Environ. Sci. Technol. 2006, 40, 823-829
Dichlorodifluoromethane Decomposition to CO2 with Simultaneous Halogen Fixation by Calcium Oxide Based Materials
important for initiation of the decomposition reaction. However, partially fluorinated MgO, which can be formed by either the decomposition reaction, or by hydrofluoric acid treatment, can selectively absorb fluorine to form MgF2 (eq 1) (13).
CCl2F2(g) + MgO(s) f
/2CO2(g) + 1/2CCl4(g) + MgF2(s); ∆H° ) - 289 kJ mol-1 (1)
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TSUKASA TAMAI, KOJI INAZU, AND KEN-ICHI AIKA* Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
The decomposition of CCl2F2 to CO2 and accompanying halogen fixation by a CaO based material was studied. To improve the low reactivity of CaO, a consequence of its low surface acidity, transition metal oxides were added. Impregnation of metal acetylacetonate followed by removal of the ligand under vacuum was found to be an effective method. This method resulted in the formation of carbonaceous species and the reduction of metal oxide to metal, both of which were thought to initiate the decomposition reaction. The reactivity of these materials (MOx(a)/CaO-vac) was found to be in the following order: M ) Ni > Cu > V ) Fe > Mn > Co > Ca. In particular, nickel supported on CaO was most effective for the decomposition of CCl2F2. During the preparation, nickel oxide was reduced to the metal phase. CCl2F2 was decomposed to CO2 with a small amount of CO, and halogens were fixed as CaFCl, without significant deactivation at 723 K.
Introduction Chlorofluorocarbons (CFCs) are environmentally toxic materials. CFCs are also a major cause of ozone depletion and probably contribute to global warming. Many methods to decompose CFCs have been investigated, but most of them require hydrolysis. Through hydrolysis, corrosive products such as HCl and HF are formed. Mineralization of such toxic organohalogenated compounds is a desirable solution, because halogens are directly fixed as metal halides and formation of HCl and HF does not occur. Although this method is simple and useful, such direct mineralization of CFCs has not been well studied. Burdenic and Crabtree have reported that sodium oxalate can decompose CCl2F2 effectively at 616-636 K (1). Martyanov and Klabunde have reported CCl3F decomposition using a vanadium oxide-magnesium oxide system (2). Destructive fixation of chlorocarbon and hydrochlorocarbon into metal chlorides, which is easier than the fixation of CFCs, has been well investigated using La2O3, CeO2 (3), alkaline earth metal oxides (4), MgO, and CaO (5-11). In particular, transition metal oxide loaded MgO and CaO, (called second-generation particles) are able to effectively decompose CCl4 (8-11). We have studied CCl2F2 decomposition by MgO based materials (12-14). The reactivity of MgO was found to be markedly enhanced by either fluorination or metal oxide addition, where an increase in the acidity of MgO was * Corresponding author fax: +81-45-924-5441; phone: +81-45924-5416; e-mail:
[email protected]. 10.1021/es050139f CCC: $33.50 Published on Web 12/20/2005
2006 American Chemical Society
MgO based materials have a low reactivity for CCl4 and the low absorption ability of chlorine is the reason for this phenomenon. Partially fluorinated MgO sites can be formed once decomposition starts, then selective fluorine fixation of MgO occurs automatically. Among the metal oxides supported on MgO, vanadium oxide has a characteristic feature of promoting chlorine fixation to some extent, in addition to fluorine fixation (12, 14). However, the MgCl2 that is formed is unstable and easily converted to MgF2 under the presence of excess CCl2F2 (eq 2).
