Article pubs.acs.org/Langmuir
Chemisorption of Cyanogen Chloride by Spinel Ferrite Magnetic Nanoparticles T. Grant Glover,*,† Jared B. DeCoste,† Daniel Sabo,‡ and Z. John Zhang*,‡ †
SAIC, Gunpowder Branch, P.O. Box 68, APG, Maryland 21010, United States School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States
‡
ABSTRACT: Spinel ferrite magnetic nanoparticles, MnFe2O4, NiFe2O4, and CoFe2O4, were synthesized and used as gas-phase adsorbents for the removal of cyanogen chloride from dry air. Fixedbed adsorption breakthrough experiments show adsorption wave behavior at the leading edge of the breakthrough curve that is not typical of physically adsorbed species. Fourier transform infrared spectroscopy (FTIR) results indicate that CK is reacting with the spinel ferrite surface and forming a carbamate species. The reaction is shown to be a function of the hydroxyl groups and adsorbed water on the surface of the particles as well as the metallic composition of the particles. The surface reaction decreases the remnant and saturation magnetism of the MnFe2O4 and CoFe2O4 particles by approximately 25%.
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INTRODUCTION
In general, toxic industrial inhalation hazards comprise a wide variety of chemicals and include organic molecules, such as benzene, as well as smaller molecules, such as ammonia, sulfur dioxide, and cyanogen chloride.10,11,16 The adsorption of large organic molecules typically relies on the available surface area of the adsorbent for physical adsorption, whereas smaller molecules, such as sulfur dioxide and cyanogen chloride, are more difficult to adsorb by physisorption alone.17 For example, it was recently shown that the adsorption of ammonia on activated carbon was limited but the adsorption of ammonia on MOF-74, which contains open metal sites for adsorption, was significant.11 Similarly, spinel ferrite magnetic nanoparticles were shown to have significantly higher sulfur dioxide loading than BPL activated carbon even though activated carbon has approximately 10 times the surface area of the magnetic particles.14 Regarding sulfur dioxide adsorption, it was shown via adsorption breakthrough experiments that spinel ferrites can remove sulfur dioxide from dry air because the surface provides chemisorption sites for sulfate formation. In some applications, such as the adsorption of toxic gases from breathing air, chemisorption of the toxic materials is appealing because it reduces the possibility of subsequent desorption of the toxic species back into the air stream as air passes over the adsorbent. Cyanogen chloride falls into the class of small toxic chemicals that are difficult to remove from air, and a variety of studies have examined the adsorption and reaction of cyanogen chloride on carbons and metal oxides.18−26 Traditional activated carbons have limited adsorption capacity for cyanogen
Significant effort has focused on developing adsorbents with tailored surface functionalities to solve specific gas adsorption challenges, such as toxic gas filtration or carbon dioxide storage.1−11 Of the nanoscale materials that can be tailored, magnetic nanoparticles are appealing as adsorbents or impregnants because they offer not only a controlled surface, but also a thermally robust crystalline structure that exhibits magnetic changes as the surface of the particle changes.12,13 For example, Vestal et al. showed that magnetic properties can be changed by binding benzene ligands to the surface of the particles and that the magnetic properties could be manipulated by changing the functional group attached to the benzene ligand. In addition, it was recently shown that magnetic nanoparticles exhibit approximately a 20% change in magnetism upon adsorption of sulfur dioxide.14 Both of these works are important because they highlight that magnetic nanoparticles are sensitive to the events that occur on the surface of the particles. Others have also examined the reactive nature of ferrites and have presented work highlighting the application of these materials as chemical sensors. A variety of gases have been considered, including hydrogen sulfide and chlorine, where changes in the conductivity of the material are observed upon adsorption.15 Although these works do not concentrate specifically on adsorption capacities or gas separations, the results are significant because they provide a precedence for the consideration of ferrites as adsorbents that can provide feedback about nanoscale surface chemistry events. It is noteworthy that many of the gases examined are also considered to be toxic industrial chemicals and are inhalation hazards. © 2013 American Chemical Society
Received: January 28, 2013 Revised: March 28, 2013 Published: March 29, 2013 5500
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The microbreakthrough system as configured for this study does not allow for in situ characterization of the adsorbent during the adsorption process. Therefore, the particles were removed from the microbreakthrough system after adsorbate exposure to conduct attenuated total reflectance Fourier transform infrared spectroscopy and magnetic measurements. Additional Experimental Techniques. Nitrogen adsorption isotherms were measured at 77 K using a Quantachrome Autosorb 1A sorption analyzer. The spinel ferrites were outgassed at 120 °C overnight under vacuum prior to analysis. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) of the materials was performed on a Bruker Tensor 27 FTIR with a Bruker platinum ATR accessory equipped with a single reflection diamond crystal. Sixteen scans were averaged over a range of 4000 to 600 cm−1 with background subtraction at a resolution of 4 cm−1. FTIR data was gathered at ambient humidity. The magnetic properties of CoFe2O4 spinel ferrite nanoparticles were studied using a Quantum Design MPMS-5S SQUID magnetometer with a magnetic field of up to 5 T. Samples were placed in the sample holder at ambient humidity. X-ray diffraction data was gathered using a Panalytical X’pert MPD powder diffractometer using Cu Kα radiation. A Perkin-Elmer OPTIMA 7300DV inductively coupled plasma optical emission spectrometer (ICP-OES) was used to determine the composition of the particles.
