Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 293-298
Acknowledgments All adsorption measurements were made by J. E. Carpenter. We also benefited from discussions with R. M. Sinkevitch. Literature Cited Adawi, M. K., Briggs, A. D., Ilelosh, R. G., Smith, C. S.,US. Patent 4 024 076 (May 24, 1977). Canale, R. P., Carlson, C., Kseener, D., Miles, D., presented at SOC.Auto. Eng. Cong., Detroit, MI, March 1978, Paper No. 780205. Gandhi, H. S., Piken, A. G., Sheief, M., Deiosh, R. G., presented at SOC.Auto. Eng. Cong., Detroit, MI, Feb 1976, Paper No. 760201. Gandhi, H. S.,Plken, A. G., Stepien, H. K., Shelef, M., presented at SOC. Auto. Eng. Cong., Detroit, MI, March 1977, Paper No. 770196. Hegedus, L. L., Summers, J. C., Schbtter, J. C., Baron, K., J . Catal., 58. 321 (1979). Joy, G. C., Molinaro, F. S.,Lester, G. R., presented at 6th N. Amer. Mtg. Catal. Soc., Chicago, IL, March 1979a. Joy, G. C., Lester, G. R., Molinaro, F. S.,presented at SOC.Auto. Eng. Mtg., Houston, TX, Oct 1979b, Paper No. 790943. Schlatter, J. C., presented at SOC.Auto. Eng. Cong., Detroit, MI, March 1978, Paper No. 780199; GM Research Publication GMR-2566. Schlatter, J. C., Sinkevitch, R. M., Michell, P. J., presented at 6th N. Amer. Mtg. Catal. SOC., Chicago, IL, March 1979a; GM Research Publication GMR-29 11.
293
Schlatter, J. C., Taylor, K. C., Sinkevitch, R. M., presented at Advances In Catalytic Chemistry Symposium I, Snowbird, UT, Oct 1979b; to be published in Symposium Proceedings. Seiter, R. E., Clark, R. J. presented at SOC.Auto. Eng. Cong.. Detroit, MI, March 1978, Paper No. 780203. Summers, J. C., Ausen, S . A,, J. Catal., 58, 131 (1979). Summers, J. C., Baron, K., J . Catal., 57, 380 (1979). Summers, J. C., Monroe, D. R., Chang, C. C., Gaarenstroom, S. W., presented at Mater. Res. SOC. Ann. Mtg., Cambridge, MA, Nov 1979. Taylor, K. C., in "The Catalytic Chemistry of Nitrogen Oxides", R. L. Kllmlsch, J. G. Larson, Ed., Plenum Press, New York, 1975. Taylor, K. C., Sinkevitch, R. M., Klimisch, R. L., J. Catal.. 35, 34 (1974). Wallman, S., Engh, G. T., presented at SOC. Auto. Eng. Cong., Detroit, MI, March 1977, Paper No. 770295. Yao. Y.-F. Yu, presented at 6th N. Amer. Mtg. Catal. SOC.,Chicago, IL, March 1979.
Received for review November 8, 1979 Accepted March 7, 1980 Presented at the Symposium on Automobile Exhaust Catalysis, 178th National Meeting of the American Chemical Society, Washington, D.C., Sept 10-13, 1979, Division of Industrial and Engineering Chemistry.
Oxidation of Alkanes over Noble Metal Catalysts YungFang Yu Yao Engineering and Research Staff, Ford Motor Company, Dearborn, Michigan 48 72 I
The total oxidation of C, to C4 n-alkanes at 200-500 O C over Pd and Rh catalysts, unsupported and supported on AI,,O, or CeO2/AI2O3,is fractional order with respect to the hydrocarbon and zero order with respect to oxygen. The specific rates per Pd or Rh surface area increase moderately with increasing chain length. Over the Pt catalysts, the partial reaction orders vary from 0.6 to 3 for the alkanes and from -1 to -3 for oxygen depending on the hydrocarbon chain length. The specific rates per Pt area increase by an order of magnitude for each increase in C number. These differences in reaction kinetics among the three metals can be explained with a proposed reaction mechanism assuming that the chemisorption of the alkanes is dissociative and competing with O2 for the same metal sites. The dispersion of the metals also plays an important role.
