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The similarities and differences in structural characteristics and physico-chemical properties of AgAlBEA and AgSiBEA zeolites
Accepted Manuscript The similarities and differences in structural characteristics and physico-chem‐ ical properties of AgAlBEA and AgSiBEA zeolites Stanislaw Dzwigaj, Nataliia Popovych, Pavlo Kyriienko, Jean-Marc Krafft, Sergiy Soloviev PII: DOI: Reference:
S1387-1811(13)00385-5 http://dx.doi.org/10.1016/j.micromeso.2013.08.009 MICMAT 6166
To appear in:
Microporous and Mesoporous Materials
Received Date: Revised Date: Accepted Date:
29 April 2013 10 July 2013 10 August 2013
Please cite this article as: S. Dzwigaj, N. Popovych, P. Kyriienko, J-M. Krafft, S. Soloviev, The similarities and differences in structural characteristics and physico-chemical properties of AgAlBEA and AgSiBEA zeolites, Microporous and Mesoporous Materials (2013), doi: http://dx.doi.org/10.1016/j.micromeso.2013.08.009
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1 The similarities and differences in structural characteristics and physico-chemical properties of AgAlBEA and AgSiBEA zeolites Stanislaw Dzwigaja, b, *, Nataliia Popovychc, *, Pavlo Kyriienko c, Jean-Marc Kraffta, b, Sergiy Solovievc a
UPMC Univ Paris 06, Laboratoire de Réactivité de Surface, Case 178, Site d’Ivry-Le Raphaël,
3 rue Galilée, 94200, Ivry sur Seine, France b
CNRS-UMR 7197, Laboratoire de Réactivité de Surface, Case 178, Site d’Ivry-Le Raphaël,
3 rue Galilée, 94200, Ivry sur Seine, France c
L.V. Pisarzhevsky Institute of Physical Chemistry of the NAS of Ukraine,
31 Prosp. Nauky, 03028 Kyiv, Ukraine
Figures: 9 Scheme: 1 Table: 1
Keywords: SiBEA, AlBEA, silver, acidity, CO, pyridine
* Corresponding authors: S. Dzwigaj, E-mail address: [email protected], fax: +33 01 44 27 60 33. N. Popovych, E-mail address: [email protected], fax: +38 (044) 525 65 90.
2 Abstract AgAlBEA and AgSiBEA zeolites have been prepared by two preparation procedures. The conventional wet impregnation is used to obtain AgAlBEA with silver present mainly in extra-framework position. The two-step postsynthesis procedure which consists of first creating the vacant T-atom sites with associated silanol groups by treatment of TEABEA zeolite with nitric acid and then impregnating of resulting SiBEA zeolite with an aqueous solution of AgNO3 is used to obtain AgSiBEA zeolites with silver present mainly in framework position. The incorporation of Ag ions into the vacant T-atom sites of the framework of SiBEA zeolite as mononuclear Ag(I) has been evidenced by combined use of P-XRD, diffuse reflectance UV-vis, FTIR and XPS. The consumption of OH groups has been monitored by FTIR. The Brønsted and Lewis acidity of AlBEA, AgAlBEA, SiBEA and AgSiBEA has been identified by FTIR spectroscopy of adsorbed pyridine and CO as probe molecules. The catalytic activities of AgAlBEA and AgSiBEA in SCR of NO with ethanol have been investigated. These studies show that silver environment and acid-base properties play a significant role in catalytic activities of these Ag containing BEA zeolites.
3 1. Introduction Recent legislation in industrialized countries places more stringent regulations on the emissions of vehicles and mobile energy generators powered by diesel. Selective catalytic reduction of nitrogen oxides (NOx) in these emissions with hydrocarbon or oxygenates as reducing agents seems to be a perspective approach, which brings about the requirement for a novel catalyst that is able to reduce NOx in an excess of oxygen. Extensive research has been done on the development of de-NOx catalysts and many types of catalysts have been reported [110]. Silver-based materials (generally silver-alumina) were found to be the most active and selective catalytic systems for SCR of NOx with hydrocarbon or oxygenated organic reducing agents [11-19]. As known, materials based on BEA zeolites are widely studied as catalysts for industry and environmental protection due to their large surface area, shape selectivity, strong acidity and efficient activity [20-29]. Many works are devoted to the study of M-BEA (M: Fe, Cu, Co) zeolites as promising catalysts for SCR of NOx with hydrocarbons and oxygenates [2329]. Nevertheless, there are only little research data in literature about Ag-containing zeolite catalysts applied in the SCR of NOx [17-19,30]. According to previous reports, well dispersed silver species (cations and small clusters) seem to be active catalytic centers of SCR of NO on Ag/Al2O3 and Ag-MFI zeolites [7,15,16,18,31,32]. However, dispersing of Ag in these catalytic systems are non selective and they contain silver in the form of various species (ions, nanoclusters, metallic nanoparticles) [33-36]. To obtain well dispersed silver species as isolated mononuclear Ag(I) we have used in the present work the two-step postsynthesis method developed earlier for preparation metalcontaining BEA zeolites [37-40]. This postsynthesis procedure allowed for low metal content (< 2-3 wt.%) incorporating of metal ions in the framework of zeolite mainly as isolated and homogeneously distributed metal species without formation of metal oligomers or metal oxides. The incorporation of silver ions in the vacant T-atom sites of BEA zeolite as isolated mononuclear Ag(I) species has been evidenced by P-XRD, FTIR, diffuse reflectance (DR) UV-
4 vis and XPS. The silver-containing BEA zeolites have been also prepared by a conventional wet impregnation with aqueous AgNO3 solution. Thus, we have used in this work two preparation procedures, the two-step postsynthesis method [37-46] in order to obtain AgSiBEA zeolites with silver present mainly in framework position and the conventional wet impregnation in order to obtain AgAlBEA with silver present mainly in extra-framework position. The present work is aimed to investigate the effect of the preparation way on the physicochemical properties of silver-containing BEA zeolites.
2. Experimental 2.1. Materials Silver-containing SiBEA zeolite was prepared by the two-step postsynthesis method developed earlier for preparation of V-loaded BEA zeolite [37,41-43] allowing control the incorporation of Ag ions in the zeolite framework. In the first step, the TEABEA zeolite provided by RIPP (China) was treated in a 13 mol·L−1 HNO3 aqueous solution (4 h, 353 K) to obtain a dealuminated and organic-free SiBEA support (Si/Al =1000) with the vacant T-atom sites (T = Al). SiBEA was then recovered by centrifugation, washed with distilled water and dried at 353 K. To incorporate Ag ions in the vacant T-atom sites, 2 g of SiBEA was firstly stirred under aerobic conditions for 2 h at 298 K in 200 mL of AgNO3 (Fluka silver nitrate with high Ph Eur purity with Ag > 99.8 %) aqueous solution (pH = 2.3) with concentration of 5.4 × 10 −3 mol·L−1. Then, the suspension was stirred in evaporator under vacuum of a water pump for 2 h in air at 353 K until the water was completely evaporated. The resulting solid containing 3.0 Ag wt. % was labeled as Ag3.0SiBEA. To prepare AgAlBEA zeolite, firstly the TEABEA was calcined at 823 K for 15 h in air to obtain AlBEA (Si/Al = 12.5) and secondly, the latter was impregnated at 298 K for 2h with 200 mL of aqueous AgNO3 solution at pH = 3.1 with concentration of 2.6 and 9.8 ×10 −3 mol·L−1.
5 Then, the suspensions were stirred in evaporator under vacuum of a water pump for 2 h in air at 353 K until the water was completely evaporated. The resulting solids containing 1.5 and 4.0 Ag wt. % were labeled as Ag1.5AlBEA and Ag4.0AlBEA, respectively.
