Abstract:
The invention concerns a process for the preparation of a vanadia SCR-catalyst supported on titania. The process is characterized in that the catalyst is prepared by dispersing titania in an ammonium metavanadate (NH 4 VO 3 ) solution, adjusting the pH of the solution to a value of 7.0-7.1 by NH 4 OH and HNO 3 , stirring the resulting suspension for a time sufficient for complete adsorption of the vanadium compound on titania, filtering the suspension and drying and calcining the resulting catalyst compound.

Description:
INTRODUCTION AND BACKGROUND  
         [0001]    The present invention concerns an improved process for the preparation of a vanadia SCR-catalyst supported on titania.  
           [0002]    In view of the US 2002/2004 and EU IV/V legislation, the major concern of exhaust gas aftertreatment for the future heavy duty diesel vehicles is reducing the NO x  and/or particulate emissions. At the same time fuel efficiency is a major engineering target as the end users will see a clear effect on their operation costs. A fuel efficient operation gives high NO x  raw emissions and low particulate emissions. To reduce the NOx emission an efficient aftertreatment system is needed. The SCR (Selective Catalytic Reduction) exhaust gas aftertreatment system using urea as reductant has a high NO x  reduction potential. Especially supported vanadium oxide catalysts have been widely investigated in recent years as they represent an important group of highly active catalysts for the selective catalytic reduction of NO to N 2  with NH 3 . The results of activity measurements (I. Georgiadou, Ch. Papadopoulou, H. K. Matralis, G. A. Voyiatzis, A. Lycourghiotis, Ch. Kordulis, J. Phys. Chem., 102 (1998) 8459 and A. Burkardt, Diplomarbeit, Inst. f. Chem. Technik, Universität Karlsruhe (1998)) reveal a varying activity and selectivity for V 2 O 5 /TiO 2  catalysts resulting from both different preparation methods and varying vanadia loading. Different surface vanadia structures with varying catalytic properties were supposed to be responsible for the observed effects. According to Topsoe et. al. (N. -Y. Topsøe, CatTech, Vol 1 (1997) 125 and J. A. Dumesic, N. -Y. Topsøe, H. Topsøe, Y. Chen, T. Slabiak, J. Catal., 163 (1996)409) the existence of both a V—OH Brønsted site and an adjacent V═O redox site are necessary for a selective reduction of NO to N 2 . IR studies with ammonia revealed a direct correlation between the concentration of V—OH groups and the Brønsted acidity (N. -Y. Topsøe, H. Topsøe, J. A. Dumesic, J. Catal., 151 (1995)22). Therefore the mentioned differences in activity/selectivity could be an effect of different surface vanadia structures with varying Brønsted acidities.  
           [0003]    Both surface structure and active phase dispersion in supported vanadia catalysts were intensively studied by many different characterization methods such as Laser Raman spectroscopy,  51 V MAS-NMR, V-K-XANES, XRD, XPS, TPR and by oxygen chemisorption methods in recent years. It turned out, that the results were not always in agreement. A different catalyst preparation with regard to the used method as well as the vanadium oxide precursor, varying active phase loading and the use of either a high or low surface area TiO 2  (anatase) could result in different observed vanadia structures. The combined results of these investigations suggest, that within the so called monolayer coverage the vanadia species are primarily present as isolated and polymerized VO 4  units anchored to the TiO 2  surface by strong interaction. However, the existence of surface VO 6  units may also be possible. At higher vanadia loading a disordered or even “paracrystalline V 2 O 5 ” phase is assumed.  
           [0004]    The object of the present invention is to supply a process for the preparation of vanadia-SCR catalysts on titania which yields reliably highly disperse and amorphous vanadia deposits on titania. As a result of these highly disperse and amorphous deposits the activity and aging stability of the catalyst is increased. In addition the catalyst reveals a higher selectivity than conventionally prepared catalysts.  
