Ind. Enp. Chem. Prd. Res. Dev. 1982, 21, 570-575
570
The results of these experiments are given in Table III and are shown in Figure 3. The catalysts ranked in decreasing order of activity were ruthenium on carbon > copper(1) chloride > rhodium on alumina > copper(I1) acetate > copper(I1) sulfate. In addition, a comparison of the available copper(1) halides also was made, of which copper(1) chloride gave the best results (Table V). In general, the oxidation of tetraehlorohydrcquinone proceeded in greater yield than in the case of hydroquinone itself, with a maximum yield of 95%. No polymeric materials were isolated in any of the experiments. In those experiments with lower yields, a portion of the starting material was recovered. Literature Cited CAThLIST EUPLO"E0
Figure 3. Comparison of catdystaolvent effectiveness in the oxidation of tetrachlorohydroquinoine with molecular oxygen at elevated pressures.
To investigate the use of these substituted derivatives, several experiments were run in which tetrachlorohydroquinone (IV) was oxidized catalytically at elevated pressures (eq 6). OH
n
OH
0
BuBhltK. C. A.; Pearson. D. E. "Survey of m n l c Synlhases": Wlley-Inlersclence: New Ywk. 1970;VOl. 1. p 730. Fenton. D. M.: Stalnwand.P. J. J . Ckg. Chem. 1974. 39. 701. James. T. H.: Snell. J. M.; Welsaberger. A. J . Am. Chem. SOC. 1998. BO.
2084. MCKillop. A.: Ray. S. J. Synfhesk 1977.847. MeshRsuka.S.: Ichlkawa.M.: Tamaru. K. J . Chem. SOC. Chem. Commwr. 1975,0, 360. Patai. S.. Ed. "The Chemistry 04 oulnom Gomwnds. Parts 1 and 2"; Wlley: New Voh, 1974. Radel. R. J.: Sullivan. J. M.: Hallleeld. J. D. Ind. Eng. Chem. Rod. Res. B v . 1982,I" press. Rhone-Poubnc, S. A. French Patonl 1388462. 1983. Rhone-Podenc. S. A. French Patent 83 108. 1964. Rhone-Podenc, S.A. French Patent 1388869. 1965. Zehner. L. R. US. Palem 4005 130. 1977.
Received for review February 16, 1982 Accepted May 21, 1982
Wet Oxidation of Acetic Acid Catalyzed by Co-Bi Complex Oxides Sei-lchlro Imamura,' Aklhlro Hlrano, and Narlyoohl Kawabata oepamnent of C.%mb!sfry, Kyoto InsfflMeof Technology.Matswsaki, Sakyo-ku. Kyoto 80s. Japan
Wet oxidation of acetic acid was carried out in the presence of various heterogeneous catalysts. Bismuhcontainlng catalysts were found to be active, of which cobalt-bismuth complex oxide with a molar ratio of Co to Bi of 5 (Co/Bi(S/l))exhibited the highest activity. Calcination temperature in the preparation of Co/Bi(S/l) affected its
surface area: however, specific activity of Co/Bi(5/1) was indifferent to calcination temperature. Co/Bi(5/1) apparently had basic sites where acetic acid was preferentially adsorbed by an acid-base interaction in the first step of the reaction. Co/BYS/l) also exhibited a remarkable activity in the decomposition of hydrogen peroxide, and, therefore, it was assumed that the redox property of Co/Bi(S/l)contributed to its high actvity in the oxidation of acetic acid.