2.2. Techniques X-Ray Fluorescence chemical analysis was performed at room temperature on a SPECTRO X-LabPro apparatus. Powder XRD (P-XRD) were recorded at room temperature and ambient atmosphere on a Bruker D8 Advance diffractometer using the CuKa radiation (λ = 154.05 pm). Specific surface area and adsorption isotherms of nitrogen at 77 K were measured on a Sorptomatic 1990 apparatus. All samples were outgassed first at room temperature, then at 623 K to a pressure < 0.2 Pa. The surface areas were determined from nitrogen adsorption values for five relative pressures (P/P0) ranging from 0.006 to 0.074 using the BET method. The microporous pore volume was determined from the amount of N2 adsorbed up to P/P0 = 0.1. TEM micrographs were obtained on SELMI TEM-125K microscope operating at 100 kV. For TEM measurements samples were dispersed in acetone with ultrasound and deposited on Cu grid covered with carbon. Interplanar spacings (d hkl) calculated from the diffraction ring pattern were compared with the ASTM data. DR UV–vis spectra were recorded under ambient conditions on a Specord M40 (Carl Zeiss) with a standard diffuse reflectance unit. X-ray photoelectron spectroscopy (XPS) measurements were performed with a hemispherical analyzer (PHOIBOS 100, SPECS Gmbh) using MgKα (1253.6 eV) radiation. The power of the X-ray source was 300 W. The area of the sample analyzed was ~ 3 mm2. The powder samples were pressed on an indium foil and mounted on a special holder. Binding energy (BE) for Si and Ag was measured by reference to the O 1s peak at 532.5 eV, corresponding to the binding energy of oxygen bonded to silicon. Before analysis, samples were
6 outgassed at room temperature to a pressure of 10-7 Pa. All spectra were fitted with a Voigt function (a 70/30 composition of Gaussian and Lorentzian functions) in order to determine the number of components under each XPS peak. Analysis of the acidic properties of samples was performed by adsorption of pyridine (Py) and CO followed by infrared spectroscopy. Before analysis, the samples were pressed at ~ 1 ton·cm-2 into thin wafers of ca. 10 mg·cm-2 and placed inside the IR cell. Before CO adsorption experiment, the wafers were activated by calcination at 723 K for 2 h in flowing 2.5 % O2/Ar and then outgassed at 573 K (10-3 Pa) for 1 h. Following thermal treatment, the samples were cooled down to 100 K. CO was introduced in increasing amounts up to an equilibrium pressure of 133 Pa. Infrared spectra were recorded using a Bruker Vertex 70 spectrometer (resolution 2 cm-1, 128 scans). The spectra were obtained after subtraction of the spectrum recorded after calcination and prior to CO adsorption. Before pyridine adsorption/desorption experiments, the wafers were activated by calcination in static conditions at 773 K for 1 h in O2 (2 · 104 Pa) and then outgassing under secondary vacuum at 673 K (10 -3 Pa) for 1 h. These wafers were contacted at 423 K with gaseous pyridine. The spectra were recorded after pyridine desorption at 423, 573 and 673 K using a Spectrum One FTIR spectrometer (resolution 1 cm-1, 12 scans). The reported spectra were obtained after subtraction of the spectrum recorded after calcination and prior to pyridine adsorption. Catalytic activity tests were carried out in a fixed-bed flow quartz reactor at atmospheric pressure. Samples with grains of 1-2 mm (0.5 cm3, 0.3 g) were loaded into the reactor. Gas feed for the reaction was 500 ppm NO, 1000 ppm C2H5OH, 10 % O2 in He, and the gas hour space velocity was 24 000 h−1. The gas feed was adjusted by mass-flow controllers (Chromatek-Crystal FGP). Before reaction, the catalyst was heated to 773 K at a heating rate of 20 K·min−1 in a flow of O2/He and held for 1 h, then was cooled to 453 K with a further step-heating in reaction gas feed to a temperature of conversion measurement. The steady-state activity was measured in the
7 temperature range of 473-773 K after 30 min reaction at a certain temperature. The temperature was controlled through an Autonics TZN4S temperature controller using a Chromel-Alumel thermocouple. The concentration of NO was continuously monitored using a chemiluminescence gas analyzer (344HL04, Ukraine). The products were analyzed by gas chromatograph (TCD) (Kristallyuks 4000M, Metachrom, Russia) with a CaA column (for NO, N2, CO) and a Polisorb1 column (for N2O, CO2, С2Н4, С2Н4О, ethanol). Catalytic activity was characterized by NO conversion and temperatures of its achievement. Conversion of NO was calculated as X(NO)=(1[NO]out /[NO]in)⋅100, where the subscript ‘‘in’’ refers to initial concentration whereas ‘‘out’’ means after reaction.
3. Results and discussion 3.1. Structural and textural properties The P-XRD patterns of AlBEA, Ag1.5AlBEA, Ag4.0AlBEA, SiBEA and Ag3.0SiBEA are typical of BEA zeolite. As shown in Figure 1, the P-XRD patterns are very similar which indicates that the crystallinity of BEA zeolite is preserved after dealumination and introduction of silver ions in AlBEA and SiBEA. This behavior of BEA zeolite upon dealumination is related to the presence in its origin structure of some amounts of vacant T-atom sites. Because of this the zeolite structure is flexible and allows removing almost all Al atoms from the BEA zeolite without losing of its crystallinity. A narrow diffraction peak near 22.5-22.6° is generally taken as evidence of lattice contraction/expansion of the BEA structure [47,48]. The d 302 spacing related to this peak decreases from 3.950 (AlBEA) (with 2θ of 22.48°) to 3.930 Å (SiBEA) (with 2θ of 22.60°) upon dealumination, indicating some contraction of the BEA matrix. However, it increases after silver introduction to 3.961 Å (Ag3.0SiBEA) (with 2θ of 22.42°), which indicates some expansion of the BEA structure and suggests that silver is incorporated into the vacant-T-atom sites of the framework of SiBEA zeolite. In contrast, the introduction of silver ions into AlBEA by
8 conventional wet impregnation does not lead to any change in the BEA structure, as evidenced by the same value of d302 spacing (3.950 Å) for AlBEA and Ag4.0AlBEA (with 2θ of 22.48°), indicating that silver is not incorporated into the framework of BEA zeolite. The texture of samples has been characterized by nitrogen adsorption/desorption and TEM. The acid treatment of AlBEA induced a slight decrease of SiBEA BET surface area (Table 1) due to the decreasing of microporous volume. The acid treatment leads to slight increasing of the contribution of the mesopore volume (isotherms of AlBEA and SiBEA have been reported in our earlier work [49]). Adsorption and desorption isotherms of Ag4.0AlBEA and Ag3.0SiBEA (Fig. 2) indicate that the incorporation of silver does not affect the crystallinity and does not lead to formation of mesopores. The similar specific surface areas for AlBEA and Ag4.0AlBEA, SiBEA and Ag3.0SiBEA (Table 1) and the absence of extra-framework crystalline compounds or long-range amorphization of the Ag4.0AlBEA and Ag3.0SiBEA zeolites indicate that silver is incorporated as well dispersed species. On the micrographs of Ag4.0AlBEA and Ag3.0SiBEA calcined at 773 K no silver particles are seen and no clear ring diffraction patterns of silver are visible (Figures in supplementary material) which may indicate that silver is highly dispersed in the zeolites. The interplanar dhkl spacings for the most contrast electron diffraction rings are equal to 3.949 Å for Ag4.0AlBEA and 3.970 Å for Ag3.0SiBEA. They can be attributed to the interplanar d302 spacing of BEA zeolite, which corroborate the P-XRD results. Silver in the zeolite can be seen only after reduction of Ag4.0AlBEA and Ag3.0SiBEA with hydrogen (5% H2/Ar, 773 K, 1 h). Diffraction rings appeared on pattern of Ag4.0AlBEA (with d hkl of 2.354, 2.007, 1.242 Å) and Ag3.0SiBEA (2.353, 1.175, 0.934 Å) after H2 reduction can be respectively assigned to (111), (200), (311) and (111), (222), (331) planes of the face-centered cubic metallic silver.
3.2. Nature of the silver species
9 The electronic structure of a silver-containing zeolite material can be regarded as a superposition of the electronic structure of the framework, of the solvent molecules (water) and of the silver species, in line with earlier reports [34,50]. The results of DR UV-vis investigations of AlBEA, Ag1.5AlBEA, Ag4.0AlBEA, SiBEA and Ag3.0SiBEA zeolites are displayed in Fig. 3. Silver-free AlBEA and SiBEA absorb light within the spectral range of 200-320 nm. Absorption bands at 282 nm for AlBEA, 208 and 284 nm for SiBEA are probably related to charge transfer transitions in BEA framework. Different shapes of AlBEA and SiBEA absorption spectra can be related to the Al3+ centers in AlBEA zeolite, causing a negatively charged framework (AlO2-), which influence the band structure to some extent [50]. The Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA samples exhibit two bands at 214-215 and 233-238 nm attributed to the charge transfer transition between 4d10 and 4d95s1 level of highly dispersed Ag+ species, in line with earlier report [18,51-53]. Furthermore, as was shown in earlier report [34,50], the band around 215 nm can be attributed to a charge transfer transition from Ag+ coordinated water molecules to the empty 5s state of the Ag+. The position of absorption band is also dependent on silver sites. Three different Ag+ sites were identified by Gellens et al. [54], namely Ag+ at 6- and 8-ring sites, and at 4-ring sites in the sodalite cage. Ag+ coordinated to 6- and 8-ring oxygen give rise to electronic transition in the range of 220-255 cm1
. These results indicate that in as-prepared Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA samples
well dispersed Ag+ species are present. The absence of bands at higher wavelength of 260-350 and 450-500 nm suggests that silver nanoclusters (Agnδ+) and metallic silver nanoparticles are not present in Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA. Our preliminary EPR results show that for activated Ag3.0SiBEA (outgassed at 773 K for 2 h under dynamic vacuum of 10 -3 Pa) a paramagnetic signal with giso = 2.00149 appears characteristic of Ag(H2O)2 species, in line with earlier report of R.A. Schoonheydt [55]. In activated Ag3.0SiBEA, the Ag atoms have isotropic g-values slightly below the free atom value.
