Sonochemical and sonoelectrochemical conversion of CO2 into hydrocarbons

Abstract

Carbon dioxide (CO2) is one of the major greenhouse gas (GHG) contributors to global climate change. Combustion of fossil fuels accounts for approximately 80-90 % of the global CO2 emission, which over the last decade, has been increasing by 2.7 % annually. There is an urgent need to significantly reduce these CO2 emissions into the atmosphere if mankind is to avoid irreparable damages to the world’s ecosystems. Currently, there are two methods available for reducing CO2 emissions into the atmosphere. One is the carbon capture and storage (CCS) method in which CO2 is captured and stored for extended periods. Another method is the carbon capture and utilization (CCU), where captured CO2 is used to yield economically valuable products. Currently, there are several methods available for CO2 utilization. Among them, the conversion of CO2 into hydrocarbons is of specific interest since this process helps to recycle CO2 as energy carrier by reducing its accumulation in the atmosphere while producing valuable and useful compounds. For the conversion of CO2 into hydrocarbons, several methods namely, chemical, electrochemical, biochemical and photochemical methods are available. However, most of these processes are energy intensive and inefficient to be used commercially. In this study, we investigated an alternative method in which power ultrasound was used to carry out the Sabatier process at ambient conditions i.e., at room temperature and pressure and without the use of catalysts to produce methane (CH4) from CO2. We named this process as the “sono-Sabatier process”. In this process, a small quantity of CO2 (<3 %) and molecular hydrogen (H2) gas mixture was used to saturate a solution such as either pure water, artificial seawater or NaCl (of low concentrations, from 0.5 to 1.0 M) in a specially designed sonochemical reactor, equipped with a 488 kHz ultrasonic transducer. After 1 hour of ultrasonication, the gas samples were collected and analyzed by gas chromatography (GC). It was found that a portfolio of various hydrocarbons such as CH4, C2H4 and C2H6 were formed by the reduction of CO2. We found that there are several parameters governing the sono-Sabatier process. One of the most important parameters is the effect of molecular hydrogen gas concentration. It was observed that yields of hydrocarbons increased significantly with the increase of hydrogen concentration. We also witnessed that hydrogen gas played two different roles. The first role is the supply of hydrogen to the CO2 methanation reaction. In the second role, hydrogen acts as a reducing agent where it scavenges the hydroxyl radicals (OH•) formed during water sonolysis (water dissociation into radicals under ultrasonication) creating a strong reducing environment. Another important parameter that governs the sono-Sabatier process is the concentration of NaCl in the ultrasonicated solution. Yields of hydrocarbons increased with increasing concentration of NaCl up to 1.0 M and then decreased. It is well known in sonochemistry that increasing NaCl concentration decreases cavitational activity. However, at 1.0 M NaCl concentration and 98 % H2 mixed with 2 % CO2, optimal conditions were obtained where the highest reduction environment was seen, due to the synergistic effects of molecular hydrogen and 1.0 M NaCl solution. These findings were applied to the CO2 to hydrocarbon conversion from synthetic industrial flue gases. However, since the flue gas contains around 13 % of CO2, it requires to be diluted with molecular hydrogen for efficient conversion. It was also found that synthetic seawater could be used as the ultrasonicating media for the CO2 conversion where ca. 40 % methane yield was obtained (Paper 4). Moreover, the effects of ultrasound on the electrochemical reduction of CO2 (CO2RR) into hydrocarbons were also studied. We have named this approach as the sono-CO2RR process. It was found that the cathodic current density for the CO2 reduction increased significantly in the presence of ultrasound when compared to silent conditions (absence of ultrasound). It was observed that ultrasound increased significantly the faradaic efficiency of CO, CH4 and C2H4 formation. Under ultrasonication, 40 % higher faradaic efficiencies of methane were observed that in the absence of ultrasound for identical mass transport conditions. Interestingly, the faradaic efficiency of hydrogen gas formation decreased in the presence of ultrasound. We postulated that (i) hydrogen gas was consumed in the sono-CO2RR process giving rise to higher amounts of hydrocarbons, and (ii) hydrogen initiated new reaction pathways yielding new products such as ethylene (C2H4) and ethanol (C2H5OH) (Paper 3). Further investigations are necessary in order to improve the state-of-the-art of these processes. For example, the use of a catalyst may significantly improve the sono-Sabatier process. For the sono-CO2RR process, using non-cavitating coupling fluid such as silicon oil at 1.0 bar of over pressure can greatly increase the transmission of ultrasound to the electrolytes as well as the faradaic efficiencies of the CO2 reduced products. Finally, a combined process could be designed whereby the hydrogen produced in the sono-CO2RR process could be used in the sono-Sabatier process in turn reducing the overall consumption of hydrogen.