Rapid Evaluation of Automotive Exhaust Oxidation Catalysts with a Differential Scanning Calorimeter Brent Wedding" and Robert J . Farrauto Corning Glass Works. Research and Development Laboratory, Corning, New York 74830
A differential scanning calorimeter screening test for automotive exhaust oxidation catalysts is described. The sample is heated in a flowing gas environment, and its temperature rise relative to an inert reference is used as a measure of activity. Use of the technique for routine screening, as well as for studies of reaction kinetics and catalyst degradation or poisoning, is discussed.
Introduction The use of differential thermal analysis as a catalyst screening tool is not new, and the advantages and limitations of the method are discussed by Locke and Rase (1960). We have found the differential scanning calorimeter (DSC) to be useful in testing automotive exhaust oxidation catalysts on a routine basis. The data are used to determine which experimental materials merit more realistic testing. Also, performance trends resulting from treatments of a given catalyst can he found. In this paper we describe our routine evaluation procedure. Methods of determining reaction kinetics and of studying catalyst poisoning and other degradation mechanisms are also discussed to indicate the versatility of the DSC. DSC testing has many attractive features. some of the most obvious being economy of time spent and the small amount of material needed. An experimental run requires 0.5 hr, and a laboratory technician can easily attend more than one instrument during the routine analyses.
Experimental Section The instrument used is a Du Pont Model 900 thermal analyzer, and is different from the Stone DTA apparatus employed by Locke and Rase or Papadatos and Shelstad (1973), since the sample is in an open pan placed in a continuously replenished gas environment, rather than in a cavity in the gas stream. A cross section of the DSC cell is shown in Figure 1. The sample pan is of thin aluminum. about 6-mm diameter and 1.5-mm deep. A temperature scan rate of 20"/min and chart display of .iO"/in. are used. The faster rate was found to give results identical with those from a 10"/min scan. The experimental gas mixture is made from the laboratory air supply and a tank of either carbon monoxide or hexane in nitrogen; however, any compatible gas mixture could be used. Our usual concentrations are 4300 ppm of CO or 260 ppm of hexane. with a flow rate of about 600 cm3 min. Separate runs are made for testing CO and hydrocarbon activity. The method of Altshuller and Cohen (1960) was used to produce a hydrocarbon blend for some runs. This technique works well, but is not as convenient. The sample pan is filled about half full with powdered catalyst. An empty pan is used as a reference. We do not weigh the sample for routine testing. The results are not strongly dependent on the amount of material used, and small variations about the "half-full" mark have negligible effect on the response of interest. Variations in catalyst particle size from 420 to less than 149 Fm had no effect on the data. Some catalysts were also examined with a bench test catalytic reactor with the sample supported by a cylindrical ceramic monolith, 25 mm diameter and 75 mm long.
The synthetic exhaust mixture was 0.025% HC ( & H E ) , 1.0% CO, 1.25% 0 2 , 10% H20 vapor, and NZ balance. A space velocity of 15,000 hr-I was used. Our response variable is the same as that of major automobile manufacturers: the temperature where 50% of the specified reagent is oxidized. Nondispersive infrared was used to monitor CO, and a flame ionization detector measured C ~ H G . Base-metal oxide catalyst samples were prepared by thermal decomposition of appropriate reagent grade nitrates or nitrate mixtures. The copper-chromium oxide mixture had excess CuO which could be removed by HC1 leaching to yield CuO .Crz03. Results from leached and unleached samples are given later in this paper. For routine screening tests the base line is assumed to be a linear extension of the flat portion of the curve before oxidation begins. Some specimens pick up mosture readily, giving a skewed "base-line'' as the sample is heated and the water desorbs. When this happens, the sample can be cooled in the cell and the run immediately repeated.
Results The temperature rise A T due to CO oxidation by a Cu0 . C r z 0 3 catalyst is shown as a function of sample temperature in Figure 2 for various amounts of catalyst. A similar scan was made with no CO present in order to establish a base line. Mass spectrographic analysis of gas samples before and after passing over the catalyst at three different temperatures demonstrated a direct proportion to within 5% between exotherm height and amount of G O z produced. About 20% of the carbon monoxide was oxidized a t the peak. indicating a bypass which seems reasonable when one considers the cell geometry. A number we use for a figure of merit is the temperature at which the exotherm reaches one-half of its maximum, analogous to the 50% conversion numbers from the bench test. The values for the data of Figure 2 are 239" for 3.9 mg of sample, 225" for 7.5 mg, 211" for 15 mg, and 202" for 30 mg. The pan is filled with 30 mg of this catalyst, while 3.9 mg did not cover the bottom completely. At low temperatures and with catalyst weights of 15 mg or less, the exotherm A T is roughly proportional to the amount of catalyst, indicating that the conversion depends on the number of sites available. The conversions appear independent of sample size above 15 mg, possibly due to a leveling effect of heat transfer to the detector. The initial conversion is an exponential function of increasing temperature. As the temperature is increased, the AT curve undergoes an inflection, reaches a maximum, and then slowly decreases. This flattening out occurs because the available supply of CO is limited, and the slight decrease in AT at higher temperature may be an indication of changes in heat transfer mechanisms in Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 1 , 1974
45
Silver Rinq
Therrnoelectrlc DISC
W
Figure 1. Cross section of Du Pont DSC cell.
