Application of electrocatalysis of anodic oxygen-transfer reactions: development of a coulometric method for the determination of chemical oxygen demand

Voltammetric and coulometric results are described for the electrocatalytic oxidation of Q(III) to Q(V1) at the mixed bismuth(V)-lead(IV) dioxide film electrodes (designated "Bi-PbOj") that are electrodeposited from acidic solutions of Pb(II) containing Bi(ni). A current efficiency of 99.2 % (o = 1.1%, N = 12) is obtained for galvanostatic generation of Cr(VI) at a preconditioned Bi-PbOj film electrode. Preconditioning of the Bi-PbOj film merely involves the generation of Cr(VI) which, based on scanning electron micrographs, is concluded tentatively to achieve chemical stripping of non-catalytic portions of the Bi-PbOz films to produce films having greater electrocatalytic activity for the desired reaction. Electrochemical stripping of the Bi-PbOj electrode had a similar preconditioning effect. Based on wavelength dispersive spectroscopy, electrochemical stripping resulted in an increase in surface Bi concentration of 33% relative to Pb. * Reprinted with permission of J. Electrochem. Soc., 143,2119 (1996). Graduate student and Professor, respectively. Department of Chemistry and Ames Laboratory, Iowa State University. ' Primary researcher and author ^ Author for correspondence 23 Introduction Much research in this laboratory has been directed to the electrocatalysis of anodic reactions that require the concomitant transfer of oxygen from HjO to the oxidation productCs).'"^ Our use of the term electrocatalysis is intended to imply involvement of specific surface sites within these anodic 0-transfer mechanisms to increase the rate of the heterogeneous reactions. Compounds typically investigated include inorganic and organic substances that are not generally observed to be electrochemically active at the conventional anodes (Pt, Au, PbOz and C). Electrocatalytic activity has been substantially increased for various anodic 0-transfer reactions at p-PbO, film electrodes that are heavily doped with various altervalent metallic ions. Anodic 0-transfer mechanisms require the concomitant anodic discharge of H2O to produce adsorbed hydroxyl radicals (•OHjjs).' Therefore, the catalytic function of altervalent dopants in the doped metal oxide film electrodes is commonly attributed to a diminished overpotential for discharge of H2O at the dopant sites. Furthermore, it is speculated that a moderately low surface coverage by dopant sites (0 ca, 10 HM) produced a momentary electrocatalytic activity followed by a loss of electrode activity."* This was explained by Larew etoL as the consequence of the formation of a monolayer of the electrosorbed Bi(V) which covered PbOi sites necessary for preadsorption of the DMSO." Research described here is motivated by the goal of automating the determination of chemical oxygen demand (COD) of polluted wateis on the basis of an indirect coulometric procedure that utilizes an electrochemically generated oxidation mediator. Chromium(VI) is a powerful oxidizing agent, as indicated by the large standard reduction potential given by: CrjO,'+ 14H" + 6e2Ct ' + 7H2O; = 1.33 V vs. NHE [1] and this compound has been used frequently for oxidation of organic compounds.'* The standard method for determination of COD in organically polluted waters utilizes Cr(VI) with back titration of excess oxidant using a standard solution of Fe(II)." 