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Application of electron probe micro-analysis to the estimation of chlorine in alumina-based heterogeneous catalysts
Talon~a. Vol. 39. No. I. Pnnted tn Great Bntain
pp. 17-19.
1992
0039-9140/92 S5.00 + 0.00 Pergamon Press plc
APPLICATION OF ELECTRON PROBE MICRO-ANALYSIS TO THE ESTIMATION OF CHLORINE IN ALUMINA-BASED HETEROGENEOUS CATALYSTS V. J. KOSHY; K.
V. RAO,G. KALPANAand V. N. GARG
Research Centre, Indian Petrochemicals Corporation Limited, PO: Petrochemicals, Gujarat, India
391 346, Vadodara,
(Received 24 July 1990. Revised 2 July 199 1. Accepted 3 July 1991)
Summary-Chlorine in alumina-based catalysts has been determined with a scanning electron microscope attached to an energy dispersive x-ray analyser (SEM-EDX). The method is less time consuming compared to conventional methods involving sample dissolution followed by titrimetry, absorption spectrophotometry or ion chromatography. The spectrometer is calibrated with laboratory pmpared standards. This technique IS found suitable for the estimation of chlorine in the range 0.1~1.0% (w/w) wrth a relative standard deviation < 10% for chlorine levels above 0.2%.
Noble metal catalysts, mainly platinum supported catalysts, are employed in several petrochemical processes like dehydrogenation, reforming and isomerization. It is well known that the chlorine ion which gets incorporated during impregnation of platinum into the alumina support has profound influence on the physicochemical properties of the catalyst.’ Chlorine balance in the catalyst is critically important for optimum activity, selectivity and stability of the catalyst during reforming operations. It is therefore important to monitor the level of chlorine in the catalysts during their preparation as well as usage in commercial plants. Argentometric titration,’ x-ray fluorescence spectrometry,’ absorption spectromet$ and ion-chromatography’ are the techniques reported for the estimation of chlorine in catalysts. In the present study, the possibility of using the Scanning Electron Microscope in conjunction with an Energy Dispersive X-ray Analyser (SEM-EDX) is investigated. This technique has been widely used for qualitative identification of elements, but rarely for their quantitative estimation in catalysts. The accurate quantification of elements by x-ray emission methods is based on availability of reference materials whose matrix and composition match fairly well with those of the samples to be analysed.’ Since these standards
were not available, they were prepared in our laboratory and their chlorine content was ascertained by spectrophotometry. EXPERIMENTAL
A Scanning Electron Microscope (JEOL, JSM-35C) in conjunction with an energy dispersive x-ray analyser (Kevex, model 7000-77) was employed for this study. EDX spectra were recorded on a Hewlett-Packard 7015 B X-Y recorder. Absorbance measurements were recorded on a Varian Superscanspectrophotometer with matched l-cm silica cells. All reagents used were of GR grade. yAlumina pellets (3.2 mm) from Alfa Products were used as the support material for prep aration of standards. Doubly distilled water was used in all the experiments. Preparation of standard samples Sodium chloride was dried at 100” for 4 hr. About 1.65 g was weighed accurately into a lOO-ml standard flask and made up with water to prepare a stock solution containing 10 mg/ml chloride. A 5.0-g portion of finely powdered and dried ( < 2 10 mesh, 200” for 12 hr) y-alumina was taken in six separate lOO-ml round bottom flasks. To each of them was added 0, 1, 2, 3, 4 and 5 ml, respectively, of sodium chloride solution. The total volume of solution in each flask was adjusted to 15 ml with water and the contents were kept at 40” for 4 hr with constant stirring. Subsequently, the samples were dried
IPLC Communicatron No.: 173. *Author for correspondence. 17
V. J. KOSHYet
18
al.
