Accurate analysis of precious metals Pt, Rh and Pd and other elements in car catalytic converters

Catalytic converters are designed to remove toxic gases and pollutants from car exhaust gases. Therefore, they contain the precious metals Pt, Pd and Rh. When the car reaches end of life the converter is scrapped. The precious metals are recovered in a recycling chain comprising several different stakeholders, from scrap yards to toll refineries. Due to their high value, accurate analysis and processing of the precious metal content is critical to ensure maximum return in recycling. This application note shows a safe, easy and accurate method for providing data giving full insight in the composition of the to-be-processed converter.

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Introduction

Catalytic converters are designed to remove toxic gases and pollutants from car exhaust gases. Therefore, they contain the precious metals Pt, Pd and Rh. When the car reaches end of life the converter is scrapped. The precious metals are recovered in a recycling chain comprising several different stakeholders, from scrap yards to toll refineries. Due to their high value, accurate analysis and processing of the precious metal content is critical to ensure a maximum return in recycling. This application note shows a safe, easy and accurate method for providing data giving full insight into the composition of the to-be-processed converter .

Instrumentation

Measurements were performed using a Malvern Panalytical Epsilon 4 EDXRF spectrometer, equipped with a 15W, 50 kV silver (Ag) anode X-ray tube, 6 software-selectable filters, a high-resolution SDD30 silicon drift detector and a 10 position sample carousel.

Creating calibrations

Fourteen car catalyst samples, supplied by several customers, were used to create the calibration. The precious metal content of each calibration sample was determined by fire assay or inductive plasma coupled spectroscopy (ICP). The majors and minors of the calibration samples were certified using one of Malvern Panalytical’s standardless solutions WROXI or Omnian.

Sample preparation

The samples were dried for 2 hours before pressing. To make 32 mm pellets, 10 grams of sample was mixed with 1 gram of Ultrawax as a binder. The pressing was done by applying 20 tons for 60 seconds.

[Figure 1 AN220401-accurate-analysis-precious-metals-catalytic-converters.jpg] Figure 1 AN220401-accurate-analysis-precious-metals-catalytic-converters.jpg

Figure 1. Image of one of the prepared pressed pellets.

CompoundskVuAFilterMediumMeas. time (s)
Pt, NiO, ZnO, SrO, Y2O3, ZrO, WO3, PbO,5080AgAir60
Rh, Pd, BaO50300Cu-thickAir180
K2O, CaO2, TiO2, MnO, Fe2O3, La2O3, CeO2, Nd2O315370Al-thinAir30
MgO, Al2O3, SiO2, SO372100TiAir30

Table 1. Measurement conditions

Measurement procedure

Four different measurement conditions were used to measure the 3 precious metals and other compounds in the samples. Measuring the total matrix improves the flexibility of the method and makes it more accurate for different catalytic converter materials. The total measurement time was set at 5 minutes per sample.

Calibration results

Figures 2, 3 and 4 show the calibration graphs for Pt, Rh and Pd in the customer-certified calibration samples. The graphs show good correlations between certified concentrations and measured XRF intensities. Detailed calibration results for the precious metals and matrix elements are listed in Table 2. 

The RMS (Root Mean Square) value is equivalent to 1 sigma standard deviation.

CompoundsConcentration rangeRMS (ppm)*Correlation Coefficient
Pt (ppm)566 - 199249.50.9942
Rh (ppm)64.5 - 2657.20.9949
Pd (ppm)485 - 240747.40.9954
MgO (wt-%)4.73 - 7.200.180.96158
Al2O3 (wt-%)27.59- 41.781.110.9592
SiO2 (wt-%)29.00 - 46.791.420.9728
SO4 (wt-%)1.05 - 4.890.230.9874
K2O (wt-%)0.25 - 0.450.020.9605
CaO2 (wt-%)0.35 - 1.660.040.9953
TiO2 (wt-%)0.55 - 4.650.040.9994
MnO (wt-%)0.00 - 0.340.020.9809
Fe2O3 (wt-%)1.21 - 2.360.040.9922
NiO (wt-%)0.02 - 0.110.0010.9642
ZnO (wt-%)0.11 - 0.290.010.9865
SrO (wt-%)0.16 – 0.410.020.9939
Y2O3 (wt-%)0.01 – 0.130.0010.9997
ZrO (wt-%)1.44 – 6.020.120.9974
BaO (wt-%)0.22 - 0.810.040.9853
La2O3 (wt-%)0.14 - 0.730.020.9928
CeO2 (wt-%)1.59 - 4.830.130.9947
Nd2O3 (wt-%)0.06 – 0.380.040.9497
WO3 (wt-%)0.00 - 0.440.0040.9996
PbO (wt-%)0.00 - 2.660.070.9977

