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Relating adatom emission to improved durability of Pt-Pd diesel oxidation catalysts

Journal of Catalysis

Johns, Tyne R.; Goeke, Ronald S.; Ashbacher, Valerie; Thüne, Peter C.; Niemantsverdriet, J.W.; Kiefer, Boris; Kim, Chang H.; Balogh, Michael P.; Datye, Abhaya K.

Sintering 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 ≤ 800°C). Pt sinters readily in air, and at high temperatures (≥800°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.

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TYPE Journal Article YEAR 2015
ScopusOSTIDOI

Mechanical and electrical performance of thermally stable Au-ZnO films

Acta Materialia

Schoeppner, R.L.; Goeke, Ronald S.; Moody, Neville R.; Bahr, D.F.

The mechanical properties, thermal stability, and electrical performance of Au-ZnO composite thin films are determined in this work. The co-deposition of ZnO with Au via physical vapor deposition leads to grain refinement over that of pure Au; the addition of 0.1 vol.% ZnO reduces the as-grown grain size by over 30%. The hardness of the as-grown films doubles with 2% ZnO, from 1.8 to 3.6 GPa as measured by nanoindentation. Films with ZnO additions greater than 0.5% show no significant grain growth after annealing at 350°C, while pure gold and smaller additions do exhibit grain growth and subsequent mechanical softening. Films with 1% and 2% ZnO show a decrease of approximately 50% in electrical resistivity and no change in hardness after annealing. A model accounting for both changes in the interface structure between dispersed ZnO particles and the Au matrix captures the changes in mechanical and electrical resistivity. The addition of 1-2% ZnO co-deposited with Au provides a method to create mechanically hard and thermally stable films with a resistivity less than 80 n-m. These results complement previous studies of other alloying systems, suggesting oxide dispersion strengthened (ODS) gold shows a desirable hardness-resistivity relationship that is relatively independent of the particular ODS chemistry.

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TYPE Journal Article YEAR 2015
ScopusOSTIDOI

On the thermal stability of physical vapor deposited oxide-hardened nanocrystalline gold thin films

Journal of Applied Physics

Argibay, Nicolas A.; Mogonye, J.E.; Michael, Joseph R.; Goeke, Ronald S.; Kotula, Paul G.; Scharf, T.W.; Dugger, Michael T.; Prasad, Somuri V.

We describe a correlation between electrical resistivity and grain size for PVD synthesized polycrystalline oxide-hardened metal-matrix thin films in oxide-dilute (<5 vol. % oxide phase) compositions. The correlation is based on the Mayadas-Shatzkes (M-S) electron scattering model, predictive of grain size evolution as a function of composition in the oxide-dilute regime for 2 μm thick Au-ZnO films. We describe a technique to investigate grain boundary (GB) mobility and the thermal stability of GBs based on in situ electrical resistivity measurements during annealing experiments, interpreted using a combination of the M-S model and the Michels et al. model describing solute drag stabilized grain growth kinetics. Using this technique, activation energy and pre-exponential Arrhenius parameter values of Ea = 21.6 kJ/mol and Ao = 2.3 × 10-17 m2/s for Au-1 vol. % ZnO and Ea = 12.7 kJ/mol and Ao = 3.1 × 10-18 m2/s for Au-2 vol. % ZnO were determined. In the oxide-dilute regime, the grain size reduction of the Au matrix yielded a maximum hardness of 2.6 GPa for 5 vol. % ZnO. A combined model including percolation behavior and grain refinement is presented that accurately describes the composition dependent change in electrical resistivity throughout the entire composition range for Au-ZnO thin films. The proposed correlations are supported by microstructural characterization using transmission electron microscopy and electron diffraction mapping for grain size determination.

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TYPE Journal Article YEAR 2015
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Results 26–50 of 90
Results 26–50 of 90