Introduction A significant portion of the hydrocarbons emitted from automotive exhaust consists of saturated hydrocarbons. They are usually more difficult to oxidize than olefins and aromatics. As emission control requirements become more stringent, the removal of these saturated hydrocarbons becomes more crucial. However, there are relatively few reports in the literature on the total oxidation of saturated hydrocarbons, and even less is known about the oxidation kinetics which are of importance for the modelling and design of catalytic converters. In this investigation, the oxidation kinetics of C1 through C4alkanes over the precious metal (PM) catalysts, Pt, Pd, and Rh, will be examined. The parameters of interest are the following: (1)the nature of the noble metal used, (2) the hydrocarbon chain length, (3) the effect, of the A1203 support and Ce02 additive, and (4) the effect of the metal dispersion and thermal aging. Experimental Met hods The catalyst preparation and experimental methods are similar to those reported previously (Yu Yao, 1977). The P M wires (Pt, Pd, and Rh) used as the unsupported catalysts were wound into spirals to facilitate good heat 0196-4321/80/1219-0293$01.00/0
dissipation and to minimize gas by-passing of the sample. The supported catalysts of PM/A1,03 and PM/Ce02/ A1203were prepared by the conventional method of wetting the -pAl,O, or Ce02 precoated A1,0, powders with a minimum amount of the P M salt solution of known concentration to obtain incipient wetness. The catalysts were calcined in air a t increasing temperature steps of 600,700, 800, and 900 "C for 4 h or more each. The reaction was conducted in a quartz flow reaction maintained at the desired temperature with a tube furnace. Helium containing the reacting gases was passed through the catalyst at atmospheric pressure. The composition of the gas, which by-passed the sample or passed through the sample, was monitored with an on-line mass spectrometer. Prior to each series of measurements, the sample was heated to 500 "C in He containing about 1% O2to remove any combustible contaminants on the surface. The dispersion of the PM on the catalyst was determined by CO chemisorption a t 25 "C after pre-reduction with Hz at 500 "C. Results Examination of the mass balance for each alkane/PM couple showed that complete conversion of the hydro-
0 1980 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
Table I. Kinetic Parameters over Pd Wire AE,
hydro- temp range, carbon "C CH, C,H, C,H, C,H,,
350-500 325-400 275-400 250-350
m 0.1 0.1
0 0
n 0.8 0.6 0.4 0.6
kcal/ mol 17 22 23 26
R(T.N.)s (T," C )
IOoo
E----
s-'
5.4 ( 4 0 0 ) 13.6(400) 8.9(350) 37.5(350)
Table 11. Kinetic Parameters over Rh Wire A E,
hydro- temp range, carbon "C
m
n
kcal/ mol
CH, C,H, C,H, C,H,,
0 0.3 0.1 0.1
0.6 0.5 0.5 0.6
24 18 22 21
450-550 375-450 350-450 350-450
R(T.N.),, (T," C )
S'
0.52(500) 1.21(450) 1.18(400) 1.93 (400)
,
loo
I
2
3
4
5
Table 111. Kinetic Parameters over Pt Wire hydro- tern? O,/HC carbon range, C range CH, C,H, C,H, C,H,,
475-550 325-400 225-300 175-250
1-4 5.5-13 5-17 8-20
A E,
m
-0.6 -2.5 -1 -0.6
kcal/ R(T.N.)* n mol s-' (T," C )
1.0 3.0 1.2 1.0
21 0.13(500) 26 0.93(350) 22 1 0 . 0 ( 2 5 0 ) 25 1 0 . 4 ( 2 2 5 )
carbon (HC) to C02 occurred in all cases. In contrast, partial oxidation is observed for unsaturated hydrocarbons (Yu Yao, 1977). The kinetics of the reaction was treated with a simple empirical power law: rate of COz formation = kpOzmPHC e The partial reaction orders, m and n, were derived from data obtained at low conversions, usually less than 30%. The partial pressures of O2 and HC are the average values of the inlet and exit concentration of each component. Most of the kinetic parameters included in this report were obtained with the 02/HC ratio ranging from 0.7 to 5 times the stoichiometric ratio for each alkane. For the purpose of comparison, the rates obtained under the arbitrarily chosen conditions of 1% O2 and 0.1% HC were taken. They are expressed in terms of turnover numbers as: (T.N.), = moles of COz produced/s - total moles of PM in the catalyst, or (T.N.), = moles of COZ produced/s - moles of PM on the catalyst surface = (T.N.),/d, where d (the dispersion of PM on the support) = moles of CO chemisorbed/moles of PM in the catalyst. The activation energy, AE,was evaluated from the temperature variation of these rates. PM Wires. The kinetic parameters obtained over the wire catalysts are shown in Tables 1-111. Over the Pd and Rh wires, the reactions for all four alkanes are nearly independent of the O2 partial pressure and 0.5 to 0.8 order Table IV. PM Dispersion of PM/.Al,O, . -Catalysts pretreatment catalyst T," C dispersion Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd
