Interactions of NO2 with Amine-Functionalized SBA-15: Effects of

New York, New York 10031, United States. Langmuir , 2012, 28 (13), pp 5703–5714. DOI: 10.1021/la300371m. Publication Date (Web): March 20, 2012...
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Interactions of NO2 with Amine-Functionalized SBA-15: Effects of Synthesis Route Benoit Levasseur, Amani M. Ebrahim, and Teresa J. Bandosz* The City College of New York and The Graduate School of CUNY, 160 Convent Avenue, New York, New York 10031, United States ABSTRACT: SBA-15 mesoporous silica was modified using (3-aminopropyl)trimethoxysilane (APTMS) following cocondensation or grafting methods and then used as a NO2 adsorbent at room temperature. The samples were characterized before and after exposure to NO2 by SEM-EDX, N2 adsorption at 77 K, potentiometric titration, thermal analysis, and FTIR spectroscopy. Even though, regardless of the synthesis route, the addition of propylamine groups leads to a significant enhancement in the amount of NO2 adsorbed (from 21 to 124 mgNO2/g), a higher retention of NO2 and NO (released as a result of surface reactions) was measured on the grafted silica than on all of the co-condensed samples. In the case of the latter materials, improvements in both NO2 adsorption capacity and NO retention were found for the samples treated with NaOH. This behavior is related to the higher reactivity of deprotonated propylamine groups (formed during NaOH treatment) with NO2, the presence of silanol groups, and the residual amount of sodium present in the samples. The mechanism of NO2 adsorption on propylamine groups involves the formation of nitramine and/or nitrosamine. Analysis of the spent materials indicates that the porosity of co-condensed materials is not affected to the same extent by adsorption of NO2 as that of the grafted silica.

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INTRODUCTION Over the past several decades, advances have occurred in NOx removal technologies, especially those based on adsorption and/or chemical transformation processes. Selective catalytic reduction (SCR) by hydrocarbons1−3 and NOx storage and reduction (NSR)4,5 catalysis are among the most frequently used approaches. Nevertheless, techniques based on adsorption phenomena have become increasingly attractive, because, in contrast to SCR and NSR, they do not require high temperatures or any treatments for the unreacted reagents.6 The large specific surface areas of activated carbons7−9 and the possibility of modifying their surfaces by introducing heteroatoms10−12 or metals/metal oxides13−16 make them good candidates for this process. However, numerous studies have reported a significant release of NO during the adsorption of NO2, because of the oxidation of the carbon surface.13,14,16,17 Despite a high capacity for NO2 removal, that marked release of NO can compromise the usefulness of such adsorbents. Therefore, to overcome this major drawback, various methods to enhance the retention of NO have been explored. Introduction of −OH groups onto the surface of activated carbons by KOH treatment is a good example of a treatment that results in an increase in NO retention on the surface.18,19 NO is oxidized, and potassium nitrate is formed on the surface of such adsorbents.19 The functionalization of the carbon surface with nitrogen-containing groups such as urea,10 amine,11 or dimethylamine12 also represents a promising way to lower the © 2012 American Chemical Society

release of NO during NO2 adsorption. Specific surface chemistry leads to better stabilization of NO on positively charged nitrogen center.10,12 Moreover, Abe et al. reported that the strength of the interactions between NO and −NH2 groups are stronger than those on the unmodified activated carbon surface.11 Aside from carbonaceous materials, mesostructured silicas are excellent candidates for amino group functionalization, because of their high surface areas and tunable pore sizes (from 2 to 50 nm). Amine-functionalized silicas have been studied for a wide range of applications such as catalysis,20−23 sensing,24 and adsorption.25−29 These materials were found to be very attractive for the removal of heavy metals from wastewater25,26 and for volatile organic compounds from the gas phase.27−29 Numerous studies have addresed the functionalization of silica surfaces with amino groups, and two principal routes have been identified: postsynthesis grafting26,30−32 and direct cocondensation.25,33−35 The former method consists of a modification of an inner surface of the silica by reaction with various organosilanes.36 This method presents the advantage of retaining the porosity of the parent silica materials, but the slow diffusion of the organosilane molecules to the center of the pores might lead to a nonhomogeneous distribution of the Received: January 25, 2012 Revised: February 29, 2012 Published: March 20, 2012 5703

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organic groups in a porous network and a blockage of mesopores.37,38 In the co-condensation route, the silica source and the organosilane are simultaneously condensed during the synthesis of the materials.36 By contrast, in the grafting method, the surface coverage by organic groups is much more uniform.39 In the case of the grafting method, some interactions between the organic groups and the silanol groups of the silica surface have been reported,37,40 and in some cases, they decrease the number of adsorption sites.40 Taking into account the above summary, the objective of this study was to evaluate the effect of ordered silica modification with propylamine groups on the NO2 adsorption at ambient conditions. Functionalized silicas prepared by co-condensation or grafting were tested for NO2 adsorption and conversion. The results are discussed in terms of NO2 adsorption capacity and percentage of NO released during the adsorption process. The structure and surface chemistry of the composites were investigated before and after exposure to NO2 to derive a mechanism of reactive adsorption and thus to identify the important features required for efficient NOx adsorbents applied at ambient conditions. The ambient environment of the removal process is the application challenge. High adsorption capacity and irreversible retention of molecules on the surface at room temperature are the requirements for efficient filtering media for toxic industrial compounds (TICs).