chloride. As a result, transition-metal impregnants such as Cu, Zn, and Cr, as well as bases such as 1,4-diazabicyclo[2.2.2]octane, pyridines, and ethylendiamine, are impregnated on carbon to alter its chemistry and improve its adsorption capacity.26 Spinel ferrites have not been examined as a means of adsorbing cyanogen chloride. However, the tailored size and composition of the materials makes the surface chemistry of spinel ferrites appealing. This work evaluates the use of spinel ferrites as adsorbent materials by examining the adsorption of cyanogen chloride on, and subsequent magnetic changes of, MnFe2O4, NiFe2O4, and CoFe2O4 nanoparticles.
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EXPERIMENTAL SECTION
Synthesis of Magnetic Nanoparticles. CoFe2O4 nanoparticles were synthesized via a solution reaction.27 Cobalt acetate salt and iron acetate salt with a 1 to 2 ratio were dissolved in a mixture of oleylamine and dibenzyl ether and stirred for 1 h. While purging with argon, the mixed acetate solution was heated to 120 °C and kept at that temperature for 1 h. Then the temperature was raised to 240 °C, and the reaction solution was agitated for 30 min. Upon cooling to room temperature, the nanoparticles were precipitated by the addition of absolute ethanol. The nanoparticles were collected and then washed using ethanol. MnFe2O4 and NiFe2O4 samples were made using the same procedure with corresponding manganese and nickel acetate salts. The particles were washed with a 1 M sodium hydroxide solution to deposit OH groups on the surface of the particles. Washes were completed using 1 and 3 M sodium hydroxide solutions. X-ray diffraction measurements were completed before and after the sodium hydroxide wash, and no changes were observed in the diffraction patterns after washing. Microbreakthrough Experiments. Cyanogen chloride microbreakthrough experiments were conducted using a well-established apparatus.11 Cyanogen chloride was injected into a ballast and subsequently pressurized. The contents of the ballast were then mixed with a dry air stream to achieve a concentration of 1000 mg/m3. The completely mixed stream passed through a sorbent bed in a 4 mm internal diameter glass tube that was submerged in a temperaturecontrolled water bath at 25 °C. The sorbent bed was filled on a volumetric basis to a height of 4 mm, resulting in an average of 41 mg of spinel ferrite magnetic nanoparticles being used for each test. The samples were tested without outgassing or regeneration. Clean air, with the humidity matching the conditions of the experiment, was passed through the bed to evaluate the desorption behavior of the material after breakthrough had occurred. The dry air used in these experiments had a dew point of approximately −35 °C. In all cases, the effluent stream was continuously sampled using an HP5890 series II gas chromatograph equipped with a flame ionization detector (FID). The data were plotted on a normalized time scale of minutes per gram of adsorbent. Breakthrough tests were also performed using 20 × 40 mesh glass beads to ensure that the adsorption bed and system were not influencing the breakthrough behavior. No significant time delay was observed for the glass bead runs nor was any adsorption observed. Details of the experimental conditions are shown in Table 1.
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RESULTS AND DISCUSSION After synthesis, the crystallinity of the spinel ferrites, as measured via X-ray diffraction, was consistent with a spinel structure.28,29 Also, ICP-OES verified that the molar ratio of the metals contained in the nanoparticles was consistent with the expected spinel molecular formula, as shown in Table 2. To Table 2. Composition of Magnetic Nanoparticles transition metal/iron MnFe2O4 6 nm 7 nm 9 nm NiFe2O4 7 nm 9 nm 11 nm CoFe2O4 8 nm
value
challenge concentration temperature RH % bed height bed volume flow rate residence time detector
4000 mg/m3 20 °C 0 4 mm 50 mm3 20 mL/min at 20 °C 0.15 s GC/FID
1:1.6 1:1.8 1:2.5 1:1.9
evaluate the adsorption capacity of spinel ferrites, cyanogen chloride (CK) breakthrough testing was completed, and the results are shown in Figures 1 and 2. Breakthrough experiments have been used extensively to characterize novel adsorbent materials and are complementary to adsorption isotherms. Specifically, when novel materials are evaluated to remove chemicals from gas streams, if the adsorbate reacts with the adsorbent then the measurement of physical adsorption gas isotherms may not be appropriate. In this case, a breakthrough experiment can quantify the ability of the novel material to remove the adsorbate from the gas stream by measuring the outlet concentration of the bed as a function of time. If the gas is physically adsorbed, chemically adsorbed, or reacted into products, then the breakthrough device will show that the target adsorbate has been removed from the gas stream regardless of the removal mechanism. However, because the microbreakthrough system does not definitively identify the mechanism of removal, it is necessary to characterize the
Table 1. Cyanogen Chloride Breakthrough Conditions breakthrough parameter
1:2.4 1:2.5 1:2.2
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Figure 1. (a) Breakthrough data for cyanogen chloride adsorption on MnFe2O4. (b) Breakthrough data for cyanogen chloride on MnFe2O4 after exposure to cyanogen chloride and subsequent regeneration via heating and sodium hydroxide washing. (c) Expected adsorption wave breakthrough behavior.
