Nanostructured metal-based oxides gained considerable attention on the research front due to their practical applications in catalysis [1], biomedical field [2], optoelectronics [3], etc as compared to their counterparts. Substantially, research on the fabrication of nano sized metal oxides having fine shape, uniform morphology, well-defined crystal structure, and phenomenal catalytic properties has become a current hotspot. For the synthesis of metal oxide nanomaterials, one of the two approaches viz. top-down approach and bottom-up approach has been followed. The bottom-up approach provides the assembling of small atoms or molecules by operating physical/chemical forces to get the mesoporous product of high crystallinity [4, 5]. Of various synthetic routes such as templating, combustion, cathodic corrosion, aerogel formation, etc., the template-assisted synthetic strategy is considered one of the most advanced bottom-up approaches for the fabrication of metal oxides having nanometre to micrometer size of the structural units [6]. The template-assisted synthesis is a convenient strategy unresponsive to the synthetic conditions and furnished the controlled morphology, structure, and particle size of the resultant material. Template materials are categorized into the hard template (zeolites, anodic alumina membranes, etc.) and soft templates (ammonium ions, amphophilic surfactants, ionic liquids, biopolymers, etc.). In literature, a variety of nano-sized metal oxides were synthesized using different template materials such as iron oxide nanoparticles using polysaccharide (chitosan, starch, alginate, agarose, dextran, gelatin) [7, 8], CuO nanosheets using Sodium dodecyl sulfate [9], CeO2-ZrO2 mixed oxides using N-Hexadecyl-N,N,N-trimethylammoniumbromide [10], Mn3O4 nanoparticles using 1-n-butyl-3-methylimidazolium chloride and 1-n-butyl-3-methylimidazolium bromide [11], Co3O4 hollow nanostructures using Co-based zeolitic imidazolate framework (ZIF-67) [12], SnO2 nanotubes using anodized aluminium oxide [13], etc.
Cerium, one of the most abundant rare earth metals belonging to lanthanides possesses a strong oxidation/reduction behavior during cycling between + 3 and + 4 ionic states of cerium, by virtue of the presence of ground-state electron in 4f orbital [14]. Pure ceria exhibits a fluorite structure having some defects rely on the intrinsic property i.e., partial pressure of oxygen. The synthesis of nanostructured CeO2 having controlled shape and size, uniform morphology, significant crystallinity with pure phase and desired composition is an indispensable attribute for their performance. The assorted synthetic methodologies such as template-assisted [15–17], flame spray pyrolysis [18, 19], reversed microemulsion method [20], hydrothermal [21–22], wet chemical [23], rapid precipitation [24], co-precipitation [25], solvothermal [26, 27], sonochemical [28], microwave [29], sol-gel [30], etc. have been employed for the synthesis of well-defined CeO2 nanostructures. The curtailment of the size of the ceria to the nanometre range resulted in their high bandgap and significant catalytic properties. Nanostructured CeO2 possesses the plethora of applications such as polishing agents [31], sunscreens [32], solid electrolytes [33], catalysis [34], ultraviolet absorber [35, 36], fuel additives [37], electrochemistry [38], etc. Among the various applications, catalytic performance heavily reliant upon the surface to volume ratio, oxygen storage capacity, particle size, and morphology of the ceria nanostructures [39–41]. CeO2 is a significant redox catalyst and the relatively few literature reports exemplify the remarkable catalytic efficiency of CeO2 nanostructures [42–46].
Xanthan gum, a high molecular weight exo-saccharide biopolymer produced by Xanthomonas campestris bacterium by fermentation of simple sugars. Xanthan gum is composed of pentasaccharide units that constitutes the trisaccharide chain linked with the glucose moiety. The trisaccharide chain consists of two units of mannose (α-mannose and β-mannose) and a single unit of glucuronic acid (Fig. 1). Xanthan gum is a highly stable biopolymer and the high stability is attributed to the formation of a double-helical structure [47]. Xanthan gum is comprised of several functional moieties such as primary and secondary alcohols, ester, carboxylic acid, acetate and pyruvate groups, etc.
Xanthan gum is employed in chemical sensing [48], agriculture [49], wastewater treatment [50], pharmaceuticals [51], etc. owing to their unique properties viz. high viscosity at low concentrations, biodegradability, non-toxicity, biocompatibility, etc. Additionally, xanthan gum is exploited as base fluid [52], capping agent [53], stabilizing agent [54], flocculant [55], polymerization agent [56], for the synthesis of metal oxide nanostructures. Xanthan gum can act as the template by providing favourable sites for the growth of the particles in a controlled manner and prohibited their extensive growth due to the presence of different polar functional moieties such as carboxylic group, ester group, and primary and secondary hydroxyl groups. The polar functionalities of the template interact with the particle surface consequently impact the size and shape of the particle. The particle formation process is remarkably influenced by template via controlling the nucleation and growth of nanoparticles during the oxide crystallization process. In the present study, we report the adequate and economical template-assisted synthetic methodology for the fabrication of CeO2 nanostructures and its catalytic response for the reduction of nitroaromatic compounds. The preparation of nanoparticles has been carried out using a template-assisted sonication synthetic strategy by employing a biopolymer, xanthan gum as a template. The authors do not find any reported data in the literature related to the exploitation of xanthan gum as a template for the synthesis of nanosized metal oxides. We have first time employed and reported the xanthan gum as a template for nanostructured CeO2 synthesis.
The synthesized nanosized CeO2 has the potential to be applied as catalytic support for reduction reactions. Here, we report the utilization of CeO2 nanoparticles for the reduction of two nitroaromatic compounds viz. nitrobenzene and 2-nitrophenol as the nitro compounds are suspected to be carcinogenic and high-risk chemical. Presently, the nitroaromatic compounds are one of the largest and most important groups acting as a reactant or the intermediate for the production of drugs, synthetic dyes, pesticides, etc., and also formed as by-products during chemical reactions on the industrial level [57]. Exposure of a high concentration of nitroaromatic compounds from industrial waste to the water system causes many toxic effects on human as well as aquatic lives [58]. So, the reduction of nitroaromatic compounds viz. nitrobenzene and 2-nitrophenol to the amines is a major issue from the environmental point of view. It was investigated that merely sodium borohydride does not lead to nitro compounds reduction under ordinary reaction conditions [59, 60]. So, the combination of sodium borohydride with a catalytic amount of metal oxides Cu2O/NaBH4 [61], titania-supported gold catalyst/NaBH4 [62], Co3O4/NaBH4 [63], CuO/γ-Al2O3/NaBH4 [64], reduced graphene oxide-ZnO/NaBH4 [65], multiple Au cores in CeO2/NaBH4 [42], etc. in the protic or aqueous solvent system has been adopted results to the reduction reactions.