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Catalytic Reduction of NO with CO on Active Carbon-Supported Copper, Manganese, and Copper–Manganese Oxides
Journal of Colloid and Interface Science 241, 439–447 (2001) doi:10.1006/jcis.2001.7726, available online at http://www.idealibrary.com on
Catalytic Reduction of NO with CO on Active Carbon-Supported Copper, Manganese, and Copper–Manganese Oxides Neli B. Stankova, Mariana S. Khristova, and Dimitar R. Mehandjiev1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bontchev Str. bl. 11, 1113 Sofia, Bulgaria Received March 15, 2001; accepted May 25, 2001; published online August 1, 2001
EXPERIMENTAL The catalytic activity of copper, manganese, and copper– manganese oxide catalysts with respect to NO reduction with CO has been investigated in the temperature range 25–300◦ C. Copper– manganese oxide catalysts with Cu:Mn ratios of 1 : 2 and 1 : 1 where the two metals form spinel-like phases have shown the highest activity. With the three kinds of catalysts NO reduction proceeds to N2 formation over the whole temperature range. The texture changes of the support after heat treatment and the adsorption characteristics of all samples have been evaluated by means of adsorption studies. Chemical and X-ray phase analyses and EPR investigations have been performed. °C 2001 Academic Press Key Words: Cu–Mn oxides; active carbon; catalytic NO reduction with CO.
INTRODUCTION
One of the not yet solved problems associated with air pollution is the neutralization of nitrogen oxides in waste gases. In this respect it is especially important to develop new more effective catalysts working at low temperatures in the presence of oxygen. Transition metal oxides, which are known to have high activity in NO reduction with CO (1), are especially promising. Investigations on γ -Al2 O3 -supported (2) and unsupported Cu–Mn spinels (3) in NO reduction with CO show them to be active even in the presence of oxygen. In previous papers (4–6) active-carbonsupported cobalt and nickel oxides were also found to possess high activity toward NO purification. In some cases the activity was even higher than that of the alumina-supported oxides. Nickel oxide on active carbon has also proved to be active in direct NO decomposition to nitrogen in the presence of oxygen (7). For that reason, the present work is aimed at studying the catalytic activity toward NO reduction with CO shown by two other oxides of the 3d transition metals, i.e., active-carbon-supported copper and manganese oxides and their two-component oxide.
1 To whom correspondence should be addressed. Fax: (*359) 2 705 024. E-mail: [email protected].
Materials The support used was active carbon with a specific surface area of 910 m2 /g, a pore volume of 0.64 cm3 /g, and a micropore volume of 0.42 cm3 /g. The support characteristics are given in (8). A 0.8- to 1.2-mm fraction of active carbon (AC) preliminary dried at 110◦ C was subjected to a 2-h impregnation at 95◦ C in a water bath with solutions containing different concentrations of copper or manganese nitrate. After the impregnation the samples were dried at 110◦ C for 2 h. The copper oxide samples were decomposed at 200◦ C. The manganese oxide and the two-component (manganese and copper) oxide decomposition occurred at 300◦ C. The copper and manganese contents of AC-supported samples varied between 2 and 7 wt%. With two-component samples, three Cu:Mn ratios were chosen: 1 : 1, 1 : 2, and 2 : 1. Three series of sample were obtained: (i) active carbon-supported copper oxide catalysts (denoted as Cu/AC); (ii) active carbon-supported manganese oxide catalysts (denoted as Mn/AC), and (iii) active carbon-supported copper–manganese oxide catalysts (denoted as Cu–Mn/AC). Methods Chemical analysis. The Cu and Mn concentrations in the impregnating solution were determined by chemical analysis. The Cu and Mn contents in the impregnated samples were determined after extraction with HCl by atomic absorption analysis using an atomic absorption spectrometer (Pye Unicam SP90V). X-ray powder diffraction. The catalyst samples were characterized by X-ray powder diffraction (TUR, Germany, CuK α radiation). EPR. The EPR spectra of the samples were registered with an ERS 220/Q spectrometer at 100–400 K. Adsorption studies. The texture characteristics were determined by low-temperature (77.4 K) nitrogen adsorption in an adsorption apparatus. The specific surface area was calculated by the BET method. The total pore volume (Vt ) was determined at a relative pressure p/ p0 = 0.95. The mesopore volume (Vme ) was found from Vt − Wo , where Wo is the micropore volume
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as calculated according to the Dubinin–Radushkevich equation (9). The micropore volume (Vmi ) and the supermicropore volume (Vsmi ) were calculated by a modified t/F method (10). The micropore size distribution was obtained using the SE method (11, 12) and the method of Pierce (13) for mesopores. Catalytic studies. The catalytic investigations were performed in a flow apparatus (14). The catalyst (about 0.3 g) was placed in a quartz reactor heated by an electric furnace. Prior to the experiments, the catalyst was heated at 200◦ C for Cu samples or 300◦ C for Mn and CuMn samples in an Ar flow for 1 h. A gas mixture (1200 ppm NO, 1200 ppm CO and Ar as a carrier gas) was passed through the catalyst with a gas flow rate of 440 ml/min. Continuous gas analysis of the reagents and the reaction products was carried out as follows: the concentrations of NO and CO were analyzed by a UNOR 5N NDIR (Maihak, Germany) gas analyzer; the CO2 obtained was measured by a Infralyt 2106 spectrometer (Germany) and the analysis of N2 O was performed with a Specord 75 IR spectrometer (Specac, UK), using a 1-m folded path gas cell. The data were collected by a CSY-10 personal data station. The N2 concentration in the outlet gas was determined on the basis of the material balance with respect to NO consumption. The transient response method (15) was used to study the interaction of the gas phase with the catalyst surface. Temperature-programmed desorption (TPD) experiments were carried out with the same apparatus as in the case of the catalytic experiments in an argon flow at a heating rate of 20◦ C /min. RESULTS AND DISCUSSION
To elucidate the effect of the heat treatment on the texture of the active carbon, adsorption studies were carried out on samples of the initial active carbon heated in air at temperatures and for times used for the preparation of metal oxide catalysts. The results are presented in Table 1. There, AC is the initial active carbon dried for 2 h at 110◦ C. AC-200 and AC-300 are active carbon samples calcined at 200 and 300◦ C, respectively, for 2 h. The adsorption isotherms of the three samples (Fig. 1) are IV type, which is typical of adsorbents of a mixed micromesopore kind with predominating microporosity. The texture character-
TABLE 1 Texture Characteristics before and after Calcination of the Initial Active Carbon Used as a Support ABET Vt Vmi Vsmi Vme x0 δ Sample (m2 g−1 ) (cm3 g−1 ) (cm3 g−1 ) (cm3 g−1 ) (cm3 g−1 ) (nm) (nm) AC AC-200 AC-300
910 890 990
0.64 0.80 0.87
0.01 0.18 0.26
0.41 0.19 0.16
0.22 0.45 0.45
1.2 1.1 1.0
0.05 0.4 0.4
Note. x0 , the half-width of slit-shaped micropores for the distribution curve maximum.
