pubs.acs.org/Langmuir © 2010 American Chemical Society
Effect of Carbon Surface Modification with Dimethylamine on Reactive Adsorption of NOx Eleni Deliyanni and Teresa J. Bandosz* Department of Chemistry, The City College of New York, New York, New York 10031, United States, and The Graduate School of City University of New York, New York, New York 10031, United States Received October 22, 2010. Revised Manuscript Received December 6, 2010 The wood-based activated carbon, either as received or oxidized with nitric acid, was exposed to dimethylamine vapors. This modification was expected to introduce nitrogen groups. Then, the modified samples were used as adsorbents of NO2 under dynamic conditions. Both NO2 breakthrough curves and the NO concentration curves were recorded. The samples before and after exposure to NO2 were characterized using adsorption of nitrogen, elemental analysis, potentiometric titration, FTIR, and thermal analysis. Modifications with amines resulted in an increase in NO2 adsorption and in a decrease in NO emission. The effects were more visible when oxidation was used as a pretreatment of the carbon surface. This process increased the incorporation of nitrogen to the carbon matrix via acidbased reactions resulting in the formation of amides and amine carboxylic salts. Besides this, dimethylamine was strongly adsorbed on the carbon surface via hydrogen bonding with oxygen-containing groups. When the samples were exposed to nitrogen dioxide, there was an indication that nitramine and nitrosoamine were formed in the reactions of NO2 with either amides or amines. In the reactions of amines with NO, nitrosoamines are the likely products. As a next step, the surface of the carbon matrix is reoxidized by NO2, which is accompanied by the release of NO.
Introduction Activated carbons are considered to be very good adsorbents for the removal of pollutants and contaminants from ambient air.1-6 Toxic industrial compounds (TICs) are examples of species whose separation from air under ambient conditions recently has drawn the attention of various research groups.7-9 Among those species, nitrogen oxides are considered to be difficult to remove on activated carbons. This is owing to their involvement in redox reactions with carbon surfaces and weak adsorption potential at room temperature.2,3,5,6 Moreover, the presence of water can visibly interfere with the separation process and result in the formation of nitric and nitrous acid, which, when deposited in the pore system, change the chemical character of the carbon surface.3 To improve the capacity of the carbon surface to retain NO2/ NO or to even convert the oxides to N2, various modifications *To whom correspondence should be addressed. Tel: (212) 650-6017. Fax: (212)650-6107. E-mail:
[email protected]. http://www.sci.ccny. cuny.edu/∼tbandosz. (1) Zhang, W. J.; Rabiei, S.; Bagreev, A.; Zhuang, M. S.; Rasouli, F. Appl. Catal., B 2008, 83, 63–71. (2) Zhang, W. J.; Bagreev, A.; Rasouli, F. Ind. Eng. Chem. Res. 2008, 47, 4358– 4362. (3) Pietrzak, R.; Bandosz, T. J. Carbon 2007, 45, 2537–2546. (4) Teng, H.; Suuberg, E. M. Ind. Eng. Chem. Res. 1993, 32, 416–423. (5) Shirahama, N.; Moon, S. H.; Choi, K. H.; Enjoji, T.; Kawano, S.; Korai, Y.; Tanoura, M.; Mochida, I. Carbon 2002, 40, 2605–2611. (6) Jeguirim, M.; Tschamber, V.; Brilhac, J. F.; Ehrburger, P. J. Anal. Appl. Pyrolysis 2004, 72, 171–181. (7) Britt, D.; Tranchemontagne, D.; Yaghi, O. M. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 11623–11627. (8) Petit, C.; Bandosz, T. J. Adv. Funct. Mater. 2010, 20, 111–118. (9) Bandosz, T. J. J. Colloid Interface Sci. 2002, 246, 1–20. (10) Kante, K.; Deliyanni, E.; Bandosz, T. J. J. Hazard. Mater. 2009, 165, 704– 713. (11) Kaneko, K. Langmuir 1987, 3, 357–363. (12) Illan-Gomez, M. J.; Solano, A. L.; Salinas-Martinez de Lecea, C. Energy Fuels 1995, 9, 976–983. (13) Illan-Gomez, M. J.; Solano, A. L.; Radovic, L. R.; Salinas-Martinez de Lecea, C. Energy Fuels 1995, 9, 540–548.