10 It is probably related to strong interaction of Ag species with zeolite structure and suggests that silver atoms are present in the framework of zeolite as mononuclear Ag(I). It confirms above DR UV-vis results. XPS was used to determine the chemical state of the silver species present in silvercontaining AlBEA and SiBEA. Figure 4 shows the Ag 3d 3/2 and 3d5/2 bands of Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA zeolites. One peak at binding energy (BE) of 368.3-368.4 eV (Ag 3d 5/2) and at BE of 374.3-374.4 eV (Ag 3d3/2) for Ag1.5AlBEA and Ag4.0AlBEA, as well as at 368.0 eV (Ag 3d 5/2) and 374.0 eV (Ag 3d 3/2) for Ag3.0SiBEA indicates that Ag+ is the main silver species in both AgAlBEA and AgSiBEA zeolites, in line with earlier reports for silver in zeolites [56,57]. Differences in binding energies of Ag 3d5/2 and Ag 3d3/2 of AgAlBEA and AgSiBEA are related to different silver environment in both zeolites, in extra-framework and framework positions respectively, as proposed in Scheme 1.
3.3. FTIR characterization of the hydroxyl groups The FTIR spectrum of AlBEA (Fig. 5) exhibits bands attributed to Al-OH (bands at 3781 and 3667 cm-1), zeolite acidic hydroxyls Al-O(H)-Si (band at 3615 cm-1) and Si-OH (band at 3750 cm-1 with a shoulder at 3740 cm-1) groups, in line with earlier work [58,59]. The band at 3750 cm-1 is sharp, indicating that the Si-OH groups are isolated [46,60]. Comparison of the spectrum of Ag1.5AlBEA with that of AlBEA shows that the Al-OH bands at 3781 and 3667 cm-1 have decreased in intensity, indicating that some of the Al-OH groups have been consumed during their reaction with silver precursor upon conventional wet impregnation. Upon dealumination, the three bands attributed to Al-OH and Al-O(H)-Si groups are eliminated, suggesting that aluminum is fully removed, and bands at 3739, 3709 and 3529 cm-1 appear (SiBEA, Fig. 5), attributed to isolated internal, terminal internal and hydrogen bonded SiOH, respectively, located in the vacant T-atom sites of the framework of SiBEA zeolite forming hydroxyl nests (Scheme 1), in line with earlier assignments [37,46,61,62].
11 Introduction of silver into SiBEA markedly changes the spectra (Ag3.0SiBEA, Fig. 5) with almost total disappearance of the FTIR bands at 3739, 3709 and 3529 cm-1 of isolated internal, terminal internal and hydrogen bonded Si-OH groups suggesting that these silanol groups react with the Ag precursor (Scheme 1). The consumption of silanol groups of the vacant T-atom sites monitored by FTIR is very high and after that the band at 3750 cm-1 of isolated external silanol groups and the bands at 3739 and 3709 cm-1 with very low intensity are only present in Ag3.0SiBEA (Fig. 5).
3.4. FTIR characterization of acidic centers: adsorption of CO and pyridine CO is often used to simultaneously probe Lewis and Brønsted acidic sites. Difference spectra between FTIR spectra recorded after and before CO adsorption on AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA at 100 K are given in Figs 6 (a, b) and 6 (a, b). Due to H-bonding, CO induces a broadening and a red shift of the OH bands. The higher the OH acidity, the larger is the shift of the OH modes and the higher the carbonyl stretching frequency [63]. However, the weak CO–OH interaction requires the experiments to be performed at low-temperature. In the case of AlBEA and Ag1.5AlBEA introduction of CO at 100 K (100 Pa equilibrium pressure) leads to appearance of intense positive bands at 3650, 3595, 3445, 3290 and negative bands at 3781, 3750, 3715 and 3615 cm-1 (Fig. 6a). The observed shift of 100 cm-1 from 3750 to 3650 cm-1 for isolated external Si-OH groups and 120 cm-1 from 3715 to 3595 cm-1 for terminal internal Si-OH groups indicate that both silanol groups have very weak acidic character. The positive bands at 3445 and 3290 cm-1 (Fig. 6a), related to red shifted bands of perturbed Al-OH and Al-O(H)-Si groups [58], suggests that both types of acidic hydroxyls groups occur in AlBEA and Ag1.5AlBEA. A large red shift of the band related to bridged AlO(H)-Si groups from 3615 cm-1 to 3290 cm-1 (∆ν = 325 cm-1) proves strong acidity of the proton of these groups. Moreover, the second FTIR band at 3445 cm-1 seems to be related to perturbed extra framework Al-OH groups (3667 cm-1) (∆ν = 222 cm-1), in agreement with recent work of
12 Chakarova and Hadjiivanov [64]. Since intensities and positions of all bands are similar for AlBEA and Ag1.5AlBEA, one can conclude that the hydroxyl covering of these samples is similar. For dealuminated SiBEA zeolite, intense bands appear at 3647, 3595 and 3450 cm-1 and negative bands at 3739 and 3709 cm-1. The intensity of the bands at 3647 and 3595 cm-1 quickly decreases during outgassing of CO (Fig. 6b), while the bands at 3739 and 3709 cm-1 is restored (Fig. 6b). A very small shifts from 3739 to 3647 cm-1 (92 cm-1) for isolated internal Si-OH groups and from 3709 to 3595 cm-1 (114 cm-1) for terminal internal Si-OH groups indicate that both silanol groups have a very weak acidic character. The very low intensity of positive band at 3450 cm-1 for SiBEA (Fig. 6b) suggests very low content of these particular Si-OH groups in SiBEA. For Ag3.0SiBEA the adsorption of CO at 100 K leads to the appearance of only one main positive FTIR band at 3647 cm-1 and two negative bands at 3750 and 3739 cm-1 with a low shoulder at 3709 cm-1 (Fig. 6b). A very small shift from 3750 to 3647 cm-1 (103 cm-1) in Ag3.0SiBEA for bands assigned to isolated external Si-OH groups indicates that this silanol groups have a very weak acidic character. Figure 7a shows the changes in the carbonyl region when CO is adsorbed on AlBEA and Ag1.5AlBEA. For convenience, the same set of spectra as that presented in Fig. 6a is given. In the case of AlBEA, under CO equilibrium pressure of 100 Pa, carbonyl bands are detected at 2175, 2157, 2140 and 2135 cm-1 (Fig. 7a, spectrum a). The bands at 2140 and 2135 cm-1 are assigned to weakly bonded physically adsorbed CO or polarized by oxygen anions [63,65] and disappear first upon outgassing. The next band to disappear is that at 2157 cm-1. It changes simultaneously with the band at 3650 cm-1 allowing assigning the 2157 cm-1 band to CO bonded to silanol groups, as reported earlier [43,66]. Further sample outgassing provokes disappearance of the band at 2175 cm-1 (Fig. 7, spectra b-f), typical of CO interacting with bridging zeolite hydroxyls.
13 Extra-framework Al sites were shown to be present in AlBEA by a carbonyl band at 2227 cm-1, which is in line with earlier report [63]. For Ag1.5AlBEA, in CO equilibrium, the main bands appear at 2186, 2175, 2157, 2140 and 2135 cm-1 (Fig. 7a, spectrum a). However, after outgassing of the sample (10-3 Pa), the main FTIR band appears at 2186 cm-1 which do not correlate with any OH band and thus could be assigned to silver carbonyls. Fig. 7b gives the spectra of CO adsorbed at 100 K on SiBEA and Ag3.0SiBEA. For SiBEA four main bands at 2174, 2157, 2140, 2135 cm-1 appear assigned to CO interacting with bridging hydroxyls, silanol groups and physically bonded CO, respectively. The low intensity band at 2174 cm-1 is due to CO interacting with acidic hydroxyls present as traces in SiBEA after dealumination of BEA zeolite. The main FTIR band remained in the spectrum of SiBEA after outgassing (10 -3 Pa) is that at 2157 cm-1 corresponding to CO bonded to Si-OH groups of the vacant T-atom sites. It confirms that in SiBEA mainly silanol groups in the vacant T-atom sites are present. For Ag3.0SiBEA four main bands at 2170, 2157, 2140 and 2135 cm-1 appear. The positions of three bands at 2157, 2140, 2135 cm-1 are the same as those observed for SiBEA and can be assigned to CO interacting with silanol groups (band at 2157 cm-1) and to physically bonded CO (bands at 2140 and 2135 cm-1) respectively. In contrast, the intensity of the band at 2157 cm-1 for Ag3.0SiBEA is much lower that that observed for SiBEA. It proves the consumption of silanol groups upon incorporation of silver ions in the vacant T-atom sites of SiBEA zeolite. The significant decreasing of the intensities of the bands of silanol groups after incorporation of silver ions in the vacant T-atom sites is accompanied by the appearance of the band at 2170 cm-1 which could be assigned to CO bonded to ionic Ag species. As seen in Figure 7, the frequency of CO adsorbed on AgAlBEA and AgSiBEA zeolites differs and is equal to 2186 and 2170 cm-1, respectively. The adsorption of CO on different Ag+containing samples has been studied by several research groups [67-69] and the frequency of the
14 Ag+-CO surface species has been observed within the 2200-2150 cm-1 spectral range. The ν(CO) stretching vibration of Ag+-CO complexes depends on the type of the support, as reported earlier [67]. The carbonyls on Ag+/SiO2 are characterized by ν(CO) at 2169 cm-1 [70]. This value is close to that observed for CO adsorbed on AgSiBEA sample. The frequency of CO adsorbed on different Ag-exchanged zeolites also differs. It was reported that adsorption of CO on AgNa-A sample produced a band at 2174 cm-1 attributed to CO vibration which frequency is related to the donation of σ(CO) electron density to the Ag+ in the Ag+-(CO) complex [71]. When the carbonyl species are formed on Ag-ZSM-5 they absorb at 2192 cm-1. However, when dicarbonyl species are present on this zeolite, C-O stretching mode occur at 2186 cm-1 [70]. The acidity of AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA was also investigated by adsorption of pyridine on the samples calcined at 773 K. Difference spectra between IR spectra recorded after and before pyridine adsorption are shown in Fig. 8 (a, b). The FTIR bands typical of pyridinium cations are observed at 1637 and 1546 cm-1 for AlBEA and Ag1.5AlBEA (Fig. 8a), indicating presence of Brønsted acidic sites. The other observed bands at about 1621, 1490 and 1450–1455 cm-1 correspond to pyridine interacting with Lewis acidic sites, in line with earlier data [40,46,72]. Pyridinium cations (bands at 1637 and 1546 cm-1) and pyridine bonded to Lewis acidic sites (1622, 1490 and 1450–1455 cm-1) remain even after outgassing at 673 K, suggesting the presence in both AlBEA and Ag1.5AlBEA of strong Brønsted and Lewis acidic sites. Moreover, on the spectra of Ag1.5AlBEA new FTIR band appear at 1605 cm-1, indicating the presence of pyridine bonded to silver Lewis acidic sites and more intense bands at 1462 and 1455 cm-1 remain after outgassing at 673 K (Fig. 8a, spectrum c), suggesting that silver increases the strength and amount of Lewis acidic sites of AlBEA. Only the bands at 1600, 1491 and 1448 cm-1 appear for SiBEA (Fig. 8b) related to pyridine interacting with weak Lewis acidic sites and/or pyridine physisorbed [73-75]. The intensities of these bands sharply decrease with increasing desorption temperature. The
15 incorporation of silver in SiBEA leads to appearance of new bands at 1605 and 1450 cm-1, as shown for Ag3.0SiBEA in Fig. 8b, suggesting the formation of new silver Lewis acidic sites.
