Determination of Platinum in Reforming Catalyst by X-Ray Fluorescence E. L. GUNN Humble Oil & Refining CO, Baytown, lex.
A n x-ray fluoreseenee m e t h o d has been developed a n d applied to the routine d e t e r m i n a t i o n of platinum in petroleum r e f o r m i n g catalyst. The specimen is prepared for analysis b y briquetting the sample with 5% of an organic binder. A t the 0.6v0 level the precision of m e a s u r e m e n t is expressed as a s t a n d a r d deviation of 0.006~0platinum. Excellent a g r e e m e n t with c h e m i c a l m e t h o d s is shown. Instrumental d r i f t w i t h time is m i n o r b u t detectable; a m e t h o d of eorreeting f o r it is described. O p t i m u m tube c o n d i t i o n s me seleeted for m a x i m u m peak-to-background ratio. The influence of oarhon, iron, and w a t e r c o n t a m i n a t i o n in a catalyst is minor or negligible. H e a t treatment does not affect the value o b t a i n e d on a catalyat. H e l i u m offers no m a r k e d a d v a n t a g e over air as an atmosphere in the optical path of the i n s t r u m e n t .
T.
HE initiation of plant reforming operations in the petroleum
mdustry hy means of platinum catalyst has made necessary the consideration of several methods of analysis for routine inspection of this catalyst. One of the inspections required is the determination of platinum ‘content in the fresh catalyst. The platinum content of a commeroially available fresh reforming a t d y 8 t is 0.50 to 0.60%. After depletion through plant use, the spent catalyst may be forwarded to a processor for reworking and recovery of platinum. Because platinum is an expensive commodity, an accurate evaluation of platinum content in both the fresh and spent inventory is very important. An inspection error of only 0.02% on a 100,000-pound quantity has monetary value equivalent to approximately $34,000 (platinum current market value of $120 per troy ounce is assumed). Thus the incentive is strong for employing the best anrtlytical techniques available in the inspections. Several techniques are under investigation or have been developed ( 5 ) . X-ray fluorescence has been demonstrated to he precise and accurate for the determination of trace metals in cracking catalyst ( 3 ) . This article discusses the development and applimtion of an x-ray fluorescence technique for the analysis of platinum reformer catalyst.
methods, including x-ray fluorescence. The fact that the sample is not expended in x-ray analysis enables the laboratory t o accumulate a reliable set of reference standards. Scope. The method is applicable to the determination of platinum in platinum-alumina catalyst in the range of 0.05 to 1%.
Sample Preparation. A portion of ground platinum catalyst is ignited in a muffle furnace for 3 hours a t 1000”F., then cooled in B desiccator to room temperature. A 3.8-gram portion is admired thoroughly with 0.2 gram of Sterotex, a hydrogenated corn oil product (Capital City Products Co., Columbus, Ohio), preparatory to hnquetting. A 5qb Stemtex addition helps prevent shattering of the pellet upon ejection from the hriquetting press. Apparatus. The pelleta are fabricated in an Applied Research Laboratories Model 3502 bri uetting press, using a 1-inch die and a pressure of approximat& 40,000 pounds per square inoh. These conditions yield pellets 1 inch in diameter and ‘/a inch thick. A North American Phillips (Norelco) x-ray fluorescence Geiger counter spectrometer is used for the analysis of the platinum reforming catalyst pellets. A lithium fluoride crystal ( d = 2.0138 A,) is used in the goniometer diffracting the. fluorescence spectrum. A leaf collimator with 0.02-inch sDacmm is used before oned in the x-ray beam by means of a modification of the sample holder supplied with the instrument, as shown in Figure 1. Xirradiation is generated bv 8 Machlett tunmten target source
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the spectral coincidence of several tungsten Lo lines ( 2 ) a t the Pt LeI position is rather troublesome, precluding the use of Pt Lm,. Background is read a t 29.0’ 28, counting 12,800 impulses, and subtracted from the platinum line to give net counts per second contributed by the platinum, The variance of counting statistics, introduced both in line and in hitokground measurement. should he considered in selecting the number of counts to
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analysis:
Each fate of two pelletshr one face of four pellets is
EXPERIMENTAL WORK
Calibration standards. The x-ray fluorescence technique of analysis is dependent upon the use of reference standards of accurately known composition for calibration. The standards used for calibration should he very similar in composition to the unknown sample. The value of a result obtained in an analysis obviously is no better than the accuracy to which the eomponenta of the calibrating standards are known. For the present purpose two types of standards have been empolyed in calibration: commercial catalysts, the platinum content of which is known, and synthetic samples containing known amounts of added platinum. Methods of rtnalysis which have been employed on commercial catalysts or other platinum-hew ing substances include the fire asmy, gravimetric, and photometric methods. The difficulty of the technique or the state of the art of each chemical method is such at present that further development is necessary t o adapt the method to as rapid routine use as is attainithle with x-ray fluorescence. To select a “best” set of standards, judicious consideration must he given to the consistency as well as the quality of values obtained by several
Figure 1. Modified sample holder 1433
ANALYTICAL CHEMISTRY
1434
Table I. Analysis of Platinum Reforming Catalyst Sample No.
Spectrophotometric
Platinum, % X-ray fluorescence
0,592 0.587 0,583 0.581 0.573 0.571 0,586 0.583 0.587 0.583 0.580 0.589
0,583 0,590 0,582 0,583 0.575 0.582 0.575 0.573 0.581 0,584 0 574 0.586
1 2 3 4 5 ti 7
8
9 10 11 12
DISCUSSION Difference - 0 009 +0.003 -0.003 +o 002 0.000 +0.011 - 0 011
-0.009 - 0 006
+ o . 001
-0.012 -0,003
4 v . 0 006
Table 11. Influence of Specimen Preparation on X-Ra? Fluorescence Measurement of Platinum in a Sample of Reforming Catalyst” Specimen Preparation
Cell Fillings
Rleasureinents
Standard Deviation. % Pt
Powder
1’7 1 1
12 11 9
0.0092 0,0202 0.0170
1
7 I? ti ti 0
0 0 0 0 0 0 0
Briquet
1
1
fi
G ”
imum difference between methods is 0.0127, and the average difference is 0.00670 platinum.
