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Effects of Metal Particle Size and Carbon Fouling on the Rate of Heptane Oxidation Over Platinum
R.K. Crasselli and A.W. Sleight (Editors), Structure-Actiuity and Selectiuity Relationships in Heterogeneous Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam
127
EFFECTS OF METAL PARTICLE SIZE AND CARBON FOULING ON THE RATE OF HEPTANE OXIDATION OVER PLATINUM R.F. HICKS, R.G. LEE, W.J. HAN AND A.B. KOOH Chemical Engineering Department, University of California, Los Angeles, CA, 90024- 1592 ABSTRACT A series of platinum catalysts, with metal dispersions ranging from 10 to 81%, have been tested for heptane oxidation in 5% excess oxygen and at temperatures between 90 and 140OC. The rate of reaction declines exponentially with time because carbon fouls the metal surface. The deactivation rate is first order in the concentration of active sites. The rate of coke formation depends on the metal particle size, the support composition, and the density of metal particles on the support. Small particles are inactive for carbon deposition. On large particles, the amount of carbon deposited per surface platinum atom is higher on alumina than on zirconia, and increases as the number of particles decreases. Most of this carbon resides on the support. The rate of catalyst deactivation follows a trend opposite to that of coke formation. Small particles deactivate very slowly. On large particles, the deactivation rate is higher on zirconia than on alumina, and increases as the number of particles increases. Evidently, the faster the metal transfers carbon to the support, the slower it deactivates. The turnover frequency for heptane oxidation on sites not fouled by carbon (A sites) depends on metal particle size. On average, large crystallites are 23 times more active than small ones. At long reaction times, the turnover frequency for heptane oxidation is obscured by carbon fouling, and the rate appears insensitive to particle size. INTRODUCTION Catalytic oxidation is used extensively to reduce hydrocarbon emissions from automobiles
(1,2) and industrial processes (3). However, there have been few published studies of the catalytic oxidation of hydrocarbons other than methane. Prior research on C,C, alkane oxidation has shown that the rate of reaction depends on catalyst snucture (4-8). Platinum is more active than palladium, and large metal crystallites are more active than small ones. Nevertheless, it is difficult to tell the magnitude of these effects, because the rates were often measured at high conversion, where transport resistances may have influenced the results. Also, the effect of reaction time on the rate was not considered.
128
In this paper, we report the results of our study of heptane oxidation over supported platinum.
Samples were prepared using alumina and zirconia as supports and with metal
dispersions ranging from 10 to 81%. The inmnsic rate of reaction was measured as a function of time at low temperature in 5% excess oxygen and at low conversion. We found that the rates fell rapidly with time because carbon fouls the catalyst. The effect of the metal particle size on the rate of heptane oxidation and the rate of deactivation is described below. A more complete analysis of our results is presented elsewhere (9). EXPERIMENTAL The samples used in this study and their method of preparation are shown in Table 1. The supports were Degussa, flame-synthesized aluminum oxide "C" and zirconia. The alumina was calcined at lO00"C for 24 h and had a surface area of 83 m2/g. The zirconia was calcined at 600°C for 24 h and had a surface area of 40 m2/g. The metal was deposited by ion exchange on sample
a and by incipient-wetness impregnation on samples b throughf. The samples were dried at 125°C for 2 h, then calcined as shown in the Table. Metal loadings were determined by inductively coupled plasma emission spectroscopy.
The platinum dispersion was measured by hydrogen
titration of preadsorbed oxygen at 25"C, assuming an adsorption stoichiometry of 1.5 H, per PtO, (10).
TABLE 1 Physical characteristics of the platinum catalysts. Sample designation
a b C
d e
f
Support
no2 no2
no2 A1203 -41203
Metal salt
Pt(NH,),Cl, Pt(NH3),Cl2 Pt(NH,),Cl, Pt(NH,),Cl, H2PtC1, H,PtCl,
Calcined 2 h in air at ("C)
Metal loading
500 500 500 500 600 600
0.4 0.3 5.0 0.3 0.8 5.0
(%)
Dispersion (96)
81 56 19 58 13
10
The rate of heptane oxidation was determined in a fixed-bed microreactor, equipped with on-line gas chromatography. Carbon dioxide was the only reaction product. The method of testing the catalysts was as follows. Between 0.1 to 0.7 g of sample (32-60 mesh pellets) was loaded into
a 4 mm-LD. glass tube, oxidized in 50 cm3/min oxygen at 500°C for 20 min, and reduced in 50 cm3/min hydrogen at 300°C for 20 min. Then the sample was cooled to the reaction temperature
129
and the feed switched to 25 Torr heptane, 282 Torr oxygen and 918 Tom helium at 235 cm3/min (1 Torr = 133 N/m*). The temperature and the amount of catalyst were adjusted to keep the conversion below 1%. The reaction was continued for 4 to 16 h. At the end of this period, the feed was switched to 30 cm3/min helium, and the reactor was cooled to 60°C. The number of metal atoms exposed was determined by hydrogen-oxygen titration and carbon monoxide adsorption. On some samples, the amount of carbon deposited during reaction was determined by recording the carbon dioxide evolution upon heating in oxygen. In this case, the sample was cooled to 50°C in
50 cm3/min helium. Then 22 cm3/min oxygen and 120 cm3/min helium was introduced, and the sample was heated at 5"C/min to 500°C. The amount of carbon dioxide evolved was measured every 3 min with the gas chromatograph. RESULTS Shown in Fig. 1 is the effect of time on the turnover frequencies for heptane oxidation over five platinum catalysts. The turnover frequencies decrease exponentially with time.
