Nucleation and growth of catalytically active Pd islands on Au(111)-(22 x Ö3) studied by scanning tunnelling microscopy

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 fee 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.</p

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

The first two layers of Sn deposited on Pd(lll) 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 --&gt; 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 approximate to 0.7 eV. Depending on the initial Sn loading, heating generates stable monolayer (Pd,Sn) or multilayer (Pd,Sn) surface alloys exhibiting root 3 and (2 x 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. (C) 1997 Elsevier Science B.V.</p

Tilt the molecule and change the chemistry: mechanism of S-promoted chemoselective catalytic hydrogenation of crotonaldehyde on Cu(111).

(Figure Presented) Tilt it a little: X-ray absorption spectroscopy and photoemission were used to elucidate the mechanism of promoter action in the Cu-catalyzed chemoselective hydrogenation of crotonaldehyde. Sulfur adatoms electronically perturb the C=O bond and tilt the C=C bond away from the surface, thus activating the former and rendering the latter inert to hydrogenation. Quantitative conversion of the reactant with 100% selectivity may be achieve

Tilt the molecule and change the chemistry: mechanism of S-promoted chemoselective catalytic hydrogenation of crotonaldehyde on Cu(111).

(Figure Presented) Tilt it a little: X-ray absorption spectroscopy and photoemission were used to elucidate the mechanism of promoter action in the Cu-catalyzed chemoselective hydrogenation of crotonaldehyde. Sulfur adatoms electronically perturb the C=O bond and tilt the C=C bond away from the surface, thus activating the former and rendering the latter inert to hydrogenation. Quantitative conversion of the reactant with 100% selectivity may be achieve

Adsorption of ethyne on Cu(110): Experimental and theoretical study

High-resolution electron energy loss spectroscopy and angle-resolved ultraviolet photoelectron spectroscopy data indicate that ethyne adopts a low symmetry (most likely C-1) adsorption geometry on Cu(110). Detailed ab initio Hartree-Fock cluster calculations identify a minimum on the potential energy surface for ethyne in a C-1 adsorption geometry. This structure also provides the best agreement between the experimental and calculated vibrational frequencies of the geometries investigated. In addition, the calculations show that the internal structure of the ethyne molecule is relatively insensitive to the adsorption site and that the adsorbed molecule is essentially sp(2) hybridized.</p