M. Kaltchev

Reflection-absorption infrared spectroscopy of ethylene on palladium (111) at high pressure

The adsorption of ethylene adsorbed on Pd(111) at ∼ 300 K is studied using reflection-absorption infrared spectroscopy which confirms the formation of an ethylidyne species because of the presence of vibrational mode at 1329 cm−1 with a less-intense peak at 1089 cm−1. The 1329 cm−1 methyl mode is well away from any vibrational modes of gas-phase ethylene, which allows the spectrum of the surface species to be collected in the presence of high pressures (up to ∼ 1 torr) of ethylene. These results reveal that ethylidyne is present on the surface in the presence of gas-phase ethylene and that there may be a slight increase in coverage. The width of the line, however, increases substantially by 5.3±0.4 cm−1 torr−1. This effect is ascribed to a loss of order in the ethylidyne layer probably caused by co-adsorption of ethylene.

On the reaction pathway for the formation of benzene from acetylene catalyzed by palladium

The reaction between gas‐phase acetylene and alumina‐supported palladium saturated with 13C‐labelled vinylidene is studied using both one‐pulse, 13C magic‐angle spinning, nuclear magnetic resonance (NMR) spectroscopy and by mass spectroscopic analysis of the reaction products to probe the reaction pathway. The presence of vinylidene on alumina‐supported palladium is confirmed by comparing the infrared spectra of the species formed on the supported sample with those found on a Pd(111) single crystal. It is shown using NMR that a high pressure (∼350 Torr) of gas‐phase acetylene reacts with adsorbed vinylidene at the same rate at which benzene is formed catalytically on the same sample. The resulting benzene incorporates two 13C atoms. This indicates that benzene is formed by a slow reaction between gas‐phase (12C‐labelled) acetylene and adsorbed vinylidene (13CH2=13C=) to form a C4 intermediate which reacts rapidly with further acetylene to yield benzene. There are precedents for such reactions in homogeneous phase. The proposed reaction pathway differs from that elucidated previously from ultrahigh vacuum studies on clean Pd(111), where it was found that benzene synthesis also proceeds via a C4 intermediate, in this case formed from two adsorbed acetylenes.

An investigation of the tribological properties of thin KCl films on iron in ultrahigh vacuum: modeling the extreme-pressure lubricating interface

The frictional properties of thin KCl films deposited onto clean iron are measured in ultrahigh vacuum using a tungsten carbide tribotip, where the observed initial rapid decrease in friction coefficient with film thickness is proposed to be due to the formation of a complete KCl monolayer where the friction coefficient of this film is ∼0.27. A 1800 A thick KCl film shows a hardness and friction coefficient similar to those for bulk KCl when the width of the surface height distribution of the tribotip measured by atomic force microscopy (AFM) is 2000–3000 A. This implies that the KCl film behaves like the bulk material when the film thickness exceeds the roughness of the interfaces.

A reflection-absorption infrared spectroscopic study of the adsorption of ethylene and ethylene oxide on oxygen-covered Ag( 111)

The adsorption of ethylene and ethylene oxide has been studied on clean and oxygen-covered Ag(1 1 1) using temperature-programmed desorption and reflection–absorption infrared spectroscopy (RAIRS). Ethylene adsorbs weakly on Ag(1 1 1) at 80 K with the molecular plane oriented parallel to the surface. The effect of adsorbed oxygen (Θ(O)∼0.1) is to increase the heat of adsorption slightly and to cause the ethylene to tilt. Ethylene oxide also adsorbs weakly at 80 K with the molecular plane oriented perpendicularly to the surface, where the heat of adsorption also increases due to the presence of adsorbed oxygen. The RAIR spectra of both ethylene and ethylene oxide adsorbed on oxygen-activated Ag(1 1 1) at 300 K under a pressure of 1 Torr show the formation of a number of surface species. An η2(C,O) bonded acetaldehyde species is found, where the infrared features decrease coincident with acetaldehyde/ethylene oxide desorption. A species persists on heating to 450 K which exhibits a single infrared peak at 1004 cm−1. Based on the frequency shifts observed on isotopic substitution (with D and 18O), it appears to contain C, O and H. This feature disappears on heating to 550 K correlating with the desorption of CO2 in temperature-programmed desorption. Finally, a series of features is detected which may be due to an adsorbed formate or strongly distorted ethylene oxide. These results emphasize that good quality infrared spectra can be collected for adsorbed species formed at high pressures on a model, oxygen-activated Ag(1 1 1) catalyst and that the surface chemistry is completely different to that found when dosing at 80 K under ultrahigh vacuum conditions.

