Ind. Eng. Chem. Res. 1993,32, 2863-2865
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Catalytic Effects of Non-Oxide Metal Compounds on the Thermal Decomposition of Sodium Chlorate Yunchang Zhang,' Girish Kshirsagar, John E. Ellison, and James C. Cannon Puritan-Bennett Corporation, 10800 Pflumm Road, Lenexa, Kansas 66215
Thermal decompositions of NaC103 in the presence of various non-oxide metal compounds were studied using thermogravimetric analysis. The non-oxide additives show catalytic activities similar to those of the corresponding metal oxides. The correlations between electron configurations and catalytic activities are discussed. It is found that metal cations with partially filled d orbitals are all active catalysts. Metal cations with doconfigurations are moderately active. Metal cations with dl0 or noble gas configurations have the lowest activity.
Introduction Alkali metal chlorates and perchlorates can generate oxygen upon thermal decomposition. Thermal decomposition of alkali metal chlorates and perchlorates catalyzed by metal oxide additives has been extensivelystudied (Rudloff and Freeman, 1970; Shimokawabe et al., 1977; Said et al., 1983; Iwakura et al., 1991; Morishima et al., 1991; Feng et al., 1988). However, not much has been reported on the decomposition of NaC103 catalyzed by non-oxide metal compounds. It is reported that sodium dichromate catalyzes the decomposition of KC103 (Udupa, 1981). It is also reported that when FeCl3 and CuC12 were doped into KC103, the decomposition temperature of KC103 decreased significantly (Rudloff and Freeman, 1980). This may also be a catalytic effect rather than an effect of doping, because no structural changes were detected by X-ray diffraction analysis, and the chlorides have very different crystal structure from KC103. It is likely that other non-oxidemetal compounds are also active for the decomposition of the chlorates. Previous authors have explained the catalytic activities in several different or even conflicting ways based on the band structures and electronic properties of the metal oxides (Rudloff and Freeman, 1970; Shimokawabe et al., 1977; Said et al., 1983; Iwakura et al., 1991). Feng et al. (1988) have proposed that the catalytic activity may be related to d electronsand considered that oxides containing metal cations with half-filled d orbitals have the highest activity. If the activity is related to the electronic properties, non-oxide compounds such as sulfates would have very much different activity from the corresponding oxides. If the activity is related only to metal cations, on the other hand, non-oxide metal compounds should show behaviors similar to those of the corresponding oxides. Therefore, it is the purpose of this study to compare the relative activities of the non-oxide metal compounds on the decomposition of sodium chlorate, and to compare the catalytic activities of non-oxide metal compounds with those of metal oxides. The relationship between the catalytic activity and electron configurations of the metal cations will be discussed on the basis of the relative activity. Experimental Section Sodium chlorate (NaC1031, sodium dichromate (Na2Cr20,-2H20), manganese(I1)chloride (MnCL4H20), iron(111) chloride (FeCly6Hz0), cobalt(I1) chloride (CoCly 6H2O),cobalt(III)fluoride (CoFs),nickel(II)sulfate (NiSOq 7&0), copper(1) chloride (CuCl), copper(I1) chloride (CuCl2+2H20),copper(I1) sulfate (CuS04*5H20),lithium silicate (LizSiOa),and sodium stannate (NazSnOy3Hz0)
as purchased were in the form of crystalline particles. Calcium hydroxide (Ca(OH)2),lanthanum hydroxide (La(OHI3), and manganese(I1) carbonate (MnC03) were powders. Manganese oxide, cobalt oxide, and copper oxide were prepared by decomposing the corresponding carbonates at 425,350, and 260 OC, respectively. The oxide products were analyzed by X-ray diffraction analysis using a Rigaku D/maxII diffractometer. The surface areas of these oxides were measured with a BET sorptometer using the multipoint measurement. Each sample was heated at 150 OC in vacuum for 30 min to drive off moisture and other adsorbed gases. Nitroen was used as adsorbent gas. Each of the non-oxide and oxide additives was mixed with sodium chlorate in a molar ratio of 4 % corresponding to a ratio of 1mol of metal cations to 24 mol of NaC103. Each of the mixtures was intimately mixed using a mortar and pestle. Thermogravimetric (TG) analysis was carried out using a Netzsch thermal analyzer Model STA 409. Approximately 100 mg of sample was heated up 20 OC/min to 700 "C in an oxygen stream of 150mL/min. The sample weight was recorded against ita temperature.
