Anal. Chem. 1994,66, 2226-2231
High-Performance Evolved Gas Analysis System for Catalyst Characterization P. A. Barnes,’ 0. M. B. Parkes, and E. L. Charsley Chemistry Group, Leeds Metropolitan Univers@, Calverley Street, Leeds, LSI 3HE, UK A new, high-resolutionsystem for analyzingand characterizing materials is described. Based on evolved gas analysis (EGA), it can perform both controlled transformation rate thermal analysisand stepwise isothermalanalysis as well as conventid (linear temperature program) EGA. Features include a fastresponse furnace, direct temperature measurement, sensitive specific detector, and sophisticated temperature control. The system permits, for the first time, a comparison of the three modes of operation under an identical sample environment. The advantages and limitations of each are illustrated using a sample chosen to model a type of reaction often found in the preparation of catalysts by calcination, the thermal decomposition of hydroxides. It is shown that these methods used with our system have potential advantages in analysis and in the study of complex thermal decomposition processes, by increasing the resolution of consecutive events and providing a more closely controlled sample environment. In conventional thermal analysis the sample is made to follow a predetermined, usually linear, temperature program while one or more of its properties, e.g., mass loss, energy change, or gas evolution, is measured. To improve resolution and provide better conditions for kinetic studies, a number of workers have developed techniques in which variable heating rates are used.l-12 Thesediffer from normal thermoanalytical procedures in that the heating rate is not predefined but is altered as a consequence of some change in a monitored parameter of the sample, e.g., in TG it would be the reaction rate as determined from the rate of change of sample mass. As the reaction rate increases the heating rate is decreased, the result being a lowered heating rate through each reaction step. This improves the resolution because of the lower temperature and concentration gradients which exist across the sample under these conditions. The work described in this paper is part of a program to develop new temperatureprogrammed techniques for characterizing materials and for preparing catalysts with enhanced properties. Previous Work. In 1962 Paulik and Paulik’ patented a new approach which they called “quasi-isothermal and quasi(I) Erden, L.; Paulik, F.; Paulik, J . Hungarian patent 152197, 1962. (2) Paulik, J.; Paulik, F. Thermochim. Acfa 1986. 100, 23. (3) Rouquerol, J. Bull. Soc. Chim. Fr. 1964, 31. (4) Reading, M.; Rouquerol, J. Thermochim. Acfa 1985, 85, 299. (5) Stacey, M. Langmuir 1987, 3, 681. (6) Real, C.; Alcala, M. D.; Criado, J. M. J. Therm. Anal. 1992, 38, 797. (7) Rouquerol, J. Thermochim. Acfa 1989, 144, 209. (8) Sorensen, 0. T. Thermochim. Acfa 1982, SO, 263. (9) Sorensen. 0. T. J. Therm. Anal. 1992, 38, 213. (10) Gill, P. S.; Sauerbrunn, R.; Crowe, B. S.J . Therm. Anal. 1992, 38, 255. ( 1 1) Reading, M. In Thermal Analysis-Techniques and Applicafions:Charsley, E. L., Warrington, S. B., Eds.; Royal Society of Chemistry: London, UK, 1992; pp 226-255. (12) Barnes, P. A.; Parkes, G. M. B., unpublished work.
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Ana~kaIChemlstry,Vol. 66, No. 14, July 15, 1994
isobaric” thermal analysis. They used a feedback mechanism from a thermobalance to change the heating rate in such a way as to force the sample to react at a constant rate. The partial pressure of the product gas over the sample was also controlled, at approximately 1 atm pressure, by using “convoluted” pans which restricted the escape of product gas from the sample. For reversible dissociation processes, the effect was that the major part of the reaction occurred at a virtually constant temperature, giving rise to the term “quasiisothermal”. A full review of this approach is given in ref 2. A similar technique, but operating at reduced pressure, was developed independently by Rouquerol3 who called his method “controlled transformation rate thermal analysis”. He utilized a vacuum thermobalance connected to a mass spectrometer via a constriction valve which maintained a fixed pressure of product gas above the sample, thus ensuring a constant reaction rate. Later work by Reading and Rouqueror used infrared detectors, while Stacey5 used a fluidized bed system, coupled to a katharometer, to investigate the decomposition of boehmite and gibbsite. Real et a1.6 have examined the temperature-programmed oxidation of catalysts using constant reaction rate. Rouquerol has given a detailed review of the technique in a recent paper.7 Sorensen8v9modified the Pauliks’ approach to develop a method called “stepwiseisothermal thermal analysis”. Instead of a heating rate which changes as the measured sample response varies, Sorensen arranged the apparatus so that the heating rate was switched between zero, when the sample responseexceeded an upper preset threshold, and a fixedvalue, when the response fell below a lower preset threshold. In this approach no attempt is made to keep the reaction rate constant, although it is constrained within limits. A commercial development, “dynamic rate TGA”, which uses a proprietary relationship between the reaction and heating rates, has become available from TA Instruments.’O Because neither the heating rate nor the reaction rate is kept constant, it has been proposed” that this method be called “constrained rate thermal analysis”. There is some confusion in the literature concerning the nomenclature for the above methods. The phrase “constant rate” is somewhat ambiguous as it could refer to the rate of either the reaction or the heating employed. To be absolutely clear in this paper we use the expression “constant reaction rate thermal analysis” (CRRTA) to describe the approach of Rouquerol and the Pauliks. Conventional (linear temperature programmed) evolved gas analysis (EGA) is referred to here as LTP-EGA, and Sorensen’s stepwise isothermal thermal analysis method is abbreviated to SITA. QQQ3-27QQI 941Q366-2226$Q4.5Q/Q