c
4
'
lO4O
100
200 300 Time (rnin)
400
500 600
Fig. 1. The dependence of the turnover frequency on time at 90°C for samples b (W), c e (O),andf(0).
(A),
d
(A),
Over samples b, d and e, the slope of the line of log (turnover frequency) versus time remains constant over the whole run. However, over samples c andf, the slope of the line drops after a certain period of time. The decay of the rate with time can be attributed to poisoning of the active sites.
130
These sites are lost at a rate proportional to the number present, ie., first-order deactivation (11,12): d8 dt
- -k8,
which upon integration yields,
8
- exp(-kt),
PI
where 8 is the fraction of active sites not poisoned, t is time (s), and k is the apparent rate constant for deactivation (s-'). The change in the slope of the line of log (turnover frequency) versus time, observed for some samples, suggests that there are two sites of high and low intrinsic activity: TOFo,
- TOF,8,
+ TOF,8,,
where TOF is the turnover frequency (dl), and the subscripts obs, A and B refer to observed, A site and B site, respectively. The A site may be thought of as a site on the "clean" platinum surface, whiIe the B site may be thought of as a site on the poisoned platinum surface. Substitution of Eq. 2 into Eq. 3 yields:
-
TOPObS TOF,exp(-k,t)
+ TOF,exp(-k,t).
The curves drawn through the data points in Fig. 1 are the best fit of Eq. 4 to the results. Shown in Table 2 are the values of the turnover frequencies and rate constants for deactivation of the A and B sites, which are obtained from fitting Eq. 4 to the rate data. The turnover frequency of the A site varies over a wide range, from 0.06 ~ 1 0s". ~for sample d to 3.2 xlO-' 8' for sample e. The change in the specific activity of the A site correlates with the platinum particle size. Samples containing small metal particles, ie., dispersions greater than 50%, exhibit low activity, while samples containing large metal particles, ie., dispersions less than 20%, exhibit high activity. The influence of temperature on the reaction rate was determined on samples b andf. The apparent activations energies for heptane oxidation on samples b and f are 20 f 4 and 19 f 2.5
kcaVmole (1 kcal = 4.186 kJ) (9). These activation energies are the same within experimental error.
This indicates that the large difference in specific activity of the samples is not due to a difference in the reaction mechanism. Instead, the specific rate of heptane oxidation on the A site increases with platinum particle size, because the large particles contain a greater fraction of active sites than the small ones.
131
TABLE 2 Rates of heptane oxidation and deactivation over the platinum catalysts.
Sample designation
Composition
Initial dispersion
Turnover frequency ( X ~ O ~ ~ at S -90°C ')
Deactivation rate (xlOd s-') at 9OoC
(%)
U
b C
d e
f
0.4% Pt/ZrO, 0.3% Pt/ZrOz 5.0% Pt/zro, 0.3% Pt/Al,O, 0.8% Pt/A1203 5.0% Pt/Al,O,
81 56 19 58 13 10
TOF,
TOF,
k*
k,
0.07" 0.14 1.10 0.06 3.20 2.10
0.05" 0.00 0.14 0.00 0.00 0.24
0.41' 0.07 11.00 0.53 0.38 3.20
-0.02"
~~
1.10
0.16 ~~
Torrected from a reaction temperature of 14OOC to 90°C using an activation energy of 19 kcal/mole for the turnover frequency and 11 kcal/niole for the deactivation rate (9). The turnover frequency of the B site does not change nearly as much as the turnover frequency of the A site.