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Table 1. Preparation of NH2-Functionalized Silica Materials sample

template extraction

SBA-15 CoA

calcination 550 °C for 6 h ethanol at room temperature for 48 h ethanol at room temperature for 48 h ethanol in Soxhlet at 100 °C for 48 h ethanol in Soxhlet at 100 °C for 48 h calcination 550 °C for 6 h

CoAB CoS CoSB G

NH2 functionalization

posttreatment

n/a co-condensation

n/a n/a

co-condensation

0.1 M NaOH for 1 h n/a

co-condensation co-condensation grafting

0.1 M NaOH for 1 h n/a

samples were packed into a glass column (length, 370 mm; internal diameter, 9 mm). About 2 cm3 of glass beads were mixed with adsorbents to avoid a pressure drop. The concentrations of NO2 and NO in the outlet gas were measured using an electrochemical sensor (RAE Systems, MultiRAE Plus PGM-50/5P). The adsorption capacity of each adsorbent was calculated in milligrams per gram of adsorbent by integration of the area above the breakthrough curve. The tests were conducted until the concentrations of NO2 and NO reached the electrochemical sensors’ upper limit values of 20 and 200 ppm, respectively. After the breakthrough tests, all samples were exposed to a flow of carrier air (180 mL/min) to desorb weakly adsorbed NOx and thus to evaluate the strength of NO2 retention. The suffix ED was added to the names of the samples after exposure to NO2. Surface pH. The samples were first dried, and then 0.4 g of sample was added to 20 mL of distilled water and stirred overnight. The pH of the suspension was then measured. Elemental CHN Analysis. The carbon, hydrogen, and nitrogen contents were determined on the samples after synthesis. The analysis was carried out by Micro Analysis Inc., Wilmington, DE. SEM-EDX Analysis. Scanning electron microscopy (SEM) and electron-dispersive X-ray (EDX) spectroscopy were performed on a Zeiss Supra 55 instrument with a resolution of 5 nm at 30 kV. Analyses were performed on a sample powder previously dried. For EDX spectroscopy, the analysis was done at magnification 2.5K, and the contents of elements on the surface were calculated based on the means of three different single-point analyses. Adsorption of Nitrogen. Nitrogen adsorption isotherms were measured at −196 °C using an ASAP 2010 or ASAP 2050 apparatus (Micromeritics). Prior to each measurement, initial and spent samples were outgassed at 120 °C. The surface area, SBET; total pore volume, Vt; micropore volume, Vmic (Dubinin−Radushkevich method42); and mesopore volume, Vmes, were obtained from the isotherms. Pore size distributions were calculated using density functional theory.43 Potentiometric Titration. Potentiometric titration experiments were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set in the mode for collecting the equilibrium pH. Approximately 0.100-g subsamples of the materials studied in 50 mL of 0.01 M NaNO3 were placed in a container thermostatted at 298 K and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The sample suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were performed in the pH range of 3−10. Each sample was titrated with base after the sample suspension had been acidified. The experimental data were transformed into proton binding curves, Q, representing the total amount of protonated sites.44 In this experiment, the population of sites can be described by f(pKa), or the continuous pKa distribution. The proton binding isotherm, Q, can be found by transforming the experimental data. The

