Characterization of alumina-supported Pt and Pt–Pd NO oxidation catalysts with advanced electron microscopy

Alumina-supported Pt and Pt–Pd catalysts for NO oxidation were fabricated from two different precursors, acetylacetonate and nitrate. A variety of electron microscopy techniques, notably aberration-corrected scanning transmission electron microscopy, were used to investigate both sets of catalysts in the fresh state (air calcined at 300 °C, then reduced in 1% H2 at 300 or 500 °C) and after oxygen-rich hydrothermal aging at 500 °C and 900 °C. Aberration-corrected microscopy and spectroscopic analysis revealed a significant difference in particle size and composition between the two fresh bimetallic catalysts, with the catalyst made from nitrate precursors containing relatively large (3–10 nm) Pd-rich particles in addition to smaller (1–2 nm) Pt-rich particles that were typical of the acetylacetonate precursors. Regardless of initial state, however, bimetallic particles underwent similar degrees of growth and homogenization (alloying) with aging. In particular, particle growth in the bimetallic catalysts from both precursors was limited relative to that in the pure Pt catalysts, where anomalously large (∼500 nm) particles formed at 900 °C, and the bimetallic catalysts from both precursors thus showed similar improvements in thermal stability, with little effect on measured NO reaction rates.