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

Abstract

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(H 2 O) x , and siloxane, SiOSi(H 2 O) 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(H 2 O) x and SiOSi(H 2 O) y vibrations and the excitation spectra of the SiOH(H 2 O) x and SiOSi(H 2 O) 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 Å layer of water on the silica surface. CO 2 quenches the SiOH(H 2 O) x , and SiOSi(H 2 O) y excited states by a proton transfer mechanism which initiates the CO 2 /CH 4 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/H 2 O surface sustaining the CO 2 /CH 4 conversion.