with a Buchi Rotavapor and the final drying was carried out in an air oven at 100” for 4 hr. These samples were cooled and transferred into airtight containers and preserved in a desiccator. The chlorine contents of laboraestimated were standards tory-prepared by the spectrophotometric method,6 using Fe(III)-Hg(SCN)2 reagent. Preparation of samples for SEM-EDX men ts
measure-
All standards and catalyst samples were ground to pass through a 210-mesh sieve. Pellets of 13-mm diameter and 2-mm thickness were prepared with a hydraulic press at a pressure of 10 tons/cm2 and a retention time of 2 min. The flat surfaces of the spacers that came into contact with the pellet were mirror-polished so that the specimen (pellet) had a very smooth surface. The pellet was cut into four quadrants and one quadrant was mounted on a carbon stub with silver conducting paste with the smooth surface facing upwards. Since all the specimens were nonconducting, a carbon layer of 400 8, thickness was coated by using a JEOL JEE4X vacuum evaporator in order to avoid charge build-up and specimen heating during the analysis. Spectra acquisition
The sample, after mounting and carbon coating, was kept in the specimen chamber of the scanning electron microscope. The electron flux was kept constant by maintaining the x-ray count rate of 4000 cps on a clean aluminium stub. The count rate did not change throughout the spectral acquisition, indicating that there was no loss of surface chlorine due to electron beam impingement. The other conditions used in operating the x-ray analyser are given in Table 1. RESULTS AND DISCUSSION
The spectra obtained from the various samples have been analysed by different comTable 1.Operating conditions for the acquisition of EDX spectrum of chlorine on y-Alumina Operating voltage Specimen surface area Specimen tilt Magnification Spectrometer range Acqmsition time Dead time
I8 KV 2 mm2 zero degree 100X
10 eV/channel 200 set 20%
In
E
a
0
A
, 1A4
Energy, keV
4. PJ
Fig. 1. EDX spectra of (A) support (y -Al,O, ), (B) standard Cl/y-AI,O>) (C) catalyst (Cl/Pt/y-Al,O,-3 in Table 2) and (D) catalyst (Cl/Pt-Re/y-AI,O,-5 in Table 2). (0.95%
puter programs like ASAP, ZAF and LSQ supplied by Quantex software. The LSQ program was found to be the most suitable in the present case. It uses a background filtering routine and determines the area of peak in terms of counts per second in the range of interest, i.e., 2.62 keV for chlorine K, emission. Figure 1 shows the x-ray emission spectrum of alumina samples in the presence and absence of chlorine. Evaluation of standardr
The uniformity as well as stability of the laboratory-prepared standards have been tested by recording their EDX spectra and analysing them by the LSQ method. Four standard samples having chlorine contents of 0.14, 0.36, 0.61 and 0.95% (w/w) were chosen and six spectra were acquired from different portions of each sample. This exercise was carried out on the basis of the assumption that non-uniformity of chlorine distribution in the sample would result in a difference in the analyte signals obtained from different portions of the same sample specimen. The variation in the results was less than 3%, suggesting that the samples were uniform. The chlorine peak intensity measurement of each standard sample was performed over a period of three months and the variation observed from time to time was S-8%
Estimation of chlorine in alumina-based
heterogeneous
catalysts
19
obtained by SEM-EDX agree well with the spectrophotometric results. The method has been applied successfully in the case of platinum on alumina as well as platinum-rhenium on alumina catalysts. However, under the experimental conditions employed, i.e., channel width of lOeV, Ru (L, = 2.56 keV) and Rh (L,= 2.70 keV) can cause interferences. Chlorine content
[% (w/w)]
Fig. 2. Relationship between counts per second and chlorine content with LSQ program. Table 2. Chlorine content of various laboratoryprepared Alumina supported Pt-Re catalysts with different composition Sample No.