CCl2F2(g) + MgCl2(s) f CCl4(g) + MgF2 (s);
∆H° ) - 100 kJ mol-1 (2)
Fixation of chlorine is difficult for MgO based materials, especially in CCl2F2 rich conditions. Thus, MgO is thought to be a useful candidate for a selective fluorine absorbent. In this study, CaO was selected as the base absorbent for enhancement of chlorine fixation ability. The expected decomposition reaction of CCl2F2 by CaO is as follows:
CCl2F2(g) + 2CaO(s) f
CO2(g) + CaCl2(s) + CaF2(s); ∆H° ) - 661 kJ mol-1 (3)
It has been reported that for the decomposition of CCl4, the reactivity of CaO is higher than that of MgO (4). CaO is often used as an absorbent for HCl, which means that chlorine is easily absorbed to the CaO bulk. However, there are not many reports concerning the direct mineralization of CFCs using CaO. Imamura et al. have reported that CCl2F2 decomposition by limestone (main content is CaCO3 or CaO at high temperature) does not occur effectively (15). They have concluded that it is more useful to use CaO based materials as an absorbent for HF and HCl formed by catalytic hydrolysis of CFCs. The lower reactivity of CaO to CFCs is considered to be due to its strongly basic nature. In this report, we tried to modify CaO for CCl2F2 decomposition in the same way as has been done for MgO. Fluorination and metal oxide addition (in this case metal separation may be necessary after use) were used in an attempt to enhance the reactivity of CaO, and the results were compared with those for MgO.
Experimental Section Materials. CaO (Aldrich, 99.9%, 26 m2 g-1) was suspended in water and vigorously stirred for 10 h at room temperature, then dried at 353 K for several hours. The sample was further dried at 393 K overnight in a drying oven, and the formed Ca(OH)2 was heated under vacuum at 723 K for 2 h and finally at 873 K for 4 h to form CaO. The resultant CaO had a surface area of 65 m2 g-1. CaF2-CaO was obtained by adding hydrofluoric acid to an aqueous suspension of CaO, heating the suspension until dry, and then finally heating under vacuum at 873 K for 4 h. The degree of fluorination was 10 mol % to CaO. Vanadium oxide supported on CaO was VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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prepared by the following three methods; CaO was added to an aqueous solution containing 0.5 wt % NH4VO3 (Kanto Kagaku, 99%) and 1 wt % NH4OH at 343 K. The sample suspension was stirred, dried, and calcined in air at 973 K for 3 h (16). The resultant product is denoted as VOx(n)/ CaO-air. CaO was added to a tetrahydrofuran (THF) solution containing vanadium acetylacetonate (V(acac)3, Aldrich, 97%). The sample suspension was stirred at room temperature overnight, dried with a rotary evaporator, and then dried in an oven at 373 K. The V(acac)3/CaO obtained was treated under vacuum or air (denoted as VOx(a)/CaO-vac or VOx(a)/CaO-air, respectively). The temperature program for ligand removal of the VOx(a)/CaO-vac sample was as follows: 2 h increase up to 473 K and held for 1 h; 1 h increase up to 673 K and held for 1 h; and 1 h increase to 873 K and held for 6 h. The same program was applied for VOx(a)/ CaO-air, in addition to a final calcination at 1073 K. Vanadium loading was 5 wt % (V/CaO ) 5 wt %). Acetylacetonate precursors for transition metal oxides of MOx(a)/CaO-vac (M: Ca, V, Mn, Fe, Co, Ni, Cu) were treated in the same manner as VOx(a)/CaO-vac. Metal precursors used were Ca(acac)2 (Tokyo Kasei Kogyo), V(acac)3 (Aldrich), Mn(acac)3 (Wako), Fe(acac)3 (Wako), Co(acac)3 (Wako), Ni(acac)2‚2H2O (Wako), and Cu(acac)2 (Wako). CCl2F2 Decomposition. 1% CCl2F2, diluted with helium (Takachiho Chemical Industry Co., Ltd.), was fed to 0.28 g (5.00 mmol) of CaO-based samples placed in a tubular reactor at a total flow rate of 30 mL/min, after the sample was pretreated with helium at 873 K for 3 h. Analysis of the outlet gas was performed using a gas chromatograph-thermal conductivity detector (GC-TCD, Shimadzu GC-14B) with Porapak-Q column. Characterization. The samples were evacuated in a quartz U-tube reactor at 873 K for 2 h before the temperatureprogrammed desorption of ammonia (NH3-TPD). After the sample was cooled to 373 K, 100 Torr of ammonia was introduced to the system, then an adsorption equilibrium was attained within 30 min. NH3-TPD was performed with a TCD detector by increasing the temperature from 373 to 873 K (Shimadzu GC-8A) after removing physisorbed ammonia under vacuum at 373 K for 0.5 h. XRD patterns of the fresh and reacted samples were obtained in the range of 20-70 degrees with a Rigaku Multiflex S with Cu KR as an incident X-ray source. Thermogravimetric differential thermal analysis (TG-DTA) was performed using an SSC/5200 (Seiko Instruments) from room temperature to 1173 K with a ramp rate of 10 K min-1 under O2/N2 (1:4) flow at 250 mL min-1. Elemental analysis was performed to estimate the amount of carbon deposition for the fresh samples using a Yanaco MT-5 analyzer. Raman spectra of the fresh samples were obtained with a JASCO RMP-200.
