Reduction and Adsorption Kinetics of Nitric Oxide on Cobalt Perovskite Catalysts M. W. Chien, Irving M. Pearson, and Ken Nobe* School of Engineering and Applied Science, Universify of California,
Los Angeles, California 90024
The catalytic activities of the perovskites, Lac003 and doped LaCoOs, for the reduction of NO with CO at 100-500°C under plug flow reactor conditions, have been investigated. Comparisons were made with other Co oxides. The order of catalytic activity after pretreatment with CO was found to be Co304 > Ce02-Co304 > La0,85Bao,~&o03 =;: CuC0204 > LaCo03 = L ~ O . ~ ~ H ~ > ~ .Lao.85Ca0.15CoO3 ~ ~ C O O ~ > Lao.~&ro,l5CoO3. In all cases, the reaction rates were zero order in CO and fractional order in NO. The latter was converted principally to N2 between 380 and 500°C; at lower temperatures more N 2 0 was formed. The kinetics of the chemisorption of NO at low pressures on LaCo03 and the Ba-doped material was studied from 21 to 4OO0C. Pretreatment of t h e adsorbents with CO increased the adsorption rates greatly.
There has been much interest in efficient, durable and low-cost exhaust or flue-gas catalysts which can foster the reduction of NO with CO. Many metal and metal oxides have been studied for this purpose. The most promising supported catalysts were reported to be precious metals and Cu oxides or Cu chromite (Shelef and Kummer, 1967). Taylor (1959) claimed more than 80% removal of NO over various chromites and Cr-promoted iron oxide a t 200-350°C. Roth and Doerr (1961) concluded that these catalysts can exist in either oxidized or reduced states. Bauerle et al. (1972) showed that CO-reduced Cu chromite gives better reproducibility and greater NO conversions than the unreduced catalyst. Otto and Shelef (1969, 1970) found that NO adsorption on reduced forms of supported chromia and iron oxide was greater than on the oxidized forms. Presumably, this would also be true of other tansition metal oxides which have readily attainable reduced states. Terenin and Roev (1961) established that the chemisorption of NO on Cr3and Co2+, and Mn2+ ion is especially large because of formation of covalent bonds between NO and the metal ions in the adsorbent. Inasmuch as chemisorption of NO is probably an important step in the reduction of NO (Otto and Shelef, 1969), the Terenin and Roev data suggest that catalysts containing these transition element ions could be useful for the reduction of NO with CO. Libby (1971) reported that L a c 0 0 3 shows good activity in catalyzing the isomerization, cracking, and hydrogenation of hydrocarbons. This binary oxide may incorporate the redox capability of Co into the stable perovskite structure. Consequently, we decided to study the efficacy of activated (CO-treated) LaCoO3, and p- and n-doped LaCoO3, as catalysts for the NO reduction with CO, in comparison with Co304, CuC0204, and CeOz-CosO4. The chemisorption of NO by two of the perovskites, before and after reduction with CO, was also examined in an attempt to learn more about this important kinetic step. Experimental Section Kinetics Study. The experimental apparatus and techniques have been described previously (Bauerle et al.,
1972, 1974). High-purity Matheson gases were employed. N2 (99.998% min.) was the carrier gas which was passed through the reactor during the kinetics experiments. Plugflow reactor conditions were maintained by adjusting the N2 flow rate to 300 hr-1 (25°C) giving a space velocity of 15,000 h r - l for the 20 ml catalyst volumes (cat. wt = 14 9). NO and CO were 99.0'70 and 99.5% in purity, respectively. The initial concentrations of NO were varied from 500 to 2500 ppm while the initial CO concentration was maintained at 9900 ppm in some experiments and ranged from 3700 to 16,000 ppm in others. The reaction temperatures were varied from about 100 to 500°C. Adsorption Study, NO adsorption experiments were carried out in a constant volume Pyrex apparatus. The equipment and procedures were similar to those described by Blumenthal and Nobe (1966). All stopcocks were greased with Dow Coming Silicone high vacuum greases. The volumes of the gas sample storage bulk and the adsorption bulb were 2330 and 2340 ml, respectively. The adsorption bed (2.5 g) was maintained a t *2"C, the temperature being varied from 21 to 400°C in the various experiments. The NO pressure ranged from about 0.04 to 0.02 Torr. Two of the catalytic materials (LaCoOj and Lao.85Bao.