Precious metal analysis of fresh reforming catalysts are typically performed by both the catalyst manufacturer and buyer to arrive at a financial settlement on the quantity of metal in each lot of commercial catalyst: Traditional assay methods involve a variety of fire assay or wet chemical acid digestion schemes coupled with gravimetric, colorimetic, or titrimetric measurement for precious metals. Methods must have sufficient precision and accuracy to afford interlaboratory agreement of within one half of one percent relative between the catalyst supplier and purchaser. To meet this requirement many laboratories rely on classical methods. Unfortunately these procedures are labor intensive and time consuming.
X-ray fluorescence has the inherent instrument precision to achieve typical intralaboratory precision of 0.5% RSD on a wide variety of elements and numerous sample types. We have developed an X-ray fluorescence method for the assay quality analysis of fresh reforming catalysts containing platinum, rhenium, and iridium. This method was applied to numerous samples over the past five years.
The XRF method uses chemically assayed fresh catalysts as comparison standards to quantify the X-ray fluorescence response on unknown samples. Samples and standards of the same matrix and nominal elemental concentrations are ground, dried overnight, and then allowed to air equilibrate. Standards and samples are pressed into briquettes with alumina cups in a pellet press to 20 tons. Net counts for each element are collected at the appropriate wavelengths for both standards and samples. Analysis is performed on a minimum of three sample briquettes and four briquettes for each of three standards. The ratio of the net counts to the precious metal concentration on each standard is averaged to obtain a calibration factor. Platinum alone and platinum in combination with other promoter precious metal concentrations are calculated by taking the average sample net counts for each element and dividing by the appropriate calibration factor. Intralaboratory and interlaboratory results are presented showing the applicability of X-ray fluorescence as an assay quality procedure for precious metal analysis.