Indian Journal of Chemistry Vol. 42A, October 2003, pp. 2475-2479

Co-precipitation of a mixture of CuO and Cr203 through NaN03-KN03 eutectic mixture and its catalytic activity

N B Singh* & A K Ojha

Department of Chemistry, DDU Gorakhpur University, Gorakhpur 273009, India

Received 17 January 2003; revised 24 July 2003

When a mixture of basic copper carbonate and chromium nitrate in 1: 1 molar ratio has been heated at about 570°C a mixture of CuO and Cr203 is obtained. Further, when basic copper carbonate and chromium nitrate are mixed in 1: 1 molar ratio in NaN03-KN03 eutectic mixture and heated at ==270°C i.e. at a much lower temperature, again a mixture of the same oxides has been obtained. The formation of the oxides is confirmed by powder X-ray diffraction technique. The mixture of CuO and Cr203 obtained through the two routes is found to be an effective catalyst for the decomposition of ammonium perchlorate.

It has been reported!·5 that when metal salts were heated in the presence of eutectic mixtures, precipitation of metal oxides occurred as a result of decomposition reactions at a much lower temperature. The mechanism of such reactions has been discussed. However, co-precipitation of mixture of two metal oxides through eutectic mixture has not been reported so far. In this paper we report our results wherein a mixture of CuO and Cr203 has been co-precipitated when a mixture of CuC03.Cu(OHh.H20 and Cr(N03h.9H20 was heated in an eutectic mixture of NaNOr KN03• Number of oxides, prepared by different routes, have been used as catalysts in the decomposition of ammonium perchlorate and other materials6•11 • However, the mixture of oxides co-percipitated through eutectic melts have not been used asa catalyst. The catalytic activity of the mixture of CuO and Cr203 during the decomposition of ammonium perchlorate has also been reported in this paper.

Materials and Methods NaN03, KN03, CuC03.Cu(OHh.H20 and

Cr(N03h9H20 all from Qualigenes were used. Ammonium perchlorate (AP) of 99.1 % purity was

received from Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India. It was crushed gently in an agate mortar to avoid explosion and sieved through 100-200-400 mesh.

CuO, Cr203 and mixture of the two oxides prepared directly by thermal decomposition are referred to as CuO, Cr203 and CUO+Cr203' On the other hand,

oxides prepared through nitrate eutectic are referred to as ECuO, ECr203. E (CuO + Cr203 ).

Dried samples of sodium nitrate and potassium nitrate were mixed in 45:55 percent ratio (wt. %) in a clean test tube. The mouth of the test tube containing the nitrate mixture was sealed and kept in a furnace at a temperature slightly higher than the melting point of the components. When the entire mass melted, the molten liquid was quenched suddenly to room temperature. The process of heating and cooling was repeated several times and ultimately the solidified material was removed from the test tube and crushed into fine powder for obtaining a homogenous mixture of the components. The melting point of the eutectic mixture was found to be 226°C.

Simultaneous TG-DSC studies on basic copper carbonate. chromium nitrate and mixture of copper carbonate and chromium nitrate in the presence and absence of NaN03-KN03 eutectic, ammonium perchlorate with different oxides (2%); CuO, Cr20], CuO-Cr203 and ammonium perchlorate with these oxides (2%) prepared through NaN03-KN03 eutectic were performed with thermal analyzer ST A 409 EP apparatus at HEMRL, Pune under an atmosphere of air at a heating rate of lOoC min,l.

The powder X-ray diffraction patterns of the decomposition products after TG experiments were recorded with a X-ray diffractograph (XRD-5 General Electric, USA) using CuKa radiation.

Surface areas of CuO prepared by thermal decomposition of basic copper carbonate and through

2476 INDIAN J CHEM. SEC A. OCTOBER 2003

NaN03-KN03 eutectic were measured by BET method at BARC, Mumbai.

The evolved gases were tested in the usual way and found to be CO2 and water vapour. The former was identified by passing through lime water and the latter by anhydrous copper sulphate.

Solid reaction products obtained from thermogravimetric studies were washed with water to separate insoluble oxides. Copper and chromium were estimated gravimetrically as an oxide by standard methodl2.

Ingition delay (DE) studies of ammonium perchlorate in the absence and presence of 2.0 wt. % CuO, Cr203 and a mixture of CuO + Cr203 were carried out by tube furnace technique. The sample (15 mg, 100-400 mesh) was taken in ignition tube (length 5 cm, and diameter 0.4 cm) and the time interval, (DE) between the insertion of the tube into the tube furnace and the time of ignition was noted with the help of stop watch with an accuracy of 0.1 second.

