Removal of Chlorine Gases from Streams of Air Using Reactive

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Removal of Chlorine Gases from Streams of Air Using Reactive Zirconium Hydroxide Based Filtration Media Gregory W. Peterson† and Joseph A. Rossin*,‡ †

RDCB-DRP-F, U.S. Army Edgewood Chemical Biological Center, 5183 Blackhawk Road, Aberdeen Proving Ground, Maryland 21010-5423, United States ‡ Guild Associates, Inc., 5750 Shier-Rings Road, Dublin, Ohio 43016, United States

bS Supporting Information ABSTRACT: Zirconium hydroxide and zirconium hydroxide impregnated with triethylenediamine (TEDA) were evaluated for their ability to remove toxic chlorine gases, namely Cl2, COCl2, and HCl from streams of air in respirator applications. Zirconium hydroxide displayed a high capacity for the removal of HCl; however, the ability of zirconium hydroxide to remove Cl2 and COCl2 was relatively low. The removal of Cl2 and COCl2 greatly improved upon impregnation of zirconium hydroxide with TEDA. The improved performance was attributed to the ability of TEDA to promote the hydrolysis of Cl2 and COCl2, leading to the formation of HCl which was subsequently removed via reaction with hydroxyl groups associated with zirconium hydroxide. XPS analysis revealed the presence of both terminal and bridging hydroxyl groups associated with zirconium hydroxide, with only the terminal hydroxyl groups participating in the removal of reactant or reaction product HCl.

1. INTRODUCTION General-purpose respirators are designed to provide safe breathing in a chemically contaminated environment, such as those encountered by military personnel and first responders. Because of the unknown nature of the chemical threat, the respirator must be able to provide protection versus a wide range of toxic compounds, examples of which include high volatility gases (e.g., hydrogen cyanide, cyanogen chloride, phosgene, and chlorine) and persistent vapors (e.g., chemical warfare agents, pesticides, and pesticide precursors). The filtration material employed by general-purpose respirators typically consists of activated carbon impregnated with basic metal salts plus triethylenediamine.1,2 Although effective, there are some significant drawbacks associated with carbon-based filtration media. First, the capacity for the removal of several high volatility gases is low, being limited by the impregnant loading of approximately 10% metals by weight.1 Higher metal loadings result in a decreased metal dispersion, which leads to the physical blocking of pores and an overall reduced effectiveness. Second, the metal impregnants are not well dispersed,3 leading to poor utilization of the metals. Third, the metal impregnants have a tendency to migrate to the external surface of the carbon granule as a result of prolonged environmental exposure, leading to decreased filtration performance.3,4 We have previously postulated that a reactive substrate will have significant advantages over an impregnated material in chemical filtration applications.5,6 A reactive substrate is defined as a material in which the reactive functional groups are incorporated into the structure, rather than impregnated onto the surface as per activated, impregnated carbon. This will allow for a high concentration of well-dispersed reactive sites. Further, because the sites are associated with the substrate, loss of site dispersion as a result of prolonged environmental exposure is expected to be minimized/eliminated. r 2011 American Chemical Society

The filtration of chlorine (Cl2) and phosgene (COCl2) has not been widely reported in the open literature. Noyes7 reported that COCl2 removal proceeds via adsorption and hydrolysis within the pores of the carbon granule, leading to the formation of hydrogen chloride (HCl). HCl is subsequently neutralized by reactions with metal impregnants, such as copper oxide. Lodewyckx and Verhoeven8 reported chlorine breakthrough curves using activated carbon, activated carbon impregnated with copper chromate, and activated carbon impregnated with copper chromate plus triethylenediamine (TEDA). Results demonstrated that the addition of copper chromate or copper chromate plus TEDA to the formulation decreased the breakthrough time. The decreased breakthrough time was correlated to a decrease in the micropore volume. From these data, Lodewyckx and Verhoeven8 concluded that the removal of Cl2 was governed by physical adsorption within the micropores, followed by subsequent hydrolysis. In previous work, we reported the use of zirconium hydroxide (Zr(OH)4) as a reactive substrate for the removal of SO2 from streams of dry and humid air.5 SO2 was removed via reactions involving surface hydroxyl groups leading to the formation of sulfite (SO32
) species. Although Zr(OH)4 yielded a high capacity filtration medium, only a portion of the hydroxyl groups contributed to the removal of SO2. In a later study, we evaluated the use of TEDA-impregnated Zr(OH)4 to remove cyanogen chloride (ClCN) in filtration applications.6 The TEDA-impregnated Zr(OH)4 displayed a significantly greater capacity for the removal of ClCN than did a copper
TEDA-impregnated carbon.