Introduction Wet-air oxidation, k e d out in air under high pressure and at elevated temperature, is particularly effective for the treatment of wastewaters containing organic chemicals which are resistant to biological treatment. It has been applied successfully to the treatment of wastewaters discharged from petroleum and petrochemical industries (Tagashira et al., 1976) and to the treatment Bfpttlp and paper mill wastes (Teletzke, 1964). Organic pollutants can be removed completely under appropriate conditions of 019&4321/82/1221-0570$01.25/0
treatment. Recovery of mechanical energy is also possible when the process is applied to a highly contaminated wastewater such as that discharged from coal gasification process (Chou and Verhoff, 1981). Therefore, the importance of the wet-oxidation process will increase in the future. However, the severe reaction conditions require high operating and installation costs and practical applications of this process are limited. The development of various catalysts has been attempted in order to mitigate the severe reaction conditions (Katzer et al., 1976; Levec 0 1982 American Chemical Soclely
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
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Table I. Wet Oxidation of Acetic Acid Catalyzed b y Various Metal Saltsa ([AcOH], = 5000 ppni; Po, = 1.0 MPa; PN, = 3.0 MPa; 248 "C; pH = 3.5-4.1) method cat. calcination of concn, ATOC,, min,e temp,"C mMd % remarks run catalyst (molar ratio)b 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
without catalyst WNO,),
cu co
Cu/Mn (1:l) Cu/Ni (1:1) cu/v (1:l) cu/Y-Al,o, cu/co (1:l) Cu/Zn (1:l) Cu/Bi (1:0.05) Cu/Co/Bi (1: 1:O. 1) Cu/Co/Bi ( 1 : l : O . l ) Cu/Co/Bi (3:1:0.05) Cu/Co/Bi (1:3:0.05) Cu/Mn/Bi ( 2 : l : O . l ) Cu/Mn/Bi ( 2 : l : O . l ) Cu/Bi/y-Al,O, (1: 1) Bi Bi Bi /y-Al ,O, Bi/y-Al,O, Co/Bi (1:0.05) Co/Bi (1:l) Co/Bi/y-Al,O, (1:l) Co/Bi/y-Al,O, (1: 1) Sn/Bi (1:l)
Sn/Bi (1:l) Sn/Bi ( 1 : O . l ) Sn/Bi ( 4 : l ) Sn/Bi (1:4) Zn/Bi (1:l) Ni/Bi (1:l)
700 700 550 550 550 800 700 700 700 550 7 00 700 7 00 550 7 00 550 550 7 00 550 7 00 700 550 550 7 00 550 7 00 7 00 7 00 7 00 700 7 00
A A A A A C A A D
D D D
D D D E A A B B D A E E A A A A A A A
20 20 20 40 40 40 20 40 40 20 40 40 20 20 30 30 40 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
15.0 90.0 7.0 16.0 8.8 8.8 2.0 14.6 34.3 0 19.0 46.2 23.0 16.0 17.0 35.0 21.2 25.1 21.3 11.3 33.7 17.1 30.0 31.5 16.8 26.3 8.4 38.0 16.7 14.0 19.0 26.3 16.1
homogeneous, pH = 3.1
6.5 mM of Cu was eluted 5 mM of Cu was eluted
1mM of Cu was eluted 4 mM of Cu was eluted
See Experimental Section. Molar ratio of r-Al,O, is not included. Size of the catalvsts was smaller than 100 mesh. Total metal concentrations; concn of y-Al,O, is not included. e Percentage decrease in TOC after 60 min: ATOC,, m h = ([TOClo- [TOCI6omin)/[TOCIoX 100. a
et al., 1976; Tagashira et al., 1976). Although homogeneous copper catalysts are highly active (Morisaki et al., 1980; Imamura et al., 1982), heterogeneous catalysts are recommended in the actual wastewater treatment. However, data on active heterogeneous catalysts seem to be meager (Katzer et al., 1976). Previously wet oxidation of various organic compounds including dyes, amides, and water-soluble polymers was carried out, and it was found that acetic acid was one of the most refractory compounds (Imamura et al., 1979, 1980a,b, 1981a). Organic pollutants degrade to lower molecular weight compounds and refractory acetic acid accumulates at the later stage of wet oxidation (Higashijima, 1978). Therefore, complete oxidation of acetic acid is an important problem. This paper deals with the wet oxidation of acetic acid catalyzed by various heterogeneous catalysts with particular attention paid to the catalytic action of cobalt-bismuth complex oxides.
