Observation of Aluminate Whiskers and Nanotubes in Dynamometer-Aged Three-Way Automotive Catalysts

Abstract

Short nanotubes and whiskers of aluminates of different compositions, NixAl2O3+x, SrAl12O19 and (Nd,La)AlO3, with a noble metal particle attached to one end, are observed in dynamometer-aged automotive three-way catalysts (TWCs). The samples were from different production formulations for the US market that were aged for 120 h to simulate 100 000 miles of driving with a peak temperature of 1000-1050 °C in the middle of the catalyst monolith. NixAl2O3+x appears as nanotubes in both Pt/Rh- and Pd-based catalysts with NiO additions. The nanotubes suspend the precious metal catalyst particles in the gas stream. Solid whiskers of SrAl12O19 have been found in a Pd-based catalyst with SrO additions in both pores and within alumina agglomerates. (Nd,La)AlO3 whiskers have thus far been found only within alumina agglomerates in a Pd-based catalyst. The aspect ratio of the whiskers and nanotubes observed after aging is 4:1 or more. The solid noble metal particles are cuboctohedral in shape and average 90-150 nm in diameter, depending on the sample. Catalyst particle size controls the nanotube and whisker diameter, although some conical whiskers have also been observed.