A molecular beam study of the tribological chemistry of dialkyl disulfides

Molecular beam studies carried out in ultrahigh vacuum show that dimethyl disulfide reacts with initially clean iron to evolve methane. The reaction is proposed to proceed via a methyl thiolate intermediate. Reaction ceases at ∼600 K, an effect that is proposed to be due to the surface being blocked by an overlayer of sulfur and carbon. Reaction recommences above ∼950 K as sulfur diffuses into the iron. The activation energy for the film-forming reaction is 52.5±2.1 kcal/mol, in good agreement with the activation energy for the growth of FeS films from dimethyl disulfide at higher pressures measured using a microbalance. A depth profile of the film grown in ultrahigh vacuum shows that the sulfur-containing film grows on a Fe+C underlayer. Similar molecular beam experiments with diethyl disulfide suggest the formation of an intermediate ethyl thiolate species which decomposes via a β-hydride elimination reaction to evolve ethylene. The activation energy for film growth, in this case, is 60±2.4 kcal/mol. The results of tribological experiments using a pin and v-block apparatus are consistent with FeS forming the anti-seizure film.

Molecular beam and infrared spectroscopic studies of the thermodynamics of CO on clean and vinylidene-covered Pd(111)

A differentially pumped, capillary array molecular beam source is used to study the reversible adsorption of CO on CO/Pd(111) [Θ(CO)=0.55] and vinylidene/Pd(111) [Θ(vinylidene)=1.0] at 300 K. Differentially pumping allows the beam to equilibrate rapidly (in ∼2 s) while maintaining good beam uniformity. The isosteric heat of adsorption of reversibly held CO on a surface precovered with 0.55 monolayers of chemisorbed CO is 5.6±0.2 kcal/mol at low excess coverages but decreases linearly with coverage so that an additional 2% of a monolayer of CO reduces the isosteric heat of adsorption to 3.0±0.2 kcal/mol. CO adsorbs reversibly on vinylidene-saturated Pd(111) with an isosteric heat of adsorption of 1.0±0.5 kcal/mol for coverages up to ∼1% of a monolayer of CO. Infrared spectra of CO on vinylidene-covered Pd(111) at higher pressures (several Torrs) reveal that CO adsorbs on the metal surface. Assuming that the heat of adsorption of CO on vinylidene-covered Pd(111) decreases with CO coverage at higher coverage...

The Tribological Properties of Monolayer KCl Films on Iron in Ultrahigh Vacuum: Modeling the Extreme-Pressure Lubricating Interface

The friction coefficients of thin KCl films deposited onto clean iron in ultrahigh vacuum are measured using a tungsten carbide tip. A rapid decrease is found in the friction coefficient from ∼ 2 for clean iron to 0.27 ± 0.03 after the deposition of ∼40 Å of KCl. Based on previous contact resistance measurements, this was proposed to be due to the completion of the first layer of KCl. The first-layer KCl coverage was measured by adsorbing deuterium onto an iron surface partially covered by KCl, where deuterium selectively adsorbs onto the iron. This revealed that the first monolayer is complete after the deposition of ∼40 Å of KCl and that the first-layer KCl film coverage ΘKCl(1) is given by ΘKCl(1) = 1 - exp(-0.39±0.02t), where t is the film thickness. XPS data suggest that heating a KCl film to ∼550 K causes it to wet the surface. This leads to decreases in the friction coefficients for thin KCl films in accord with the idea that friction is reduced by the first monolayer of KCl on iron. Temperature-programmed desorption data indicate that KCl in the first monolayer is ∼5 kJ/mol more stable than the multilayer consistent with the wetting behavior. Finally, the kinetic data are analyzed to suggest that the first-layer film is ∼2.6 Å thick.

