Promoting Zeolite NaY as Efficient Nitrosamines Trap by Cobalt Oxide

That is to say, both cobalt-modified zeolite samples trap the all nitrosamines in ...... A former chemistry PhD candidate at Queen's University in Can...
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J. Phys. Chem. C 2007, 111, 538-548

Promoting Zeolite NaY as Efficient Nitrosamines Trap by Cobalt Oxide Modification Yi Cao, Ting Ting Zhuang, Jing Yang, Hua Dao Liu, Wei Huang, and Jian Hua Zhu* Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: August 2, 2006

To develop a new trapper of nitrosamines with high efficiency, cobalt oxide incorporated zeolite NaY samples were prepared by different methods and characterized by XRD, DRS, and H2-TPR techniques. The influence of preparation on the dispersion and distribution of cobalt species in NaY zeolite were examined, involving the migration of cobalt oxide from the sodalite cage of zeolite. An adsorption experiment of N-nitrosopyrrolidine (NPYR) indicated the impact of appropriate loading amount and calcination temperature on the capability of zeolite NaY and revealed two properties of the 3CoY sample: the highest adsorptive capacity at 338 K and the potential reversible adsorbent of nitrosamines. As revealed by thermogravimetric analysis-mass spectroscopy (TG-MS) results, cobalt-modified zeolite NaY had high activity to catalyze the degradation of NPYR at relatively low temperature.

Introduction It is necessary to design and synthesize effective functional materials for adsorption and conversion of environment pollutants, especially for removal of carcinogenic pollutants. Nitrosamines are well-recognized teratogens and carcinogens in animals and are considered potentially carcinogenic in humans. They exist widely in environments such as the industrial workplace, beer, diet, and bacon as well as cigarette smoke, causing serious health hazards even in trace amounts.1-3 Apart from tobacco-specific nitrosamines in smoke, volatile nitrosamines like N-nitrosodimethylamine (NDMA) and N-nitrosopyrrolidine (NPYR) can induce tumors. After metabolic activation, they react with DNA to cause mutations and cancer. Many efforts have been made to reduce the amount of nitrosamines in the environment. Apart from the zeolites catalyst added in cigarettes to reduce the level of nitrosamines and polycyclic aromatic hydrocarbons (PAHs) in smoke,4,5 adsorption by zeolites was studied in detail.6,7 The pore size, surface area, and acid--base properties of zeolites determine the adsorptive capacity of the microporous materials,6 and among common zeolites, the sample of NaY possesses the highest performance for trapping volatile nitrosamines in a gas stream due to large pore size and pore volume.7 The cation of zeolite is another important factor affecting the adsorption of nitrosamines. The N-NO groups of nitrosamines possess a negative charge so that they are easily attached to the cations in zeolite through an electrostatic interaction; therefore, the nitrosamine molecule is adsorbed by zeolite in the way the N-NO group inserts inside the zeolite channels.8,9 Accordingly, for the zeolite with definite pore structure, the feasible way to enhance its efficiency to capture nitrosamines is to introduce more cations inside the channels. Copper oxide was used to modify zeolite NaY, NaZSM-5, and NaA, significantly promoting the adsorption of volatile nitrosamines and enabling zeolite to degrade NPYR at a lower temperature.10 However, these guest metal oxides also occupied definite space in the channel of the zeolite, *To whom correspondence should be addressed. Fax: (+)0086-2583317761. Tel: (+)0086-25-83595848. E-mail: [email protected].

and partially hindered the diffusion of adsorbate, especially in the case of adsorbing bulky N-nitrosohexamethyleneimine (NHMI) or N-nitrosonornicotine (NNN).10 So, it is crucial to optimize the modification of zeolite to enable these cations to exert their optimal function in trapping nitrosamines, which is not only useful for a deeper understanding of the selective removal of nitrosamines but also for seeking a new way to create novel functional materials. However, it is unclear what strategy should be adopted to prepare the efficient nitrosamines trap. First, there are two common techniques, ion exchange and impregnation, which begs the question, what is the preferred method to modify the zeolite? The former only replaces the original cation by another while the latter adds to the amount of cations in the zeolite but changes the curvature of the channels. It is unknown which cation plays the predominant role for adsorption of nitrosamines, those in ionic sites or those coated on the wall of pore? Thus, a second question arises, should we use a solvent to load the guest compounds on the zeolite? The solvent can bring the solute to cover the channel wall of the zeolite equally but causes competitive adsorption with the solute. Besides, the migration and evaporation of solvent will affect the distribution of solute during the calcination of the resulting composite. To answer these questions, we employed cobalt oxide to modify the zeolite NaY through different ways, including impregnation, ionexchange, and solvent-free methods. Cobalt oxide has been widely utilized as a versatile catalyst for Fischer-Tropsch syntheses or hydrodesulfurization,11,12 and the zeolite catalysts containing cobalt oxide have also been used in industrial process.13 The number of cobalt sites available for catalytic reaction depends on the Co content of the catalysts, the sizes of Co particles, and their reducibility. The ion-exchange method is a common technology to introduce cobalt ions into zeolites, in which Co2+ occupies different exchangeable sites of zeolites to minimize the energy of the unit cell.14 The openshell Co2+ plays an important role in catalytic reduction of nitric oxide because of its partly filled d orbitals and coordinative versatility,15 therefore cobalt ion-exchanged zeolite Y and ZSM-5 have been used in catalytic reduction of nitric oxide,16-18

