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alumina catalysts for carbon monoxide oxidation and nitric oxide reduction
Applied Catalysis, 71 (1991) 275-282 Elsevier Science Publishers B.V., Amsterdam
275
Calcination conditions on copper/alumina catalysts for carbon monoxide oxidation and nitric oxide reduction Ta-Jen Huang* and Tai-Chiang Yu Department
of Chemical Engineering,
(Taiwan ROC), tel. (+886-35) (Received 6 September
National
Tsing Hua University,
713691, fax. (+886-35)
1989, revised manuscript
Hsinchu 30043
715408
received 5 December 1990)
Abstract A commercial copper oxide/alumina catalyst with 6.5 wt.-% copper metal was used to study the effect of calcination conditions on carbon monoxide oxidation and nitric oxide reduction with ammonia. Calcination in a reducing atmosphere, at temperatures up to 9OO”C, may induce a strong metal-support interaction which leads to enhanced activities not only for carbon monoxide oxidation but also for nitric oxide reduction. This strong metal-support interaction effect was stronger for higher calcination temperatures. It was found that high temperature treatment in a reducing atmosphere was beneficial to carbon monoxide oxidation while an oxidizing atmosphere was usually preferred for nitric oxide reduction. In addition, copper species at higher oxidation states may lead to a higher activity of nitric oxide reduction, but an optimum state may exist. It can be concluded that nitric oxide reduction is a structuresensitive reaction over copper catalysts. Keywords: copper oxide/alumina, carbon monoxide oxidation, nitric oxide reduction, calcination, strong metal-support interaction, structural sensitivity.
INTRODUCTION
Base-metal oxide catalysts have received considerable attention because of their use in automobile emission control systems. During carbon monoxide oxidation, base-metal oxide catalysts, especially copper oxide, exhibit activities per unit surface area similar to those of noble-metal catalysts such as platinum [ 11. However, sintering is usually considered to be a problem during the use of copper catalysts in automobile emission control. Huang et al. [2] reported that calcination at 800’ C in a reducing atmosphere produces a sintered copper surface but also a largely enhanced activity of carbon monoxide oxidation. As temperatures higher than 800’ C may be encountered in automobile emission, further investigations were needed and these were carried out in this study. In contrast, alumina-supported copper oxide catalysts have been used in many commercial processes in order to carry out a selective catalytic reduction
0166-9834/91/$03.50
0 1991 Elsevier
Science
Publishers
B.V.
276
to remove nitric oxide from flue gases in a reaction with ammonia [ 31. When removing carbon monoxide from exhaust gases, Huang et al. [2] found that carbon monoxide oxidation is a structure-sensitive reaction over copper catalysts. In this study the authors have attempted to clarify the structural sensitivity of selective nitric oxide reduction which has not yet been explained, to the knowledge of the authors. The effect of strong metal-support interaction (SMSI) has been studied extensively for supported metal catalysts [ 2,4-61. Foger [ 51 reported that metals in the SMSI state exhibit the general feature of having an enhanced activity for reactions involving carbon monoxide, such as Fischer-Tropsch synthesis, methanation, and carbon monoxide-nitric oxide reactions. Huang et al. [2] have confirmed this for carbon monoxide oxidation over copper catalysts. It should be interesting to see whether a similar feature is at work for selective nitric oxide reduction. In this paper we have proved this to be the case, for the first time as far as we are aware. In our experiments, calcination was carried out at a series of temperatures (450-1050°C) and in different gaseous atmospheres (oxidizing or reducing) in order to induce a large variation in the surface properties of a commercial copper oxide/y-alumina catalyst. The effect of these calcination conditions was studied by the reactive chemisorption of nitrous oxide and by activity tests for carbon monoxide oxidation and nitric oxide reduction. EXPERIMENTAL
A copper oxide/y-alumina catalyst in the form of 1/8x l/8 in. cylindrical pellets was obtained from Strem Chemicals, Newburyport, MA, U.S.A. It contained 6.5 wt.-% copper metal, which is an optimum copper content for carbon monoxide oxidation as was reported by Huang et al. [ 71. The catalyst pellets were calcined under the following conditions: (A) Oxidizing atmosphere: Calcination took place in a flow of air at 450900°C overnight and in stagnant air at 1050°C overnight. These are denoted as catalyst A. (B) Reducing atmosphere: Calcination took place in a flow of 10% hydrogen in nitrogen at 450-900°C overnight. Then, the temperature was decreased to 60°C before an air flow was introduced. The last step was taken in order to reduce or avoid any possible destruction of the metal-support interaction by oxidation [8]. These are denoted as catalyst B. After calcination, the pellets were crushed and screened to a size under 200 mesh before carrying out subsequent dispersion measurements and activity tests. The number of accessible surface copper atoms was determined by applying a nitrous oxide pulse method as described by Evans et al. [ 91. Some comments on this method are described elsewhere [ 21.
