Determination of the Platinum, Rhenium and Chlorine ...

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ANALYST, AUGUST 1991, VOL. 116

Determination of the Platinum, Rhenium and Chlorine Contents of Alumina-based Catalysts by X-ray Fluorescence Spectrometry

Published on 01 January 1991. Downloaded by Indian Institute of Technology New Delhi on 25/04/2016 07:33:12.

Rao. V. C. Peddy, G. Kalpana and Valsamma J. Koshy Research Centre, Indian Petrochemicals Corporation Limited, Baroda-397 346, India

N. V. R. Apparao, M. C. Jain and R. V. Patel Research Centre, Indian Oil Corporation, Faridabad, India

An X-ray fluorescence (XRF) method is described for the determination of the platinum, rhenium and chlorine contents of y-alumina supported catalysts. Calibration graphs were linear in the range 0.02-0.80,0.20-1 .I0 and 0.1-1.0% m/m for platinum, rhenium and chlorine, respectively. In order to determine the accuracy of the proposed method, the results obtained were compared with those given by ultraviolet-visible (UVNIS) spectrophotometry, inductively coupled plasma atomic emission spectrometry (ICP-AES), electrothermal atomic absorption spectrometry (ETAAS) and energy dispersive X-ray analysis (ED-SEM). Statistical analysis showed no significant errors at the 95% confidence interval for the chlorine content obtained by XRF, U V N l S spectrophotometry and ED-SEM. The determination of platinum in monometallic catalysts was carried out by XRF and the results obtained were compared with those given by ICP-AES, ETAAS and U V N l S spectrophotometry. Similarly, for bimetallic catalysts, the results obtained using XRF and U V N l S spectrophotometry for the determination of platinum and rhenium were compared. Statistical evaluation showed that there was n o significant bias between the analytical methods used. Keywords: X-ray fluorescence spectrometry; platinum; rhenium; chlorine; catalyst

Platinum supported catalysts have the ability to rearrange and transform the molecular structure of hydrocarbons; hence they have become very important in the petrochemical industry. The determination of platinum in catalysts requires a higher accuracy and precision than the normal analysis of materials owing to economic considerations. In the production of reforming catalysts on a commercial scale, small variations in the platinum assay will lead to heavy financial losses either by the producer or by the customer. Because of its importance, there are many different methods and techniques described in the literature for the determination of platinum. Most of these methods involve instrumental techniques: e.g., wavelength dispersive X-ray fluorescence spectrometry,1-4 X-ray Diffraction (XRD) ,5 atomic absorption spectrometry (AAS) ,637 emission ~pectrography~s.9 neutronactivation analysis10 and spectrophotometry .11-14 However, little work has been carried out on the determination of the elements present in bimetallic reforming catalysts. For work on catalyst development, a fast and reliable method of analysis is required, particularly when there is a need to analyse various batches of catalysts. X-ray fluorescence (XRF) provides an analytical facility that meets most of the requirements for the determination of the elements present over a wide range of concentration levels in alumina-based catalysts. 15 The reliability of XRF spectrometric results depends on the extent of correlation between the measured XRF intensities of the sample and calibration standards. The main source of error is likely to be matrix differences between the samples and standards. The aim of this work was to develop an XRF method for determining the platinum, rhenium and chlorine contents of y-alumina supported catalysts. As there were n o certified reference materials available for these elements, synthetic standards were prepared in this laboratory and the results obtained with the XRF method were compared with those given by other complementary techniques such as ultravioletvisible (UVNIS) spectrophotometry , AAS, inductively coupled plasma atomic emission spectrometry (ICP-AES) and energy dispersive X-ray analysis (ED-SEM) .

Experimenta1 Regents and Chemicals All reagents used were of analytical-reagent grade. Doubly distilled water was used throughout for dilution purposes. The y-alumina spheroids used for preparing the catalyst standards were 99.99% pure (Condea, USA). Stock solution of platinum, lo00 pg ml-1. Prepared by dissolving 0.5000 g of pure platinum wire in 20 ml of aqua regia [HC1-HNO3 ( 3 l)]. Oxides of nitrogen were removed by three successive evaporations with concentrated HCl. The residue was then dissolved in 10ml of HCI, the solution transferred into a 500ml calibrated flask and diluted to volume with water. Stock solution of rhenium, lo00 pg ml-1. Prepared by weighing 1.3007g of Re207 (Alfa Products) in a beaker, adding 25 ml of concentrated HCI to dissolve the contents and diluting to 11in a calibrated flask. Stock solution of chloride, 10mgml-1. Prepared by dissolving 1.65 g of dried sodium chloride (Merck) in water and making up to 100 ml in a calibrated flask.

