Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 179-186
179
CATALYST SECTION
Reduction of NO with NH, on A1203- and Ti0,-Supported Metal Oxide Catalysts Wing C. Wong+ and Ken Nobe' Department of Chemical Engineering, University of Caiifornla, Los Angeles. California 90024
NO reduction with NH, in the presence of excess 0,on VZO,, CrzO,, FezO,, and mixtures of these metal oxides supported on TiO, and AI,O, has been studied. Ti0,-supported catalysts are generally more active than AI,O,supported catalysts. Production of N,O is significant at T > 300 O C and more N,O is produced on Ti0,-supported catalysts than on A1,03-supported catalysts. Substantial N 2 0 is produced on Cr,O3 catalysts even at low temperatures. The kinetic data of the NO-NH,-O, reaction on the metal oxide catalysts, after correcting for pore diffusion effects, indicate that the catalytic reaction is first order (NO) and zero order (NH,). The Inomata-Miyamoto-Murakami and Otto-Shelef-Kummer mechanisms have been modified to interpret our results and those of others for the NO-NH,-O, reaction on V205;the two modified mechanisms are shown to be equivalent for V,O5 catalysts.
Introduction One promising approach to controlling nitrogen oxide emissions from stationary sources is selective catalytic reduction (SCR). The most effective reducing agent is NH3, which is added to the flue gas and selectively reacts with NO in the presence of excess oxygen. 4N0
+ 4NH3 + O2
-
4N2 + 6H20
(A)
A literature review, which included patents, and further experimental tests of reported promising catalysts and catalytic systems point to V205and Fe-Cr oxides as two very effective non-noble-metal catalysts for SCR of NO with NH3 even in the presence of sulfur oxides (Nobe and Bauerle, 1974). Subsequent studies that showed these catalysts to be highly active for the reaction include, for example, A1203-supportedV205(Wu, 1977; Inomata et al., 1982); Cr2O3 (Niiyama et al., 1977); and Fe2O3 (Kato et al., 1981; Nobe et al., 1976). Work in this laboratory showed that the addition of small amounts of Cr203and V205to A1203-supportedFe203catalyst substantially increases its activity (Nobe et al., 1976). Kasaoka and Yamanaka (1977) reported that Fe203,Cr203,V205,and other metal oxides are generally significantly more active on T i 0 2supports than on A1203 supports. Recently, Inomata et al. (1982) and Wong (1984) have also shown that Ti02 is a more effective support than A1203for V205catalysts. Nakajima et al. (1978) in the Hitachi patent claimed that Ti02-supported catalysb such as V, Fe, and Cr oxides had exceptionally high activity and resistance to SO, poisoning. More recently, Shikada et al. (1981) reported that V20, on a SO2-Ti02 support had very high activity and good
* To whom correspondence should be addressed. 'Preeent address: Energy TechnologyDivision, TFtW, Redondo Beach, CA 90278. 0196-432 1/ 86/ 1225-0 179$0 1.50/ 0
resistance to SO2 poisoning in simulated flue gas. The Hitachi group also reported that A1203-supportedFe203 and Fe-Cr oxide catalysts were readily poisoned by SO, in long-term tests (Matsuda et al., 1978). In contrast, our group found that V205-A1203and Fe203-Cr203-A1203 catalysts maintained long-term high activity in simulated flue gas containing SO, (Nobe et al., 1976). Although V205catalysts, which have been extensively studied by the Nagoya University group (e.g., Inomata et al., l978,1980,1982,1983a,b, and Miyamoto et al., 1981a,b, 1982a,b), appear to be the most effective for the NO-NH3-02 reaction, Fe203 catalysts are very attractive for large-scale industrial use because of their high activity and low cost (Naruse et al., 1980; Kat0 et al., 1981, 1983). Furthermore, Kato et al. (1983) reported that a Fe203-TiO, catalyst maintained its high activity for 1200 h even in the presence of SO3, while an unsupported Fe203-Cr203catalyst quickly declined in activity in the same environment. A careful examination of the literature indicates that catalyst preparation procedures and the type of catalyst support have a great effect on activity and resistance to SO, poisoning and that the optimum catalyst for SCR of NO with NH3 in flue gas containing sulfur oxides has not yet been developed. The present paper reports on the results of a study that examines the relative catalytic activities of oxides and oxide mixtures of V, Cr, and Fe supported on both Ti02 and A1203for the NO-NH3-02 reaction.
