Energy & Fuels 1999, 13, 903-906
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Impact of Pretreatments on the Selectivity of Carbon for NOx Adsorption/Reduction Bo Xia, Jonathan Phillips,* and Chun-Ku Chen Department of Chemical Engineering, The Pennsylvania State University, 133 Fenske Laboratory, University Park, Pennsylvania 16802-4400
Ljubisa R. Radovic Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, 205 Hosler Building, University Park, Pennsylvania 16802
Isabel F. Silva New University of Lisbon, Departamento de Quimica, Portugal
J. Angel Mene´ndez Instituto Nacional del Carbon (INCAR), C.S.I.C., Apartado 73, 33080 Oviedo, Spain Received December 15, 1998. Revised Manuscript Received March 24, 1999
At present the primary difficulty with employing carbon for the purposes of NOx removal is the lack of selectivity. Carbon generally reacts as readily with oxygen as with NO. Thus, carbons in lean-burn exhausts (O2/NO > 100) generally “burn” rather than selectively removing NO. Here we report on microcalorimetric studies which show that high temperature (950 °C) hydrogentreated carbons will adsorb NO at room temperature, but will not adsorb significant quantities of oxygen. In contrast, the same activated carbon treated at high temperature in nitrogen will strongly adsorb both species. The selective nature of the hydrogen-treated material and the lessselective nature of the nitrogen-treated material is fully consistent with an earlier model of carbon surface chemistry which highlights the contrast in the character and concentration of active surface species created by each of these treatments.
Introduction Great effort continues in the search for technologies capable of removing NOx from lean-burn exhaust streams. Current technologies are not considered effective enough to meet pending environmental standards, for either electricity generating plants, which burn hydrocarbons in excess oxygen, or energy-efficient leanburn vehicle engines. The key to both current and future technologies is the ability to selectively reduce NOx species. That is, the reducing agent must have a strong preference for interaction with NOx, rather than oxygen, as the oxygen concentration in these exhaust streams, typically 40 000 ppm or more, is much higher than that of the NOx (ca. < 1000 ppm). As discussed and reviewed elsewhere,1 carbon has been tested in a variety of manners as a potential selective adsorbent2 and/or reducing agent3-12 for NOx in lean burn exhausts. Recent studies are focused on * Author to whom correspondence should be addressed. (1) Illa´n-Go´mez, M. J.; Salinas-Martı´nez de Lecea, C.; LinaresSolano, A.; Radovic, L. R. Energy Fuels 1998, 12, 1256. (2) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.; Tanizawa, T. Catal. Today 1996, 27, 63.
carbon as a reducing agent in the presence of oxygen.13,14 One message is clear: not all carbons are equivalent. The origin, natural impurities, added catalytic agents,1 and heat treatments15 of a carbon affect its selectivity, both as an adsorbent and as a reducing agent. For example, the relative rates of carbon combustion by NOx (3) Kapteijn, F.; Stegenga, S.; Dekker, N. J. J.; Bijsterbosch, J. W.; Moulijn, J. A. Catal. Today 1993, 16, 273. (4) DeGroot, W. F.; Richards, G. N. Carbon 1991, 29, 179. (5) Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1991, 78, L1. (6) Mochida, I.; Sun, Y. N.; Fujitsu, H.; Kisamori, S.; Kawano, S. Nippon Kagaku Kaishi 1991, 885. (7) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992, 6, 398. (8) Yamashita, H.; Yamada, H.; Kyotani, T.; Tomita, A.; Radovic, L. R. Energy Fuels 1993, 7, 85. (9) Illa´n-Go´mez, M. J.; Salinas-Martı´nez de Lecea, C.; Calo, J. M.; Linares-Solano, A. Energy Fuels 1993, 7, 146. (10) Kominami, H.; Sawai, K.; Hitomi, M.; Abe, I.; Kera, Y. Nippon Kagaku Kaishi 1994, 582. (11) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840. (12) Tabor, K.; Gutzwiller, L.; Rossi, M. J. J. Phys. Chem. 1994, 98, 6172. (13) Illa´n-Go´mez, M. J.; Salinas-Martı´nez de Lecea, C.; LinaresSolano, A.; Phillips, J.; Radovic, L. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 174. (14) Garcı´a-Garcı´a, A. Ph.D. Thesis, University of Alicante, Spain, 1997. (15) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475.
