Nitric Oxide Reduction on Copper-Nickel Catalysts George L. Bauerle, Lee L. C. Sorensen, and Ken Nobe* School of Engineering and Applied Science, University of Caiifornia, Los Angeles, California 90024
Catalytic activity for the reduction of nitric oxide with carbon monoxide on copper-nickel catalysts of varying composition has been investigated. The catalysts were prepared by impregnating preformed alumina pellets with aqueous solutions of copper and nickel nitrates, calcining at 500" and activating in CO at 350". The results indicate that the catalytic activity is dependent on the nicke1:copper ratio. In general, activity increased with increasing copper content. Above 60 atom % copper (carrier-free basis), which corresponds to filled 3d band vacancies of nickel, the reduction activity did not vary substantially with catalyst composition. At the higher copper levels, the reaction is first order with respect to NO; reaction order decreased with decreasing copper content below 24%. More severe activation of the catalysts in CO at 500" led to the opposite trend in activity; that is, activity increased with increase in nickel content. Furthermore a general decrease in the relative level of activity occurred with catalysts reduced at 500".
Introduction Recent works indicate that copper-nickel alloys show promise as catalysts for the reduction of nitric oxide. Bernstein, et al. (1971), studied the use of unsupported Monel (copper-nickel alloys) catalysts for reduction of NO in automobile exhaust and reported complete conversion of NO above 700". Meguerian and Lang (1971) found that copper and Monel catalysts had approximately equal activity for reduction of NO a t 580". They found that optimum activation of the catalyst involved partial oxidation in moist air. Copper-nickel alloy catalysts have been investigated for a variety of other reactions. It has been established by Conway, e t al. (1961), that the electrocatalytic activity for hydrogen evolution on copper-nickel alloys varied with composition. The exchange current was shown to change the least with composition when the copper content was increased beyond 60 atom 70, which corresponds to the filled 3d bands of nickel. A change in catalytic activity for orthohydrogen-parahydrogen conversion a t this critical composition was reported by Couper and Eley (1950). Dowden and Reynolds (1950) observed a sharp decrease in the rate of styrene hydrogenation on copper-nickel alloys with increasing copper content. A similar decrease in activity was observed for decomposition of methanol. For decomposition of formic acid, it was observed that CO and Hz evolution decreased by a factor of 4 from pure nickel to 70% nickel and then increased gradually until the rate of gas evolution on pure copper was about half that on pure nickel. Decomposition of hydrogen peroxide was observed to decrease with increasing nickel content. Reynolds (1950) studied hydrogenation of benzene on two types of nickel-copper catalysts and hydrogenation of methanolic styrene on reduced nickel-copper oxide catalyst. The metal alloy catalysts were powdered Raney type prepared by exhaustive extraction of copper-nickel-aluminum alloy powders with caustic and a granular catalyst prepared from similar alloys by surface activation in caustic. The reduced oxide catalysts were prepared by coprecipitation of a mixture of metal nitrates on Kieselguhr followed by calcination a t 400" and reduction in hydrogen a t 500". All catalysts showed a decrease in activity with copper content. The critical copper level above which activity was very low, appeared to be about 60%. In each case, the decrease in activity could not be attributed solely to the observed decrease in surface area with increasing copper content. The rather low temperatures used by Reynolds
(1950) in preparing the reduced-oxide catalysts (as compared with temperatures required to prepare the alloys by fusion methods) indicated that the relationship between activity and d-band vacancies was applicable not only to the metallic alloys but also to reduced binary metal oxide catalysts. I t should be noted that the latter was not established to be completely in metallic form. This paper reports on a study of the activity of reduced copper-nickel oxide catalysts for the complete reduction of NO by CO to determine if there was a correlation between activity and metal composition as observed for other reactions.