0.038%) 0.038%) 0.038%) 0.155%) 0.155%) o.i55%j 0.71%) 0.71%) 0.71%) 1.07%) 1.07%) 1.07%)
600 800 900 600 800 900 600 700 800 600 800
900
0.41 0.55 0.64 0.51 0.67 0.66 0.30 0.21 0.20 0.18 0.16 0.05
pretreatment catalyst Rh (0.03%) Rh (0.153%) Rh (0.153%) Pt (0.023%) Pt (0.023%) Pt io. 023%j Pt (0.22%) Pt (0.22%) Pt (0.22%)
T,"C
dispersion
600 600 800 600 800 900 600 800 900
0.69 0.57 0.07 0.87 0.36 0.13 0.19 0.07 0.06
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
295
(0.155%) 8 0 0 @ C
::I (y,///G] 0
100
1 /
-_I_-
I50
200
250 300 350 TEMPERATURE ( V I
400
450
500
-.--i___L-400 450 500 TEMPERATURE ( O C ) 1
Figure 2. Conversions of alkanes over Pt catalysts: inlet Po2 = 5%; 02/HC = 2 X S.R.
and C,Hlo. Because of the large difference in kinetics, a general comparison of activity among the three metals is unwarranted. Although the majority of the study was conducted under O2 rich conditions, some data were obtained in the O2 deficient region. Some results are shown in Figure 1. A reversal of the sign of m and n was found for C2Hs oxidation over Pt. Supported Catalysts. The dispersion of PM on the PM/A1203catalysts was determined by CO chemisorption at 25 "C. The results are shown in Table IV. The dispersion of PM in PM/Ce02/A120, catalysts could not be determined because Ce02 can be reduced by H2 or CO under the conditions used to reduce the PM and because CO can also adsorb on the Ce02 surface (Yu Yao, 1979). For Pd/A1203catalylsts containing relatively low concentration of Pd, the dispersion of Pd increases slightly with increasing calcination temperature from 600 to 800 "C and remains unchanged from 800 to 900 "C. Catalysts of higher Pd concentration lose their dispersion more easily. Pt on A1203sinters readily at 600 "C or above even for catalysts of very low Pt conceintration. CO chemisorption on Rh/ Alz03 catalysts also decreases sharply with increasing calcination temperature. The results for the oxidation of the four alkanes over the supported catalysts are shown in Tables V-VII. In general, the kinetic parameters of m, n, and AI3 for each PM/Al2O3 catalyst are quite similar to those found for the corresponding wire, but some changes were found for catalysts with CeOz added. The rates expressed as (T.N.), or (T.N.), are dependent on the nature of the nobel metal, the hydrocarbon, the dispersion, and the presence of Ce02 The ratio of (T.N.), for the A1203supported catalysts to (T.N.), for the wire is given for a variety of dispersions in Table VIII. The effect of thermal aging at different temperatures on (T.N.It and dispersion is shown in Table IX. Of interest to the practical application of these catalysts for the removal of hydrocarbons in vehicle exhaust gas is the variation of catalyst activity with increasing and decreasing temperature over the entire range of conversion efficiency from zero to 100%. In our study of this, the inlet composition of the reactant gases was fixed at an 02/HC ratio of 2 times the stoichiometric ratio for the alkane. The oxidation rate was monitored with slowly increasing temperature followed by slowly decreasing temperature. Typical sets of such curves are shown in Figures 2-4. Except for a small hysteresis loop, the heating and cooling curves for each case generally coincide. Only the heating curves are shown to avoid cluttering. There is no discontinuity, which suggests no change of reaction mechanism
250
300
350
Figure 3. Conversions of alkanes over Pd catalysts: inlet Po,= 5%; OP/HC = 2 X S.R. 100
CI-Rh/AP203--
80
60
,/
'
450
500
TEMPERATURE I T )
Figure 4. Conversions of alkanes over Rh catalysts: inlet Pop= 5%; 02/HC = 2 X S.R.
