Pd model catalysts: Effect of air pulse length during redox aging on Pd redispersion

Nano-sized noble metal particles are an essential component in automotive catalytic converters. However, during customer in-use operation they agglomerate into larger particles and the performance of the catalytic converter is negatively affected. Engine control methods are needed to provide an environment capable of redispersing noble metal catalyst particles while the particle size is still small. In this study, Pd model powder catalysts, supported on three-way catalytic converter washcoat components of ceria-zirconia (Pd/CZO), 4 wt% La2O3 stabilized gamma-Al2O3 (Pd/AI) and unstabilized)gamma-Al2O3 (Pd/A) were exposed to redox cycling at 700 degrees C for 16 h (to simulate customer in-use operation engine exhaust) either continuously or interrupted with brief air pulses. The two goals of this research were to determine the air pulse length required so that any Pd redispersion gains achieved by air treatment in each 20 min redox cycle will be retained by the end of the 16 h aging, and to determine if these supports enable rapid Pd redispersion on the time scale of a common similar to 10 s engine fuel cut. Pd particle size and dispersion measurements were performed with three complimentary techniques of H-2 chemisorption, XRD and electron microscopy since not one technique could be used exclusively. Catalyst activity was determined by CO oxidation with the Water Gas Shift (WGS) reaction and Oxygen Storage Capacity (OSC) measurements to probe the contact between the noble metal and support at a given state of catalyst deterioration, and a CO light-off test to confirm trends observed with Pd metal dispersion. We found that air pulses lasting for 1/6 min applied to interrupt short redox aging spans enabled Pd redispersion only on the Pd/Al catalysts. Strong metal support interaction on the Pd/CZO catalysts by decoration of the Pd surface with support oxide was suspected to slow Pd redispersion by limiting metal mobility. Support oxides that promote the Pd2+ state may have been required on the Pd catalysts in order to exhibit Pd redispersion. The insight gained from this work could be used to develop engine control and aftertreatment systems that actively intervene and regenerate a catalyst capable of rapid Pd redispersion. (C) 2017 Elsevier B.V. All rights reserved.