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Physical Characterization of Industrial Catalysts: The Mechanism of Ammonia Synthesis
T. Inui (Editor), Successful Design of Catalysts © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
315
PHYSICAL CHARACTERIZATION OF INDUSTRIAL CATALYSTS: THE MECHANISM OF AMMONIA SYNTHESIS
G. ErtJ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 0-1000 Berlin 33 (W. Germany)
ABSTRACT The application of various physical methods enabled detailed characterisation of the properties of the industrial catalyst for ammonia synthesis, as well as identification and determination of structural, energetic and kinetic parameters of the various reaction intermediates on iron single crystal surfaces. Based on the resulting reaction mechanism industrial ammonia yields could successfully be evaluated without any adjustable parameters. In 1913, e.g. 75 years ago, based on the work by Haber, Bosch, and Mittasch the first plant for the catalytic synthesis of ammonia from the elements started its operation in Ludwigshafen-Oppau and initiated a new era of industrial chemistry (ref. 1). Despite considerable progress in technology, the promoted iron catalyst as well as the reaction conditions remained basically unchanged over all these years. Extensive research by using conventional methods yielded a view about the mechanism of this reaction which even in the mid-seventies was still rather incomplete and controversial: While soon general agreement was reached that nitrogen adsorption represents the rate- 1imit ing step, it remained unclear if this species undergoing subsequent hydrogenation was atomic or molecular in nature (ref. 2). This situation changed markedly during the past decade by the advent and application of new physical methods which permitted atomic level studies of the surface properties of industrial catalysts as well as of well-defined single crystal samples serving as proper model systems. The industrial ammonia synthesis is prepared from magnetic (Fe304) with sma 11 percentages of K and Alas the most important promoters. Under work i ng cond it ions it consi sts essent i ally of meta11 i c Fe (with traces of unreduced iron). The promoters A1203 and CaO stabilize the small Fe particles against sintering, while potassium spreads over the Fe surface and forms a K+O overlayer acting as 'electronic' promoter (ref. 3). This adlayer interacts strongly with the Fe substrate and is quite different from all bulk compounds between potassium and oxygen. The presence of 0 increases the thermal stability
316
of adsorbed K, while in turn the strong K-O interaction prevents complete reduction. Since this potassium overlayer merely enhances the catalytic activity but would per se be inactive, the first step towards appropriate model systems consists in studying the behaviour of clean iron surfaces to which the promoter may be added in a well-defined manner at a later stage. Next, in order to investigate the influence of the atomic structure of the surface, experiments with well-defined single crystals rather than with polycrystalline material are feasible. Kinetic studies with clean Fe single crystal surfaces at high pressures (20 atm.) by Somor ja i et a1. (ref. 4) revealed that at 773 K with a 3: 1 H2: N2 mixture the activity varies in the sequence (111»(100»(110) by more than two orders of magnitude. The (110) surface is the most ~ensely packed plane of bcc Fe, while the (111) surface exhibits so-called C7-sites for which already in earlier work indications for particular reactivity had been found (ref. 5). If the quoted kinetic data are transformed into reaction probabilities for a N2 molecule striking the surface to become converted into a NH3 molecule, values of the order lO-6±1 result. These numbers as well as the sequence of activites are in good agreement with the data for the sticking coefficients of dissociative nitrogen adsorption measured with Fe single crystal surfaces under low pressure conditions (ref. 6). This result is of particular importance, since direct identification and characterization of the adsorbed surface species by application of modern surface spectroscopic techniques based on the interaction with electrons or ions can only be performed at very low pressures. This 'pressure gap' between the conditions of industrial catalysis and those of surface analysis may in general, of course, represent a serious problem. The large body of experimental evidence of the various aspects of ammonia synthesis demonstrates, however, that in this case this difficulty could be overcome. Since nitrogen adsorption is the rate-limiting step this short outline will mainly concentrate on some of the features of this process. Fig. 1 shows XPS data from the Nls core levels as 'fingerprints' of the three nitrogen species which may exist on a Fe(lll) surface (ref. 7): The molecular y-state is weakly held with an adsorption energy of 24 kJ/mole. Its N-N stretch frequency is close to that of free N2 (ref. 8), and the molecular axis is perpendicular to the surface plane (ref. 9). This y-state may convert into the a-state, which is still molecular in nature. It exhibits a higher adsorption energy (31 kJ/mol) and a strongly reduced N-N stretch frequency. Its molecular axis is inclined towards the surface plane. This a-state is the immediate 'precursor' for the formation of atomic nitrogen. The latter species is very strongly bonded and was also denoted as a 'surface nitride'. (The formation of bulk iron nitrides will never take place during ammonia synthesis for thermodynamic reasons). The structure of the c2x2-phase formed by atomic nitrogen on the Fe(lOO) surface is
317
shown in Fig. 2 (ref. 10). The N atoms occupy fourfold hollow-sites with their plane about 0.3 A above the topmost plane of Fe atoms. With the Fe(lll) and (110) surfaces pronounced displacements of the surface atoms (reconstruction) take place under the influence of adsorbed N atoms.
