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Selective catalytic reduction of NOx with C3H6 under lean-burn conditions on activated carbon-supported metals
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
1427
Selective Catalytic Reduction of NOx with C3H6 under lean-burn conditions on activated carbon-supported metals J. M. Garcia-Cort6s, M. J. Ill6.n-G6mez, A. Linares-Solano and C. Salinas-Martinez de Lecea Dpto. Quimica Inorganica. Facultad de Ciencias. Universidad de Alicante. Ap. 99, E-03080. Alicante. Spain
The Selective Catalytic Reduction of NOx with C3H6 under excess of oxygen has been investigated using different transition metals supported on a carbonaceous material. For this purpose, 1 wt % Pt, Pd, Fe, Co, Ni and Cu supported on an activated carbon, ROXN, have been tested by Temperature Programme Reaction (TPR). A ~as mixture containing 1000 ppm of NO, 1500 ppm of C3H6 and 5 vol. % of O2 (SV = 3600 h- ) has been used to carry out the experiments. In terms of NOx conversion and Nz selectivity, a diversity of behaviours have been obtained. Pt/ROXN presents the highest NOx conversion, almost 100%, but exhibits a moderate N: selectivity. On the other hand, Fe/ROXN presents a 100% N: selectivity, but exhibits low NOx conversions. The results have been discussed attending to the characteristics of the active phase and to the behaviour of the catalysts for C3H6 combustion. Conventional alumina-supported catalysts have been used for comparative purposes: an identical activity trend has been obtained for the studied metals. Nevertheless, the activated carbon-supported metal catalysts show two advantages; i) higher NOx conversions at lower temperatures and ii) the achievement of their maximum values for N2 selectivities, which are similar to the obtained for the alumina-supported catalysts, at lower temperatures.
1. Introduction. The Selective Catalytic Reduction using hydrocarbons as reductant agents (HCSCR) (see reaction l) is, at the moment, one of the most promising solutions to deplete atmospheric NOx emissions. In the last five years, a lot of new catalytic systems have been developed for this purpose. In this way, the authors have recently demonstrated that Pt supported on activated carbons, using C3H6, are an efficient alternative due to its high NOx conversions at low temperature compared with conventional catalytic systems as Pt/Al203 [ 1]. In that study, it was established that the chemical surface properties of the activated carbon play an important role, allowing the support to participate in the reaction. The highest NOx conversion was obtained for the activated carbon with the highest content in CO:-type oxygen groups on the carbon surface. Also, for Pt/AC catalysts, an enhancement of the C3H6 combustion was observed, with a decreasing in the temperature for the maximum NOx conversion respect to the other conventional catalytic systems. The very stable performance shown by these Pt/AC systems indicate that Pt supported on activated carbon can be Cnrre.~nnndin~ alJthnr Tel" +34-965=9093 50" fax" +34=965-903454
1428 successfully used in the reaction under study. Unfortunately, these catalysts present the disadvantage of having a low N2 selectivity, being the undesired N20 a reaction product, as for most of the Pt based catalytic systems [2]. This drawback has motivated the search for alternative catalytic systems, including different metals able to directly reduce NOx to N2 without N20 formation. The literature presents some promising catalysts to solve this problem, for example, transition metals as Pd, Fe, Co, Cu, etc. as active phase [3]. However, the NOx conversions obtained for all of them diminish considerably regarding the corresponding platinum catalysts. This paper, considering the advantages of an activated carbon support, attemps to improve N2 selectivities using the above mentioned transition metals. For comparative purposes, y-Al203 support and Pt catalysts will be also used. NO + C3H 6 + 402 ~ ~ N 2 + 3CO2 + 3H20
2.
(1)
Experimental.
