Carbon Dioxide Reduction on Photocatalytic Nanomaterials

Joint research project

Project leaders: Nicola Armaroli, Ezequiel Wolcan
Agreement: ARGENTINA - CONICET - Consejo Nacional de Investigaciones Científicas y Técnicas
Call: CNR/CONICET 2015-2016
Department: Chemical sciences and materials technology
Thematic area: Chemical sciences and materials technology
Status of the project: New

Research proposal

The massive combustion of fossil fuels over the last century has caused the quickest increase in the atmospheric concentration of carbon dioxide ever experienced in the history of Earth.[1] Global CO2 production is already beyond the ability of the biosphere to process it by photosynthesis, hence the gas accumulates in the atmosphere, increases the greenhouse effect with respect to its natural level, and alters the Earth's climate.[2] An active response to this global problem is to capture CO2 at large stationary sources (e.g. power plants) and chemically convert it into fuels or basic C1 chemicals, a dramatically growing field of research that is the core of the present proposal.[3-8] Obviously, this conversion must be made by using non-fossil energy sources to achieve a net reduction of atmospheric carbon dioxide. Interest in the chemical fixation of CO2 has been stimulated in recent years by the idea of transforming this abundant source of carbon into fuels and/or chemical feedstocks.[9,10] Several reduction reactions of CO2 are thermodynamically viable, since the energy required per electron transferred is quite low.[11] For example, the process described below has a redox potential of -0.52 V vs. NHE at pH 7 in aqueous solution:
CO2 + 2H+ + 2e ---> CO + H2O
In practice, however, the electrochemical reduction of CO2 occurs at considerably more negative potentials due to the presence of a substantial kinetic barrier (overpotential).[11] Therefore, any feasible system for reducing CO2 requires the use of one or more redox catalysts that mediate this multi-electronic process and provide low energy paths that lead to the desired products. Many catalysts containing metal complexes are capable of mediating the chemical conversion of CO2 to higher energy products, the most successful are based on Ru(II), Re(I), Ni(II), Co(II), and Fe(II).[12,13] When a catalyst such as ReBr(CO)3(2,2'-bipyridine) is irradiated using a solvent saturated with CO2, the main products formed are CO and formic acid.
Various strategies have been implemented toward catalytic CO2 reduction. Among them, the work of Bocarsly[14] stands out for its use of a simple organic electrocatalyst (pyridine) to produce methanol (a 6-electron reduced product) instead of the more common CO and formate (both 2-electron reduced products). It is believed that this six-electron process occurs in a sequentially manner: first the reduction of the pyridinium cation (pyH+) to the neutral radical (pyHo), followed by insertion of CO2 into the NH bond to give a radical carbamate intermediate.[14] MacDonnell et al. recently showed that pyridine and [Ru(phen)3]2+ can catalyze the reduction of CO2 to produce both formate and methanol in an homogeneous photochemical process, using ascorbic acid as the sacrificial donor.[14]
In the present project we target the preparation and investigation of three different types of nano materials to be used as scaffold for metallic photocatalytic centers.

COLLABORATION
The two research groups are leaders in the study of photoactive coordination compounds in supramolecular systems, polymeric structures, and hybrid nanomaterials. The strategies proposed in this project for the photoinduced reduction of CO2 are highly innovative and require the contribution of research groups experts in (a) the synthesis and photochemistry of Re(I) and Ru(II) complexes (CONICET); (b) the preparation and in-depth structural and photophysical characterization of photoactive nanomaterials (CNR). This blend of complementary expertise can be perfectly matched through the close collaboration between the research groups of La Plata and Bologna. As a matter of fact, the CONICET group has extensive experience in the synthesis and photochemical and photophysical characterization of complexes with general formula [Re(CO)3L]+ (both alone and coordinated to polymers),[15-25] whereas the group at CNR is expert in the study of nanomaterials related to energy conversion and conservation, such as those targeted in this project.[26-29] The two laboratories have all the equipment needed to carry out the project.

