On the orientation of benzene adsorbed on Cu{110}

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).</p

Ethyne Cyclization to Benzene over Cu(110)

Ethyne cyclization to benzene over Cu(110) is an efficient reaction that proceeds at low temperatures with close to 100% efficiency. On the clean surface, C2H2 adsorbs into islands, there is no threshold coverage for the onset of reaction, and benzene evolution into the gas phase occurs in a single TPR peak due to a surface reaction rate limited process. In each of these four respects the behavior is very different from that found on Pd(111). Isotope tracing experiments show that cyclization occurs by an associative mechanism, and the use of cis-1,2-dichlorocyclobutene indicates that C4H4 is the key reaction intermediate, as it is on Pd(111). Additional data, including results of experiments with C8H8 and with C4H4Cl2 + C2D2, demonstrate that cyclooctatetraene is not a reaction intermediate in this system, and the possible scheme 2C2H2(a) → C4H4(a); 2C4H4(a) → C8H8(a); C8H8(a) → C6H6(a) + C2H2(a) is ruled out. The mechanism 2C2H2(a) → C4H4(a); C4H4(a) + C2H2(a) → C6H6(a) is established, and it is shown that the first step is rate limiting overall.</p

Adsorption Geometry Determines Catalytic Selectivity in Highly Chemoselective Hydrogenation of Crotonaldehyde on Ag(111)

ABSTRACT The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature programmed desorption/reaction, high resolution XPS and NEXAFS. The organic molecule adsorbed without decomposition, all three possible hydrogenation products were formed and desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were found under TPR conditions. The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO 2 catalysts and the present findings permit a rationalization of the catalytic performance in terms of pronounced coverage-dependent changes in adsorption geometries of the reactant and the products. Thus at low coverages the C=O bond in crotonaldehyde lay almost parallel to the metal surface whereas the C=C was appreciably tilted, favoring hydrogenation of the former and disfavoring hydrogenation of the latter. With increasing coverage of reactants, the C=C bond was forced almost parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing selectivity towards formation of crotyl alcohol. Butanol formation was the result of an overall twostep process: crotonaldehyde crotyl alcohol butanol, further hydrogenation of the desired product crotyl alcohol being promoted at high hydrogen coverage due to the C=C bond in the unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to C=O π bond, as suggested by C 1s XPS binding energies