Ind. Eng. Chem. Res. 2007, 46, 3073-3077
3073
Catalytic Effects of Metals on Thermal Decomposition of Sodium Chlorate for Emergency Oxygen Generators Evgeny Shafirovich,† Changjun Zhou,‡ Sambandan Ekambaram,† Arvind Varma,*,† Girish Kshirsagar,§ and John E. Ellison§ School of Chemical Engineering, Purdue UniVersity, 480 Stadium Mall DriVe, West Lafayette, Indiana 47907, Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and B/E Aerospace Inc., 10800 Pflumm Road, Lenexa, Kansas 66215
Chemical compositions for emergency oxygen generators typically include sodium chlorate (NaClO3) as the oxygen source, transition metal oxide as catalyst for NaClO3 decomposition, and metal fuel to provide energy for self-sustained combustion. To clarify the role of metal fuel in the combustion mechanism, potential catalytic effects of metals on NaClO3 decomposition are investigated. Thermal analysis was conducted in argon flow for binary NaClO3/Me mixtures, where Me ) Al, Fe, Co, Ni, and Sn, as well as in oxygen flow for metal powders alone and mixtures of NaClO3 with Sn and Co. The experiments show that catalytic effects of metals on NaClO3 decomposition range from negligible for Al and Sn, to significant for Co and Ni, while Fe exhibits moderate catalytic activity. Thermodynamic calculations indicate that replacement of Sn with Co or Ni does not significantly alter combustion characteristics, which determine product oxygen flow rate. Replacing Sn with Co or Ni may decrease the number of ingredients in oxygen generators, since these metals both catalyze and fuel the process. 1. Introduction Chemical compositions for emergency oxygen generators typically include sodium chlorate (NaClO3) as the oxygen source. Their ignition leads to exothermic decomposition of sodium chlorate and evolution of oxygen. The reactive mixtures also include transition metal oxide, such as cobalt (Co3O4) or nickel (NiO) oxide, as catalyst for sodium chlorate decomposition1-5 and metal fuel, such as tin (Sn) or iron (Fe), which provides additional energy required for self-sustained combustion.6 The process is characterized by pulsations of front propagation and oxygen flow rate.7-9 A possible reason for the instability is involvement of two exothermic reactions (decomposition of sodium chlorate and oxidation of the metal fuel) with different rate and temperature characteristics. Specifically, the ignition temperature of tin particles in oxygen is higher than the onset temperature of catalyzed NaClO3 decomposition, and this may cause instabilities.7,8 To understand the combustion mechanisms and to ensure stable oxygen generation, it is important to investigate potential catalytic effects of Sn and Fe on sodium chlorate decomposition. Prior studies1-5 have focused on the catalytic activity of metal oxides. It is known, however, that metal powders not only serve as fuel but can also catalyze decomposition of substances such as sodium chlorate. For example, significant catalytic effects of magnesium on decomposition of sodium chlorate and perchlorate were reported recently.10 On the other hand, the use of transition metals, such as Co or Ni, simultaneously as fuels and catalysts is of interest for eliminating metal oxides and reducing the number of ingredients.11 Oxidation heats of Co and Ni are sufficiently large to consider them as potential fuels (see Table 1). It is also interesting to investigate aluminum (Al) due to its exceptionally high oxidation heat and low cost. * To whom correspondence should be addressed. Tel.: (765) 4944075. Fax: (765) 494-0805. E-mail:
[email protected]. † Purdue University. ‡ University of Notre Dame. § B/E Aerospace.
