On the behaviour of hydrous ceria as an ion exchanger: Surface properties, structural features, capacity and apparent pk values

Colloids and Surfaces, 49 (1990) 211-218 Elsevier Science Publishers B.V., Amsterdam

211

On the Behaviour of Hydrous Ceria as an Ion Exchanger: Surface Properties, Structural Features, Capacity and Apparent pK Values NADIA Sh. PETRO’, NASR Z. MISAK’** and IBRAHIM M. EL-NAGGAR2 ‘Surface Chemistry and Catalysis Unit, National Research Centre, Dokki, Cairo (Egypt) 2Chemistry Department, Nuclear Research Centre, Atomic Energy Establishment, Atomic Energy Post 13759, Cairo (Egypt) (Received 30 October 1987; accepted 19 December 1989)

ABSTRACT For three prepared hydrous ceria samples, it was found that the surface properties can be affected by the preparation procedure details. A substantial crystahinity improvement occurs at 400 ’ C. Infrared and capacity data show that OH condensation starts at 100°C and that Hz0 is involved in anion exchange. Surface structure, and not surface area, is the factor determining the capacity. The acidity and basicity decrease with heating temperature.

INTRODUCTION

Hydrous oxides are now finding applications in the fields of chemical separations and nuclear energy [l-7]. Much work has been performed in this laboratory [f&11] on the ion exchange behaviour of hydrous oxides that is still not clearly understood. The present paper deals with some of the properties that are apt to affect this behaviour for the little studied [8,9] hydrous ceria. EXPERIMENTAL

Three samples (Ce-I, Ce-II and Ce-III) were prepared. Ce-I was prepared by rapid addition while stirring of 4 M NHIOH to an equal volume of a saturated ceric sulphate solution. After standing overnight, the solid was washed with distilled water for about 3 months and then dried at 50°C. The solid, found to release SO:- when contacted with alkaline solutions, was washed with 4 M NH,OH till the washings were free of SO:-. It was then washed with water till the washings were free of NH: and redried at 50°C. Ce-II was similarly prepared but the solid was washed with 4 M NH,OH directly after overnight standing. Ce-III was prepared as Ce-II but with the use of 0.4 M NH40H. ‘To whom all correspondece should be addressed.

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Portions of the samples were heated for 2 h at 100,150,200 and 400” C, and all the samples were stored in a nitrogen atmosphere over saturated NH&l. The samples were found to be insoluble in NaOH and KOH and in up to 0.01 N HCI and HzS04. The average particle size of Ce-II heated at 50,200 and 400’ C was found (as measured by an optical Reichert microscope) to remain the same after immersion overnight in water, indicating almost no swelling. X-ray analysis was carried out with a Philips 1010 X-ray diffractometer, using Cu K, radiation. A Pye-Unicam infrared spectrometer and a DuPont thermal analyser (10°C mine1 heating rate) were used for spectral and thermal analyses. Determination of weight losses at different temperatures involved heating for 2 h, cooling in a desiccator over CaCI,, and weighing. Nitrogen adsorption-desorption isotherms were measured with a conventional volumetric apparatus. The samples heated at 50,100,150,200 and 400” C were degassed under vacuum at room temperature, 60,80,130 and 23O”C, respectively. Isotherms were also measured for some heated Ce-II samples after immersion for one week in water and then drying in air and for some heated Ce-I, Ce-II and Ce-III samples after about 3 months of storage over NH&l (all the ion exchange experiments were finished by this time). Besides, isotherms were measured for Ce-II heated at 100,200 and 400” C after evacuation at room temperature. These isotherms are not given here and the surface parameters obtained from them [ SBET ( m2 g- ’ ) , VP ( cm3 g-l ) and mean pore radius (nm) are equal to 17.5,0.04 and 4.6 at lOO”C, 21.8,0.03 and 2.8 at 200°C and 15.6, 0.02 and 2.5 at 4OO”C, respectively] are different by < 20% from those obtained from the other isotherms. The surface areas are always lower but the trend of variation of all parameters with heating temperature is always the same. The apparent pK values of the acid and base behaviour of the samples were determined from the variation with pH of the Na+ capacity (from NaCl and NaCl + NaOH) and NO; capacity (from NaNO, + HN03) at a total ion concentration of 0.1 M. The sorption values were reproducible to ? 3%. The sorption of Na+ and NO; was followed by measuring these ions in both solution and solid, and by titration of solution. 22Na tracer was used and its y-activity was determined by the scintillation assembly of Nuclear Enterprises, United Kingdom, with ST6 scaler timer. NO; was determined spectrophotometritally [ 121 on a Beckman spectrophotometer, and was removed from the solid by NaCl+ HCl. Insignificant changes in weight occurred in transforming ceria to the Na+ and NO; forms. The sorption values on ceria remain practically the same after its storage for about 3 months over saturated NH&l. The pH was measured with a digital Orion Research pH meter, with an accuracy of ? 0.01 pH units.