2.1. Catalyst preparation All catalysts have been prepared by the excess-solution impregnation method using an activated carbon, ROXN, as catalyst supports. Details of the support preparation and characteristics are found in the literature [1]. H2PtCI6 has been used as platinum precursor while metal nitrate is the precursor for the other metals, Pd, Fe, Co, Ni and Cu. To obtain a 1 wt % metal loading, 10 ml of an aqueous solution with the appropriate concentration, has been added per 1 g of activated carbon. After that, mixture was first submitted to a N2 flow, for water evaporation, and then dried in an oven at 110~ under vacuum, for a period of 12 h. the nomenclature includes the corresponding metal followed by the support, M/ROXN. In addition, for comparative purpose, four catalysts were prepared, following the same preparation process described above but using 3t-A1203 as catalyst support. The metals selected are Pt, Co, Ni and Cu; their nomenclature is similar to the previous one, M/Ai203.
2.2. Catalyst Characterization The metal content of the carbon-supported catalysts has been obtained using Atomic Absorption Spectroscopy. For this purpose, the samples were converted to ash in a muffle furnace at 900~ for a period of 12 hours and the ashes were subsequently dissolved in aqua regia and analyzed. Approximately 1 wt. % of metal has been obtained for all the samples.
2.3. Catalyticperformance measurements The NOx conversion experiments have been performed at atmospheric pressure in a quartz flow reactor (15 mm, i.d.) connecting outlet gases to a gas chromatograph (HP Model 5890 Series II) and to a Chemiluminiscence NOx analyzer (Thermo Environmental Inc, Model 42H). The chromatograph is equipped with a switched dual columns system and with two serial columns (Poropak Q 80/100, for separation of CO2, N20, 1-120and C3H6, and Molecular Sieve 13X, for 02, N2, and CO) joined by a six-way valve. The feed consists of a gas mixture
1429 containing 1000 ppm NO, 1500 ppm C3I-I6 and 5% 02 balanced with He. The amount of catalyst and the total flow rate is maintained constant, 300 mg and 60 ml/min, respectively. The space velocity for these experimental conditions corresponds to 3600 hz. To study the NOx reduction by C3H6, Temperature Programmed Reaction (TPR) experiments, with a heating rate of 3~ up to 500~ have been carried out. Before to the reaction the samples were reduced at 350"C in 1-12overnight.
3. Results and conclusions. Figure 1 shows the NOx conversions obtained from the TPR experiments for the carbon-supported samples studied. Two sets of curves, corresponding to two different behaviours, can be clearly distinguished. The catalysts containing noble metals (Pt/ROXN and Pd~OXN) present high NOx conversions at low temperatures. However, the non-noble metal (Fe/ROXN, Co/ROXN, Ni/ROXN and Cu~OXN) present moderate NOx conversions at higher temperatures. Furthermore, the curves shape is also different for the two groups of catalysts. The P t ~ O X N and Pd/ROXN samples show a maximum of NOx conversion at 200~ while the other catalysts do not present any maximum in the range of temperature studied. In fact, at 200~ they do not present any appreciable activity, being the NOx conversion significant at temperatures over 240~
20000 -
100 80 =o
, ,t,,,,l
"~' 15000 -
60 "~10000 40
O
9 Z 20
O
0
-
5000 0 r
100
200 T (~
300
400
100
-"
9
~
..... ,
200 T (~
300
400
.... i
-.- Pt/ROXN -~ Co/ROXN
-o-Pd/ROXN -~ Ni/ROXN
Figure 1. TPR curves.
---Fe/ROXN -,-Cu/ROXN
--- Pt/ROXN -,- Co/ROXN
§ Pd//ROXN -~-Ni/ROXN
-,- Fe/ROXN I -.- Cu/ROXN
Figure 2. Evolution of CO2 during TPR experiments.
