Mechanistic and spectroscopic identification of initial reaction intermediates for prenal decomposition on a platinum model catalyst.

The prediction of a reaction mechanism and the identification of the corresponding chemical intermediates is a major challenge in surface science and heterogeneous catalysis, due to a complex network of elementary steps and surface species. Here we demonstrate how to overcome this difficulty by tracking the temperature dependent formation of the initial reaction intermediates and identifying the decomposition pathways in the case of prenal, an α,ß-unsaturated aldehyde, on the Pt(111) model catalyst surface by combining vibrational spectroscopy, thermal reaction/desorption spectroscopy (TPRS) experiments and detailed theoretical analysis. TPRS characterization of this reaction up to 600 K shows a series of desorption states of H(2) (∼280 K, 410 K and 473 K) and CO (∼414 K), giving valuable insights into the sequence of elementary steps suggesting that the loss of hydrogen and the carbonyl functions are among the first elementary steps. HREELS experiments recorded after annealing to specific temperatures result in complex spectra, which can be assigned to several subsequently formed and transformed surface intermediates. Starting from stable prenal adsorption structures, complementary DFT calculations allow the determination of the most likely reaction pathway for the initial decomposition steps and the identification of the corresponding intermediates by comparison with HREELS. The decomposition occurs from the strongly bonded prenal adsorption structures via a dehydro-η(3)-triσ(CCC)-H1 intermediate to the highly stable η(1)-isobutylidyne species at high temperatures.

Adsorption and reaction of Rh(CO)2(acac) on Al2O3/Ni3Al(111).

The Al(2)O(3)/Ni(3)Al(111) surface has been used as a template for the nucleation and growth of rhodium clusters using an organometallic precursor: Rh(CO)(2)(acac). When Rh(CO)(2)(acac) is deposited on the Al(2)O(3)/Ni(3)Al(111) surface, the molecule is observed to bind preferentially to specific sites associated with the film superstructure (known as the dot structure) and appears to be stable at temperatures up to 473 K at which point some sintering and aggregation processes begin. Annealing the sample to 673 K results in further sintering of the metal deposits as well as an apparent loss in the coverage of rhodium species possibly due to a combination of desorption and deligation. After annealing to 873 K the coverage of rhodium species decreases by about 50% with respect to the initial deposited coverage. Our results suggest that using an organometallic precursor rather than metal atoms to form deposited metal particles on oxide substrates may result in increased resistance to sintering processes.

Structural analysis of wheat wax (Triticum aestivum, c.v. 'Naturastar' L.): from the molecular level to three dimensional crystals.

In order to elucidate the self assembly process of plant epicuticular waxes, and the molecular arrangement within the crystals, re-crystallisation of wax platelets was studied on biological and non-biological surfaces. Wax platelets were extracted from the leaf blades of wheat (Triticum aestivum L., c.v. 'Naturastar', Poaceae). Waxes were analysed by gas chromatography (GC) and mass spectrometry (MS). Octacosan-1-ol was found to be the most abundant chemical component of the wax mixture (66 m%) and also the determining compound for the shape of the wax platelets. The electron diffraction pattern showed that both the wax mixture and pure octacosan-1-ol are crystalline. The re-crystallisation of the natural wax mixture and the pure octacosan-1-ol were studied by scanning tunnelling microscopy (STM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Crystallisation of wheat waxes and pure octacosano-1-ol on the non polar highly ordered pyrolytic graphite (HOPG) led to the formation of platelet structures similar to those found on the plant surface. In contrast, irregular wax morphologies and flat lying plates were formed on glass, silicon, salt crystals (NaCl) and mica surfaces. Movement of wheat wax through isolated Convallaria majalis cuticles led to typical wax platelets of wheat, arranged in the complex patterns typical for C. majalis. STM of pure octacosan-1-ol monolayers on HOPG showed that the arrangement of the molecules strictly followed the hexagonal structure of the substrate crystal. Re-crystallisation of wheat waxes on non-polar crystalline HOPG substrate showed that technical surfaces could be used to generate microstructured, biomimetic surfaces. AFM and SEM studies proved that a template effect of the substrate determined the orientation of the re-grown crystals. These effects of the structure and polarity of the substrate on the morphology of the epicuticular waxes are relevant for understanding interactions between biological as well as technical surfaces and waxes.

Formation of supramolecular cavitands on copper electrode surfaces.

The adsorption of 1,1'-dibenzyl-4,4'-bipyridinium molecules (dibenzyl-viologen or DBV(2+) for the sake of simplicity) on chloride precovered Cu(100) has been studied in an electrochemical environment by means of cyclic voltammetry and in situ scanning tunneling microscopy. DBV(2+) spontaneously forms a highly ordered phase on the chloride c(2 x 2) adlayer at potentials close to the onset of the copper dissolution reaction when the pure supporting electrolyte (10 mM HCl/5 mM KCl) is exchanged by one also containing DBV(2+). This ordered phase can be described by a ( radical 53 x radical 53)R15.9 degrees unit cell relating the organic adlayer to the chloride c(2 x 2) structure underneath or alternatively by a ( radical 106 x radical 106)R29.05 degrees unit cell relating the organic layer to the Cu(1 x 1) substrate structure. Thus, the negatively charged chloride layer acts as a template for the adsorption and phase formation of DBV(2+). Compared to the copper-chloride interaction, the DBV(2+)-chloride interaction appears to be weaker since the organic layer can be easily removed from the surface by the tunneling tip when drastic tunneling conditions (low bias voltage, high tunneling current) are applied. A key structural element of the DBV(2+) adlayer is an assembly of four individual DBV(2+) molecules forming square-shaped supramolecular units with pronounced cavities in their center. Characteristically, the supramolecular assemblies reveal a preferential rotational orientation resulting in the appearance of two chiral forms of these assemblies. Furthermore, these two chiral supramolecular assemblies occur in two mirrored domains of the ( radical 53 x radical 53)R15.9 degrees structure. It can be assumed that these viologen-based supramolecular architectures can be used as potential host cavitands for the inclusion of smaller organic molecules.

Cadmium underpotential deposition on Cu(111) in situ scanning tunneling microscopy.

Atomically resolved in situ STM images are presented for an underpotentially deposited (upd) cadmium layer on a Cu(111) electrode from a 10(-4) M CdCl2/10(-2) M HCl solution. The observed moiré-like structure seen in the images is analysed by means of an algebraic model for this long-range superstructure. A structure model for the upd layer is developed which reflects all features of the observed moiré pattern. Furthermore the height modulation was simulated by a hard-sphere model for the Cd overlayer and shows remarkable agreement with the detailed tunneling current density distribution of the measured STM images. The existence of translational and rotational domains is demonstrated. The results are also compared and shown to be fully consistent with previous (ex situ) low-energy electron diffraction (LEED) observations of this system. The mechanism of Cd upd involves a dynamic site exchange between preadsorbed Cl- anions and adsorbing Cd2+ cations as previously concluded from ex situ X-ray photoelectron (XPS) and low-energy ion scattering (LEIS) measurements.