         SUMMARY OF THE INVENTION  
         [0005]    This object is achieved by a process for the preparation of a vanadia SCR-catalyst supported on titania, characterised in that the catalyst is prepared by dispersing titania in an ammonium metavanadate (NH 4 VO 3 ) solution, adjusting the pH of the solution to a value of 6.8-7.3 by NH 4 OH and HNO 3 , stirring the resulting suspension for a time sufficient for complete adsorption of the vanadium compound on titania, filtering the suspension and drying and calcining the resulting catalyst compound.  
           [0006]    It was found that this adsorption procedure leads to highly disperse and amorphous deposits of vanadia on titania. Especially important is the precise control of the pH-value of the suspension in the interval between 6.8 and 7.3, preferably between 7.0 and 7.1. The suspension of titania in the solution of ammonium metavanadate is stirred until the adsorption of the ammonia compound on titania is complete. The adsorption proceeds slowly and may take 24 to 50 hours. The completion of the adsorption can be easily controlled by watching the change in color of the suspension. At the start the suspension is yellow. As the adsorption proceeds the color fades away. Alternatively the completion of the adsorption process may be monitored by analyzing repeatedly the ammonium metavanadate content of the liquid phase of the suspension. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0007]    The invention will be further understood with reference to the drawings, wherein:  
         [0008]    [0008]FIG. 1 is a graph showing NO x  conversion (left) and N 2 O concentration (right) for model gas measurements as a function of temperature  
         [0009]    [0009]FIG. 2 is a graph showing NH 3  concentration for model gas measurements as a function of temperature  
         [0010]    [0010]FIG. 3 is a graph showing NO x  conversion and gas outlet concentrations for model gas measurements of the 4.6 wt.-% V 2 O 5 /TiO 2  (left) and the 1.2 wt.-% V 2 O 5 /TiO 2  (right) catalyst with or without water in the feedgas  
         [0011]    [0011]FIG. 4 is a Laser-Raman spectra of anatase (- - -),  
         [0012]    1.2 wt.-% V 2 O 5 /TiO 2 (a),  
         [0013]    2.3 wt.-% V 2 O 5 /TiO 2 (b),  
         [0014]    2.5 wt.-% V 2 O 5 /TiO 2 (c),  
         [0015]    3.4 wt.-% V 2 O 5 /TiO 2 (d),  
         [0016]    4.6 wt.-% V 2 O 5 /TiO 2 (e)  
         [0017]    [0017]FIG. 5 is the Laser-Raman spectra of the 4.6 wt.-% V 2 O 5 /TiO 2  catalyst: hydrated (a), 45 min 573 K (b), 573-773 K β=5 K/min (c), 45 min 500 K (d), anatase/hydrated (—), anatase/dehydrated 45 min 773 K (- - -)  
         [0018]    [0018]FIG. 6 is the  51 V MAS-NMR spectra of the hydrated and dehydrated 4.6 wt.-% V 2 O 5 /TiO 2  catalyst sample and the model compound V 2 O 5 . The center bands are indicated by an asterisk.  
         [0019]    [0019]FIG. 7 is the  51 V MAS-NMR spectra of the 2.5 and 1.2 wt.-% V 2 O 5 /TiO catalyst samples and the model compound Na 6 V 10 O 28 . The center bands are indicated by an asterisk.  
         [0020]    [0020]FIG. 8 shows the Vanadium K edge XANES of the 4.6 wt.-% V 2 O 5 /TiO 2  catalyst sample in a hydrated and dehydrated state together with two reference compounds  
         [0021]    [0021]FIG. 9 shows the Vanadium K-edge XANES of the 1.2 wt.-% V 2 O 5 /TiO 2  catalyst sample together with two reference compounds  
         [0022]    [0022]FIG. 10 is the I.R. spectra of pyridine absorbed on the 4.6 wt.-% and 1.2 wt.-% V 2 O 5 /TiO 2  catalyst sample followed by 30 minutes evacuation at 323 K  
         [0023]    [0023]FIG. 11 represents Crystallographic matching between a (010) V 2 O 5  plane and the (001) anatase surface. Two representative bridging V—O—V groups being possibly replaced by two V—OH groups upon hydration are marked by an asterisk.  
