Vanadium-titanium oxide aerogel catalysts

J O U R N A L OF

ELSEVIER

Journal of Non-Crystalline Solids 186 (1995) 408-414

Vanadium-titanium oxide aerogel catalysts Ronald J. Willey *, Chien-Tsung Wang, John B. Peri Department of Chemical Engineering, Northeastern University, Boston, MA 02115, USA

Abstract

Selective catalytic reduction of nitric oxide by ammonia in the presence of oxygen was studied over titania-supported vanadia aerogels promoted by the addition of cerium oxide and tungsten oxide. Conversion of NO by NH 3 was determined for temperatures ranging from 423 to 723 K at a space velocity of 200 000/h (NTP). The highest activity (95% conversion) was observed for a W / V / T i (1.0/2.5/97.5 at.%) aerogel at 723 K. The best activities for the C e / V / T i (0.1/2.5/97.5) and V / T i (2.5/97.5) aerogels were 89% and 83% conversion respectively at 723 K. Arrhenius plots gave an activation energy of 42-71 kJ/mol dependent on catalyst investigated. Surface groups identified by Fourier transform infrared (FTIR) characterization demonstrated strong absorption bands of nitric oxide on pre-reduced surfaces. Both strong Lewis and weak Brcnsted acid sites were detected by ammonia adsorption on pre-oxidized samples containing vanadium, cerium, and tungsten.

1. Introduction

Nitric oxides (NO x) contribute to environmental pollution and formation of acid rain. NO x removal systems are necessary to meet standards for the emissions of NO x in the stack gases from stationary sources. Generally, a combination of combustion modification and non-catalytic post NO x control reduces NOx emissions by 75%. However, to reach 90% removal, the selective catalytic reduction (SCR) of NO x by NH 3 is required. Excellent background about SCR can be found in Ref. [1]. The overall reaction stoichiometry in the presence of oxygen is 4NO + 4NH 3 + O 2 catalyst) 4N2 + 6H20"

* Corresponding author. Tel: + 1-617 373 3962. Telefax: + 1617 373 2501, E-mail: [email protected].

Commercially, flue gas is fed through a packed catalytic reactor with an ammonia injection system located upstream of the reactor. The catalyst must possess high activity, superior selectivity, good resistance to SO x poisoning and inactivity for SO 2 oxidation to SO 3. Titania-supported vanadia catalysts meet these requirements. Typically, trace amounts of additional metal a n d / o r metal oxides are added to catalysts to further enhance activity or provide stability. One example is cerium oxide used in automotive exhaust catalysts to stabilize Pt and Rh [2-4]. Another example is tungsten oxide which alone is an active SCR catalyst [5,6]. However, a further understanding of the role of tungsten oxide in the commercial SCR catalysts is still needed. Aerogels are ideal for fundamental and catalytic studies because of their small particle size [7]. Thus, a series of vanadia-titania, c e r i a - v a n a d i a - t i t a n i a , and tungsten o x i d e - v a n a d i a - t i t a n i a aerogel catalysts were prepared to investigate the nature of the sur-

0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0022-3093(95)00063-1

R.J. Willeyet al, /Journal of Non-Crystalline Solids 186 (1995)408-414 face-active sites, and to evaluate the catalytic activity of these catalysts for the SCR of NO by NH 3.

2. Experimental procedure 2.1. Aerogel preparations Aerogels were prepared by the sol-gel method using supercritical drying. The synthesis procedure consisted of four steps: preparation of the colloidal solution, hydrolysis, gelation and supercritical drying. As an example, consider the co-gel procedure used for the vanadia-titania mixed oxide aerogel (VT01). Two separate solutions were made according to appropriate metallic atomic ratio of 97.5 Ti to 2.5 V atoms. Solution 1 was composed of 4.3 wt% titanium 1-butoxide (3.890 g) in 1-butanol (87.002 g). Solution 2 was 5.0 wt% vanadium acetylacetonate (0.108 g) in methyl alcohol (1.99 g). Solution 2 was slowly poured into a rapidly stirring Solution 1. Next, water was added to the colloidal solution at 1.4 times (total 1.164 g) the stoichiometric amount required to hydrolyze vanadium acetylacetonate and titanium 1-butoxide to their respective hydroxides. A mixed oxide gel (brown colored solution) formed after mixing for 10 min. Complete gelation was attained by continuous stirring for 12 h. The final gel solution was then transferred into a Pyrex glass liner that was then placed inside a 300 cm 3 autoclave. 1-butanol solvent was added in order to perform supercritical drying. The autoclave was then heated at about 2 - 3 K / m i n . Critical properties of 1-butanol (Tc = 560 K, Pc = 49.0 bar) were used to judge the termination point. After reaching the supercritical region, the pressure was then slowly released maintaining a constant temperature. Finally, the autoclave was purged with a nitrogen flow and cooled to room temperature overnight. The resultant aerogel (cocoa brown colored powder) was then removed from the autoclave. Another V - T i aerogel (VT02) was prepared by first making a titania aerogel. Then this aerogel was placed in a methanolic solution of vanadium acetylacetonate. Next the supercritical drying process was repeated (a two-step preparation procedure).

