Adsorption of Ammonia by Sulfuric Acid Treated Zirconium

(1) However, activated carbon shows a limited capacity for some adsorbates, ... has focused on the adsorption of acidic gases, such as sulfur dioxide...
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Adsorption of Ammonia by Sulfuric Acid Treated Zirconium Hydroxide T. Grant Glover,*,† Gregory W. Peterson,‡ Jared B. DeCoste,‡ and Matthew A. Browe‡ †

SAIC, P.O. Box 68, Gunpowder, Maryland 21010, United States Edgewood Chemiacal and Biological Center, 5183 Blackhawk Road, Aberdeen Proving Ground, Maryland 21010, United States

‡

ABSTRACT: The adsorption of ammonia on Zr(OH)4, as well as Zr(OH)4 treated with sulfuric acid, were examined. The results show that treating Zr(OH)4 with sulfuric acid leads to the formation of a sulfate on the surface of the material, and that the sulfate contributes to the ammonia adsorption capacity through the formation of an ammonium sulfates species. Calcination of Zr(OH)4 decreases the ammonia adsorption capacity of the material and limits the formation of sulfate species. NMR and FTIR spectroscopy results are presented that show the presence of two distinct ammonium species on the surface of the material. The adsorption capacity of the materials is shown to be a complex phenomenon that is impacted by the surface area, the sulfur content, and the pH of the material. The results illustrate that Zr(OH)4, which is known to adsorb acidic gases, can be modified and used to adsorb basic gases.

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INTRODUCTION A variety of inhalation hazards in both industrial and military situations require the use of filtration to ensure air for breathing meets appropriate health specifications, and many fundamental questions must be answered in order to develop an adsorbent material that can work as an effective air filtration material. In general, the material must be able to adsorb a broad spectrum of toxic chemicals including low volatility organics, such as pesticides and solvents, as well as filter smaller molecules such as chlorine, hydrogen cyanide, and cyanogen chloride.1 In addition, the adsorbent material must also be stable in ambient humidity, and readily formulated into an engineered pellet or granule.2−4 Activated carbon meets many of the criteria necessary to function as an air purification medium, and with impregnation of metals and organic catalysts, can filter a broad range of chemicals.1 However, activated carbon shows a limited capacity for some adsorbates, such as ammonia, and other traditional materials, such as zeolites, which have good capacity for ammonia in dry air streams, have effectively no capacity for ammonia in humid conditions.5 Ammonia has been identified as a particular safety and military concern by the U.S. Department of Defense, as well as the National Institute for Occupational Safety and Health (NIOSH), as a result of the toxicity of ammonia, and also the pervasive use of ammonia in industries including the manufacturing of nitric acid, explosives, synthetic fibers, and as a refrigerant.6−9 Therefore, significant effort has been invested recently to the development of novel porous materials for the adsorption of ammonia as well as other toxic gases, and a several materials have been investigated including metal−organic frameworks © 2012 American Chemical Society

(MOFs), graphitic nanostructures, MOF−graphite composites, and organosilicates.5,10−13 Additionally, studies have specifically focused on ammonia adsorption and have examined the adsorption of ammonia on templated silicas, covalent organic frameworks (COFs), graphite, graphite−polyoxometalate composites, MOF−graphite composites, carbons impregnated with transition metal chlorides, acid treated carbons, MOF− silica composites, as well as carbons modified with zirconium and other metals.14−28 Among the unique materials that have been examined as adsorbents for toxic gas adsorption, zirconium hydroxide is interesting because of the high density of hydroxyl groups and large surface area. However, historically, much of the work in the literature has focused on the development of zirconium oxide as a catalyst.29−32 In particular, sulfated zirconia has been shown as a solid acid catalyst for the isomerization of n-butane to isobutane at room temperature. The catalytic activity of sulfated zirconia at room temperature is noteworthy because other materials, such as zeolites, do not promote isomerization at room temperature.32 Additionally, works have identified that the properties of sulfated zirconia catalyst, such as surface area, sulfur content, and reactivity, depend heavily on the steps used to synthesize the material.29,32 The sensitivity of the material to these various conditions is not surprising given the structure of the material. For example, zirconium oxide materials can be composed of tetragonal, monoclinic, and amorphous phases, and it is not uncommon for commercial zirconium hydroxide to have two phases present in the solid.32−34 Additionally, the crystal phases of Received: May 24, 2012 Published: June 11, 2012 10478