         [0024]    [0024]FIG. 12 represents postulated vanadia species anchored to the (001) anatase TiO 2  plane consisting of isolated (a) or polymerized (b) VO 4  units  
         [0025]    [0025]FIG. 13 represents two dimensional vanadia species consisting of polymerized VO 4  units  
         [0026]    [0026]FIG. 14 represents polymerized V 3 O 9  clusters consisting of three edge combined square pyramidal vanadia units 
     
    
     DETAILED DESCRIPTION OF INVENTION  
     EXAMPLE  
       [0027]    Five supported vanadia on titania (WO x  stabilized anatase) catalysts with the following loading of V 2 O 5  were manufactured:  
         [0028]    a) 1.2 wt.-% V 2 O 5 /TiO 2    
         [0029]    b) 2.3 wt.-% V 2 O 5 /TiO 2    
         [0030]    c) 2.5 wt.-% V 2 O 5 /TiO 2    
         [0031]    d) 3.4 wt.-% V 2 O 5 /TiO 2    
         [0032]    e) 4.6 wt.-% V 2 O 5 /TiO 2    
         [0033]    The titania support material was dispersed in a solution of ammonium metavanadate (NH 4 VO 3 ). The pH of the resulting suspension was adjusted to a value of 7.0-7.1 by NH 4 OH and HNO 3 . After 48 h of stirring the suspension was filtered. The wet catalyst sample was dried at 333 K for 24 h and then calcined in air at 773 K for 2 h.  
         [0034]    The catalyst powders were coated on substrates. The catalysts thus obtained were thoroughly investigated for their various properties as given below.  
         [0035]    Activity and Selectivity:  
         [0036]    Activity and selectivity of the catalysts were measured under stationary conditions in a temperature range of 423 K to 773 K. Unless otherwise reported, the standard gas composition given in table 1 was used with a space velocity of 30.000 h −1 .  
                                             TABLE 1                       Composition of the gas mixture used in the model gas experiments                                    Component   NO   NH 3     O 2     H 2 O   N 2             [Vol-ppm]   500   450   5000   13000   Balance                      
 
         [0037]    Both inlet and outlet gases were analyzed by a FTIR spectrometer. The temperature (443 K) and the pressure (1080 mbar) in the used gas cell were kept at a constant level. To minimize problems with H 2 O in the quantitative FTIR gas analysis the water concentration was kept at a relatively low level of 1.3 vol.-%. Typically, diesel exhaust gas contains 6 vol.-% water. However, above H 2 O concentrations of about 0. 5 vol.-% no effect on the catalytic reaction is observed other than a dilution effect.  
         [0038]    Laser Raman Studies:  
         [0039]    The Laser Raman Spectroscopy (LRS) was done using a BRUKER RFS 100. The hydrated samples were measured in the standard sample holder at room temperature in air. The dehydrated samples were prepared ex-situ, stored and measured in an argon atmosphere using a quartz glass reactor.  
         [0040]    Nuclear Magnetic Resonance Studies:  
         [0041]    [0041] 51 V-MAS solid-state NMR measurements were carried out at 65,8 MHz (5,87 T) on a Bruker AM-250 spectrometer, equipped with a 7 mm double bearing MAS probe (zirconia spinners). The magic angle was set to 54,7°. To differentiate the main peak from spinning side bands, the measurements were obtained at least at two sample spinning speeds in the range of 3 to 5 kHz. The isotropic chemical shifts were determined from the position of the main peak, which does not change with the sample spinning speed. The chemical shift is expressed in terms of the δ scale with respect to V 2 O 5  as an external reference with δ=−609 ppm (H. Eckert, I. E. Wachs, J. Phys. Chem., 93 (1989)6796).  