There is a fivefold increase in the B-site activity as the platinum
dispersion decreases from 81 to 10%. Also, the turnover frequency of the B site is approximately equal to the turnover frequency of the A site on the small platinum particles. At long reaction times, the observed rate on small particles is close to the turnover frequency of the A site, while the observed rate on large particles is close to the turnover frequency of the B site. Since these turnover frequencies are nearly equal, the reaction rate at long times appears to be insensitive to the platinum particle size. The rate constants for deactivation of the A and B sites vary widely over samples u through
f. On the samples with platinum dispersion greater than 50%, the rate constant for deactivation of the A site is low. Whereas, on the samples with platinum dispersions less than 20%, the rate constant for deactivation of the A site depends on the metal loading and support composition. The deactivation rate constant increases 8.4 times as the amount of metal on the alumina increases from 0.8 to 5.0% at constant dispersion (samples e and fi. For a metal loading of 5%, switching from
an alumina to a zirconia support increases the deactivation rate constant by 3 times (samplesfand
c). These data indicate that the deactivation rate is sensitive to the platinum particle size, the nature of the support and the amount of platinum in contact with it. Carbon deposits on the platinum samples during heptane oxidation. Shown in Table 3 is the amount of carbon deposited as determined by temperature-programmed oxidation before and
132
after reaction. Before reaction, the samples were oxidized at 500°C and reduced at 300 C three times before recording the TPO spectrum. The reaction temperature was 9O"C, and the reaction
time was 4 h for the platinum catalysts and 2 h for the supports. The alumina and zirconia supports accumulate a significant quantity of carbon, but do not convert any heptane to carbon dioxide during exposure to heptane oxidation. The amount of carbon oxidized off the supports after reaction equals 7.5 xlO-' mole/g. The extent of carbon deposition on the platinum catalysts depends on the metal particle size. On samples b and d, with dispersions of 56 and 58%, the amount of carbon deposited during reaction is not much greater than that deposited on the supports. By contrast, on samples c, e andf, with dispersions between 10 and 19%, the carbon deposited during reaction is ten times greater than that deposited on the supports. Evidently, small platinum crystallites are inactive for the conversion of heptane into coke. As shown above, small crystallites are also inactive for the conversion of heptane into carbon dioxide. TABLE 3 The amount of carbon deposited on the platinum catalysts.
Sample designation
b C
d e
f
Composition
Initial dispersion
(%I
0.3% Pt/zro, 5.0% Pt/zro, 0.3% Pt/A1,0, 0.8% Pt/A1203 5.0% Pt/A1,03
zro2
'41203
56 19 58 13 10
Metal exposed (xio-' moIes/g)
0.9 4.9 0.9
0.5 2.6
Carbon oxidized (x104 mole/g) Before reaction
After reaction
0.3 1.1 0.2 0.0 1.1
2.4 6.4 1.o 8.9 11.6
0.3
0.3
0.8
0.7
In Table 3, the moles of metal exposed on each sample is compared to the amount of carbon deposited. In every case, the carbon accumulated during reaction greatly exceeds the amount of metal exposed. Most of this carbon is probably on the support. On the three active platinum catalysts, samples c, e and$ the mole ratio of carbon to surface platinum equals 13, 178 and 45, respectively. The mole ratio of carbon to surface platinum is higher on Pt/A120, than on PtErO,, and it increases as the platinum loading goes down. These ratios may be compared to the rate constants for deactivation of the A sites, which are 11.00, 0.38 and 3.20 x104 s-', respectively. These rate constants follow an inverse correlation with the mole ratio of carbon to surface platinum.
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This correlation can be rationalized as follows. Carbon produced on the metal migrates to the support. The surface diffusion of carbon is consistent with an observed square-root dependence of the amount of carbon deposited on reaction time (9). If the rate of carbon migration to the support is high, as evidenced by a high mole ratio of carbon to platinum exposed, then deactivation is slow. Conversely, if the rate of carbon migration is slow, as evidenced by a low mole ratio of carbon to platinum exposed, then deactivation is fast. The rate of carbon migration appears to be faster on alumina than on zirconia, and increases as the number of platinum particles on the support decreases. Titration of the catalysts after reaction indicates that the metal surfaces are covered by carbon and a small amount of oxygen (9). Between 30 and 100% of the carbon on the metal is displaced by hydrogen and carbon monoxide. The fraction displaced increases with decreasing particle size. After oxidation at 500°C and reduction at 3OO0C, the adsorption capacities of all the catalyst samples recover to their initial values. The number of sites which can be titrated with hydrogen, oxygen and carbon monoxide does not change with reaction time. This is true in spite of the fact that carbon continuously builds up throughout the run. The ability of hydrogen and
carbon monoxide to displace the carbon from the platinum suggests that only one layer of carbon sits on the metal surface. This carbon is a small fraction of the total amount deposited on the catalyst. Most of the carbon migrates to the support. The distribution of carbon and oxygen on the platinum surface may depend on the particle size and the reaction time.