FIG. 1. Nitrogen isotherms (77.4 K) of samples AC, AC-200, and AC-300.
istics exhibit with a certain approximation that at 200◦ C ABET does not change, whereas at 300◦ C a change is observed. The heat treatment leads to a change of the remaining parameters. Thus, the total volume (Vt ) of samples AC-200 and AC-300 shows an increase as compared to that of the initial AC, which is associated with the change in volume of the microand mesopores. This process, which can be called reactivation, is undoubtedly associated with combustion of part of the supermicropores and fine mesopores, which is confirmed by the increase in mesopore volume and the shift of the distribution to the finer micropores (Fig. 2). The data in Table 1 indicate that the initial sample contains a large amount of supermicropores and mesopores. The heat-treated samples show a trend toward transformation of the supermicropores into micropores. The supermicroporosity of the initial AC is characterized by considerable uniformity (δ = 0.05 nm), whereas the reactivation reduces the uniformity of the micropore distribution of the remaining two samples (δ = 0.4 nm) (Fig. 2). Part of the adsorption isotherms of the supported samples are given in Fig. 3, while the pore size distribution can be seen in Fig. 4. All isotherms are IV type. They show a high plateau characteristic of samples containing a large amount of microand mesopores. Table 2 presents the chemical analysis data, the texture characteristics, and the phases found by X-ray analysis in the supported copper, manganese, and mixed oxide catalysts. Obviously, the three types of samples display different texture characteristic
REDUCTION OF NO WITH CO ON SUPPORTED Cu–Mn OXIDES
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increase in the total amount of the metal ions upon the active carbon support, the total pore volume, the mesopore volume, and the specific surface area decrease. This shows that the active phase is in the mesopores of the support, and the relatively small changes in the micropore volume can be ascribed to partial blocking of the micropores. The general conclusion is that, during the deposition of the active phase, which is also accompanied by reactivation due to the thermal treatment, the deposition and formation of transition metal oxides proceed in the space of fine mesopores and supermicropores. With copper oxide catalysts this deposition is more uniform. According to data from X-ray analysis, a CuO phase is formed on the surface of copper-containing catalysts. With manganesecontaining samples the phase is γ -Mn3 O4 , and with Cu–Mn samples it is a mixture of two oxides, CuO and γ -Mn3 O4 .
FIG. 2. Pore size distribution of samples AC, AC-200, and AC-300.
changes. With Mn oxide catalysts the type and character of the isotherms remains unchanged but a decrease of adsorption proportional to the active phase content is observed. This finding reflects on the adsorption structure parameter values, respectively. The significant decrease in specific surface area (about 20%) even at the lowest concentration (2 wt% Mn) shows that during the phase deposition pores having narrowing are blocked. The pore size distribution curves indicate that the fine mesopores are filled and the distribution maximum is shifted to supermicropores. With rising active phase concentration some larger pores (6–15 nm) are also filled, which results in transformation of the distribution curve shoulder into a broad peak (2– 4 nm). The isotherms of the copper samples do not exhibit substantial adsorption differences irrespective of the active phase amount. A comparison with the isotherm of the initial active carbon shows much smaller changes found at relative pressures of 0.5–0.9. The adsorption characteristics and the isotherms show no essential texture changes with these samples. No specific surface area decrease is observed. With some samples there is even a certain increase in ABET with the copper concentration. It should be noted that the presence of metal ions could lead to catalytic reactivation of the support. The micropore distribution curves show that, during heating of active carbon impregnated with Cu, the number of typical micropores increases as a result of filling of the supermicropores with active phase. The mesopore distribution shows that the fine mesopores are also filled. With mixed oxide catalysts the changes in texture characteristics are analogous to those with manganese catalysts. With the
FIG. 3. Nitrogen isotherms (77.4 K) of manganese (a), copper (b), and mixed (c) oxides.
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TABLE 2 Data on the Metal (Cu, Mn) Content in Impregnated AC, Adsorption Data on AC, Mn/AC, Cu/AC, CuMn/AC-Supported Catalysts, and X-ray Powder Diffraction Data Csup (wt%) Sample
Cu
Mn
ABET (m2 g−1 )
Vt (cm3 g−1 )
Vmi (cm3 g−1 )
Vsmi (cm3 g−1 )
W0 (cm3 g−1 )
Phase composition
Mn/AC-1 Mn/AC-2 Mn/AC-3 Mn/AC-4 Cu/AC-1 Cu/AC-2 Cu/AC-3 Cu/AC-4 CuMn/AC-1 CuMn/AC-2 CuMn/AC-3
— — — — 2.10 3.02 5.15 7.10 2.52 4.72 7.16
2.02 3.73 5.05 7.50 — — — — 4.22 4.28 4.13
780 782 622 666 810 814 839 830 890 828 694
0.76 0.77 0.66 0.67 0.72 0.70 0.72 0.70 0.85 0.70 0.67
0.09 0.13 0.09 0.03 0.22 0.22 0.22 0.23 0.06 0.12 0.12
0.33 0.23 0.20 0.27 0.13 0.12 0.15 0.13 0.34 0.23 0.22
0.42 0.36 0.24 0.30 0.35 0.35 0.37 0.35 0.40 0.35 0.32
γ –Mn3 O4 γ –Mn3 O4 γ –Mn3 O4 γ –Mn3 O4 CuO CuO CuO CuO CuO+γ –Mn3 O4 CuO+γ –Mn3 O4 CuO+γ –Mn3 O4
The EPR data show that the copper-containing catalysts have two types of signals due to the isolated Cu2+ ions and exchangecoupled Cu2+ ions (Fig. 5). When the total amount of Cu increases from 2 to 7 wt%, both types of EPR signals decrease
FIG. 4. Pore size distribution of manganese (a), copper (b), and mixed (c) oxides.