10.1021/jp064978f CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006

Zeolite NaY as Efficient Nitrosamines and vis-NIR absorption spectra provide much useful information on the sites of cobalt ions in the zeolite to reveal their electronic structure, coordination, and local geometry.19 Impregnation of zeolite with a solution of Co(NO3)2 is another convenient way to support cobalt oxide, both sodium and cobalt cations in the modified zeolite NaY exhibit mobility during the calcination process, similar to cesium exchanged and impregnated zeolite Y and X.20 In the present research, we investigate the incorporation of cobalt oxide in zeolite NaY and characterize the resulting composites by X-ray diffraction (XRD), differential reflectance spectroscopy (DRS), and H2-TPR (temperature-programmed reduction) methods. To assess the actual function of the Comodified zeolite on the adsorption and catalysis of nitrosamines, NPYR, one volatile nitrosamine that exists in both cigarette smoke and beer, is chosen as the target. 2. Experimental Section 2.1. Material Preparation. NPYR was purchased from Sigma and dissolved in CH2Cl2 (1:19, v/v). Zeolite NaY, a commercially available powder with a Si/Al ratio of 2.86 and surface area of 766 m2 g-1, was stirred in 1 M NaCl 6 times, then washed to free Cl-, and finally calcinated in air prior to usage. Cobalt-oxide modified zeolites were prepared by the impregnated method. Typically, 220 mg of Co(NO3)2‚6H2O was dissolved in deionized water at first to achieve a concentration of 0.02 M. An amount of 1.94 g of zeolite NaY was then added into the solution and stirred at 303 K for 24 h, followed by evaporation at 353 K. After the sample was dried at 373 K for 12 h and calcined in nitrogen flow at 673 K for 3 h, the color of the solid changed from pink to dark gray and the obtained sample was denoted as nCoY where n represented the weight percent of cobalt oxide. To study the influence of the synthetic method and calcination on the structure-function relationship between resulting composites, some cobalt-modified samples were prepared by the grinding or ion-exchange method besides impregnation. For the grinding method, 1.94 g of zeolite NaY was ground thoroughly with 220 mg of Co(NO3)2‚6H2O in mortar for about 0.5 h. For ion exchange, 2 g of NaY was stirred in a 0.02 M solution of Co(NO3)2 for 2 h. After the exchange was repeated 6 times, the sample was filtered and washed thoroughly and air-dried. These obtained samples were calcined at 773 K in N2 for 5 h then in air for another 5 h and denoted as 3CoY-i, 3CoY-g, and CoY-ex, respectively, where the suffixes i, g, and ex represent the impregnation, grinding, and ion-exchanging methods. According to inductively coupled plasma (ICP) analysis, the content of cobalt in the CoY-ex sample was 1.2 mmol g-1, while the contents of Al and Na were 3.2 and 1.0 mmol g-1, respectively. In similar procedures, the other samples of zeolite NaY modified with other metal oxides, named as 3CuY, 3FeY, or 3ZnY, were prepared for comparison. 2.2. Characterization. Powder XRD patterns were recorded with a set of D/MAX-RA X-ray diffractometers with Cu KR radiation in which the X-ray tube was operated at 40 kV and 100 mA, over the 2θ range from 5 to 80˚. DRS were taken on a Lambda 900 (Perkin-Elmer) UV-visNIR spectrophotometer at room temperature. The powder sample was mounted onto a round disk whose thickness was greater than 3 mm, and the spectra were recorded in the 4002500 nm wavelength range using BaSO4 as a reference. The reduction behaviors of the Co-modified NaY sample were examined by H2-TPR. About 100 mg of sample (20-40 mesh) was heated in nitrogen flow at 673 or 773 K (determined by the calcination temperature used in preparation of the samples)

J. Phys. Chem. C, Vol. 111, No. 2, 2007 539 for 2 h. After the sample was cooled to 303 K and purged by reducing gas (H2/Ar ) 1/9, v/v) with a rate of 30 mL min-1 for 1 h, the temperature was raised to 1273 K at a rate of 10 K min-1 meanwhile the hydrogen consumption caused by the reduction was measured by an “on line” Varian 3380 gas chromatograph (GC). Before the gas reached the thermal conductivity detector (TCD), it was led via a CaA zeolite trap to remove the product water. The equipment of NO2-TPD is similar to that of H2-TPR. A 100 mg sample (20-40 mesh) was first pretreated in a U-shape quartz reactor with a nitrogen gas flow at 673 or 773 K for 2 h and then cooled down to 338 K and contacted with NO2 (20 mL) for 0.5 h. When the weakly adsorbed NO2 was purged thoroughly, the sample was heated again with a ratio of 10 K min-1, while the released NO2 was detected by the “on line” Varian 3380 GC. Adsorption of NPYR was performed in a stainless steel microreactor with 3 mm diameter and 150 mm length. A 5 mg sample (20-40 mesh) was filled in one end of the reactor and sealed by glass wool to fix the position. This part was inserted deeply into the injector port of the Varian 3380 GC and another end was connected with the separation column in the GC.21 The sample was directly heated to 338 K in He flow with a rate of 30 mL min-1, and the NPYR solution was pulse injected with amounts of 2 µL each time. The TCD of the GC analyzed the gaseous effluent, and the decrement in the ratio of solute to solvent was utilized to calculate the adsorbed amount.10,22 Temperature programmed surface reaction (TPSR) experiments were performed in the manner reported previously.7 A 40 mg sample (20-40 mesh) was first activated in nitrogen gas flow at 673 or 773 K for 2 h, then cooled down to 338 K upon contact with the NPYR solution (100 µL). After the sample was purged, the temperature was increased at the rate of 10 K min-1, and the formed NOx was detected by a spectrophotometric method to represent the amount of NPYR decomposed.7 A thermogravimetric analysis-mass spectroscopy (TG-MS) test was performed on a Netzsch STA449C TG/DSC-MS instrument. The sample (20-40 mesh) was activated at 673 or 773 K at first, in N2 flow of 30 mL min-1 for 2 h. Then, 100 µL of NPYR solution was injected on the sample under N2 flow at 338 K. After the sample was transferred into the crucible of the TG-MS instrument, it was degassed at 303 K for 0.2 h. Then, the TGA was operated from 303 to 773 K at a rate of 10 K min-1 under the protection of Ar flow, meanwhile the released components were sent to the mass spectrometer by carrier gas. The gas line between TG and MS was heated to 453 K to avoid cold points and thus condensation of some gaseous products. 3. Results and Discussion 3.1. Characterization of Cobalt-Modified Zeolite NaY. Figure 1 shows the XRD patterns of zeolite NaY modified with cobalt by different methods. The diffraction peaks of the 1CoY, 3CoY, and 5CoY samples were identical to that of NaY, but the intensity slightly decreased as the loading amount of cobalt oxide increased. It appears that cobalt oxide can be adequately dispersed on zeolite NaY provided the loading amount is around 5 wt %, which coincided with the report of Tang et al.23 in which the threshold was 6 wt %. As the loading amount of cobalt oxide increased to 10 wt %, equaling the cobalt nitrate of 18.5 wt % loaded on NaY, two peaks of cobalt nitrate appeared with 2-e` values of 29.4°, 31.8°, and 38.9° on the XRD patterns of the sample prepared by impregnation. Changing the loading method from impregnation to grinding did not improve the dispersion of cobalt salt on zeolite NaY (Figure 1, curves e

