XRF ANALYSIS OF AUTOMOTIVE CATALYSTS BY FLUX/FUSION

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1 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume XRF ANALYSIS OF AUTOMOTIVE CATALYSTS BY FLUX/FUSION A. R. Drews Physical and Environmental Sciences Department, Ford Research Laboratories 2101 Village Road, Dearborn MI The classic approach to preparation of XRF specimens of inhomogeneous materials is the method of flux/fusion. This technique has not been applied to automotive catalysts because they utilize large quantities of precious metals (Pt, Pd, Rh) that are reduced at typical fusion temperatures and agglomerate and alloy with the crucible walls, leading to erratic losses of the precious metals. This effect is pronounced in automated fusion machines that vigorously agitate the crucible over a gas flame. In this report I describe a fusion technique that overcomes the problem of precious metal alloying and that yields good results for the broad range of elements found in modern automotive catalysts. INTRODUCTION Automotive exhaust-gas treatment catalysts have evolved considerably since their introduction in the 1970's. Modern catalysts for gasoline engine applications commonly employ formulations that simultaneously oxidize CO and hydrocarbons while reducing NO x in what has become known as "three-way" catalysts. An example of a catalyst brick and an electron backscatter image of a polished cross section of a brick are shown in Figure 1. Of particular interest to the present discussion is the contrast apparent in the electron image between the washcoat layers and between the washcoat layers and the cordierite substrate. Contrast in back-scatter electron imaging is sensitive to the average Z of the material, and thus, the contrast apparent in the image represent a gross inhomogeneity of the material's chemistry that can have implications to XRF analysis if it is not completely removed. Figure 1. A catalyst brick (left) removed from its packaging. The washcoat is deposited onto the cordierite substrate that forms a honeycomb of gas passage channels. A backscattered electron image (right) of a polished cross-section clearly shows two washcoat layers with different average Z. The composition of typical three-way catalysts has evolved from early formulations that primarily used Pt as the catalytic material and little else, to current technology that relies heavily on Pd and Rh in combination with larger amounts of (Ce,Zr)O 2 for oxygen storage, BaCO 3 for

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3 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume NO x storage/reduction and NiO for S trapping. Concentration ranges for the catalytic materials and typical poisons are shown in Table 1. Washcoat Elements Concentration Range (wt%) Poison Elements Concentration Range (wt%) Substrate Elements Concentration Range (wt%) Ce 2-10 S 0-5 Mg 4-8 Zr 2-8 P 0-8 Al Ba 0-5 Zn 0-4 Si La 0-2 Pb 0-2 Fe Nd 0-3 Na 0-1 Ti Ni Ca 0-1 Sr Pd Pt Rh Table 1. Concentration ranges typically found in three-way catalysts for gasoline engines. Although pressed powder specimens of ground catalysts is a convenient preparation method, reliable analyses are limited to cases where a suite of calibration standards for that particular catalyst type are available, i.e., no universal calibration will yield accurate results for all catalyst types and no amount of grinding will produce a truly homogeneous specimen. This can be illustrated by comparing the relative difference between the results from XRF analysis of pressed pellets of multiple catalyst types to the results derived from standard chemical assay techniques (ICP-OES, fire assay) of the same powders (Table 2). Pt Pd Rh Ni Ce Zr Ba La Nd Sr Catalyst A Catalyst B Catalyst C Catalyst D Catalyst E % In-House Reference Table 2. Relative deviations (%) between XRF concentrations of pressed pellets and the concentrations determined by an independent, outside laboratory using standard chemical assay techniques. The classic solution to analysis of inhomogeneous specimens is to dissolve the specimen into a liquid matrix (flux) 1. Dissolution of oxides near room temperature is possible using strong acids (microwave acid digestion) and has been used for catalyst materials with good results, including the precious metals (PM's). However, this method is very time consuming and involves handling of very hazardous materials (concentrated HF and aqua regia). In the context of a laboratory that handles >1000 specimens/year, such a method is not very attractive. Fluxing complex oxide materials for XRF analysis using alkaline borates at high temperatures which are fused into solid, non-corrosive glass disks is very common practice and is applicable to a wide range of materials. Typically, the flux and sample are loaded into a crucible which is agitated over a gas flame for several minutes and then cast into dish.

