ARTICLE pubs.acs.org/JPCC
Surface Chemistry and Morphology of Zirconia Polymorphs and the Influence on Sulfur Dioxide Removal Gregory W. Peterson,*,† Joseph A. Rossin,‡ Christopher J. Karwacki,† and T. Grant Glover§ †
Edgewood Chemical Biological Center, 5183 Blackhawk Rd., APG, Maryland 21010-5424, United States Guild Associates, Inc., 5750 Shier Rings Road, Dublin, Ohio 43016, United States § SAIC, Inc. P.O. Box 68, Gunpowder, Maryland 21010-0068, United States ‡
ABSTRACT: Zirconium hydroxide was calcined at discrete temperatures up to 900 °C to study the effect of thermal treatment on the structure and surface chemistry related to the filtration of SO2. As-received and calcined materials were characterized using multiple techniques that included thermal gravimetric analysis, X-ray diffraction, and nitrogen porosity to determine changes to the zirconium hydroxide structure. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy were used to characterize changes in surface hydroxyl groups. Changes in the morphology and surface chemistry were correlated to sulfur dioxide filtration through chemical breakthrough studies. It was found that as calcination temperature increased, materials become more crystalline, which led to the loss of terminal hydroxyl groups and a decrease in the efficacy for sulfur dioxide removal.
’ INTRODUCTION Zirconium hydroxide is an important precursor that is used in the synthesis of zirconium dioxide, a material widely used in the ceramics industry. Zirconium hydroxides are also used as catalyst supports and/or additives to catalytic materials, owing to its highly hydroxylated surface.1,2 Zirconium hydroxide is an amorphous material that, when heated, becomes crystalline through the formation of oxygen bridges. Yet, the hydroxide material itself is highly porous, with a specific surface area of several hundred square meters per gram. The porosity and hydroxyl groups associated with zirconium hydroxide result in highly reactive sites for sorption.3 The combination of porosity and surface functionality has led to its possible use in gas sorption based technologies, including respirators for the removal of toxic gases.3,4 Several studies have documented the effect of calcination temperature on the morphology and surface chemistry of zirconia.58 However, it is also well-known that the porosity and surface functionality depend greatly on the initial precursor used as well as the treatment conditions.9,10 For example, Davies and co-workers examined the activity and structural differences of zirconia using zirconium oxychloride, ZrOCl2, and zirconium n-propoxide, Zr(OC3H7)4.10 They found that the subsequent precipitated hydroxide from the oxychloride contained more hydroxyl groups than that precipitated from the alkoxide and in addition contained Lewis acid sites. Hertl investigated the surface chemistry of amorphous, monoclinic, and tetragonal phases of zirconia polymorphs and determined that the crystalline phases reacted with alcohol to form alkoxide species, whereas the amorphous material did not.11 Guo and co-workers precipitated zirconium hydroxide from zirconium oxychloride at different pH levels and found that the structural and thermal properties varied greatly from hydroxides precipitated under basic conditions.12,13 Bachiller-Baeza et al. determined that the chemistry of carbon r 2011 American Chemical Society
dioxide sorption on zirconia polymorphs changes based on the crystal phase, indicating that this behavior may be typical for a wider range of chemicals.6 Furthermore, Ouyang and co-workers determined that terminal hydroxyl groups are a crucial component of the substrate for interaction with chemicals such as pyridine, carbon dioxide, and formic acid.7 This indicates that materials containing less crystallinity, and therefore more terminal hydroxyl groups, may be preferred for reactive sorption of toxic chemicals. Sulfur dioxide is employed in a number of chemical manufacturing processes, and thus large amounts are used, manufactured, and transported on an annual basis. As a result, sulfur dioxide has recently been identified as a high priority toxic industrial chemical for inhalation and ocular hazards,14 and its removal is a requirement for several air purification filters, including those certified by NIOSH. Previous investigations into the removal properties of sulfur dioxide by zirconium hydroxide indicate that the chemical interacts with hydroxyl groups to form a sulfite species,3 a behavior supported by work conducted on other metal oxyhydroxides.15,16 In our previous paper, we identified zirconium hydroxide as a reactive substrate with a high capacity for the removal of SO2.3 Zirconium hydroxide is comprised of both bridging and terminal hydroxide groups;9 yet, despite the high capacity relative to carbonbased media, only a fraction (∼10%) of the hydroxyl groups contributed to the removal of SO2. The objective of this study was to evaluate the effects of the hydroxyl groups, namely terminal and bridging, on the SO2 filtration capability of Zr(OH)4. The fraction of terminal hydroxyl groups and the nature of the surface were varied via thermal treatment at temperatures to 900 °C. The Received: February 4, 2011 Revised: April 11, 2011 Published: April 22, 2011 9644
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Figure 1. TGA curves for as-received zirconium hydroxide.
