Christopher J. Baddeley

A photoelectron diffraction study of ordered structures in the chemisorption system Pd{111}-CO

The adsorption system Pd{111}-CO gives rise to a series of at least seventeen, sometimes complicated, ordered overlayers. The R30° and c(4×2) phases are formed at coverages of θ=0.33xa0ML and 0.50xa0ML, respectively. The results from surface vibrational spectroscopy have hitherto been interpreted in terms of CO adsorbing on threefold symmetric hollow sites in the R30° phase, but occupying bridge sites in the c(4×2) phase. We show in a quantitative photoelectron diffraction study in the scanned energy mode that, whereas hollow sites are indeed occupied in the ()R30° structure, CO adsorbs in a mixture of fcc and hcp hollows in the c(4×2) phase. Several structural parameters, in particular the Pd-C layer spacings, have been determined. The result is essentially identical to that obtained for the Ni{111}c(4×2)-CO system. It is also inferred that at coverages between 0.33 and 0.50xa0ML “domain wall”, or weakly incommensurate, phases occur in which the percentage of occupied hcp sites increases monotonically as the coverage is raised.

A spectroscopic study of the chemistry and reactivity of SO2 on Pt{111}: Reactions with O2, CO and C3H6

The chemisorption and reactivity of SO2 on Pt{111} have been studied by HREELS, XPS, NEXAFS and temperature-programmed desorption. At 160 K SO2 adsorbs intact at high coverages, with η2 Sue5f8O coordination to the surface. On annealing to 270 K, NEXAFS indicates the SO2 molecular plane essentially perpendicular to the surface. Preadsorbed Oa reacts with SO2 to yield adsorbed SO4, identified as the key surface species responsible for SO2-promoted catalytic alkane oxidation. Coadsorbed CO or propene efficiently reduce SO2 overlayers to deposit Sa, and the implications of this for catalytic systems are discussed.

Nucleation and growth of catalytically active Pd islands on Au(111)-22 × studied by scanning tunnelling microscopy

Abstract The nucleation and growth of Pd films on Au(111)-(22 × substrates from 0.07 ML coverage has been characterized using an ultra high vacuum scanning tunnelling microscope. Initially, polygonal islands nucleate and grow at sites near the surface edge dislocations in the elbows of the herringbone reconstruction. At low coverage, the herringbone reconstruction remains relatively undisturbed and most Pd islands are found on fcc regions of the Au(111) substrate. Increasing coverage leads to distortion of the underlying reconstruction which in turn affects the surface morphology of the Pd islands. Atomic resolution images of Pd island surfaces show that they are well ordered and close packed. Second-layer growth is also in the form of polygonal islands on the underlying layers. The morphological evolution of the system with increasing Pd coverage provides a good explanation for its catalytic behaviour.

Structural and electronic properties of Sn overlayers and PdSn surface alloys on Pd(111)

Abstract The first two layers of Sn deposited on Pd(111) at 300 K grow in layer-by-layer fashion after which crystallite formation commences. The electronic properties of these overlayers are dependent on the size of the 3D Sn islands. The occurrence of Sn→Pd valence charge-transfer is inferred, due allowance being made for initial and final state effects in the photoemission data. Evidence is presented for a significant Pd surface core-level shift enhancement by Sn of ≈0.7 eV. Depending on the initial Sn loading, heating generates stable monolayer (Pd2Sn) or multilayer (Pd3Sn) surface alloys exhibiting √3 and (2 × 2) periodicities, respectively. The very different CO adsorption capacity of these two phases indicates that on Pd Sn alloy surfaces, only pure Pd threefold hollow-sites are capable of strongly chemisorbing CO.

Surface crystallography of three catalytically important structures in the Au{111}-Pd system

Automated Tensor LEED has been used to determine the detailed crystallography of structures formed during Pd deposition on Au{111}. At 200 K, deposition of 0.2 monolayer Pd results in a complete lifting of the clean surface Au{111} reconstruction, with excess Au atoms being squeezed into a disordered adlayer with the added Pd adatoms. At monolayer coverage at this temperature a disordered structure is formed, which orders on heating to 300 K to a (1 × 1) structure with a predominantly Pd top layer. Further deposition of Pd produces a disordered overlayer. On annealing to ⩾ 500 K, this converts to an ordered alloy structure with (3×3)R30° periodicity. This structure consists of at least 2 layers of Pd2Au alloy. It is unique in showing a constant splitting of all Bragg beams, attributed to facetting into domains oriented only 0.75° from the planar Au{111} surface. This rumpled structure possesses domains which are larger than 100 A in width. The results are discussed in relation to the known catalytic activity for the various structures in the cyclisation of ethyne to benzene.