213 RESULTS AND DISCUSSION

Structural features X-ray analysis has shown that all the samples dried at 50°C are microcrystalline and give the main reflections of cerianite (ceric oxide) [ 131, indicating that the samples are hydrous ceric oxide, in accordance with the model of England et al. [ 141. Crystallinity improves significantly only at 400’ C. The infrared spectra of the original samples differ in detail. For example, very small peaks around 2850 cm-‘, due to stretching vibrations of OH groups combined with H+ [ 31 or strongly H-bonded [ 15 1, are present in Ce-I and CeII but not in Ce-III. The spectra of the samples heated to 100, 150, 200 and 400°C show that the broad band at 3600-1800 cm-‘, due to stretching vibrations of OH groups and water molecules [ 3,16,17], decreases in all the samples on heating. This is also the case with the large broad band at 1700-1100 cm-‘, which is due to water molecules [ 8,161 or incorporates peaks due to carbonate ions [ 181. The peaks at 1050 and 940 cm- ‘, due to bending Ce-OH vibrations [ 18,191, decreases with heating (even to 100’ C) and the first peak disappears (Ce-II and Ce-III) or is largely reduced (Ce-I) at 400°C. It is deduced from the spectra of Ce-I saturated with Na+, Cl- and NO, that the main sites for cation exchange are the OH groups. For anion exchange, sites originally occupied by water molecules or carbonate ions (bands at 1500 and/or 1330 cm- ’ ) seem to be involved also. The samples gave almost the same thermogravimetric curve. This curve shows, in the light of the infrared data, that the rapid main weight loss up to 200’ C is due to removal of water in the interparticle space [ 141 together with some OH condensation. DTA shows a large endothermic peak pointing at about 125’ C and extending to 200’ C in Ce-I, 240’ C in Ce-II and 290’ C in Ce-III, representing the major loss of water (H20 and OH groups). Ce-I has a sharp exothermic peak at about 32O”C, probably due to the combustion of an adsorbed ammonia impurity. Since Ce-I, Ce-II and Ce-III are all hydrous ceric oxide, the differences in the details of their infrared and DTA data may reflect differences in their surface structure, which will be considered later. Surface properties The adsorption isotherms given in Figs l-3 are type II [ 201. Almost no change is observed on immersion in water or storage, indicating no significant aging. s BET> total pore volume, VP (from sorption at P/PO=O.95), and mean pore radius (F) are given in Table 1. Comparing the values of VP and r obtained from sorption at P/P” =0.95 with those obtained from sorption at P/P” = 1 (obtained by extrapolation and not given here), it was found that the former

214

60.

q

Atter storage

0.3

0.4

0.5

0.6

0.7

0.8

0.9

I

1.0

PIP0

Fig. 1. Adsorption-desorption isotherms on Ce-I heated at different temperatures.

