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Unusual adsorption and desorption behaviors of NO and CO on nanoporous nickel phosphate VSB-5: In situ FT-IR and TPD study
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Unusual adsorption and desorption behaviors of NO and CO on nanoporous nickel phosphate VSB-5: In situ FT-IR and TPD study Zhi Chen a,b,∗ , Dantong Zhou a , Tong Gao a , Weihua Shen c , Xiaoping Dong d , Shuichi Naito b , Laishun Qin a , Yuexiang Huang a a College of Materials Science and Engineering, China Jiliang University, No. 258 Xueyuan Street, Xiasha Higher Education District, Hangzhou 310018, Zhejiang Province, PR China b Department of Material & Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan c School of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China d Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 12 October 2014 Received in revised form 5 June 2015 Accepted 8 June 2015 Available online xxx Keywords: Nanoporous nickel phosphate VSB-5 NO adsorption/desorption CO adsorption/desorption Isotopic labeled adsorption In situ FTIR TPD
a b s t r a c t To develop new catalyst for controlling NO emission is important for pollution control. The investigation of NO or CO adsorption could contribute the fundamental understanding the reaction mechanism of de-NOx process and the coordinate state of the specific catalyst. Nanoporous nickel phosphate VSB-5 is an attractive catalyst and is firstly employed as the active component for the adsorption of NO and/or CO. In situ FT-IR and temperature programmed desorption (TPD) are used to characterize the adsorption behaviors. A large amount of NO and/or CO can be adsorbed in the pores of VSB-5 and strong bands from different adsorbed species are observed by in situ FT-IR investigation, which is confirmed by isotopic labeled adsorption. The amount of adsorption at room temperature unusually increases by raising the adsorption temperature to 100–200 ◦ C. The possible reason may be explained by the generation of more active sites at elevated temperature which plays a more important role than corresponding desorption. The adsorption of CO and NO is competitive and the adsorbed CO may be replaced by NO due to a stronger bonding strength of the latter. This report contributes to monitor the active sites in the pores of VSB-5 and also makes VSB-5 to be an active candidate for NO or CO related catalytic process. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The removal of NO is one of the hottest topics around the world since it is one of the main atmospheric pollutants. A large amounts of efforts have been focused on the exploitation of novel kind of catalysts to realize the practical applications or to illuminate the reaction mechanism [1,2]. Among these proposed catalysts, zeolites have shown particularly high catalytic activities for this removal and demonstrate high catalytic performance at low temperature [3–5]. Additionally, notable attentions have been concentrated on the fundamental studies on the mechanism of the process by using in situ FT-IR, XPS, TPD and so on. A large number of fundamental researches using supported noble or transition metal catalysts have been carried out to study the nature of catalytic removal of NO
∗ Corresponding author at: College of Materials Science and Engineering, China Jiliang University, No. 258 Xueyuan Street, Xiasha Higher Education District, Hangzhou 310018, Zhejiang Province, PR China. Tel.: +86 571 86875612; fax: +86 571 86835738. E-mail address: [email protected] (Z. Chen).
by H2 or CO without the presence of O2 and H2 O [6–8]. Transition metal containing zeolites are the most promising SCR catalysts due to the high performance of selective catalytic reduction of nitrogen oxides by hydrocarbons on Ni-ZSM-5 and Co-ZSM-5, etc. [9–11]. In these reports, NO and CO have been employed as the probe molecule to monitor the surface of transition metal ions [9]. The adsorbed species with high stability have been identified as the reaction intermediates contributing to the catalytic efficiency. On the other side, the IR spectroscopy, especially in situ IR, provides significant information about the nature of intermediates, the surface of the catalyst and the bonds formed between the surface of catalyst and adsorbate [11–14]. Great efforts have been developed to exploit the potential substitutes as active components for the replacement of noble metals due to their scarcity and high cost. Nickel phosphate VSB-5 (Versailles-Santa Barbara) is a nanoporous material with 24-ring open framework and high thermal stability [15]. In contrast to traditional aluminosilicate zeolites, VSB-5 has 1-D channels with larger pore size of 1.1 nm which make it to be the host for constructing nanostructure or the pathway for large molecules. Additionally, the different valences and various coordination
http://dx.doi.org/10.1016/j.cattod.2015.06.003 0920-5861/© 2015 Elsevier B.V. All rights reserved.
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numbers of the nickel ions make VSB-5 one of the most promising materials for wide applications. For example, the unsaturated Ni2+ coordinate sites in the framework of VSB-5 are accessible to H2 molecules in the pores making VSB-5 one of the candidate materials for hydrogen storage in comparison with ZSM-5 and active carbon [15–20]. However, although more and more attentions have been attracted on VSB-5, the situation of active unsaturated Ni2+ sites is far from clarity which may hinder the further application of VSB5 material. Additionally, to the best of our knowledge, the use of NO or CO as probe molecular for investigating active sites in the pores of VSB-5 has never been reported, which may also benefit the application of VSB-5 in de-NOx process. In this work, NO or CO has been firstly used as probe molecular for monitoring the surface of nanoporous nickel phosphate VSB-5 which demonstrates unusual adsorption behaviors. In situ FT-IR and temperature-programmed desorption (TPD) techniques are employed to investigate the adsorption/desorption behaviors of NO or CO. NO or CO can be adsorbed on different types of nickel positions in the framework which is confirmed by isotopic labeled adsorption. The amounts of adsorbed NO or CO at elevated temperature are larger than that at ambient temperature which results from more sites are accessible for NO or CO at elevated temperature. The adsorption between CO and NO is competitive and adsorbed CO could be replaced by NO due to the stronger interaction between NO and substrate. This report helps clarifying surface state in the pores of VSB-5 and also makes it to be an active candidate for NO or CO related catalysis. 2. Experimental methods Nickel phosphate VSB-5 was synthesized in hydrothermal conditions according to the literature [17]. Self-standing pellets (50.0 mg) were pressed after 120 ◦ C heating and treated in situ in the IR cell. The FT-IR spectra were recorded at room temperature on JASCO FT/IR 660 instrument equipped with a MCT detector at a resolution of 4 cm−1 with 512 scans. Before adsorption, NO was further purified by passing through a liquid nitrogen cold trap and additional fraction distillation. The wafer was heated at 250 ◦ C under vacuum for 1 h as the pretreatment. The adsorption/desorption time is set as 30 min for all the FTIR experiments. For TPD measurement, 300 mg VSB-5 powder was put into a quartz tube. As a pretreatment, the sample was heated to 250 ◦ C with a ramp of 5 ◦ C min−1 and kept for 1 h under He flow (50 mL min−1 ). The adsorption of NO (0.1 vol% NO balanced with He) was operated at room temperature or at 150 ◦ C for 30 min. The amount of adsorbed NO was calculated from the D-value between the input and output. The following TPD was operated as below: 50 mL min−1 He flowed for 15 min to remove the freely gaseous NO; then, the temperature was raised (5 ◦ C min−1 ) under He flow (50 mL min−1 ); the out-coming species were recorded by quadruple mass spectrometer (QME200, PFEIFFER). 