Results and Discussion The manganese oxide, cobalt oxide, and copper oxide are identified as MnO2, Co304, and CuO, through X-ray diffraction analysis. Surface areas of the MnO2, Co304, and CuO are 58,61, and 63 m2/g,respectively. The nonoxide additives were not measured for their surface areas because of their low stability. Actually, surface area has less effect on these non-oxide compounds because they decompose at relatively low temperatures. The TG profiles of NaC103 with 4 mol % of CoClz-6Hz0, MnCL4H20, CuC1~2H20,FeCb6H20, NiS04.7H20, Na2Cr207-2H20,La(OH)3, NazSnOr3H20, Ca(OH)2,and Li2Si03 are given in Figure 1. Since the decomposition onset temperature is relatively difficult to determine unambiguously, the temperature at which 50% of the sodium chlorate has decomposed (50%DT) is used in comparing the relative activity of each additive. The non-oxide additives in Figure 1can be divided into three groups according to their catalytic activities. The first group includes chloride of manganese(II1, iron(III), cobalt(I1) and copper(II), and nickel(I1) sulfate. The compounds of this group are all very active catalysts for NaC103 decomposition and they bring down the 50% DT by 260 OC or more. The second group includes lanthanum hydroxide and sodium dichromate. These two compounds are moderately active, and they bring down the 50% DT by 120and 150 OC, respectively. The third group includes
0888-5885/93/2632-2863$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
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Figure 1. TG curves of NaC103 catalyzed by non-oxide additives. (1) CoClr6Hz0, (2) MnClz.4Hz0, (3) CuClr2Hz0, (4) FeC1~6Hz0, (5)NiSO44Hz0, (6)Na2Crz0~2Hz0,(7) La(OH)3,(8)NazSnO3.3H~0, (9)Ca(OH)2, (10) Li~Si03,and (11)no catalyst. 120 I
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Figure 2. TG curves of NaC103 catalyzed by cobalt compounds and manganese compounds. (1) CoC12.6Hz0, (2) c0304, (3) CoF3, (4) MnClr4Hz0, ( 5 ) MnOz, and (6) MnCos.
sodium stannate, calcium hydroxide, and lithium silicate. These compounds have very low activities and can bring down the 50% DT by less than 20 OC. The manganese(I1) chloride, iron(I1) chloride, and cobalt(I1) chloride are even more active than the corresponding metal oxides as given in Figure 2 and as reported (Wydeven, 1970; Iwakura et al., 1991; Morishima et al., 1991). This is because all these chlorides have low melting temperatures. MnCly4H20, FeCl~6Hz0, and CoCly6Hz0 melt at 58, 37, and 86 OC, respectively. Once melted, an additive can have very intimate contact with NaC103 particles and thus produce higher activity. CoC12.6H20 has the highest apparent activity. NaC103 with 4 mol % CoCl~6H20decomposes completely by 245 OC compared to the melting temperature 260 OC for NaC103. Therefore, most, if not all, of the NaC103 decomposes in the solid state. Figure 2 is a catalytic activity comparison of three cobalt compounds and three manganese compounds. Even though CoC12.6H20, Co304,and COF3have different crystal structures, different anions, and different oxidation states of cobalt, their activities are similar. The 50% DTs are all within a range of 25 "C. MnCly4H20, MnO2, and MnC03 also have similar activities. The 50% DTs of NaC103 catalyzed by these three compounds are located in a range of 30 OC. Once again, the differences in crystal structures, anions, and oxidation states have not caused much difference in the catalytic activity.
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Figure 3. TG curves of NaC103 catalyzed by copper compounds. (1) CuClr2H20, (2) CuS046Hz0, (3) CUO,and (4) CuCl.
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Figure 4. Thermal decomposition of some non-oxide metal compounds. (1)CuCl, (2) CoF3, (3)La(OH)s,(4) MnCOs, (5) CuClrlHzO, (6) CuSO4.5Hz0, (7) MnC12.4H20, (8) NiSOda7Hz0, and (9) CoClz.6H20.