in intensity. Since CuO is EPR “silent”, these Cu2+ ions can be related to the carbon support. After the Mn-containing samples were heated at 300◦ C, the EPR spectrum (Fig. 6) shows a single line with a Lorentzian shape whose width depends on the Mn amount: 1Hpp ≈ 45 mT and 1Hpp ≈ 69 mT for Mn/AC-1 and Mn/AC-4, respectively. With the increase in amount of Mn from 1 to 6 g/100 ml in the solutions, the EPR line intensity increases about three times. This signal is most probably due to exchangecoupled Mn4+ . In the case of Cu–Mn/AC samples the intensity of the EPR signal is strongly decreased, which can be due to the appearance of additional magnetic interactions (Fig. 7).
FIG. 5. EPR spectra of Mn/AC-1 and Mn/AC-4 samples.
REDUCTION OF NO WITH CO ON SUPPORTED Cu–Mn OXIDES
FIG. 6. EPR spectra of Cu/AC-1 and Cu/AC-4 samples.
Nevertheless, the signals corresponding to the isolated and nonisolated Cu2+ ions contribute mainly to the EPR profile. Hence, according to texture studies, differently bonded Cu2+ and Mn2+ , Mn3+ , and Mn4+ ions are formed on the active carbon surface
FIG. 7. EPR spectra of CuMn/AC-1 and CuMn/AC-2 samples.
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FIG. 8. Dependence of the degree of NO conversion on temperature for copper samples: curve 1, Cu/AC-1; curve 2, Cu/AC-2; curve 3, Cu/AC-3; and curve 4, Cu/AC-4.
and in the space between the supermicropores and the fine mesopores. This, on its part, allows formation of different adsorption sites and, on their basis, of a catalytically active complex in which there are different oxidation states of active phase ions as well as surface groups of the support (16). Figures 8–10 show the results of NO conversion depending on temperature for the three series of samples. Pure AC without an active phase on it shows no activity in the reduction of NO. Obviously, Cu-containing catalysts (Fig. 8) are the most active ones and above 160◦ C 90% NO purification is attained. With rising copper concentration in the samples their activity toward NO conversion increases. Manganese-containing catalysts (Fig. 9) have a much lower activity as compared to copper-containing ones. The Mn/AC-1 and Mn/AC-2 samples which have the lowest Mn content (2.02 and 3.73 wt%, Table 2) are most effective in NO reduction as compared to the other Mn-containing catalysts. This probably depends on the more uniform distribution of the manganese oxide on the support surface, which can be judged by the fact that the specific surface area remains large (Table 2). Figure 10 shows the results of NO conversion depending on temperature for the two-component samples. The picture observed with two-component samples is similar to that with copper-containing samples. The presence of manganese even leads to a certain decrease of the catalyst efficiency toward NO conversion. It is interesting that the most active catalysts are those with a copper: manganese ratio of 1 : 2 or 1 : 1. This once
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FIG. 9. Dependence of the degree of NO conversion on temperature for manganese samples: curve 1, Mn/AC-1; curve 2, Mn/AC-2; curve 3, Mn/AC-3; and curve 4, Mn/AC-4.
FIG. 10. Dependence of the degree of NO conversion on temperature for mixed samples: curve 1, CuMn/AC-1; curve 2, CuMn/AC-2; curve 3, CuMn/ AC-3.
more confirms that when the ratio between two metals permits the formation of a spinel-like structure, the catalytic activity in redox reactions can be increased sharply (17, 18). Simultaneously, NO reduction with CO leads to N2 ; i.e., no N2 O is found over the whole temperature range investigated. The activity of the deposited phase is demonstrated by the amount of reduced NO per 1 g of Cu or Mn. The same dependence for Cu–Mn catalysts with respect to the Cu content has also been plotted. The results are presented in Fig. 11. As was to be expected, the increase in metal concentration led to a drop in the catalyst effectiveness due to the particle size increase. Copper shows the most uniform distribution, which determines the maximum at ∼2 wt% Cu. This once more confirms that with two-component catalysts the best indices belong to those which have a Cu:Mn ratio of 1 : 2. It should be taken into account that the formation of two spinels, CuMn2 O4 and Cu1.5 Mn1.5 O4 , is possible in the Cu–Mn system. On the active carbon surface there are regions where the two metals form a spinel-like phase. The transient response method was used to analyze the process mechanism. Figures 12a, 12b, 13a, 13b, 14a, and 14b show the transient response curves of the reactants NO and CO and the reduction products N2 and CO2 obtained at temperatures of 100, 130, and 170◦ C for Cu/AC-1, Cu/AC-2, Mn/AC-1, Mn/AC-2, CuMn/AC-1, and CuMn/AC-2 in the presence of an NO + CO + Ar gas mixture. All three catalysts exhibit a NO response at 100
FIG. 11. Amount of reduced NO per 1 g of Cu or Mn depending on the Cu or Mn concentration in catalysts: curve 1, manganese samples at 200◦ C; curve 2, manganese samples at 250◦ C; curve 3, copper samples at 200◦ C; curve 4, mixed samples at 200◦ C; and curve 5, mixed samples at 250◦ C.
