Anal. Chem. 2007, 79, 6383-6389
Determination of Platinum, Palladium, and Rhodium in Automotive Catalysts Using High-Energy Secondary Target X-ray Fluorescence Spectrometry Katleen Van Meel,*,† Anne Smekens,‡ Marc Behets,‡ Paul Kazandjian,‡ and Rene´ Van Grieken†
Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerpen, Belgium, and Analytical Laboratory, Umicore Precious Metals Refining, A. Greinerstraat 14, B-2660 Hoboken, Belgium
A fast and direct determination procedure for precious metals in spent automotive catalyst was developed using the novel high-energy polarized-beam XRF. A sample preparation method working directly on the ground material was optimized. The material was pressed as a pellet using wax as a binder; no internal standard was added. The standards for this application were available spent automotive catalyst, previously analyzed by ICP-OES to verify their concentration, prepared in the same way as the unknown samples. The investigated concentration ranged from nearly 0 to approximately 2700 ppm for Pt, to 500 ppm for Rh, and to 7500 ppm for Pd. The repeatability of the XRF measurement appeared to be better than 0.5%, while the precision of the whole method was ∼1%. The accuracy of the XRF method was verified with the well-established (but very time-consuming) ICPOES method; a good agreement (no difference when using the 95% confidence interval) was found for the results. When using an irradiation time of 500 s for the CsI secondary target and the Zr secondary target, the detection limits for Pt, Pd, and Rh were found to be better than 5 ppm. The so-called, three-way types of automotive catalytic converters (ACCs), which deal with the three most important pollutants in vehicle exhaust (CO, NOx, unburned or partially burned hydrocarbons), are usually based on a honeycomb monolith made of cordierite (2MgO‚2Al2O3‚5SiO2). This material is wash coated with a mixture of alumina (usually γ-Al2O3), CeO2, ZrO2, precious metals (Pt, Pd, Rh) and Ba- or La-oxides. The precious metals are the most important components of the ACCs, since the catalytic activity occurs at the precious metal center. It is generally agreed that Rh promotes the NOx dissociation, while Pt and Pd promote the oxidation reaction.1 Umicore Precious Metals Refining (UPMR) is a world market leader in recycling complex materials containing precious metals. The plant in Hoboken, Belgium, is designed to recover precious * Corresponding author. E-mail:
[email protected]. Tel.: +32 3 820 23 46. † University of Antwerp. ‡ Umicore Precious Metals Refining. (1) Kaspar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419-449. 10.1021/ac070815r CCC: $37.00 Published on Web 07/13/2007
© 2007 American Chemical Society
and non-ferrous metals from a broad range of industrial byproducts and secondary precious metals bearing raw materials from sectors such as electronics, photography, and catalysts. The feed includes among others spent automotive catalysts, which is the focus of this research. In the analytical laboratory, there is a continuous research for enhanced, reliable, and automated analysis methods. Key drivers are high-quality, optimized lead times, and customer-oriented solutions. Quantitative analyses of precious metal in ACCs have not been reported frequently in the literature.2-6 Gaita and Al-Bazi2 optimized an ion-exchange method for leaching Pt, Pd, and Rh from the ACC matrix. The ACC was ground and treated with HCl and NaClO3. The optimal concentrations for both acids were investigated, as well as the effect of the temperature; final recoveries for the precious metals were good (61-99% depending in the element). Angove et al.3 investigated the wash coat layer of spent ACCs with particle-induced X-ray emission. They identified the possible contaminants and their distribution in the wash coat and determined the uniformity of the wash coat composition. A total of 20 elements were analyzed. Borisov et al.4 optimized a digestion procedure to determine Pt, Pd, Rh, and Ti in ACCs with inductively coupled plasma mass spectrometry (ICPMS) with liquid nebulization. The results were compared with a previously optimized wavelength dispersive X-ray fluorescence (WDXRF) procedure. The detection limits in the newly optimized procedure appeared to be better than the ones from WDXRF. Borisov et al.5 investigated the possibilities to work directly on a solid sample for the determination of Rh and Pt in ACCs applying ICPMS spark ablation. To make the samples conductive, the ACCs were pressed together with graphite powder. The sample preparation time could be reduced drastically and significant reduction of polyatomic species was achieved. Yoon et al.6 worked out a method to determine Pt group elements (PGE) with WDXRF; the sample preparation consisted of grinding and pelletizing the ACCs. A (2) Gaita, R.; Al-Bazi, S. J. Talanta 1995, 42, 249-255. (3) Angove, D. E.; Cant, N. W.; Bailey, G. M.; Cohen, D. D. Nucl. Instrum. Methods Phys. Res. 1996, B109/110, 563-568. (4) Borisov, O. V.; Coleman, D. M.; Oudsema, K. A.; Carter, R. O., III. J. Anal. At. Spectrom. 1997, 12, 239-246. (5) Borisov, O. V.; Coleman, D. M.; Carter, R. O., III. J. Anal. At. Spectrom. 1997, 12, 231-137. (6) Yoon, H.; Park, C. S.; Yoon, C.; Hong, J.; Kim, N. S.; Han, K. N. Miner. Metall. Process. 2005, 22, 101-106.
