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Hydrocarbon synthesis and carbon dioxide adsorption on iron catalysts
63
Fuel Processing Technology, 3 (1980) 63-70 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
HYDROCARBON SYNTHESIS AND CARBON DIOXIDE ADSORPTION ON IRON CATALYSTS
S. PARKASH, S.K. CHAKRABAR’ITY Fuel Sciences Division, Alberta Research Council, I1 315 - 87th Avenue, Edmonton, Alberta, T6G 2C2 (Canada) and MICHAEL P. ROSYNEK Department of Chemistry, Texas A & M University, College Station, Texas, 77843 (U.S.A.) (Received June 28th, 1979; accepted August 2&h, 1979)
ABSTRACT Adsorption isotherms have been measured for carbon dioxide on iron synthetic-NH, catalyst and on low-grade iron ore at six temperatures in the range lOO-350°C and at pressures up to one atmosphere. The results indicate that at least two dissimilar types of adsorbed CO, exist on the surface of the iron synthetic-NH, catalyst, one of which is generated only at temperatures >25O”C. By contrast, only one form of adsorbed CO, was detected on the iron ore catalyst at all temperatures up to 350°C. The kinetics of carbon monoxide hydrogenation are comparable over the two catalysts, and both exhibit virtually identical selectivities for methane formation at equivalent total CO conversions.
INTRODUCTION
As a result of expected declines in world-wide petroleum reserves, renewed interest has been shown during the past several years in the possibility of economically producing hydrocarbons by classical Hz/CO synthesis gas reactions, i.e., methanation, Fischer-Tropsch syntheses, and the isosynthesis. Consequently, catalytic hydrogenation of carbon monoxide has become the topic of considerable recent research [l-4], the primary aim in most cases being the establishment of activity/selectivity behaviors of various supported metals and alloys over an appropriate range of reaction conditions. However, in all such processes, the formation and subsequent adsorption/desorption properties of carbon dioxide may play important roles in governing the overall behavior of the system, particularly as regards hydrocarbon selectivity patterns. Generation of CO2 during HJCO conversions occurs by at least three significant routes, viz., the water-gas-shift reaction: CO + H,O + CO2 + Hz Alberta Research Council Contribution 1002.
(1)
64
disproportionation of CO reactant (the so-called Boudouard reaction) : 2co -+ C(s) + co2
(2)
and as a by-product of hydrocarbon formation: 2n CO + @+I) Hz + C,Hzn+2 + n COz
(3)
and/or 2nCO+nH, +C,H,,+nCOz
(4)
Despite both extensive current research and the existence of a large body of previously-published data on H&O conversions [ 5,6] , however, information about the extents of carbon dioxide adsorption on metal catalysts over a significant range of coverages is lacking. The present investigation was undertaken in an effort to elucidate the markedly different COz adsorption properties exhibited by an iron synthetic-ammonia catalyst and a low-grade iron ore, as well as the comparative catalytic behaviors of these materials for the hydrogenation of carbon monoxide. EXPERIMENTAL
Materials The iron synthetic-NH3 catalyst was kindly provided by Sherrit Gordon Mines, Ltd. of Fort Saskatchewan, Alberta, and the native iron ore was obtained from the Geology Division of the Alberta Research Council. Table 1 TABLE 1 Chemical composition of iron synthetic-NH, catalyst and iron ore Iron synthetic-NH, catalyst
Iron ore
Constituent
wt.%
as
Constituent
Wt.%
as
Iron Silica Alumina Potassium Calcium Nickel Chromium Manganese Zinc
84.9
Fe SiO, AbO, R,O CaO NiO (X0, MnO ZnO
Iron Silica Alumina Potassium Calcium Magnesium Phosphorus Manganese Sulphur Ignition loss
41.4 31.0 6.9 0.8 2.2 1.2 0.8 0.2 0.1 16.4
Fe SiO, AJO, IGO CaO MgO P MnO S H,O, CO,
1
9.2 1.2 0.9 1.1 1.0 0.9 0.8
65
