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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Photoluminescence of Porous Vycor Glass; Surface Enhanced Photocatalyzed Conversion of CO to CH 2
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Harry D. Gafney, Luat T Vuong, Robert L Neuweiler, and Edward G. Look J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09981 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019
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Photoluminescence of Porous Vycor Glass; Surface Enhanced Photocatalyzed Conversion of CO2 to CH4 Robert L. Neuweiler, Edward G. Look, Luat T. Vuong and Harry D. Gafney* Department of Chemistry Department of Physics City University of New York Queens College Flushing, NY 11367
Abstract. Removing water from porous Vycor glass (PVG) resolves the near-IR (NIR) spectrum revealing 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 water content imply 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. While 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.
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Introduction. A variety of porous silicas have been found to be emissive.1-10 Excitation of xerogels derived from the polymerization of tetramethoxysilane/methanol/water (TMOS/CH3OH/H2O) and tetraethoxysilane/ethanol/water (TEOS/EtOH/H2O) with 200 to 300 nm light, for example, leads to broad, weak emissions in the 300-450-nm region.1-4 Water is ubiquitous in xerogels and the increase in emission intensity with drying and loss of emissivity on consolidation to a nonporous glass led to the proposal that emissivity derives from silanol groups on the silica surface, or surface states possessing a geometry not typical of glassy silica.3 Whereas xerogels are partially polymerized silicas, porous Vycor glass (PVG) derives from a melt heated above its sintering temperature that is acid leached after cooling.11-13 Nonetheless, UV excitation of PVG also leads to a weak emission in the 400 nm region. In spite of differences in surface area, pore size, and consolidation temperatures, at the molecular level, xerogels and PVG are chemically and structurally similar materials composed of SiO2 nodules with the intervening spaces defining a randomly dispersed porosity.11-13 Both possess hydroxylated silica surfaces composed of individual (free), hydrogen bonded (associated) and vicinal silanols, siloxane functionalities, as well as chemi- and physi-sorbed water. 1113 And like the xerogels, consolidation of PVG leads to a loss of emissivity. Our interest in the excited states of porous silica stems from photocatalyzed conversion of CO2 to CH4.14,15 UV excitation of PVG and TMOS/CH3OH/H2O xerogels doped with tungsten or molybdenum oxides leads to CH4 evolution.14,15 Stoichiometric measurements, H/D and 13C/12C labeling, dependence on coabsorbed water, and the detection of 32-74% of the stoichiometrically expected O2 correspond to the overall reaction14,15 CO2[ads] + 2H2O[ads]→ CH4[g] + 2O2[g]
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O2 evolution and H/D labelling point to adsorbed water as the source of the protons and reducing equivalents.14,15 If viewed as a redox process, however, the reducing potential of an electron in the conduction band (CB) of WO3, +0.2V (vs. NHE),16 is not sufficient to reduce CO2, Eo = -1.90 V (vs. NHE) 17,18 thereby precluding a direct photoinduced electron transfer mechanism. One mechanistic idea postulates the hole created in the metal oxide valence band (VB) on populating the CB initiates the oxidation of water. The standard potentials for the reduction of CO2 to CH4, Eo = 0.169V19, and that for the oxidation of H2O to O2, Eo = -1.23V20 establish the oxidation of water as the energy demanding step. And, the oxidizing potential of the hole in the VB of WO3, +2.7 V (vs. NHE)16 is sufficient to oxidize water under standard conditions. Nevertheless, it is difficult to see how the oxidation of water, the product of which is expected to be an oxidant, initiates the reduction of CO2, and, in view of the Stark-Einstein law,21,22 how the CO2 to CH4 conversion which, as redox processes, involves the transfer of eight electrons and four protons from water, appears to be driven by a single photon.14 Equally perplexing is that the CO2/CH4 conversion occurs in PVG in the absence of the metal oxides. CH4 evolution occurs from PVG (50%T @ 259 nm) with 254-nm excitation implying the metal oxides are neither necessary for the conversion, acting as the source of the reducing equivalents, nor essential to the conversion as an oxidant. In addition, the band gap of bulk and nanoparticle SiO2, 8.8 eV,23-27 is close to twice the energy of the exciting, 254-nm photon, 4.9 eV. While multi-electron, multi-proton conversions lie at the heart of storing solar energy as combustible fuels, understanding how light initiates and sustains these 2
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conversions, particularly on wide band gap materials, remain challenging experimental objectives. These experiments were under taken to identify the excited state(s) responsible for the emission from porous Vycor glass28 and to explore their interactions with CO2 and H2O. At “Nonlinear Schroedinger equation: generalized Darboux transformation and rogue wave solutions” J. Exp. Theor. Phys. Lett. (1970), 11, 05. 76. Akhmediev, N.; Ankiewicz, A.; Taki, M. “Waves that appear from nowhere and disappear without a trace” Phys. Lett. A (2009), 373, 675-678. 77. Chabchoub, A.; Hoffmann, N.P.; Akhmediev, N. “Rogue Wave Observation in a Water Wave Tank” Phys. Rev. Lett. (2011), 106, 204502-1 - 204502-4. 78.
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