CARL E. LOCKE' and HOWARD F. RASE Department of Chemical Engineering, University of Texas, Austin 12, Tex.
Differential Thermal A n a l y s i s .
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Rapid Definition of Catalyst Characteristics With this new technique, optimum pretreating and reduction conditions can be determined rapidly, catalyst poisons can changes can be followed
CATALYST
STUDIES require tedious and repetitive evaluation tests, but the tedium can be reduced by using screening techniques which define limits of certain variables, and prove others insignificant. This procedure saves time and expense but, more importantly, it encourages a creative-type investigation. If ideas can be sifted rapidly by tests requiring only minutes, the range of possibilities seems much greater. One such screening technique, differential thermal analysis (DTA), has been used only occasionally on catalysts, and deserves more careful attention. T o evaluate the full range of its usefulness this technique has been applied to a commercial nickel-on-kieselguhr hydrogenation catalyst which is widely used and has been studied extensively in these laboratories (7, 3-5). DTA tests were made to evaluate the method for catalyst studies and to define and understand better the nickel catalyst. The reduction step was studied together with characteristics of both the reduced and unreduced catalyst. The data and qualitative conclusions have proved useful in planning and executing other research programs using this catalyst.
A controlled-pressure, controlled-atmosphere apparatus (Model No. DTA 11 MC; R. L. Stone Co., Austin, Tex., was used. This is similar to the equipment described previously (7) but has an improved valving system which ensures accurate timing of gas cycling. The catalyst was Harshaw Chemical Co. Ni-0101 T-1/8, a supported nickel hydrogenation catalyst prepared from basic nickel carbonate [xNiCOa:yNi( OH)z] deposited on kieselguhr. T h e l/a-inch tablets were ground to -48 100 mesh, and Type VI11 Johns Manville Co. Celite catalyst carrier, ground to -48 100 mesh, was used as the inert comparison sampIe. Electrolytic hydrogen was passed through an Englehard deoxo purifier and an alumina dryer before use. The method of operation has been described (7). Two main types of tests were used: straight DTA, where thermal
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* Present address, Continental Oil Co., Ponca City, Okla.
effects caused by changes in the catalyst were observed, while heating the sample at temperatures which were increased at a uniform rate; also two gases were cycled at constant temperature over the sample and thermal effects of adsorption and desorption were observed. The cycling runs were made after pretreatment of the sample in various ways. The flow time for each gas was regulated. By maintaining a constant rate of heating and using equal sample sizes, straight DTA runs were reproducible. The cycling runs, however, were slightly more difficult to reproduce. The degree of sample packing and flow rate for both hydrogen and nitrogen affected height of the hydrogen absorption peak. But satisfactory reproducibility (2 to 5% deviation from the average) was possible, and runs not within this percentage were discarded. Reduction and Catalyst Characteristics
Unreduced Catalyst. Thermograms on the unreduced catalyst in atmospheres of nitrogen, hydrogen, carbon dioxide, and water vapor revealed some interesting facts that otherwise would have been difficult to obtain. Two major endothermic peaks (downward deflections) occur in a nitrogen atmosphere beginning at 60' or 70' and 250' C., respectively, as illustrated. Neither peak is observed after cooling the catalyst and retreating. The first peak is probably caused by desorption of water vapor and/or oxygen from the catalyst. The second peak is particularly interesting because it occurs in a temperature range which investigators have used for reduction. Its significance is clarified by the thermograms in other gaseous atmospheres. Both carbon dioxide and water vapor retarded the second peak. The effect became more pronounced a t higher pressures, indicating that carbon dioxide and water are products of decomposition. Since the unreduced catalyst contains basic nickel carbonate which decomposes upon heating to nickel monoxide, carbon dioxide, and water, it may be concluded that on the catalyst this decomposition begins a t 250' C. in nitrogen atmospheres.
be detected, and phase
The catalyst also probably contains hydrosilicate formed from the reaction of basic nickel carbonate with kieselguhr (8,9).T h e hydrosilicate is known to decompose endothermically yielding silicon dioxide, nickel monoxide, and water, and this probably occurs in the same temperature range as the nickel carbonate decomposition. T h e second peak in a hydrogen atmosphere is shorter and of different shape than in nitrogen or carbon dioxide, indicating that another reaction may occur. This fact was confirmed by testing pure basic nickel carbonate which exhibited an endothermic peak a t 260' C. followed by a sharp exothermic peak probably the result of the exothermic reaction COz
+ CO + 7Hz
+ 2CH4
+ 3H20
which occurs in presence of reduced nickel ( 2 ) . Carbon monoxide is formed from carbon dioxide and water. O n the catalyst the exothermicity is masked by the endothermic decomposition of the carbonate and the nickel hydrosilicate. These speculations suggest a possible advantage in pretreating the catalyst in nitrogen atmospheres to decompose the carbonate and hydrosilicate fully prior to reduction with hydrogen. Reduction and Reduced Catalyst. Most optimum conditions of pretreatment and the effect of reactor feed contaminants can be obtained from a simple and rapid series of DTA tests. Chemisorbed hydrogen is essential for the hydrogenation reaction. Hence, the peak height, produced by hydrogen when cycling nitrogen and hydrogen, was selected to measure catalyst activity. Preliminary reaction tests confirm the validity of this basis. Cycling tests with nitrogen and hydrogen showed that maximum hydrogen adsorption (5-cm. peak heights) occurred on samples reduced from 400' to 500" C. for 2 hours. Above or below these temperatures, lower adsorption was observed, equivalent to 1.4 cm. VOL. 52, NO. 6