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have been proposed.1,11-19 Impregnation of carbon with iron species was indicated as a very promising method to increase the NO2 adsorption capacity.11-14,20 Alca~niz-Monge and coworkers found metallic and partially reduced iron to be the most active in adsorption of NO on iron-containing activated carbon fibers.16 Iron in the form of γ-FeOOH and R-Fe2O3 was also indicated to be an enhancing factor for adsorption of both NO2 and NO.20 As surface reaction products, nitrates and nitrites were proposed. Besides iron, a positive role of metals such as sodium, potassium, lanthanum, and cerium was indicated.10 The latter two are active in the redox reactions with nitrogen oxides and in the formation of nitrates. Another method of carbon modification, which was shown as enhancing the NO2 adsorption capacity, is an introduction of nitrogen functionalities to the surface via an impregnation with urea and heat treatment.21 Nitrogen in the form of amines was also suggested as the main sites for NOx adsorption on dimethylurea/ZSM-5 zeolite.22 Taking into account the previous findings, the objective of this Article is to investigate the effects of surface modification with dimethylamine (DMA) on the capacity of the carbon to retain nitrogen oxide at ambient temperatures. As described in the literature, amines can be strongly retained on the carbon surface (14) Illan-Gomez, M. J.; Solano, A. L.; Radovic, L. R.; Salinas-Martinez de Lecea, C. Energy Fuels 1995, 9, 97–103. (15) Lee, Y. W.; Choi, D. K.; Park, J. W. Carbon 2002, 40, 1409–1417. Illan-Gomez, (16) Alca~niz-Monge, J.; Bueno-Lopez, A.; Lillo-Rodenas, M. A.; M. J. Microporous Mesoporous Mater. 2008, 108, 294–302. (17) Carabineiro, S. A.; Bras Fernandes, F.; Vital, J. S.; Ramos, A. M.; Silva, I. F. Catal. Today. 1999, 54, 559–567. (18) Carabineiro, S. A.; Bras Fernandes, F.; Vital, J. S.; Ramos, A. M.; Fonseca, I. M. Appl. Catal., B 2003, 44, 227–235. (19) Carabineiro, S. A.; Bras Fernandes, F.; Vital, J. S.; Ramos, A. M.; Silva, I. F. Catal. Today 2000, 57, 305–312. (20) Bashkova, S.; Bandosz, T. J. Ind. Eng. Chem. Res. 2009, 48, 10884–10891. (21) Bashkova, S.; Bandosz, T. J. J. Colloid Interface Sci. 2009, 333, 97–102. (22) Feaver, W. B.; Karwacki, C. J.; Rossin, J. U.S. Patent 7,238,332, 2007.
Published on Web 12/28/2010
DOI: 10.1021/la1042537
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via polar and electrostatic interactions or acid-base reactions.23 Their amount adsorbed is strongly enhanced when the polarity of a carbon surface increases and acidic groups are introduced via oxidation. Therefore, the carbon chosen for this study is a woodbased relatively reactive material, which can be easily oxidized to a great extent.24 Exposing the oxidized and initial samples to amines should result in differences in the amount of amine groups retained on the surface and generally in differences in the chemistry of nitrogen- and oxygen-containing surface groups. These groups are expected to affect the amount of NO2 adsorbed and NO formed as a result of NO2 reduction. Moreover, the amount of the NO emitted from the surface is expected to be altered by the presence of amines. The results are analyzed in terms of the adsorption capacity and the effects of surface features on the adsorptive performance. Although there are numerous studies in the literature, which focus on catalytic NOx reductions, the processes are usually investigated at elevated temperatures, where the catalytic activity is enhanced.12-17 The broad objective of our study is the investigation of the surface features that are important for NOx reactive adsorption under ambient conditions. The intended application of the adsorbents we study is in gas mask filters, which are supposed to remove the variety of gases. So far, activated carbon is considered to be the most versatile media.
Experimental Section Materials. A wood-based activated carbon BAX-1500 (Westvaco) was used in this study. Before experiments, the initial carbon was washed in a Soxhlet apparatus to remove water-soluble species and then dried. Part of the carbon was oxidized with 70% nitric acid for 4 h (1 g of carbon/10 mL of acid). After oxidation, the sample was washed in a Soxhlet apparatus to remove any excess oxidizing agent and any water-soluble compounds. No significant change in the weight of the sample was recorded after oxidation. The washing was done with periodic changes of water until constant, close-to-neutral pH of water was measured. The initial sample was designated as B, and the sample after oxidation was designated as BO. Both initial and oxidized carbon samples were modified to introduce nitrogen groups into their structure. The DMA treatment was performed in a vessel with a glass frit above the bottom and a gas inlet at the side of the vessel below the glass frit. A 60 g carbon sample was placed on the glass frit and formed a bed, through which DMA vapors were introduced to the vessel from the pressurized bottle. The excess of the vapors was collected in the balloon attached to the outlet of the vessel in a way that when the balloon expanded, the flow of DMA was stopped, and the system was left for equilibration. When the visible excess of vapors was adsorbed, the flow of DMA was open again. The experiments were performed until no changes in the balloon volume were observed. After the modification, the system was left for 24 h, and after that time it was assumed that the adsorption equilibrium was reached. The samples obtained in this way were designated as BAin and BOAin following the names of the precursors. It is important to mention that during the exposure to DMA, strong exothermic effects were noticed, especially for BO with the visible deposition of clear liquid on the walls of the vessels. During equilibration, that liquid was adsorbed by carbons. Before being tested for NO2 adsorption, 5 g of BAin or BOAin was put in a glass column 370 mm long by 9 mm diameter and purged with air (0.45 L/min) to remove weakly adsorbed DMA. The experiments were run until no change in weight was detected. The resulted samples were designated as BA and BOA, respectively. The samples after adsorption of NO2 are referred to with an additional letter E. (23) Elsayed, Y.; Bandosz, T. J. Langmuir 2002, 18, 3213–3219. (24) Salame, I. I.; Bandosz, T. J. Ind. Eng. Chem. Res. 2000, 39, 301–306.