10.1021/ef9802680 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/28/1999
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and oxygen at elevated temperatures have been extensively studied. “Amorphous” forms of carbon are more rapidly burned by oxygen,16,17 whereas graphitic material appears to react more rapidly with any NOx species.18 The present work describes the impact of different treatments on the adsorption selectivity of one activated carbon. In particular, the focus of the study was to contrast the impact of high-temperature (950 °C) nitrogen treatment and high-temperature hydrogen treatments on NO vs oxygen adsorption. Using microcalorimetry it is shown that an activated Norit C carbon treated at a high temperature in nitrogen adsorbed large quantities of both NO and oxygen at 30 °C. In contrast, the same material treated at high temperature in hydrogen adsorbed virtually no oxygen, but a very significant quantity of NO. Calorimetric studies also reveal the existence of both “strong” and “weak” forms of NO chemisorption. This correlates with earlier studies that show the existence of irreversible and reversible NO adsorption.19,20 Experimental Section For the microcalorimetric work, the samples were prepared in flowing gases, either nitrogen or hydrogen, at 950 °C for 4 h in a quartz reactor. The samples were then cooled to 30 °C in flowing gas, gravity transferred to the microcalorimeter,21 and then evacuated for 6 h. After this treatment differential dosing with either oxygen or NO was initiated. The process described permits samples to be treated at high temperature and transferred to the calorimeter cell with no danger of air exposure. The 6 h evacuation is required to allow the system to fully thermally equilibrate following sample transfer. Norit C carbon is a relatively “clean”, high surface area material. After a high-temperature treatment in nitrogen (N950) the BET surface area is >1200 m2/g and the composition is approximately 94.8 wt % carbon, 4.1 wt % oxygen, 0.5 wt % hydrogen, and 0.6 wt % nitrogen. Sulfur (0.02 wt %) and phosphorus (0.9 wt %) levels are very low. The H950 materials also have a BET surface area >1200 m2/g. It is 98.5 wt % carbon, 0.8 wt % oxygen, 0.4 wt % hydrogen, and approximately 0.3 wt % nitrogen. More detail regarding the composition and structure of Norit C activated carbon following different treatments, including pore volume and impurity concentrations, is available elsewhere.21-24
Results and Discussion As shown in Figure 1, the sample treated in nitrogen adsorbs a great deal of both oxygen and NO. The measured amount of oxygen adsorbed and the measured heats are consistent with prior studies of this same material treated in this fashion.22-24 In particular, the amount of oxygen strongly adsorbed (approximately 400 µmol/g) and the pattern of the heats, including initial (16) Radovic, L. R.; Walker, P. L. Fuel Process. Technol. 1984, 8, 149. (17) Suuberg, E. M.; Teng, H.; Calo, M. Symp (Int’l) Combust. 1990, 23, 1199. (18) Chu, X.; Schmidt, L. D. Ind. Eng. Chem. Res. 1993, 32, 1359. (19) Teng, H.; Suuberg, E. M. J. Phys. Chem. 1993, 97, 478. (20) Teng, H.; Suuberg, E. M. Ind. Eng. Chem. Res. 1993, 32, 416. (21) Phillips, J.; Xia, B.; Mene´ndez, J. A. Thermochim. Acta 1998, 312, 87. (22) Mene´ndez, J. A.; Phillips, J.; Xia, B.; Radovic, L. R. Langmuir 1996, 12, 4404. (23) Mene´ndez, J. A.; Radovic, L. R.; Xia, B.; Phillips, J. J. Phys. Chem. 1996, 100, 17243. (24) Mene´ndez, J. A.; Xia, B.; Phillips, J.; Radovic, L. R. Langmuir 1997, 13, 3414.
Figure 1. (a) Heat of adsorption (30 °C) of NO vs uptake on an N950 sample. Also plotted is the normalized adsorption parameter or NAP, which is discussed elsewhere.29,30 The NAP is roughly inversely proportional to the inverse of rate, normalized to the rate of the first process. (b) Comparison of NO and O2 uptakes on identically prepared N950 samples: heat vs uptake. (c) Comparison of NO and O2 isotherms (30 °C) on N950 sample.
heats greater than 120 kcal mol-1, are very similar to the values measured previously. The amount of NO adsorbed by the nitrogen-treated surface is remarkable. It is seen that on these samples almost four times as much NO as oxygen adsorbs with a heat greater than 10 kcal mol-1. Indeed, almost 1500 µmol/g of NO adsorbs with a heat of adsorption greater than 10 kcal, whereas at most 400 µmol/g of oxygen adsorbs with a heat greater than this value. A likely
Selectivity of Carbon for NOx Adsorption/Reduction
explanation for this difference can be found in our earlier studies.21-24 To explain calorimetric observations, it was postulated that two main classes of active sites exist on the surface of a nitrogen-treated carbon. One class of active surface site was postulated to be the highly unsaturated carbon atoms (“dangling carbons”). Strongly adsorbed oxygen (heat of adsorption greater than 90 kcal mol-1) was associated with these sites. The second class of active site was postulated to be the (unsaturated) edge sites, with associated electrons, which are highly basic. That oxygen which adsorbed with relatively low heat, and only at elevated temperatures (ca. 150 °C), was associated with these sites. The present results can thus be explained if it is assumed that NO, in contrast to oxygen, adsorbs on both dangling carbons and the edge sites at 30 °C. This hypothesis is strengthened by consideration of the character of the NO adsorption revealed in the present work. Specifically, the observed heat pattern also suggests that there are roughly two types