Experimental Procedures The schematic drawing of the experimental apparatus and instrumentation, which includes an integral, upflow reactor operating under isothermal, steady-state conditions, has been shown previously (Wong and Nobe, 1984). N2 (99.998% purity) was used as the carrier gas and was maintained at 300 L(NTP)/h. Catalytic activities were 0 1986 American Chemical Society
180
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
Table I. Phvsical ProDerties of Catalysts specific surface pellet pore area, density, volume, catalyst m2/g g/cm3 cm3/g 1.57 0.32 26 Vz05-Ti02 1.79 0.28 46 FezO3-TiOZ 1.61 0.31 75 Crz03-Ti02 1.68 0.30 80 V206-Fe203 (20:1)-T102 1.75 0.29 84 V205-Cr203 (20:1)-Ti02 1.54 0.33 91 FezO3-Cr2O3-VzO5 ( 18:1:1)-TiO2 1.85 0.27 42 Fe2O3-Cr2O3 (9:1)-Ti02 1.08 0.46 200 Vz06-Alz03 1.00 0.50 Fez03-A1203 1.01 0.50 CrzO3-Al2O3 1.44 0.35 180 V205-Fe203 (20:1)-A1203 1.47 0.34 190 Vz06-Cr203 (20:1)-A1203 1.04 0.48 Fez03-Crz03-Vz06 (18:1:1)-A1203 1.11 0.45 Fe203-Cr203 (9:1)-A&O3 ~~~~~
mean pore radius, A 250 120 83 74
70
-
60
-
68 72 130
46 53 53 38
-8
-
I T~OZ 0 A1203
-
z 50-
DASH LINE
3
N 2 0 CONCENTRATION
=
SOLID LINE
i
NO CONVERSION
0 Y
-
z > 8
400
- 7 :
36
30
-
20
-
51 47
determined with a gas mixture containing 1000 ppm NO and 1000 ppm NH, in N2 in the absence and presence of O2 (2.2%). The temperatures were varied from 100 to 450 "C. Seven 10 wt % metal oxides and mixtures of metal oxides of Fe, Cr, and V catalysts were impregnated on Ti02 pellets. TiOz was precipitated by hydrolysis of TiC14with H20 (Chertov et al., 1978). The filtered residue was dried overnight at 110 "C and pressed into cylindrical pellets in. X l/s in.). The Ti02 pellets were then calcined in flowing air at 400 "C for 4 h. The impregnation method involved soaking the pellets in aqueous solutions containing appropriate amounts of dissolved metal salts (NH4V03,CrO,, and Fe(N03),.9H20). The catalysts were calcined at 400 "C for 4 h in a flowing stream of dry air. A second set of seven 10 wt % metal oxide catalysts were impregnated on A1203support (Filtrol grade 86 alumina pellets, l/s in. X 1/8 in.). These catalysts were calcined as described above. A more detailed description of catalyst preparation procedures is given elsewhere (Wong, 1982). Table I lists the catalysts and gives pertinent physical properties.
Results Effect of Support on Catalytic Activity of V2OP Figure 1compares the global activities of V205on the two different supports, Ti02 and A1203,for the reduction of NO with NH3 in the presence of excess 02.The results in the form of NO conversion over the temperature range of 150-375 "C indicate that Ti02-supported V205is more active than Al,O,-supported V205;neither TiOz nor Al2O3 has significant catalytic activity. Inomata et al. (1982) also found that an equivalent 10 wt % V205-Ti02 catalyst was more active than 10 wt % v205-&03 for the NO-NH3-02 reaction. For Ti02-supported V205 catalyst, NO conversion reached a maximum at 300 "C. Experimental results indicated that the ratio of the removal of NO to NH3is about unity, which is in agreement with the stoichiometry of reaction A. NO is selectively reduced by NH, to N2 at temperatures below 300 "C, but above 350 "C NO conversion starts to decrease and NzO formation increases substantially, indicating that another reaction (or reactions) becomes important.
10
P
J
-E
--
-6:
- 5
5
-4;
z -3:
-
8 0 N
- I
t '100
150
I 200
9250 TEMPERATURE
I 350
300 [ 'C
400
z
10 450
I
Figure 1. Activities of V205catalysts.
For A1203-supportedV205catalyst, the NO reduction attains a maximum at about 400 "C, as shown in our earlier study (Wu and Nobe, 1977). The amount of N20 produced was relatively small, about one-quarter of that produced by the Ti02-supported V206at 400 "C. Catalytic Activity of Ti0,Supported Metal Oxides. The activity, expressed as NO conversion vs. temperature, of Ti02-supported V2O5, Cr203,and Fe203catalysts for the NO-NH, reaction in the presence and absence of O2 is given in Figure 2. In the absence of 02,the order of increasing catalytic activity is V205 Fe2O3 CrzO3. In the presence of excess 02,NO conversion for all three catalysts is substantially enhanced. The accelerating effect of O2 on the reaction rate has been reported previously (e.g., Markvart and Pour, 1967, for Pt-A1203 catalysts and Bauerle et al., 1975, for A1203-supported V205 and Fe203-Cr203catalysts). The greatest enhancement is observed for the V205 catalyst (Figure 2). The temperatures of maximum NO conversion are 300 and 280 "C for V205and Cr203catalysts, respectively, and 400 "C for Fe203. The order of increasing activity in the presence of O2 is Fe203 < Cr203< V205. The amount of N20 produced is also shown in Figure 2. The Fez03catalyst produced only a small amount of N20 over the range of temperatures studied. Substantial increases in N20 production were observed for the V205 catalyst at temperatures above that of maximum NO conversion. Cr203produced large amounts of N20 even at temperatures below that of maximum NO conversion. Catalytic Activity of A1203-SupportedMetal Oxides. Figure 3 compares the activities of the Vz05,Cr203, and Fe203catalysts supported on A1203for the NO-NH, reaction in the presence and absence of oxygen. In the absence of 02,the A1203-supportedmetal oxide catalysts are relatively inactive and the order of increasing activity is V205 < Fe203< Cr203. A comparison of the results in Figures 2 and 3 indicates that in the absence of oxygen the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
181
I00
- 10 9
-
E - 8 4
- 7 ?
