First rare-earth phosphate aerogel: sol–gel synthesis of monolithic ceric hydrogen phosphate aerogel

Abstract

AnnotationSince the late 1960s, ceric hydrogen phosphates have attracted the attention of scientists due to remarkable ion exchange, sorption, proton-conduction and catalytic properties. In this work, through the application of various solvents, we, for the first time, have obtained monolithic aerogels based on ceric hydrogen phosphates with high porosity (~99%) and extremely low density (~10 g/cm 3 ). The composition and structure of aerogels were thoroughly studied with XRD, TEM, SEM, XPS, low temperature nitrogen adsorption methods, TGA/DSC, FTIR and SANS. The aerogels were found to belong to the fibrous macroporous aerogels family. IntroductionAerogels, being highly porous materials with low density and high specific surface area, are gels in which the liquid phase is completely replaced by a gaseous phase [1]. Aerogels are typically used as catalysts, sensors, ion-exchange materials, and as heat and sound insulators [2].They can be produced from a number of simple substances, both inorganic and organic compounds; hybrid organic-inorganic aerogels are also known [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].The most studied SiO 2 -based aerogels, obtained by hydrolysis of silicon alkoxides, with subsequent supercritical drying, consist of isotropic SiO 2 nanoparticles (0D) forming a spatial network [18]. Recently, a significant number of papers have also been published on the production of aerogels containing anisotropic 1D and 2D building blocks, including carbon nanotubes and graphene [19][20][21][22]. Special attention has been paid to monolithic materials having a similar architecture, but containing not only micropores (<2 nm) and mesopores (2-50 nm), but also macropores, since they have a highly accessible surface, which is guaranteed by macroporosity. In addition to the high permeability of such materials, which is important for their application as catalysts and sensors, they can be easily recovered for repetitive use [23].One of the most challenging tasks in the design of new multifunctional materials is the production of aerogels from orthophosphates of transition and rare-earth elements. Such materials could attract a great deal of interest, due to their extended applications in various possible applications, such as ion-exchange for water purification, catalysts, proton conductors, etc. At the same time, the data on the synthesis of such materials is extremely scarce. Thus, methods are reported for the synthesis of aerogels of the compositions Ti 3 (PO ) 4 [24]. Zhu et al. [23] described the synthesis of a monolithic aerogel based on zirconium phosphate, which can be used for the purification of water as a heavy metals sorbent.Studies concerning the preparation of surface-modified, phosphate-containing oxide aerogels have been also reported [25][26][27][28]. In particular, Boyse et al. [25] obtained Nb 2 O 5 -based aerogels containing 5 or 10 mol.% of niobium phosphate, which showed a high level of catalytic activity in the reaction of butene-1 isomerisation.The synthesis of rare-earth ph...

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