Pt is an active catalyst for diesel exhaust catalysis but is known to sinter and form large particles under oxidizing conditions. Pd is added to improve the performance of the Pt catalysts. To investigate the role of Pd, we introduced metallic Pt nanoparticles via physical vapor deposition to a sample containing PdO nanoparticles. When the catalyst was aged in air, the Pt particles disappeared, and the Pt was captured by the PdO, forming bimetallic Pt-Pd nanoparticles. The formation of metallic Pt-Pd alloys under oxidizing conditions is indeed remarkable but is consistent with bulk thermodynamics. The results show that mobile Pt species are effectively trapped by PdO, representing a novel mechanism by which Ostwald ripening is slowed down. The results have implications for the development of sinter-resistant catalysts and help explain the improved performance and durability of Pt-Pd in automotive exhaust catalytic converters.

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The time and resources of this participatory evaluation process enabled successful navigation of two important issues: (1) increased attention to statewide accountability of collaborative public health initiatives, and (2) increased expectation by health councils or other community partnerships to have a recognized voice in defining measures for this accountability.

As more efficient combustion engines are developed for transportation, it is expected that less heat will be wasted in the exhaust, leading to lower exhaust temperatures. Hence DOE has set a goal of achieving 90% conversion of target pollutants by 150 °C [1]. To meet exhaust emission standards, it is necessary to develop catalysts that provide light off at lower temperatures than the current generation of catalysts (which become active at ~200 °C). The new targets cannot be achieved simply by increasing the loading of noble metals. One way to achieve higher reactivity at low temperatures is by control of the crystallite size of the platinum group metal (PGM) nanoparticles [2]. Smaller particles and subnanometer clusters show higher reactivity, and in the limit, we can envision single atom catalysts, which provide the highest atom efficiency to reduce noble metal usage, since every atom is involved in the catalytic cycle. The challenge is to make these single atom and sub-nm structures durable so they can survive high temperature aging protocols and demonstrate performance under realistic conditions. This presentation will highlight approaches to enhance the reactivity and thermal durability of emissions control catalysts.The primary process for the degradation of PGM-based heterogeneous catalysts is Ostwald ripening, where mobile species emitted from the nanoclusters migrate over the support surface (or through the vapor phase), to form large particles [3]. After aging, only a fraction of the precious metal is available for catalysis. For example, when Pt/alumina diesel oxidation catalysts are aged in air at 800 °C for one week (the DOE aging protocol), the initially dispersed Pt (Fig. 1a) forms aggregates exceeding 100 nm in diameter, where less than 1% of the Pt is on the surface (Fig. 1b). This results in the loss of catalytic activity. Building on our study of the fundamentals of catalyst sintering [3,4] we reasoned that one way to slow the processes of Ostwald ripening would be to trap the mobile species emitted from the PGM nanoparticles. We found that PdO is able to trap mobile Pt species very effectively, forming Pt-Pd nanoparticles [5]. We have concluded that trapping of PtO2 by PdO constitutes an important mechanism for the improved durability of diesel oxidation catalysts. Continuing our search for metal oxides that are effective at trapping Pt, we discovered that cerium oxide powders were very effective at trapping Pt. As described by us recently [6], a physical mixture of Pt/alumina with ceria helped preserve the catalytic activity of the Pt catalyst for CO oxidation, even after aging at 800 °C for one week (Fig. 1c) since all of the Pt after aging was present in atomically dispersed form (Fig. 1d). Atom trapping constitutes a facile approach for preparing single atom catalysts that are thermally durable and can withstand high temperatures (Fig. 1e). Besides ceria and PdO, we have seen other examples of atom trapping which help in achieving greater durability and improved regeneration of heterogeneo...