- 6
2
0
- 5 I-
- 4
E
i - 3 "
TEMPERATURE
-: K 0
V 2 0 , -A1203
0
Cr20,-A1203
A A
90
CLOSED P O I N T S OPEN POINTS
Fe203- AI2O3 3
NO-NH,-02
' NO-NH3
eo
70
60
f
4
v1
50
Y
-
.
>
40-
0
30
-
20
-
10
-
'100
I
I
IS0
200
I 250 TEMPERATURE
I
300 ['C
350
'C 1
Figure 4. Catalytic activities of Ti02-supportedmetal oxide mixtures.
400
0 450
NO conversion the order of increasing activity is Fez03 < Vz05 < Cr203. The temperature at maximum NO conversion is 310 "C for CrzO3 and 400 "C for V2O5 and Fez03 catalysts. Below 280 "C lower activities were observed for A1203-supportedcatalysts than for Ti02-supported Cr2O3 and V205catalysts; however, higher activities were obtained for A1203-supportedFe203,which is in agreement with Kasaoka and Yamanaka's (1977) observation that the maximum NO conversion on 10 wt % Fe203-A1203catalysts is greater than on Fe203-Ti02catalysts. In the presence of oxygen, N20formation is much larger on Cr203-A1203than on V205-A1203;no N20was formed on Fe203-A1203catalyst. Comparison of the results in Figures 2 and 3 clearly shows that significantly less N20 forms on A1203-supported catalysts than on Ti02-supported catalysts. Catalytic Activity of Metal Oxide Mixtures. The above results show V205-Ti02as the most active of the catalysts for the NO-NH3-02 reaction. Although the Fe203-Ti02 prepared in this study was the least active of the Ti02-supportedcatalysts, Naruse et al. (1980) and Kato et al. (1983) have shown that the activity of iron oxide catalysts is strongly affected by the preparation procedure. Also, previous studies (Nobe et al., 1976) indicate substantial improvements in activity can be achieved by addition of V2O5 and/or Cr203to Fez03 catalysts. NO conversion-temperature data for several oxide mixtures of Fe, Cr, and V with different composition are given in Figures 4 (Ti02-supported)and 5 (Alz03-supported). The catalysts examined are Fe203-Cr203(9:1), Fe203-Cr203-V205(18:1:1), V205-Cr203(201), and V205-Fe203 (20:l). The results for the V205catalysts are included for comparison as well as the amount of N2O produced by the NO-NH3-02 reaction. The order of increasing activity for the Ti0,-supported metal oxide catalysts, as shown in Figure 4, is as follows:
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
182
Table 11. Calculated Intrinsic Kinetic Parameters A , mol/g of E, catalvst n m cat.h.atm"+'" kcal/mol 5.19 X lo6 12.9 VzOS-TiO2 1 0 1 o 3.45 x 104 10.1 FezO3-TiOZ 1 0 2.41 X lo8 14.6 CrZO3-TiOz 4.25 X lo6 11.9 VzO5-FezO3-TiO2 a b 4.25 X lo6 11.9 VzO5-CrZO3-TiOz a b 1 0 5.36 X lo5 11.4 Fez03-Crz03-Vz05-Ti0z 1.09 X lo6 12.4 FezO3-CrzO3-TiO2 a b 1 0 1.26 X lo7 14.8 Vz05-A1203 a b 1.09 X lo6 13.2 Fe203-A1203 Cr203-A1203 a b 2.70 X lo7 15.0 V205-Fe203-A1203 1 0 1.26 X lo6 11.0 1 0 2.28 X lo6 11.3 VzO5-Cr2O3-Al2O3 a b 4.29 X lo5 11.4 Fez03-Crz03-Vz05-A1,03 5.80 X lo5 12.3 Fe203-Cr203-A1203 a b
401
"Assumed to be 1. *Assumed to be 0. V 2 0 5 -A1203
30
00
Fe-Cr-A1203
mn
Fc-Cr-V- A1203
A A
V - C r - AI 0
V V
V-Fe-AI2O3
2 3
POINTS
201
1
'100
d J'
CLOSED POINTS
NO CONVERSION N 2 0 CONCENTRATION
I
I 150
200
I 250 TEMPERATURE
300
350
400
0 450
kinetic data are corrected for pore diffusion effects, at the lower temperatures where reaction A is essentially the sole reaction. Kinetic Analysis. The intrinsic reaction rates were determined from the experimental kinetic data for NO reduction with NH3 in the presence of excess O2 (2.2%) in the temperature region T < 350 "C; NO and NH, concentrations were varied from about 200 to 2000 ppm. The overall reaction in this temperature range is assumed to be 4N0
+ 4NH3 + O2 = 4N2 + 6Hz0
(A)
i°C I
Figure 5. Catalytic activities of AlZO3-supportedmetal oxide mixtures.