409

The promoted cerium and tungsten oxide V / T i aerogels were prepared by mixing the proper ratio of three precursor solutions together followed by supercritical drying. The titanium and vanadium precursor solutions were prepared as described above. For the cerium precursor solution, cerium hydroxide (0.025 g) was dissolved in nitric acid (1.240 g) under gentle heating. The resultant cerium nitrate (Ce(NO3) 4) was rinsed with methanol (2.03 g). Then, the three precursor solutions were mixed together. The three levels of Ce studied were 0.1 (CVT001), 0.5 (CVT005), and 1.0 (CVT010) atomic ratios based on the atomic portions of 2.5 V plus 97.5 Ti. For the tungsten oxide, tungsten hexacarbonyl 1 (0.041 g) was placed in methyl alcohol (2.065 g). The three solutions were combined and allowed to mix for 24 h. Then, supercritical drying followed. The three levels of W investigated were 0.1 (TVT001), 0.5 (TVT005) and 1.0 (TVT010) atomic ratios based on atomic portions of 2.5 V plus 97.5 Ti. Also a pure Ti aerogel was prepared as a control (T100). All materials were calcined in a furnace starting at ambient temperature and heating to 500°C. Then, the materials were held at 500°C for 2 h.

2.2. Infrared characterization The infrared system consisted of five primary parts: an infrared spectrometer (Perkin-Elmer Model #1600 Fourier transform infrared spectrometer), a vacuum system, a furnace, a gas manifold and a sample cell (NaCI windows). An enclosed sample holder could be moved between NaC1 windows and the furnace by an external magnet. The manifold system supplied probe molecules (NO, NH3, CO, CO 2 and H 2) for surface reactions on samples. Self-supporting wafers were made by compressing approximately 200 mg of the powdered aerogels in a 1¼ inch die. These were then cut to fit a sample holder and then placed inside the infrared cell.

2.3. Evaluation of activity for the SCR of NO by NH~ The catalytic performance of the aerogels was evaluated in a specially designed Pyrex flow reactor. 1Tungsten hexacarbonyl is highly toxic: care must be taken with its usage.

R.J. Willey et al. / Journal of Non-Crystalline Solids 186 (1995) 408-414

410

The reactor consisted of a set of concentric Pyrex tubes with frits that held the powdered aerogels (about 150 mg) in place. The reactor flow rate was set at about 0.1 m 3 / h at normal temperature and pressure of air. Inlet concentrations of NO and NH 3 were held constant at about 2000 ppm and 2500 ppm, respectively. The temperature range was from 423 to 723 K at 25 K increments. Product and feed analyses for nitric oxide concentrations were determined by a chemiluminescent analyzer (Thermal Electron Corporation Model 44).

3. Results

Table 1 lists elemental analysis (by energy-dispersive spectroscopy (EDS)), surface area (by Brunauer, Emmett and Teller method (BET)), packing density (by volumetric measurement) and calculated particle size (from BET area) of the nine aerogels synthesized. The co-gel prepared vanadia-titania aerogel (VT01) gave a higher surface area (61.7 m 2 / g ) compared with the two-step method (51.1 mZ/g) (VT02). The cerium-containing mixed oxides had higher surface areas than the others (from 72.5 to 115.4 m2/g). Thus the addition of cerium enhanced the surface area and textural properties. The surface areas for the tungsten-containing mixed oxides decreased with increasing tungsten loadings (from 67.0 to 53.5 m2/g).

0.9

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2200

2000

1800

1600

i

~-~i

I

I

1400

1200

~'

WAVENUMBER (CM-1 ) Fig. 1. Infrared spectra for NO adsorbed on oxidized vanadiatitania aerogel (VT01): (a) after adsorption of NO (20 Torr for 30 min); (b) after evacuation at room temperature for 5 min; (c) after evacuation at 373 K for 10 min; (d) after evacuation at 473 K for 10 min.