Results and Discussion CCl2F2 Decomposition by CaO. The temperature dependence of CCl2F2 decomposition by CaO was first studied as a reference and is shown in Figure 1. CCl2F2 starts to react with CaO at 823 K, and the initial conversion is increased as the temperature increases. However, at 973 K the activity is soon decreased, although sufficient CaO bulk is remaining. This is probably due to the sintering of CaO at high temperatures. In this way, pure CaO cannot be highly reactive. Time course plots of product yields at 873 K, with the CaO conversion calculated from the amount of evolved CO2, are shown in Figure 2. The average CO2 selectivity is approximately 70% and CCl3F and CCl4 are formed as byproducts. CCl2F2 conversion is increased from the 5-min (initial data) to 30min run. Figure 3 shows XRD patterns of the CaO before and after the reaction. Calcium halides, CaF2, and CaFCl, were observed 824
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FIGURE 1. Temperature dependence of CCl2F2 decomposition by CaO. Reaction temperature: 773 K (O), 823 K (0), 873 K (4), 923 K (]), 973 K (×).
FIGURE 2. CCl2F2 decomposition by CaO at 873 K: CCl2F2 conversion (O), CO2 selectivity (0), CCl4 selectivity (]), CCl3F selectivity (×), CaO conversion (b). after the reaction, but CaCl2 was not observed. This agrees with the results of the gaseous products, where only chlorine rich molecules, CCl3F and CCl4, were observed. Under these conditions, larger amounts of fluorine rather than chlorine are absorbed by CaO. It was confirmed by separate experiments that physically mixed CaCl2 and CaF2 heated at this temperature resulted in the formation of CaFCl (eq 4). At this temperature, halogens are mixed well in this system.
CaCl2 + CaF2 f 2CaFCl
(4)
In the case of MgO (13), reaction with CCl2F2 starts at 773 K after an induction period of 2.5 h, and finally CCl2F2 conversion reaches 100%. The induction period tends to be shorter as the reaction temperature increases. MgO is activated during the reaction and the resulting MgO has higher reactivity than CaO. This activation effect is derived from fluorination of the MgO during decomposition, which produces sufficient Lewis acid sites in MgO (13). The effect of fluorination of CaO was also studied. NH3-TPD profiles of 10% fluorinated CaO and pure CaO samples are shown in Figure 4 together with the results of MgO and partially fluorinated MgO. From Figure 4, it is obvious that the acid amount (the area of the profiles) and strength (the peak temperature) of fluorinated CaO is small and weak, compared
FIGURE 3. XRD patterns of CaO before (a) and after (b) CCl2F2 decomposition at 873 K for 5 h: CaO (O), CaFCl (1), CaF2 (9).