of NO chemisorption on this sample: “strongly” chemisorbed NO (∆Hads > 40 kcal mol-1) and “weakly” chemisorbed NO (10 kcal mol-1 < ∆Hads < 20 kcal mol-1). The first type (approximately 700 µmol/g) is associated, according to this model, with adsorption on the highly unsaturated sites, which are only present on the nitrogen-treated surface. The more weakly adsorbing NO is associated with the edge sites. As shown by Teng and Suuberg19,20 chars will adsorb NO both in a reversible manner with a ∆Hads of approximately 10 kcal mol-1, and irreversibly (∆Hads not determined). Irreversible adsorption was only found on chars cleaned of oxygen groups by a high-temperature treatment in an inert gas. The analogy to the present experimental results is clear. That is, this study also indicates that there are two types of chemisorption, one strong and one weak. The weak form of adsorption has a ∆Hads approximately equal to that estimated in the earlier studies for reversible adsorption. Moreover, as shown below, the strongly adsorbing sites are found only on samples treated in inert gas at an elevated temperature. As shown in Figure 2, the hydrogen-treated carbon adsorbs virtually no oxygen, as anticipated on the basis of earlier studies, but a significant amount of NO, most of which adsorbs “weakly”. This can be explained on the basis of the earlier model of the hydrogen-treated surface.21 It was postulated that hydrogen-treated surfaces contain no “dangling carbons”, as these are gasified (probably to form methane) by hydrogen during treatment. These surfaces do contain a high concentration of (unsaturated) edge sites which were associated with relatively weak oxygen adsorption, a process which takes place only at an elevated temperature. The same explanation for the low heat form of NO adsorption on nitrogen-treated surfaces suffices to explain low-heat NO adsorption on hydrogen-treated surfaces. That is, (unsaturated) edge sites will reversibly adsorb NO at 30 °C. Thus, one observes that at 30 °C the surface of the high-temperature hydrogen-treated carbon selectively adsorbs NO, primarily with a low heat of adsorption. This is not surprising. NO is generally considered much more reactive than oxygen as the N-O bond is far easier to activate than the O-O bond.
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Figure 2. (a) Heat of adsorption (30 °C) of NO vs uptake on an H950 sample. Also plotted is the NAP. (b) Comparison of NO and O2 uptakes on identically prepared H950 samples. (c) Comparison of NO and O2 isotherms (30 °C) on H950 sample.
The difference between “reversible” and “irreversible” adsorption was experimentally studied using the calorimeter. As shown in Figure 3, there is no “strong” adsorption (∆H > 40 kcal mol-1) associated with a second isotherm. The “weak” adsorption component of the heat profile is unchanged. The explanations for “extra” NO adsorption on the nitrogen-treated sample and selective, low-heat NO
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Figure 3. Heat of adsorption (30 °C) of NO vs uptake on an H950 sample, 1st and 2nd isotherms. Note that there are no “high heats” of adsorption on the second isotherm. This supports the argument that the strongly adsorbed NO (>40 kcal mol-1 of NO adsorbed) is associated with irreversible adsorption.
adsorption on the hydrogen-treated sample are two independent tests of our earlier model of nitrogen- and hydrogen-treated carbon surfaces. For both surfaces the NO adsorption behavior is completely consistent with expectations arising from that model. Thus, the experimental results collected in the present case add further evidence to the validity of that model. One interesting observation regarding NO adsorption on both carbons is that the adsorption process in both cases is clearly nonequilibrium. As shown in Figures 1a and 2a, on both surfaces the heat initially increases and
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then decreases. In contrast, no such observation was made regarding oxygen adsorption on samples treated in the same fashion and studied in the same calorimeter.21-24 Previous explanations for nonequilibrium adsorption behavior observed in microcalorimetric studies focus on the impact of kinetics.25-28 That is, if there is no linear free energy relationship between heats and rates of adsorption, and mass transfer is also not rate controlling, then the sequence of adsorption on a heterogeneous surface will be controlled by relative rates of adsorbate arrival at the different sites. The heat of adsorption in these cases will not decrease linearly with coverage. Kinetic control appears to be the only satisfactory explanation for the NO adsorption process in the present case. The increasing value of the normalized adsorption parameter25,29 with coverage is consistent with a decreasing rate of adsorption with coverage, and hence with kinetic control of the process. Finally, it must be noted that the selectivity of adsorption from mixtures of gaseous NO and oxygen cannot be determined using the calorimeter. Further work, using appropriate instrumentation, must be carried out to determine the relative adsorption selectivity from mixtures. EF9802680 (25) O’Neil, M.; Phillips, J. J. Phys. Chem. 1987, 91, 2867. (26) Gow, A. S.; Phillips, J. Ind. Eng. Chem. Res. 1992, 31, 193. (27) Brennan, D.; Hayward, D. O.; Trapnell, B. M. W. Proc. R. Soc. London 1960, A256, 81. (28) Brennan, D.; Hayes, F. H. Proc. R. Soc. London 1965, A258, 347. (29) Cobes, J.; Phillips, J. J. Phys. Chem. 1991, 95, 8776.