Figure 5. Oxidation of C4HIo(upper) and C2H,(lower) over Pt/ Ce02/A1203catalyst.
up to nearly 100% conversion. Under the reaction conditions of Figures 2-4, each Ce02/A1203supported Pt and Pd catalyst is less active than its counterpart without Ce02. On the contrary, the activity of the Rh catalysts is slightly higher with Ce02 than without. For the CeOZ/Al2O3supported Pt and Pd catalysts, the kinetic parameters are different from those for the wire or for the metals supported on A1203. However, this difference is removed by reduction of the PM/CeO2/Al2O3 catalysts by CO for a few minutes at 300 "C or above or by the hydrocarbons at a temperature at which the oxidation rate is measurable. The reduced state of the Ce02 containing P M catalysts reverts back gradually to that before reduction. The life of the reduced state is de-
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Table V. Pd Catalysts cat. (Pd %)" (pret. T,' C)
m
n
kcalimol
CH4
0.038% (800) 0.038% (900) 1.0% (800) 0.037%/23% CeO, 0.154%/20% CeO, 0.038% (800) 0.155% (800) 1.0% (800) 0.154%/20% CeO, 0.038% (800) 0.038% (900) 0.155% (800) 1.0% (800) 0.037%/23% CeO, 0.038% (800) 0.038% (900) 0.155% (800) 1.0% (800) 1.0% (900) 0.03'7%/23% CeO, 0.154%/20% CeO, 0.154%/20% CeO,
0 0 0.1 0 0 0 0.2 0.1 0.1 0.1 0.2
0.6 0.8 0.7 0.3 0.5 0.6 0.4 0.5 0.4 0.6 0.6 0.5 0.5 0.6 0.5 0.3 0.4 0.6 0.5 0.4 0.5 0.4
17 17 20 15 13 16 16 23 20 15 17 18 22 15 18 20 17 21 23 14 15 18
CZH,
C3H8
a
A E,
alkane
(900) (800)
(900)
(900)
(900) (800) (900)
0.1 0.1 0.1 0.1 0 0.2 0 0.1 0 0 0
(T.N.)t,S - ' (T,"C) 0.013 (400) 0.0075 (400) 0.048 (400) 0.011 (500) 0.03 (500) 0.062 (400) 0.053 (400) 0.06 (400) 0.011 (400) 0.072 (350) 0.039 (350) 0.046 (350) 0.034 (350) 0.007 (350) 0.065 (350) 0.051 (350) 0.058 (350) 0.048 (350) 0.034 (350) 0.03 (400) 0.014 (400) 0.028 (400)
0.024 (400) 0.012 (400) 0.31 (400) 0.113 (400) 0.078 (400) 0.37 (400) 0.13 (350) 0.062 (350) 0.068 (350) 0.21 (350) 0.12 (350) 0.08 (350) 0.087 (350) 0.30 (350) 0.68 (350)
Wt %, balance A1,03.
