Automotive catalyst

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. Rigaku's ZSX Primus II high-performance WDXRF spectrometer 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 dried and pelletized in a laboratory hydraulic press, the performance data of Tables 1-3 for calibration accuracy, repeatability and detection limits were compiled using the ZSX Primus II in the analysis of fresh automotive catalyst. Rigaku's fundamental parameters (FP) method based on first principles of X-ray spectrometry and physical constants of the elements was used for calibration purposes due to the ease and sophistication with which FP mathematically accounts for interelement influences or "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 intensities and theoretical intensity predictions of the FP method, and thus the accuracy of instrument calibration. Correlation coefficients (R²) of ≥0.999 were common throughout this work.

Calibration accuracy
Table 1: Calibration accuracy
*Root mean square (RMS) deviation

Repeatability data
Table 2: Repeatability data
Results are based on 10 consecutive measurements of the sample.
RSD = relative standard deviation

Lower Limits of Detection for selected counting times
Table 3: Lower Limits of Detection for selected counting times
Lower limit of detection (LLD) = 3/M x Sqrt (Rb/Tb), where:
  • M = measurement sensitivity (cps/ppm)
  • Rb = count rate at background (cps)
  • Tb = counting time at background (sec)
  • cps = counts per second

FP calibration graph for Pd  FP calibration graph for Pd
Figure 1: FP calibration graph for Pd
Correlation coefficient = 0.99992
Figure 2: FP calibration graph for Rh
Correlation coefficient = 0.99974
FP calibration graph for Ce FP calibration graph for Zr
Figure 3: FP calibration graph for Ce
Correlation coefficient = 0.99917
Figure 4: FP calibration graph for Zr
Correlation coefficient = 0.99918

Whether you're a producer, researcher or recycler, the ZSX Primus II delivers the ultimate in consistent, high accuracy, high precision elemental analysis of automotive catalysts in a simplistic, cost-effective framework. The performance demonstrated here extends to numerous other testing applications in the field of heterogeneous catalysis. Please contact Rigaku Americas Corporation for details about your specific testing needs.