Ethanol Photocatalysis on TiO₂-Coated Optical Microfiber, Supported Monolayer, and Powdered Catalysts:  An in Situ NMR Study

Abstract

In situ solid-state NMR methodologies have been employed to investigate the photocatalytic oxidation of ethanol (C 2 H 5 OH) over a TiO 2 -coated optical microfiber catalyst and two other TiO 2 -based catalysts. Adsorption of ethanol on the surface of the TiO 2 /optical microfiber catalyst formed a strongly hydrogen-bonded species and a Ti ethoxide species. In situ UV irradiation experiments under 13 C magic angle spinning (MAS) conditions reveal the formation of two main reaction intermediates, 1,1-diethoxyethane (CH 3 CH(OC 2 H 5 ) 2 ) and acetic acid, under dry conditions. The catalyst was shown to be highly effective for the degradation of ethanol as complete photooxidation of ethanol was observed to form acetic acid and CO 2 . These results were compared to those using a monolayer catalyst supported on porous Vycor glass and powdered TiO 2 . Solid-state NMR investigations on TiO 2 powder modeled after temperature-programmed desorption experiments confirm the identities of the hydrogen-bonded and Ti ethoxide species and show that the strongly bound ethoxide species has a number of adsorption sites. Kinetic experiments indicate this latter species reacts much more rapidly. Studies of the effect of surface hydration show that the presence of water decreases the rate of ethanol photodegradation. Water and ethanol compete for the same adsorption sites on the surface of the TiO 2 catalysts.

Hydrogen Production by Photoreforming of Renewable Substrates

“…The alcohol may adsorb both in a molecular way or forming alcoxy species, which are more readily oxidized than the H-bonded ones [196,197]. A correlation between H 2 yield and the polarity of alcohols was also proposed over TiO 2 -based photocatalysts [151].…”

Characterization of Surface and Photooxidative Properties of Supported Metal Oxide Photocatalysts Using Solid-State NMR

“…downfield resonances were identified as the Ti bound ethoxide species. 9,20 The intensity of the upfield methylene resonance decreased with increasing temperature and disappeared by 473 K, and is identified as the hydrogen-bonded ethanol species. The downfield methylene resonance eventually disappears by 623 K, and is identified as the surface bound ethoxide species in agreement with previous work on polycrystalline TiO2 powders.…”

“…9,20 The reactive intermediates identified from in situ SSNMR investigations of the photocatalytic oxidation of ethanol over TiO2 powder and single monolayer TiO2/PVG catalysts are acetaldehyde (CH3CHO, 198.7 and 28.8 ppm), acetic acid (CH3COOH, 171.2 and 19.2 ppm), acetate (CH3COO -, 179.0 and 21.9 ppm), formic acid (HCOOH, 161.2 ppm), and formaldehyde (CH2O) in agreement with other photocatalytic oxidation studies. [25][26][27] Figure 8 shows the time progression of the photooxidation of 48 µmol of 1,2-13 C-ethanol and 96 µmol of O2 over the one monolayer TiO2/PVG catalyst.…”

LED‐Illuminated NMR Studies of Flavin‐Catalyzed Photooxidations Reveal Solvent Control of the Electron‐Transfer Mechanism

“…These structural aspects and timescales are the classical domain of NMR spectroscopy, which in turn is a very slow and insensitive method and in many aspects complementary to UV/Vis spectroscopy. NMR spectroscopy can directly detect diamagnetic intermediates but neither excited states nor most of the radical species (Figure 1 blue highlights), which is reflected in many elaborate NMR studies of photoreactions, for example, in solid-state NMR spectroscopy, [3] protein folding, [4] organometallic photochemistry, [5] and photocatalysis. [6] Through the use of modern NMR photo-CIDNP techniques (CIDNP = chemically induced dynamic nuclear polarization) [7] with laser illumination, also transient radical pairs in photochemical reactions can be studied indirectly (purple frames in Figure 1).…”