Fe203-Cr203 < Fez03-Cr203-V205 < V20s-Cr203 = V20,-Fe203 < V205. The effect of adding a small amount of Fe203 or Cr203to the V205-Ti02 catalyst has little influence on conversion of NO at temperatures below 200 OC, but slightly lower conversions than V205are observed at higher temperatures. However, the addition of Cr2O3 and V205to Fe203substantially increases its activity (see Figure 2 for Fe203-Ti02). The conversion-temperature profiles of the catalyts are quite similar except for Fe2OpCr2O3-TiO2,which reached a maximum conversion of NO a t about 270 OC, but then NO conversion sharply declined as the temperature was increased further, somewhat like the behavior of Cr203-TiOz (Figure 2). The conversion of NO on Fe203-Cr203-Ti02at 270 OC is about 70%, and the amount of N20 produced is nearly 450 ppm. A nitrogen balance indicates that the formation of N20 on these catalysts probably results from NH, oxidation with 02.It is apparent from these results that N 2 0production is enhanced by the presence of Cr203in the catalyst. The addition of Cr203or Fe203causes a noticeable increase in NO conversion for the A120,-supported Vz05 catalyst, as shown in Figure 5. This result is in contrast to the TiOz-supported VzO, catalysts. The order of increasing activity for the A120,-supported metal oxide mixture catalysts is as follows: Fe203-Cr203< Fe203Cr203-V205< V205 < V205-Fe20, < V205-Cr203. Only small amounts of N20 are formed on the A1203-supported catalysts, substantially less than on Ti02-supported catalysts. On the basis of the results given in Figures 4 and 5, below the temperature of maximum conversion the Ti02-supported metal oxide mixture catalysts are more active than the corresponding A1203-supportedcatalysts. The experimental results presented above provide a comparison of the relative global activities of the 14 catalysts over the wide range of temperatures studied. Relative intrinsic activities are compared below, after the
The rates were correlated with the power law expression
r = kPNOnPNHam
(1)
Intrinsic kinetic parameters were determined by following calculation procedures used in a previous paper (Wong and Nobe, 1984) to correct for pore diffusion effects. The results of these calculations are summarized in Table 11. The intrinsic rates are determined to be first order with respect to NO and zero order with respect to NH, on catalysts for which experimental data at various NO and NH, concentrations were obtained. Reaction orders were not determined experimentally for Cr203-A1203and Fez03-A1203. In the absence of 02,Niiyama et al. (1977) reported reaction orders of one (NO) and zero (NH,) on a 13 wt % Cr203-A1203catalyst; Naruse et al.'s (1980) results indicated first order (NO) and zero order (NH,) for unsupported iron oxides in excess oxygen. Also, Powell (1981) obtained a rate expression for a 10 wt '70 Fe-Cr (5/1) oxide-A1203 catalyst where NO and NH, reaction orders were essentially one and zero, respectively. Inomata et al. (1980) studied the NO-NH, reaction on an unsupported V205catalyst and reported that the reaction rate is first order with respect to NO and zero order with respect to NH3 in both the absence and presence of oxygen. In the light of the experimental results obtained in the current study and the results reported by others, calculations to determine intrinsic kinetic parameters have been based on first-order (NO) and zero-order (NH,) kinetics for the NO-NH3-02 reaction on all catalysts studied. The intrinsic Arrhenius parameters A and E are given in Table 11. The activation energies ranged from 10.1 to 15.0 kcal/(g mol) for the 14 catalysts. The Cr203catalysts, in general, have higher activation energies than the other catalysts; the activation energy is 14.6 kcal/mol for the Cr203-Ti02catalyst and 15.0 kcal/mol for the Crz03-A1203
Ind. Eng. Chem. Prod.Res. Dev., Vol. 25, No. 2, 1986 183
-
TtO2 -SUPPORTED
-
0 Fe203
n
CATALYSTS
Cr2 03
I FC203-Cr203
19 I 1
Y\
0 F e 2 0 3 - C r Z 0 3 - V 2 0 5 118 I I 1
A 0
V205
v205
-
Fe203 I 2 0 I1
' 4 2 0 5 - C r Z 0 3 1 2 0 I1
ld4
I
I
1
- Fe203 ll8
0
v205
0
v 2 0 5 - 0 2 0 3 118 I 1
I1
A I
I
I
I
I
conversion of NO (XNo) catalyst 100 OC 125 OC 150 OC 175 OC V205-Ti02 0.15 0.39 0.72 0.95 0.11 0.21 0.37 Fe203-TiOz 0.05 Crz03-Ti02 0.13 0.36 0.70 0.93 VzO5-Fe2O3-TiOZ 0.40 0.76 0.97 1.00 V205-Cr203-Ti02 0.40 0.76 0.97 1.00 Fez03-Cr203-V206-Ti02 0.12 0.29 0.55 0.81 Fe203-Cr203-Ti02 0.07 0.18 0.39 0.67 V205-Al203 0.03 0.10 0.28 0.58 0.07 0.17 0.36 Fe203-A1203 0.02 0.16 0.42 0.77 Crz03-A1203 0.05 V2O5-FezO3-Al2O3 0.40 0.73 0.95 1.00 V205-Cr203-A1203 0.46 0.80 0.98 1.00 0.24 0.47 FezO3-Cr2O3-V2O6-Al2O3 0.10 0.74 0.04 0.11 0.25 Fe2O3-Cr2O3-Al2O3 0.48 "Reaction conditions: PNo0= 1000 X 10" atm; PNH,O = 1000 X Po: = 0.022 atm; Nz balance, total gas flow rate = 300 L(NTP)/h; catalyst weight = 14 g. 10" atm;
