Bicarbonate reduction with semiconductor photocatalysts: study of effect of positive hole scavengers
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Research Letter
Bicarbonate reduction with semiconductor photocatalysts: study of effect of positive hole scavengers Hanqing Pan and Michael D. Heagy, Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Address all correspondence to Michael D. Heagy at [email protected] (Received 28 May 2018; accepted 13 August 2018)
Abstract Two semiconductors (Cu2O and TiO2) were chosen for the photocatalytic reduction of bicarbonate to formate in order to perform a systematic study on the effect of six different hole (h+) scavengers. The six h+ scavengers selected for the study include glycerol, ethylene glycol, 2-propanol, sodium sulfite, triethanolamine, and ethylenediaminetetraacetic acid. Glycerol proved to be the most efficient h+ scavenger, and TiO2 in glycerol showed the highest quantum efficiency of 5.04 ± 0.3%. This finding bodes well as a sustainable one because glycerol is environmentally benign, a low-cost material, and is derived from plants, as opposed to petroleum sources like 2-propanol or ethylene glycol.
Introduction In semiconductor photocatalysis, the process is initiated by a photon of energy greater or equal to the band gap of the semiconductor. These photons excite valence band electrons (e−) of the semiconductor and create photogenerated e− and holes (h+) which migrate to the semiconductor surface to subsequently cause oxidation and reduction reactions. In our study, the excited e− react with bicarbonate to form the CO•− 2 radical anion, which undergoes further reduction to formate, an intermediate to methanol—a high-octane-number fuel. While semiconductor photocatalysis has been widely studied, one major drawback is the rapid e−/h+ recombination rate. In most cases, the e− and h+ recombine too rapidly for the semiconductor to be efficient.[1] Prompt charge carrier recombination can be mitigated by the addition of a h+ scavenger, a sacrificial agent which undergoes oxidation in the photocatalytic process.[2] The h+ scavengers are important in photocatalytic reactions because they have been reported to enhance the photoactivity of TiO2.[3–7] The role of a h+ scavenger is to refill the photogenerated h+, thus reducing h+/e− recombination, and to prevent the interaction of h+ with precursors of the desired products.[8] Figure 1 shows a schematic representation of the oxidation and reduction processes on the surface of a photoexcited semiconductor, which also highlights the role of a h+ scavenger. The h+ scavengers used in previous studies include organic compounds such as methanol, ethanol, formic acid, acetic acid, triethanolamine (TEA), and disodium ethylenediamine tetraacetic acid (EDTA). One of the earlier studies on h+ scavengers started with Prairie et al. in 1993 as they investigated the effect of various organic compounds (EDTA, citric acid, salicylic acid, acetic acid, methanol, and ethanol) for the photoreduction
of chromate.[3] Since then, other h+ scavengers such as sucrose, sodium sulfite, and sodium chloride have been used.[6,9–12] The novel idea of an
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