Light-Off Temperature Determination of Oxidation Catalyst Using

Chapter 22

Light-Off Temperature Determination of Oxidation Catalyst Using Fourier Transform IR Technique

Downloaded by UNIV LAVAL on September 25, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch022

Chen C . Hsu U.S. Army Chemical Research, Development, and Engineering Center, Aberdeen Proving Ground, M D 21010-5423

Light-off temperatures of oxidation catalysts are considered as one of the important parameters for catalyst performance evaluation. In this study, the in-situ FTIR technique was developed and used to determine the light-off temperatures and reaction products of three three-way automotive catalysts with 20 torr monomethylamine in a i r at 0.5 1/min flow rate. Light-off temperatures were found to be 140, 143, and 170°C for Davison, Allied-Signal, and Degussa oxidation catalysts, respectively. CO, CO , H 0, and n i t r i c acid were found to be the major oxidation products. The activation energies of formation of CO and CO on the catalysts were also determined. 2

2

2

There are various techniques to evaluate the performance o f o x i d a t i o n c a t a l y s t s . Measurements o f surface composition, surface s t r u c t u r e , surface area, p o r o s i t y , a c i d i t y , and d i s p e r s i o n o f a c t i v e metals o f o x i d a t i o n c a t a l y s t s , e t c . , can be r e l a t e d to the c a t a l y s t performance (1). Other methods such as the decoloration o f the indigo carmine s o l u t i o n t o c o r r e l a t e the c a t a l y t i c a c t i v i t y w i t h the f l a s h point (2) and the recorderequipped calorimeter technique t o determine the i g n i t i o n time, maximum temperature and maximum combustion temperature (3) were used. For an o x i d a t i o n c a t a l y s t , however, one d i r e c t measurement o f i t s performance i s to determine i t s l i g h t - o f f temperature, the temperature a t which s i g n i f i c a n t o x i d a t i o n reactions occur. I n general, i t i s true that the lower the l i g h t - o f f temperature, the more e f f e c t i v e w i l l be the c a t a l y s t performance (4-5). Indeed, t h i s c o r r e l a t i o n was used i n the past t o s e l e c t the optimum reduction temperature and the duration o f reduction f o r the preparation of the best Ni-containing c a t a l y s t (6) . We have developed an i n - s i t u FTIR technique f o r simultaneously determining the l i g h t - o f f temperatures and i d e n t i f y i n g c a t a l y t i c o x i d a t i o n products o f o x i d a t i o n catalysts. Using this newly developed technique, three three-way automotive c a t a l y s t s were evaluated. The d e t a i l s o f the subject technique and the r e s u l t s o f c a t a l y s t performance evaluation are described below. This chapter not subject to U.S. copyright Published 1989 American Chemical Society

In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Light-Off Temperature Determination

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Experimental Anhydrous monomethylamine (MMA) used i n t h i s study was supplied by Matheson Gas Products w i t h a p u r i t y o f 99.99%. MMA was used as received without f u r t h e r p u r i f i c a t i o n . Three three-way automotive o x i d a t i o n c a t a l y s t s were provided by A l l i e d - S i g n a l (UOP# 4576-136), Davison (alumina c a t a l y s t s code 701), and Degussa (TWC-1, MS-599) . Bead-type c a t a l y s t samples were d r i e d i n s i d e an oven a t 110°C overnight before use. The schematic diagram o f the i n - s i t u FTIR c a t a l y s t l i g h t - o f f temperature measurement system i s shown i n Figure 1. The measurement system c o n s i s t s of a flow IR c e l l , a K-type thermocouple w i t h a chart recorder and d i g i t a l thermometer d i s p l a y , a mass flow c o n t r o l l e r , a l e c t u r e b o t t l e o f MMA, an MKS pressure gauge, and N i c o l e t FTIR Model 60SX. The d e t a i l s of the flow IR c e l l are i l l u s t r a t e d i n Figure 2. This flow IR c e l l has 10 cm o p t i c a l pathlength and 25x4 mm NaCl window, the heating element and sample holder, the gas i n l e t and o u t l e t , and the thermocouple. The thermocouple i s p o s i t i o n e d i n the proximity of the sample to measure the temperature of an o x i d a t i o n r e a c t i o n . For accurate determination o f the l i g h t - o f f temperature o f a c a t a l y s t , a chart recorder was used along w i t h a d i g i t a l thermometer display. I n i t i a l l y , a c o r r e l a t i o n between the temperature s e t t i n g on the heater c o n t r o l l e r and the temperature reading on the d i g i t a l thermocouple d i s p l a y was obtained. I t was found that the temperature reading on the d i g i t a l thermometer was about 30 to 40% of the temperature s e t t i n g on the heater c o n t r o l l e r . To run an experiment, the IR background spectra were taken at various temperatures w i t h the flow IR c e l l purged w i t h d r i e d a i r at 0.5 1/min flow rate which was automatically c o n t r o l l e d by an Matheson mass flow c o n t r o l l e r . I t i s to be noted that the d r i e d a i r was not preheated before purging the IR c e l l . Background spectra were taken i n the mixture of d r i e d a i r and 20 t o r r MMA i n the absence o f c a t a l y s t . The background spectra e s s e n t i a l l y remain the same i n the temperature range of 25-363°C, i n d i c a t i v e of no c a t a l y t i c a c t i v i t y due to the heating element. For c a t a l y s t evaluation, three c a t a l y s t beads were placed on the sample holder (the heating element w i t h a V-shape) and the thermocouple was p o s i t i o n e d i n the proximity of c a t a l y s t beads. After the lighto f f temperature measurement system was assembled, the flow IR c e l l was evacuated w i t h a vacuum pump and MMA was i n j e c t e d and maintained a t 20 t o r r . The system was disconnected w i t h the vacuum pump and the IR c e l l was brought to ambient pressure w i t h compressed a i r at 0.5 1/min flow r a t e . The heater c o n t r o l l e r was set at around 200°C (equivalent to the thermocouple reading o f 80°C) and the s e t t i n g was increased a t ten degrees increment every 15-20 minutes. During t h i s p e r i o d of time, an IR spectrum was taken i n the range between 4000 and 400 cm"-*- w i t h 4 cm"l r e s o l u t i o n . Normally, 32 scans were taken f o r each IR spectrum a t every temperature setting. Significant oxidation reaction was detected around 140°C thermocouple reading on the d i g i t a l thermometer display. The determination of l i g h t - o f f temperature was c a r r i e d out using the chart o f the temperature recorder. The l i g h t - o f f temperatures were found reproducible w i t h repeated on and o f f cycles o f MMA. These procedures described above were used f o r three three-way automotive o x i d a t i o n c a t a l y s t s provided by Davison, A l l i e d S i g n a l , and Degussa.