3.5. Catalytic properties Figures 9a and 9b show the temperature dependences of NO conversions in the SCR of NO with ethanol on AlBEA, AgxAlBEA, SiBEA and AgxSiBEA zeolite materials. The SCR activity of AlBEA and SiBEA is low, i.e. NO conversion does not exceed 10% in the large temperature range. The introduction of 1.5 and 4.0 wt % of Ag in the AlBEA zeolites enhances NO conversion to 29 and 36% (Fig. 9a). As shown in Fig. 9b, the incorporation of 1.5 and 3.0 wt % of silver in the SiBEA zeolite also increases the catalytic activity: NO conversion increases to 29 and 33%, respectively. It should be noted here that increases of silver loading from 1.5 to 3.0 wt % hardly changes the activity of the catalyst. In the SCR of NO with ethanol on the AgxAlBEA and AgxSiBEA, complete conversion of ethanol was observed in almost all temperature range of the reaction (473-773 K), and the identified products of its conversion were CO2, CO, ethylene, diethyl ether and acetaldehyde (traces). The conversion of NO into N2O did not exceed 1%. For AgxAlBEA and AgxSiBEA the different temperature range of maximum NO conversion are observed: much wider range for AgxAlBEA than for AgxSiBEA. This may be due to the different temperature of reducing agent activation on the various silver species of Agzeolites. It is probable that different acid-base properties of the materials have significant influence on their catalytic activity in SCR of NO with ethanol. A more detailed study on the catalytic properties of these materials will be presented in our future article.
Conclusions The two-step postsynthesis method allows to incorporate Ag ions into the vacant T-atom sites of the framework of SiBEA zeolite as mononuclear Ag(I) as evidenced by combined use of P-XRD, diffuse reflectance UV-vis, FTIR and XPS.
16 The conventional wet impregnation method allows obtaining AgAlBEA with silver present mainly in the extra-framework position, as evidenced by XPS and FTIR. The consumption of OH groups of vacant T-atom sites upon reaction with Ag precursor has been monitored by FTIR. The Brønsted and Lewis acidity of AlBEA, AgAlBEA, SiBEA and AgSiBEA have been identified by FTIR spectroscopy of adsorbed pyridine and CO as probe molecules. Our preliminary studies of SCR of NO with ethanol on AlBEA, AgAlBEA, SiBEA and AgSiBEA zeolites show that silver environment and acid-base properties play a significant role in catalytic activity of Ag-containing BEA zeolites.
Acknowledgments This work was partly supported by the program of National academy of sciences of Ukraine “Fundamental problems of nanostructure systems, nanomaterials, nanotechnologies”.
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21
Table 1. Textural properties of zeolite samples Specific Sample
Micropore
Micropore
surface area
surface area
volume
(m2⋅g-1)
(m2⋅g-1)
(cm3⋅g-1)
Median diameter of micropores
Mesopore volume (cm3⋅g-1)
(nm) AlBEA
625
453
0.237
0.95
0.52
Ag4.0AlBEA
605
436
0.233
0.94
0.48
SiBEA
580
403
0.219
0.98
0.55
Ag3.0SiBEA
567
387
0.210
0.97
0.49
22
Si
Si O TEA
O O
O
Si
Si
O
O H 3615 Al3+
Calcination in air
Al3+
Si
Si
at 823 K, 15 h
Si
TEABEA
O
O
Si Impregnation with AgNO
Si
3
O H Si
AlBEA
O
Ag+O
Si
Al3+ O
Si
AgxAlBEA
Conventional wet impregnation
Si
Si O TEA Si
O
O
+
TEABEA
Si 3+
+ 3H , -Al , -TEA
Al3+ O
(1)
Si
Dealumination with HNO3
+
Si
3709 Si H H O
O 3739
H 3529 O O Si 3529 H
(2) +
+ Ag , + H2O -H3O+
SiBEA
Si H H O O 3739 Ag+ O O Si Si H
Si
AgxSiBEA
Two-step postsynthesis method
Scheme 1. Proposed ways of preparation of AgxAlBEA and AgxSiBEA by conventional wet impregnation and two-step postsynthesis method, respectively. The numbers quoted correspond to cm-1. The wavenumbers 3739, 3709, 3615 and 3529 cm-1 correspond to isolated internal SiO-H, terminal internal SiO-H, Al-O(H)-Si and H-bonded SiO-H vibrators respectively.
23 Figure captions
Figure 1. X-ray diffractograms recorded at room temperature and ambient atmosphere of AlBEA, Ag4.0AlBEA, SiBEA and Ag3.0SiBEA. Figure 2. Adsorption isotherms of nitrogen at 77 K on Ag4.0AlBEA and Ag3.0SiBEA. Full symbols – adsorption; empty symbols – desorption. Figure 3. DR UV–vis spectra recorded at room temperature and ambient atmosphere of asprepared AlBEA, Ag1.5AlBEA, Ag4.0AlBEA, SiBEA and Ag3.0SiBEA. Figure 4. XP spectra recorded at room temperature of Ag 3d core level of as-prepared Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA. Figure 5. FTIR of AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA recorded at room temperature after calcination at 773 K for 3 h in flowing air and then outgassing at 573 K for 2 h (10-3 Pa). Figure 6. FTIR difference spectra (OH stretching region) of AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA after adsorption of CO at 100 K: equilibrium CO pressure of 100 Pa (a) and development of the spectra during evacuation at 100 K (b–f). Figure 7. FTIR difference spectra (carbonyl stretching region) of CO adsorbed at 100 K on AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA: equilibrium CO pressure of 100 Pa (a) and development of the spectra during evacuation at 100 K (b–g). The spectra are background corrected. Figure 8. FTIR difference spectra recorded at room temperature of AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA after calcination at 773 K, 1 h in O2 (2 · 104 Pa), outgassing at 10-3 Pa for 1 h at 673 K, adsorption of pyridine at 423 K and desorption of pyridine at (a) 423 K, (b) 573 K, (c) 673 K. Figure 9. Temperature-dependence of NO conversion in SCR of NO with ethanol over AgxAlBEA (a) and AgxSiBEA (b).
24
Fig. 1.
25
Fig. 2.
26
Fig. 3.
27
Fig. 4.
28
Fig. 5.
29
Fig. 6.
30
Fig. 7.
31
Fig. 8.
32
Fig. 9.