Experimental Section Materials. Commercial acetic acid, metal nitrates, hydrogen peroxide, and other reagents were used without further purification. Commercial 300-mesh y-alumina (7-A1203)with a BET surface area of 330 m2/g was used as a support. Preparation of Catalysts. Catalysts were prepared according to the following methods (methods A-E) and were crushed to sizes smaller than 100- or 200-mesh. Metal nitrates were used as starting materials except tin(I1) chloride. Calcination of the catalysts was carried out in air for 3 h. Calcination temperature and molar ratio of
metal salts in the composite catalysts are shown in Table I. Although most catalysts were assumed to be oxides or complex oxides of the corresponding metals, their compositions were not determined except for cobalt-bismuth complex oxide with a molar ratio of cobalt to bismuth of 5. Method A. Most of the catalysts were prepared by precipitation from an aqueous solution of a metal salt or coprecipitation from a mixed aqueous solution of metal salts. When bismuth was employed as a component, it was dissolved in concentrated nitric acid. The method of preparation of cobalt-bismuth composite catalyst is typically described as follows. Cobalt(I1) nitrate and bismuth(II1) nitrate with a known molar ratio were dissolved in a minimum amount of concentrated nitric acid, and the solution was poured into an excess of sodium hydroxide solution. The resultant precipitate was washed with water and was dried a t 100 "C, followed by calcination at a prescribed temperature. Method B. (Bi/T-A1203).A known amount of y-A1203 was mixed with bismuth(I1)nitrate dissolved in concentrated nitric acid. The mixture was heated to dryness and calcined. Method C. (Cu/7-A1,O3). y-A1203was impregnated with aqueous copper(I1) nitrate, and the composite was dried at 100 "C and calcined. Method D. (Co/Bi, Cu/Bi, Cu/Co/Bi, and Cu/ Mn/Bi). Corresponding oxides or complex oxides prepared by method A (calcination temperature of 550 or 700 "C) were mixed with bismuth(II1) nitrate in concentrated nitric acid. Nitric acid was removed under heating and
572
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
-i_.
0
5C R
- d o
u
93
Co+B (mol%
Figure 1. Effect of the composition of Co/Bi: [AcOH], = 5000 ppm; [Co/Bi] = 20 mM (total metal concentration), calcined at 550 "C and crushed to the size smaller than 100 mesh; Po, = 1.0 MPa; F'N~ = 3.0 MPa; 248 "C: (0)ATOCm (0) Ri; ( 8 )Ri/S: PercentagF aecrease in TOC after 60 min; initial rate of TOC decrease: specific activity
the composite was calcined a t the same temperature as that of the calcination in the preparation of the starting oxides. Method E. (Co/Bi/y-Al,O, and Cu/Bi/y-AlzO,). -y-Al,O, was mixed with cobalt(I1) nitrate and bismuth(II1) nitrate or copper(I1) nitrate and bismuth(II1) nitrate in concentrated nitric acid. Nitric acid was evaporated, and the residual composite was calcined. Apparatus and Procedure. Deionized water, catalyst, nitrogen (3.0 MPa) and oxygen (1.0 MPa) were charged into the reaction vessel, a 270-mL autoclave equipped with a sample injector and a valve for sampling. After the vessel was heated to 248 "C with an electric furnace, a known amount of aqueous acetic acid was added through the injector under pressure. The solution was stirred using a magnetic agitator. Constancy of the rate of reaction with the change of agitation speed showed that the reactions were not controlled by diffusion of oxygen into the liquid phase. A t appropriate time intervals, an aliquot of the solution was withdrawn through a cooling jacket and was submitted to analysis. Unless otherwise stated, pH of the solution, ranging from 3.4 to 4.1 in all cases, was not controlled. Analysis. Total organic carbon (TOC) and total nitrogen analyses were carried out with a Sumitomo GCT12N total organic carbon-total nitrogen analyzer. Acetic acid was determined with a Hitachi 063 gas chromatograph equipped with a flame ionization detector using Chromosorb 101, 3 mm x 1 m, and carbon dioxide with a Shimadzu GC-6A gas chromatograph using activated charcoal, 3 mm X 1 m. DTA and X-ray analyses were carried out by a Rigaku Denki Thermoflex Cat. No. 8001 DTA analyzer and a Rigaku Denki Geigerflex 2012 X-ray analyzer, respectively. BET surface area of the catalysts was determined by a Shimadzu Sorptgraph ADS-1B surface area analyzer. Cobalt ion in the solution was determined by a Jarrel-Ash AA-782 atomic absorption spectrophotometer. Copper(I1) ion and hydrogen peroxide were determined hv an iodometric titration. Results and Discussion Wet Oxidation of Acetic acid Catalyzed by Various Heterogeneous Catalysts. Activities of representative heterogeneous catalysts are shown in Table I, the result of noncatalyzed reaction and the activity of homogeneous copper salt being also shown in the table. The activity of catalyst was expressed on the basis of the percentage decrease in TOC after 60 min (ATOC60,). Homogeneous copper salt was remarkably active (run a), while heterogeneous copper salt showed no activity (run 3). Some of the multicomponent copper catalyst exhibited considerable activity (runs 9, 12. 16, and 18).the activity of Cu/Mn/Ri
Time (min)
Figure 2. Effect of calcination temperature of Co/Bi(5/1): [AcOH], = 5000 ppm; [Co/Bi(5/1)] = 20 mM (total metal concentration); size is smaller than 200 mesh; PO,= 1.0 MPa; PNB = 3.0 MPa; 248 "C: (w) without catalyst; ( X ) Cu(NOJ2, 20 mM. Table 11. Effect of Calcination Temperature o n the Surface Area and Specific Activity of Co/Bi (511) calcination temp, "C 250 350 450 5 50
S,
m'lg
RiIS!& ppmlmin m 2
127.1 61.6 45.1 23.1
3.50 4.11 3.64 4.59
,a
a BETsurface area. Specific activity of Co/Bi (5/1) in the oxidation of acetic acid. Reaction condition is shown in Figure 2.
