Presentation Information
[U17-11]Effect of Cl– ion on photocatalytic chemical evolution in CO world
*Yamada Yusuke1 (1.Osaka Metroplitan University)
Keywords:
ultraviolet light,metal complexes,metal sulfide,sea water
Gas-phase chemical reactions in the atmosphere played a major role in chemical evolution, which is the origin of the emergence of life. However, the processes of chemical evolution occurring on land or in water may be assisted by various metals working as catalysts. Considering that CO and CO2 in the atmosphere served as carbon sources for the formation of numerous organic compounds, various photosemiconductors may function as catalysts capable of driving these reduction reactions using ultraviolet and visible light.
Research on semiconductor photocatalysts has long been conducted, primarily as a subfield of artificial photosynthesis focused on water splitting. These studies have been revealed that many metal oxide semiconductors can perform reduction reactions using water as an electron source under ultraviolet irradiation, and that metal sulfides exhibit responsiveness to visible light. Furthermore, chloride ions (Cl-) present in such reaction systems plays an important role, because photocatalytic oxidation of Cl- proceeds (2Cl- + 2H2O + 4h+ -> 2HClO + 2H+), generating hypochlorous acid (HClO). HClO has been regarded as an efficient redox mediator for water oxidation in the field of water photolysis, because HClO decomposes upon ultraviolet irradiation to produce oxygen (2HClO -> O2 + 2Cl- + 2H+).
HClO and other chlorine oxoacids possess strong oxidizing power. For example, chlorine dioxide (ClO2), which has drawn considerable attention in recent years, can oxidize methane at room temperature and pressure to form methanol or formic acid. Among these species, HClO is known to exhibit the strongest oxidizing ability. It may have oxidized methane to generate CO, methanol, and formic acid on the prebiotic Earth. Therefore, we examined whether the formation of CO which is the core species in the "CO world" as well as methanol and formic acid could be observed by immersing metal oxide and metal sulfide semiconductor photocatalysts in saline solution under methane flow while irradiating with light.
If a certain amount of CO was present in the atmosphere of the prebiotic Earth, it may have reacted with metals such as iron and nickel on the Earth's crusts to produce iron carbonyl and nickel carbonyl complexes. CO ligands of these carbonyl complexes dissociate from central metals with ultraviolet light, indicating open metal sites which can be afforded for substances. As a result, these species can function as catalysts for chemical reactions such as CO transformation reactions. Mononuclear iron carbonyl and nickel carbonyl complexes have low boiling points (103 and 42 C, respectively) so that they easily evaporated to be decomposed in air. In contrast, multinuclear iron carbonyls are insoluble, non-volatile solids, so after forming, they could have remained on the surface and subsequently been exposed to seawater. In seawater, ultraviolet irradiation can cause CO ligands to dissociate, allowing chloride ions (Cl-) and other ligating molecules present in seawater to coordinate to the metal. It has also been proposed that cyanide ions (CN-) existed in seawater under prebiotic conditions, making the formation of cyanide complexes plausible. Metal cyanide complexes including Prussian Blue are known to catalyze various chemical reactions.
Thus, we first investigated the chemical species generated when Fe2(CO)9 and Fe3(CO)12 complexes were immersed in saline solution designed to simulate seawater, using an electrospray mass spectrometer (ESI-MS). As a result, multiple new chemical species were detected for both Fe2(CO)9 and Fe3(CO)12 upon immersion in saline solution. Furthermore, when these solutions were irradiated with ultraviolet light, the peaks corresponding to these newly formed chemical species disappeared. This finding indicates that, under UV irradiation in the presence of chloride ions, highly reactive new species are generated. The details of the reaction are currently under investigation.
Research on semiconductor photocatalysts has long been conducted, primarily as a subfield of artificial photosynthesis focused on water splitting. These studies have been revealed that many metal oxide semiconductors can perform reduction reactions using water as an electron source under ultraviolet irradiation, and that metal sulfides exhibit responsiveness to visible light. Furthermore, chloride ions (Cl-) present in such reaction systems plays an important role, because photocatalytic oxidation of Cl- proceeds (2Cl- + 2H2O + 4h+ -> 2HClO + 2H+), generating hypochlorous acid (HClO). HClO has been regarded as an efficient redox mediator for water oxidation in the field of water photolysis, because HClO decomposes upon ultraviolet irradiation to produce oxygen (2HClO -> O2 + 2Cl- + 2H+).
HClO and other chlorine oxoacids possess strong oxidizing power. For example, chlorine dioxide (ClO2), which has drawn considerable attention in recent years, can oxidize methane at room temperature and pressure to form methanol or formic acid. Among these species, HClO is known to exhibit the strongest oxidizing ability. It may have oxidized methane to generate CO, methanol, and formic acid on the prebiotic Earth. Therefore, we examined whether the formation of CO which is the core species in the "CO world" as well as methanol and formic acid could be observed by immersing metal oxide and metal sulfide semiconductor photocatalysts in saline solution under methane flow while irradiating with light.
If a certain amount of CO was present in the atmosphere of the prebiotic Earth, it may have reacted with metals such as iron and nickel on the Earth's crusts to produce iron carbonyl and nickel carbonyl complexes. CO ligands of these carbonyl complexes dissociate from central metals with ultraviolet light, indicating open metal sites which can be afforded for substances. As a result, these species can function as catalysts for chemical reactions such as CO transformation reactions. Mononuclear iron carbonyl and nickel carbonyl complexes have low boiling points (103 and 42 C, respectively) so that they easily evaporated to be decomposed in air. In contrast, multinuclear iron carbonyls are insoluble, non-volatile solids, so after forming, they could have remained on the surface and subsequently been exposed to seawater. In seawater, ultraviolet irradiation can cause CO ligands to dissociate, allowing chloride ions (Cl-) and other ligating molecules present in seawater to coordinate to the metal. It has also been proposed that cyanide ions (CN-) existed in seawater under prebiotic conditions, making the formation of cyanide complexes plausible. Metal cyanide complexes including Prussian Blue are known to catalyze various chemical reactions.
Thus, we first investigated the chemical species generated when Fe2(CO)9 and Fe3(CO)12 complexes were immersed in saline solution designed to simulate seawater, using an electrospray mass spectrometer (ESI-MS). As a result, multiple new chemical species were detected for both Fe2(CO)9 and Fe3(CO)12 upon immersion in saline solution. Furthermore, when these solutions were irradiated with ultraviolet light, the peaks corresponding to these newly formed chemical species disappeared. This finding indicates that, under UV irradiation in the presence of chloride ions, highly reactive new species are generated. The details of the reaction are currently under investigation.