1 2 3 4 5 6
Chlorine content (% w/w) by SEM-EDX Spectrophotometry 0.13 If: 0.04 0.15 f 0.03 0.60 _+0.02 0.78 + 0.02 0.69 It 0.03 0.77 f 0.03
0.11 + 0.13 f 0.65 f 0.77 f 0.66 f 0.74 f
0.01 0.01 0.02 0.02 0.01 0.01
when each of the standards was analysed with the stored calibration graph. Hence it is considered essential to acquire fresh standard spectra and construct a calibration graph each day, before sample analysis. The calibration graph obtained on a particular day is presented in Fig. 2. Analysis of alumina-based catalyst samples
The EDX spectrum A in Fig. 1 shows the absence of any x-ray emission in the region of 2.62 keV energy. The chlorine peak is clearly shown in spectrum B corresponding to the calibration standard. Spectra C and D show the well-defined chlorine peak sufficiently separated from all concomitant emission lines, enabling accurate computation. Catalyst samples, designated 1 through 6, were analysed by the LSQ routine and the results are presented in Table 2. The standard deviations in the order of 0.04% in chlorine content show good reproducibility of the method. The mean values of chlorine
CONCLUSION
A simple, rapid and direct method has been developed for the estimation of chlorine in catalysts with Quantex software LSQ method. Laboratory prepared standards are suitable for calibration purposes. It is essential that spectra of calibration standards be acquired prior to the analysis of catalyst samples. Noble metals like Ru and Rh can cause interference by virtue of their L, emission. The spatial resolutions of this technique make it useful for profiling studies, an objective in our laboratory. Acknowlcdgmnfs-The authors wish to thank Dr 1. S. Bhardwaj, Direotor (R&D), IPCL for encouragement and permission granted to publish this paper. They also thank Dr K. R. Krishnamurthy for the supply of catalyst samples. The skilled assistance of Mr A. R. Shah, Mr H. N. Vaidya and MS R. H. Pate1 is gratefully acknowledged. REFERENCES 1. A. A. Castro, 0. A. Scelza, E. R. Benvenuto, G. T. Baronetti, S. R. De Miguel and J. M. Parera, in Preparation of Caralysts III, G. Poncelet, P. Grouge and P. A. Jacobs (eds), p. 47. Elsevier, Amsterdam, 1983. 2. R. J. Verderoane, C. L. Pieck, M. R. Sad and J. M. Parera, Appl. Cafal., 1986, 21, 239. 3. M. N. Edgar, in Applied Industrial Catalysts, B. E. Loach (cd.), Vol. I, pp. 141, 147. Academic Press, New York, 1983. 4. M. Sittig, Ho&book of Cafalysr Manufacture, p. 230. Noyes Data Corporation, New Jersey, 1978. 5. J. P. Bouronville and G. Martino, in Caralysr Deactiuation, B. Delmon and G. F. Froment (eds), p. 159. Else&r, Amsterdam, 1980. 6. V. J. Koshy and V. N. Garg, Talanla, 1987, 34, 905. 7. R. P. Sit@, K. Alam, D. S. Redwan and N. M. Abbas, Anal. Chem., 1989, 61, 1924. 8. J. Goldstein, D. Newbury, P. Echlin, D. Joy, E. Fiori and E. Lifshin, Scanning Electron Microscopy and X-ray Microanalysis, Plenum Press, New York, 1981.
pp. 17-19.
1992
0039-9140/92 S5.00 + 0.00 Pergamon Press plc
APPLICATION OF ELECTRON PROBE MICRO-ANALYSIS TO THE ESTIMATION OF CHLORINE IN ALUMINA-BASED HETEROGENEOUS CATALYSTS V. J. KOSHY; K.
V. RAO,G. KALPANAand V. N. GARG
Research Centre, Indian Petrochemicals Corporation Limited, PO: Petrochemicals, Gujarat, India
391 346, Vadodara,
(Received 24 July 1990. Revised 2 July 199 1. Accepted 3 July 1991)
Summary-Chlorine in alumina-based catalysts has been determined with a scanning electron microscope attached to an energy dispersive x-ray analyser (SEM-EDX). The method is less time consuming compared to conventional methods involving sample dissolution followed by titrimetry, absorption spectrophotometry or ion chromatography. The spectrometer is calibrated with laboratory pmpared standards. This technique IS found suitable for the estimation of chlorine in the range 0.1~1.0% (w/w) wrth a relative standard deviation < 10% for chlorine levels above 0.2%.
