A novel photo-electrochemical approach for the chemical recycling of carbon dioxide to fuels

Carbon dioxide is one of the principal greenhouse gases that enter the atmosphere because of natural processes and human activities. The quantity of CO2 coming from fossil fuel combustion has accounted for approximately 80% of the global warming potential weighted emissions since 1990. In order to limit the consequences due to the climate change, it is urgent to develop new technologies for greater energy saving and efficiency, and to find in a medium term a sustainable solution to CO2 emissions. However CO2 is recently turning image and there are increasing attempts to consider it a resource and a business opportunity rather than a waste with a cost of disposal. Increasing amounts of low-cost and relatively pure CO2 will be soon available from current and planned plants for carbon sequestration and storage (CSS). Therefore, CO2 will be a feedstock of nearly zero (or even negative) cost for conversion to fuels and chemicals, in addition to the many benefits in terms of positive image for companies, which will adopt politics of reduction of CO2 emissions. The other factor stimulating the interest in CO2 chemical recycling is the presence of many emissions for which the CSS option is unsuitable: distance from safe sequestration sites, diluted concentration of CO2 in the emitting gas, small-medium size sources, and uncertainty of the environmental impact [1]. There are different options to convert CO2. In this work we focused the attention on the reduction of CO2 to liquid fuels, by using a novel photo-electrochemical (PEC) approach [2]. This process, carried out at room temperature and atmospheric pressure using solar light, represents a highly challenging approach to close the CO2 cycle and develop photosynthesis mimic approaches. The core of the PEC system is a particular homemade photo-reactor, built in Plexiglas and equipped with a quartz window. The PEC reactor was realized in a highly compact configuration in order to minimize scattering phenomena and increase the photo-efficiency [3]. It has a two-electrode configuration with two compartments and it can be used either for H2 production by water splitting or for CO2 reduction back to liquid fuels (hydrocarbons, alcohols with C>2). The photo-anode is a nanostructured TiO2 thin film supported over a porous titanium foil [4]. The cathode consists of Pt(or Fe)/carbon nanotubes (CNT) supported on carbon cloth. The two electrodes are joint together by a protonic membrane (Nafion). The simplified process is as follows: i) light crosses the quartz window and reaches the nanostructured film (photoanode) where photo-generated electron and hole pairs are generated and O2 evolves, ii) protons pass through the protonic membrane, while electrons are collected and reach the cathode through an external wire, and iii) protons react with CO2 in the presence of electrons on the CNT based electrocatalyst to give liquid fuels, or recombine with electrons over Pt nanoparticles supported on carbon cloth to give H2. The physical separation of the two reactions of water oxidation and CO2 reduction in a photoanode and electrocathode respectively, is necessary to increase the efficiency of the process and limit charge recombination. Moreover, this device could be used to produce renewable H2 by photocatalytic reforming of chemicals present in waste streams from agro-food or agro-chemical production, such as diluted streams of ethanol, glycerol, etc. These processes are only at an early stage, but with relevant potential in the future. In a long term, the photoelectrocatalytic reduction of CO2 under solar irradiation (together with other new potential technologies, such as thermal CO2 reduction) could greatly increase carbon recycling and reduce fossil fuel consumption. Here we reported on the results obtained separately by experiments with photoanode and cathode in the frame of two distinct EU projects (NATAMA and ELCAT, respectively). Up to now the full assembled PEC reactor was tested only for water splitting with separate H2 and O2 production and results are very promising. Work is in progress for the case of CO2 reduction. [1] G. Centi, S. Perathoner. Catalysis Today, Vol. 148 (2009), pp. 191-205. [2] C. Ampelli, G. Centi, R. Passalacqua, S. Perathoner. Energy & Environmental Science, Vol. 3 (2010), pp. 292-301. [3] C. Ampelli, R. Passalacqua, S. Perathoner, G. Centi. Chemical Engineering Transactions, Vol. 17 (2009), pp. 1011-1016. [4] C. Ampelli, R. Passalacqua, S. Perathoner, G. Centi, D.S. Su, G. Weinberg. Topics in Catalysis, Vol. 50 (2008), pp. 133-144.