3. Results and discussion 3.1. Adsorption of NO at different temperatures After the 250 ◦ C pretreatment by 1 h evacuation, 30 Torr NO was introduced and the spectrum was recorded, as shown in Fig. 1 (curve a). Two bands (1873 and 1858 cm−1 ) and a shoulder (1886 cm−1 ) are observed in the in situ FTIR spectra. These three bands may be assigned to the mononitrosyl species adsorbed on 3 kinds of Ni2+ at specific positions corresponding to three kinds of Ni in the framework of VSB-5 [9,15,21]. The main band may be assigned to NO adsorbed on Ni2+ while the weak bands at 1858 and 1886 cm−1 may be attributed to the NO adsorbed on Ni2−ı and
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o
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(d) 200 C o
(c) 150 C o (b) 100 C (a) RT 1900
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Ni2+ı , respectively. The Ni2−ı may be resulted from the Ni2+ coordinated with more electrically negative species such as oxo-species while the Ni2+␦ may come from the Ni2+ linked with less electrically negative species [11,22]. The nitrite or nitrate species might be also formed during these processes. However, there exists vigorous signal noise in the region between 1700 cm−1 and 1000 cm−1 overlapping with the adsorption region of nitrite/nitrate complexes (spectrum not shown). This results in the difficulty of identifying them from FTIR spectra. A weak band at 2242 cm−1 is observed on the spectrum adsorbed at 250 ◦ C and could be assigned to NO+ species formed at that temperature [23]. No band is observed in the region of 2300–2100 cm−1 for all other spectra at lower temperatures. This suggests that NO+ species should come from the decomposition of adsorbed NO at high temperature. The sample was heated to different temperatures under NO atmosphere and kept for 30 min at each temperature. Then, the FT-IR spectra were recorded after the temperature was lowered down to room temperature in the presence of NO, as shown in Fig. 1 (curves b–e). The integrated intensity of absorbed species increases about 31% when the temperature is raised to 100 ◦ C suggesting that more NO is adsorbed on some sites at high temperatures, which are inaccessible at lower temperatures. It is known that 5 kinds of OH groups exist in the channels of VSB-1 and interact with each other during heating process [24]. Considering the similarity of the structure between VSB-1 and VSB-5, it is reasonable to suppose that the heating process may cause the interactions between hydroxyl groups of VSB-5. These interactions have an influence on the coordination of nickel ions and produce some unsaturated sites providing more active locations for the adsorption of NO inside the channels. However, the integral intensities decrease when the temperature is higher than 150 ◦ C. Furthermore, the integral intensities are still larger than that at room temperature. On the contrary, the intensity is lower than that at room temperature after heating at 250 ◦ C. This may result from the destruction of some NO adsorption sites during the further heating process since the adsorption is unfavorable to heat. Additionally, decomposition of NO occurs and the formation of N2 and N2 O is also observed during the NO adsorption at higher temperature from the online analysis of GC. This indicates that there has reaction between NO and VSB-5. 3.2. Adsorption of NO under different gas pressures at room temperature Different amounts of NO with varied pressures were introduced after pretreatment and kept for 30 min at room temperature, respectively; then, the FT-IR spectra were recorded, as shown in Fig. 2. Two bands and a shoulder were observed at 1875, 1858 and 1886 cm−1 under 7 × 10−2 Torr NO. The adsorption increases with
Please cite this article in press as: Z. Chen, et al., Unusual adsorption and desorption behaviors of NO and CO on nanoporous nickel phosphate VSB-5: In situ FT-IR and TPD study, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.003
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Wavenumber(cm ) Fig. 2. FT-IR spectra of NO adsorption under different pressures at room temperature.
the increase of pressure and the integrated intensity at 10 Torr is 1.54 times more than that at 7 × 10−2 Torr. However, there is no much difference between the adsorption at 10 Torr and 30 Torr indicating the balance may be attained. Additionally, the position of main band shifts from 1875 to 1873 cm−1 when the pressure is increased to 1 Torr; however, it keeps consistently even the pressure is further increased. This shift suggests that the adsorbed NO species may interact with the surrounding species from the framework of VSB-5 and form different kinds of mononitrosyl species [21]. It is reported that NO preferred to form dinitrosyl species on the Ni2+ surface; however, no dinitrosyl species are observed in above experiments [9,25]. One possible reason may be that the Ni2+ ions are highly occupied and there are no enough vacancies for dinitrosyls. The other possible reason is that the dinitrosyl complexes may be quickly decomposed into linear mononitrosyl species [9,25]. At the same time, the shoulder at 1858 cm−1 of 7 × 10−2 Torr changes into a clear band when the pressure is increased to 1 Torr. These bands have very narrow full width at half maximum (FWHM) which suggests that the Ni sites are homogeneously distributed corresponding to the reported literature [15].
Fig. 4. FT-IR spectra of 10 Torr 14 NO adsorption at different temperatures on VSB-5 after pre-adsorption with labeled 15 NO at room temperature.
pre-adsorbed at room temperature is shown as curve (d). The adsorbed species could be desorbed by evacuation and the desorption is accelerated by raising evacuation temperature due to thermal effects [25,26]. The weak band at 1886 cm−1 is unstable and firstly converts into a weaker shoulder after evacuation at 100 ◦ C. 75% of the integral intensity is lost by evacuating 30 min at 150 ◦ C (curve c). As for the species adsorbed without the heating process, similar behaviors are observed during the desorption process. However, the adsorbed species are less stable comparing those adsorbed at high temperature and 94% of integral intensity loses after evacuating 30 min at 150 ◦ C (curve d). It can be seen that the IR band almost vanish when the evacuation temperature is elevated to 200 ◦ C (curve e). This indicates that complete removal of the species could be achieved by 200 ◦ C evacuation. These results indicate that the species adsorbed at raised temperature have higher stability and the stability should come from the components adsorbed at the sites inaccessible at low temperature. This confirms that heating process produces more accessible sites for NO adsorption.
