Peter Hollins

The influence of surface defects on the infrared spectra of adsorbed species

Abstract Defect sites on a surface frequently give rise to small wavenumber shifts in the infrared spectrum of an adsorbed species. When this effect is combined with the influence of dipolar coupling interactions, serious distortions can occur in the spectra and may lead to erroneous interpretations, particularly in spectra from supported catalysts and other relatively poorly defined substrates. The problem is especially acute in the case of carbon monoxide, the molecule most commonly employed as a probe of substrate morphology. The influence of defect sites is shown to have a major effect on the interpretation of spectra from the CO/Cu and CO/Pt adsorption systems, and its relevance to other substrates and adsorbates is discussed.

Adsorption of carbon monoxide on the gold(332) surface

The adsorption of carbon monoxide on the Au(332) surface has been investigated by low-energy electron diffraction, surface-potential measurements, temperature-programmed desorption and infrared spectroscopy. The surface potential was found to increase monotonically with coverage, while the isosteric enthalpy of adsorption derived from isobaric desorptions was found to be 55 kJ mol–1 at low coverages, falling progressively to 20 kJ mol–1 at saturation. No ordered overlayer structures were obtained. Infrared studies showed a band appearing above 2120 cm–1 initially and moving smoothly to lower wavenumber with increasing coverage. Although some structure was evident in the band envelope, distinct components could not be resolved in single-isotope spectra. Spectra from C16O–C18O mixtures, however, indicated the presence of four separate components.

Interaction of CO molecules with the Au(332) surface

Reflection-absorption infrared spectra of CO adsorbed on Au(332) show a single band which first appears above 2120 cm -1 , and shifts to lower wavenumber with increasing coverage. The band is broad and consists of at least three imperfectly resolved components.

A combined STM/molecular beam study of formic acid oxidation on Cu(110)

Abstract In this paper, we describe a powerful combination of methods used to elucidate the mechanisms and kinetics of surface reactions, namely, molecular beam reactor work and scanning tunnelling microscopy (STM). These two combine microkinetic measurements with a technique which has atomic spatial resolution for structural determination during the course of a reaction. This gives the ability to link the worlds of macroscale and nanoscale surface chemistry. A particular application is highlighted here, namely, the adsorption and oxidation of formic acid on Cu(110), in principle, a simple reaction, which turns out to be very complicated. The findings can be summarised as follows: (i) predosed oxygen enhances formic acid sticking by at least an order of magnitude; (ii) oxygen predosed to a coverage below 0.25 monolayers is completely removed as water by the reaction with formic acid to produce formate; (iii) above this oxygen coverage, some oxygen remains on the surface after reaction and is compressed up into the locally higher coverage c (6×2) structure, otherwise only obtained by extremely high exposures of oxygen; (iv) the reaction stoichiometry changes from 2 HCOOH: 1 O (a) to 1:1 at high temperature where the formate is unstable; (v) gross rearrangement and redistribution of Cu atoms occurs during the course of the reaction; and (vi) the formate adsorbate structure varies considerably with substrate temperature, oxygen precoverage and formic acid exposure, five distinct structures being observed with STM.

The photodimerisation of trans-cinnamic acid and its derivatives: a study by vibrational microspectroscopy.

Infrared and Raman spectroscopies have been used to monitor the [2 + 2] photodimerisation reactions of alpha-trans-cinnamic acid and of a number of its derivatives. The principal changes observed in the spectra upon dimerisation are decay of a band around 1637 cm(-1), which is assigned to v(C=C) of the ethene bond of the monomer, and growth of bands just above 3000 cm(-1), which result from v(C-H) of saturated carbon atoms of the dimer. The use of microscope attachments has allowed us to follow the reactions of single crystals and we conclude that the reactions are topotactic in nature. We have carried out preliminary kinetic experiments in which spectra of one single crystal are recorded after sequential periods of photolysis. We find that the rates of dimerisation of differently substituted cinnamic acids are similar, with physical effects, such as the thickness of an individual crystal, outweighing any observed electronic effects (inductive or mesomeric).

