Cobalt(II) tetrasulfophthalocyanine on titanium dioxide: A new efficient electron relay for the photocatalytic formation and depletion of hydrogen peroxide in aqueous suspensions

Abstract

A novel synthesis for the covalent linkage of cobalt(II) tetrasulfophthalocyanine (Co^(II)TSP) to the surface of titanium dioxide (TiO_2) particles (d ≤ 0.5 µm) is described. Upon irradiation with light that exceeds the bandgap energy (E_g) of TiO_2 (i.e., ⋋ 5 380 nm), Co^(II)TSP is reduced to Co^ITSP under anoxic conditions both as a dry powder and in aqueous suspension. The photochemical reduction is shown to be fully reversible in the presence of molecular oxygen (O_2). Hydrogen peroxide (H_2O_2) is produced upon irradiation of an aerated aqueous suspension of the 'hybrid" catalyst, TiO__2-Co^(II)TSP. Formation kinetics are followed in situ with a polarographic detector (detection limit ≃ 10^(7-) M H_2O_2); quantum yields, ϕ_(H_2O_2) between 0.16 and 0.49 have been determined. The reactive photocatalytic center appears to be generated by the attachment of molecular oxygen in the open apical coordination site of the hybrid Co^(II)TSP complex. Formation of Co^(II)TSP-O_2^(•-) is enhanced by the binding of TiO^- surface groups in the opposite apical position. Hydrogen peroxide is produced in a two-step electron transfer from the conduction band via the Co(III) center. The involvement of free radical intermediates in this mechanism appears to be highly unlikely. On the basis of its observed chemical and photochemical stability and the high quantum yields for O_2 reduction, the newly developed hybrid material is proposed to be applicable as a potent and stable oxidation catalyst.

Additional details