Table 1. Standard Enthalpies of Formation for Main Oxides of Aluminum, Iron, Cobalt, Nickel, and Tin per Mole of Oxide and per Gram of Metal metal
metal oxide
-∆Hf, kJ/mol
-∆Hf, kJ/g
Al Fe Co Ni Sn
Al2O3 Fe2O3 Co3O4 NiO SnO2
1676 822 887 240 581
31.03 7.34 5.01 4.07 4.88
To study the catalytic effects of Al, Fe, Co, Ni, and Sn on decomposition of sodium chlorate, we carried out thermal analysis of binary mixtures NaClO3/Me, where Me is a metal from the list above, in argon flow. Experiments with NaClO3/ Sn and NaClO3/Co mixtures in oxygen atmosphere were also conducted. In addition, thermal analysis was made for oxidation of metal powders in O2 gas flow. 2. Experimental Section The thermal analysis was carried out using SDT 2960 Simultaneous DTA/TGA (TA Instruments), in Alundum crucibles, under 50 mL/min gas (Ar or O2) flow at a heating rate of 10 °C/min. The characteristics of used reagents are given in Table 2. The condensed combustion products were studied using X-ray diffraction analysis (Scintag X1 Advanced Diffraction System). 3. Results and Discussion 3.1. Thermal Analysis of NaClO3/Metal Mixtures in Argon Flow. Figure 1a shows thermogravimetric (TG) curves for mixtures of 80 wt % NaClO3 and 20 wt % metal powder, obtained in argon flow. The results for pure NaClO3 are also shown where the TG curves were recalculated for the theoretical mixture 80 wt % NaClO3/20 wt % inert powder. Thermogravimetric analysis (TGA) shows that Al and both fractions of Sn slightly promote NaClO3 decomposition in the range from 400 to 500 °C and the decomposition is completed at the same temperature as
10.1021/ie060585r CCC: $37.00 © 2007 American Chemical Society Published on Web 08/12/2006
3074
Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007
Figure 1. Thermal analysis of 80% NaClO3/20% Me mixtures in Ar flow, where Me ) Al, Fe, Co, Ni, and Sn: (a) TGA; (b) DTA. The NaClO3/inert values were obtained in experiments with NaClO3, followed by calculations accounting for inert. Table 2. Characteristics of Used Reagents reagent
purity, %
size
BET surface area, m2/g
manufacturer
Al Fe Co Ni Sn (1) Sn (2) NaClO3 Ar O2
99.5 99 99.8 99.9 99.9 99.9 99.5 99.999 99
-325 mesh -325 mesh -325 mesh -325 mesh -325 mesh 1-3 µm -100 mesh
0.53 0.40 0.44 0.59 0.17 0.86
Alfa Aesar SCM Metal Cerac Atlantic Equipment Engineers Atlantic Equipment Engineers Atlantic Equipment Engineers Kerr-McGee Airgas Airgas
for pure salt. The values of weight loss at 600-750 °C show no Al oxidation and noticeable Sn oxidation, which is more significant for its finer fraction. Iron exhibits catalytic activity at 300-370 °C, with no further weight loss up to ∼470 °C. At higher temperatures, the curve is close to that obtained for pure NaClO3. The final product weighs more, clearly due to Fe oxidation. The largest catalytic effect was observed for Co, where decomposition starts at 250 °C and is complete at 400 °C. A distinct plateau is observed at temperatures from 320 to 350 °C, the result of a sequence of steps involved in sodium chlorate decomposition.1-5 Catalytic activity of Ni starts at higher temperatures for Co, while the decomposition is complete at the same temperature (400 °C) with no plateau observed. The weight curves at 400-750 °C indicate that oxidation of Co and Ni occurs in parallel with sodium chlorate decomposition. Figure 1b shows differential thermal analysis (DTA) curves for the same mixtures. The endothermic peak at 265 °C corresponds to melting of sodium chlorate. The mixture of NaClO3 with Al shows an additional endothermic peak at 660 °C due to Al melting. Pure NaClO3 and NaClO3/Sn mixtures exhibit an exothermic peak at 500-550 °C related to decomposition heat release (the data for the coarser and finer Sn fractions are similar to each other). Two exothermic peaks (480 and 550 °C) are observed for NaClO3/Fe. Cobalt and nickel decrease NaClO3 decomposition temperature below 400 °C level.
