I n d . E n g . Chem. Res. 1995,34,3146-3148
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Catalytic and Inhibiting Effects of Lithium Peroxide and Hydroxide on Sodium Chlorate Decomposition James C. Cannon and Yunchang Zhang* Puritan-Bennett Corporation, 10800 Pflumm Road, Lenexa, Kansas 66215
Catalytic decomposition of sodium chlorate in the presence of cobalt oxide, lithium peroxide, and lithium hydroxide is studied using thermal gravimetric analysis. Lithium peroxide and hydroxide are both moderately active catalysts for the decomposition of sodium chlorate when used alone, and inhibitors when used with the more active catalyst cobalt oxide.
Introduction Chemical oxygen generators based on sodium chlorate and lithium perchlorate are used in airplanes, submarines, diving, and mine rescue. The chlorate or perchlorate typically is mixed with iron powder as a fuel and cobalt oxide as a catalyst. When heated the chlorate and perchlorate decompose to generate oxygen. A small amount of chlorine is produced together with the oxygen through side reactions. Chlorine is toxic and must be removed or reduced. A common practice is to add an alkaline compound to the mixture to adsorb the chlorine. Barium peroxide has been used in many oxygen generators. It is reported that barium peroxide has some interesting properties during the decomposition of sodium chlorate. Barium peroxide by itself is a catalyst for the decomposition of sodium chlorate. When used together with cobalt oxide, however, it is an inhibitor (Zhang et al., 1993). It is this property that makes barium peroxide able to smooth the oxygen generation rates. Although barium peroxide has many good properties, it is a toxic compound. Disposal of scrap and expended generators is costly. Some people have reported that lithium peroxide can be used in the chemical oxygen generator formulation t o suppress the formation of chlorine gas (Gao et al., 1989; Markowitz et al., 1964). However, little has been reported concerning the effects lithium peroxide has on the decomposition of the chlorate or perchlorate. It is the purpose of this work to see what effects lithium peroxide has on decomposition of sodium chlorate and t o see whether the effects of lithium peroxide are parallel to those of barium peroxide.
Experimental Section Cobalt oxide was prepared by decomposing cobalt carbonate at 260 "C. The product was analyzed as C0304 by X-ray diffraction analysis using a Rigaku D/maxII diffractometer. NaC103 was used in the form of crystalline particles. Lithium peroxide and hydroxide were used as purchased from Aldrich Chemical. The surface area of the cobalt oxide was measured using a BET Sorptometer. The sample was heated at 150 "Cfor 30 min in vacuum to drive off any adsorbed moisture and other gases prior to measurements. The specific surface area of cobalt oxide was measured as 150 m2/g using multipoint technique with nitrogen as the adsorbent gas. Chemical mixtures for thermal analysis were made based on weight percentages. Each mixture was intimately mixed using an agate pestle and mortar for 0888-588519512634-3146$09.00/0
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Figure 1. Thermal decomposition of Liz02 and LiOH.HZ0 in oxygen. (1)TGA trace of Li202, (2) TGA trace of LiOHsH20, (3) DTA trace of Li202, and (4)DTA trace of LiOHqH20.
about 5 min. The total weight of each mixture was approximately 2 grams. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out using a Netzsch Model 409 thermal analyzer. The sample was heated in the furnace at 20 "C/ min in an oxygen stream of 150 mumin. The TGA and DTA data were recorded simultaneously. The sample size was approximately 50 mg for the mixtures of lithium hydroxide (LiOHaH20)and sodium chlorate and 100 mg for all other samples. When 100 mg was used, the mixture of LiOH.Hz0 and NaC103 splashed out from the crucible during the decomposition and no accurate sample weight change could be obtained.
Results and Discussion Even though lithium peroxide is thermally stable it reacts slowly with moisture t o form lithium hydroxide. During the oxygen generator manufacturing process a few percent water is usually added to the chemical mixture, to facilitate the mixing and the formation of the chemical core. The water is subsequently removed by drying at 100-140 "C. Lithium peroxide may be converted t o the hydroxide during the process, and the hydroxide may have different effects on generator performance. Therefore, a parallel study was also carried out for lithium hydroxide. Thermal Stability of Li202and LiOH-HZO. The TGA and DTA results of the decomposition of Liz02 and LiOHaH20 in oxygen are given in Figure 1. The decompositionof lithium peroxide to lithium oxide starts at about 350 "C, and the decomposition is complete at about 440 "C. The operation temperature of a chemical oxygen generator is usually higher than 500 "C. Therefore, all the active oxygen from Liz02 will be released. This is an advantage lithium peroxide has over barium 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 9,1995 3147
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Figure 2. TGA of NaC103 catalyzed by lithium peroxide. (1)20% Li202, (2) 8% Li202, (3) 4% Li202, (4) 2% LizOz, (5) 1%Li202, and ( 6 ) 0% Li202.