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Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra of the materials before and after ammonia exposure were taken 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. All ATR-FTIR experiments on ammonia exposed samples were conducted on samples exposed to dry ammonia. Microbreakthrough Experiments. Ammonia adsorption experiments were conducted using a microbreakthrough system that has been documented previously.5 Briefly, analyte was injected into a ballast and subsequently pressurized; this chemical mixture was then mixed with an air stream containing the required moisture content to achieve a predetermined concentration. The completely mixed stream was then passed through a sorbent bed submerged in a temperaturecontrolled water bath. The sorbent bed was filled on a volumetric basis in a 4 mm internal diameter tube to a height of 4 mm resulting in approximately 29 mg of adsorbent material being used for each dry test and 28 mg for each humid test. The flow rate delivered to the adsorbent was 20 mL/min at 20 °C. The sample bed was constructed of glass so that the bed height could be measured. For humid test conditions, the samples were prehumidified in air at 80% relative humidity at 20 °C for 2 h. The dry air used in these experiments had a dew point of approximately −35 °C. The materials were stored in air and evaluated as prepared without regeneration. In all cases, the effluent stream then passed through a continuously operating HP5890 Series II Gas Chromatograph equipped with a photoionization detector. All of the data was plotted on a normalized time scale of minutes per gram of adsorbent. Breakthrough data were used to calculate the capacity to saturation using a mass balance as described previously.5 Nuclear Magnetic Resonance Experiments. 15N Magic angle spinning (MAS) NMR spectroscopy experiments were performed by exposing each Zr(OH)4 to 10% 15 N labeled ammonia (15NH3) under the same conditions as described earlier in the microbreakthrough section. All NMR experiments were conducted on samples exposed to dry ammonia. The 15NH3 exposed zirconium hydroxide sample was loaded into a 5 mm zirconia rotor with Kel-F caps. The selection of rotor materials was free of nitrogen to accurately identify 15N peaks associated with the sample. The packed sample was spun at 10 kHz to minimize broadening effects as well as eliminate any spinning side bands. Each 15N MAS NMR spectra were obtained at 60.811 MHz using a 7 μs pulse (90°), a relaxation delay of 1.000 s, followed by an acquisition time of 2.000 s and repeated for 7500 transients on a Varian 600 NMR spectrometer with a magnetic field of 14.1 T and equipped with a DOTY Scientific XC-5 5 mm VT-MAS NMR probe. All spectra were referenced to an external standard of NH4 Cl at δ = −352.9 ppm. For each sample, the sweep width was 60 kHz, exploring the entire range of known chemical shifts from approximately 500 to −450 ppm; however, only chemical shifts were observed in the region from −390 to −350 ppm. Potentiometric Titration. Potentiometric titrations were completed using a Metrohm Titrando 907 with Dosino 800 20 mL dosing units. Prior to titration, approximately 100 mg of sample was mixed with 50 mL of 0.01 M NaNO3 for approximately 5 h. Each titration solution was adjusted using HCl to a pH of 3 and titrated with NaOH until a pH of 10. The materials that were treated with sulfuric acid as the last step of material preparation, samples S and CS, did not require titration with HCl to a pH of 3 given the low pH of the sample when mixed in NaNO3 solution, 2.84 and 3.00, respectively. The initial pH of the Zr(OH)4, SC, and C samples when mixed in NaNO3 solution was 5.84, 3.53, and 7.79, respectively. The samples were titrated with 0.1 M NaOH, which was freshly prepared each time prior to a titration experiment in order to eliminate the influence of CO2 on the quality of titrant. The quality of the NaOH preparation was verified by titrating the NaOH base with a Sigma Aldrich 0.2071 M HCl standard. To prevent the influence of CO2 during the measurement, the titration was completed in a plastic enclosure under a continuous stream of Ultra High Purity helium to blanket the sample. The plastic enclosure