EXPERIMENTAL SECTION

Materials. SBA-15 Synthesis. SBA-15 mesoporous silica was synthesized following the procedure presented in ref 41. Precisely 2.0 g of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (also known as P123) was dissolved in 60 mL of 2.0 M HCl at room temperature, and then 8.8 g of KCl and 4.2 g (20 mmol) of tetraethyl orthosilicate (TEOS) were added under vigorous stirring. After being stirred at room temperature for 1 day, the mixture was transferred into an autoclave and heated at 100 °C for 24 h. The product was then collected by filtration, washed with both ethanol and water (to remove unreacted P123 and KCl), and dried. The as-made powder was calcined at 550 °C for 6 h to remove the organic template. NH2−SBA-15 Prepared by Co-condensation Method. In this method, 1 g of (3-aminopropyl)trimethoxysilane (APTMS) (6 mmol) was added 1 h after the addition of TEOS following the regular synthesis of SBA-15.25 The molar ratio of APTMS to TEOS was 0.3. After being stirred at room temperature for 1 day, the mixture was transferred into an autoclave and heated at 100 °C for 24 h. The product was collected by filtration and dried overnight at 100 °C. Then, the template was extracted with ethanol for 48 h either at room temperature or in a Soxhlet apparatus at 120 °C. The samples obtained using the former approach are referred to as CoA, and those obtained using the latter approach are referred to as CoS. Finally, fractions of CoA and CoS samples were treated with NaOH (0.1 M) for 1 h and are referred to as CoAB and CoSB following the names of the corresponding unmodified samples. NH2−SBA-15 Prepared by Postgrafting Method. Amino groups were grafted onto the silica surface by mixing a batch of dried SBA-15 (calcined at 550 °C for 6 h) with 1 g of APTMS in 200 mL of toluene. The mixture was then heated to 110 °C under nitrogen for 24 h. The material was then collected, washed with toluene, and finally dried under a vacuum at 50 °C for 24 h. This sample is referred to as G. The identities of the samples and their synthesis routes are summarized in Table 1. Methods. NO2 Breakthrough Capacity. The NO2 breakthrough capacities were measured in a laboratory-scale fixedbed dynamic adsorption system at room temperature. In a typical test, 1000 ppm NO2 in nitrogen went through a bed of an adsorbent at a total inlet flow rate of 225 mL/min. The 5704

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proton binding isotherm represents the total amount of protonated sites, which is related to the pKa distribution by the equation

Q (pH) =

breakthrough times between the amine-functionalized samples indicate that the synthesis route can be optimized to obtain the best-performing adsorbents. With the exception of sample G, the shapes of the curves are very steep, which indicates the fast kinetics of NO2 retention on the surface. The NO2 adsorption capacities calculated from the breakthrough curves are collected in Table 2. On pure silica, 21 g of

∞

∫−∞ q(pH, pKa)f (pKa)dpKa

The solution of this equation was obtained by using the numerical SAIEUS procedure,44 which includes regularization combined with non-negative constraints. The detailed surface chemistry was evaluated on the spectrum of acidity constants and the history of the samples. Thermal Analysis. Thermogravimetric (TG) curves and their derivatives (DTG) were obtained using a TA Instruments thermal analyzer. The samples (initial and spent) were previously dried in oven at 100 °C to remove the adsorbed moisture and then heated to 1000 °C at a heating rate of 10 °C/min under a nitrogen flow of 100 mL/min. FTIR Spectroscopy. Fourier transform infrared (FTIR) spectroscopy was carried out on a Nicolet Magna-IR 830 spectrometer using the attenuated total reflectance method. The spectrum was generated, collected 32 times, and corrected for background noise. The experiments were performed on powdered samples without addition of KBr.

Table 2. NO2 Adsorption Capacity, NO Released, and pH before and after Exposure to NO2 on NH2-Functionalized Silica Materials sample pHinitial SBA-15 CoA CoAB CoS CoSB G

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RESULTS AND DISCUSSION NO2 breakthrough curves measured on our materials are presented in Figure 1. The breakthrough times visibly differ,

6.87 4.76 7.99 4.28 8.78 9.21

NO2 adsorption (mgNO2/gadsorbent) 21 ± 1 45 ± 1 66 ± 3 71 ± 1 100 ± 4 124 ± 1

NO NO2 adsorption released (mmolNO2/mmolNH3) (%) pHfinal n/a n/a 1.59 n/a 1.98 1.28

12 >15 10 >20 12 4

6.52 3.77 7.69 3.61 8.27 8.81

NO2 per gram of adsorbent was retained, whereas the capacities of the NH2-functionalized materials ranged between 45 mg/g on CoA and 124 mg/g on G. Lower NO2 adsorption capacities were, however, measured on the co-condensed materials than on the grafted one. Differences in the capacities for CoAB and CoSB, compared to CoA and CoSB, indicate that both template extraction and the basic post-treatment have an impact on the retention of NO2. The performances of CoA and CoS suggest that different amounts of template might be present in the structures of the two materials and that the template has an effect on the NO2 adsorption capacity. The NO concentration curves are shown in Figure 1, and the percentages of NO released during the tests are summarized in Table 2. On the pure silica and co-condensed materials without any treatment with NaOH, we observed an immediate release of NO, in amounts ranging from 12% to more than 20% of the NO2 adsorbed. On the other materials, NO appeared in the outlet gas after 12 min/g for CoAB and CoSB and 22 min/g for G. The treatment with NaOH caused a decrease in the percentage of NO released from 14% on CoA to 10% on CoAB and from >20% on CoS to 12% on CoSB. Finally, on sample G, only 4% of the NO was released during the NO2 adsorption test. CHN contents in CoAB, CoSB, G, and SBA-15 are summarized in Table 3. The G silica has more carbon and nitrogen than do the two co-condensed materials (CoAB and CoSB). Based on this analysis, it was estimated that 2.1 mmol/g of propylamine groups existed on the surface of sample G, and about 1 mmol/g existed on both the CoAB and CoSB materials. This indicates that the amount of propylamine groups incorporated onto the silica surface depends on the synthesis route. This finding provides a new perspective on the reactivity and/or capacity of the surface to retain NO2. Thus, simple calculations show that 1.98 mmolNO2/mmolNH3 was adsorbed on CoSB silica, whereas these amounts were 1.59 and 1.28 mmolNO2/mmolNH3 on CoAB and G silica, respectively. Another interesting fact is the difference between the C/N molar ratios. This ratio is higher for CoAB silica than for the G and CoSB samples, which suggests that there is another source of carbon in the CoAB structure, not coming from the propylamine groups.

