966
Ind. Eng. Chem. Res. 1993,32, 966-969
Functions of Barium Peroxide in Sodium Chlorate Chemical Oxygen Generators Yunchang Zhang,' Girish Kshirsagar, and James C. Cannon Puritan-Bennett Corporation, 10800 Pflumm Road, Lenexa, Kansas 66215
Catalyzed decomposition of NaC103and the effects of BaOz and Ba(OH)z*HzOon the decomposition are studied using thermal gravimetric analysis and differential thermal analysis. BaOz and Ba(OH)z-HzOare moderately active catalysts when used alone and inhibitors when used with the more active catalyst cobalt oxide for NaC103 decomposition. The smoothing effect of BaOz on NaC103 decomposition is explained, and the mechanism of chlorine suppressing during NaC103decomposition is discussed.
Introduction NaC103 chemical oxygen generator is also called chemical oxygen candle. It is used in passenger airplanes, military airplanes, space flights, submarine, mine rescue, diving, and other circumstances where oxygen supply is needed. A NaC103 chemical oxygen candle generally consists of NaC103 as oxygen source; a fuel to supply the extra heat needed to sustain the decomposition, a catalyst to bring down the decompositiontemperature, a chlorine adsorbent, and some other fillers such as glass powder, fiber glass, ceramic fibers, or steel wool to modify the physical and chemical properties of the generator. The chemical ingredients are mixed uniformly. A few percent water is used as a binder and lubricant. The mixture is then pressed to form acoherent cylindrical block with a diameter of 1-2 in. and a length of 3-8 in. depending on application. After being dried at around 130 "C, the block is then installed in a metal can. It can be ignited from one end through a primer or a fuse. The reaction front is incandescent and propagates along the block to the other end. A uniform reaction front is essential to have stable oxygen generation. When the operation of a generator is initiated, NaC103 decomposes exothermically to NaCl and 02.During the decomposition a few to several hundred ppm chlorine gas can also be evolved through side reactions. Chlorine is a highly toxic gas and must be removed to make the oxygen breathable. The common practice is to add an alkaline metal oxide to the candle body to reduce chlorine. Alkali metal oxides such NazO, Na202, LizOz,and KO2 have been used for this purpose, but BaOz is far more popularly used because of its higher chemical and thermal stability. BaOz was used as a chlorine absorbent as early as when the candle itself was invented during World War 11. It is still used in most chemical oxygen generators today. BaOz is generally considered a chlorine suppressant, but it has other useful functions in the chemical oxygen candles. Several authors have described effects of BaOz other than suppressing chlorine formation. However, the functions and effects reported by these authors are inconsistent and even contradictory. Pappenheimer (1946) reported an oxygen candle formulation with 2 4 % BaOz. He observed that a candle with higher BaOz loading can reach a higher maximum temperature and a higher oxygen evolution rate, and the effect of 2% BaOz on the maximum temperature was equivalent to the effect of 2% iron powder. He concluded that increasing BaOz loading resulted in more iron being oxidized. Gustafson (1965) described BaOz as a catalyst. Littman and Prince (1969) reported that BaOz provided smoother burning in addition to suppressing chlorine.
Heintz (1978) described a cobalt oxide catalyzed oxygen generating formulation containing BaOz. He considered BaOz a diluent, because it tended to slow down the rate of oxygen evolution. Gao et al. (1989) reported that BaOz caused slow initiation of the oxygen-generating candles. BaOz is an important component in the chemical oxygen candle, and a better understanding of this chemical can help to improve chemical oxygen candle performance. In addition, a better understanding of its functions can help us to find a nontoxic chemical as a substitute in the candle.