3.2. Thermal Analysis of Metal Powders in O2 Flow. To clarify the potential influence of metals oxidation on their catalytic effect in NaClO3/metal mixtures, thermal analysis of the metal powders with no sodium chlorate was conducted in oxygen flow. Figure 2 shows that iron is already significantly oxidized at temperatures between 400 and 500 °C. This implies that the plateau in the TG curve for NaClO3/Fe mixture (Figure 1a) may be caused by oxidation of iron, masking the weight loss due to NaClO3 decomposition. Oxidation of cobalt is also significant at these temperatures but decomposition of NaClO3 in mixtures with Co is complete by 400 °C; thus cobalt oxidation cannot significantly influence the process. The same conclusion can be made for nickel. Note, however, that direct reactions between Co (or Ni) and liquid NaClO3 may occur at lower temperatures than the metal oxidation by oxygen gas, and oxide films on the surface of Co and Ni particles may play an important role in their catalytic effects. Oxidation of tin is most significant at temperatures 550-600 °C, somewhat higher than sodium chlorate decomposition in mixture with tin. Thus, Sn oxidation cannot significantly influence the NaClO3 decomposition. As for Al, relatively little oxidation occurs below 750 °C. 3.3. Influence of Metal Fraction on Decomposition of NaClO3/Sn and NaClO3/Co Mixtures. The effects of Sn and Co were studied in more detail because Sn is currently the fuel of choice for chemical oxygen generators used in aircraft6 and
Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007 3075
Figure 2. Thermal analysis of Al, Fe, Co, Ni, and Sn powders in O2 flow: (a) TGA; (b) DTA.
Figure 3. Thermal analysis of NaClO3/Sn mixtures in Ar flow: (a) TGA; (b) DTA.
Co exhibits the largest catalytic effect (Figure 1). For this purpose, experiments were conducted with NaClO3/Sn and NaClO3/Co mixtures in wide ranges of sodium chlorate-metal mass ratios. Figure 3a shows TG curves for NaClO3/Sn mixtures in argon flow. The curves demonstrate the slight catalytic effect of Sn (see curves for 0, 20, and 40% Sn). In addition, Table 3 compares the measured weight at 600 °C with theoretical values calculated for two different assumptions: no and complete Sn oxidation. The TG results indicate that oxidation is small at 20% Sn and significant at 40-80% Sn. DTA shows (see Figure 3b) a significant increase in the exothermic peak height for the mixture with 60 wt % Sn, obviously caused by tin oxidation. Figure 4 shows TG curves for NaClO3/Sn mixtures in oxygen
Table 3. Weight Loss of NaClO3/Sn Mixtures After Heating in Argon Flow Sn fraction, wt %
theor wt, no Sn oxidn, %
theor wt, complete Sn oxidn, %
exptl weight at 600 °C, %
80 60 40 20 0
91.0 82.0 73.0 63.9 54.9
100 98.1 83.7 69.3 54.9
96.0 87.3 77.5 62.0 51.3
flow. It is interesting that oxidation of Sn in the mixtures occurs at lower temperatures than for Sn alone (see curves for 80, 90, and 100% Sn). This implies that tin particles react easier with liquid sodium chlorate than with O2 gas. The measured weights of the samples at 750 °C agree well with the values calculated
3076
Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007
Figure 4. TGA of NaClO3/Sn mixtures in O2 flow.
885 K for the mixture with Sn (combustion products NaCl, O2, and SnO2), 855 K for the mixture with Co (combustion products NaCl, O2, and Co3O4), and 950 K for the mixture with Ni (combustion products NaCl, O2, and NiO). Thus, replacement of tin with cobalt or nickel will not significantly alter the combustion temperature and front propagation velocity characteristics, which determine the product oxygen flow rate. In addition, the studied metal fuels were compared by the amount of oxygen they would consume during combustion of the actual mixture. Note that in current commercial chemical oxygen generators the metal fuel (Sn) consumes