3.4. Isotopic 15 NO adsorption 3.3. NO desorption by evacuation
Absorbance(a.u)
Fig. 3 shows the desorption spectra of pre-adsorbed NO (30 Torr) at specific temperature. The desorption spectra of those after the above heating-up/cooling-down adsorption process at 250 ◦ C are shown as curve (a)–(c) and (e). The desorption spectrum of NO
0.1 (a) (b)
(a) RT(T) o (b) 100 C(T) o (c) 150 C(T) o (d) 150 C(R) o (e) 200 C(T)
(c) (d) (e)
1950
1900
1850
-1
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Wavenumber(cm ) Fig. 3. FT-IR desorption spectra of pre-adsorbed NO (30 Torr) by evacuating at specific temperature: those pre-adsorbed in the above heating-up/cooling-down adsorption process at 250 ◦ C (curve (a)–(c) and (e)); NO pre-adsorbed at room temperature (curve (d)).
Labeled 15 NO is employed after the pretreatment to further investigate the adsorption/desorption behaviors. 10 Torr 15 NO is introduced at room temperature and the spectrum is shown in Fig. 4 (curve a). Two main bands and a shoulder with narrow FWHM are observed at 1842, 1826 and 1853 cm−1 which is similar as those of normal NO (14 NO). The positions of these bands show about 30 cm−1 red shift comparing with those of 14 NO which confirms that these adsorbed bands should be from the NO species [27]. These bands could be assigned to the mononitryl species and no dinitrosyl species are observed which is in correspondence with the results of 14 NO adsorption. Increase of the pressure or temperature under the NO atmosphere results in the analogous behaviors as those of 14 NO adsorption/desorption. After the adsorption of 15 NO and the following removal of gaseous 15 NO by evacuation at ambient temperature, 10 Torr 14 NO was introduced and the adsorption was operated at different temperatures. The spectra are recorded at room temperature, shown in Fig. 4 (curves b–e). Intense adsorption bands attributed to the characteristics of 14 NO are observed and the bands intensity of isotopic 15 NO simultaneously decreases. The decrease of isotopic 15 NO intensity after introducing 14 NO indicates that adsorbed isotopic 15 NO species is replaced by the gas phase 14 NO. The intensity loss enlarges with the temperature rising and almost all the absorbed 15 NO species may be replaced at 200 ◦ C. Accordingly, the intensities of adsorbed 14 NO species increases with temperature since more
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Wavenumber(cm ) Fig. 5. FT-IR spectra of 10 Torr 14 NO adsorption at different temperatures on VSB-5 after pre-adsorption with labeled 15 NO at 150 ◦ C.
available sites are released by the dissociation of labeling 15 NO at high temperature. However, the absorbed 15 NO species become stronger once the adsorption is operated at 150 ◦ C under 10 Torr of 15 NO atmosphere, as shown in Fig. 5 (curve a). Then, 14 NO was introduced after the removal of gaseous isotopic 15 NO by evacuation and the spectra are shown in Fig. 5 (curve b–f). Adsorption bands from the 14 NO species emerge and the intensity of 15 NO species simultaneously decreases a little. However, the intensity of 15 NO species is still stronger than that of 14 NO species adsorbed at room temperature, which is different from the result shown in Fig. 4 (curve b). The intensities of 14 NO increase with the temperature rise and the maximum is obtained by heating at 200 ◦ C, while at the same time the intensities of 15 NO decrease at the expense. However, obvious bands of 15 NO are still observed even the temperature is raised to 250 ◦ C. This further confirms the above conclusion that the species absorbed at raised temperature are much more stable than those adsorbed at room temperature. 3.5. CO and labeled 13 CO adsorption Fig. 6 shows the in situ FT-IR spectra of adsorbed CO by similar operations as those of NO adsorption. After the adsorption at room temperature for 60 min, the sample was then heated for 30 min at corresponding temperatures and then it was cooled down to room temperature. One main band at 2194 cm−1 is observed after introduction of 10 Torr CO at room temperature, as shown in Fig. 6 (curve a). The deconvolution curve indicates that this main band is composed of two bands at 2185 and 2195 cm−1 , as shown in the inset of
2250 2225 2200 2175 2150 2125 2100 2075 -1
Wavenumber(cm ) Fig. 7. FT-IR spectra of 10 Torr 12 CO adsorption at different temperatures after 13 CO pre-adsorbed at room temperature (the 13 CO gas phase was removed).
Fig. 6. These bands may be assigned to the Ni2+ -CO species adsorbed on different nickel sites [9,12]. No additional carbonyl is observed attributing to its weaker bond strength in comparison with that of nitrosyl species. The integral intensity of the bands increases with temperature rise and reaches the maximum at 200 ◦ C. This indicates that more active sites are available for CO at elevated temperature; on the contrary, the intensity decreases when the temperature is at 250 ◦ C implying that desorption is dominant at that situation. Additionally, the intensities of main bands increase with adsorption time or CO pressure indicating that more CO could be adsorbed with the prolongation. These results are in well accordance with the behaviors of NO adsorption. The intensity for species adsorbed at ambient temperature sharply decreases after evacuation at room temperature and complete removal of the adsorbed species is finished after 150 ◦ C evacuation. This verifies that the CO species adsorbed at room temperature are unstable and also weaker than NO ones. These results, together with the fact that stretching frequency is higher than that of gaseous CO (2143 cm−1 ), indicate that the VSB5 matrix has high electrophilicity and -carbonyls may be formed [9,12]. Fig. 7 shows the adsorption experiment of labeled 13 CO and the following experiments of replacing 13 CO (a) (adsorbed 13 CO) with 12 CO at different temperatures. First of all, 10 Torr labeled 13 CO was introduced and kept at ambient temperature for 60 min. Strong band at 2146 cm−1 with about 50 cm−1 red-shift is observed comparing with that of 12 CO. After the gas phase of 13 CO was removed by evacuation, 10 Torr of 12 CO was introduced for 60 min adsorption at room temperature. The temperature was then raised under 12 CO atmosphere and kept for 30 min at corresponding temperatures, as shown in Fig. 7. The red shift of the band confirms the band at 2196 cm−1 comes from the adsorbed CO species. The integral intensity of adsorbed 13 CO species decreases when 12 CO is introduced and further loss of the intensity is obviously observed with the temperature rise. The decrease of the integral intensity indicates that the adsorbed isotopic 13 CO may be replaced by 12 CO and the replacement accelerates during the heating-up process. 3.6. Co-adsorption of NO and CO
Fig. 6. FT-IR spectra of 12 CO adsorption at different temperatures for 30 min and the inset shows deconvolution of spectrum at room temperature.