Figure 1. NO2 breakthrough curves and NO concentration curves for NH2-functionalized silica materials.

and all of the NH2-functionalized silica samples, regardless of their synthesis and post-treatment, showed better retention of NO2 than the unmodified material. The differences in the 5705

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Table 3. Surface Composition Determined by EDX and Elemental Analyses on NH2-Functionalized Silica Materials EDX analysis

elemental analysis

sample

O (at. %)

Si (at. %)

C (at. %)

K (at. %)

Na (at. %)

C (wt %)

H (wt %)

N (wt %)

C/N (at. ratio)

−NH3 groups (mmol/g)

SBA-15 CoAB CoSB G

59.8 42.6 46.9 45.3

40 27.6 27.7 32.3

0 29.3 24.6 22.4

0.2 0 0 0

0 0.5 0.8 0

0.6 5.6 4.8 9.8

0.3 1.5 1.4 1.9

0 1.2 1.5 2.9

0 5.2 3.8 3.9

0 0.9 1.1 2.1

Figure 2. SEM images of (A) SBA-15, (B) CoAB, (C) CoSB, and (D) G.

aggregation of rodlike particles of approximately 50−100 μm in length can be seen. For the co-condensed silicas (CoAB and CoSB),

SEM images of the surfaces of the SBA-15, CoAB, CoSB, and G silicas are shown in Figure 2. In the case SBA-15 and sample G, 5706

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Figure 3. EDX maps of NH2-functionalized silica materials before exposure to NO2.

the images show a more disorganized structure than those for SBA-15 and G silica. This disorganization was also noticed previously after the treatment of SBA-15 with NaOH and is related to a partial dissolution of silica by sodium hydroxide.45 “Petal”-shaped particles are visible on the images of the CoAB and CoSB silicas with sizes between 2 and 10 μm. The contents (in atomic percentages) of silica, oxygen, carbon, sodium, and potassium obtained by EDX analysis of the SBA-15, CoAB, CoSB, and G surfaces are listed in Table 3. For those four samples, the O/Si atomic ratio was close to 1.5. In the case of C/Si ratio, some differences are visible. The O/Si ratio was lower for G silica (0.7) than for the co-condensed samples CoAB and CoSB (1.1 and 0.9, respectively) indicating that there were more carbon atoms on the surface of cocondensed materials. The higher C/Si ratio found for CoAB is consistent with the elemental analysis and confirms that a secondary source of carbon, not coming from the propylamine groups, was present on the surface. Even though the samples were deposited on a carbon tape, the atomic percentages of the elements were calculated from the means of three single-point analyses of the silica surfaces. Thus, the carbon tape did not influence the results of the elemental analyses. One should notice that a trace of potassium (0.2 at. %) was detected on the surface of SBA-15 and is related to the addition of KCl during the synthesis of this material. In the case of the two cocondensed silicas treated with NaOH (CoAB and CoSB), some residual sodium was detected (less than 0.8 at.%). The element maps are collected in Figure 3. They indicate that the particles were homogeneously composed of silica and oxygen, regardless of the sample type. For the co-condensed silicas treated with NaOH, some small spots of sodium are visible and likely represent the residue after the treatment had been applied. In the case of carbon, the maps were apparently affected by visible carbon zones around the particles (especially for SBA-15). As explained above, carbon was not counted in the elemental analysis. Small spots of carbon were observed on the three functionalized silicas, indicating the presence of the propylamine functionality on their surface. The porous structure of our materials was evaluated from nitrogen adsorption isotherms measured at 77 K. The isotherms and the pore size distributions calculated from

them are presented in Figures 4 and 5, respectively. The parameters of the porous structure are summarized in Table 4.

Figure 4. Nitrogen adsorption isotherms on SBA-15 and the functionalized materials before (solid lines) and after (dotted lines) exposure to NO2.

