- Email: [email protected]
Detection of catalytic oscillations by differential thermal analysis
-77xrmochimica
@ Ekevicr
Ada,
15 (l976)238440
S&entific PubIishing Company.
Amsterdam
- Printed in Belgium
Note
Detection of catalytic oscillations by differential thermal analysis P. IL
GALLAGHER
AND
D. W. JOHNSON,
JR
Bell laboratories, Murray Hill, N. I. 07974 (CL S. A.) (Reseivai
5
September1975)
We have discussed the applicability of DTA to studies of catalysis in earlier work1-3_ The technique is conceptually simple, is quick and sensitive, and commercial equipment is readily availabIe_ In this brief note we wish to report that we have observed by DTA certain oscillatory or instability phenomena”.’ which involve rapid changes in the cataIytic activity. We also rXustrate how its rapid response and high sensitivity make it suitable for the study of these oscillations which may he more difficult to detect by other technique?. EXPERF&ZXTAL
PROCEDURES
AXD RESULTS
Nominally 15 mg sampIes were placed in the sample pan of the DSC cell of a DuPont Model 900 DTA apparatus. An atmosphere of 2% CO, 2% 02, 96% N2, and with or without 150 ppm of SO, was flowed through the cell at a rate of O-5 1min- *. The heating rate was 10°C min- ‘. Some typical examples of DTA curves exhrbiting oscillations are shown in Fig. !. In order to be sure the effect was not instrumental, we aIso performed an experiment using the Mettler 200 DTA system which confirmed our observations. DISCUSSION
Danchot and CakenbergheJ observed oscihations for the oxidation of CO on Pt wires and thin films by following electrical resistance or photo-voltaic effects. The sampIes in which we observe oscillations in the DTA pattern all contain Pt in s&r&ant amounts. Those illustrated in Fig_ 1 have Pt contents 2 2400 ppm so that a significant amount of the observed activity, particularly in the presence of SOL, is due to Pt6*7. Danchot and [email protected] attribute these oscillations to the apphcability of the Langmuir-Hinselwood mechanism in which the reaction takes pIace between CO and 0, moIecuIes adsorbed upon the surface of the Pt. The exothermicity of the reaction markediy raises the temperature of the catalyst desorbing the product CO,. IIowever, at higher temperatures O2 is preferentially adsorbed and the sample must cool in order to adsorb enough CO before reacting again. This gives rise to cycles whose period and amplitude depend upon many things, e.g., thermal transport, concentrations in the -gas phase, etc.
239
i
L
Fig. 1. DTA Curves for 15 mg of the indicated catalysts and atmospheres. Heating rate, 10“C rnin- ’ and ftow-sate, 0.5 I min‘ ‘. (a) La~.~Pb~,~MnO3+-2400 ppm Pt, ground single crystzds, 2% CO, 2% 01, 96% N=_ (b) La,_7Pbe_&n0,+2500 ppm Pt, freeze dried, 2% CO, 2% Oz. 96% Ntf 300 ppm SOZ.
The examples in Fig. i were selected in order to ilfustrate a wide range in the observation of this effect. Figure la shows essentially the tieshold of the observable eff’ It is characteristicaliy more obvious on the cooling curve as can also be seen in Fig. Zb where the eft’ect has become greater. The exact nature of the oscillations is not precisely reproducible under our experimental conditions as illustrated by successiye heating curves shown in Fi g- Ic. Figure Id is our most pronounced example of this effect, The oscillations are as Iarge as 4°C in amplitude with a varying period of Z-30 sec. They, cf course, represent a combination of the actual catalytic. effect and the experimental and instrumental response characteristics. Obviously further work ;: would be necessary to establish the actual cat@tic response curves. n
The authors are gratefiA to Mrs. E, M, Vogel for some experimental assistance and to Mr. R, 3. H. Voorhoeve for helpfu1 discussions.
240 REFERENCES 1 D_ W_ iobnsos Jr_ and P_ EC Gallagher, tiA_ Acts 7 (1973) 303. 2 D_ W. Johnson, Jr, P_ K. Gallagixr, E M. Vo8cl and F. Schrey, 4th InrernarionalConjkence on W Mysi~, Buhpesr. lGng~, July I974 Therm. AI&. 3 (1975) 18I_ 3 P_ K. Gallagher* D_ W_ Johnsan, Jr- and E MS Vogel. 5th ConfaMcc on Catalysis in Organic Syn2kexi.s.Bcrrron,April 1975, to be publishedin the proceedingp thereof. 4 J. P. Danchot and J. Van Cakenbeqhe, Nature Phps. Sci_, 246 (1973) 61, 5 K_ M_ Ya and 3. M. Douglas. Chem- i%g_ Sci-, 29 (1974) 163. 6 P. IL Gailaghcr, D. W. Johnson. Jr., J. P. Rcmcikc, F_ Schrcy, L E Trintble. E M. Vogel and R. J. H. Vwriwee, Mater_ Rcs. &?I., IO (1975) 529. 7 P. K Gallaglur, D. W. Johnson, Jr., E M. Vogel and F. Schrey, Mafcr. Rcs. Bull., 10 (1975) 623.
