Stability and reactivity of copper clusters with five atoms

FELIX G. REQUEJO; LISANDRO J. GIOVANETTI; JOSE M. RAMALLO LÓPEZ; CRISTIÁN HUCK IRIART; CARLOS ESCUDERO; DAVID BUCETA; ARTURO LOPEZ QUINTELA

Campinas

Congreso; 27th Annual User Meeting LNLS/CNPEM (RAU); 2017

CNPEM

We report the XAFS (X-ray absorption fine structure) and NAP-XPS (Near Ambient Pressure XPS) characterization and catalytic properties of subnanometric Cu atomic clusters (atomic quantum clusters: AQC) in water without any surfactant or protective agent. The original synthesis is based on the kinetic control of the reaction, which is achieved with an electrochemical method and using a solution with almost no conductivity (i.e., without added electrolytes commonly used in electrochemical methods). AQC with five cupper atoms (Cu5) were initially characterized by ultraviolet ? visible (UV?vis) and fluorescence spectroscopies, atomic force microscopy and electrospray ionization time-of-flight mass spectrometry. For further studies focused on structure and stability of Cu AQC, we performed Cu K-edge XANES and EXAFS and NAP- XPS experiments under controlled oxygen pressure and temperature. Contrary to what should be expected, such clusters are very stable and remain unaltered in water solution and supported on different substrates like graphite or gold in presence of oxygen at higher temperatures than 200°C. The reason for such stability resides on their huge band gap (4.07 eV). Moreover, such Cu5 clusters are extremely stable to UV irradiation, temperature, and wide variations of pH values. In present contribution we show the particular features of Cu-K XANES spectra, away from the usual concept referred to the position of the edge according to the oxidation state of the absorbing element. Additionally, through NAP-XPS, an ?anomalous? behavior can be observed for the oxidation and reversible reduction of the Cu state in oxygen atmosphere. These particular characteristics can be only explained because the unique electronic structure of Cu5 AQC. The same electronics of Cu5 may have significant catalytic implications, such as high oxidative capacity, as it is also shown by a control experiment of cysteine oxidation by S-K XANES experiments.