Optical properties of anatase TiO2: synergy between transition metal doping and oxygen vacancies.

Charge carriers (electrons and holes) are generated on the TiO2 using UV radiation; this excitation energy can be reduced by modifying the material electronic structure, for example, by doping or creating oxygen vacancies. Here, the electronic structure of a transition metal-doped anatase, bulk and surface, and their interaction with oxygen vacancies are studied using density functional theory. The visible light response of metal-doped TiO2 (101) is also determined. Transition metals generate intra-band gap states, which reduce the excitation energy but may also act as charge recombination sites. Dopants Fe, Co, and Ni remarkably enhance the visible light response due to the states in the middle of the gap. However, Co and Ni create heavier charge carriers. Our results show that Pd and Pt-doped TiO2 generate states near the valence and conduction band with a "clean" band gap (without states in the middle of the gap). Moreover, Pt-doped TiO2 maintains the charge mobility because it presents a small charge carriers mass. Hence, Pt-doped TiO2 represents the best alternative to activate TiO2 under visible light. The optical response of transition metal-doped TiO2 follows the order 3d > 4d > 5d. The oxygen vacancies reduce the response of metal-doped TiO2 to visible light because the unpaired electrons generated occupy the empty states of metal-doping. Graphical Abstract Density of states of TiO2 (101) surface doped with transition metals and oxygen vacancies.

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

CO 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 Abstract CO oxidation mechanism on small Pd clusters.

The N2O activation by Rh5 clusters. A quantum chemistry study.

Nitrous oxide (N2O) is a by-product of exhaust pipe gases treatment produced by motor vehicles. Therefore, the N2O reduction to N2 is necessary to meet the actual environmental legislation. The N2O adsorption and dissociation assisted by the square-based pyramidal Rh5 cluster was investigated using the density functional theory and the zero-order regular approximation (ZORA). The Rh5 sextet ground state is the most active in N2O dissociation, though the quartet and octet states are also active because they are degenerate. The Rh5 cluster spontaneously activates the N2─O cleavage, and the reaction is highly exothermic ca. -75 kcal mol(-1). The N2─O breaking is obtained for the geometrical arrangement that maximizes the overlap and electron transfers between the N2O and Rh5 frontier orbitals. The Rh5 high activity is due to the Rh 3d orbitals are located between the N2O HOMO and LUMO orbitals, which makes possible the interactions between them. In particular, the O 2p states strongly interact with Rh 3d orbitals, which finally weaken the N2─O bond. The electron transfer is from the Rh5 HOMO orbital to the N2O antibonding orbital.