Nano- and macro-structured cerium oxide – carbon nanotubes composites for the catalytic ozonation of organic pollutants in water

Abstract A novel methodology for the preparation of nanostructured layers of mechanically processed carbon nanotubes (CNT) on macro-structured supports was investigated as a means to prepare macro-structured composite catalysts consisting of a variety of different ceria – CNT combinations as ozonation catalysts. Two different synthesis routes were used for the preparation of the ceria: precipitation and sol-gel. The composite materials were prepared using different approaches: introduction of ball-milled CNT during the synthesis procedure; ball-milling the pure ceria together with as-received CNT; decoration of ball-milled CNT with well-dispersed ceria through incipient wetness impregnation. The powder composite catalysts were then transferred onto macro-structured supports by using the aforementioned novel methodology for the preparation of slurries suitable to be used in the dip-coating of the macro-structures. A further structured composite catalyst was also prepared using a two-step approach by dipping a previously prepared CNT structured catalyst in a ceria solution. The powder and structured composite catalysts were then tested as ozonation catalysts in semi-batch and continuous experiments. It was found that the different ceria synthesis routes and the composite preparation methods affected both the catalytic performance of the powder composites and also the characteristics of the slurry used for dip-coating of the macro-structured supports. The ball-milling of sol-gel ceria together with CNT resulted in the most active powder catalyst, but the impregnated CNT with well-dispersed ceria was found to be the most active structured catalyst prepared from coating with the powder composites. The two-step structured catalyst prepared by dipping in a ceria-sol was able to achieve the highest removal of oxalic acid (85% at steady-state continuous operation); however, due to the more energy intensive preparation procedure, its use in practical applications might not be the most recommendable.