         [0042]    XANES Studies:  
         [0043]    X-Ray Absorption Near Edge Structure (XANES) measurements were performed at the Synchrotronstrahlungslabor HASYLAB at DESY, Hamburg, Germany.  
         [0044]    The spectra at the V K-edge (5464 eV) were taken at beamline E4 using a Si (111) double crystal monochromator. Efficient rejection of higher harmonics was obtained by use of a Au coated focussing mirror together with a Ni coated plain mirror. During measurements the primary intensity was set to 70% of the maximum Bragg peak intensity by means of a piezo stabilized feedback loop. All measurements were performed in vacuum at room temperature in transmission mode. A thin layer of catalyst powder (ca. 10 mg/cm 2 ) enclosed between two layers of tape was used as sample. Simultaneously, the absorption of a V metal foil reference was measured to accurately define the energy scale. Background substraction of the spectra was done by substracting the fitted Victoreen of the pre-edge region from the energy corrected data. Due to the strong EXAFS signal of titanium at the vanadium K-edge of the samples with low vanadium loading, background substraction of these samples was done by substraction of the linear fit of the pre-edge region. Finally the spectra were normalized.  
         [0045]    Determination of the Acidic Sites with DRIFTS:  
         [0046]    The measurements were done in a heatable DRIFTS cell with two variable gas inlets and one high vacuum connection. The sample (approx. 50 mg) was placed in the flat receptacle of the cell (10 mm diameter) and evacuated at 473 K for 30 minutes to dry the catalyst. After pyridine was added at room temperature the cell was evacuated at 423 K for 30 minutes to remove non adsorbed probe molecules. The I.R. spectra were recorded at room temperature.  
         [0047]    Activity Measurements with Water in Feedgas:  
         [0048]    [0048]FIG. 1 shows the NO x  conversion and N 2 O concentration in the model gas experiments for the various V 2 O 5  loadings as a function of temperature.  
         [0049]    The catalysts with a V 2 O 5  loading above 3 wt.-% show a high NO x  conversion above 550 K and a high selectivity to N 2 . With decreasing V 2 O 5  concentration the NO x  conversion decreases. At lower temperatures (&lt;500 K) this decreased activity is due to the decreased activity of the SCR reaction as both the NO and the NH 3  conversion are reduced to the same extent (FIGS. 1,2). However, at higher temperatures (&gt;600 K) a complete conversion of ammonia is observed (FIG. 2). The NO x  conversion level is relatively low and can be attributed to the partial oxidation of NH 3  to NO x . N 2 O is preferentially formed in a temperature range between 500 and 700 K. This leads to the observed low selectivity.  
         [0050]    Activity Measurements without Water in the Feedgas:  
         [0051]    [0051]FIG. 3 shows the NO x  conversion and gas outlet concentrations as a function of temperature for two differently loaded catalysts with and without water in the inlet gas. However, as water is one of the products of the SCR reaction and its side reactions, an absolute water free atmosphere can never be achieved.  
         [0052]    The selectivity of the 4.6 wt.-% V 2 O 5 /TiO 2  sample is clearly decreased in the absence of water at high temperatures (&gt;650 K). NH 3  is partially oxidized and a considerable amount of N 2 O is formed. In contrast, the selectivity of the 1.2 wt.-% V 2 O 5 /TiO 2  sample is not influenced by the presence or absence of water. For this sample only a slightly increased activity can be observed.  