Fig. 8 illustrates the spectra of co-adsorption experiments of NO and CO. CO and NO (5 Torr each) were simultaneously introduced to the wafer after pretreatment. Strong band at 1876 cm−1 and two shoulders attributed to the mononitrosyl species are observed, which is same as those NO adsorption in the absence of CO. Only very weak band at 2196 cm−1 attributed to carbonyl is observed indicating the adsorption of CO is blocked by NO. The intensity of
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Ni2+ -NO species increases when the temperature is raised to 150 ◦ C; nevertheless, the intensity of Ni2+ -CO species remains constantly. This confirms that the adsorption of NO and CO is competitive and the formed nitrosyl species are stronger than the carbonyls [28]. To further confirm this conclusion, pre-adsorption of NO were performed by firstly introducing 10 Torr NO and kept at ambient temperature for 30 min. After removing the gaseous NO by evacuation, 10 Torr CO was introduced accompanying with liquid nitrogen trap in whole process, as shown in Fig. 9. Carbonyl species are formed at the beginning and intense band at 2196 cm−1 is observed at room temperature. The intensities of carbonyls increase with heating-up, while the intensities of nitrosyls fall at the mean time. The reappearance of the carbonyls demonstrates that CO adsorption could take place under low equilibrium NO pressure situation as liquid nitrogen cold trap (PNO = 7 × 10−2 Torr). This further verifies that the adsorption of NO and CO is competitive which takes place at the same site. NO can be catalytically reduced by CO at high temperature from results of the on-line GC analysis in a closed gas circulation system. A certain amount of N2 , N2 O and CO2 are produced at 250 ◦ C. The intensities of adsorbed NO do not significantly decrease even the temperature is higher than 200 ◦ C suggesting the reaction rate is low and the adsorbed compounds should act as the reservoirs of NO-CO reaction [29]. 3.7. TPD profiles
Absorbance(a.u)
Fig. 10 illustrates the TPD profile of NO after the adsorption at room temperature. The amount of NO adsorbed after pretreatment at 250 ◦ C for 1 h is calculated from the mass spectra and the value is
0.1
o
(d)250 C o
(c)200 C o
(b)150 C (a)RT 2100
2000
1900
-1
100
150 o
200
Temperature( C)
Fig. 8. FT-IR spectra of co-adsorption of NO and CO: the simultaneous adsorption of 5 Torr NO and 5 Torr CO at different temperatures.
2200
1.00E-13
1800
Wavenumber(cm ) Fig. 9. FT-IR spectra of co-adsorption of NO and CO: 10 Torr CO adsorption at different temperatures on VSB-5 after pre-adsorption with NO at room temperature.
Fig. 10. NO TPD profile on VSB-5 after adsorption of NO at room temperature.
about 0.1 mmol g−1 corresponding to the occupancy of 1/3 of nickel sites with adsorbed NO. The corresponding TPD spectra demonstrate 5 characteristic peaks of desorbed NO centered at 60, 90, 120, 140 and 170 ◦ C from deconvolution. Complete removal of the adsorbed NO species may be achieved at 200 ◦ C corresponding to the results of FTIR desorption experiments. Three peaks of them are assigned to the adsorbed NO species at different Ni2+ ions considering there are one main band and two shoulders in the in situ FT-IR spectra. The other two peaks might come from the decomposition of nitrite or nitrate species formed during the adsorption process which involves dissociation of NO during the TPD process. The existence of nitrite or nitrate complexes is confirmed by the fact that N2 and N2 O, analyzed by online GC, are formed during the NO decomposition at the high temperature in the presence of only NO atmosphere. To obtain the adsorbed amount at higher temperature, the flow of NO is started at 150 ◦ C and kept for 30 min to reach equilibrium. Then, the TPD process is carried on after flowing with He for 15 min and the desorbed amount is 0.05 mmol g−1 . This desorbed amount at high temperature without the cooling down process is 50% of that at room temperature since the adsorption is an exothermal process favoring low temperature. It must point out that this phenomenon is not conflict with the FTIR data which are recorded at room temperature after the heating-up/cooling-down process. The heating process produces more available adsorption sites, inaccessible at low temperature, and more NO can be adsorbed during the cooling-down process. Additionally, some weakly adsorbed NO may be blown away by He flow. It may be deduced that the IR intensity at higher temperature could be lower than that of room temperature if the FTIR spectra are taken at high temperature. The corresponding TPD process demonstrates similar 5 peaks and the complete removal of adsorbed species may be achieved at 200 ◦ C. The other two peaks may also come from the nitrite or nitrate species formed at high temperature adsorption. This is in line with the IR results. The CO TPD presents two CO desorption peaks at 70 and 100 ◦ C, respectively, indicating there are two kinds of adsorbed CO, which is in accordance with two kinds of adsorbed CO in the FT-IR spectrum shown in inset of Fig. 6. The complete removal of adsorbed CO could be finished at 150 ◦ C which is the same with the FTIR spectra. This further verifies that the bond strength of adsorbed CO is weaker than that of adsorbed NO. No more CO peaks are observed because carbonate is difficult to be formed at present conditions. 4. Conclusions In the present study, VSB-5 is firstly used as active components for the adsorption of NO and/or CO and unusual adsorption
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behaviors of NO or CO have been observed for the first time. Large amount of NO or CO can be adsorbed at different nickel sites in the micropores of VSB-5. Strong and sharp IR adsorption bands of mononitrosyl species are observed at ambient temperature which is also confirmed by adsorption of isotopic labeled 15 NO or 13 CO. Increase of the temperature might produce more available sites which are inaccessible for the adsorption at low temperature; this results in the unusual increase of the IR intensity and quantity of adsorbed species. Species adsorbed at high temperature have stronger stability than those adsorbed at ambient temperature which needs high temperature to be replaced by the labeled gas. The adsorption between CO and NO is competitive and the adsorption of CO may be blocked by NO since the latter has stronger bond strength. This study undoubtedly contributes to understand coordinate state of unsaturated Ni ions in the framework of VSB-5 which benefits the further application of VSB-5 material. It also indicates that VSB-5 can be used as active compound for NO or CO adsorption which may play a role in catalytic de-NOx process. The application of VSB-5 in catalytic reduction of NO will be discussed in the next step. Acknowledgments This work is financially supported by Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Sci-Tech University (No. 2014C31026) Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Sci-Tech University (YR2015003), International S&T Cooperation Program of China (No. 2013DFG52490) and the National Natural Science Foundation of China (No. 51372237). References [1] R. Gopalakrishnan, C.H. Bartholomew, ACS Sympos. Ser. 7 (58) (1995) 56. [2] M. Shelef, Chem. Rev. 95 (1995) 209–225.
