pubs.acs.org/Langmuir © 2010 American Chemical Society
Interactions of NO2 and NO with Carbonaceous Adsorbents Containing Silver Nanoparticles Mykola Seredych,† Svetlana Bashkova,† Robert Pietrzak,‡ and Teresa J. Bandosz*,† †
Department of Chemistry, The City College of New York, 160 Convent Avenue, New York, New York 10031, and ‡Laboratory of Coal Chemistry and Technology, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Pozna~ n, Poland Received October 28, 2009. Revised Manuscript Received April 29, 2010
Interactions of NO2 and NO (the product of NO2 reduction by carbon) with biomass-based carbonaceous materials with silver nanoparticles deposited on the surface were studied. The surface of the materials was characterized using adsorption of nitrogen, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR), and thermogravimetric analysis (TGA). The results showed that the amount of NO2 adsorbed, its conversion to NO, and the amount of NO released from the carbon surface depend on the carbon’s content of silver. More silver results in a better performance of the adsorbent. The products of NO2 interactions with silver include surface chelates such as Ag2-O-NO or Ag-O2-NO. Another element, active in the surface reactions with NO2, is phosphorus. Both silver and phosphorus species are oxidized by NO2. The product of NO2 reduction, NO, is either retained on the carbon surface by its interactions with metallic silver or is further reduced to N2O or N2. Besides silver, carbon support is also active in the reduction of NO2 to NO. Carbon monoxide formed in such a processes can reduce silver oxide nanoparticles, and thus, it provides more metallic silver for interactions with NO.
Introduction NOx emissions having their origin in mobile exhausts and stationary power generating sources are considered as environmentally detrimental. To minimize/eliminate these emissions, adsorption-based methods are often used with activated carbon as adsorbents of choice.1-6 It was shown that surface modifications of activated carbons, using deposition of metals, improve their catalytic properties toward conversion/retention of NOx.7-17 It is well-known that carbon surface reduces NO2 to NO and this process is accompanied by the formation of CO and CO2.2,3,5,6 It *To whom correspondence should be addressed. Telephone: 212-650-6017. Fax: 212-650-6107. E-mail:
[email protected]. (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) Kaneko, K. Langmuir 1987, 3, 357–363. (8) Illan-Gomez, M. J.; Solano, A. L.; Salinas-Martinez de Lecea, C. Energy Fuels 1995, 9, 976–983. (9) Illan-Gomez, M. J.; Solano, A. L.; Radovic, L. R.; Salinas-Martinez de Lecea, C. Energy Fuels 1995, 9, 540–548. (10) Illan-Gomez, M. J.; Solano, A. L.; Radovic, L. R.; Salinas-Martinez de Lecea, C. Energy Fuels 1995, 9, 97–103. (11) Lee, Y. W.; Choi, D. K.; Park, J. W. Carbon 2002, 40, 1409–1417. (12) Illan-Gomez, M. J.; Solano, A. L.; Radovic, L. R.; Salinas-Martinez de Lecea, C. Energy Fuels 1995, 9, 112–118. (13) Carabineiro, S. A.; Bras Fernandes, F.; Vital, J. S.; Ramos, A. M.; Silva, I. F. Catal. Today 1999, 54, 559–567. (14) Carabineiro, S. A.; Bras Fernandes, F.; Vital, J. S.; Ramos, A. M.; Fonseca, I. M. Appl. Catal., B 2003, 44, 227–235. (15) Carabineiro, S. A.; Bras Fernandes, F.; Vital, J. S.; Ramos, A. M.; Fonseca, I. M. Appl. Catal., B 2005, 59, 181–186. (16) Carabineiro, S. A.; Bras Fernandes, F.; Silva, R. J. C.; Vital, J. S.; Ramos, A. M.; Fonseca, I. M. Catal. Today 2008, 133-135, 441–447. (17) Carabineiro, S. A.; Bras Fernandes, F.; Vital, J. S.; Ramos, A. M.; Silva, I. F. Catal. Today 2000, 57, 305–312.