summarizes the chemical compositions of the two catalysts. Prior to each adsorption or reaction experiment, a sample of the desired material was evacuated overnight at 400”Cto a residual pressure of - 10e4 Torr, reductively activated in situ in a rapid (2 l/h) flow of hydrogen for 8 h, and finally re-evacuated for several hours at the same temperature. All gases (- 99.9% purity) were obtained from Matheson of Canada, Ltd. Before use hydrogen and carbon monoxide were further purified by passage through a trap at liquid nitrogen temperature, while research purity carbon dioxide was used without further purification. Apparatus Adsorption isotherms for COZ on the two iron-containing materials were determined using a Micromeritics Co. Model 2100D gas adsorption apparatus. The glass sample vessel attached to the instrument was filled with a weighed amount of catalyst in each case, and then installed in a specially designed and insulated furnace chamber that maintained the sample temperature constant to within +O.l”C. An arbitrary adsorption time of 90 min was allowed for each point on the isotherm. All adsorption calculations employed the weight of the catalyst sample as measured after completion of measurements. Fractional surface coverages (6 ) were estimated from the weight of adsorbed gas by assuming a value of 16.4 A* for the effective area occupied by a CO2 molecule in a filled monolayer [ 71. Catalytic behaviors of the two materials for CO hydrogenation were determined using a fixed-bed integral flow reactor. Rates of formation of methane and carbon dioxide were measured for 2:l H2:C0 reactant ratios at 250°C and at total pressure near or below one atmosphere. RESULT AND DISCUSSION
Catalyst characteristics Experimentally-measured physical characteristics of the two iron catalysts are shown in Table 2. Monolayer coverages of hydrogen were determined from H2 adsorption isotherms at 25”C, using the zero-pressure intercept method described by Vannice [ 81. Corresponding values for carbon monoxide were obtained for each catalyst by measuring the difference in total adsorbate uptake between CO and N2 isotherms at -196°C and at equal relative pressures [ 91. Percentages of exposed metal are baaed on weights of reduced catalysts and were calculated assuming dissociative adsorption of H2 and linearly chemisorbed CO. As seen, results from the two types of adsorption measurements were in good agreement for both catalysts. Investigations by means of X-ray indicate that in the case of iron syntheticNH3 catalyst, the presence of promoters (K20, CaO, A1203) caused no separate
66 TABLE 2 Physical and surface parameters of iron catalysts Parameter
Iron syntheticNH, catalyst
Iron ore
BET-N, surface area (m’lg) N, pore volume (cm’/g) Apparent density* (g/cm”) True density* * (g/cm”) H, monolayer at 25°C (&mole/g) percentage exposed Fe CO monolayer at -196°C (pmole/g) percentage exposed Fe
10.60 0.07 1.49 4.72 15.50 0.20 32.60 0.21
93.80
0.24 0.73 2.92 29.10 0.63 55.30 0.62
* By Hg displacement at 25°C. ** By He displacement at 25°C.
phases which suggest that the promoters formed a solid solution with magnetite. However, this is not exactly the case for iron ore. Carbon dioxide adsorption Figure 1 presents adsorption isotherm obtained for carbon dioxide on samples of the iron ore catalyst at six temperatures in the range lOO-350°C. Total uptakes of CO2 decrease in the expected manner with increasing temperature at all equilibrium pressures up to one atmosphere. Surface coverage by CO2 at the lowest temperature (100°C) appears to asymptotically
Equilibrium pressure
(tow
Fig.1. Adsorption isotherm for CO, on iron ore.
67
approach total saturation of the available metal sites, viz., - 0.6 pmole of exposed iron atoms per m2, with increasing pressure. For the iron synthetic-NH3 catalyst (Fig. Z), isotherms in the temperature range 100-250” C show large initial uptakes of CO2 at very low pressures (
01
0
’
I
loo
200
I I I 300 4 0 0 6 0 0
1 600
1 ]
766
Equilibrium pressure, torr
Fig.2. Adsorption isotherms for CO, on iron synthetic-NH, catalyst.
