Promotive Effect of Bicarbonate Ion on Two‐Electron Water Oxidation to Form H₂O₂ Catalyzed by Aluminum Porphyrins

Abstract

Two‐electron water oxidation initiated by one‐electron oxidation of aluminum porphyrins (AlTMPyP) is an alternative water oxidation to the conventional four‐electron pathway and could help to avoid the bottleneck subject of photon‐flux density in artificial photosynthesis. Here, a dramatic enhancement of the reactivity by bicarbonate ion in the two‐electron water oxidation to form H2O2 is reported. An addition of sodium carbonate (Na2CO3) controlled both catalytic current and product selectivity of the two‐electron water oxidation to enhance the activity of AlTMPyP at pH≈10–11. Controlled potential electrolysis experiments at different concentrations of Na2CO3 (10–100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of [Na2CO3] added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H2O2 from the hydroperoxy complex of AlTMPyP through an attack of a bicarbonate ion. The detailed electrochemical studies and product selectivity indicated that the bicarbonate ion served as a good cofactor for producing H2O2 from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na2CO3 on the catalytic reactivity was observed.

Cobalt Carbonate as an Electrocatalyst for Water Oxidation

“…It is suggested that this process involves the oxidation of the Co III species formed in the first process to Co IV species, the observed peak potential fits DFT calculations for the redox potential of the Co IV/III (CO 3 ) 3 2−/3− couple (see below), which oxidize bicarbonate through reactions analogous to the reactions in Equations (11)–(13) that consist of the catalytic process observed. Very recently, a similar catalytic process has been reported for an Al porphyrin complex at 1.1 V (vs. Ag/AgCl) . The dependence of the second process on the pH is presented in Figure S12 (in the Supporting Information).…”

“…Very recently,asimilarc atalytic process has been reported for an Al porphyrin complex at 1.1 V( vs. Ag/AgCl). [71] The dependence of the second process on the pH is presented in Figure S12 (in the Supporting Information). Twoe ffects are observed: a) the peak potentialshifts considerably to lower potentials with the increase in the pH;b )the current increases considerably with the increasei nt he pH.…”

“…[70] Av ery recent study on an aluminium-porphyrin complexs howed the catalytic formation of peroxo-monocarbonate in the presence of bicarbonate/carbonate. [71] The role of carbonate in these processes is attributed mainly to two properties of carbonate: [72] a) it is as trong base and as trong s donor, therefore it stabilizes transition-metal complexes in high oxidation states; [15,45,70] b) its relativelyl ow oxidation potential [68,69] enhances electron transfer to the central cation. [45,70] Several mechanismsf or the role of carbonate in thesep rocesses can be proposed [Equations (5)-(8)]:…”

“…Furthermore, a recent study pointed out that carbonate ligands also facilitate electrocatalytic water oxidation in the presence of a nickel complex . A very recent study on an aluminium‐porphyrin complex showed the catalytic formation of peroxo‐monocarbonate in the presence of bicarbonate/carbonate . The role of carbonate in these processes is attributed mainly to two properties of carbonate: a) it is a strong base and a strong σ donor, therefore it stabilizes transition‐metal complexes in high oxidation states; b) its relatively low oxidation potential enhances electron transfer to the central cation .…”

CO2/carbonate-mediated electrochemical water oxidation to hydrogen peroxide

“…Inspired by this phenomenon and previous studies in electrogenerated chemiluminescence 29 , 30 and peroxycarbonate synthesis 31 , here we hypothesize that CO 2 (or carbonate as its ionic form in water) may serve as an effective mediator to promote the H 2 O 2 pathway in electrochemical water oxidation (Fig. 1a ) 32 – 34 . Using fluorine doped tin oxide (FTO) as a model catalyst electrode, we demonstrated high H 2 O 2 selectivity up to 87%, delivered high H 2 O 2 partial current densities up to 1.3 A cm −2 , and achieved a long-term stable and continuous H 2 O 2 generation for 250 hours with over 80% H 2 O 2 selectivity at 150 mA cm −2 current density in carbonate solutions.…”