Study On Novel TiO₂ Photocatalysts Used For NADH Regeneration

As many redox-enzymes depend on the common NADH-cofactor and the factor isusually expensive, research interest in regeneration of NADH continues to grow veryrapidly and various methods have been invesitgated. Among them photochemicalmethod captures more and more interest in recent years because of the efficientutilization of cheap and clean solar energy. Herein we studied the preparation of novelTiO2 photocatalysts and used them in the photoregeneration of NADH under visiblelight.Carbon nanotube doped TiO2 could be excited by visible light and showed highphotocatalytic activity in the regeneration of NADH. Graphite doped TiO2 couldabsorb about 80% visible light, but the negligible photoreaction could be found undervisible light. This could be attributed to the fact that carbon nanotube with exceptionalsemiconductor properties could accept photoelectron as the electron trap center. Onthe other hand, there was a substantial interaction between TiO2 and carbon nanotube.The exsitence of carbon nanotube enhanced the binding energy of Ti and O to 461.00eV and 532.25 eV respectively. While the binding energy of graphite doped TiO2remained unchanged and the photoenergy was not transformed to chemical energy.Compared with organic-carbon containing TiO2, it was found that the photocatalyticactivity of TiO2 samples increased with the increase of their binding energy and theshifting of binding energy was contributed to the presence of C=O bond.Zinc meso-tetra porphyrin (ZnP) modified and carbon nanotube doped TiO2(ZnP-CN/TiO2) had strong absorption towards visible light in the range of 400nm to1100nm. The ZnP on TiO2 surface facilliated the absorption of visible light, especiallyin the 400nm~700nm region while the presence of carbon nanotube reinforced theabsorption of light in the 700nm~1100nm region. XPS results indicated that theinfluence of carbon nanotube on binding energy was much greater than that of ZnP.The binding energy of Ti and O in ZnP-CN/TiO2 was shifted to 461.25eV and532.48eV respectively. In the photoregeneration of NADH, ZnP-CN/TiO2 exhibitedhigh photocatalytic activity and the conversion achieved 77.3% at 10 h.Lower pH value and higher temperature resulted in higher conversion and reactionrate. The influence of [Cp*Rh(bpy)(H2O)]2+ initial concentration on thephotochemical regeneration of NADH was much greater than that of NAD+ initialconcentration. The optimum condition for photoregeneration of NADH wasdetermined as follows: pH6.0, 37℃, NAD+ initial concentration 0.2mM,[Cp*Rh(bpy)(H2O)]2+ initial concentration 0.3mM~0.5mM. The conversion usingdifferent electron donors was in the order of mercaptoethanol> EDTA> ascorbic acid>format sodium> H2O. The oxygen in atmosphere inhibited the reduction of NAD+ atTiO2 surface while the reductive H2 in photosystem improved the formation ofNADH . In presence of H2, the conversion could reach 94.29% using mercaptoethanolas electron donor at 31℃, pH7.0.