These results suggest that a least two dissimilar forms of adsorbed CO2 exist on the iron synthetic-NH, catalyst, one of them being generated only at temperatures 2250” C, presumably due to a considerably higher activation energy of adsorption. In this connection, we note that two forms of adsorbed CO2 have been reported to exist on the surface of activated y-alumina at loo-300°C [lo]. Further confirmation of the differing behaviors exhibited by the two catalysts for CO2 adsorption was obtained from isobaric data. Each catalyst
68
- lo- 0 T e m p . i n c r e a s e P 8- l Temp decrease Synthetic- NH, iron
OOIO Temperoture ( ‘Cl
Fig.3. Adsorption isobars at 400 Torr initial pressure for CO, on iron ore and on iron synthetic-NH, catalysts.
was exposed to an initial pressure (- 400 Torr) of CO2 at 25”C, then heated in 50°C increments to 350°C at constant CO2 pressure, and finally cooled in 50°C decrements to lOO”C, a measurement of total CO? uptake being made following equilibration at each temperature. The results are shown for both catalysts in Fig.3. It is evident that the isobar observed for the iron ore sampling during temperature decrease is coincident with that measured at increasing temperature, consistent with the presence of a single, thermally equilibrated, adsorbed species over the entire temperature range investigated. With the iron synthetic-NH3 catalyst, on the other hand, although total COz uptake decreased, as expected, with increasing temperature up to 25O”C, a sharp increase in adsorption, due to formation of a second type of COz surface species, occurred when the temperature was raised to 300°C and then 350” C, in agreement with the isothermal adsorption data for this catalyst shown in Fig.2. This presumably strongly-bound form of adsorbed COz remained on the surface when the sample temperature was subsequently lowered in 50” C steps to 100” C. The small increases in CO2 uptake during temperature decrease are caused by increasing surface concentrations of the first type of adsorbed CO2 that predominated at G25O”C during temperature increase.
69
The large uptake of CO2 (- 29 pmole/m’) by synthetic-NH, catalyst above 250” C corresponds to over two monolayers of CO2 on the entire catalyst surface. It is possible that at these higher temperatures, chemical reactions (CaO + CO2 = CaCO,, AG” = -130 kJ mole-’ and K?O + CO2 = K&OJ, AG” = -460 kJ mole”) may be occurring, leading to carbonate formation*. In chemisorption studies on reduced magnetities by Brunauer and Emmett [ 111 it was assumed that COZ was exclusively adsorbed by the surface alkali molecules. Collins and Trapnell [12] and Dry and coworkers [13,14] have shown that CO2 chemisorbs not only on the oxides of alkali metals but also on the free iron surface as well. We concur with the view [13,14] that large uptake of CO? is not necessarily due to alkalis themselves, but also on the exposed iron sites generated due to the presence of the alkali on the surface, e.g., by altering the chemical potential of the neighboring surface iron atoms, especially when two dissimilar forms of CO* have been observed on the synthetic-NH3 catalyst. Because of the lack of similar observations for iron ore, we believe that in this case potassium and calcium are present as silicates and do not react with carbon dioxide. Carbon monoxide hydrogenation Kinetic data for the hydrogenation of CO over both the iron ore and the iron synthetic-ammonia catalyst are summarized in Table 3. The data presented were obtained under steady-state conditions for the near-atmospheric pressure reaction at 250” C. It is evident from the results in Table 3 that, at equivalent total CO conversions, methane turnover frequencies at 250“ C were comparable and selectivities for CH4 formation were virtually identical (- 19%) over both catalysts. Under these conditions, however, the amount of COZ appearing in the gaseous reaction products was much less (35% vs. 60% of all CO converted) with the iron synthetic-NH, catalyst than with the iron ore. The fact that the two catalysts demonstrated different adsorption behaviors for CO2 without affecting the synthesis kinetics strongly suggest that the COZ adsorption sites are not the reaction sites. Some reversible restructuring of the synthetic-NH3 catalyst is more than likely taking place at temperatures > 250” C, which manifests itself in a change in the kinetics of COZ adsorption. An alternate possibility is just that adsorbed COZ does not affect the reaction kinetics on the argument that both HZ and CO chemisorb much more strongly than CO* and thereby displace CO, from the surface when all three gases are present. ACKNOWLEDGEMENT
One of the authors (M.P.R.) gratefully acknowledges financial support by the Robert A. Welch Foundation. *Theoretically the amount of CO, needed for complete conversion of present in synthetic-NH, catalyst to carbonates is - 27 pmole/m’.
