Deactivation Mechanism Of Photocatalytic H₂ Evolution On TiO₂

TiO2, a classic photocatalyst, has always been a hot area of research. The studies about it are extensive and intensive including preparation, doping, dye sensitization, modification and so on. P25, a mix-phase TiO2, is used as a standard for the evolution of other photocatalysts due to its high activity and excellent photostability. The H2 evolution experiment on P25, however, reveals that there is a sharp decrease of the activity after an extended running. The results of repeat experiments show that this deactivation is not an exception. This profile breaks the common knowledge of P25’s stability. In this article, the study of this deactivation will allow for the further understanding of photocatalytic mechanism and the nature of TiO2.The H2 evolution activity on P25 in methanol-water solution decreases significantly after about 6 cycling experiments. In the following two circles, the activity has quickly fallen to one tenth of the original activity. Also in long-term continuous experiment, the deactivation took place after 3 hour’s irradiation as well. All the experiments indicate that P25 indeed suffers from obvious deactivation for H2 evolution.In the photocatalytic cycle, photocatalysts may undergo unexpected physical and chemical change upon the effects of light, (photo)chemical reaction. The electronic state, crystal structure, defect density of P25 may be varied after a long term UV light irradiation and therefore lead to a deactivation. For example, CdS is an attractive material for its response to visible light and a suitable conduction band potential to produce H2, but its photocorrosion is a greatest challenge for its advancements. Generally, the deactivation result from the structure collapse of the photocatalysts, which can be detected by several characterization techniques, is common. In the case of TiO2 the accumulation of Ti species with lower oxidation state may lead to the decreased activity. Apart from the changes of photocatalysts, the sacrificial electron donor of CH3OH applied in the half reaction of H2 evolution is decomposed step by step to form CO2 via the formation of HCHO and HCOOH intermediates, resulting in compositional variation of the reaction solution. It is therefore the decomposition intermediates of CH3OH may also greatly influence the H2 evolution rate, by competitive adsorption with CH3OH on the P25 surface and occupying the H2 production active sites.The causes of deactivation are many and varied. In this study, we checked the changes of suspensions, P25 samples and effects of intermediates, in order to figure out the intrinsic deactivation mechanism of H2 evolution on P25. The results show that the composition (or more accurately, HCHO) change of the solution after a long period of reaction is responsible for the decreased H2 evolution activity on P25. By replacing the reacted solution with fresh aqueous methanol solution and washing deactivated sample with distilled water we have succeeded to recover the H2 evolution activity on P25, which supports our conclusion. In addition, further deactivation investigation by using the anatase TiO2 and rutile TiO2 as the photocatalysts respectively indicate that anatase TiO2 is more tolerant to HCHO and the rutile phase constituent in P25 is responsible for the sharp decrease of the H2 production activity.