zirconia have also been shown to impact the incorporation of sulfur onto the zirconia surface. For example, incorporating sulfur into zirconium hydroxide followed by calcination is a more effective means of incorporating sulfur into the material than efforts to incorporate sulfur onto the surface of crystalline zirconia.31,35,36 In the hydroxide phase of the material, zirconium atoms can be bound to terminal hydroxyl groups or bridging hydroxyl groups, and, in general, the terminal hydroxyl groups are basic while the bridging hydroxyl groups are acidic. Also, because the nature of the hydroxyl groups impacts the reactivity of the material, techniques such as NMR and X-ray photoelectron spectroscopy (XPS) have been used to quantify each of these species.34,37,38 Although extensive work has been invested to understand the performance of zirconia as a catalyst, only limited work has been completed detailing the use of the material as an adsorbent. For example, Peterson et al. have investigated zirconia polymorphs as air purification adsorbents and have documented the ability of zirconia to adsorb sulfur dioxide, chlorine, and cyanogen chloride, all acid gases, from air.1,34,39,40 Given the acidic properties of sulfated zirconia, the surface chemistry is hypothesized to be well suited to adsorb basic gases such as ammonia. Additionally, the use of a sulfur to increase the ammonia adsorption capacity in porous materials has also been explored in traditional activated carbons. For example, Petit et al. have shown that treating activated carbon with sulfur-containing groups improves ammonia adsorption performance, which is consistent with the work of others in the literature that have also found that sulfuric acid treated carbons improve ammonia adsorption capacity.41−43 However, the use of sulfur treated zirconia as a gas phase ammonia adsorbent material has not been detailed. The work done previously on the catalytic properties of sulfated zirconia has focused on the crystalline metal oxide, and the work using zirconium hydroxide as an adsorbent has focused on the adsorption of acidic gases, such as sulfur dioxide. Therefore, the purpose of this work is to investigate the ability of zirconium hydroxide to remove ammonia, a basic gas, from air by treating the adsorbent with sulfuric acid.

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EXPERIMENTAL SECTION

Materials Synthesis. Amorphous zirconium hydroxide (Zr(OH)4), was purchased from Magnesium Electron Limited (MEL, Flemington, NJ). The average particle diameter of the material was 1 μm. Materials were prepared by treating 1.5 g of Zr(OH)4 in 10 mL of 3.0 N sulfuric acid overnight (Sigma Aldrich). The materials were collected by vacuum filtration and allowed to air-dry. Calcination was completed using a Lindberg Blue M tube furnace under an air flow of approximately 45 mL/min at an 18 °C/min ramp to 450 °C and then held at 450 °C for 3 h. In total, five samples were investigated: an untreated zirconium hydroxide designated Zr(OH)4, zirconium hydroxide treated with sulfuric acid designated S, a calcined zirconium hydroxide designated C, zirconium hydroxide calcined and then treated with sulfuric acid designated CS, and zirconium hydroxide treated with sulfuric acid and then calcined designated SC. Adsorption Isotherms. Nitrogen isotherms at 77 K were gathered using a Quantachrome Autosorb-1. The samples were outgassed under vacuum at 85 °C overnight prior to analysis. Thermal Gravimetric Analysis. Thermal gravimetric analysis (TGA) was performed using a TA Instruments Q500 Thermogravimetric Analyzer. Samples were heated in a platinum pan under a 20 mL/min flow of nitrogen at a 3 °C/min ramp to 900 °C. 10479