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Unusual adsorption and desorption behaviors of NO and CO on nanoporous nickel phosphate VSB-5: In situ FT-IR and TPD study Zhi Chen a,b,∗ , Dantong Zhou a , Tong Gao a , Weihua Shen c , Xiaoping Dong d , Shuichi Naito b , Laishun Qin a , Yuexiang Huang a a College of Materials Science and Engineering, China Jiliang University, No. 258 Xueyuan Street, Xiasha Higher Education District, Hangzhou 310018, Zhejiang Province, PR China b Department of Material & Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan c School of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China d Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 12 October 2014 Received in revised form 5 June 2015 Accepted 8 June 2015 Available online xxx Keywords: Nanoporous nickel phosphate VSB-5 NO adsorption/desorption CO adsorption/desorption Isotopic labeled adsorption In situ FTIR TPD
a b s t r a c t To develop new catalyst for controlling NO emission is important for pollution control. The investigation of NO or CO adsorption could contribute the fundamental understanding the reaction mechanism of de-NOx process and the coordinate state of the specific catalyst. Nanoporous nickel phosphate VSB-5 is an attractive catalyst and is firstly employed as the active component for the adsorption of NO and/or CO. In situ FT-IR and temperature programmed desorption (TPD) are used to characterize the adsorption behaviors. A large amount of NO and/or CO can be adsorbed in the pores of VSB-5 and strong bands from different adsorbed species are observed by in situ FT-IR investigation, which is confirmed by isotopic labeled adsorption. The amount of adsorption at room temperature unusually increases by raising the adsorption temperature to 100–200 ◦ C. The possible reason may be explained by the generation of more active sites at elevated temperature which plays a more important role than corresponding desorption. The adsorption of CO and NO is competitive and the adsorbed CO may be replaced by NO due to a stronger bonding strength of the latter. This report contributes to monitor the active sites in the pores of VSB-5 and also makes VSB-5 to be an active candidate for NO or CO related catalytic process. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The removal of NO is one of the hottest topics around the world since it is one of the main atmospheric pollutants. A large amounts of efforts have been focused on the exploitation of novel kind of catalysts to realize the practical applications or to illuminate the reaction mechanism [1,2]. Among these proposed catalysts, zeolites have shown particularly high catalytic activities for this removal and demonstrate high catalytic performance at low temperature [3–5]. Additionally, notable attentions have been concentrated on the fundamental studies on the mechanism of the process by using in situ FT-IR, XPS, TPD and so on. A large number of fundamental researches using supported noble or transition metal catalysts have been carried out to study the nature of catalytic removal of NO
∗ Corresponding author at: College of Materials Science and Engineering, China Jiliang University, No. 258 Xueyuan Street, Xiasha Higher Education District, Hangzhou 310018, Zhejiang Province, PR China. Tel.: +86 571 86875612; fax: +86 571 86835738. E-mail address: [email protected] (Z. Chen).
by H2 or CO without the presence of O2 and H2 O [6–8]. Transition metal containing zeolites are the most promising SCR catalysts due to the high performance of selective catalytic reduction of nitrogen oxides by hydrocarbons on Ni-ZSM-5 and Co-ZSM-5, etc. [9–11]. In these reports, NO and CO have been employed as the probe molecule to monitor the surface of transition metal ions [9]. The adsorbed species with high stability have been identified as the reaction intermediates contributing to the catalytic efficiency. On the other side, the IR spectroscopy, especially in situ IR, provides significant information about the nature of intermediates, the surface of the catalyst and the bonds formed between the surface of catalyst and adsorbate [11–14]. Great efforts have been developed to exploit the potential substitutes as active components for the replacement of noble metals due to their scarcity and high cost. Nickel phosphate VSB-5 (Versailles-Santa Barbara) is a nanoporous material with 24-ring open framework and high thermal stability [15]. In contrast to traditional aluminosilicate zeolites, VSB-5 has 1-D channels with larger pore size of 1.1 nm which make it to be the host for constructing nanostructure or the pathway for large molecules. Additionally, the different valences and various coordination
http://dx.doi.org/10.1016/j.cattod.2015.06.003 0920-5861/© 2015 Elsevier B.V. All rights reserved.
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numbers of the nickel ions make VSB-5 one of the most promising materials for wide applications. For example, the unsaturated Ni2+ coordinate sites in the framework of VSB-5 are accessible to H2 molecules in the pores making VSB-5 one of the candidate materials for hydrogen storage in comparison with ZSM-5 and active carbon [15–20]. However, although more and more attentions have been attracted on VSB-5, the situation of active unsaturated Ni2+ sites is far from clarity which may hinder the further application of VSB5 material. Additionally, to the best of our knowledge, the use of NO or CO as probe molecular for investigating active sites in the pores of VSB-5 has never been reported, which may also benefit the application of VSB-5 in de-NOx process. In this work, NO or CO has been firstly used as probe molecular for monitoring the surface of nanoporous nickel phosphate VSB-5 which demonstrates unusual adsorption behaviors. In situ FT-IR and temperature-programmed desorption (TPD) techniques are employed to investigate the adsorption/desorption behaviors of NO or CO. NO or CO can be adsorbed on different types of nickel positions in the framework which is confirmed by isotopic labeled adsorption. The amounts of adsorbed NO or CO at elevated temperature are larger than that at ambient temperature which results from more sites are accessible for NO or CO at elevated temperature. The adsorption between CO and NO is competitive and adsorbed CO could be replaced by NO due to the stronger interaction between NO and substrate. This report helps clarifying surface state in the pores of VSB-5 and also makes it to be an active candidate for NO or CO related catalysis. 