Peroxide fusion solution for the determination of platinum, palladium and rhodium in automotive catalytic converters by ICP analysis

Introduction

Since the 1990s, the automotive industry has become the largest consumer of platinum (Pt), palladium (Pd), and rhodium (Rh) on account of the arrival of catalytic converters in the car exhaust systems. Approximately 40% of the total Pt, 74% of the total Pd and 80% of the total Rh were consumed by the auto catalyst industry in 2012.^[1] 

A catalytic converter generally consists of catalytically active metals, such as Pt, Pd and Rh, deposited onto the large surface area of a substrate, such as alumina or cordierite. Both are contained in a metallic case coupled to the car exhaust system^[2] to transform the toxic carbon monoxide (CO) and nitrogen oxides (NOx) into environmentally-friendly carbon dioxide (CO₂), water (H₂O), oxygen (O₂) and nitrogen (N₂). Depending on the cubic capacity of the engine and type of fuel used^[3], the content of Pd, Pt and Rh in an automotive catalytic converter varies, and may consist of Pt only, or various ratios of Pt-Pd-Rh, Pt-Rh, and Pd-Rh.^[4] Platinum and/or palladium are used for hydrocarbon and CO oxidation, while rhodium is used for NOx reduction^[5]. 

Since Pt, Pd and Rh metals are limited natural resources and that the investment needed to extract and to refine them are huge, another alternative to acquire them is by recycling old, unused, automotive catalysis converters. Important factors to consider when purchasing and recycling autocatalyst scraps, are the highly variable compositions and the highly volatile prices of Pt, Pd and Rh. To avoid considerable financial losses, it is very important to determine the content of Pt, Pd and Rh in the autocatalyst scraps quickly and accurately at the collector’s site and in the refineries.^[6] 

Analytical laboratories mostly use an Aqua Regia digestion procedure, either on a hot plate or using a microwave digestion system to render the crushed and milled catalytic converters’ substrates amenable for the determination of their Pt, Pd and Rh content by inductively coupled plasma spectrometry. However, these digestion procedures are time consuming and do not allow a complete dissolution of the samples. The addition of concentrated hydrofluoric (HF) and sulfuric (H₂SO₄) acids could lead to total dissolution of the samples.^{[7, 8]} Nevertheless, handling such hazardous acids is dangerous. Also, these multi-steps digestion procedures require highly skilled chemists and are not economic for laboratories that handle hundreds of samples per year. 

Malvern Panalytical has developed a simple sodium peroxide fusion method, for the dissolution of automotive catalytic converters substrates samples. Combined with a simultaneous ICP-OES, it yields good accuracy and precision for the determination of Pt, Pd and Rh contents in autocatalysts. The Claisse^® fusion method is quick, allows total sample dissolution, eliminates the use of dangerous acids and demonstrates good accuracy, precision and recovery for Pt, Pd and Rh.

Method

1. Sample Preparation by Peroxide Fusion

Claisse® Peroxide^TM Fluxer
• Zirconium crucibles and covers
• 0.25 g of ground sample
• 3.0 g of Sodium peroxide flux
• 3.5 minutes of heating at 600°C
• 4.0 minutes of cooling
• Dissolution in a HCl/HNO₃ solution.

2. Analysis by ICP-OES

2.1. PerkinElmer® OptimaTM 7300DV 

2.3. Analytical Method

Results

^{Dilution Factor: 1000 
Parenthesis = BQL} 

^{Spike additions: 50% of the initial concentration is added to each solution
Corrected Dilution Factor: 1000}

Discussion

Sample preparation by sodium peroxide takes only a few minutes and the dissolution is always complete. It is a very high throughput method and allows various advantages versus other dissolution methods that are mostly brought on by their versatility and temperature stability. Using sodium peroxide causes higher salt concentrations in the solutions which then need to be further diluted to prevent the salts from damaging the spectrometer. Solutions from alkaline fusion injected in an optical ICP should have a total dissolved salt concentration of 1.5% or less.

Using matrix matching and Cd 226.502 as an internal standard as well as strict interference management schemes, we were able to obtain recoveries of 100 ± 11% for the elements in both CRMs, as shown in table 6. Tables 4 and 5 reveal RSDs that are lower than 4% for all the elements showing a good reproducibility during the entire analytical process.

The dilution level used in the method did not allow us to quantify Rhodium in CRM NIST SRM 2556 as its concentration in the diluted sample was lower than the quantification limit. However, for the recovery measurements, a spike was added to the solutions which allowed representative recovery measurements, even on the Rhodium.

Conclusion

Autocatalysts materials are difficult to prepare for analysis by ICP and common hot plates, while microwave acid digestion recipes are laborious, time-consuming and may not provide total dissolution of the sample. Peroxide fusion has proven to be a valid alternative to prepare these samples for determination of PGM content. Combined with a simultaneous ICP-OES, it has the analytical capabilities to perform the analysis of low PGM concentrations on catalyst supports. In this study, platinum, palladium and rhodium were measured in two automotive reference materials, with good accuracy, precision and recovery. Important benefits resulting from using peroxide sample preparation by fusion include:

1. Complete sample dissolution in less than 10 minutes;
2. Easy to use compared to other sample dissolution recipes;
3. No HF or other hazardous acids needed.
   

References

[1] Johnson Matthey PLC, “Platinum 2012 Interim Review”, http://www.platinum.matthey.com/media/1393522/platinum_2012_interim_review.pdf
[2] Puig, A.I., Alvarado, J.I., “Evaluation of four sample treatments for determination of platinum in automotive catalytic converters by graphite furnace atomic absorption spectrometry”, Spectrochimia Acta Part B, Vol. 61, (2006), pp. 1050-1053.
[3] Thermo Scientific, “Determination of Platinum, Palladium, and Rhodium in Spent Automotive Catalytic Converters with Thermo Scientific Niton XL3t Series Analyzers”, http://www.niton.com/docs/literature/autocatalyticconverter_hirez_2012july18.pdf?sfvrsn=0
[4] Drews A.R., “XRF analysis of automotive catalysts by flux/fusion”, International Centre for Diffraction Data, Advances in X-ray Analysis, Vol. 45, (2002), pp. 472-477.
[5] Lucena, P., Vadillo, J.M., Laserna, J.J., “Mapping of platinum group metals in automotive exhaust three-way catalysts using laser-inducted breakdowns spectrometry”, Analytical Chemistry, Vol. 71, No. 19, (1999), pp. 4385-4391.
[6] BASF The Chemical Company, “Typical ceramic auto catalyst processing and refining”, http://www.converter-recycling.basf.com/p02/USWeb-Internet/converter-recycling/en_GB/function/conversions:/publish/content/microsites/converter-recycling/BASF_9492_Autocat_Flowchart_USLtr_2012-02-10.pdf
[7] ASTM, Standard D5153-04, “Standard Test Method for Palladium in Molecular Sieve Catalyst by Atomic Absorption”.
[8] ASTM, Standard D4782-04, “Standard Test Method for Palladium in Molecular Sieve Catalyst by Wet Chemistry”.