Effect of reactor irradiation on adsorption properties of REM oxides

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0969"806X(95)

Radiat. Phys. Chem. Vol. 46, No. 4-6, pp. 537-539, 1995 Copyright © 1995 Elsevier Science Ltd 00212°x Printed in Great Britain. All rights reserved 0969-806X/95 $9.50 + 0.00

EFFECT OF REACTOR IRRADIATION ON ADSORPTION PROPERTIES OF REM OXIDES T. I. AKSENOVA and D. K. DAUKEEV

Institute of Atomic Energy, National Nuclear Centre, Almaty, 480082

ABSTRACT It is determined that irradiation in reactor at neutron fluence significantly changes the adsorption properties of REM oxides.The oxides of the metals, having anomalous properties (variable valency, high cross-section capture) revealed espesial adsorption properties after irradiation. KEYWORDS Adsorption, rare earth metal, reactor irradiation. Rare earth metal (REM) oxides are widely spreaded in technological applications because they have high catalitic activity in transformation reactions of many organic molecules. This is due to pecularities of their surface states, that is reflected on corresponding irregularities of gas adsorption. The last is very sensitive to irradiation. In this paper the results of comparative investigation of adsorption curves versus REM order number (La, Ce, Nd, Pr, Gd) and dose of irradiation are presented. It is discovered that the original REM oxides have high adsorption capacity both for donor gases and for acceptor ones.The adsorption capacity for oxygen and hydrogen becomes lower when the REM order number is higher, but it is contrary for water molecules. (Ermatov et al.; 1984). The study of initial REM oxides by infrared- and thermodesorption methods showed that their surface has been covered by adsorbed molecules of water and carbon dioxide (fig.l); the form of adsorption and durability of links are distinguished for different oxides. Thus in IR spectrum of Y203 the molecular adsorbed water (adsorbed band 3240 cm -I) is observed while on surface of N d 2 0 3 the hydroxyl groups are presented ( 3640 cm'l). The molecular water is removed at slow temperature of heating ( 200-300 °C) with the exception of CeO 2 where Tde s more lower ( l l 0 ° C ) . Thermodesorption analysis of oxides surface revealed that ~'elative quantity of water is minimum for Y203 and CeO 2 and maximum - for Gd20 3. Characteristic peculiarity of REM oxides is presence of the chemical activity as regards CO and CO 2. Therefore the carbonate structures are presented in different forms on Ln20 3 surface. More widespreaded form of carbonate structure is monodentat carbonate, which is corresponded by appearence in IR spectrum of absorption bands 1070,1420,1540 cm-I (Evans et al., 1967). Such adsorption form is typical for Y, La, Nd, Gd, Pr, with the exeption of Ce. There are the small adsorption bands 1060, 1120 1170 cm "1 on CeO 2 and the large absorption near 1 ! 00cm" 1.

Desorption of carbon dioxide is accompanied by appearence of one peak in temperature interval 400-600°C for all oxides; as for CeO 2 there are three peaks of CO 2 desorption (100,320,840oc). 537

538

T. I. Aksenova and D. K. Daukeev

1

I

I

i

2800

I,

I

3200

~6QQ

I

i .

g era

-/

Fig. 1. IR spectrum initial R E M oxides.

It is shown that in case of CeO 2 there are both more weak linked H 2 0 and CO 2 and more durability linked CO2.(Fig.2 ).

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~q

0

~00

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Fig.2. Desorption of water (1) and carbon dioxide (2) molecules from R E M oxides during linear heating.

Analysis of thermodesorption date showed that adsorbed CO 2 has high thermostability and they are always located on a surface. The regime of vacuum treatment dosn't permit to remove CO 2 completely; part of CO 2 molecules are remaining in a strong linked form. The results of research of initial R E M oxides are obliged us to take into consideration the real state of oxides surface under research of neutron irradiation effect on their properties.

9th International Meeting on Radiation Processing

539

Irradiation in reactor at neutron fluence 0,2-1,2 10 18 n/m 2 significantly changes the adsorption properties of REM oxides; dose dependences of adsorption centres quantity N(D) being different for donor and acceptor gases For hydrogen the N(D) changes unmonotonously and maximum N(D) shifts from dose 3.1017 n/m 2 for CeO 2 to 1.10 17 n/m2for La203. Pr40 7 has revealed contrary irregularity. For aceeptor gas (O2~ the quantity of adsorption centres depends on neutron dose lineary in the interval 0,2-2.10 t a rdm 2. Irradiation at fluences more then 2,4.1018 rdm 2 causes the intensive desorption from surface, so that the pressure in quartz cell increases. The gadolinium oxide is the exlusion since there has been the fast growth of N(D) for fluences higher then 1.1017 n/m 2. It is obviosly due to that gadolinium has big cross-section capture of thermal neutrons and so there is an additional internal source of gamma irradiation in this case (Fig. 3).

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le "

Zs

led

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!

o

qa

/,e

/,g

I

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0

/,2"/(7

Fig. 3. Dose dependenses of absorption centers quantity for neutron irradiated REM oxides.

The study of desorption processes from irradiated surface at linear heating gives an opportunity to define the activation energy of desorption and other characteristics of the process for different oxides. The activation energy ofdesorption is defined to be 7-10 kkal/mol. Therefore one can consider that the chemosorption bond between adsorbate and surface takes place. The obtained results showed that adsorption properties of REM oxides were changing on identical laws. But oxides of the metals, having anomalous properties (variable valency, high cross- section capture) revealed espesial adsorption properties under irradiation. In the time of consideration of possible mechanism of radiation - stimulated gas adsorption on oxide surface it is necessary to take in attention the formation of radiation defects both in crystal lattice and in carbonate structures as well. REFERENCES Ermatov S.E., T.I. Aksenova and A. Tuleuova (1984). Effect of gamma-irradiation on adsorption properties of REM oxides and yttrium. Journal Phys. Chem. USSR, 58, pp. 1135-1138. Evans J.W. and T.L. Whately (1967). IR-spectroscopic study of adsorption CO 2 and H 2 0 on MgO. Trans. Farad. Soc., 63, pp.2769-2774.