4 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume Unfortunately, the precious metal components of catalysts (Pt, Pd, Rh) are not stable as oxides at typical borate fusion temperatures (PdO is unstable above 800C and PtO 2 is unstable above ~450C) and are not soluble in traditional fluxes. In their reduced form, these metals agglomerate and alloy with the walls of a Pt/5Au crucible (this composition is commonly used because of its exceptional wetting characteristics). The amount of PM that will be lost from the specimen in a fusion in a Pt/Au crucible depends on the agitation of the mix, the initial distribution of the materials in the mix and the fluxing temperature and time. Fluxing in carbon crucibles is also possible (PM agglomerates will not alloy with the crucible walls), although the carbon readily oxidizes at typical fusion temperatures, leading to a reducing environment and a short lifetime. To overcome these problems with the fusion method when applied to materials containing precious metals, an alternate high temperature borate flux/fusion technique was developed that uses a rapid fusion in a viscous flux without agitation or casting. The method described here is intended to provide significantly improved accuracy of XRF analyses of catalyst materials using a "universal" calibration in a simple, fast method. This report will describe the method and results from measurements to test its applicability to a wide range of catalyst formulations as well as synthetic standards. EXPERIMENTAL METHOD Specimens of catalyst materials for this study were drawn from three sources: 1) Lab synthesized model catalysts; 2) Calibration standards developed by two catalyst suppliers for four different catalysts formulations; 3) Production catalysts. The lab synthesized model catalysts were developed by mixing ground and dried oxides, carbonates and hydroxides of the individual elements needed to model the washcoat with appropriate amounts of ground cordierite (Mg 2 Al 4 Si 5 O 18 ) in a WC grinding dish for a minimum of 5 minutes. Calibration standards from catalyst manufacturers were supplied as powders produced from production starting materials and developed using the same deposition techniques that are used to produce the particular catalyst type that they are intended to model. Production catalyst powders were produced by crushing and grinding whole catalyst bricks. Concentrations for production catalysts were determined by an outside testing laboratory (Ledoux) using a variety of techniques (not XRF), including ICP-OES and sulphide collection/fire assay (for the PM's). Three specimen preparations methods were examined: Solid phase sintering; low temperature flux/fusion; high temperature, fast fusion without agitation. Of these three, only the third was found to be suitable and is described below. Specimens for fusion were prepared by mixing 1.0 g of catalyst powder with 10 g of Li 2 B 4 O 7 (JMI Puratronic) for 5 minutes in a polystyrene vial (SPEX Spectrovial) with two 11 mm methyl methacrylate balls using a SPEX ball mill. The mixed powder was loaded into a home-made Pt/Au flat-bottom crucible (inner diameter at the base: 32 mm; height: ~15 mm; inner diameter at the top: 40 mm). The crucible was placed directly into a laboratory muffle furnace maintained at 1000C and allowed to fuse undisturbed for 10 minutes, after which the crucible was removed and placed on an insulating ceramic block to cool. Loss on ignition was determined by comparing the

5 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume net weight of the loaded crucible before and after heating. Since no de-wetting agent was used during fluxing, the specimens required mechanical encouragement to release from the crucible. XRF data were collected on a Philips PW 2400 sequential spectrometer equipped with a Cr tube. Data from in-house synthetic catalysts standards, certified production catalysts and standards from two catalyst suppliers were used to determine a calibration for fresh catalyst materials within the Philips SuperQ semi-quantitative (regression) analysis software. Each measurement line was chosen to be relatively free from line-overlap events and incorporating at least two background points, one on either side. A list of measurement conditions is shown in Table 3. El Line Xtal Coll. Det. kv ma Time (s) CSE (%) Pd Kα LiF µm Scint Rh Kα LiF µm Scint Zr Kα LiF µm Scint Sr Kα LiF µm Scint Pt Lα LiF µm Scint Ni Kα LiF µm Duplex Fe Kα LiF µm Duplex Nd Lβ 1 LiF µm Duplex Ce Lα LiF µm Duplex La Lα LiF µm Flow Ti Kα LiF µm Flow Ba Lα LiF µm Flow Hf Lα LiF µm Duplex Table 3. Measurement conditions used in the regression calibration. "Duplex" refers to the simultaneous use of the sealed Xe and flow proportional counters. The cordierite contribution to the matrix is calculated by difference and the matrix corrections were calculated using a fundamental parameters algorithm. One limitation of a regression analysis is the difficulty of preparing a full suite of uncorrelated standards. For aged three-way catalysts, up to 22 elements may be needed and more if other types of catalysts are included. Furthermore, by its nature, a regression calibration is relatively inflexible with respect to the nature of the specimen (its matrix, weight, dilution, etc.). For these reasons a "fundamental parameters" based analysis using the UniQuant program is being developed. In the UniQuant method, primary sensitivities, line overlap corrections and a background model are determined from a suite of single element standards. The details of the calibration procedure within the UniQuant method will not be discussed here, although selected results will be presented that were determined from it. Aging a catalyst in a vehicle exposes it to contamination by fuel and engine oil additives/impurities. Once the contaminants are transported to the catalyst, they can alter the washcoat components by reaction or impede their functionality by coating the surface of the catalyst. Quantifying the poisons is important to understanding how to make catalysts more resistant to aging effects. To be useful for analysis of aged catalysts, the fusion method must not lead to significant losses due to volatilization that depends on the form of the poison on the catalyst. Fused specimens containing MgSO 4 (decomposes above 1100C) were used for calibrating UniQuant. To test for potential loss of S during fusion, two comparison specimens