Figure 3. TGA of NaC103 catalyzed by lithium hydroxide. (1)20% LiOHeH20, (2) 8% LiOHsH20, (3) 4% LiOHmH20, (4)2%LiOHsH20, (5) 1%LiOHwH20, and ( 6 )0% LiOHeH20.
peroxide. Barium peroxide has less active oxygen and only partially decomposes and produces only a few percent oxygen during the operation of a chemical oxygen generator (Zhang et al., 1993). It has been reported that lithium peroxide can exist in two forms, a-LizO2 and P-Li202. a-LizOs transforms t o P-Li202 at 225 "C, P-Li202 is stable up to 315 "C, and a polymorphous phase transformation can be observed on thermal study (Rochow, 1966). The phase transition was not observed in this study. Only one endothermic peak corresponding to the decomposition of lithium peroxide is observed on the DTA trace in Figure 1. Therefore, it is unlikely that there exists a polymorphic phase transition. However, our lithium peroxide is not very pure. The impurities may diminish the phase transition. LiOH*H20loses its hydration water between 70 and 190 "C. The anhydrous lithium hydroxide formed starts decomposing very gradually at 500 OC,and only a small fraction has decomposed t o Liz0 by 700 "C. DTA trace of LiOHmH20 shows two endothermic peaks at around 160 and 485 "C respectively corresponding to the dehydration of the original sample and melting of the anhydrous lithium hydroxide formed, which is consistent with the data given in handbooks. This indicates that the hydration water can be driven off during the drying between 100 and 140 "C if lithium hydroxide hydrate is used in chemical oxygen generating cores. Effect of Lithium Peroxide on the Decomposition of Sodium Chlorate in the Absence of Other Catalysts. In order t o see the effect of lithium peroxide on sodium chlorate decomposition, sodium chlorate is loaded with 0, 1, 2, 4, 8, and 20% Liz02 by weight, respectively. The TGA profiles are presented in Figure 2. Since the onset decomposition temperature is somewhat difficult to determine and depends on particle size of the mixture, the temperature at which 50% of the sodium chlorate has decomposed (50% DT) is used in comparing the relative activity. Sodium chlorate by itself loses 50% by weight at 566 "C, and the decomposition is complete at about 600 "C. When 1%by weight of lithium peroxide is mixed with NaC103, the mixture loses 50%weight at approximately 500 "C and the decomposition is complete at 560 "C. Loading of 1%lithium peroxide has resulted in sodium chlorate decomposing at significantly lower temperatures. Therefore, lithium peroxide is a catalyst for sodium chlorate decomposition. The TGA profiles for the samples of sodium chlorate with 2, 4, 8, and 20% lithium peroxide all show two-step decomposition. The
first step between about 370 and 420 "C is the decomposition of the lithium peroxide as can be inferred from Figure 1, and the second step is the decomposition of sodium chlorate. Therefore, Liz02 has decomposed before the decomposition of sodium chlorate and the decomposition of sodium chlorate is catalyzed by Li20. The catalytic effect appears to saturate at 1% lithium peroxide loading. Increased lithium peroxide loading does not shift the decomposition to much lower temperatures. Samples of sodium chlorate loaded with 2,4, 8, and 20% lithium peroxide decompose only at slightly lower temperatures. The 50% DT of sodium chlorate mixed with 20% lithium peroxide is 484 "C, which is only 16 "C lower than the 50% DT of the sample containing 1%lithium peroxide. This is further evidence that lithium peroxide is a catalyst rather than a reactant. The effect of a reactant is usually proportional to its loading. Effects of LiOHBH20 on Sodium Chlorate Decomposition in the Absence of Other Catalysts. The TGA profiles of sodium chlorate mixed with 0, 1 , 2 , 4,8, and 20% weight LiOHsH20 are presented in Figure 3. Decomposition temperature of sodium chlorate decreases progressively with increased loading of lithium hydroxide without saturation at least to 20% loading. The 50% DTs for sodium chlorate loaded with 0, 1,2,4, 8, and 20% by weight of LiOHaH20 are 566, 550, 537, 518, 494 and 458 "C, respectively. There is a weight loss below 120 "C for the samples containing LiOHoH20. This corresponds to the dehydration of LiOHoH20 as shown in Figure 1. Since LiOHaH20 is highly dispersed in these samples, the dehydration occurs at a little lower temperature than in the pure sample. Effect of Lithium Peroxide on Sodium Chlorate Decomposition in the Presence of Cobalt Oxide. Sodium chlorate catalyzed by 1%by weight C03O4 starts losing weight at 225 "C, and the decomposition proceeds rapidly and is complete by 300 "C (Figure 4). This result is consistent with the catalytic effect of cobalt oxides reported by Rudloff and Freeman, and Wydeven, both in 1970. Lithium peroxide by itself is a catalyst for the decomposition of sodium chlorate. When lithium peroxide is added to the system of sodium chlorate containing 1% Co304, however, the catalytic effect of cobalt oxide is partially suppressed o r deactivated. TGA profiles of the samples containing 1%cobalt oxide and 0, 1, 2, 4,and 8% lithium peroxide are presented and compared with decomposition profile of that of pure
3148 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995
Similar to the peroxide, lithium hydroxide also inhibits the catalytic activity of cobalt oxide. The inhibiting effect saturates at 4%LiOHmH20 loading. The 50% DTs for the samples of sodium chlorate containing 1% C0304 with 1 , 2 , 4 ,and 8% LiOHoH20 are 382,395,430, and 437 respectively compared to the 50% DT 273 "C for the sample of sodium chlorate with 1% Co304 and without lithium hydroxide. The catalytic and inhibiting effects of lithium peroxide and hydroxide on the decomposition of sodium chlorate are parallel to the effects of barium peroxide and hydroxide as previously reported (Zhang et al., 1993).
