The interaction of oxygen with small gold clusters

Abstract

Presented in this work are the results of a quantum chemical study of oxygen adsorption on small Aun and Au−n (n=2,3) clusters. Density functional theory(DFT), second order perturbation theory (MP2), and singles and doubles coupled clustertheory with perturbative triples [CCSD(T)] methods have been used to determine the geometry and the binding energy of oxygen to Aun. The multireference character of the wave functions has been studied using the complete active space self-consistent field method. There is considerable disagreement between the oxygen binding energies provided by CCSD(T) calculations and those obtained with DFT. The disagreement is often qualitative, with DFT predicting strong bonds where CCSD(T) predicts no bonds or structures that are bonded but have energies that exceed those of the separated components. The CCSD(T) results are consistent with experimental measurements, while DFT calculations show, at best, a qualitative agreement. Finally, the lack of a regular pattern in the size and the sign of the errors [as compared to CCSD(T)] is a disappointing feature of the DFT results for the present system: it is not possible to give a simple rule for correcting the DFT predictions (e.g., a useful rule would be that DFT predicts stronger binding of O2 by about 0.3 eV). It is likely that the errors in DFT appear not because of gold, but because oxygen binding to a metal cluster is a particularly difficult problem. Presented in this work are the results of a quantum chemical study of oxygen adsorption on small Au n and Au n Ϫ (nϭ2,3) clusters. Density functional theory ͑DFT͒, second order perturbation theory ͑MP2͒, and singles and doubles coupled cluster theory with perturbative triples ͓CCSD͑T͔͒ methods have been used to determine the geometry and the binding energy of oxygen to Au n . The multireference character of the wave functions has been studied using the complete active space self-consistent field method. There is considerable disagreement between the oxygen binding energies provided by CCSD͑T͒ calculations and those obtained with DFT. The disagreement is often qualitative, with DFT predicting strong bonds where CCSD͑T͒ predicts no bonds or structures that are bonded but have energies that exceed those of the separated components. The CCSD͑T͒ results are consistent with experimental measurements, while DFT calculations show, at best, a qualitative agreement. Finally, the lack of a regular pattern in the size and the sign of the errors ͓as compared to CCSD͑T͔͒ is a disappointing feature of the DFT results for the present system: it is not possible to give a simple rule for correcting the DFT predictions ͑e.g., a useful rule would be that DFT predicts stronger binding of O 2 by about 0.3 eV͒. It is likely that the errors in DFT appear not because of gold, but because oxygen binding to a metal cluster is a particularly difficult problem.

Binding of propene on small gold clusters and on Au(111): Simple rules for binding sites and relative binding energies

“…[21][22][23][24][25][26] No such even-odd oscillation of the desorption energies with the cluster size is observed for the systems considered here.…”

“…This is indeed the case for both neutral and negative clusters. [21][22][23][24][25][26][27][28][29] The calculations presented here find no correlation between the desorption energy of the propene and the electron affinity of the naked cluster ͑or the number of electrons in the cluster͒.…”

“…The binding of O 2 to the Au clusters involves donation from the cluster to the 2 * MO of the O 2 molecule. [23][24][25] This will make the Au cluster more positive, hence more able to take electrons from propene. Our rules predict that the binding of propene to Au n O 2 will be stronger than the binding to the Au n .…”

“…This behavior is similar to that observed for the adsorption of O 2 on small neutral and anionic gold clusters. [23][24][25] Whether the bonding is at a corner or bridging, the length of the CuC double bond increases and the CuCuC angle decreases. When propene is adsorbed at a corner site, the length of the CuC double bond varies between 1.36 and 1.40 Å and the CuCuC angle is between 123.3 and 125.4°.…”

“…Since oxygen is an electron acceptor it binds more strongly to the clusters having lower ionization potential. [21][22][23][24][25][26][27][28][29] Since the ionization potentials of the naked Au clusters are smaller when the cluster has an odd number of electrons, the desorption energy of oxygen to such clusters should be higher. This is indeed the case for both neutral and negative clusters.…”