TG curves of NaC103 catalyzed by four different copper compounds are compared in Figure 3. The 50% DTs of NaC103 catalyzed by CuO, CuS0~5H20, and CuCl~2Hz0 fall in a range of less than 10 "C. CuC1, however, has much lower activity. The 50% DT of NaC103 catalzed by CuCl is approximately 80 "C higher than when catalyzed by the other copper compounds. Figure 4 shows the TG curves of the decompositions of La(OH13,MnC03, MnCl~4Hz0, COF3, CoCL6H20, CuC1, CuCly2Hz0, CuS04.5H20, and NiS0~7H20.This figure shows that these non-oxide metal compounds have not been converted to the corresponding oxides before the decomposition of NaC103 in the presence of each of the compounds. The decomposition temperature of calcium hydroxide is given in handbooks as 580 OC, which is higher than the 50% DT of sodium chlorate. MnC03 starts decomposing gradually at about 300 "C to MnO2 as verified by X-ray diffraction analysis, and the MnO2 starts decomposing at 525 OC to Mn304 as reported (Bamford and Tipper, 1980). MnCly4HzO loses four hydration water molecules by about 260 OC. The anhydrous MnC12 formed is stable to about 400 "C. CoCL6H20 loses its six hydration water molecules below 225 OC. The anhydrous cobalt(I1)chlorideformed through the dehydration was rather stable. No observable oxidation occurred up to about 550 "C. There is no observable reaction between CoF3 and oxygen up to 600 OC, except a small amount of weight loss below 200 OC. This weight loss is due to moisture adsorption.
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2865 N i S 0 ~ 7 H 2 0loses six of its hydration water molecules below 200 "C and loses is seventh water molecule between 350 and 450 "C. No further decomposition can be observed up to 700 "C. CuCl is stable up to about 350 "C in oxygen. A t about 350 "C it starts gaining weight. This is due to the reaction of CuCl and oxygen. Cu+ in CuCl is partially oxidized to Cu2+. CuC1~2H20decomposes in two steps. The first step below 200 OC corresponds to the loss of two water molecules, and the second step between 450 and 550 "C corresponds to the decomposition of CuC12 to CuC1. CuS0~5H20loses ita five hydration water molecules in two steps by 300 "C. The anhydrous copper(I1) sulfate formed is stable a t least up to 700 "C. Non-oxide compounds MnC03, MnC12, FeC13, CoC12, COF3, CuC12, CuSO4, and NiSO4 are all as active as their corresponding oxides for NaC103 decomposition. The electrical conductivities of these compounds are not available to our knowledge. However, it is unlikely that the ionic compounds such as MnC03, NiSO4, and CuSO4 are conductors or semiconductors. Therefore, the catalytic activity of a compound is not related to its conductivity or semiconductivity. Conductivity and semiconductivitydepend on the band structure and, therefore, on the oxidation state of metal cation, the crystal structure, and the anions associated with the metal caton. However, the catalytic activity is independent of crystal structure, oxidation state of the metal cation, and anions associated with the cation as shown in Figure 2. This is more evidence that the catalytic activity is not related to the electronic properties, namely semiconductivity and conductivity, of the catalysts. The catalytic activity is primarily related only to the identity of the metal cations and their electron configurations. The metal compounds of the first group in Figure 1and those in Figures 2 and 3 except CuCl all have a metal cation with partially filled d orbitals such as Mn2+,Mn4+, Fe3+,Co2+,Co3+,Ni2+,and Cu2+,and they all have a high activity for the thermal decomposition of sodium chlorate. Compounds containing transition metal cations with partially filled d orbitals such as Mn203, Fe203, and NiO have also been reported to have high activity (Morishima et al., 1991). The second group includes compounds containing transition metal cations with completely empty d orbitals (ndo)such as La3+and Cr6+. Compounds of this group are moderately active. Their activities are lower than those of the first group additives and higher than those of the third group. Lanthanide metal oxides such as La203, CeO2, and Gd2O3 all have empty d orbitals, and they are also moderately active as reported (Iwakura et al., 1991). The third group includes compounds containing metal cations with noble gas configurations, such as Li+, Na+, Si4+,and Ca2+,and completely filled d orbitals (ndlo),such as