porosity of the material following thermal treatment is determined via nitrogen adsorption, and the hydroxyl species on the surface are quantified using XPS techniques. Resulting sulfur species are further investigated using XPS to determine the effect of surface functionality on sulfur dioxide removal.
’ EXPERIMENTAL SECTION Materials. Zirconium hydroxide powder was purchased from Magnesium Electron Limited (MEL) and had a mean particle diameter of ∼1 μm.17 The powder was heated statically in air at discrete temperatures of 150, 300, 500, and 900 °C at a rate of 18 °C/min in a Fischer Scientific Isotemp programmable forced draft furnace. The final temperature was maintained for 3 h. Following treatment, the material was cooled to room temperature and then placed in a glass jar. The resulting materials are denoted as Zr(OH)4, Z-150, Z-300, Z-500, and Z-900. Media were prepared as 12 30 mesh particles by pressing the powder into a wafer using a carver press, with the wafers crushed and sieved to yield 12 30 mesh particles. X-ray Diffraction. X-ray diffraction patterns were obtained using a Philips diffractometer with XCelerator detector, Cu KR radiation, a scan range of 20°120° 2Θ, and a spinning sample stage. Samples were placed on zero background sample slides. Nitrogen Adsorption. Samples were evaluated for nitrogen adsorption using a Quantachrome Autosorb-1 instrument. Samples were off-gassed at 85 °C for ∼16 h, achieving a final vacuum of less than 10 μmHg. The lower than traditional off-gassing temperature was employed so as not to dehydroxylate the surface. The surface area was calculated using the BET method, and the pore size distribution was calculated using a non local density functional theory (NLDFT) method. Thermal Gravimetric Analysis. Thermal gravimetric analyses were conducted on the baseline zirconium hydroxide material to determine appropriate calcination temperatures corresponding to changes in the structure. The TGA experiment was run using a TA Instruments TGA Q500 instrument and at a rate of 2 °C/min from 25 to 900 °C.
Figure 2. XRD patterns for as-received and calcined zirconium hydroxide samples. Peaks corresponding to tetragonal phase zirconia are denoted with a “T”. All other peaks are monoclinic phase.