2. Experimental methods Nickel phosphate VSB-5 was synthesized in hydrothermal conditions according to the literature [17]. Self-standing pellets (50.0 mg) were pressed after 120 ◦ C heating and treated in situ in the IR cell. The FT-IR spectra were recorded at room temperature on JASCO FT/IR 660 instrument equipped with a MCT detector at a resolution of 4 cm−1 with 512 scans. Before adsorption, NO was further purified by passing through a liquid nitrogen cold trap and additional fraction distillation. The wafer was heated at 250 ◦ C under vacuum for 1 h as the pretreatment. The adsorption/desorption time is set as 30 min for all the FTIR experiments. For TPD measurement, 300 mg VSB-5 powder was put into a quartz tube. As a pretreatment, the sample was heated to 250 ◦ C with a ramp of 5 ◦ C min−1 and kept for 1 h under He flow (50 mL min−1 ). The adsorption of NO (0.1 vol% NO balanced with He) was operated at room temperature or at 150 ◦ C for 30 min. The amount of adsorbed NO was calculated from the D-value between the input and output. The following TPD was operated as below: 50 mL min−1 He flowed for 15 min to remove the freely gaseous NO; then, the temperature was raised (5 ◦ C min−1 ) under He flow (50 mL min−1 ); the out-coming species were recorded by quadruple mass spectrometer (QME200, PFEIFFER). 3. Results and discussion 3.1. Adsorption of NO at different temperatures After the 250 ◦ C pretreatment by 1 h evacuation, 30 Torr NO was introduced and the spectrum was recorded, as shown in Fig. 1 (curve a). Two bands (1873 and 1858 cm−1 ) and a shoulder (1886 cm−1 ) are observed in the in situ FTIR spectra. These three bands may be assigned to the mononitrosyl species adsorbed on 3 kinds of Ni2+ at specific positions corresponding to three kinds of Ni in the framework of VSB-5 [9,15,21]. The main band may be assigned to NO adsorbed on Ni2+ while the weak bands at 1858 and 1886 cm−1 may be attributed to the NO adsorbed on Ni2−ı and
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o
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(d) 200 C o
(c) 150 C o (b) 100 C (a) RT 1900
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Ni2+ı , respectively. The Ni2−ı may be resulted from the Ni2+ coordinated with more electrically negative species such as oxo-species while the Ni2+␦ may come from the Ni2+ linked with less electrically negative species [11,22]. The nitrite or nitrate species might be also formed during these processes. However, there exists vigorous signal noise in the region between 1700 cm−1 and 1000 cm−1 overlapping with the adsorption region of nitrite/nitrate complexes (spectrum not shown). This results in the difficulty of identifying them from FTIR spectra. A weak band at 2242 cm−1 is observed on the spectrum adsorbed at 250 ◦ C and could be assigned to NO+ species formed at that temperature [23]. No band is observed in the region of 2300–2100 cm−1 for all other spectra at lower temperatures. This suggests that NO+ species should come from the decomposition of adsorbed NO at high temperature. The sample was heated to different temperatures under NO atmosphere and kept for 30 min at each temperature. Then, the FT-IR spectra were recorded after the temperature was lowered down to room temperature in the presence of NO, as shown in Fig. 1 (curves b–e). The integrated intensity of absorbed species increases about 31% when the temperature is raised to 100 ◦ C suggesting that more NO is adsorbed on some sites at high temperatures, which are inaccessible at lower temperatures. It is known that 5 kinds of OH groups exist in the channels of VSB-1 and interact with each other during heating process [24]. Considering the similarity of the structure between VSB-1 and VSB-5, it is reasonable to suppose that the heating process may cause the interactions between hydroxyl groups of VSB-5. These interactions have an influence on the coordination of nickel ions and produce some unsaturated sites providing more active locations for the adsorption of NO inside the channels. However, the integral intensities decrease when the temperature is higher than 150 ◦ C. Furthermore, the integral intensities are still larger than that at room temperature. On the contrary, the intensity is lower than that at room temperature after heating at 250 ◦ C. This may result from the destruction of some NO adsorption sites during the further heating process since the adsorption is unfavorable to heat. Additionally, decomposition of NO occurs and the formation of N2 and N2 O is also observed during the NO adsorption at higher temperature from the online analysis of GC. This indicates that there has reaction between NO and VSB-5. 3.2. Adsorption of NO under different gas pressures at room temperature Different amounts of NO with varied pressures were introduced after pretreatment and kept for 30 min at room temperature, respectively; then, the FT-IR spectra were recorded, as shown in Fig. 2. Two bands and a shoulder were observed at 1875, 1858 and 1886 cm−1 under 7 × 10−2 Torr NO. The adsorption increases with
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Wavenumber(cm ) Fig. 2. FT-IR spectra of NO adsorption under different pressures at room temperature.
the increase of pressure and the integrated intensity at 10 Torr is 1.54 times more than that at 7 × 10−2 Torr. However, there is no much difference between the adsorption at 10 Torr and 30 Torr indicating the balance may be attained. Additionally, the position of main band shifts from 1875 to 1873 cm−1 when the pressure is increased to 1 Torr; however, it keeps consistently even the pressure is further increased. This shift suggests that the adsorbed NO species may interact with the surrounding species from the framework of VSB-5 and form different kinds of mononitrosyl species [21]. It is reported that NO preferred to form dinitrosyl species on the Ni2+ surface; however, no dinitrosyl species are observed in above experiments [9,25]. One possible reason may be that the Ni2+ ions are highly occupied and there are no enough vacancies for dinitrosyls. The other possible reason is that the dinitrosyl complexes may be quickly decomposed into linear mononitrosyl species [9,25]. At the same time, the shoulder at 1858 cm−1 of 7 × 10−2 Torr changes into a clear band when the pressure is increased to 1 Torr. These bands have very narrow full width at half maximum (FWHM) which suggests that the Ni sites are homogeneously distributed corresponding to the reported literature [15].
Fig. 4. FT-IR spectra of 10 Torr 14 NO adsorption at different temperatures on VSB-5 after pre-adsorption with labeled 15 NO at room temperature.
pre-adsorbed at room temperature is shown as curve (d). The adsorbed species could be desorbed by evacuation and the desorption is accelerated by raising evacuation temperature due to thermal effects [25,26]. The weak band at 1886 cm−1 is unstable and firstly converts into a weaker shoulder after evacuation at 100 ◦ C. 75% of the integral intensity is lost by evacuating 30 min at 150 ◦ C (curve c). As for the species adsorbed without the heating process, similar behaviors are observed during the desorption process. However, the adsorbed species are less stable comparing those adsorbed at high temperature and 94% of integral intensity loses after evacuating 30 min at 150 ◦ C (curve d). It can be seen that the IR band almost vanish when the evacuation temperature is elevated to 200 ◦ C (curve e). This indicates that complete removal of the species could be achieved by 200 ◦ C evacuation. These results indicate that the species adsorbed at raised temperature have higher stability and the stability should come from the components adsorbed at the sites inaccessible at low temperature. This confirms that heating process produces more accessible sites for NO adsorption.
