Monitoring the interaction of adsorbates on metal surfaces by surface site engineering: the case of ethoxy on Cu, Pd, Ag and Au regular and stepped surfaces.

The interaction of ethoxy with the (111), (100), (511) and (310) surfaces of Cu, Pd, Ag and Au has been studied by means of periodic density functional calculations with the main aim to investigate, in a systematic way, the effect of the coordination number of surface metal atoms directly interacting with the adsorbate on the adsorption properties. The geometry of the adsorbed molecule is only slightly affected by the type of surface but the adsorption energy may change up to 40% on going from the (111) surface with atoms with coordination number of 9 to the (310) one where the coordination number decreases to 6. Analysis of the work function of the different surfaces and of the charge density reveals that the enhancement of the interaction is not due to variations in the charge transfer. However, a well defined trend between the interaction energy and the coordination number is clearly observed which is interpreted in terms of the d-band center model. These results strongly suggest that it is possible to tune the interaction energy by surface engineering.

Active sites for H2 adsorption and activation in Au/TiO2 and the role of the support.

The catalytic activity toward H2 dissociation of several Au nanoparticles of different shape, supported on stoichiometric and reduced TiO2 surfaces, has been investigated by means of periodic DF calculations. Gold nanoparticles become positively charged when supported on stoichiometric TiO2 and negatively charged when adsorbed on the reduced surface, although this finding does not appear to be relevant for H2 dissociation activity. It is shown that Au atoms active for H2 dissociation must be neutral or with a net charge close to zero, and be located at corner or edge low coordinated positions and not directly bonded to the support. The particles with the largest number of potentially active sites for H2 dissociation are 2L isomers consisting of at least one bottom layer of gold atoms in contact with the support and therefore inactive, and one top layer with low coordinated gold atoms on which H2 is adsorbed and activated. The presence of Ovacancy defects in reduced surfaces preferentially stabilizes the most active 2L particles, while less active 1L isomers are the most stable on the stoichiometric surfaces.

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A Molecular mechanism for the chemoselective hydrogenation of substituted nitroaromatics with nanoparticles of gold on TiO2 catalysts: a cooperative effect between gold and the support.

Nanoparticles of gold on TiO2 are highly chemoselective for the reduction of substituted nitroaromatics, such as nitrostyrene. By combining kinetics and in situ IR spectroscopy, it has been found that there is a preferential adsorption of the reactant on the catalyst through the nitro group. IR studies of nitrobenzene, styrene, and nitrostyrene adsorption, together with quantum chemical calculations, show that the nitro and the olefinic groups adsorb weakly on the Au(111) and Au(001) surfaces, and that although a stronger adsorption occurs on low-coordinated atoms in gold nanoparticles, this adsorption is not selective. On the other hand, an energetically and geometrically favored adsorption through the nitro group occurs on the TiO2 support and in the interface between the gold nanoparticle and the TiO2 support. Such preferential adsorption is not observed with nanoparticles of gold on silica which, contrary to the Au/TiO2 catalyst, is not chemoselective for the reduction of substituted nitroaromatic compounds. Therefore, the high chemoselectiviy of the Au/TiO2 catalyst can be attributed to a cooperation between the gold nanoparticle and the support that preferentially activates the nitro group.