Article pubs.acs.org/JPCC
Incorporation of Silver Atoms into the Vacant T‑Atom Sites of the Framework of SiBEA Zeolite as Mononuclear Ag(I) Evidenced by XRD, FTIR, NMR, DR UV−vis, XPS, and TPR Stanislaw Dzwigaj,*,†,‡ Yannick Millot,†,‡ Jean-Marc Krafft,†,‡ Nataliia Popovych,§ and Pavlo Kyriienko§ †
Laboratoire de Réactivité de Surface, UPMC Univ Paris 06, Case 178, Site d’Ivry-Le Raphaël, 3 rue Galilée, 94200 Ivry sur Seine, France ‡ Laboratoire de Réactivité de Surface, CNRS-UMR 7197, Case 178, Site d’Ivry-Le Raphaël, 3 rue Galilée, 94200 Ivry sur Seine, France § L.V. Pisarzhevsky Institute of Physical Chemistry of the NAS of Ukraine, 31 Prosp.Nauky, 03028 Kyiv, Ukraine ABSTRACT: An AgSiBEA zeolite has been prepared by a two-step postsynthesis procedure that 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. 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 XRD, NMR, FTIR, diffuse reflectance UV−vis, XPS, and TPR. The consumption of OH groups has been monitored by FTIR. The Brønsted and Lewis acidities of AlBEA, SiBEA, and AgSiBEA have been identified by FTIR spectroscopy of adsorbed pyridine and CO as probe molecules.
1. INTRODUCTION Selective catalytic reduction (SCR) of NOx in the oxygen-rich exhaust gases of lean-burn and diesel engines remains among the major challenges of environmental catalysis. Extensive research has been done on development of de-NOx catalysts, and many types of catalysts have been reported.1−7 Among them silver-based materials (generally Ag/Al2O3) were found to be the most active and selective catalytic systems for the SCR of NOx with hydrocarbon or oxygenated organic reducing agents.8−16 Besides the metal oxide catalysts, zeolite-based catalysts have received much attention due to its high activity and relatively wide temperature window.1 However, there are only little research data in literature about Ag-containing zeolite catalysts applied in the SCR of NOx.12−16 It is believed, on the basis of the previous reports,17,18 that well-dispersed silver species (Ag+ ions and Agnδ+ clusters) could be active centers of the SCR of NO process on Ag catalysts. On the other hand, it has been recently reported that addition of small amount of H2 promotes the formation of cationic Ag clusters that strongly enhances NO reduction activity in the SCR process over Ag/ Al2O3 and Ag ion-exchanged MFI zeolites.8,12,13,19 Since small Agnδ+ (n ≤ 8) clusters seems to be more active centers in the SCR of NO than both Ag+ ions and agglomerated bigger Agnδ+ (n > 8) clusters, responsible for reducing agent deep oxidation,15 therefore the formation of well-dispersed small Agnδ+ clusters in the zeolite structure would be the key way to obtain a good SCR catalyst. However, a major problem is that current methods used for dispersing metals in zeolites are nonselective and result in the introduction of metals in the form of various species. As a matter of fact, depending on the preparation method, different © 2013 American Chemical Society
metal species can be formed in zeolites: isolated framework and extra framework metal species, metal oligomers, and/or metal oxides. This problem has been recently greatly reduced by the development of a two-step postsynthesis procedure which consists first of creating the vacant T atom sites by dealumination of BEA zeolite with nitric acid and then impregnating the resulting SiBEA with metal precursors.5,20,21 As shown earlier,20−28 this postsynthesis procedure allows for low metal content (lower than 2 wt %), incorporating of metal mainly as isolated and homogeneously distributed metal species without formation of metal oligomers or metal oxide. Thus, in the present work, we have used a silver precursor (AgNO3) and the two-step postsynthesis procedure developed earlier for the incorporation of vanadium,20−24 cobalt,25 iron,26 copper,27 or chromium28 to incorporate silver ions into BEA zeolite. The incorporation of silver in the vacant T atom sites of BEA zeolite as isolated mononuclear Ag(I) species has been evidenced by XRD, FTIR, NMR, diffuse reflectance UV−visible (DR UV− vis), XPS, and TPR.
2. EXPERIMENTAL SECTION 2.1. Materials. AlBEA zeolite was prepared by calcination of a tetraethylammonium BEA (TEABEA) zeolite (Si/Al = 12.5) provided by RIPP (China) at 823 K for 15 h in air under static condition to remove the organic template. Silver-containing BEA zeolite was prepared by a two-step postsynthesis procedure described earlier,20−23 allowing control Received: February 21, 2013 Revised: May 30, 2013 Published: May 30, 2013 12552
dx.doi.org/10.1021/jp401849e | J. Phys. Chem. C 2013, 117, 12552−12559
The Journal of Physical Chemistry C
Article
rotation (12 kHz) was carefully cleaned with ethanol and dried in air at room temperature. The proton signals from probe and rotor were subtracted from the total free induction decay. UV−vis diffuse reflectance 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 Mg Kα (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, the samples were 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. The TPR-H2 measurements were carried out on an AutoChem 2910 apparatus (Micromeretics) in the temperature range 298−1173 K with a linear heating rate of 7 K/min. Samples (weight about 0.1 g) were reduced in hydrogen stream (5% H2/Ar) with a gas volume velocity of 40 mL min−1. Hydrogen consumption was monitored with a thermal conductivity detector (TCD).
the incorporation of Ag ions in the zeolite framework. In the first step, the TEABEA zeolite 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 first stirred under aerobic conditions for 2 h at 298 K in 200 mL of aqueous AgNO3 solution (pH = 2.3) with different concentrations of 0.9 × 10−3 and 2.7 × 10−3 mol L−1 to obtain the zeolites with various Ag content. 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 0.5 and 1.5 Ag wt % were labeled as Ag0.5SiBEA and Ag1.5SiBEA. 2.2. Techniques. X-ray fluorescence chemical analysis of all samples used in this work was performed at room temperature on a SPECTRO X-LabPro apparatus. Specific surface area and adsorption isotherms of nitrogen at 77 K were measured on a Sorptomatic 1990 apparatus. All samples were outgassedfirst at room temperature and then at 623 K to a pressure
