Motor vehicle catalytic converters provide an environment for chemical reactions in which toxic combustion by-products such as nitrous oxides, carbon monoxide and unburnt hydrocarbons are converted to safe or less toxic substances including oxygen, nitrogen, water vapor and carbon dioxide. A refractory ceramic monolith with a honeycomb structure forms the core of the converter to which an alumina “washcoat” is applied at 10-50 μm for increased surface area to boost efficiency. The catalyst itself, typically the noble metals platinum, palladium and rhodium, are incorporated into the washcoat in suspension before it is applied to the core. Barium sulfate and rare earth compounds such as a ceria-zirconia solid solution, lanthana and hafnia are also commonly added as oxygen storage materials, thermal and surface area stabilizers, and promoters.
Increasingly stringent global emission standards are leading researchers and manufacturers to place greater emphasis on optimizing automotive catalyst formulations for new lean-burning fuels and reducing the cost and complexity of new products without sacrificing performance. Reliable chemical characterization of catalyst formulations is critical to these efforts. Wavelength-dispersive x-ray fluorescence spectrometry (WDXRF) replaces arduous and precarious traditional chemical test methods for noble metal determinations such as fire assay and acid digestion with a fast, direct and highly reliable method of analysis having excellent detection sensitivity and measurement precision. The WDXRF method is also fully applicable to quantification of the rare earth additives and a wide range of other constituents in the catalyst formulation, as well as substrate components. The method accommodates both fresh and spent catalyst with suitable calibration.
Using a set of calibration standards prepared from slurried powder blends that were dried and then pelletized in a laboratory hydraulic press, the performance data of Tables 1-3 for calibration accuracy, repeatability and detection limits were compiled using a Rigaku ZSX Primus II WDXRF spectrometer (info here) for the analysis of fresh automotive catalyst. A fundamental parameter (FP) mathematical method based on first principles of x-ray physics, physical constants of the elements, and instrumental characteristics was used for calibration purposes due to the ease and sophistication with which FP accounts for interelement influences (matrix effects) that arise from variations in concentration of the elemental constituents of the samples. Plots of representative FP calibration graphs provided in Figures 1-4 graphically illustrate the outstanding linear correlation between measured x-ray intensities and the theoretical predictions of x-ray intensities by the FP method, and thus the accuracy of instrument calibration. Correlation coefficients (R2) of ≥0.999 were common throughout this work.
The performance demonstrated here extends to numerous other testing applications in the field of heterogeneous catalysis. Please contact ICON XRF Consulting for details about your specific testing needs.
Table 1. Calibration Accuracy
Table 2. Repeatability Data
Table 3. Lower Limits of Detection for Selected Counting Times
Figure 1. FP Calibration Graph for Pd
R2 = 0.99992
Figure 2. FP Calibration Graph for Rh
R2 = 0.99974
Figure 3. FP Calibration Graph for Ce
R2 = 0.99917
Figure 4. FP Calibration Graph for Zr
R2 = 0.99918