Noble metal catalysts, mainly platinum supported catalysts, are employed in several petrochemical processes like dehydrogenation, reforming and isomerization. It is well known that the chlorine ion which gets incorporated during impregnation of platinum into the alumina support has profound influence on the physicochemical properties of the catalyst.’ Chlorine balance in the catalyst is critically important for optimum activity, selectivity and stability of the catalyst during reforming operations. It is therefore important to monitor the level of chlorine in the catalysts during their preparation as well as usage in commercial plants. Argentometric titration,’ x-ray fluorescence spectrometry,’ absorption spectromet$ and ion-chromatography’ are the techniques reported for the estimation of chlorine in catalysts. In the present study, the possibility of using the Scanning Electron Microscope in conjunction with an Energy Dispersive X-ray Analyser (SEM-EDX) is investigated. This technique has been widely used for qualitative identification of elements, but rarely for their quantitative estimation in catalysts. The accurate quantification of elements by x-ray emission methods is based on availability of reference materials whose matrix and composition match fairly well with those of the samples to be analysed.’ Since these standards
were not available, they were prepared in our laboratory and their chlorine content was ascertained by spectrophotometry. EXPERIMENTAL
A Scanning Electron Microscope (JEOL, JSM-35C) in conjunction with an energy dispersive x-ray analyser (Kevex, model 7000-77) was employed for this study. EDX spectra were recorded on a Hewlett-Packard 7015 B X-Y recorder. Absorbance measurements were recorded on a Varian Superscanspectrophotometer with matched l-cm silica cells. All reagents used were of GR grade. yAlumina pellets (3.2 mm) from Alfa Products were used as the support material for prep aration of standards. Doubly distilled water was used in all the experiments. Preparation of standard samples Sodium chloride was dried at 100” for 4 hr. About 1.65 g was weighed accurately into a lOO-ml standard flask and made up with water to prepare a stock solution containing 10 mg/ml chloride. A 5.0-g portion of finely powdered and dried ( < 2 10 mesh, 200” for 12 hr) y-alumina was taken in six separate lOO-ml round bottom flasks. To each of them was added 0, 1, 2, 3, 4 and 5 ml, respectively, of sodium chloride solution. The total volume of solution in each flask was adjusted to 15 ml with water and the contents were kept at 40” for 4 hr with constant stirring. Subsequently, the samples were dried
IPLC Communicatron No.: 173. *Author for correspondence. 17
V. J. KOSHYet
18
al.
with a Buchi Rotavapor and the final drying was carried out in an air oven at 100” for 4 hr. These samples were cooled and transferred into airtight containers and preserved in a desiccator. The chlorine contents of laboraestimated were standards tory-prepared by the spectrophotometric method,6 using Fe(III)-Hg(SCN)2 reagent. Preparation of samples for SEM-EDX men ts
measure-
All standards and catalyst samples were ground to pass through a 210-mesh sieve. Pellets of 13-mm diameter and 2-mm thickness were prepared with a hydraulic press at a pressure of 10 tons/cm2 and a retention time of 2 min. The flat surfaces of the spacers that came into contact with the pellet were mirror-polished so that the specimen (pellet) had a very smooth surface. The pellet was cut into four quadrants and one quadrant was mounted on a carbon stub with silver conducting paste with the smooth surface facing upwards. Since all the specimens were nonconducting, a carbon layer of 400 8, thickness was coated by using a JEOL JEE4X vacuum evaporator in order to avoid charge build-up and specimen heating during the analysis. Spectra acquisition
The sample, after mounting and carbon coating, was kept in the specimen chamber of the scanning electron microscope. The electron flux was kept constant by maintaining the x-ray count rate of 4000 cps on a clean aluminium stub. The count rate did not change throughout the spectral acquisition, indicating that there was no loss of surface chlorine due to electron beam impingement. The other conditions used in operating the x-ray analyser are given in Table 1. RESULTS AND DISCUSSION
The spectra obtained from the various samples have been analysed by different comTable 1.Operating conditions for the acquisition of EDX spectrum of chlorine on y-Alumina Operating voltage Specimen surface area Specimen tilt Magnification Spectrometer range Acqmsition time Dead time
I8 KV 2 mm2 zero degree 100X
10 eV/channel 200 set 20%
In
E
a
0
A
, 1A4
Energy, keV
4. PJ
Fig. 1. EDX spectra of (A) support (y -Al,O, ), (B) standard Cl/y-AI,O>) (C) catalyst (Cl/Pt/y-Al,O,-3 in Table 2) and (D) catalyst (Cl/Pt-Re/y-AI,O,-5 in Table 2). (0.95%
puter programs like ASAP, ZAF and LSQ supplied by Quantex software. The LSQ program was found to be the most suitable in the present case. It uses a background filtering routine and determines the area of peak in terms of counts per second in the range of interest, i.e., 2.62 keV for chlorine K, emission. Figure 1 shows the x-ray emission spectrum of alumina samples in the presence and absence of chlorine. Evaluation of standardr
The uniformity as well as stability of the laboratory-prepared standards have been tested by recording their EDX spectra and analysing them by the LSQ method. Four standard samples having chlorine contents of 0.14, 0.36, 0.61 and 0.95% (w/w) were chosen and six spectra were acquired from different portions of each sample. This exercise was carried out on the basis of the assumption that non-uniformity of chlorine distribution in the sample would result in a difference in the analyte signals obtained from different portions of the same sample specimen. The variation in the results was less than 3%, suggesting that the samples were uniform. The chlorine peak intensity measurement of each standard sample was performed over a period of three months and the variation observed from time to time was S-8%
Estimation of chlorine in alumina-based
heterogeneous
catalysts
19
obtained by SEM-EDX agree well with the spectrophotometric results. The method has been applied successfully in the case of platinum on alumina as well as platinum-rhenium on alumina catalysts. However, under the experimental conditions employed, i.e., channel width of lOeV, Ru (L, = 2.56 keV) and Rh (L,= 2.70 keV) can cause interferences. Chlorine content
[% (w/w)]
Fig. 2. Relationship between counts per second and chlorine content with LSQ program. Table 2. Chlorine content of various laboratoryprepared Alumina supported Pt-Re catalysts with different composition Sample No.