+

Preparation of Standards y-Alumina was used as the support for preparing the catalyst standards. The support was dried at 200 "C for 4 h to remove moisture and volatile matter. Platinum standards were prepared to achieve a final concentration of platinum in the range 0.02-0.80% m/m on a 20 g batch of y-alumina when impregnated using the pore volume technique. 16 Rhenium standards were prepared in a similar fashion from the rhenium stock solution to achieve a final concentration of rhenium in the range 0.20-1.10% d m . For bimetallic standards, platinum and rhenium solutions were mixed in various proportions from the stock solutions in the working range 1: 1and 1:2 (by mass) of platinum and rhenium, respectively. They were made up to slightly more than the estimated pore volume of alumina and these solutions were impregnated on 20g batches of y-alu-

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mina. Each batch was then dried at 200 "C for 4 h. The XRD patterns of these samples were checked for uniform distribution of the active elements and were found to match well with the diffractograms of commercial catalyst samples. For the preparation of chlorine standards , y-alumina was finely powdered in an agate mortar with a pestle, sieved (200 mesh) and dried at 200 "C for 4 h in an air oven. The dried samples (5.0 g) were placed in separate 100 ml round-bottomed flasks. To these, the appropriate volume of the sodium chloride stock solution was added to attain a final concentration of chlorine in the range 0.1-1 .O% d m . The total volume of the solution was adjusted to 15 ml using doubly distilled water. The contents of the flasks were maintained at 40 "C for 4 h with constant stirring and then dried on a Biichi Rotavapor. The final drying stage was carried out in an air oven at 100 "C for 4 h . These standard samples were stored in clean, air-tight bottles, kept in a desiccator. Instrumentation and Measurement Procedure

All the measurements were made using a Philips PW 1410 XRF spectrometer and associated equipment. For determinations using U V N I S spectrophotometry , absorbance values were measured on a Varian Superscan-3 spectrophotometer with matched 1cm silica cells. The AAS measurements were carried out using a Varian Techtron 1200 atomic absorption spectrometer with a Varian CRA-63 graphite furnace accessory. A Kevex Model 7000-77 energy dispersive X-ray analyser attached to a scanning electron microscope (Jeol JSM 35C) was used for ED-SEM measurements. A Jarrell-Ash Atom Comp 1100-Mark I11 emission spectrometer was used for ICP-AES measurements. All synthetic standards and catalyst samples were finely powdered and sieved through 200 mesh and dried at 200 "C for 2 h in an air oven. Finely ground samples (2.5 g; 200 mesh) were placed in individual spex cells (1.5 x 2.5cm i.d.) and covered with Mylar film. The samples were loaded into the X-ray spectrometer and were compacted in situ for analysis. A portion (2.5 g) of each of these powdered samples was placed in individual spex cells and covered with Mylar film. These cells were loaded into the X-ray spectrometer for analysis. The experimental conditions are summarized in Table 1. X-ray fluorescence intensity data were obtained for all the standards of platinum, rhenium and chlorine. Six replicate measurements were made on each sample. The mean measured XRF intensities for each set were subjected to linear regression against the concentration of the corresponding analyte. Catalyst solutions were prepared and the tin(I1) chloride method was applied12 for the determination of platinum by UVNIS spectrophotometry . The reference solutions were treated in the same way as the samples. The absorbance of the orange-yellow platinum(1v)-tin(1r) chloride complex was measured at 403 nm. Similarly, thiourea and tin(r1) chloride were used for the determination of rhenium17 in the platinumrhenium bimetallic catalysts. The chloride content of the laboratory prepared standards was determined by a spectrophotometric method18 using iron(iI1)-mercury thiocyanate,

and these values were used for further calculations. Platinum was separated as the platinum-dithizone complex by extraction into isobutyl methyl ketone and determined by electrothermal atomic absorption spectrometry (ETAAS). l9 The chlorine contents of the catalyst samples were determined by ED-SEM.20 The chlorine K a X-ray energy falls between 2.58 and 2.68 keV (window). The determination of platinum in the monometallic catalysts was carried out using ICP-AES at 265.95 nm in the sequential mode.

Results and Discussion Evaluation of Laboratory Prepared Standards

Acquisition of the XRF data for the laboratory prepared standards was carried out under the experimental conditions listed in Table 1. The LiF analysing crystal was found to separate the platinum and rhenium peaks clearly. Although the normal method of preparing the powdered sample is first to grind it and then to form a pellet at high pressure, it was found in this work that when the finely ground samples were subjected to analysis, reproducible results were obtained [relative standard deviation (RSD) S 1 .O%]. Hence the powdered samples were used as such for analysis. The bulk of the scattered continuum emanated from the Mylar window fitted to the base of the sample cup. The spectral background, which was found to be constant for the samples, was subtracted from the peak counts for each measurement. The counts corresponding to the alumina blank were negligible. New calibration graphs were constructed each time by plotting signal intensity against analyte concentration. The time required for the calibration procedure is about 40min. The calibration graphs for platinum, rhenium and chlorine were linear (Fig. 1) for the concentration ranges covered by the standards: the regression parameters are listed in Table 2. The bimetallic catalyst reference material prepared by the procedure described above was analysed by spectrophotometry. 17 This reference material was also analysed by XRF using monometallic standards, and it was found that the values for platinum obtained with XRF and those given by UVNIS spectrophotometry were comparable. For rhenium, there was a slight deviation which might be due to interelement interferences caused by platinum. The values for rhenium obtained by XRF were higher than those afforded by spectrophotometry. In order to account for this deviation, a correction factor ( K f )was employed based on the equation:

