There and Back Again: The Unique Nature of Copper in Ambient Pressure Dried-Silica Aerogels

摘要

We have previously reported the formation of framework single-site copper in silica ambient-pressure-dried (APD) aerogels and xerogels, for which the metal (2—11 wt 96) was added during the sol—gel stage (Kristiansen, T.; Mathisen, K.; Einarsrud, M.-A.; Bjorgen, M.; Nicholson, D. G. J. Phys. Chetn. C 2011, 115, 19260—19268). We here present a fundamental study on the formation of ultrasmall metal nanOclusters in hydrogen and complete restoration of the initial surroundings in nitric oxide/oxygen, using in situ X-ray absorption spectroscopy (XAS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We coupled density functional theory (DFT) with the Cu—Cu shell multiplicities and distances from extended X-ray absorption fine structure (EXAFS) to investigate the dimensions and structure dynamics of the copper nanoclusters. Their average cluster size did not exceed dimensions above 1.6 nm in either gel system up to 450 °C in hydrogen; however, a significant drop in the first Cu—Cu shell multiplicity suggests a change of morphology and structure. A subsequent treatment in nitric oxide/oxygen resulted in reoxidation to single-site copper(II) species. We believe the driving force for complete redisperSion is framework vacancies containing acidic silanol clusters, created during cation removal as confirmed by DRIFTS. This unique reversibility of the copper(II) single sites establishes the silica aerogel and xerogel systems as highly functional supports for ultrasmall clusters and in redox applications.