Results and Discussion T y p i c a l IR spectra a t various temperatures are i l l u s t r a t e d i n Figures 3-5 w i t h Davison three-way c a t a l y s t samples. At 94°C, the IR features are



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CHARACTERIZATION AND CATALYST DEVELOPMENT

Figure 1. I n - s i t u FTIR c a t a l y s t l i g h t - o f f temperature measurement system. 1. compressed a i r ; 2. mass flow c o n t r o l l e r ; 3. gas l e c t u r e b o t t l e ; 4. MKS pressure gauge; 5. exhaust; 6. IR c e l l ; 7. IR window; 8. c a t a l y s t sample; 9. thermocouple; 10. sample compartment of N i c o l e t FTIR Model 60SX; 11. chart recorder; 12. d i g i t a l thermometer d i s p l a y .

I

1 isodoo/i f

—

f

t

Figure 2. Schematic diagram o f the flow IR c e l l . 1. gas i n l e t ; 2. gas o u t l e t ; 3. IR window; 4. thermocouple; 5. c a t a l y s t sample; 6. e l e c t r i c a l leads f o r heating; 7. heating element and sample holder; 8. IR beam.



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tOOO

237


3600

3200

2800

2t00 2000 WAVENUMBER

I BOO

1200

800

'tOO

Figure 3. IR spectrum o f MMA w i t h Davison c a t a l y s t a t 94°C.

u

DC • CD m

^000

3600

3200

2800

2^00 2000 WAVENUMBER

1600

1200

800

Figure 4. IR spectrum o f MMA w i t h Davison c a t a l y s t a t 190°C.


*00



Figure 5. IR spectrum o f MMA with Davison c a t a l y s t a t 261°C.


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239



very much the same as MMA as shown i n Figure 3. When the temperature was increased to 190°C as shown i n Figure 4, peaks around 2154 and 2351 cm"l r e s p e c t i v e l y corresponding t o CO and CO2, and peaks around 1540 and 3770 cm"-'- corresponding t o h^O grow s u b s t a n t i a l l y . As the temperature reaches to 261°C, the formation o f HNO3 and the e f f e c t o f water on n i t r i c a c i d (79) corresponding t o the bands centered around 2964, 1700, 1320, 760 c n f l appear to be the major r e a c t i o n products, Figure 5. For each sample, a t l e a s t three experiments were conducted and the l i g h t - o f f temperatures were determined u s i n g the charts from the temperature recorder. The average l i g h t - o f f temperatures f o r three three-way automotive c a t a l y s t s are shown i n Table 1. I n the increasing l i g h t - o f f temperature, the order i s Davison, A l l i e d - S i g n a l , and Degussa c a t a l y s t s . For Davison and A l l i e d - S i g n a l

Table 1. L i g h t - O f f Temperatures o f Three-Way Automotive C a t a l y s t s