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was operated at approximately 1 in. water column of overpressure relative to the pressure of the laboratory, and helium was allowed to flow continuously through the plastic enclosure. The solution was blanketed in helium during both the titration and the equilibration with the inert NaNO3 electrolyte. XPS data verified that no nitrates are adsorbed by Zr(OH)4 after being stirred with NaNO3 for five hours. All titration experiments on ammonia exposed samples were conducted on samples exposed to dry ammonia.

material is calcined, more porosity is lost as the material condenses to a metal oxide. In addition to a BET structure analysis, FTIR data were gathered and are presented in Figure 2a,b. All of the materials treated with sulfuric acid show peaks between 1200 and 950 cm−1, which are typically associated with sulfur species.33,44−50 FTIR peaks identified at approximately 1200 cm−1, 1115 cm−1, 1050 cm−1, and 1000 cm−1 are consistent with a sulfate species on the surface.33 Although it is possible to identify peaks and shoulders in the FTIR spectra of the CS sample that are consistent with sulfur species, in general only a limited amount of sulfur is present on the calcined Zr(OH)4 sample surface when compared to sample S. Peaks near 1600 cm−1 as well as 3300 cm−1 indicate adsorbed water on the materials. The presence of a sulfate species on the surface of Zr(OH)4 is supported by previously conducted XPS experiments.1 Specifically, Peterson et al. exposed Zr(OH)4 to sulfuric acid and showed that the sulfur 2p XPS peak occurs at 169.5 eV, which is consistent with a sulfate species. TGA data were also gathered on each of the samples, as shown in Figure 3, and a significant weight loss for the pristine Zr(OH)4 is observed as water is desorbed as temperature is increased, which is consistent with data reported previously.34 The TGA data for the calcined samples show less weight loss prior to 150 °C. The materials treated with sulfuric acid, except sample CS, exhibit a weight loss between 600 and 700 °C consistent with zirconium sulfate decomposition.51 The CS material reflects limited zirconium sulfate decomposition, which is consistent with the FTIR data showing less sulfur present on the surface of this material. The exact route of sulfate binding on the surface of the Zr(OH)4 is complex. A variety of reaction schemes have been presented, and a full review of each scheme is available elsewhere.32 Briefly, models have been presented that show the sulfate as a bidentate ligand bound to the surface of Zr(OH)4 by one or two Zr atoms. It has also been proposed that the sulfate binds to three zirconium atoms via three of the sulfate oxygen atoms at low sulfate concentrations, and at higher sulfate concentrations, a polysulfate structure forms. The influence of water on the state of the sulfate binding has been considered, and the influence of hydroxyl groups next to bidentate sulfate ligands on the acidity of the Zr(OH)4 has been discussed. The dual Brønsted/Lewis site structures model of Clearfield et al., which show sulfates and hydroxyl groups bridging two Zr atoms, has been particularly helpful in qualitatively explaining the ability of sulfated Zr(OH)4 to catalyze reactions. In this model when sulfated Zr(OH)4 is calcined, Lewis acid sites are formed, which withdraw electrons from the bisulfate and weaken the SO−H bond. The weakened bond results in a highly acidic Brønsted site.32,52 To determine the ammonia adsorption capacity of these materials, fixed-bed microbreakthrough experiments were conducted. In general, adsorbent materials used in air filters will be utilized once and not regenerated after use. Although these materials may be applicable to regenerative adsorption systems, the results presented here will focus on single use without considering any regenerative applications. As shown by others, breakthrough testing is well suited to evaluate single use, or regenerative, novel adsorbent materials.10,13,48,53 For this work, all materials were stored in ambient air and evaluated without regeneration. The results, shown in Figure 4 and Table 1, illustrate that treating Zr(OH)4 with sulfuric acid

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RESULTS AND DISCUSSION After treating each material the porosity was measured using nitrogen adsorption at 77 K, as shown in Figure 1 and detailed

Figure 1. Nitrogen adsorption isotherms for the calcined and sulfuric acid-treated Zr(OH)4 samples. Open symbols reflect desorption, and closed symbols reflect adsorption.