Experimental Section Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out using a Netzsch thermal analysis unit, Model 409. The sample was heated in the furnace at 20 OC/min in an oxygen stream of 150 mL/min. The TGA and DTA data were recorded simultaneously. The sample size was approximately 240 mg for BaOz and Ba(0H)rHzO and 100 mg for other samples. Cobalt oxide was prepared by decomposing cobalt carbonate at 260 "C. The product was analyzed as Co304 by X-ray diffraction analysis using a Rigaku D/maxII diffractometer. NaC103 was used in the form of crystalline particles. BaOz and Ba(0H)rHzO were used as purchased from Aldrich. Chemical mixtures for thermal analysiswere intimately mixed using an agate pestle and mortar for about 5 min for each mixture. The total weight of each mixture was approximately 2 g. Surface areas of the BaOz and cobalt oxide were measured using a BET Sorptometer. The samples were heated at 150 "C for 30 min in vaccum to drive off any adsorbed moisture and other gases prior to measurements. The specific surface area of BaOz was measured as 1.0 mz/g using multipoint measurement with krypton as the adsorbent gas, and the surface area of cobalt oxide was measured as 150 m2/gusingthe multipoint technique with nitrogen as the adsorbent gas.
Results and Discussion Thermal Stability of BaOz and Ba(OH)2*H&. BaOz usually contains a small amount of BaO. BaO is not stable and decomposes to form Ba(0H)z when it meets water or moisture. BaOz itself also reacts with water or moisture to form barium hydroxide even though the reaction is relatively slow. When BaOz was kept over water in a desiccator overnight at room temperature, approximately 15% BaOz was converted to Ba(OH)2-3HzO. Barium hydroxide may have different effects on candle performance. Therefore, a parallel study was also carried out for barium hydroxide Ba(0H)rHzO.
0888-5885/93/2632-0966$04.oo/o 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32,No. 5, 1993 967
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BaO2 starts losing weight at around 650 "C, and the decomposition is complete by 930"C (Figure 1). The total weight loss was 9.0% compared to the theoretical weight loss of 9.45% for the decomposition of BaO2 to BaO. Therefore, the BaO2 as purchased is determined to contain approximately 95% BaO2. The remainders are likely to be BaO and BaC03. Barium peroxide has been considered by many researchers as an oxygen source, but it has only 9% of its total weight as releasable oxygen when completely decomposed. It lost only 2% of its weight when heated to 800 "C at 20 "Urnin, and 7% of ita weight after being held at 800 "Cfor 1 h. The temperature of an operating candle body is usually lower than 800 "C, and the duration of the operation is usually shorter than 20 min. Therefore, oxygen generation from BaO2 is secondary compared with ita other functions. TGA and DTA results of Ba(OH)2-H20are also given in Figure 1. The TGA profile shows a weight loss of 9.5% between 120 and 230 "C, which corresponds to the dehydration of Ba(OH)2-H20to Ba(OH)2. Ba(OH)2starts decomposing very gradually around 600"C. Only a small fraction of Ba(OH)2 decomposes to BaO up to 880 "C as indicated by the weight loss. Therefore,barium hydroxide basically stays undecomposed during the operation of a oxygen candle. Since barium hydroxide is very corrosive at high temperatures, the thermal analysisis not attempted to temperatures higher than 880 "C. Two endothermic peaks are evident in the DTA curve. The first endothermic peak around 205 "C correspondsto the dehydration of Ba(OH)2*H20,and the second peak at 408 "C is assigned to the melting of Ba(OH)2. Effect of Ba02 on Uncatalyzed NaC103 Decomposition. In order to see the effect of barium peroxide on sodium chlorate decomposition,sodium chlorate is loaded with 0, 1,2,4,8,and 20% by weight, BaO2 respectively. The TGA profiles are presented in Figure 2. Since the onset decomposition temperature is somewhat difficult to determine and depends on the particle size of the mixture, 50 % decomposition and complete decomposition temperatures are used in comparing the relative activity. When no BaO2 is loaded, 50% of the NaC103decomposes below 566 "C and the decomposition is complete by 600 "C. When 1% by weight BaO2 is mixed, 50% of the NaC103 decomposes below 533 "C and the decomposition is completeat 562 "C. When mixed with 27% by weight BaO2, 50% of the NaC103 decomposes by 512 "C and the decomposition finishes at 535 "C. Loading of BaO2 has resulted in sodium chlorate decomposing at a much lower temperature. Therefore, BaO2 is a catalyst for NaC103 decomposition.