Figure 2. Breakthrough data for cyanogen chloride on (a) NiFe2O4 and (b) CoFe2O4.
adsorbent after the breakthrough test. This approach is well suited for the evaluation of novel adsorbent materials and has been used extensively by others.10,11,16,25 The results in Figure 1a show adsorption wave behavior for the adsorption of CK on MnFe2O4 nanoparticles that is not typical of physically adsorbed fixed-bed adsorption wave fronts. Expected adsorption wave behavior is shown in Figure 1c, showing the effluent concentration of the fixed bed increasing with time until the concentration of the effluent equals that of the bed inlet.30 However, in the current work, CK breaks through the bed at approximately 70% of the feed concentration near time zero, begins to drop, reaching a minimum at nearly 1000 min/g, and finally proceeds toward the feed in a more typical fixed-bed adsorption wavefront. This behavior is observed for all three sizes of MnFe2O4 nanoparticles. As shown previously, the adsorption wavefront can also be integrated and the capacity of the material can be quantified as shown in Table 3.11 The breakthrough data show that approximately 1 mol/kg CK is loaded onto the MnFe2O4
Table 3. Cyanogen Chloride Loadings on Magnetic Nanoparticles sample MnFe2O4 6 nm 7 nm 9 nm 7 nm after heating 7 nm after NaOH wash NiFe2O4 7 nm 9 nm 11 nm CoFe2O4 8 nm
loading (mol/kg)
loading with desorption (mol/kg)
1.42 1.14 0.99 0.64 0.88
1.39 1.12 0.97 0.58 0.84
0.72 0.72 0.71
0.68 0.68 0.68
1.03
0.94
particles and the particles have limited desorption. The adsorption loadings are significant given that the nanoparticles 5502
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are nonporous and therefore have limited surface area for adsorption when compared to microporous adsorbents. Additionally, the loadings of the MnFe2O4 particles are consistent with the size of the particles, with the large particles having decreased surface area per unit mass. The unique wave behavior implies that physisorption is not the only behavior taking place in the fixed bed. Therefore, to gain insight into any possible reaction products that were adsorbed on the particle surface, FTIR data were gathered on the MnFe2O4 particles before and after exposure to CK as shown in Figure 3 and summarized in Table 4. Prior to
Table 4. FTIR Data sample
physisorbed CK, C−N stretch
molecular water, OH bend
MnFe2O4
1633
MnFe2O4 + CK
1624
MnFe2O4 after heating+CK MnFe2O4 after NaOH wash MnFe2O4 after NaOH wash + CK MnFe2O4 after NaOH wash + CK + 12 days CoFe2O4 CoFe2O4 + CK NiFe2O4 NiFe2O4 + CK
2203
1621
1633
2203
1538, 1336 1534 (weak) 1336 (weak) 1524 (weak) 1336 (weak) 1538 (weak) 1336 (weak)
carbamate COO− symmetric stretch
1423
1629
1423
1629
1419
1633 2203
CO32− species
1621
1514, 1332 1407
1629 1617
metal oxides, such as zeolites, have shown the ability to stabilize carbamates on their surfaces, which may be removed by washing with a strong base.32 The analysis of the surface of the magnetic particles indicates the presence of the carbamate species; however, a portion of the carbamate could have decomposed to ammonia and carbon dioxide but would not be detected by the microbreakthrough system, which was monitoring only for CK in the effluent of the fixed bed. To support the reaction scheme proposed, the MnFe2O4 particles that had been exposed to CK were then regenerated by heating the particles to 200 °C under vacuum overnight to remove adsorbed CK species, surface hydroxyl groups, and adsorbed water. The regenerated particles, which were packed under nitrogen and not exposed to ambient air after regeneration, were then exposed to CK on the breakthrough apparatus. The results of the breakthrough experiment on the regenerated samples are shown in Figure 1b, and the total loading of the regenerated particles was 0.64 mol/kg as shown in Table 3. The FTIR spectra for these particles, shown in Figure 3 as “MnFe2O4 after heating + CK”, shows the total loss of the OH group and adsorbed water FTIR signals in the 3200 cm−1 region. There is a disappearance of the FTIR modes of CO3− as well as H2O because these species are liberated upon heating. A new stretch occurs at ν2 ≈ 2200 cm−1 corresponding to the CN stretch of adsorbed CK.43,44 These results show that surface hydroxyl groups and adsorbed water groups play a key role in the reaction of CK with spinel ferrites and that removing the groups and water from the surface results in a lower CK capacity and no reactive products observed on the surface of the particles.