In the isotherms, large hysteresis loops typical for mesoporous materials (type IV isotherm) are visible. The loop is especially 5707

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(Vmic/Vt) increased from 5% for SBA-15 to 20% after grafting, indicating that the loss in porosity affected only the mesopores. The pore size distribution for sample G was narrow, and the average pore size shifted from 100 Å for SBA-15 to 54 Å after grafting. In the case of the co-condensed materials (CoA and CoS), any potential differences in the values of the parameters of the porous structure would be related to the efficiency of template removal during the extraction step. Nevertheless, both the specific surface areas and volumes of the pores were of the same magnitude regardless of the type of extraction. For CoA and CoS, the specific surface areas were 386 and 451 m2/g, respectively. The volume of pores was 0.344 cm3/g for CoA and 0.360 cm3/g for CoS. Both extractions resulted in a major increase in the contributions of the volume of micropores from 5% for SBA-15 to 36% and 44% for CoA and CoS, respectively. The average pore size for these two materials ranged from 20 to 70 Å (Figure 5). A significant difference with respect to the narrow pore size distribution of SBA-15, where the template was removed by calcination, suggests that some quantities of P123 were left in the CoA and CoS materials and, consequently, impacted the NO2 adsorption capacity. The treatment with NaOH applied to the co-condensed materials led to an increase in the specific surface area and volume of pores, which is likely associated with the removal of the residual template left in the structures of the CoA and CoS samples.25 However, slight differences in terms of porosity were observed between CoAB and CoSB. The larger specific surface area and volume of pores found for CoSB than for CoAB might be related to the higher carbon content detected for the latter sample by EDX and elemental analyses. We attribute this higher carbon content to the presence of a residual amount of template left in the structure of CoAB. After exposure to NO2, some changes in the porous structure were found for pure silica and its functionalized counterparts. For SBA-15, a 37% loss in the specific surface and a 21% decrease in the volume of pores were observed. The reactive adsorption of NO2 on the SBA-15 surface also led to a strong modification of its pore network, as a bimodal distribution was revealed after the breakthrough test. Such a strong alteration of the porous structure after adsorption of NO2 suggests that silanol groups are active centers for the NO2 retention. The effects of these groups were also reported in the case of NO2 adsorption on silica modified with copper or ceria−zirconium mixed oxides.45,46 A high density in silanol groups, such as that found on pure silica SBA-15, enhances the retention of NO2 as a result of the formation of weakly bound nitric acid.46 Therefore, that consumption of silanol groups is hypothesized to be the reason for the bimodal distribution of the pore sizes observed after exposure of SBA-15 to NO2. On the grafted material (G), the loss of porosity after exposure to NO2 was more visible. The specific surface area decreased from 237 to 119 m2/g, and the volume of the pores changed from 0.416 to 0.200 cm3/g. The pores remained relatively homogeneous in their sizes (narrow distribution) despite the decrease in their volume. The porosity of the cocondensed materials not treated with NaOH (CoA and CoS) remained stable after exposure to NO2, which suggests that the adsorption process on these specific materials might be physical in nature. This hypothesis is supported by the highest degree of microporosity for these materials (36% and 44% for CoA and CoS, respectively). For the co-condensed silicas treated with NaOH, we observed decreases in the specific surface area and

Figure 5. Pore size distributions of SBA-15 and the functionalized materials before (solid lines) and after (dotted lines) exposure to NO2.

Table 4. Parameters of the Porous Structure of NH2-Functionalized Silica Materials sample

SBET (m2/g)

Vμp (cm3/g)

Vtot (cm3/g)

Vμp/Vtot (%)

Dp (Å)

SBA-15 SBA-15 ED CoA CoA ED CoS CoS ED CoAB CoAB ED CoSB CoSB ED G G ED

626 397 386 366 451 426 398 285 460 381 237 119

0.045 0.053 0.124 0.107 0.160 0.141 0.056 0.050 0.129 0.111 0.085 0.040

0.961 0.761 0.344 0.347 0.360 0.332 0.507 0.481 0.570 0.499 0.416 0.200

5 7 36 31 44 42 11 10 22 22 20 20

100 76 34 34 31 34 67 65 48 51 70 67

pronounced for SBA-15. The isotherms for the co-condensed materials before NaOH treatment show a flattened hysteresis loop indicating differences in the porosity. The treatment with NaOH resulted in typical type IV isotherms. Even though both grafted and co-condensation methods led to a loss in the specific surface area and in the volume of pores compared to those for SBA-15, the extent of the changes differed depending on the synthesis method. For sample G, an about 60% decrease in the specific surface area and pore volume was observed. Interestingly, the degree of microporosity 5708