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10 0
100 200 300 400 500 600 700 800 900 1000 Temperature ( " C )
Figure 1. Thermal decomposition of BaOz and Ba(OH)2.H20 in oxygen. (1)TGA trace of BaOz, (2) TGA trace of Ba(OH)Z.H20,and (3) DTA trace of Ba(OH)Z.HzO.
50
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Figure 2. TGA of NaC103 catalyzed by BaOz. (1)20, (2) 8,(3) 4, (4) 2, (5)1,and (6) 0% BaOz. 110 100
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Figure 3. TGA of NaC103 catalyzed by barium hydroxide. (1) 20, (2) 8,(3) 4, (4) 2, (5) 1,and (6)0% Ba(0H)THzO.
The increased catalytic effect with increasing BaO2 loading appears to saturate at 2% BaOa. NaC103 loaded with 4 and 8% BaO2 decomposes only at slightly lower temperatures. When mixed with 20% BaO2,50% of the NaC103 decomposes below 478 "C, which is only 34 "C lower than the 50% decomposition temperature of NaC103 loaded with 2% BaO2. This is another evidence that BaO2 is a catalyst rather than a reactant. Effects of Ba(0H)rHzOon NaClOsDecomposition. The TGA profiles of NaC103 mixed with 0, 1,2,4,8,and 20% by weight Ba(OH)2*H20are presented in Figure 3. The decomposition temperature decreases progressively with increased loading of Ba(OH12eH20 without saturation at least to 20% loading. The 50% decomposition temperatures for NaC103 loaded with 0, 1, 2,4,8,and 20% by weight Ba(OH)2*H20are 566,545,512,476,450, and 422 "C, respectively. It is clear by comparing Figures 2 and 3 that Ba(OH)2 is a more active catalyst than BaO2 for NaC103 decomposition. Therefore, conversion of BaO and BaO2 to barium hydroxide during wet mixing, pressing, and drying can increase the catalytic activity. There is a weight loss starting around 125 "C for the samples containing Ba(OH)yH20. This corresponds to the dehydration of Ba(OH)yH20 as shown in Figure 1. Ba(OH)2 does not decompose appreciably up to 800 "C. Therefore, water contribution from Ba(OH12 decomposition is not serious. The hydrated water can be driven off by drying at 110-130 "C. Chlorine can react with Ba(OH)2 to release water. However, the amount of water would be insignificant because the concentration of chlorine in the oxygen is very low.
968 Ind. Eng. Chem. Res., Vol. 32,No. 5, 1993 100
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Figure 4. TGA of NaC103 catalyzed by cos04 and BaOz. (1) 1% Co304, (2) 1%Co304 and 1%BaOz, (3) 1%Co304 and 4 % BaOz, (4) 1%Co3Ol and 8% BaOn, and (5) no Co304 or BaOz.