Experimental Section The experimental apparatus has been described previously (Bauerle, et al., 1972). Nitric oxide was analyzed using a Beckman Model 315 nondispersive infrared (NDIR) analyzer. Carbon monoxide was determined with a Mine Safety Appliances LIRA 300 NDIR analyzer and carbon dioxide was determined using a Beckman Model 15A NDIR analyzer. Nitrogen dioxide was determined with a Beckman Model 77 flow colorimeter. Nitrous oxide was determined by gas chromatography using a 3.3-m column (3.2-mm 0.d.) of stainless steel containing Porapak &. The catalysts were prepared by impregnation of dry, cylindrical alumina pellets (Filtrol Grade 86, 3.2 mm in diameter by 3.2 mm in length, surface area 190 m2/g) which were heated to 180" and then soaked in hot, aqueous solutions of nickelous nitrate and cupric nitrate. The impregnated pellets were air-dried overnight a t 180" and then heated a t 500" in flowing air in a combustion tube. The catalysts were then activated in pure CO a t 350" for 16 hr. Table I gives the catalyst compositions. The total amount of metal on each catalyst was 13.8%. Total catalyst weight was 16.5 g in each case. Surface areas of the catalysts were determined using a flow adsorption system. With this system, adsorption data agreed reasonably well, for reference cases, with standard, static BET techniques. The test procedure involved setting the proper nitrogen carrier gas flow rate, adjusting the temperature controls, and zeroing the analyzers. The NO and CO streams were then set a t approximately the desired rates. During the thermal equilibration period, the NO and CO flows were readjusted accurately using a soap-bubble buret. The analyzer span controls were adjusted to provide readings conInd. Eng. Chern., Prod. Res. Develop., Vol. 13, No. 1, 1974
61
Table I. Compositions of Copper-Nickel Catalysts
(Carrier-Free Basis) Catalyst
Atom % Ni
Catalyst
Atom 5% Ni
A
100 76 52
D E F
26 14 0
B C
sistent with earlier calibrations by admitting a sample of the gas stream from the reactor inlet sample port. The outlet concentrations were recorded after isothermal conditions in the reactor has been attained and the analyzers showed steady-state operation. Periodic checks of the analyzer span settings were made during each day's operation. Total flow rate in each test was 300 l./hr a t room temperature and atmospheric pressure. Nitric oxide concentration was varied from 200 to loo0 ppm. The CO concentration was maintained at 10,000 ppm. Temperature was varied from about 100 to 500".
0 100
200
300
400
TEMPERATURE ("C 1
Figure 1. Conversion of NO as a function of temperature with catalysts activated at 350"; flow rate 300 l./hr; loo0 ppm of NO, 1O,oo0 ppm of CO: v, catalyst A; 0 ,B; A, C;O, D: 0 ,E; 0,F
Results and Discussion Data points were taken in order of decreasing temperature and, in the kinetic studies to be described later, in order of decreasing NO concentration. Such operation was necessary to ensure optimal attainment of steady-state conditions for individual tests and for maximum reproducibility of the overall activity-temperature-inlet concentration relationships. In general, it was found that during the initial run with each catalyst, operation of the reactor for a considerable length of time was necessary to stabilize the catalyst. The duration required to attain stable operation appeared to be a function of the nickel-copper ratio. The copper-rich catalysts required periods of about 1 hr to stabilize, while the nickel-rich catalysts required considerably longer times. The 100% nickel catalyst (carrier-free basis), for instance, required over 8 hr to attain stable, steady-state operation during the initial run. Then, with all the catalysts, subsequent data points could be taken usually within about 1 hr after new operating conditions were set. It has not been established that the slow attainment of steady state with the nickel-rich catalysts was due to the relative difficulty in reducing nickel oxide compared to copper oxide. I t seems reasonable to suppose that the excess CO causes further reduction of nickel oxide simultaneously with NO reduction resulting in the very slow drift in conversion that was observed. However, the drift was in the direction of decreased conversion (at times, to values less than half of the initial value). Thus, it would follow that an initial, higher oxidation state for nickel would appear to be more active for NO reduction if such in situ reduction were occurring. Figure 1 shows.typica1 results with the catalysts for tests in which NO and CO inlet concentrations were maintained a t 1000 and 10,000 ppm, respectively. I t is seen that activity decreases, in general, with increasing nickel content. This trend is shown more clearly in Figure 2, the points of which were cross-plotted from Figure 1. In some cases, a slight extrapolation of data in Figure 1 was necessary; such extrapolated points are shown as solid symbols in Figure 2. Activity expressed in terms of conversion was approximately constant up to 26 atom % nickel. Above this nickel content, conversion decreased approximately linearly with concentration for temperatures of 300" or lower and somewhat less abruptly a t 350". The nickel content above which conversion of NO started to decrease, a t each temperature, was somewhat lower than the critical composition for filled 3d-band vacancies 62
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 1, 1974
Table 11. Surface Area Calcined and Activated Catalysts (Activation for 16 hr in CO at 350")
Surface area, m2/g After calcination in air a t
After activation in CO a t
Catalvst
350'
350 O
A1203(carrier only) 0 % Ni 14% Ni 26% Ni 52% Ni 76% Ni 100% Ni
190 149 148 144 143 144 148
185 144 149 142 141 151 146
of nickel (theoretically, 40% Ni). As in Reynold's studies of the hydrogenation of methanolic styrene on reduced copper-nickel oxide catalysts (1950), it was not established that the catalysts had been completely reduced to the metallic state. The decrease in activity above 26% nickel cannot be attributed to a decrease in surface area. Table I1 summarizes results of surface area determinations using the flow adsorption system. Direct reduction of the catalysts in the adsorption apparatus was made precluding exposure to air between the activation step and surface area determinations. Table I1 shows that relatively little variation in surface area occurred by varying the nickel content; furthermore, reduction a t 350" caused little change in the surface areas. Conversion data for each catalyst were correlated by use of the empirical "power law" rate equation
r = kP,," (1) and the mass balance equation for integral, plug-flow reactors
The integrated forms of eq 2 after substitution of eq 1 are
k=
QPX~'-~ [l - (1 - x , ' - n ] W(1 - n )
(3)
for n # 1 and k = - lQnP-
w
for n = 1.
