Single-Site Copper by Incorporation in Ambient Pressure Dried Silica Aerogel and Xerogel Systems: An X-ray Absorption Spectroscopy Study

Copper incorporated silica aerogels and xerogels were prepared by adding the metal during the sol-gel stage and applying the ambient pressure drying (APD) method. The materials were characterized using inductively coupled mass spectrometry (ICP-MS), the Brunauer-Emmet-Teller (BET) method, X-ray diffraction (XRD), thermograviometric analysis-differential scanning calorimtery (TGA-DSC), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The introduction of copper led to changes in the aerogel architecture, seen by considerable expansion of average pore size, while maintaining a hydrophobic nature. The valence state and local chemical environment of copper was studied by X-ray absorption spectroscopy (XAS) before and after annealing to evaluate the interaction with the gel systems. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra showed that copper was divalent and present in a tetragonally distorted environment in both aerogels (1.8-11 wt %) and xerogels (11 wt %) before and after annealing, seen by 4 + 2 Cu-O distances at 2 angstrom and 2.25-2.35 angstrom. EXAFS also showed a third and fourth shell contribution attributed to one or more Cu center dot center dot center dot Si backscattering pairs at 2.9-3.2 angstrom. This confirmed that copper was coordinated to siloxy groups in the gels. Copper(II) oxide was not formed during annealing in any of the materials in this study, even at relatively high copper loadings. Clearly, the silica aerogels and xerogel showed great flexibility and capacity for copper cation uptake. The sol-gel incorporation route combined with the APD drying yielded a highly porous hydrophobic material able to stabilize single-site copper cations up to 450 degrees C, a prerequisite for optimal and stable catalytic performance.