This different catalytic performance can be explained considering the reaction mechanism. For the reaction under study (HC-SCR), a two-step mechanism is generally accepted. In the first step, one molecule of hydrocarbon reduces a fraction of metal surface M-O into metallic phase M (oxygen 'clean-off' step). In a consecutive second stage, the adsorption/dissociation of NO proceeds in these reduced sites. The adsorbed NO can recombine with an N-atom, which explains the formation of N20 or, at appropriate temperature, the NO dissociation is favoured and then N2 is formed. Thus, the determining
1430 step rate seems to be the oxygen 'clean-off'. This hypothesis is supported by experimental results presented in the literature in which is concluded that the metal needs to be in a reduced state to start the NOx reduction [4]. Information about the reduced state of the metal can be obtained following the CO2 evolution because it is formed as a consequence of the reductant consumption. Thus, the CO2 evolution is a useful tool to indicate the easiness to 'clean' the active phase surface of oxygen. Figure 2 shows the evolution of CO2 during TPR experiments. For the Pt/ROXN and Pd/ROXN catalysts, the propene combustion proceeds at low temperatures and, therefore, oxygen 'clean-off' is expected to take place also at low temperatures. On the other hand, for non-noble metal catalysts, the temperatures at which the metal surface is cleaned is observed at higher temperatures because propene combustion is delayed from 200~ to 300~ These results support the previous hypothesis about the determining step of the reaction: until the metal surface is not 'cleaned' of oxygen by the hydrocarbon (then CO2 comes out), the global reaction do not proceds and, consequently, appreciable NOx conversions are not obtained. The discontinuous line of Figure 2 corresponds to the total combustion of the hydrocarbon, i.e., 1500 ppm of C3I-I6, produces 4500 ppm of CO2. Figure 2 also features that the CO2 level surpass the stoichiometric line at certain temperatures. This fact indicates that the activated carbon support is being consumed by O2 combustion. Thus, the evolution of CO2 during TPR experiments is a useful method to analyze the behaviour of the carbon supports versus combustion and therefore, to select the reaction temperature to avoid any loss of carbon. It is remarkable the low support combustion presented by Pd/ROXN, even at high temperatures. For the alumina-supported metal catalysts (Figures 3 and 4), an identical behaviour respect to the activated carbon-supported metal has been obtained. In terms of NOx conversion also two behaviours can be distinguished, corresponding to a noble and non-noble metals: the catalyst containing noble metal (Pt/AI203) presents high NOx conversions at low temperatures, while the non-noble metal catalysts (C0/A1203, Cu/AI203 and Ni/A1203) present moderate NOx conversions at high temperatures. Furthermore, the same NOx conversion trend is followed for the alumina-supported metals studied than the observed for corresponding activated carbon-supported metals (Pt > Cu > Co > Ni).
100-
-,-Pt/A1203 N1/AI203
80 -
~- Co/Al203 + Cu/Al203
4-Pt/~ +IW~
15(XX)-
-,-CNAI203 -,-(MAI203
.o 60l-q ~0
>
o
40-
0 Z 20
O o - " .
100
.
.
.
.
200
Figure 3. TPR curves.
.
.
.
300
T(oc)
.
.
-
"1
w
-
I
I
I
300
400
500
I
400
500
100
2~30
Figure 4. Evolution of CO2 during TPR experiments.
1431 In terms of C3I-I6 combustion Figure 4 presents the CO2 evolution curves. The lowest temperature for the hydrocarbon consumption corresponds to the Pt/A1203 catalyst, followed by Cu/A1203 and Co/A1203, and finally, the Ni/A1203 catalyst. As for M/AC catalysts, the same sequence is followed by NOx conversion and by C3H6 combustion, indicating that the observed behaviour depends on the metal properties. Thus, for A1203 supported catalysts, the rate determining step seems to be also the oxygen 'clean-off' step. The comparison of Figure 1 and 3, reveals that the NOx conversions obtained for the activated carbon-supported catalysts are higher than those obtained for the aluminasupported catalysts. The greatest difference is observed for Pt samples, in which an increment of NOx conversion from 55 % at 240~ for Pt/A1203 catalyst, to 95 % at 200~ for Pt/ROXN sample, is achieved. These results can be explained regarding the easiness for the hydrocarbon combustion; the activated carbon catalysts exhibit the complete C3H6 combustion at approximately 50~ lower than alumina catalysts (Figures 2 and 4). This finding shows the effect of support properties in C3H6 combustion and, consequently, in NOx conversion. An additional important point to recall in the selective catalytic reduction of NOx, is the N2 selectivity, usually defined as: N2selectivity-
%N2~ This (% N2 + % N20) oulet selectivity is related with the intrinsic properties of metal used as catalyst and it seems to be independent of the support used [2]. According to the mechanism explained above, once the surface of the active phase is 'oxygen cleaned', the different capacity of metals for dissociative chemisorption of NO will determine the selectivity toward N2. Table 1 shows the maximum N2 selectivities found during TPR experiments, the temperature at which the maximum is observed and the NOx conversion at this temperature for the activated carbonsupported metal catalysts. Table 1. M / A C catalysts N2 selectivity results N2 Selectivity a T (~ NOx conversion c
Pt/ROXN 50 200 Pd/ROXN 70 225 Fe/ROXN 100 200-400 Co/ROXN 50 300 Ni/ROXN 40 300 Cu/ROXN 90 300 a MaximumN2 Selectivitypresentedin the TPR. bTemperatureat the ~ u m N2 Selectivity. ~NOx conversionat the maximumN2 Selectivity.