1 1 - x
(4)
I
-
/
a
0
-
1
0.2
01 0
I
I
I
20 ATOMIC
I
40
I
I
60
I
I
80
I
1
I00
PERCENT NICKEL
ATOMIC
Figure 2. Effect of catalyst composition on NO conversion with catalysts activated at 350". Points cross-plotted from Figure 1 (solid points represent slight extrapolation of data)
A computer program was used which performed a leastsquares fit of log k us. l / T for determination of the constants in the Arrhenius equation
k = A exp(-E/RT)
(5 1
The order n was varied incrementally until the minimum in the sum of squares of the difference between the experimental values of k (eq 3 or 4) and those calculated by eq 5 was determined. T o provide data for apparent reaction order determinations, additional data in which NO concentration was varied from 200 to 1000 ppm were obtained. Carbon monoxide concentrations were maintained a t least a factor of 10 in excess of NO concentrations. Figure 3 shows apparent reaction order and activation energy as functions of nickel content. An empirical first order was determined for all catalysts up to 76% nickel. With the pure nickel catalyst, reaction order was 0.8. Activation energy was approximately 6.5 kcal/mol up to 76% nickel and 8.8 kcal/mol for pure nickel. The trend in activity with composition shown in Figure 2 was in the opposite direction to that reported by Bernstein, et al. (1971), for NO reduction on unsupported copper-nickel alloys in the presence of C02 and H20. With the alloys it was found that conversion, in general, increased with increasing nickel content. However, a direct comparison with the work of Bernstein, et al., with auto exhausts (1971) is not possible since in our study the catalysts used were probably only partially reduced to the metallic state and its exact nature was not known and the gases used consisted of only NO, CO, and Nz. A series of catalysts1 identical with those listed in Table I was prepared. However, in this series a more severe activation of the catalysts was performed. In this case, catalysts were activated in CO a t 500" for up to 96 hr rather than a t 350" for 16 hr. Figure 4 shows conversion-composition data with the catalyst subjected to the more severe activation procedure. It is seen that the activity of this series was less than for the catalysts activated a t 350". With pure nickel catalyst, conversion was almost equivalent for the two cases (see data for 350 and 360" in Figures 2 and 4, respectively). (1) Experimental work performed b y G . R. Service (1971)
PERCENT
NICKEL
Figure 3. Activation energy and reaction order with respect to NO as functions of catalyst composition: open points, catalysts activated at 350"; solid points, catalysts activated at 500"
loo
F
ATOMIC PERCENT
NICKEL
Figure 4. Conversion of NO as a function of composition of nickel with catalysts activated at 500". Solid points represent slight extrapolation of data similar to Figure 1
There was no indication, as noticed earlier, that catalytic activity remained constant for compositions corresponding to filled 3 d-band vacancies of nickel. In fact, a minimum in activity occurred a t 14% nickel, well below the theoretical, critical composition. Reaction order for NO and activation energy were determined, as before, for the latter series of catalysts. Figure 3 also. summarizes these results. Reaction order increased with increasing nickel content and was 0.94 for pure nickel, as compared with 0.80 for the nickel catalyst reduced a t 350". Apparent activation energies for the two nickel catalysts were identical. Surface areas for this second series of catalysts are summarized in Table 111. Surface areas appeared to increase somewhat with nickel content in this case. Carbon deposits formed from the disproportionation of carbon monoxide during the extended, high-temperature activation step may have been the cause of the lower, overall activity of the latter catalyst series. This reason is suggested by the work of Hightower and White (1928), who reported disproportionation of CO to COz and carbon Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 1, 1974
63
0
e
ATOMIC
PERCENT
Surface area, m2/g
!
