- Email: [email protected]
Kinetics of wet atmosphere reduction of a fused iron catalyst for ammonia synthesis
Applied Catalysis, 71 (1991) Ll-L4 Elsevier Science Publishers B.V., Amsterdam
Ll
Kinetics of wet atmosphere reduction of a fused iron catalyst for ammonia synthesis A. Baranski Regional Laboratory of Physicochemical Analyses and Structural University, Karasia 3, PL-30-060 Cracow (Poland)
Research,
Jagiellonian
A. Kotarba Department
of Chemistry,
Jagiellonian
University,
Karasia 3, PL-30-060
Cracow (Poland)
and J.M. tagan Regional Laboratory of Physicochemical Analyses and Structural University, Karasia 3, PL-30-060 Cracow (Poland)
Research,
Jagiellonian
(Received 16 January 1991, revised manuscript received 25 February 1991) Keywords: ammonia synthesis, fused iron, kinetics, water vapour.
An iron catalyst, used for ammonia synthesis, is activated by means of the reduction of its oxide precursor. The catalyst is reduced in situ by a mixture of gases containing hydrogen in the industrial reactors or in especially built installations. Water vapour retards the reduction rate at conditions far beyond the thermodynamic expectations, and, hence, its partial pressure is an essential factor in influencing the technology of the catalyst activation [ 11. Surprisingly, an explanation of the retardation effect at the molecular level is not known, though the catalyst has been used for almost 80 years [ 21. This is also in contrast with the fact that the mechanism of ammonia synthesis on the already reduced catalyst is known in detail [ 31. The effect of water vapour has been studied in our laboratory for many years. The dramatic decrease of the reduction rate of the iron catalyst [4] was linked with the presence of promoters [ 51. The role of alumina [ 61 and, later on, the role of a magnetite-alumina catalyst sub-system [7] was emphasized. This agrees well with the earlier data of Dry [ 81 and Rozowskii [ 91. It has also been found that the effect was reversible [lo] in the sense that the removal of water from the gas-phase instantaneously accelerated the retarded reduction rate.
0166-9834/91/$03.50
0 1991 Elsevier Science Publishers B.V.
L2
Let us note that all the information outlined above is of a phenomenological type. Our tentative explanation of the effect [lo] emphasized - let us quote “blocking the catalyst surface usually enriched with alumina. Water may interact with the hydroxyl groups of alumina thus preventing an access of hydrogen to the surface”. Recently, Schlijgl proposed a more detailed hypothetical interpretation [ 111. Water supports the segregation of alumina from magnetite and enables the formation of a thin, dense shell of alumina oxohydroxides which hinder the hydrogen diffusion and block the progress of the reduction. At present it seems worthwhile testing the kinetic implications of the hypothesis concerning the blockage of the surface by a thin, dense shell. We used our old published data [4] for this purpose. The reduction kinetics of KM-l type iron catalysts in the presence of 1% water vapour in the gas phase were reconsidered. The partial pressure in question is sufficiently high so that the effect of water on the reduction rate is fully revealed. The kinetic equation
lOOR= kt"
(1)
(where R is a reduction degree (dimensionless) 0
O.IzO I-’
o.oob
100
Fig. 1. Application
’
c
200 300 Time/min
of the parabolic
1
400
’
500
law to the description
of the reduction
kinetics
of an iron
catalyst in a wet atmosphere. Experimental values (dashed line) are taken from Fig. 2. ref. set C. Calculated values (solid line) are based on eqn. 1.
[ 41,
L3
calculated values of the reduction degree amounts to 0.300. The following values of parameters were obtained: k= 1.27. 10V2, n = 0.428. Undoubtedly, the value n= 0.428 is close to 0.5. Hence, the parabolic law, as a first approximation, is suitable for describing the data. The parabolic law is based on the assumption that the diffusion through a layer is the rate determining step of the process. The diffusion is inversely proportional to the thickness of the layer [ 121. Obviously, the concepts of the blocking layer and the parabolic law are useful in studying the effect of water on the reduction rate of the iron catalyst. What is the process in question? What layer is involved in this process? These questions are awaiting answers. Let us remember that magnetite and wustite are the components of the iron catalyst. Pattek-Janczyk et al. have proved that wustite was reduced faster and earlier than magnetite [ 131. One can expect that the beginning of the kinetic curve corresponds to the reduction of wustite. Kohl and Engell [ 141 observed the validity of the parabolic law in the case of the reduction of wustite in a (hydrogen+ water) mixture with 2.5% water vapour. They postulated the formation of a dense iron layer. However, the kinetic curve seen in Fig. 1 cannot only represent the reduction of wustite. The wustite content amounts to ca. 11% [15]. The final experimental value of the reduction degree R=0.16 cannot be achieved at its expense. Additionally, the reduction rate at the end of the curve, although small, is not approaching zero. Taking into account the absence of wustite, let us consider the reduction of the magnetite phase. The iron catalyst singly promoted with alumina and oxidized to magnetite is the model catalyst here. It cannot be reduced at 500°C in the presence of 1% water [ 71. This means that the blocking surface layer (aluminum oxohydroxy layer, we believe) is impermeable at the conditions listed above. One can conclude that a simple interpretation of the kinetic curve is lacking. Undoubtedly, further systematic studies are needed in order to understand the reduction of the iron catalyst under wet atmospheric conditions.