The photodimerisation of the α- and β-forms of trans-cinnamic acid: a study of single crystals by vibrational microspectroscopy

Abstract Infrared and Raman microspectroscopy have been used to follow the photodimerisation reactions of single crystals, the α- and β-forms of trans-cinnamic acid. This approach allows the starting materials and products—α-truxillic acid that has Ci symmetry and β-truxinic acid, which has Cs symmetry—to be identified. It also allows the topotactic nature of the reaction to be confirmed. Attempts to produce the poorly-defined unreactive γ-form of trans-cinnamic acid resulted only in a mixture of the α- and β-forms. The findings suggest a wide role for these spectroscopic methods in monitoring solid-state organic reactions.

The adsorption of ethene on clean and chlorine-precovered Ag(100)

Abstract RAIRS, in conjunction with LEED and electron spectroscopies, has been used to characterise both the low-temperature adsorption of ethene on the Ag(100) surface and the influence of preadsorbed chlorine on this adsorption. On the clean surface, ethene is found to be weakly τ-bonded at all coverages with dynamic pressures being required to saturate the monolayer adsorption state at 100 K. At low coverages, the molecule lies parallel to the surface plane, but further adsorption induces a reorientation involving rotation about the CC axis. The influence of preadsorbed chlorine depends critically on the Cl coverage. At low coverage values adsorption is enhanced and LEED shows the presence of ordered ethene-chlorine structures, although no substantial re-hybridisation of the ethene is induced. At high Cl coverage a progressive passivation of the surface is produced.

Infrared spectroscopy of the mineralogy of coprolites from Brean Down: evidence of past human activities and animal husbandry

The mineralogy of 11 concretions from the Bronze Age settlement horizons at Brean Down near Weston-super-Mare, Somerset, UK, has been examined by infrared spectroscopy. The concretions are found to contain calcite and apatite and, in some cases, quartz. Four further concretions from the later Iron Age Meare Village, soil samples from Brean Down and mineralised samples of known faecal origin from a cesspit within the Tudor Merchants house in Tenby have been similarly examined. It is found that all samples contain calcite, but only the concretions and the Tenby cesspit samples contain apatite. None of the soil samples contain apatite, although these are relatively high in quartz. This suggests that the concretions are coprolites and that the apatite has a biological origin in small bone fragments. The infrared study is backed up by scanning electron microscopy which confirms the presence of phosphorus in the coprolite samples and shows a morphology suggestive of the presence of bone fragments; it is likely, therefore, that the coprolites result from a carnivore--most probably from dogs. The findings show the usefulness of infrared spectroscopy for the rapid identification of mineralised coprolitic material from archaeological sites.

The photodimerisation of the chloro-, methoxy- and nitro-derivatives of trans-cinnamic acid: a study of single crystals by vibrational microspectroscopy

The techniques of infrared and Raman microscopy have been used to monitor the photochemical dimerisation reactions of β-2-chloro-trans-cinnamic acid, β-3-chloro-trans-cinnamic acid, β-4-chloro-trans-cinnamic acid, α-2-methoxy-trans-cinnamic acid, β-2-nitro-trans-cinnamic acid, β-3-nitro-trans-cinnamic acid and β-4-nitro-trans-cinnamic acid. In this way the topotactic nature of the reactions has been confirmed. These reactive derivatives have been compared with the unreactive γ-forms of these and related compounds.

Gas-solid reactions of single crystals: a study of the reaction of bromine with single crystals of trans-cinnamic acid and a range of its derivatives by infrared and Raman microspectroscopy

Single crystals of trans-cinnamic acid and of a range of derivatives of this compound containing halogen substituents on the aromatic ring have been reacted with 165 Torr pressure of bromine vapour in a sealed desiccator at 20 degrees C for 1 week. Infrared and Raman microspectroscopic examination of the crystals shows that bromination of the aliphatic double bond, but not of the aromatic ring, has occurred. It is demonstrated also that the reaction is truly gas solid in nature. A time-dependent study of these reactions shows that they do not follow a smooth diffusion-controlled pathway. Rather the reactions appear to be inhomogeneous and to occur at defects within the crystal. The reaction products are seen to flake from the surface of the crystal. It is shown, therefore, that these are not single crystal to single crystal transitions, as have been observed previously for the photodimerisation of trans-cinnamic acid and several of its derivatives. It is shown that there are no by-products of the reaction and that finely ground samples react to form the same products as single crystals.