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Monolayers of vanadia were immobilized on pure titania and silica, and on mixed gel carriers of titania and silica containing 1, 10, 20 and 50 mol% of titania ...
Applied Catalysis, 35 (1987) 365-380 Elsevier Science Publishers B.V., Amsterdam

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Selective Catalytic Reduction of Nitric Oxide with Ammonia II. Monolayers of Vanadia Immobilized on Titania-Silica Mixed Gels A. BAIKER*,

P. DOLLENMEIER

and M. GLINSKI*

Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology (ETH), CH-8092 Ziirich (Switzerland) and A. RELLER Institute of Inorganic Chemistry, University of Ziirich, Ziirich (Switzerland) (Received

4 May 1987; accepted

14 May 1987)

ABSTRACT Monolayers of vanadia were immobilized on pure titania and silica, and on mixed gel carriers of titania and silica containing 1, 10, 20 and 50 mol% of titania using the selective reaction of vanadyl triisobutoxide with surface hydroxyl groups of the carriers. The catalysts were investigated with regard to their structural properties and their activity in selective catalytic reduction (SCR) of nitric oxide with ammonia. The textural properties of both pure and impregnated carriers depended strongly on their chemical composition. Low titania content (1 mol%) led to,a marked increase of the BET surface area and the pore volume, whereas with a higher titania content ( > 10 mol%) these properties decreased drastically. X-ray diffraction and high-resolution electron microscopy indicated that silica was present as an amorphous phase in all carriers, whereas crystalline domains of titania (anatase) were found in carriers containing 10 mol% and more of titania. On all carriers the immobilized vanadia species were well dispersed and disordered. Temperature programmed reduction showed for all samples only a single peak for the reduction of the immobilized vanadia layer. The temperature of maximum reduction rate which reflects the ease of reduction of the supported vanadia layer decreased with increasing titania content of the carrier. It was highest for vanadia supported on pure silica ( 790 K) and lowest for vanadia supported on titania (700 K) . This behavior was attributed to the markedly stronger support interaction of titania compared with silica. The highest activity for SCR was found for vanadia supported on mixed gels containing 20 and 50 mol% titania. As a result of the weak support interaction, the vanadia species supported on pure silica tended to agglomerate when exposed to higher temperatures under SCR conditions. This agglomeration was suppressed when titania was added to the silica matrix of the carrier. Of all the catalyst preparations only the vanadia layer supported on equimolar titania-silica and on pure titania maintained stable activity for SCR when exposed to sulfur dioxide-containing feed. *On leave from the Polytechnical

0166-9834/87/$03.50

University

of Warsaw.

0 1987 Elsevier Science Publishers

B.V.

366 INTRODUCTION

Several studies [l-6] have shown that the catalytic properties as well as the chemical and physical structure of supported vanadium oxide are markedly influenced by parameters such as the method of preparation, the concentration of the active component, and the nat.ure of the support. Titania, used either as a carrier or as the major component in coprecipitated catalysts, was found to improve the activity and selectivity of supported vanadia in selective catalytic reduction (SCR) of nitric oxide with ammonia and to make it resistant to poisoning by sulfur dioxide [ 71. Recently, we have studied the properties of vanadia species immobilized on titania using the selective reaction of vanadyl triisobutoxide with surface hydroxyl groups [ 81. It was shown that the vanadia species existing in singly impregnated catalysts (monolayer) were markedly less active in SCR than the VO, species obtained after four- to five-fold impregnation ( multilayer). This behavior was attributed to differences in the structure and support interaction of the VO, species in the monolayer and the multilayers. The support interaction is expected to be largest for the VO, species in the first layer and to become weaker in successive layers. In contrast to titania, silica was found to exhibit unfavourable properties when used as a carrier for vanadia in SCR catalysts [g-11]. It was suggested that the silica-vanadia interaction is weak [ 3,4,10]and consequently the VO, species agglomerate and form relatively large particles [ 3,101. The sol-gel process has been known as a method of preparation of homogeneous multicomponent metal oxides for more than a decade. The method consists of complexation, hydrolysis and polycondensation of mixed metal alkoxides (coprecipitation) and subsequent calcination [ 12,131. Generally such chemically mixed oxides are used for the production of glasses or ceramics at temperatures well below the melting point, but other applications are gaining interest [ 141. To our knowledge reports concerning the application of mixed gels as catalysts or as carriers for active components are still scarce [ 11,15,16]. In the present work we report the preparation and chemical and physical properties of titania-silica mixed gels used as carriers for monolayers of vanadia. It is shown that by application of mixed gels with appropriate silica-titania ratios the strong interaction of the immobilized VO, species with the titania can be weakened in such a way that optimal carriers, i.e. optimal interaction of the carrier with the immobilized VO, species, are obtained for SCR. EXPERIMENTAL