where XI and X 2 are the rhenium concentrations obtained by XRF and the standard spectrophotometric method, respectively, and X3 is the concentration of platinum obtained by XRF. This factor was employed in the determination of rhenium in bimetallic catalyst samples.

Table 1 Instrumental parameters of the XRF spectrometer

Cr tube Element Pt Re

c1

kV 40 40 30

20ldegrees

mA 30 30 20

* PK: Peak. 7 BG: Background. $ SC: Scintillation counter.

5 PET: Pentaerythritol.

7 FC: Gas flow proportional counter.

Crystal LiF (200) LiF (200) PETS

Collimator Coarse Coarse Fine

Line Pt La Re Lor CI Kor

Detector

scs scs FCll

PK* 38.15 41.74 66.39

BGS -1.15 +1.26 1-1.61

Counting tirne/s

PK* 40 40 40

BGS 20 20 20

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I

0.02

0.80

5

25

Pt (%)

1

Pt/pg m1-I

5 W p g ml-1

I

10.1

0.76

m

z X

0.06

4.6 0.45

1.05

2

25

0.24

0.9

0.1

CI (%)

Re/pg ml-'

Re (%)

-(g)

a C

e

r = 0.9989

2

2 5.3

0.03 0.5

5.0 CVpg rnl

0.9

0.1 CI (%)

Fig. 1 Calibration graphs for Pt, Re and C1. ( a ) Pt (XRF); (b) Pt (UVNIS) (Pt : Re molar ratio, A, 1:O; B, 1: 1; and C, 1:2); (c) Pt (ETAAS); (d) Re (XRF); (e) Re (UVNIS); (f) C1 (XRF); ( g ) C1 (UVNIS); and (h) C1 (ED-SEM) Table 2 Regression parameters for calibration graphs

Concentration No. of range of Method Element r standards* points 6 0.02-0.8 Pt 0.9961 XRF 5-25 5 0.9999 UV/VIS Pt 5 0.9963 1-5 ETAAS Pt 0.9994 5 5-25 UVNIS Ptt 5-25 0.9971 5 UVNIS PtS 6 0.20-1.1 Re 0.9953 XRF 2-25 0.9999 UVNIS 6 Re 4 0.10-1 .o 0.9945 XRF c1 0.9989 6 UVNIS C1 0.50-5.0 ED-SEM 4 0.9935 Cl 0.10-1 .o * Values in % m/m for XRF and ED-SEM and in pg ml-* for the other methods. t For Pt : Re (1: 1) bimetallic catalyst standards. S For Pt : Re (1 : 2) bimetallic catalyst standards.

Table 3 Chlorine contents of commercial catalysts determined by using three spectrometric methods

Chlorine (Yo m/m) Sample description* XRFt UV/VIS ED-SEM 0.78 0.77 0.83 P- 1 0.13 0.11 0.14 P-2 0.09 0.10 0.12 P-3 0.60 0.65 0.60 P-4 0.11 0.10 0.12 P-5 0.15 0.13 P-6 0.14 0.77 0.74 0.76 PR- 1 0.71 0.78 PR-2 0.72 0.69 0.66 PR-3 0.67 * P-1-P-6 are Pt/y-A1203monometallic catalysts. PR-1-PR-3 are Pt-Re/y-A1203 bimetallic catalysts. n = 6; RSD 0.97) and rhenium (r > 0.99) data obtained with XRF and UVNIS spectrophotometry (Table 4) indicate that the methods are not significantly different.

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Table 4 Correlation parameters for the different methods of catalyst analysis

Sample No. 1 2 3 4 5 6 7 * Bimetallic catalyst. t Monometallic catalyst.

Method XRF versus UVNIS XRF versus UVIVIS XRF versus ICP-AES XRF versus UVIVIS XRF versus ETAAS XRF versus UVIVIS XRF versus ED-SEM

Element Pt* Re Ptt Ptt Ptt C1 C1

Table 5 Platinum contents of Pt/y-A1203 monometallic catalysts determined by using four different methods

Platinum (YO m/m) Sample No. P- 1 P-2 P-3 P-4

XRF* 0.61 0.36 0.34 0.60 P-5 0.33 P-6 0.42 * n = 6; RSD So.7%.