Examples are shown in Figs. 1 and 2 of infrared spectra obtained when probe molecules (NO and NH 3) are added to VT01 after a pretreatment calcination at 500°C in 200 Torr air. Figs. 3 - 5 show activity results for eight of the nine aerogels prepared for the SCR of NO by NH 3. The results are presented as conversion as a function of temperature. Conversion in these figures are defined as (cone NO in - conc NO o u t ) / ( c o n c NO in) × 100.

Table 1 Composition and bulk properties of aerogels Metallic atomic ratios (at.%) Ti (as prep.) VT01 VT02 CVT001 CVT005 CVT010 TVT001 TVT005 TVT010 T100

97.5 97.5 97.5 97.5 97.5 97.5 97.5 97.5 100.0

Ti (by EDS) (+0.1) 97.0 97.8 97.0 97.2 97.6 97.6 97.2 96.0 -

V (as prep.)

V (by EDS a) (+0.2)

Ce or W b (as-prep.)

Ce or W c (by EDS)

2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 0

2.8 1.9 2.2 1.9 2.0 2.0 1.8 2.3 -

0 0 0.1 0.5 1.0 0.1 0.5 1.0 0

0.3 0.4 0.4 0.2 0.7 1.1 -

Packing

BET

Ea d

A d

density (kg/m3)

surface (m2/g)

(kJ/mol)

(Mm3/h/kgcat)

278 482 196 155 277 158 298 213 510

61.7 51.1 72.5 115.4 98.4 67.0 62.0 53.5 70.0

49.07 41.53 56.90 53.02 42.96 70.61 54.52 53.18 -

10.0 2.1 53.0 16.0 2.5 700.0 35.0 33.0 -

EDS analysis performed by Dr Steven A. Oliver, CEM, Northeastern University. b A low background of Ce (0.2-0.4%) was detected in all samples. c For Ce the error is +0.4, for W the error is +0.2. d For use in the rate equation: RateNo (kmol/h/kgca t) = A exp(-E,/RT)CNo, units of Cso are k m o l / m 3. a

R.J. Willey et al. /Journal of Non-Crystalline Solids 186 (1995) 408-414 100

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. - - ~ ~i 1 I I 423 473 523 573 TEMPERATURE

I I 623

I I 673

I 723

(K)

Fig. 5. Activities of tungsten oxide-vanadia-titania aerogels for the SCR of NO by NH 3 in air.

Conversion as a function of temperature for T100 (pure titanium aerogel) was essentially zero at all temperatures and thus these results are not shown.

100

80 -

o

(CM "1)

Fig. 2. Infrared spectra for NH 3 adsorbed on oxidized vanadiatitania aerogel (VT01): (a) after adsorption of NH 3 (20 Torr) for 30 min; (b) after evacuation at room temperature for 5 min; (c) after evacuation at 373 K for 10 min; (d) after evacuation at 473 K for 10 min; (e) after evacuation at 573 K for 10 min.

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4. Discussion

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TEMPERATURE

Fig. 3. Activities of vanadia-titania aerogels for the SCR of NO by NH 3 in air. 100

80 Z O

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~ i 423 473

CV'I'O01

I 523

I 573

TEMPERATURE

I

I I 623

.,~ _~

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I 723

(K)

Fig. 4. Activities of cerium oxide-vanadia-titaniaaerogels for the SCR of NO by NH 3 in air.

Table 1 shows that the packing densities of aerogels are well below 1000 k g / m 3. The v a n a d i a - t i t a n i a aerogel prepared by the two-step method (VT02) gave a packing density, 482 k g / m 3, almost equal to that of pure titania, 510 k g / m 3. However, the cogel-synthesized v a n a d i a - t i t a n i a aerogel (VT01) and other aerogels gave a much lower packing density ranging from 155 to 298 k g / m 3. This indicates that the addition of vanadium, cerium and tungsten by the co-gel synthesis approach changes the textural formation of the titania gel support and may happen because of co-gelation. If vanadium is added during the formation of titania gel, the resultant aerogel has a lower packing density (about one-half) compared with pure titania. If vanadium is added after the formation of titania, the resultant aerogel has textural properties very similar to that of pure titania. Further evidence about textural development can be seen in the column for surface areas. Addition of a small amount of Ce increased the surface area by 1 0 - 4 0 % to a high of 115 m 2 / g . The remaining aerogels ranged in surface areas from 51 to 72 m 2 / g .