Table VI. Rh Catalysts ~
alkane
cat. ( R h % ) a (pret. T,"C)
m
n
kcal/mol
CH4
0.03% (600) 0.153% (600) 0.153% (800) 0.03%/20% CeO, (600) 0.155%/20% CeO, (600) 0.153% (600) 0.153% (800) 0.155%/20% CeO, (600) 0.153% (600) 0.153% (800) 0.155%/20% CeO, (600) 0.03% (600) 0.153% (600) 0.153% (800) 0.03%/20% CeO, (600) 0.155%/20% CeO, (600)
0 0.1.
0.5 0.4
0 0.1 0.2
0.4 0.5 0.4
23 22 22 20 20 21
0.2 0
0.4 0.5
19 24
0.1 0 0 0 0 0.2
0.4 0.5 0.5 0.5 0.5 0.3
20 18 21 20 18 19
CZH, C3H*
a
A E,
(T.N.bt, S-' (T, C)
(T.N.)s,s-' (T,"C) 0.048 (500) 0.072 (500) 0.14 (500)
0.033 (500) 0.041 (500) 0.010 (500) 0.039 (500) 0.043 (500) 0.055 (450) 0.0076 (450) 0.133 (450) 0.040 (400) 0.0065 (400) 0.13 (400) 0.114 (400) 0.06 (400) 0.004 (400) 0.21 (400) 0.13 (400)
0.10 (450) 0.11 (450)
0.07 (400) 0.09 (400) 0.17 (400) 0.11 (400) 0.06 (400)
Wt % balance A1,0,.
pendent on the noble metal, the reaction conditions, the 02/HC ratio, and the temperature. In general, it is more stable for Pt than for Pd or Rh. A scenario of this process is shown in Figure 5. The kinetic parameters of the reduced Pt/Ce02/A1203catalysts are shown in Table VII. The life of the reduced state of the Pd and Rh catalysts is too short for the kinetic parameter evaluation, but a transient enhancement of activity was observed after reduction for both Pd and Rh supported on Ce0,/A1,03 (Figure 6).
Discussion The differences in reaction kinetics over the three metals or over the PM/Al2O3 catalysts can be explained by the following mechanism. The slow step of the alkane oxidation has been postulated as the dissociative chemisorption of the alkane on the bare metal surface with the breakage of the weakest C-H bond (Cullis et al., 1970; Schwartz et al., 1971; Hiam et al., 1968) followed by its interaction with O2 adsorbed on an adjacent site. If the P M surface atoms are sites for both O2and hydrocarbon chemisorption, then they compete with each other and the surface coverage of both reactants is interdependent. Because of its size, each hydrocarbon molecule is expected
OpOFF
,02
L
I
0
IN
7----------
S
Pd/AP203 (0.038%) 350.C
I
IO TIME LMIN.)
,
I
IS
TIME IMIN.)
Figure 6. Oxidation of C4Hloover Pd/AI2O3 (upper) and Pd/ CeO2/AI2O3(lower) catalysts.