Received: April 15, 2011 Accepted: December 21, 2011 Revised: July 25, 2011 Published: December 21, 2011 2675

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Industrial & Engineering Chemistry Research Table 1. Physical Properties of Zr(OH)4

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Table 2. Chlorine Gas Test Parameters

property

parameter

surface area

367 m2/g

particle size

12  30 mesh

pore volume

0.274 cm3/g

bed depth

2.0 cm

micropore volume

0.130 cm3/g

volume

26.4 cm3

air flow velocity

9.6 cm/s

density Zr(OH)4

0.97 g/cm3

temperature

25 °C

Zr(OH)4-6T

1.01 g/cm3

relative humidity

15%

TEDA promoted the hydrolysis of ClCN, with product HCl interacting with the hydroxyl groups associated with Zr(OH)4. In the present paper, we extend the Zr(OH)4 studies to include the removal of chlorine gases, namely Cl2, COCl2, and HCl. The removal of chlorine and phosgene from air streams presents a unique challenge in that both chemicals do not react directly with alkaline surfaces at ambient temperature. Rather, Cl2 and COCl2 must first undergo hydrolysis reactions prior to removal.7
11 This effort represents part of an ongoing effort aimed at developing high capacity filtration media to be employed in respirator applications. The objective of this effort is to determine the ability of Zr(OH)4, and Zr(OH)4 impregnated with TEDA, to remove Cl2, COCl2, and HCl from streams of air. An additional objective is to assess the role of the hydroxyl groups in the removal of the chlorine gases.

Figure 1. Cl2, COCl2, and HCl breakthrough curves recorded using Zr(OH)4.

2. EXPERIMENTAL SECTION 2.1. Materials. Zirconium hydroxide was purchased from

MEL Chemicals. Triethylenediamine (TEDA) was purchased from Aldrich. Cl2, COCl2, and HCl were obtained as neat chemicals in compressed gas cylinders. Zr(OH)4 was impregnated with TEDA by adding approximately 200 g of Zr(OH)4 powder (dried) into a V-blender (internal volume = 400 cm3) along with the desired quantity of TEDA. The V-blender was sealed, and the blender was placed into a forced convection oven at 60 °C and rotated for 2 h in order to sublime the TEDA into the pores of the Zr(OH)4. Following sublimation, the V-blender was removed from the oven and allowed to cool to room temperature prior to removing the contents. The TEDA-impregnated powder (referred to as Zr(OH)4-xT, where x is the weight percent TEDA loading) was pressed into 2.5 cm diameter wafers using a Carver press. The resulting wafers were crushed and sieved to 12  30 mesh granules. Table 1 reports the physical properties of the Zr(OH)4 media employed in this effort. 2.2. Materials Characterization. X-ray photoelectron spectra (XPS) were recorded using a Perkin-Elmer Phi 570 ESCA/SAM system employing Mg Kα X-rays. All binding energies are reported referenced to the C 1s photoelectron peak at 284.6 eV. When recording XPS spectra for media following exposure to the chlorine gas, the particles were crushed into a fine powder. In this manner, the internal pore structure (rather than the external surface of the granule) is exposed to the X-ray source, providing an analysis more consistent with the bulk sample. Sensitivity factors employed are those supplied by the instrument manufacturer. Nitrogen adsorption isotherms were recorded using a Micromeritics ASAP 2000 system. Data were used to determine the surface area and pore volume of the Zr(OH)4 powder. Off-gassing of the Zr(OH)4 powder was performed for approximately 16 h at 80 °C.

