Chapter 3
Selective Catalytic Reduction of NO by N H over Aerogels of Titania, Silica, and Vanadia 3
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M. D. Amiridis , B. K. Na , and E. I. Ko 1
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W. R. Grace & Co.-Connecticut, 7379 Route 32, Columbia, MD 21044 Korea Institute of Science and Technology, Chemical Processes Lab, P.O. Box 131, Cheongryang, Seoul, Korea Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213
Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0587.ch003
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Aerogels of titania, titania-silica, and titania-vanadia were prepared with titanium butoxide, silicon ethoxide, and vanadium triisopropoxide as precursors. The titania and titania-silica aerogels were then used as supports for vanadia, introduced by the incipient wetness impregnation of vanadium triisopropoxide and the subsequent calcination at 773 K . The structures of vanadia and titania in these samples were characterized by X-ray diffraction and Raman spectroscopy, and their catalytic properties by the selective catalytic reduction (SCR) of N O with NH . With H O and SO in the feed stream, the SCR activity decreased with increasing vanadia loading. Samples of high activity all contained anatase TiO , but an active vanadia species only needed to be in close proximity and interacting with, and not necessarily deposited on the surface of crystalline titania. Co-gelling was thus an effective way to prepare an active sample in a single step, as demonstrated by the SCR data of the titania-vanadia aerogel. The addition of niobia, up to 10 weight %, did not appreciably change the surface area, structure, or SCR activity of the titania aerogel supported vanadia. 3
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Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperatureprogrammed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al. (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al. have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace & Co.-Conn., SYNOX™, is based on a titania/silica support (7).
0097-6156/95A)587-O032$12.00A) © 1995 American Chemical Society
In Reduction of Nitrogen Oxide Emissions; Ozkan, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
3. AMIRIDIS ET AL.
SCR of NO by NH over Aerogels 3
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Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0587.ch003
Recently Handy and co-workers have used the sol-gel method to prepare gels containing various combinations of vanadia, titania and silica (8, 9). Solgel synthesis is a versatile approach in preparing catalytic materials because it offers better control over their textural properties and sample homogeneity in multicomponent systems. Aerogels, materials that are obtained by drying wet gels with supercritical extraction, offer additional advantages such as high specific surface area and thermal stability (10). We have explored some of these advantages by preparing aerogels of vanadia, titania and silica and evaluating their SCR activities. The specific issues that we have addressed are: (1) the use of a high-surface-area titania aerogel as a support, (2) the different support behavior between aerogels of titania and titania-silica, (3) the effect of having vanadia in rather than on titania, and (4) the effect of niobia addition. Our key findings are reported below. Experimental Sample Preparation. The preparation of the titania aerogel is described in detail elsewhere (11). Briefly, we prepared a titania gel from a solution of methanol, titanium n-butoxide, nitric acid and doubly distilled water and subsequently removed the solvent by supercritical drying with carbon dioxide. To prepare a titania-silica aerogel containing 40 weight % (33 mol %) titania, we prehydrolyzed tetraethylorthosilicate (TEOS, same as silicon ethoxide) in a separate solution of methanol, nitric acid and water for 10 minutes before combining it with a solution containing the titanium precursor. A similar procedure, but without prehydrolysis, was used to prepare a titania-vanadia aerogel containing 10 weight % vanadia by using vanadium triisopropoxide as a precursor. A l l aerogels were calcined in flowing oxygen at 773 K for 2 h before the introduction of supported vanadia and niobia. Four vanadia/titania samples, containing 4, 10,15 and 20 weight % vanadia, were prepared by the incipient wetness impregnation of a titania aerogel with a solution of vanadium triisopropoxide in either methanol or 1-propanol. These samples were heated under vacuum at 383 K for 3 h, then at 573 K for 3 h and finally in flowing oxygen at 773 K for 2 h. The same sequence of heat treatment was used after the sample containing 4 weight % vanadia was impregnated with a methanol solution of niobium ethoxide. Three niobiapromoted samples, containing 2, 5 and 10 weight % niobia, were prepared. Table I summarizes all the samples used in this study. We use the notation A/B to denote that A is supported on B, and A-B to denote that A and B are mixed in the bulk. Physical Characterization. The specific B E T surface areas of all aerogels were determined from nitrogen adsorption data with a commercial Autosorb-1 instrument (Quantachrome Corp.). Powder X-ray diffraction patterns were obtained with a Rigaku D/Max diffractometer with Cu Kcc radiation. Raman spectra were obtained with the 514.5-nm line of a Spectra Physics Model 20505W argon ion laser (12). Samples were dried in an in situ cell at 383 K for 2 h in flowing oxygen before Raman measurements. SCR Activity. In a typical run, 0.1 g of sample, after being pressed at 15,000 psi and sieved into 40/60 mesh particles, was tested in a stainless steel one pass flow reactor at atmospheric pressure, a temperature of 623 K and a flowrate of 200 l(STP)/h. The reactant concentrations were 400 ppm NO, 400 ppm NH3, 4% O2, 800 ppm SO2, 8 % H2O and the balance N2- The N O concentration at both the inlet and the outlet of the reactor was analyzed by the use of a
In Reduction of Nitrogen Oxide Emissions; Ozkan, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
In Reduction of Nitrogen Oxide Emissions; Ozkan, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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Ti02-V205 aerogel containing 10 wt % vanadia
2 wt % Nr^Os on 4V/T
5 wt % Nb205 on 4V/T
10 wt % Nb20 on 4V/T
(V-T) 10
2Nb/4V/T
5Nb/4V/T
10Nb/4V/T
(DAfter calcination at 773 K for 2 h. ( )(s) = strong; (w) = weak.
10 wt % V 0 on Ti02-Si0 aerogel containing 40 wt. % TiC>2
10V/(T-S)40
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340
4 wt % V2O5 on TiC>2-Si02 aerogel containing 40 wt % Ti02
4V/(T-S)40
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anatase (s) 145
anatase (s)
anatase (s)
X-ray amorphous
X-ray amorphous
anatase (s), rutile (w), vanadia (w)
anatase (s), rutile (w), vanadia (w)
