Conversion of CO₂ in Methanol into HCOOCH₃ and CO By Photoelectrochemical Reduction with TiO₂ Photoanode

Abstract

Introduction Carbon dioxide is a linear and very stable molecule in which the oxygen atoms are weak Lewis (and Brønsted) bases and the carbon is electrophilicCarbon dioxide (CO2) is one of the main contributors to the global warming. Consequently, the conversion of CO2 into value-added products and/or high-energy content fuel will become one of the goals in environmental research. Among the various possible approaches, the photoelectrochemical reduction technique is used to reduce the CO2. It has been found that the electrochemical technique may not only convert CO2 from the environment but also the products by photoelectrochemical reduction of CO2 may be used as renewable energy. Methanol is better solvent of CO2 than water, particularly at a low temperature. Literature data for the solubility of CO2 in pure methanol and water, at 15 oC, were of 4.6 and 1.07 cm3 cm-3, respectively. Therefore, methanol has been used as a physical absorbent of CO2. In the present study, we investigated the performance of TiO2 nanotube prepared by anodization for the photoelectrochemical reduction of CO2 in the methanol. Also, the emphasis is placed on evaluating and comparing the current density and Faradaic efficiencies at different calcination temperatures, calcination times and different cathodes (Pb and Ag). Experimental The TiO2 photoanode was prepared by anodization Ti plate (30 × 20 mm) in hydrogen fluoride solution (1% in 10% CH3OH + 90% water) under ultrasound and calcination at 300-900 oC for improvement crystallinity of TiO2. The electrochemical reactor was operated using a potentiostat (HA-3001A) under ambient pressure and temperature. The 3.0 M KHCO3 and 0.3 M KOH in CH3OH solution was used as catholyte and anolyte, respectively, in a standard three-electrode H-type cell equipped with cathodes (Pb and Ag) as the counter electrode and a Ag/AgCl as reference electrode. Then, CO2 gas was bubbled into the methanol catholyte for 2 hours. Aldrich Nafion 117 ion exchange membrane was used as the diaphragm between photoanode and cathode. The working electrode (TiO2 photoelectrode) was irradiated by a UV light (365nm) for the photoelectrochemical reduction of CO2 in the methanol. The gaseous products obtained during electroreduction were collected in a gas collector and were analyzed by gas chromatography. Products soluble in the catholyte were analyzed by using high performance liquid chromatography. A scanning electron microscope (SEM) was used to characterize TiO2 electrode. Results and discussion As a result of the observation of TiO2 photoelectrode (calcination at 700 oC for 2 hours) before and after the photoelectrochemical reduction use with a scanning electron microscope (SEM), the electrode surface became a nanotube. TiO2 photoelecrode after the photoelectrochemical reduction had the broken part, but we were able to confirm a nanotube in a considerable part. To examine the electrochemical behavior with the photoelectrode, the photoreaction of TiO2 photoelectrode (calcination at 700 oC for 2 hours) with Pb and Ag cathode is observed at an applied potential of 2.0 V vs. Ag/AgCl for the anode. Because an electric current flowing at the potential (2.0 V vs. Ag/AgCl) under the illumination was different from that observed without the light with both Ag and Pb cathode, it was confirmed that the TiO2 photoelectrode could use the light energy. Also, because no remarkable changes of current density were observed for the Ag and Pb cathodes, the influence of metal materials on current density could not be observed. In order to identify the optimum program of calcination temperature and time, the photoelectrochemical reduction of CO2 was performed for evaluating and comparing the current density and Faradaic efficiencies at different calcination temperatures (300-900 oC) and calcination times (1-10 hours). A result for calcination temperature suggested that the current density and Faradaic efficiency for TiO2 photoelectrode at the 700 oC calcination for 2 hours with Pb cathode, at an applied potential of 2.0 V vs. Ag/AgCl, were J = 2.2 mA cm-2 and FE (HCOOCH3) = 107.6 %. Furthermore, a result for calcination time suggested that the current density and Faradaic efficiency for TiO2 photoanode at the calcination at 700 oC during 2 hours with Pb cathode, at an applied potential of 2.0 V vs. Ag/AgCl, were J = 2.1 mA cm-2 and FE (HCOOCH3) = 89.0%. The main product by photoelectrochemical reduction of CO2 with Pb cathode was HCOOCH3 whereas the main product with Ag cathode was CO. Consequently, the use of TiO2 photoanode may be possible for the application for the photoelectrochemical reduction of CO2, although further research is still required in order to develop highly active, selective and stable catalysts.