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Enhanced chlorine evolution from dimensionally stable anode by heterojunction with Ti and Bi based mixed metal oxide layers prepared from nanoparticle slurry > MOx+1 + Cl− → > MOx + ClO− (6) Therefore, the selectivity towards ClER should be an important consideration for an energy-efficient electrochemical RCS mediated water treatment, especially considering limited Cl− concentration (<50 mM) in (waste)water electrolyte [11]. Specific group of metal oxides such as PbO2, SnO2, and TiO2 could be characterized by a weak bond strength between metal and active oxygen to predominantly exist in the >MO(·OH) form which would prefer ClER due to stronger oxidation power than MOx+1. In particular, TiO2 is non-toxic and widely-used earth-abundant catalysts to provide > Ti(·OH) under an anodic potential with high surface density. Nevertheless, relatively large kinetic barrier for the water discharge has limited the utilization of TiO2 as an electrocatalyst. We previously proposed a heterojunction architecture with coating TiO2 layers with or without mixing Bi (BixTi1−xOz) on top of Ir7Ta3Oy DSA for an enhanced RCS generation in dilute aqueous solutions [11]. In these configurations, conductor-like nature of IrTaOy could serve as an ohmic contact to Ti substrate [11], [12], while surface hydrous TiO2 (>Ti(OH–)) provided elevated quasi-stationary concentration of >MOx(∙OH). In spite of limited consideration for limited active sites and different kinetic parameters to predict selectivity in parallel reactions, density functional theory (DFT) calculations on model RuO2/TiO2 architecture [13] further suggested that the TiO2 heterojunction layer could shift locations in volcano plots of OER and ClER, which in-turn influences the ClER selectivity. In addition, mixing Bi in the outer layer was evinced to increase electrostatic sorption of anions (Cl−). In our earlier contribution, nevertheless, the peroxo-route for aqueous Ti-glycolate complex and solution casting of the BixTi1−xOz layer were rather dangerous and labor consuming. This study reports simplified preparation methods for coating TiO2 based heterojunction layers (on Ir7Ta3Oy DSA) which was either decorated by Bi2O3 micro-particles or mixed with Bi3+. In particular, employing nanoparticle slurry precursors was expected not only to augment the catalytic edge sites [3], but also to allow an effective passivation owing to greater viscosity than aqueous precursor [14]. In dilute (50 mM) NaCl solutions, the activity and selectivity for ClER were comparatively evaluated for the heterojunction anodes with variable outer layer loading (thickness) and mixing levels of Bi. In addition, the roles of >MOx(∙OH) and >MOx+1 on the RCS generation were interrogated by correlation with formate ion degradation. 2. Experimental section 2.1. Preparation and characterization of the anodes All chemical reagents were analytical grade and obtained from Sigma-Aldrich or Daejung chemicals to be used without further purification. Solutions were prepared with Millipore-Q water (Millipore) with a specific resistivity of 18 MΩ cm−1. For pretreatment of the anode base, Ti foils (99.5% Alfa-Aesar) were cleaned with SiC paper, sonicated in organic solvent solutions (equivolumic deionized water, ethanol, and acetone) for 0.5 h, and immersed in 10 wt% oxalic acid for 50 min at 80 °C. The IrTaOy layer was prepared by soaking the Ti substrate (3 × 2 cm2, distance = 5 mm) into solutions with 73 mM H2IrCl6, and 27 mM TaCl5. 4 N HCl and equivolumic ethanol/isopropanol solutions were compared as solvents. The loaded precursor was sequentially dried at 80 °C and annealed at 525 °C for 10 min, which was repeated five times https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7539370/?report=printable[10/12/2020 8:49:16 AM]PDF Image | Enhanced chlorine evolution
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