energy (2500 eV) leads to the disproportionation of the adsorbed CO (2 COad → Cad + CO2g ). The rate of oxidation of this surface carbide is , where hc is the carbon 272 eV Auger peak height, n=0.5 and the apparent activation energy Eact =13.3 kcal/mole. This relation is valid at 200–400°C and at oxygen pressures of 5 × 10−9−8 × 10−7 Torr.

Dissolution of carbon into nickel through the (110) surface

Abstract The dissolution of carbon through the Ni(110) surface has been studied by means of AES and LEED. Reproducible kinetic data were obtained in the temperature range 615–660 K after the crystal had been annealed at 870 K. These data are interpreted with a model in which bulk diffusion from a constant plane source just beneath the surface is rate limiting. The calculated carbon concentrations are compared with literature data on the solubility of graphite in nickel.