&oOa) described below were used as adsorbents for NO without admixture of the A1203 support utilized in the NO-CO reaction kinetics study. Portions of these oxides were activated by reduction with CO a t 450°C for 48 hr. Surface Area a n d Pore Volume Measurements. Physical properties of the catalysts are given in Table I. A standard BET adsorption apparatus was used to determine the surface area of the AlzOs-supported catalysts; N2 or K r gas (Matheson Research Grade) was used as the adsorbate. The specific pore volumes were determined from the skeletal densities and particle densities of the catalysts which were measured by pycnometer methods using Hg and isopropyl alcohol, respectively. Catalyst Preparation. The eight catalysts utilized in this study were prepared as follows from either Baker or MCB reagent grade chemicals. 1. LaCo03-La203 and COCO3 in 1:2 mole ratio were Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
131
-5
i
0 CO Initial Concentration
1370Oppm
0 8OOOppm 0 6700ppm
A LoCo03 La0.85ca0.1Sc003
INITIAL CONCENTRATION
OF NITRIC OXIDE (opm)
Figure 1. Effect of initial CO and NO concentrations on the reduction of NO on La0.s~Ba0.1&003 at 438°C.
IO 400
1
I
I
425
450
475
I 500
TEMPERATURE P C l
Figure 2. Temperature dependence of activities of catalysts for the reduction of NO with CO. POco = 9900 ppm, PONo = 1500 PPm.
Table I. Physical Characteristics of Catalyst Pellets Surface area, m2/g
Catalysta LaCo03 8 tiBa,
With Pore Active A1,03 volume, material support cm3/g 1.3
1
La0.85Sr0.t5C003 La0.85Ca0.i5C003
C0304* CeOp-Co30, cucopo,
... ...
24
... ...
230 235 196 203 9 . .
Mean pore radius,
0.401 0.465 0.385 0.622
...
A
35 40 39 61
...
177 0.535 61 181 0.569 63 La0.85Hf0. iBCo03 ... 218 0.700 64 "All catalysts contained 15% active material and 85% A1&. After reaction, the pellet crumbled into powder.
mixed with a mortar and pestle and then heated a t 950°C for 15-18 hr in an electric furnace a t ambient pressures. For the NO-CO reaction study, Filtrol Grade 86 A1203 pellets were ground to a fine powder and dried. The A1203 powder was mixed with the Lac003 (15 w t % of the latter) and 10 g of stearic acid was added to each 100 g of mix which was then pressed into Vs x %-in. cylindrical pellets with a Stokes single punch tablet machine. The stearic acid lubricant was burned off by passing air through the pellets a t 400°C. A portion of those pellets were pre-reduced with CO for 16 hr @ 450°C. The same procedures for forming the supported catalyst, pelletizing, and reduction were followed for all the other catalytic materials described below. 2. La0.85Mo 15C003 (where M = Ba, Sr, Ca, respectively). These were made in the same manner as in 1, from mixes containing La203, MC03, and COCO3 in mole ratios of 0.85:0.3:2. 3. L a o . ~ ~ H f o . 1 ~ C oThis 0 3 . material and the supported catalyst were similarly prepared beginning with a mix of La203, HfO2 and COCOSin a 0.85:0.3:2 mole ratio. 4. Co304. This oxide was obtained by the decomposition of C o c o 3 a t 950°C for 15-18 hr. 5. CeOz-Co304. This catalyst was similarly prepared from a mix of CeOl and Co304 in a 1:2 mole ratio. 6. CuC0204. C u ( N 0 3 ) ~and Co(NO3)2 in 1:2 mole ratio were dissolved together in a minimum amount of distilled water. The solid mixture obtained by evaporation of the solution was heated a t 850°C for 5 hr. 132
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
X-Ray diffraction analysis showed that Lac003 and all the La0.85Mo& 0 0 3 compounds (where M includes Hf) had the perovskite structure, based on comparison with literature data for interatomic spacing, and relative intensity. The Ce02-Co304 product did not have the perovskite structure but consisted of a mixture of the two polycrystalline oxides. Cos04 and CuC0204 had the reported crystal structures. Except where otherwise noted below all the catalysts were reduced with CO as described above before the experiments were carried out.