Fourier Transform Infrared Spectroscopy. Spectra were collected using a Nicolet 6700 (Thermo Electron Corp.) FTIR in attenuated total reflectance mode. A diamond crystal was used with baseline correction. Samples were off-gassed at 80 °C for 4 h before analysis to minimize bulk water content. Peaks were assigned using a second derivative algorithm. X-ray Photoelectron Spectroscopy. XPS analyses were performed using a Perkin-Elmer Phi 570 ESCA/SAM system. All binding energies were referenced to the C 1s photoelectron peak at 284.6 eV. XPS analyses of sulfur dioxide exposed samples employed material removed from the entrance region of the filter bed. Breakthrough Testing. Sulfur dioxide breakthrough curves were recorded by storm filling 12 30 mesh granules of filtration media into a 4.1 cm inside diameter jacketed glass tube. All tests were performed employing a 2.0 cm deep bed of filtration material, with testing performed at 25 °C, 15% RH employing a feed concentration of 2000 mg/m3 (765 ppm) of SO2. Details of the testing methodology have been reported elsewhere.3
’ DISCUSSION Thermal Gravimetric Analysis. Figure 1 illustrates the TGA curve for the as-received Zr(OH)4 material obtained from MEL. The TGA curve indicates a total mass loss of ∼33%, which corresponds well to previous studies.18 This initial mass loss is attributed to bulk water offgassing at temperatures below 75 °C. At ∼100 °C, the differential mass loss curve indicates another weight loss, likely from more strongly bound surface or coordinated water species. Between 400 and ∼450 °C, two slighter weight losses are observed and are attributed to bridging and terminal hydroxyl groups forming oxide bonds during crystallization to zirconium dioxide. Following TGA, zirconium oxyhydroxide samples were calcined at temperatures corresponding to changes in the material. The first material was calcined at 150 °C such that bulk and some 9645
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Figure 3. (a) Nitrogen isotherms, (b) NLDFT (silica cylindrical pore kernel) pore size distributions, and (c) changes in porosity and surface area for samples studied. Samples calcined up to 300 °C show significant nitrogen uptake and are highly microporous. Above the crystallization temperature, samples lose almost all microporosity.
coordinated water were removed. The next material, Z-300, was calcined at 300 °C to produce a material without coordinated water, but not yet crystallized. Another material was calcined at 500 °C, a slightly higher temperature than where crystallization occurs. The final material was calcined at 900 °C to produce a highly crystalline polymorph (vide infra). Structure and Surface Analysis. Figure 2 compares the XRD patterns of the five materials studied. Note that the as-received Zr(OH)4 material as well as the Z-150 and Z-300 samples are relatively amorphous, with very broad peaks at approximately 30° and 50°60° 2Θ. As the material is calcined at higher temperatures, crystallization begins to take place, with the Z-900 species exhibiting the highest degree of crystallinity. The Z-500 sample
exhibit a mixture of tetragonal and monoclinic crystal phases, with peaks at approximately 30°, 35°, 50°, and 60° 2Θ, corresponding to the tetragonal phase. The Z-900 material in particular is highly monoclinic but is still comprised of some tetragonal phase zirconia. Nitrogen isotherm data were collected to determine the changes in porosity due to calcination. Isotherm data and pore size distributions are shown in Figure 3, and BET surface area and porosity measurements are summarized in Table 1. The asreceived Zr(OH)4 shows significant nitrogen uptake, as does the sample calcined at 150 °C. In fact, the Z-150 sample has a slightly higher surface area and pore volume than the baseline material. This behavior is likely due to the removal of coordinated water molecules, allowing for more nitrogen to adsorb on the surface. 9646
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Table 1. Surface Area and Pore Volumes for Samples Studied material
SBET (m2/g)
VMicro,DR (cm3/g)
VTotal (cm3/g)
% micropores
DP,1/2 (Å)
Zr(OH)4
533
0.19
0.54
34.4
20.3
Z-150
537
0.19
0.63
29.4
23.4
Z-300
314
0.11
0.42
25.4
26.8
Z-500
66
0.03
0.25
10.8
73.8
Z-900
7
0.00
0.07
3.9
197.0
Table 2. Sulfur Dioxide Removal Capacities Calculated for Baseline and Calcined Samples
Figure 4. ATR spectra of as-received Zr(OH)4 and calcined samples.