REDUCTION OF NO WITH CO ON SUPPORTED Cu–Mn OXIDES
445
Figures 15a, 15b, and 15c show the TPD spectra of NO and CO obtained during the increase of temperature from 25 to 200◦ C for Cu/AC-1 and Cu/AC-2 and from 25 to 300◦ C for Mn/AC-1, Mn/AC-2, Cu–Mn/AC-1, and Cu–Mn/AC-2 in the presence of Ar after a NO+CO reaction at 25◦ C. The TPD spectra of the remaining samples are analogous to the ones above. It is evident that during NO desorption all catalysts produce one peak at about 50◦ C in the case of Cu-containing samples, whereas with Mncontaining samples the peak is at 75◦ C and with Cu–Mn catalysts it is at 60◦ C. In all cases the same form of nitrogen oxide is probably adsorbed on the surface of these samples. However, with CO adsorption there is a different situation. Copper-containing samples show two forms of CO adsorption: a more weakly bound one which corresponds to the temperature region of the NO adsorption form and a more strongly bound form which is desorbed at higher temperatures (150◦ C). The same is observed with twocomponent catalysts but the second peak for the CO desorption is less distinct at 150◦ C. Manganese-containing catalysts produce no CO desorption peak. The fact that simultaneous adsorption of NO and CO is possible below 130◦ C indicates that the rate-controlling step is their surface interaction. The presence of different phases on the support surface and the existence of surface groups indicate that
FIG. 12. Transient response curves of NO, CO, CO2 , and N2 on Cu/AC-1 (a) and Cu/AC-2 (b).
and 130◦ C. This type of response is an indication that the surface reaction or the reactant adsorption is the rate-controlling step of the reaction. The absence of NO desorption curves at the stop stage (when the reaction mixture is replaced by Ar) with all samples investigated shows that the surface reaction at 130◦ C is the rate-controlling step. Above this temperature, e.g., at 170◦ C, the response curves of NO are of an instantaneous type with all samples. The presence of this type of response curves shows that the rate-controlling step is the active site regeneration. The change of the rate-controlling step is probably associated with a change in reaction mechanism. A similar change has been found with alumina-supported oxide catalysts (14). With these catalysts the mechanism changes from associative below 130◦ C to redox above this temperature. The mechanism of the reaction proceeding on these catalysts is of the same type but on the active carbon there may be free Cu2+ and Mn4+ ions and oxygencontaining groups, which also determines different intermediate steps and side reactions. This is confirmed by the fact that on these catalysts (Cu/AC, Mn/AC, and CuMn/AC) no formation of N2 O is observed, contrary to the case when it is found on the same oxide, both pure and alumina-supported. The microporous structure of the active carbon also plays a role here, as has been shown in some papers of (19–21). The above mechanism is additionally confirmed by TPD studies.
FIG. 13. Transient response curves of NO, CO, CO2 , and N2 on Mn/AC-1 (a) and Mn/AC-2 (b).
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the catalytic activity is after all determined by the formation of the so-called catalytically active complex (16). Thus, at temperatures above 130◦ C the probable mechanism of NO reduction with CO over the catalysts under consideration can be presented as follows, NO + CAC → N2 + CAC CAC [O] + CO → CO2 + CAC,
[O]
[1] [2]
where CAC is a catalytically active complex on the catalyst surface. The results obtained by the transient response method show that the rate-controlling step is the regeneration of CAC and confirm that during redox reactions such as NO reduction with CO the activity depends on the formation in CAC of pairs of transition metal ions in different oxidation states. With activecarbon-supported copper these may be Cu2+ –Cu+ or Cu+ –Cu0 (22). With manganese compounds the pair is Mn3+ –Mn4+ (23). In the case of mixed massive oxides (23), one of the most active catalysts for NO reduction even at room temperature (including during direct decomposition of NO) has Cu+ Mn4+ – Cu2+ Mn3+ pairs participating in CAC and a spinel-like phase is
FIG. 15. TPD spectra of copper, Cu/AC-1 and Cu/AC-2, manganese, Mn/AC-1 and Mn/AC-2, and mixed, CuMn/AC-1 and CuMn/AC-2, samples.
formed. Despite there being no direct evidence for the presence of Cu+ ions analogous to the results in (22), one can suppose that areas with a spinel-like structure are formed and the CAC are formed of Cu–Mn pairs of ions. This can be indirectly proved by the higher activity of copper ions in the mixed oxide catalysts and by the possibility of the presence of Cu ions, which do not participate in CuO phase. SUMMARY
FIG. 14. Transient response curves of NO, CO, CO2 , and N2 on CuMn/ AC-1 (a) and CuMn/AC-2 (b).
During the preparation of copper, manganese, and mixed catalysts, changes in their texture characteristics are observed. This is due to reactivation of the support as a result of the heat treatment and the active phase deposition. The formation of transition metal oxides occurs in the space between the fine mesopores and supermicropores. With copper oxide catalysts the deposition is more uniform than in the case of the other two types of samples. The samples investigated have high activity in NO reduction with CO at temperatures up to 200◦ C. The copper manganese oxides with a Cu:Mn ratio of 1 : 2 and 1 : 1 where the two metals form spinel-like phases are the most active ones.
REDUCTION OF NO WITH CO ON SUPPORTED Cu–Mn OXIDES
Investigation of NO reduction has shown that on all three kinds of samples over the whole temperature range the reduction proceeds to N2 formation. By means of the transient response method the ratecontrolling step of NO reduction below 130◦ C has been found to be the surface interaction of the adsorbed reactants. At higher temperatures the reduction takes place according to a redox mechanism. The present investigations show that active-carbon-supported catalysts are very promising for applications to reactions of nitrogen oxide purification. ACKNOWLEDGMENT Financial support for this work from the Foundation for Scientific Investigations at the Ministry of Science and Education, Project X-807, is gratefully acknowledged.
REFERENCES 1. Panayotov, D., and Mehandjiev, D., Appl. Catal. 34, 65 (1987). 2. Panayotov, D., React. Kinet. Catal. Lett. 58, 73 (1996). 3. Khristova, M., Panaytov, D., and Mehandjiev, D., in “Prep. 7th International Symposium on Heterogeneous Catalysis,” Part 2, p. 1025, Burgas, Bulgaria, 1991. 4. Mehandjiev, D., Bekyarova, E., and Khristova, M., J. Colloid Interface Sci. 179, 509 (1996).
