Energy Fuels 2010, 24, 3321–3328 Published on Web 04/09/2010
: DOI:10.1021/ef901455v
NO Reduction on Carbon-Supported Chromium Catalysts† Juana M. Rosas, Jose Rodrı´ guez-Mirasol,* and Tomas Cordero Chemical Engineering Department, School of Industrial Engineering, University of M alaga, Campus de Teatinos, s/n, 29071 M alaga, Spain Received December 1, 2009. Revised Manuscript Received March 9, 2010
Carbon-supported chromium catalysts were obtained from activated carbons prepared by chemical activation of orange skin with phosphoric acid. These catalysts present a high development of the porous structure, with a relatively large contribution of micro- and mesopores. The presence of chromium produces a considerable improvement of the NO reduction compared to that obtained for the corresponding support. The removal of the carbon-oxygen complexes by thermal treatment is unfavorable for the NO reduction in both activated carbons and chromium catalysts. Oxygen surface groups improve the chromium dispersion and seem to play an important role in the NO reduction mechanism. A linear relationship between the apparent reaction rate of NO reduction and the amount of acidic surface groups was found. The presence of C3H6 and CO considerably increases the NO reduction on the chromium catalyst. This catalyst exhibited a high resistance toward SO2 poisoning at different experimental conditions. The presence of oxygen increases the oxygen surface groups of the catalyst that seem to be responsible for the enhancement of the NO reduction at low temperatures.
porous development on this reaction.8-12 Low NO conversions are found for activated carbons, in general, compared to other inorganic catalysts; despite the fact that the former presents good behavior in SO2 atmospheres. The addition of different metals increases the NO conversion considerably,13 although the best results have been obtained with noble metals.14,15 However, transition metals have been proposed as a less expensive alternative.16-18 Direct NO decomposition with transition-metal-activated carbon catalysts has been investigated by different authors, with an important increase of the NO conversion. This enhancement has been attributed to a redox mechanism, where the oxidized catalyst is reduced by the carbon. The use of some kind of reducing agent, such as CO, C3H6, or NH3, has also been widely studied;19-21 the reductant intensifies the catalyst activity because it favors the regeneration of active sites occupied by oxygen. However, in most of these
1. Introduction The world demand of activated carbons is steadily increasing because of their wide range of applications, playing an important role in those related to the control of air pollution, such as adsorbents, catalysts, and catalyst supports.1,2 They present a very high specific surface area, high chemical stability, surface oxygen-containing functional groups, and they can also be obtained from different raw carbonaceous materials,3-5 including lignocellulosic waste, which provide an economic and environmental benefit. The possibility of using activated carbon for nitric oxide removal from gaseous effluents has been broadly studied, because of the fact that carbons can be used not only as an adsorbent and catalyst support but also as a reductant and catalyst themselves.6,7 There are many works analyzing the influence of the porous structure and surface chemistry of carbons on the NO reduction, and it is generally established that the oxygen surface groups are more relevant than the
(10) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993, 7, 85–89. (11) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840–2845. (12) Xue, Y.; Guo, Y.; Zhang, Z.; Guo, Y.; Wang, Y.; Lu, G. Appl. Surf. Sci. 2008, 255, 2591–2595. (13) Illan-G omez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´ nez de Lecea, C. Energy Fuels 1996, 10, 158–168. (14) Garcı´ a-Cortes, J. M.; Perez-Ramı´ rez, J.; Illan-G omez, M. J.; Kapteijn, F.; Moulijn, J. A.; Salinas-Martı´ nez de Lecea, C. Appl. Catal., B 2001, 30, 399–408. (15) Gonc-alves, F.; Figueiredo, J. L. Appl. Catal., B 2004, 50, 271– 278. (16) Illan-G omez, M. J.; Linares-Solano, A.; Salinas-Martı´ nez de Lecea, C. Energy Fuels 1995, 9, 976–983. (17) Grzybek, T.; Rog oz, M.; Papp, H. Catal. Today 2004, 90, 61–68. (18) Boyano, A.; Galvez, M. E.; Moliner, R.; Lazaro, M. J. Fuel 2008, 87, 2058–2069. (19) Stegenga, S.; van Soest, R.; Kapteijn, F.; Moulijn, J. A. Appl. Catal., B 1993, 2, 257–275. (20) Kapteijn, F.; Stegenga, S.; Dekker, N. J. J.; Bijsterbosch, J. W.; Moulijn, J. A. Catal. Today 1993, 16, 273–287. (21) Marban, G.; Antu~ na, R.; Fuertes, A. B. Appl. Catal., B 2003, 41, 323–338.