95 50 40 (300~ 30 20 45
According to these results, Pt shows the highest NOx conversion at low temperatures and a intermediate N2 selectivity while Pd, Fe and Cu feature a higher N2 selectivity but low NOx conversion at low temperature. It has to be pointed out that all the samples are stable in terms of support consumption, at temperatures around 200 ~ and 300~ which are practical temperatures for NOx conversion with noble and non-noble metal catalysts, respectively. An interesting behaviour is observed for the activated carbon catalysts, the temperature for the maximum of N2 selectivity coincides with the temperature at which the C3H6 is completely consumed (see Figure 2) that is, the temperature for oxygen 'cleanoff', with the exception of sample Ni/ROXN, probably because for this catalyst the
1432 hydrocarbon combustion is delayed and consequently shows low NOx conversions even at high temperature The maximum value for N2 selectivities encountered for activated carbonsupported metal catalysts are quite similar compared with the corresponding aluminasupported metal catalysts, confirming that selectivity is an intrinsic property of the metal. However, the temperature at which these N2 selectivities are reached are significantly higher (between 50 and 100~ for alumina than for activated carbon samples. This behaviour, which is also observed for temperature of NOx conversion maximum, has to be related with the temperature for metal reduction. It is well known [5] that activated carbon favours the metal reduction because is also a reductor. Therefore, the easier metal reduction in activated carbon than in alumina catalysts has to explain the great performance of M/ROXN catalysts.
4. Conclusions A series of transition metals supported on an activated carbon has been used for the selective catalytic reduction of NOx with propene under lean burn conditions with TPR technique. The results discussed in this paper reveal two behaviours in terms of NOx conversion and N2 selectivity. The metal properties seem to determine the catalyst performance for, both the NOx conversions and the N2 selectivity. The easiness of hydrocarbon to carry out the oxygen 'clean-off" step is the most important point to determine the global performance of the catalysts. For this reason, noble metal catalysts present the highest NOx conversions at low temperature, while non-noble metal catalysts present low NOx conversion even at high temperatures. Conventional catalysts (M/AI203 catalysts) have also been tested, obtaining similar trends. However, the NOx conversions are appreciably lower and they are achieved at temperatures higher than with activated carbon catalysts. Among the catalysts tested, Pt/ROXN shows the highest NOx conversion at the lowest temperature and a medium N2 selectivity, while Pd~OXN, Fe/ROXN and Cu/ROXN feature the highest N2 selectivity and a medium NOx conversion. From these results, it can be deduced that the N2 selectivity of the Pt catalyst can be improved by using other metals, as Pd, Fe and Cu, although with some decrease in the NOx conversion.
Acknowledgement The authors want to thanks CYCIT (project AMB96-0799) for the financial support.