Effect of BaOz on NaC103 Decomposition in the Presence of Cobalt Oxide. Cobalt oxide has a very active catalytic effect on NaC103 decomposition. NaC103 catalyzed by 1 % by weight Co304starts losing weight at 225 "C, and the decomposition proceeds rapidly and is complete by 300 "C (Figure 4). The 50% decomposition temperature is 272 "C compared to 566 "C for NaC103 alone. The decomposition starts below the melting temperature of 260 "C for sodium chlorate. Thus, a substantial portion of the sodium chlorate decomposes in the solid phase. This result is consistent with the catalytic effect of cobalt oxides reported by Rudloff and Freeman (1970)and Wydeven (1970). When BaO2 is added to NaC103 containing 1 % Co304, however, the decomposition temperature does not remain unchanged or decrease further as one would expect. Instead, the catalytic effect of cobalt oxide is partially quenched or deactivated by the loading of another catalyst, BaO2. TGA profiles of the samples containing 1% cobalt oxide and 0, 1,4,and 8% Ba02 are presented and compared with the decomposition profile of pure NaC103 in Figure 4. NaC103 mixed with both 1% cobalt oxide and 1 % BaO2 startslosingweight at 300 "C and has a 50% decomposition temperature of 355 "C. The 50% decomposition temperature is 83 "C higher than when BaO2 is absent. Thus, BaO2 functions as a inhibitor in the presence of cobalt oxide. Increased loading of BaO2 enhances the inhibiting effect only slightly. NaC103 containing 1%cobalt and 8% BaO2 has a 50% decomposition temperature only 27 "C higher than that of NaC103 containing 1% Cos04 and 1% BaO2. Figures 2 and 4 show why inconsistent results were reported for the role of Ba02 in NaC103 decomposition. When no other active catalysts are used, BaOe brings down the decomposition temperature and acts as a catalyst. When catalyzed by cobalt oxides, however, BaO2 brings up the decomposition temperature and acts as an inhibitor. Figure 5 clearly shows why BaO2 can smooth NaC103 decomposition. DTA profiles 1,2,and 3 in the figure are for the samples of NaC103, NaC103 with 1% cobalt oxide, and NaC103 with 1% cobalt as well as 4% BaO2, respectively. The DTA profile of pure NaCl03 has two distinct peaks. The first at 270 "C is endothermic, which corresponds to the melting of NaC103. The second peak at 575 "C is exothermic and is composed of two overlapped peaks. This exothermic peak corresponds to the decomposition of NaC103 and of NaC104 formed through disporportionation of NaC103. When catalyzed by cobalt oxide,the apparent size of the endothermic peak of NaC103 melting is much smaller and overlaps with the subsequent
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Figure 6. DTA of NaC103 decomposition. (1)NaC103, (2) NaCl03 with 1%Co304,and (3) NaC103 with 1%Cos04 and 4 % BaOn.
exothermic peak due to the decomposition of NaC103. Although one cannot unambiguously distinguish by DTA between melting that does not occur and melting that is masked by simultaneous decomposition, it is clear that NaC103 does not melt completely before it starts decomposing. It can be estimated by comparing the size of the endothermic peaks that a significant proportion of NaC103 decomposes below the melting temperature of NaC103, that is, in the solid state. When both cobalt oxide and BaO2 are loaded, however, the endothermic peak and the exothermic peak are separated. The melting temperature does not change much, but the decompositionstdvtaat higher temperaturea. The size of the endothermic peak is about the same as the endothermic peak in Figure 5 (profile 1)and about twice the size of the endothermic peak in Figure 5 (profile 2). This reveals that the decomposition takes place above the melting temperature, and little or no sodium chlorate decomposes in the solid phase. In the solid state,the contact between NaC103 and cobalt oxide is poor and irregular. The oxygen produced from the decomposition has to build up pressure and force ita way out from inside of the candle block. This will result in irregular oxygen evolution. In addition, pressure buildup inside a candle block can also result in cracks, which would cause irregular reaction front