Design of 3-dimensionally self-assembled CeO2 hierarchical nanosphere as high efficiency catalysts for toluene oxidation
Introduction
Volatile organic compounds (VOCs) are causing more and more attentions not only for their environmental toxicity but also because they are precursors of ozone and photochemical smog [1], [2]. Toluene, a typical kind of VOCs, are widely used as solvents, gasoline additives and chemical raw materials, hence can pose a high risk to both the environment and human health [3]. Catalytic combustion of toluene into water and CO2 is a very promising method in VOCs abatement, considering its economically available and practically effective. Last decades, multiple catalysts, especially non-noble metal catalysts, have been developed to get a highly catalytic performance and low cost [4], [5].
In recent years, much work has been done to study the catalytic oxidation of toluene [6], [7], [8]. Wang et al. investigated the toluene elimination on ceria, vanadia/ceria and gold/ceria by operando technics, and found that the nature of the active sites, configurational adsorption of toluene and the reactive oxygen species play important roles in the catalyst activity and selectivity [9]. Ren et al. synthesized 3D hierarchical Co3O4 nanocatalysts with various morphologies for catalytic oxidation of toluene, and concluded that the large specific surface area, highly defective structure with abundant surface adsorbed oxygen species and rich high valence Co ions in the Co3O4 sample were responsible for its excellent catalytic performance [5].
Among all of the non-noble metal catalysts, CeO2 exhibited superiority for its abundant oxygen vacancies and excellent redox ability [10], [11]. Engineering the shape of CeO2 at nanoscale actually alter their performances, and it has been attracted enormous interests to modify ceria nanocrystals morphologies over the past years [12], [13], [14], [15], [16]. CeO2 with different morphologies always expose different crystal planes and atomic arrangements. Precise control of morphology may modify the CeO2 surface atomic arrangement, which may in turn influence the oxygen vacancies contents and then the catalytic performance [17], [18], [19], [20]. Li et al. [21] employed shape-controlled CeO2 nanoparticles (cubes, octahedrons and rods) with facets of preferential orientations((1 0 0), (1 1 1), (1 1 0)), and studied their activity and stability in the catalyzed HCl oxidation reaction. They found that both activity and stability of CeO2 are structure-sensitive. Peng et al. [22] investigated Pt supported on different morphologies of CeO2 to catalytic toluene oxidation, and found that CeO2 rod which exposed facet (1 1 0) exhibited best catalytic performance.
Typically, 3-Dimensionally hierarchical structure materials always exhibit superior performance to bulk materials due to their enhanced multiple functionalities [23], [24], [25]. Imagawa et al. [26] synthesized monodisperse CeO2 nanoplates developed from assembled nanoparticles, and found that the formation of this polycrystalline structure is preferable to induce stability by decreasing the surface energy and positive charge of the NPs. Deori et al. [19] designed 3‑Dimensionally self-assembled CeO2 nanocube for alkylarene oxidation, and the as-synthesized catalysts showed breakthrough in catalytic performance and selectivity, owing to higher concentrations of oxygen vacancies and larger surface area. Cui et al. [12] prepared CeO2 nanosphere composed of numerous nanoflakes, and discussed the possible formation mechanism. They found that as-prepared nanosphere exposed more active (2 0 0) and (2 2 0) planes, and exhibited lower T50 and T90 (50% and 90% conversion temperature of CO) of CO oxidation.
But until very recently, the effect of 3-Dimensionally hierarchical structure on CeO2 catalysts in the toluene catalytic combustion behavior was hardly reported in detail. Our previous work has found that compared with bulk CeO2, as-synthesized CeO2 hollow sphere exhibited very excellent toluene catalytic activities for its higher specific surface area and abundant oxygen vacancies [27]. To the best of our knowledge, the effect of 3-Dimensionally hierarchical structure on CeO2 catalysts in the toluene catalytic combustion behavior are still unclear.
Herein, 3-Dimensionally (3D) hierarchical CeO2 nanospheres with different composition were synthesized via hydrothermal methods without the use of templates. The samples are self-assembled by nanoparticle, nanorod and small nanosphere respectively. And their toluene catalytic performances are characterized using various techniques, to correlate the catalytic activities with physical-chemical properties.
Section snippets
Catalyst preparation
All the chemicals were analytical grade and used as received without further purification.
Study of the CeO2 nanosphere formation process
A complete understanding of the CeO2 nanosphere formation and growth process is essential to optimizing the synthesis method and provides guidance for the design of CeO2 nanocrystals. SEM was employed to investigate the growth process of CeO2 nanosphere. Fig. 1 shows the SEM images of samples obtained in hydrothermal process for different time periods.
In the initial stage of hydrothermal process, the products demonstrate an amorphous state. Subsequently, uniform single-crystalline CeO2
Conclusions
In summary, 3D hierarchical CeO2 nanospheres, self-assembled by nanoparticle, nanorod and small nanosphere (named as PS, RS and HS) respectively, were synthesized via hydrothermal methods without the use of templates. The formation processes of CeO2 nanospheres were deeply studied. They primarily exposed (1 1 1), (1 1 0) and (1 0 0) crystal planes, respectively. The HS sample exhibited largest specific surface area and possessed most contents of Ce3+ and oxygen vacancies, which were responsible
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Science and Technology Planning Project of Guangdong Province, the Fundamental Research Funds for the Central Universities, and Natural Science Foundation of Guangdong Province, China (Grant No. 2015B020236002, 2014A030310431 and 2016A030311003), the National Natural Science Foundation of China (No. 51578245, 51678245).
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