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was hypothesized by Xia et al.18 that on the surface of activated carbons NO is adsorbed on the dangling carbons and edge sites. Various methods have been investigated to improve the ability of a carbon surface to bind NO.1,7-14,17 Kaneko7 reported that on highly dispersed γ-FeOOH NO molecules are first chemisorbed and deposited near the entrances of the micropores and, as a next step, they move into the micropores, which results in the formation of NO dimers. The enhancement in NO2 reduction and NO retention was also found on graphite oxide samples modified with iron species, where γ-FeOOH and R-Fe2O3 were identified as active centers.19 The reduction of NO on activated carbons was also investigated in the presence of transition metals,8,9 potassium10 and calcium.12 Iron, cobalt, and nickel were found to be the most effective catalysts for NO dissociative chemisorption at low temperatures.8 On the other hand, potassium and iron were indicated as the most effective catalysts in the redox reaction of NO with carbon.9,10 Conversion of NO and N2O at room temperature on transition metal oxides and their binary mixtures, supported on the activated carbons, was investigated by Carabineiro and co-workers.13-17 They found that for this process NO has to be dissociatively adsorbed and the surface has to have the sites to retain oxygen atoms. Metal species were suggested as the ones that provide such sites. NOx was also successfully removed by mineral oxide particles20-23 and zeolites.24,25 (18) Xia, B.; Phillips, J.; Chen, C. H.-K.; Radovic, L. R.; Silva, I. F.; Menedez, J. A. Energy Fuels 1999, 13, 903–906. (19) Bashkova, S.; Bandosz, T. J. Ind. Eng. Chem. Res. 2009, 48, 10884-10891. (20) Underwood, G. M.; Miller, T. M.; Grassian, V. H. J. Phys. Chem. A 1999, 103, 6184–6190. (21) Toops, T. J.; Smith, D. B.; Partridge, W. P. Catal. Today 2006, 114, 112– 124. (22) Zhu, R.; Guo, M.; Ci, X.; Ouyang, F. Catal. Commun. 2008, 9, 1184–1188. (23) Raj, A.; Le, T. H. N.; Kaliaguine, S.; Auroux, A. Appl. Catal., B 1998, 15, 259–267. (24) Chen, H. Y.; Wang, X.; Sachtler, M. H. Appl. Catal., A 2000, 194, 159–168. (25) Valyon, J.; Hall, W. K. J. Phys. Chem. 1993, 97, 1204–1212.
Published on Web 05/20/2010
DOI: 10.1021/la101175h
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The enhanced catalytic activity of metal/metal oxide nanoparticles for various reactions is a well-known phenomenon.26-28 The extent of this enhancement is affected by the chemistry of species and their size, crystalline phase, and surface dispersion. When silver is the active species, the interaction with NO may happen either via direct contact with silver or, according to Carabineiro and co-workers,13,14 via interaction with the reduced (by carbon) silver oxide, where NO is further reduced to N2. An investigation of the adsorption of NO on silver-exchanged microporous zeolites indicated that such species as Ag(I)-NO, Ag(I)(NO)2, Ag(II)-NO2, N2O, N2O3, and Ag-(NO3) complexes were the products of surface reactions.29 The mononitrosyl species were found to be the most abundant species on the surface, and silver nitrite and nitrate were the most stable species on the surface. The interactions of a NO/O2 mixture with different silver surfaces were also investigated by several research groups.30,31 It was indicated that the interactions of silver foil with a NO/O2 mixture resulted in the formation of a complicated “sandwich” structure consisting of superimposed AgNO3 and Ag2O layers on metallic silver. In another study, the surface of Ag(110) was exposed to the mixture of NO/O2 and it was found that oxygen enhances the adsorption of NO by forming NO2 and NO3 species, and NO enhances the cleavage of dioxygen bonds on Ag(110) at elevated temperatures.31 Exposure of the Ag(111) surface to the same mixture at about 47 °C resulted in the formation of a thick layer of AgNO3;32 this process was suggested to proceed via the reaction between silver and NO2, where the latter one was formed by the conversion of NO in the excess of oxygen. The objective of this paper is to investigate the adsorption of NO2 and the retention of NO on carbonaceous materials with silver nanoparticles deposited on the surface. Silver is expected to increase the reduction capability of the carbon surface and to enhance the interactions with NO. To understand the effects of silver, the surfaces of the adsorbents are extensively characterized before and after exposure to NO2.