@ Ekevicr
Ada,
15 (l976)238440
S&entific PubIishing Company.
Amsterdam
- Printed in Belgium
Note
Detection of catalytic oscillations by differential thermal analysis P. IL
GALLAGHER
AND
D. W. JOHNSON,
JR
Bell laboratories, Murray Hill, N. I. 07974 (CL S. A.) (Reseivai
5
September1975)
We have discussed the applicability of DTA to studies of catalysis in earlier work1-3_ The technique is conceptually simple, is quick and sensitive, and commercial equipment is readily availabIe_ In this brief note we wish to report that we have observed by DTA certain oscillatory or instability phenomena”.’ which involve rapid changes in the cataIytic activity. We also rXustrate how its rapid response and high sensitivity make it suitable for the study of these oscillations which may he more difficult to detect by other technique?. EXPERF&ZXTAL
PROCEDURES
AXD RESULTS
Nominally 15 mg sampIes were placed in the sample pan of the DSC cell of a DuPont Model 900 DTA apparatus. An atmosphere of 2% CO, 2% 02, 96% N2, and with or without 150 ppm of SO, was flowed through the cell at a rate of O-5 1min- *. The heating rate was 10°C min- ‘. Some typical examples of DTA curves exhrbiting oscillations are shown in Fig. !. In order to be sure the effect was not instrumental, we aIso performed an experiment using the Mettler 200 DTA system which confirmed our observations. DISCUSSION
Danchot and CakenbergheJ observed oscihations for the oxidation of CO on Pt wires and thin films by following electrical resistance or photo-voltaic effects. The sampIes in which we observe oscillations in the DTA pattern all contain Pt in s&r&ant amounts. Those illustrated in Fig_ 1 have Pt contents 2 2400 ppm so that a significant amount of the observed activity, particularly in the presence of SOL, is due to Pt6*7. Danchot and [email protected] attribute these oscillations to the apphcability of the Langmuir-Hinselwood mechanism in which the reaction takes pIace between CO and 0, moIecuIes adsorbed upon the surface of the Pt. The exothermicity of the reaction markediy raises the temperature of the catalyst desorbing the product CO,. IIowever, at higher temperatures O2 is preferentially adsorbed and the sample must cool in order to adsorb enough CO before reacting again. This gives rise to cycles whose period and amplitude depend upon many things, e.g., thermal transport, concentrations in the -gas phase, etc.
239
i
L
Fig. 1. DTA Curves for 15 mg of the indicated catalysts and atmospheres. Heating rate, 10“C rnin- ’ and ftow-sate, 0.5 I min‘ ‘. (a) La~.~Pb~,~MnO3+-2400 ppm Pt, ground single crystzds, 2% CO, 2% 01, 96% N=_ (b) La,_7Pbe_&n0,+2500 ppm Pt, freeze dried, 2% CO, 2% Oz. 96% Ntf 300 ppm SOZ.
The examples in Fig. i were selected in order to ilfustrate a wide range in the observation of this effect. Figure la shows essentially the tieshold of the observable eff’ It is characteristicaliy more obvious on the cooling curve as can also be seen in Fig. Zb where the eft’ect has become greater. The exact nature of the oscillations is not precisely reproducible under our experimental conditions as illustrated by successiye heating curves shown in Fi g- Ic. Figure Id is our most pronounced example of this effect, The oscillations are as Iarge as 4°C in amplitude with a varying period of Z-30 sec. They, cf course, represent a combination of the actual catalytic. effect and the experimental and instrumental response characteristics. Obviously further work ;: would be necessary to establish the actual cat@tic response curves. n
The authors are gratefiA to Mrs. E, M, Vogel for some experimental assistance and to Mr. R, 3. H. Voorhoeve for helpfu1 discussions.
240 REFERENCES 1 D_ W_ iobnsos Jr_ and P_ EC Gallagher, tiA_ Acts 7 (1973) 303. 2 D_ W. Johnson, Jr, P_ K. Gallagixr, E M. Vo8cl and F. Schrey, 4th InrernarionalConjkence on W Mysi~, Buhpesr. lGng~, July I974 Therm. AI&. 3 (1975) 18I_ 3 P_ K. Gallagher* D_ W_ Johnsan, Jr- and E MS Vogel. 5th ConfaMcc on Catalysis in Organic Syn2kexi.s.Bcrrron,April 1975, to be publishedin the proceedingp thereof. 4 J. P. Danchot and J. Van Cakenbeqhe, Nature Phps. Sci_, 246 (1973) 61, 5 K_ M_ Ya and 3. M. Douglas. Chem- i%g_ Sci-, 29 (1974) 163. 6 P. IL Gailaghcr, D. W. Johnson. Jr., J. P. Rcmcikc, F_ Schrcy, L E Trintble. E M. Vogel and R. J. H. Vwriwee, Mater_ Rcs. &?I., IO (1975) 529. 7 P. K Gallaglur, D. W. Johnson, Jr., E M. Vogel and F. Schrey, Mafcr. Rcs. Bull., 10 (1975) 623.