RESUMEN

The search for a catalyst for the reduction of nitrous oxide (N2O) is now imperative, as this molecule is a very dangerous pollutant. We found that the low-symmetry Pt8 cluster presents multiple reaction pathways for N2O rupture, which are regioselective. This result was revealed by means of density functional theory calculations within the zero-order-regular approximation, ZORA, explicitly including relativistic effects. It is further proved that Pt8 is a competitive N2O catalyst compared to sub-nanometric rhodium clusters, obtaining similar reaction barriers. The hot adsorption site, a tip atom of Pt8, and the rotation of the N2O molecule over the metallic cluster promote the formation of a frustrated bridge activated transition state, Pt8-N2O. This transition structure yields to spontaneous dissociation of N2O without bridge formation. Along this catalytic process, rearrangements within the metal cluster take place, preserving its stability. Moreover, in addition to being important attributes of the Pt8 particle in the N2O reduction, fluxionality and multiple reaction pathways may also prevent poisoning effects. Overall, this differs from reported results for more symmetric metal particles also used as catalysts.

RESUMEN

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.

RESUMEN

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.

Asunto(s)

Gases/química , Óxido Nitroso/química , Restauración y Remediación Ambiental , Vehículos a Motor , Teoría Cuántica

RESUMEN

Nitrous oxide (N(2)O) is an intermediate compound formed during catalysis occurring in automobile exhaust pipes. Atomic Au in its ground state is unable to react with N(2)O, however, several Au excited states are bound to N(2)O, but not all of these states are able to activate N(2)O bonds. In this work, N(2)O capture and activation by a single Au atom are studied considering Au in the ground and excited states with multiplicities = 2, 4 and 6. The Au + N(2)O reactions are studied at multireference second-order perturbation level of theory using C(s) symmetry. The AuN(2)O ((4)A', (4)A'', (6)A' and (6)A'') adducts are spontaneously created from Au excited states. From these complexes, only the (4)A', (6)A' and (6)A'' states exhibit N(2)O activation reaction paths yielding N(2,) NO and O atoms as end products when N(2)O approaches Au excited states side-on. Cations both ground and excited states, capture N(2)O although only the Au(+) ((5)A') + N(2)O ((1)Σ(+)) → NAuNO(+) ((5)A') reaction (for the end-on and side-on approaches) shows N(2)O activation with N-N bond breaking. In the case of Au anions, the ground state and most of the excited states capture N(2)O and activation takes place according to Au(-) ((3)A', (5)A', (5)A'') + N(2)O ((1)Σ(+)) → AuO(-) ((3)A', (5)A', (5)A'') + N(2)(g) for the N(2)O end-on approach by the oxygen atom. The reaction paths show a metal-gas dative covalent bonding character. Mulliken charge population analysis obtained for the active states shows that the binding is done through charge donation and retro-donation between the metal and the N(2)O molecule.