propagation and thus erratic reaction rate. Decomposing in liquid phase does not have those problems and thus has a smoother oxygen generation. Effect of Ba(OH)z*HzOon NaClOs Decomposition in the Presence of Cobalt Oxide, The effect of barium hydroxide on cobalt oxide catalyzed NaC103 decomposition is given in Figure 6. Curve 5 is the TGA profile of NaC103 by itself. Curves 1,2,3,and 4 are NaC103 mixed with 1% cobalt oxide and 0, 1,4,and 8% Ba(OH)2*H20,respectively. TGA curves in Figure 6 look very much like those in Figure 4. Therefore, barium hydroxide has a similar effect in inhibiting cobalt oxide catalyzedNaC103decomposition. When as little as 1 % Ba(OH)2-H20is mixed into the system of sodium chlorate containing 1% cobalt oxide, the decomposition temperature is raised by 90 "C. The inhibiting effect appears to saturate at 1%Ba(OH)2-H20. Higher loading of barium hydroxide raises the decomposition temperature only slightly. BaOz Loading and Chlorine Level. It is generally considered that BaO2 can suppress chlorine by reacting with it to form BaC12 and 0 2 according to eq 1 and thus BaO,
+ C12= BaCl, + 0,
(1)
reduce chlorine level in the generated oxygen. However,
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 969 form either through the reverse reaction of eq 4 or as an intermediate compound from the stepwise decomposition of NaC103. Combining eqs 3 and 4 can produce the total reaction as eq 5. The concentration of chlorine can be expressed as eq 6, where K is a constant. A working generator involves 3Cl2+ 60H- + 5C1- + c10;
+ 3H20
(5)
40 K
[C10~1°~33[H201 (6) [OH-12 both molten salts and solid phases and is different from a solution. However, the same trend would hold true. It is clear from eq 6 that the chlorine level is proportional to the concentration of water and inversely proportional to the square of hydroxide concentration. This is consistent with the facts that higher moisture level tends to produce more chlorine and loading of a basic compound can reduce the chlorine level. When a acidic compound such as B203 or Si02 is added, the strength of the base is reduced and more chlorine is produced as reported by Markowitz et al. (1964). In a working candle the reverse reactions as in eqs 3 and 4 can occur. The chlorate can oxidize the chloride to form hypochlorite. The hypochlorite can then react with chloride to evolve chlorine. BaOz and Ba(OH)rH20 can suppress this process and thus reduce the chlorine level.
30
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Literature Cited
C1,
+ H 2 0* H+ + C1- + HClO
(2)
chlorine will disporportionate according to eq 3. The C1,
+ 20H- + C1- + C10- + H 2 0
(3)
hypochlorite formed can further disproportionate to form chloride and chlorate according to eq 4 if the solution is hot. If a base is added to the solution, the equilibria of 3C10- + 2C1- + C10;
(4)
both eqs 2 and 3 move to the right-hand side. Any chlorine present will disproportionate. If an acid is added to the solution, however, the equilibria will move to the lefthand side and thus evolve chlorine. The hypochlorite can
Gao, H.; Ma, W.; Zhang, Y.; Shi, F. Sodium Chlorate Oxygen Candle, China Patent, CN 1035248,1989. Gustafson, P. R. Chlorate Candle for Oxygen Production. US Patent 3,207,695,1965. Gustafson, P. R.; Smith, S. H., Jr.; Miller, R. R. "Chlorate-Candle Fabrication by Hot Pressing"; NRL Report 5732, 1962. Heintz, C. E. Cobalt Oxide Chlorate Candle. US Patent 4,073,741, 1978. Littman, J.; Prince, R. N. "Portable Life Support Systems"; Proceedings of NASA Ames Research Center, April 1969. Markowitz,M. M.;Boryta, D. A.; Stewart, H., Jr. Lithium Perchlorate Oxygen Candle. Ind. Eng. Chem. Prod. Res. Dev. 1964,3, 321. Pappenheimer, J. R. "Development of Oxygen Candle Apparatus for Use in Aircraft"; Report to the Committee on Medical Research of the Office of the Scientific Research and Development, June 1946. Rudloff, W. K.; Freeman, E. S. The Catalytic Effect of Metal Oxide on Thermal Decomposition Reactions. J. Phys. Chem. 1970,74 (18),3317. Wydeven,T.Catalytic Decomposition of Sodium Chlorate. J.Catal. 1970,19, 162.
Received for review November 23, 1992 Accepted February 10,1993