1 2 3 4 5 6
Chlorine content (% w/w) by SEM-EDX Spectrophotometry 0.13 If: 0.04 0.15 f 0.03 0.60 _+0.02 0.78 + 0.02 0.69 It 0.03 0.77 f 0.03
0.11 + 0.13 f 0.65 f 0.77 f 0.66 f 0.74 f
0.01 0.01 0.02 0.02 0.01 0.01
when each of the standards was analysed with the stored calibration graph. Hence it is considered essential to acquire fresh standard spectra and construct a calibration graph each day, before sample analysis. The calibration graph obtained on a particular day is presented in Fig. 2. Analysis of alumina-based catalyst samples
The EDX spectrum A in Fig. 1 shows the absence of any x-ray emission in the region of 2.62 keV energy. The chlorine peak is clearly shown in spectrum B corresponding to the calibration standard. Spectra C and D show the well-defined chlorine peak sufficiently separated from all concomitant emission lines, enabling accurate computation. Catalyst samples, designated 1 through 6, were analysed by the LSQ routine and the results are presented in Table 2. The standard deviations in the order of 0.04% in chlorine content show good reproducibility of the method. The mean values of chlorine
CONCLUSION
A simple, rapid and direct method has been developed for the estimation of chlorine in catalysts with Quantex software LSQ method. Laboratory prepared standards are suitable for calibration purposes. It is essential that spectra of calibration standards be acquired prior to the analysis of catalyst samples. Noble metals like Ru and Rh can cause interference by virtue of their L, emission. The spatial resolutions of this technique make it useful for profiling studies, an objective in our laboratory. Acknowlcdgmnfs-The authors wish to thank Dr 1. S. Bhardwaj, Direotor (R&D), IPCL for encouragement and permission granted to publish this paper. They also thank Dr K. R. Krishnamurthy for the supply of catalyst samples. The skilled assistance of Mr A. R. Shah, Mr H. N. Vaidya and MS R. H. Pate1 is gratefully acknowledged. REFERENCES 1. A. A. Castro, 0. A. Scelza, E. R. Benvenuto, G. T. Baronetti, S. R. De Miguel and J. M. Parera, in Preparation of Caralysts III, G. Poncelet, P. Grouge and P. A. Jacobs (eds), p. 47. Elsevier, Amsterdam, 1983. 2. R. J. Verderoane, C. L. Pieck, M. R. Sad and J. M. Parera, Appl. Cafal., 1986, 21, 239. 3. M. N. Edgar, in Applied Industrial Catalysts, B. E. Loach (cd.), Vol. I, pp. 141, 147. Academic Press, New York, 1983. 4. M. Sittig, Ho&book of Cafalysr Manufacture, p. 230. Noyes Data Corporation, New Jersey, 1978. 5. J. P. Bouronville and G. Martino, in Caralysr Deactiuation, B. Delmon and G. F. Froment (eds), p. 159. Else&r, Amsterdam, 1980. 6. V. J. Koshy and V. N. Garg, Talanla, 1987, 34, 905. 7. R. P. Sit@, K. Alam, D. S. Redwan and N. M. Abbas, Anal. Chem., 1989, 61, 1924. 8. J. Goldstein, D. Newbury, P. Echlin, D. Joy, E. Fiori and E. Lifshin, Scanning Electron Microscopy and X-ray Microanalysis, Plenum Press, New York, 1981.