FIGURE 5. TG-DTA analysis of V(acac)3/CaO. Flow gas: O2 50 mL/ min and N2 200 mL/min. Temperature ramp rate: 10 K/min. Temperature and DTA (a), and DTG-TG curves (b) as a function of time.
FIGURE 4. NH3-TPD profiles of fluorinated CaO and MgO: 10% MgF2-MgO (a), MgO (b), 10% CaF2-CaO (c), CaO (d). to those of the pure CaO and MgO samples. Active site formation due to acid site formation is not the case for CaO. Characterization of CaO with Vanadium Oxide Additive. In our previous research, vanadium oxide addition to MgO was found to be effective in enhancing CCl2F2 decomposition and halogen absorption (12). Therefore, vanadium oxide addition to CaO was examined in this study. Three different samples were prepared: VOx(n)/CaO-air, VOx(a)/CaO-air, and VOx(a)/CaO-vac. The surface areas of these samples were 19, 7, and 86 m2g-1, respectively. TG-DTA analysis of V(acac)3/CaO in air is shown in Figure 5a and b. Oxidative decomposition of acetylacetonate occurs at 473 and 673 K. These processes were extremely exothermic (Figure 5a) with accompanying CO2 formation. Most of the CO2 must be adsorbed on to CaO, because of the weight decrease observed at approximately 1073 K (CaCO3 decomposition). Phase shift of the sample at such a high temperature causes severe sintering. The resulting VOx(a)/CaO-air sample, which was calcined under air at 1073 K as described in Experimental Section, had a surface area of 7 m2g-1. On the other hand, for the case of thermal decomposition of acetylacetonate under an inert atmosphere or vacuum, only a small amount of CO2 may be produced. Even if CO2 is evolved and adsorbed on CaO, the decomposition tem-
perature of CaCO3 must be shifted to a lower value (17, 18). The resulting VOx(a)/CaO-vac, which was heated under vacuum at 873 K, had a high surface area of 86 m2g-1 with no CaCO3 present. The reason for the high surface area is assumed to be due to the low-temperature treatment and also to deposition of carbonaceous species (on the surface), which may protect the morphological change of CaO. It has been reported that thermal decomposition of acetylacetonate complexes in an inert atmosphere or vacuum causes carbon deposition in some cases, and reduction of the metal oxide (19, 20). XRD patterns of the three samples were obtained. VOx(a)/CaO-vac showed only diffraction peaks for CaO, and small peaks of Ca(OH)2, but no CaCO3 peaks. VOx(n)/CaOair and VOx(a)/CaO-air had diffraction peaks for Ca3(VO4)2 and unidentified calcium vanadate peaks. In the case of vanadium oxide supported on MgO calcined in air, Mg3(VO4)2 is observed with a high vanadium loading (14). VOx(n)/CaO-air, which has a higher surface area than VOx(a)/CaO-air, was then selected as being representative of the two similar samples and was compared with VOx(a)/CaO-vac. The Raman spectra of the two VOx/CaO samples are shown in Figure 6. VOx(a)/CaO-vac shows two peaks at 1578 and 1354 cm-1, which are assigned to G-carbon (graphitic) and D-carbon (disordered), respectively. Therefore, carbon deposition during evacuation was confirmed, and no vanadium oxide species were observed, in agreement with the results of XRD. On the other hand, VOx(n)/CaO-air had no such carbonaceous-derived peaks, but did show two peaks at 858 and 801 cm-1. These spectra are similar to those of authentic Ca3(VO4)2 (21) and Mg3(VO4)2 samples assigned to VO4 tetrahedra (22-24). Similar spectra have already been VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Raman spectra of VOx(a)/CaO-vac (a) and VOx(n)/CaOair (b).
FIGURE 8. XRD pattern of VOx(a)/CaO-vac after CCl2F2 decomposition for 5 h at 723 K: CaO (O) CCaCO3 (0), CaClF (1), CaF2 (9).