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calibration was done using artificially made standards, and the effect of the matrix was investigated. More recent papers dealing with the quantification of precious metals in materials other than ACCs usually involve ICP-related techniques.7-12 These kinds of analyses are very time-consuming due to various sample preparation steps, such as fire assay and acid digestion. Zereini et al.7 determined Pt and Rh in atmospheric particulate matter with adsorptive-stripping voltammetry (ASV). The sample was digested in a mixture of HNO3 and HCl with a high-pressure asher. Pd was quantified using total reflection X-ray fluorescence (TXRF) after preconcentration with Hg. Locatelli et al.8 determined Pt, Pd, and Rh in (laurel) leaves using squarewave (SW)-ASV and made a comparison with graphite furnace atomic absorption spectrometry (GFAAS). For both techniques, sample digestion was required with a mixture of HNO3, HCl, and H2SO4 after thoroughly cleaning, drying, and lyophilizing the leaves. GFAAS was conducted directly on the remaining solution while for the analysis by SW-ASV additional products had to be added. Fragnie`re et al.9 determined Pt in environmental samples using inductively coupled plasma sector field mass spectrometry. The samples were freeze-dried and microwave (MW)-digested with HNO3 and H2O2 in pressure vessels. Niemela¨ et al.10 also used MW-assisted digestion for the determination of Pt, Pd, and Rh in dust samples with inductively coupled plasma mass spectrometry (ICPMS). Dimitrova et al.11 applied GFAAS to determine Pd in environmental and biological samples. Flow injection was used for preconcentration of Pd. Sample digestion based on addition of HNO3, HF, and HClO4 was performed after drying the sample. According to the review of Bencs et al.,12 ICPMS is the most commonly used technique for determination of PGE in various types of samples. Among the possible X-ray spectrometric techniques, TXRF is used most often; however, most common X-ray fluorescence spectrometry methods are not very popular in this field. In this work, an energy-dispersive (ED)XRF method was developed to determine PGEs in ACCs. The primary goal was to obtain a fast method with as simple sample preparation as possible, without losing analytical performance compared to ICP-optical emission spectrometry (ICP-OES), which is now used in the daily routine at the UPMR laboratory. Apart from the accurate and precise (1%) quantification of Pt, Pd, and Rh, analyses of most major and minor constituents in the catalytic matrix were also requested; their determination has however been very well established in the past and there is no innovative aspect. Therefore, the quantification of those elements is beyond the scope of this paper and will not be treated. EDXRF has the benefit of (in most cases) easy sample preparation and multielement analysis. When using a high-energy (7) Zereini, F.; Alt, F.; Messerschmidt, J.; Wiseman, C.; Feldmann, I.; Von Bohlen, A.; Mu ¨ ller, J.; Liebl, K.; Pu ¨ ttmann, W. Environ. Sci. Technol. 2005, 39, 2983-2989. (8) Locatelli, C.; Melucci, D.; Torsi, G. Anal. Bioanal. Chem. 2005, 382, 15671573. (9) Fragnie`re, C.; Haldimann, M.; Eastgate, A.; Kra¨henbu ¨ hl, U. J. Anal. At. Spectrom. 2005, 20, 626-630. (10) Niemela¨, M.; Kola, H.; Pera¨ma¨ki, P.; Piispanen, J.; Poikolainen, J. Microchim. Acta 2005, 150, 211-217. (11) Dimitrova, B.; Benkhedda, K.; Ivanova, E.; Adams, F. J. Anal. At. Spectrom. 2004, 19, 1394-1396. (12) Bencs, L.; Ravindra, K.; Van Grieken, R. Spectrochim. Acta 2003, B58, 17231755.
