Nanoscience and Microsystems ETDs

Publication Date

Summer 7-11-2017

Abstract

This work provides a fundamental understanding on the vapor phase processes that govern the sintering of supported nanoparticles in relation to the diesel oxidation catalyst (DOC). Sintering is a deactivation process that affects this catalyst significantly, and many other catalyst systems. Therefore, it is important to understand the sintering mechanisms in order to improve the long term catalytic reactivity.

Pt is an active catalyst in the DOC but it sinters via Ostwald ripening to form large particles under the accelerated aging conditions (800 oC) recommended by the Department of Energy (DOE). At 800 oC in the presence of oxygen, Pt forms PtO2 with a high vapor pressure. Therefore, vapor phase ripening via transport of PtO2 is significant under these conditions and leads to the growth of large Pt nanoparticles. It is well known that adding Pd improves the durability of Pt in DOCs and that the support influences the rate of Pt sintering. However, there is no clear consensus in the literature for the responsible mechanisms. In conventional powder catalysts, it is impossible to study the effect of Pd and the support on the vapor pressure of PtO2 because the pores trap the volatilized metal. In this study, an open system such as planar model catalysts were utilized to gain mechanistic insights and to directly measure the vapor pressure of PtO2.

With the use of model catalysts, it was found that the mechanism by which Pd improves the durability of Pt is two-fold: (1) Pd lowers the vapor pressure of PtO2 and (2) PdO is regenerated by the emission of Pt from Pt-Pd nanoparticles and serves as trapping sites for mobile Pt species. The trapping effect is enhanced when there is excess PdO. Lastly, model catalysts allowed for studying the role of the support on the sintering of Pt. Three supports were investigated: La-Al2O3, MgAl2O4, and polyhedral CeO2. By depositing thin films of powder onto planar model TEM grids, it was possible to measure the vapor pressure of PtO2 and relate it to the rate of sintering.

The last study in this dissertation was performed at General Motors Global Research and Development at the Chemical and Materials Systems Lab. The work provides a fundamental study on the kinetics of propane oxidation over Pd/La-Al2O3 catalysts. The work aims to develop low temperature catalysts for hydrocarbon oxidation that will meet the 150 oC challenge as proposed by the DOE.

Keywords

Pt, Pd, Ostwald ripening, diesel oxidation catalyst

Document Type

Dissertation

Language

English

Degree Name

Nanoscience and Microsystems

Level of Degree

Doctoral

Department Name

Nanoscience and Microsystems

First Committee Member (Chair)

Abhaya K. Datye

Second Committee Member

Fernando Garzon

Third Committee Member

Hien Pham

Fourth Committee Member

Gongshin Qi

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