Table 1. Ammonia Breakthrough Capacity for Zr(OH)4 Materials sample

BET area (m2/g)

initial pH

dry loading (mol/kg)

humid loading (mol/kg)

Zr(OH)4 C S CS SC

522 135 67 92 43

5.84 7.79 2.84 3.00 3.53

2.2 1.0 3.9 1.5 0.7

1.9 0.3 3.9 1.6 1.0

in Table 1. These results show that exposing Zr(OH)4 to sulfuric acid reduces the porosity of the material. Calcination also reduces the porosity of the material as hydroxyl groups are condensed and the material changes to zirconium oxide.34 When the Zr(OH)4 treated with sulfuric acid, sample S, and sample CS are compared, it is clear that calcining Zr(OH)4 before treating the material with sulfuric acid reduces the impact of the acid on the porosity of the material. The more limited impact of the acid on sample CS likely results from the decrease in reactive hydroxyl groups of the metal oxide versus the metal hydroxide. For sample SC, treating Zr(OH)4 with acid reduces the porosity of the hydroxide, and when the 10480

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Figure 2. FTIR spectra for Zr(OH)4, Zr(OH)4 treated with sulfuric acid (S), calcined Zr(OH)4 (C), Zr(OH)4 treated with sulfuric acid and then calcined (SC), and Zr(OH)4 calcined and then treated with sulfuric acid (CS) prior to ammonia exposure (a,b), and samples after exposure to ammonia (c,d).

improves ammonia adsorption performance. From the data it is clear that the sulfuric acid treated Zr(OH)4, sample S, shows a significant improvement in ammonia capacity, and calcining the material, as shown with sample C, decreases ammonia capacity compared to the pristine Zr(OH)4. Treating the calcined material with sulfuric acid, sample CS, improves the performance over sample C. However, treating the pristine Zr(OH)4 with sulfuric acid prior to calcination, shown with sample SC, does not improve the performance and decreases the capacity of the material as compared to the pristine Zr(OH)4. To investigate the mechanism of ammonia adsorption on the surface of the materials, 15N MAS NMR experiments were completed, and the results are shown in Figure 5. The results show the presence of two distinct species from the reactions of ammonia with peaks between δ = −380 and −370 ppm, which corresponds to the region where NH4+ salts typically appear.54 In particular, the dominant peak in the pristine Zr(OH)4 material occurs at δ = −376 ppm, and can be assigned to an ammonium ion.55

One possible scheme for this species is presented in Scheme 1, where ammonia has been bound by the formation of a surface ammonium salt via the removal of an acidic surface proton of Zr(OH)4. The formation of the ammonium species on the bridging group is supported by others that have identified bridging hydroxyl groups as Brønsted acid sites and terminal hydroxyl groups as Brønsted basic sites.38 Also, the NMR results show only ammonium species are present, which indicates that ammonia-zirconium complexes are not likely. The dominant peak in the spectra for sample S occurs at approximately δ = −371 ppm, and is in good agreement with literature values for an ammonium sulfate species.54 A combination of these two species can be seen on sample CS, and it is clear that the NH4+ salt species bound to the surface of the zirconium hydroxide makes the greatest contribution to the adsorption capacity. A minor contribution can be seen from the ammonium sulfate species, which is consistent with only a limited amount of sulfur present on the CS sample. However, for sample SC, the ammonium sulfate species makes the dominant contribution to the ammonia 10481

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Figure 3. TGA data for Zr(OH)4, Zr(OH)4 treated with sulfuric acid (S), calcined Zr(OH)4 (C), Zr(OH)4 treated with sulfuric acid and then calcined (SC), and Zr(OH)4 calcined and then treated with sulfuric acid (CS).

