Pd model catalysts: Effect of aging duration on lean redispersion

Abstract

An automotive three-way catalyst (TWC) deteriorates as a function of temperature, time and aging environment. While much effort has gone into formulating durable exhaust catalysts, relatively little attention has been paid to controlling the aging environment on the vehicle with techniques currently in use to protect the catalyst and counteract the increasing demands for higher fuel economy (e.g., overfueling to reduce exhaust temperatures). New engine control methods that are designed to minimize aging may be possible that can reduce the extent of catalyst deactivation and provide a lean environment capable of redispersing many of the precious metal particles. To develop improved engine control methods, detailed experimental information is needed to map the response of the catalyst to different aging conditions. In this report, we examine palladium-based model powder catalysts on ceria-zirconia, which were exposed to three different exhaust compositions, lean-only, rich-only and redox, each at 700°C for three different durations, 20 minutes, 2 hours and 16 hours. Residual catalyst activity and metal-support interaction were probed with Water Gas Shift (WGS) reaction and Oxygen Storage Capacity (OSC) measurements. The Pd metal particle size and dispersion were estimated by H 2 chemisorption and XRD line broadening. Lean catalyst treatments at 550°C and 700°C were applied to determine the effect on Pd size and catalyst activity. An infrared study of CO adsorption onto the catalysts was used to identify whether Pd crystallite facets were covered by the support after exposure to the redox aging environment then again after the lean treatments were applied. The aging temperature and reducing gas environment significantly deteriorated catalytic activity through a combination of metal oxidation state effects and support interactions, while the aging duration was linked to the extent of Pd sintering reversibility through lean treatments. These insights provide a basis to develop engine control and aftertreatment design strategies to avoid severe aging modes and determine how often to actively intervene to regenerate the catalyst.

Thermally Induced Restructuring of Pd@CeO2 and Pd@SiO2 Nanoparticles as a Strategy for Enhancing Low-Temperature Catalytic Activity

“…The growth of the Pd 3d peak suggests that Pd has migrated closer toward the outer perimeter of the shell so that more Pd species exist within the pertinent photoelectron escape depth for the XPS experiment. Although further study is warranted, based on the previous literature − and the aging conditions used in this work, an Ostwald Ripening mechanism is most likely responsible for Pd mobility, as opposed to transport in the vapor phase.…”

Advances in Catalytic Oxidation of Volatile Organic Compounds over Pd-Supported Catalysts: Recent Trends and Challenges

“…On the other hand, high-temperature reduction could decrease the surface Pd particle size by encapsulating and trapping it with the support and subsequently promoting the activation of O 2 , which is beneficial to the degradation of VOCs (Li et al, 2017). Lupescu et al stated that Pd below 8.8 nm was essential for the rapid formation of PdO and efficient re-dispersion of Pd (Lupescu et al, 2016). Lowering the size of Pd NPs can increase the interface between the Pd and support, offering more available mobile oxygen (Giraudon et al, 2008a).…”

A sinter-resistant catalyst using an alumina support recycled from AlP fumigation residue: trash to treasure

“…For the two fresh catalysts, supported Pd species were exclusive in the form of PdO according to their binding energies of Pd 3d 5/2 . 26,27 Pd is converted to PdO at a temperature over 325 1C and undergoes a wetting and spreading action to form thin flat crystallites. 28 Our fresh catalysts were obtained by heat treatment in flowing air at 400 1C for 5 h, thus most Pd species were oxidized to small PdO grains in our prepared catalysts.…”