Quantitative Pt and Rh distributions in pollution-control catalysts

Abstract

In the control of automobile emissions Pt is utilized to facilitate the oxidation of CO and hydrocarbons while Rh is added to selectively catalyze the reduction of NO x to N 2. These separate functions suggest that an optimum catalyst might have these elements at different locations within the support. Two levels of noble-metal distribution are important: (a) the distribution across the 1.2 mm wall of the γ-alumina support monolith and (b) the distribution within individual 1–20 nm particles of reduced catalyst. An electron probe microanalyzer (EPMA) was used for quantitative analysis of the Pt and Rh distributions across the support wall. The EPMA can detect a minimum of 200–500 ppm of these metals in alumina, but at a spatial resolution of 10 μm. An analytical electron microscope (AEM) with a field emission electron source was used to measure the composition of small metal particles down to 1–2 nm in diameter. The high spatial resolution of the AEM analysis allowed detection of surface segregation in 1–20 nm particles containing both Pt and Ph. Platinum segregated to the particle surface as expected for Pt—Rh particles reduced in H 2.