Figure 3. FTIR spectra of MnFe2O4 exposed to cyanogen chloride.
exposure to cyanogen chloride, the Mn particles show a broad band corresponding to surface hydroxyl groups, resulting from the sodium hydroxide wash during synthesis, and adsorbed water at νO−H ≈ 3370 cm−1 as well as a band corresponding to the bending mode of molecular water at ν2 ≈ 1630 cm−1. The presence of the CO32− species adsorbed onto the surface is also observed at ν ≈ 1570 and 1340 cm−1.31 Upon exposure of MnFe2O4 to CK, the FTIR modes of CO32− disappear as CO2 is liberated for more strongly bound surface species. The spectra indicate the formation of a surface carbamate species. One possible reaction mechanism between CK and MnFe2O4 to form a carbamate species has been speculated by others and is shown in Scheme 1.33−35 The modes of the NH2 group on the surface-bound carbamate species can be seen as masked shoulder peaks in the OH stretching region between 3000 and 3400 cm−1.36,37 Vibrational modes of carbamate NH2 species in this region are typically too weak to be accurately assigned, with competing OH stretches from water and surface hydroxyl groups of metal oxides.32 Also, the band at 1430 cm−1 corresponds to the νs mode of a bidentate carboxylate group of the surface-bound carbamate species.38,39 The conspicuous absence of the antisymmetric stretch of the carboxylate group has been observed by others for surface-bound carboxylate groups and has been explained to indicate that the N−COO− bond is nearly perpendicular to the surface.40−42 Although many carbamate species have been shown to decompose readily into ammonia and carbon dioxide, 5503
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Scheme 1
stability of the particles should be examined in more detail to determine the applicability of these materials in the cyclic adsorption process. The hydrolysis mechanism of CK on Zr(OH)4 has been detailed previously using NMR.25 However, the magnetic properties of the particles in the current study make it difficult to utilize NMR to determine reaction products. NMR, in addition to XPS, has also been used to quantify surface hydroxyl groups, and the application of XPS techniques may provide a means of quantifying the contributions of hydroxyl groups versus adsorbed water on the surface of the nanoparticles.47,48 However, a well-controlled set of experiments is necessary to determine the complete reaction mechanism and the roles of surface adsorbed water and OH groups in the reaction but is beyond the scope of the current work. To verify that that the band at ν2 ≈ 2200 cm−1 corresponds to physically adsorbed CK, the particles were allowed to sit in a capped glass vial, under ambient air, for 12 days, and the FTIR spectra were gathered again, the results of which are shown in Figure 3. After 12 days, the band at approximately ν2 ≈ 2200 cm−1 is essentially absent from the spectra, which supports the proposed physical adsorption of CK. The bands attributed to the surface carbamate species are still present, indicating that the species is covalently bound. Adsorption experiments were also conducted on manganese oxide particles to determine the impact of the iron and the spinel structure on the adsorption. As shown in Table 5, the MnO and Mn3O4 spinel particles showed effectively no adsorption. Typical fixed-bed wave behavior was observed for MnO and Mn3O4.
