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Search This Site | People | Departments | Penn State
Robert M. Rioux
Department of Chemistry and Chemical Biology
Harvard University
Tuesday, February 12, 2008
102 Chemistry Building
10:00 a.m. - 11:00 a.m.
Abstract
Heterogeneous catalysts are complex materials with chemical and physical interactions on numerous length scales.
Current methods to produce catalysts rely on physically-driven assembly, enabling little control on the atomic to nanometer level.
In this talk, we will introduce a synthetic method that enables the design of catalysts by individually synthesizing the building blocks of the catalyst.
Colloidal methods to produce size- and shape-controlled nanoparticles and their encapsulation in well-ordered mesoporous silica materials will be demonstrated.
Platinum-based materials are characterized by a number of chemical and physical techniques, and their inherent activity in a variety of probe reactions has been measured.
Macroscopic kinetic measurements on these samples yield more insight into structure-function relationships in catalysis than their classically-prepared counterparts.
As an example, we will demonstrate the utility of these well-defined catalytic structures to examine the nanoscale size dependence of the catalyst in an environmentally-relevant reaction. The oxidation of NO to NO2 on Pt nanoparticles is an important step in nitrogen oxides (NOx) storage-reduction (NSR) technology. The state-of-the-art NSR materials have a significantly higher adsorption capacity for NO2 than NO; however, >90% of the nitrogen-containing compounds are in the form of NO. Larger Pt nanoparticles are much more active and stable under NO oxidation conditions. It appears that the ability of larger nanoparticles to maintain metallic character under NO oxidation conditions is critical for high activity and stability. X-ray absorption spectroscopy measurements demonstrated small particles quickly oxidize with time-on-stream, while larger particles surface oxidized - confirmed by high pressure x-ray photoelectron spectroscopy - but remained metallic in the bulk. The kinetic behavior of these silica-supported Pt catalysts is used to develop a reaction mechanism that is consistent with spectroscopic measurements and supported by theoretical calculations.
This work demonstrates that the coupling of nanoparticles synthesis by colloidal chemistry with in-situ characterization methods allows the elucidation of nanoscale phenomena that are masked in ensemble measurements of dispersed catalytic materials.
*This work was performed at the University of California, Berkeley and in collaboration between Purdue University and R. M. R.