Results and Discussion TG studies have shown that basic copper carbonate

when heated alone, decomposes (230-320°C) with the precipitation of CuO. But in the presence of NaN03-KN03 eutectic mixture, the reaction occurs at a lower temperature with the precipitation of CuO the

formation of which has been confirmed by powder X-ray diffraction technique (Fig. 1).

DSC curve (Fig. 2a) for the decomposition of basic copper carbonate exhibits a small endothermic peak at 60°C due to removal of water molecule, sharp

I 0 0 Z iii "

100 200 300 1000 500 600 700

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100 200 300 400 500

TEMPERATURE 'c

Fig. 2-DSC curve for (a) basic copper carbonate; and (b) basic copper carbonate in NaN03-KN03 eutectic.

eo 7& 72 &8 eo sa 20.

Fig. I-XRD pattern for (a) CuO obtained from solid state reaction of basic copper carbonate; and (b) CuO obtained from decomposition of basic copper carbonate in NaN03,KN03 eutectic.

SINGH et al. : CO-PRECIPITATION OF CuO & Cr203 THROUGH NaN03-KN03 EUTECTIC MIXTURE 2477

endothermic peak at 297.6°C and an endothermic hump at 315°C due to decomposition of carbonate and hydroxide components of the material. The data show that there is very little temperature difference between the decomposition of carbonate and hydroxide part of the salt. But the DSC curve (Fig. 2b) of the above material in NaN03-KN03 eutectic shows two endothermic peaks at 125 and 227.4°C, the former is due to phase transformation13

of KN03 whereas the latter corresponds to melting of eutectic as well as decomposition of basic copper carbonate.

Cr(N03h. 9H20 decomposed14 at 100°C but in case

of mixture of CuC03.Cu(OH)2.H20 and Cr(N03h9H20; continuous weight loss took place up to 350°C followed by slow weight loss (58.5%) up to 570°C though the theoretical weight loss for the formation of a mixture of CuO and Cr203 is 60%. The end product (600°C) was analysed by powder X-ray diffraction technique as a mixture of copper oxide and chromium oxide. Trace amount of K2Cr04 and Na2Cr04 were also formed.

DSC data for a mixture of CuC03.Cu(OHh.H20 and Cr(N03)3.9H20 are represented in Fig.3a. showing four endothermic peaks at 90.6, 121, 183.7, 257.4°C, the first two are due to removal of water molecules and decomposition of Cr(N03).9H20 respectively whereas the last two peaks correspond to the decomposition of basic copper carbonate.

Data on DSC for a mixture of basic copper carbonate, chromium nitrate and eutectic mixture are represented in Fig. 3b, which shows four endothermic peaks at 78.4, 125, 180.2 and 220.4°C. The first peak is due to removal of water molecule and the second one may be related to decomposition of chromium nitrate and phase transformation of KN03. The peak at l80.2°C may correspond to the decomposition of carbonate part of basic copper carbonate, whereas the peak at 220.4°C may be attributed to melting of the eutectic mixture and the decomposition of hydroxide part of the basic copper carbonate. The results show that there is lowering of decomposition temperature for the mixture of two salts in eutectic mixture. Powder X-ray diffraction studies have shown that a mixture of CuO and Cr203 is formed in both the cases.

Ammonium perchlorate plays a very important role during the combustion of number of composite propellants. The data indicate that the decomposition and burning properties of the salt are changed considerably in the presence of catalysts. Pure AP is

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\ 100 200 300 400

TEMPERATURE ·c 500

Fig. 3-DSC curve for mixture of (a) basic copper carbonate and chromium nitrate; (b) basic copper carbonate and chromium nitrate in NaNOr KN03 eutectic.

quite stable at room temperature but when heated at 249°C undergoes a crystallographic modification as indicated by an endothermic peak in the DSC curve (Fig. 4a). In this temperature range orthorhombic form is converted to cubic form and the process is accompanied by a simultaneous dissociati ve sublimation. The thermal decomposition of AP occurs in two steps15.16, the low temperature decomposition (LTD) up to about 300°C (30%) and the higher temperature decomposition (HTD) (>300°C) with a second exothermic peak at 455°C. The thermal stability of AP is extremely sensitive to additives and catalysts influence the reaction both in acceleratory and decceleratory periods. Copper oxide prepared directly from basic copper carbonate decreases (246.4°C) the phase transformation temperature and the two exothermic peaks come much closer to each other and the temperatures are lowered (Fig. 4b).