References [1] J.M. Garcia-Cort6s, M.J. Illan-G6mez, A. Linares-Solano and C. Salinas-Martinez de Lecea, Appl. Catal. B, submitted Appl. Catal. B, (1999). [2] G.R. Bamwenda, A. Obuchi, A. Ogata, J. Oi, S. Kushiyama, K. Mizuno. Journal of Molecular Catalysis A: Chemical 126 (1997) 151-159. [3] V.I. P~.rvulescu, P. Grange and B. Delmon. Catal. Today 46 (1998) 233-316. [4] R. Burch, J.A. Sullivan and T.C. Watling. Catalisis Today, 42 (1998) 13-23. [5] M.C. Rom~in-Martinez, D. Cazorla-Amor6s, A. Linares-Solano, C. Salinas-Marinez de Lecea. Current Topics in Catalysis 1 (1997) 17.
1427
Selective Catalytic Reduction of NOx with C3H6 under lean-burn conditions on activated carbon-supported metals J. M. Garcia-Cort6s, M. J. Ill6.n-G6mez, A. Linares-Solano and C. Salinas-Martinez de Lecea Dpto. Quimica Inorganica. Facultad de Ciencias. Universidad de Alicante. Ap. 99, E-03080. Alicante. Spain
The Selective Catalytic Reduction of NOx with C3H6 under excess of oxygen has been investigated using different transition metals supported on a carbonaceous material. For this purpose, 1 wt % Pt, Pd, Fe, Co, Ni and Cu supported on an activated carbon, ROXN, have been tested by Temperature Programme Reaction (TPR). A ~as mixture containing 1000 ppm of NO, 1500 ppm of C3H6 and 5 vol. % of O2 (SV = 3600 h- ) has been used to carry out the experiments. In terms of NOx conversion and Nz selectivity, a diversity of behaviours have been obtained. Pt/ROXN presents the highest NOx conversion, almost 100%, but exhibits a moderate N: selectivity. On the other hand, Fe/ROXN presents a 100% N: selectivity, but exhibits low NOx conversions. The results have been discussed attending to the characteristics of the active phase and to the behaviour of the catalysts for C3H6 combustion. Conventional alumina-supported catalysts have been used for comparative purposes: an identical activity trend has been obtained for the studied metals. Nevertheless, the activated carbon-supported metal catalysts show two advantages; i) higher NOx conversions at lower temperatures and ii) the achievement of their maximum values for N2 selectivities, which are similar to the obtained for the alumina-supported catalysts, at lower temperatures.
1. Introduction. The Selective Catalytic Reduction using hydrocarbons as reductant agents (HCSCR) (see reaction l) is, at the moment, one of the most promising solutions to deplete atmospheric NOx emissions. In the last five years, a lot of new catalytic systems have been developed for this purpose. In this way, the authors have recently demonstrated that Pt supported on activated carbons, using C3H6, are an efficient alternative due to its high NOx conversions at low temperature compared with conventional catalytic systems as Pt/Al203 [ 1]. In that study, it was established that the chemical surface properties of the activated carbon play an important role, allowing the support to participate in the reaction. The highest NOx conversion was obtained for the activated carbon with the highest content in CO:-type oxygen groups on the carbon surface. Also, for Pt/AC catalysts, an enhancement of the C3H6 combustion was observed, with a decreasing in the temperature for the maximum NOx conversion respect to the other conventional catalytic systems. The very stable performance shown by these Pt/AC systems indicate that Pt supported on activated carbon can be Cnrre.~nnndin~ alJthnr Tel" +34-965=9093 50" fax" +34=965-903454
1428 successfully used in the reaction under study. Unfortunately, these catalysts present the disadvantage of having a low N2 selectivity, being the undesired N20 a reaction product, as for most of the Pt based catalytic systems [2]. This drawback has motivated the search for alternative catalytic systems, including different metals able to directly reduce NOx to N2 without N20 formation. The literature presents some promising catalysts to solve this problem, for example, transition metals as Pd, Fe, Co, Cu, etc. as active phase [3]. However, the NOx conversions obtained for all of them diminish considerably regarding the corresponding platinum catalysts. This paper, considering the advantages of an activated carbon support, attemps to improve N2 selectivities using the above mentioned transition metals. For comparative purposes, y-Al203 support and Pt catalysts will be also used. NO + C3H 6 + 402 ~ ~ N 2 + 3CO2 + 3H20
2.