CaO and K,O
70 TABLE 3 Kinetic parameters for CO hydrogenation over iron catalysts* Catalyst
% co conversion
% Selectivity to CO,
CH,
Rate of CH, formation bmolelgls)
Methane turnover frequency (X 10’ s-1)
Apparent activation energy for CH, formation (kJ/mole)
Synthetic-NH, iron catalyst
41.4
35.0
19.3
0.16
5.2
88
Iron ore
40.1
60.1
18.7
0.17
3.0
75
*Reaction conditions: H,:CO = 2:l; T = 250°C;~ = 700 Torr; VHSV = 3000 h-l.
REFERENCES
10 11 12 13 14
Mills, G.A. and Steffgen, I.W., 1973. Catal. Rev. Sci. Eng., 8: 159. Vannice, M.A., 1976. Catal. Rev. Sci. Eng., 14: 153. Dry, M.E., 1976. Ind. Eng. Chem. Prod. Res. Dev., 15: 282. Vannice, M.A., 1977. J. Catal., 59: 228. Starch, H.H., Golumbic, N. and Anderson, R.B., 1951. The Fischer+‘ropsch and Related Syntheses, Wiley-Interscience, New York. Anderson, R.B., 1956. In: P.H. Emmett (Ed.), Catalysis, Vol. IV, Reinhold, New York. Ross, S. and Oliver, J.P., 1964. In: On Physical Adsorption, Wiley-Interscience, New York. Vannice, M.A., 1975. J. Catal., 37: 449. Anderson, R.B., Hall, W.K. and Hofer, L.J.E., 1948. J. Am. Chem. Sot., 70: 2465. Rosynek, M.P., 1975. J. Phys. Chem., 79: 1280. Brunauer, S. and Emmett, P.H., 1940. J. Am. Chem. Sot., 62: 1732. Collins, A.C. and Trapnell, B.M.W., 1975. Trans Faraday Sot., 53: 1467. Dry, M.E. and Oosthuizen, G.J., 1968. J. Catal., 11: 18. Dry, M.E., Shingles, T., Boshoff, L.J. and Oosthuizen, G.J., 1969. J. Catal., 15: 190.
Fuel Processing Technology, 3 (1980) 63-70 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
HYDROCARBON SYNTHESIS AND CARBON DIOXIDE ADSORPTION ON IRON CATALYSTS
S. PARKASH, S.K. CHAKRABAR’ITY Fuel Sciences Division, Alberta Research Council, I1 315 - 87th Avenue, Edmonton, Alberta, T6G 2C2 (Canada) and MICHAEL P. ROSYNEK Department of Chemistry, Texas A & M University, College Station, Texas, 77843 (U.S.A.) (Received June 28th, 1979; accepted August 2&h, 1979)
ABSTRACT Adsorption isotherms have been measured for carbon dioxide on iron synthetic-NH, catalyst and on low-grade iron ore at six temperatures in the range lOO-350°C and at pressures up to one atmosphere. The results indicate that at least two dissimilar types of adsorbed CO, exist on the surface of the iron synthetic-NH, catalyst, one of which is generated only at temperatures >25O”C. By contrast, only one form of adsorbed CO, was detected on the iron ore catalyst at all temperatures up to 350°C. The kinetics of carbon monoxide hydrogenation are comparable over the two catalysts, and both exhibit virtually identical selectivities for methane formation at equivalent total CO conversions.
INTRODUCTION
As a result of expected declines in world-wide petroleum reserves, renewed interest has been shown during the past several years in the possibility of economically producing hydrocarbons by classical Hz/CO synthesis gas reactions, i.e., methanation, Fischer-Tropsch syntheses, and the isosynthesis. Consequently, catalytic hydrogenation of carbon monoxide has become the topic of considerable recent research [l-4], the primary aim in most cases being the establishment of activity/selectivity behaviors of various supported metals and alloys over an appropriate range of reaction conditions. However, in all such processes, the formation and subsequent adsorption/desorption properties of carbon dioxide may play important roles in governing the overall behavior of the system, particularly as regards hydrocarbon selectivity patterns. Generation of CO2 during HJCO conversions occurs by at least three significant routes, viz., the water-gas-shift reaction: CO + H,O + CO2 + Hz Alberta Research Council Contribution 1002.
