All-carbon microporous graphitic photocatalyst-promoted reduction of CO₂ to CO in the absence of metals or dopant elements

Microporous graphitic carbon (mp-C) derived from the pyrolysis of α-, β-, and γ-cyclodextrins exhibited photocatalytic activity in CO₂-saturated acetonitrile-water upon irradiation with UV-Vis light and in the presence of triethanolamine, forming H₂ (19 μmol h^-1) and CO (23 μmol h^-1) accompanied by a lesser proportion of CH₄ (4 μmol h^-1). The most efficient was the mp-C material derived from α-cyclodextrin (mp-C_α) and having a pore dimension of 0.68 nm. The process also occured, although to a much lesser extent, under simulated sunlight or with UV-Vis irradiation in the absence of a sacrificial agent, with H₂O being the electron donor. The origin of the CO was proved by isotopic ¹³C labelling experiments. Photocurrent measurements proved the occurrence of charge separation and the increase in photocurrent intensity in the presence of CO₂. Transient absorption spectroscopy was used to detect the charge separate state decay in the microsecond time scale and proved that a fraction of the photogenerated electrons were able to react with CO₂.