The lower than typical off-gas temperature was employed so as to not dehydrate the Zr(OH)4. 2.3. Breakthrough Testing. Details of a test apparatus employed in performing breakthrough tests have been reported previously.12 Cl2, COCl2, or HCl from a compressed gas cylinder was delivered to a humidified air stream using a calibrated mass flow controller. The humidified air stream was generated using a Miller-Nelsen HCS-401 system, which controlled the temperature and humidity of the process stream. The chlorine gas laden humidified process stream was delivered to a manifold using constant concentrations on an atomic chloride basis, meaning that the concentration of HCl was approximately twice that of Cl2 and COCl2. Test cells containing filtration media were located downstream of the manifold. Process gas from the manifold was drawn through each test cell using a vacuum source in conjunction with a calibrated mass flow controller. A carbon filter was located between the test cell and the mass flow controller for the purpose of removing any residual chlorine gas from the process stream prior to the mass flow controller. Breakthrough testing was performed by first storm filling each test cell with filtration media to a bed depth of 2.0 cm, equivalent to a bed volume of approximately 26.4 cm3. Details of the test cell have been reported elsewhere.12 Once loaded, test cells were returned to the test apparatus. Testing was initiated by generating the chlorine gas process stream. Once the feed concentration stabilized, a portion of the process stream was diverted to the test cell by switching an on/off valve to the flow-controlled vacuum. During testing, the effluent concentrations of Cl2 and HCl were measured using Interscan electrochemical voltametric sensors, and the COCl2 effluent was measured using a HP5890 Series 2 gas chromatograph equipped with an electron capture detector. Breakthrough test parameters are summarized in Table 2. 2676

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Table 3. Summary of Breakthrough Times material Zr(OH)4

Cl2 (min)

COCl2 (min)

HCl (min)

3

21

90

50

86

Zr(OH)4-3T

31

Zr(OH)4-6T

34

Figure 2. Cl2, COCl2, and HCl breakthrough curves recorded using Zr(OH)4-6T.

3. RESULTS AND DISCUSSION 3.1. Chemical Filtration. Figures 1 and 2 compare breakthrough curves for Cl2, COCl2, and HCl recorded using Zr(OH)4 and Zr(OH)4-6T particles, respectively. Data were recorded employing feed concentrations of 1400 ppm Cl2 and COCl2, and 2700 ppm HCl. The nearly twice higher feed concentration of HCl was selected so that breakthrough curves for the different chlorine gases could be recorded on a near-constant atomic chlorine basis. Testing with HCl was performed for the purpose of determining the total number of sites active toward acid gas removal of each material. When testing with Cl2, only Cl2 was detected in the effluent stream. No HCl was detected. The lack of HCl is attributed to the greater reactivity of HCl versus Cl2 with the Zr(OH)4 substrate. The effluent stream was not evaluated for the presence of HCl during COCl2 testing. Upon termination of the feed, the effluent concentrations of both HCl and Cl2 rapidly decrease to baseline levels. This result indicates that both HCl and Cl2 are removed by chemical reaction rather than by physical adsorption. If physical adsorption were contributing to the removal of either chemical, the decrease in the effluent concentration of chemical would have occurred over a much longer period of time. As results presented in Figure 1 demonstrate, the ability of Zr(OH)4 to remove Cl2 and COCl2 is significantly less than that of HCl, with Cl2 being the most difficult of the gases to remove. HCl is expected to react directly with Zr(OH)4 according to

ZrðOHÞ4 þ 2HCl f 3H2 O þ ZrOCl2

ð1Þ

yielding zirconium oxychloride as the final product. Note from eq 1 that two molecules of HCl results in the removal of three hydroxyl groups from the zirconium atom. Cl2 and COCl2 are not expected to react directly with the hydroxyl sites, but must first be hydrolyzed (to HCl),7
11 with product HCl being removed as described above. The decreased filtration capabilities of Zr(OH)4 versus Cl2 and COCl2 relative to HCl suggest that these gases are not readily hydrolyzed within the pores of Zr(OH)4. This result is not surprising, as hydrolysis reactions involving Cl2 and COCl2 in neutral water are relatively slow.9,11 The addition of TEDA to the filtration media greatly improved the performance versus Cl2 and COCl2, as illustrated by the data presented in Figure 2. One possible explanation for the improved performance is that Cl2 and COCl2 are able to react directly with TEDA. Another possible explanation is that TEDA is promoting