However, this
distribution could not be determined by the titration experiments, because uncontrolled amounts of heptane and oxygen were exposed to the catalyst upon switching from reaction to gas-pulsing (9).
DISCUSSION The deactivation of the catalysts is most likely due to carbon fouling of the metal particles. First-order deactivation, Eq. 1, is consistent with the surface sites becoming progressively covered by some species during reaction (12). Titration of the samples after reaction reveals that the surface of the platinum crystallites is covered with carbon. Once the carbon has been removed by oxidation and reduction, the adsorption capacity of the platinum recovers to its initial value, and so does the rate of heptane oxidation. The rate of carbon deposition depends on the metal particle size. Large platinum crystallites are active for converting heptane into coke, whereas small platinum crystallites are not. The rate of coke deposition during hydrocarbon reforming over platinum also increases with increasing particle size (13). However, both large and small particles deposit significant amounts of carbon during reforming. On the large crystallites, the rate of carbon accumulation per exposed metal atom depends on the support composition and the number of metal particles in contact with the support.
134
This is because coke formation is a self-poisoning reaction. Carbon remaining on the metal surface blocks the sites for converting heptane into coke. However, if the support contains sites for carbon adsorption, then the carbon can migrate to these sites, and free up the metal surface for further reaction. Decreasing the number of metal particles, increases the number of support adsorption sites relative to the metal atoms exposed, and increases the rate of carbon migration from the metal to the support. The rate of deactivation of the catalysts also depends on metal particle size, support composition, and the number of metal particles in contact with the support. Small platinum crystallites are inactive for carbon deposition and consequently, deactivate slowly. Large platinum crystallites are active for carbon deposition, and in this case, the rate of deactivation depends on how fast the carbon migrates to the support. On sample e, the platinum deactivates slowly because the low concentrations of metal on the alumina promotes a rapid rate of carbon diffusion to the support. Conversely, on sample c, the platinum deactivates quickly because the high concentration of metal on the zirconia promotes a slow rate of carbon diffusion to the support. These results have important implications for the design of hydrocarbon oxidation catalysts. To improve the resistance of the catalyst to carbon fouling, the surface area of the support should be increased as much as possible, and the support should contain a maximum number of carbon adsorption sites. The turnover frequency for heptane oxidation on the A site (ie., the surface not fouled by carbon) is moderately affected by the platinum particle size. Catalysts with dispersions below 20% are on average 23 times more active than catalysts with dispersions above 50%. These results qualitatively agree with other studies of hydrocarbon oxidation over platinum (4,s). The rate of heptane oxidation and the rate of coke deposition show similar dependencies on platinum crystallite size. This suggests that these products come from common intermediates on the metal surface. The slow step in both mechanisms may be the reaction of adsorbed oxygen with adsorbed carbon. Both carbon and oxygen are present on the metal surface during reaction. The rate law for hydrocarbon oxidation is zero order in hydrocarbon and oxygen partial pressures, and is consistent with a surface reaction between carbon and oxygen (5,9). Also, very little carbon accumulates on the platinum catalysts when they are exposed to heptane at 90°C and no oxygen. Thus, the effect of particle size on the rates of heptane conversion to carbon dioxide and coke may be to alter the distribution of carbon and oxygen on the metal surface. The dependence of the rate of heptane oxidation on the platinum particle size is obscured by carbon fouling. At long reaction times, the observed turnover frequency is close to the turnover frequency of the B site. The B-site activity is not very sensitive to particle size, increasing by a factor of five as the dispersion decreases from 81 to 10%. These results show that the deactivation kinetics must be accounted for in order to correctly interpret the effect of catalyst structure on the
135
hydrocarbon oxidation rate. ACKNOWLEDGEMENT This work was supported by the National Science Foundation Engineering Research Center for Hazardous Substance Control at UCLA. REFERENCES
7 8 9 10 11 12 13
J. Wei, Advan. Catal., 24 (1975) 57. G. Kim, Ind. Eng. Chem. Prod. Res. Dev., 21 (1982) 267. J.J. Spivey, Ind. Eng. Chem. Res., 26 (1987) 2165. M.L. Carballo and E.E. Wolf, J. Catal., 53 (1978) 366. Y.F. Yu Yao, Ind. Eng. Chem. Prod. Res. Dev., 19 (1980) 293. V.A. Drozdov, P.G. Tsyrulnikov, V.V. Popovskii, N.N. Bulgakov, E.M. Moroz and T.G. Galeev, React. Kinet. Catal. Lett., 27 (1985) 425. J. Volter, G. Lietz, H. Spindler and H. Lieske, J. Catal., 104 (1987) 375. S.K. Gangwal, M.E. Mullins, J.J. Spivey and P.R. Caffrey, Appl. Catal., 36 (1988) 231. A.B. Kooh, W.J. Han, R.G. Lee and R.F. Hicks, (J. Catal.) submitted for publication. J.E. Benson and M. Boudart, J. Catal., 4 (1965) 704. 0. Levenspiel, Chemical Reaction Engineering”, Wiley, New York, 1972. J.B. Butt and E.E. Petersen, Activation, Deactivation and Poisoning of Catalysts, Academic Press, San Diego, 1988. J. Barbier, Appl. Catal., 23 (1986) 225.