A molecular-beam study of the tribological chemistry of carbon tetrachloride on oxygen-covered iron

Dc molecular-beam methods are used to examine the reactivity of carbon tetrachloride with oxide films grown on iron in ultrahigh vacuum. The incident CCl4 beam flux is sufficiently low that the nature of the surface oxide is dictated by the annealing temperature allowing the reactivity of Fe2O3, Fe3O4 and FeO films to be examined. Carbon tetrachloride reacts rapidly with Fe2O3 and reaction with Fe3O4 commences at ∼620 K to evolve CO. The activation energy for this process is 20.6±1.0 kcal/mol. CCl4 reacts with FeO above ∼790 K, also to evolve CO, and the activation energy for this reaction is 5.7±0.4 kcal/mol. X-ray photoelectron spectroscopy shows the formation of a halide after reaction at 900 K. These results are in accord with film-growth kinetics measured using a microbalance at high pressures, where it was found that it was not necessary to remove the oxide layer prior to reaction. This contrasts with the behavior of sulfur-containing molecules, where the oxide layer had to be removed before a film would grow. This effect may contribute to the additive synergies commonly found in extreme-pressure lubricant additives where one of the roles of the chloride may be to reduce the oxide layer.

Preparation and characterization of alumina‐supported metal‐carbonyl‐derived model catalytic systems in UHV conditions

Active catalysts for metathesis of alkenes, hydrodesulfurization, and hydrogenation can be prepared by exposing a high‐surface‐area alumina support to molybdenum hexacarbonyl at room temperature. This strategy is mimicked in ultrahigh vacuum by adsorbing molybdenum hexacarbonyl onto an ultrathin hydroxylated alumina film grown onto a Mo(100) substrate. In contrast to results found on high surface area, no Mo(CO)6 is found to adsorb on alumina at 300 K, and significant molybdenum deposition is only found by heating the sample to above 670 K. Alternatively, molybdenum hexacarbonyl adsorbs on alumina when cooled to 80 K. In this case the majority of the carbonyl desorbs intact and temperature‐programmed desorption and X‐ray photoelectron spectroscopy indicate that ∼2% of a monolayer of the carbonyls undergoes decarbonylation. Auger and X‐ray photoelectron spectroscopy measurements reveal that molybdenum carbide (MoC) is deposited onto the alumina surface heated to 700 K forming a monolayer after an exposure of ∼50 L. This layer is reduced to the metal by heating to ∼1500 K by reaction with the alumina substrate to evolve CO and form metallic molybdenum. The carbide can be reformed by heating the metal‐covered alumina sample in ethylene at 900 K, and the carbide can once again be reduced to the metal by heating to 1500 K. This process can be repeated so that the carbide can be regrown by reaction with ethylene and reduced by annealing in vacuo to 1500 K. Subcarbonyl species are detected after adsorbing Mo(CO)6 on hydroxylated alumina at 80 K as the sample is heated to ∼200 K. At higher temperatures, the molybdenum is oxidized to an approximately 4+ oxidation state and deposits primarily oxalate species on the surface. The adsorbed oxalates thermally decompose at ∼300 K to evolve CO to form surface formates. These are stable to ∼560 K and react to evolve CO at this temperature. It is also found that the extent of decarbonylation depends on the degree of alumina hydroxylation so that heating hydroxylated alumina to 900 K, which removes ∼50% of the surface hydroxyls, decreases the both CO desorption yield and the oxalate coverage by 50%.