25 Experimental Chemicals All chemicals were analytical reagent grade from Fisher Scientific, Alfa Products, or Aldrich Chemicals. Water was distilled and purified further by passage through a NANOpure-n system (Bamstead). Electrodes Deposition of Bi(V)-doped P-PbOj films followed procedures described previously.These films, designated as "Bi-PbOj/Au", were deposited on a gold rotated disk electrode (RDE, 0.196 cm^; Pine Instrument Co.) under quiescent conditions at 1.7 V in 1.0 M HQO4 containing 10 mM Pb(N03)2 and 0 to 20 mM Bi0^O3)3-5H2O for up to 5 min. Deposition of films on the Pt gauze electrode (5-cm length, 4-cm diameter and ca. 126-cm^ area, ca. 40 mesh woven from ca. 180-(im diameter wire) was achieved under potentiostatic (1.5 to 1.7 V, 5 to 20 min) or galvanostatic (50 mA for 15 to 20 min) conditions from 1 M HNO3 or 1 M HCIO4 containing 0.5 M Pb(N03)2 and 0.05 to 0.2 M Bi(N03)3-51120. These films are designated as "Bi-PbOj/Pt". The Bi-PbOj films were dissolved from the metal substrates by immersion for several minutes in a 50/50 mixture of glacial acetic acid and 30% hydrogen peroxide solution. The Pt gauze electrode was frequently cathodized (500 mA for 10 min) which seemed to result in the best films. The Au RDE was polished after removal of Bi-Pb02 films using Buehler Microcloth with alumina slurries (1.0 |im, 0.3 jim, 0.05 |a,m; Buehler). 26 All Pb(II) and Bi(III) compounds are toxic substances. Therefore, waste solutions containing these metals were disposed according to accepted procedures. A saturated calomel electrode (SCE, Fisher Scientific) provided the reference potential for all voltammetric and potentiostatic coulometric experiments and all potential values are given vs. the SCE. The coimter electrode was a coiled Ft wire (cc. 7 cm^). Instrumentation Voltammetric procedures have been described.' The Au RDE was used with a model MSRX rotator and speed controller (Pine) and a model RDE-4 bipotentiostat (Pine). The cylindrical Pt gauze electrode was used with a coulometer, potentiostat, and power supply (Electrosynthesis Co., Inc.) for coulometric measurements. The scanning electron microscope (SEM) was a model JSM 840-A (JEOL). Determination of current efficiency The percent current efficiency (%,^) for conversion of Cr(III) to Cr(VI) was based on potentiometric titrations using 0.2 M Fe(NH4)2(S04)2*6H20 in 1.0 M H2SO4 standardized daily against reagent grade KiCrjO,. The Pt-gauze working electrode coated with designated films was placed in the central working chamber of a three-compartment cell. Reference and counter electrodes were placed in side-chambers separated from the working chamber by fritted glass disks (medium porosity). The working chamber was filled with 150 mL of 1.0 M H2SO4 containing 0.10 M CrK(S04)2'12H20 and the side chambers were filled with 1.0 M H2SO4. Oxidation of Cr(in) to Cr(VI) was performed galvanostatically at 1(X) mA, with magnetic stirring, and the total charge passed was 27 monitored using the coulometer. The potentiostat was switched to "standby" when the charge reached ca. 400 C, Le., conversion of 10 % of the initial Cr(III), and the working electrode was raised above the level of the solution and rinsed with deionized water. The contents of all three chambers were quantitatively transferred to a beaker for titrimetric determination of Cr(VI). Values of were calculated as follows; ~ ^Fe(n) ^Fe(n) F100%/Q^ [2] where and are the concentration (mol L"') and volume (L) at the endpoint, respectively, of the titrant; Q,g, is the total charge passed (C); and F is the Faraday constant. Results and Discussion Voltainmetric response of Cr(III) Based on i-E curves (not shown) obtained at Au and Pt RDEs in 1.0 M H2SO4 with and without the presence of 10 mM Cr(III), it is readily apparent that, for both electrode materials, there is no significant change in voltammetric response that can be attributed to the oxidation of Cr(III). Figure 1 contains the cyclic voltammetric curves obtained at a VhOJAu RDE {A,B) and Bi-PbOj/Au RDE (CJ)) in 1.0 M H2SO4 with (A,C) and without (BJ3) the presence of 10 mM Cr(III). For both electrodes, anodic production of Oj occurs at a significant rate for E> ca. 1.75 V. Following the addition of Cr(in), very little increase in current is observed for the PbOj/Au electrode (B); however, a significant increase in anodic current is observed at the Bi-Pb02 electrode (C) in comparison to the residual response (D).