Figure 4. Breakthrough data for Zr(OH)4, Zr(OH)4 treated with sulfuric acid (S), calcined Zr(OH)4 (C), Zr(OH)4 treated with sulfuric acid and then calcined (SC), and Zr(OH)4 calcined and then treated with sulfuric acid (CS). Dry breakthrough is shown in panel a, and humid breakthrough data are shown in panel b.

regions. The samples that have not been exposed to sulfuric acid, Zr(OH)4 and sample C, show the presence of CO32− species adsorbed onto the surface with ν2 ≈1570 cm−1 and 1340 cm−1.56 Ammonia exposed Zr(OH)4 materials show differences in IR bands, especially between those that were treated with sulfuric acid and those that were not. The pristine Zr(OH)4 shows new IR bands at 3400 and 1100 cm−1 corresponding to the ν1 and ν4 bands of adsorbed ammonia respectively.57 These bands are not as pronounced in sample C, but are still present. Samples S, CS, and SC all show the distinctive IR bands of ammonium sulfate. Bands at 3220, 3060, and 2840 cm−1 all correspond to the ν3 mode of NH4+, while the band at 1430 cm−1 corresponds to the ν4 mode of NH4+.58,59 The sulfate region is much more convoluted and not only shows the presence of new bands but also shows the loss of

capacity. The material that has only been calcined and not treated with sulfuric acid, sample C, shows no ammonium sulfate species present on the material and the adsorption capacity can be attributed entirely to NH4+ salt formed with the Zr(OH)4 surface. FTIR data were also gathered on the samples after ammonia exposure as shown in Figure 2c,d. With respect to the IR spectral features, stark differences can be seen upon the introduction of ammonia to each sample. Each of the samples before exposure to ammonia shows a broad band corresponding to the bridging and terminal hydroxyl groups of Zr(OH)4, as well as adsorbed surface water, with ν2 ≈ 3370 cm−1 and a medium to strong band corresponding to the bending mode of molecular water with νO−H ≈1635 cm−1. Samples that have not been calcined, Zr(OH)4 and sample S, show a much larger content of hydroxyl groups and molecular water in these 10482

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samples both before and after ammonia adsorption as shown in Figure 6. The proton binding curves of the samples prior to ammonia adsorption, shown in Figure 6a, allow for the relative acidities of the samples to be identified. Specifically, as defined by the mass balance, the negative values reflect the presence of acidic sites, whereas the positive values reflect basic sites.62 The Zr(OH)4 and the calcined Zr(OH)4 samples show that both acid and basic sites are present on the material as the binding curve has both positive and negative values. The materials treated with sulfuric acid are dominated by acidic sites. When the proton binding curves are evaluated using the SAIEUS method, distinct acid sites are presented as peaks at different values of pKa as shown in Figure 6b.65 For the pristine Zr(OH)4, the results show peaks that are consistent with previous work, although some variation is expected given differences in the Zr(OH)4 preparation and method of titration.66 The calcined material, sample C, shows less adsorption sites, as is expected for a metal oxide compared to a metal hydroxide. The Zr(OH)4 sample treated with sulfuric acid, sample S, shows an increase in the number of acidic sites on the surface of the material. Sample CS and SC each show varying degrees of surface acidity between the pristine Zr(OH)4 and sample S. To verify that the sulfate remains on the surface of the sulfuric acid treated samples during the titration by washing a portion of sample S 10 times in water and then placing it in excess water overnight. The sample was collected and dried under vacuum, without heat, and the FTIR spectra was gathered. The washed sample showed no differences in the sulfate region of the spectra when compared to unwashed sample S, which indicates that, although sulfates are soluble in water, the sulfate is retained on the surface of the adsorbent during the titration experiment. Repeating the titration experiments on the materials exposed to ammonia, as shown in Figure 6c,d, produces proton binding curves showing more weakly acidic species present on the adsorbents. The increase of weakly acidic species is expected given the adsorbed ammonium species identified with the NMR data. The proton binding curves show the most dramatic change for the Zr(OH)4 sample treated with sulfuric acid, sample S, which shows acidic sites being neutralized and significantly more weakly acidic species present on the material. The pristine Zr(OH)4 sample also shows an increase in the number of weakly acidic species on the surface after adsorption of ammonia. Consistent with the low ammonia loading of the calcined Zr(OH)4 sample, only minimal changes are seen in the proton binding curve of sample C. The distribution of acid sites from the titration data for the materials exposed to ammonia, shown in Figure 6d, indicate the presence of a new weakly acidic species at a pKa near 9.2, which is consistent with NH4+ species.17,26,41 The presence of the ammonium species is consistent with the NMR data. The species at pKa of 9.2 is most prevalent on the sulfuric acid treated sample, but is also present on all the materials treated with sulfuric acid, specifically samples CS and SC. In addition, it is possible to clearly identify that the peak at 9.2 pKa was not previously present on the CS or SC samples, which identifies a new species on the surface of the material after adsorption. In general, several of the acidic sites that are detailed in the titration data for the sulfur acid treated Zr(OH) 4 samples show changes in intensity and width upon adsorption of ammonia.