slightly more active than the V205catalyst; however, the excessive production of N20 may make the CrzO3 catalyst undesirable for controlling NO emissions. Fe203is shown to be the least active (Figure 6), but the addition of small amounts of Cr203and Vz05 significantly enhances the intrinsic activity of the Fe203catalyst. The catalytic activities of the A1203-supportedcatalysts, as shown in Figure 7, follow roughly the same order as for the TiO,-supported catalysts except that Vz05-Cr203A1203is slightly more active than V2O5-FeZO3-Al2O3and that at lower temperatures Fe2O3-Cr2O3-V2O5-Al2O3is more active than the V205-Alz03catalyst. Conversion of NO on the catalysts studied between 100 and 175 "C is compared in Table 111. These calculations are based on fmt-order (NO) and zero-order (NH,) kinetics and the intrinsic kinetic parameters given in Table 11.
184
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
These NO conversion results provide another relative measure of the catalytic activity. Addition of either Fez03 or Cr203increases substantially the intrinsic activity of the Vz05catalyst for both Ti02and Alz03supports, with much greater enhancement obtained for the AlzO,-supported catalysts. These catalysts are the most active and the Fez03catalysts the least active, as also shown in Figures 6 and 7. Table I11 shows that catalysts supported on Ti02 are, in general, more active than on Alz03.Previous studies (Cole et al., 1976; Tauster et al., 1978 Vejux and Courtine, 1978; Roozeboom et al., 1981; Inomata et al., 1982) suggest that the superior activity of catalysts supported on TiOz compared to A120, appears to be the result of strong interaction between the active metal oxide catalyst and the TiOz support.
Discussion Mechanism of the NO-NH3-02 Reaction on V205TiOz and V2O5-AlZO3 Catalysts. Takagi et al. (1976) proposed a mechanism for NO reduction with NH, on Vz05-A1203 in the presence of oxygen based on IR, XPS, MS, adsorption, and reaction experiments. The surface step involved reaction between NH4+and NOz adsorbed species. In a series of papers, Murakami and his coworkers describe the results of studies in which they performed rectangular pulse, isotope labeling, XRD, and IR, ESR, and UV-visible spectroscopy experiments on unsupported Vz05,V205-Ti02,and V205-A120, to elucidate the catalyst structure and the mechanism of the NO-NH, and NO-NH3-02 reactions (Inomata et al., 1978, 1980, 1982; Miyamoto et al., 1981a,b, 1982a,b); the results of these experiments are summarized by Inomata et al. (1983a,b). On the basis of these experimental results, the Murakami group identified V=O as the active surface site responsible for the enhancement of the NO-NH, reaction. As the result of reaction in the absence of gas-phase 02, the active site is reduced to V-OH which is then reoxidized by lattice oxygen which diffuses to the surface from the bulk solid. In the presence of 02,the reduced surface site (V-OH) can also be reoxidized by gas-phase 02,and the lattice oxygen depleted by diffusion to the surface is replenished by diffusion of gas-phase oxygen into the bulk solid. 15N isotope labeling of NO in the NO-NH, reaction provides evidence that below the temperature of maximum NO conversion, V205is highly selective for the reduction of NO with NH, to Nz yhere the major product species (NN) forms from both NO and NH, (Miyamoto et al., 1982). 15Nisotope labeling of NH, in the NO-NH, reaction on Pt-A1,03 (Otto et al., 1970), CuO (Otto and Shelef, 19721, CrzO,-Alz0, (Niiyama et al., 1977),and FezO3-AlZO3 (Niiyama et al., 1975) also shows NN as the major product species. Above the temperature of maximum NO conversion on V20B,Miyamoto et al. (1982) concluded from their isotope-labeling studies that product species (Nz and N20) form from NH, oxidation (lattice oxygen in the absence of O2 and gas-phase Oz in the presence of Oz). Thus, the decline in NO conversion above 400 "C is due to lower concentrations of NH3 and not the result of NO formation by oxidation of NH,. On the basis of the above results, the Inomata-Miyamoto-Murakami (IMM) mechanism (1980) can be modified to describe the results obtained in the current study for NO reduction with NH3 on V205TiOz and VzO5-AlZ0,in the presence of excess oxygen, as shown in eq la-d and 2b,d where S.S, are adjacent V=O and V-OH surface sites. S' represents V-OH due to reduction of V=O and is reoxidized to V-0 by gas-phase
/H
H\ H-N
4 S . S ' t 4NH3
-
:
/
(la)
4S.3'
H H\
H-N
/ 4N2 t 4 H 2 0 t 4s'.
