Understanding the electron-accepting sites on the surface of cage zirconium phosphates of nasicon type doped with cobalt, nickel and copper ions

This paper looks at the synthesis of new type of catalysts — solid electrolytes with cation-type sodium ion conductivity that fall within the NASICON family. The paper also examines their surface adsorption properties. Complex sodium zirconium phosphates doped with ions of 3d metals — i. e. cobalt (II), nickel (II) and copper (II), were synthesized by sol gel method. Sodium ions are substituted with doping ions in the cationic part of the structure. Detailed analysis of the physico-chemical properties of the synthesized structures has been carried out that involved X-ray diffraction analysis, scanning electron-ion spectroscopy, scanning microscopy and thermal desorption of nitrogen. It was found that the composition and structural parameters of the synthesized phosphates conform with the literature. The study looked at the surface electron-accepting properties of the sodium zirconium phosphates of the following composition: Na(1 – 2х) MхZr2(PO4)3 withх = 0.125 and 0.25, whereM2+M — Co2+, Ni2+, Cu2+. The study procedure involved spectrophotometric method based on adsorption of pyridine and spectrokinetic method based on the indicator test showing how fast the blue aniline colour forms from the adsorbed paraphenylenediamine. Several acid sites have been identified on the surface and in the micropores of the phosphates. It was established that the nature and number of such sites are governed by the nature and amount of the introduced doping ions. It is shown that the colour generation rate tends to increase in the row Ni ~ Co < Cu, which has a qualitative correlation with the growth of ion radii. It is demonstrated that the ions of 3dd metals introduced in the NASICON structure enable to control the acidic properties of their surface and obtain catalysts with predefined properties. This research was funded by the Russian Foundation for Basic Research under the Project No. 18-33-20101. This publication was prepared with support from the Programme “5-100” implemented by Peoples’ Friendship University of Russia. © 2019, "Ore and Metals" Publishing house. All rights reserved.