NICKEL
Figure 5 . Extent of carbon deposition during activation: open points, catalyst activated at 350"; solid points, catalyst activated at 500". Data linearly normalized to 16-hr activation period on nickel catalyst above 300" during the synthesis of methane from water gas. The analyses of carbon on the catalysts in this study were determined by an ASTM method (1964). The results are given in Figure 5, and it is seen that substantially more carbon was deposited on catalysts that were activated a t the higher temperature. Although the general trends shown in Figures 4 and 5 appear somewhat similar, a correlation between carbon deposition and catalytic activity is not readily discernible. This is evident by a comparison of the results in Figures 2 and 4 showing the 100% copper catalyst to have a substantially higher activity for the lower activation temperature. On the other hand, the change in the activation temperature did not affect appreciably the activity of the 100% nickel catalysts. In contrast, Figure 5 shows that the amounts of carbon formed a t the two activation temperatures were small and not significantly different for the 100% copper catalyst while there was a very large difference for 100% nickel. A final series of tests in this study was to determine if the two groups of catalysts ( i e . , those activated in CO a t 350 and 500°, respectively) could be reversibly converted from one activation state to the other. Catalysts C, D, and E, which had been activated in CO a t 350" and tested, were subsequently activated in CO a t 500" for 16 hr. Resultant activity was intermediate between the values shown in Figures 2 and 4 for the given compositions. Activation of catalyst E in CO a t 500" for an additional 36 hr resulted in NO conversion very close to that shown in Figure 4. Similarly, after recalcination of the 14% nickel catalyst, which had previously been activated in CO a t 500°, in air a t 500°, and subsequently in CO a t 350" for 16 hr, the conversion data closely resembled those obtained for a similar catalyst initially reduced a t 350".
64
Table 111. Surface Areas of Calcined and Activated Catalysts (Activation for 18 hr in CO at 500')
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 1, 1974
After calcination in air at
After activation in CO at
Catalyst
500 O
500 O
A1203(carrier only) 0 % Ni 14% Ni 26% Ni 52% Ni 76% Ni 100% Ni
190 149 148 144 143 144 148
177 152 162 145
166 189
256
Conclusions Activity of supported, reduced copper-nickel oxide catalysts for the reduction of NO with CO is a function of the nicke1:copper ratio. Activity is also a function of the catalyst activation procedure. Catalysts activated in CO a t 500" are significantly less active than those activated a t lower temperatures except for the 100% nickel catalyst which appeared to have similar activity after either activation scheme. Those activated a t 350" show a decrease in activity with nickel content above about 26% while those activated a t 500" show a general improvement in activity with increasing nickel content. Carbon deposits can result from disproportionation of CO on the copper-nickel catalysts. Maximum carbon deposits were obtained with the nickel-rich catalysts. There appears to be a relationship between the per cent of 3d-band vacancies of nickel and activity with catalysts activated in CO a t 350". With activation in CO at 500", no clear correlation was obtained, although a minimum in activity occurred a t the copper-rich end of the composition range. Literature Cited ASTM, Book ASTM Stand., Part 32 (1964). Bauerle, G . L., Service, G . R., Nobe, K . , Ind. Eng. C h e m . , Prod. Res. Develop., 11, 54 (1972). Bernstein, L. S., Kearby, K. K., Rarnan, A . K . S., Vardi, J., Wigg, E . E., Paper 710014, Automotive Engineering Congress, Detroit, Mlch , Jan 1971. Conway, 6.E., Beatty, E. M., De Maine, P. A . D., Electrochim. Acta, 7, 39 (1961). Couper, A , , Eley, D. D., Discuss. Faraday Soc., 8, 172 (1950). Dowden, D. A , , Reynolds, R. W . . Discuss. Faraday Soc.. 8, 184 (1950). Hightower, F. W . , White, A . H . , Ind. Eng. Chem., 20, 10 (1928) Meguerian, G. H.. Lang, C. R . , Paper 710291, Automotive Engineering Congress, Detroit, Mich., Jan 1971. . Reynolds, R. W . , J. Chem. Soc.. 265 (1950). Service, G. R., University of California at Los Anoeles. unDublished results, 1971.
Received foireuiew May 16, 1973 Accepted October 20, 1973
This work was supported in part by Grant AP00913, Air Programs Office, Environmental Protection Agency, and, in part, by Air Resources Board Project No. 2-009, State of California.