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groups. This is supported by the fact that the synthesis by cocondensation was performed under strongly acidic conditions. After treatment with NaOH, the surfaces of the organosilicates became much more basic than that of pure silica. This increase of the surface pH can be related, on one hand, to the extraction of the residual P123 block copolymer during the basic treatment or/and, on the other hand, to the deprotonation of the propylamine groups by NaOH and to residual NaOH present in the samples. Whereas the former hypothesis is supported by an increase in the specific surface area and pore volumes found for CoAB and CoSB in comparison with CoA and CoB, support for the deprotonation of propylamine groups is in their pKa value, which is 10.6.48 The fact that the grafted sample had the most basic surface among all samples studied is, in addition, supported by the larger amount of propylamine groups found on its surface, as indicated by elemental analysis. After exposure to NO2, the surfaces of all samples became much more acidic (Table 2, Figure 6). Usually, such acidification is related to the surface oxidation.10,12,46 This suggests that the propylamine groups were oxidized by NO2. As seen, the effects of NO2 on the surface acidity of our materials varied. On the pure silica and co-condensed samples not exposed to NaOH treatment, a smaller increase in the acidity was found than on the co-condensed materials treated with NaOH. This behavior is related to the reactivity of the protonated propylamine groups toward NO2. Compared to CoAB and CoSB, smaller NO2 adsorption capacities were measured on both CoA and CoS (Table 2). This suggests that protonated amine groups are less reactive than deprotonated ones, as an increase of about 30% in NO2 adsorption capacity was measured on co-condensed materials after basic treatment. The increase in acidity after exposure to NO2 was also different on the grafted material and the co-condensed materials treated with NaOH. Despite a higher NO2 adsorption capacity, the acidification of the surface was less marked for G than for CoAB and CoSB. This difference is directly related to the amount of NO2 adsorbed per propylamine group (Table 2). Because CoSB and CoAB were the two best adsorbents, with 1.97 and 1.54 mmolNO2/mmolNH3, respectively, the acidification of their surfaces was the strongest because of the oxidation of the propylamine groups. Despite the higher content in the propylamine groups, the proton binding curves of G silica indicate that only a fraction of those groups reacted with NO2, which is consistent with the smaller amount of NO2 adsorbed per millimole of propylamine present on the G material compared to those on CoAB and CoSB. Taking into account this finding and the smaller volume of pores in sample G compared to the other silicas, it is possible that some pores were blocked during the postgrafting procedure, as has already been reported in the literature.37,38 . Figure 7 shows the pKa distributions of the functional groups present on the silica surfaces before and after exposure to NO2. For SBA-15, three peaks appear at about 6.5, 7.8, and 9. The first peak is related to the deprotonation of Si−OH groups,49 whereas the last two represent the first step of deprotonation of Si(OH)350 and Si(OH)2,51 respectively. These peaks are visible in the distributions for all of the functionalized silicas regardless their synthesis and the treatment applied. After functionalization, the numbers of groups assigned to Si(OH) increased, whereas the numbers of Si(OH)3 and Si(OH)2 groups decreased. This is especially visible for sample G, suggesting that this decrease in the amounts of Si(OH)3 and Si(OH)2 is related to the consumption of silanol during the functionaliza-

pore volume after exposure to NO2. Nevertheless, this loss of porosity was limited because it represented only about 25% of the specific surface area and 10% of the pore volume for both CoAB and CoSB. Even though the differences in porosity, discussed above, can partially explain some variations in the reactivity of our samples toward NO2 adsorption, the surface chemistry was investigated to have more complete understanding of the NO2 adsorption mechanism. Proton binding curves for each material before and after exposure to NO2 are shown in Figure 6. Positive Q values

Figure 6. Proton binding curves of SBA-15 and the functionalized materials before and after exposure to NO2.

represent proton uptake (basic surface), whereas negative Q values are related to proton release (acidic surface). Thus, before exposure to NO2, the co-condensed materials (CoA and CoS) had more acidic surfaces than pure silica, whereas the surfaces of their counterparts treated with NaOH (CoAB and CoSB) and the grafted (G) sample were more basic in chemical nature. This is in agreement with the samples’ surface pH values collected in Table 2. The surface acidity of CoA and CoS might be partially related to the remains of P123 template left in their structures after extraction, as the pKa of P123 ranges between 5.5 and 6.5.47 Incomplete removal of the template was also suggested by the porosity analysis presented above and confirmed by the higher contents of carbon in these materials. Neverthless, the acidity of P123 combined with the small amount of this compound present in the CoA and CoS cannot completely explain their high acidity. Therefore, another plausible explanation is the protonation of the propylamine 5709

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Figure 8. DTG curves of SBA-15 and the functionalized materials before (solid lines) and after (dotted lines) exposure to NO2.