\::;~'t'<\"'c!.'..y.........,...·;.....V";",,,,,·,,~j~'i:;'-':'i';~·"f.~,\.,
; -t,,...
~'/~"'-'i!{",/~"~;yl:":.".,;"".,.,,,,,,.;
§
412
406 400 energy below E-Ferml (eV)
394
Fig. 1. X-ray photoelectron spectroscopy (XPS) data of the N1s-core levels for three different nitrogen species which may exists on a Fe(lll) surface: a) y-N2,ad, b) a-N2,ad, c) Nad .
d =0.27 N
0 0
r········ 1
=1~54
d,
e
j I
O~$9
_.. ..
1,'43
L
a
b
~---4,05----"
Fig. 2. The structure (top and side views) of the c2x2-phase formed by N atoms adsorbed on a Fe(100) surface as determined by low energy electron diffraction (LEED) .
The difference in activity between the various single crystal planes is essentially due to differences in the net activation energy for the overall process N2+2Nad, which ranges (in the limit of zero coverage) from 27 kJ/mol for Fe(110) to -0 kJ/mol for Fe(lll). These sticking coefficients may be markedly enhanced by the presence of coadsorbed K atoms which reflects the role of the electronic promoter. It was found that the adsorption energy of a-N2 is locally enhanced in the vicinity of an adsorbed K atom, and this stabilization
318
is in turn connected with a lowering of the activation energy for dissociation and hence the overall sticking coefficient increases. The combined experimental evidence on the various surface intermediates leads to the formulation of the following sequence of elementary steps involved in the overall reaction: 2Had H2 + * 2N ad N2 + * ~ N2,ad Nad + Had NHad NHad + Had NH2,ad NH2,ad + Had NH3,ad ~ NH3 + * (* denotes schemactially an ensemble of atoms forming an adsorption site).
This scheme was of course also amongst those proposed in previous studies, but only the application of surface physical methods enabled direct identification of the surface species and, in addition, determination of the relevant kinetic parameters of the elementary steps. Conceptual insight into the progress of the reaction is achieved by its energy profile as reproduced in fig. 3: The energy gain associated with the formation of the surface -N and -H bonds overcompensates the dissociation energies of N2 and H2, and the consecutive hydrogenation steps are energetically uphill. Dissociative nitrogen adsorption is rate-limiting, however primarily not because of a high activation energy but since the preexponential factor is rather unfavourable. N+3H
NH+2H 112,9
1400
-950
Nod+ 3 Had Fig. 3. Schematic energy profile for synthesis on iron. (Energies in kJ/mol).
the
progress of catalytic ammonia
319
The assumption of nitrogen adsorption as being the rate-limiting step was already underlying the derivation of the Temkin formalism which proved to be quite successful in modelling the kinetics of industrial ammonia synthesis if the parameters were properly adjusted (ref. 11). This formalism includes also the non-uniformity of the surface, viz. the variation of the adsorption parameters with coverage. As was recently pointed out by Boudart (ref. 12), the resulting equation exhibits a close formal analogy to an expression derived for a uniform surface with the kinetic parameters taken at the limit of low coverages. For this reason, successful approximate mod~lling of the kinetics of an industrial catalytic process based on data obtained with low-pressure, single crystal studies might become feasible. This way was recently followed by two groups (refs. 13, 14). Particularly successfully was the treatment by Stoltze and N0rskov (ref. 14) who developed a kinetic model with the results from the quoted single crytsal work as input parameters from which they calculated the yields with an industrial catalyst under high pressure conditions without any further adjustment of parameters. Fig. 4 represents a comparison of calculated and measured data under different conditions which demonstrates the excellent agreement. Also various trends of the rate upon variation of temperature, gas flow, pressure, as well as the influence of potassium coverage or water content in the gas phase are successfully reproduced by this model.