to take up several adjacent sites. Several reports (Ertl and Koch, 1973; Campbell et al., 1977) have shown that Pd and Rh surfaces under O2rich conditions are covered with a layer of O2 as expected from thermodynamic considera-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
297
Table VII. Pt Catalysts .A E ,
alkane
cat. ( P t %)" (pret. T, "C)
CH,
0.22% (600) 0.22% (900) 0.30%/23% CeO, 0.22% (600) 0.22% (900) 0.23%/20% CeO, 0.05%/23% CeO, 0.05%/23% CeO, 0.022% (600) 0.022% (800) 0.22% ( 6 0 0 ) 0.22% (900) 0.42% (900) O.O5%/23% CeO, 0.05%/23% CeO, 0.30%/23% CeO, 0.022% ( 6 0 0 ) 0.022% (800) 0.22% (600) 0.22% (900) 0.05%/23% CeO, 0.05%/23% CeO, 0.23%/20% CeO, 0.30%/23% CeO,
CZH,
C3H8
C4H1 0
1.2 1.1
-3 - 2.7 -3 0 -2 -1 - 0.8 -2 -1 -1 0 - 0.8 -1 - 0.9 -1 -1 -0.5 0 - 0.8 .- 1 - 0.7
2 3 2.5 0.9 2 2.0 1.7 2.0 2 2 1.4 1.0 2 1.0 1.0 1.0 0.9 0.6 1.2 1.0 0.7
(900) (900) (R)b
(900) (R)b (900)
(900) (R)b
(900) (900)
Pd
Rh
D 0.87 0.36 0.19 0.06 0.04 0.64-0.67 0.5 1-0.55 0.16 0.05 0.57 0.07
0.016 0.075 0.130
0.58 0.75
0.18 0.33
0.002 0.004 0.057
0.006 0.008 0.027
0.254 0.007 0.015 0.024
0.138 0.269
0.083 0.091
0.059 0.076
0.12 0.75 0.22 0.51 0.70 0.0023 0.004 0.008 0.018 0.057 0.031
Table IX. Thermal AgGng Effect on C,H," Oxidation ~~
--____ PM/Al,O, Pd (0.038%)
(T.N.)t/D, s-' ( T , C)
6 0 0 "CY
800 Ca
0.065/0.55 (350) Pd (0.155%) 0.085/0.51 0.058/0.67 (350) (350) Pd (1%) 0.048/0.16 (350) Rh (0.03%) 0.114/0.69 0.028/(?) (400) (400) Rh (0.153%) 0.06110.57 0.0041/0.07 (4010) (400) 0.92/0.36 Pt (0.022%) 1.07/10.87 (225) (225) Pt (0.22%) 0.42/0.19 0.44/0.07 (22,5) (225) a Calcination in air for 4 h.
20 19 20 16 22 22 25 20 23 22 11 16 23 18 18 17 18 16 18
19 19
(T.N.)t, s-' (T," C ) 0.0145 (500) 0.0057 (500) < 0.0002 (500) 0.033 (350) 0.021 (350) 0.0096 ( 3 5 0 ) 0.0029 (350) -0.01 (350) 0.136 (250) 0.268 (250) 0.25 (250) 0.19 ( 2 5 0 ) 0.052 (250) 0.027 (350) 0.107 (350) 0.034 (250) 1.07 (225) 0.92 ( 2 2 5 ) 0.42 ( 2 2 5 ) 0.31 (225) 0.0063 ( 3 0 0 ) 0.046 (225) 0.028 (225) 0.078 (225)
( T . N . ) , s-' (T,"C) 0.076 (500) 0.098 (500) 0.17 (350) 0.31 (350)
0.16 (250) 0.75 (250) 1.30 (250) 1.46 ( 2 5 0 ) 2.54 ( 2 5 0 )
1.75 ( 2 2 5 ) 2.54 ( 2 2 5 ) 2.22 (225) 5.18 ( 2 2 5 )
R = reduced with respective hydrocarbon a t 2 3 5 0 ° C .
Table VIII. Effect of PM Dispersion o n the Relative Activity of PM/Al,O, (Catalysts
Pt
kcal/mol 24 24
(900)
W t % balance A1,0,, pret. in air a t T > 4 h.
