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.

A DFT comparative study of carbon adsorption and diffusion on the surface and subsurface of Ni and Ni3Pd alloy.

Carbon diffusion in transition metal nanoparticles is assumed to be a key factor in the catalyzed growth of carbon nanotubes (CNT). Aiming at designing more efficient catalysts, we have compared this carbon diffusion process in the near surface and in the bulk of Ni and Ni(3)Pd by means of density functional theory (DFT) calculations. Ni nanoparticles are indeed the most largely used catalysts and the alloying with Pd could modify and improve their properties. The alloy has the same crystal structure as pure Ni, with a slight lattice expansion due to the presence of palladium. For both systems, the subsurface octahedral site is the most stable adsorption site, but the thermodynamic trend favoring the penetration to the subsurface is larger on the alloy than on the Ni. As a result, in the conditions of temperature and pressure for nanotube growth, the population of the subsurface sites is a more exothermic process on the alloy. In addition, while on pure nickel the diffusion over the (111) surface is easy, on the alloy the vertical process leading the carbon to the subsurface is preferred. Palladium atoms have the double effect to expand the lattice parameter providing more adapted diffusion channels for the carbon and to create new adsorption sites less stable than the all-nickel ones. The results can be related to more selective formation of nanotubes on the alloy at low temperature, where Ni produces fibers.

A TD-DFT investigation of two-photon absorption of fluorene derivatives.

We report time-dependent density functional theory (TD-DFT) calculations on the two-photon absorption (TPA) properties of fluorene and derivatives. The influence of donor and acceptor groups and of dimerisation is investigated. Firstly the choice of a DFT functional and of the basis set is performed by comparison of experimental and calculated excitation energies and two-photon cross sections. Then, the calculations display an enhancement of the cross section with acceptor groups or with a combination of one donor and one acceptor groups (push-pull), at some positions on the cycles. Moreover, the largest cross section is obtained for bifluorene. The replacement of carbon atoms by nitrogen atoms, giving heterocycles, is not efficient. In chloroform as solvent, the excitation energy decreases and the two-photon cross section increases, mainly with a polar molecule. Finally, a rationalization of the results is given based on the three-level model by analysis of the transition moments and of the molecular orbitals.

Asymmetric synthesis of 2-methyl cyclohexane carboxylic acids by heterogeneous catalysis: mechanistic aspects

The catalytic hydrogenation of (S)-alkyl-N-(2-methylbenzoyl)pyroglutamates was studied over supported rhodium and ruthenium catalysts at room temperature and a pressure of 5 MPa. The reaction was diastereoselective with the predominant formation of (1S,2R)-2-methylcyclohexane carboxylic acid with a diastereomeric excess (de) of up to 96%. The most stable conformation was determined by means of a combination of modelling calculations, NMR spectroscopy and X-ray structural determination. In this conformation, the carbonyl group of the pyroglutamate auxiliary shields one face of the aromatic ring. The observed selectivity may thus be explained by a preferential adsorption at the unshielded face which avoids steric repulsion by the C=O group to result in a cis hydrogenation. The addition of an amine, the nature of the support (alumina or active carbon) or of the metal (Rh or Ru) were shown to give additional stabilisation of the adsorption at the unshielded face to increase the diastereoisomeric excess.