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Figure 1. XRD patterns of the cobalt-oxide-modified NaY samples prepared by the following different methods: (a) NaY, (b) 1CoY, (c) 3CoY, (d) 5CoY, (e)18.5% Co(NO3)2/NaY-g, (f)18.5% Co(NO3)2/NaYi, (g) 10CoY, (h) 3CoY-g, (i) CoY-ex, and (j) Co3O4.

Figure 2. Diffuse reflectance spectra of (a) Co3O4, (b) 10CoY, (c) 5CoY, (d) 3CoY, (e) 1CoY, (f) NaY, and (g) Co(NO3)2/NaY samples.

and f). After calcination, the obtained 10CoY sample possessed the characteristic 2θ value of 36.8° for Co3O4, confirming the existence of bulk cobalt oxide in the composite. The characteristic XRD line of crystalline Co3O4 could be identified on the ground sample 3CoY-g but is absent on the ion-exchange sample CoY-ex, while both two samples possessed weaker zeolitic XRD peaks than the impregnation samples, due to the different calcination conditions. After impregnation, the nCoY sample was calcined at 673 K in N2 for 3 h while 3CoY-g and CoY-ex were calcined at 773 K for 10 h in N2 and air. The higher temperature and the longer time of calcination cause the decline of the order of zeolite. Figure 2 depicts the DRS of the cobalt-modified zeolite NaY along with Co3O4 in visible and NIR regions. Bulk Co3O4 (curve a in Figure 2) shows two spectral adsorption peaks in the NIR region. The first one consists of two components, the peak near 1520 nm originates from the crystal field 4A2(F) f 4T1(F) transition in the Co3O4 structure24 and another peak around 1270 nm is attributed to an “intervalence” charge-transfer Co(II) T Co(III).25 The second peak also contains two adsorption bands near 730 and 500 nm, assigned to ligand-metal charge-transfer events O(-II) f Co(III) and O(-II) f Co(II), respectively.26,27

Cao et al.

Figure 3. Diffuse reflectance spectra of (a) NaY, (b) CoY-ex, (c) 3CoY, (d) 3CoY-i, and (e) 3CoY-g samples.

Zeolite NaY (curve f in Figure 2) has six obvious adsorption peaks at 2234, 1906, 1400, 926, 514, and 454 nm. Among them, the broad peak near 1400 nm can be attributed to the first overtone of the fundamental O-H stretching vibration, while the sharp peak at 1906 nm is the combination stretching and bending modes of the water molecules. In addition, the weak peak centered at 2234 nm is the combination stretching and bending of Al-OH or Si-OH groups.28 Impregnating cobalt nitrate on zeolite NaY did not cause an obvious change in the vis-NIR spectrum (curve g in Figure 2). Once the cobalt salt was converted to cobalt oxide, some new bands appeared. In the vis region, a board adsorption band in the range of 400-600 nm (curves b-e) could be assigned to the d-d transition of cobalt oxide;29 the band near 442 nm and the shoulder around 737 nm (curves b-e in Figure 2) reflected the presence of octahedral (Oh) cobalt. The band near 678 nm (curve d in Figure 2) only appeared on the 3CoY sample to indicate the existence of tetrahedral (Td) symmetrical cobalt.30 As the loading amount of cobalt oxide increased, the reflectance signals of the composite decreased and the band of zeolite at 1400 nm declined gradually to a broad peak similar to that of Co3O4, indicating the coverage of cobalt oxide on the surface of zeolite NaY. For the ion-exchange sample CoY-ex, its visNIR spectrum (curve b in Figure 3) showed three continuous peaks at 518, 566, and 638 nm, mirroring the Co(II) ions in distorted tetrahedral symmetry.31 The similarity between 3CoY and CoY-ex samples is the tetrahedral symmetry of cobalt, but the microenvironment of cobalt is different. On the sample of 3CoY, the guest exists in the form of oxide and most of the cobalt combines with oxygen as CoO or Co2O3. For the CoYex sample, however, each Co2+ cation has exchanged two Na+ cations and keeps the ionic state on the ionic sites of zeolite NaY. Therefore, the two samples show different colors, the former is dark gray and the latter is baby blue. On the other hand, calcination temperature has an obvious impact on the DRS of the sample modified with cobalt. Higher temperature calcination favors the formation of more Co(III) than Co(II), along with the octahedral symmetry that becomes exclusive. Compared with the spectra of 3CoY, the 678 nm band of the 3CoY-i sample (curve d in Figure 3) vanished and other reflectance signals diminished. A similar spectrum was found on 3CoY-g (curve e in Figure 3), indicating that the state of cobalt oxide