†
This paper has been designated for the special section Carbon for Energy Storage and Environment Protection. *To whom correspondence should be addressed. Telephone: þ34951952385. Fax: þ34-951952385. E-mail:
[email protected]. (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988; Chapter 2. (2) Rodrı´ guez-Reinoso, F. Activated carbon: Structure, characterization, preparation and applications. In Introduction to Carbon Technologies; Marsh, H., Heintz, E. A., Rodríguez-Reinoso, F., Eds.; Secretariado de Publicaciones, Universidad de Alicante: Alicante, Spain, 1997. (3) Gonz alez-Serrano, E.; Cordero, T.; Rodrı´ guez-Mirasol, J.; Cotoruelo, L.; Rodrı´ guez, J. J. Water Res. 2004, 38, 3043–3050. (4) Tancredi, N.; Cordero, T.; Rodrı´ guez-Mirasol, J.; Rodrı´ guez, J. J. Fuel 1996, 15, 1701–1706. (5) Rosas, J. M.; Bedia, J.; Rodrı´ guez-Mirasol, J.; Cordero, T. Ind. Eng. Chem. Res. 2008, 43, 1288–1296. (6) Aarna, I.; Suuberg, E. Fuel 1997, 76, 475–4791. (7) Tomita, A. Fuel Process. Technol. 2001, 71, 53–70. (8) Ill an-G omez, M. J.; Linares-Solano, A.; Salinas-Martı´ nez de Lecea, C.; Calo, J. M. Energy Fuels 1995, 9, 976–983. (9) Yang, J.; Mestl, G.; Herein, D.; Schl€ ogl, R.; Find, J. Carbon 2000, 38, 729–740. r 2010 American Chemical Society
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2.2. Characterization. The porous structure of the activated carbons and catalysts was characterized by N2 adsorptiondesorption at -196 °C, performed in Omnisorp 100cx equipment (Coulter), and by CO2 adsorption at 0 °C, carried out with an Autosorb-1 apparatus (Quantachrome). Samples were previously outgassed for at least 8 h at 150 °C. From the N2 isotherm, the apparent surface area (ABETN2) was determined applying the BET equation. The Dubinin-Radushkevich (DR) method was used to obtain the values of the micropore volume (VDRN2). The narrow mesopore volume was determined as the difference between adsorbed volume at a relative pressure of 0.95 and microporous volume (VDRN2). From the CO2 adsorption data, the narrow micropore volume (VDRCO2) and the apparent surface area (ADRCO2) were calculated using the DR equation. The surface chemistry of the samples was analyzed by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). TPD profiles were obtained in a customized quartz fixed-bed reactor placed inside an electrical furnace. The samples were heated from room temperature to 900 °C at a heating rate of 10 °C/min in helium flow (200 cm3 STP/min). The amounts of CO and CO2 evolved from the samples were monitored with a Siemens ULTRAMAT 22 apparatus. XPS analyses were obtained using a 5700C model Physical Electronics apparatus with Mg KR radiation (1253.6 eV). For the analysis of the XPS peaks, the C 1s peak position was set at 284.5 eV and used as a reference to locate the other peaks. The fitting of the XPS peaks was performed by the least squares using Gaussian-Lorentzian peak shapes. The surface texture of the samples was analyzed by transmission electron microscopy (TEM), carried out by a Philips CM200 instrument. 2.3. NO Reduction Experiments. The reduction experiments were performed at atmospheric pressure and different temperatures, in a fixed-bed reactor with 4 mm of internal diameter, using 300 mg of sample (80 mg of catalyst diluted with 220 mg of SiC). The total flow rate was 200 cm3 STP/min, for different concentrations of NO ranging from 200 to 800 ppm NO, 1% CO, 2000 ppm C3H6, 2000 ppm SO2, and 3% O2. NO and NO2 concentrations were measured by a chemiluminiscent analyzer (EcoPhysics, CLD 700 AL model), and CO and CO2 concentrations were measured by means of a nondispersive infrared analyzer (Ultramat 22, Siemens model). N2O and N2 were analyzed with a mass spectrometer analyzer (Balzers MsCube).
