Photocatalytic Reduction of Fumarate to Succinate on ZnS Mineral Surfaces

The reductive tricarboxylic acid (rTCA) cycle is an important central biosynthetic pathway that fixes CO₂ into carboxylic acids. Among the five reductive steps in the rTCA cycle, the two-electron reduction of fumarate to succinate proceeds nonenzymatically on the surface of photoexcited sphalerite (ZnS) colloids suspended in water. This model reaction is chosen to systematically study the surface photoprocess occurring on ZnS in the presence of [Na₂S] (1-10 mM) hole scavenger at 15°C. Experiments at variable pH (5-10) indicate that monodissociated fumaric acid is the primary electron acceptor forming the monoprotic form of succinic acid. The following reaction scheme is proposed: (1) photoexcitation of ZnS generates conduction band electrons and valence band holes, (2) the hole scavenger donates electrons while producing sulfur-containing intermediates en route to sulfate formation, (3) a first electron transfer occurs at the conduction band converting chemisorbed monoprotic fumaric acid at surface zinc sites into an adsorb radical anion, and (4) the radical anion accepts a second electron and forms an adsorbed carbanion, which (5) abstracts two protons consecutively from either hydronium ion (acidic condition) or water (neutral and basic condition) to be desorbed as monodissociated succinic acid. The apparent quantum yield measurement of succinate production (Pdbl_s) under periodic irradiation at λ ≥ 305 nm shows that the time scale of electron transfer on the conduction band (t₁) and valence band hole loss (t₂) are in the order of hundred microseconds and a few milliseconds, respectively. These transitions (t₁ and t₂) become undistinguishable at 520 μs for a zeta potential ζ = -22.09 mV corresponding to [Na₂S] = 0.57 mM. Overall, this work provides new insights to model heterogeneous processes such as the reduction of CO₂ occurring on the surface of photocatalysts and advance present understanding of photocatalytic reactions.