0126 0053 0082 0018 0018 0038 0030
I n connection with the development of the x-ray fluorescence technique for the analysis of platinum reforming catalyst, an investigation was carried out on several variables a hich were believed to influenre the accuracy of the method. Powder us. Pellet Specimen. Bulk density, surface, homogeneity, particle size, and area irradiated are physical factors that are known to influence the intensity of emitted x-ray fluorescence ( 4 ) . The preparation of the sample specimen in a ieproducible manner to minimize the effects of these factors is of utmost importance in an analysis h comparison was made of preparing the specimen as a powder, which was hand-packed into a rectangular cell, and as a briquetted wafer. The smaller irradiated area for the pellet than for the hand-packed powder results in a slight decrease in peak-to-background ratio for the pellet as compared with the powder. The precision of a series of measurements is shoFn in Table 11, in nhirh the standard deviation 1s expicssed as per cent platinum.
Table 111.
(0 03’2c; P t )
Siiecmen A B
Platinum content 0.632%.
C D
measured and an average net value of counts per m w n t i is iised for the interpretation for one determination. RESULTS
Calibration. A straight-line regression curve was eztatilirhed to relate concentration of platinum with the intensity in routits per second of the fluorescent x-radiation. I n this 1almr:itory :L \rorlting curve has been espresed by the equation platinum
=
Yariation of ?leasuretnent hetween Specimen Preparations
1 450.3 448.6 443.4 444.1
Intensity Measurement Counts per Second 9 3 4 5 448 2 449.4 430.5 446.6 438.8 466.4 440.0 440.5 442 7 440.8 443.7 447.3 449.4 430.0 441.5 442.0 Av. 4413 7 0
___________~______ Analysis of Variance So11rce Wirliin Between means Total
Slim of square 810.7
Degrees of freedom 20
137.2 9G7,9
3 23
T-ariance 40.54 Fo.ss(3,20) = 3 . 10 52.4 F = 1.29
Conclusion. The difference between m w n s on different specimrn preparations is not statistically significant.
Table IY. \-ariation of 1Ieasurement on a Specimen with Time (0 (iR2C, P t )
0.00164 (counts per second - G O ) 1);ite of
The curve passes through the ordinate rather than the origin for zero platinum. Several aluminas were tested to verify this property of the curve and the measurements !?ere found to he i n good agreement for differmt aluminas. A slight hiit charncteristic “bump” above kiackgronnd in the spectrum at the position of the platinum peak callws the curve to pass through tho peak at :32” 28 itccounts ordinate. The occurrence of the for this observation ( 2 ) . The time requirement for measurement on an unli110\\-11normally is about 15 minutes per ssmple. The use of one or mow reference samples with the llnknowns gives greatest acruracy. Precision. The estimated precision of fluorescence measurement may be obtaincd by replicate values on a given sample. The data of Table 111, for esample, are replicates on one sample. The standard deviation of four replicates is 3.2 counts per second; this is equiwlent to 0.005(jL platinum. Comparison of Methods. Table I compares PpecBtrophotometric and x-ray fluorescence methods of analysis on t>-pical ctommercial catalyst samples. The former method measures the intensity of the comples of chloroplatinic. ion with stannous (ahloride in hydrochloric acid medium ( 2 ; 5 ) ; in this laboratorj- a \\-:ive length of either 400 or 403 nip has been used. The max-
6 461.4 443.1 450.0 443.2
Intensity Illeasrirement, llpa-iirein~nt 1 ‘2 3 4 5 ?-13-19c53 4 4 9 . 9 4 5 3 . 7 4 5 5 . 7 4 5 4 . 2 4 5 2 . 1 .,-4-195$- 4 5 0 . 6 4 5 0 . 3 4 5 2 . 1 4 4 9 . 3 4 3 3 . 3 .i-l9-19.>., 4 4 4 . 1 449 4 445 0 4 4 1 . 5 447 0
Source IVi t liin Means
Total
Sum of squares 2 2 7 . (i 161 . G 389 2
Counts per Second _ _ _ ~ G 7 8 4 5 2 . 2 445 9 447 9 451.3 . , , , , , 448.2 , , .,, ~
.-
.$I.,
451.8 431.2 444.2
Analysis of Variance Degrees of freedoill Variance i 113.8 Fo.ra(2.17) = 3 59 17 9 51 1$1
.F’
_.
= 1 1 46
Conclusion. T h e differenre betn-ern nienns f o r n sprciinrn nienniirrd a t ditfrrt,iit times is Iiiglily significant.
\Tit11 powder filling, t h r mrasiirements tended to diift with time of residence in the instiument, although the data that illustrate this are not included in this paper. This effect was attributed to heating of the sample, 17-hich changed the level of the radiated surface. Actually the data of Table I1 indicate that replicates ohtained on different fillings arc more precise than replicates on the m n e filling, ~1 here t h r spetbimen reninins in the instrument during inwsii! rment8.
V O L U M E 2 8 , NO. 9, S E P T E M B E R 1 9 5 6
Table 1'. Influence of Tube T-oltage and Aniperage on Peak-to-Background Ratio for Six Samples" Tube power setting -41.. intensity, c.g.s. h Background Peak Line/background b
15 ma., 4.5 I
