서강대학교

초록/요약

Artificial photosynthesis (AP), namely, the sun light driven photocatalytic conversion of carbon dioxide (CO2) and water into fuel, is currently an important area of research. Among various attempts the ‘one-pot’ AP systems which attempt to produce fuels by irradiation of reactors containing CO2, H2O, and metal-loaded TiO2 with proper light sources have received most attention during the last three decades. We define such AP systems as self-contradictory AP systems because they contain water oxidation sites which can oxidize the carbon-containing organic fuel, if produced, more readily than water. Methanol has been the most desired product from such self-contradictory AP systems because it is liquid under the ambient condition and can be easily transformed into various useful compounds. However, the major products that have been observed from such self-contradictory AP systems under normal conditions (1 bar of CO2, 1 sun, neutral condition, and the irradiation wavelength > 350 nm) have been the gaseous products, methane (CH4) and carbon monoxide (CO). From a systematic study aimed to elucidate the fate of methanol in such self-contradictory AP systems with novel metal nanoparticle loaded TiO2 (Mn-TiO2, M = Pd, Pt, Cu and Au) as the catalyst we found that methanol and the related less reduced products, formaldehyde and formic acid are not produced from such self-contradictory AP systems, indicating that the gaseous products should be produced from the pathways which do not involve the formation of methanol and the less reduced products. We also elucidated the important fact that during irradiation of the self-contradictory AP system with methanol as the added reagent, fifteen different reactions take place, which are photocatalytic dehydrogenation of methanol to formaldehyde (R1), thermal hydration of formaldehyde to methanediol (R2), photocatalytic dehydrogenation of methanediol to formic acid (R3), thermal esterification of formic acid and methanol to MF (R4), photocatalytic dehydrogenation of formic acid to carbon dioxide (R5), thermocatalytic coupling of formaldehyde and methanol to methoxymethanol (R6), photocatalytic (R7) and thermocatalytic (R8) dehydrogenation of methoxymethanol to MF in the presence of O2, thermocatalytic coupling of methoxymethanol and methanol to DMM (R9), photocatalytic hydrolysis of DMM to methanol and formaldehyde (R10), photocatalytic decomposition of dry DMM to dimethyl ether (DME) and formaldehyde (R11), photocatalytic hydrolysis of MF to formic acid and methanol (R12), and photocatalytic decomposition of dry MF to methanol and carbon monoxide (heterolysis, R13), to two formaldehyde (homolysis, R14), and to methane and carbon dioxide (heterolysis, R15), respectively. Their flow and interconnectivity is shown in Scheme 1. The reaction is initiated by photoinduced excitation of the charge-transfer (CT) band from methanol to TiO2 surface which appears in the UV region by the UV part of the solar light. These reactions can also be used for production of various useful compounds. The oxidation of alcohols (methanol, ethanol, propanol, benzyl alcohol etc.) to their oxidized product is well study reaction using oxygen as an oxidant under the thermal condition. This is one of the alternative approach for the preparation of ester and other important compound for the industrial application. However, using oxygen as an oxidant is an expensive approach because under this condition only the oxidized products were obtained however, the most valuable products i.e. hydrogen is not possible at all. Steam reforming is the method of choice if hydrogen has to be produced from methanol in high yields. Under ideal conditions, the reaction converts methanol and water into carbon dioxide and three moles of hydrogen in a moderately endothermic transformation. The production of hydrogen from alcohols is more promising for the fuel cell technology as well as “Hydrogen economy”. In order for hydrogen fuel cells to have a large impact on reducing greenhouse gas emissions, the hydrogen needs to be derived from sunlight, either directly or indirectly from biomass through photosynthesis. The use of hydrogen fuel cells in vehicles or in portable power plants will require lightweight H2 storage or “on-board” reforming of hydrogen- containing compounds into H2. We have conducted a series of photooxidation of alcohols (methanol and ethanol) on several metal oxide nanoparticle-loaded TiO2 [(MO)n-TiO2], with M = Pd, Pt, Au, and Cu, by feeding methanol or ethanol and water in the vapor phase into the reactor charged with (MO)n-TiO2 powder. The reason (MO)n-TiO2 was used as the photocatalyst instead of Mn-TiO2 is because we later found that (MO)n-TiO2 is first converted to Mn-TiO2 under the reaction condition, and this is more convenient to carry out the reaction. Irradiation was carried out using a solar simulated light with the power of 100 mW cm- 2 under the standard AM 1.5 conditions. We found that the photocatalytic reaction of methanol and water on (MO)n-TiO2 involves fifteen different reactions (denoted as Rn with n = 1-15 and the corresponding rate as rRn) and thirteen different compounds (CH3OH, HCHO, H2C(OH)2, HCO2H, HCO2CH3, CH3OCH2OH, (CH3O)2CH2, CH3OCH3, CH4, CO2, CO, H2, and H2O). We found that the photocatalytic reaction of ethanol and water on (MO)n-TiO2 involves sixteen different reactions (denoted as Rn with n = 1-16 and the corresponding rate as rRn) and thirteen different compounds (C2H5OH, CH3CHO, H2CH4(OH)2, CH3CO2H, CH3CO2C2H5, C2H5OC2H5OH, (C2H5O)2C2H4, C2H5OC2H5, CH4,C2H6,C2H4, CO2, CO, H2, and H2O). Photocatalytic dehydrogenation of ethanol to acetaldehyde (R1), thermal hydration of acetaldehyde to ethanediol (R2), photocatalytic dehydrogenation of ethanediol to acetic acid (R3), thermal esterification of acetic acid and ethanol to EA (R4), photocatalytic decomposition of acetic acid to carbon dioxide and methane (R5), thermocatalytic coupling of acetaldehyde and ethanol to ethoxyethanol (R6), photocatalytic (R7) and thermocatalytic (R8) dehydrogenation of ethoxyethanol to EA in the presence of O2, thermocatalytic coupling of ethoxyethanol and ethanol to DEE (R9), photocatalytic hydrolysis of DEE to ethanol and acetaldehyde (R10), photocatalytic decomposition of dry DEE to diethyl ether and acetaldehyde (R11) in the presence of Ar and O2, thermal and photocatalytic hydrogenation of DEE to diethyl ether and ethanol (R13) in the presence of hydrogen. Photocatalytic hydrolysis of EA to acetic acid and ethanol (R12), and photocatalytic decomposition of dry EA to carbon dioxide, methane and ethene (heterolysis, R14), to two acetaldehyde (homolysis, R15) and dry decomposition of acetaldehyde to carbonmonoxide and methane (R16).