357
Ind. Eng. Chem. Fundam. 1983,22, 357-358 Kalm, P. A. Ph.D. Dissertation, University of Michigan, Ann Arbor, MI, 1981. Ranr, W. E.; Marshall, R. E. Jr. Chem. fng. frog. 1952, 48, 173. sherwood. T. K.; Pigford, R. L.; Wllke, c. R. “Mass Transfer”; McGraw-Hill: New York, 1975; Chapter 7, pp 257-8. Sprouse, K. M. AIChE 87th Natlonai Meeting, Boston, MA, Aug 1979. Ubhayakar, S. K.; Stickler, D. B.; Gannon, R. E. Fuel 1977, 56(3), 281.
’ Faculty of Chemical Engineering, Technion-Israel Institute of Technolo-
gy, Haifa 32000, Israel.
Department of Chemical Engineering The University of Michigan Ann Arbor, Michigan 48109
Philip A. Kalson*
Received for review March 5 , 1982 Accepted February 28, 1983
Influence of Thermal Shocks on the Catalytic Activity of Palladium Pretreatment steps involving abrupt cooling increased for several hours the catalytic activity of palladium wires toward the oxidation of hydrogen. This “Superactivity” decayed slowly under reaction conditions and may disguise the intrinsic catalytic activity.
Introduction Measurements of the electrical power required to maintain a catalytic wire at a preset resistance (temperature) is a convenient method of determining the isothermal catalytic activity (Zuniga and Luss, 1978; Rajagopalan et al., 1980). When the wire is heated electrically abrupt changes in the temperature may occur. The purpose of this work is to study the influence of thermal shocks on the oxidation activity and resistance of palladium wires. Experimental System and Methods A. Reaction Rate Measurements. A 5 cm long and 0.005 cm diameter palladium wire (Goodfellow Metals, Cambridge, England) of 99.9% purity was placed in a flow reactor and maintained at a constant preset resistance and hence temperature by a constant-temperature anemometer (Thermo-Systems, Inc., Minneapolis, MN). The resistance of the catalytic wire can be set by the anemometer to within 0.01 R. Extra-dry grade oxygen and hydrogen and high-purity grade nitrogen from cylinders (Linde Inc.) were passed through activated charcoal beds, mixed, and dried by passing through two beds packed with glass beads and Drierite pellets, respectively. The dry, mixed gases were then fed to the reactor. In all the experiments nitrogen was used as the inert gas. The isothermal reaction rate was determined by measuring the electric power required to maintain the wire at a preset resistance with and without reaction. All reaction rate measurements reported here are normalized per unit geometric area of the catalyst. Details of the experimental system and instrumentation are described by Zuniga and Luss (1978). B. Resistance Measurements. The electrical resistance of the catalytic wire was measured by passing a small current ( w 100 mA) through it from a 6-V battery with a calibrated decade resistance box in series. The voltage drop across the catalyst and the known resistance were compared to compute the catalyst resistance. All the resistance measurements were made with the catalyst kept in a stream of nitrogen flowing at a velocity of 3 cm/s at room temperature. Hence, the reference temperature of the catalyst for all resistance measurements is the same. Estimated values of the heat transfer coefficient and the current through the wire were used to compute the reference temperature as 304 K. The exact temperature is not of concern since we only compare resistance values at the same standard temperature. The estimated time constant for cooling of the wire is 4 s. Resistance measurements were made 30 s after the electirc heating was stopped. This time was considered 0196-4313/83/1022-0357$01.50/0
sufficient for the catalyst to thermally equilibrate with the nitrogen stream. The voltage drop values were measured by a digital voltmeter (Non-Linear Systems Inc., Del Mar, CA). The estimated maximum error in the resistance measurement is 0.01 52. C. Activation of the Catalyst. The commercial cold drawn palladium wire exhibited immeasurably small activity under the conditions employed in this work (T,= 85 to 200 “C, H2= 0 to 2% v and excess 02). It was activated using the procedure described by Zuniga and Luss (1978). The activated catalytic wire was always kept in an atmosphere of flowing N2between experimental runs to prevent poisoning.