TABLE 1. Surface Area and Carbon Deposition of MOx(a)/ CaO-vac Samples
FIGURE 7. CCl2F2 decomposition by VOx(a)/CaO-vac at 723 K: CCl2F2 conversion (O), CO2 yield (0), CO yield (4), and CaO conversion (b). observed for the VOx(a)/MgO-air sample with a high vanadium content (14). A small peak at approximately 1200 cm-1 is thought to be due to CaCO3, which must be formed during sample preparation. Since acidity is important for activation of the absorbent, and therefore the decomposition of CCl2F2, the NH3-TPD of VOx(n)/CaO-air was also measured. The acidity was as weak, and the amount of acid sites was as small as those of the reference CaO. This was in contrast to the case of MgO, where vanadium oxide addition and formation of magnesium vanadate significantly increased the acidity of MgO (14). Generation of acid sites is not caused with vanadium oxide itself, but is due to the combination of vanadium and magnesium oxide. The reason for the low acidity of calcium vanadate is unknown, although it is thought that the stronger base character of CaO, compared with MgO, would be the main reason. Reaction of CCl2F2 with CaO and Vanadium Oxide Additive. CCl2F2 decomposition was performed using the two obtained materials, VOx(n)/CaO-air and VOx(a)/CaOvac, at a reaction temperature of 723 K. Only VOx(a)/CaOvac exhibited reactivity. VOx(a)/CaO-air was not reactive at this temperature, as was CaO. Time on stream decomposition of CCl2F2 by VOx(a)/CaO-vac is shown in Figure 7. An initial high CCl2F2 conversion (100%) was obtained, but deactivation occurred quickly. The reaction products observed were CO and a small amount of CO2. The lack of carbon mass balance and no observation of CO2 at longer reaction times could be due to the formation of CaCO3. Other byproducts such as CCl3F and CCl4, which were formed in the case of vanadium 826
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MOx(a)/CaO-vac (M))
surface area m2 g-1
carbon deposit wt %
Ca V Mn Fe Co Ni Cu
68 87 94 91 69 95 97
1.2 3.4 3.0 2.7 3.3 3.6 3.4
oxide supported on MgO sample, were not observed. For this reason, the conversion of CaO was calculated based on the amount of decomposed CCl2F2. The XRD pattern of VOx(a)/CaO-vac after the reaction is shown in Figure 8. Diffraction peaks of CaFCl and CaCO3 were observed, but the diffraction peaks of CaO were quite weak. From these results, the reaction with VOx(a)/CaO-vac can be expressed approximately as follows (eq 5):
CCl2F2(g) + CaO(s) f CO(g) + CO2(g) (CaCO3(s)) + CaFCl(s) (5) CCl2F2 Decomposition by Metal Oxide or Metal Loaded on CaO. Since VOx(a)/CaO-vac showed high reactivity, other metal oxides supported on CaO (MOx(a)/CaO-vac; M ) Ca, Mn, Fe, Co, Ni, Cu) were prepared in the same manner as VOx(a)/CaO-vac, and the reactivity was compared. The reason for preparing CaOx(a)/CaO-vac was to clarify the effect of carbon deposition on CaO (without any other metallic species present). Simple diffraction patterns of CaO were observed for the four MOx(a)/CaO-vac (M ) Ca, V, Mn, Co) samples. Peaks due to a metallic phase were also observed in addition to the CaO pattern for the three MOx(a)/CaO-vac (M ) Fe, Ni, Cu) samples. These oxides are known to be easily reduced to the metallic phase. The BET surface area and the amount of deposited carbon are listed in Table 1. All of the samples bear 1-4 wt % of carbon, and have a specific surface area of 70-100 m2 g-1. Figure 9 shows the results of the activity test for conversion of CCl2F2 (a), conversion of CaO (b), CO yield (c), and CO2 yield (d). The results for all the MOx(a)/CaO-vac samples are similar to those of VOx(a)/CaO-vac; CO and CO2 were the only products and no other byproducts were formed. CaO
