Spontaneous Reduction of Transition Metal Ions by One Electron in Water Microdroplets and the Atmospheric Implications

Abstract

Freshman chemistry teaches that Fe 3+ and Cu 2+ ions are stable in water solutions, but their reduced forms, Fe 2+ and Cu + , cannot exist in water as the major oxidation state due to the fast oxidation by O 2 and/or disproportionation. Contrary to these well-known facts, significant fractions of dissolved Fe and Cu species exist in their reduced oxidation states in atmospheric water such as deliquesced aerosols, clouds, and fog droplets. Current knowledge attributes these phenomena to the stabilization of the lower oxidation states by the complexation of ligands and the various photochemical or thermal pathways that can reduce the higher oxidation states. In this study, by spraying the water solutions of transition metal ions into microdroplets, we show the results of the spontaneous reduction of ligated Fe(III) and Cu(II) species into Fe(II) and Cu(I) species, presenting a previously unknown source of reduced transition metal ions in atmospheric water. It is the spontaneously generated electrons in water microdroplets that are responsible for the reduction. Control experiments in the atmosphere and in a glove box filled with precisely controlled gaseous contents reveal that O 2 , CO 2 , and NO 2 are the major competitors for the electrons, forming O 2 − , HCO 2 − , and NO 2 − , respectively. Taking these findings together, we opine that microdroplet chemistry might play significant but previously underestimated roles in atmospheric redox chemistry.

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

“…Historically, the most unstable anion radical made by water microdroplets is the pyridyl anion because the EA of pyridine was reported to be negative . Another study of ours provided evidence that O 2 , NO 2 , and CO 2 compete for the electron in water microdroplets, forming O 2 – , NO 2 – , and HCO 2 – , respectively . Since the electron is viewed as the strongest reducing reagent, this work provides another example of the strong reducing power of water microdroplets.…”

“…In the recent several years, water microdroplet chemistry is emerging as an exciting new field for the abilities 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 observation that water microdroplets exhibit strong reducing and oxidizing powers is especially intriguing. − It is been postulated that there is a high electric field (∼10 9 V/m) at the surface of microdroplets either due to the formation of electric double layers or the alignment of the free O–H bonds of water molecules. − This high field can even pull an electron out of a hydroxide ion, − forming a hydroxyl radical and an electron (Figure ). The strong reducing power of water microdroplets is considered to be a result of the electron, and the oxidizing power of water microdroplets is a result of the hydroxyl radical.…”

“…It is worth mentioning that on the aerial surface of air bubbles in water, the oxidation potential of OH – to OH was found to occur at least 0.7 V below the redox table values as measured by electrochemical methods, so the reaction in Figure should be easier at the air–water interface compared to the bulk. The power of water microdroplets to promote reduction chemistry has been demonstrated in the formation of pyridyl anion in spraying of 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 viologen, 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 •– and the subsequent C–I bond cleavage, and the reduction of Fe 3+ and Cu 2+ complexes into Fe 2+ and Cu + complexes . In most of these cases, the reduction reactions were finished by solely the electrons, but some others needed the participation of other species such as protons in water.…”

“…Since the electron and the hydroxyl radical coexist as shown in Figure , water microdroplets should presumably exhibit reducing and oxidizing powers simultaneously. However, most of the existing publications show either only reduction reactions − or only oxidation reactions. − This phenomenon might be a result of the redox properties of the substrates per se , since some compounds are easy to oxidize but difficult to reduce and some compounds are easy to reduce but difficult to oxidize. There have been only two cases , where reduction and oxidation reactions occurred simultaneously to the same substrate.…”

“…The power of water microdroplets to promote reduction chemistry has been demonstrated in the formation of pyridyl anion in spraying of an aqueous solution containing dissolved pyridine, 3 the reduction of dissolved chloroauric acid in water microdroplets to yield gold nanoparticles and nanowires, 4 the reduction of doubly charged viologen compounds to singly charged viologen, 5 the reduction of organic compounds by hydrogenation, 6 the reduction of CO 2 into formate, 7,8 the reduction of Hg 2+ into atomic Hg, 9 the reduction of C 6 F 5 I into C 6 F 5 I •− and the subsequent C−I bond cleavage, 10 and the reduction of Fe 3+ and Cu 2+ complexes into Fe 2+ and Cu + complexes. 11 In most of these cases, the reduction reactions were finished by solely the electrons, but some others needed the participation of other species such as protons in water. The power of water microdroplets to promote OH oxidation chemistry has been demonstrated in the oxidation of salicylic acid, 6 the oxidation of I − anions into I atoms, 12 the oxidation of caffeine and melatonin, 13 the detection of the combination of hydroxyl radicals to yield hydrogen peroxide (H 2 O 2 ), 14−17 and the direct capture of the OH radical in the form of H 4 O 2 + cations, whose structure is essentially a hydroxyl radical combined with a hydronium via a hydrogen bond: OH••• H 3 O + .…”