M.R. Albert

The mechanism of the decomposition of methanethiol on Fe(100)

Abstract The mechanism of decomposition of methanethiol on the clean Fe(100) surface, and the Fe(100) surface modified by sulfur has been investigated. Temperature programmed reaction spectroscopy, high resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy have been used to investigate the adsorption and decomposition of methanethiol on these surfaces. At low coverage, methanethiol undergoes SH bond scission upon adsorption at temperatures as low as 102 K. In the case of multilayer adsorption, associative CH 3 SH is also detected. Thermal decomposition of methanethiol on the Fe(100) surface results in the formation of methane (CH 4 ) and hydrogen (H 2 ). The amount of CH 4 formed increases as methanethiol coverage is increased, while the production of H 2 shows a maximum as a function of coverage. On a c(2 × 2)S-Fe(100) surface almost all adsorbed CH 3 SH desorbs without decomposition. Thiomethoxy (−SCH 3 ) and methyl (−CH 3 ) are identified as surface intermediates during thermal decomposition of CH 3 SH, using HREELS. The existence of these intermediates is supported by XPS data. A mechanism for the decomposition involving the thiomethoxy and methyl intermediates is proposed.

The adsorption of oxygen on the Fe(100) surface

Abstract The adsorption and dissociation of oxygen on the Fe(100) surface has been studied using a combination of methods. High resolution electron energy loss spectroscopy, Auger electron spectroscopy, low energy electron diffraction and temperature programmed reaction spectroscopy have been used to monitor the O 2 /Fe(100) adsorption system for various oxygen coverages and annealing conditions. These studies indicate that adsorption of O 2 on the Fe(100) surface at 103 K occurs through a mobile precursor followed by disordered dissociative adsorption. Initially, four-fold hollow sites are occupied by O atoms, followed by bridging sites at higher coverages. A vibrational band near 650 cm −1 is observed at still higher coverages, which may be related to O-O bonding in a highly-perturbed non-dissociated adsorbed oxygen species. The disordered overlayer can be ordered by heating to 923 K. Diffusion-segregation processes are observed on annealing.

The mechanism of methanol decomposition on Fe(100): Comparison to methanethiol decomposition

Abstract The mechanism and rate determining step of the decomposition of methanol on Fe(100) is characterized by TPRS and HREELS. At low temperature (113 K) methanol undergoes facile O-H bond scission and forms an overlayer of adsorbed methoxy. At 450 K, the methoxy adsorbate bends, bringing one hydrogen atom close to the surface. This leads to C-H bond scission and the decomposition of methoxy to H 2 and CO. A comparison of this process to the decomposition of methanethiol on the same surface is presented.

Decomposition of methanol on oxygen-modified Fe(100) surfaces. II, Preadsorbed oxygen as poison, selectivity modifier and promoter

Decomposition of methanol (CH3OH) on the Fe(100) surface modified by low temperature adsorption of oxygen has been studied, using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). Fe(100) surfaces studied were modified by adsorption of O2 at 113 K, and methanol decomposition as a function of oxygen coverage was monitored. The effect of pre-heating the oxygen overlayers on the methanol decomposition was also examined. Decomposition of methanol on these O-modified surfaces passes through a methoxy (-OCH3) intermediate. The thermal stability of methoxy increases in the presence of pre-adsorbed oxygen. At low coverage, atomic oxygen occupies four-fold hollow sites. In this case, the effect of oxygen on the methanol decomposition is similar to that observed previously on the annealed O-modified surfaces. At higher oxygen coverage, a more weakly bound non-hollow site oxygen also exists on the surface, which reacts with hydroxyl (-OH) hydrogen of the CH3OH, promoting the formation of methoxy. At high oxygen coverage (close to saturation coverage at 113 K), decomposition of methanol results in the formation of formaldehyde (H2CO), without production of carbon monoxide (CO). This is very different from the decomposition of methanol on the clean Fe(100) surface, where decomposition leads to the formation of CO without H2CO. The effect of oxygen modification is discussed in terms of changing relative probabilities of competing reaction pathways.

