Efficiency enhancement of the electrocatalytic reduction of CO 2 : fac-[Re(v-bpy)(CO) 3 Cl] electropolymerized onto mesoporous TiO 2 electrodes Francesca Cecchet a, * , Monica Alebbi b , Carlo Alberto Bignozzi b , Francesco Paolucci a a Dipartimento di Chimica ‘‘G. Ciamician’’, Universita ` degli studi di Bologna, via F. Selmi 2, 40126 Bologna, Italy b Dipartimento di Chimica, Universita ` di Ferrara, via L. Borsari 46, 44100 Ferrara, Italy Received 6 January 2006; accepted 19 April 2006 Available online 3 May 2006 Abstract As the greenhouse effect increases, the development of systems able to convert with high efficiency CO 2 to energetically rich molecules owns a crucial weight in the technological and environmental domain. As catalyst, rhenium complexes, of the type fac-[Re(L)(CO) 3 Cl] (i.e. L = 2,2 0 -bipyridyl or 4,4 0 -bipyridyl), have attracted a large interest demonstrating promising catalytic properties. fac-[Re(v-bpy)- (CO) 3 Cl]-based polymer deposited onto a solid support has been already investigated as heterogeneous catalyst in the reduction of CO 2 . Here, we deposited by electrochemical polymerization fac-[Re(v-bpy)(CO) 3 Cl] onto a nanocrystalline TiO 2 film on glass and we investigated by cyclic voltammetry the properties of such heterogeneous catalyst in the electrochemical reduction of CO 2 . We demon- strated that the nanoporous nature of the substrate allows to increase the two-dimensional number of redox sites per surface area and hence to get a significant enhancement of the catalytic yield. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Metal transition complexes; Nanoporous titanium dioxide (TiO 2 ); Carbon dioxide (CO 2 ) reduction; Heterogeneous catalyst; Cyclic voltam- metry (CV) 1. Introduction The development of molecular systems for the conver- sion of CO 2 to energetically rich molecules, such as CO, represents an important technological and environmental challenge due to the abundance of CO 2 in the biosphere that contributes significantly to the greenhouse effect [1]. A wide range of transition metal complexes have been stud- ied for their photo- and electrocatalytic properties in the reduction of CO 2 [2–6]. In particular, rhenium complexes, of the type fac-[Re(L)(CO) 3 Cl] (where L is a bidentate ligand such as 2,2 0 -bipyridyl or two monodentate ligands such as 4,4 0 -bipyridyl), have attracted a large interest due to their promising catalytic properties [7–17]. The successful results obtained in solution encouraged to develop methods to anchor the rhenium-based catalyst onto semiconducting or conducting supports, with the pur- pose of creating a heterogeneous catalyst. The most inves- tigated pathways to immobilize the above catalyst onto a solid support consider either the insertion of the molecule in a polymer matrix [9,18] or the chemical modification of the molecule with a function able to polymerize [10– 14]. The latter strategy leads to the formation of a so-called redox polymer. Onto a surface like in solution, the catalytic mechanism proceeds via the co-ordination of a CO 2 molecule by a rhe- nium atom followed by the reduction of CO 2 to CO [12– 17]. Therefore, the catalytic yield per unitary area is strictly related to the two-dimensional number of redox sites per surface area. By increasing the two-dimensional density of redox sites, we can therefore improve the catalytic yield. 0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.04.037 * Corresponding author. E-mail address: francesca.cecchet@fundp.ac.be (F. Cecchet). www.elsevier.com/locate/ica Inorganica Chimica Acta 359 (2006) 3871–3874