values are generally lower than the latter ones by about 13-20% and that the same trend of variation with heating temperature is displayed. The micropore volume ( VmiJ given in Table 1 is determined by the cu,-method [ 20,211. The data in Table 1 and earlier work [ 221 show that the texture parameters of ceria can be largely affected by the preparation procedure. The changes occurring in these parameters with heating are the resultants of the processes of particle shrinking [ 23 1, surface annealing [ 22,241 and sintering (at 400°C). The decrease of SBETand VP accompanied by an apparent increase in pore size on heating Ce-II to 100” C may be due to shrinkage, surface annealing and aging, leading to blocking of the finer pores. This is almost the case with CeIII on heating to 150°C. Vmic is not given in Table 1 for the cases of absent (Ce-I) or very low (Ce-II and Ce-III) microporosity. Capacity and apparent pK values The apparent pK values were obtained from the capacity-pH curves, as previously described [ 25,261, and are given in Table 2. The presence of relatively strong acid sites in ceria (Table 2) may be due to the presence of a sulphate impurity, and is unexpected from the model of England et al. [ 14 ], which seems to be an oversimplification of the actual structure of hydrous oxides. The decrease of cationic and, particularly, anionic capacity at 100 oC is in accordance

216

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.6

0.9

I

1.0

P/PO

Fig. 3. Adsorption-desorption

isotherms

on Ce-III heated at different

temperatures.

TABLE 1 Surface properties Sample

Ce-I

Ce-II

Ce-III

of hydrous ceria samples

Heating temperature (“C)

s l3ET (m’ g-‘)

“P (cm3 g-i)

50 100 200 400

62 II 17 75

0.12 0.13 0.13 0.10

50 100 150 200 400

40 20.8 24.3 25.7 17.2

0.06 0.04 0.03 0.03 0.02

0.007

50 100 150 200 400

36 27.3 20.0 27.3 21.7

0.03 0.04 0.04 0.04 0.04

0.010

“mic

r

(cm3 g-i)

(nm)

3.9

0.010 0.008

0.008

0.009

3.4 3.4 2.1 3.0 3.9 2.5 2.3 2.3 1.7 2.9 4.0 2.9 3.7

217 TABLE 2 pK values of acid and base sites in the different ceria samples Sample

Heating temperature (“C)

Base behaviour

Acid behaviour More acidic sites

Less acidic sites

Concn (meq g-‘)

PK

Concn (meq g-‘1

PK

Concn (meq g-‘)

PK

Ce-I

50 100 200 400

0.11 0.07 0.04

4.8 7.0 8.7

0.43 0.32 0.18 0.15

11.0 11.4 11.6 12.5

Ce-II

50 100 150 200 400

0.09 0.08 0.02

5.3 6.0 8.0

0.46 0.43 0.09 0.09 0.07

10.9 11.1 11.7 12.4 12.7

0.77 0.31 0.18 0.16 0.076

3.5 3.1 3.0 2.8 2.3

Ce-III

50 100 150 200 400

0.11 0.09 0.05

5.7 6.3 7.4

0.33 0.32 0.17 0.15 0.09

11.1 11.2 11.4 12.2 12.7

0.81 0.43 0.31 0.21 0.053

3.5 3.3 3.2 3.1 2.4

show that the cationic and anionic capacity cannot be correlated with surface area. The surface site density is different, at the same heating temperature, for the different cerias. It also changes (mostly decreases) on increasing the heating temperature for any of the samples. Surface structure seems, therefore, to be the dominant factor governing the capacity. If the surface structure is the same, the amount of H,O and OH present directly on the surface is expected to be higher for the sample with the higher surface area. Since H,O and OH are similarly interacting with surfaces of similar structure, their different amounts in samples of different surface areas may be expected to lead to different thermogravimetric curves. As mentioned before, the different ceria samples give the same thermogravimetric curve regardless of surface area differences. This may indicate a different surface structure. The capacity would be affected, not only by the population of potential exchange sites (OH, H,O ) , but also by the degree of their ionization (and hence ability to participate in ion exchange), which may depend on their environment. Site density, site acidity and pore size can affect ionic mobility inside ion exchangers [ 26,281, and these together with surface site density, porosity and surface structure affect selectivity [ 28,291. These situations have been discussed for ferric oxide gels [ 281.

218 ACKNOWLEDGEMENT

The authors wish to thank deeply the authorities of the International Atomic Energy Agency, Vienna, for financial support under research contract No 2716/ RB.

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