         [0053]    Laser-Raman Spectroscopy:  
         [0054]    [0054]FIG. 4 shows the Raman spectra of the different catalysts with V 2 O 5  concentrations from 1.2 to 4.6 wt.-%. The broad band at 975 cm −1 , which is found in all spectra, can be assigned to tungsten oxide species present on the surface of the used WO x  doped anatase TiO 2 . At high vanadia loading (&gt;3 wt.-%) a sharp band at 995 cm −1  is observed which can be assigned to the stretching mode of a surface monoxo vanadia species. At vanadia concentrations below 3 wt.-% this band disappears. This indicates that two different vanadia structures exist respectively at high and low active phase surface loading. The ν V═O  band found at higher vanadia concentrations accidentally coincides with the most intense band of crystalline V 2 O 5 . To discriminate between bulk oxide and surface vanadyl absorptions a measurement of the dehydrated sample is required.  
         [0055]    [0055]FIG. 5 shows the spectra of the 4.6 wt.-% V 2 O 5 /TiO 2  catalyst after dehydration in vacuum at different temperatures. The sharp band at 995 cm −1  for the hydrated sample shifts to 1035 cm −1  with increasing dehydration temperature. This excludes the presence of bulk V 2 O 5  at the catalysts surface. The occurring band at 1015 cm −1  can be assigned to dehydrated WO x  surface species.  
         [0056]    Solid State  51 V MAS-NMR:  
         [0057]    [0057]FIG. 6 shows the spectra of the hydrated and dehydrated 4.6 wt.-% V 2 O 5 /TiO 2  sample and V 2 O 5  as a reference. The clear MAS-NMR patterns observed for the sample signals in both hydrated and dehydrated form show the formation of a distinct, well defined vanadium (V) oxide species. The MAS-NMR side band patterns especially those of the dehydrated catalyst sample are strikingly similar to those observed for V 2 O 5 . Almost the same result was found by Fernandez and Guelton (C. Fernandez, M. Guelton, Catal. Today, 20 (1994) 77) for a V 2 O 5 /TiO 2  catalyst sample used within the Eurocat project. The position of the MAS center band positions is also quite similar, located at −609 ppm for V 2 O 5 , at −611 ppm for the dehydrated and at −624 ppm for the hydrated sample.  
         [0058]    [0058]FIG. 7 shows the spectra of samples with a V 2 O 5  loading below 3 wt.-%. As a result of the strong background signal and the relative low vanadium concentration the patterns observed for the signals are not as well defined as for the highly loaded samples. Therefore the anisotropic chemical shift could only be roughly determined to a broad region at −500±10 ppm. Nevertheless the spectra are quite similar to the spectrum of Na 6 V 10 O 28* aq for both the chemical shift and the MAS sideband patterns. In a  51 V-NMR study done by Eckert and Wachs (H. Eckert, I. E. Wachs, J. Phys. Chem., 93 (1989)6796) a very similar spectrum of a V 2 O 5 /TiO 2  catalyst sample was observed with a chemical shift determined at −510±10 ppm.  
         [0059]    XANES Investigations:  
         [0060]    Absorption measurements were done for all catalyst samples. Both the highly and lowly loaded samples exhibit a prepeak which coincides in position and height to that of the shown references (FIGS. 8,9) and is therefore attributed to a 5+ configuration of the V atoms. The near edge absorption fine structure of the 4.6 wt.-% V 2 O 5 /TiO 2  catalyst especially in the dehydrated state reveals a significant similarity to that of V 2 O 5  (FIG. 8). However, the XANES of the 1.2 wt.-% V 2 O 5 /TiO 2  sample shown in FIG. 9 is more comparable to that of Na 6 V 10 O 28* aq. This result is in good agreement with the FT-Raman and  51 V Solid State MAS-NMR studies shown before.  
         [0061]    Measurement of the Acidic Properties with DRIFTS:  
         [0062]    [0062]FIG. 10 shows the I.R. spectra of pyridine adsorbed on two different catalysts samples after evacuation at 323 K. The bands at 1602, 1572, 1484 and 1442 cm −1  can be assigned to the vibrational modes of Lewis-coordinated pyridine (L-Py). The bands at 1636, 1578 and 1537 cm −1  being assigned to the corresponding modes of the pyridinium ion (B-Py). The 1537 and 1442 cm −1  bands are characteristic of B-Py and L-Py, respectively. The intensity ratio of Brønsted to Lewis acid sites varies with the vanadia loading.  