Table 2. Calibration details (*RMS: The more accurate calibrations have the smaller RMS values).

[Figure 2 AN220401-accurate-analysis-precious-metals-catalytic-converters.jpg] Figure 2 AN220401-accurate-analysis-precious-metals-catalytic-converters.jpg

Figure 2. Calibration graph for Pt in catalytic converters.

[Figure 3 AN220401-accurate-analysis-precious-metals-catalytic-converters.jpg] Figure 3 AN220401-accurate-analysis-precious-metals-catalytic-converters.jpg

Figure 3. Calibration graph for Rh in catalytic converters.

[Figure 4 AN220401-accurate-analysis-precious-metals-catalytic-converters.jpg] Figure 4 AN220401-accurate-analysis-precious-metals-catalytic-converters.jpg

Figure 4. Calibration graph for Pd in catalytic converters.

Sample name (repeats)CompoundsCertified conc.Average conc.RMSRel. RMS (%)
NIST 2556 (20x)Pt (ppm)697.4 +/- 6.3692.81.30.2
Rh (ppm)51.2 +/- 0.552.50.71.3
Pd (ppm)326 +/- 1.6322.62.80.9
NIST 2557 (20x)Pt (ppm)1131 +/- 111152.22.40.2
Rh (ppm)135.1 +/- 1.9143.51.41.0
Pd (ppm)233.2 +/- 1.9225.61.30.6
ERM-EB504 a (5x)Pt (ppm)1414 +/- 9.01392.02.60.2
Rh (ppm)210 +/- 2.2207.82.51.2
Pd (ppm)1596 +/- 111640.9113.40.8

Table 3. Accuracy and precision testing on three commercially available certified reference materials.

CompoundUnitMeasured concentrationRMSRel. RMS (%)CompoundUnitMeasured concentrationRMSRel. RMS (%)
MgOwt-%9.670.040.4Rhppm143.51.41.0
Al2O3wt-%45.780.040.1Pdppm225.61.50.7
SiO2wt-%34.340.050.1Ptppm1152.22.40.2
TiO2wt-%0.60.0010.2BaOwt-%0.3440.0010.3
Fe2O3wt-%1.9690.0030.2La2O3wt-%0.0930.0022.2
NiOwt-%0.9240.0010.1CeO2wt-%1.6940.0030.2
ZnOwt-%0.2170.00050.2PbOwt-%1.8640.0020.1

Table 4. Result of validation measurement with a car catalytic sample prepared as a pressed pellet.

Accuracy and precision for precious metals

The method accuracy and instrument precision were tested by measuring multiple commercial certified reference materials (CRMs) 20 times (NIST 2556 & NIST 2557) or 5 times (ERM-EB504a) consecutively. The certified and average measured concentrations, RMS and relative RMS values are presented in Table 3, all demonstrating excellent accuracy and precision.

Precision

Besides the precious metal content, the Epsilon 4 can help the user to obtain a complete overview of the sample. Knowing the full elemental composition enables you to determine the optimal strategy for efficient processing, and to detect the presence of any rogue elements. An example is shown in Table 4, where a typical sample was measured 20 times.

Conclusion

The results clearly demonstrate the capability of Epsilon 4 to determine the concentrations of valuable precious metals Pt, Rh and Pd and other elements in catalytic converters. The repeatable sample positioning and outstanding sensitivity of the optical components combined with powerful software deconvolution algorithms allows the Epsilon 4 to provide accurate analysis for every measurement in only 5 minutes.

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