Results and Discussion The pertinent reactions in the reduction of NO with CO are as follows. 2N0 + CO 2N0 + 2CO
-
--+
N,O + CO, N, + 2COp
(1) (2)
Reaction 2 is favored a t higher temperatures. The following observations were made in the catalytic NO-CO experiments. 1. A t constant temperature ( T ) and initial concentration of CO (Poco), the reduction of NO decreased as the initial NO concentration (P O N O ) increased, as shown in Figure 1 for the typical case of supported Lao.85Ba0,~&oO3(complete data given elsewhere, Chien, 1973). 2 . A t constant T and PONo, the rate of NO reduction was essentially independent of the CO concentration (Figure 1). 3. At constant POco and PONo, NO reduction increased as T increased (Figure 2 ) . 4. The amount of N2O produced between about 200-300°C was almost equivalent to the amount of C02 produced (Figure 3). Above about 360"C, N2 predominates increasingly over N20, as is shown in Figure 3 for supported Lao 85Bao.l&oO3 (no NO2 was detected in any of the experiments by flow colorimeter tests). The calculations of the outlet concentrations of N2 produced (exclusive of the nitrogen carrier gas) were based on a material balance of the other reaction products. The comparative effectiveness of the activated supported catalysts in promoting the reduction of NO with CO principally to N2 at 400-500°C is shown in Figure 2 . The following order of catalytic activity is evident: Cos04 >
W
uoz 100
200
300
REACTION
500
400
TEMPERATURE
(‘C1
Figure 3. Temperature dependence of N 2 0 , C02, and N2 formation, and NO reduction on Lao.saBao.1&003.POCO = 9900 ppm, PON= ~ 2300 ppm.
CeOz-Co3O4 > Lao,ssBa0.1&003 CuCozO4 > Lac003 = L ~ O . S ~ H ~ O . I ~ C> O O Lao.ssCa0.1~CoO3 ~ > L~o.ssS~O.ISCOO~. The oxide Cos04 (which is not a durable catalyst) gave the largest reduction rate of NO. Ce02-Co304 was somewhat superior to the perovskite catalysts and CuC0204. Lao.ssBao.1~CoO3seemed to be the best of the perovskites. It has been reported (Jonker, 1969) that the mixed La cobalt oxides doped with Ca, Sr, and Ba are p-type semiconductors; the Hf-doped material should be n-type. However, no significant correlation between the alleged semiconductor type and the catalytic properties was established. Shelef et al. (1968) also found no indication in their single oxide series that p - or n-type materials were more active catalysts for the NO-CO reaction. Conversion data obtained for complete reduction of NO a t the higher temperatures were correlated for each catalyst by use of the empirical “power law” rate equation Y = kPNo”
(3)
and the Arrhenius equation
0
II 1.3
I
I
I
1.4
1.45
1.5
1.55
1(
IO’
(OK-’)
Figure 4 . Arrhenius plots.
Table 11. Kinetic Parameters for NO reduction with CO on Supported Catalysts A , pre-exponen-
Catalyst LaCoO, 8 5Ba0.
85sr0.