0 1994 Amdcan Chemical Society
Hygrometer Control Box
Valve
Hygrometer RS232C
The furnace is controlled by a Eurotherm 8 18P temperature programmer, which is capable of “self-tuning”; Le., it can alter internal control algorithms to match the particular furnace in use, by optimizing the PID parameters. Any desired temperature or heating rate can be set by means of commands sent from the computer, via an RS232C serial link to the 818P. The EGA detector used is an electrolytic hygrometer (Mark 2) manufactured by Salford Electrical Instruments. As it operates coulometrically, it is intrinsically highly accurate. It is a water-specific device and is very sensitive. It can provide a full-scale response to as little as 10 ppm water vapor in a gas stream. In this application, it is used on the least sensitive setting and with a reduced carrier gas flow rate to avoid overloading the detector. Although scaled in ppm water (by volume), the instrument response is flow-rate dependent and the nominal reading only applies at a flow rate of 100 cm3 m i d . In this paper, we use a calibration factor which corrects for the different flow rate used and converts the output directly into the rate of evolution of water, expressed in milligrams per minute. This can be related directly to the rate of reaction for the decomposition of the hydroxides. The hygrometer is mounted immediately above the furnace, to keep the swept volume to a minimum, and is connected to it by 15 cm of 3-mm stainless steel tubing which is heated to >lo0 OC to reduce errors caused by adsorption of water vapor. Helium is used as a carrier gas because of its good thermal conductivity, which is necessary to minimize the control-loop time constant. An accurate, constant flow of gas is maintained by a Bronkhorst Hi-Tec mass-flow controller. The output of the hygrometer is sent to the microcomputer via a Comark Compuface 16-bit analog-to-digital-convertor (ADC). This has four thermocouple inputs, four analog inputs, software-selectable voltage ranges, and internal filtering. Connection to the computer is made with a second RS232C serial link. The apparatus is controlled by a 33 MHz 486DX PC, running a custom-designed program, which controls the temperature programmer, acquires the data, and processes the results. Sophisticated temperature control is used, based on algorithms designed to minimize overshoot, ringing, and response time. The control and data-processing software was written in ANSI C using a C+ compiler (Borland) and provides three principal types of thermal analysis: (a) LTPEGA, Le., using conventional preset linear heating rates; (b) CRRTA, i.e., experiments where the temperature is adjusted to keep the rate of reaction constant; (c) SITA, Le., where the heating rate is switched between a fixed value and zero as the reaction rate crosses preset thresholds. Depending on the mode selected, the software allows the user to choose and/or record experimental parameters such as maximum and minimum heating rates, various reaction rate threshold values, sample weight and description, data sampling rate, etc. The operational parameters are stored on the computer’s hard disk and so may be retrieved later for subsequent examination or printing. During acquisition, data are recorded in real time in ASCII (text) format, which permits it to be reprocessed later, or, alternatively, loaded into a spreadsheet (such as Excel (Microsoft) or the FigP graphics program (Biosoft)) for subsequent analysis and plotting.
I Helium I
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486 DX
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Figure 1. Diagram of the CRRTA and SITA apparatus.
As each variable heating rate technique has its own advantages, the aim of the work described here was to develop a new system which could provide not only conventional LTPEGA but also CRRTA and SITA, so allowing the operator to select the most appropriate method for the work in hand. Although a specific detector is used in this work, it is equally possible to employ more generally applicable detectors such as a katharometer or mass spectrometer.’*
EXPER IMENTAL SECT1ON Equipment. The apparatus, which is described in detail elsewhere,’3 is illustrated in Figure 1. The evolved gas and temperature signals are processed, and according to the type of thermal analysis experiment in use, the computer instructs the temperature programmer to change the sample temperature or its heating rate appropriately. The new system has two key features. First, it provides the three types of thermal analysis technique described above. Second, it is designed to minimize the control-loop time constant, which is the time the system takes to adjust the sample temperature in response to an alteration in the reaction rate. The fast response means that good control can be maintained even at high reaction rates. Details of the various components are given below. Good temperature control, which is essential in CRRTA and SITA measurements, requires a furnace which can be heated and cooled quickly. This is particularly important if run times in CRRTA are to be minimized because this entails operating at high reaction rates, where a rapid furnace response is required to maintain control of the reaction. It is necessary that the system can produce both positive and negative heating rates to obtain true constant reaction rates. The nichrome wound furnace (Stanton Redcroft Model 76 1) has a maximum operating temperature of 1000 OC. It has a thin ceramic lining and a small swept volume (