33
Calcination in air
Al
at 823 K for 15 h
Al HAlBEA
TEABEA
1st step
Al TEABEA
AgNO3
HNO3
Al
Ag
AgxAlBEA
2nd step OH OH OH
SiBEA
AgNO3
Ag AgxSiBEA
34
Highlights
*Two-step postsynthesis method allowed to incorporate silver into the vacant T-atom sites *Conventional wet impregnation introduced silver mainly in extra-framework position *Consumption of OH groups upon reaction with Ag precursor has been monitored by FTIR *Brønsted and Lewis acidity were identified by FTIR spectroscopy of adsorbed pyridine and CO * Significant role of silver state and acid-base properties for catalytic activity of AgBEA zeolites
S1387-1811(13)00385-5 http://dx.doi.org/10.1016/j.micromeso.2013.08.009 MICMAT 6166
To appear in:
Microporous and Mesoporous Materials
Received Date: Revised Date: Accepted Date:
29 April 2013 10 July 2013 10 August 2013
Please cite this article as: S. Dzwigaj, N. Popovych, P. Kyriienko, J-M. Krafft, S. Soloviev, The similarities and differences in structural characteristics and physico-chemical properties of AgAlBEA and AgSiBEA zeolites, Microporous and Mesoporous Materials (2013), doi: http://dx.doi.org/10.1016/j.micromeso.2013.08.009
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1 The similarities and differences in structural characteristics and physico-chemical properties of AgAlBEA and AgSiBEA zeolites Stanislaw Dzwigaja, b, *, Nataliia Popovychc, *, Pavlo Kyriienko c, Jean-Marc Kraffta, b, Sergiy Solovievc a
UPMC Univ Paris 06, Laboratoire de Réactivité de Surface, Case 178, Site d’Ivry-Le Raphaël,
3 rue Galilée, 94200, Ivry sur Seine, France b
CNRS-UMR 7197, Laboratoire de Réactivité de Surface, Case 178, Site d’Ivry-Le Raphaël,
3 rue Galilée, 94200, Ivry sur Seine, France c
L.V. Pisarzhevsky Institute of Physical Chemistry of the NAS of Ukraine,
31 Prosp. Nauky, 03028 Kyiv, Ukraine
Figures: 9 Scheme: 1 Table: 1
Keywords: SiBEA, AlBEA, silver, acidity, CO, pyridine
* Corresponding authors: S. Dzwigaj, E-mail address: [email protected], fax: +33 01 44 27 60 33. N. Popovych, E-mail address: [email protected], fax: +38 (044) 525 65 90.
2 Abstract AgAlBEA and AgSiBEA zeolites have been prepared by two preparation procedures. The conventional wet impregnation is used to obtain AgAlBEA with silver present mainly in extra-framework position. The two-step postsynthesis procedure which consists of first creating the vacant T-atom sites with associated silanol groups by treatment of TEABEA zeolite with nitric acid and then impregnating of resulting SiBEA zeolite with an aqueous solution of AgNO3 is used to obtain AgSiBEA zeolites with silver present mainly in framework position. The incorporation of Ag ions into the vacant T-atom sites of the framework of SiBEA zeolite as mononuclear Ag(I) has been evidenced by combined use of P-XRD, diffuse reflectance UV-vis, FTIR and XPS. The consumption of OH groups has been monitored by FTIR. The Brønsted and Lewis acidity of AlBEA, AgAlBEA, SiBEA and AgSiBEA has been identified by FTIR spectroscopy of adsorbed pyridine and CO as probe molecules. The catalytic activities of AgAlBEA and AgSiBEA in SCR of NO with ethanol have been investigated. These studies show that silver environment and acid-base properties play a significant role in catalytic activities of these Ag containing BEA zeolites.
3 1. Introduction Recent legislation in industrialized countries places more stringent regulations on the emissions of vehicles and mobile energy generators powered by diesel. Selective catalytic reduction of nitrogen oxides (NOx) in these emissions with hydrocarbon or oxygenates as reducing agents seems to be a perspective approach, which brings about the requirement for a novel catalyst that is able to reduce NOx in an excess of oxygen. Extensive research has been done on the development of de-NOx catalysts and many types of catalysts have been reported [110]. Silver-based materials (generally silver-alumina) were found to be the most active and selective catalytic systems for SCR of NOx with hydrocarbon or oxygenated organic reducing agents [11-19]. As known, materials based on BEA zeolites are widely studied as catalysts for industry and environmental protection due to their large surface area, shape selectivity, strong acidity and efficient activity [20-29]. Many works are devoted to the study of M-BEA (M: Fe, Cu, Co) zeolites as promising catalysts for SCR of NOx with hydrocarbons and oxygenates [2329]. Nevertheless, there are only little research data in literature about Ag-containing zeolite catalysts applied in the SCR of NOx [17-19,30]. According to previous reports, well dispersed silver species (cations and small clusters) seem to be active catalytic centers of SCR of NO on Ag/Al2O3 and Ag-MFI zeolites [7,15,16,18,31,32]. However, dispersing of Ag in these catalytic systems are non selective and they contain silver in the form of various species (ions, nanoclusters, metallic nanoparticles) [33-36]. To obtain well dispersed silver species as isolated mononuclear Ag(I) we have used in the present work the two-step postsynthesis method developed earlier for preparation metalcontaining BEA zeolites [37-40]. This postsynthesis procedure allowed for low metal content (< 2-3 wt.%) incorporating of metal ions in the framework of zeolite mainly as isolated and homogeneously distributed metal species without formation of metal oligomers or metal oxides. The incorporation of silver ions in the vacant T-atom sites of BEA zeolite as isolated mononuclear Ag(I) species has been evidenced by P-XRD, FTIR, diffuse reflectance (DR) UV-
4 vis and XPS. The silver-containing BEA zeolites have been also prepared by a conventional wet impregnation with aqueous AgNO3 solution. Thus, we have used in this work two preparation procedures, the two-step postsynthesis method [37-46] in order to obtain AgSiBEA zeolites with silver present mainly in framework position and the conventional wet impregnation in order to obtain AgAlBEA with silver present mainly in extra-framework position. The present work is aimed to investigate the effect of the preparation way on the physicochemical properties of silver-containing BEA zeolites.
2. Experimental 2.1. Materials Silver-containing SiBEA zeolite was prepared by the two-step postsynthesis method developed earlier for preparation of V-loaded BEA zeolite [37,41-43] allowing control the incorporation of Ag ions in the zeolite framework. In the first step, the TEABEA zeolite provided by RIPP (China) was treated in a 13 mol·L−1 HNO3 aqueous solution (4 h, 353 K) to obtain a dealuminated and organic-free SiBEA support (Si/Al =1000) with the vacant T-atom sites (T = Al). SiBEA was then recovered by centrifugation, washed with distilled water and dried at 353 K. To incorporate Ag ions in the vacant T-atom sites, 2 g of SiBEA was firstly stirred under aerobic conditions for 2 h at 298 K in 200 mL of AgNO3 (Fluka silver nitrate with high Ph Eur purity with Ag > 99.8 %) aqueous solution (pH = 2.3) with concentration of 5.4 × 10 −3 mol·L−1. Then, the suspension was stirred in evaporator under vacuum of a water pump for 2 h in air at 353 K until the water was completely evaporated. The resulting solid containing 3.0 Ag wt. % was labeled as Ag3.0SiBEA. To prepare AgAlBEA zeolite, firstly the TEABEA was calcined at 823 K for 15 h in air to obtain AlBEA (Si/Al = 12.5) and secondly, the latter was impregnated at 298 K for 2h with 200 mL of aqueous AgNO3 solution at pH = 3.1 with concentration of 2.6 and 9.8 ×10 −3 mol·L−1.
5 Then, the suspensions were stirred in evaporator under vacuum of a water pump for 2 h in air at 353 K until the water was completely evaporated. The resulting solids containing 1.5 and 4.0 Ag wt. % were labeled as Ag1.5AlBEA and Ag4.0AlBEA, respectively.