FIGURE 9. CCl2F2 decomposition by MOx(a)/CaO-vac (M ) Ca (O), V (4), Mn (0), Fe (]), Co (b), Ni (2), Cu (9)) at 723 K: CCl2F2 conversion (a), CaO conversion (b), CO yield (c), CO2 yield (d). conversion is calculated based on the amount of CaO and decomposed CCl2F2. In all of the samples, except for CaOx(a)/CaO-vac, the initial conversion of CCl2F2 was 100%. However, for CuOx(a)/CaO-vac and NiOx(a)/CaO-vac, the CCl2F2 conversion decreased drastically with time (Figure 9a). NiOx(a)/CaO-vac maintained the highest reactivity (100%) to CCl2F2 for more than 2.5 h, where the CaO conversion exceeds 70%. This result is surprising, because these types of solid-gas reactions tend to be deactivated with increased reaction time. Product yields are shown in Figure 9c and d. It is clear that the formation of CO is smallest for NiOx(a)/CaO-vac, and the formation of CO2 was the largest for this sample. This suggests that the reaction scheme is similar, in that it is independent of the supported metal species, except in the case of Ni. CaO conversion for NiOx(a)/CaO-vac finally converged to 85%. The actual value of CaO conversion should be higher, because CaO conversion is calculated by disregarding the amount of carbonaceous species deposited on CaO and the amount of nickel. The detection of CO2 is observed only after prolonged reaction periods, due to the CO2 formed being readsorbed on CaO. For any samples on which CO2 was detectable, CO2 only became observable when the conversion of CaO was over 50%. Reaction Mechanism for CCl2F2 Decomposition using CaO Containing Carbon. The reactivity of CaO itself to CCl2F2 was low, as shown in the former section. This is due to the low acidity of the CaO surface during the reaction (even after absorbing some amount of fluorine). We were not successful in creating acidic sites to improve the reactivity of CaO. Partial fluorination and vanadium oxide addition (with formation
of composite oxide compounds, which were effective in introducing acid sites on MgO to increase reactivity) were not effective in the case of CaO. This is probably due to the strongly basic nature of CaO compared with MgO. In this study, the deposition of carbonaceous species on the CaO surface was found to drastically improve the reactivity of CaO. The reaction performance (Figure 9) is different from that of pure CaO (Figure 2). It is obvious that carbonaceous species on CaO are the active species, because CaOx(a)/ CaO-vac that has only carbonaceous species on the surface becomes reactive even at 723 K. Colussi and Amorebieta have reported chlorofluorocarbon (in this case CCl3F) decomposition by active carbon (25). In this case the reaction is discussed as being initiated by the homogeneous dissociation of a C-Cl bond. Although CCl2F2 is more difficult to decompose than CCl3F, because of its stronger carbonhalogen bond, CCl2F2 decomposition is probably initiated in a similar way by the homogeneous dissociation of the C-Cl bond, which is weaker than the C-F bond. Carbon containing CaO is considered to decompose CCl2F2, forming CO2 and CO without forming any other toxic halogenated byproducts such as CCl4 or CCl3F. No halogen exchange reaction occurred in our system, and dissociation of the halogen-carbon bond in CCl2F2 continues to change CCl2F2 to CO and CO2. Formation of CO means that CCl2F2 is reduced during the reaction. There is no indication of the formation of oxidized product such as oxygen, chlorine, and fluorine during the decomposition; therefore, carbon must be involved in the reaction (eq 6).
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TABLE 2. Results of CCl2F2 Decomposition by MOx(a)/CaO-vac Samples at 723 K for 5 h and the Calculated Reaction Coefficient from Eq 10 MOx/(a)CaO-vac (M))
100y/% (CO select.)
100z /% (CO2 select.)
