Novel Intelligent Particle Design for CO₂ Fixation Using TiO₂ Combined with Biocatalyst

Abstract

A CO₂ fixation system in which a photocatalysis in a light stage is separated from a biocatalysis in a dark stage can be expected to perform highly selective production through the use of light-energy without denaturation of enzyme due to light irradiation.

To realize the concept as particle design, an electron-transfer system which combines semiconductor photocatalyst particles (TiO₂) with two biocatalysts was proposed. In this study, formate dehydrogenase (FDH) was used as a catalyst of CO₂ fixation. Methyl viologen (MV), NAD⁺ and diaphorase were used to mediate electron-transfer from TiO₂ to FDH.

To confirm the feasibility of the electron-transfer system, the kinetics of the photocatalytic reaction side of the system was studied with time course of the reduction of MV²⁺ by TiO₂ particles under UV-irradiation. It was found that MV⁺ production depends on the concentrations of MV²⁺ and TiO₂, and the initial rates can be described by Langmuir-Hinshelwood expression.

Experiments in a batch operation showed that the total system works, and both concentrations of MV and NADH affected the rate of CO₂ fixation. In the new photo-bioreactor system with a ceramic membrane inserted between TiO₂ particles and the two enzymes, it was shown that the CO₂ fixation progressed although the intensity of light is high enough to degrade enzyme's activities. From these results, it was indicated that the proposed system works without denaturation of enzyme due to light irradiation. Next, to immobilize the biocatalytic reaction side of the system, NAD⁺ -bonded gel beads with entrapped FDH and DAH were prepared by allowing droplets of sodium-alginate solution containing FDH, DAH and polymerized NAD⁺　fall into a CaCl₂ solution. Activity of the beads was confirmed as the internal electron-transfer from formate to MV²⁺. Therefore, it was found that the coupled reaction of FDH and DAH was successfully immobilized in the gel beads.