Decomposition of methanol on oxygen-modified Fe(100) surfaces

Abstract Decomposition of methanol on the clean and oxygen-modified Fe(100) surfaces has been studied, using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). The Fe(100) surface was modified by adsorption of O 2 at 113 K, heating to 923 K, followed by cooling back to 113 K. Chemisorbed O atoms following this treatment are located on four-fold hollow sites. These preadsorbed O atoms selectively poison the decomposition of methanol into CO, which dominates on the clean Fe(100) surface, and open up a new reaction pathway which results in the formation of formaldehyde. The decomposition of CH 3 OH involves a stable methoxy intermediate for both decomposition products. The thermal stability of the methoxy increases as the O coverage is increased. The presence of preadsorbed four-fold hollow O atoms reduces the amount of methoxy which can be formed on the surface, poisoning the surface for the decomposition of methanol. A mechanism for the decomposition reaction on clean and modified Fe(100) surfaces which accounts for these observations is proposed.

High resolution electron energy loss spectroscopic characterization of ordered atomic overlayers on Fe(100)

Several ordered atomic overlayers, c(2 × 2)-C, c(2 × 2)-N, p(1 × 1)-O, c(2 × 2)-C,O, and c(2 × 2)-S on the Fe(100) surface, have been prepared and studied by High Resolution Electron Energy Loss Spectroscopy (HREELS). The following vibrational peaks were observed: a peak at 520 cm −1 for c(2 × 2)-C, a peak at 490 cm −1 for c(2 × 2)-N, two peaks at 280 cm −1 and 430 cm −1 for p(1 × 1)-O, two peaks at 395 cm −1 and 515 cm −1 for c(2 × 2)-C,O and a peak at 270 cm −1 for c(2 × 2)-S. The metal-atom stretching frequencies are ordered as ṽ Fe-C > ṽ Fe-N > ṽ Fe-O on the Fe(100) surface. Differences between these observations and previous measurements on the Fe(111) surface are discussed.

The decomposition of methanol on the sulfur-modified Fe(100) surface

Decomposition of methanol (CH3OH) on sulfur-modified Fe(100) surfaces has been studied under ultrahigh vacuum (UHV) conditions, using temperature-programmed reaction spectroscopy (TPRS) and high-resolution electron energy loss spectroscopy (HREELS). Preadsorbed sulfur overlayers, prepared by thermal decomposition of CH3SH, poison the decomposition of CH3OH on the Fe(100) surface. The decomposition of methanol occurs by way of a methoxy (−OCH3) intermediate on the sulfur-modified surface. The amount of methoxy intermediate formed in the decomposition process decreases as sulfur coverage increases. Preadsorbed sulfur atoms also modify the selectivity of the methanol decomposition. The formation of formaldehyde (H2CO) is enhanced in a rather narrow sulfur coverage range, while the amount of carbon monoxide (CO) product decreases monotonically with increasing sulfur coverage. The effect of sulfur modification on the decomposition of methanol appears to be primarily a localized site blocking effect. Sulfur modification of the methanol decomposition on this surface is compared with that of the effect of oxygen modification.

Adsorption state conversion of CO on Fe(100)

Abstract The conversion of adsorbed CO among different adsorption states on an Fe(100) surface was investigated through the coadsorption of labelled 13CO and unlabelled CO. In these experiments selected adsorption states were initially populated by one type of CO and the surface was then exposed to the other CO adsorbate. While no conversion occurred between the β-CO states and the α states, or between the α2 and α1 states, significant conversion was observed between the α2 and α3 states. A relative energy profile of the adsorption state conversions is proposed based on these observations.

The decomposition of surface methoxy on clean and oxygen post dosed Fe(100); control of reaction selectivity

Pure methoxy overlayers on the Fe(l00) surface were prepared and characterized by high resolution electron energy loss spectroscopy (HREELS). Reactions of the methoxy overlayer on the clean surface and in the presence of coadsorbed oxygen have been studied using temperature programmed reaction spectroscopy (TPRS). Post-dosed oxygen alters dramatically the selectivity of methoxy decomposition. Carbon monoxide and hydrogen are the major decomposition products from a pure methoxy overlayer, while formaldehyde is the major decomposition product from an oxygen coadsorbed methoxy overlayer. Clear evidence for production of formaldehyde via a disproportionation of methoxy is presented. Coadsorbed oxygen raises the decomposition temperature of methoxy by 200 K.

Sulfur deposition on Fe(100) from methanethiol decomposition

Sulfur overlayers on the Fe(100) surface were deposited by decomposition of methanethiol (CH3SH), and characterized by Auger electron spectroscopy and low‐energy electron diffraction. Thermal decomposition of CH3SH results in the formation of c(2×2) sulfur overlayers. In the presence of electron beam irradiation, high coverage (greater than half monolayer) sulfur overlayers with antiphase domain structures can be produced.