- Email: [email protected]
Scanning Auger Microscopy Studies on Industrial Ammonia Synthesis Catalysts
B. Imelik et al. (Editors),Metal-Support and Metal-Additive Effects in Catalysis
271
0 1982 Elsevier ScientificPublishing Company,Amsterdam - Printed in The Netherlands
SCANNING AUGER MICROSCOPY STUDIES ON INDUSTRIAL AMMONIA SYNTHESIS CATALYSTS
M. WEISS* and G. ERTL**
*
Perkin-Elmer Verkauf GmbH, Physical Electronics Division - Europe, Munchen-Vaterstetten (G.F.R.)
** Institut fur Physikalische Chemie, Universitat Munchen (G.F.R.)
RESUME
La composition de surface d'un catalyseur industriel de synthese d'ammoniac (BASF S 6 - 1 0 )
a Pte etudige au moyen des microscopies Blectroniques Auger 2
balayage PHI 590 - SAM et 595 - SAM, de resolution laterale respective de 2000 et 500 A. La surface est fortement enrichie en promoteurs, oxydes de Al, K
et Ca
:
A1203 et CaO tendent d former des particules separges, et de ce fait
agissent c o m e des promoteurs structuraux, alors que le potassium couvre plus ou moins uniformgment la surface de fer. La nature chimique et l'action promo-
trice du potassium sont discutees en relation avec les rgsultats d'autres techniques, et les modifications de composition de surface et de topographie causees par la reduction ou l'empoisonnement par le soufre sont decrites.
ABSTRACT The surface composition of an industrial ammonia synthesis catalyst (BASF S 6 - 10) was investigated by means of scanning Auger electron microscopy (PHI 590-SAM, resp. 595-SAM), enabling lateral resolution of 2000
8
and 500
8,
respectively. The promoter oxides of Al, K and Ca are strongly enriched at the surface: A1203 and CaO tend to form separate particles and thus act as 'structural' promoters, while potassium covers more or less uniformly the Fe areas. The chemical nature and promoter action of the K overlayer will be discussed also in view of results with other techniques, and changes in the surface composition and topography caused by reduction or sulfur poisoning will be described.
218
INTRODUCTION The activity of the iron catalysts used for ammonia synthesis (Haber-Bosch process) is significantly enhanced by the addition of A1203, CaO, and K20 (1). Alumina and calcium oxide are 'structural' promoters which ascertain essentially a high surface area even under prolonged reaction conditions, while potassium is considered as an 'electronic' promoter directly affecting the kinetics of the rate-limiting step (2). In the present work scanning Auger electron spectroscopy was used in order to characterize the surface compcsition of industrial catalysts with high lateral resolution.
EXPERIMENTAL The measurements were performed with a BASF S 6 - 10 ammonia synthesis catalyst. First a certain typical section of the surface area was selected on the basis of the secondary electron image and then analysed by Scanning AES. Subsequently the sample was reduced in a separate vacuum system at 4 5 0 ° C and 1 atm of a 2 : l N - H gas mixture. The progress of reduction could be monitored in situ by re2' 2 cording the Fe 2p - core level by means of XPS (3). Since the catalyst was afterwards transferred through air superficial reoxidation took place, of course, which, however, does not affect the conclusion to be reached here. Sulfur poisoning was achieved by exposure to H S at 500°C. 2 Two different commercial Scanning Auger systems (PHI 590-SAM and 595-SAM) were used, enabling lateral resolution of 2000
fl and
500
fl,
respectively. Quan-
titative evaluation of the surface composition was achieved by recording the AES peak-to-peak amplitude of the respective elements and by using the corresponding sensitivity factors ( 4 ) . The latter were derived from point analysis of practically pure particles of A1 0 CaO and K 0 (resp. K CO ) on the catalyst surface. 2 3' 2 2 3 RESULTS AND DISCUSSION Fig. 1 shows the secondary electron image of the selected area of the (reduced) catalyst surface. Flat regions as well as various separate particles are clearly discernible. The corresponding 'Auger maps' exhibiting the lateral distributions of the elements Fe, K, A1 and Ca are reproduced in Fig. 2. With these images the brightness is a relative measure of the local surface concentration, i.e. dark areas denote low concentration of the respective element. Comparison between Figs. 1 and 2 clearly demonstrates that the flat areas are associated with Fe which is more or less uniformly covered by K (Potassium is strongly enriched at the surface as will be outlined below). On the other hand aluminium and calcium
279
Fig.
