Читать книгу High-Performance Materials from Bio-based Feedstocks - Группа авторов - Страница 68

A further aspect of Starbon catalysis relates to the photocatalytic decomposition of water pollutants. Colmenares and colleagues have recently published work [32] that illustrates the excellent activity of Starbon‐based titanium dioxide photocatalysts for the total oxidation of phenol, one of the more toxic of water‐borne pollutants.

Figure 3.10 Preparation of a chiral bis‐oxalolidinone catalyst via surface bromination/displacement.

Figure 3.11 Selective acylation of a diol by a supported Cu‐bis‐oxalodinone/Starbon catalyst.

Part of the rationale for choosing Starbon as a support for the titania catalyst is that the TiO₂ particles need to be nanoscale in order to have sufficient surface area to function efficiently. This raises two issues. First, separation of the nanoparticles from water is a major challenge, and second, agglomeration of the nanoparticles is a significant issue, which reduces the activity and efficiency of the catalytic system.

In their work, Colmenares et al. [32] utilised ultrasound‐assisted formation and deposition of titania onto thermally pre‐treated Starbon, avoiding the use of surfactants, and adding to the green credentials of the process. The materials were then dried and re‐calcined to produce a hybrid material containing 25 wt% of TiO₂. The authors suggest that carboxylic groups on the surface act as nucleation sites for the titania particles, which is further discussed in a second paper [33]. Interestingly, only the anatase form of titania was formed on Starbon, whereas similar amounts of anatase and rutile were produced on Norit, a standard activated carbon. This is important because anatase is the catalytically active form. The crystallinity was also found to be excellent on Starbon, meaning few defects are present – defects reduce the efficiency of the photocatalytic process by facilitating electron‐hole recombination. Graphene oxide seemed to produce almost no discernible crystalline material. Titania particle sizes for Starbon‐ and Norit‐derived materials were largely similar and in the range 20–30 nm. Loading of the inorganic phase resulted in a reduction in surface area by a factor of 1.6 for Starbon, and 1.3 for graphene oxide, but the microporous Norit suffered a sevenfold decrease in surface area.

Photochemical mineralisation of phenol was carried out using 50 ppm solutions of phenol in water, a Hg lamp (365 nm) and without bubbling of air through the solution (i.e. passive diffusion of oxygen into the solution). The Starbon materials had substantially greater activity than either Norit or graphene oxide‐derived materials, with the mineralisation rate constant being approximately 3 times higher for Starbon‐titania than for either of the other two systems. This was ascribed mainly to the pure, highly crystalline anatase phase formed on Starbon, as well as the mesoporosity of the system that allowed much better transport of phenol. Runs utilising Starbon itself also indicated that Starbon’s ability to adsorb phenol from the aqueous solutions was very good, meaning that it can pick up phenol from water and deliver it to the catalytic sites efficiently. Physical mixtures of Starbon and commercial anatase (Evonik P25) likewise showed excellent results, having approximately 50% higher rate constant than the ultrasound synthesised materials. Rate curves for the various systems indicated that Starbon‐based systems reduced phenol levels to 0.3% of the initial (i.e. to 0.3 ppm, well below the 1 ppm target) compared to 0.5 ppm for Norit and 0.7 ppm for graphene oxide. Not only were the final levels lower, but the rate of removal was faster as well.