E. Poulain

The catalytic activity of supported platinum: A theoretical study of the activation of H2

Abstract Theoretical calculations, which resemble the supported catalyst, were done for different geometric approaches and electronic states of a platinum dimer interacting with a hydrogen molecule. The Pt 2 +H 2 reaction curves have been analyzed and compared with previous platinum monomer plus hydrogen molecule reaction using very carefully theoretical ab-initio methods, including relativistic effective core potential in a multi configuration self consistent field (MC-SCF) and a multi reference configuration interaction (MR-CI) variational and perturbative. From the different H 2 to Pt 2 approaches considered, the parallel one is the most interesting: the A 1 symmetry singlet and triplet states of the system lead to dissociatively capture of H 2 . These captures present deep wells, 47 and 30 kcal/mol, respectively, and important activation barriers, 18 and 14 kcal/mol. As a consequence of that, they do not allow easy exit channels; but in the minima of the wells, the Pt–H bond is weak, allowing the hydrogen to participate in catalytic reactions.

C-, N-, S-, and F-Doped Anatase TiO2 (101) with Oxygen Vacancies: Photocatalysts Active in the Visible Region

Anatase TiO2 presents a large bandgap of 3.2 eV, which inhibits the use of visible light radiation (  > 387 nm) for generating charge carriers. We studied the activation of TiO2 (101) anatase with visible light by doping with C, N, S, and F atoms. For this purpose, density functional theory and the Hubbard approach are used. We identify two ways for activating the TiO2 with visible light. The first mechanism is broadening the valence or conduction band; for example, in the S-doped TiO2 (101) system, the valence band is broadened. A similar process can occur in the conduction band when the undercoordinated Ti atoms are exposed on the TiO2 (101) surface. The second mechanism, and more efficient for activating the anatase, is to generate localized states in the gap: N-doping creates localized empty states in the bandgap. For C-doping, the surface TiO2 (101) presents a “cleaner” gap than the bulk TiO2, resulting in fewer recombination centers. The dopant valence electrons determine the number and position of the localized states in the bandgap. The formation of charge carriers with visible light is highly favored by the oxygen vacancies on TiO2 (101). The catalytic activity of C-doping using visible radiation can be explained by its high absorption intensity generated by oxygen vacancies on the surface. The intensity of the visible absorption spectrum of doped TiO2 (101) follows the order: C > N > F > S dopant.

The CO oxidation mechanism on small Pd clusters. A theoretical study.

AbstractCO is a pollutant that is removed by oxidation using Pd, Pt or Rh as catalysts in the exhaust pipes of vehicles. Here, a quantum chemistry study on the CO + O2 reaction catalyzed by small Pdn clusters (n ≤ 5) using the PBE/TZ2P/ZORA method is performed. The limiting step in this reaction at low temperature and coverage is the O2 dissociation. Pdn clusters catalyze the O=O bond breaking, reducing the energy barrier from 119 kcal mol-1 without catalyst to ∼35 kcal mol-1. The charge transfer from Pd to the O2,ad antibonding orbital weakens, and finally breaks the O─O bond. The CO oxidation takes place by the Eley-Rideal (ER) mechanism or the Langmuir-Hinshelwood (LH) mechanism. The ER mechanism presents an energy barrier of 4.10-7.05 kcal mol-1 and the formed CO2 is released after the reaction. The LH mechanism also shows barrier energies to produce CO2 (7-15 kcal mol-1) but it remains adsorbed on Pd clusters. An additional energy (7-25 kcal mol-1) is necessary to desorb CO2 and release the metal site. The triplet multiplicity is the ground states of studied Pdn clusters, with the following order of stability: triplet > singlet > quintet state. Graphical AbstractCO oxidation mechanism on small Pd clusters