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5. Mehandjiev, D., Bekyarova, E., and Khristova, M., J. Colloid Interface Sci. 192, 440 (1997). 6. Bekyarova, E., Khristova, M., and Mehandjiev, D., J. Colloid Interface Sci. 213, 400 (1999). 7. Khristova, M., and Mehandjiev, D., Carbon 36, 1379 (1998). 8. Mehandjiev, D., Stankova, N., and Dimitrova, P., J. Colloid Interface Sci. 230, 53 (2000). 9. Dubinin, M. M., and Radushkevich, L. B., Zh. Fiz. Khim. 23, 469 (1949). 10. Nickolov, R., Thesis, MTSRI, Sofia, Bulgaria, 1988. 11. Nickolov, R., and Mehandjiev, D., in “Proc. of the 9th International Symposium”, p. 193. Varna, Bulgaria, 2000. 12. Mehandjiev, D., Bekyarova, E., and Nickolov, R., Carbon 32, 372 (1994). 13. Pierce, C., J. Phys. Chem. 57, 149 (1953). 14. Panayotov, D., Khristova, M., and Mehandjiev, D., Appl. Catal. 34, 48 (1987). 15. Kobayshi, M., Chem. Eng. Sci. 37, 393 (1982). 16. Mehandjiev, D., in “Proc. of the 9th International Symposium”, p. 19. Varna, Bulgaria, 2000. 17. Spassova, I., Khristova, M., Panayotov, D., and Mehandjiev, D., J. Catal. 185, 43 (1999). 18. Spassova, I., Khristova, M., Nyagolova, N., and Mehandjiev, D., in “Studies in Surface Science and Catalysis 130” (A. Corma, F. V. Melo, S. Mendioroz, and J. L. Fierro, Eds.) p. 1313. 2000. 19. Kaneko, K., Kosugi, N., and Kuroda, H., J. Chem. Soc. Faraday Trans. 1 85, 869 (1989). 20. Kaneko, K., Fukuzaki, N., Kakei, K., Suzuki, T., and Ozeki, S., Langmuir 5, 960 (1989). 21. Bekyarova, E., Thesis, IGIC, Sofia, Bulgaria, 1995. 22. Tsoncheva, T., Nickolov, R., and Mehandjiev, D., React. Kinet. Catal. Lett. 72, 389 (2001). 23. Spassova, I., and Mehandjiev, D., React. Kinet. Catal. Lett. 58, 57 (1996).
Catalytic Reduction of NO with CO on Active Carbon-Supported Copper, Manganese, and Copper–Manganese Oxides Neli B. Stankova, Mariana S. Khristova, and Dimitar R. Mehandjiev1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bontchev Str. bl. 11, 1113 Sofia, Bulgaria Received March 15, 2001; accepted May 25, 2001; published online August 1, 2001
EXPERIMENTAL The catalytic activity of copper, manganese, and copper– manganese oxide catalysts with respect to NO reduction with CO has been investigated in the temperature range 25–300◦ C. Copper– manganese oxide catalysts with Cu:Mn ratios of 1 : 2 and 1 : 1 where the two metals form spinel-like phases have shown the highest activity. With the three kinds of catalysts NO reduction proceeds to N2 formation over the whole temperature range. The texture changes of the support after heat treatment and the adsorption characteristics of all samples have been evaluated by means of adsorption studies. Chemical and X-ray phase analyses and EPR investigations have been performed. °C 2001 Academic Press Key Words: Cu–Mn oxides; active carbon; catalytic NO reduction with CO.
INTRODUCTION
One of the not yet solved problems associated with air pollution is the neutralization of nitrogen oxides in waste gases. In this respect it is especially important to develop new more effective catalysts working at low temperatures in the presence of oxygen. Transition metal oxides, which are known to have high activity in NO reduction with CO (1), are especially promising. Investigations on γ -Al2 O3 -supported (2) and unsupported Cu–Mn spinels (3) in NO reduction with CO show them to be active even in the presence of oxygen. In previous papers (4–6) active-carbonsupported cobalt and nickel oxides were also found to possess high activity toward NO purification. In some cases the activity was even higher than that of the alumina-supported oxides. Nickel oxide on active carbon has also proved to be active in direct NO decomposition to nitrogen in the presence of oxygen (7). For that reason, the present work is aimed at studying the catalytic activity toward NO reduction with CO shown by two other oxides of the 3d transition metals, i.e., active-carbon-supported copper and manganese oxides and their two-component oxide.
1 To whom correspondence should be addressed. Fax: (*359) 2 705 024. E-mail: [email protected].
Materials The support used was active carbon with a specific surface area of 910 m2 /g, a pore volume of 0.64 cm3 /g, and a micropore volume of 0.42 cm3 /g. The support characteristics are given in (8). A 0.8- to 1.2-mm fraction of active carbon (AC) preliminary dried at 110◦ C was subjected to a 2-h impregnation at 95◦ C in a water bath with solutions containing different concentrations of copper or manganese nitrate. After the impregnation the samples were dried at 110◦ C for 2 h. The copper oxide samples were decomposed at 200◦ C. The manganese oxide and the two-component (manganese and copper) oxide decomposition occurred at 300◦ C. The copper and manganese contents of AC-supported samples varied between 2 and 7 wt%. With two-component samples, three Cu:Mn ratios were chosen: 1 : 1, 1 : 2, and 2 : 1. Three series of sample were obtained: (i) active carbon-supported copper oxide catalysts (denoted as Cu/AC); (ii) active carbon-supported manganese oxide catalysts (denoted as Mn/AC), and (iii) active carbon-supported copper–manganese oxide catalysts (denoted as Cu–Mn/AC). Methods Chemical analysis. The Cu and Mn concentrations in the impregnating solution were determined by chemical analysis. The Cu and Mn contents in the impregnated samples were determined after extraction with HCl by atomic absorption analysis using an atomic absorption spectrometer (Pye Unicam SP90V). X-ray powder diffraction. The catalyst samples were characterized by X-ray powder diffraction (TUR, Germany, CuK α radiation). EPR. The EPR spectra of the samples were registered with an ERS 220/Q spectrometer at 100–400 K. Adsorption studies. The texture characteristics were determined by low-temperature (77.4 K) nitrogen adsorption in an adsorption apparatus. The specific surface area was calculated by the BET method. The total pore volume (Vt ) was determined at a relative pressure p/ p0 = 0.95. The mesopore volume (Vme ) was found from Vt − Wo , where Wo is the micropore volume