1838 DOI: 10.1021/la1042537
Methods. Evaluation of NO2 Adsorption Capacity. Evaluation of NO2 adsorption capacity was conducted in a laboratoryscale, fixed-bed reactor system. The dried carbon samples were loaded into the glass column, 370 mm long by 9 mm diameter, such that the volume of the carbon bed was ∼2 cm3. A dry air with 0.1% of NO2 was passed through the column of an adsorbent. These conditions reflect an accelerated test and were chosen to simulate the NO2 removal in the industrial processes or in the case of it accidental/intended release to the atmosphere. The flow rates were controlled by the flow meters (Cole Palmer). All experiments were run at room temperature (∼23 C). The breakthrough concentration of NO2 and the concentration of NO were monitored using a multiple gas monitor with the electrochemical sensors (RAE Systems, MultiRAE Plus PGM-50/5P). The tests were conducted until the concentrations of NO2 and NO reached the sensors’ upper limit values of 20 and 200 ppm, respectively. When the NO2 concentration reached 20 ppm, the NO2 flow was turned off, and the carbon bed was purged with air (0.45 L/Min) to evaluate the strength of NO2 adsorption. This is reflected by a decrease in NO2 concentration on the breakthrough curves. The breakthrough capacities of the samples for NO2 were calculated per volume and per mass of a sample by integration of the area above the NO2 breakthrough curve, taking into account the initial concentration of NO2, total flow rate, breakthrough time, and mass of the sample. The breakthrough time used to calculate the NO2 capacity is defined here as the elapsed time from the beginning of NO2 challenge until the concentration of NO2 in the effluent gas had reached 20 ppm. The experiments were conducted at room temperature and atmospheric pressure with a total gas flow rate of 0.45 L/min. The measurement error for the breakthrough curves was between 10 and 20% Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm, Brinkmann Instruments, Westbury, NY). The instrument was set in the equilibrium mode, and the pH was collected when no change in the value was observed for 60 s. Approximately 0.100 g of samples was placed in a container thermostatted at 298 K with 50 mL of 0.01 M NaNO3 and equilibrated overnight. To eliminate any interference by dissolved CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred throughout the measurement. Each sample was titrated with 0.1 M NaOH titrant. Experiments were carried out in the pH range 3-10.25 It was assumed that the population of surface sites is described by a continuous pKa distribution, f(pKa). The experimental data were transformed into a proton binding isotherm, Q, representing the total amount of protonated sites, which is related to the pKa distribution by the following integral equation Z QðpHÞ ¼
¥
-¥
qðpH, pKa Þf ðpKa Þ dpKa
The solution of this equation was obtained using SAIEUS numerical procedure,26 which applies regularization combined with non-negativity constraints. On the basis of the spectrum of acidity constants and the history of the samples, the detailed surface chemistry was evaluated. pH. The pH of a carbon sample suspension provides information about the acidity and basicity of the surface. A sample of 0.4 g of dry carbon powder was added to 20 mL of water, and the suspension was stirred overnight to reach equilibrium. Then, the sample was filtered and the pH of solution was measured. Sorption of Nitrogen. The nitrogen adsorption isotherms on the materials studied were determined at -196 C using an ASAP 2010 instrument (Micromeritics). The samples were outgassed at 120 C to a constant vacuum (10-4 Torr) immediately (25) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994, 32, 1026–1028. (26) Jagiello, J. Langmuir 1994, 10, 2778–2785.
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Figure 1. NO2 breakthrough curves for the carbons studied. prior to the isotherm measurements. The specific surface area (SBET) was calculated from the isotherm data using the Brunauer, Emmet, and Teller (BET) model. The total pore volume (Vtot), the mesopore volume (Vmes), the micropore volume (Vmic), and the volume in pores