Ca V Mn Fe Co Ni Cu
51.5 68.3 62.8 61.0 60.1 12.9 52.4
0.0 11.8 0.0 21.7 0.0 78.9 37.0
reaction coefficient for eq 10 CaO C CaCO3 5.49 5.08 5.37 4.96 5.40 4.29 4.74
0.51 0.68 0.63 0.61 0.60 0.13 0.52
1.49 1.08 1.37 0.96 1.40 0.29 0.74
CO formation was not observed for the CaO sample without carbonaceous species (Figure 2). The formation of CO2 can be explained by the reaction given in eq 7. If any CO2 is actually formed, it is easily readsorbed or absorbed by CaO at a reaction temperature of 723 K (eq 8). The main reaction can be described as given in eq 9.
CCl2F2 + 2CaO f CO2 + 2CaFCl
(7)
CaO + CO2 f CaCO3
(8)
CCl2F2 + 3CaO f 2CaFCl + CaCO3
(9)
Using these equations, a total reaction equation can be written as follows (eq 10) with y as CO selectivity and z as CO2 selectivity.
2CCl2F2 + (6 - y - 2z)CaO + yC f 4CaFCl + 2yCO + 2zCO2 + (2 - y - 2z)CaCO3 (10) Using this equation, CCl2F2 decomposition (for 5 h) was analyzed for each sample and the results are listed in Table 2. Except for NiOx(a)/CaO-vac, the reaction coefficients for CaO, C, and CaCO3 are much the same for all samples, with ranges of 4.7-5.5, 0.5-0.7, and 0.74-1.49, respectively. As an example, the reaction equation for CaOx/CaO-vac can be approximately described as follows:
2CCl2F2 + 5.5CaO + 0.5C f 4CaFCl + CO + 1.5CaCO3 (11) In this case, no CO2 was formed, and consumption of carbon was 83% of the original C source after 5 h. This high consumption of carbon (surface active species) must be one of the reasons for deactivation (Figure 9a). On the other hand, NiOx(a)/CaO-vac shows quite a different result; a low reaction coefficient for C and CaCO3 and a coefficient of almost 4 for CaO. In short, the reaction equation approximates eq 7, which does not incorporate deposited carbon. For the NiOx(a)/CaO-vac sample, the formation rate of CO was low. Ni as a catalyst has been known to bear carbon easily as reported by various reactions, such as the reforming of methane (26, 27) and methane decomposition to obtain hydrogen (28). Thus, carbon deposition from the CO disproportionation reaction (eq 12) can decrease the formation of CO on NiOx(a)/CaO-vac.
2CO f CO2 + C
(12)
To confirm the above hypothesis, the CO disproportion reaction was carried out at 723 K for MOx(a)/CaO-vac samples, and the results are shown in Figure 10. In fact, CO conversion on NiOx(a)/CaO-vac was over 70% and showed the highest reactivity. CoOx(a)/CaO-vac had the second highest reactivity, but it was soon deactivated. The other 828
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FIGURE 10. CO disproportionation reaction (2CO f CO2 +C) over 0.1 g of MOx(a)/CaO-vac sample at 723 K: M ) V (O), Mn (4), Fe (0), Co (]), Ni (b), Cu (2). CO: 1% (helium balanced), 30 mL/min. samples showed almost no reactivity. The phenomenon that CoOx(a)/CaO-vac has low CO selectivity at the initial stage of CCl2F2 decomposition (Figure 9c) is likely to be related to CO disproportionation. After some time on stream the CO selectivity increased, which could be due to the change of the CoOx species during the reaction. Metallic Ni is known to dissociate the CO bond, so that once CO is formed during the reaction, it can be converted into C and CO2. Thus, in the case of NiOx(a)/CaO-vac, the carbonaceous species (active sites) can be regenerated. For the other samples, carbonaceous species are consumed without regeneration during CCl2F2 decomposition. This is likely to be a reason for quick deactivation, in addition to CaO consumption to form CaCO3. A detailed reaction mechanism is unclear at present. However, it is clear that carbonaceous species and metallic species have important roles, and the reaction mechanism is quite different in the case of transition metal doped MgO materials (MOx(a)/MgO: M ) V, Mn, Fe, Co, Ni, Cu), where the reactivity is controlled by the transition metals; V > Cu ) Fe > Mn > Co > Ni (12). These materials had no carbonaceous species present, because residual carbonaceous species were combusted in air during the ligand removal process, and metallic