6 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume were fused from NI&HSOb (5 wt.% S and 10 wt.% S), which decomposes at -23X. pre-fusion procedures were followed. No special Additional checks of the validity of the calibration for Pt and Pb were provided by analysis of fused specimens of NIST SRM 2557 (Used Automotive Catalyst). RESULTS AND DISCUSSION The overall effectiveness of the present method can be judged by the quality of the calibration curves (line overlap and matrix corrected count rate vs. concentration). Specifically, the homogeneity of the specimen will affect the amount of scatter of the calibration curves. For the PM s, the scatter in the calibration regressions derived from multiple, uncorrelated sources are characterized by average relative errors of: 5.0 % (PdO, Figure 2), 2.7 % (Rh203) and 3.1 % 2.3 PHILIPS PW /24/2001 XRF spectrometer ii PF, t, Flyer-:- - Ii?- 1.5 X-Axis: LoC car. C(Chem) Y-Axis: Mat&LoR car. Rate Pd4(kcps) RE K: RMS: F: D: E: :5 PdO(%) Figure 2. Calibration curve for PdO plotted as the line overlap, background and matrix corrected count rate vs. concentration. Flyers are points that lie more than two standard deviations from the calibration line. (PtO,). Similar quality calibration plots are obtained for most of the base metals, with the relative errors derived from the calibration regressions all falling below 6%. To determine if progressive loss of Pd occurs from convective mixing, several specimens were made from a catalyst standard and fused for different times (Figure 3). Fitting the measured Pd concentration with a linear g 0.34 z 0.32 k 2 : z: Fusion 10 Time (min.) Figure 3. Palladium concentration (wt. 5%)for different fusion times, fitted to a linear dependence.

7 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume dependence yielded a measured loss rate of ~1.3%/minute. Since the fusion time is typically controlled to within ± 10 seconds, the loss of Pd due to variation in fusion time can be kept to negligible levels. The estimated lower limits of detection and average relative deviations from the calibration regressions are shown in Table 4. For all elements except Nd, the LLD is less 100 ppm, and the average relative error is less than 6% for all elements other than Sr. Compound LLD (ppm) Relative Error (%) Compound LLD (ppm) Relative Error (%) BaCO PdO CeO PtO La2O Rh2O Nd2O SrO NiO ZrO Table 4. Estimated Lower Limits of Detection (LLD) and the average relative error (%) based off of the regression calibration. Loss of sulfur on fusion of NH 4 HSO 4 is apparently negligible, with the UniQuant analysis of fused specimens of NH 4 HSO 4 yielding 4.76% and 9.77% S (compared to the nominal 5% and 10%), despite an obvious loss of weight. The weight loss observed was consistent with the loss expected from volatization of NH 4 and H 2 O on decomposition of the NH 4 HSO 4. Apparently, the liberated SO 2 reacts quickly with Li 2 O and is stabilized in the fusion. Analysis of the NIST SRM 2557 also yielded good agreement with the certified concentrations of Pt (0.110 % vs. the certified value of 0.113%) and Pb ( 1.32% vs. the certified concentration of 1.39%). CONCLUSIONS The fusion method described herein does not yield the accuracy found with traditional fusion preparations but does yield improved accuracy for base metals and precious metals over results obtained from pressed pellets without special calibrations. By thoroughly premixing Li 2 B 4 O 7 with ground catalyst powder (10:1 dilution), fusing in a flat bottom dish in a muffle furnace without agitation and by minimizing the fusion time, the average relative errors were less than 6% with LLD's generally less than 100 ppm. This fusion method when applied to NIST SRM 2557 also yields values in good agreement with the certified values for Pt and Pb. Volatile loss of S does not appear to be a problem, even for an easily decomposed sulphate, suggesting that this method will be useful for aged catalysts. ACKNOWLEDGMENTS I would like to thank Ann Chen for the SEM micrograph used in this report, Dave Benson for his assistance in all aspects of XRF, Charlotte Lowe-Ma for her critical reading of this manuscript and Roc Carter for his pesky questions that stimulated me to move beyond the obvious. REFERENCES 1 Methods of Decomposition in Inorganic Analysis, by Zdenek Sulcek and Pavel Povondra, CRC Press, Boca Raton, Florida, 1989.