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C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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as calculated according to the Dubinin–Radushkevich equation (9). The micropore volume (Vmi ) and the supermicropore volume (Vsmi ) were calculated by a modified t/F method (10). The micropore size distribution was obtained using the SE method (11, 12) and the method of Pierce (13) for mesopores. Catalytic studies. The catalytic investigations were performed in a flow apparatus (14). The catalyst (about 0.3 g) was placed in a quartz reactor heated by an electric furnace. Prior to the experiments, the catalyst was heated at 200◦ C for Cu samples or 300◦ C for Mn and CuMn samples in an Ar flow for 1 h. A gas mixture (1200 ppm NO, 1200 ppm CO and Ar as a carrier gas) was passed through the catalyst with a gas flow rate of 440 ml/min. Continuous gas analysis of the reagents and the reaction products was carried out as follows: the concentrations of NO and CO were analyzed by a UNOR 5N NDIR (Maihak, Germany) gas analyzer; the CO2 obtained was measured by a Infralyt 2106 spectrometer (Germany) and the analysis of N2 O was performed with a Specord 75 IR spectrometer (Specac, UK), using a 1-m folded path gas cell. The data were collected by a CSY-10 personal data station. The N2 concentration in the outlet gas was determined on the basis of the material balance with respect to NO consumption. The transient response method (15) was used to study the interaction of the gas phase with the catalyst surface. Temperature-programmed desorption (TPD) experiments were carried out with the same apparatus as in the case of the catalytic experiments in an argon flow at a heating rate of 20◦ C /min. RESULTS AND DISCUSSION
To elucidate the effect of the heat treatment on the texture of the active carbon, adsorption studies were carried out on samples of the initial active carbon heated in air at temperatures and for times used for the preparation of metal oxide catalysts. The results are presented in Table 1. There, AC is the initial active carbon dried for 2 h at 110◦ C. AC-200 and AC-300 are active carbon samples calcined at 200 and 300◦ C, respectively, for 2 h. The adsorption isotherms of the three samples (Fig. 1) are IV type, which is typical of adsorbents of a mixed micromesopore kind with predominating microporosity. The texture character-
TABLE 1 Texture Characteristics before and after Calcination of the Initial Active Carbon Used as a Support ABET Vt Vmi Vsmi Vme x0 δ Sample (m2 g−1 ) (cm3 g−1 ) (cm3 g−1 ) (cm3 g−1 ) (cm3 g−1 ) (nm) (nm) AC AC-200 AC-300
910 890 990
0.64 0.80 0.87
0.01 0.18 0.26
0.41 0.19 0.16
0.22 0.45 0.45
1.2 1.1 1.0
0.05 0.4 0.4
Note. x0 , the half-width of slit-shaped micropores for the distribution curve maximum.
FIG. 1. Nitrogen isotherms (77.4 K) of samples AC, AC-200, and AC-300.
istics exhibit with a certain approximation that at 200◦ C ABET does not change, whereas at 300◦ C a change is observed. The heat treatment leads to a change of the remaining parameters. Thus, the total volume (Vt ) of samples AC-200 and AC-300 shows an increase as compared to that of the initial AC, which is associated with the change in volume of the microand mesopores. This process, which can be called reactivation, is undoubtedly associated with combustion of part of the supermicropores and fine mesopores, which is confirmed by the increase in mesopore volume and the shift of the distribution to the finer micropores (Fig. 2). The data in Table 1 indicate that the initial sample contains a large amount of supermicropores and mesopores. The heat-treated samples show a trend toward transformation of the supermicropores into micropores. The supermicroporosity of the initial AC is characterized by considerable uniformity (δ = 0.05 nm), whereas the reactivation reduces the uniformity of the micropore distribution of the remaining two samples (δ = 0.4 nm) (Fig. 2). Part of the adsorption isotherms of the supported samples are given in Fig. 3, while the pore size distribution can be seen in Fig. 4. All isotherms are IV type. They show a high plateau characteristic of samples containing a large amount of microand mesopores. Table 2 presents the chemical analysis data, the texture characteristics, and the phases found by X-ray analysis in the supported copper, manganese, and mixed oxide catalysts. Obviously, the three types of samples display different texture characteristic
REDUCTION OF NO WITH CO ON SUPPORTED Cu–Mn OXIDES
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increase in the total amount of the metal ions upon the active carbon support, the total pore volume, the mesopore volume, and the specific surface area decrease. This shows that the active phase is in the mesopores of the support, and the relatively small changes in the micropore volume can be ascribed to partial blocking of the micropores. The general conclusion is that, during the deposition of the active phase, which is also accompanied by reactivation due to the thermal treatment, the deposition and formation of transition metal oxides proceed in the space of fine mesopores and supermicropores. With copper oxide catalysts this deposition is more uniform. According to data from X-ray analysis, a CuO phase is formed on the surface of copper-containing catalysts. With manganesecontaining samples the phase is γ -Mn3 O4 , and with Cu–Mn samples it is a mixture of two oxides, CuO and γ -Mn3 O4 .
FIG. 2. Pore size distribution of samples AC, AC-200, and AC-300.
changes. With Mn oxide catalysts the type and character of the isotherms remains unchanged but a decrease of adsorption proportional to the active phase content is observed. This finding reflects on the adsorption structure parameter values, respectively. The significant decrease in specific surface area (about 20%) even at the lowest concentration (2 wt% Mn) shows that during the phase deposition pores having narrowing are blocked. The pore size distribution curves indicate that the fine mesopores are filled and the distribution maximum is shifted to supermicropores. With rising active phase concentration some larger pores (6–15 nm) are also filled, which results in transformation of the distribution curve shoulder into a broad peak (2– 4 nm). The isotherms of the copper samples do not exhibit substantial adsorption differences irrespective of the active phase amount. A comparison with the isotherm of the initial active carbon shows much smaller changes found at relative pressures of 0.5–0.9. The adsorption characteristics and the isotherms show no essential texture changes with these samples. No specific surface area decrease is observed. With some samples there is even a certain increase in ABET with the copper concentration. It should be noted that the presence of metal ions could lead to catalytic reactivation of the support. The micropore distribution curves show that, during heating of active carbon impregnated with Cu, the number of typical micropores increases as a result of filling of the supermicropores with active phase. The mesopore distribution shows that the fine mesopores are also filled. With mixed oxide catalysts the changes in texture characteristics are analogous to those with manganese catalysts. With the
FIG. 3. Nitrogen isotherms (77.4 K) of manganese (a), copper (b), and mixed (c) oxides.
