Designing Catalysts for Meeting the DOE 150 °C Challenge for Exhaust Emissions

Abstract

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...

Kinetic Synergy between Supported Ir Single Atoms and Nanoparticles during CO Oxidation Light-Off

“…For this, we will consider an inlet blend that is more commonly expected for engine exhaust and is consistent with the 150 °C DOE challenge. 5,6 We chose conditions of 1% CO and 10% O 2 with 99% conversion being the target conversion at 150 °C. From the general definition of C (eq 3), C = 13.5 at 150 °C for the Ir single atoms and nanoparticles on MgAl 2 O 4 .…”

“…With the understanding of which parameters affect the kinetic synergy between Ir single atoms and nanoparticles, we can now optimize a catalyst for a given set of reaction conditions. For this, we will consider an inlet blend that is more commonly expected for engine exhaust and is consistent with the 150 °C DOE challenge. , We chose conditions of 1% CO and 10% O 2 with 99% conversion being the target conversion at 150 °C. From the general definition of C (eq ), C = 13.5 at 150 °C for the Ir single atoms and nanoparticles on MgAl 2 O 4 .…”

“…This has been driven in part by the 150 °C U.S. Department of Energy (DOE) challenge, 5 which strives for a catalyst that is able to treat 90% of automobile exhaust pollutants at or below 150 °C with a focus on cold startup which is when most of the pollutants are released. 6 This focus has led to many studies using CO (and other pollutants) light-off experiments to understand how to increase the catalyst activity and decrease the temperature required for 90% conversion.…”

“…Not only are the catalysts allowing for full usage of the expensive precious metals, they also can have different properties leading to improved activity and/or selectivity for some reactions. − CO oxidation is one such reaction that has seen much research into the ability of single atoms to improve the activity and metal utilization compared to small clusters and nanoparticles. This has been driven in part by the 150 °C U.S. Department of Energy (DOE) challenge, which strives for a catalyst that is able to treat 90% of automobile exhaust pollutants at or below 150 °C with a focus on cold startup which is when most of the pollutants are released . This focus has led to many studies using CO (and other pollutants) light-off experiments to understand how to increase the catalyst activity and decrease the temperature required for 90% conversion.…”

The carrier effect of ternary solid solution derived by trimetallic CeZrNi MOFs on the Pd dispersion stability for NGVs exhaust purification

“…In order to satisfy the increasingly stringent regulations, such as China VI, Europe VI, the United States 2021-2027 second-stage heavy greenhouse gas emission standards, [5] the precious metal component in the commercial TWC catalyst for natural gas engine exhaust purification is still an indispensable main active component, especially the Pd component is widely used. [6] However, a long-standing problem is that under engine operating conditions, palladium nano-particles tend to aggregate (or sintering) and grow larger, [7][8][9][10] and the availability of palladium active components and the chemical property of palladium is partly determined by the partical size, [11][12] so improving the dispersion stability of palladium components can effectively improve the catalytic activity of palladium catalysts and reduce their production costs. Therefore, in order to achieve the effective removal of pollutants (CO, NO x and CH 4 , etc) from the natural gas vehicles exhaust, it is necessary to continuously develop the highly dispersed Pd-based catalyst technology with good sintering resistance.…”