' I
(lb)
t 4NO=activated complex I
4S.S'
4N20 t 4HzO t 4 s ' ' S ' (2bl N
\ I
4 S ' * S ' t 02 a 2 H 2 0 t 4 S . S '
H
H-N
' I
4s.:'
(IC)
2N20 t 6H20 t 4 S . 3 ' (Id)
t 202 activated complex I1 1
2N2 t 6H2O t 4s.:'
(2d)
Oz or lattice oxygen. According to Inomata et al. (1980), oxidized to V=O due to the crystal structure of V205(see Figure 10, Inomata et al., 1980). The Otto-Shelef-Kummer (OSK) mechanism (1970) devised for the NO-NH, reaction can also be modified for the NO-NH3-O2 reaction on V205by taking into account Miyamoto et al.'s (1982) results for the NO conversiontemperature behavior above the temperature of maximum NO conversion, as shown in eq la'-d' and 2b',d'. It should S' cannot be
4NH3(ads)
-
4NH2(ads) t 4H(ads) 4N2
4NH2(ado) t 4NO(ads)zsurface complex
t 4H20
(la') (16)
I 4Nz0 t 4 H z 0 (2b')
4H(ads) t 02 = 2H20
4NHdads) t 202=surface complex 11'
2
(1 c') 2N20 t 6H20 ( I d ' ) 2N2 t 6H2O
(2d')
be noted that the formation and transformation of the activated and surface complexes can be readily expressed in terms of more elementary reaction steps. The modified and the original form of the OSK mechanism can be used to describe the kinetics of the NO-NH3-Oz reaction over a variety of catalyst types without detailed specification of the identity and nature of the reaction sites at the catalyst surface. In contrast, the modified IMM mechanism identifies and describes the nature of the reaction sites but is limited to Vz05catalysts at present, although it may be applicable to other catalyst types. An examination of the reaction steps in the modified OSK and IMM mechanisms indicates that the two mechanisms are equivalent for Vz05catalysts. NO-NH3-02 Reaction on Ti0,- and Al,O,-Supported Fe and Cr Oxides and Oxide Mixture Catalysts. The modified OSK mechanism can be applied to describe the NO conversion-temperaturebehavior and NzO formation results obtained in the current studies for the NO-NH& reaction on TiOz-and A1203-supportedFez03, Cr203,FeZO3-CrzO3,FezO3-CrZ0,-V2O5,V205-Cr203,and V205-Fe203 catalysts. Thus, at lower temperatures the higher activity of TiOz-supported catalysts relative to Alz03-supportedcatalysts for NO reduction to Nz and NzO may be attributed to the larger rate constants for the formation of surface complex I and/or its transformation in reaction steps lb' and 2b'. Reaction step la' is very fast since NH, follows zero-order kinetics. At higher temperatures (above the temperature of maximum NO conversion), the rate of the decline of NO conversion with temperature may be ascribed to the magnitude of the rate constants in the NH,-02 reaction steps. For example, the lower temperature of the maxima,
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
100
r
"2'5-'2'3
185
r
Fe203- TI O2 2 7 9 *C
2
"[ 00
0
IO
2 0 x ) 40 50 60 70 TIME (mln)
0
IO 2 0 30 40 TIME (mh )
-
Fa-Cr OXIDES-TiO2
Fe Cr -VcD(IDES-Tio2
2719:
0 20
0
40
TIME (mln
60
Do
80
IO 2 0 30 TIME ( mln )
0
IO 2 0 30 40 TIME (mln)
50
Figure 9. Decline of NO conversion with time after shut-off of 0.i (a)Cr203-Ti02; (b) Fe203-Ti02; (c) Fe203-Cr203-Ti02;(d) Fe203Cr203-V205-Ti0,.
0'
'
0
'
'
20
'
'
'
'
40 60 TIME ( m l n )
'
;
'
80
'
1
IO0
TIME (min)
70 r
Figure 8. Decline of NO conversion with time after shut-off of 02: (a) V206-A1203;(b) V205-Ti02.
