Pd Nanoparticles Decorated 3D Printed Hierarchically Porous TiO₂ Structure For Catalytic Reduction Of Highly Concentrated 4-Nitrophenol Solution

With the rapid development of economy,the consumption of chemicals is increasing year by year,which produces huge organic pollutants,which causes serious pollution to water resources and endangers the ecological environment and human health.In the numerous pollutants,one of aromatic nitro compound,4-Nitrophenol?4-NP?,produced by the dyes,pesticides and pharmaceutical industries has become one of the main pollutants due to its high toxicity,non-biodegradability,and large discharge characteristics.How to efficiently eliminate the 4-NP in industrial wastewater is a significant environmental issue,and has attracted great attentions all over the word.Among the 4-NP removal technologies,the catalytic reduction of 4-NP using noble metal nanoparticles as catalysts has been recognized as a promising and sustainable method to remove the 4-NP in industrial wastewater.However,the agglomeration of the noble metal nanoparticles always results in the poor catalytic activity and stability.To address this problem,the immobilization of noble metal nanoparticles onto various carrer materials have been investigated extensively.This method not only solves the problem of poor stability of catalyst,but also improves the catalytic activity through the synergistic reaction of noble metal nanoparticles and the carrier materials.So fat,the reported composite catalysts are mostly limited to the treatment of low concentration?<1 mM,?0.139 g·L-1?4-NP wastewater which is far from meeting the requirements of direct industrial wastewater treatment.Moreover,the composite catalysts are generally in powder state and difficult to be separated from the wastewater,causing undesirable recyclability and secondary contamination.In view of the above problems,this work focuses on the design and preparation of three-dimensional catalyst carrier with both microscopic and macroscopic pore structures to support noble metal nano catalysts.On one hand,the abundant microscopic pores provide adequate sites for the catalyst loading;on the other hand,while the macroscopic interconnected channels are beneficial to the mass transfer during the catalytic reaction.The rapid development of 3D printing provides a powerful tool for the fast construction of complex three-dimensional structures with controllable macro-and micro-structures.In this work,we developed an extrusion-based 3D printing strategy to directly construct TiO₂ 3D scaffolds with porous structure in multiscale,and to function as a noble metal nanocrystal carrier.We have analyzed and characterized the morphology and structure of the composite catalyst,verified the catalytic reduction performance of 4-NP in high concentration industrial wastewater and given an explanation of the corresponding mechanism.The research results are presented as follows:?1?In order to solve the problem of poor recovery ability of the catalyst prepared by powder carrier,we made use of the advantage of 3D printing,two kinds of industrial-grade TiO₂ powders were used to prepare TiO₂ scaffold carrier.This method can improve the disadvantages of titanium salt precursor used in traditional printing,such as complex synthesis process,little output,toxicity and high price,and the TiO₂ scaffold with both macro and micro pore structure can be printed in an air atmosphere by adjusting the ratio of two kinds of TiO₂,the rheological properties,and the printing parameters.The 3D TiO₂ scaffold with certain mechanical strength and hierarchically porous structure is expected to be an ideal catalyst carrier.?2?Pd nanoparticles were grown on the TiO₂ scaffold with hierarchically porous structure by hydrothermal method for catalytic reduction of high concentration 4-NP solution.Through detailed characterization of the morphology and loading of Pd nanoparticles,the effects of modification of hydroxyl and carboxyl groups on the surface of TiO₂ scaffold and hydrothermal growth process on the growth of Pd nanoparticles and their effects on catalytic performance were discussed.?3?With the flexibility of 3D printing structure design,we printed different layers TiO₂ scaffold and loaded Pd nanoparticles respectively and explored the effect of shape and size of the 3D printing structure on catalytic reduction of high concentration of 4-NP solution.The experimental results show that the reduction performance of 4-NP is different with different layers of composite catalyst scaffold and the catalytic performance increases nonlinearly with the increase of layers.The turnover frequency of hierarchically porous Pd/TiO₂-scaffold?16 layers?is up to 2.69 min-1,which is 1063 times higher than that of Pd/TiO₂-bulk material with the same size?0.00253 min-1?.The synergistic effect of Pd nanoparticles and TiO₂ scaffold carrier on 4-NP catalytic reduction ability was also studied.The 3D Pd/TiO₂ composite catalyst structure prepared in this study helps to promote the practical application of noble metal nanoparticles for the catalytic reduction of 4-NP industrial wastewater.