412

R.J. Willey et al. /Journal of Non-Crystalline Solids 186 (1995) 408-414

4.2. Infrared characterization Adsorption of NO on pure titania aerogel (T100) showed no significant absorption bands; however, when vanadium was added (VT01) several NO absorption bands appeared, as shown in Fig. 1. One broad absorption band appeared at 1700-2200 cm 1 that progressively increased to a strong band at 1922 cm -~. This broad absorption band probably corresponds to nitrosonium ions (NO +) multiply coordinated with several vanadium centers. These similar results resembled the absorption observed by Ramis et al. [8] and Primet et al. [9] after contact of HCl-treated TiO 2 with NO. Upon increasing the contact time, a composite band with maxima at 1616, 1551 and 1520 cm -1 and two bands at 1302 and 1247 cm ~ were observed. The original weak band at 1616 cm -~ that shifted to 1627 cm -~ is assigned to nitric oxide adsorption on the surface oxygen ( V - O = N O ) . The band pairs at 1551 and 1302 cm ~ and at 1520 and 1247 cm 1 are assigned to nitrate species with different coordination and thermal stability. Evacuation at 373 K caused the disappearance of the broad band in the region 2 2 0 0 1700 cm 1, and the decrease of the intensity of the bands due to nitrate species (spectrum c, Fig. 1). Spectrum (a) in Fig. 1 shows a negative absorption band at 2045 cm 1 that is associated with the perturbation of coordinately unsaturated vanadyls (VO 2+), and is assigned to the first overtone of the fundamental V = O stretching [8,10,11]. An absorption band at 1035 c m - 1 in spectrum (d) of Fig. 1 appears after all of the NO is removed by evacuation at 473 K. This band is assigned to the fundamental V = O stretching of surface isolated vanadyl groups. The V = O group is suggested as an active site for the SCR reaction. In conclusion, vanadyl sites ( V = O ) and not titanium sites are involved in the formation of adsorbed nitrate ions on vanadia-titania mixed oxides. At low temperature, NO + is the active species. At high temperature, NO is an active reacting species. The spectra of nitric oxide absorption on the reduced vanadia-titania showed two bands at 1902 and 1831 cm -1. These are assigned to nitrosonium ions (NO +) coordinated on vanadyl groups. Also, a strong sharp band at 1758 cm -1 was reflected by nitric oxide adsorption onto vanadyl group. The band at 1627 cm -~ was associated with nitrate species

with different coordination and thermal stability [8]. A weak band at 1492 cm 1 was assigned to nitrate ( M O - N O 2 ) , and the NO 3 species [8]. In addition, two weak absorption bands at 1195 and 1054 cm -1, distinguishable following evacuation at 373 K, were assigned to coordinated nitrosyl ion ( N O - ) species. Upon evacuation at 473 K, these high-frequency absorption bands disappeared, whereas the lowfrequency bands due to nitrosyl ion species became strong. Accordingly, it indicates that the rate of reaction began to take off around 473 K at which N O - was the active reacting species on the reduced sample. Again, the V = O vanadyl group is suggested as an active site for nitric oxide reduction over reduced vanadia-titania aerogel. Fig. 2 shows that ammonia was strongly adsorbed on one type of Lewis site on vanadia-titania and originated at 1169 (NH 3 symmetric bending mode), 1600 (NH 3 asymmetric bending mode), 3356 (not shown) and 3274 c m - I (not shown representing NH asymmetric and symmetric stretching). The negative absorption bands at 2045 and 1035 cm -1 and the broad positive absorption at 1920 cm -1 (due to V = O in the perturbed form) indicate that ammonia was strongly coordinated on vanadyl groups ( O = V NH3). The addition of vanadium apparently produced Br0nsted acid sites ( V - O H ) at the surface of vanadia-titania as indicated by ammonium ion detection at 1453 (NH~- asymmetric bending), 1670 with a shoulder (NH~- symmetric bending), 3407 and 3302 cm -1 (NH stretching frequencies are not shown on the figure). Upon evacuation at 473 K, the bands due to ammonium ions decreased, those of coordinated ammonia were still observed and a shoulder near 1537 cm -1 emerged. These results indicated a greater thermal stability for coordinated ammonia than ammonium ions (NH~-). The band at 1537 cm -1 (NH 2 scissoring) and the stretching absorption at 3448 cm-~ were assigned to an amide species N H f according to the following equation

[121: NH 3 +

0 2-

)

NHf + OH-.

The detection of these special bands on vanadiatitania and not on titania suggests that vanadium centers are involved in the formation of the amide species.