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Figure 4. H2-TPR curves of the cobalt-modified NaY zeolite samples and cobalt oxide.

was close to that of the 3CoY-i sample. Both the 3CoY-g and 3CoY-i samples have the same cobalt content and the same calcination temperature, and the only difference is the use of solvent. On the basis of the results, it is clear that the impact of solvent on the final state of cobalt in zeolite NaY is minor. The influence of solvent competitive adsorption and migration/ evaporation on the distribution of cobalt cations is covered up by the calcination at 773 K for 10 h. Figure 4 illustrates the H2-TPR curves of cobalt-modified NaY samples along with bulk Co3O4. Cobalt oxide exhibited two-stage reductive process with two peaks around 585 and 622 K, originating from the reduction of Co3O4 to CoO and CoO to cobalt metal, respectively.32,33 Owing to the strong interaction between cobalt species and the framework of zeolite along with the reduction of cobalt species inside the pores and/or small cages of zeolite NaY,23 dispersion of cobalt oxide on NaY enhanced the reduction temperature significantly. For instance, the reduction began around 544 K on the sample of 3CoY, about 60 K higher than that on bulk Co3O4. Besides, the well-dispersed Co3+ and Co2+ had strong interaction with the support to make the reduction of cobalt oxide difficult;30 the time required for the reduction of nCoY samples was thus increased. The reduction of 3CoY was mainly divided into three peaks: the first was centered at 623 K owing to the dispersion of cobalt species in supercage,34 the second had a cap sheaf at 832 K originating from the reduction of cobalt in the sodalite cage, and the third overlapped with the second and its peak was around 937 K that resulted from the reduction of cobalt in hexagonal prisms.34 The quantitative evaluations of these H2-TPR peaks revealed that the proportion of the first peak at 623 K was ca. 23% among all three peaks, while those of the second and third peaks were ca. 44% and 33%, respectively. This suggests that ca. 23% of cobalt oxide clusters are within the supercages while a large part is inside the sodalite cages and hexagonal prisms. Figure 4a reveals the impact of the calcination condition on the reduction of cobalt species in NaY. The reduction curve of the 3CoY-i sample differed from that of 3CoY; the first peak near 629 K became stronger and its proportion increased to ca. 38%, contrary to the second peak around 832 K which declined to 25%, while the third shifted to 961 K and its intensity varied slightly, and the corresponding proportion raised to 36%. It appears that calcination with long time causes the re-distribution of cobalt species, and part of the guest in the sodalite cage migrates toward the supercage. The absence of solvent in preparation of cobalt-modified zeolite also had an influence on the reduction of cobalt-modified NaY. The 3CoY-g sample had

a strong peak at 623 K with a proportion of ca. 38% among all three peaks, nonetheless the second peak at 832 K became very inconspicuous and its proportion was only 21%; meanwhile, the peak at 947 K became very strong and its proportion achieved 41.0%. Without competitive adsorption of solvent, more cobalt species could enter the hexagonal prisms. Together with XRD patterns in which the crystalline Co3O4 was found on the sample of 3CoY-g, it seems that the peaks at 629 K probably include the reduction of supported Co3O4 particles in zeolite apart from those in the supercage, since supported cobalt oxide possesses peaks at 603-673 K corresponding to the stepwise reduction of Co3O4 particles to CoO and then to metallic Co on the support.33,35 The ion-exchanged sample CoY-ex exhibited the highest reduction temperature, 1228 K, among the samples used in the experiments; two reduction peaks at 695 and 840 K could also be identified on CoY-ex, but their intensities were very low. During the ion-exchange process, the Co2+ ion gradually replaced the Na+ ion to obtain a very steady state; therefore, the CoY-ex sample required a high reduction temperature. However, the amount of H2 consumed during the TPR process of the CoY-ex sample corresponded to an H2/Co ratio of 0.23. This value does not obey the stoichiometric complete reduction of CoO (1.0) and Co3O4 (1.33) to Co metal. That is to say, only about 18% of the cobalt cations in the sample were reduced. In contrast, the amount of H2 consumed in the impregnation sample is close to the amount of cobalt loaded, say, 110% in the 3CoY-i sample while 102% in 5CoY-i. It is clear that Co species in the CoY-ex sample do not exist as CoOx clusters or crystallites but as Co2+ cations which reside at the cation exchange sites and hence part of them are very difficult to reduce. Similar results are also found on Co/H-ZSM-5 samples.36 Figure 5 illustrates the results of the NO2-TPD experiment on various cobalt-modified zeolites. Coinciding with that reported by Wang et al.,37 there is a strong desorption of NO2 centered around 412 K on the NaY zeolite, accompanied with a weak one near 608 K while only one desorption peak centered at 551 K emerges on Co3O4 with a low intensity (0.22 mmol/ g). The Si/Al ratio of zeolite NaY is 2.86, and the content of sodium cation in the zeolite can thus be calculated as 2.88 mmol/ g. Upon NO2 adsorption on solid catalyst surfaces, NO+ and NO3- species are formed in disproportionation of NO2:2NO2 T NO+ + NO3- and the formed NO+ binds to an O- site of the zeolite framework, replacing a charge-compensating Na+ ion, while the NO3- binds to a Na+ ion.38 However, in the experiment, zeolite NaY only adsorbs 2.31 mmol/g of NO2,

