F.M. Leibsle

Methanol oxidation on Cu(110)

The mechanism of reaction and surface structures for the adsorption and partial oxidation of methanol on the oxygen precovered Cu(110) surface have been studied with a variety of surface sensitive techniques. Molecular beam reaction measurements and TPD have been used to determine the chemical nature of the adsorbed species and the products formed. LEED and STM have been employed to determine the surface structure of both the oxygen covered surface and the structures formed by reaction with methanol. The kinetics are shown to be dependent on the initial oxygen precoverage and the surface temperature during reaction. The mechanism shows three distinct temperature regimes; at low temperatures (< 330 K) stable methoxy species are formed on the surface in a 2:1 ratio to preadsorbed oxygen atoms while at intermediate temperatures (330–450 K) the stoichiometry is the same, but the methoxy is unstable, decomposing to formaldehyde and hydrogen. At high temperatures (

STM observations of Cu(100)−c(2×2)N surfaces: evidence for attractive interactions and an incommensurate c(2 × 2) structure

450 K) the stoichiometry of reaction changes to 1:1 with no hydrogen production. STM and LEED show a previously unreported (5 × 2) structure for methoxy adsorbed on this surface. It is clear from the combination of techniques that a dual site combination is crucial for high reactivity. This dual site consists of an oxygen atom and a Cu0; the fully oxygen covered surface with partially oxidised Cu atoms is poisoned in activity for the reaction, whereas the partly covered surface is the most active. The proposed active sites are located at the short side of the rectangular oxygen islands.

The adsorption and decomposition of formic acid on Cu {110}

Abstract STM studies of N on Cu (100), adsorbed via activated ion bombardment, reveal behaviour consistent with attractive interactions and an incommensurate c(2×2)N structure. For extremely low N doses, images of the resultant surface show roughly square shaped islands with island edges running along the 〈001〉 directions. The islands have an internal c(2 × 2) periodicity and while tending to group together do not coalesce. For higher N doses, island coalescence occurs simultaneously with the appearance of trench-like defect structures running in the 〈011〉) directions and small one-atomic layer high islands. Lack of island coalescence at low coverages and the appearance of trench-like defects at higher coverages are interpreted as strain-relief mechanisms for an incommensurate c(2×2)N structure. To derive the N adsorption site, S has also been coadsorbed along with the c(2×2)N islands to form coexisting p(2×2)S domains. The spatial relationships between features in both domains show that the protrusions observed in the c(2 × 2)N areas are located above four-fold hollow sites. Finally, in order to compare our work with previous studies, we have obtained detailed LEED I–V data from the various Cu(100)−c(2×2)N surfaces studied with STM.

Scanning tunnelling microscopy studies of α-Fe2O3(0001)

Abstract The reactive adsorption of formic acid (HCOOH) on oxygen dosed Cu(110) has been studied using a molecular beam system, TPD, LEED and STM. At low temperature the reaction is strongly oxygen coverage dependent. All coverages result in high reaction probability (0.8 at room temperature) for formic acid and, for less than 0.25 monolayers of oxygen there is complete oxygen clean-off, leaving formate on the surface in a c(2 × 2) structure. At higher coverages the situation is more complex, with some oxygen remaining coadsorbed with the formate. The two adsorbates are then mainly phase separated into islands of c(6 × 2) oxygen and (3 × 1) formate. The two phases mutually compress each other due to pressure at the phase boundaries. The reaction stoichiometry is 2:1 formic acid:oxygen atoms in this temperature range. At higher temperatures (> 450 K) the formate itself is unstable and decomposes during adsorption which results in a change of stoichiometry of the reaction; one molecule of formic acid removes an oxygen atom as water, and hydrogen evolution ceases. There is a range of temperature between 350 and 420 K for which the reaction becomes very difficult, and the reaction probability drops to ∼ 0.1. It is proposed that this is due to rapid compression of much of the oxygen adlayer into the unreactive c(6 × 2) structure by small amounts of formate. The reaction proceeds through a highly mobile, weakly held, “precursor” state on the surface, which is able to seek out the active sites on the surface, which are low in coverage at high levels of oxygen. These active sites are the terminal oxygen atoms in the oxygen islands (in the [001] direction), which are only present at step edges or phase boundaries at 0.5 monolayers coverage of oxygen.

Fe3O4(111) termination of α-Fe2O3(0001)

Abstract Scanning tunnelling microscopy (STM) images of two different reconstructions of an α-Fe2O3(0001) crystal are presented. Annealing the sample to 1000 K creates a selvedge stabilised by a thin film of Fe3O4, with its (111) plane parallel to the basal plane of the underlying substrate. The STM images confirm that this surface is structurally equivalent to that previously reported for the surface of Fe3O4(111) single crystals, in that two coexisting terminations, denoted A and B, are present separated by alternate steps. Termination A has been identified with 1 4 ML of O atoms capping 3 4 ML of Fe atoms, while termination B consists of 1 2 ML of Fe atoms overlaying a close-packed O layer. Some regions of the sample are disordered but contain small triangular islands of termination A. This structure is attributed to Ar ion induced sputter damage. A different termination, created by annealing the sample at 1100 K in 1 × 10−6 mbar O2, has a distinctive hexagonal LEED pattern, with all the main beams floreted, being surrounded by a hexagon of smaller spots. The STM results show that this surface is stabilized by coexisting α-Fe2O3(0001) and Fe1−xO(111) phases, with each phase existing in atomically well ordered islands of mesoscopic dimensions. The islands themselves are arranged to form a superlattice. The formation of this superlattice can be explained in terms of the lattice mismatch between the two types of oxygen sub-lattices.

