3 X-Ray Photoelectron Spectroscopy of Cobalt Catalysts Correlation with Carbon Monoxide Hydrogenation Activities D. G. CASTNER and D. S. SANTILLI Downloaded by PENNSYLVANIA STATE UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch003
Chevron Research Company, Richmond, CA 94802-0627
A series of supported cobalt catalysts (Co/Al O , Co/K-Al O , Co/SiO , Co/TiO ) have been examined by X-ray photoelectron spectros copy (XPS) and microreactor studies. A catalyst treatment system attached to the XPS spectrome ter was used to prepare in situ treated (air, H , 1% H S/H ) catalysts for determining the identity, concentration, and reducibility of the cobalt species. At least three different types of cobalt species were present on the calcined catalysts. These included large particles of 2
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Co O , various Co species, and CoAl O . The Co O particles were more readily reduced to metallic cobalt in H than the Co species were. After H reduction at 480°C, the CO hydrogenation activity in 10 atmospheres of 3H :1CO at 260°C for supported 5 wt % cobalt decreased as Co/SiO > Co/TiO > Co/Al O > Co/K-Al O . The determination of the types of cobalt species present on each support and their reduction properties were used to explain the catalysts' CO hydrogenation activities. 3
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We undertook t h i s i n v e s t i g a t i o n i n order t o examine the r e l a t i o n s h i p of p h y s i c a l s t r u c t u r e and composition of cobalt c a t a l y s t s t o c a t a l y t i c a c t i v i t y . Several d i f f e r e n t cobalt species have been detected on supported cobalt c a t a l y s t s (lf-7); the type, amount, and r e a c t i v i t y of the cobalt species v a r i e d with support, metal loading, and preparation procedures. For t h i s i n v e s t i g a t i o n , the supports were v a r i e d and the other parameters were held constant. S i 0 , T i 0 , A 1 0 , and K-A1 0 were used as 2
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0097-6156/84/0248-0039$06.00/0 © 1984 American Chemical Society
In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
CATALYTIC MATERIALS
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supports. Since cobalt c a t a l y s t s are known to be good CO hydrogénation c a t a l y s t s (8) and the CO/H2 r e a c t i o n has been shown to be s e n s i t i v e to metal-support i n t e r a c t i o n s (5, 9j 10), we s e l e c t e d CO hydrogenation as our r e a c t i o n probe.
Downloaded by PENNSYLVANIA STATE UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch003
Experimental C a t a l y s t P r e p a r a t i o n . The supported c a t a l y s t s were a l l prepared by pore f i l l impregnation w i t h an aqueous s o l u t i o n of ΟοίΝΟβ^ to give a nominal Co l o a d i n g of 5 wt %. The supports were f i r s t c a l c i n e d at 510°C f o r one to two hours, then impregnated, d r i e d at 65°C, and r e c a l c i n e d at 510-540°C f o r one to two hours. The supports used were γ-Α^Ο^ ( C a t a p a l ) , s i l i c a g e l (Davison), and T1O2 ( g ) » K-AI2O3 was made from γ-Α^Οβ by pore f i l l impregnation w i t h K2CO3 followed by d r y i n g (65°C, s e v e r a l hours) and c a l c i n i n g (870°C, four hours). Κ a d d i t i o n was done p r i o r to impregnating w i t h Co. D e
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X-Ray Photoelectron Spectroscopy (XPS or ESCA) A n a l y s i s . The c a t a l y s t treatment-surface a n a l y s i s system employed to charac t e r i z e and t r e a t the cobalt samples has been described p r e v i ously ( 4 ) . B r i e f l y , i t i s a modular system c o n s i s t i n g of a Hewlett-Packard 5950A ESCA spectrometer, two quartz r e a c t o r s , a metal evaporator, an Auger e l e c t r o n spectroscopy (AES) and t h e r mal desorption spectroscopy (TDS) s t a t i o n , a sample storage area, and a r a p i d sample entry p o r t . A 3.5-m long, 10-cm diameter t r a n s f e r tube connects these s t a t i o n s . The samples are moved between s t a t i o n s by a combination of a c h a i n - d r i v e n t r o l l e y and m a g n e t i c a l l y coupled rods. The t r a n s f e r tube i s always maintained under u l t r a h i g h vacuum (UHV) w i t h a base p r e s sure of 1 χ 10"~^ t o r r a f t e r bakeout. The r e a c t o r s and sample i n l e t s t a t i o n s are c o n t i n u a l l y c y c l e d between atmospheric p r e s sure and UHV and have working base pressures of 10 torr, although pressures i n the 10""^ t o r r range can be a t t a i n e d a f t e r a bakeout. For c a t a l y s t treatments, the sample i s t r a n s f e r r e d i n t o the quartz r e a c t o r i n vacuo, the r e a c t o r i s o l a t e d , the gas flow commenced, and temperature l i n e a r l y ramped to the d e s i r e d v a l u e . A f t e r the sample has been at temperature f o r the d e s i r e d time p e r i o d , the sample i s cooled to room temperature i n the gas flow, the gas flow i s stopped, and the r e a c t o r i s evacuated. The sample i s then t r a n s f e r r e d i n vacuo to the XPS spec trometer. For ease of handling, a few m i l l i g r a m s of each sample i s pressed i n t o e i t h e r a 400- or 200-mesh gold g r i d and then mounted i n a gold sample h o l d e r .