Figure 5. 15N MAS NMR of Zr(OH)4 materials including calcined Zr(OH)4 treated with sulfuric acid(S), Zr(OH)4 (C), Zr(OH)4 treated with sulfuric acid and then calcined (SC), and Zr(OH)4 calcined and then treated with sulfuric acid (CS).

Scheme 1. One Possible Mechanism for the Adsorption of Ammonia on Zr(OH)4

certain bands, when compared to the materials not exposed to ammonia. Sample S shows a significant decrease in intensity for the band at 1210 cm−1, while little other information can be extracted from the masked sulfate region. Sample SC is also relatively unperturbed in the sulfate region. The most evident differences in the sulfate region come from sample CS with the disappearance of bands at 1230 and 965 cm−1, and the appearance of a shoulder at 1210 cm−1 and a strong band at 1110 cm−1. The strong band at 1110 cm−1 corresponds to the ν3 modes of SO42−.58 These results coupled with the 15N MAS NMR data show evidence of an ammonium sulfate species and adsorbed ammonia/ammonium ion species on the Zr(OH)4 samples. X-ray diffraction patterns were gathered on the samples both before and after exposure to ammonia, and no changes in the diffraction patterns were observed after ammonia exposure. The temperature dependence of the diffraction patterns has been shown elsewhere.34 Given the important role of pH in the performance of these materials, potentiometric titration was used to provide insight into the role of surface acidity and the adsorption of ammonia. This technique has been used extensively by others as a means to understand the surface of porous materials and ammonia adsorption.17,19,20,27,60−64 In this approach, protons are assumed to adsorb on specific sites on the adsorbent, and via a mass balance and the assumption of a Langmuir style of proton loading, the loading of protons on each of these sites can be determined.60,61 Titration data was gathered on the 10483

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Figure 6. Potentiometric titration data for various Zr(OH)4 samples both before (a,b) and after (c,d) exposure to ammonia.

when compared to other adsorbent materials, such as MOFs, which may degrade upon adsorption of ambient water.4,67 Several factors contribute to the complex breakthrough results, such as the pH of the material after acid treatment, the surface area, and the amount of sulfur species on the surface of the materials. As shown in Table 1, a decreased pH contributes to the ammonia adsorption capacity, but does not directly correlate to the ammonia adsorption capacity results. For example, the pristine Zr(OH)4 has the highest pH but exhibits the second highest ammonia capacity. Additionally, the presence or absence of sulfur species does not completely correlate the results. For example, by examining the FTIR data, it is clear that more sulfur species are present on sample SC than sample CS. However, the sample CS has a much higher ammonia capacity than sample SC. Also, the pristine Zr(OH)4 has no sulfur species present and maintains a higher ammonia capacity than all of the treated materials, except sample S. Compared to the pristine Zr(OH)4 material, the sulfuric acid treated material, sample S, has nearly twice the loading of ammonia even though the surface area of sample S is 8 times lower than the pristine Zr(OH)4. As with many physisorption