Figure 3. Effects of TEDA loading on the shape and position of Cl2 breakthrough curves.

the hydrolysis of Cl2 and COCl2, consistent with results reported by Peterson et al.6 for the removal of cyanogen chloride. In comparing results presented in Figures 1 and 2, the addition of TEDA to the formulation did not significantly affect the filtration of HCl. It is likely that there is some interaction between HCl and TEDA; however, the overall reactivity is minimal as compared to the removal capacity of the substrate. As we reported previously, TEDA will react with HCl to form the adduct complex.6 One possible explanation is that TEDA is physically blocking pores and/or reactive sites, thereby neutralizing its contributions to chemical reactivity. This behavior is supported by the HCl breakthrough curve corresponding to Zr(OH)4-6T (Figure 2), which exhibits a slightly shallower breakthrough curve than the TEDAfree sample. Breakthrough times (to 0.5 ppm for each chemical) are summarized in Table 3. Note that, in the cases of Cl2 and COCl2, the addition of TEDA to the formulation greatly increased the breakthrough time. Figure 3 illustrates the effects of the TEDA loading on the shape and position of Cl2 breakthrough curves. Data were recorded for the purpose of further assessing the role of TEDA in the removal of Cl2. Cl2 was selected for further evaluation because the addition of TEDA to the formulation appeared to have the greatest impact on filtration performance, increasing the breakthrough time (to 0.5 ppm) from 3 to 34 min (see Table 3). As results demonstrate, the addition of 3% TEDA to the formulation greatly improves the Cl2 removal efficiency of Zr(OH)4. Further increasing the TEDA loading to 6% only slightly increases the ability of the material to remove Cl2. Data indicate that TEDA may be directly reacting with Cl2. However, if a stoichiometric reaction between TEDA and Cl2 were occurring, one would expect that doubling the TEDA loading would nearly double the filtration performance of the material, which is not the case. Further, for Zr(OH)4 impregnated with 3% TEDA, the ratio of the moles of Cl2 removed per mole of TEDA (calculated at the time the test was terminated) is slightly greater than 2.5, which far exceeds the stoichiometric ratio of 1 (assuming each chlorine atom reacts with 2677

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Table 4. Summary of Atomic XPS Elemental Ratios for Samples Following Chlorine Gas Exposure sample