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Figure 5. NO2-TPD spectra of the cobalt-modified NaY samples prepared by different methods.

similar to a previous report in which the NO2 cationic site coverage achieved 41 %.37 It is clear that not all of the Na+ in the zeolite NaY interacts with the NO2 molecule owing to the space limitation provided by the framework of the zeolite. As the loading amount of cobalt oxide on zeolite NaY increases from 1 to 5 wt %, the second desorption peak of NO2 shifts from 596 to 578 K, mirroring the contribution of the cobalt guest on which desorption of NO2 occurs mainly around 551 K, while the first peak remains unchangeable. Through impregnation, the cobalt species cover some sodium cations in NaY, which decreases NO2 adsorption because of the weak ability of cobalt oxide in adsorption of NO2 (0.22 mmol/g). As the loading amount of cobalt oxide on NaY raises from 1 to 5 wt %, the detected amount of NO2 from the cobalt-modified sample during the TPD process decreases from 2.34 to 1.76 mmol/g. Owing to the inefficient dispersion of cobalt oxide by grinding and partial block of the zeolite channel, the 3CoY-g sample exhibits an inferior adsorption ability of NO2 (1.84 mmol/g), similar to that of 5CoY-i. On the contrary, exchange of NaY with cobalt cation lowers the number but enhances the accessibility of the cation in the zeolite so that the adsorption of NO2 on the sample of CoY-ex increased from 2.31 to 2.86 mmol/g. 3.2. Trapping Volatile Nitrosamine in Airflow. Figure 6 shows the adsorption of nitrosamines on NaY zeolites. All samples exhibited the same ability in the initial stage of adsorption, and all of the nitrosamines could be trapped provided that the total amount of NPYR was below 1.0 mmol/g. As the amount of NPYR continuously rose, variation emerged on different samples. Zeolite NaY could capture about two-thirds of the NPYR in the gaseous phase when the amount of NPYR passed through the adsorbent accumulated to 2.66 mmol/g, while loading 1 wt % of cobalt oxide on NaY enhanced this proportion to 74 % under the same conditions, due to the electrostatic interaction provided by the guest cations to the N-NO group of nitrosamines.9 The 3CoY sample showed the highest adsorptive capability and 92% of NPYR was trapped at 338 K, one-third more than that of the parent zeolite. This high adsorption capacity enables 3CoY to exceed 3% CuO/NaY that was the best adsorbent of NPYR.10 However, loading more cobalt oxide on NaY could not elevate the adsorptive capability further. The 5CoY sample only adsorbed 72% of NPYR, which is the same as that of 1CoY within the experimental error, whereas the adsorption ability of 10CoY was lower than that of the parent zeolite (Figure 6A). Consulting with the XRD patterns of 10CoY where the diffraction peak of Co3O4 emerged, it is clear that loading 10 wt % of cobalt oxide on zeolite NaY

Cao et al. is too excessive so that the guests jam the channel to hinder the penetration and diffusion of nitrosamine molecules. Figure 6B reveals the adsorption of cobalt-modified NaY prepared in different methods. Calcination at 773 K weakened the ability of sample to adsorb NPYR. Under the same conditions mentioned above, the 3CoY-i sample adsorbed 85% of the NPYR in the gas phase while the 3CoY sample trapped 92%. These samples calcined at 773 K, no matter if they were made by the ion-exchange or grinding method, and did not exhibit a higher adsorption ability than their analogues calcined at 673 K in the experiments. The reason may be assigned to the migration of cobalt cations toward sodalite cages in zeolite NaY. During the ion-exchange process, Na+ cations at sites I, I', II, and II' could be replaced by Co2+, whereas sites III and III' were not easily occupied (Scheme 1).39 To maintain the balance of charge, each Co2+ ion will replace two Na+ ions, which will make the total amount of cations decrease. However, cations in zeolites take charge in attracting NPYR so that the CoY-ex sample possesses a weaker adsorption ability than the 3CoY or 3CoY-i samples which have more cations for the adsorption; likewise, the difference in dispersion and accessibility of cobalt cations between the impregnation and ground samples, as revealed by the XRD and NO2-TPD results, can be used to explain why 3CoY-g has a similar inferior adsorption ability. Figure 6C reveals the impact of calcination temperature on the adsorption of 3CoY-i samples, for which all the samples were calcined at different temperatures in N2 for 5 h then in air for another 5 h. As the calcination temperature increased from 673 to 873 K, the resulting sample exhibited a decreased adsorption capacity. When the sample was calcined at 1073 K, its adsorptive capability declined so badly that only about 0.20 mmol/g of NPYR could be trapped, which resulted from the detriment of the zeolite structure by the high-temperature calcination. Figure 6D depicts the adsorption of NPYR by the sample of NaY zeolite modified with different metal oxides. As the accumulated amount of NPYR exceeded 1.2 mmol/g, all of the modified zeolites showed an adsorptive ability superior to NaY itself. When the amount of NPYR achieved 1.8 mmol/ g, the cobalt-modified sample could capture more nitrosamines than the sample modified with copper, iron, or zinc under the same conditions. Figure 7 presents the TG-DSC results of cobalt-modified NaY zeolites. The sample of 3CoY only had a one-step weight loss of about 8% owing to the desorption of physically adsorbed water. After adsorbed NPYR, 3CoY possessed a new exothermic process centered at 554 K in which degradation of NPYR occurred (Figure 7A). The sample of 3CoY-i also exhibited the exothermic process centered at 564 K once it adsorbed NPYR (Figure 7B), and the new-formed weight loss achieved about 13%, similar to that on 3CoY. This result implies that both samples have adsorbed about 1.3 mmol/g of NPYR, equal to the total amount of NPYR used in the experiment to contact with the composite. That is to say, both cobalt-modified zeolite samples trap the all nitrosamines in airflow, confirming their excellent adsorption capability. Figures 8-10 demonstrate the degradation of NPYR on cobalt-modified NaY zeolites. Cleavage of the N-NO band is the first step in nitrosamine degradation, either through a homolytic cleavage of the bond or from NO+ through a heterolytic cleavage of the relevant bond;40 therefore, in Figure 8, the NOx products detected in the reaction represent the decomposed nitrosamines. The 3CoY sample not only decomposed more NPYR (0.172 mmol/g) than NaY (0.152 mmol/g) but also performed the reaction mainly near 573 K, 20 K lower

