Q. Guo

CO and O2 adsorption on Rh(110)

The adsorption of CO and oxygen on Rh(110) has been studied using a thermal molecular beam, thermal desorption and LEED. CO adsorbs with an initial sticking coefficient of 0.68 and shows a coverage dependence which is well described by the Kisliuk formalism for precursor kinetics. The desorption shows a main peak at 485 K, which shifts to lower temperature with increasing coverage up to ~ 0.4 monolayers. Above this coverage a shoulder appears at ~ 425 K and a further shoulder at 390 K above about 0.75 monolayers. These effects are due to repulsive lateral interactions in the adlayer, though the only clear ordered pattern seen in the LEED at 320 K is a (2 × 1)p1g1 pattern above 0.9 monolayers. For oxygen the initial sticking coefficient is 0.62 (±0.01) at 310 K which diminishes relatively slowly with increasing oxygen coverage. We believe that this is due to row pairing in the adlayer which produces stretched” rows of higher reactivity adjacent to them. The row pairing is seen in a wide sequence of LEED patterns with increasing coverage — (1 × 3), (2 × 2)plgl, (1 × 2)(1 × 3), (2 × 3)p1g1, c(2 × 6), c(2 × 8). Much higher exposures are require latter 3 structures since the sticking coefficient has diminished to a low value by then. The apparent “saturation” point in the sticking curves is 0.65 monolayers, but oxygen can continue to adsorb slowly. We propose that the last three structures are due to completely restructured surfaces, possibly with subsurface oxygen, though this requires further work. Desorption from these high coverage states begins at the relatively low temperature of 750 K.

The orientation of acetate on a TiO2(110) surface

The adsorption of acetic acid on a TiO2(110) surface has been studied using electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED). The acetate intermediates arising from the dissociative adsorption of acetic acid form an ordered (2×1) overlayer at saturation coverage. The H+ ESD ion angular distributions can be resolved into two contributions: Those ions desorbing from hydrogen atoms bonded at the oxide substrate, and those ions desorbed via the rupture of the C–H bonds of the acetate. The geometry of the ESDIAD pattern led us to propose that the acetates are bridge bonded with the five-fold coordinated Tiu20094+ ions, with their molecular plane perpendicular to the surface. Decomposition of acetate at room temperature occurs under electron beam irradiation, resulting in the desorption of CH2CO and CH3/CH4.

The adsorption of benzoic acid on a TiO2(110) surface studied using STM, ESDIAD and LEED

The adsorption of benzoic acid on a TiO2(110) surface at room temperature has been studied using scanning tunneling microscopy (STM), electron stimulated desorption ion angular distribution (ESDIAD), and low energy electron diffraction (LEED). The adsorption is dissociative, forming benzoate and surface hydroxyl. The adsorbed benzoate is bonded through the car☐yl group with the five-fold coordinated Ti4+ cations, and forms an ordered pseudo-(2 × 1) overlayer at a saturation coverage of 0.5 ML. This (2 × 1) structure is mainly determined by the relatively strong absorbate-substrate interaction. Attractive interactions between aromatic rings of the benzoates lead to the formation of dimerised benzoate rows along the [001] direction.

NO adsorption on Rh(110)

Abstract NO adsorption on Rh(110) has been investigated using a molecular beam system and TPD. NO appears to adsorb initially dissociatively at 300 K, followed by molecular adsorption at higher exposures. The initial sticking coefficient is high (0.67) at all temperatures, declines only slowly with increasing coverage and the saturation coverage at 300 K is 1.1 ( ± 0.1) monolayers. This seems to be composed of a mix of atomic adsorption in the trough sites and molecular adsorption on on top sites. If the adsorption measurements are performed above 380 K, N 2 is seen to evolve during the experiments, with kinetics which depend on the binding strength of the N atoms to the surface. TPD shows that is dramatically weakened above a total atomic coverage of ~ 0.5 monolayers, due to the presence of coadsorbed oxygen atoms. Since the saturation atomic coverage under these circumstances can be as high as 1.5 monolayers it appears that this destabilisation is due to a structural rearrangement of the surface which displaces nitrogen atoms to low coordination sites (perhaps on top) while the oxygen remains in the more favourable sites.

The effect of adsorbate–adsorbate interaction on the structure of chemisorbed overlayers on TiO2(110)

Abstract The adsorption of acetic and benzoic acid on a TiO 2 (110) surface has been studied using scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Both acids dissociate upon adsorption at room temperature and with saturation coverage for both species, LEED identifies a (2×1) pattern. Under the same conditions, STM shows a (2×1) form for acetate, but a predominance of (2×2) with benzoate. It is proposed that rotation of the phenyl ring takes place, allowing the formation of dimers of benzoate at the surface.