Figure 1. Schematic diagram of the procedures and nomenclature used for sample preparation.
works, NO reduction has been studied over transition metals supported on activated carbons obtained from coal-derived carbon.16,22 In the present work, carbon-supported chromium catalysts were prepared from activated carbons obtained by chemical activation of orange skin with phosphoric acid. A large amount of orange skin waste is derived from the industrial activities of juice and jam production. In Spain, about 15% of the citrus production (5 000 000 tons of citrus/year) is used for these activities, generating more than 90% of the weight residue. Direct decomposition of NO and/or reduction with CO and C3H6 have been studied on a carbon-supported chromium catalyst and the corresponding activated carbon supports over different gas atmospheres. The influence of the surface chemistry and the porous structure of the carbons on the NO decomposition/reduction has been analyzed. 2. Experimental Section 2.1. Activated Carbon and Catalyst Preparation. Skin from the peeling of oranges harvested from Guadalhorce Valley (M alaga, Spain) was used as the raw material for the preparation of activated carbons. The skin was washed with water, air-dried at room temperature for about 1 month, and ground and sieved to a particle size between 100 and 200 μm. This precursor was impregnated with 85% (w/w) aqueous H3PO4 at room temperature and dried for 24 h at 60 °C in a vacuum dryer. The impregnation ratio (R = weight of H3PO4/weight of dry precursor) used was 3. Figure 1 shows an outline of the procedures and nomenclature used for sample preparation. The impregnated citrus skin was thermally treated under continuous N2 flow [150 cm3 standard temperature and pressure (STP)/min], in a conventional tubular furnace. The activation temperature, 500 °C, was reached at a 10 °C/min heating rate and maintained for 2 h. The activated samples were cooled inside the furnace, maintaining the N2 flow, and then washed with distilled water at 60 °C until neutral pH and negative phosphate analysis in the eluate.3 The resulting activated carbon was dried at 100 °C and weighed to determine the yield of the activation process. The nomenclature used for this activated carbon is citrus-activated carbon (CAC). CAC was thermally treated to remove the surface oxygen functional groups. The treatment was carried out at 900 °C in a conventional tubular furnace; the temperature was reached at a 10 °C/min heating rate, with a continuous flow of N2 (150 cm3 STP/min). This activated carbon was denoted by CAC-TT. The catalysts were prepared by pore-volume impregnation of the dried activated carbons (CAC and CAC-TT) with an aqueous solution of chromium nitrate, Cr(NO3)3 3 9H2O. After drying under vacuum at 100 °C for 24 h, a thermal treatment at 400 °C in a flow of nitrogen (150 cm3 STP/min) for 6 h was carried out. The catalysts obtained were denoted by CAC-Cr and CAC-TT-Cr, respectively. The chromium catalyst CAC-Cr was also submitted to a thermal treatment at 900 °C at the same previous conditions to obtain the catalyst denoted by CAC-Cr-TT.