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Figure 4. TGA of NaC103 catalyzed by Co304 and Li20z. (1)1% Co304, (2) 1% Co304 and 1%Li202, (3) 1%Cos04 and 2%Li202, (4) 1% Cos04 and 4% Li202, (5) 1%Co304 and 8% Li202, and (6) no Cos04 or Li202. 120
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Figure 5. TGA of NaC103 catalyzed by Cos04 and LiOHeH20. (1) 1%Co304, (2) 1%Co304 and 1%LiOHsH20, (3) 1%Co304 and 2% LiOHeH20, (4) 1% Cos04 and 4%LiOH*H20,(5) 1%Cos04 and 8% LiOHsH20, and (6) no Co304 or LiOH.Hz0.
sodium chlorate in Figure 4. Sodium chlorate mixed with both 1%cobalt oxide and 1%lithium peroxide has a 50% DT at 380 "C, which is 107 "C higher than the 50% DT of sodium chlorate with 1% cobalt oxide and without lithium peroxide. Thus, lithium peroxide functions as an inhibitor in the presence of cobalt oxide. This inhibiting effect increases with increased loading of lithium peroxide and saturates at 2% loading of the peroxide. The sample with cobalt oxide and 2%lithium peroxide has a 50% DT at 417 "C and the 50% DTs of the samples with cobalt oxide and 4 and 8% peroxide are only marginally higher. Effect of LiOH*H20on NaC103 Decomposition in the Presence of Cobalt Oxide. The effects of lithium hydroxide on cobalt oxide catalyzed sodium chlorate decomposition is given in Figure 5. Curve 6 is the TGA profile of sodium chlorate by itself. Curves 1-5 are sodium chlorate mixed with 1% cobalt oxide and 0, 1, 2, 4, and 8% LiOHmH20, respectively.
In conclusion, lithium peroxide shows very close similarities to the effects previously reported for barium peroxide (Zhang et al., 1993). It is a catalyst when used alone and an inhibitor when used with cobalt oxide for the decomposition of sodium chlorate. Since the hydroxide has properties similar to those of the peroxide, the performance would not be very much different when the peroxide is partially converted to the hydroxide. Since lithium peroxide has lower molecular weight than that of barium peroxide, it produces more effect in both the catalytic decomposition and inhibition. Suppression of chlorine is based on the reaction between lithium peroxide and chlorine. Therefore, lithium peroxide will be more effective than barium peroxide in suppressing chlorine when the same weight is loaded. Therefore, lithium peroxide can be a good substitute for barium peroxide.
Literature Cited Gao, H. C.; Ma, W. X.; Zhang, Y.; Shi, F. Y. Sodium Chlorate Oxygen Candle China Patent, CN 1035248, 1989. Markowitz, M. M.; Boryta, D. A.; Stewart, H. Lithium Perchlorate Oxygen Candle Ind. Eng. Chem. Prod. Res. Dev. 1964,3, 321. Rochow, E. G . Peroxides, Superoxides, and Ozonides ofAlkali and Alkaline Earth Metals; Plenum Press: New York, 1966;Chapter 2. Rudloff, W. K.; Freeman, E. S. The Catalytic Effect of Metal Oxide on Thermal Decomposition Reactions. J . Phys. Chem. 1970,74 (181,3317. Wydeven, T.Catalytic Decomposition of Sodium Chlorate. J . Catal. 1970,19, 162. Zhang, Y.; Kshirsagar, G.; Cannon, J. C. Functions of Barium Peroxide in Sodium Chlorate Chemical Oxygen Generators. Ind. Eng. Chem. Res. 1993,32 (51, 966 (1993).
Received for review November 1, 1994 Accepted May 11, 1995 @
IE940634C
EI Abstract published in Advance A C S Abstracts, July 15, 1995.
