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

Neuweiler, Robert L., Look, Edward G., Vuong, Luat T., Gafney, Harry D.
Journal of physical chemistry 2019 v.123 no.11 pp. 6464-6476
absorption, carbon dioxide, energy, fluorescence, glass, hydrides, hydrogen, ions, methane, nanopores, photoluminescence, silica, water content
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 Å 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.