3.4. Isotopic 15 NO adsorption 3.3. NO desorption by evacuation
Absorbance(a.u)
Fig. 3 shows the desorption spectra of pre-adsorbed NO (30 Torr) at specific temperature. The desorption spectra of those after the above heating-up/cooling-down adsorption process at 250 ◦ C are shown as curve (a)–(c) and (e). The desorption spectrum of NO
0.1 (a) (b)
(a) RT(T) o (b) 100 C(T) o (c) 150 C(T) o (d) 150 C(R) o (e) 200 C(T)
(c) (d) (e)
1950
1900
1850
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Wavenumber(cm ) Fig. 3. FT-IR desorption spectra of pre-adsorbed NO (30 Torr) by evacuating at specific temperature: those pre-adsorbed in the above heating-up/cooling-down adsorption process at 250 ◦ C (curve (a)–(c) and (e)); NO pre-adsorbed at room temperature (curve (d)).
Labeled 15 NO is employed after the pretreatment to further investigate the adsorption/desorption behaviors. 10 Torr 15 NO is introduced at room temperature and the spectrum is shown in Fig. 4 (curve a). Two main bands and a shoulder with narrow FWHM are observed at 1842, 1826 and 1853 cm−1 which is similar as those of normal NO (14 NO). The positions of these bands show about 30 cm−1 red shift comparing with those of 14 NO which confirms that these adsorbed bands should be from the NO species [27]. These bands could be assigned to the mononitryl species and no dinitrosyl species are observed which is in correspondence with the results of 14 NO adsorption. Increase of the pressure or temperature under the NO atmosphere results in the analogous behaviors as those of 14 NO adsorption/desorption. After the adsorption of 15 NO and the following removal of gaseous 15 NO by evacuation at ambient temperature, 10 Torr 14 NO was introduced and the adsorption was operated at different temperatures. The spectra are recorded at room temperature, shown in Fig. 4 (curves b–e). Intense adsorption bands attributed to the characteristics of 14 NO are observed and the bands intensity of isotopic 15 NO simultaneously decreases. The decrease of isotopic 15 NO intensity after introducing 14 NO indicates that adsorbed isotopic 15 NO species is replaced by the gas phase 14 NO. The intensity loss enlarges with the temperature rising and almost all the absorbed 15 NO species may be replaced at 200 ◦ C. Accordingly, the intensities of adsorbed 14 NO species increases with temperature since more
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available sites are released by the dissociation of labeling 15 NO at high temperature. However, the absorbed 15 NO species become stronger once the adsorption is operated at 150 ◦ C under 10 Torr of 15 NO atmosphere, as shown in Fig. 5 (curve a). Then, 14 NO was introduced after the removal of gaseous isotopic 15 NO by evacuation and the spectra are shown in Fig. 5 (curve b–f). Adsorption bands from the 14 NO species emerge and the intensity of 15 NO species simultaneously decreases a little. However, the intensity of 15 NO species is still stronger than that of 14 NO species adsorbed at room temperature, which is different from the result shown in Fig. 4 (curve b). The intensities of 14 NO increase with the temperature rise and the maximum is obtained by heating at 200 ◦ C, while at the same time the intensities of 15 NO decrease at the expense. However, obvious bands of 15 NO are still observed even the temperature is raised to 250 ◦ C. This further confirms the above conclusion that the species absorbed at raised temperature are much more stable than those adsorbed at room temperature. 3.5. CO and labeled 13 CO adsorption Fig. 6 shows the in situ FT-IR spectra of adsorbed CO by similar operations as those of NO adsorption. After the adsorption at room temperature for 60 min, the sample was then heated for 30 min at corresponding temperatures and then it was cooled down to room temperature. One main band at 2194 cm−1 is observed after introduction of 10 Torr CO at room temperature, as shown in Fig. 6 (curve a). The deconvolution curve indicates that this main band is composed of two bands at 2185 and 2195 cm−1 , as shown in the inset of
2250 2225 2200 2175 2150 2125 2100 2075 -1
Wavenumber(cm ) Fig. 7. FT-IR spectra of 10 Torr 12 CO adsorption at different temperatures after 13 CO pre-adsorbed at room temperature (the 13 CO gas phase was removed).
Fig. 6. These bands may be assigned to the Ni2+ -CO species adsorbed on different nickel sites [9,12]. No additional carbonyl is observed attributing to its weaker bond strength in comparison with that of nitrosyl species. The integral intensity of the bands increases with temperature rise and reaches the maximum at 200 ◦ C. This indicates that more active sites are available for CO at elevated temperature; on the contrary, the intensity decreases when the temperature is at 250 ◦ C implying that desorption is dominant at that situation. Additionally, the intensities of main bands increase with adsorption time or CO pressure indicating that more CO could be adsorbed with the prolongation. These results are in well accordance with the behaviors of NO adsorption. The intensity for species adsorbed at ambient temperature sharply decreases after evacuation at room temperature and complete removal of the adsorbed species is finished after 150 ◦ C evacuation. This verifies that the CO species adsorbed at room temperature are unstable and also weaker than NO ones. These results, together with the fact that stretching frequency is higher than that of gaseous CO (2143 cm−1 ), indicate that the VSB5 matrix has high electrophilicity and -carbonyls may be formed [9,12]. Fig. 7 shows the adsorption experiment of labeled 13 CO and the following experiments of replacing 13 CO (a) (adsorbed 13 CO) with 12 CO at different temperatures. First of all, 10 Torr labeled 13 CO was introduced and kept at ambient temperature for 60 min. Strong band at 2146 cm−1 with about 50 cm−1 red-shift is observed comparing with that of 12 CO. After the gas phase of 13 CO was removed by evacuation, 10 Torr of 12 CO was introduced for 60 min adsorption at room temperature. The temperature was then raised under 12 CO atmosphere and kept for 30 min at corresponding temperatures, as shown in Fig. 7. The red shift of the band confirms the band at 2196 cm−1 comes from the adsorbed CO species. The integral intensity of adsorbed 13 CO species decreases when 12 CO is introduced and further loss of the intensity is obviously observed with the temperature rise. The decrease of the integral intensity indicates that the adsorbed isotopic 13 CO may be replaced by 12 CO and the replacement accelerates during the heating-up process. 3.6. Co-adsorption of NO and CO
Fig. 6. FT-IR spectra of 12 CO adsorption at different temperatures for 30 min and the inset shows deconvolution of spectrum at room temperature.
