(Invited) Effect of Particle Size on Photocatalytic Reaction Kinetics Including Multielectron-Transfer Processes

Abstract

Oxidation of water into molecular oxygen (O2 + 4H+ + 4e– → 2H2O) must be the largest-scale chemical reaction occurring on the earth as a part of photosynthesis by plants, and the corresponding backward half-cell reaction, reduction of oxygen into water (O2 + 4H+ + 4e– → 2H2O) might proceed in the same scale by the activity of plants and animals to get energy by combustion of chemicals keeping the concentration of oxygen on the earth. It is well known that the forward and backward reactions are counter oxidation and reduction reactions, respectively, of photocatalytic water splitting/carbon-dioxide fixation and photocatalytic decomposition of organic compounds by semiconductor particles. Standard electrode potential (SEP) of this four electron-transfer redox (O2 + 4H+ + 4e– = 2H2O) is 1.23 V (versus SHE = standard hydrogen electrode potential (0 V) in definition). Almost all the papers reporting oxygen evolution use this SEP, while those of photocatalytic organics decomposition tend to use SEP for one-electron transfer, O2 + e– = O2•– (–0.28 V), for discussion. One of the possible reasons for such difference is that it is convenient to presume the production of superoxide anion radical which may produce "reactive oxygen species (ROS)" to decompose organic compounds. Anyway, it can be stated that those proposed number of transferred electrons seems speculative without any experimental evidence. We have been studying light-intensity dependence (LID) of photocatalytic reaction rates using highly intense UV-LEDs with intensity maximally ca. 3 W cm-2. For oxygen evolution from aqueous titania suspensions in the presence of electron acceptors such as iodate (IO3 –) or iron(III) ions under deaerated conditions, small anatase (8 nm) and rutile (13 nm) particles show bimodal, at first second and then first order, LID in the lower and middle intensity range while large anatase and rutile particles show simple first-order LID in the whole intensity range. This is attributable to two-electron (positive hole) transfer process in the initial step. An interesting feature is that forth-order LID was observed with small anatase particles at high (2.5 W cm-2) intensity to suggest shift of the kinetics to four-electron transfer process [1]. For oxygen reduction, LID analyses for the rate of oxidative decomposition of acetic acid by a bismuth tungstate photocatalyst suggested that two-electron oxygen reduction predominantly proceeds [2]. On the basis of these results, "effective particle size" defined as size of particle (or particle aggregates) in which multiple electrons (positive holes) are accumulated to induce multielectron processes is proposed. This "effective particle size" may govern the effect of size on photocatalytic reaction rates. [1] Takeuchi, S.; Takashima, M.; Takase, M.; Ohtani, B. Chem. Lett. 2018, 47, 373-376. [2] Hori, H.; Takashima, M.; Takase, M.; Ohtani, B. Catal. Today 2018, 313, 218-223.