(1)
Experimental.
2.1. Catalyst preparation All catalysts have been prepared by the excess-solution impregnation method using an activated carbon, ROXN, as catalyst supports. Details of the support preparation and characteristics are found in the literature [1]. H2PtCI6 has been used as platinum precursor while metal nitrate is the precursor for the other metals, Pd, Fe, Co, Ni and Cu. To obtain a 1 wt % metal loading, 10 ml of an aqueous solution with the appropriate concentration, has been added per 1 g of activated carbon. After that, mixture was first submitted to a N2 flow, for water evaporation, and then dried in an oven at 110~ under vacuum, for a period of 12 h. the nomenclature includes the corresponding metal followed by the support, M/ROXN. In addition, for comparative purpose, four catalysts were prepared, following the same preparation process described above but using 3t-A1203 as catalyst support. The metals selected are Pt, Co, Ni and Cu; their nomenclature is similar to the previous one, M/Ai203.
2.2. Catalyst Characterization The metal content of the carbon-supported catalysts has been obtained using Atomic Absorption Spectroscopy. For this purpose, the samples were converted to ash in a muffle furnace at 900~ for a period of 12 hours and the ashes were subsequently dissolved in aqua regia and analyzed. Approximately 1 wt. % of metal has been obtained for all the samples.
2.3. Catalyticperformance measurements The NOx conversion experiments have been performed at atmospheric pressure in a quartz flow reactor (15 mm, i.d.) connecting outlet gases to a gas chromatograph (HP Model 5890 Series II) and to a Chemiluminiscence NOx analyzer (Thermo Environmental Inc, Model 42H). The chromatograph is equipped with a switched dual columns system and with two serial columns (Poropak Q 80/100, for separation of CO2, N20, 1-120and C3H6, and Molecular Sieve 13X, for 02, N2, and CO) joined by a six-way valve. The feed consists of a gas mixture
1429 containing 1000 ppm NO, 1500 ppm C3I-I6 and 5% 02 balanced with He. The amount of catalyst and the total flow rate is maintained constant, 300 mg and 60 ml/min, respectively. The space velocity for these experimental conditions corresponds to 3600 hz. To study the NOx reduction by C3H6, Temperature Programmed Reaction (TPR) experiments, with a heating rate of 3~ up to 500~ have been carried out. Before to the reaction the samples were reduced at 350"C in 1-12overnight.
3. Results and conclusions. Figure 1 shows the NOx conversions obtained from the TPR experiments for the carbon-supported samples studied. Two sets of curves, corresponding to two different behaviours, can be clearly distinguished. The catalysts containing noble metals (Pt/ROXN and Pd~OXN) present high NOx conversions at low temperatures. However, the non-noble metal (Fe/ROXN, Co/ROXN, Ni/ROXN and Cu~OXN) present moderate NOx conversions at higher temperatures. Furthermore, the curves shape is also different for the two groups of catalysts. The P t ~ O X N and Pd/ROXN samples show a maximum of NOx conversion at 200~ while the other catalysts do not present any maximum in the range of temperature studied. In fact, at 200~ they do not present any appreciable activity, being the NOx conversion significant at temperatures over 240~
20000 -
100 80 =o
, ,t,,,,l
"~' 15000 -
60 "~10000 40
O
9 Z 20
O
0
-
5000 0 r
100
200 T (~
300
400
100
-"
9
~
..... ,
200 T (~
300
400
.... i
-.- Pt/ROXN -~ Co/ROXN
-o-Pd/ROXN -~ Ni/ROXN
Figure 1. TPR curves.
---Fe/ROXN -,-Cu/ROXN
--- Pt/ROXN -,- Co/ROXN
§ Pd//ROXN -~-Ni/ROXN
-,- Fe/ROXN I -.- Cu/ROXN
Figure 2. Evolution of CO2 during TPR experiments.