chemical

O/Zr

Cl/Zr

Zr(OH)4

none

3.55

0

Zr(OH)4-6T Zr(OH)4

Cl2 COCl2

3.14 2.94

0.15 0.28

Zr(OH)4-6T

COCl2

2.66

0.44

Zr(OH)4

HCl

2.55

0.88

Zr(OH)4-6T

HCl

2.40

0.88

each nitrogen atom associated with TEDA). Therefore, TEDA does not appear to be directly reacting with Cl2. An alternative explanation is that TEDA is facilitating a catalytic hydrolysis reaction. The ability of TEDA to facilitate the catalytic hydrolysis of cyanogen chloride has recently been reported,6 and it is believed that TEDA is acting in a similar manner here. The greater filtration performance of the TEDA-impregnated sample versus COCl2 is attributed to the enhanced hydrolysis activity. The likely cause of Cl2 and COCl2 breakthroughs occurring at significantly shorter times than HCl breakthrough is attributed to TEDA being poisoned by the hydrolysis product HCl. Breakthrough times corresponding to data presented in Figures 1
3 are presented in Table 3. Breakthrough times presented in Table 3 correspond to a breakthrough concentration of 0.5 ppm for each chemical. Although the Zr(OH)4 base medium yields an extended HCl breakthrough time, the breakthrough time is far less than that expected based on the reaction stoichiometry leading to the formation of zirconium oxychloride. In the case of the HCl challenge, the ratio of moles of HCl removed per mole of Zr upon termination of the test was 0.44. Theoretically, 1 mol of zirconium hydroxide reacts with 2 mol of HCl to yield zirconium oxychloride plus water. Thus, only 22% of the zirconium was able to contribute to the removal of HCl. Interestingly, in a study involving the filtration of SO2 over Zr(OH)4, Peterson et al.5 reported that only 21% of the zirconium is contributing to the removal mechanism, similar to results reported here for HCl. These results indicate that not all hydroxyls are able to contribute to the removal of acid gases. 3.2. Spectroscopic Analysis. XPS spectra were obtained for Zr(OH)4 and Zr(OH)4-6T samples following exposure to Cl2, COCl2, and HCl for the purpose of assessing changes in the surface composition and surface speciation of zirconia. Following completion of the breakthrough test, the filter bed was purged for approximately 20
30 min with air at 15% relative humidity. Samples were then removed from the inlet portion of the filter bed, as material from this location is expected to be fully saturated with reactant chemical. Once removed, samples were subjected to XPS analysis by grinding the particles into fine powders. Table 4 reports the quantitative XPS analysis for each sample. The Cl2-exposed Zr(OH)4 did not contain sufficient atomic chlorine to yield useful results. No change in either the shape or position of the Zr 3d photoelectron peak was observed for the exposed samples. Note from Table 4 that all samples display a decrease in the XPS atomic O/Zr ratio following exposure. The decrease in the atomic O/Zr ratio is accompanied by an increase in the Cl/Zr ratio. The extent of oxygen loss (and chlorine increase) is consistent with the order of removal: HCl was the most readily removed, followed by COCl2 and then Cl2. Results presented in Table 4 are consistent with a mechanism involving the removal of

hydroxyl groups leading to the formation of a zirconium chloride species. Zr(OH)4 is comprised of both bridging and terminal hydroxyl groups.13
15 Figure 4 compares the XPS spectra of the oxygen 1s photoelectron region for Zr(OH)4-6T prior to and following exposure to Cl2, COCl2, and HCl. For the unexposed Zr(OH)46T, two types of oxygen peaks are present, located at approximately 529.9 and 531.6 eV. The peak at 531.6 eV is assigned to terminal hydroxyl groups, while the peak at 529.9 eV is assigned to the bridging hydroxyl groups. These peak assignments are based on those of van den Brand et al.,16 who reported the presence of terminal hydroxyl groups associated with aluminum oxide. Results show that the intensity of the peak associated with the terminal hydroxyl group (531.6 eV) decreases following exposure to each chlorine gas. Following HCl exposure, virtually no trace of terminal hydroxyl groups is evident. As a note, there is an observed increase in the peak width of the oxygen 1s photoelectron peaks corresponding to the bridging hydroxyl group with an increase in the chlorine loading of the sample. This increase is attributed to the accumulation of chlorine on the sample. Table 5 summarizes the changes in the percent of terminal hydroxyls and the ratio of the bridging hydroxyls to zirconium (br-OH/Zr) as a function of the chemical exposure for Zr(OH)4 and Zr(OH)4-6T. Note from Table 5 that, in all cases, the brOH/Zr ratio has remained essentially constant at approximately 2.1
2.2, while the percentage of terminal hydroxyl groups has decreased, with the decrease becoming more significant as the removal capacity of the chlorine gas is increased. Results presented in Figure 4 and summarized in Table 5 suggest that only the terminal hydroxyl groups are able to contribute to the removal of chlorine gases. Evidently, despite the strong acidity of HCl, the bridging hydroxyl groups are very stable and do not contribute to the reaction. Figure 5 illustrates the relationship between the accumulation of chlorine (XPS atomic Cl/Zr ratio) and the hydroxyl groups (bridging and terminal) following chlorine gas exposure. Results demonstrate a linear decrease in the percentage of terminal hydroxyl groups with an increase in the chlorine accumulation on the material; however, the ratio of the bridging hydroxyl groups to zirconium (as determined by XPS analysis) remains essentially constant. This result further demonstrates that HCl is displacing the terminal hydroxyl groups, and that only the terminal hydroxyl groups participate in the removal of the chlorine gas. XPS spectra were recorded using the spent materials following chlorine gas exposure in an effort to assess the fate of TEDA. Samples were removed from the inlet portion of the filter bed. Figure 6 reports the N 1s photoelectron spectra for Zr(OH)4-6T samples following exposure to Cl2, COCl2, and HCl. A reference sample corresponding to TEDA titrated with HCl is shown for comparison. The reference sample was prepared by slurrying TEDA in deionized water and then titrating to pH 7 with a dilute HCl solution. As a note, approximately 1 mol of HCl was required to titrate 1 mol of TEDA to pH 7, suggesting that only one of the nitrogen atoms associated with TEDA was interacting with HCl, forming a TEDA 3 HCl adduct. This result is consistent with pKa values for the conjugate acids of TEDA. These values are 2.97 and 8.82, indicating that a lower pH would be required to protonate the second N atom (to form the second HCl adduct). Following titration, the water was evaporated and the solids were subjected to XPS analysis. The peak at 399.2 eV is assigned to the bare nitrogen atom, while the peak at 401.2 eV is assigned to 2678