Both carbon monoxide oxidation and nitric oxide reduction activities were measured in a conventional microreactor made from a quartz tube. For carbon monoxide oxidation, the reactant gas was a mixture of 2% carbon monoxide and 1.9% oxygen in nitrogen; for nitric oxide reduction, it was 4500 ppm nitric oxide and 4880 ppm ammonia in nitrogen. Forty ml/min of the reactant gas was passed over 0.1 g of the catalyst. The reaction temperature was 210’ C. All activity results were obtained under steady-state conditions. Preliminary tests showed that both external and internal diffusional resistances were negligible under our experimental conditions. RESULTS
AND DISCUSSION
Copper dispersion Copper dispersion is defined as being the ratio of the number of surface copper atoms to the total number of copper atoms in the catalyst. Fig. 1 shows that copper dispersion increased as the temperature increased from 600 to 900’ C during oxidative calcination. However, as the temperature further increased to 105O”C, the copper area decreased. This is due to the transformation of y-alumina in to a-alumina which caused the closure of micropores [ 10,111. As a result, copper oxides were wrapped inside the alumina phase and the accessible copper species were reduced. During reductive calcination, copper dispersion decreased as the temperature increased from 450 to 750’ C. This was presumably caused by the sintering of copper atoms [ 21. Nevertheless, the sintering effect decreased at approxi-
402
6ml
SD3 Calclnotlon
loo0
IDI
Temperature
Fig. 1. The effect of calcination conditions on copper dispersion. Dispersion is the percentage exposed, temperature in “C!. (w ) Oxidizing atmosphere; (0 ) reducing atmosphere.
278
mately 750” C and copper dispersion was kept constant or might have increased, instead, as the temperature further increased to 900°C. This was possibly caused by the induction of SMSI which led to a redispersion of the copper atoms [ll]. Carbon monoxide oxidation activity Carbon monoxide oxidation rates per unit catalyst weight are presented in Fig. 2a. During oxidative calcination, the carbon monoxide oxidation rate increased as the calcination temperature increased from 450 to 900°C and then decreased as the calcination temperature further increased. This indicates that calcination temperatures in the region of 1000°C or above may be detrimental to the catalyst. During reductive calcination, the activities are at a minimum when the cal-
(b)
Fig. 2. The effect of calcination conditions on CO oxidation activity. min.g catalysts; (b) specific rate, in 10m5 g mol/min*m* Cu area.
(a) Rate, in lop5 g mol/
279
cination temperature is approximately 600” C. As the calcination temperature increased from 600 to 9OO”C, the activity increased. This indicates that high temperature treatment in a reducing atmosphere may have a beneficial effect which is consistent with the findings of Huang et al. [ 2 J. It should, however, be noted that our highest calcination temperature was 100” C higher than that of Huang et al. [ 21. Although the effectiveness of a commercial catalyst is dependent on its activity per unit catalyst weight, the activity in terms of specific surface area usually gives a better understanding of its surface properties. The latter are presented in Fig. 2b. It can be seen that the specific rate, i.e., the rate per unit of copper surface area, of catalyst B was always higher than that of catalyst A. This is consistent with the findings of Huang et al. [2] and thus may be ascribed to the SMSI effect, which has also been observed above. Additionally, the enhancement of the specific rate, defined as the extent of the specific rate of catalyst B divided by that of catalyst A, always increased, indicating that
(b)
4
2 lopper
Dispersion
(Z
Exposed)
Fig. 3. The effect of copper dispersion on specific rate. (a) CO oxidation, Fig. 2 for the descriptions.