R.J. Willey et al. /Journal of Non-Crystalline Solids 186 (1995) 408-414

It is concluded that ammonia is strongly coordinated with both titanium and vanadium cations through Lewis acid sites. The Br0nsted acid sites are created by the presence of different vanadyl structure. Ammonia weakly bound to Brcnsted acid sites seems to be correlated with the high SCR activity. 4.3. Activity evaluation f o r the SCR o f N O by N H 3

Pure titania, a good support, reflected no activity for the SCR of NO by NH 3 and served an important reference as compared with other mixed oxide aerogels. These results are similar to those reported by Chen and Yang [13]. The titania-supported vanadia catalysts, VT01 and VT02 in Fig. 3, showed essentially the same conversion for VT01 and VT02 below 633 K and better conversions for VT01 above 633 K. At 723 K, the maximum conversion of VT01 (82.9%) was significantly higher than that of VT02 (66.5%). One possible reason for lower conversion with VT02 is that less vanadium was present (Table 1, 2.8% for VT01 compared with 1.9% for VT02). Compared with the results of pure titania, one can see the enhanced catalytic conversion is due to the addition of vanadium. The effect of cerium added to a vanadia-titania catalyst, as shown in Fig. 4, promoted increased conversion of nitric oxide (NO) for only the CVT001 (0.1Ce/2.5V/97.5Ti) catalyst. CVT005 had a narrower reaction temperature window from 573 to 723 K. CVT010 had lowered catalytic conversion compared with the original vanadia-titania catalyst (VT01). At 723 K, the three cerium-containing catalysts ranked as follows in activity: CVT001 (89.2%) > CVT005 (83.0%) > CVT010 (66.6%). In summary, the addition of very small amount of cerium enhances the conversion of nitric oxide by ammonia. Higher cerium loadings become a detriment to the catalytic activity. The addition of tungsten gave a significant increase in nitric oxide conversion for the three WO 3VzOs-TiO2 catalysts, as indicated in Fig. 5. The overall activity was enhanced as loadings of tungsten increased. TVT010 had the highest NO conversion.

413

At 723 K, the three tungsten-containing catalysts ranked as follows: TVT010 (94.6%) > TVT001 (93.4%) > TVT005 (89.2%). In conclusion, the maximum conversions obtained for the SCR of NO with NH 3 over all aerogels are in the following decreasing order: TVT010 (94.6%) > TVT001 (93.4%) > TVT005 (89.2%) = CVT001 (89.2%) > CVT005 (83.0%) > VT0a (82.9%) > CVT010 (66.6%) > VT02 (66.5%) >> T100 (0.5%). Table 1 lists the apparent activation energy and pre-exponential constant for each catalyst assuming a first-order reaction in NO concentration. Overall a compensation effect is seen and the isokinetic temperature is 591 K [14]. Higher activation energies had higher pre-exponential constants. Within a promoter series, the activation energy is lower as the per cent amount of promoter is increased. This is exactly what a catalyst is supposed to do, namely lower the activation energy or barrier to reaction. However, because of the compensating effect, the benefit is not fully realized. Understanding the meaning of these results is still a goal.

5. Conclusions

Mixed oxide aerogels composed of V-Ti, C e - V Ti and W - V - T i were synthesized. The addition of small percentage of V to Ti changed the resultant aerogel substantially creating lower bulk densities by a factor of two. Further, it created a catalytically active material for the SCR of NO by NH 3. Addition of a small amount of Ce (0.1%) increased the catalytic activity; however, higher levels of Ce (1%) retarded the catalytic activity compared with the V-Ti-based aerogels. Addition of tungsten (up to 1%) further enhanced catalytic activity. Infrared studies of NO and NH3 adsorbed on these materials showed many complex surface groups and bands. Br0nsted activity may correlate with the SCR reac-

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R.J. Willey et al. /Journal of Non-Crystalline Solids 186 (1995) 408-414

tion; h o w e v e r , as the t e m p e r a t u r e a p p r o a c h e s w h e r e a c t i v i t y b e g i n s , o n l y N H 3 a n d N H ~ r e m a i n o n the surface.

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[6] J.P. Chen and R.T. Yang, Appl. Catal. A80 (1992) 135. [7] G.M. Pajonk, Appl. Catal. 72 (1991) 217. [8] G. Ramis, G. Busca, F. Bregani and P. Forzatti, Appl. Catal. 64 (1990) 259. [9] M. Primer, C. Naccache, M.V. Mathieu and B. Imelik, J. Chim. Phys. 67 (1970) 1629. [10] G. Ramis, G. Busca and P. Forzatti, Appl. Catal. B1 (1992) L9. [11] M. Schraml-Marth, A. Wokaun and A. Balker, J. Catal. 124 (1990) 86. [12] L.H. Little, Infrared Spectra of Adsorbed Species (Academic Press, New York, 1967). [13] J.P Chen and R.T. Yang, J. Catal. 125 (1990) 411. [14] M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions (Princeton University Press, Princeton, NJ, 1984) p. 49.