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polarizing-beam EDXRF (HE-P-EDXRF) equipped with secondary targets, additional benefits regarding low background13 and better excitation of heavy elements are obtained.14 In this work, a method was developed to analyze the ACC in the form of pressed powder pellets using a HE-P-EDXRF equipment. For the sake of simplicity toward the sample preparation, no internal standard was added and Compton correction for matrix effects was used instead. MATERIALS AND METHODS Sample Preparation. (1) Incoming Material. The ACC material is first removed from its metallic housing and then broken and ground in different steps, until the powder can pass through a sieve of 112 µm. The process is fully automated and hardly requires any manpower. For loads between 3 and 6 tonnes, the crushing time is 2-3 days. This powder is used as the starting material in the further sample preparation processes for both applied techniques. (2) For XRF. For XRF analysis, the samples were prepared in the form of pressed powder pellets. Before doing any other handling, the powder was dried for 2 h at 105 °C to evaporate any remaining water. Because low-Z elements should also be determined in this preparation, different grinding times were investigated in order to avoid problems from grain size variability effects. Several grinding times were tested ranging up to 10 min with intervals of 2 min, and the intensities were mapped. The optimum grinding time (i.e., no additional changes in intensity) appeared to be 6 min, and hence, this grinding time was applied to all samples. The majority of the particles (more than 70%) has a particle size below 10 µm as a result of the grinding process. Grinding occurred in a disk mill with disks from agate material (Siebtechnik, Mu¨lheim-an-der-Ruhr, Germany); the cleaning of this device was performed with sand. For the preparation of the pellets, 3 g of wax (Hoechst wax pulver C, Sanofi-Aventis, Frankfurt, Germany) was added to 15 g of ground material and everything was mixed during 2 min using an automatic mixer (Spex Mixer/Mill, SPEXCertiPrep, Metuchen, NJ) without mixing balls. The pellets were pressed in an Al support (PANalytical, Almelo, The Netherlands) with an automatic hydraulic press (Herzog, Osnabru¨ck, Germany) applying a pressure of 20 tonnes during 10 s. A 6-µm Mylar foil (supplied by PANalytical) was used to avoid contamination, and the equipment is cleaned with an ethanol-water mixture after every use. The resulting pellets are ∼8.5 mm thick and have a diameter of 40 mm; they proved to be “infinitely thick” for the relevant X-ray radiation (intensity of Ba KR was monitored at various thicknesses to verify this). All pellets have a uniform mass of 18 ( 0.01 g. (3) For ICP-OES. The results of the newly developed method are compared to the analysis method that is daily applied in the UPMR laboratory: NiS fire assay, followed by ICP-OES. The precious metals in the ground material were preconcentrated using the NiS fire assay. The material was molten at 1150 °C for 75 min with a flux containing 30 g of sodium carbonate, 60 g of borax, 10 g of white sand, 12.5 g of S and 20 g of NiO. After cooling, the first NiS bullet was separated from the slag. The slag was again molten at 1150 °C, this time with a smaller amount (50%) of the same flux for 45 min, resulting in a second smaller (13) Standzenieks, P.; Selin, E. Nucl. Instrum. Methods 1979, 165, 63-65. (14) Spolnik, Z.; Belikov, K.; Van Meel, K.; Adriaenssens, E.; De Roeck, F.; Van Grieken, R. Appl. Spectrosc. 2005, 59, 1465-1469.
Table 1. Summary of the Experimental Conditions for All Elements of Interest analytes and their lines
secondary target
tube conditions
filter used
excitation time (s)
Mg KR Al KR, Si KR, S KR, P KR, Ca KR, K KR Fe KR, Zn KR, Ni KR, Cu KR Pt LR Pb LR Pd KR, Rh KR Ce KR, Ba KR, Zr KR
Al Ti Ge Zr Mo CsI W
25 kV, 24 mA 35 kV, 17 mA 65 kV, 9 mA 90 kV, 6.66 mA 90 kV, 6.66 mA 50 kV, 12 mA 100 kV, 6 mA
none none none Cu Cu none none
200 100 100 500 100 500 100
NiS bullet. The two NiS bullets were ground and transferred to a beaker, where they were covered with 40 g of NH4Cl crystals. To this mixture, 400 mL of concentrated HCl was added and the beaker was heated on a hot plate at 180 °C. The mixture was left to react, and every time the dissolution stopped (indicated by the lack of escaping gas), the temperature was raised. At 280 °C, the mixture was left to boil for 10 min and the solution was diluted with hot distilled water to 500 mL. The solution was allowed to cook, and 2 mL of Te solution (2.5 g of 99.99% TeO2 in 50% HCl) and 10 mL of SnCl2 solution (225 g of SnCl2·H2O in 50% HCl) were added. After 15 min of boiling, the precipitation should be complete. The temperature was then set to 130 °C, and the material was left to rest for 15 min. The precipitate was filtered and washed with 30% HCl. A 500 mg/L NaCl solution was added, and everything was dissolved in aqua regia. The remaining solution was evaporated to dryness. The residue was then dissolved with 30% HCl, and Y was added as an internal standard in a concentration of 10 mg/L. This procedure is very complex and time-consuming ( ∼5 working days), but it is necessary and routinely used in the precious metal industry. Instrumentation. (1) EDXRF. For HE-P-EDXRF analysis, the Epsilon 5 instrument (PANalytical) was used. Epsilon 5 is equipped with a 600-W Gd anode with a voltage ranging from 25 to 100 kV and a current from 0.5 to 24 mA. There are 13 secondary targets (W, CeO2, CsI, Ag, Mo, Zr, KBr, Ge, Co, Fe, Ti, CaF2, Al) and two Barkla scatterers (Al2O3 and B4C). It is possible to use a primary beam filter between tube and target; the following filter materials are present: Al 100 µm, Al 500 µm, Cu 250 µm, Zr 125 µm, and Mo 250 µm. Detection is performed with a high-purity Ge detector (HPGe) with an energy range from 0.7 to 200 keV and a resolution at Mn KR of