To examine the impact of humidity on these samples, breakthrough experiments were also conducted at 80% relative humidity at 25 °C. The results, designated as humid loading and shown in Table 1 and Figure 4b, show that when the materials are stored at ambient conditions, additional humidity in the adsorbate gas stream has little impact on the capacity of these materials. The humid breakthrough tests show that the samples contain preadsorbed water such that preconditioning the samples in a humid air stream did not increase the water content in the adsorbent. The amount of water preadsorbed on the samples is quantified with the TGA results, which show the pristine Zr(OH)4 sample showing a 30% loss in sample weight as water is desorbed. These results are important when considering potential applications of these materials as adsorbents for ammonia. Specifically, the results show that the adsorbent material can be stored in air without degradation of performance and that additional humidity in the adsorbate gas stream will not cause a large variance in total ammonia adsorption capacity. The ability to store these materials in ambient conditions is noteworthy 10484

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events, the surface area impacts total capacity by providing more adsorption surface for ammonia loading, as well as more sites for sulfur incorporation. Many of these trends can be explained by considering the impact of each treatment on the adsorption surface of the Zr(OH)4. For example, when the materials are treated with sulfuric acid, the sulfate on the surface of the material provides an adsorption site for ammonia, and similar to sulfur treated carbons, ammonia is removed via the formation of an ammonium sulfate at the adsorption site.1,41 Although the acid treatment increases the capacity by providing more sites for chemisorption, the physisorption capacity of the material is decreased as a result of a decrease in surface area. Calcining the material significantly impacts the ammonia adsorption capacity as the material transitions to a zirconium oxide.34 The decrease in surface area limits the physical adsorption capacity of the material, as well as prevents significant sulfur loading. There is also a loss of acidic bridging hydroxyl groups upon calcination, which likely impacts ammonia capacity. The results presented in Table 1 imply that an optimum of these parameters likely exists that can provide a large surface area for adsorption, a low pH, and a high sulfur content.

REFERENCES

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CONCLUSION The adsorption of ammonia by Zr(OH)4 can be significantly improved by treatment with sulfuric acid. Additionally, calcination of the material decreases the ammonia adsorption capacity and decreases the effectiveness of sulfuric acid treatments to increase ammonia adsorption. The treatment of Zr(OH)4 with sulfuric acid leads to the formation of a sulfate on the surface that contributes to the adsorption of ammonia through the formation of an ammonium sulfate species as identified by NMR. The pristine Zr(OH)4 also provides a mechanism for the adsorption of ammonia without sulfur through the formation of an ammonium species with the zirconium hydroxide surface. The adsorption capacity is complex and is a function of the pH and the surface area of the material, as well as the amount of sulfate present on the adsorption surface. The results show that Zr(OH)4 is an tailorable substrate that can be modified to adsorb basic gases and may be well suited as an adsorbent in respiratory or industrial filters.

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AUTHOR INFORMATION

Corresponding Author

*Address: SAIC, Gunpowder, MD 21010. Tel.: (410) 4369408. FAX: (410) 436-3764. e-mail: thomas.g.glover6.ctr@mail. mil. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Defense Threat Reduction Agency for the support of this research under project BA07PRO104. This research was performed while J.B.D. held a National Research Council Research Associateship Award at the Edgewood Chemical Biological Center. We gratefully acknowledge the assistance of Professor Teresa Bandosz at the City College of New York for guidance in executing the potentiometric titration experiments. 10485

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