1 5c003 i5c003
La0.85Ca0.15C003 CeO,-Co,O, c030,
(4)
I
1.35
I/T
cuco,o, k = A exp(-E/RT)
I
La0.85Hf0.i5Co03
tial factor, E, g-mol activation n, reaction of NO/hr-g energy, order of cat. -atm cal/g-mol 0.75 0.75 0.75 0.75 0.75 0.75 0.85 0.85
875 2277 1630 230 1079 1348 104 11150
13,647 14,840 15,153 12,120 16,830 13,548 9,372 16,317
according to the calculation procedure described previousAll the supported catalysts were reduced with CO bely by Bauerle et al. (1974). The results of these calculafore use as described because this procedure greatly imtions are shown in the Arrhenius plots of Figure 4 and proved the rate of NO reduction far more than was attribTable I1 which summarizes te kinetic parameters for the utable to the 5% increase of surface area which resulted various supported catalysts. An NO reaction order of 0.85 was obtained for the CuC0204 and L ~ o . s s H ~ ~ . ~ scatC O O ~from the pre-reduction. For example, trials with supported Lac003 a t 450°C showed that pre-reduction gave as alysts. All the other supported catalysts gave a reaction much as a fourfold increase in NO conversion, indicating order of 0.75. The linear Arrhenius plots and effectiveness that this technique yields a considerable increase in active factor calculations indicate that pore diffusion effects catalytic sites. This subject is explored further in the NO were not significant. Also, analysis has shown that exteradsorption study. nal diffusion effects can be neglected, NO Adsorption Kinetics. The experimental kinetic An order of oxide catalytic activity for the NO-CO data obtained for the adsorption of N O on the two unsupreaction is reported to be (Shelef and Kummer, 1967): ported catalysts were correlated with a simplified inteFez03 > CuCr204 > Cu20 > CrZ03 > NiO > Pt > Co304 > A1203 (5% SiO2) > MnO > V 2 0 6 . Inasmuch a s the per- grated form of the Elovich equation (Otto and Shelef, 1969) ovskite materials are inferior in activity to eo304 it is evident that they are relatively poor catalysts for the NO-CO = (2.3/c~) log t - (2.3/c~) log t o (5) reaction. Several different concentrations of COz (1000 and 2500 where q is the amount of NO adsorbed a t time t , N is a ppm) were injected into the reactant streams to detercoefficient which is dependent on temperature and presmine the C02 effect on the NO-CO reaction on sure, and t >> t o . La0.~5Hfo,o&oO3.The reaction rate decreased somewhat Semilog plots of q and t for the adsorption of NO a t with increasing initial COZ concentration suggesting that various temperatures on activated (reduced) and unactivCO2 adsorbs to a limited extent on active catalyst sites. ated Lao,85Ba0.~CoO3are given in Figures 5 and 6, reInd. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
13s
%'
0 400.C
n
I I B'C
21oc
0
I
A
A
A
.
, I I
2
05
3
4
5
7
IO
TIME
jmin 1
TIME
Figure s. A d s o r p t i o n of N O o n a c t i v a t e d Lao.ssBao 1sC003. 5 .
A 0
e
5
i10
20i
Irnin.1
Figure 7. C o m p a r i s o n of NO a d s o r p t i o n o n a c t i v a t e d a n d u n a c t i v a t e d Lac003a n d Lao.ssBao.lsCoOs a t 400°C.
A two-site model of the catalyst surface similar to Shelef et al. (1968) can be utilized to explain the NO adsorption results obtained for LaCoOa and the Ba-doped material and the greater activity for the NO-CO reaction of the various supported catalysts after activation. Catalyst surfaces activated with CO have type I and type I1 sites as follows
21.C 219-C 316T 4005:
reduction
Site I (cos+)
0 5
I
2
TIME
3
4
5
7
IO
20
(rnin.)