z
o i= u « a: u, ~
o
10-1
•
::.; M
:r: z
!:: x
UJ
o
W
10- 2
~
...J
::J
U
...J
o
t ctm
u
•
rso atm
«
10- 3 IL.,-10-3
o
300atm
~_=_---__.~----__:
10-2
10-1
EXPERIMENTAL EXIT NH3 MOLE FRACTION
Fig. 4. Comparison of ammonia production with an industrial catalyst measured under varying conditions of pressures, temperature and gas flow, with the corresponding data calculated with a kinetic model based on single-crystal, low pressure data for the elementary steps (ref. 14).
320
For the first time the kinetics of a process of industrial catalyts can thus be modelled on the basis of detailed knowledge about its mechanism.
REFERENCES
2
3 4 5 6
7 8 9 10 11 12 13 14
a) B. Timm. Proc, 8th Int. Congr. on Catalysis. Berlin. July 2-6. 1984. Verlag Chemie. Vol. I. p. 7. b) S. a. Topham, in J. R. Anderson and M. Boudart (Eds .}, Catalysis. Science and Technology. Springer-Verlag, Vol. 7. 19B5, p. 1. a) P. H. Emmett, in E. Drauglis and R. I. Jaffee (Eds.), The physical basis for heterogeneous catalysis. Plenum Press, 1975, p. 3. b) A. Ozaki and K. Aika. in J. R. Anderson and M. Boudart (Eds.), Catalysis. Science and Technology. Springer-Verlag. Vol. 1. 1981. p. 88. G. Ertl. D. Prigge. R. Schlagl and M. Weiss. J. Catal •• 79 (1983) 359. a) N. D. Spencer, R. C. Schoonmaker and G. A. 'somorjai, J. Catal., 103 (1987) 129. b) D. D. Strongin, J. Carrazza, S. R. Bare and G. A. Somorjai, J. Catal., 103 (1987) 213. a) R. Brill, E. L. Richter and E. Ruch, Angew. Chern., 6 (1967) 882. b) J. A. Dumesic, H. Topsoe , S. Khammouma and M. Boudart, J. Catal., 37 (1975) 503; 513. The work from the author's laboratory has been reviewed e.g. a) G. Ertl, in J. R. Anderson and M. Boudart (Eds.), Catalysis. Science and Technology. Springer-Verlag. Vol. 4, 1983, p. 209. b) G. Ertl, J. Vac. Sci. Techn., Al (1983) 1247. c) G. Ertl, in J. R. Jennings (Ed.), Catalytic ammonia synthesis. Plenum Press (in press). M. Grunze, M. Golze, W. Hirschwald, H. J. Freund, H. Pulm, U. Seip, M. C. Tsai, G. Ertl and J. Kuppers, Phys. Rev. Lett •• 53 (1984) 850. L. J. Whitman. C. E. Bartosch, W. Ho, G. Strasser and M. Grunze. Phys. Rev. Lett., 56 (1986) 1984. H. J. Freund, B. Bartos, R. P. Messmer, M. Grunze, H. Kuhlenbeck and M. Neumann, Surface Sci., 185 (1987) 187. R. Imbihl, R. J. Behm, G. Ertl and W. Moritz, Surface Sci., 123 (1982) 129. M. I. Temkin and V. Pyzhev, Acta Physicochem. USSR, 12 (1940) 489. M. Boudart, Catal. Lett., 1 (1988) 21. M. Bowker. I. Parker and K. Waugh, Apple Catal., 14 (1985) 101; Surface Sci., 197 (1988) L 223. P. StoHze and J. K. Ni1lrskov, Phys. Rev. Lett., 55 (1985) 2502; Surface Sci,. 197 (1988) L230.