As with the other samples, the FTIR experiment for the heatregenerated sample was conducted after the breakthrough test and at ambient humidity. The sample was quickly moved from the breakthrough sample holder and placed on the FTIR stage to minimize the impact of water adsorption from the air. The regenerated magnetic nanoparticles after CK exposure were collected and washed with 3 M sodium hydroxide overnight to regenerate surface OH and water groups. The acidity of the particles was measured by placing 25 mg of particles in 2 mL of water and measuring the solution pH. The pH of the pristine particle solution was 10. The regenerated samples were washed until the acidity was the same for the regenerated sample and the pristine sample. It is important to note the pH of the samples having consistent acidity because the adsorption of gases, such as ammonia, has been shown to be a function of the material pH.45,46 FTIR data in Figure 3 show the presence of OH groups and adsorbed water on the surface after being washed with sodium hydroxide. These particles were then exposed to CK, and the FTIR spectra were collected again. In this case, after returning the OH groups and water to the surface of the particles and then exposing the particles to CK, the 3000 cm−1 region has been perturbed, and the three modes of the NH2 as well as the peak corresponding to a bidentate carboxylate are observed. The sodium hydroxide-washed particles produced a loading of 0.88 mol/kg, or 77% of the original capacity, which is lower than the pristine particles but higher than the particles without OH groups and adsorbed water present on the surface. The breakthrough curve for the sodium hydroxide-washed particles is shown in Figure 1b. It is possible that the full capacity of the pristine spinel ferrites was not fully recovered as a result of residual CK reaction products being retained on the surface of the particles, which reduces the total number of CK surface reaction sites. The reduction of available CK reaction sites is also reflected in the breakthrough wave behavior of the sodium hydroxidewashed sample and the regenerated sample not showing the unique wave behavior seen with the pristine samples. The lack of the unique wave behavior in the adsorption data indicates that the removal mechanisms of the pristine and regenerated samples are not identical. It has been reported by others that the hydrolysis of CK produces hydrogen chloride; furthermore, the presence of a metal provides a sink for the removal of hydrogen chloride and that the consumption of the metal terminates the reaction.25 Likewise, the reactions of chlorine on the surface of the nanoparticles may erode the nanoparticle surface and reduce the activity of the metal surface even after regeneration in base. We speculate that the reaction of CK with the surface OH groups and adsorbed water, coupled with possible reactions of the nanoparticles with chlorine from CK hydrolysis, may be complex and take place in multiple steps, resulting in the peculiar shape of the breakthrough curves. The impact of chlorine and particle regeneration on the performance and
Table 5. CK Loadings on Manganese Oxide Particles sample MnO 16 nm 22 nm 30 nm Mn3O4 18 nm 20 nm 22 nm
loading (mol/kg)
loading with desorption (mol/kg)
0.11 0.13 0.02
0.10 0.12 0.01
0.03 0.08 0.03
0.02 0.08 0.02
To examine the impact of composition on the CK breakthrough capacity, NiFe2O4 and CoFe2O4 particles were also examined, and the breakthrough data is shown in Figure 2a,b and quantified in Table 3. The NiFe2O4 particles show typical fixed-bed adsorption wave behavior, less adsorption capacity, and effectively no desorption. The NiFe 2 O 4 adsorption results show no sensitivity to the size of the particles with all sizes showing limited capacity. The breakthrough capacity for the CoFe2O4 particles falls in between those of the Ni and Mn samples. Initially, some breakthrough is observed up to approximately 30% of the feed 5504
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Figure 4. FTIR spectra for (a) CoFe2O4 and (b)NiFe2O4 exposed to cyanogen chloride.
Figure 5. Magnetic measurements of magnetic nanoparticles before and after cyanogen chloride exposure.
Table 6. Magnetization of Magnetic Nanoparticles before and after Cyanogen Chloride Exposure sample
remnant magnetization emu/g
MnFe2O4 MnFe2O4 + CK CoFe2O4 CoFe2O4 + CK NiFe2O4 NiFe2O4 + CK
10.47 8.16 40.64 29.53 11.00 11.50
percent change
coercivity g
22 27 4
249.85 245.20 15 714 12 249 267.09 257.03
percent change 2 22 4
saturation magnetization emu/g 41.78 32.09 64.20 44.41 41.33 45.15
percent change 23 31 8
The FTIR and breakthrough results are noteworthy because they identify one possible CK reaction mechanism and show that the metal comprising the spinel ferrite has a strong impact on the adsorption performance. Additionally, the FTIR results are consistent with the microbreakthrough loadings of the MnFe2O4 materials, showing more reaction products and producing higher loading as compared to the Co and Ni samples. The adsorption wave behavior does not follow typical physical adsorption behavior as a result of the surface reaction on the nanoparticles. After regeneration, the capacity of the spinel ferrite particles is reduced and the reaction has less impact on the wave shape.
concentration, followed by a minimum, and then more typical fixed-bed wave behavior. The FTIR spectra for the NiFe2O4 sample and the CoFe2O4 samples are shown in Figure 4. The FTIR spectra of NiFe2O4 exhibits changes in the adsorbed water region and the CO3− regions, but the reaction products and physically adsorbed species that were detected in the MnFe2O4 particles are absent. The lack of significant changes to the FTIR spectra is consistent with the lower breakthrough capacity. The CoFe2O4 particles have FTIR spectra that show carbamate formation as well as physically adsorbed CK; however, the reaction product FTIR signals are weaker relative to those of the MnFe2O4 particles. 5505
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diminished. MnO does not show any reactivity with CK nor do Mn3O4 particles even though Mn3O4 particles contain the same spinel structure. Significant magnetic changes are observed after the nanoparticles are exposed to CK. The Mn and Co samples showed a reduction of approximately 25% in saturation and remnant magnetization. The Co sample also showed a 22% reduction in coercivity. The Ni samples showed a slight increase in magnetic behavior after CK exposure. The results show that cyanogen chloride can be adsorbed on the particle surface in the absence of significant porosity, the reaction is specific to the functionality of the magnetic nanoparticle surface, the reaction is sensitive to the composition of the particles, and magnetic changes are observed upon reaction.