REFERENCES
[1] N. Armaroli, V. Balzani, Energy for a Sustainable World - From the Oil age to a Sun Powered Future, Wiley-VCH, Weinheim, 2011.
[2] N. Armaroli, V. Balzani, Chem.-Asian J., 2011, 6, 768.
[3] M.E. Boot-Handford et al., Energ. Environ. Sci., 2014, 7, 130.
[4] N. Armaroli, V. Balzani, ChemSusChem, 2011, 4, 21.
[5] M. Cokoja et al., Angew. Chem. Int. Ed., 2011, 50, 8510.
[6] A.J. Hunt et al., ChemSusChem, 2010, 3, 306.
[7] M. Mikkelsen et al., Energ. Environ. Sci., 2010, 3, 43.
[8] W. Wang et al., Chem. Soc. Rev., 2011, 40, 3703.
[9] C. Ampelli et al., Energ. Environ. Sci., 2010, 3, 292.
[10] J. Barber, Chem. Soc. Rev., 2009, 38, 185.
[11] B. Kumar et al., Annu. Rev. Phys. Chem., 2012, 63, 541.
[12] E. Fujita, Coord. Chem. Rev., 1999, 185, 373.
[13] J. Agarwal et al., J. Am. Chem. Soc., 2012, 134, 5180.
[14] D.J. Boston et al., Inorg. Chem., 2014, 53, 6544.
[15] L.L.B. Bracco et al., J. Photochem. Photobiol. A, 2010, 210, 23.
[16] E. Wolcan et al., J. Phys. Chem. B, 2005, 109, 22890.
[17] E. Wolcan, M.R. Féliz, Photochem. Photobiol. Sci., 2003, 2, 412.
[18] E. Wolcan et al., Inorg. Chem., 2006, 45, 6666.
[19] E. Wolcan, G. Ferraudi, J. Phys. Chem. A, 2000, 104, 9285.
[20] E. Wolcan et al., Supramol. Chem., 2003, 15, 143.
[21] G.T. Ruiz et al., Dalton Trans., 2007, 2020.
[22] E. Wolcan et al., J. Chem. Soc., Dalton Trans., 2002, 2194.
[23] H.H. Martinez Saavedra et al., J. Organomet. Chem., 2013, 745-746, 470.
[24] F. Ragone et al., J. Phys. Chem. A, 2013, 117, 4428.
[25] E. Wolcan, Spectrochim. Acta, Part A, 2014, 129, 173.
[26] R.D. Costa et al., Angew. Chem. Int. Ed., 2012, 51, 8178.
[27] N. Armaroli, V. Balzani, Energ. Environ. Sci., 2011, 4, 3193.
[28] V.K. Praveen et al., Angew. Chem. Int. Ed., 2014, 53, 365.
[29] V.K. Praveen et al., Chem. Soc. Rev., 2014, 43, 4222.

Research goals

The key goal of this project is the preparation of new nanomaterials onto which we will graft photoactive metal centers to be used as photocatalysts in the reduction of CO2. Two of the systems to be investigated, (i) and (iii) below, share the common feature of having accessible pyridine sites near the photochemical center that will initiate the catalysis. We expect them to work even more effectively than the simple system [Ru(phen)3]2+/pyridine due to the controlled accumulation of catalytic sites onto the nanomaterials.
The photochemical and photophysical properties of these new materials will be investigated in detail in order to elucidate the reaction mechanisms involved in the reduction process. In particular, we plan to apply the following three systems to study the photoinduced reduction of CO2:
(i) Hybrid organic-inorganic polymers based on poly-4-vinylpyridine and poly-2-vinylpyridine and containing coordination complexes of Re(I) and/or Ru(II) in their structures.
(ii) Supramolecular nanostructures containing covalently functionalized carbon nanotubes (CNT) externally decorated with Re(I) complexes.
(iii) Modified SiO2 nanoparticles containing coordinated [Re(I)(CO)3(bpy)]+ groups.
The variety of materials we target - hybrid or fully inorganic - characterized by different intrinsic physical properties, will enable catalytic exploitation in different environments and under different conditions.

Last update: 19/05/2025