Experimental Section Materials. The carbonaceous adsorbents used in this study are biomass-based activated carbons homemade (using phosphoric acid activation) at the Instituto Zuliano de Investigaciones Tecnological, in Venezuela. On those carbons, various amounts of silver were deposited using the Tollens method.33,34 According to this method, silver in the form of silver diammine complex ([Ag(NH3)2]þ) is supposed to react with aldehyde groups on the carbon surface. As a result, [Ag(NH3)2]þ is reduced to Ag and the aldehyde groups are oxidized to carboxylic groups. In all cases, the same initial carbon was used as a support. The materials are referred to as CAg1, CAg2, and CAg3, where increasing numbers represent an increase in the silver content. After exposure to NO2, the letter E is added to the names of the samples. The carbon sample without silver is referred to as C. (26) Zheng, N.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 14278–14280. (27) Wachs, I. Catal. Today 2005, 100, 79–94. (28) Mrabet, D.; Zahedi-Niaki, M. H.; Do, T.-O. J. Phys. Chem. C 2008, 112, 7124–7129. (29) Akolekar, D. B.; Bhargava, S. K. J. Mol. Catal. A: Chem. 2000, 157, 199–207. (30) Zemlyanov, D.Yu.; Nagy, A.; Schl€ogl, R. Appl. Surf. Sci. 1998, 133, 171–183. (31) Bao, X.; Wild, U.; Muhler, M.; Pettinger, B.; Schl€ogl, R.; Ertl, G. Surf. Sci. 1999, 425, 224–232. (32) Zemlyanov, D.; Schl€ogl, R. Surf. Sci. 2000, 470, L20–24. (33) Panacek, A.; Kvı´ tek, L.; Prucek, R.; Kolar, M.; Vecerova, R.; Pizurova, N.; Sharma, V. K.; Nevecna, T.; Zboril, R. J. Phys. Chem. B 2006, 110, 16248–16253. (34) Chen, Z.; Chen, X.; Zheng, L.; Gang, T.; Cui, T.; Zhang, K.; Yang, B. J. Colloid Interface Sci. 2005, 285, 146–151.