Preparation

of titania-silica

mixed gels

The mixed gels of titania and silica used as carriers were prepared by coprecipitation of the corresponding

for the vanadia layers hydrosols according to

369

.

carrier

s impregnated

g

151 20

40

mol

60

carrier

80

96 titania

Fig. 1. Change of textural properties of pure and impregnated titania-silica carriers.

Fig. 2. XRD patterns of impregnated mixed gels of titania and silica. Percentages quoted correspond to titania content (mol% ) of mixed gels.

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Fig. 3. High-resolution electron micrographs and electron diffraction patterns of impregnated titania-silica carriers. (A) 0% titania; (B) 10% titania, arrows point to the crystalline titania (anatase) particles; (C) 50% titania, revealing the perfectly crystalline anatase particles within the amorphous silica matrix.

the electron micrographs due to their disordered structure and their low concentration. The reduction profiles of the catalysts as measured by TPR are presented in Fig. 4. The pure carriers did not exhibit any significant reduction below 900 K. All impregnated catalysts exhibited one single peak with the peak maximum shifted to a lower temperature with increasing titania content of the carrier. The hydrogen consumption measured over the whole temperature range was used to estimate the vanadia load assuming that reduction from vanadium(V) to vanadium( III) only occurred. X-ray fluorescence analysis of the catalysts with regard to their vanadia content confirmed that the above assumption was justified, i.e. the vanadium(V) concentration could be determined with reasonable accuracy from the hydrogen consumptions measured during TPR. The density of vanadium (V) on the carrier surfaces was estimated from the BET surface areas and the hydrogen consumptions. The changes in the ease of reduction (expressed as the temperature of maximum reduction rate, Z’,,,, in the TPR profiles ) and of the monolayer capacity of the supported vanadia resulting from addition of titania to the silica matrix of the carrier are plotted in Fig. 5. ESR was used to gain some information about the chemical structure of the supported vanadia layers. Samples containing 0 or 1 mol% of titania showed hardly any signals after reaction at 570 K, while after reaction at 380 K a strong signal due to vanadium (IV) ions appeared. After reaction at 570 K samples with a higher titania content showed signals due to vanadium (IV) which were

372

Y

E”

Y

!-

L 600

600

TEMPERATURE

20

40

6C

80

I

1000 /

K

mol

% titania

Fig. 4. TPR profiles of vanadia layers supported on mixed gels of titania and silica. Percentages quoted correspond to mol% of titania in the carriers. Ti-V and Si-V denote vanadia layers immobilized on pure titania and silica, respectively. Conditions are given in the experimental section. Fig. 5. Reducibility and silica.

and monolayer

capacity

of vanadia

layers supported

on mixed gels of titania

assigned to V02+ species. However, the signals observed for the same samples after reaction at 380 K were markedly stronger. Characteristic ESR spectra recorded for catalysts with a higher titania content are presented in Fig. 6. The samples with more than 10 mol% of titania exhibited spectra due to V02+ ions in more than one axial site. The sample with 50 mol% of titania, for example, showed the presence of VO*+ ions in two types of axial site, as shown by the differing “parallel” features and the following spin Hamiltonian parameters:A,,=0.0162T,A~,=0.0210T,g,,=1.934,g~=1.922,A,=0.0083T, g, = 1.985. The spin Hamiltonian parameters of “average” axial symmetry, as determined from the more prominent features of the spectra at room temperature, were quite similar. All the catalysts, except the monolayer on silica, showed the presence of V02+ species at 120 K in a number of different sites as indicated by the lower and higher field “parallel” features. This made it difficult to obtain the parameters, even assuming axial symmetry.