(b) NO reduction.
See
280
the SMSI effect may also increase, as the calcination temperature increased from 450 to 900°C. However, the enhancement leveled off at calcination temperatures in the region of 900°C. It could further be seen that the rate of enhancement reached a maximum at a calcination temperature in the region of 700°C. This may indicate that such a temperature induced the SMSI effect with the highest rate. On the other hand, Carberry [ 121 has stated that structural sensitivity can be identified by plotting specific rate against metal dispersion. This is shown in Fig. 3a, especially in relation to catalyst B. Thus, carbon monoxide oxidation is a structure-sensitive reaction [ 121, consistent with the findings of Huang et al. [ 2 1. Nitric oxide reduction activity Selective nitric oxide reduction rates per unit catalyst weight are presented in Fig. 4a. It can be seen that catalyst A has a higher activity than catalyst B up to a calcination temperature in the region of 750°C. This is the same temperature range as that usually encountered by industrial processes for selective nitric oxide reduction, Thus, an oxidizing atmosphere is not only preferred for the reaction (as it is well-known that the presence of some oxygen enhances the reduction of nitric oxide with ammonia) but also for the catalyst pretreatment. In contrast, as is observed above, a reducing atmosphere is preferred for the pretreatment of the catalyst during carbon monoxide oxidation. The specific rates for nitric oxide reduction are presented in Fig. 4b. Activity always increased with increasing calcination temperature and the rate of increase was greatest at a calcination temperature in the region of 700’ C. This finding is similar to that for carbon monoxide oxidation. In addition, the activity also leveled off at calcination temperatures in the region of 900’ C. This result confirms the fact that a calcination temperature of approximately 700 oC may induce the SMSI effect with the highest rate. At calcination temperatures higher than 700’ C, an enhancement of the specific rate occurred. However, at lower calcination temperatures, where the SMSI should be weaker or even not induced, the oxidizing atmosphere led to higher activity than that of the reducing atmosphere. X-ray diffraction results showed that the surface copper species of catalyst A were at higher oxidation states than those of catalyst B. It can therefore be concluded that copper species at higher oxidation states may lead to higher activity. Nevertheless, an oxidation state which is too high may reduce the activity, as is shown by the decrease of the specific rate with increasing temperature of the oxidative calcination. An optimum oxidation state may thus exist. On the other hand, as is shown in Fig. 3b, nitric oxide reduction is also a structure-sensitive reaction. The structural sensitivity of nitric oxide reduction was very similar to that of carbon monoxide oxidation over catalysts treated with reducing atmosphere.
281
800
600 Colcinotion
IMO
Go0
Temperature
Fig. 4. The effect of calcination conditions on NO reduction activity. (a) Rate, (b) specific rate. See Fig. 2 for the descriptions. CONCLUSIONS
A large variation in the surface properties of a commercial copper oxide/yalumina catalyst was induced by calcinating at a series of temperatures between 450 and 1050 oC as well as in either oxidizing or reducing atmospheres. The effects of these calcination conditions were clarified by the measurement of accessible surface copper atoms and by activity tests for carbon monoxide oxidation and nitric oxide reduction with ammonia. Calcination at temperatures up to 900’ C in a reducing atmosphere may induce an SMSI effect which leads to enhanced activities not only for carbon monoxide oxidation but also for nitric oxide reduction. This SMSI effect was stronger when the calcination temperature was higher. High temperature calcination in a reducing atmosphere was beneficial to carbon monoxide oxidation while an oxidizing atmosphere was usually pre-
282
ferred for nitric oxide reduction. Copper species at higher oxidation states may produce a higher activity of nitric oxide reduction but an optimum state may exist. The structural sensitivity of carbon monoxide oxidation was confirmed. Nitric oxide reduction was also a structure-sensitive reaction over copper catalysts.