2.2. Techniques X-Ray Fluorescence chemical analysis was performed at room temperature on a SPECTRO X-LabPro apparatus. Powder XRD (P-XRD) were recorded at room temperature and ambient atmosphere on a Bruker D8 Advance diffractometer using the CuKa radiation (λ = 154.05 pm). Specific surface area and adsorption isotherms of nitrogen at 77 K were measured on a Sorptomatic 1990 apparatus. All samples were outgassed first at room temperature, then at 623 K to a pressure < 0.2 Pa. The surface areas were determined from nitrogen adsorption values for five relative pressures (P/P0) ranging from 0.006 to 0.074 using the BET method. The microporous pore volume was determined from the amount of N2 adsorbed up to P/P0 = 0.1. TEM micrographs were obtained on SELMI TEM-125K microscope operating at 100 kV. For TEM measurements samples were dispersed in acetone with ultrasound and deposited on Cu grid covered with carbon. Interplanar spacings (d hkl) calculated from the diffraction ring pattern were compared with the ASTM data. DR UV–vis spectra were recorded under ambient conditions on a Specord M40 (Carl Zeiss) with a standard diffuse reflectance unit. X-ray photoelectron spectroscopy (XPS) measurements were performed with a hemispherical analyzer (PHOIBOS 100, SPECS Gmbh) using MgKα (1253.6 eV) radiation. The power of the X-ray source was 300 W. The area of the sample analyzed was ~ 3 mm2. The powder samples were pressed on an indium foil and mounted on a special holder. Binding energy (BE) for Si and Ag was measured by reference to the O 1s peak at 532.5 eV, corresponding to the binding energy of oxygen bonded to silicon. Before analysis, samples were
6 outgassed at room temperature to a pressure of 10-7 Pa. All spectra were fitted with a Voigt function (a 70/30 composition of Gaussian and Lorentzian functions) in order to determine the number of components under each XPS peak. Analysis of the acidic properties of samples was performed by adsorption of pyridine (Py) and CO followed by infrared spectroscopy. Before analysis, the samples were pressed at ~ 1 ton·cm-2 into thin wafers of ca. 10 mg·cm-2 and placed inside the IR cell. Before CO adsorption experiment, the wafers were activated by calcination at 723 K for 2 h in flowing 2.5 % O2/Ar and then outgassed at 573 K (10-3 Pa) for 1 h. Following thermal treatment, the samples were cooled down to 100 K. CO was introduced in increasing amounts up to an equilibrium pressure of 133 Pa. Infrared spectra were recorded using a Bruker Vertex 70 spectrometer (resolution 2 cm-1, 128 scans). The spectra were obtained after subtraction of the spectrum recorded after calcination and prior to CO adsorption. Before pyridine adsorption/desorption experiments, the wafers were activated by calcination in static conditions at 773 K for 1 h in O2 (2 · 104 Pa) and then outgassing under secondary vacuum at 673 K (10 -3 Pa) for 1 h. These wafers were contacted at 423 K with gaseous pyridine. The spectra were recorded after pyridine desorption at 423, 573 and 673 K using a Spectrum One FTIR spectrometer (resolution 1 cm-1, 12 scans). The reported spectra were obtained after subtraction of the spectrum recorded after calcination and prior to pyridine adsorption. Catalytic activity tests were carried out in a fixed-bed flow quartz reactor at atmospheric pressure. Samples with grains of 1-2 mm (0.5 cm3, 0.3 g) were loaded into the reactor. Gas feed for the reaction was 500 ppm NO, 1000 ppm C2H5OH, 10 % O2 in He, and the gas hour space velocity was 24 000 h−1. The gas feed was adjusted by mass-flow controllers (Chromatek-Crystal FGP). Before reaction, the catalyst was heated to 773 K at a heating rate of 20 K·min−1 in a flow of O2/He and held for 1 h, then was cooled to 453 K with a further step-heating in reaction gas feed to a temperature of conversion measurement. The steady-state activity was measured in the
7 temperature range of 473-773 K after 30 min reaction at a certain temperature. The temperature was controlled through an Autonics TZN4S temperature controller using a Chromel-Alumel thermocouple. The concentration of NO was continuously monitored using a chemiluminescence gas analyzer (344HL04, Ukraine). The products were analyzed by gas chromatograph (TCD) (Kristallyuks 4000M, Metachrom, Russia) with a CaA column (for NO, N2, CO) and a Polisorb1 column (for N2O, CO2, С2Н4, С2Н4О, ethanol). Catalytic activity was characterized by NO conversion and temperatures of its achievement. Conversion of NO was calculated as X(NO)=(1[NO]out /[NO]in)⋅100, where the subscript ‘‘in’’ refers to initial concentration whereas ‘‘out’’ means after reaction.
3. Results and discussion 3.1. Structural and textural properties The P-XRD patterns of AlBEA, Ag1.5AlBEA, Ag4.0AlBEA, SiBEA and Ag3.0SiBEA are typical of BEA zeolite. As shown in Figure 1, the P-XRD patterns are very similar which indicates that the crystallinity of BEA zeolite is preserved after dealumination and introduction of silver ions in AlBEA and SiBEA. This behavior of BEA zeolite upon dealumination is related to the presence in its origin structure of some amounts of vacant T-atom sites. Because of this the zeolite structure is flexible and allows removing almost all Al atoms from the BEA zeolite without losing of its crystallinity. A narrow diffraction peak near 22.5-22.6° is generally taken as evidence of lattice contraction/expansion of the BEA structure [47,48]. The d 302 spacing related to this peak decreases from 3.950 (AlBEA) (with 2θ of 22.48°) to 3.930 Å (SiBEA) (with 2θ of 22.60°) upon dealumination, indicating some contraction of the BEA matrix. However, it increases after silver introduction to 3.961 Å (Ag3.0SiBEA) (with 2θ of 22.42°), which indicates some expansion of the BEA structure and suggests that silver is incorporated into the vacant-T-atom sites of the framework of SiBEA zeolite. In contrast, the introduction of silver ions into AlBEA by
8 conventional wet impregnation does not lead to any change in the BEA structure, as evidenced by the same value of d302 spacing (3.950 Å) for AlBEA and Ag4.0AlBEA (with 2θ of 22.48°), indicating that silver is not incorporated into the framework of BEA zeolite. The texture of samples has been characterized by nitrogen adsorption/desorption and TEM. The acid treatment of AlBEA induced a slight decrease of SiBEA BET surface area (Table 1) due to the decreasing of microporous volume. The acid treatment leads to slight increasing of the contribution of the mesopore volume (isotherms of AlBEA and SiBEA have been reported in our earlier work [49]). Adsorption and desorption isotherms of Ag4.0AlBEA and Ag3.0SiBEA (Fig. 2) indicate that the incorporation of silver does not affect the crystallinity and does not lead to formation of mesopores. The similar specific surface areas for AlBEA and Ag4.0AlBEA, SiBEA and Ag3.0SiBEA (Table 1) and the absence of extra-framework crystalline compounds or long-range amorphization of the Ag4.0AlBEA and Ag3.0SiBEA zeolites indicate that silver is incorporated as well dispersed species. On the micrographs of Ag4.0AlBEA and Ag3.0SiBEA calcined at 773 K no silver particles are seen and no clear ring diffraction patterns of silver are visible (Figures in supplementary material) which may indicate that silver is highly dispersed in the zeolites. The interplanar dhkl spacings for the most contrast electron diffraction rings are equal to 3.949 Å for Ag4.0AlBEA and 3.970 Å for Ag3.0SiBEA. They can be attributed to the interplanar d302 spacing of BEA zeolite, which corroborate the P-XRD results. Silver in the zeolite can be seen only after reduction of Ag4.0AlBEA and Ag3.0SiBEA with hydrogen (5% H2/Ar, 773 K, 1 h). Diffraction rings appeared on pattern of Ag4.0AlBEA (with d hkl of 2.354, 2.007, 1.242 Å) and Ag3.0SiBEA (2.353, 1.175, 0.934 Å) after H2 reduction can be respectively assigned to (111), (200), (311) and (111), (222), (331) planes of the face-centered cubic metallic silver.
3.2. Nature of the silver species
9 The electronic structure of a silver-containing zeolite material can be regarded as a superposition of the electronic structure of the framework, of the solvent molecules (water) and of the silver species, in line with earlier reports [34,50]. The results of DR UV-vis investigations of AlBEA, Ag1.5AlBEA, Ag4.0AlBEA, SiBEA and Ag3.0SiBEA zeolites are displayed in Fig. 3. Silver-free AlBEA and SiBEA absorb light within the spectral range of 200-320 nm. Absorption bands at 282 nm for AlBEA, 208 and 284 nm for SiBEA are probably related to charge transfer transitions in BEA framework. Different shapes of AlBEA and SiBEA absorption spectra can be related to the Al3+ centers in AlBEA zeolite, causing a negatively charged framework (AlO2-), which influence the band structure to some extent [50]. The Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA samples exhibit two bands at 214-215 and 233-238 nm attributed to the charge transfer transition between 4d10 and 4d95s1 level of highly dispersed Ag+ species, in line with earlier report [18,51-53]. Furthermore, as was shown in earlier report [34,50], the band around 215 nm can be attributed to a charge transfer transition from Ag+ coordinated water molecules to the empty 5s state of the Ag+. The position of absorption band is also dependent on silver sites. Three different Ag+ sites were identified by Gellens et al. [54], namely Ag+ at 6- and 8-ring sites, and at 4-ring sites in the sodalite cage. Ag+ coordinated to 6- and 8-ring oxygen give rise to electronic transition in the range of 220-255 cm1
. These results indicate that in as-prepared Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA samples
well dispersed Ag+ species are present. The absence of bands at higher wavelength of 260-350 and 450-500 nm suggests that silver nanoclusters (Agnδ+) and metallic silver nanoparticles are not present in Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA. Our preliminary EPR results show that for activated Ag3.0SiBEA (outgassed at 773 K for 2 h under dynamic vacuum of 10 -3 Pa) a paramagnetic signal with giso = 2.00149 appears characteristic of Ag(H2O)2 species, in line with earlier report of R.A. Schoonheydt [55]. In activated Ag3.0SiBEA, the Ag atoms have isotropic g-values slightly below the free atom value.
10 It is probably related to strong interaction of Ag species with zeolite structure and suggests that silver atoms are present in the framework of zeolite as mononuclear Ag(I). It confirms above DR UV-vis results. XPS was used to determine the chemical state of the silver species present in silvercontaining AlBEA and SiBEA. Figure 4 shows the Ag 3d 3/2 and 3d5/2 bands of Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA zeolites. One peak at binding energy (BE) of 368.3-368.4 eV (Ag 3d 5/2) and at BE of 374.3-374.4 eV (Ag 3d3/2) for Ag1.5AlBEA and Ag4.0AlBEA, as well as at 368.0 eV (Ag 3d 5/2) and 374.0 eV (Ag 3d 3/2) for Ag3.0SiBEA indicates that Ag+ is the main silver species in both AgAlBEA and AgSiBEA zeolites, in line with earlier reports for silver in zeolites [56,57]. Differences in binding energies of Ag 3d5/2 and Ag 3d3/2 of AgAlBEA and AgSiBEA are related to different silver environment in both zeolites, in extra-framework and framework positions respectively, as proposed in Scheme 1.
