Photoluminescence of a Porous Vycor Glass; Surface-Enhanced Photocatalyzed Conversion of CO2 to CH4

Removing water from porous Vycor glass resolves the near-IR (NIR) spectrum revealing that the emissivity of nanoporous silicas derives from two energetically distinct silanol, SiOH(H2O)(x), and siloxane, SiOSi(H2O)(y), surface sites. The occurrence of isosbestic points and emission lifetimes independent of a water content implies that these sites do not randomly interact with water but are associated with a specific number of water molecules on the silica surface. Integer relationships between NIR absorptions that are overtones of the SiOH(H2O)(x) and SiOSi(H2O)(y) vibrations and the excitation spectra of the SiOH(H2O)(x) and SiOSi(H2O)(y) fluorescence indicate a facile exchange of electronic and vibrational energy with the majority of the excitation energy stored as O-H vibrational energy in a 14-15 angstrom layer of water on the silica surface. CO2 quenches the SiOH(H2O)(x), and SiOSi(H2O)(y) excited states by a proton transfer mechanism which initiates the CO2/CH4 conversion. Although individual O-H vibrations do not have sufficient energy to promote the more energetic steps in the conversion, higher-order composite overtones indicate the presence of nonlinear processes that promote the transfer of hydrogen atoms and hydride ions from the silica/H2O surface sustaining the CO2/CH4 conversion.