REFERENCES 1 2 3
4
5 6 7 8 9 10 11 12
J.T. Kummer, Prog. Energy Combust. Sci., 6 (1980) 177. T.-J. Huang, T.-C. Yu and S.-H. Chang, Appl. Catal., 52 (1989) 157. H.L. Faucett, J.D. Maxwell and T.A. Burnett, Technical Assessment of NO, Removal Process for Utility Application, Industrial Environmental Research Laboratory, NC, U.S.A., 1977. H.C. Yao, H.S. Gandhi and M. Shelef, in B. Imelik, C. Naccache, G. Condurier, H. Praliaud, P. Meriandeau, P. Gallezot, G.A. Martin and J.C. Verdrine (Editors), Metal-Support and Metal-Additive Effects in Catalysis, Elsevier, Amsterdam, 1982, p. 159. K. Foger, Dispersed Metal Catalysts (Catalysis - Science and Technology, Vol. 6), SpringerVerlag, Berlin, 1984, p. 291. W.R.A.M. Robinson and J.C. Mol, Appl. Catal., 44 ( 1988) 165. T.-J. Huang, W.-G. Jang and S.-H. Chang, J. Chin. Inst. Chem. Eng., 18 (1987) 77. A. Kadkhodayan and A. Brenner, J. Catal., 117 (1989) 311. .J.W. Evans, M.S. Wainwright, A.J. Bridgewater and D.J. Young, Appl. Catal., 7 (1983) 75. C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, New York, 1980, p. 136. T.-J. Huang, J. Chin. Inst. Chem. Eng., 21 (1990) 305. J.J. Carberry, Chemical and Catalytic Reaction Engineering, McGraw-Hill, New York, 1976, p. 423.
275
Calcination conditions on copper/alumina catalysts for carbon monoxide oxidation and nitric oxide reduction Ta-Jen Huang* and Tai-Chiang Yu Department
of Chemical Engineering,
(Taiwan ROC), tel. (+886-35) (Received 6 September
National
Tsing Hua University,
713691, fax. (+886-35)
1989, revised manuscript
Hsinchu 30043
715408
received 5 December 1990)
Abstract A commercial copper oxide/alumina catalyst with 6.5 wt.-% copper metal was used to study the effect of calcination conditions on carbon monoxide oxidation and nitric oxide reduction with ammonia. Calcination in a reducing atmosphere, at temperatures up to 9OO”C, may induce a strong metal-support interaction which leads to enhanced activities not only for carbon monoxide oxidation but also for nitric oxide reduction. This strong metal-support interaction effect was stronger for higher calcination temperatures. It was found that high temperature treatment in a reducing atmosphere was beneficial to carbon monoxide oxidation while an oxidizing atmosphere was usually preferred for nitric oxide reduction. In addition, copper species at higher oxidation states may lead to a higher activity of nitric oxide reduction, but an optimum state may exist. It can be concluded that nitric oxide reduction is a structuresensitive reaction over copper catalysts. Keywords: copper oxide/alumina, carbon monoxide oxidation, nitric oxide reduction, calcination, strong metal-support interaction, structural sensitivity.
INTRODUCTION
Base-metal oxide catalysts have received considerable attention because of their use in automobile emission control systems. During carbon monoxide oxidation, base-metal oxide catalysts, especially copper oxide, exhibit activities per unit surface area similar to those of noble-metal catalysts such as platinum [ 11. However, sintering is usually considered to be a problem during the use of copper catalysts in automobile emission control. Huang et al. [2] reported that calcination at 800’ C in a reducing atmosphere produces a sintered copper surface but also a largely enhanced activity of carbon monoxide oxidation. As temperatures higher than 800’ C may be encountered in automobile emission, further investigations were needed and these were carried out in this study. In contrast, alumina-supported copper oxide catalysts have been used in many commercial processes in order to carry out a selective catalytic reduction
0166-9834/91/$03.50
0 1991 Elsevier
Science
Publishers
B.V.
