Electron Donation from Cu Atoms to Al Oxide Clusters upon Mixing Revealed by Thermal Desorption Spectrometry

It is well-known that the reactivity of a Cu atom is sensitive to its charge state. We discussed the charge states of the Cu atoms mixed with Al oxide clusters (Al 4 O 6 + and Al 4 O 7 + ) at an atomic level based on gas-phase thermal desorption spectrometry (TDS) and density functional theory (DFT) calculations. By comparing the compositions of thermally durable Cu/Al oxide cluster ions with those of Al oxide cluster ions experimentally, it was found that mixing three Cu atoms with the stable Al oxide clusters accompanied one additional O atom, and namely, Cu 3 O (and not Cu 3 or Cu 3 O 3 ) was added to Al 4 O 6 + and Al 4 O 7 + . Stable geometrical isomers were obtained for Al 4 O 6 +, Al 4 O 7 +, Cu 3 Al 4 O 7 +, and Cu 3 Al 4 O 8 + by DFT calculations. Analysis of the geometries and the charge distributions on the atoms indicates that the low-energy isomers with the same compositions have essentially similar local structures and charge distributions. Hence, it is considered that the stability of the clusters is determined by the local structure and the charge distribution on the atoms. When the Cu atoms are mixed with the Al oxide clusters, the Cu atoms can afford to donate electrons to the electron-deficient O atoms and become positively charged.