315
PHYSICAL CHARACTERIZATION OF INDUSTRIAL CATALYSTS: THE MECHANISM OF AMMONIA SYNTHESIS
G. ErtJ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 0-1000 Berlin 33 (W. Germany)
ABSTRACT The application of various physical methods enabled detailed characterisation of the properties of the industrial catalyst for ammonia synthesis, as well as identification and determination of structural, energetic and kinetic parameters of the various reaction intermediates on iron single crystal surfaces. Based on the resulting reaction mechanism industrial ammonia yields could successfully be evaluated without any adjustable parameters. In 1913, e.g. 75 years ago, based on the work by Haber, Bosch, and Mittasch the first plant for the catalytic synthesis of ammonia from the elements started its operation in Ludwigshafen-Oppau and initiated a new era of industrial chemistry (ref. 1). Despite considerable progress in technology, the promoted iron catalyst as well as the reaction conditions remained basically unchanged over all these years. Extensive research by using conventional methods yielded a view about the mechanism of this reaction which even in the mid-seventies was still rather incomplete and controversial: While soon general agreement was reached that nitrogen adsorption represents the rate- 1imit ing step, it remained unclear if this species undergoing subsequent hydrogenation was atomic or molecular in nature (ref. 2). This situation changed markedly during the past decade by the advent and application of new physical methods which permitted atomic level studies of the surface properties of industrial catalysts as well as of well-defined single crystal samples serving as proper model systems. The industrial ammonia synthesis is prepared from magnetic (Fe304) with sma 11 percentages of K and Alas the most important promoters. Under work i ng cond it ions it consi sts essent i ally of meta11 i c Fe (with traces of unreduced iron). The promoters A1203 and CaO stabilize the small Fe particles against sintering, while potassium spreads over the Fe surface and forms a K+O overlayer acting as 'electronic' promoter (ref. 3). This adlayer interacts strongly with the Fe substrate and is quite different from all bulk compounds between potassium and oxygen. The presence of 0 increases the thermal stability
316
of adsorbed K, while in turn the strong K-O interaction prevents complete reduction. Since this potassium overlayer merely enhances the catalytic activity but would per se be inactive, the first step towards appropriate model systems consists in studying the behaviour of clean iron surfaces to which the promoter may be added in a well-defined manner at a later stage. Next, in order to investigate the influence of the atomic structure of the surface, experiments with well-defined single crystals rather than with polycrystalline material are feasible. Kinetic studies with clean Fe single crystal surfaces at high pressures (20 atm.) by Somor ja i et a1. (ref. 4) revealed that at 773 K with a 3: 1 H2: N2 mixture the activity varies in the sequence (111»(100»(110) by more than two orders of magnitude. The (110) surface is the most ~ensely packed plane of bcc Fe, while the (111) surface exhibits so-called C7-sites for which already in earlier work indications for particular reactivity had been found (ref. 5). If the quoted kinetic data are transformed into reaction probabilities for a N2 molecule striking the surface to become converted into a NH3 molecule, values of the order lO-6±1 result. These numbers as well as the sequence of activites are in good agreement with the data for the sticking coefficients of dissociative nitrogen adsorption measured with Fe single crystal surfaces under low pressure conditions (ref. 6). This result is of particular importance, since direct identification and characterization of the adsorbed surface species by application of modern surface spectroscopic techniques based on the interaction with electrons or ions can only be performed at very low pressures. This 'pressure gap' between the conditions of industrial catalysis and those of surface analysis may in general, of course, represent a serious problem. The large body of experimental evidence of the various aspects of ammonia synthesis demonstrates, however, that in this case this difficulty could be overcome. Since nitrogen adsorption is the rate-limiting step this short outline will mainly concentrate on some of the features of this process. Fig. 1 shows XPS data from the Nls core levels as 'fingerprints' of the three nitrogen species which may exist on a Fe(lll) surface (ref. 7): The molecular y-state is weakly held with an adsorption energy of 24 kJ/mole. Its N-N stretch frequency is close to that of free N2 (ref. 8), and the molecular axis is perpendicular to the surface plane (ref. 9). This y-state may convert into the a-state, which is still molecular in nature. It exhibits a higher adsorption energy (31 kJ/mol) and a strongly reduced N-N stretch frequency. Its molecular axis is inclined towards the surface plane. This a-state is the immediate 'precursor' for the formation of atomic nitrogen. The latter species is very strongly bonded and was also denoted as a 'surface nitride'. (The formation of bulk iron nitrides will never take place during ammonia synthesis for thermodynamic reasons). The structure of the c2x2-phase formed by atomic nitrogen on the Fe(lOO) surface is
317
shown in Fig. 2 (ref. 10). The N atoms occupy fourfold hollow-sites with their plane about 0.3 A above the topmost plane of Fe atoms. With the Fe(lll) and (110) surfaces pronounced displacements of the surface atoms (reconstruction) take place under the influence of adsorbed N atoms.
