JOURNAL OF MATERIALS SCIENCE LETTERS 5 (1986) 1279-1280

Reduct ion of Ni z+ in chlor i te

M. PATEL* Regional Research Laboratory, CSlR, Bhopal 462026, India

Frenkel [1] has recently conceived of a new process for the storage of hydrogen, by encapsulating it in zeo- lites. He has shown that many of the disadvantages of a hydride storing system can be eliminated by storing in zeolite. Some of the clay minerals, such as mont- morillonite, vermiculite and chlorite, have properties similar to zeolite, i.e. void spaces with metal-ion exchange properties. These clay minerals are consti- tuted of units containing an octahedral layer embedded between two tetrahedral layers. These units, termed normally silicate layers, are separated by an additional interlayer space containing mainly Na + , K + , or Ca 2+ , coordinated to water molecules, of thickness --~ 0.5 nm.

These ions are exchangeable and quantified by a parameter called the cation exchange capacity (CEC). Chlorite mineral possesses a supplementary exchange- able layer, known as brucitic layer. The CEC values of montmorillonite, vermiculite and chlorite range from 100 to more than 200meq/100g solid, comparable to that of zeolite. Therefore, it is conceivable that these clay minerals can also encapsulate hydrogen, similar to zeolites, and find application for the storage of hydrogen.

Perusal of the literature showed that there were no basic data available on the H2-chlorite system. The objective of the present study is to report the hydrogen uptake values in chlorite mineral, containing N 2+ in the brucitic layer.

The chlorite was obtained from a Wyoming mont- morillonite following a method given by Longuet- Escard [2]: i.e. by reacting the mineral with NiCI2 under highly alkaline conditions (pH 10) in a glass vessel and then washing the centrifuged mass repeat- edly with water to eliminate excess alkalinity and chloride ions. It was found at the end of this operation that some Ni 2+ remained deposited over the external surface of the mineral, apart from forming a brucitic layer.

The X-ray and infrared studies, made on the initial chlorite sample and after heat treatment in vacuum, clearly indicate that the product was a chlorite mineral. The reflections of quartz observed in the X-ray dia- gram were obviously from the little impurity present in montmorillonite. As this would not interfere with the hydrogen uptake of chlorite, no attempt was made to remove it. The hydrogen uptake was measured in the same apparatus as used for montmorillonite [3]. Here also, the liquid nitrogen trap, placed near to the rector, was kept filled during measurement of the hydrogen uptake.

The structural formula of the montmorillonite

mineral is as follows:

(Al1.79 Fe2~5 Mg0.22)

( Si3.85 Fe3.~0 a10.05)O,0 (OH)= Ca0.2, XH20

The total amount of Ni 2+ was 22.07%. Such a great amount of Ni 2÷ was intentionally allowed as this can lead to higher hydrogen uptake. The pretreatment temperatures were 355 and 255 ° C. Montmorillonite was pretreated at 255°C. The vacuum achieved in both cases was of the order of 0.133 Pa and the time of treatment was 20h. The hydrogen treatment was made at a hydrogen pressure of 6.665 kPa and a tem- perature of 252°C in both experiments.

The amount of moisture determined after pretreat- ment at 255 and 355 ° C was 11.39 and 11.66%, respec- tively, showing the percentage water loss to be more or less the same at these two temperatures. The amount of water condensed at the end of one reduc- tion process was measured (at 252 ° C) and found to be 2.26mm. The formation of water at the end of the reduction operation tends to indicate that reduction of Ni 2÷ to Ni ° is through the same mechanism as proposed in montmorillonite [3]. The fraction of Ni 2+ reduced at various temperatures and times is shown in Table I. The reduction mechanism can be represented by:

Ch "+ OH + 1H 2 ~ C h (n-l)+ + H 2 0

T A B L E 1 Fraction of Ni 2+ reduced at different temperatures

Time (rain) Fraction of Ni 2+ reduced

255°C 355°C

3 0.226 0.488 6 0.254 0.570

15 0.285 0.670 30 0.322 0.730 45 0.350 0.775 60 0.373 0.813 75 0.400 0.845 90 0.42 0.875

120 0.45 0.925 150 0.482 0.962 180 0.510 0.995 240 0.55 1.042 300 0.582 1.078 360 0.610 1.100 420 0.630 1.120 480 0.650 1.135 540 0.670 1.150 600 0.685 1.162 900 0.768 1.230

1200 0.82 1.280

*Permanent address: National Aluminium Company Ltd, Damanjodi, 763 008 Orissa, India.

0261-8028/86 $03.00 + .12 © 1986 Chapman andHall Ltd. 1279

40

30

2O 0

x

t _

=z

10

J

I I I 5 lO ~5

Time ( h ) ~ 20

Figure I Hydrogen uptake in nickel chlorite.

where Ch represents chlorite and n is the charge on the mineral.

The increase in pretreatment temperature of chlorite mineral induces an enhanced adsorption capacity of the product, as the two curves in Fig. 1 indicate. As in montmorillonite and vermiculite, the hydrogen uptake reaches almost a state of saturation after about 8 h reaction, the major adsorption having occurred in the first 4 to 5 h.

The overall hydrogen uptake values in chlorite are higher than that in montmorillonite and comparable to that of vermiculite [3]. The maximum value attain- able at a reduction temperature of 250°C is

0.33 x 102°atomg -~ for montmorillonite and 0.45 x 1020 for vermiculite, and is 0.49 × 1020 for chlorite at 355 ° C. The higher hydrogen uptake value in chlorite is possible as the mineral has a higher thermal resis- tance than the other two minerals.

References 1. S. FRENKEL, Chemtech. 12 (1982) 278. 2. J. LONGUET-ESCARD, Mere. Ser. Chim. 3 (1951) 279. 3. M. PATEL, Clay Clay Miner. 30 (1982) 397.

Received 17 January and accepted 30 June 1986

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