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titania. The microcrystals of anatase were intimately mixed with the amorphous silica matrix and their size increased as the titania content of the mixed gels increased. As to the structure of the supported vanadia layer, we concluded from our structural investigations only that the immobilized VO, species were well dispersed and strongly disordered. More detailed studies concerning the structure of the immobilized vanadia species are presently being undertaken in our laboratory. As regards the vanadia layer capacity of the titania-silica carriers we found that it depended on the titania content, in particular, in the composition range up to 20 mol% of titania (Fig. 5). Upon addition of only 1 mol% of titania to the silica, the vanadium (V) density decreased markedly from 1.1 ~mol/m2 (pure silica) to 0.6 pmol/m2. However, at higher titania content ( > 10 mol% ) , the vanadia layer capacity increased steadily reaching a maximum with pure titania. We note that the theoretical capacity of about 8 /lrnol V5+/m2 [l] estimated by assuming a vanadium ( V) density of 4.8 nmp2 for the (010) face of unsupported vanadium pentoxide was not even remotely reached. The considerably lower vanadia layer capacity is not surprising in view of the fact that our carriers were calcined at 873 K before impregnation. This calcination led to partial dehydroxylation of the surface and consequently the resulting vanadia layer capacities were considerably smaller [ 81. The relatively high calcination temperature was used to eliminate possible side reactions caused by the presence of strongly adsorbed water on the carrier surface. TPR can discriminate between different phases of supported vanadia as was shown for multilayers of vanadia deposited on titania [1,8]. The TPR profiles of the vanadia layers supported on the pure silica and titania carriers as well as on the mixed gels of these materials exhibited a single reduction peak (Fig. 4). The appearance of a single reduction peak in the TPR profiles of the vanadia layers supported on the titania-silica carriers was surprising considering that the reducibility of vanadia supported on silica was largely different from that supported on titania (Fig. 4). The single reduction peaks indicate that the mixed gel carriers prepared by the sol-gel process can be regarded as homogeneous binary oxides. Heterogeneous mixtures of titania and silica would be expected to yield two distinguishable reduction peaks in the TPR profiles. The ease of reduction of the immobilized vanadia layer was reflected by the temperature of maximum reduction rate ( T,,,) in the TPR profiles (Fig. 4). We note that both vanadia on silica ( T,,,, 790K) as well as vanadia on titania than unsupported (Trnax, 700 K) were reduced at a much lower temperature pure vanadia ( T,,,, 940 K ) [ 81. Upon addition of only 1 mol% of titania to silica T,,, was lowered by about 30 K (Fig. 5). Further addition of titania resulted in a less pronounced, but steady decrease of T,,,. Taking the ease of reduction as a measure of the interaction of the vanadia with the titania-silica carrier we can state that this carrier interaction increases steadily with in-

creasing titania content of the carrier. The interaction is weak with silica and considerably stronger with titania. Similar behavior has been observed earlier by Roozeboom et al. [ 31 for silica supported vanadia prepared by wet impregnation and ion-exchange. These authors found by means of Raman spectroscopy that crystalline vanadium pentoxide already coexisted with a dispersed phase at low vanadia content. On calcining the catalysts at a higher temperature they observed that the Raman lines of vanadium pentoxide at 996 and 703 cm-l disappeared while the band around 1020 cm-l increased. This behavior was explained by gradual depolymerization of the crystalline vanadium pentoxide at temperatures near the melting point of vanadium pentoxide (943 K) . It is well known that oxygen in vanadium pentoxide becomes very mobile near its melting point, the exchange rate of gaseous oxygen and lattice oxygen being very high [ 211. Since this exchange occurs through shear planes of oxygen in the vanadia lattice, the bonds between two lamellae are essentially weakened with increasing temperature until depolymerization occurs at the melting point. The ESR investigations (Fig. 6) of the catalysts after reaction at 380 K and 570 K indicated that the reoxidation of the surface was very slow at 380 K. This conclusion emerged from the much weaker spectra of VO” of the catalysts after reaction at 570 K compared with those after reaction at 380 K. The fact that the room temperature spectra obtained for samples with higher a titania content exhibited two or more types of axial site indicated that the vo*+ ions became more and more attached to titanium (IV) with increasing titania content. Thus it seems that there was no significant preference of the vanadyl precursor species as concerns the site for immobilization on the surface of the mixed gels. Catalytic acticity The peculiar activity behavior observed for the silica supported vanadia layer (Fig. 7, Si-1V) is presently only little understood. However, it is clear that it originates from the weak vanadia-silica interaction which facilitates changes in the dispersion of the vanadia species under SCR conditions. It appears that the unusual activity behavior has to be attributed to reversible changes in the vanadia dispersion caused by the interplay between thermally induced agglomeration (polymerization) and the redox processes occurring on the vanadia surface during SCR. Takagi et al. [ 101 det.ected by XPS partial reduction of vanadia by ammonia at temperatures as low as 473 K. This behavior was only observed for vanadia supported on silica and not for alumina and titania supported vanadia. At lower temperatures reoxidation of the surface appears to be a slow process, as evidenced by our ESR investigations, and bulk oxygen from underlying shear planes moved onto the surface, leaving reduced vanadium (IV) sites (dark colour ) in t.he shear planes. This behavior may enable the small ammonia molecules to fill the empty space between the vanadia la-