The changes in the magnetic properties of the spinel ferrite nanoparticles upon CK exposure are shown in Figure 5. For the MnFe2O4 sample, the saturation magnetization of the particles was reduced 23%, the remnant magnetization decreased 22%, and the coercivity changed minimally upon exposure to CK as summarized in Table 6. The CoFe2O4 particles show significant reductions in all three magnetic properties with the coercivity, saturation, and remnant magnetization reduced by 22, 31, and 27%, respectively. The difference in the shape of the magnetization plots arises as a result of the differences in the composition of the samples. It is assumed that the accumulation of strongly bound CK reaction products makes a more significant contribution to the magnetic properties of the ferrites than water adsorbed from the air while preparing samples for magnetic characterization. Changes in magnetic properties upon modification of a magnetic nanoparticle surface have been documented by others.12,14 Vestal and Zhang highlight the importance of considering the impact of surface modifications on the surface anisotropy of the particles and state that others have performed theoretical calculations and have shown that coercivity decreases with a decrease in surface anisotropy. It is possible that upon adsorption of CK that a reduction in the surface anisotropy and spin−orbital couplings results in a reduction in coercivity and saturation magnetization. A detailed study focusing on surface modification and magnetic measurements is required to verify this hypothesis. Unlike the Mn and Co samples, the NiFe2O4 samples show that upon exposure to CK the saturation magnetization increases by 8% and the remnant magnetization increases by 4%. However, the Ni sample showed only limited reactivity, and the magnetic data may be reflecting more than one surface event as a result of exposure to CK. In the absence of clear reaction products accumulated on the surface of the particles, the magnetic changes may be reflecting the removal of carbonate species as well the accumulation of small amounts of adsorbed species. The FTIR data support changes in the NiFe2O4 surface chemistry with the spectra near 1340 and 1630 cm−1, showing the possible removal of carbonates from the particle surface and the hydroxyl region perturbed upon exposure, relative to the pristine particles. Thus, in the absence of significant adsorption loadings of CK products, it is difficult to correlate the NiFe2O4 magnetic changes to CK loadings directly.
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AUTHOR INFORMATION
Corresponding Author
*(T.G.G.) Tel: (410) 436-6744. Fax: (410) 436-3764. E-mail:
[email protected]. (Z.J.Z.) Tel: (404) 894-6368. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Paulette Jones for her assistance with the breakthrough device and the Defense Threat Reduction Agency project BA07PRO105 for funding.
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REFERENCES
(1) Furtado, A. M. B.; Wang, Y.; Glover, T. G.; LeVan, M. D. MCM41 impregnated with active metal sites: synthesis, characterization, and ammonia adsorption. Microporous Mesoporous Mater. 2011, 142, 730− 739. (2) Schindler, B. J.; LeVan, M. D. The theoretical maximum isosteric heat of adsorption in the Henry’s law region for slit-shaped carbon nanopores. Carbon 2008, 46, 644−648. (3) Millward, A. R.; Yaghi, O. M. Metal-organic frameworks with exceptionally high capacity for storage of carbon dioxide at room temperature. J. Am. Chem. Soc. 2005, 127, 17998−17999. (4) Wu, H.; Zhou, W.; Yildirim, T. High-capacity methane storage in metal-organic frameworks M2(dhtp): the important role of open metal sites. J. Am. Chem. Soc. 2009, 131, 4995−5000. (5) Murray, L. J.; Dinca, M.; Long, J. R. Hydrogen storage in metalorganic frameworks. Chem. Soc. Rev. 2009, 38, 1294−1314. (6) Dillon, A. C.; Heben, M. J. Hydrogen storage using carbon adsorbents: past, present and future. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 133−142. (7) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M. Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage. Science 2002, 295, 469−472. (8) Morris, R. E.; Wheatley, P. S. Gas storage in nanoporous materials. Angew. Chem., Int. Ed. 2008, 47, 4966−4981. (9) Britt, D.; Furukawa, H. Highly efficient separation of carbon dioxide by a metal-organic framework replete with open metal sites. Proc. Nat. Acad. Sci. U.S.A. 2009, 106, 20637−20640. (10) Britt, D.; Tranchemontagne, D.; Yaghi, O. M. Metal-organic frameworks with high capacity and selectivity for harmful gases. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 11623−11627. (11) Glover, T. G.; Peterson, G. W.; Schindler, B. J.; Britt, D.; Yaghi, O. M. MOF-74 building unite has a direct impact on toxic gas adsorption. Chem. Eng. Sci. 2011, 66, 163−170. (12) Vestal, C. R.; Zhang, Z. J. Synthesis and magnetic characterization of Mn and Co spinel ferrite-silica nanoparticles with tunable magnetic core. Nano Lett. 2003, 3, 1739−1743.