Figure 6. A d s o r p t i o n of S O on u n a c t i v a t e d Lao,s5Bao.l5CoOa.
spectively. The plots are fairly linear. The rate of adsorption of the unactivated material decreases with increasing temperature while the reverse tends to be true of the activated materials. The latter also tend to show a much faster rate of chemisorption than the unactivated compounds. The plot for activated Lao 85Bao 15Co03 (Figure 5 ) shows a sharp break and consists of two intersecting line segvs. 0.24 mol/g-min), and q and dq/dt at ments (~0.053 the intersection point, in general, increase with increasing temperature (except for a small inversion between the 313 and 400°C results). Although Shelef et al. utilized A1203 supports, higher pressures and different time and temperature ranges in their studies of K O chemisorption on Cr2O3 (1969), Fez03 and Fe304 (1970), NiO (1972) and Cu oxides (1973), the reduction effect and the segmented integrated Elovich plot for reduced Lao SEBao 15Co03 are similar t o their results obtained for the more strongly NO-adsorbing single oxides. A t 400"C, the temperature at which almost complete reduction of NO with CO was found to occur (see above), Figure 7 shows that the activated forms of Lac003 and Lao 85Bao 15CoO3 adsorb a t much faster rates than the unactivated forms. The adsorption rate for the Ba-doped material, which was found to be a better catalyst for the NO-CO reaction than Lac003 exceeds somewhat the rate for the latter, suggesting that the NO-CO reaction rate was influenced by the chemisorption rate of NO on the respective catalysts.
134
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
7 Site I1 oxidation
(6)
(~02')
Type I sites which are predominant in the unreduced catalysts are less active for the NO-CO reaction and NO adsorption and exhibit a negative temperature dependence for NO adsorption. Type I1 sites are formed after activation with CO and show better reactivity and a positive temperature dependence for NO adsorption. Type I1 sites increase the rate of NO adsorption a t the higher temperatures, based on the assumption that NO adsorbs more strongly than CO, and readily displaces the latter from the catalyst surface as in the case for NiO (Alexeyev and Terenin, 1965). The two-site model is consistent with the results obtained by Otto and Shelef (1969, 1970) for Cr203 and Fe oxides. Literature Cited Alexeyev, A,, Terenin, A. N., J. Catal., 4, 440 (1965). Bauerle, G. L., Service, F. R., Nobe, K.. Ind. Eng. Chem., Prod. Res. Dev., 11, 54 (1972). Bauerle, G. L., Sorensen. L. L. C.. Nobe, K., Ind. Eng. Chem.. Prod. Res. Dev., 13, 61 (1974). Blumenthal, J. L., Nobe, K., Ind. Eng. Chem., Proc. Des. Dev., 5, 177 (1966), Chien, M. W., Ph.D. Dissertation, UCLA, Dec 1973. Gandhi, J. S.,Shelef, M., J. Catal., 24, 241 (1972). Gandhi, J. S.,Shelef, M., J. Catal., 28, 1 (1973). Jonker, G. H . , Philips Res. Rep., 24, 1 (1969). Libby, W. F., Science, 171,499 (1971). Otto, K . , Shelef, M., J. Catal., 14, 266 (1969). Otto, K . , Shelef, M., J. Catal.. 18, 184 (1970). Roth, J. F. Doerr, R . C., lnd. Eng. Chem., 53, 293 (1961). Shelef, M.. Kummer, J. T.. Proceedings, 62nd Annual Meeting, A.I.Ch.E., Nov 16-20, 19-28. Washington, 0. C . , 1967. Shelef. M., Otto, K., Gandhi, H . S., J . Catal., 12, 361 (1968). Taylor, F. R., Air Pollution Foundation Report (L.A.) No. 28 (1959). Terenin, A. N., Roev. L. M . , Actes Congr. lnt. Catalyse, 2e, Paris, 2, 2183 (1961).
Received for reuiew July 29, 1974 Accepted J a n u a r y 22, 1975 W o r k s u p p o r t e d in p a r t by G r a n t P r o t e c t i o n Agency.
No. AP00913, E n v i r o n m e n t a l