1: Secondary e l e c t r o n image of t h e s e l e c t e d c a t a l y s t a r e a (Mag.: 1 o o o X )
Fig. 2a: Fe
- Auger map of F i g .
280
Fig. 2b: K - Auger map of Fig. 1
Fig. 2 c : Ca - Auger map of Fig. 1
281
Fig. 2d: A1 - Auger map of Fig. 1
are essentially present in the form of separate particles of A1203 or CaO. These conclusions are supported by Fig. 3 which shows two Auger spectra (point analysis) : (a) - from a flat area and (&) from a Ca-rich region. The rate limiting step in ammonia synthesis is dissociative nitrogen adsorption on iron ( 5 ) , and therefore the flat Fe-rich areas are identified with the catalytically active regions. These areas are uniformly covered with an appreciably high concentration of potassium, which is in agreement with the general view of the promoter action of this element ( 5 ) : The dissociative nitrogen adsorption is accelerated by stabilization of the molecularly adsorbed nitrogen by the vicinity of electropositive elements. The actual catalyst surface is, however, covered by a composite K
+
0 adlayer rather than by potassium alone.
The presence of oxygen increases the thermal stability but on the other hand largely cancels the promoter effect ( 6 ) , so that the overall increase of the specific activity is not very dramatic (7).
282 10 9 8 7
6
5 4 3
2 1
0
200
600
800 1000 1200 1400 1600 1800 2000 KINETIC ENERGY, E V
600
800
400
0 :A 4-
32-
0
1-
400
KINETIC
1000 1200 1400 1600 1800 2000
ENERGY , E V
F i g . 3: Auger p o i n t a n a l y s i s from a f l a t a r e a ( & ) a n d a Ca-rich r e g i o n (b)
283 Quantitative evaluation of the surface composition on an 'active' area as well as averaged over the whole surface is summarized in tables 1 a and b for the unreduced and reduced samples, respectively. With the reduced catalyst potassium is enriched at the surface by a factor of about 100, if compared with the bulk content. The Ca-concentration remains essentially unchanged, while A1 is enriched by a factor of about 3. The reduced catalyst exhibits a higher K-concentration on the 'active' areas, while the overall concentration is smaller than with the unreduced sample. The latter contained small particles of K2C03 and KOH which disappeared under the high temperature reduction treatment. These particles are considered to act as 'internal' potassium sources during the reduction process. K spreads over the reduced iron particles and builds up a fairly high concentration.
TABLE 1 Surface compositions (atom % ) of an unreduced (a) and reduced (b) ammonia synthesis catalyst (BASF S6 - 10. Bulk composition, A1:2, K:0.35, Fe:40.5, 0:53.2, Ca:1.7, Si:O.25) _____~
TABLE 1 a
K
Whole Surface Fig. 1
36.1
' Active ' Area
22
TABLE 1 b
~
0
Ca
S
A1
40
4.7
-
10.7
27
45.9
-
1.1
K
Fe
0
Ca
S
A1
Whole Surface Fiq. 1
27
11
41
4
-
17
' Active ' Area
29
30
32.9
1.0
0.3
i)
Fe
8.6
~-
4
6.8
The effect of H S treatment was found to be twofold: 2 Relatively high sulfur concentrations were found to be present on the 'active'
areas after exposure to H S. 2
ii) The potassium concentration was lowered by about 30%. XPS studies (8) indicated that this is due to a decrease of the desorption energy of the K-adlayer. Both effects decrease the rate of dissociative nitrogen adsorption and are thus responsible for the poisoning action of sulfur.
284 REFERENCES 1 A . M i t t a s c h , Adv. C a t a l y s i s ,
2
( 1 9 5 0 ) , 81.
2 A. Ozaki a n d K. Aika, i n " C a t a l y s i s : S c i e n c e and Technology", ( J . R . Anderson and M. B o u d e r t , e d s . ) , V o l . ( 1 9 8 1 ) , 87.
1
3 G . E r t l a n d N.
T h i e l e , Appl. S u r f . S c i .
3
(19791, 99.
4 Handbook o f Auger E l e c t r o n S p e c t r o s c o p y ( P h y s i c a l E l e c t r o n i c s I n d u s t r i e s , Eden P r a i r i e , MN, 1 9 7 6 ) . 5 G. E r t l , S . B .
Lee a n d M.
6 Z . P a h l , G. E r t l and S.B.
Weiss, S u r f . S c i .
114 ( 1 9 8 2 ) ,
L e e , Appl. S u r f . S c i .
7 R . K r a b e t z and C . P e t e r s , Angew. Chem.
8 D. P r i g g e , u n p u b l i s h e d r e s u l t s .
527.