Davison Allied-Signal Degussa

140°C ± 5% 143°C ± 3 . 5 % 170°C ± 2 . 3 %

samples, the l i g h t - o f f temperatures are very close t o each o t h e r . However, for Degussa sample, t h e l i g h t - o f f temperature i s about 30°C h i g h e r t h a n that of Davison and Allied-Signal. The absorbance of CO and CO2 were t a k e n from the IR s p e c t r a a t v a r i o u s temperatures from which A r r h e n i u s p l o t s were obtained as shown i n Figures 6-7. The a c t i v a t i o n energies o f f o r m a t i o n of CO and CO2 were found using the r e l a t i o n of slope = - E /R, where E i s the a c t i v a t i o n energy and R i s t h e i d e a l gas c o n s t a n t which i s e q u a l t o 1.987 cal/mol/K. Table 2 shows the a c t i v a t i o n e n e r g i e s o f f o r m a t i o n o f CO and CO2 f o r three three-way automotive c a t a l y s t s . I n general, t h e a c t i a

a

Table 2. A c t i v a t i o n Energy o f Formation, Kcal/mol

co

Davison Allied-Signal Degussa

2

2.29 2.68 5.76

CO

6.41 12.07 21.60

v a t i o n energy of formation of CO2 i s lower than t h a t o f CO c o n s i s t e n t w i t h the f a c t that the free energy of formation of CO2 i s l o w e r t h a n t h a t o f CO. I t i s to be noted that the lower the l i g h t - o f f t e m p e r a t u r e , t h e l o w e r w i l l be the a c t i v a t i o n energy of formation f o r b o t h CO and CO2 from t h e t h r e e way a u t o m o t i v e c a t a l y s t s i n v e s t i g a t e d . I t was a l s o o b s e r v e d t h a t t h e c o n d e n s a t i o n o f w a t e r a n d n i t r i c a c i d i n s i d e t h e IR c e l l d u r i n g t h e c a t a l y t i c oxidation of MMA may be a t t r i b u t e d t o t h e s c a t t e r i n g o f t h e d a t a points i n F i g u r e s 6-7.




240


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Figure 7. Arrhenius p l o t of

on Davison o x i d a t i o n

catalyst.


242



Summary The r e s u l t s can be summarized as f o l l o w s . 1. I n - s i t u FTIR technique was developed and demonstrated f o r the l i g h t - o f f temperature determination and the i d e n t i f i c a t i o n o f r e a c t i o n p r o d u c t s of oxidation c a t a l y s t s . 2. L i g h t - o f f t e m p e r a t u r e s w i t h 20 t o r r MMA f o r Davison, A l l i e d - S i g n a l , and Degussa three-way automotive c a t a l y s t s were determined t o be 140, 143, and 170°C i n a i r with 0.5 1/min flow r a t e , r e s p e c t i v e l y . 3. The r e a c t i o n p r o d u c t s were i d e n t i f i e d t o be CO, CO2, t^O, and HNO3 f o r three three-way automotive c a t a l y s t s . 4. The a c t i v a t i o n e n e r g i e s o f f o r m a t i o n f o r CO and CO2 on t h r e e t h r e e way c a t a l y s t s were determined. For CO, they a r e 6.41, 12.07, and 21.60 K c a l / m o l ; and f o r C 0 , t h e y a r e 2.29, 2.68, a n d 5.76 Kcal/mol, f o r Davison, A l l i e d - S i g n a l , and Degussa three-way automotive c a t a l y s t s , respectively. 2

Acknowledgments Sincere appreciation i s due t o Marie A. Yeh o f C. M i l t o n High S c h o o l , MD, who performed the experiments during her tenure as a DoD Science A p p r e n t i c e i n t h e summer o f 1987. S p e c i a l thanks a r e due t o Dr. G. R. L e s t e r o f Allied-Signal, Dr. R. F. Freese of Degussa, and Dr. P. A. Smith of Davison who provided the three-way automotive catalysts used i n t h i s s t u d y .

Literature Cited 1.

2. 3.

4. 5. 6. 7. 8. 9.

Delannay, F.; Delmon, B. i n C h a r a c t e r i z a t i o n of Heterogeneous Catalysts, F. Delannay, Ed.; Marcel Dekker, Inc., New York and Basel, 1984. Krause, A.; Nowakowski, E. Chem.-Zig. Chem. App. 1968, 92, 774-5. Guveev, A. A.; Dovlator, I. A.; Kazaryan, S. A.; Naftulin, I. S. Determination of the Octane Number of Gasoline from Catalytic Reforming According to the Characteristics of Low-Temperature Gas-Phase Oxidation. Neftepererab. Neftekhim., Moscow, 1987, Vol. 9, pp 5-8. P f e f f e r l e , L, D.; Pfefferle, W. C. Catal. Rev.-Sci. Eng. 1987, 29, 219-267. Prasad, R.; Kennedy, L. A.; Ruckenstein, E. Catal. Rev.-Sci. Eng. 1984, 26, 1-58. A n d e r l o h r , A.; Hedden, K. Chem.-Ing.-Tech. 1979, 51, 811. McGraw, G. E.; B e r n i t t , D. L.; Hisatsune, I. C. J . Chem. Phys. 1965, 42, 237-244. Ingold, C. K.; Millen, D. J . J . Chem. Soc. 1950, 2612. S t e r n , S. A.; Mullhaupt, J . T.; Kay, W. B. Chem. Revs. 1960, 60, 185.

R E C E I V E D July 27, 1989


Light-Off Temperature Determination of Oxidation Catalyst Using

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