In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
3. CASTNER AND SANTILLI
Cobalt Catalysts and CO Hydrogénation
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The 5950A ESCA spectrometer i s i n t e r f a c e d to a desktop computer f o r data c o l l e c t i o n and a n a l y s i s . S i x hundred watt monochromatic ΑΙ Κα X-rays are used to e x c i t e the photoelectrons and an e l e c t r o n gun set at 2 eV and 0.3 mAmp i s used to reduce sample charging. Peak areas are numerically i n t e g r a t e d and then d i v i d e d by the t h e o r e t i c a l p h o t o i o n i z a t i o n c r o s s - s e c t i o n s (11) to o b t a i n r e l a t i v e atomic compositions. For the supported c a t a l y s t samples, a l l binding energies (BE) are referenced to the A l 2p peak at 75.0 eV, the S i 2p peak at 103.0 eV, or the T i 2 p / peak at 458.5 eV. Downloaded by PENNSYLVANIA STATE UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch003
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Reactor S t u d i e s . The m i c r o r e a c t o r system c o n s i s t s of a 9.5-mm diameter, 0.9-m long s t a i n l e s s s t e e l reactor tube mounted i n a tube furnace and connected to s t a i n l e s s s t e e l feed l i n e s equipped w i t h pressure and flow c o n t r o l l e r s and an o n - l i n e gas chromatograph (GC). The reactor tube i s packed w i t h 0.2 g of c a t a l y s t i n the middle (-6 mm i n length) and alundum on both ends. The c a t a l y s t bed i s separated from the alundum w i t h glass wool plugs. The premixed CO/H2 feed gas (Linde Custom grade) i s passed through a -78°C cold trap to remove metal carbonyls before i n t r o d u c i n g i t i n t o the m i c r o r e a c t o r system. The o n - l i n e GC i s equipped w i t h an OV 101 c a p i l l a r y column and flame i o n i z a t i o n detector (FID) f o r hydrocarbon product d e t e c t i o n . (The hydrocarbon values are not corrected f o r the small d i f ferences i n detector e f f i c i e n c i e s . ) The e x i t i n g gases are analyzed f o r CO, CO2, and CH4 w i t h a gas p a r t i t i o n e r equipped w i t h a thermistor d e t e c t o r . The peak areas obtained from the gas p a r t i t i o n e r are converted to weight percentages by using the appropriate s e n s i t i v i t y f a c t o r s ( 1 2 ) . The c a t a l y s t runs are conducted as f o l l o w s . F i r s t , the system i s flushed w i t h helium, then hydrogen i s passed over the c a t a l y s t at 150 p s i and ca. 50 ml/min. as the reactor i s heated from room temperature to 480°C over ~40 min. A f t e r 30 min. at 480°C, the r e a c t o r i s cooled to the desired operating tempera ture ( u s u a l l y 260°C); and the gas flow i s switched from H t o 3H :1C0. F i n a l l y , the flow i s adjusted to 20 ml/min. During each run, the products are analyzed to determine "pseudosteady s t a t e c o n d i t i o n s " and the temperature i s v a r i e d t o o b t a i n an approximate measure of a c t i v i t y changes. These measurements are made during a span of s e v e r a l hours. 2
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Temperature Programmed Reduction (TPR) S t u d i e s . I n the TPR s t u d i e s , a gas mixture of 2% H2 i n Ar i s passed over powdered samples of the c a l c i n e d c a t a l y s t s . The c a t a l y s t s are held i n the middle of a 5-mm diameter, 0.4-m long quartz reactor w i t h
In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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CATALYTIC MATERIALS
g l a s s wool plugs and quartz c h i p s . The samples are heated at a rate of 4.5°C/min. by a tube furnace mounted around the r e a c t o r . A t h e r m a l c o n d u c t i v i t y d e t e c t o r i s used t o monitor the composition of the gas phase a f t e r i t passes through the r e a c t o r and a water removal t r a p . Results C0/AI2O3, C0/K-AI2O3, Co/SiQ , and C o / T i 0 were a l l t e s t e d f o r CO hydrogenation to determine the e f f e c t of the supports on the a c t i v i t y and s e l e c t i v i t y of cobalt (Table I ) . The data i n Table I were taken a f t e r the c a t a l y s t s had been onstream f o r s e v e r a l hours. Since c a t a l y s t p r e p a r a t i o n , c a l c i n a t i o n , reduc t i o n , and s t a r t u p procedures can a f f e c t c a t a l y s t performance, procedures were kept as constant as p o s s i b l e t o a l l o w meaningful comparisons. For C0/K-AI2O3, the r e a c t i o n temperature was r a i s e d t o 315°C because of i t s low a c t i v i t y at 260°C. A l l c a t a l y s t s had to be exposed to 00/Η f o r 10-30 min. before CO hydro génation a c t i v i t y was detected. The onset of r e a c t i o n was accompanied by a l a r g e i n c r e a s e i n the water content of the product gas.
Downloaded by PENNSYLVANIA STATE UNIV on July 19, 2012 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch003
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Table I .
CO Hydrogénation A c t i v i t i e s and S e l e c t i v i t i e s of Cobalt C a t a l y s t s 1
Temp. Catalyst Co/Si0 Co/Ti0
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Co/K-Al 0 2
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CH
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Propene
CO Conv. to HC (%)
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HC
HC
Propane
260 260 260 260 315
0.51 0.48 0.54
0.19 0.24 0.18
0.2 1.1 0.5
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0.34
3
0.32
73 23 17