catalyst having already been reported (Box and Farha, 1977). However, in all cases, copper(I1) ion was detected in the solution. Therefore, high activity of these catalysts was attributable to copper(1I) ion eluted during the reaction. Bi/y-Alz03 (run 21), Co/Bi (runs 23 and 24), Co/Bi/y-Alz03 (run 26), Sn/Bi (run 28), and Zn/Bi (run 32) were found to be active. The common component in these composite catalysts is Bi, and it is interesting to see that Bi catalyst calcined at 550 " C had some activity (run 19). Variation in the method of preparation of Bi/y-Alz03, Co/Bi/y-A1203, Sn/Bi, and Zn/Bi, however, did not result in the further increase in their activity. Oxidation of Acetic Acid Catalyzed by Co/Bi Catalysts. Activity of Co/Bi catalysts changed when the molar ratio of Bi to Co was varied (Figure 1). Addition of Bi to Co increased both ATOCm min and initial rate of TOC decrease (Ei)to maxima a t the molar percentage of Bi around 17%, further increase in Bi rather decreasing the activity. As surface area of the catalyst changed depending upon the composition of the catalyst, specific activity (initial rate divided by a total surface area: R J S ) was examined. The relation between Ri/S and the catalyst composition also indicated that the catalyst with the highest activity contained 16.7% of Bi (Co:Bi = 5:l). The catalyst with this composition was designated as Co/Bi(5/1). After the reaction, however, a small amount of nitrogen component was detected in the soliton, which seemed to elute from the catalysts. Therefore, the catalysts were subsequently prepared by washing the coprecipitated Co/Bi mixed hydroxides until no nitrogen component was detected in the washings by the total nitrogen analyzer. Oxidation of Acetic acid Catalyzed by Co/Bi(5/1). As the temperature of the calcination of Co/Bi(5/1) was lowered, the apparent activity of the catalyst increased; Co/Bi(5/1) calcined a t 250 "C exhibited even higher activity than homogeneous copper(I1) nitrate (Figure 2). However, surface area of the catalysts increased remarkably as the calcination temperature was lowered, and there was no remarkable difference among the specific activities
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
Table 111. Wet Oxidation of Acetic Acid ([AcOH], = 5000 ppm; [Catalyst] = 20 mM;a PO,= 1.0 MPa; PN. = 3.0 MPa; 248 "C) catalyst (molar ratio)
1 2 3
Co(N03)2C Co/Bi ( 5 : l ) d Co + Bi(5:lV' Co/Bi( 5 i l ) f ' Co( OH),g Bi(OH),g
4
5 6
l l
171hl>b
AT0C60
run
%
15.0 57.0 25.0 100 35.7 18.8
Percentage decrease in a Total metal concentration. TOC after 60 min. Homogeneous. Prepared by method A with a calcination temperature of 550 "C (see Experimental Section). Size of the catalyst is smaller than 200 mesh. e Mixture of Co,O, and Bi,O, prepared by method A with a calcination temperature of 550 "C, in which molar ratio of Co t o Bi is 5. Size of the catalyst Coprecipitated and dried at is smaller than 200 mesh. room temperature. g Precipitated and dried at room temperature.