(1)
64
disproportionation of CO reactant (the so-called Boudouard reaction) : 2co -+ C(s) + co2
(2)
and as a by-product of hydrocarbon formation: 2n CO + @+I) Hz + C,Hzn+2 + n COz
(3)
and/or 2nCO+nH, +C,H,,+nCOz
(4)
Despite both extensive current research and the existence of a large body of previously-published data on H&O conversions [ 5,6] , however, information about the extents of carbon dioxide adsorption on metal catalysts over a significant range of coverages is lacking. The present investigation was undertaken in an effort to elucidate the markedly different COz adsorption properties exhibited by an iron synthetic-ammonia catalyst and a low-grade iron ore, as well as the comparative catalytic behaviors of these materials for the hydrogenation of carbon monoxide. EXPERIMENTAL
Materials The iron synthetic-NH3 catalyst was kindly provided by Sherrit Gordon Mines, Ltd. of Fort Saskatchewan, Alberta, and the native iron ore was obtained from the Geology Division of the Alberta Research Council. Table 1 TABLE 1 Chemical composition of iron synthetic-NH, catalyst and iron ore Iron synthetic-NH, catalyst
Iron ore
Constituent
wt.%
as
Constituent
Wt.%
as
Iron Silica Alumina Potassium Calcium Nickel Chromium Manganese Zinc
84.9
Fe SiO, AbO, R,O CaO NiO (X0, MnO ZnO
Iron Silica Alumina Potassium Calcium Magnesium Phosphorus Manganese Sulphur Ignition loss
41.4 31.0 6.9 0.8 2.2 1.2 0.8 0.2 0.1 16.4
Fe SiO, AJO, IGO CaO MgO P MnO S H,O, CO,
1
9.2 1.2 0.9 1.1 1.0 0.9 0.8
65
summarizes the chemical compositions of the two catalysts. Prior to each adsorption or reaction experiment, a sample of the desired material was evacuated overnight at 400”Cto a residual pressure of - 10e4 Torr, reductively activated in situ in a rapid (2 l/h) flow of hydrogen for 8 h, and finally re-evacuated for several hours at the same temperature. All gases (- 99.9% purity) were obtained from Matheson of Canada, Ltd. Before use hydrogen and carbon monoxide were further purified by passage through a trap at liquid nitrogen temperature, while research purity carbon dioxide was used without further purification. Apparatus Adsorption isotherms for COZ on the two iron-containing materials were determined using a Micromeritics Co. Model 2100D gas adsorption apparatus. The glass sample vessel attached to the instrument was filled with a weighed amount of catalyst in each case, and then installed in a specially designed and insulated furnace chamber that maintained the sample temperature constant to within +O.l”C. An arbitrary adsorption time of 90 min was allowed for each point on the isotherm. All adsorption calculations employed the weight of the catalyst sample as measured after completion of measurements. Fractional surface coverages (6 ) were estimated from the weight of adsorbed gas by assuming a value of 16.4 A* for the effective area occupied by a CO2 molecule in a filled monolayer [ 71. Catalytic behaviors of the two materials for CO hydrogenation were determined using a fixed-bed integral flow reactor. Rates of formation of methane and carbon dioxide were measured for 2:l H2:C0 reactant ratios at 250°C and at total pressure near or below one atmosphere. RESULT AND DISCUSSION
Catalyst characteristics Experimentally-measured physical characteristics of the two iron catalysts are shown in Table 2. Monolayer coverages of hydrogen were determined from H2 adsorption isotherms at 25”C, using the zero-pressure intercept method described by Vannice [ 81. Corresponding values for carbon monoxide were obtained for each catalyst by measuring the difference in total adsorbate uptake between CO and N2 isotherms at -196°C and at equal relative pressures [ 91. Percentages of exposed metal are baaed on weights of reduced catalysts and were calculated assuming dissociative adsorption of H2 and linearly chemisorbed CO. As seen, results from the two types of adsorption measurements were in good agreement for both catalysts. Investigations by means of X-ray indicate that in the case of iron syntheticNH3 catalyst, the presence of promoters (K20, CaO, A1203) caused no separate
66 TABLE 2 Physical and surface parameters of iron catalysts Parameter
Iron syntheticNH, catalyst
Iron ore
BET-N, surface area (m’lg) N, pore volume (cm’/g) Apparent density* (g/cm”) True density* * (g/cm”) H, monolayer at 25°C (&mole/g) percentage exposed Fe CO monolayer at -196°C (pmole/g) percentage exposed Fe
10.60 0.07 1.49 4.72 15.50 0.20 32.60 0.21
93.80
0.24 0.73 2.92 29.10 0.63 55.30 0.62
* By Hg displacement at 25°C. ** By He displacement at 25°C.