3. Results and Discussion 3.1. Characterization of Activated Carbons and Chromium Catalysts. Figure 2 shows the N2 adsorption-desorption isotherms for the activated carbons and the carbon-supported chromium catalysts. All of the samples show a modified type-I isotherm with important uptake at low relative pressure and an appreciable hysteresis loop at relative pressure values higher than 0.4, characteristic of solids with a well-developed microporous structure and an important contribution of mesoporosity. Citrus skin-based activated carbon presents higher N2 adsorption values in the whole relative pressure range, which indicates a larger presence of micro- and mesopores in this solid. The thermal treatment of CAC at high temperatures (CAC-TT) produces a decrease in the amount of adsorbed nitrogen in the whole range of relative pressures, as a consequence of an important shrinkage of the porous structure. This reduction with the increase of the temperature is generated by the breakdown of the cross-links obtained during the activation process with phosphoric acid, which are responsible for the porous development.23,24 (23) Jagtoyen, M.; Derbyshire, F. Carbon 1998, 36, 1085–1097. (24) Molina-Sabio, M.; Rodrı´ guez-Reinoso, F. Colloids Surf., A 2004, 241, 15–25.
(22) L azaro, M. J.; Suelves, I.; Moliner, R.; Vassilev, S. V.; Braekman-Danheux, C. Fuel 2003, 82, 771–782.
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Figure 2. N2 adsorption-desorption isotherms at -196 °C of activated carbons and carbon-supported chromium catalysts.
Figure 3. CO evolution during the TPD of activated carbons and carbon-supported chromium catalysts.
Table 1. Structural Characteristics of Activated Carbons and Chromium Catalysts
CAC CAC-TT CAC-Cr CAC-TT-Cr CAC-Cr-TT
ABETN2 (m2/g)
VDRN2 (cm3/g)
VmeN2 (cm3/g)
ADRCO2 (m2/g)
VDRCO2 (cm3/g)
881 683 669 598 440
0.256 0.227 0.228 0.195 0.155
0.782 0.542 0.396 0.549 0.267
417 398 351 384 323
0.159 0.151 0.133 0.146 0.123
CAC-Cr and CAC-Cr-TT N2 adsorption isotherms present the same relation as the ones observed between those corresponding to CAC and CAC-TT. There is a reduction of the porous structure with the thermal treatment as a consequence of the breakdown of the cross-links, even when the activated carbon has been impregnated with chromium oxide. The N2 isotherms of CAC-TT-Cr show an important reduction on the nitrogen adsorption capacity because of the blockage of the porous structure by the chromium impregnation. However, this decrease is not as pronounced as that found for CAC-Cr. While CAC-Cr undergoes an important decrease of both micro- and mesopore volumes, only a significant reduction of the micropore volume is detected for CAC-TT-Cr. Kang et al.25 found that an oxidized carbon surface can have larger adsorption capacities of chromium ions and that these ions can diffuse more easily into the interior of the porous structure. This behavior may be responsible for the larger chromium dispersion and the uniform porous structure decrease for CAC-Cr. The structural parameters derived from N2 and CO2 adsorption isotherms of activated carbons and chromium catalysts are shown in Table 1. The values of specific surface area obtained from the N2 adsorption isotherm, ABETN2, are higher than those obtained from the CO2 adsorption isotherm, ADRCO2, which is indicative of a wide porous structure. The large mesopore volumes of the activated carbons and chromium catalysts provide excellent properties for catalytic applications. Incorporation of Cr on the carbon support surface reduces the meso- and micropore volumes, as can be observed in Table 1. Figures 3 and 4 show the CO and CO2 TPD profiles for the activated carbons and the carbon-supported chromium catalysts. Table 2 shows the amount of CO and CO2 evolved from TPD analyses. CO desorption takes place mainly at higher temperatures (from 700 °C) for the CAC carbon
Figure 4. CO2 evolution during the TPD of activated carbons and carbon-supported chromium catalysts. Table 2. CO and CO2 Evolved from TPD Analyses CAC CAC-TT CAC-Cr CAC-TT-Cr CAC-Cr-TT
CO evolved (mmol/g)
CO2 evolved (mmol/g)
2.79 1 5.59 3.89 1.5
0.11 0.04 1.01 0.71
support. This desorption is related to decomposition of carbonyl and quinone surface groups and the decomposition of phosphorus complexes, such as C-O-PO3 and C-PO3, which are relatively stable at high temperatures.5,26 Moreover, a little evolution of CO is observed at lower temperatures, related to phenol and anhydride surface group decomposition. Activated carbons prepared by chemical activation with H3PO4 produce lower amounts of CO2 during TPD because of a small presence of carboxyl, lactone, and anhydride groups. The samples present a CO2 desorption peak at high temperatures, probably caused by the secondary reaction of CO. CO and CO2 evolved amounts decrease considerably for CAC-TT, as a consequence of the thermal treatment. However, desorption of CO still takes place at high temperatures, because of the decomposition of very stable C-O-PO3 and, mainly, C-PO3 surface groups.5 The chromium incorporation on both supports (CAC and CAC-TT) produces different surface reactions that generate a large amount of surface oxygenated groups, as observed in
(25) Kang, M.; Lee, C. Appl. Catal., A 2004, 266, 163–172.