\::;~'t'<\"'c!.'..y.........,...·;.....V";",,,,,·,,~j~'i:;'-':'i';~·"f.~,\.,
; -t,,...
~'/~"'-'i!{",/~"~;yl:":.".,;"".,.,,,,,,.;
§
412
406 400 energy below E-Ferml (eV)
394
Fig. 1. X-ray photoelectron spectroscopy (XPS) data of the N1s-core levels for three different nitrogen species which may exists on a Fe(lll) surface: a) y-N2,ad, b) a-N2,ad, c) Nad .
d =0.27 N
0 0
r········ 1
=1~54
d,
e
j I
O~$9
_.. ..
1,'43
L
a
b
~---4,05----"
Fig. 2. The structure (top and side views) of the c2x2-phase formed by N atoms adsorbed on a Fe(100) surface as determined by low energy electron diffraction (LEED) .
The difference in activity between the various single crystal planes is essentially due to differences in the net activation energy for the overall process N2+2Nad, which ranges (in the limit of zero coverage) from 27 kJ/mol for Fe(110) to -0 kJ/mol for Fe(lll). These sticking coefficients may be markedly enhanced by the presence of coadsorbed K atoms which reflects the role of the electronic promoter. It was found that the adsorption energy of a-N2 is locally enhanced in the vicinity of an adsorbed K atom, and this stabilization
318
is in turn connected with a lowering of the activation energy for dissociation and hence the overall sticking coefficient increases. The combined experimental evidence on the various surface intermediates leads to the formulation of the following sequence of elementary steps involved in the overall reaction: 2Had H2 + * 2N ad N2 + * ~ N2,ad Nad + Had NHad NHad + Had NH2,ad NH2,ad + Had NH3,ad ~ NH3 + * (* denotes schemactially an ensemble of atoms forming an adsorption site).
This scheme was of course also amongst those proposed in previous studies, but only the application of surface physical methods enabled direct identification of the surface species and, in addition, determination of the relevant kinetic parameters of the elementary steps. Conceptual insight into the progress of the reaction is achieved by its energy profile as reproduced in fig. 3: The energy gain associated with the formation of the surface -N and -H bonds overcompensates the dissociation energies of N2 and H2, and the consecutive hydrogenation steps are energetically uphill. Dissociative nitrogen adsorption is rate-limiting, however primarily not because of a high activation energy but since the preexponential factor is rather unfavourable. N+3H
NH+2H 112,9
1400
-950
Nod+ 3 Had Fig. 3. Schematic energy profile for synthesis on iron. (Energies in kJ/mol).
the
progress of catalytic ammonia
319
The assumption of nitrogen adsorption as being the rate-limiting step was already underlying the derivation of the Temkin formalism which proved to be quite successful in modelling the kinetics of industrial ammonia synthesis if the parameters were properly adjusted (ref. 11). This formalism includes also the non-uniformity of the surface, viz. the variation of the adsorption parameters with coverage. As was recently pointed out by Boudart (ref. 12), the resulting equation exhibits a close formal analogy to an expression derived for a uniform surface with the kinetic parameters taken at the limit of low coverages. For this reason, successful approximate mod~lling of the kinetics of an industrial catalytic process based on data obtained with low-pressure, single crystal studies might become feasible. This way was recently followed by two groups (refs. 13, 14). Particularly successfully was the treatment by Stoltze and N0rskov (ref. 14) who developed a kinetic model with the results from the quoted single crytsal work as input parameters from which they calculated the yields with an industrial catalyst under high pressure conditions without any further adjustment of parameters. Fig. 4 represents a comparison of calculated and measured data under different conditions which demonstrates the excellent agreement. Also various trends of the rate upon variation of temperature, gas flow, pressure, as well as the influence of potassium coverage or water content in the gas phase are successfully reproduced by this model.