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Figure 6. Adsorption of NPYR on the cobalt-containing zeolite NaY samples at 338 K.

SCHEME 1: Zeolite NaY Supercage and Cation Locations

than NaY (593 K). Under the same conditions, the sample of 3CoY-i exhibited catalytic activity similar to that of NaY but the desirable temperature of reaction was considerably lower. However, both the amount of nitrosamines degraded and the reaction temperature on the sample of 3-CoY-g were close to that of NaY, hence further study was carried out on the 3CoY and 3CoY-i samples. For degradation of NPYR, the functional group N-NO was first ruptured into the NO· radical and then changed to NO2 so that the signal intensity of NO2 was a lot higher than that of NO. As seen in parts B and C of Figure 8,

NO2 formed on both 3CoY and 3CoY-i from 500 to 675 K, while the signals of NO emerged early around 450 K and ended at 773 K. Judging by the amount of nitrogen oxide products, the sample of 3CoY possesses a higher catalytic activity than its analogue. Figure 8D illustrates the degradation of NPYR on the other sample of NaY modified with different metal oxides, in comparison with the cobalt-containing composite. Judging by the amount of NOx released in the TPSR process of NPYR that represents the amount of nitrosamines decomposed, and the Tmax, the temperature at which the desorption of NOx reaches the maximum, it is clear that the samples containing copper, iron, or zinc exhibit more inferior catalytic behavior than the sample of 3CoY under the same conditions. MS analysis reveals another difference between the two cobalt-modified NaY zeolites. There was an obvious desorption of NPYR which occurred near 402 K on the 3CoY sample while it was faint on the 3CoY-i sample. For the spectrophotometric method used in the TPSR test to monitor the production of nitrogen oxides, the desorbed nitrosamines are invisible so that they cannot be detected in Figure 8A. However, this desorption can be sensitively determined by MS, including NPYR molecules (m/e ) 100) and their ionic fragments such as NO (m/e ) 30) formed during MS measurement. Comparing Figure 9B and Figure 8C,it is clear that the NO signal that emerged on the cobalt-modified NaY around 404 K comes from the desorbed instead of the degraded NPYR. Such desorption was not observed on NaY itself (Figure 9A) and absent in the zeolites modified with copper or ferric oxides,10,41 which implies that such desorption of NPYR around 402 K results from the inherent

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Figure 7. TG-DSC curve of (A) 3CoY and (B) 3CoY-i samples before (-‚-‚-‚) and after (ss) adsorbed NPYR.

Figure 8. Profile of (a) NOx, (b) NO2, and (c) NO released in the TPSR process of NPYR on cobalt-containing zeolite NaY samples; (D) the NOx released from the samples modified with copper, iron, or zinc in the TPSR process of NPYR.

property of cobalt oxide and involves the distribution of the guest. Calcination at 773 K enables the 3CoY-i sample to adsorb the nitrosamines more strongly than the analogue calcined at 673 K, hence the desorption of NPYR below 500 K is suppressed. Figure 10 illustrates the degradation products of NPYR detected by MS on the cobalt-modified NaY zeolites. There are various nitrogen oxides such as NO, NO2 (m/e ) 46), and N2O

(m/e ) 44) in the products, along with nitrogen (m/e ) 28); among them, NO and N2O were detected in two temperature regions, 300-500 and 500-650 K. Referring to Figure 8, the MS signals detected below 500 K originate from the desorbed instead of degraded NPYR so that only those detected above 500 K reflect the degradation of nitrosamines on the composites. On the basis of the results demonstrated in Figures 9B and 10, it is clear that the Tmax value of either NO or N2O desorbed on

Zeolite NaY as Efficient Nitrosamines

J. Phys. Chem. C, Vol. 111, No. 2, 2007 545

Figure 9. MS signal of (A) NPYR and (B) NO detected on 3CoY and 3CoY-i as well as the copper or ferric modified samples in the TPSR process of NPYR.

Figure 10. MS signals of the decomposition products of NPYR that desorbed from cobalt-modified zeolite NaY samples.

the 3CoY sample, at which temperature the maximum concentration of the nitrogen oxide is detected, is 20 K lower than that on the 3CoY-i sample, which indicates the higher activity of the former. Under the same conditions, the main desorption of NO emerged around 543 K on the copper-containing NaY sample while the desorption occurred near 540 K on the sample of ferric-oxide-modified NaY zeolite (Figure 9B). Both the temperatures are slightly higher than that on the 3CoY sample, implying the high catalytic activity of the cobalt-containing

composite in the degradation of volatile nitrosamines. Owing to the specific catalysis of cobalt and isolated octahedral Co2+ sites to decompose nitrous oxide,15,42 the N2 signal appeared in Figure 10A, especially at 739 K on which the NO signal vanished while the N2O and NO2 signals declined. Adsorption of NPYR in cobalt-modified NaY is usually carried out at relative low temperatures, and incorporation of cobalt oxide strengthens the function of the zeolite to trap nitrosamines. Elevated temperature induces desorption of NPYR

546 J. Phys. Chem. C, Vol. 111, No. 2, 2007

Cao et al.