(26) Wu, X.; Radovic, L. R. Carbon 2006, 44, 141–151.
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3.2. NO Reduction Experiments. Figure 6 shows the steadystate NO conversions as a function of the reaction temperature for different chromium catalysts and supports, with a NO inlet concentration of 600 ppm and a space time of 0.98 g s μmol-1 (200 ppm NO and a W/FNO = 2.88 g s μmol-1 also for the best catalyst). The activated carbons CAC and CAC-TT present similar NO conversion values compared to those found in the literature for the uncatalyzed reaction in the presence of activated carbons13,30,31 and significantly lower values than those observed for chromium catalysts, despite the fact that the carbon supports present a higher development of the porous structure. The presence of chromium increases notably the NO reduction, in agreement with the literature.16 The removal of oxygen surface groups with the thermal treatment produces a considerable decrease of the NO conversion. A thermal treatment previous to the chromium impregnation significantly reduces the oxygen surface groups, producing apparently lower chromium dispersion.25 A thermal treatment after the chromium incorporation removes the surface carbon-oxygen complexes, which seem to play an important role in NO reduction.17,30-32 Therefore, the incorporation of chromium on the carbon support produces an important increase in the NO conversion caused by both the catalytic effect of chromium itself and the significant generation of oxygen surface groups during the impregnation step. To evaluate the importance of the oxygen surface groups, another chromium catalyst (with the same Cr loading) was obtained by doping an activated carbon obtained by exactly the same methodology but from Alcell lignin. This catalyst, denoted by CAL-Cr, presented a higher porous structure development, ABET ≈ 1000 m2/g, and very similar chromium content. However, the catalyst contains a lower amount of carbon-oxygen complexes that decompose at low temperature (carboxylic acid, lactone, anhydride, and phenolic groups) and a similar amount of those that decompose at temperatures higher than 700 °C (carbonyl and quinone groups). The activity of this catalyst, shown in Figure 6, is considerably lower than that of CAC-Cr. The different results obtained for these two chromium catalysts seem to point to the high relevance of some kind of oxygen surface groups, especially those of acidic character, on the NO reaction. To analyze the influence of these oxygen surface complexes on the NO reduction, the CAC-Cr catalyst was submitted to thermal treatments at different final temperatures (from 500 to 900 °C). Figure 7 shows the steady-state NO conversion as a function of the reaction temperature for the CAC-Cr catalyst after different thermal treatments. Steady-state NO conversion curves undergo a considerably shift at higher temperatures with the increase of the thermal treatment temperature. Zhu et al.33 studied the effects of acid treatments of carbon-supported copper catalysts over the NO reduction. They found that the treatment with HNO3 enhanced the catalytic activity because of the formation of carboxyl and lactone groups (which decompose as CO2 in the range of reaction temperatures) that increase the copper dispersion. L azaro et al.22 reported that the higher the CO2/CO ratio determined by TPD, the higher the NO
Table 3. Mass Surface Concentration (%) Determined by XPS Quantitative Analysis of Activated Carbons and Chromium Catalysts CAC CAC-TT CAC-Cr CAC-TT-Cr CAC-Cr-TT