Figure 5. Sulfur dioxide breakthrough. Breakthrough testing was conducted on 1.5 cm beds of 12 30 mesh samples at a challenge concentration of 2000 mg/m3, airflow velocity of 9.6 cm/s, and a relative humidity of 15% referenced to 25 °C.
material
Pparticle (g/cm3)
mg SO2/g material
Zr(OH)4
0.78
110
Z-150
0.80
89
Z-300
0.97
48
Z-500
1.10
13
Z-900
1.38
2
Yet the average pore diameter increases in the Z-150 material compared to the as-received Zr(OH)4, indicating an increase in meso- or macroporosity. The Z-300 sample has a significantly lower nitrogen uptake but still exhibits extensive microporosity. Those samples calcined above the crystallization temperature display almost no microporosity, and the surface area and porosity decrease considerably as compared to material treated at lower temperatures. The surface area of the Z-500 sample decreases by ∼90% compared to the as-received Zr(OH)4, while the surface area of the Z-900 sample decreases by 99%, resulting in an essentially nonporous media. Figure 4 reports the FTIR spectra obtained in attenuated total reflectance (ATR) mode for Zr(OH)4 samples calcined at discrete temperatures. Above 2500 cm1, all information pertaining to the hydroxylated surface of the materials is masked by atmospheric moisture and is therefore not shown. However, much information has been obtained at wavenumbers less than 2000 cm1. In particular, Guo and co-workers have shown that carbon dioxide (CO2) provides valuable information on the zirconia polymorphic structure through interactions with various surface species. The peak at ∼1630 cm1 is indicative of either the bending mode of coordinated water species or CO2 interactions with terminal hydroxyl groups, forming a bicarbonate species on the surface.13,19 Note that the relative intensity of this peak is reduced as the calcination temperature increases. Similarly, the peak centered at ∼1331 cm1 is evidence of “sideon” coordination of CO2 to unsaturated Zr4þ sites.13 Dodson and McQuillan also assign the peaks centered around 1550 and 850 cm1 to surface carbonate species.19 In both cases, the peaks almost completely disappear for the Z-900 sample, indicating that those surface functionalities are no longer present on the material. Guo attributes the peak at ∼1000 cm1 to ZrdO species and is only present in the crystalline Z-500 and Z-900 samples. Similarly, the peak at ∼740 cm1 must also be related to crystalline ZrO species, as they are only present in the samples that were calcined above the crystallization temperature. Sulfur Dioxide Removal. Figure 5 compares the sulfur dioxide breakthrough behavior of the as-received Zr(OH)4 and calcined zirconium polymorph samples, and Table 2 summarizes the sulfur dioxide removal capacity of each material. Upon feed 9647
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Figure 6. XPS oxygen 1s spectra with deconvolution for (a) as-received Zr(OH)4, (b) Z-150, (c) Z-300, (d) Z-500, and (e) Z-900. The first four samples show two peaks corresponding to bridging and terminal hydroxyl groups. A third peak is present in the Z-900 sample and is attributed to the presence of ZrdO bonds.
termination, the sulfur dioxide effluent concentration decreases almost immediately to the baseline for all samples, indicating any sulfur dioxide removed by the materials is strongly retained. In particular, the as-received Zr(OH)4 takes the longest time to reach saturation, indicating that the as-received material retains the most sulfur dioxide as compared to the calcined samples. As the calcination temperature increases, the resulting sulfur dioxide time to saturation reduces and the overall removal capacity decreases accordingly. Of particular note is the difference between the as-received Zr(OH)4 and Z-150 materials. Nitrogen isotherm data indicate that the Z-150 material actually has a higher surface area than the as-received Zr(OH)4, yet the asreceived Zr(OH)4 provides greater sulfur dioxide removal. One explanation is that the adsorbed moisture in the baseline material may enhance the sulfur dioxide removal capabilities. Another explanation is that the surface chemistry of the material changes even with the relatively mild temperature treatment. Clearly, the higher temperatures have a more dramatic effect on sulfur dioxide removal, likely due changes in surface chemistry accompanying the reduced porosity and surface area. To elucidate the cause of reduced sulfur dioxide removal capacity with increased calcination temperature, the oxygen 1s region of the XPS spectra was analyzed. Figure 6 illustrates the oxygen 1s photoelectron region for each material following thermal treatment.