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TABLE 2 Data on the Metal (Cu, Mn) Content in Impregnated AC, Adsorption Data on AC, Mn/AC, Cu/AC, CuMn/AC-Supported Catalysts, and X-ray Powder Diffraction Data Csup (wt%) Sample
Cu
Mn
ABET (m2 g−1 )
Vt (cm3 g−1 )
Vmi (cm3 g−1 )
Vsmi (cm3 g−1 )
W0 (cm3 g−1 )
Phase composition
Mn/AC-1 Mn/AC-2 Mn/AC-3 Mn/AC-4 Cu/AC-1 Cu/AC-2 Cu/AC-3 Cu/AC-4 CuMn/AC-1 CuMn/AC-2 CuMn/AC-3
— — — — 2.10 3.02 5.15 7.10 2.52 4.72 7.16
2.02 3.73 5.05 7.50 — — — — 4.22 4.28 4.13
780 782 622 666 810 814 839 830 890 828 694
0.76 0.77 0.66 0.67 0.72 0.70 0.72 0.70 0.85 0.70 0.67
0.09 0.13 0.09 0.03 0.22 0.22 0.22 0.23 0.06 0.12 0.12
0.33 0.23 0.20 0.27 0.13 0.12 0.15 0.13 0.34 0.23 0.22
0.42 0.36 0.24 0.30 0.35 0.35 0.37 0.35 0.40 0.35 0.32
γ –Mn3 O4 γ –Mn3 O4 γ –Mn3 O4 γ –Mn3 O4 CuO CuO CuO CuO CuO+γ –Mn3 O4 CuO+γ –Mn3 O4 CuO+γ –Mn3 O4
The EPR data show that the copper-containing catalysts have two types of signals due to the isolated Cu2+ ions and exchangecoupled Cu2+ ions (Fig. 5). When the total amount of Cu increases from 2 to 7 wt%, both types of EPR signals decrease
FIG. 4. Pore size distribution of manganese (a), copper (b), and mixed (c) oxides.
in intensity. Since CuO is EPR “silent”, these Cu2+ ions can be related to the carbon support. After the Mn-containing samples were heated at 300◦ C, the EPR spectrum (Fig. 6) shows a single line with a Lorentzian shape whose width depends on the Mn amount: 1Hpp ≈ 45 mT and 1Hpp ≈ 69 mT for Mn/AC-1 and Mn/AC-4, respectively. With the increase in amount of Mn from 1 to 6 g/100 ml in the solutions, the EPR line intensity increases about three times. This signal is most probably due to exchangecoupled Mn4+ . In the case of Cu–Mn/AC samples the intensity of the EPR signal is strongly decreased, which can be due to the appearance of additional magnetic interactions (Fig. 7).
FIG. 5. EPR spectra of Mn/AC-1 and Mn/AC-4 samples.
REDUCTION OF NO WITH CO ON SUPPORTED Cu–Mn OXIDES
FIG. 6. EPR spectra of Cu/AC-1 and Cu/AC-4 samples.
Nevertheless, the signals corresponding to the isolated and nonisolated Cu2+ ions contribute mainly to the EPR profile. Hence, according to texture studies, differently bonded Cu2+ and Mn2+ , Mn3+ , and Mn4+ ions are formed on the active carbon surface
FIG. 7. EPR spectra of CuMn/AC-1 and CuMn/AC-2 samples.
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FIG. 8. Dependence of the degree of NO conversion on temperature for copper samples: curve 1, Cu/AC-1; curve 2, Cu/AC-2; curve 3, Cu/AC-3; and curve 4, Cu/AC-4.
and in the space between the supermicropores and the fine mesopores. This, on its part, allows formation of different adsorption sites and, on their basis, of a catalytically active complex in which there are different oxidation states of active phase ions as well as surface groups of the support (16). Figures 8–10 show the results of NO conversion depending on temperature for the three series of samples. Pure AC without an active phase on it shows no activity in the reduction of NO. Obviously, Cu-containing catalysts (Fig. 8) are the most active ones and above 160◦ C 90% NO purification is attained. With rising copper concentration in the samples their activity toward NO conversion increases. Manganese-containing catalysts (Fig. 9) have a much lower activity as compared to copper-containing ones. The Mn/AC-1 and Mn/AC-2 samples which have the lowest Mn content (2.02 and 3.73 wt%, Table 2) are most effective in NO reduction as compared to the other Mn-containing catalysts. This probably depends on the more uniform distribution of the manganese oxide on the support surface, which can be judged by the fact that the specific surface area remains large (Table 2). Figure 10 shows the results of NO conversion depending on temperature for the two-component samples. The picture observed with two-component samples is similar to that with copper-containing samples. The presence of manganese even leads to a certain decrease of the catalyst efficiency toward NO conversion. It is interesting that the most active catalysts are those with a copper: manganese ratio of 1 : 2 or 1 : 1. This once
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FIG. 9. Dependence of the degree of NO conversion on temperature for manganese samples: curve 1, Mn/AC-1; curve 2, Mn/AC-2; curve 3, Mn/AC-3; and curve 4, Mn/AC-4.
FIG. 10. Dependence of the degree of NO conversion on temperature for mixed samples: curve 1, CuMn/AC-1; curve 2, CuMn/AC-2; curve 3, CuMn/ AC-3.
more confirms that when the ratio between two metals permits the formation of a spinel-like structure, the catalytic activity in redox reactions can be increased sharply (17, 18). Simultaneously, NO reduction with CO leads to N2 ; i.e., no N2 O is found over the whole temperature range investigated. The activity of the deposited phase is demonstrated by the amount of reduced NO per 1 g of Cu or Mn. The same dependence for Cu–Mn catalysts with respect to the Cu content has also been plotted. The results are presented in Fig. 11. As was to be expected, the increase in metal concentration led to a drop in the catalyst effectiveness due to the particle size increase. Copper shows the most uniform distribution, which determines the maximum at ∼2 wt% Cu. This once more confirms that with two-component catalysts the best indices belong to those which have a Cu:Mn ratio of 1 : 2. It should be taken into account that the formation of two spinels, CuMn2 O4 and Cu1.5 Mn1.5 O4 , is possible in the Cu–Mn system. On the active carbon surface there are regions where the two metals form a spinel-like phase. The transient response method was used to analyze the process mechanism. Figures 12a, 12b, 13a, 13b, 14a, and 14b show the transient response curves of the reactants NO and CO and the reduction products N2 and CO2 obtained at temperatures of 100, 130, and 170◦ C for Cu/AC-1, Cu/AC-2, Mn/AC-1, Mn/AC-2, CuMn/AC-1, and CuMn/AC-2 in the presence of an NO + CO + Ar gas mixture. All three catalysts exhibit a NO response at 100
FIG. 11. Amount of reduced NO per 1 g of Cu or Mn depending on the Cu or Mn concentration in catalysts: curve 1, manganese samples at 200◦ C; curve 2, manganese samples at 250◦ C; curve 3, copper samples at 200◦ C; curve 4, mixed samples at 200◦ C; and curve 5, mixed samples at 250◦ C.