276
to remove nitric oxide from flue gases in a reaction with ammonia [ 31. When removing carbon monoxide from exhaust gases, Huang et al. [2] found that carbon monoxide oxidation is a structure-sensitive reaction over copper catalysts. In this study the authors have attempted to clarify the structural sensitivity of selective nitric oxide reduction which has not yet been explained, to the knowledge of the authors. The effect of strong metal-support interaction (SMSI) has been studied extensively for supported metal catalysts [ 2,4-61. Foger [ 51 reported that metals in the SMSI state exhibit the general feature of having an enhanced activity for reactions involving carbon monoxide, such as Fischer-Tropsch synthesis, methanation, and carbon monoxide-nitric oxide reactions. Huang et al. [2] have confirmed this for carbon monoxide oxidation over copper catalysts. It should be interesting to see whether a similar feature is at work for selective nitric oxide reduction. In this paper we have proved this to be the case, for the first time as far as we are aware. In our experiments, calcination was carried out at a series of temperatures (450-1050°C) and in different gaseous atmospheres (oxidizing or reducing) in order to induce a large variation in the surface properties of a commercial copper oxide/y-alumina catalyst. The effect of these calcination conditions was studied by the reactive chemisorption of nitrous oxide and by activity tests for carbon monoxide oxidation and nitric oxide reduction. EXPERIMENTAL
A copper oxide/y-alumina catalyst in the form of 1/8x l/8 in. cylindrical pellets was obtained from Strem Chemicals, Newburyport, MA, U.S.A. It contained 6.5 wt.-% copper metal, which is an optimum copper content for carbon monoxide oxidation as was reported by Huang et al. [ 71. The catalyst pellets were calcined under the following conditions: (A) Oxidizing atmosphere: Calcination took place in a flow of air at 450900°C overnight and in stagnant air at 1050°C overnight. These are denoted as catalyst A. (B) Reducing atmosphere: Calcination took place in a flow of 10% hydrogen in nitrogen at 450-900°C overnight. Then, the temperature was decreased to 60°C before an air flow was introduced. The last step was taken in order to reduce or avoid any possible destruction of the metal-support interaction by oxidation [8]. These are denoted as catalyst B. After calcination, the pellets were crushed and screened to a size under 200 mesh before carrying out subsequent dispersion measurements and activity tests. The number of accessible surface copper atoms was determined by applying a nitrous oxide pulse method as described by Evans et al. [ 91. Some comments on this method are described elsewhere [ 21.
Both carbon monoxide oxidation and nitric oxide reduction activities were measured in a conventional microreactor made from a quartz tube. For carbon monoxide oxidation, the reactant gas was a mixture of 2% carbon monoxide and 1.9% oxygen in nitrogen; for nitric oxide reduction, it was 4500 ppm nitric oxide and 4880 ppm ammonia in nitrogen. Forty ml/min of the reactant gas was passed over 0.1 g of the catalyst. The reaction temperature was 210’ C. All activity results were obtained under steady-state conditions. Preliminary tests showed that both external and internal diffusional resistances were negligible under our experimental conditions. RESULTS
AND DISCUSSION
Copper dispersion Copper dispersion is defined as being the ratio of the number of surface copper atoms to the total number of copper atoms in the catalyst. Fig. 1 shows that copper dispersion increased as the temperature increased from 600 to 900’ C during oxidative calcination. However, as the temperature further increased to 105O”C, the copper area decreased. This is due to the transformation of y-alumina in to a-alumina which caused the closure of micropores [ 10,111. As a result, copper oxides were wrapped inside the alumina phase and the accessible copper species were reduced. During reductive calcination, copper dispersion decreased as the temperature increased from 450 to 750’ C. This was presumably caused by the sintering of copper atoms [ 21. Nevertheless, the sintering effect decreased at approxi-
402
6ml
SD3 Calclnotlon
loo0
IDI
Temperature
Fig. 1. The effect of calcination conditions on copper dispersion. Dispersion is the percentage exposed, temperature in “C!. (w ) Oxidizing atmosphere; (0 ) reducing atmosphere.