For material treated at temperatures up to 300 °C, two oxygen photoelectron peaks are evident. The peak at ∼529.7 eV is attributed to bridging oxygen and/or bridging hydroxyl groups. The peak present at approximately 531.4531.6 eV is attributed to the presence of terminal hydroxyl groups. These assignments are supported by XPS data collected on other metal oxides/ hydroxides.20,21 Increasing the calcination temperature to 500 and 900 °C shifts the position of the terminal hydroxyl groups to a greater binding energy. This shift is attributed to a difference in the structure of the polymorph (amorphous versus crystalline as per XRD patterns illustrated in Figure 2. Although most of the surface terminal hydroxyls have converted to bridging groups at these higher temperatures, it is possible that there are still terminal groups within the pore structure of the material.8 Furthermore, studies on other crystalline metal oxides have shown that terminal hydroxyl groups exhibit binding energies around 532.4.20 Following treatment at 900 °C, a third oxygen peak is present at at ∼527.3 eV. This peak is attributed to the formation of ZrdO species and is supported by the FTIR spectra (Figure 3), which show a peak forming at ∼1000 cm1, which is attributed to the ZrdO species.13 Table 3 summarizes the relative amounts of terminal and bridging oxygen species as well as the XPS atomic oxygen/zirconium ratio. As the calcination temperature increases to 300 °C, the fraction of terminal hydroxyl species decrease in favor of bridging species. This 9648
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Table 3. Various Oxide and Hydroxide Species on Zirconia Polymorphs Studied
a
material
% terminal OH
% bridging oxygena
O/Zr
S/Zr
Zr(OH)4
36.1
63.9
3.28
0.09
Z-150 Z-300
26.2 22.8
73.8 77.2
2.94 2.97
0.11 0.04
Z-500
8.7
91.3
2.68
0.01
Z-900
8.3
85.5
2.58
0.00
Refers to either bridging hydroxyl or oxide.
Figure 8. Sulfur dioxide capacity as a function of percent terminal hydroxyl groups. The capacity increases linearly with increasing terminal hydroxyl groups.
Results suggest that the removal of SO2 over zirconia-based media is due to an interaction with terminal hydroxyl groups. From Table 3, as the percentage of terminal hydroxyl groups decreases, the capacity to remove SO2 is greatly reduced (Table 2). Figure 8 illustrates the linear relationship. Although material calcined at 500 °C and above still retains small fraction of terminal hydroxyl groups, decrease in porosity reduces the accessibility.
Figure 7. Sulfur 2p photoelectron spectra. A clear peak centered around 167.1 eV is seen for the Zr(OH)4 and Z-150 materials. As calcination temperature increases, the peak disappears, indicating sulfur species are not deposited on the surface after SO2 exposure.
is not surprising as XRD data (Figure 2) illustrate the formation of crystalline species in the Z-500 and Z-900 samples, which take place through the formation of oxide bridges from terminal hydroxyl groups. In fact, the Z-500 and Z-900 samples are comprised of almost all bridging oxygen species, present as either oxides or hydroxides. Temperature-programmed desorption also indicated that three different sites may be responsible for sulfur dioxide removal.3 To determine if the same chemistry occurs in the calcined samples, sulfur 2p XPS data were collected for all samples. Figure 7 illustrates the XPS spectra of the sulfur 2p photoelectron region, and sulfur/zirconium ratios are summarized in Table 3. The as-received and Z-150 materials both exhibit significant peaks centered on 167.1 eV, indicating the presence of sulfite.3 The Z-300 material also shows a slight hump in that vicinity, but in general, the amount of sulfur strongly bound to the surface decreases with increasing calcination temperature. This is not surprising as the breakthrough curves show a significant decrease in capacity with increasing calcination temperature.