DSC curve for the decomposition of AP in the presence of CuO prepared through NaN03-KN03 eutectic is shown in (Fig. 4c). The phase transition temperature of AP is shifted to lower temperature (246.3°C). The first exothermic peak at 293.1 °C in the case of pure AP is splitted into two peaks (287 and 304.8°C) indicating that probably CuO modifies the LTD process of AP, resulting decrease in temperature from 293.l oC to 287°C. Since the surface area of CuO prepared from eutectic is expected to be higher, (specific surface area of CuO = 42.0 m2/g, ECuO = 82.8 m2/g) , the gases evolved may get adsorbed and when heated further desorption occurs at 304.8°C.

2478 INDIAN J CHEM, SEC A, OcrOBER 2003

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Fig. 4-DSC curve for (a) ammonium perchlorate; (b) ammonium perchlorate in the presence of CuO prepared from thermal decomposition of basic copper carbonate; (c) ammonium perchlorate in presence of CuO prepared from thermal decomposition of basic copper carbonate in NaN03-KN03 eutectic (d) ammonium perchlorate in presence of Cr203 prepared from thermal decomposition of chromium nitrate; (e) ammonium perchlorate in presence of Cr203 prepared from thermal decomposition of chromium nitrate in eutectic; (f) ammonium perchlorate in presence of CuO + Cr203 prepared from thermal decomposition of mixture of basic copper carbonate and chromium nitrate; and (g) ammonium perchlorate in presence of CuO + Cr203 prepared from thermal decomposition of mixture of basic copper carbonate and chromium nitrate in eutectic.

The second exothermic peak (HTD) is also lowered in the presence of CuO but the lowering is slightly less than that in the presence of CuO prepared directly. The results show that CuO prepared through nitrate eutectic have slightly higher catalytic activity towards the decomposition of AP.

Decomposition data of AP in the presence of Cr20 -, prepared directly from the decomposition of Cr(N03h9H20 are presented in Fig. 4d indicating that the phase transformation temperature of the material is lowered (246.3°C). Both LTD and HTD are lowered in the presence of Cr203. However, the first exothermic peak is split into two peaks, 283.5 and 319. 7°C similar to that obtained in the presence of CuO prepared through eutectic mixture. However, the peak temperatures are slightly higher than that in the presence of CuO. The HTD also shifted to lower temperature in the presence of Cr203.

The DSC curve for the decomposition of AP in the presence of Cr203 prepared through eutectic mixture is shown in (Fig. 4e). The major difference in the decomposition of AP in the presence of Cr203 prepared through two routes is that the area of the exotherm for HTD is decreased considerably indicating that major decomposition of AP occurs in the temperature range of LTD.

Data for the decomposition of AP in the presence of mixture of CoO of Cr203 (prepared directly mixing basic copper carbonate and chromium nitrate) are represented in Fig. 4f. The phase transformation temperature is decreased and the two exothermic sharp peaks corresponding to LTD and HTD are shifted to lower temperatures (280.3 and 335.0°C), which is maximum lowering as compared to that in the presence of individual oxides. The result indicates that the mixture of two oxides is a very effecti ve catalyst for the decomposition of AP. The DSC curve for the decomposition of AP in the presence of mixture of CuO and CrZ03 (through eutectic mixture) is shown in (Fig. 4g). There is a considerable lowering in the phase transformation temperature,

Table 1-Ignition delay of AP at different temperatures in the presence of different oxides

Temp AP AP+CuO AP+ECuO AP+ Cr203 AP+ ECr20 3 AP + CuO + Cr203 AP + E CuO + E Cr20, (0C)

350 67.4 72.4 74.1 64.5 64.5 57.5 400 79.4 44.7 51.3 51.3 46.7 47.8 42.6 450 42.6 33.9 35.5 41.7 34.7 35.4 36.3 500 33.9 25.7 27.5 33.1 30.2 29.5 28.8 600 18.2 17.8 19.05 23.4 21.9 19.9 20.4

SINGH et al. ; CO-PRECIPITATION OF CuO & Cr20 3 THROUGH N aNOr KN03 EUTECTIC MIXTURE 2479

LTD and HTD (280.0, 305.3 and 340.0°C). The overall results indicate that the mixture of CuO and Cr20 3 is a better catalyst for the decomposition of AP and the catalytic activity of the mixture of two oxides obtained through two routes is almost the same.

The igmtIOn delays of AP at different temperatures and in the presence of different oxides are incorporated in Table 1. It is seen that the DE is decreased with increase in temperature in all the cases. The data suggest that catalysts decrease the ignition delay of AP at all the temperatures. However, as the efficiency of different catalysts is different. it is difficult, to interpret the data.

The overall results have shown that the mixture of CuO and Cr203 is co-precipitated through NaN03-KN03 eutectic mixture at lower temperature and the catalytic activity of the mixture of CuO and Cr203 obtained through the two routes is almost the same.

Acknowledgement The authors are thankful to the CSIR, New Delhi

for financial support.

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