278
mately 750” C and copper dispersion was kept constant or might have increased, instead, as the temperature further increased to 900°C. This was possibly caused by the induction of SMSI which led to a redispersion of the copper atoms [ll]. Carbon monoxide oxidation activity Carbon monoxide oxidation rates per unit catalyst weight are presented in Fig. 2a. During oxidative calcination, the carbon monoxide oxidation rate increased as the calcination temperature increased from 450 to 900°C and then decreased as the calcination temperature further increased. This indicates that calcination temperatures in the region of 1000°C or above may be detrimental to the catalyst. During reductive calcination, the activities are at a minimum when the cal-
(b)
Fig. 2. The effect of calcination conditions on CO oxidation activity. min.g catalysts; (b) specific rate, in 10m5 g mol/min*m* Cu area.
(a) Rate, in lop5 g mol/
279
cination temperature is approximately 600” C. As the calcination temperature increased from 600 to 9OO”C, the activity increased. This indicates that high temperature treatment in a reducing atmosphere may have a beneficial effect which is consistent with the findings of Huang et al. [ 2 J. It should, however, be noted that our highest calcination temperature was 100” C higher than that of Huang et al. [ 21. Although the effectiveness of a commercial catalyst is dependent on its activity per unit catalyst weight, the activity in terms of specific surface area usually gives a better understanding of its surface properties. The latter are presented in Fig. 2b. It can be seen that the specific rate, i.e., the rate per unit of copper surface area, of catalyst B was always higher than that of catalyst A. This is consistent with the findings of Huang et al. [2] and thus may be ascribed to the SMSI effect, which has also been observed above. Additionally, the enhancement of the specific rate, defined as the extent of the specific rate of catalyst B divided by that of catalyst A, always increased, indicating that
(b)
4
2 lopper
Dispersion
(Z
Exposed)
Fig. 3. The effect of copper dispersion on specific rate. (a) CO oxidation, Fig. 2 for the descriptions.
(b) NO reduction.
See
280
the SMSI effect may also increase, as the calcination temperature increased from 450 to 900°C. However, the enhancement leveled off at calcination temperatures in the region of 900°C. It could further be seen that the rate of enhancement reached a maximum at a calcination temperature in the region of 700°C. This may indicate that such a temperature induced the SMSI effect with the highest rate. On the other hand, Carberry [ 121 has stated that structural sensitivity can be identified by plotting specific rate against metal dispersion. This is shown in Fig. 3a, especially in relation to catalyst B. Thus, carbon monoxide oxidation is a structure-sensitive reaction [ 121, consistent with the findings of Huang et al. [ 2 1. Nitric oxide reduction activity Selective nitric oxide reduction rates per unit catalyst weight are presented in Fig. 4a. It can be seen that catalyst A has a higher activity than catalyst B up to a calcination temperature in the region of 750°C. This is the same temperature range as that usually encountered by industrial processes for selective nitric oxide reduction, Thus, an oxidizing atmosphere is not only preferred for the reaction (as it is well-known that the presence of some oxygen enhances the reduction of nitric oxide with ammonia) but also for the catalyst pretreatment. In contrast, as is observed above, a reducing atmosphere is preferred for the pretreatment of the catalyst during carbon monoxide oxidation. The specific rates for nitric oxide reduction are presented in Fig. 4b. Activity always increased with increasing calcination temperature and the rate of increase was greatest at a calcination temperature in the region of 700’ C. This finding is similar to that for carbon monoxide oxidation. In addition, the activity also leveled off at calcination temperatures in the region of 900’ C. This result confirms the fact that a calcination temperature of approximately 700 oC may induce the SMSI effect with the highest rate. At calcination temperatures higher than 700’ C, an enhancement of the specific rate occurred. However, at lower calcination temperatures, where the SMSI should be weaker or even not induced, the oxidizing atmosphere led to higher activity than that of the reducing atmosphere. X-ray diffraction results showed that the surface copper species of catalyst A were at higher oxidation states than those of catalyst B. It can therefore be concluded that copper species at higher oxidation states may lead to higher activity. Nevertheless, an oxidation state which is too high may reduce the activity, as is shown by the decrease of the specific rate with increasing temperature of the oxidative calcination. An optimum oxidation state may thus exist. On the other hand, as is shown in Fig. 3b, nitric oxide reduction is also a structure-sensitive reaction. The structural sensitivity of nitric oxide reduction was very similar to that of carbon monoxide oxidation over catalysts treated with reducing atmosphere.