REFERENCES 1. G.D. Honti, The Nitrogen Industry, Akademiai Kiado, Budapest, 1976, p. 124. 2 S.A. Topham, in I.R. Anderson and M. Boudart (Editors), Catalysis, Vol. 7, Akademie Verlag, Berlin, 1986, p. 1. 3 G. Ertl, Catal. Rev. Sci. Eng., 21 (1980) 201. 4 A. Baranski, J.M. tagan, A. Pattek and A. Reizer, Appl. Catal., 3 (1982) 201. 5 A. Baranski, J.M. tagan, A. Pattek and A. Reizer, Appl. Catal., 3 (1982) 207. 6 A. Barariski, A. Reizer, A. Kotarba and E. Pyrczak, Appl. Catal., 19 (1985) 417. 7 A. Barafiski, A. Reizer, A. Kotarba and E. Pyrczak, Appl. Catal., 40 (1988) 67. 8 M. Dry, J. South African Chem. Inst., 15 (1962) 11.
L4 9 10
11 12 13
A.Ya. Rozovskii,
Heterogeneous
Chemical
Reactions:
Kinetics
and Macrokinetics,
Nauka,
Moscow, 1980, pp. 220-222. A. Baranski, A. Reizer, A. Kotarba and E. Pyrczak, in D. Shopov, A. Andreev, A. Palazov and L. Petrov (Editors), Proc. 6th Int. Symp. on Heterogeneous Catalysis, Sofia, 1987, Bulg. Acad. Sci., Sofia, 1987, Part 2, p. 420. R. Schlogl, in J.R. Jennings (Editor), Catalytic Ammonia Synthesis: Fundamentals and Practice, Plenum, London, to be published. L. von Bogdandy and H.-J. Engell, The Reduction Verlag Stahleisen, Dusseldorf, 1971, p. 139. A. Pattek-Janczyk, 35.
A.Z. Hrynkiewicz,
of Iron Ores, Springer Verlag, Berlin,
J. Kraczka and D. Kulgawczyk,
14
H.K. Kohl and H.-J. Engell, Arch. Eisenhtittenwes,
15
A. Pattek-Janczyk
and A.Z. Hrynkiewicz,
34 (1963) 411.
Appl. Catal., 6 (1983)
27.
Appl. Catal., 6 (1983)
Ll
Kinetics of wet atmosphere reduction of a fused iron catalyst for ammonia synthesis A. Baranski Regional Laboratory of Physicochemical Analyses and Structural University, Karasia 3, PL-30-060 Cracow (Poland)
Research,
Jagiellonian
A. Kotarba Department
of Chemistry,
Jagiellonian
University,
Karasia 3, PL-30-060
Cracow (Poland)
and J.M. tagan Regional Laboratory of Physicochemical Analyses and Structural University, Karasia 3, PL-30-060 Cracow (Poland)
Research,
Jagiellonian
(Received 16 January 1991, revised manuscript received 25 February 1991) Keywords: ammonia synthesis, fused iron, kinetics, water vapour.
An iron catalyst, used for ammonia synthesis, is activated by means of the reduction of its oxide precursor. The catalyst is reduced in situ by a mixture of gases containing hydrogen in the industrial reactors or in especially built installations. Water vapour retards the reduction rate at conditions far beyond the thermodynamic expectations, and, hence, its partial pressure is an essential factor in influencing the technology of the catalyst activation [ 11. Surprisingly, an explanation of the retardation effect at the molecular level is not known, though the catalyst has been used for almost 80 years [ 21. This is also in contrast with the fact that the mechanism of ammonia synthesis on the already reduced catalyst is known in detail [ 31. The effect of water vapour has been studied in our laboratory for many years. The dramatic decrease of the reduction rate of the iron catalyst [4] was linked with the presence of promoters [ 51. The role of alumina [ 61 and, later on, the role of a magnetite-alumina catalyst sub-system [7] was emphasized. This agrees well with the earlier data of Dry [ 81 and Rozowskii [ 91. It has also been found that the effect was reversible [lo] in the sense that the removal of water from the gas-phase instantaneously accelerated the retarded reduction rate.
0166-9834/91/$03.50
0 1991 Elsevier Science Publishers B.V.