Sn4+. Compounds of this group all have low activity. Other compounds containing cations with noble gas or dl0 configurations such as Si02, A1203, MgO (Iwakura et al., 1991),ZnO (Rudloff and Freeman, 1970),KC1, CsC1, CaC12, and CdC12 (Rufloff and Freeman, 1980) have also been reported to have low activity. Cu+ also has the 3d1° configuration, and, as one would predict, CuCl has much lower activity than Cu2+ compounds. The higher activity of Cu+compared to other d10 cations such as Sn4+ can be explained by the partial oxidation of Cu+ to Cu2+as indicated by the weight gain in Figure 4, curve 1. Metal cations with partially fiied d orbitals have unfilled valence orbitals. These cations have a relatively small size because of less electrical shielding. Therefore, they have a higher tendency to attract extra electrons and
behave as Lewis acid sites. Each of the oxygen atoms in the chlorate group c103- has two unshared electron pairs and can behave as a Lewis base. Therefore, the oxygen can donate one of the unshared electron pairs into the empty valence orbitals of the metal cation, and thereby form a complex. Formation of the Mn+-O coordination bond weakens the 0-C1 bond in the chlorate group and thus results in decomposition of the chlorate. Compounds containing cations with d10 or noble gas configurations, however, are spherically symmetrical and have the lowest energy. The positive nuclear charges of these cations are effectively shielded by electrons. Therefore, they have a lower tendency to attract extra electrons to form extra bonds. The electron pair from the chlorate will also have to go into higher energy orbitals, which is not favored for the formation of the complex. Transition metal cations with doconfigurations are also spherical and have low energies. However, they have empty d orbitals to accommodate the electron pairs from the chlorate. Therefore, do cations are more active than metal cations with dl0or noble gas configurations and less active than transition metal cations with partially filled d orbitals. Catalytic activity depends on the interaction between the chlorate group and the metal cation. The structure and anions of the additive are not important. Therefore, non-oxide metal compounds have the same activities as their corresponding oxides. In summary, non-oxide metal compounds show similar catalytic activities toward sodium chlorate decomposition. The catalytic activity of a compound depends on the electron configuration of the metal cation. Transition metal cations with partially filled d orbitals are all very active, transition metal cations with doconfigurations are moderately active, and metal cations with noble gas or dl0 configurations have only minimal activity.
Literature Cited Bamford, C. H.;Tipper, C. F. H. Comprehensive Chemical Kinetics, Elsevier: New York, 1980;Vol. 22,p 173. Feng, ZhengYuan; Pan, Yunxiang; Wu, Yansun; Zhang, Jianhua Catalyzed Thermal Decomposition of Potassium Chlorate. Huarue Tongbao 1988,No. 9,43. Iwakura, Hideaki; Kakuta, Noriyoshi; Ueno, Akifumi; Kishi, Kazuo; Kato, Jun Oxygen Evolution from KC103 Catalyzed by Metal Oxides as Air Bag Inflators. Ind. Eng. Chem. Res. 1991,30,778. Morishima, Shingo; Iwakura, Hideaki; Kakuta, Noriyoshi; Ueno, AWumi; Kishi, Kazjuo;Kato, Jun Catalytic Performances of Metal Oxides for Thermal Decomposition of Sodium Chlorate. Nippon Kagaku Kaishi 1991,No. 9, 1172. Rudloff, W. K.; Freeman, E. S. The Catalytic Effect of Metal Oxides on Thermal Decomposition Reactions 11. J. Phys. Chem. 1970, 74 (18),3317. Rudloff, W. K.; Freeman, E. S. The Effect of Defect Structure Due to Doping and Irradiation on the Thermal Decomposition of Potassium Chlorate. J. Thermal Anal. 1980,18,411. Said, A. A.; Hassan, E. A.; ABD El-Salaam, K. M. Effect of Oxide Additiveson the Thermal Decompositionof KClOb. Surf. Technol. 1983,20 (2),131. Shimokawabe, Masahide; Furuichi, Ryusaburo; Ishii, Tadao Effect of Metal Oxide Additives on the Thermal Decomposition of Perchlorates, Oxalates and Hydroxides. Thermochim. Acta 1977, 20,347. Udupa, M. R. Thermal Decomposition of Sodium Chlorate and Chromium(II1) Oxide Mixtures. J. Thermal Anal. 1981,21,221. Wydeven,T. Catalytic Decomposition of Sodium Chlorate. J.Catal. 1970,19, 162. Receiued for review March 15, 1993 Accepted June 22, 1993O ~~~~~~
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Abstract published in Advance ACS Abstracts, August 15, 1993.