Figure 7. pKa distributions of SBA-15 and the functionalized materials before and after exposure to NO2.

attribute the first peak at about 310 °C to the decomposition of protonated amine, whereas the second broad peak observed between 400 and 700 °C represents the decomposition of hydrocarbon chains of the silane.53 Remains of the P123 copolymer might be present in these two materials and might decompose between 200 and 650 °C,52 making this peak more complex. After NaOH treatment, the DTG curves for CoAB and CoSB changed. The peak ascribed to the decomposition of amine shifted to 430 °C, whereas the second broad peak was still detected at the same temperature as found for the samples not exposed to NaOH treatment. This shift of the decomposition of the amine groups is very interesting and suggests that the protonated form is less thermally stable. This higher stability of −NH2 groups than −NH3+ might be related either to the lone-pair electron that helps to stabilize the functional group and/or to the potential hydrogen bonding between two amines groups or amine and silanol groups. For the grafted material, only one broad peak at about 490 °C, associated with the decomposition of the propylamine groups,53 was observed. After exposure to NO2, a new peak appeared in all of the DTG curves. It appeared at about 280 °C for CoA, CoS, and G and between 300 and 350 °C for CoAB and CoSB. Another feature seen in the DTG curves of all spent silicas is a decrease in the intensity of the peak representing the decomposition of the propylamine groups (400−700 °C). This suggests their consumption during NO2 adsorption.

tion process. After functionalization, a new species representing deprotonated propylamine groups was revealed on the surface of samples at pKa about 4.5. According to Da’na and Sayari, below pH 4, the propylamine groups are protonated, whereas at pH > 4, they exist in the deprotonated form.25 After exposure to NO2, the intensities of all peaks representing different types of silanol decreased, suggesting their consumption in the NO2 adsorption mechanism. Our previous studies on silica modified with copper or ceria− zirconium mixed oxides also indicated the reactivity of these groups with NO2.45,46 The intensity of the peaks at pKa about 4.5 ascribed to the deprotonation of NH3+ also decreased after exposure to NO2, which suggests that the propylamine groups interact with NO2. No new peaks were visible in the pKa distribution after exposure to NO2, indicating that the products of the reaction between NO2 and propylamine groups were not visible in our experimental window (from pH 3 to pH 9).12 To further investigate the surface chemistry of our materials, thermal analysis was used. Weight loss derivative curves (DTG) for the functionalized and pure silica before and after exposure to NO2 are presented in Figure 8. Peaks appearing below 100 °C are related to the removal of moisture. The DTG curve for pure SBA-15 is almost featureless, whereas in the curve for SBA-15 with the template, a very broad peak is revealed between 200 and 650 °C, which corresponds to the decomposition of the template.52 In the case of the cocondensed materials untreated with NaOH, CoA and CoS, two main peaks are visible at temperatures higher than 100 °C. We 5710

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Figure 9. FTIR spectra of SBA-15 and the functionalized materials before (solid lines) and after (dotted lines) exposure to NO2.

FTIR analysis was performed on each sample before and after exposure to NO2 (Figure 9). For the unexposed samples, two characteristic vibrations due to the Si−O and Si−O−Si bonds are visible at 820 and 1070 cm−1, respectively.54 In the spectra for CoA, CoS, and CoAB, a new band was revealed at about 970 cm−1 that represents the vibration of Si−OH bond.54 The absence of this vibration for G and CoSB indicates that propylamine groups were anchored to the surface by the silanol groups. This is also supported by the presence of various NH2 vibrational modes observed between 1615 and 1715 cm−1 in the spectra for the grafted sample and co-condensed ones treated with NaOH.55 After exposure to NO2, the intensity of the bands representing vibrations of the NH2 groups decreased slightly for G, CoSB, and CoAB. This confirms the abovehypothesized involvement of the propylamine groups in NO2 adsorption. Moreover, some new peaks related to both symmetric and antisymmetric NO2 vibrations are visible in the spectra for these samples between 1380 and 1420 cm−1.56 Finally, a very weak band at 755 cm−1 for the G silica indicates skeletal bend of the N−NO2 compound.56 We hypothesize that the reaction between propylamine groups and NO2 leads to the formation of nitramine and/or nitrosoamine species. Such species were observed by Deliyanni and Bandosz on a carbon surface modified with dimethylamine after exposure to NO2.12 In addition, the reactivity of both primary and secondary amines for this reaction, due to the formation of aminyl moiety, was determined using a computational method.57 The vibrations observed by FTIR spectroscopy on spent the samples are, moreover, consistent with the formation of nitramine and/or nitrosoamine moieties. On the spectra for the co-condensed materials untreated with NaOH (CoA and CoS), only two new peaks are visible after exposure to NO2 at about 1530 and 1640 cm−1. This band likely suggests the formation of nitro compounds on these two materials.58

Based on the breakthrough tests, as well as on the changes in both porosity and surface chemistry before and after exposure to NO2, a mechanism of NO2 reactive adsorption on the functionalized-silica composites is proposed. The formation of nitramine or nitrosamine groups after exposure to NO2 on G, CoSB, and CoAB was suggested by FTIR spectroscopy and is supported by thermal analysis, where we observed their decomposition between 280 and 340 °C. Moreover, the higher acidity of nitramine functional group (pKa at about 5.559) in comparison with the amine groups (pKa = 10.648) is consistent with the decrease in the surface pH and the increase in the extent of proton release found for G, CoSB, and CoAB after exposure to NO2. Therefore, it is proposed that the following reactions represent the interactions between NO2 and/or NO and propylamine groups