S (1981),
71 ( 1 9 6 5 ) ,
333.
231.
271
0 1982 Elsevier ScientificPublishing Company,Amsterdam - Printed in The Netherlands
SCANNING AUGER MICROSCOPY STUDIES ON INDUSTRIAL AMMONIA SYNTHESIS CATALYSTS
M. WEISS* and G. ERTL**
*
Perkin-Elmer Verkauf GmbH, Physical Electronics Division - Europe, Munchen-Vaterstetten (G.F.R.)
** Institut fur Physikalische Chemie, Universitat Munchen (G.F.R.)
RESUME
La composition de surface d'un catalyseur industriel de synthese d'ammoniac (BASF S 6 - 1 0 )
a Pte etudige au moyen des microscopies Blectroniques Auger 2
balayage PHI 590 - SAM et 595 - SAM, de resolution laterale respective de 2000 et 500 A. La surface est fortement enrichie en promoteurs, oxydes de Al, K
et Ca
:
A1203 et CaO tendent d former des particules separges, et de ce fait
agissent c o m e des promoteurs structuraux, alors que le potassium couvre plus ou moins uniformgment la surface de fer. La nature chimique et l'action promo-
trice du potassium sont discutees en relation avec les rgsultats d'autres techniques, et les modifications de composition de surface et de topographie causees par la reduction ou l'empoisonnement par le soufre sont decrites.
ABSTRACT The surface composition of an industrial ammonia synthesis catalyst (BASF S 6 - 10) was investigated by means of scanning Auger electron microscopy (PHI 590-SAM, resp. 595-SAM), enabling lateral resolution of 2000
8
and 500
8,
respectively. The promoter oxides of Al, K and Ca are strongly enriched at the surface: A1203 and CaO tend to form separate particles and thus act as 'structural' promoters, while potassium covers more or less uniformly the Fe areas. The chemical nature and promoter action of the K overlayer will be discussed also in view of results with other techniques, and changes in the surface composition and topography caused by reduction or sulfur poisoning will be described.
218
INTRODUCTION The activity of the iron catalysts used for ammonia synthesis (Haber-Bosch process) is significantly enhanced by the addition of A1203, CaO, and K20 (1). Alumina and calcium oxide are 'structural' promoters which ascertain essentially a high surface area even under prolonged reaction conditions, while potassium is considered as an 'electronic' promoter directly affecting the kinetics of the rate-limiting step (2). In the present work scanning Auger electron spectroscopy was used in order to characterize the surface compcsition of industrial catalysts with high lateral resolution.
EXPERIMENTAL The measurements were performed with a BASF S 6 - 10 ammonia synthesis catalyst. First a certain typical section of the surface area was selected on the basis of the secondary electron image and then analysed by Scanning AES. Subsequently the sample was reduced in a separate vacuum system at 4 5 0 ° C and 1 atm of a 2 : l N - H gas mixture. The progress of reduction could be monitored in situ by re2' 2 cording the Fe 2p - core level by means of XPS (3). Since the catalyst was afterwards transferred through air superficial reoxidation took place, of course, which, however, does not affect the conclusion to be reached here. Sulfur poisoning was achieved by exposure to H S at 500°C. 2 Two different commercial Scanning Auger systems (PHI 590-SAM and 595-SAM) were used, enabling lateral resolution of 2000
fl and
500
fl,
respectively. Quan-
titative evaluation of the surface composition was achieved by recording the AES peak-to-peak amplitude of the respective elements and by using the corresponding sensitivity factors ( 4 ) . The latter were derived from point analysis of practically pure particles of A1 0 CaO and K 0 (resp. K CO ) on the catalyst surface. 2 3' 2 2 3 RESULTS AND DISCUSSION Fig. 1 shows the secondary electron image of the selected area of the (reduced) catalyst surface. Flat regions as well as various separate particles are clearly discernible. The corresponding 'Auger maps' exhibiting the lateral distributions of the elements Fe, K, A1 and Ca are reproduced in Fig. 2. With these images the brightness is a relative measure of the local surface concentration, i.e. dark areas denote low concentration of the respective element. Comparison between Figs. 1 and 2 clearly demonstrates that the flat areas are associated with Fe which is more or less uniformly covered by K (Potassium is strongly enriched at the surface as will be outlined below). On the other hand aluminium and calcium
279
Fig.
1: Secondary e l e c t r o n image of t h e s e l e c t e d c a t a l y s t a r e a (Mag.: 1 o o o X )
Fig. 2a: Fe
- Auger map of F i g .