REDUCTION OF NO WITH CO ON SUPPORTED Cu–Mn OXIDES
445
Figures 15a, 15b, and 15c show the TPD spectra of NO and CO obtained during the increase of temperature from 25 to 200◦ C for Cu/AC-1 and Cu/AC-2 and from 25 to 300◦ C for Mn/AC-1, Mn/AC-2, Cu–Mn/AC-1, and Cu–Mn/AC-2 in the presence of Ar after a NO+CO reaction at 25◦ C. The TPD spectra of the remaining samples are analogous to the ones above. It is evident that during NO desorption all catalysts produce one peak at about 50◦ C in the case of Cu-containing samples, whereas with Mncontaining samples the peak is at 75◦ C and with Cu–Mn catalysts it is at 60◦ C. In all cases the same form of nitrogen oxide is probably adsorbed on the surface of these samples. However, with CO adsorption there is a different situation. Copper-containing samples show two forms of CO adsorption: a more weakly bound one which corresponds to the temperature region of the NO adsorption form and a more strongly bound form which is desorbed at higher temperatures (150◦ C). The same is observed with twocomponent catalysts but the second peak for the CO desorption is less distinct at 150◦ C. Manganese-containing catalysts produce no CO desorption peak. The fact that simultaneous adsorption of NO and CO is possible below 130◦ C indicates that the rate-controlling step is their surface interaction. The presence of different phases on the support surface and the existence of surface groups indicate that
FIG. 12. Transient response curves of NO, CO, CO2 , and N2 on Cu/AC-1 (a) and Cu/AC-2 (b).
and 130◦ C. This type of response is an indication that the surface reaction or the reactant adsorption is the rate-controlling step of the reaction. The absence of NO desorption curves at the stop stage (when the reaction mixture is replaced by Ar) with all samples investigated shows that the surface reaction at 130◦ C is the rate-controlling step. Above this temperature, e.g., at 170◦ C, the response curves of NO are of an instantaneous type with all samples. The presence of this type of response curves shows that the rate-controlling step is the active site regeneration. The change of the rate-controlling step is probably associated with a change in reaction mechanism. A similar change has been found with alumina-supported oxide catalysts (14). With these catalysts the mechanism changes from associative below 130◦ C to redox above this temperature. The mechanism of the reaction proceeding on these catalysts is of the same type but on the active carbon there may be free Cu2+ and Mn4+ ions and oxygencontaining groups, which also determines different intermediate steps and side reactions. This is confirmed by the fact that on these catalysts (Cu/AC, Mn/AC, and CuMn/AC) no formation of N2 O is observed, contrary to the case when it is found on the same oxide, both pure and alumina-supported. The microporous structure of the active carbon also plays a role here, as has been shown in some papers of (19–21). The above mechanism is additionally confirmed by TPD studies.
FIG. 13. Transient response curves of NO, CO, CO2 , and N2 on Mn/AC-1 (a) and Mn/AC-2 (b).
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the catalytic activity is after all determined by the formation of the so-called catalytically active complex (16). Thus, at temperatures above 130◦ C the probable mechanism of NO reduction with CO over the catalysts under consideration can be presented as follows, NO + CAC → N2 + CAC CAC [O] + CO → CO2 + CAC,
[O]
[1] [2]
where CAC is a catalytically active complex on the catalyst surface. The results obtained by the transient response method show that the rate-controlling step is the regeneration of CAC and confirm that during redox reactions such as NO reduction with CO the activity depends on the formation in CAC of pairs of transition metal ions in different oxidation states. With activecarbon-supported copper these may be Cu2+ –Cu+ or Cu+ –Cu0 (22). With manganese compounds the pair is Mn3+ –Mn4+ (23). In the case of mixed massive oxides (23), one of the most active catalysts for NO reduction even at room temperature (including during direct decomposition of NO) has Cu+ Mn4+ – Cu2+ Mn3+ pairs participating in CAC and a spinel-like phase is
FIG. 15. TPD spectra of copper, Cu/AC-1 and Cu/AC-2, manganese, Mn/AC-1 and Mn/AC-2, and mixed, CuMn/AC-1 and CuMn/AC-2, samples.
formed. Despite there being no direct evidence for the presence of Cu+ ions analogous to the results in (22), one can suppose that areas with a spinel-like structure are formed and the CAC are formed of Cu–Mn pairs of ions. This can be indirectly proved by the higher activity of copper ions in the mixed oxide catalysts and by the possibility of the presence of Cu ions, which do not participate in CuO phase. SUMMARY
FIG. 14. Transient response curves of NO, CO, CO2 , and N2 on CuMn/ AC-1 (a) and CuMn/AC-2 (b).
During the preparation of copper, manganese, and mixed catalysts, changes in their texture characteristics are observed. This is due to reactivation of the support as a result of the heat treatment and the active phase deposition. The formation of transition metal oxides occurs in the space between the fine mesopores and supermicropores. With copper oxide catalysts the deposition is more uniform than in the case of the other two types of samples. The samples investigated have high activity in NO reduction with CO at temperatures up to 200◦ C. The copper manganese oxides with a Cu:Mn ratio of 1 : 2 and 1 : 1 where the two metals form spinel-like phases are the most active ones.
REDUCTION OF NO WITH CO ON SUPPORTED Cu–Mn OXIDES
Investigation of NO reduction has shown that on all three kinds of samples over the whole temperature range the reduction proceeds to N2 formation. By means of the transient response method the ratecontrolling step of NO reduction below 130◦ C has been found to be the surface interaction of the adsorbed reactants. At higher temperatures the reduction takes place according to a redox mechanism. The present investigations show that active-carbon-supported catalysts are very promising for applications to reactions of nitrogen oxide purification. ACKNOWLEDGMENT Financial support for this work from the Foundation for Scientific Investigations at the Ministry of Science and Education, Project X-807, is gratefully acknowledged.
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