Philip R. Davies

Surface chemistry and catalysis

Contributors. Preface To The Series. Preface. 1. Teyrnged I Meirion Wyn Roberts R. Mason. 2. Technique And Progress In Surface And Solid-State Science J.M. Thomas. 3. 50 Years In Vibrational Spectroscopy At The Gas/Solid Interface N. Sheppard. 4. High-Pressure Co Dissociation And Co Oxidation Studies On Platinum Single Crystal Surfaces Using Sum Frequency Generation Surface Vibrational Spectroscopy K. McCrea, et al. 5. Modeling Heterogeneous Catalytic Reactions I. M. Ciobica, et al. 6. Model Systems For Heterogeneous Catalysis: Quo Vadis Surface Science? H.-J. Freund, et al. 7. Surface Chemistry Of Model Oxide-Supported Metal Catalysts: An Overview Of Gold On Titania D.C. Meier, et al. 8. Nanoscale Catalysis By Gold G.U. Kulkarni, et al. 9. Catalysis From Art To Science W.M.H. Sachtler. 10. Enantioselective Reactions Using Modified Microporous And Mesoporous Materials G.J. Hutchings. 11. Molecular Description Of Transition Metal Oxide Catalysts J. Haber. 12. Selectivity In Metal-Catalysed Hydrogenation P.B. Wells. Appendix 1. M.W. Roberts Publications. Appendix 2. M.W. Roberts Students. Index.

Hydroxylation of molecularly adsorbed water at Ag(111) and Cu(100) surfaces by dioxygen: photoelectron and vibrational spectroscopic studies

Abstract Molecular water adlayers at Ag(111) and Cu(100) surfaces undergo facile hydroxylation on exposure to dioxygen at low temperatures. XP and HREEL spectra distinguish between OH(a), H 2 (a) and O − 2 (a) and in the case of Ag(111) surfaces establish that O − 2 (a) is the reactive species. At Cu(100) the evidence points to a transient O − 2 (s) species. These results establish that preadsorbed chemisorbed oxygen adatoms are not a pre-requisite for the activation of molecularly adsorbed water and provide further evidence for the high chemical reactivity of dioxygen species present transiently in the disociative chemisorption of oxygen at metal surfaces.

The chemisorption of organophosphorus compounds at an Al(1 1 1) surface

Abstract The reactive chemisorption of three organophosphorus compounds have been investigated at clean and oxidised Al(1xa01xa01) surfaces using X-ray photoelectron spectroscopy (XPS). Phosphoric acid, trimethyl ester (or trimethylphosphate, TMP, Oue605P(OCH 3 ) 3 ), and methyl phosphonic acid and dimethyl ester (or dimethylmethylphosphate, DMMP, Oue605P(CH 3 )(OCH 3 ) 2 ,) were adsorbed from the gas phase, but this approach proved unsuccessful for methyl phosphonic acid (MPA, Oue605P(CH 3 )(OH) 2 ) due to decomposition in the gas phase. MPA was therefore adsorbed from either an aqueous or an ether solution placed onto the aluminium sample in a helium atmosphere. Core binding energies for the Pue5f8OCH 3 , Pue5f8CH 3 , Pue5f8O and Pue605O groups are reported and the decomposition pathways for the three compounds discussed. The Pue5f8CH 3 bond is stable at both clean and oxidised aluminium surfaces up to temperatures of 570xa0K, whereas the Pue5f8OCH 3 bond can be broken even at room temperature. Decomposition of all three molecules requires clean aluminium sites and is hindered by high concentrations of adsorbed species.

Activation of carbon dioxide at bismuth, gold and copper surfaces

Abstract Through a combination of X-ray (XPS) and electron energy loss spectroscopies (HREELS) it has been shown that reactive chemisorption of carbon dioxide at metal surfaces (Al, Bi, Cu and Mg) is not only metal-specific ar low temperatures (80–295 K) but also can be photoinduced. The latter is also metal-specific in that it is not observed with Au and Bi surfaces. Sodium-doped gold surfaces are however intrinsically reactive to CO2 in the absence of X-irradiation.