Figure 11. MS signals of (A) pyrrolidine and (B) aniline detected on the 3CoY sample adsorbed NPYR.

SCHEME 2: Hypothesis on the Decomposition Routine of NPYR in the Cobalt-Modified Zeolitea

a

The number in brackets is the m/e value of the compound.

due to its volatility, and the catalytic decomposition will become the main process once the temperature exceeds 500 K. NPYR was first divided into a nitric oxide radical and pyrrolidine radical, which would quickly change into NO and pyrrolidine (m/e ) 71, Figure 11A). Through the disproportionation of nitric species that occurred on the cobalt-modified NaY sample, NO is converted to N2O and NO2, while the Co2+ cations in zeolites catalyze the decomposition of NOx to form N2. At the same time, dehydrogenation of pyrrolidine occurred to form 1Hpyrrole (m/e ) 67) and 2-methyl-1H-pyrrole (m/e ) 81), which involved the creaking of the five ring of NPYR (Scheme 2), because this was the only source of methyl. As the temperature raises over 600 K, new MS signals emerge with the higher m/e values such as 92, 93, and 94, mirroring the occurrence of another complicated reaction to form an aromatic cyclic compound such as aniline (Figure 11B). 3.3. Discussion on the Modification of Zeolite NaY with Cobalt Oxide. Dispersion of cobalt oxide on zeolite NaY differs from that on nonporous materials. For example, on the TiO2

with a surface area of 120 m2/g,43 the monolayer dispersion of cobalt oxide achieved 20.8 mg of CoO/g TiO2, equaling 1.4 Co/nm2. Usually, the dispersed cobalt possessed +2 valence,44 and the bond length of Co-O is 0.21 nm.43 Accordingly, the dispersion threshold of cobalt oxide on the zeolite NaY with a surface area of 766 m2/g should be about 133 mg of CoO/g NaY. However, either the report of Tang et al.23 or our experimental data was much smaller than the value and only achieves around 6 wt %. That means, only half or less of the surface area of NaY is available for the dispersion of cobalt oxide. A similar phenomenon has also observed in the dispersion of copper oxides on NaY where the threshold was near 5 wt %,10 coinciding with the report of Xie and Tang.45 The steric hindrance caused by the zeolite framework made some Na+ cations in specific positions like D6R not able to be exchanged by Rb+ or Cs+;46 these positions would be unavailable for the hydrated Co2+ or Cu2+ cations with bulky volumes. Besides, the diffusion of the impregnating solution is difficult in small micropores, especially if there are residual gas molecules and

Zeolite NaY as Efficient Nitrosamines if the wettability by the solution is poor. Furthermore, impregnated salts may not be tightly anchored on the support surface and thus can redistribute during calcination.47 However, the suspicion whether the guest locates mainly on the external surface of NaY should be excluded, because it is too small to accommodate a cobalt oxide of 5 wt %, hence most of the guest cobalt species should be located inside the channel of zeolite. Here another question arises: why is it that the 3CoY sample instead of the 5CoY sample exhibits the highest capability for trapping nitrosamines? The radii of cobalt and oxygen ions are 0.04 and 0.14 nm, respectively;48 in the case where the cobalt oxide is equally coated onto the pore wall to form the monolayer, the actual pore size of the NaY will be at least reduced from 0.74 nm to below 0.46 nm, which is harmful for fast adsorption of NPYR since the molecule size of NPYR is 0.56 nm.10 To keep the modified channel accessible for NPYR, the residual pore size should be about 0.6 nm, that is, only half of the pore wall could be coated by cobalt oxide. Otherwise, the entry and diffusion of NPYR will be obstructed. The threshold for the monolayer of cobalt oxide on NaY is about 6 wt %,23 consequently loading 3 wt % should be the best amount. In fact, apart from the 3CoY or 3CoY-i sample that showed the highest adsorptive capability (Figure 6), it was the 3% CuO/ NaY that had the highest ability for adsorbing NPYR in the copper-modified zeolite NaY samples.10 Selecting the preparation method and calcination condition can adjust the distribution of cobalt species in zeolite. Impregnation of NaY with a dilute aqueous solution of Co(NO3)2 followed by calcination at 674 K in nitrogen made the proportion of cobalt oxide located in the supercage, sodalite cage, and hexagonal prisms 1:1.9:1.4. Increasing the calcination temperature to 773 K and prolonging the time of calcination along with changing the carrier gas enabled the distribution ratio of cobalt oxide to be 1.5:1:1.4, and a considerable amount of cobalt species migrated into the supercage. A solvent-free method combined with the calcination at 773 K made a large part of cobalt enter the hexagonal prisms so that the distribution ratio changed to 1.9:1:2.0, meanwhile the large sized cobalt oxide particles existed outside the cages of NaY and caused the strongest line of Co3O4 in the XRD patterns (Figure 1). In contrast, the ionexchange process made all of the cobalt cations occupy specific ionic positions in the zeolite. Distribution of cobalt species affects their function in adsorption and catalysis. The CoY-ex sample keeps the unimpeded channel of the parent zeolite, and the exchange of Na+ with Co2+ reduces the absolute number of the cation, therefore the adsorption-desorption of NO2 is considerably improved (Figure 5). However, the molecular size of NPYR is larger than NO2, and the unapproachable position of some cations precludes the contact of the cation with the target molecules, which results in an inferior adsorptive capability on CoY-ex sample (Figure 6B) owing to lack of enough adsorptive sites to attract NPYR. The 3CoY-g sample also showed an inferior capacity in adsorption of NPYR due to the aggregation of cobalt oxide on the external surface of the zeolite and the location of a lot of cobalt species in the hexagonal prisms whose position seems not beneficial for the cation to adsorb nitrosamines. The location of cobalt species in the sodalite cage seems profitable to trap nitrosamines since 3CoY could adsorb more NPYR than 3CoY-i at 338 K, because less cobalt species located in the supercage would vacate a larger space for the adsorption of nitrosamines. However, more NPYR molecules desorbed from 3CoY at ca. 400 K than that from 3CoY-i (Figure 9B), implying that the adsorption of NPYR on the former was weaker than the latter. The reason, in our opinion, is the weaker