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mellae on the reduced sites. The strong ammonia adsorption observed with this sample at lower temperatures supported this supposition. The interplanar bond of the two vanadia lamellae (largest V-O bond) was thus broken and consequently the dispersion of the vanadium ions increased, leading to considerably higher activity. Upon returning to higher temperatures reoxidation of the catalyst occurred and the ammonia desorbed. Since the carrier interacted only weakly, it was possible for the vanadia lamellae to agglomerate and crystallize again, resulting in a decrease of dispersion and activity. We emphasize that the mechanism suggested above is speculative, and further investigations will be necessary to finally assign the reasons for the peculiar activity behavior of the silica supported vanadia layer in SCR. As regards the specific activity of the vanadia layers immobilized on the titania-silica carriers we note a distinct influence of the carrier composition on the catalytic activity. If 1 mol% of titania was present in the carrier the interaction of the vanadia monolayer with the carrier surface was strongly enhanced. The reducibility (Fig. 4, T,,,) was increased and the polymerization of the vanadia species could hardly be observed. The Arrhenius curve exhibited only a slight decrease in the slope at higher temperatures indicating that a small amount of the vanadia was still crystallizing. With higher amounts of titania, the activity and reducibility were further enhanced until with 20% of titania in the carrrier the highest activity was reached. This did not change markedly with higher titania content. It is worth mentioning that this activity was about the same as the one observed in our previous work [ 81 for titania supported vanadia multilayer catalysts obtained by four-fold impregnation of titania with vanadyl triisobutoxide. The monolayer on pure titania exhibited the highest reducibility, but the activity for SCR was rather poor suggesting that the interaction of the vanadia layer with titania was too strong for high catalytic activity. The results of the present and of the previous work [ 81 demonstrate two possible ways to weaken the vanadia-support interaction for enhancement of the catalytic activity for SCR. The vanadia-titania interaction can be weakened either by successive deposition of several vanadia layers [ 1,8] on titania, or by application of mixed gel carriers containing titania and a second component, such as silica, which weakens the titania-vanadia interaction. CONCLUSIONS

Vanadia layers were immobilized on mixed gels of titania and silica using the selective reaction of vanadyl triisobutoxide with surface hydroxyl groups. The sol-gel process is shown to be an interesting method for modifying the properties of carriers. The catalysts prepared were investigated with respect to their structural and chemical properties, and tested for the selective catalytic reduction (SCR) of nitric oxide with ammonia.

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High resolution electron microscopy and electron diffraction of the mixed gel carriers indicated that titania exists preferentially as small microcrystalline domains of anatase embedded in the completely amorphous matrix of silica. The vanadia layers supported on these carriers are considered to be welldispersed and completely disordered. On all carriers the vanadia content of the deposited layer is markedly smaller than the one necessary to cover the carriers with a complete monolayer. The strength of the vanadia-carrier interaction, as reflected by the ease of reduction of the vanadia, depends strongly on the chemical composition of the carrier. The interaction is weak for silica and strong for titania. It can be changed within this range by using titania-silica mixed gels of different composition as carriers. Vanadia layers supported on titania-silica containing 20 to 50 mol% of titania were found to be most active for SCR. However, only vanadia supported on mixed gels containing equal amounts of the constituents were resistant to poisoning by sulfur dioxide. ACKNOWLEDGEMENTS

Financial support by the Swiss National Foundation and the “Schweizerischer Schulrat” is gratefully acknowledged. Thanks are due to Dr. V.K. Sharma for performing the ESR investigations.

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