STM studies of oxygen-induced structures and nitrogen coadsorption on the Cu(100) surface : evidence for a one-dimensional oxygen reconstruction and reconstructive interactions

Scanning tunnelling microscopy (STM) has been used to investigate the structure formed on an α-Fe2O3(0001) substrate after argon ion bombardment and annealing in 1 × 10−6 mbar of O2 at 1000 K. The STM images recorded at positive sample bias reveal an hexagonal array, with a distance between (Fe) atoms of 6.0 ± 0.1 rA and steps in multiples of 4.8 A. These results are consistent with formation of an Fe3O4(111) epitaxial layer terminating in a 14 monolayer of Fe atoms.

Aspects of formaldehyde synthesis on Cu(110) as studied by STM

Abstract Scanning tunneling microscopy (STM) has been used to examine oxygen-induced structures on the (100) face of copper. These studies have resulted in images of the (√2 × 2√2)R45°O structure in which additional features not reported in prior STM studies of this system have been imaged. Surfaces containing islands of c(2 × 2)N and √2 × 2√2)R45°O structures have also been created. Images of the two structures coexisting aids in understanding the properties of both structures. These images also demonstrate the existence of reconstructive interactions: c(2 × 2)N islands appear to force the alignment of the oxygen-induced structures so that the √2 sides of the (√2 × 2√2)R45°O structures are always found alongside the c(2 × 2)N islands. Furthermore, an additional oxygen-induced structure has been observed. This structure appears to be a one-dimensional reconstruction of the Cu surface involving chains of atoms running randomly in the 〈001〉 directions. This is believed to be an added-row structure consisting of alternating Cu and O atoms similar to that found on the Cu(110)−(2 × 1)O surface.

Long-range periodicity in c(8 × 2) benzoate/Cu(110): a combined STM, LEED and HREELS study

Abstract The synthesis of formaldehyde from methanol (both CH3OH and CD3OD) on oxygen-predosed Cu(110) surfaces has been studied using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Sequential STM images show the conversion of methanol to methoxy results in the removal of O from the short sides of the ( 2 × 1 ) Cu-O islands present when the surface is dosed with oxygen to give 1 4 monolayer coverage. Methoxy forms a (5 × 2) reconstruction which incorporates added Cu atoms. Detailed observations have been made on the formation of this (5 × 2) methoxy reconstruction. (5 × 2) LEED patterns have also been observed and show diffraction spot absences characteristic of plgl or p2mg symmetries. Models for the (5 × 2) methoxy-induced structure are proposed that are consistent with both the STM and LEED data. The decomposition of methoxy to formaldehyde occurs with STM images showing as an initial step the diminishing of the (5 × 2) methoxy islands. This occurs from island edges. Upon island-breakup the added Cu atoms incorporated in the (5 × 2) reconstruction are released causing the expansion of nearby step edges.

Extended defects on TiO2(100) 1 × 3

Abstract The adsorption of benzoic acid on Cu(110) between 300 and 350K results in benzoate species oriented perpendicular to the surface with a c(8 × 2) periodicity at saturation coverage. STM images of the ordered structure and non-dipolar scattering in HREELS demonstrate alignment in the [110] azimuth. We interpret the tunnelling mechanism in STM, negative ion resonance scattering in HREELS, and assign low-frequency vibrational modes utilising ab initio molecular orbital calculations. We propose a model with four molecules per c(8 × 2) unit cell (ϑ = 0.25ML) arranged in two out-of-phase, zig-zag rows along the [001] direction with the car☐ylates bound in short bridge sites alternately three and five lattice constants apart along the [110] direction. Adsorbate-induced dipole-active phonons in the frequency range 65–185 cm −1 in HREELS support the adsorption model.

STM and SPA-LEED studies of O-induced structures on Rh(100) surfaces

Abstract STM, scanning tunneling spectroscopy (STS) and LEED have been used to investigate the interaction of structural elements associated with rutile TiO 2 (100)1 × 3. These include the O-vacancy-induced 1 × 3 reconstruction and steps in the two principal azimuths. LEED data from a 2.6° vicinal sample show splitting of the primary and 1 3 rd order beams in the [010] direction and not the vicinal off-cut [001] direction. A simple spot-profile analysis reveals that the up-down steps formed have a height of 4.59± 0.05 A and a periodicity of 45±10 A. The STM(S) results image the microfacet morphology of the 1 × 3 reconstruction suggested by diffraction measurements. They also provide an explanation for the stabilisation of up-down steps, which involves maximising the area of the (110) microfacets.