the sharp declines in the NO conversion-temperatureplots, and the higher rates of N20 formation for Cr203-Ti02, Cr203-A1203,and Fe203-Cr203-Ti02,as shown in Figures 2-4, respectively, may be attributed to larger rate constants in the "3-02 reaction steps than those of other catalysts. Also, it is likely that the rate constants of step 2b' for these three Cr catalysts are larger; evidence for the significance of this step at higher temperatures was reported for V205 by Miyamoto et al. (1982), who obtained measurable amounts of NNO above 300 "C in their isotope-labeling studies. In contrast, the lack of a sharp decline in the NO conversion-temperature plots in the temperature range studied and the lower formation rates of N 2 0 for Fez03-Ti02 and Fe203-A1203indicate that the rate constants of the above steps are much smaller than those for the other catalysts. As mentioned earlier in this paper, the Murakami group conducted a wide variety of physicochemical experiments on V205to establish their hypothesis that lattice oxygen diffusing from the bulk solid to the surface sites accelerates the NO-NH, reaction; in the presence of gas-phase oxygen, O2also accelerates the reaction. They interpret NO conversion-time behavior after shut-off of gas-phase oxygen in the N0-NH3-O2 reaction as indicating that lattice oxygen accelerates the NO-NH3 reaction (Inomata et al., 1980). Similar transients for V206-Ti02and V205-A1203, which were obtained in the present work, are shown in
"1
TIME ( m l n )
I
TIME (mln
Figure 10. Decline of NO conversion with time after shut-off of 02: (a) CrzO3-Al2O3;(b) Fe203-A1203;(c) Fe203-Cr203-A1203; (d) FezO3-Cr2O3-VZO5-Al2O3.
Figure 8. The transients are characterized by a slow decrease of NO conversion with time. Inomata et al. (1980) reported that the NO-NH, reaction on V2O5 reached steady state 60-70 h after shut-off of 02. Figures 9 and 10 show that Cr203-Ti02and Cr20,-A1203 respectively exhibit similar transient behavior as V205TiOz and V205-A1203,suggesting that lattice oxygen may also accelerate the NO-NH, reaction on Cr203as on V205 catalysts. In contrast, Fe203catalysts, as shown in Figures 9 and 10, did not exhibit the slow decrease in NO conversion with time, indicating that gas-phase oxygen alone accelerates the NO-NH3 reaction on Fez03 catalysts; lattice oxygen does not appear to be a factor in accelerating the rate of the reaction on Fe203 The addition of Cr2O3and V205to Fe203catalysts apparently provides some lattice oxygen for enhancement of the NO-NH3 reaction, as
Ind. Eng. Chem. Prod. Res. Dev.
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shown by the transient behavior of Fe203-Cr203and Fe203-Cr203-V205catalysts in Figures 9 and 10. Further comments on mechanisms and catalytic activities of Fe and Cr oxides and Fe, Cr, and V oxide mixtures are not appropriate until additional physicochemical experiments, such as those reported for unsupported and supported V205catalysts, particularly as they relate to the NO-NH, and NO-NH3-02 reactions, are performed. Registry No. A1203, 1344-28-1; TiOz, 13463-67-7; NzO, 10024-97-2; NO, 10102-43-9; NH,, 7664-41-7; V, 7440-62-2; Cr, 7440-47-3; Fe, 7439-89-6. L i t e r a t u r e Cited Bauerie, G. L.; Wu, S. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Dev. 1075, 14, 268. Chertov, V. M.; Okopnaya, N. T. Kinet. Katal. 1078, 19, 1595. Cole, D. J.; Cullis, C. F.; Hucknaii, D. J. J . Chem. SOC.,Faraday Trans. 1078, 7 2 , 2185. Inornata, M.; Miyamoto, A.; Murakami, Y. Chem. Lett. 1078, 799. Inomata, M.;Miyamoto, A,; Murakami. Y. J . Catal. 1880, 6 2 , 140. Inomata, M.; Miyamoto, A,; Murakami, Y. J. Phys. Chem. 1081, 8 5 , 2372. Inomata, M.; Miyamoto, A.; UI, T.; Kobayashi, K.; Murakami, Y. Ind. Eng. Chem. Prod. Res. Dev. 1982, 2 1 , 424. Inomata, M.; Mori, K.; Miyamoto, A,; Ui, T.; Murakami, Y. J . Phys. Chem. 10838, 8 7 , 754. Inomata, M.; Mori, K.; Miyamoto, A.; Murakami, Y. J . Phys. Chem. 1083b, 8 7 , 761. Kasaoka, S.; Yamanaka, T. Nippon Kagaku Kaishl 1077, 6 , 907. Kato, A.; Matsuda, S.; Nakajima, F.; Inamari, M.; Watanabe, Y. J. Phys. Chem. 1081, 8 5 , 1710. Kato, A.; Matsuda, S.; Kamo, T. Ind. Eng. Chem. Prod. Res. Dev. 1083, 2 2 , 406. Markvart, M.; Pour, V. C. J . Catal. 1087, 7 , 279.