The reactions between amine groups and NO2 and/or NO lead to the oxidation of the propylamine moiety and the formation of nitramine or nitrosamine compounds, respectively. Thus, the higher NO2 adsorption capacity found for the co-condensed samples CoSB and CoAB can be related to the presence and easy accessibility of the propylamine groups. In contrast, for the 5711

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groups were anchored in the pores of the silica. The functionalization by propylamine groups clearly enhanced the NO2 adsorption capacity. However, the two routes of functionalization were found to lead to different contents of propylamine groups. The higher amount of these groups was measured on the sample obtained using postgrafting synthesis. The co-condensed materials treated with NaOH exhibited the highest NO2 adsorption capacity at 1.98 mmolNO2/mmolNH3. The differences in the adsorption performance between cocondensed samples were found to be dependent on the treatment with NaOH. Without any treatment, the amine groups were protonated, and these silicas were found to be less reactive toward NO2 than the co-condensed samples treated with NaOH. The grafted material showed a lower NO2 adsorption capacity than the co-condensed silicas treated with NaOH, despite its higher content of propylamine groups. This is likely due to the blockage of some pores, which limits the accessibility of these active centers for reactions with NO2. Moreover, the absence of both silanol groups and sodium on the surface of the grafted sample might have also led to a decrease in NO2 retention compared to that of the cocondensed materials treated with NaOH. It is believed that reactions between NO2 or NO and the propylamine groups led to the formation of nitramine or nitrosamine compounds whereas, on protonated amine, nitro compounds and a significant amount of NO were formed. The silanol groups present on the surface of the samples also helped in the retention of NO2.

grafted silica, G, the limited access of NO2 to the propylamine groups, suggested by N2 adsorption and proton binding curves, causes a decrease in the NO2 adsorption capacity in comparison with CoSB and CoAB. However, reactions 1 and 2 cannot fully explain the difference of reactivity observed between the cocondensed materials treated with NaOH (CoSB and CoAB) and the grafted silica (G), since for the former materials almost two NO2 are consumed for one propylamine group. This suggests that other reactive species have to be involved in the NO2 retention mechanism. Taking into account the pKa distributions of our samples, the decrease in the intensity of the peaks ascribed to the silanol groups after exposure to NO2 indicates their reactivity toward NO2. This is also supported by the changes in the pore size distribution mentioned above for SBA-15 exposed to NO2. Thus, as proposed previously, N2O4, formed in the dimerization of NO2, might be stabilized on the silanol groups as expressed by the equation46 N2O4 (g) + 2Si−OH(s) → Si2HNO3(s) + NO2−(g) (3)

Thus, two silanol groups are required to stabilize N2O4 by (i) electrostatic interaction between the silanol oxygen doublet and the nitrogen from N2O4 and (ii) a hydrogen bond between the second silanol and a negatively charged oxygen from N2O4.60 This reaction is, in addition, supported by density functional theory calculations performed on N2O4 dimer and a silica surface with various densities of silanol groups.60 Reaction 4 is favored on a silica surface with a density of Si−OH.60 This reaction is, indeed, plausible to take place on co-condensed materials on which Si−OH vibrations were revealed by FTIR spectroscopy, regardless of any NaOH treatment. Reaction 3 is more likely to take place on CoAB and CoSB than on sample G, because silanol groups are used to anchor the propylamine moiety during the grafted synthesis. This explains the larger amount of NO2 per millimole of propylamine retained on the surfaces of CoSB and CoAB than on the surface of G silica. In addition, one has to take into account the traces of sodium (less than 0.8 at. %) found on the surfaces of CoAB and CoSB that can contribute to the formation of nitrate salt. The history of the samples and their characterization suggest that the amine groups on the surface of co-condensed silicas untreated with NaOH are protonated. In this case their interactions with NO2 can take place through the reaction

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (212)650-6017. Fax: (212)650-6107. E-mail: tbandosz@ ccny.cuny. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by ARO (Army Research Office) Grant W911NF-10-1-0030. REFERENCES

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This reaction between the protonated amine groups and NO2 leads to the formation of a nitro compound, as suggested by the results of thermal analysis and FTIR spectroscopy. It is also important to notice that this reaction involves the formation of a large amount of NO. This is supported by the high percentage of NO measured during the adsorption of NO2 on CoA and CoS. Reaction 3 can compensate for the weak reactivity of the protonated amine groups because, as indicated by FTIR spectroscopy, silanol groups are present on the surfaces of CoA and CoS silicas.

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CONCLUSIONS Surface characterization of the initial SBA-15 silica and its amine-functionalized counterparts confirms that propylamine 5712

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