The adsorption of pyridine at clean, oxidised and hydroxylated Cu(111) surfaces

Abstract The adsorption of pyridine at clean, partially oxidised and hydroxylated Cu(111) surfaces has been investigated by X-ray photoelectron and vibrational electron energy loss spectroscopies. At the clean surface at 190 K adsorption is molecular, desorption occurring by 240 K. The VEEL spectra indicate that under these conditions the molecule adsorbs with its C 2 axis parallel to the surface. In contrast in the presence of chemisorbed oxygen pyridine desorption does not occur until ca. 370 K, and the C 2 axis of the molecule is found to be perpendicular to the surface. XP spectra suggest that pyridine interacts directly with the surface oxygen rather than the metal cation, forming a species analogous to a pyridine oxide.

The oxidation of formic acid to carbonate at Cu(110) surfaces

Abstract The reaction of formic acid with pre-oxidised Cu(110) surfaces is re-evaluated on the basis of new temperature programmed desorption and X-ray photoelectron spectroscopy results. It is shown that at high oxygen coverages and for high formic acid exposures, or after co-adsorption of formic acid and dioxygen, a surface carbonate is formed. The carbonate appears to be very similar to that formed by the oxidation of CO 2 , at higher pressures, which was reported in an earlier paper. The results also account for a number of unexplained observations on this system and provide further support for the involvement of carbonate in methanol synthesis on copper.

Oxygen states at a Cu(111) surface: the influence of coadsorbed ammonia

Abstract The adsorption of dioxygen at Cu(111) surfaces has been investigated in the presence of preadsorbed ammonia and deuterated ammonia at low temperatures (100 K) using X-ray photoelectron (XP) and vibrational electron energy loss (VEEL) spectroscopies. It is shown that in contrast to the more reactive Cu(110) surface little oxydehydrogenation of ammonia occurs at the Cu(111) surface under these conditions and the oxygen transients, Oδ−(s) and Oδ−2 (s) which have been implicated in reactions at other metal surfaces [M.W. Roberts, Surface Science 299 300 (1994) 769] can be stabilised at the surface and characterised spectroscopically. The adsorbed ammonia also facilitates the formation of a further oxygen state, characterised by a high XP binding energy (531.3 eV), and of low chemical reactivity. This state is stable up to 400 K and is tentatively identified as a subsurface oxygen.

The role of a dioxygen precursor in the selective formation of imide NH(a) species at a Cu(110) surface

The coadsorption of dioxygen and ammonia results in a highly selective oxy-dehydrogenation reaction to form just chemisorbed imide NH(a) species at a Cu(110) surface at 298 K. The latter have been characterised by both core-level, N(1s), and vibrational (HREEL) spectroscopy. The role of a transient dioxygen precursor O2−(s) is discussed.

Facile hydrogenation of carbon dioxide at Al(111) surfaces: the role of coadsorbed water

The chemisorption of carbon dioxide and its reactivity in the presence of coadsorbed water has been investigated at Al(111) surfaces through a combination of in situ X-ray and electron energy-loss spectroscopies. Carbon dioxide forms surface carbonate at low temperatures, which is readily reduced to form surface carbide and oxide. However, the particular significance of the present work is that when coadsorbed with water, there is a low-energy pathway to surface formate. The vibrational loss spectra are compared with spectra of model surface formates generated at Al(111) through interaction with formic acid and also when the latter is coadsorbed with water.

An STM and XPS study of the chemisorption of methyl mercaptan at a Cu(110) surface

XP spectra of a Cu(1 1 0) surface exposed to methyl mercaptan at 290 K show the presence of an adsorbate assigned to mercaptide, CH3S(a). Scanning tunneling microscopic (STM) images show that the formation of the adsorbed mercaptide (CH3S(a)) is accompanied by a restructuring of the surface. The reconstructed surface is characterized by very narrow terraces (typically 10–15 A wide) oriented mainly in the 〈110〉 direction with a `zig-zag structure. Higher resolution images of the terraces reveal an atomic scale structure with a c(2×2) unit cell, each cell containing two bright features. The combination of STM and X-ray photoelectron spectroscopy (XPS) shows that the c(2×2) structure is complete at a surface concentration of ∼5 ×1014cm−2, consistent with the c(2×2) unit cell containing two equivalent mercaptide species. On heating to 450 K the mercaptide dissociates to give chemisorbed sulfur adatoms and the desorption of all of the surface carbon. The STM images show that following the decomposition of the mercaptide adlayer the copper surface regains its original structure of broad terraces (typically 100–200 A wide), though an adlayer of chemisorbed sulfur atoms is now present.