Fig. 8 illustrates the spectra of co-adsorption experiments of NO and CO. CO and NO (5 Torr each) were simultaneously introduced to the wafer after pretreatment. Strong band at 1876 cm−1 and two shoulders attributed to the mononitrosyl species are observed, which is same as those NO adsorption in the absence of CO. Only very weak band at 2196 cm−1 attributed to carbonyl is observed indicating the adsorption of CO is blocked by NO. The intensity of
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Ni2+ -NO species increases when the temperature is raised to 150 ◦ C; nevertheless, the intensity of Ni2+ -CO species remains constantly. This confirms that the adsorption of NO and CO is competitive and the formed nitrosyl species are stronger than the carbonyls [28]. To further confirm this conclusion, pre-adsorption of NO were performed by firstly introducing 10 Torr NO and kept at ambient temperature for 30 min. After removing the gaseous NO by evacuation, 10 Torr CO was introduced accompanying with liquid nitrogen trap in whole process, as shown in Fig. 9. Carbonyl species are formed at the beginning and intense band at 2196 cm−1 is observed at room temperature. The intensities of carbonyls increase with heating-up, while the intensities of nitrosyls fall at the mean time. The reappearance of the carbonyls demonstrates that CO adsorption could take place under low equilibrium NO pressure situation as liquid nitrogen cold trap (PNO = 7 × 10−2 Torr). This further verifies that the adsorption of NO and CO is competitive which takes place at the same site. NO can be catalytically reduced by CO at high temperature from results of the on-line GC analysis in a closed gas circulation system. A certain amount of N2 , N2 O and CO2 are produced at 250 ◦ C. The intensities of adsorbed NO do not significantly decrease even the temperature is higher than 200 ◦ C suggesting the reaction rate is low and the adsorbed compounds should act as the reservoirs of NO-CO reaction [29]. 3.7. TPD profiles
Absorbance(a.u)
Fig. 10 illustrates the TPD profile of NO after the adsorption at room temperature. The amount of NO adsorbed after pretreatment at 250 ◦ C for 1 h is calculated from the mass spectra and the value is
0.1
o
(d)250 C o
(c)200 C o
(b)150 C (a)RT 2100
2000
1900
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100
150 o
200
Temperature( C)
Fig. 8. FT-IR spectra of co-adsorption of NO and CO: the simultaneous adsorption of 5 Torr NO and 5 Torr CO at different temperatures.
2200
1.00E-13
1800
Wavenumber(cm ) Fig. 9. FT-IR spectra of co-adsorption of NO and CO: 10 Torr CO adsorption at different temperatures on VSB-5 after pre-adsorption with NO at room temperature.
Fig. 10. NO TPD profile on VSB-5 after adsorption of NO at room temperature.
about 0.1 mmol g−1 corresponding to the occupancy of 1/3 of nickel sites with adsorbed NO. The corresponding TPD spectra demonstrate 5 characteristic peaks of desorbed NO centered at 60, 90, 120, 140 and 170 ◦ C from deconvolution. Complete removal of the adsorbed NO species may be achieved at 200 ◦ C corresponding to the results of FTIR desorption experiments. Three peaks of them are assigned to the adsorbed NO species at different Ni2+ ions considering there are one main band and two shoulders in the in situ FT-IR spectra. The other two peaks might come from the decomposition of nitrite or nitrate species formed during the adsorption process which involves dissociation of NO during the TPD process. The existence of nitrite or nitrate complexes is confirmed by the fact that N2 and N2 O, analyzed by online GC, are formed during the NO decomposition at the high temperature in the presence of only NO atmosphere. To obtain the adsorbed amount at higher temperature, the flow of NO is started at 150 ◦ C and kept for 30 min to reach equilibrium. Then, the TPD process is carried on after flowing with He for 15 min and the desorbed amount is 0.05 mmol g−1 . This desorbed amount at high temperature without the cooling down process is 50% of that at room temperature since the adsorption is an exothermal process favoring low temperature. It must point out that this phenomenon is not conflict with the FTIR data which are recorded at room temperature after the heating-up/cooling-down process. The heating process produces more available adsorption sites, inaccessible at low temperature, and more NO can be adsorbed during the cooling-down process. Additionally, some weakly adsorbed NO may be blown away by He flow. It may be deduced that the IR intensity at higher temperature could be lower than that of room temperature if the FTIR spectra are taken at high temperature. The corresponding TPD process demonstrates similar 5 peaks and the complete removal of adsorbed species may be achieved at 200 ◦ C. The other two peaks may also come from the nitrite or nitrate species formed at high temperature adsorption. This is in line with the IR results. The CO TPD presents two CO desorption peaks at 70 and 100 ◦ C, respectively, indicating there are two kinds of adsorbed CO, which is in accordance with two kinds of adsorbed CO in the FT-IR spectrum shown in inset of Fig. 6. The complete removal of adsorbed CO could be finished at 150 ◦ C which is the same with the FTIR spectra. This further verifies that the bond strength of adsorbed CO is weaker than that of adsorbed NO. No more CO peaks are observed because carbonate is difficult to be formed at present conditions. 4. Conclusions In the present study, VSB-5 is firstly used as active components for the adsorption of NO and/or CO and unusual adsorption
Please cite this article in press as: Z. Chen, et al., Unusual adsorption and desorption behaviors of NO and CO on nanoporous nickel phosphate VSB-5: In situ FT-IR and TPD study, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.003
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behaviors of NO or CO have been observed for the first time. Large amount of NO or CO can be adsorbed at different nickel sites in the micropores of VSB-5. Strong and sharp IR adsorption bands of mononitrosyl species are observed at ambient temperature which is also confirmed by adsorption of isotopic labeled 15 NO or 13 CO. Increase of the temperature might produce more available sites which are inaccessible for the adsorption at low temperature; this results in the unusual increase of the IR intensity and quantity of adsorbed species. Species adsorbed at high temperature have stronger stability than those adsorbed at ambient temperature which needs high temperature to be replaced by the labeled gas. The adsorption between CO and NO is competitive and the adsorption of CO may be blocked by NO since the latter has stronger bond strength. This study undoubtedly contributes to understand coordinate state of unsaturated Ni ions in the framework of VSB-5 which benefits the further application of VSB-5 material. It also indicates that VSB-5 can be used as active compound for NO or CO adsorption which may play a role in catalytic de-NOx process. The application of VSB-5 in catalytic reduction of NO will be discussed in the next step. Acknowledgments This work is financially supported by Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Sci-Tech University (No. 2014C31026) Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Sci-Tech University (YR2015003), International S&T Cooperation Program of China (No. 2013DFG52490) and the National Natural Science Foundation of China (No. 51372237). References [1] R. Gopalakrishnan, C.H. Bartholomew, ACS Sympos. Ser. 7 (58) (1995) 56. [2] M. Shelef, Chem. Rev. 95 (1995) 209–225.
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Please cite this article in press as: Z. Chen, et al., Unusual adsorption and desorption behaviors of NO and CO on nanoporous nickel phosphate VSB-5: In situ FT-IR and TPD study, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.06.003