Y
W
3ob
20 c -
-
-
CuO.Cr, 0, CATALYST
i
10-
1
Table I. Effect of Thermal Treatment o n the Performance of Selected Catalysts
0
100
200 300 400 TEMPERATURE IN CENTIGRADE DEGREES
500
50% conversion temperature in "C
Figure 2. Temperature rise, relative to a n inert reference. of vari-
DSC
ous amounts of copper chromite catalyst heated at 20"/min in a flowing CO-air mixture.
600" Catalyst
the cell. In separate experiments we found that the maximum AT was proportional to the CO concentration for partial pressures varying from 1.9 to 4.7 m m and independent of the gas mixture flow rate when the latter was reduced from 600 to 300 cm3/min. Catalyst Degradation a n d Poisoning Studies The DSC test can be used to study the performance changes of a catalyst which has been subjected to certain treatments. We will mention some of the types of experiments that we have done and show a few results. One of the attractive features of this type of testing is that a sample batch sufficient for all runs can be prepared, thus assuring the same starting material for all specimens. Thermal degradation was investigated by testing samples prepared by heating in air a t 600", and then portions of these that were further heat-treated at 800". Table I compares 5070 conversion temperatures in centigrade degrees of DSC tests on powdered samples and bench tests on supported catalysts. Exact agreement of the numbers was not expected because of the influence of the support and since the test gases differed in composition and flow rates. The utility of using the DSC test for finding general trends in both degradation and stability is clearly shown. The influence of the exhaust components on the performance of a catalytic device is of the utmost importance for long-life efficiencies. Shelef, e t al (1973), have investigated the problems of sulfur oxide and lead poisons. The DSC is particularly useful for examining catalysts for poisoning resistance. The behavior of a catalyst in a n SOzcontaining atmosphere was studied by first heating the sample to 500" ( a typical exhaust temperature), adding Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 1, 1974
Bench test
~
~
46
1
I
- _._
+ CUO ~
CUO CizOj CuO Cr-Ole
cuo
CoaO4 a Hexane. CuO Cr2O3
*
+
800"
-
600"
- ~- --
~
800' __ -
CO HCa CO HCa CO HC" CO HC" ~-
~~-
204 288 232 315 215 307 302 360 205 288 205 260 215 315 215 315 215 327 254 377 232 343 271 427 171 271 199 321 191 277 282 410 Propylene. Prepared by HCI leaching of CuO catalyst.
100 ppm of SO2 to the gas mix after steady-state CO conversion obtained, and recording the activity as a function of time. Curve A of Figure 3 shows the performance degradation of a base-metal oxide catalyst in SOz. Little poisoning is observed for the same catalyst "spiked" with platinum (curve B). Several systems of this type were tested, and similar results were found for all. The superiority of noble-metal-containing systems for SO2 poisoning resistance is shown and is consistent with engine data of Shelef. e t a!.We also measured the change in low-temperature (lite-off) conversion after this test. Following the treatment in 100 ppm of SO2 a t 500", the catalysts of Figure 3 were cooled and DSC tested using CO. The increase of the 50% conversion temperature of the base-metal oxide sample was more than loo", while that of the noblemetal-"spiked" catalyst was only 30". Reaction Kinetics Some runs were made to determine if oxidation kinetics of carbon monoxide by a CuO.CrZO3 catalyst could be found from DSC test data. The gas mix was made up from tanks of NZ,0 2 , and CO in Nz.The major component was Nz,and the effect of variation of CO or 0 2 partial pressure was studied using the same total gas flow for each run. The O2 concentration was sufficiently high to avoid significant depletion LW chemical reaction.