C 1s (%)
O 1s (%)
84.8 85.0 77.1 75.8 83.6
10.9 10.0 15.1 15.4 9.0
Cr 2p (%)
N 1s (%)
P 2p (%)
3.2 3.2 2.4
0.6 0.7 0.9 0.9 1.0
3.7 4.3 3.7 4.6 4.3
Table 2. CAC-Cr and CAC-TT-Cr catalysts present an important evolution of CO at medium-high temperatures because of the decomposition of anhydride, phenol, and mainly carbonyl and quinone groups. XPS analyses were carried out to evaluate the surface element distribution and surface chemical structure of the samples. Table 3 shows the mass surface concentration of the different activated carbons and the carbon-supported chromium catalysts, obtained by XPS. The main components found on the external surface of the samples are carbon and oxygen because of the carbonaceous support, with lower amounts of chromium, nitrogen, and phosphorus. The presence of phosphorus arises from the phosphoric acid, used during the activation process of the citrus skin. An amount of approximately 4 wt % of phosphorus is strongly retained as stable compounds on the carbon surface, despite the washing process carried out to remove the activated agent.3,5 The high amount of oxygen observed for CAC-TT may be attributed to oxygen bonded to phosphorus as C-PO3 that remain stable on the carbon surface even after the thermal treatment at 900 °C.27 A chromium content of 3.2 wt % is obtained for the carbon catalysts (CAC-Cr and CAC-TT-Cr). The small amount of nitrogen observed for the carbon support and carbon catalysts comes from the biomass precursor. To obtain the chemical state of chromium, the band corresponding to Cr 2p was analyzed. The XPS Cr 2p region for all of the catalysts presents a doublet corresponding to Cr 2p3/2 and Cr 2p1/2. The spin-orbital splitting is close to 9.8 eV, and the Cr 2p3/2 (577.6 eV) and Cr 2p1/2 (587.4 eV) peaks are in the ratio of 1:0.5, with a full width at halfmaximum (fwhm) of 3.6 eV.28 This binding energy corresponds to the characteristic value of Cr3þ, ranging for the peak Cr 2p3/2 between 576.2 and 577.8 eV.25,28,29 These results indicate that chromium is present on the catalyst surface as Cr2O3. No evidence of a different oxidation state of chromium among the different chromium catalysts was found. Typical TEM micrographs of some activated carbons and carbon-supported chromium catalysts are shown in the panels of Figure 5: (a) CAC, (b) CAC-Cr, and (c) CAC-TT-Cr. The chromium catalyst micrograph (b) presents dark spots homogeneously distributed, with a particle size between 5 and 50 nm, not observed for the activated carbon (a). This result suggests that chromium oxide is well-dispersed on the carbon support surface. The same crystals are observed for CAC-TT-Cr, but their sizes are considerably higher. Probably, a worse dispersion was attained in this case as a consequence of the removal of surface oxygen complexes with the thermal treatment, previous to the chromium impregnation.
(27) Bedia, J.; Rosas, J. M.; Rodrı´ guez-Mirasol, J.; Cordero, T. Appl. Catal., B 2010, 94, 8–18. (28) Jagannathan, K.; Srinivasan, A.; Rao, C. N. R. J. Catal. 1981, 69, 418–427. (29) Cordero, T.; Rodrı´ guez-Mirasol, J.; Tancredi, N.; Piriz, J.; Vivo, G.; Rodrı´ guez, J. J. Ind. Eng. Chem. Res. 2002, 41, 6042–6048.
(30) Li, Y. H.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1998, 53, 1–26. (31) Li, Y. H.; Radovic, L. R.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1999, 54, 4125–4136. (32) Szymaski, G. S.; Grzybek, T.; Papp, H. Catal. Today 2004, 90, 51–59. (33) Zhu, Z. H.; Radovic, L. R.; Lu, G. Q. Carbon 2000, 38, 451–464.