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EFFECTS OF METAL PARTICLE SIZE AND CARBON FOULING ON THE RATE OF HEPTANE OXIDATION OVER PLATINUM R.F. HICKS, R.G. LEE, W.J. HAN AND A.B. KOOH Chemical Engineering Department, University of California, Los Angeles, CA, 90024- 1592 ABSTRACT A series of platinum catalysts, with metal dispersions ranging from 10 to 81%, have been tested for heptane oxidation in 5% excess oxygen and at temperatures between 90 and 140OC. The rate of reaction declines exponentially with time because carbon fouls the metal surface. The deactivation rate is first order in the concentration of active sites. The rate of coke formation depends on the metal particle size, the support composition, and the density of metal particles on the support. Small particles are inactive for carbon deposition. On large particles, the amount of carbon deposited per surface platinum atom is higher on alumina than on zirconia, and increases as the number of particles decreases. Most of this carbon resides on the support. The rate of catalyst deactivation follows a trend opposite to that of coke formation. Small particles deactivate very slowly. On large particles, the deactivation rate is higher on zirconia than on alumina, and increases as the number of particles increases. Evidently, the faster the metal transfers carbon to the support, the slower it deactivates. The turnover frequency for heptane oxidation on sites not fouled by carbon (A sites) depends on metal particle size. On average, large crystallites are 23 times more active than small ones. At long reaction times, the turnover frequency for heptane oxidation is obscured by carbon fouling, and the rate appears insensitive to particle size. INTRODUCTION Catalytic oxidation is used extensively to reduce hydrocarbon emissions from automobiles
(1,2) and industrial processes (3). However, there have been few published studies of the catalytic oxidation of hydrocarbons other than methane. Prior research on C,C, alkane oxidation has shown that the rate of reaction depends on catalyst snucture (4-8). Platinum is more active than palladium, and large metal crystallites are more active than small ones. Nevertheless, it is difficult to tell the magnitude of these effects, because the rates were often measured at high conversion, where transport resistances may have influenced the results. Also, the effect of reaction time on the rate was not considered.
128
In this paper, we report the results of our study of heptane oxidation over supported platinum.
Samples were prepared using alumina and zirconia as supports and with metal
dispersions ranging from 10 to 81%. The inmnsic rate of reaction was measured as a function of time at low temperature in 5% excess oxygen and at low conversion. We found that the rates fell rapidly with time because carbon fouls the catalyst. The effect of the metal particle size on the rate of heptane oxidation and the rate of deactivation is described below. A more complete analysis of our results is presented elsewhere (9). EXPERIMENTAL The samples used in this study and their method of preparation are shown in Table 1. The supports were Degussa, flame-synthesized aluminum oxide "C" and zirconia. The alumina was calcined at lO00"C for 24 h and had a surface area of 83 m2/g. The zirconia was calcined at 600°C for 24 h and had a surface area of 40 m2/g. The metal was deposited by ion exchange on sample
a and by incipient-wetness impregnation on samples b throughf. The samples were dried at 125°C for 2 h, then calcined as shown in the Table. Metal loadings were determined by inductively coupled plasma emission spectroscopy.
The platinum dispersion was measured by hydrogen
titration of preadsorbed oxygen at 25"C, assuming an adsorption stoichiometry of 1.5 H, per PtO, (10).