281
800
600 Colcinotion
IMO
Go0
Temperature
Fig. 4. The effect of calcination conditions on NO reduction activity. (a) Rate, (b) specific rate. See Fig. 2 for the descriptions. CONCLUSIONS
A large variation in the surface properties of a commercial copper oxide/yalumina catalyst was induced by calcinating at a series of temperatures between 450 and 1050 oC as well as in either oxidizing or reducing atmospheres. The effects of these calcination conditions were clarified by the measurement of accessible surface copper atoms and by activity tests for carbon monoxide oxidation and nitric oxide reduction with ammonia. Calcination at temperatures up to 900’ C in a reducing atmosphere may induce an SMSI effect which leads to enhanced activities not only for carbon monoxide oxidation but also for nitric oxide reduction. This SMSI effect was stronger when the calcination temperature was higher. High temperature calcination in a reducing atmosphere was beneficial to carbon monoxide oxidation while an oxidizing atmosphere was usually pre-
282
ferred for nitric oxide reduction. Copper species at higher oxidation states may produce a higher activity of nitric oxide reduction but an optimum state may exist. The structural sensitivity of carbon monoxide oxidation was confirmed. Nitric oxide reduction was also a structure-sensitive reaction over copper catalysts.
REFERENCES 1 2 3
4
5 6 7 8 9 10 11 12
J.T. Kummer, Prog. Energy Combust. Sci., 6 (1980) 177. T.-J. Huang, T.-C. Yu and S.-H. Chang, Appl. Catal., 52 (1989) 157. H.L. Faucett, J.D. Maxwell and T.A. Burnett, Technical Assessment of NO, Removal Process for Utility Application, Industrial Environmental Research Laboratory, NC, U.S.A., 1977. H.C. Yao, H.S. Gandhi and M. Shelef, in B. Imelik, C. Naccache, G. Condurier, H. Praliaud, P. Meriandeau, P. Gallezot, G.A. Martin and J.C. Verdrine (Editors), Metal-Support and Metal-Additive Effects in Catalysis, Elsevier, Amsterdam, 1982, p. 159. K. Foger, Dispersed Metal Catalysts (Catalysis - Science and Technology, Vol. 6), SpringerVerlag, Berlin, 1984, p. 291. W.R.A.M. Robinson and J.C. Mol, Appl. Catal., 44 ( 1988) 165. T.-J. Huang, W.-G. Jang and S.-H. Chang, J. Chin. Inst. Chem. Eng., 18 (1987) 77. A. Kadkhodayan and A. Brenner, J. Catal., 117 (1989) 311. .J.W. Evans, M.S. Wainwright, A.J. Bridgewater and D.J. Young, Appl. Catal., 7 (1983) 75. C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, New York, 1980, p. 136. T.-J. Huang, J. Chin. Inst. Chem. Eng., 21 (1990) 305. J.J. Carberry, Chemical and Catalytic Reaction Engineering, McGraw-Hill, New York, 1976, p. 423.