Chemisorption and reactivity of furan on Pd{111}

Abstract XPS, HREELS, ARUPS and Δo data show that furan chemisorbs non-dissociatively on Pd{111} at 175 K, the molecular plane being significantly tilted with respect to the surface normal. Bonding involves both the oxygen lone pair and significant π interaction with the substrate. The degree of decomposition that accompanies molecular desorption is a strong function of coverage: ∼40% of the adsorbate desorbs molecularly from the saturated monolayer. Decomposition occurs via decarbonylation to yield COa and Ha followed by desorption rate limited loss of H2 and CO. It seems probable that an adsorbed C3H3 species formed during this process undergoes subsequent stepwise dehydrogenation ultimately yielding H2 and Ca.

Photoelectron diffraction study of a catalytically active overlayer: C2H2 on Pd{111}

A quantitative structure determination of a newly discovered (2◊2) adsorption phase of acetylene chemisorbed on Pd{111} has been performed by scanned-energy mode photoelectron diVraction: this phase corresponds to the threshold coverage for the catalytic conversion of acetylene to benzene. The carbon atoms in the C 2 H 2 molecule are located almost over bridge sites with a C‐C bond length of 1.34+0.10 A ˚ , the centre of the molecule being positioned almost over a hollow site. Of the two hollow sites the hcp site (directly above a second layer Pd atom) is favoured, particularly by a subset of the data most sensitive to this aspect of the structure, but the full analysis indicates that the fcc site (above a third layer Pd atom) cannot formally be excluded. The adsorption site adopted by acetylene in the higher coverage (E3◊E3)R30° phase on Pd{111} is essentially identical. This is the dominant structure in the coverage regime which is catalytically active for the conversion of acetylene to benzene. The implications of these findings for acetylene coupling reactions over Pd{111} are discussed.

Chemisorption and decomposition of pyrrole on Pd{111}

Abstract XPS, TPD and HREEL results indicate that molecular pyrrole is a fragile adsorbate on clean Pd 111. At 200 K and for low coverages, the molecule remains intact and adopts an almost flat-lying geometry. With increasing coverage, pyrrole molecules tilt away from the surface and undergo Nue5f8H bond cleavage to form strongly tilted pyrrolyl (C 4 H 4 N) species. In addition, a weakly bound, strongly tilted form of molecular pyrrole is observed at coverages approaching saturation. Heating pyrrole monolayers results in desorption of ≈ 15% of the overlayer as molecular pyrrole and H a + C 4 H 4 N a recombination with formation of flat-lying pyrrole molecules. This strongly bound species undergoes decomposition to adsorbed CN, CH x and H, leading ultimately to desorption of HCN and H 2 . The implications of these results for the production of pyrrole by a heterogeneously catalysed route are discussed.

On the orientation of benzene adsorbed on Cu{110}

Abstract The orientation of benzene on Cu{110} has been determined at low and high coverages by angle-resolved photoemission and high resolution electron energy loss spectroscopy. The results clearly indicate that the molecular plane is substantially tilted with respect to the metal surface at all coverages, in disagreement with an earlier NEXAFS investigation. These findings are discussed in the light of the previously reported reaction kinetics of acetylene trimerisation over Cu{110} and comparison is drawn with the same reaction over Pd(111).

The effect of photo-oxidation on the sticking and reactivity of Ag on amorphous GeS2

Photo-oxidation of amorphous films illuminated by band-gap radiation drastically alters the growth mode and reactivity of subsequently deposited Ag. In the former case (monolayer/simultaneous multilayer growth) the Ag reacts with both Ge and S sites. In the latter case (Stranski - Krastanov growth) Ge sites are selectively oxidized and film growth proceeds by Ag nucleation at the unoxidized S sites. The behaviour is very different from that reported earlier for Zn deposition on , where photo-oxidation results in very large changes in metal sticking probability. XPS, XAES and EXAFS data provide the basis for understanding both this phenomenon and the very different photodiffusion behaviour of Zn and Ag in .