TABLE 1 Physical characteristics of the platinum catalysts. Sample designation
a b C
d e
f
Support
no2 no2
no2 A1203 -41203
Metal salt
Pt(NH,),Cl, Pt(NH3),Cl2 Pt(NH,),Cl, Pt(NH,),Cl, H2PtC1, H,PtCl,
Calcined 2 h in air at ("C)
Metal loading
500 500 500 500 600 600
0.4 0.3 5.0 0.3 0.8 5.0
(%)
Dispersion (96)
81 56 19 58 13
10
The rate of heptane oxidation was determined in a fixed-bed microreactor, equipped with on-line gas chromatography. Carbon dioxide was the only reaction product. The method of testing the catalysts was as follows. Between 0.1 to 0.7 g of sample (32-60 mesh pellets) was loaded into
a 4 mm-LD. glass tube, oxidized in 50 cm3/min oxygen at 500°C for 20 min, and reduced in 50 cm3/min hydrogen at 300°C for 20 min. Then the sample was cooled to the reaction temperature
129
and the feed switched to 25 Torr heptane, 282 Torr oxygen and 918 Tom helium at 235 cm3/min (1 Torr = 133 N/m*). The temperature and the amount of catalyst were adjusted to keep the conversion below 1%. The reaction was continued for 4 to 16 h. At the end of this period, the feed was switched to 30 cm3/min helium, and the reactor was cooled to 60°C. The number of metal atoms exposed was determined by hydrogen-oxygen titration and carbon monoxide adsorption. On some samples, the amount of carbon deposited during reaction was determined by recording the carbon dioxide evolution upon heating in oxygen. In this case, the sample was cooled to 50°C in
50 cm3/min helium. Then 22 cm3/min oxygen and 120 cm3/min helium was introduced, and the sample was heated at 5"C/min to 500°C. The amount of carbon dioxide evolved was measured every 3 min with the gas chromatograph. RESULTS Shown in Fig. 1 is the effect of time on the turnover frequencies for heptane oxidation over five platinum catalysts. The turnover frequencies decrease exponentially with time.
c
4
'
lO4O
100
200 300 Time (rnin)
400
500 600
Fig. 1. The dependence of the turnover frequency on time at 90°C for samples b (W), c e (O),andf(0).
(A),
d
(A),
Over samples b, d and e, the slope of the line of log (turnover frequency) versus time remains constant over the whole run. However, over samples c andf, the slope of the line drops after a certain period of time. The decay of the rate with time can be attributed to poisoning of the active sites.
130
These sites are lost at a rate proportional to the number present, ie., first-order deactivation (11,12): d8 dt
- -k8,
which upon integration yields,
8
- exp(-kt),
PI
where 8 is the fraction of active sites not poisoned, t is time (s), and k is the apparent rate constant for deactivation (s-'). The change in the slope of the line of log (turnover frequency) versus time, observed for some samples, suggests that there are two sites of high and low intrinsic activity: TOFo,
- TOF,8,
+ TOF,8,,
where TOF is the turnover frequency (dl), and the subscripts obs, A and B refer to observed, A site and B site, respectively. The A site may be thought of as a site on the "clean" platinum surface, whiIe the B site may be thought of as a site on the poisoned platinum surface. Substitution of Eq. 2 into Eq. 3 yields:
-
TOPObS TOF,exp(-k,t)
+ TOF,exp(-k,t).
The curves drawn through the data points in Fig. 1 are the best fit of Eq. 4 to the results. Shown in Table 2 are the values of the turnover frequencies and rate constants for deactivation of the A and B sites, which are obtained from fitting Eq. 4 to the rate data. The turnover frequency of the A site varies over a wide range, from 0.06 ~ 1 0s". ~for sample d to 3.2 xlO-' 8' for sample e. The change in the specific activity of the A site correlates with the platinum particle size. Samples containing small metal particles, ie., dispersions greater than 50%, exhibit low activity, while samples containing large metal particles, ie., dispersions less than 20%, exhibit high activity. The influence of temperature on the reaction rate was determined on samples b andf. The apparent activations energies for heptane oxidation on samples b and f are 20 f 4 and 19 f 2.5
kcaVmole (1 kcal = 4.186 kJ) (9). These activation energies are the same within experimental error.
This indicates that the large difference in specific activity of the samples is not due to a difference in the reaction mechanism. Instead, the specific rate of heptane oxidation on the A site increases with platinum particle size, because the large particles contain a greater fraction of active sites than the small ones.
131
TABLE 2 Rates of heptane oxidation and deactivation over the platinum catalysts.
Sample designation
Composition
Initial dispersion
Turnover frequency ( X ~ O ~ ~ at S -90°C ')
Deactivation rate (xlOd s-') at 9OoC
(%)
U
b C
d e
f
0.4% Pt/ZrO, 0.3% Pt/ZrOz 5.0% Pt/zro, 0.3% Pt/Al,O, 0.8% Pt/A1203 5.0% Pt/Al,O,
81 56 19 58 13 10
TOF,
TOF,
k*
k,
0.07" 0.14 1.10 0.06 3.20 2.10
0.05" 0.00 0.14 0.00 0.00 0.24
0.41' 0.07 11.00 0.53 0.38 3.20
-0.02"
~~
1.10
0.16 ~~
Torrected from a reaction temperature of 14OOC to 90°C using an activation energy of 19 kcal/mole for the turnover frequency and 11 kcal/niole for the deactivation rate (9). The turnover frequency of the B site does not change nearly as much as the turnover frequency of the A site.