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Figure 5. TEM micrographs of some of the activated carbons and carbon-supported chromium catalysts: (a) CAC, (b) CAC-Cr, and (c) CAC-TT-Cr. Table 4. Contribution of the Different Surface Oxygen Groups (mmol/g) Derived from CO and CO2 Evolution from TPD Analyses
CAC CAC-TT CAC-Cr CAC-Cr-TT CAC-TT-Cr
carboxyl
lactone
anhydride
phenol
carbonyl þ quinone
0.01 0.01 0.05 0.01 0.05
0.05 0.01 0.35 0.01 0.14
0.04 0.01 0.59 0.01 0.48
0.34 0.12 1.38 0.05 0.27
2.40 0.83 3.58 1.45 3.09
To establish correlations between the different oxygen surface functional groups and the apparent reaction rate of NO reduction, deconvolution of the CO and CO2 profiles obtained from the TPD experiments (Figures 3 and 4) has been performed, taking into account the temperature range of decomposition of the different oxygen surface groups and the fact that carbon-oxygen groups of acid character (carboxylic and lactonic) evolve as CO2 upon thermal desorption, whereas the non-acidic (carbonyl, ether, and quinone) and phenol groups decompose as CO, and anhydride evolve as both CO and CO2.34 Table 4 summarizes the amount of the different oxygen surface groups derived from these deconvolutions. We have not observed a good correlation between the apparent reaction rate of NO and the total amount of oxygen surface groups that decompose as CO2. The correlation with the oxygen surface groups that evolve as CO was not satisfactory either. However, we have obtained a linear relationship between the apparent reaction rate of NO reduction and the acidic oxygen surface groups, carboxyl, lactone, anhydride, and phenol groups (Figure 8). The only reaction products observed for the reduction of NO on CAC-Cr in the temperature range of 350-550 °C are N2 and CO2. No N2O was detected in the reaction gas. However, NO reduction on CAC for similar conversion shows mainly CO as a reaction product instead of CO2. It seems that the presence of chromium favors the product distribution toward CO2. Different authors13,17 proposed that the NO reduction catalyzed by chromium takes place through a redox mechanism, which basically consists of NO chemisorption over the catalyst surface, oxygen transfer
Figure 6. Steady-state NO conversion as a function of the reaction temperature for different chromium catalysts and supports, with 600 ppm NO, W/FNO = 0.98 g s μmol-1, and W/FNO = 2.88 g s μmol-1 (200 ppm NO for CAC-Cr at the same conditions).
Figure 7. Steady-state NO conversion as a function of the reaction temperature for CAC-Cr subjected to thermal treatments (TT) at different final temperatures (from 500 to 900 °C), 200 ppm NO, and W/FNO = 2.88 g s μmol-1.
reduction efficiency of the activated carbon-supported vanadium catalysts. Yamashita et al.10 made a distinction between stable carbon-oxygen complexes C-O and reactive surface intermediates C(O), which act as intermediates for NO reaction.
ao, J. J. (34) Figuereido, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orf~ M. Carbon 1999, 37, 1379–1389.