L2
Let us note that all the information outlined above is of a phenomenological type. Our tentative explanation of the effect [lo] emphasized - let us quote “blocking the catalyst surface usually enriched with alumina. Water may interact with the hydroxyl groups of alumina thus preventing an access of hydrogen to the surface”. Recently, Schlijgl proposed a more detailed hypothetical interpretation [ 111. Water supports the segregation of alumina from magnetite and enables the formation of a thin, dense shell of alumina oxohydroxides which hinder the hydrogen diffusion and block the progress of the reduction. At present it seems worthwhile testing the kinetic implications of the hypothesis concerning the blockage of the surface by a thin, dense shell. We used our old published data [4] for this purpose. The reduction kinetics of KM-l type iron catalysts in the presence of 1% water vapour in the gas phase were reconsidered. The partial pressure in question is sufficiently high so that the effect of water on the reduction rate is fully revealed. The kinetic equation
lOOR= kt"
(1)
(where R is a reduction degree (dimensionless) 0
O.IzO I-’
o.oob
100
Fig. 1. Application
’
c
200 300 Time/min
of the parabolic
1
400
’
500
law to the description
of the reduction
kinetics
of an iron
catalyst in a wet atmosphere. Experimental values (dashed line) are taken from Fig. 2. ref. set C. Calculated values (solid line) are based on eqn. 1.
[ 41,
L3
calculated values of the reduction degree amounts to 0.300. The following values of parameters were obtained: k= 1.27. 10V2, n = 0.428. Undoubtedly, the value n= 0.428 is close to 0.5. Hence, the parabolic law, as a first approximation, is suitable for describing the data. The parabolic law is based on the assumption that the diffusion through a layer is the rate determining step of the process. The diffusion is inversely proportional to the thickness of the layer [ 121. Obviously, the concepts of the blocking layer and the parabolic law are useful in studying the effect of water on the reduction rate of the iron catalyst. What is the process in question? What layer is involved in this process? These questions are awaiting answers. Let us remember that magnetite and wustite are the components of the iron catalyst. Pattek-Janczyk et al. have proved that wustite was reduced faster and earlier than magnetite [ 131. One can expect that the beginning of the kinetic curve corresponds to the reduction of wustite. Kohl and Engell [ 141 observed the validity of the parabolic law in the case of the reduction of wustite in a (hydrogen+ water) mixture with 2.5% water vapour. They postulated the formation of a dense iron layer. However, the kinetic curve seen in Fig. 1 cannot only represent the reduction of wustite. The wustite content amounts to ca. 11% [15]. The final experimental value of the reduction degree R=0.16 cannot be achieved at its expense. Additionally, the reduction rate at the end of the curve, although small, is not approaching zero. Taking into account the absence of wustite, let us consider the reduction of the magnetite phase. The iron catalyst singly promoted with alumina and oxidized to magnetite is the model catalyst here. It cannot be reduced at 500°C in the presence of 1% water [ 71. This means that the blocking surface layer (aluminum oxohydroxy layer, we believe) is impermeable at the conditions listed above. One can conclude that a simple interpretation of the kinetic curve is lacking. Undoubtedly, further systematic studies are needed in order to understand the reduction of the iron catalyst under wet atmospheric conditions.
REFERENCES 1. G.D. Honti, The Nitrogen Industry, Akademiai Kiado, Budapest, 1976, p. 124. 2 S.A. Topham, in I.R. Anderson and M. Boudart (Editors), Catalysis, Vol. 7, Akademie Verlag, Berlin, 1986, p. 1. 3 G. Ertl, Catal. Rev. Sci. Eng., 21 (1980) 201. 4 A. Baranski, J.M. tagan, A. Pattek and A. Reizer, Appl. Catal., 3 (1982) 201. 5 A. Baranski, J.M. tagan, A. Pattek and A. Reizer, Appl. Catal., 3 (1982) 207. 6 A. Barariski, A. Reizer, A. Kotarba and E. Pyrczak, Appl. Catal., 19 (1985) 417. 7 A. Barafiski, A. Reizer, A. Kotarba and E. Pyrczak, Appl. Catal., 40 (1988) 67. 8 M. Dry, J. South African Chem. Inst., 15 (1962) 11.
L4 9 10
11 12 13
A.Ya. Rozovskii,
Heterogeneous
Chemical
Reactions:
Kinetics
and Macrokinetics,
Nauka,
Moscow, 1980, pp. 220-222. A. Baranski, A. Reizer, A. Kotarba and E. Pyrczak, in D. Shopov, A. Andreev, A. Palazov and L. Petrov (Editors), Proc. 6th Int. Symp. on Heterogeneous Catalysis, Sofia, 1987, Bulg. Acad. Sci., Sofia, 1987, Part 2, p. 420. R. Schlogl, in J.R. Jennings (Editor), Catalytic Ammonia Synthesis: Fundamentals and Practice, Plenum, London, to be published. L. von Bogdandy and H.-J. Engell, The Reduction Verlag Stahleisen, Dusseldorf, 1971, p. 139. A. Pattek-Janczyk, 35.
A.Z. Hrynkiewicz,
of Iron Ores, Springer Verlag, Berlin,
J. Kraczka and D. Kulgawczyk,
14
H.K. Kohl and H.-J. Engell, Arch. Eisenhtittenwes,
15
A. Pattek-Janczyk
and A.Z. Hrynkiewicz,
34 (1963) 411.
Appl. Catal., 6 (1983)
27.
Appl. Catal., 6 (1983)