species are not formed in the oxygen atmosphere. Instead, acid sites must be relevant in these cases. Except for vanadium supported on MgO, these materials yielded almost an equivalent amount of CO2 and CCl4 during the reaction, because of the low reactivity of CCl4. Vanadium oxide supported on MgO, which is able to decompose CCl4, can give high CO2 selectivity. In this way the MOx(a)/CaO-vac system has quite different reaction mechanisms and characteristics compared to previously reported systems of metal oxide supported on MgO. Klabunde et al. have already reported the decomposition of CCl4, etc., with transition metal oxide loaded MgO or CaO (8-11) obtained from impregnation of metal acetylacetonate precursor and ligand removal under vacuum. In this case, they have not mentioned the possible participation of carbonaceous species and/or metal species formation and participation in decomposition. In the case of transition metal oxide loaded MgO, they reported that [Fe2O3]AP-MgO species had 4.29% of carbon according to elemental analysis (9). In the case of transition metal oxide loaded CaO, these features are not clear as there are no characterization data other than surface area. Their conclusion at that time was as follows: a transition metal oxide loaded on MgO or CaO is an active species that first reacts with CCl4 to form a metal chloride species. The resultant metal chloride species
exchanges anion with MgO or CaO, and thereby the transition metal oxide species are regenerated. In this catalytic cycle, chlorine is absorbed as MgCl2 or CaCl2. If carbonaceous species and metal species exist on MgO or CaO, as in this work, such species can assist CCl4 decomposition, and the reaction mechanism should be changed, because the C-Cl bond of CCl4 is much weaker than the C-Cl bond of CCl2F2. The role of carbonaceous species is more important in the case of CCl4, and we confirmed this fact by deposition of carbonaceous species to MgO using a magnesium acetylacetonate precursor, which had much higher reactivity than pure MgO. The activity is the highest in NiOx(a)/CaO-vac followed by CuOx(a)/CaO-vac. The lowest reactivity was observed for CaOx(a)/CaO-vac. The other four samples had almost similar reactivity. In NiOx(a)/CaO-vac and CuOx(a)/CaO-vac metallic species were observed that could also be the active species. Copper oxide and nickel oxide are easily reduced materials, so that the reducibility of the metal oxide must be an important factor. The reason for the low reactivity of CaOx(a)/CaO-vac is probably not due to the absence of other metal species, but due to the low amount of carbonaceous species (1.2 wt %), which was 1/3 of the other samples. MOx(a)/CaO-vac (M ) V, Mn, Fe, Co) had similar reactivity. This is probably due to the similar amount of carbonaceous species present, with metal oxide species not playing an important role in the reaction, contrary to the work of Klabunde et al. for CCl4. In our reaction system, the transition metal species may also have a role in the acceleration of anion exchange (halogen to oxygen in CaO) for the NiOx(a)/CaO-vac and CuOx(a)/CaO-vac systems. The importance of metallic species rather than the metal oxide species present should be stressed. During the reaction, some of the metallic species change to metal chloride or fluoride. The resultant metal chloride or fluoride can assist the halogen exchange reaction to form CaFCl and metal oxide. The metal oxide would be reduced again by CO to regenerate the metallic species. In this way, the reducibility of the metal oxide would be an important factor. This would be the reason for the high reactivity of the NiOx(a)/CaO-vac and CuOx(a)/CaO-vac systems. The detailed role of nickel metal and carbonaceous species will be published in connection with the reaction mechanism.
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Received for review January 21, 2005. Revised manuscript received October 10, 2005. Accepted November 9, 2005. ES050139F
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