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.
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.
The effect of Fe-doping on the oxygen vacancy energy formation and on the electronic structure of stoichiometric bulk and (001) surface of TiO2 anatase has been studied by means of periodic density ...functional calculations within the GGA+U approach. The vacancy energy formation is always lower for the surface than for the bulk. The presence of Fe causes only a minor perturbation of the atomic structure of anatase but strongly reduces the oxygen vacancy energy formation, especially at the surface. The present results provide additional and independent support to the claim that the enhanced catalytic activity of Au nanoparticles supported on Fe-doped TiO2 toward CO oxidation has its origin in the ease to create oxygen vacancies in the support.
New chiral CNN-pincer-type gold, palladium, and rhodium complexes containing N-heterocyclic carbene substituent and (S)-N-tert-butyl-methylpyrrolidine-2-carboxamide as chiral auxiliary have been ...synthesized and studied for asymmetric hydrogenation. The complexes were prepared by the silver carbene transfer route from the respective silver complex. The reaction with RhCl(cod)2 (cod = cycloocta-1,5-diene), PdCl2(CH3CN)2, or KAuCl4 affords the corresponding cationic Rh(cod)(ligand)Cl, PdCl(ligand)Cl, and AuCl(ligand)Cl2 complexes in which the ligand functions effectively in a CNN coordination mode. The complexes catalyze the enantioselective hydrogenation of prochiral alkenes. Enantioselectivity is very sensitive to the NHC N-substituent, resulting in a useful switch in the predominant enantiomer.
The interaction of the ethoxy radical with Cu(111), Ag(111), Pd(111) and Au(111) has been studied using a periodic density functional approach. The most stable adsorption site is the
fcc
with ...adsorption energies in the 1.1–2.2 eV interval. All analyses consistently indicate that ethoxy becomes negatively charged, that the presence of ethoxy slightly perturbs the electronic structure of the metallic surface, that the interaction is essentially of electrostatic character and not directional predicting a rather mobile species. The calculated adsorption energies are found to correlate almost linearly with the total net charge on the ethoxy moiety thus confirming that the electrostatic interactions dominate the bonding between this organic species and the underlying metallic surfaces.
The molecular structure of acrolein and crotonaldehyde adsorbed on clean and S-covered Cu(1
1
1) surfaces has been theoretically investigated in order to explain the mechanism of selective activation ...of the carbonyl group induced by the presence of small amounts of S on the metal surface. While on clean Cu(1
1
1) there is an equilibrium between a physisorbed and chemisorbed species, it is found that addition of small amounts of S enhances and modifies the metal–molecule interaction, in agreement with experimental evidence. On the S-covered Cu(1
1
1) the aldehyde is bonded to the surface through the O of the carbonyl group which is slightly stretched whereas the C
C bond remains undistorted and far away from the surface. This geometrical conformation induced by the presence of coadsorbed S activates the C
O group and renders the C
C bond inaccessible to hydrogen adatoms. We argue that this is the origin of the chemoselectivity enhancement towards the crotyl alcohol experimentally observed.
The protonation of ethene, propene, 1-butene, and iso-butene by acidic theta-1 zeolite to give covalent alkoxide intermediates has been theoretically investigated by means of cluster and periodic ...calculations. The influence of the methodology used, of the olefin size and nature, of the cluster used to model the catalyst, of the degree of geometry relaxation around the active site, and of the long-range effects caused by the Madelung potential of the crystal has been analyzed. It has been found that the stability of alkoxide intermediates is very sensitive to the local geometry of the active site and to the nature of the carbon atom involved in the C−O bond, whereas the activation energy for the process is mainly influenced by the methodology used and by electrostatic effects, due to the carbocationic nature of the transition state. Finally, the question of whether the tert-butyl carbenium ion is a reaction intermediate or just a transition state in the iso-butene protonation process has been adressed by calculating the complete reaction mechanism at different theoretical levels.
The protonation of ethene by three different acid sites of theta‐1 zeolite was theoretically studied to analyze the extent and relevance of the following aspects of heterogeneous catalysis: the local ...geometry of the Brønsted acid site in a particular zeolite, the size of the cluster used to model the catalyst, the degree of geometry relaxation around the active site, and the effects related to medium‐ and long‐range interactions between the reaction site and its environment. It has been found that while the reaction energy is very sensitive to the local geometry of the site, the activation energy is mainly affected by the methodology used and by electrostatic effects on account of the carbocationic nature of the transition state.
The effect of the local geometry of the Brønsted acid site, the size of the cluster used to model the catalyst, the degree of geometry relaxation around the active site, and medium‐range and long‐range interactions between the reaction site and its environment on the protonation of ethene catalyzed by theta‐1 zeolite was studied by ab initio calculations. The reaction energy was very sensitive to the local geometry of the site. Because of the carbocationic nature of the transition state, the activation energy was principally affected by the methodology used and by electrostatic effects.
