Spontaneous and Simultaneous Oxidation and Reduction of o-Quinones in Water Microdroplets

Abstract

Microdroplet chemistry has been an emerging new field for its large plethora of unique properties, among which an especially intriguing one is the strong oxidizing and reducing powers. The hydroxide ion in water microdroplets is considered to split into a hydroxyl radical and an electron at the air−water interface, and the former is responsible for the oxidizing capability while the latter is responsible for the reducing power, making a unity of opposites. However, to date there are only two examples showing that oxidation and reduction occur simultaneously to the same substrates, which might be a result of the redox properties of the substrate per se. In this study, we carefully chose a group of οquinone compounds as the substrates in water microdroplets and discovered that they can be both oxidized by the hydroxyl radical and reduced by the electron. These results keep pushing the limit of the unique redox properties of microdroplet chemistry.

Mechanism of Hydrogen Peroxide Formation on Sprayed Water Microdroplets

“…It should be emphasized that the opening of the ET pathway, reaction , not only accounts for the formation of H 2 O 2 , but also is the gateway to redox chemistry in general on the surface of SWMs. , Therein, HO S – becomes an effective reductant that converts H S + into H S • atoms (strong reductants as well that may otherwise recombine into H 2 ), , while being transformed into the powerful HO S • radical oxidizer. Alternatively, HO S – could directly reduce added substrates, such as pyridine, into carbanions, whereas HO S • could abstract H-atoms and oxidize other species into carbocations. , The redox potentials of these intermediates, and the competition between ET, reaction , and ion recombination, reaction , HO normalS − + H normalS + = H 2 O S are needed to delineate the range of possible redox reactions on SWMs.…”

“…It should be emphasized that the opening of the ET pathway, reaction R1, not only accounts for the formation of H 2 O 2 , but also is the gateway to redox chemistry in general on the surface of SWMs. 28,29 Therein, HO S − becomes an effective reductant that converts H S + into H S • atoms (strong reductants as well that may otherwise recombine into H 2 ), 30,31 while being transformed into the powerful HO S…”

The Spontaneous Electron-Mediated Redox Processes on Sprayed Water Microdroplets

“…Previous studies have provided evidence that there are spontaneously generated electrons on water microdroplets. − , The OH – ion in water has been considered to be the electron donor when pure water is sprayed. From the discussions in Section , we can anticipate that those molecules whose electrons are removed in microdroplets could also function as electron donors.…”

“…Water microdroplet chemistry has drawn tremendous attention in recent years for its ability to accelerate chemical reactions by several orders of magnitude compared to the same reactions in bulk water, and to trigger spontaneous reactions that cannot occur in bulk water. , Among the unique properties of water microdroplets, the strong spontaneous redox power is especially intriguing. − Several groups deduced that there is a high electric field (∼10 9 V/m) at the air–water interface of microdroplets due to the formation of electric double layers, the alignment of the free O–H bonds of the interfacial water molecules, or the partial charge transfer that results in the interfacial H 2 O + ···H 2 O – pairs. − This high field is large enough to pull electrons out of the hydroxide ions (OH – ) in water, − yielding hydroxyl radicals ( • OH) and electrons (Figure ). A combination of electrochemical experiment and theory shows that the oxidation potential of OH – to • OH is at least 0.7 V below tabled values on the aerial surface of air bubbles in water, so the reaction of OH – that yields an electron as shown in Figure should be easier to take place at the air–water interface compared to the bulk.…”

“…These results suggest that oxidation potential can be used as an indicator to predict whether or not a molecule can be oxidized by one electron in water microdroplets. The strong reducing power of water microdroplets is a result of the electron, which has been demonstrated in the formation of pyridyl anions in spraying an aqueous solution containing dissolved pyridine, the reduction of dissolved chloroauric acid in water microdroplets to yield gold nanoparticles and nanowires, the reduction of doubly charged viologen compounds to singly charged viologens, the reduction of organic compounds by hydrogenation, the reduction of CO 2 into formate, , the reduction of Hg 2+ into atomic Hg, the reduction of C 6 F 5 I into C 6 F 5 I •– , the reduction of Fe 3+ and Cu 2+ complexes into Fe 2+ and Cu + complexes, and the reduction of three ortho -quinone molecules into radical anions . In most of these cases, the reduction was achieved solely by the electrons, but some needed the participation of other species such as the protons in water.…”