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Carbon dioxide photoreduction: A great challenge for photocatalysis

Final Report Summary - CO2PHOTORED (Carbon dioxide photoreduction: A great challenge for photocatalysis)

Project PIEF-GA-2011-298740 summary report

The CO2PHOTORED project aims at the activation of CO2 followed by its photoreduction toward useful organic compounds using electron-transfer processes on heterogeneous catalysts. The main objective of the current project was to explore new catalyst formulations that can improve hydrocarbon yields from CO2 and H2O. A general characteristic that all these photocatalysts possess is the ability to generate hydrogen by photocatalytic splitting of water, which is the primary step in CO2 photoreduction as well. The second and the most crucial step is the reduction of CO2 to hydrocarbons. For this step, the energy level of the conduction band has to be more negative with respect to reduction potential for CO2. In this context, this project was focused on:
- development and characterization of several n-type semiconductors which have to maximize the CO2 photoreduction process and to reduce the charge-carrier recombination rate;
- development and characterization of p-type semiconductors which could realize the reduction of CO2 by injecting electrons from the conduction band into the CO2 molecules;
- development and characterization of some hybrid photosensitizer – n-type and p-type semiconductor systems capable to perform the CO2 photoreduction;
- development and optimization of a photoreactor in which the CO2 reduction could be performed either in solution or in gas phase.

The development of very active materials has comprised the use of different preparation techniques. Although during this stage a high number of photocatalysts were prepared and tested in the conversion of CO2 into useful compounds, the bimetallic strategy appears to be the best choice to obtain the most active material for this reaction. Since the bimetallic nanoparticles (NPs) acting as co-catalysts possess new different optical, electronic and catalytic properties comparing with those of their monometallic counterparts, then the combination of noble-metal bimetallic NPs, like Au and Cu, with TiO2 photocatalyst will boost the activity and selectivity in the carbon dioxide reduction with water.
To investigate the role of Au-Cu bimetallic NPs in the photocatalytic system, a series of (Au, Cu)-TiO2 catalysts, containing a total metallic amount of 1.5% in weight and Au/Cu ratios varying from 1:2 to 2:1, were synthesized and compared to pure 1.5% in weight Au and Cu-loaded TiO2 reference samples in the reduction of CO2. Au and Cu NPs were loaded on Evonik P25 TiO2 by using a stepwise deposition-precipitation strategy. The characterization of the resulted materials has comprised the use of different spectroscopic methods (DR-UV-Vis, in situ FTIR, TAS, Auger and XPS) or a surface analysis technique, like HRTEM. More details about the preparation and characterization procedures can be found in the already published J. Am. Chem. Soc. article. Since the photocatalytic process take place only at the surface of a solid photocatalytic material, the irradiation of CO2/H2O mixture was carried out in the presence of the thin layer of the photocatalyst deposited on 1.25 cm2 quartz plates placed in the center of an aluminium tray located inside a new constructed photoreactor perpendicularly to the light beam. During the irradiation period, the gaseous samples in the reactor were taken at desired intervals by coupling one of the photoreactor valves to a gas chromatograph.
To investigate the CO2 reduction kinetics and to propose a possible mechanism, two separate spectroscopic techniques have been used, i.e. transients absorption spectroscopy to find the relationship between the presence of Cu and the selectivity to CH4, and in situ FTIR spectroscopy to deliver information regarding the activation of CO2, reaction intermediates formation and disappearance. Based on the obtained results a reaction mechanism has been proposed, the overall schematic mechanism of the gas-phase CO2 photoreduction with H2O over (Au, Cu)/TiO2 materials apparently following the so-called “carbene pathway”. The obtained data show that Au and Cu-loaded TiO2 photocatalyst is an efficient material for the solar-light reduction of CO2 to CH4, with H2O as reducing agent exhibiting a CH4 production rate of 2.2 mmols×g-1×h-1 that is among the highest ever reported. Under optimal conditions selectivity toward the reduction of CO2 of 97% was observed.
These results might open new opportunities in the preparation of very selective materials for photocatalytic production of methane at very high conversion based on the combination in the adequate properties of two or more metals acting as co-catalysts of TiO2 semiconductor.

The project results have already been publicly disclosed at the 247th American Chemical Society Meeting & Exposition on 19 March 2014 in Dallas, Texas (oral presentation), and further will be presented to the scientific community in other articles which presently are under the final step of preparation or it have been submitted. Until today, in the framework of the presented project, two articles (S. Neatu, M. Puche, V. Fornes, H. Garcia, Chem. Commun. 2014, 50, 14643-14646; S. Neatu, J. A. Macia-Agullo, P. Conception, H. Garcia, J. Am. Chem. Soc. 2014, 136, 15969–15976), one review (S. Neatu, J. A. Macia-Agullo, H. Garcia, Int. J. Mol. Sci. 2014, 15, 5246–5262) and one book chapter comprising general considerations regarding solar light photocatalytic CO2 reduction (Chapter 14. Photocatalytic CO2 reduction, A. Primo, S. Neatu, H. Garcia in Advanced materials for clean energy (Eds.: Qiang Xu, Tetsuhiko Kobayashi), CRC Press Francis & Taylor Group, 2015, ISBN: 9781482205787, pp. 321–344) have been published.