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At high temperatures a silica skin (6) forms on the nickel surfaces of nickel-onsilica catalysts (prepared by coprecipitation). Apparently this skin forms when the catalyst is subjected to reduction temperatures for 3 hours or longer, particularly above 300' C. The low hydrogen adsorption activity at reduction temperatures below 400' C. can be attributed to incomplete decomposition of the hydrosilicate bond. The hydrosilicate is not completely decomposed below 380' to 400' C. and thus all nickel in the catalyst cannot be reduced below this temperature. When reduced for 2 hours at 400' C. and pressures of 1 to 4 atm., apparent activity of the catalyst was increased with increasing hydrogen reduction pressures. I t is possible, however, that initial activity may reach a maximum and decline when hydrogen pressure is increased further; but pressures higher than 4 atm. could not be tested because of a limitation in the hydrogen purification systems. Initial activity, of course, is not the full criterion for judging catalyst performance. Maintenance of activity is also important and requires reaction studies for thorough analysis. The DTA results, however, suggest the importance of reduction pressure and the range of interest. When first placed in the sample holder, the catalyst is at equilibrium with the room air and contains adsorbed oxygen and water vapor which must be removed to assure reproducible results. This was conveniently done in most cases by evacuating the sample cell and connecting piping at approximately 9 mm. of mercury pressure for 5 minutes. Peak heights for hydrogen adsorption on a reduced catalyst sample increased with evacuation time up to 5 minutes. Although evacuation is convenient for DTA, purging with an inert gas is more practical in bench-scale reactor studies. Tests were conducted to determine the effectiveness of a nitrogen purge on a catalyst sample prior to reduction. After a minimum of 10 hours of nitrogen purge prior to reduction, hydrogen peaks were equal to those obtained with evacuated catalyst. Thus nitrogen purging as well as evacuation removes the contaminants. These results suggest that a standard procedure for nitrogen purging of the fresh catalyst be employed for bench-scale or pilot reaction studies. Ammonia and water vapor are typical temporary poisons for nickel catalysts; sulfur dioxide is a permanent poison. These three poisons were passed over reduced catalyst samples, reduced for 2 hours a t 400' C. The temporary poisons showed strong adsorption peaks but could be easily purged from the surface by nitrogen without affecting hydrogen adsorption by the catalyst. Because other compounds, not poisons,
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NITROGEN ATMOSPHERE
CARBON DIOXIDE ATMOSPHERE
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H Y DR 0 G E N ATM 0 SPHERE
v Straight data of catalyst in different atmospheres. In a nitrogen atmosphere, two major endothermic peaks occur. Both carbon dioxide and water retard the second peak
can show large adsorption peaks, the DTA is not parricularly useful in detecting temporary poisons. The permanent poisons such as sulfur dioxide, however, can be readily detected. A freshly reduced catalyst sample showed a 9.0-cm. peak for hydrogen adsorption before sulfur dioxide treatment and a 1.44-cm. peak after treatment. Even after a half hour purge with nitrogen, the peak continued at this low level, indicating permanent poisoning of the catalyst surface. Other permanent poisons would exhibit similar characteristics and suspected compounds could be readily screened by the DTA method. Evaluation of DTA- Technique
DTA studies on catalysts are necessarily qualitative in many respects, and may require additional evidence-e.g., for an unknown sample, other techniques must be used in conjunction with DTA to define completely chemical or physical changes on the catalyst. Also, approximate catalyst activity inferred from absorption peaks of a selected reactant must be ultimately confirmed in actual reaction studies. This does not, however, limit the value of the technique in defining rather precisely the activity variables and the regions for quantitative study. Once a correlation between peak heights and reaction activity is obtained, DTA can be used as a precise quantitative measure of activities for rapid routine control tests ( 7 ) . Finally, if individual peaks are obscured because they occur at approximately the same temperature, other means must be used, but often variations in heating rate, gaseous atmosphere, or test pressure will separate the peaks. DTA requires only a small fraction of the time that would be needed for benchscale or pilot plant tests yielding the
INDUSTRIAL AND ENGINEERING CHEMISTRY
same information. Permanenr poisons can be tested rapidly before development work begins. Catalyst pretreatment is also readily studied. Purging, temperature treatment, and reduction (where applicable) all cause marked changes in thermograms which can be readily interpreted in terms of possible optimum operating conditions. Further, significant changes in the catalyst itself can be inferred when accompanying thermal efTects are measurable. Thus DTA has the advantages of any screening device. Thermograms yield not only specific data but invariably generate ideas, define areas for fruitful investigation, and add insight on the nature of the catalyst. Such background is invaluable prior to the initiation of a bench-scale or pilot plant program, because it aids in planning and reduces the amount and complexity of reaction data required. Liferatwe Cited (1) Fair, J. R., Dissertation, The University of Texas, 1955. (2) Jacobson, C. A., "Encyclopedia of Chemical Reactions," vol. 111, p. 614, Reinhold, New I'ork, 1949. (3) Oldenberg, C. C., Rase, Howard F., A.I.Ch.E. Journal 3, 462-6 (1957). (4) Perkins, T. K., Rase, Howard F., Ibid., 4, 351-5 (1958). (5) Pozzi, A. L., Rase, Howard F., IND. ENG.CHEM.50, 1075-80 (1958). (6) Schuit, 6. C. A,, van Reijen, L. L., Advances in Catalysis 10, 24-31 (1958). (7) Stone, R. L., Rase, H. F., Anal. Chern. 29, 1273-7 (1957). (8) Visser, G. H., de Lange, J. J., Ingenieur (Ulrecht) 58, 24 (1946). (9) Voorthuijsen, J. J. B. van E. van, Franzen, P., Rec. trau. chim. 70, 793 (1951).
RECEIVED for review October 12, 1959 ACCEPTED March 28, 1960 Work supported by the University of Texas Research Institute, the Humble Refining Co., and through a summer student fellowship, by the National Science Foundation.