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CONCLUSIONS The adsorption behavior of MnFe2O4, CoFe2O4, and NiFe2O4 spinel ferrites shows that these materials readily adsorb cyanogen chloride from dry air, and the adsorption wave behavior illustrates that physisorption is not the only CK removal mechanism. FTIR results illustrate that Mn and Co samples react with CK to form a carbamate species and that physically adsorbed CK is also present on the particles. The removal mechanism is shown to be a strong function of the adsorbed water and OH groups that are present on the surface of the particles, and in the absence of surface OH groups and adsorbed water, the CK capacity of the materials is significantly reduced. Furthermore, the adsorption of CK is a function of the metals that comprise the spinel structure, with the MnFe2O4 and CoFe2O4 particles being more reactive than the NiFe2O4 particles. The NiFe2O4 sample shows no clear reaction products detectable by FTIR upon exposure to CK; however, the carbonate region and hydroxyl bands of the spectrum were 5506
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(13) Vestal, C. R.; Zhang, Z. J. Atom transfer radical polymerization synthesis and magnetic characterization of MnFe2O4/polystyrene core/shell nanoparticles. J. Am. Chem. Soc. 2002, 124, 14312−14313. (14) Glover, T. G.; Sabo, D.; Vaughan, L.; Rossin, J. A.; Zhang, Z. J. Adsorption of sulfur dioxide by CoFe2O4 spinel ferrite nanoparticles and corresponding changes in magnetism. Langmuir 2012, 28, 5695− 5702. (15) Gadkari, A. B.; Shinde, T. J.; Vasambekar, P. N. Ferrite gas sensors. IEEE Sens. J. 2011, 11, 849−861. (16) Johnson, B. J.; Melde, B. J.; Peterson, G. W.; Schindler, B. J.; Jones, P. Functionalized organosilicate materials for irritant gas removal. Chem. Eng. Sci. 2012, 68, 376−382. (17) Peterson, G. W.; Karwacki, C. J.; Feaver, W. B.; Rossin, J. A. Zirconium hydroxide as a reactive substrate for the removal of sulfur dioxide. Ind. Eng. Chem. Res. 2009, 48, 1694−1698. (18) Deitz, V. R.; Rehrmann, J. A. The cumulative chemisorption of cyanogen chloride on a carbon-supported catalyst (whetlerite). Carbon 1990, 28, 363−367. (19) Biron, E.; Stavisky, R. Deactivation of ASC whetlerite charcoal upon adsorption of cyanogen chloride. Carbon 1995, 33, 1413−1416. (20) Jonas, L. A. Reaction steps in gas sorption by impregnated carbon. Carbon 1978, 16, 115−119. (21) Barnir, Z.; Aharoni, C. Adsorption of cyanogen chloride on impregnated active carbon. Carbon 1975, 13, 363−366. (22) Reucroft, P. J.; Chiou, C. T. Adsorption of cyanogen chloride and hydrogen-cyanide by activated and impregnated carbons. Carbon 1977, 15, 285−290. (23) Aharoni, C.; Barnir, Z. Efficiency of adsorbents for the removal of cyanogen chloride. Am. Ind. Hyg. Assoc. J. 1978, 39, 334−339. (24) Bailey, P. L.; Bishop, E. Hydrolysis of cyanogen chloride. J. Chem. Soc., Dalton Trans. 1973, 912−916. (25) Peterson, G. W.; Wagner, G. W.; Keller, J. H.; Rossin, J. A. Enhanced cyanogen chloride removal by the reactive zirconium hydroxide substrate. Ind. Eng. Chem. Res. 2010, 49, 11182−11187. (26) Mahle, J. J.; Peterson, G. W.; Schindler, B. J.; Smith, P. B.; Rossin, J. A.; Wagner, G. W. Role of TEDA as an activated carbon impregnant for the removal of cyanogen chloride from air streams: synergistic effect with Cu(II). J. Phys. Chem. C 2010, 114, 20083− 20090. (27) Han, M. H. Ph.D. Dissertation, Georgia Institute of Technology, 2008. (28) Zhang, Z. J.; Wang, Z. L.; Chakoumakos, C. B.; Yin, J. S. Temperature dependence of cation distribution and oxidation state in magnetic Mn-Fe ferrite nanocrystals. J. Am. Chem. Soc. 1998, 120, 1800−1804. (29) Rondinone, A. J.; Samia, A. C. S.; Zhang, J. Z. Superparamagnetic relaxation and magnetic anisotropy