There is a fivefold increase in the B-site activity as the platinum
dispersion decreases from 81 to 10%. Also, the turnover frequency of the B site is approximately equal to the turnover frequency of the A site on the small platinum particles. At long reaction times, the observed rate on small particles is close to the turnover frequency of the A site, while the observed rate on large particles is close to the turnover frequency of the B site. Since these turnover frequencies are nearly equal, the reaction rate at long times appears to be insensitive to the platinum particle size. The rate constants for deactivation of the A and B sites vary widely over samples u through
f. On the samples with platinum dispersion greater than 50%, the rate constant for deactivation of the A site is low. Whereas, on the samples with platinum dispersions less than 20%, the rate constant for deactivation of the A site depends on the metal loading and support composition. The deactivation rate constant increases 8.4 times as the amount of metal on the alumina increases from 0.8 to 5.0% at constant dispersion (samples e and fi. For a metal loading of 5%, switching from
an alumina to a zirconia support increases the deactivation rate constant by 3 times (samplesfand
c). These data indicate that the deactivation rate is sensitive to the platinum particle size, the nature of the support and the amount of platinum in contact with it. Carbon deposits on the platinum samples during heptane oxidation. Shown in Table 3 is the amount of carbon deposited as determined by temperature-programmed oxidation before and
132
after reaction. Before reaction, the samples were oxidized at 500°C and reduced at 300 C three times before recording the TPO spectrum. The reaction temperature was 9O"C, and the reaction
time was 4 h for the platinum catalysts and 2 h for the supports. The alumina and zirconia supports accumulate a significant quantity of carbon, but do not convert any heptane to carbon dioxide during exposure to heptane oxidation. The amount of carbon oxidized off the supports after reaction equals 7.5 xlO-' mole/g. The extent of carbon deposition on the platinum catalysts depends on the metal particle size. On samples b and d, with dispersions of 56 and 58%, the amount of carbon deposited during reaction is not much greater than that deposited on the supports. By contrast, on samples c, e andf, with dispersions between 10 and 19%, the carbon deposited during reaction is ten times greater than that deposited on the supports. Evidently, small platinum crystallites are inactive for the conversion of heptane into coke. As shown above, small crystallites are also inactive for the conversion of heptane into carbon dioxide. TABLE 3 The amount of carbon deposited on the platinum catalysts.
Sample designation
b C
d e
f
Composition
Initial dispersion
(%I
0.3% Pt/zro, 5.0% Pt/zro, 0.3% Pt/A1,0, 0.8% Pt/A1203 5.0% Pt/A1,03
zro2
'41203
56 19 58 13 10
Metal exposed (xio-' moIes/g)
0.9 4.9 0.9
0.5 2.6
Carbon oxidized (x104 mole/g) Before reaction
After reaction
0.3 1.1 0.2 0.0 1.1
2.4 6.4 1.o 8.9 11.6
0.3
0.3
0.8
0.7
In Table 3, the moles of metal exposed on each sample is compared to the amount of carbon deposited. In every case, the carbon accumulated during reaction greatly exceeds the amount of metal exposed. Most of this carbon is probably on the support. On the three active platinum catalysts, samples c, e and$ the mole ratio of carbon to surface platinum equals 13, 178 and 45, respectively. The mole ratio of carbon to surface platinum is higher on Pt/A120, than on PtErO,, and it increases as the platinum loading goes down. These ratios may be compared to the rate constants for deactivation of the A sites, which are 11.00, 0.38 and 3.20 x104 s-', respectively. These rate constants follow an inverse correlation with the mole ratio of carbon to surface platinum.