         [0063]    Whereas for the 1.2 wt.-% V 2 O 5 /TiO 2  sample the number of Brønsted sites is comparatively low, the B-Py to L-Py ratio for the highly loaded sample is about 1. This result might explain the different catalytic properties for the studied catalysts with either a high or low vanadia loading as both a V—OH site and an adjacent V═O redox site are necessary for the selective catalytic reduction.  
         [0064]    Results:  
         [0065]    The results of the vanadia characterization studies indicate that two different highly dispersed and homogenous vanadium oxide species can exist as a submonolayer or monolayer phase. At V 2 O 5  loadings above 3 wt.-% V 2 O 5  a vanadia structure with a V 2 O 5  like coordination was determined. As the maximum V 2 O 5  concentration using the adsorption method is about 4.6 wt.-% a more or less complete vanadia monolayer is postulated for this sample. As there is a certain degree of crystallographic fit between the (010) plane of bulk V 2 O 5  and the (001) plane of anatase TiO 2  a highly dispersed monolayer vanadia phase with a structure similar to a single (010) plane of V 2 O 5  can be assumed (FIG. 11). However, such a two dimensional V 2 O 5  phase dispersed on anatase TiO 2  would contain a certain amount of tension due to a small misalignment of the structural parameters of this vanadia phase and the (001) anatase TiO 2  plane. A hydrated structure where one bridging V—O—V group is replaced by two V—OH groups is probably more stable. In this postulated phase a V—OH site is adjacent to a V═O site resulting in a high catalytic activity and selectivity. Upon dehydration a structure with a complete two dimensional (010) V 2 O 5  plane would be formed. The results of the  51 V-NMR and XANES measurements revealing a vanadium coordination exactly like bulk V 2 O 5  in the dehydrated state support this assumption. By the shift of the Raman band assigned to the ν V═O  monoxo stretching mode the structural change upon dehydration becomes apparent and at the same time the existence of crystalline V 2 O 5  is excluded. As the dehydration is accompanied by a lower concentration of V—OH groups the catalytic selectivity is reduced as observed from the NO x  conversion in the model gas measurements without water at high temperatures.  
         [0066]    At a vanadia loading below 3 wt.-% a different vanadium oxide structure exists on the anatase TiO 2  surface. This structure has a lower number of Brønsted sites and shows a low selectivity in the selective catalytic reaction. The results of the characterization indicates the presence of vanadia species with a square pyramidal coordination as for example in Na 6 V 10 O 28* aq or Zn(VO 3 ) 2 . This vanadium oxide species could consist of isolated or polymerized VO 4  units with one terminal V═O bond anchored to the (001) anatase TiO 2  surface by strong interaction (FIG. 12). In this case a vanadium (V) would occupy roughly the position of another Ti 4+  in the bulk anatase TiO 2  structure. This would result in an almost perfect balance of the local charges on the (001) anatase plane being +⅔ on Ti 4+  and −⅔ on an unsaturated oxygen. However, a two-dimensional vanadia phase with two V—OH groups replaced by a bridging V—O—V bond (FIG. 13) accompanied by a small amount of Brønsted acid sites would be a better explanation for the low catalytic activity/selectivity. Another possible vanadium oxide structure might contain isolated or polymerized V 3 O 9  clusters consisting of three combined square pyramidal vanadia units (FIG. 14). The resulting vanadium coordination is rather comparable to that of the reference compound Na 6 V 10 O 28* aq. The formation of the assumed structure can be explained by adsorption on the (001) anatase TiO 2  surface of V 3 O 9* aq ions probably existing in the precursor solution at the adjusted pH and concentration.  
         [0067]    Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.  
         [0068]    German priority application 00 107 727.0 is relied on and incorporated herein by reference.