The reaction kinetics were found in the following way. We assume the amount of CO which is oxidized to be proportional to AT, and define the fractional conversion to be hT/AT,,,. At a given temperature, the fractional conversion is independent of CO concentration, indicating a first-order reaction. For a given CO level, the conversion was not affected by changes in the 0 2 partial pressure for 02-to-CO ratios of 1.3 to 3.5, the stoichiometric ratio being 0.5. The reaction is thus independent of the 0 2 concentration within the specifications mentioned above. To determine the activation energy, the natural logarithm of A T m a x / ( A T m a x was plotted us. 1/T. From the slope of the linear portion (20-70% conversion or 180220") a n activation energy of 17 kcal/mol was calculated. These results agree quite well with those obtained by Hertl and Farrauto (1973) using a more conventional catalytic reactor.
An
Summary The DSC test is a convenient way to evaluate the catalytic oxidative performance of materials and to determine their response to certain treatments. We have found t h a t some information about reaction kinetics can be gotten from DSC data. Although detailed agreement between
DSC results and those of more sophisticated experiments is not always found, there is good correlation of general performance trends. In general, samples which showed poor DSC response did not perform well on either bench or engine tests. Acknowledgments Some of the experiments discussed in this paper were done following suggestions of Dr. Larry E. Campbell. Mr. Norman A. Woodward performed some of the measurements. The authors are grateful for their assistance. We would also like to thank the anonymous reviewers who suggested improvements to the original manuscript. L i t e r a t u r e Cited Altshulier, A. P., Cohen, I. R . , Ana/. Chem.. 32,802 (1960). Hertl. W.. Farrauto, R. J., J. Cafai., 29, 352 (1973). Locke, C. E., Rase, H.F., lnd. Eng. Chem., 52, 515 (1960). Papadatos, K., Shelstad. K. A,, J. Catai.. 28, 116 (1973). Shelef, M., Dalla Betta, R. A,, Larson, J. A,, Otto, K., Yao, H. C., presented at the 74th National Meeting of the AIChE. New Orleans. La., March 11-15, 1973.
Received for reuieu: April 19, 1973 Accepted July 9, 1973
Correlation and Prediction of Excess Thermodynamic Functions of Strongly Nonideal Liquid Mixtures lsamu Nagata" and Toshiro Yamada Deparfment of Chemical Engineering, Kanazawa University. Kanazawa, 920, Japan
Excess enthalpy of mixing data are obtained for the ethanol-cyclohexane system at 35 and 45" and for the ethanol-methylcyclohexane system at 25, 35, and 45". The Wilson equation whose energy parameter differences are given by a quadratic function of temperature can be very useful for the prediction of binary excess heat capacity data. Good predicted results are shown for five binary alcohol-hydrocarbon systems using the parameters obtained from the excess Gibbs free energy and heat of mixing data over a moderate temperature range. This technique is easily extended to multicomponent systems. The ternary prediction of these three thermodynamic functions is presented for the ethanol-cyclohexane-nheptane mixtures over a 20-60" range.
Introduction Recently several investigations have been carried out to fit the excess enthalpy of mixing as well as the excess Gibbs free energy for completely miscible liquid mixtures using the Wilson equation. Orye (1965) and Hanks, et al. (1971), used the simplified assumption t h a t the energy parameter differences (AL, - A L L ) were independent of temperature. However, in order t o obtain the simultaneous good representation of both excess data for a great number of nonideal systems whose heat of mixing values are more than about 120 cal/mol, it is found that the Wilson parameters should be assumed to change with temperature as pointed out by Ratkcovics (1970), Ratkovics and Rehim (1970), Duran and Kaliaguine (1971). Kaliaguine and Ramalho (1972), Trinh, Ramalho, and Kaliaguine (1972), Nagata and Yamada (1972), and U'olfbauer (1972). Kaliaguine and others adopted the A[, form as the
Wilson parameters. On the contrary, other investigators used the energy parameter differences (AL, - A L L ) .Ratkovics, Rehim, Nagata, and Yamada assumed that the energy parameter differences varied with temperature linearly. Wolfbauer used the energy parameter differences and their derivatives with respect to temperature in curve fitting. Nagata, et al. (1973a,c), further demonstrated t h a t the assumption of a quadratic function of temperature for the energy parameter differences is suitable for the simultaneous correlation of excess Gibbs free energy ( g " ) and heat of mixing ( h b : ) data over a moderate temperature range and that it is also successful for the prediction of excess heat capacities Ic,E) for binary alcohol-hydrocarbon and hydrocarbon-hydrocarbon mixtures, showing the superiority of the Wilson equation over the nonrandom two liquid (Renon and Prausnitz, 1968) and Heil (Heil and Prausnitz, 1966) equations. In other words, this Ind. Eng. Chem., Process Des. Develop.,Vol. 13, No. 1, 1974
47