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70% higher NO conversion improvement and almost a 90% higher conversion for temperature (450 °C). With CO as the reducing agent, the only reaction products observed were N2 and CO2. However, with propylene, the reaction products obtained were N2, H2, H2O, and a negligible amount of CO2 that appears only at the beginning of the reaction. The fresh CAC-Cr catalyst and the catalyst after the NO reduction reaction were analyzed by XPS. Table 5 shows the mass surface concentrations determined by XPS analyses for the CAC-Cr catalyst before and after the reaction with different gases at different temperatures, in the range of 350-600 °C. Figure 10 shows the corresponding XPS spectrum of N 1s. Only the XPS spectrum of the catalyst before and after the reaction with NO and NO in the presence of CO provide evidence of the formation of nitrogen surface complexes, mainly as pyridinic, with a band at 398.7 eV, pyrrolic, with a band at 400.2 eV, and quaternary nitrogen complexes with a positive formal charge, with a band at 401.4 eV.11,35 The presence of some kind of nitrate or nitro-adsorbed species with high stability can be ruled out, independent of the reductant agent employed. No significant modifications of the mass surface concentrations are observed for the catalysts after the reaction with NO in the presence of CO at 350 °C compared to those obtained for the fresh catalyst. However, a considerable increase of the oxygen content is observed for the catalysts after the reduction of NO in the presence of CO at higher temperatures (500 °C), which does not occur at 350 °C. An increase of the surface nitrogen content has been observed after both NO direct reduction and reduction with CO, suggesting that both reactions takes place through a similar reaction pathway. This may indicate that NO is probably adsorbed before it is reduced to N2 and that part of this nitrogen adsorbed remains stable after the reaction as pyridinic and/or pyrrolic complexes. Furthermore, CO that favors the NO reduction is also chemisorbed on or interacts with the carbon surface (which has been detected by XPS analysis). On the basis of these results, it may be possible that NO reduction takes places through a Langmuir-Hinshelwood mechanism. In contrast to the case of CO, the nitrogen surface content is very similar for both the catalyst before and after the reaction with propylene. This result suggest that, in the presence of C3H6, the NO reduction goes through an Eley-Rideal mechanism, where the reaction between a molecule of propylene adsorbed and a molecule of NO in the gas phase takes place. Another important aspect to be pointed out is the slight increase of the carbon content of the catalyst after the reaction with NO in the presence of propylene (see Table 4), probably associated with the formation of pyrolytic carbon, given that no carbonaceous species (CO or CO2) were detected at the outlet of the reactor, during this reaction. The formation of pyrolytic carbon may reduce the active sites for the NO reduction, producing a slight deactivation of the catalyst (see Figure 12). The influence of the addition of oxygen on the reduction of NO was analyzed in the presence of CO and C3H6, using different addition sequences. Panels a and b of Figure 11 show the reaction gas evolution as a function of the reaction time for NO reduction on the CAC-Cr catalyst, for 200 ppm NO, 1% CO, and 3% O2, at 350 °C. The first step in both
Figure 8. Apparent reaction rate, r, obtained at 600 and 650 °C (W/FNO = 0.98 g s μmol-1) as a function of the amount of acidic surface groups obtained by TPD for the different carbons and carbon catalysts used in this work.
Figure 9. Pseudo-steady-state NO conversion as a function of the temperature for NO reduction (200 ppm) on CAC-Cr, with CO (1%) and C3H6 (2000 ppm).
from the active centers of the catalyst to the active centers of the carbon, and desorption of the surface carbon-oxygen complexes. In the present study, desorption of the oxygen product from the surface of the catalyst is limited, with significant evolution of CO2 only for high NO conversions. The total balance of oxygen for all of the analyzed reaction temperatures was not achieved. To corroborate the accumulation of oxygen as carbon-oxygen complexes, a TPD was carried out subsequently to the NO reduction experiments (with 200 ppm NO and W/FNO = 2.88 g s μmol-1 at different temperatures of 350-525 °C). The evolution of CO and CO2 was compared to that of the TPD shown in Figures 3 and 4. CO evolution was very similar for both experiments. However, a considerable increase of the CO2 evolution was obtained in comparison to that of the fresh catalyst, enough to justify the imbalance. These results suggest that NO decomposition produces very stable oxygen surface groups, in which desorption dominates the reduction process.6,30 3.3. Influence of the Reducing Agent. Figure 9 represents the pseudo-steady-state NO conversion on the CAC-Cr catalyst as a function of the reaction temperature for different NO reduction atmospheres. The presence of C3H6 or CO in the inlet gas flow produces a considerable increase of the NO conversion, with significantly higher values when using propylene as the reductant gas. At low reduction temperatures (