3.3. FTIR characterization of the hydroxyl groups The FTIR spectrum of AlBEA (Fig. 5) exhibits bands attributed to Al-OH (bands at 3781 and 3667 cm-1), zeolite acidic hydroxyls Al-O(H)-Si (band at 3615 cm-1) and Si-OH (band at 3750 cm-1 with a shoulder at 3740 cm-1) groups, in line with earlier work [58,59]. The band at 3750 cm-1 is sharp, indicating that the Si-OH groups are isolated [46,60]. Comparison of the spectrum of Ag1.5AlBEA with that of AlBEA shows that the Al-OH bands at 3781 and 3667 cm-1 have decreased in intensity, indicating that some of the Al-OH groups have been consumed during their reaction with silver precursor upon conventional wet impregnation. Upon dealumination, the three bands attributed to Al-OH and Al-O(H)-Si groups are eliminated, suggesting that aluminum is fully removed, and bands at 3739, 3709 and 3529 cm-1 appear (SiBEA, Fig. 5), attributed to isolated internal, terminal internal and hydrogen bonded SiOH, respectively, located in the vacant T-atom sites of the framework of SiBEA zeolite forming hydroxyl nests (Scheme 1), in line with earlier assignments [37,46,61,62].
11 Introduction of silver into SiBEA markedly changes the spectra (Ag3.0SiBEA, Fig. 5) with almost total disappearance of the FTIR bands at 3739, 3709 and 3529 cm-1 of isolated internal, terminal internal and hydrogen bonded Si-OH groups suggesting that these silanol groups react with the Ag precursor (Scheme 1). The consumption of silanol groups of the vacant T-atom sites monitored by FTIR is very high and after that the band at 3750 cm-1 of isolated external silanol groups and the bands at 3739 and 3709 cm-1 with very low intensity are only present in Ag3.0SiBEA (Fig. 5).
3.4. FTIR characterization of acidic centers: adsorption of CO and pyridine CO is often used to simultaneously probe Lewis and Brønsted acidic sites. Difference spectra between FTIR spectra recorded after and before CO adsorption on AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA at 100 K are given in Figs 6 (a, b) and 6 (a, b). Due to H-bonding, CO induces a broadening and a red shift of the OH bands. The higher the OH acidity, the larger is the shift of the OH modes and the higher the carbonyl stretching frequency [63]. However, the weak CO–OH interaction requires the experiments to be performed at low-temperature. In the case of AlBEA and Ag1.5AlBEA introduction of CO at 100 K (100 Pa equilibrium pressure) leads to appearance of intense positive bands at 3650, 3595, 3445, 3290 and negative bands at 3781, 3750, 3715 and 3615 cm-1 (Fig. 6a). The observed shift of 100 cm-1 from 3750 to 3650 cm-1 for isolated external Si-OH groups and 120 cm-1 from 3715 to 3595 cm-1 for terminal internal Si-OH groups indicate that both silanol groups have very weak acidic character. The positive bands at 3445 and 3290 cm-1 (Fig. 6a), related to red shifted bands of perturbed Al-OH and Al-O(H)-Si groups [58], suggests that both types of acidic hydroxyls groups occur in AlBEA and Ag1.5AlBEA. A large red shift of the band related to bridged AlO(H)-Si groups from 3615 cm-1 to 3290 cm-1 (∆ν = 325 cm-1) proves strong acidity of the proton of these groups. Moreover, the second FTIR band at 3445 cm-1 seems to be related to perturbed extra framework Al-OH groups (3667 cm-1) (∆ν = 222 cm-1), in agreement with recent work of
12 Chakarova and Hadjiivanov [64]. Since intensities and positions of all bands are similar for AlBEA and Ag1.5AlBEA, one can conclude that the hydroxyl covering of these samples is similar. For dealuminated SiBEA zeolite, intense bands appear at 3647, 3595 and 3450 cm-1 and negative bands at 3739 and 3709 cm-1. The intensity of the bands at 3647 and 3595 cm-1 quickly decreases during outgassing of CO (Fig. 6b), while the bands at 3739 and 3709 cm-1 is restored (Fig. 6b). A very small shifts from 3739 to 3647 cm-1 (92 cm-1) for isolated internal Si-OH groups and from 3709 to 3595 cm-1 (114 cm-1) for terminal internal Si-OH groups indicate that both silanol groups have a very weak acidic character. The very low intensity of positive band at 3450 cm-1 for SiBEA (Fig. 6b) suggests very low content of these particular Si-OH groups in SiBEA. For Ag3.0SiBEA the adsorption of CO at 100 K leads to the appearance of only one main positive FTIR band at 3647 cm-1 and two negative bands at 3750 and 3739 cm-1 with a low shoulder at 3709 cm-1 (Fig. 6b). A very small shift from 3750 to 3647 cm-1 (103 cm-1) in Ag3.0SiBEA for bands assigned to isolated external Si-OH groups indicates that this silanol groups have a very weak acidic character. Figure 7a shows the changes in the carbonyl region when CO is adsorbed on AlBEA and Ag1.5AlBEA. For convenience, the same set of spectra as that presented in Fig. 6a is given. In the case of AlBEA, under CO equilibrium pressure of 100 Pa, carbonyl bands are detected at 2175, 2157, 2140 and 2135 cm-1 (Fig. 7a, spectrum a). The bands at 2140 and 2135 cm-1 are assigned to weakly bonded physically adsorbed CO or polarized by oxygen anions [63,65] and disappear first upon outgassing. The next band to disappear is that at 2157 cm-1. It changes simultaneously with the band at 3650 cm-1 allowing assigning the 2157 cm-1 band to CO bonded to silanol groups, as reported earlier [43,66]. Further sample outgassing provokes disappearance of the band at 2175 cm-1 (Fig. 7, spectra b-f), typical of CO interacting with bridging zeolite hydroxyls.
13 Extra-framework Al sites were shown to be present in AlBEA by a carbonyl band at 2227 cm-1, which is in line with earlier report [63]. For Ag1.5AlBEA, in CO equilibrium, the main bands appear at 2186, 2175, 2157, 2140 and 2135 cm-1 (Fig. 7a, spectrum a). However, after outgassing of the sample (10-3 Pa), the main FTIR band appears at 2186 cm-1 which do not correlate with any OH band and thus could be assigned to silver carbonyls. Fig. 7b gives the spectra of CO adsorbed at 100 K on SiBEA and Ag3.0SiBEA. For SiBEA four main bands at 2174, 2157, 2140, 2135 cm-1 appear assigned to CO interacting with bridging hydroxyls, silanol groups and physically bonded CO, respectively. The low intensity band at 2174 cm-1 is due to CO interacting with acidic hydroxyls present as traces in SiBEA after dealumination of BEA zeolite. The main FTIR band remained in the spectrum of SiBEA after outgassing (10 -3 Pa) is that at 2157 cm-1 corresponding to CO bonded to Si-OH groups of the vacant T-atom sites. It confirms that in SiBEA mainly silanol groups in the vacant T-atom sites are present. For Ag3.0SiBEA four main bands at 2170, 2157, 2140 and 2135 cm-1 appear. The positions of three bands at 2157, 2140, 2135 cm-1 are the same as those observed for SiBEA and can be assigned to CO interacting with silanol groups (band at 2157 cm-1) and to physically bonded CO (bands at 2140 and 2135 cm-1) respectively. In contrast, the intensity of the band at 2157 cm-1 for Ag3.0SiBEA is much lower that that observed for SiBEA. It proves the consumption of silanol groups upon incorporation of silver ions in the vacant T-atom sites of SiBEA zeolite. The significant decreasing of the intensities of the bands of silanol groups after incorporation of silver ions in the vacant T-atom sites is accompanied by the appearance of the band at 2170 cm-1 which could be assigned to CO bonded to ionic Ag species. As seen in Figure 7, the frequency of CO adsorbed on AgAlBEA and AgSiBEA zeolites differs and is equal to 2186 and 2170 cm-1, respectively. The adsorption of CO on different Ag+containing samples has been studied by several research groups [67-69] and the frequency of the
14 Ag+-CO surface species has been observed within the 2200-2150 cm-1 spectral range. The ν(CO) stretching vibration of Ag+-CO complexes depends on the type of the support, as reported earlier [67]. The carbonyls on Ag+/SiO2 are characterized by ν(CO) at 2169 cm-1 [70]. This value is close to that observed for CO adsorbed on AgSiBEA sample. The frequency of CO adsorbed on different Ag-exchanged zeolites also differs. It was reported that adsorption of CO on AgNa-A sample produced a band at 2174 cm-1 attributed to CO vibration which frequency is related to the donation of σ(CO) electron density to the Ag+ in the Ag+-(CO) complex [71]. When the carbonyl species are formed on Ag-ZSM-5 they absorb at 2192 cm-1. However, when dicarbonyl species are present on this zeolite, C-O stretching mode occur at 2186 cm-1 [70]. The acidity of AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA was also investigated by adsorption of pyridine on the samples calcined at 773 K. Difference spectra between IR spectra recorded after and before pyridine adsorption are shown in Fig. 8 (a, b). The FTIR bands typical of pyridinium cations are observed at 1637 and 1546 cm-1 for AlBEA and Ag1.5AlBEA (Fig. 8a), indicating presence of Brønsted acidic sites. The other observed bands at about 1621, 1490 and 1450–1455 cm-1 correspond to pyridine interacting with Lewis acidic sites, in line with earlier data [40,46,72]. Pyridinium cations (bands at 1637 and 1546 cm-1) and pyridine bonded to Lewis acidic sites (1622, 1490 and 1450–1455 cm-1) remain even after outgassing at 673 K, suggesting the presence in both AlBEA and Ag1.5AlBEA of strong Brønsted and Lewis acidic sites. Moreover, on the spectra of Ag1.5AlBEA new FTIR band appear at 1605 cm-1, indicating the presence of pyridine bonded to silver Lewis acidic sites and more intense bands at 1462 and 1455 cm-1 remain after outgassing at 673 K (Fig. 8a, spectrum c), suggesting that silver increases the strength and amount of Lewis acidic sites of AlBEA. Only the bands at 1600, 1491 and 1448 cm-1 appear for SiBEA (Fig. 8b) related to pyridine interacting with weak Lewis acidic sites and/or pyridine physisorbed [73-75]. The intensities of these bands sharply decrease with increasing desorption temperature. The
15 incorporation of silver in SiBEA leads to appearance of new bands at 1605 and 1450 cm-1, as shown for Ag3.0SiBEA in Fig. 8b, suggesting the formation of new silver Lewis acidic sites.