phases which suggest that the promoters formed a solid solution with magnetite. However, this is not exactly the case for iron ore. Carbon dioxide adsorption Figure 1 presents adsorption isotherm obtained for carbon dioxide on samples of the iron ore catalyst at six temperatures in the range lOO-350°C. Total uptakes of CO2 decrease in the expected manner with increasing temperature at all equilibrium pressures up to one atmosphere. Surface coverage by CO2 at the lowest temperature (100°C) appears to asymptotically
Equilibrium pressure
(tow
Fig.1. Adsorption isotherm for CO, on iron ore.
67
approach total saturation of the available metal sites, viz., - 0.6 pmole of exposed iron atoms per m2, with increasing pressure. For the iron synthetic-NH3 catalyst (Fig. Z), isotherms in the temperature range 100-250” C show large initial uptakes of CO2 at very low pressures (
01
0
’
I
loo
200
I I I 300 4 0 0 6 0 0
1 600
1 ]
766
Equilibrium pressure, torr
Fig.2. Adsorption isotherms for CO, on iron synthetic-NH, catalyst.
These results suggest that a least two dissimilar forms of adsorbed CO2 exist on the iron synthetic-NH, catalyst, one of them being generated only at temperatures 2250” C, presumably due to a considerably higher activation energy of adsorption. In this connection, we note that two forms of adsorbed CO2 have been reported to exist on the surface of activated y-alumina at loo-300°C [lo]. Further confirmation of the differing behaviors exhibited by the two catalysts for CO2 adsorption was obtained from isobaric data. Each catalyst
68
- lo- 0 T e m p . i n c r e a s e P 8- l Temp decrease Synthetic- NH, iron
OOIO Temperoture ( ‘Cl
Fig.3. Adsorption isobars at 400 Torr initial pressure for CO, on iron ore and on iron synthetic-NH, catalysts.
was exposed to an initial pressure (- 400 Torr) of CO2 at 25”C, then heated in 50°C increments to 350°C at constant CO2 pressure, and finally cooled in 50°C decrements to lOO”C, a measurement of total CO? uptake being made following equilibration at each temperature. The results are shown for both catalysts in Fig.3. It is evident that the isobar observed for the iron ore sampling during temperature decrease is coincident with that measured at increasing temperature, consistent with the presence of a single, thermally equilibrated, adsorbed species over the entire temperature range investigated. With the iron synthetic-NH3 catalyst, on the other hand, although total COz uptake decreased, as expected, with increasing temperature up to 25O”C, a sharp increase in adsorption, due to formation of a second type of COz surface species, occurred when the temperature was raised to 300°C and then 350” C, in agreement with the isothermal adsorption data for this catalyst shown in Fig.2. This presumably strongly-bound form of adsorbed COz remained on the surface when the sample temperature was subsequently lowered in 50” C steps to 100” C. The small increases in CO2 uptake during temperature decrease are caused by increasing surface concentrations of the first type of adsorbed CO2 that predominated at G25O”C during temperature increase.
