Isomers of Anionic Gold Oxide Clusters, Au_nO₂^–, Investigated by Thermal Desorption Spectrometry

Abstract

Anionic gold oxide clusters, Au n O2 – (n = 2–5), were prepared by sequential O atom transfer reactions from N2O to Au n –. Response of the clusters to heat was investigated by gas-phase thermal desorption spectrometry and was compared with the behavior of Au n O2 – prepared by the reaction of Au n – with O2. No desorption of O2 was observed from Au4O2 – prepared with N2O until 1000 K, but O2 desorbed from Au4O2 – prepared with O2 at 500 K, indicating the formation of structural isomers. Theoretical calculations show that the most stable form of Au4O2 –, which contains dissociated O atoms, is produced by oxidation with N2O and that the third most stable form of Au4O2 –, which contains molecular O2, is produced by oxidation with O2. The desorption energy of O2 from Au5O2 –, in which O2 is dissociatively chemisorbed on Au5 –, was estimated from the temperature dependence of the cluster signal intensities. The experimentally determined desorption energies were consistent with the values obtained by theoretical calculation.

Adsorption and Desorption of NO and NO2 Molecules on Gold Cluster Anions Observed by Thermal Desorption Spectrometry

“…Whetten and co-workers proposed that the propensity of O 2 adsorption can be explained by the nature of the gold cluster to maintain the electron pair . This reaction was also discussed using quantum calculations from the point of view of binding energies and electronic states. − Metiu and co-workers reported calculation results and concluded that this reaction was caused by electron back-donation, which is consistent with the experiments by Whetten. , Moreover, the determination of the geometries is also important, and PES and IR spectroscopy are effective tools for the characterization of gold clusters. − We also reported the interaction of O atoms with gold clusters using a combination of thermal desorption spectrometry (TDS) and quantum calculations. , The existence of isomers of Au n O m – clusters with different thermal stabilities was implied, and different oxidants such as O 2 and N 2 O were shown to generate molecularly and dissociatively O 2 -attached clusters, respectively.…”

“…Hence, the overall reaction is expressed as 2NO 2 → 2NO + O 2 , where O 2 remains on Au 4 – . According to our previous studies, molecularly adsorbed O 2 on Au 4 – desorbs from the cluster at 400–600 K, whereas two atomically adsorbed O atoms on Au 4 – do not desorb from the cluster below 1000 K. , Hence, O 2 in the product of reaction , Au 4 O 2 – , exists as two O atoms on Au 4 – . Considering that Au 4 – catalyzes the decomposition of NO 2 , the highest activation barrier would correspond to the migration of the two O atoms on Au 4 – to encounter each other.…”