280
Fig. 2b: K - Auger map of Fig. 1
Fig. 2 c : Ca - Auger map of Fig. 1
281
Fig. 2d: A1 - Auger map of Fig. 1
are essentially present in the form of separate particles of A1203 or CaO. These conclusions are supported by Fig. 3 which shows two Auger spectra (point analysis) : (a) - from a flat area and (&) from a Ca-rich region. The rate limiting step in ammonia synthesis is dissociative nitrogen adsorption on iron ( 5 ) , and therefore the flat Fe-rich areas are identified with the catalytically active regions. These areas are uniformly covered with an appreciably high concentration of potassium, which is in agreement with the general view of the promoter action of this element ( 5 ) : The dissociative nitrogen adsorption is accelerated by stabilization of the molecularly adsorbed nitrogen by the vicinity of electropositive elements. The actual catalyst surface is, however, covered by a composite K
+
0 adlayer rather than by potassium alone.
The presence of oxygen increases the thermal stability but on the other hand largely cancels the promoter effect ( 6 ) , so that the overall increase of the specific activity is not very dramatic (7).
282 10 9 8 7
6
5 4 3
2 1
0
200
600
800 1000 1200 1400 1600 1800 2000 KINETIC ENERGY, E V
600
800
400
0 :A 4-
32-
0
1-
400
KINETIC
1000 1200 1400 1600 1800 2000
ENERGY , E V
F i g . 3: Auger p o i n t a n a l y s i s from a f l a t a r e a ( & ) a n d a Ca-rich r e g i o n (b)
283 Quantitative evaluation of the surface composition on an 'active' area as well as averaged over the whole surface is summarized in tables 1 a and b for the unreduced and reduced samples, respectively. With the reduced catalyst potassium is enriched at the surface by a factor of about 100, if compared with the bulk content. The Ca-concentration remains essentially unchanged, while A1 is enriched by a factor of about 3. The reduced catalyst exhibits a higher K-concentration on the 'active' areas, while the overall concentration is smaller than with the unreduced sample. The latter contained small particles of K2C03 and KOH which disappeared under the high temperature reduction treatment. These particles are considered to act as 'internal' potassium sources during the reduction process. K spreads over the reduced iron particles and builds up a fairly high concentration.
TABLE 1 Surface compositions (atom % ) of an unreduced (a) and reduced (b) ammonia synthesis catalyst (BASF S6 - 10. Bulk composition, A1:2, K:0.35, Fe:40.5, 0:53.2, Ca:1.7, Si:O.25) _____~
TABLE 1 a
K
Whole Surface Fig. 1
36.1
' Active ' Area
22
TABLE 1 b
~
0
Ca
S
A1
40
4.7
-
10.7
27
45.9
-
1.1
K
Fe
0
Ca
S
A1
Whole Surface Fiq. 1
27
11
41
4
-
17
' Active ' Area
29
30
32.9
1.0
0.3
i)
Fe
8.6
~-
4
6.8
The effect of H S treatment was found to be twofold: 2 Relatively high sulfur concentrations were found to be present on the 'active'
areas after exposure to H S. 2
ii) The potassium concentration was lowered by about 30%. XPS studies (8) indicated that this is due to a decrease of the desorption energy of the K-adlayer. Both effects decrease the rate of dissociative nitrogen adsorption and are thus responsible for the poisoning action of sulfur.
284 REFERENCES 1 A . M i t t a s c h , Adv. C a t a l y s i s ,
2
( 1 9 5 0 ) , 81.
2 A. Ozaki a n d K. Aika, i n " C a t a l y s i s : S c i e n c e and Technology", ( J . R . Anderson and M. B o u d e r t , e d s . ) , V o l . ( 1 9 8 1 ) , 87.
1
3 G . E r t l a n d N.
T h i e l e , Appl. S u r f . S c i .
3
(19791, 99.
4 Handbook o f Auger E l e c t r o n S p e c t r o s c o p y ( P h y s i c a l E l e c t r o n i c s I n d u s t r i e s , Eden P r a i r i e , MN, 1 9 7 6 ) . 5 G. E r t l , S . B .
Lee a n d M.
6 Z . P a h l , G. E r t l and S.B.
Weiss, S u r f . S c i .
114 ( 1 9 8 2 ) ,
L e e , Appl. S u r f . S c i .
7 R . K r a b e t z and C . P e t e r s , Angew. Chem.
8 D. P r i g g e , u n p u b l i s h e d r e s u l t s .
527.
S (1981),
71 ( 1 9 6 5 ) ,
333.
231.