9458 DOI: 10.1021/la101175h
Methods. Evaluation of NO2 Adsorption Capacity. Evaluation of NO2 adsorption capacity was conducted in a laboratory-scale, fixed-bed reactor system. The adsorbents were loaded into the glass column, 370 mm long by 9 mm diameter, such that the volume of the carbon bed was about 2 cm3. Dry air with 0.1% NO2 was passed through the column of an adsorbent. The flow rates were controlled via the flow meters (Cole Palmer). The breakthrough concentration of NO2 and the concentration of NO were monitored using a multiple gas monitor with 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 250 ppm, respectively. The experiments were conducted at room temperature and atmospheric pressure with a total gas flow rate of 0.45 L/min. Surface Area and Pore Size Distribution Measurements. 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 prior to the isotherm measurements. The specific surface area (SBET) was calculated from the isotherm data using the Brunauer, Emmett, and Teller (BET) model.35 The total pore volume (Vtot), the mesopore volume (Vmes), and the micropore volume (Vmic) were calculated using the density functional theory (DFT) approach.36 Characterization of Carbon Surface Chemistry. The pH of the adsorbent surface was obtained by mixing a sample (0.1 g) with deionized water (5 mL), stirring the suspension overnight to equilibrate, and then measuring the pH of this suspension. Fourier transform infrared (FTIR) spectroscopy was carried out on a spectrometer (Thermo Electron Corporation, Nicolet 380) using a smart diffuse reflectance mode. The spectrum was collected 16 times and corrected for the background noise. The surface chemistry of the carbons was also analyzed by thermogravimetric analysis (TGA) using a TA Instruments thermal analyzer (SDT 2960) with a nitrogen flow rate of 100 cm3/min and a heating rate of 10 °C/min. The ash content was determined by burning the samples in air up to 1000 °C. The chemical state of selected elements and surface composition of the samples were determined by X-ray photoelectron spectroscopy (XPS) using a VSW spectrometer (Vacuum Systems Workshop Ltd., England) equipped with an Al KR source and 18channel two-plate analyzer. The spectra were taken in fixed analyzer transmission mode (ΔE = const) with a pass energy of 22 eV. They were smoothed, and Shirley background was subtracted. The calibration was carried out to the main C1s peak at 284.6 eV. The concentration of elements was calculated using the intensity of an appropriate line and XPS cross sections (as given by Scofield37). Surface Texture. Scanning electron microscopy (SEM) images were obtained using a Zeiss Supra 55 VP instrument with an accelerating voltage of 15.00 kV. Scanning was performed in situ on a powder sample. SEM images with energy-dispersive X-ray (EDAX) analysis were done at 1.00 KX magnifications. X-ray diffraction (XRD) measurements were conducted using a standard powder diffraction procedure. Adsorbents were ground with methanol in a small agate mortar. The mixture was smear-mounted onto the zero-background quartz window of a Phillips specimen holder and allowed to dry in air. Samples were analyzed by Cu KR radiation generated in a Phillips XRG 300 X-ray diffractometer. A quartz standard slide was run to check for instrument wander and to obtain an accurate location of 2θ peaks. Transmission electron micrographs (TEM) were obtained using a JEM-1200 EX II (JEOL) microscope with an accelerating voltage of 15 kV, working distance of 14 mm, and digital image recording DISS. (35) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309–319. (36) Lastoskie, G.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786– 4796. (37) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129–137.
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Figure 1. NO2 Breakthrough curves and NO concentration curves on the materials studied. Table 1. Breakthrough Capacities for NO2, Retention Time of NO at 250 ppm, and Surface pH for the Materials Studied pHa
NO2 breakthrough capacity sample
[mg/g of ads]
[mg/cm3 of ads]
CAg1 23.0 7.4 CAg2 24.2 8.4 CAg3 25.5 11.7 a pH for the initial carbon is 5.26.
NO retention time [min] initial exhausted 7 21 26
5.71 8.77 8.96
3.06 5.68 5.97
Results and Discussion The NO2 breakthrough curves and NO concentration curves are collected in Figure 1. The performance of the samples differs and the breakthrough times for both NO2 and NO increase with an increase in silver content. The breakthrough capacities for NO2 and the times to reach 250 ppm in the case of NO are collected in Table 1. It has to be mentioned that the calculated NO2 capacities are lower than those reported previously at the same conditions on carbons with sodium or lanthanates,38 or even on composites with sewage sludge components.3 This fact could be related to the porosity of the materials, which is going to be discussed later. The differences in the performance of the samples are better visible when the capacity is reported per unit volume of the adsorbent; addition of a larger quantity of silver must result in a higher density of the material. The pH values of the surface increase with an increase in the content of silver as a result of neutralization of some groups by the Tollens reagent and covering the surface with the silver/silver oxide nanoparticles. After exposure to NO2, about 3 units decrease in the pH is noticed which might be caused by oxidation of the carbon surface by NO2 and deposition of surface reaction products. The very low pH values (
