Diesel oxidation catalysts (DOCs), which decrease the amount of harmful carbon monoxide (CO), nitrogen oxide (NO), and hydrocarbon (HC) emissions in engine exhaust, typically utilize Pt and Pd in the active phase. There is universal agreement that the addition of Pd improves both the catalytic performance and the durability of Pt catalysts. However, the mechanisms by which Pd improves the performance of Pt are less clear. Because these catalysts operate under oxidizing conditions, it is important to understand these catalysts in their working state. Herein, we report the microstructure of PtPd catalysts that are aged in air at 750 °C. After 10 h of aging, EXAFS and XANES analysis show that the Pt is fully reduced but that almost 30 % of the Pd species are present as an oxide. HRTEM images show no evidence of surface oxides on the metallic PtPd particles. Instead, the PdO is present as a separate phase that is dispersed over the alumina support. Within the metallic particles, Pt and Pd are uniformly distributed and there is no evidence of core–shell structures. Therefore, the improved catalytic performance is likely associated with the co‐existence of metallic Pt and Pd on the catalyst surface.

a b s t r a c tSintering of nanoparticles is an important contributor to loss of activity in heterogeneous catalysts, such as those used for controlling harmful emissions from automobiles. But mechanistic details, such as the rates of atom emission or the nature of the mobile species, remain poorly understood. Herein we report a novel approach that allows direct measurement of atom emission from nanoparticles. We use model catalyst samples and a novel reactor that allows the same region of the sample to be observed after short-term heat treatments (seconds) under conditions relevant to diesel oxidation catalysts (DOCs). Monometallic Pd is very stable and does not sinter when heated in air (T 6 800°C). Pt sinters readily in air, and at high temperatures (P800°C) mobile Pt species emitted to the vapor phase cause the formation of large, faceted particles. In Pt-Pd nanoparticles, Pd slows the rate of emission of atoms to the vapor phase due to the formation of an alloy. However, the role of Pd in Pt DOCs in air is quite complex: at low temperatures, Pt enhances the rate of Pd sintering (which otherwise would be stable as an oxide), while at higher temperature Pd helps to slow the rate of Pt sintering. DFT calculations show that the barrier for atom emission to the vapor phase is much greater than the barrier for emitting atoms to the support. Hence, vapor-phase transport becomes significant only at high temperatures while diffusion of adatoms on the support dominates at lower temperatures.

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.

Diesel oxidation catalysts (DOCs) are currently composed of platinum (Pt) and palladium (Pd). Pt catalysts are very active, but Pt sinters and has poor durability under oxidizing conditions. Reports in the literature show that Pd improves the durability of Pt catalysts but it has been unclear how Pd enhances the durability and performance of these catalysts [1,2]. Esparza et al. demonstrated that core-shell Pt-Pd structures with a surface Pd layer could be prepared via colloidal routes [3]. Ezekoye et al. suggested that in their aged Pt-Pd catalysts, particles under 2.5 nm were primarily Pt-rich while those greater than 2.5 nm were Pd-rich [2]. However, it is important to study these catalysts under oxidizing conditions, as encountered in diesel oxidation catalysts. Hence, the extent of alloying during working conditions, how Pd modifies Pt, and whether Pd is present as a surface layer or as Pd, needs further study by HRTEM and EDS. Aging under oxidizing conditions may change the morphology, in which case the core-shell structures may not be stable under working conditions. 1660

Currently, precious metals such as Pt, Pd, and Rh are used in catalytic converters for treatment of exhaust gases from gasoline and diesel engines. These catalysts help remove pollutants, such as nitrogen oxides as well as CO and hydrocarbons, all of which result in smog and respiratory problems in urban environments. The supplies of precious metals are limited worldwide, but there is increasing demand for clean energy. Hence, there is a need to develop more active catalysts that provide long-term stable performance at elevated temperatures with minimal use of precious metals such as Pt. A serious problem facing catalysts is the loss of activity during use. The primary mechanism by which automotive catalysts lose activity is through growth of nanoparticles, via a process known as Ostwald ripening In this work, we have used model catalysts to study the emission of atoms from nanoparticles, one of the key steps in Ostwald ripening. Electron beam evaporation was used to synthesize Pt, Pd, and 50% Pt-50% Pd samples on silica TEM grids. Each sample was first reduced in flowing 5% H 2 /N 2 at Figure 1 shows HAADF-STEM (High-Angle Annular Dark Field Scanning Transmission Electron Microscopy) images of the Pt and the bimetallic sample before and after aging. The Pt-Pd bimetallic sample showed significant growth in particle size after aging and a dramatic drop in the number of particles per square micron, whereas the Pt-only sample showed fewer particles but no significant particle growth. We determined the mass of metal on each sample assuming spherical particles and found that the effective thickness of the Pt layer decreased from 2.4 Å to 1.3 Å, which amounts to a 47% decrease in the mass of Pt. In contrast, the Pt-Pd sample lost only 9% of its mass. Since the loss of the metal is caused by emission of atoms, which is related to the vapor pressure of the metal (or metal oxide complex), the results show a significant decrease in atom emission due to the presence of Pd in the Pt. The vapor pressure of Pt-Pd alloys has been studied in the literature and it was reported that the system shows only a modest degree of non-ideality, implying that the drop in vapor pressure should at most be 50%, while we see over 80% decrease in metal emission rates.The composition of the bimetallic sample determined from EDS (Energy Dispersive X-Ray Spectroscopy) before aging was determined to be 39.4% Pt and 60.6% Pd. After aging, the composition was 32.7% Pt and 67.3% Pd. The images show that the nanoparticles remained in the metallic state, with only a few instances where a palladium oxide phase segregated from the Pt-Pd alloy. The similarity of compositions of the individual nanoparticles after aging to those before 1692