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Figure 4. XPS spectra of O 1s photoelectron region for Zr(OH)4-6T prior to and following Cl2, COCl2, and HCl exposure showing bridging (529.9 eV) and terminal (531.6 eV) hydroxyl groups.

Table 5. Characterization of Terminal and Bridging Hydroxyl Groups as a Function of Chlorine Gas Removal sample

chemical

% terminal OH

br-OH/Zr

Zr(OH)4

none

36.2

2.14

Zr(OH)4-6T

Cl2

30.6

2.18

Zr(OH)4

COCl2

23.9

2.28

Zr(OH)4-6T Zr(OH)4

COCl2 HCl

18.2 3.8

2.02 2.22

Zr(OH)4-6T

HCl

2.7

2.17

nitrogen associated with the HCl adduct. These peak assignments are based on TEDA 3 2HCl and TEDA/Zr(OH)4 reference samples prepared in-house. XPS spectra corresponding to these reference samples are provided in the Supporting Information. As a note, XPS spectra of neat TEDA were not attempted, as TEDA will sublime in the vacuum of the XPS introduction chamber The N 1s photoelectron region corresponding to samples exposed to the three chlorine gases reveal the presence of two atomic nitrogen peaks at positions consistent with that of the reference sample. Results indicate that at least a portion of the

Figure 5. Relationship between accumulated chlorine and hydroxyl groups (bridging and terminal) remaining on the sample following exposure to chlorine gases.

TEDA associated with each sample has interacted with chlorine, with the sample exposed to HCl the most significantly chlorinated. 2679

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Figure 6. XPS spectra of N 1s photoelectron region for Zr(OH)4-6T following Cl2, COCl2, and HCl exposure. The spectra for TEDA titrated with HCl are shown for reference.

The spectroscopic data coupled with the breakthrough test results allow us to comment on the reaction mechanism and the role of TEDA in the removal of Cl2 and COCl2. TEDA promotes the catalytic hydrolysis of both Cl2 and COCl2, leading to the formation of HCl as described by eqs 2 and 3: TEDA

Cl2 þ H2 O sf 2HCl þ ð1=2ÞO2

TEDA

COCl2 þ H2 O sf 2HCl þ CO2

ð2Þ

ð3Þ

HCl reacts with terminal hydroxyl groups associated with Zr(OH)4, leading to the formation of zirconium oxychloride, as described previously. As the reaction involving the terminal hydroxyl groups becomes consumed, product HCl begins to interact with TEDA, and the reaction (hydrolysis) rate is slowed, leading to breakthrough of the parent compound. It is also possible that

the TEDA catalyzed hydrolysis reaction is pH sensitive. As the reaction proceeds, the chlorinated surface of Zr(OH)4 becomes increasingly acidic, potentially reducing the TEDA catalyzed hydrolysis rate.

