Preparation Of Pt/Al 2 O 3 Catalysts By Sol-Gel Process and Its Application to Uranyl Ion Reduction
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A. DEPTUtA*, W. LADA*, T. OLCZAK* AND A. DI BARTOLOMEO*Institute of Nuclear Chemistry and Technology, Warsaw, Poland **C.R.E. Casaccia, ENEA, Rome, Italy.
ABSTRACT
Various kinds of active alumina supports were obtained in the following steps: (1) preparation of alumina sols by extraction of nitrates with Primene JMT from aluminium nitrate solution, (2) gelation to spherical particles (with diameter < 100 [Lm) by extraction of water from sol emulsion in 2-ethyl -1-hexanol, (3) sometimes conversion of NO3- stabilized gel to OH- form by reaction with ammonia, (4) calcination of gels to y-A120 3 with specific surface area > 100 m2 /g at 5000C . The powders were then impregnated with chloroplatinic acid and recalcined at 50000. Catalysts were also prepared by direct gelation of alumina sol containing hydroplatinic acid followed by steps (3) and (4). Pt/AI2 0 3 catalysts were used for reduction of 1M UO2 (N0 3) 2 - 0.5M HCOOH solution with hydrogen at atmospheric pressure. The course oT the reduction was controlled analytically and by measuring the U(VI)/U(IV) redox potential. It was concluded that the catalysts obtained by impregnation of supports prepared from nitrate stabilized gels exhibit the best activity and stability.
INTRODUCTION
The problem of reduction of uranyl ion in the solution is very interesting from the technological point of view. It is generally known but not fully explained that uranyl ion does not form concentrated colloidal solutions [1,2]. These colloidal uranium solutions, formed readily by U + cation contrary to urany ion, are indispensable in various variants of the sol-gel process of production of spherical grains of reactor fuels containing uranium dioxide or carbides [1-4]. The problem of reduction of uranyl nitrate solution, did not require any special attention when the precipitation- peptization method of U4+ sol preparation was used. In that method uranium (IV)hydroxide was precipitated from the starting solutions and therefore the composition of the feed solutions was not important except for the degree of uranium reduction. It was therefore possible to use stabilizers of U4+ ion (e.g. urea or hydrazine), uranyl solutions of low concentrations (< 0.5 M/l) and an excess of nitric acid. In these conditions -very favourable for the UOZ2+ reduction- the kind of catalyst was not important. Generally platinized alumina in the form of irregular grains 661 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
suspended in the reducing solutions or a fixed catalyst bed were used [1,2,5]. Alumina is widely used as catalyst support because of its physicochemical properties e.g. thermal and chemical stability, high surface area [6]. The problem of the composition of uranyl solutions and catalysts used in reduction became very important when the method of solvent extraction of nitrates was adapted for the preparation of concentrated (>1 M) urania sols (CUSP process [3,4]). First urea and hydrazine were eliminated because the formation of ammonia from those components at elevate
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