6-amino-2-mercaptobenzothiazole complexes as photocatalyst in tandem with TiO2 for CO2 reduction to alcohols

Abstract

Metal-organic materials containing 6-amino-2-mercaptobenzothiazole with Co²⁺, Ni²⁺ and Cu²⁺ cations have been tested as cocatalyst alongside TiO₂ for CO₂ photoreduction. The 6-amino-2-mercaptobenzothiazole molecule has many coordination positions and a deprotonable thiol group that facilitates the anchoring of metal centers. Cobalt(II) and nickel(II) compounds (CoAMBTZ and NiAMBTZ) present a crystal structure based on 1D-[M(μ-AMBTZ-кN1,кS1:кN2)]_n coordination polymers. Zinc(II) compound comprises [Zn₄(μ₄-O)(μ-AMBTZ-кN1:кS1)₆] discrete entities. The ligand amine groups facilitate the adsorption of CO₂, and both the metal center and sulfur atoms provide rich redox chemistries. Although neither of these coordination polymers do work alone in the photoreduction of CO₂, NiAMBTZ achieves high alcohol production rates when mixed with nanometric TiO₂. The ratio between the two components of the catalytic system can be tuned to maximize alcohol production, with methanol produced selectively at rates of 790–800 μg g⁻¹ h⁻¹. This is almost four times the value of benchmark photocatalyst TiO₂(3 %CuO) under the same experimental conditions (208 μg g⁻¹ h⁻¹). Studies using XPS to measure the low binding energy region and DRS to estimate the band gap have shown that the VBE and CBE energy values of TiO₂ and NiAMBTZ are appropriate for the photoelectron transfer that activates the CO₂ reduction mechanism. DFT calculations reveal that these transferred photoelectrons are primarily located at the Ni-S bond.