4. CONCLUSIONS Zr(OH)4 impregnated with TEDA displays a high capacity for the removal of chlorine gases from streams of air. HCl is more readily removed by Zr(OH)4-based media than Cl2 and COCl2. This is attributed to HCl being able to react directly with Zr(OH)4, while Cl2 and COCl2 must first be hydrolyzed. The addition of TEDA to Zr(OH)4 promotes the catalytic hydrolysis of both Cl2 and COCl2, resulting in the formation of HCl, which interacts with terminal hydroxyl groups. Bridging hydroxyl groups associated with Zr(OH)4 do not contribute to the removal of chlorine gases. Although the removal of both Cl2 and COCl2 is catalytic, the reaction is halted due to a combination of TEDA being poisoned by product HCl and an increase in the acidity of the Zr(OH)4 surface. 2680

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’ ASSOCIATED CONTENT

bS

Supporting Information. XPS spectra of the N 1s photoelectron region for TEDA impregnated Zr(OH)4 and for TEDA 3 2HCl prepared by titration with 2 mol of HCl/mol of TEDA. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank the Joint Science and Technology Office for Chemical and Biological Defense (JSTO-CBD) for funding this work under Project No. BA07PRO104. ’ REFERENCES (1) Doughty, D. T.; Knebel, W. J.; Cobes, J. W., III. Chrome-Free Impregnated, Activated Universal Respirator Carbon for Adsorption of Toxic Gases and/or Vapors in Industrial Applications. U.S. Patent 5,492,882, 1996. (2) Smith, S. J.; Hern, J. A. Broad Spectrum Filter System for Filtering Contaminants from Air or Other Gases. U.S. Patent 6,344,071, 2002. (3) Rossin, J. A.; Morrison, R. W. Spectroscopic Analysis and Performance of an Experimental Copper/Zinc Impregnated, Activated Carbon. Carbon 1991, 29, 887. (4) Maxwell, A. H.; Rossin, J. A. Effects of Airborne Contaminant Exposure on the Physical Properties and Filtration Performance of Activated, Impregnated Carbon. Carbon 2010, 48, 2634. (5) 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. (6) 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. (7) Noyes, W. A. Military Problems with Aerosols and Nonpersistent Gases; NDRC Report, Division 10; U.S. Army: Washington, DC, 1946. (8) Lodewyckx, P.; Verhoeven, L. Using the Modified WheelerJonas Equation to Describe the Adsorption of Inorganic Molucules: Chlorine. Carbon 2003, 42, 1215. (9) Dyson, G. M. Phosgene. Chem. Rev. 1927, 4, 109. (10) Babad, H.; Zeller, A. G. The Chemistry of Phosgene. Chem. Rev. 1973, 73, 75. (11) Wang, T. X.; Margerum, D. W. Kinetics of Reversible Chlorine Hydrolysis: Temperature Dependence and General-Acid/Base-Assisted Mechanisms. Inorg. Chem. 1994, 33, 1050. (12) Peterson, G. W.; Karwacki, C. J.; Feaver, W. B.; Rossin, J. A. H-ZSM-5 for the Removal of Ethylene Oxide: Effects of Water on Filtration Performance. Ind. Eng. Chem. Res. 2008, 47, 185. (13) Southon, P. D.; Bartlett, J. R.; Wolfrey, J. L.; Ben-Nissan, B. Formation and Characterization of an Aqueous Zirconium Hydroxide Colloid. Chem. Mater. 2002, 14, 4313. (14) Guo, G. Y.; Chen, Y. L. New Zirconium Hydroxide. J. Mater. Sci. 2004, 39, 4039. (15) Glushkova, V. B.; Lapshin, A. N. Specific Features in the Behavior of Amorphous Zirconium Hydroxide: I. Sol-Gel Process in the Synthesis of Zirconia. Glass Phys. Chem. 2003, 29, 415. (16) van den Brand, J.; Sloof, W. G.; Terryn, H.; de Wit, J. H. W. Correlation between Hydroxyl Fraction and O/Al Atomic Ratio as Determined from XPS Spectra of Aluminum Oxide Layers. Surf. Interface Anal. 2004, 36, 81. 2681

dx.doi.org/10.1021/ie200809r |Ind. Eng. Chem. Res. 2012, 51, 2675–2681