UVNIS 0.59 0.36 0.32 0.59 0.39 0.40

ICP-AES 0.60 0.34 0.36 0.60 0.37 0.40

ETAAS 0.60 0.37 0.34 0.60 0.38 0.41

Conclusions The results obtained here show that the proposed XRF method is comparable to the conventional methods for determining the platinum, rhenium and chlorine contents of mono- and bimetallic catalysts using synthetic standards. The time required for the XRF analysis of various catalyst batches is very short (6 min for each sample after the calibration has been performed) compared with the other methods in which the solution preparation step is more time consuming. The standards, once prepared, can be used several times in the XRF analysis; this is an additional advantage of the method over UVNIS spectrophotometry, ETAAS and ICP-AES. For the determination of rhenium, the composition of the reference material should be very close to that of the batch samples. The proposed method is non-destructive, sufficiently fast, sample preparation is relatively simple and the sensitivity is adequate for the determination of platinum, rhenium and chlorine in alumina-based catalysts; the method could be used as a quality control method in a production unit. The authors are grateful to Dr. I . S. Bhardwaj for permission to publish this work. They also thank R. H. Patel, N. R. Shah and M. Sunder for their technical assistance.

References 1 Coombes, R. J., and Chow, A., Anal. Chim. Acta, 1977, 91, 273. 2 Lincoln, A. J., and Davis, E. N., Anal. Chem., 1959,31, 1317. 3 Kallmann, S., Talanta, 1976,23, 579.

Slope 0.9492 0.9970 0.9893 0.9756 0.9899 0.9932 0.9969

Intercept 0.0130 0.0012 0.0050 0.0109 0.0049 0.0028 0.0014

r 0.9452 0.9960 0.9894 0.9720 0.9898 0.9924 0.9968

Table 6 Platinum and rhenium contents of Pt-Re/y-Al*03 bimetallic catalysts determined by using different methods

Platinum (Yo mlm) Rhenium (YO m/m) Sample No. XRF* UVIVIS XRF* UVNIS PR- 1 0.30 0.30 0.24 0.28 PR-2 0.30 0.30 0.72 0.69 PR-3 0.31 0.62 0.30 0.63 0.25 PR-4 0.34 0.31 0.26 PR-5 0.26 0.25 0.25 0.26 PR-6 0.25 0.21 0.36 0.37 PR-7 0.25 0.24 0.59 0.59 PR-8 0.27 0.29 0.27 0.26 * n = 6; RSD (Pt) S1.0%;RSD (Re) So.7%.

4 Beamish, F. E., Lewis, C. L., and Van Loon, J. C., Talanta, 1969, 16, 1. 5 Van Norstrand, R. A., Lincoln, A. J., and Carnevole, A., Anal. Chem., 1964,36, 819. 6 Potter, N. M., Anal. Chem., 1976, 48, 531. 7 Rubeska, I., and Stupar, J., At. Absorpt. Newsl., 1966, 5, 69. 8 Talalaen, B. M., Zh. Anal. Khim., 1964. 19, 1163. 9 Cooley, E. F., Curry, K. J., and Carlson, R. R., Appl. Spectrosc., 1976,30, 52. 10 Venovking, A. V., Gilbert, E. N., and Mckhailev, V. A., J. Radioanal. Chem., 1977, 36, 359. 11 Sandell, E. B., Colorimetric Determination of Traces of Metals, Interscience, New York, 3rd edn., 1959. 12 Ayres, G. H., and Meyer, A. S., Jr., Anal. Chem., 1951, 23, 299. 13 Conrad, A. J., and Evans, J. K., Anal. Chem., 1960,32, 47. 14 Okubo, T., and Kojima, M., Bunseki Kagaku, 1966, 15, 845. 15 Labrecque, J. J., X-Ray Spectrom., 1980, 9, 28. 16 Castro, A. A., Scelza, 0. A., Bencenuto, E. R., Baronetti, G. T., De Miguel, S. R., and Parere, J. M., Preparation of Catalysts IZZ, Elsevier, Amsterdam, 1983. 17 Olivera, G., Garcia, S., and Bezombe, A., Rev. Fac. Ing. Quim., 1987,47, 35. 18 Koshy, V. J., and Garg, V. N., Talanta, 1987,34,905. 19 Kalpana, G., Koshy, V. J., and Garg, V. N., Indian J . Chem., submitted for publication. 20 Koshy, V. J., Kalpana, G., Rao, K. V., and Garg, V. N., Talanta, in the press. 21 Ripley, B. D., and Thompson, M., Analyst, 1987, 112, 377.

Paper 1I00166 C Received January 14th, 1991 Accepted April 2nd, 1991