of these catalyst (Table 11). A discrepancy was observed between the specific activities of these catalysts and those of the catalysts used in the previous runs shown in Figure 1; the catalysts used in the previous runs were more active (compare the specific activities of Co/Bi(5/1) calcined at 550 "C in both cases). This may be due to the presence of impurities in the former catalysts, because the state of the catalyst may be affected by impurities. For example, surface areas were much smaller in the former series of the catalysts; BET surface area of Co/Bi(5/1) calcined at 550 OC was 4.86 m2/g for the former catalyst and 23.1 m2/g for the latter. Co(OH), changes into Co304upon heating to 265 "C in air (Snell and Ettre, 1970). It is assumed that dehydration of Bi(OH)3 occurs at 100 "C to form BiO(OH), and an additional water molecule is lost above 400 "C to give a-Biz03(Meller, 1964). From the observation of the weight change of the catalyst during calcination, an apparent structural formula of the catalyst was deduced as Co3O4*3/5Bi0(OH)(I 350 "C) and Co304*3/10Bi203 (450 "C 5 ) . As the specific activity of Co/Bi(5/1) was almost indifferent to calcination temperature (Table 111, the structural formula of the catalyst is not an important factor controlling the activity. It was found that 2.4-3.6 ppm of Co was detected in the solution at the end of the reaction, depending upon the kind of Co/Bi(5/1) employed. However, such a small amount of Co ion does not accelerate the reaction; homogeneous cobalt(I1) nitrate was utterly inactive (Table 111). Activity of the mixture of Co304and Bi203,each prepared by method A, with a molar ratio of Co to Bi of 5 was far lower than that of Co/Bi(5/1) (compare run 2 with run 3). Coprecipitated Co/Bi(5/1) even without heat treatment (run 4) exhibited a remarkable activity which was much higher than that of Co(OH), or Bi(OHI3 prepared by the same method (runs 5 and 6). These results, together with the fact that the structural formula of Co/Bi(5/1) had a rather minor effect on the activity, indicate that the most important factor to induce the catalytic activity is a formation of some interaction between Co and Bi as a result of coprecipitation from a mixed solution of Co and Bi. It is interesting to note that coprecipitated Co/Bi(5/ 1)without heat treatment was as active as Co/Bi(5/1) calcined at 250 "C (see also Figure 2). However, the catalyst without heat treatment might be aged in the reaction vessel during the period of preheating up to 248 "C, and the interaction between Co and Bi might become strong during this period.
573
20
30
40
50
60
70
20 (CuKa)
Figure 3. X-ray diffraction patterns of Co304,Bi203,and Co/Bi(5/ l),each prepared by calcinating the corresponding hydroxides at 550 "C. Table IV. Durability of Co/Bi(5/1) Calcined at 350 "C ( [ AcOH], = 5000 ppm; [Co/Bi(5/1)] = 20 mM;a Po-= 1.0 MPa; PN. = 3.0 MPa; 248 "C) b
run 1st 2nd 3rd
4th 5th
R6o min, ppm/min
27.7 27.5 29.1 25.1 26.3
a Total m e M concentration. Size of Co/Bi( 5 / 1 ) is smaller than 200 mesh. Average rate of TOC decrease for 60 min.
Figure 3 shows X-ray diffraction patterns of Co/Bi(5/1), Co304and Bi203,each prepared by calcinating the corresponding hydroxides at 550 "C. Bi203 exhibited high degree of crystallinity, while Co304 was considerably amorphous. X-ray diffraction pattern of Co/Bi(5/1), however, was different from those of Co30, and Bi203. Peaks at 6 = 27.5" (d = 3.24 A) and 33.0' (d = 2.71 A) of Bi203disappeared and a new peak appeared at 28.0" in the X-ray diffraction pattern of Co/Bi(5/1), suggesting the existence of some interaction between Co and Bi in the catalyst No decomposition product (e.g., formic acid or oxalic acid) was detected in the oxidation of acetic acid catalyzed by Co/Bi(5/1) calcined at 350 "C, and the TOC decrease corresponded exactly to the decrease in the carbonaceous part of acetic acid determined by gas chromatography. Formic acid and oxalic acid were found to be highly reactive; therefore, even if produced, they must disappear instantaneously under the reaction condition, and the decomposition of acetic acid is a rate-determining step in the reaction. It was found that more than 80% of the consumed acetic acid was transformed into carbon dioxide. As the accuracy of the determination of carbon dioxide was rather poor, stoichiometry of the reaction was not examined. Apparent activation energy was found to be 105.9 kJ/mol. Durability of Co/Bi(5/1) calcined at 350 "C was examined by a pulse method. Another 5000 ppm of acetic acid was injected at 60 min after the first addition of acetic acid, and this procedure was repeated five times. Average rates of TOC decrease for 60 min in each run are listed in Table IV. Even after the fifth run, the catalyst retained almost the same activity as that in the first run. The net activity of Co/Bi(5/1) was compared with that of homogeneous copper(I1) nitrate. The maximum number of Co and Bi atoms exposed above the surface of the catalyst is roughly calculated for Co/Bi(5/1) calcined above 450 "C on the assumption that Bi203exists as monoclinic a-Biz03and Co304has a spinel structure (Malmros, 1970; Yasui, 1979) and that the composition is uniform throughout the catalyst. The maximum number of surface metal atoms is obtained when the (200) plane of a-Bi203
.