69
The large uptake of CO2 (- 29 pmole/m’) by synthetic-NH, catalyst above 250” C corresponds to over two monolayers of CO2 on the entire catalyst surface. It is possible that at these higher temperatures, chemical reactions (CaO + CO2 = CaCO,, AG” = -130 kJ mole-’ and K?O + CO2 = K&OJ, AG” = -460 kJ mole”) may be occurring, leading to carbonate formation*. In chemisorption studies on reduced magnetities by Brunauer and Emmett [ 111 it was assumed that COZ was exclusively adsorbed by the surface alkali molecules. Collins and Trapnell [12] and Dry and coworkers [13,14] have shown that CO2 chemisorbs not only on the oxides of alkali metals but also on the free iron surface as well. We concur with the view [13,14] that large uptake of CO? is not necessarily due to alkalis themselves, but also on the exposed iron sites generated due to the presence of the alkali on the surface, e.g., by altering the chemical potential of the neighboring surface iron atoms, especially when two dissimilar forms of CO* have been observed on the synthetic-NH3 catalyst. Because of the lack of similar observations for iron ore, we believe that in this case potassium and calcium are present as silicates and do not react with carbon dioxide. Carbon monoxide hydrogenation Kinetic data for the hydrogenation of CO over both the iron ore and the iron synthetic-ammonia catalyst are summarized in Table 3. The data presented were obtained under steady-state conditions for the near-atmospheric pressure reaction at 250” C. It is evident from the results in Table 3 that, at equivalent total CO conversions, methane turnover frequencies at 250“ C were comparable and selectivities for CH4 formation were virtually identical (- 19%) over both catalysts. Under these conditions, however, the amount of COZ appearing in the gaseous reaction products was much less (35% vs. 60% of all CO converted) with the iron synthetic-NH, catalyst than with the iron ore. The fact that the two catalysts demonstrated different adsorption behaviors for CO2 without affecting the synthesis kinetics strongly suggest that the COZ adsorption sites are not the reaction sites. Some reversible restructuring of the synthetic-NH3 catalyst is more than likely taking place at temperatures > 250” C, which manifests itself in a change in the kinetics of COZ adsorption. An alternate possibility is just that adsorbed COZ does not affect the reaction kinetics on the argument that both HZ and CO chemisorb much more strongly than CO* and thereby displace CO, from the surface when all three gases are present. ACKNOWLEDGEMENT
One of the authors (M.P.R.) gratefully acknowledges financial support by the Robert A. Welch Foundation. *Theoretically the amount of CO, needed for complete conversion of present in synthetic-NH, catalyst to carbonates is - 27 pmole/m’.
CaO and K,O
70 TABLE 3 Kinetic parameters for CO hydrogenation over iron catalysts* Catalyst
% co conversion
% Selectivity to CO,
CH,
Rate of CH, formation bmolelgls)
Methane turnover frequency (X 10’ s-1)
Apparent activation energy for CH, formation (kJ/mole)
Synthetic-NH, iron catalyst
41.4
35.0
19.3
0.16
5.2
88
Iron ore
40.1
60.1
18.7
0.17
3.0
75
*Reaction conditions: H,:CO = 2:l; T = 250°C;~ = 700 Torr; VHSV = 3000 h-l.
REFERENCES
10 11 12 13 14
Mills, G.A. and Steffgen, I.W., 1973. Catal. Rev. Sci. Eng., 8: 159. Vannice, M.A., 1976. Catal. Rev. Sci. Eng., 14: 153. Dry, M.E., 1976. Ind. Eng. Chem. Prod. Res. Dev., 15: 282. Vannice, M.A., 1977. J. Catal., 59: 228. Starch, H.H., Golumbic, N. and Anderson, R.B., 1951. The Fischer+‘ropsch and Related Syntheses, Wiley-Interscience, New York. Anderson, R.B., 1956. In: P.H. Emmett (Ed.), Catalysis, Vol. IV, Reinhold, New York. Ross, S. and Oliver, J.P., 1964. In: On Physical Adsorption, Wiley-Interscience, New York. Vannice, M.A., 1975. J. Catal., 37: 449. Anderson, R.B., Hall, W.K. and Hofer, L.J.E., 1948. J. Am. Chem. Sot., 70: 2465. Rosynek, M.P., 1975. J. Phys. Chem., 79: 1280. Brunauer, S. and Emmett, P.H., 1940. J. Am. Chem. Sot., 62: 1732. Collins, A.C. and Trapnell, B.M.W., 1975. Trans Faraday Sot., 53: 1467. Dry, M.E. and Oosthuizen, G.J., 1968. J. Catal., 11: 18. Dry, M.E., Shingles, T., Boshoff, L.J. and Oosthuizen, G.J., 1969. J. Catal., 15: 190.