Molecular-beam studies of methanol partial oxidation on Cu(110)

A thermal molecular-beam system has been used to examine the oxidative dehydrogenation of methanol on the Cu(110) surface. The initial sticking probability for oxygen is 0.21 (±0.01) at room temperature and shows a near linear dependence of sticking coefficient on atomic coverage due to an island growth mechanism with dissociation on an oxygen dilute inter-island phase. Beam temperature variations show this adsorption to be activated. The reaction of methanol in the beam with a predeposited patch of oxygen depends strongly on surface temperature and oxygen coverage. There is a change in stoichiometry in the reaction from 2CH3OH + O(a)→ 2H2CO + H2+ H2O at 330 K to CH3OH + O(a)→ H2CO + H2O above ca. 450 K. Oxygen promotes the methanol adsorption and reaction at low coverages, but shows poisoning effects at half a monolayer [saturation, p(2 × 1) structure] where the rate of reaction is very much reduced, and there is an induction time before products are seen in the gas phase (ca. 30 min at 333 K under these conditions). This is explained by stabilisation of the methoxy species when the adjacent (110) trough sites are blocked by either adsorbed oxygen or hydroxyl groups; a kinetic model is being developed to describe these complex kinetics, based on a slow production of vacant sites in these (110) troughs. This is shown to describe the kinetics quite well in a semi-quantitative manner. Use of CD3OD in the beam shows a marked isotope effect, whereas CH3OD does not, again indicating that it is methoxy decomposition which limits the product evolution rate.

ESDIAD studies of the structure of TiO2(110)(1 × 1) and (1 × 2) surfaces and interfaces in conjunction with LEED and STM

The TiO2(110)-(1 × 1) surface and its reconstruction as a (1 × 2) form have been studied with low energy electron diffraction (LEED), electron stimulated desorption ion angular distribution (ESDIAD) and scanning tunnelling microscopy (STM). Oxygen ion desorption occurs within a lobe perpendicular to the (1 × 1) surface, changing to two off-normal lobes for the (1 × 2) reconstruction. This transformation in the ESDIAD pattern is consistent with the added Ti2O3 row model of the (1 × 2) reconstruction proposed by Onishi and Iwasawa. STM studies of the stoichiometric and electron irradiated surfaces reinforce the association of the O+ ESD contribution with majority sites at the surface. Adsorption of acetic acid on the (1 × 1) surface produces a (2 × 1) overlayed and induces a reconstruction of the underlying substrate. ESDIAD reveals H+ ions emitted off-normally from dissociatively adsorbed acetate, and along the surface normal from surface hydroxyls. Adsorption of acetic acid on the (1 × 2) surface does not modify the LEED pattern, but ESDIAD reveals H+ desorption with a weaker off-normal contribution consistent with the Ti2O3 model of the reconstruction.

Structure sensitivity in CO oxidation over rhodium

The rates of CO oxidation on the (110) and (111) planes of rhodium have been directly compared using a thermal molecular beam reactor. When the surfaces are largely covered by CO the reaction rate is the same on the two crystal planes. At higher temperatures, where CO desorbs and the surface becomes oxygen covered, the reaction becomes structure sensitive, being markedly faster on the more open (110) plane.

The structure of TiO2(110) (1 × 1) and (1 × 2) surfaces with acetic acid adsorption — a PES study

Abstract The adsorption of acetic acid on the TiO 2 (110) surface has been studied with PES, using synchrotron radiation ∼47 eV, both at the stoichiometric (1 × 1) surface and the (1 × 2)-reconstructed surface. The results indicate a similar bonding configuration for each surface, and that the saturation acetate coverage at room temperature on the (1 × 2)-reconstructed surface is approximately half that of the (1 × 1) surface. The results are interpreted in favour of the added Ti 2 O 3 -row model of the (1 × 2) reconstruction at the clean surface.

The adsorption and bonding of chlorine at silicon (100) investigated using ESD/ESDIAD with Cl+ and Cl− ions

Abstract The adsorption of chlorine on Si(100) 2 × 1 surface has been studied using electron stimulated desorption (ESD) and electron stimulated desorption ion angular distribution (ESDIAD) in conjunction with AES and gas uptake techniques. ESDIAD and ESD measurements were performed on negative as well as positive atomic chlorine species, and the responses with the different polarity of charged species are not seen as complementary. Gas uptake at the surface proceeds initially with a high sticking probability with the atomic chlorine resulting from dissociation not being limited to single dimer sites. ESDIAD studies with positive Cl + ions reveal normal and off-normal beams associated with symmetric and asymmetric dimers, with relative contributions depending on surface coverage and temperature. Transformations between bonding configurations seen in positive ion ESDIAD are linked with lateral interactions in the adsorbate layer, and their influence is also evident in the forms of the ion yields of both polarity of species with changing coverage. Negative chlorine ions exhibit a predominance of emission around the surface normal, and are produced via a dipolar dissociation process. Missing atom defect sites with an associated high electron density are postulated as playing a central role in their production. The desorption of positive chlorine ions follows mainly from a two-hole, one-electron (2h1e) repulsive state initiated by the ionisation of the Cl 3s level.