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Ind Eng Chem Prod Res Dev , Vol 21, No 4, 1982
Table V. Adsorption of Acetic Acid and Sodium Acetate on Co/Bi(5/1)Calcined at 350 "C ([AcOH], [AcONal 33 mM; C ~ / B i ( f i / l ) ~0.4 g; H.0 4 0 mL; 30°C) __ - _____ percentage of pH adsorption. "r ____ ____ - - _ _ __ AcOH 3.3 24 h AcONa 5.4 11 ____I-
a
Figure 4. Adsorption isotherm of acetic acid on r-Al,O, and Co/ Bi(5/1) calcined at 350 OC: 30 "C: (0) r-Al,O, ( S , = 330 m2/g); ( 0 ) Co/Bi(5/1) (S, = 61.6 m2/g).
catalysts (runs 4-6), Co304,and Biz03 prepared from the corresponding hydroxides at a calcination temperature of 550 "C (runs 7 and 8) were utterly inactive. A series of Co/Bi(5/1) calcined at various temperature (runs 13-16) exhibited remarkable activities, and calcination at lower temperat-me resulted in the higher activity of the catalysts. However, as calcination temperature affected the surface area of the catalysts, activities per unit surface area did not differ so much among these catalysts. Decomposition of hydrogen peroxide may proceed through an ionic mechanism catalyzed by the basic sites of Co/Bi(5/1). However, hydrogen peroxide was not decomposed at all in alkaline media (run 17). Therefore, redox reaction presumably occurred between Co/Bi(5/1) and hydrogen peroxide as in the case of heterogeneous silver catalyst (Ono et al., 1977). Although Co/Bi(S/ 1)without heat treatment (run 11) exhibited an activity higher than those of Co(OHI2 and Bi(OH), prepared by the same method (runs 9 and lo), its activity was far lower than that of Co/Bi(5/1) calcined at 250 " C (run 13). This is a different phenomenon from that observed in the oxidation of acetic acid, in which Co/Bi(5/1) without heat treatment was as active as Co/ Bi(5/1) calcined at 250 "C (see Table 111). Moreover, heat treatment at 100 "C did not increase the activity (run 12). Therefore, calcination above 100 "C (possibily around 250 " C ) may be necessary to obtain a catalyst with enough redox ability as a result of the formation of a strong interaction between Co and Bi. Although the mechanism of oxidation of acetic acid is not clarified as yet, hydroperoxide may be formed during the reaction. However, catalytic activity of Co/Bi(5/1) is not due to its action to decompose hydroperoxide and
Table VI. Decomposition of Hydrogen Peroxide ( [ H 2 0 2 ] ,-, 0.25 M; [Catalyst] catalyst (molar ratio)
--
1 mM;' 0 "C
calcination temp, "C
method of prepnb
B C D A
Ri, mM/min 0 0 0 0 0
C u ( N 0 , ) '' CUO
1 2 3
tii20,j
5 ti
Bi/y-Al,O,c C u / r -A1,0, Cu/Mn/Bi ( 2 :1:0.1
550 550
i
GO
550 550
4
..
Bi " Co(0H);' Bi( OH),' Co/Bi (5:1)? Co/Bi ( 5 : l ) " Co/Bi ( 5 : l ) h Co/Bi (5: l ) h Co/Bi (5:1)'* Co/Bi $