This different catalytic performance can be explained considering the reaction mechanism. For the reaction under study (HC-SCR), a two-step mechanism is generally accepted. In the first step, one molecule of hydrocarbon reduces a fraction of metal surface M-O into metallic phase M (oxygen 'clean-off' step). In a consecutive second stage, the adsorption/dissociation of NO proceeds in these reduced sites. The adsorbed NO can recombine with an N-atom, which explains the formation of N20 or, at appropriate temperature, the NO dissociation is favoured and then N2 is formed. Thus, the determining
1430 step rate seems to be the oxygen 'clean-off'. This hypothesis is supported by experimental results presented in the literature in which is concluded that the metal needs to be in a reduced state to start the NOx reduction [4]. Information about the reduced state of the metal can be obtained following the CO2 evolution because it is formed as a consequence of the reductant consumption. Thus, the CO2 evolution is a useful tool to indicate the easiness to 'clean' the active phase surface of oxygen. Figure 2 shows the evolution of CO2 during TPR experiments. For the Pt/ROXN and Pd/ROXN catalysts, the propene combustion proceeds at low temperatures and, therefore, oxygen 'clean-off' is expected to take place also at low temperatures. On the other hand, for non-noble metal catalysts, the temperatures at which the metal surface is cleaned is observed at higher temperatures because propene combustion is delayed from 200~ to 300~ These results support the previous hypothesis about the determining step of the reaction: until the metal surface is not 'cleaned' of oxygen by the hydrocarbon (then CO2 comes out), the global reaction do not proceds and, consequently, appreciable NOx conversions are not obtained. The discontinuous line of Figure 2 corresponds to the total combustion of the hydrocarbon, i.e., 1500 ppm of C3I-I6, produces 4500 ppm of CO2. Figure 2 also features that the CO2 level surpass the stoichiometric line at certain temperatures. This fact indicates that the activated carbon support is being consumed by O2 combustion. Thus, the evolution of CO2 during TPR experiments is a useful method to analyze the behaviour of the carbon supports versus combustion and therefore, to select the reaction temperature to avoid any loss of carbon. It is remarkable the low support combustion presented by Pd/ROXN, even at high temperatures. For the alumina-supported metal catalysts (Figures 3 and 4), an identical behaviour respect to the activated carbon-supported metal has been obtained. In terms of NOx conversion also two behaviours can be distinguished, corresponding to a noble and non-noble metals: the catalyst containing noble metal (Pt/AI203) presents high NOx conversions at low temperatures, while the non-noble metal catalysts (C0/A1203, Cu/AI203 and Ni/A1203) present moderate NOx conversions at high temperatures. Furthermore, the same NOx conversion trend is followed for the alumina-supported metals studied than the observed for corresponding activated carbon-supported metals (Pt > Cu > Co > Ni).
100-
-,-Pt/A1203 N1/AI203
80 -
~- Co/Al203 + Cu/Al203
4-Pt/~ +IW~
15(XX)-
-,-CNAI203 -,-(MAI203
.o 60l-q ~0
>
o
40-
0 Z 20
O o - " .
100
.
.
.
.
200
Figure 3. TPR curves.
.
.
.
300
T(oc)
.
.
-
"1
w
-
I
I
I
300
400
500
I
400
500
100
2~30
Figure 4. Evolution of CO2 during TPR experiments.