PM/ A1,0,
n
rn -- 0.6 -0.5
900 O c a 0.051/0.64 (350) 0.06110.66 (350) 0.034/0.05 (350) 0.001 7/(?) (400) 0.00 2 1/( ?) (400) 0.3U0.06 (225)
tions and thus the oxidation reaction is zero order with respect to 0,. Compared to Pd and Rh, Pt has a higher ionization potential and its oxide is of lower stability. Therefore, the 0, surface coverage on Pt is expected to be less than that of Pd and Rh and more dependent on the Oz/hydrocarbon ratio, and on the ease of chemisorption and reaction of the hydrocarbon. For long-chain hydrocarbons which will chemisorb and react readily, one would
expect that Pt surface to be more reduced and the reaction rate to be higher. The heat of adsorption of O2 on Pt was reported to vary from 70 kcal/mol at 0 = 0 to 36 kcal/mol at 0 = 0.63 and presumably to lower values at higher 0 (Brennan et al., 1960). Dissociative adsorption of hydrocarbons including methane (the most difficult one) on Pt has been reported (Cullis et al., 1970; Clarke and Rooney, 1976). Thus, the adsorption of hydrocarbons over Pt could be energetically competitive with O2 under proper conditions. Such a Langmuir-Hinshewood reaction mechanism involving interaction between two competitively adsorbed species could lead, depending on the reaction conditions, to inhibition by either one of the reactants as found for C2H, oxidation over Pt. Recently, Frennet and his co-workers (1978) have treated the case of hydrocarbon interaction with H, or D2 over metal surface by means of a competitive chemisorption mechanism with the hydrocarbon taking up several reactive sites. They showed that the partial reaction order could change from a large negative value to a large positive value for one reactant with concomitant changes in the opposite direction for the other reactant when the relative surface concentration of the two reactants changed. It is also possible that the oxidation state of Pt changes with the 0 2 / H C ratio and thereby changes the kinetic parameters. The increase in oxidation rate with increasing chain length of the alkanes is consistent with the postulation of H abstraction as the slow step, because the ease of breaking the C-H bond increases in the same order (Schwartz et al., 1971). The kinetic parameters for each alkane/PM couple are not changed by the presence of the Alz03support, but the rate per surface PM atom, (T.N.),, over each PM/Al2O3 catalyst is less than that over its corresponding wire. This reduction in rate for the supported catalysts, or deactivation by support, is shown in Table VI11 as a function of dispersion. Over Pt and Pd catalysts, the ratio of (T.N.), for the supported catalyst to that for the wire increases with decreasing dispersion. A t the same dispersion, the deactivation by the AlzO, support for each PM is less for CHI than for the larger molecules. This is consistent with the idea that more adjacent sites are needed for the ad-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
sorption of the larger alkanes. The proposition that highly dispersed sites are not active for alkane oxidation can also be used to explain the results in Table IX. The rate per total PM present, (T.NJt, decreases only slightly with increasing aging temperature, despite a much larger decrease in dispersion. It has been reported (Yates et al., 1979; Yao et al., 1979a) that the noble metals can exist on A1203surfaces as both particles and isolated ions or atoms. The former contributes heavily to the oxidation activity for the alkanes while the latter is more important in the CO chemisorption used for dispersion measurements. During high temperature pretreatment the isolated ions or atoms will coalesce to form metallic particles and cause a large loss in CO chemisorption capacity with little change in activity. The (T.N.),'s over Pt/A120, catalysts are less than that of Pt wire by a factor of less than 8, while those over Pd/AI2O3 catalysts are at least two orders smaller than that of Pd wire. The use of the same catalysts for CO oxidation is reported in a separate paper (Yu Yao, 1977). It was found that the Pd/A1203 catalysts are of comparable activity (rate/Pd surface area) as the Pd wire. Therefore, the lower activity for alkane over the Pd/A1203catalysts can also be attributed at least partly to the larger size of the alkanes than CO. Another contributing factor would be the tendency of dispersed Pd to be in the ionic state, which will be discussed later. The (T.N.), of Rh/A1203appears to be less dependent on dispersion as measured by CO chemisorption (Table VIII). The (T.N.), for the Rh/A1203catalyst decreases sharply with