3.5. Catalytic properties Figures 9a and 9b show the temperature dependences of NO conversions in the SCR of NO with ethanol on AlBEA, AgxAlBEA, SiBEA and AgxSiBEA zeolite materials. The SCR activity of AlBEA and SiBEA is low, i.e. NO conversion does not exceed 10% in the large temperature range. The introduction of 1.5 and 4.0 wt % of Ag in the AlBEA zeolites enhances NO conversion to 29 and 36% (Fig. 9a). As shown in Fig. 9b, the incorporation of 1.5 and 3.0 wt % of silver in the SiBEA zeolite also increases the catalytic activity: NO conversion increases to 29 and 33%, respectively. It should be noted here that increases of silver loading from 1.5 to 3.0 wt % hardly changes the activity of the catalyst. In the SCR of NO with ethanol on the AgxAlBEA and AgxSiBEA, complete conversion of ethanol was observed in almost all temperature range of the reaction (473-773 K), and the identified products of its conversion were CO2, CO, ethylene, diethyl ether and acetaldehyde (traces). The conversion of NO into N2O did not exceed 1%. For AgxAlBEA and AgxSiBEA the different temperature range of maximum NO conversion are observed: much wider range for AgxAlBEA than for AgxSiBEA. This may be due to the different temperature of reducing agent activation on the various silver species of Agzeolites. It is probable that different acid-base properties of the materials have significant influence on their catalytic activity in SCR of NO with ethanol. A more detailed study on the catalytic properties of these materials will be presented in our future article.
Conclusions The two-step postsynthesis method allows to incorporate Ag ions into the vacant T-atom sites of the framework of SiBEA zeolite as mononuclear Ag(I) as evidenced by combined use of P-XRD, diffuse reflectance UV-vis, FTIR and XPS.
16 The conventional wet impregnation method allows obtaining AgAlBEA with silver present mainly in the extra-framework position, as evidenced by XPS and FTIR. The consumption of OH groups of vacant T-atom sites upon reaction with Ag precursor has been monitored by FTIR. The Brønsted and Lewis acidity of AlBEA, AgAlBEA, SiBEA and AgSiBEA have been identified by FTIR spectroscopy of adsorbed pyridine and CO as probe molecules. Our preliminary studies of SCR of NO with ethanol on AlBEA, AgAlBEA, SiBEA and AgSiBEA zeolites show that silver environment and acid-base properties play a significant role in catalytic activity of Ag-containing BEA zeolites.
Acknowledgments This work was partly supported by the program of National academy of sciences of Ukraine “Fundamental problems of nanostructure systems, nanomaterials, nanotechnologies”.
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21
Table 1. Textural properties of zeolite samples Specific Sample
Micropore
Micropore
surface area
surface area
volume
(m2⋅g-1)
(m2⋅g-1)
(cm3⋅g-1)
Median diameter of micropores
Mesopore volume (cm3⋅g-1)
(nm) AlBEA
625
453
0.237
0.95
0.52
Ag4.0AlBEA
605
436
0.233
0.94
0.48
SiBEA
580
403
0.219
0.98
0.55
Ag3.0SiBEA
567
387
0.210
0.97
0.49
22
Si
Si O TEA
O O
O
Si
Si
O
O H 3615 Al3+
Calcination in air
Al3+
Si
Si
at 823 K, 15 h
Si
TEABEA
O
O
Si Impregnation with AgNO
Si
3
O H Si
AlBEA
O
Ag+O
Si
Al3+ O
Si
AgxAlBEA
Conventional wet impregnation
Si
Si O TEA Si
O
O
+
TEABEA
Si 3+
+ 3H , -Al , -TEA
Al3+ O
(1)
Si
Dealumination with HNO3
+
Si
3709 Si H H O
O 3739
H 3529 O O Si 3529 H
(2) +
+ Ag , + H2O -H3O+
SiBEA
Si H H O O 3739 Ag+ O O Si Si H
Si
AgxSiBEA
Two-step postsynthesis method
Scheme 1. Proposed ways of preparation of AgxAlBEA and AgxSiBEA by conventional wet impregnation and two-step postsynthesis method, respectively. The numbers quoted correspond to cm-1. The wavenumbers 3739, 3709, 3615 and 3529 cm-1 correspond to isolated internal SiO-H, terminal internal SiO-H, Al-O(H)-Si and H-bonded SiO-H vibrators respectively.
23 Figure captions
Figure 1. X-ray diffractograms recorded at room temperature and ambient atmosphere of AlBEA, Ag4.0AlBEA, SiBEA and Ag3.0SiBEA. Figure 2. Adsorption isotherms of nitrogen at 77 K on Ag4.0AlBEA and Ag3.0SiBEA. Full symbols – adsorption; empty symbols – desorption. Figure 3. DR UV–vis spectra recorded at room temperature and ambient atmosphere of asprepared AlBEA, Ag1.5AlBEA, Ag4.0AlBEA, SiBEA and Ag3.0SiBEA. Figure 4. XP spectra recorded at room temperature of Ag 3d core level of as-prepared Ag1.5AlBEA, Ag4.0AlBEA and Ag3.0SiBEA. Figure 5. FTIR of AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA recorded at room temperature after calcination at 773 K for 3 h in flowing air and then outgassing at 573 K for 2 h (10-3 Pa). Figure 6. FTIR difference spectra (OH stretching region) of AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA after adsorption of CO at 100 K: equilibrium CO pressure of 100 Pa (a) and development of the spectra during evacuation at 100 K (b–f). Figure 7. FTIR difference spectra (carbonyl stretching region) of CO adsorbed at 100 K on AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA: equilibrium CO pressure of 100 Pa (a) and development of the spectra during evacuation at 100 K (b–g). The spectra are background corrected. Figure 8. FTIR difference spectra recorded at room temperature of AlBEA, Ag1.5AlBEA, SiBEA and Ag3.0SiBEA after calcination at 773 K, 1 h in O2 (2 · 104 Pa), outgassing at 10-3 Pa for 1 h at 673 K, adsorption of pyridine at 423 K and desorption of pyridine at (a) 423 K, (b) 573 K, (c) 673 K. Figure 9. Temperature-dependence of NO conversion in SCR of NO with ethanol over AgxAlBEA (a) and AgxSiBEA (b).
24
Fig. 1.
25
Fig. 2.
26
Fig. 3.
27
Fig. 4.
28
Fig. 5.
29
Fig. 6.
30
Fig. 7.
31
Fig. 8.
32
Fig. 9.
33
Calcination in air
Al
at 823 K for 15 h
Al HAlBEA
TEABEA
1st step
Al TEABEA
AgNO3
HNO3
Al
Ag
AgxAlBEA
2nd step OH OH OH
SiBEA
AgNO3
Ag AgxSiBEA
34
Highlights
*Two-step postsynthesis method allowed to incorporate silver into the vacant T-atom sites *Conventional wet impregnation introduced silver mainly in extra-framework position *Consumption of OH groups upon reaction with Ag precursor has been monitored by FTIR *Brønsted and Lewis acidity were identified by FTIR spectroscopy of adsorbed pyridine and CO * Significant role of silver state and acid-base properties for catalytic activity of AgBEA zeolites