1986,25, 186-192
Matsuda, S.;Takeuchl, M.; Hishinuma, Y.; Nakajim, F.; Narita, T.; Watanabe, Y.; Inamarl, M. J. Air Pollut. Control Assoc. 1078, 2 8 , 350. Miyamoto, A.; Yamazaki, Y.; Inomata, M.; Murakami, Y. J. Phys. Chem. 1081, 8 5 , 2366. Miyamoto, A.; Yamazaki, Y.; Hattori, T.; Inomata, M.; Murakami, Y. J. Catal. 1082a, 7 4 , 144. Miyamoto, A.; Kobayashi, K.; Inomata, M.; Murakami, Y. J. Phys. Chem. 1082b, 8 6 , 2945. Nakajima, F.; Takeuchi, M.; Matsuda, S.; Uno, S.; Mori, T.; Watanabe, Y.; Inamari, M. US. Patent 4085 193, 1978. Naruse, Y.; Ogasawara, T.; Hata, T.; Kishitake, H. Ind. Eng. Chem. Prod. Res. Dev. 1080, 19, 57. Niiyama, H.; Ookawa, T.; Echigoya, E. Nippon KagakuKaishi 1075, 2 , 1871. Niiyama, H.; Murata, K.; Echigoya, E. J. Catal. 1077, 48, 201. Nobe, K.; Bauerie, 0. L. "Technical Assessment of Catalysts for Control of NOx from Stationary Power Plants", UCLA-ENG-7456, June 1974. Nobe, K.; Bauerle. G. L.; Wu, S. C. "Parametric Studies of Catalyst for NO, Control from Stationary Power Plants", EPA-80017-76-026, 1976. Otto, K.; Sheief, M. J. Phys. Chem. 1072, 76, 37. Otto, K.; Sheief, M.; Kummer, J. T. J. Phys. Chem. 1070, 7 4 , 2690. Powell, D. E.; Nobe, K. Chem. Eng. Commun. 1981, 10, 103. Roozeboom, F.; van Diiien, A. J.; Geus, J. W.; Geiiings. P. J. Ind. Eng. Chem. Prod. Res. Dev. 1081, 2 0 , 304. Shikada, T.; Fujimoto, K.; Kunugi, T.; Tominaga, H.; Kaneko, S.;Kubo, Y. Ind. Eng. Chem. Prod. Res. Dev. 1981, 2 0 , 91. Takagi, M.; Kawai, T.; Soma, M.; Onishi, T.; Tamaru, K. J . Phys. Chem. 1078, 80. 430. Tauster, S.J.; Fung, S. C.; Garten, R. L. J. Am. Chem. SOC. 1078, 100, 170. Vejux, A.; Courtine, P. J. SolM State Chem. 1078, 2 3 , 93. Wong, W. C. Dissertation, University of California at Los Angeies, Feb 1982. Wong, W. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Dev. 1084, 2 3 , 564. Wu, S. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Dev. 1077, 16, 136.
Receiued for review November 5 , 1984 Revised manuscript received October 4, 1985 Accepted November 22, 1985
Model of Temperature Dependence of a Vanadia-Alumina Catalyst for NO Reduction by NH,: Fresh Catalyst In-Slk Nam,t John W. Eldrldge," and J. R. Klttrellt DepaHment of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 0 1003
A vanadla-alumina catalyst (10% V,O, on AI,O,) was employed in a packed-bed, tubular reactor to obtain kinetic data for the modeling of NO reduction by NH3 in the absence of SO2. The net NO conversion with this catalyst decreases at high reaction temperatures (>400 "C), due to the competing reaction of ammonia oxidation, characteristic of most catalysts for the selective catalytic reduction of NO, by NH,. A two-parameter model (based upon two simultaneous and competitive reactions) is proposed for the analysis of the data at and above the temperature of maximum conversion of NO. Below that temperature this model reduces to the pseudo-first-order behavior in NO commonly observed for this reaction system. The model adequately f i s all experimental data over the entire range of reaction temperature and space velocity. The activation energy of the NH, oxidation reaction (53.9 kcal/mol) was found to be over 4 times that of the NO reduction reaction (12.8 kcal/mol).
Introduction
Selective catalytic reduction (SCR) of NO by NH3 involves two dominant reactions competing for the NH3: the reduction of NO and an oxidation producing NO. These reactions may be written as 6N0 + 4NH3 5N2 6H20 (1) 502 + 4NH3 4N0 + 6H20 (2)
-
+
-+
Other stoichiometries of the NO reduction reaction have been observed on various catalysts. *Author to whom correspondence should be addressed. 'Present address: Chemical Engineering Department, Wayne State University, Detroit, MI 48202. * KSE Inc., Amherst, MA 01004. 0196-432118611225-0186$01.50/0
Reaction 1provides the desired conversion of NO to N2 Reaction 2 is undesirable, not only because it produces NO but also because it consumes NH3. Reaction 2 becomes particularly important as reaction temperature is increased. Their relative rates depend largely upon reaction temperatures. The effects of these two reactions on overall reaction rates for nondeactivating systems have been the subject of many studies (Miyamoto et al., 1982; Ganti, 1980; Seiyama et al., 1979; Mizumoto et al., 1979; Eng, 1977; Pavao, 1977; Tsang, 1977; Nobe et al., 19761, but they have not yet been successfully modeled. Furthermore, the study of deactivation behavior for this reaction system has not yet been reported, since the modeling of such deactivation is made even more complicated by the complex primary reactions. 0 1986 American Chemical Society