increasing aging temperature. The supported catalysts are much less active than the wire. All these suggest that the aging of Rh may be different from that for Pt even though both show large decrease of dispersion. Yao et al. (1977) reported that Rh supported on A1203can be incorporated into the sublayer of A1203upon heating to above 700 "C. In a previous report (Yao et al., 1979b), we have studied the C4HI0 oxidation over a series of Rh/A1203catalysts of high dispersion (pretreated at 500 "C and d N 1). The (T.N.), obtained was independent of the Rh loading and of the same order as that obtained in this study. Therefore, on high temperature (800 "C) pretreated Rh/A1,03 catalysts the Rh is most likely in the dispersed ionic state and not present as small metal particles. Ceria Containing Catalysts. The following characteristics observed for the alkane oxidation over the Pt/ Ce02/A1203catalyst of 0.05% Pt suggest that Pt is in the oxidized state after interaction with CeOs and that the oxidized state is less active than that of Pt metal or Pt particles on A1203. These characteristics are as follows: (1)zero order with respect to O2 instead of O2 inhibition, (2) less positive dependence on the hydrocarbon than over Pt metal, (3) less active under the standard conditions (Figure 4), and (4) a similarity of the kinetic behavior to Pt metal or Pt/A120, after a brief reduction together with
a slow reversion to the oxidized state upon exposure to 0, rich conditions. A difference between Pd/CeOz/A1203and Pt/Ce02/ A 1 2 0 3 is the higher stability of the reduced state for Pt than Pd. A transient high activity for Pd/Ce02/A1203was observed immediately after the reduction (Figure 6). This confirms that the reduced PM is more active for the alkane oxidation. The highly dispersed PM is easier to oxidize and is stable as ions and thus gives lower activity. In addition, the multiple chemisorption sites requirement is not met for highly dispersed PM. A previous study (Kummer et al., 1970) of CO oxidation has shown that the CO oxidation activity increases with the addition of Ce02 to the Pd/A1203catalysts. Therefore, the lower activity for alkane oxidation over the Pd/Ce02/A1203catalysts could not be attributed to lower dispersion of Pd on the CeO2/AI2O3support. The Rh/Ce02/A1203activity for the alkane oxidation can also be momentarily increased by reduction, but the life of the reduced state is even shorter than that of the Pd/Ce02/A1203. Because Rh is expected to exist as dispersed ions, with or without CeO, (as shown by the same kinetic parameters), the slightly higher activity for Rh/ CeO2/AI20,can only be attributed to the stabilization of Rh on the surface by CeO, and a consequent inhibition of its solution into the A120, support. Unfortunately, we could not determine the Rh surface concentration in the Ce02 containing catalysts.
Acknowledgment The author is indebted to Dr. J. T. Kummer for many helpful discussions throughout the course of this work. Literature Cited Brennan, D., Hayward, D. O., Trapnell, B. M. W., Roc. R. Soc. London, Ser. A . 258. 81 (1960). CampbelL'C. T.', Fo+, D. C., White, J. W., J . Phys. Chem., 81, 491 (1977). Clarke. J. K. A . , Rooney, J. J., Adv. Catal., 25, 144 (1976). Cullis. C. F., Keene. D. E., Trimm, D. L., J. Catal., 18, 864 (1970). Ertl, G. L., Koch, J., "Proceedings of 5th Internatbnal Congress on Catalysis", p 967, American Elsevier, New York, 1973. Firth, J. G., Trans. Faraday SOC., 82, 2566 (1966). Frennet, A., Lienard, L., Crucq, A., Degois, L., J . Catal., 53, 150 (1978). Hiam, L.,Wise, H., Chaikin, S., J . Catal., 9-10, 272 (1968). Kummer, J. T., Yu Yao, Y. F., McKee D., Proceedings of Automotive Engineering Congress, Paper No. 760143, Detroit, Mich, Feb 23-27, 1976. Schwartz, A., Holbrook, L. L., Wlse, H., J . Catal., 21, I 9 9 (1971). Yao, H. C., Japar, S., Shelef, M., J . Catal., 50, 407 (1977). Yao. H. C., Sieg, M., Plummer. H. K., J . Catal., 58, 365 (1979a). Yao, H. C., Yu Yao, Y. F., Otto, K., J. Catal., 58, 21 (1979b). Yates, D. J. C., Murrell, L. L., hestridge. E. E., J . Catal., 57, 41 (1979). Yu Yao, Y. F., Paper No. 069 Colloid and Surface Chemistry Divlslon 174th National Meeting of American Chemical Society, Chicago, Aug 28-Sept 2, 1977. Yu Yao, Y. F., Presented at the 6th North American Meeting of the Catalysis Society, Chicago, Mar 18-22, 1979.
Receiued for review November 16, 1979 Accepted April 18, 1980 Presented at the Symposium on Automobile Exhaust Catalysis, Division of Petroleum Chemistry, 178th National Meeting of the American Chemical Society, Washington, D.C. Sept 9-15, 1979.