J. Phys. Chem. C, Vol. 111, No. 2, 2007 547 attraction provided by the cobalt cations in the sodalite cage than that in the supercage. This phenomenon provides a clue to prepare the reversible adsorbent of nitrosamines. Conclusions The following conclusions can be drawn from this work: (1) Modification of zeolite NaY with cobalt oxide can improve its efficiency in adsorption of nitrosamines, and among various methods, impregnation is suitable to disperse the guest equally in the zeolite and to effectively attract NPYR in airflow. (2) Calcination conditions affect the distribution of cobalt cations in zeolite NaY. If the sample was calcined in nitrogen at 673 K, then the cobalt cations would exist in both octahedral and tetrahedral symmetries and a lot of them would be located in the sodalite cage of zeolite NaY. However, a higher calcination temperature like 773 K accompanied with plenty of oxygen would simplify the state of the cobalt cations, only octahedral symmetry existed, and the cations migrated to the supercage of NaY. (3) The location of cobalt cations in zeolite NaY affects their adsorption capability for NPYR in airflow, loading 3 wt % of cobalt oxide exhibits the highest adsorption capability of NPYR at 338 K. (4) The catalytic ability of cobalt-oxidemodified zeolite NaY is remarkable, not only catalyzing NPYR degraded to less harmful fragments but also converting the NOx products to nitrogen. Acknowledgment. NSF of China (20273031, 20373024, and 20673053) and Analysis Center of Nanjing University financially support this research. References and Notes (1) Magee, P. N.; Barnes, J. M. J. Ind. Med. 1956, 10, 114-122. (2) Shank, R. C.; Magee, P. N. In Mycotoxins and N-nitroso Compounds: EnVironmental Risks; CRC Press: Boca Raton, Florida, 1981; Vol. 1, p 186. (3) Lijinsky, W. Mutat. Res. 1999, 443, 129-138. (4) Meier, M. W.; Siegmann, K. Microporous Mesoporous Mater. 1999, 33, 307-310. (5) Xu, Y.; Wang, Y.; Zhu, J. H.; Ma, L. L.; Liu, L. Stud. Surf. Sci. Catal. 2002, 142, 1489-1496. (6) Zhu, J. H.; Yan, D.; Xia, J. R.; Ma, L. L.; Shen, B. Chemosphere 2001, 44 (5), 949-956. (7) Xu, Y.; Zhu, J. H.; Ma, L. L.; Wei, Y. L.; Shang, X. Y. Microporous Mesoporous Mater. 2003, 60, 125-138. (8) Xu, Y.; Yun, Z. Y.; Zhou, C. F.; Zhou, S. L.; Xu, J. H.; Zhu, J. H. Stud. Surf. Sci. Catal. 2004, 154, 1858-1865. (9) Zhou, C. F.; Yun, Z. Y.; Xu, Y.; Wang, Y. M.; Chen, J.; Zhu, J. H. New J. Chem. 2004, 28 (7), 807-814. (10) Xu, Y.; Liu, H. D.; Zhu, J. H.; Yun, Z. Y.; Xu, J. H.; Wei, Y. L. New J. Chem. 2004, 28, 244-252. (11) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic Press: New York, 1984; Chapter 4. (12) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979; Chapter 5. (13) Ono, Y. Stud. Surf. Sci. Catal. 1990, 54, 185. (14) Maes, A.; Cremers, A. AdV. Chem. Ser. 1973, 121, 230-239. (15) Drago, R. S.; Jurczyk, K.; Kob, N. Appl. Catal., B 1997, 13, 6979. (16) Shuichi, N.; Kenzi, T. J. Phys. Chem. 1983, 87, 315-319. (17) Stakheev, A. Yu.; Lee, C. W.; Park, S. J.; Chong, P. J. Appl. Catal., B 1996, 9, 65-76. (18) Indovina, V.; Campa, M. C.; De Rossi, S.; Ferraris, G. Appl. Catal., B 1996, 8, 315-331. (19) Klier, K. Langmuir 1988, 4, 13-25. (20) Hunger, M.; Schenk, U.; Buchholz, A. J. Phys. Chem. B 2000, 104, 12230-12236. (21) Cao, Y.; Shi, L. Y.; Zhou, C. F.; Yun, Z. Y.; Wang, Y.; Zhu, J. H. EnViron. Sci. Technol. 2005, 39, 7254-7259. (22) Xu, Y.; Jiang, Q.; Cao, Y.; Wei, Y. L.; Yun, Z. Y.; Xu, J. H.; Wang, Y.; Zhou, C. F.; Shi, L. Y.; Zhu, J. H. AdV. Funct. Mater. 2004, 14, 1113-1123. (23) Tang, Q. H.; Zhang, Q. H.; Wang, P.; Wang, Y.; Wan, H. L. Chem. Mater. 2004, 16, 1967-1976.

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