energy distribution in CoFe2O4 Spinel Ferrite Nanocrystallites. J. Phys. Chem. B 1999, 103, 6876−6880. (30) LeVan, M. D.; Carta, G.; Yon, C. M. Adsorption and Ion Exchange. In Perry’s Chemical Engineers’ Handbook, 7th ed.; Green, D. W., Maloney, J. O., Eds.; McGraw-Hill: New York, 1997. (31) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 6th ed.; John Wiley & Sons: Hoboken, NJ, 2009. (32) DeCoste, J. B.; Doetschman, D. C.; Lahr, M. J.; Kanyi, C. W.; Schulte, J. T. The room temperature chemistries of isocyanates with zeolite NaX. Microporous Mesoporous Mater. 2011, 139, 110−119. (33) Bodrikov, I. V.; Danova, B. V. Reactions of cyanogen chloride with olefins in alcoholic solutions of Lewis acids. J. Org. Chem. USSR 1968, 4, 1611. (34) Bodrikov, I. V.; Danova, B. V. Chemistry of cyanogen halides. J. Org. Chem. USSR 1969, 5, 1558−1562. (35) Smith, M. B.; March, J. March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th ed.; Wiley-Interscience: New York, 2001. (36) Knöfel, C.; Martin, C. l.; Hornebecq, V.; Llewellyn, P. L. Study of carbon dioxide adsorption on mesoporous aminopropylsilane-
functionalized silica and titania combining microcalorimetry and in situ infrared spectroscopy. J. Phys. Chem. C 2009, 113, 21726−21734. (37) Bacsik, Z.; Atluri, R.; Garcia-Bennett, A. E.; Hedin, N. Temperature-induced uptake of CO2 and formation of carbamates in mesocaged silica modified with n-propylamines. Langmuir 2010, 26, 10013−10024. (38) Nara, M.; Torii, H.; Tasumi, M. Correlation between the vibrational frequencies of the carboxylate group and the types of its coordination to a metal ion: an ab initio molecular orbital study. J. Phys. Chem. 1996, 100, 19812−19817. (39) Vestal, C. R.; Zhang, Z. J. Effects of surface coordination chemistry on the magnetic properties of MnFe2O4 spinel ferrite nanoparticles. J. Am. Chem. Soc. 2003, 125, 9828−9833. (40) Osawa, M.; Ataka, K. I.; Yoshii, K.; Nishikawa, Y. Surfaceenhanced infrared spectroscopy: the origin of the absorption enhancement and band selection rule in the infrared spectra of molecules adsorbed on fine metal particles. Appl. Spectrosc. 1993, 47, 1497−1502. (41) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Characterization of benzoic and phenolic complexes at the goethite/ aqueous solution interface using cylindrical internal reflection Fourier transform infrared spectroscopy. Part 1. Methodology. Langmuir 1990, 6, 979−987. (42) Suzuki, O.; Shibata, Y.; Inoue, M. Adsorption of benzoic compounds onto stainless steel particles. J. Colloid Interface Sci. 1997, 193, 234−241. (43) Kim, S.; Sorescu, D. C.; Yates, J. T. Infrared spectroscopic study of ClCN adsorption on clean and triethylenediamine-precovered γAl2O3. J. Phys. Chem. C 2007, 111, 18226−18235. (44) James, O. J. Vibrational frequencies and structural determination of cyanogen fluoride hydrofluoride. THEOCHEM 2005, 728, 135− 139. (45) Seredych, M.; Bandosz, T. J. Mechanism of ammonia retention on graphite oxides: role of surface chemistry and structure. J. Phys. Chem. C 2007, 111, 15596−15604. (46) Glover, T. G.; Peterson, G. W.; DeCoste, J. B.; Browe, M. A. Adsorption of ammonia by sulfuric acid treated zirconium hydroxide. Langmuir 2012, 28, 10478−10487. (47) DeCoste, J. B.; Glover, T. G.; Mogilevsky, G.; Peterson, G. W.; Wagner, G. W. Trifluoroethanol and 19F magic angle spinning nuclear magnetic resonance as a basic surface hydroxyl reactivity probe for zirconium(IV) hydroxide structures. Langmuir 2011, 27, 9458−9464. (48) Mogilevsky, G.; Karwacki, C. J.; Peterson, G. W.; Wagner, G. W. Surface hydroxyl concentration on Zr(OH)4 quantified by 1H MAS NMR. Chem. Phys. Lett. 2011, 511, 384−388.
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dx.doi.org/10.1021/la400385b | Langmuir 2013, 29, 5500−5507