Results Heating the activated catalyst to 900 “C for 10 min in N2 and quenching it to room temperature increased the resistance at the reference temperature of 304 K by 0.1 9. Scanning electron micrographs showed that the heat treatment increased the average size of the crystalline grains on the Pd surface. In all the quenching experiments, cooling was by radiation and it lasted about 0.5 s. Thus, the quench rate was less than 1.8 X lo3 K/s. After quenching from 900 “C, the catalyst was left at room temperature in an atmosphere of N2. The resistance at the reference temperature dropped exponentially until all the residual resistance disappeared. The response of the resistance to quenching from different temperatures is shown in Figure 1. In all these cases the resistance of the catalyst before the heat treatment was 2.46 9 and the residual resistance due to quenching disappeared completely at room temperature. The durations of heating for the different temperatures are also specified in Figure 1. The processes that increase the resistance are slower at temperatures lower than 900 “C. Hence the catalyst was kept for a longer time at the lower temperatures before quenching. The transient activity of the catalyst toward H2 oxidation could be influenced by its thermal history. Quenching from a high temperature to the reaction temperature caused a temporary activity enhancement, hereafter referred to as transient superactivity. The higher activity disappeared in about 2 h under reaction conditions. The influence of the thermal history on the catalytic activity is shown in Figure 2. The superactivity due to the quenching from 900 “C and the subsequent deactivation under reaction conditions occurred for all reactant compositions in the range studied, i.e., O2= 10 to 40% v and H2 = 0 to 1% V. 0 1983 American Chemical Society
Ind. Eng. Chem. Fundam. 1983, 22, 358-360
358
-
I I -
2,55!k
_, 50Cci,3Gmio
TIME, MIN
Figure 1. Response of catalyst resistance following heat and quenching to 25 "C. Catalyst resistance before heat treatment was 2.46 s2. I
to creation and annealing of point defects. Quenching of the catalyst from 900 "C resulted in a transient superactivity which was followed by a slow deactivation for about 2 h (Figure 2). Increase in grain size observed due to heat treatment suggests that the observed superactivity cannot be explained by changes in active surface area. Resistance measurements indicate that the mechanism suggested by Duell and Robertson (1961) may be applicable to the reaction system studied here. The experiments show that pretreatment steps involving abrupt temperature changes can alter the catalytic activity of a palladium wire toward H2oxidation for several hours. This phenomenon may disguise the intrinsic catalytic activity in kinetic studies. A sufficiently slow cooling of the catalyst from the pretreatment temperature can help the realization of steady, reproducible catalytic activity. Acknowledgment
This work was supported by a grant from the Robert A. Welch Foundation. Nomenclature
1 0
' 20
- 1
40
60
80
100
I20
T I M E , MIN
Figure 2. Response of catalytic activity to heating and quenching to 105 "C; T,= 105 "C, O2 = 3570, H 2 = 1.1%; v = 3 cm/s. Discussion
Transient superactivity due to quenching from 1330 "C was reported for Ni and Cu wires by Duell and Robertson (1961). Creation of point defects during the quenching process was the suggested cause of superactivity. Creation of point defects or dislocations in a metal increases its electrical resistivity (Broom, 1954). Dislocations caused by quench strain are likely to be insignificant at quenching rates smaller than lo4 K/s for the Pd wire employed in our studies (Damask and Dienes, 1971). Resistance changes observed in Figure 1are probably due
T,= temperature of the catalyst surface O2 = concentration of oxygen, in percentage volume, to the reactor feed H2 = concentration of hydrogen, in percentage volume, to the reactor feed u = velocity of the gas stream in the reactor Registry No. Palladium, 7440-05-3. L i t e r a t u r e Cited Broom, T. Adv. Phys. (Phil. Meg. Suppl.) 1954, 3 , 26. Damask, A. C.; Dienes, G. J., "Polnt Defects In Metals"; Gordon and Breach: New York, 1971. Duell, M. J.; Robertson, A. J. Trans. Faraday Soc. 1861, 57, 146. Rajagopalan, K.; Sheintuch, M.; Luss, D. Chem. Eng. Commun. 1980, 7, 335. Zunlga, J. E.; Luss, D. J . Catel. 1976, 53, 312.
Department of Chemical Engineering University of Houston Houston, Texas 77004
K. Rajagopalan Dan Luss*
Received for review July 22, 1982 Accepted March 4, 1983
Empirical Equation for Estimation of Critical Pressures for Organic Compounds An empirical equation which employs the critical temperature T,, K, and the normal boiling point T,, K, to predict the critical pressure P, for organic substances has been obtained. This equation accurately predicts the values of P, for organic compounds including the various isomers in the families alkanes, alkenes, aromatics, alicyclic hydrocarbons, alkyl ethers, and alkyl ketones.
Lydersen (1955) has given relations which employ structural contributions to estimate the critical volume V,, the critical pressure P,, and the critical temperature T, for organic compounds. Although Lydersen's relations predict the values of the critical constants for organic compounds to a good accuracy, these cannot estimate the varying magnitudes of the values of V, and P, for isomers. Nath (1982) has recently obtained an empirical equation which requires the knowledge of T,, P,, and the acentric factor w to predict V,, and he has found that this equation accurately estimates V , for numerous normal fluids. Nath's equation also predicts accurately the varying magnitudes 0196-4313/83/1022-0358$01.50/0
of V , for isomers. In the present program, it is thought to have a relation which can accurately predict values of P, for organic compounds including the isomers in various families. Hence, the data on critical constants as reported by Kudchadker et al. (1968) and by Reid et al. (1977) and the data on normal boiling points Tb (Reid et ai., 1977) for numerous organic compounds have been examined, and the following equation for P, has been obtained.
P, = T , [ A + B (Apb)
c(Apb)2]-2
(1)
In eq 1,A, B, and C are paramete? which are characteristic of compounds of a family, and AT, is equal to the reduced 0 1983 American Chemical Society