1431 In terms of C3I-I6 combustion Figure 4 presents the CO2 evolution curves. The lowest temperature for the hydrocarbon consumption corresponds to the Pt/A1203 catalyst, followed by Cu/A1203 and Co/A1203, and finally, the Ni/A1203 catalyst. As for M/AC catalysts, the same sequence is followed by NOx conversion and by C3H6 combustion, indicating that the observed behaviour depends on the metal properties. Thus, for A1203 supported catalysts, the rate determining step seems to be also the oxygen 'clean-off' step. The comparison of Figure 1 and 3, reveals that the NOx conversions obtained for the activated carbon-supported catalysts are higher than those obtained for the aluminasupported catalysts. The greatest difference is observed for Pt samples, in which an increment of NOx conversion from 55 % at 240~ for Pt/A1203 catalyst, to 95 % at 200~ for Pt/ROXN sample, is achieved. These results can be explained regarding the easiness for the hydrocarbon combustion; the activated carbon catalysts exhibit the complete C3H6 combustion at approximately 50~ lower than alumina catalysts (Figures 2 and 4). This finding shows the effect of support properties in C3H6 combustion and, consequently, in NOx conversion. An additional important point to recall in the selective catalytic reduction of NOx, is the N2 selectivity, usually defined as: N2selectivity-
%N2~ This (% N2 + % N20) oulet selectivity is related with the intrinsic properties of metal used as catalyst and it seems to be independent of the support used [2]. According to the mechanism explained above, once the surface of the active phase is 'oxygen cleaned', the different capacity of metals for dissociative chemisorption of NO will determine the selectivity toward N2. Table 1 shows the maximum N2 selectivities found during TPR experiments, the temperature at which the maximum is observed and the NOx conversion at this temperature for the activated carbonsupported metal catalysts. Table 1. M / A C catalysts N2 selectivity results N2 Selectivity a T (~ NOx conversion c
Pt/ROXN 50 200 Pd/ROXN 70 225 Fe/ROXN 100 200-400 Co/ROXN 50 300 Ni/ROXN 40 300 Cu/ROXN 90 300 a MaximumN2 Selectivitypresentedin the TPR. bTemperatureat the ~ u m N2 Selectivity. ~NOx conversionat the maximumN2 Selectivity.
95 50 40 (300~ 30 20 45
According to these results, Pt shows the highest NOx conversion at low temperatures and a intermediate N2 selectivity while Pd, Fe and Cu feature a higher N2 selectivity but low NOx conversion at low temperature. It has to be pointed out that all the samples are stable in terms of support consumption, at temperatures around 200 ~ and 300~ which are practical temperatures for NOx conversion with noble and non-noble metal catalysts, respectively. An interesting behaviour is observed for the activated carbon catalysts, the temperature for the maximum of N2 selectivity coincides with the temperature at which the C3H6 is completely consumed (see Figure 2) that is, the temperature for oxygen 'cleanoff', with the exception of sample Ni/ROXN, probably because for this catalyst the
1432 hydrocarbon combustion is delayed and consequently shows low NOx conversions even at high temperature The maximum value for N2 selectivities encountered for activated carbonsupported metal catalysts are quite similar compared with the corresponding aluminasupported metal catalysts, confirming that selectivity is an intrinsic property of the metal. However, the temperature at which these N2 selectivities are reached are significantly higher (between 50 and 100~ for alumina than for activated carbon samples. This behaviour, which is also observed for temperature of NOx conversion maximum, has to be related with the temperature for metal reduction. It is well known [5] that activated carbon favours the metal reduction because is also a reductor. Therefore, the easier metal reduction in activated carbon than in alumina catalysts has to explain the great performance of M/ROXN catalysts.
4. Conclusions A series of transition metals supported on an activated carbon has been used for the selective catalytic reduction of NOx with propene under lean burn conditions with TPR technique. The results discussed in this paper reveal two behaviours in terms of NOx conversion and N2 selectivity. The metal properties seem to determine the catalyst performance for, both the NOx conversions and the N2 selectivity. The easiness of hydrocarbon to carry out the oxygen 'clean-off" step is the most important point to determine the global performance of the catalysts. For this reason, noble metal catalysts present the highest NOx conversions at low temperature, while non-noble metal catalysts present low NOx conversion even at high temperatures. Conventional catalysts (M/AI203 catalysts) have also been tested, obtaining similar trends. However, the NOx conversions are appreciably lower and they are achieved at temperatures higher than with activated carbon catalysts. Among the catalysts tested, Pt/ROXN shows the highest NOx conversion at the lowest temperature and a medium N2 selectivity, while Pd~OXN, Fe/ROXN and Cu/ROXN feature the highest N2 selectivity and a medium NOx conversion. From these results, it can be deduced that the N2 selectivity of the Pt catalyst can be improved by using other metals, as Pd, Fe and Cu, although with some decrease in the NOx conversion.
Acknowledgement The authors want to thanks CYCIT (project AMB96-0799) for the financial support.
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