Determination of Precious Metal Content in Catalytic Converters Using EDXRF

Automotive catalytic converters (ACCs) play an important role in the exhaust system to minimize the emission of harmful pollutants such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). A converter comprises a honeycomb substrate (Fig. 1). Such structures contain many fine channels very similar in appearance to the honeycombs bees produce in nature. The majority of ACCs are ceramic based and consist of a substrate made of cordierite (2MgO.2Al2O3.5SiO2). This material is wash-coated with alumina (Al2O3) to which a combination of platinum group metals (Pt, Pd, Rh) and rare earth oxides such as CeO2, ZrO2, etc. are added. The recovery of precious metals by recycling spent ACCs has a substantial economical value. Energy dispersive X-ray fluorescence (EDXRF) offers a rapid and precise determination of Pt, Pd and Rh content in spent ACCs.

Figure 1. Schematic of a typical automotive catalytic converter (ACC).

Sample Preparation

ACCs are inhomogeneous at the microscopic level. A thorough sample preparation is required before reproducible analysis is possible. The initial step includes grinding the material using a ring or a vibratory disk and puck mill. It is essential to check the element characteristic line intensities as a function of grinding time and to assure that these have reached a constant value. This should not only be checked for the precious metals but also for the presence of other elements such as Al, Zr, Ce, etc. It is important to avoid grinding sets made of tungsten carbide (WC) since sample contamination by tungsten will results in characteristic tungsten lines interfering with the platinum lines. After grounding, the powders can be pressed into a pellet with the aid of a binder. Just like geological samples, the inhomogeneous nature of ACC will still lead to mineralogical effects. In order to avoid these, laboratories prefer to fuse the sample but this results in other difficulties related to the instability of precious metal oxides and their tendency to agglomerate or alloy with Pt/Au crucibles. In this article, pressed samples of 32 mm diameter using an automatic press applying 20 tons of pressure for 15 s were prepared.

Instrument Sensitivity

The Thermo Scientific ARL QUANT’X consists of a Peltier-cooled Si(Li) detector of 3.5-mm thickness (Fig. 2). This thickness ensures high stopping power for higher energy X-rays produced by heavier elements like precious metals. Table 1 shows the detection limits obtained for Rh, Pd and Pt in a standard ACC matrix.

Figure 2. Thermo Scientific ARL QUANT’X Series.

Table 1. Typical ARL QUANT’X minimum detection limits for Rh, Pd and Pt in an ACC matrix.

Rh Pd Pt
MDL (100s), ppm 2.2 2.7 4.9

Quantitative Analysis using UniQuant Program

As explained, ACCs are complex samples and need a large suite of calibration standards to cover the variation in the material composition. Even though this is preferred, such a set of standards is not always available or the standards are not fully characterized, which makes the right estimation of inter-element effects impossible. This article presents an alternative based on standard-less fundamental parameters. Sample composition has been determined using Thermo Scientific UniQuant, a standard-less fundamental parameters program popular for wavelength dispersive X-ray fluorescence (WDXRF) and now also offered for EDXRF. The UniQuant program has been pre-calibrated with a set of pure elements and compounds and can correct not only for matrix effects but also for spectral interferences. It uses all eight primary beam filters available with the ARL QUANT'X and pre-set voltage settings to obtain the best possible profile of any unknown sample without user intervention or optimization. A complete analysis takes about 10 minutes per sample and provides the entire composition of the sample.

Results

A set of 10 ACC samples have been studied in order to determine Rh, Pd and Pt concentrations independently using ICP-OES and EDXRF combined with UniQuant. Figure 3 shows the results of this study. A good agreement is obtained between both techniques with average relative differences below 3%. Given the intrinsic heterogeneity of the samples and the uncertainty, this is an acceptable result.

Figure 3 shows comparisons of ACC analysis results obtained using ICP-OES and the ARL QUANT’X EDXRF combined with UniQuant standardless FP software.

Figure 3.  Rhodium (top), Palladium (middle) and Platinum (bottom) concentrations measured on the target ACC samples using ICP-OES and EDXRF combined with UniQuant.

Repeatability

In order to determine the repeatability of this analysis method a catalyst sample was analysed consecutively eleven times in a row. In between measurements, the sample was removed from the sample holder and placed back again. The measurement time was set to 30 s for the conditions involving the elements of interest. Table 2 shows the test results. The average relative standard deviation at 1 sigma turns out to be less or equal to 2%.

Table 2. Repeatability results obtained upon analyzing NIST 2557.

Rh Pd Pt
Counting time 30 s 30 s 30 s
Run nr ppm ppm ppm
01 133 224 1040
02 136 227 1060
03 137 225 1060
04 135 226 1040
05 136 228 1080
06 136 230 1040
07 136 230 1070
08 140 228 1090
09 137 230 1080
10 133 226 1020
11 134 225 1070
Average, ppm 136 227 1059
St.Dev (1), ppm 2 2 22
Relative St.Dev., % 1.5 1.0 2.0

Conclusions

The ARL QUANT’X along with the UniQuant software is used to analyze automotive catalytic converters (ACCs) and determine Rh, Pd and Pt content. Combination of EDXRF with UniQuant standard-less fundamental parameters yields results which show excellent agreement with those obtained via ICP-OES.

About Thermo Fisher Scientific-Elemental Analysis

For over 75 years, Thermo Fisher Scientific has been a worldwide supplier of spectrochemical instrumentation to major industries including steel, transportation, cement, construction, food, pharmaceuticals, chemicals, academic research, petroleum and electronics. They offer unsurpassed capabilities in the areas of optical emission (OE), X-ray fluorescence (XRF), X-ray diffraction (XRD) and automation of spectrometers.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific- Elemental Analysis.

For more information on this source, please visit Thermo Fisher Scientific- Elemental Analysis.

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