’ CONCLUSIONS The effects of calcination on the structure and surface chemistry of zirconium hydroxide have been investigated. Results indicate that, as the structure is heated, surface hydroxyl groups are transformed into oxide bridges, resulting in the formation of crystallites with reduced porosity. An initial decrease in sulfur dioxide filtration performance from the asreceived material to the Z-150 material results from a decrease in terminal hydroxyl groups. Further increasing the calcination temperature to 300 °C does not significantly reduce the fraction of terminal hydroxyl groups, yet the capacity decreases by ∼50% due to the associated loss in surface area. At 500 °C, all accessible terminal hydroxyl groups have been removed, and the sulfur dioxide capacity is reduced to 10% of the original material. The material calcined at 900 °C has lost all accessible terminal groups and porosity, resulting in immediate sulfur dioxide breakthrough. ’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected]. Phone: (410) 436-9794. Fax: (410) 436-5513.
’ ACKNOWLEDGMENT We thank Alex Balboa and Matt Browe for their contributions to the experimental data. This work was completed under the 9649
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Joint Science and Technology Office for Chemical Biological Defense (JSTO-CBD) Project No. BA07PRO104.
’ REFERENCES (1) Idakiev, V.; Tabakova, T.; Naydenov, A.; Yuan, Z.-Y.; Su, B.-L. Appl. Catal., B 2006, 63, 178–186. (2) Zhang, X.; Wang, H.; Xu, B.-Q. J. Phys. Chem. B 2005, 109, 9678–9683. (3) Peterson, G. W.; Karwacki, C. J.; Feaver, W. B.; Rossin, J. A. Ind. Eng. Chem. Res. 2009, 48, 1694–1698. (4) Peterson, G. W.; Wagner, G. W.; Keller, J. H.; Rossin, J. A. Ind. Eng. Chem. Res. 2010, 49, 11182–11187. (5) Navio, J. A.; Colon, G.; Sanchez-Soto, P. J.; Macias, M. Chem. Mater. 1997, 9, 1256–1261. (6) Bachiller-Baeza, B.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Langmuir 1998, 14, 3556–3564. (7) Ouyang, F.; Nakayama, A.; Tabada, K.; Suzuki, E. J. Phys. Chem. B 2000, 104, 2012–2018. (8) Chadwick, A. V.; Mountjoy, G.; Nield, V. M.; Poplett, J. F.; Smith, M. E.; Strange, J. H.; Tucker, M. G. Chem. Mater. 2001, 13, 1219–1229. (9) Southon, P. D.; Bartlett, J. R.; Woolfrey, J. L.; Ben-Nissan, B. Chem. Mater. 2002, 14, 4313–4319. (10) Davies, L. E.; Bonini, N. A.; Locatelli, S.; Gonzo, E. E. Lat. Am. Appl. Res. 2005, 35, 23–28. (11) Hertl, W. Langmuir 1989, 5, 96–100. (12) Guo, G. Y.; Chen, Y. L. Journal of Materials Science, 2004, 39 (12), 4039–4043. (13) Guo, G. Y.; Chen, Y. L.; Ying, W. J. Mater. Chem. Phys. 2004, 84, 308–314. (14) TIC/TIM Task Force Prioritization & Application Recommendations. MFR #1. 24 February 2009. (15) Baltrusaitis, J.; Cwiertny, D. M.; Grassian, V. H. Phys. Chem. Chem. Phys. 2007, 79, 5542. (16) Waqif, M.; Saad, A. M.; Bensitel, M.; Baceller, J.; Saur, O.; Lavelley, J. C. J. Chem. Soc., Faraday Trans. 1992, 88, 2931. (17) http://www.zrchem.com/Data/Product_Downloads/UNDOPED4. pdf. (18) Huang, C.; Tang, Z.; Zhang, Z. J. Am. Ceram. Soc. 2001, 84 (7), 1637–38. (19) Dobson, K. D.; McQuillan, A. J. Langmuir 1997, 13, 3392–3396. (20) van den Brand, J.; Sloof, W. G.; Terryn, H.; de Wit, J. H. W. Surf. Interface Anal. 2004, 36, 81–88. (21) Veith, G. M.; Lupini, A. R.; Dudney, N. J. J. Phys. Chem. C 2009, 113, 269–280.
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