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This correlation can be rationalized as follows. Carbon produced on the metal migrates to the support. The surface diffusion of carbon is consistent with an observed square-root dependence of the amount of carbon deposited on reaction time (9). If the rate of carbon migration to the support is high, as evidenced by a high mole ratio of carbon to platinum exposed, then deactivation is slow. Conversely, if the rate of carbon migration is slow, as evidenced by a low mole ratio of carbon to platinum exposed, then deactivation is fast. The rate of carbon migration appears to be faster on alumina than on zirconia, and increases as the number of platinum particles on the support decreases. Titration of the catalysts after reaction indicates that the metal surfaces are covered by carbon and a small amount of oxygen (9). Between 30 and 100% of the carbon on the metal is displaced by hydrogen and carbon monoxide. The fraction displaced increases with decreasing particle size. After oxidation at 500°C and reduction at 3OO0C, the adsorption capacities of all the catalyst samples recover to their initial values. The number of sites which can be titrated with hydrogen, oxygen and carbon monoxide does not change with reaction time. This is true in spite of the fact that carbon continuously builds up throughout the run. The ability of hydrogen and
carbon monoxide to displace the carbon from the platinum suggests that only one layer of carbon sits on the metal surface. This carbon is a small fraction of the total amount deposited on the catalyst. Most of the carbon migrates to the support. The distribution of carbon and oxygen on the platinum surface may depend on the particle size and the reaction time.
However, this
distribution could not be determined by the titration experiments, because uncontrolled amounts of heptane and oxygen were exposed to the catalyst upon switching from reaction to gas-pulsing (9).
DISCUSSION The deactivation of the catalysts is most likely due to carbon fouling of the metal particles. First-order deactivation, Eq. 1, is consistent with the surface sites becoming progressively covered by some species during reaction (12). Titration of the samples after reaction reveals that the surface of the platinum crystallites is covered with carbon. Once the carbon has been removed by oxidation and reduction, the adsorption capacity of the platinum recovers to its initial value, and so does the rate of heptane oxidation. The rate of carbon deposition depends on the metal particle size. Large platinum crystallites are active for converting heptane into coke, whereas small platinum crystallites are not. The rate of coke deposition during hydrocarbon reforming over platinum also increases with increasing particle size (13). However, both large and small particles deposit significant amounts of carbon during reforming. On the large crystallites, the rate of carbon accumulation per exposed metal atom depends on the support composition and the number of metal particles in contact with the support.
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This is because coke formation is a self-poisoning reaction. Carbon remaining on the metal surface blocks the sites for converting heptane into coke. However, if the support contains sites for carbon adsorption, then the carbon can migrate to these sites, and free up the metal surface for further reaction. Decreasing the number of metal particles, increases the number of support adsorption sites relative to the metal atoms exposed, and increases the rate of carbon migration from the metal to the support. The rate of deactivation of the catalysts also depends on metal particle size, support composition, and the number of metal particles in contact with the support. Small platinum crystallites are inactive for carbon deposition and consequently, deactivate slowly. Large platinum crystallites are active for carbon deposition, and in this case, the rate of deactivation depends on how fast the carbon migrates to the support. On sample e, the platinum deactivates slowly because the low concentrations of metal on the alumina promotes a rapid rate of carbon diffusion to the support. Conversely, on sample c, the platinum deactivates quickly because the high concentration of metal on the zirconia promotes a slow rate of carbon diffusion to the support. These results have important implications for the design of hydrocarbon oxidation catalysts. To improve the resistance of the catalyst to carbon fouling, the surface area of the support should be increased as much as possible, and the support should contain a maximum number of carbon adsorption sites. The turnover frequency for heptane oxidation on the A site (ie., the surface not fouled by carbon) is moderately affected by the platinum particle size. Catalysts with dispersions below 20% are on average 23 times more active than catalysts with dispersions above 50%. These results qualitatively agree with other studies of hydrocarbon oxidation over platinum (4,s). The rate of heptane oxidation and the rate of coke deposition show similar dependencies on platinum crystallite size. This suggests that these products come from common intermediates on the metal surface. The slow step in both mechanisms may be the reaction of adsorbed oxygen with adsorbed carbon. Both carbon and oxygen are present on the metal surface during reaction. The rate law for hydrocarbon oxidation is zero order in hydrocarbon and oxygen partial pressures, and is consistent with a surface reaction between carbon and oxygen (5,9). Also, very little carbon accumulates on the platinum catalysts when they are exposed to heptane at 90°C and no oxygen. Thus, the effect of particle size on the rates of heptane conversion to carbon dioxide and coke may be to alter the distribution of carbon and oxygen on the metal surface. The dependence of the rate of heptane oxidation on the platinum particle size is obscured by carbon fouling. At long reaction times, the observed turnover frequency is close to the turnover frequency of the B site. The B-site activity is not very sensitive to particle size, increasing by a factor of five as the dispersion decreases from 81 to 10%. These results show that the deactivation kinetics must be accounted for in order to correctly interpret the effect of catalyst structure on the
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hydrocarbon oxidation rate. ACKNOWLEDGEMENT This work was supported by the National Science Foundation Engineering Research Center for Hazardous Substance Control at UCLA. REFERENCES
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