z
o i= u « a: u, ~
o
10-1
•
::.; M
:r: z
!:: x
UJ
o
W
10- 2
~
...J
::J
U
...J
o
t ctm
u
•
rso atm
«
10- 3 IL.,-10-3
o
300atm
~_=_---__.~----__:
10-2
10-1
EXPERIMENTAL EXIT NH3 MOLE FRACTION
Fig. 4. Comparison of ammonia production with an industrial catalyst measured under varying conditions of pressures, temperature and gas flow, with the corresponding data calculated with a kinetic model based on single-crystal, low pressure data for the elementary steps (ref. 14).
320
For the first time the kinetics of a process of industrial catalyts can thus be modelled on the basis of detailed knowledge about its mechanism.
REFERENCES
2
3 4 5 6
7 8 9 10 11 12 13 14
a) B. Timm. Proc, 8th Int. Congr. on Catalysis. Berlin. July 2-6. 1984. Verlag Chemie. Vol. I. p. 7. b) S. a. Topham, in J. R. Anderson and M. Boudart (Eds .}, Catalysis. Science and Technology. Springer-Verlag, Vol. 7. 19B5, p. 1. a) P. H. Emmett, in E. Drauglis and R. I. Jaffee (Eds.), The physical basis for heterogeneous catalysis. Plenum Press, 1975, p. 3. b) A. Ozaki and K. Aika. in J. R. Anderson and M. Boudart (Eds.), Catalysis. Science and Technology. Springer-Verlag. Vol. 1. 1981. p. 88. G. Ertl. D. Prigge. R. Schlagl and M. Weiss. J. Catal •• 79 (1983) 359. a) N. D. Spencer, R. C. Schoonmaker and G. A. 'somorjai, J. Catal., 103 (1987) 129. b) D. D. Strongin, J. Carrazza, S. R. Bare and G. A. Somorjai, J. Catal., 103 (1987) 213. a) R. Brill, E. L. Richter and E. Ruch, Angew. Chern., 6 (1967) 882. b) J. A. Dumesic, H. Topsoe , S. Khammouma and M. Boudart, J. Catal., 37 (1975) 503; 513. The work from the author's laboratory has been reviewed e.g. a) G. Ertl, in J. R. Anderson and M. Boudart (Eds.), Catalysis. Science and Technology. Springer-Verlag. Vol. 4, 1983, p. 209. b) G. Ertl, J. Vac. Sci. Techn., Al (1983) 1247. c) G. Ertl, in J. R. Jennings (Ed.), Catalytic ammonia synthesis. Plenum Press (in press). M. Grunze, M. Golze, W. Hirschwald, H. J. Freund, H. Pulm, U. Seip, M. C. Tsai, G. Ertl and J. Kuppers, Phys. Rev. Lett •• 53 (1984) 850. L. J. Whitman. C. E. Bartosch, W. Ho, G. Strasser and M. Grunze. Phys. Rev. Lett., 56 (1986) 1984. H. J. Freund, B. Bartos, R. P. Messmer, M. Grunze, H. Kuhlenbeck and M. Neumann, Surface Sci., 185 (1987) 187. R. Imbihl, R. J. Behm, G. Ertl and W. Moritz, Surface Sci., 123 (1982) 129. M. I. Temkin and V. Pyzhev, Acta Physicochem. USSR, 12 (1940) 489. M. Boudart, Catal. Lett., 1 (1988) 21. M. Bowker. I. Parker and K. Waugh, Apple Catal., 14 (1985) 101; Surface Sci., 197 (1988) L 223. P. StoHze and J. K. Ni1lrskov, Phys. Rev. Lett., 55 (1985) 2502; Surface Sci,. 197 (1988) L230.