Sarah Tupy, Dionisios G. Vlachos, Jingguang G. Chen, University of Delaware

Aaron Jenkins, Angelica Sanchez, Abhaya K. Datye, University of New Mexico

Soenke Seifert, Argonne National Laboratory

Sergei Ivanov, CINT/Los Alamos National Laboratory

The design of highly active and selective catalysts will have a great impact on our energy future, the economy and the environment. For example, it is estimated that the US can sustainably replace one third of its fossil based transportation fuels with biofuels from biomass. However, this requires the development of new processes and catalysts that are highly active, selective and stable under extreme conditions. In order to design more active, and especially more selective catalysts, the synthesis of well-defined nanostructures and correlating their structure under reaction conditions with the reaction kinetics is required. The talk will cover two topics: 1) mechanisms of colloidal Pd nanoparticles synthesis in solution and 2) structure evolution of carbon supported PtNi and PtCo nanoparticles during aqueous phase reforming conditions.

For the colloidal nanoparticles synthesis, a combination of in situ XAFS and SAXS in a microfluidic reactor showed that the synthesis is limited by slow, continuous nucleation. The growth of the nanoparticles goes through two autocatalytic phases, a fast phase followed by a very slow growth phase. The transition from fast to slow growth was determined to be caused by rapid increase in bonding with the capping agent. Surprisingly, despite the overlap of nucleation and growth phases, small nanoparticles with narrow size distributions were still formed. The insights into the synthesis mechanisms gained from XAFS and SAXS will be presented and the origin of the narrow size distribution will be explained.

Using, operando XAFS, the PtNi and PtCo/C structures will be shown to be dependent on the reaction environments. The PtNi nanoparticles reversibly re-structure from a Pt rich to a Ni rich core-shell structure under hydrogen and aqueous phase reforming conditions. On the other hand, PtCo nanoparticles go through irreversible changes caused by leaching of 60% of the Co in solution during aqueous phase reforming of glycerol. Insights into the restructuring using DFT calculations will be presented and the catalyst structures from XAFS will be correlated with the reaction kinetics.

This research was funded, in part, by the Chemical Imaging Initiative at Pacific Northwest National Laboratory. It was conducted under the Laboratory Directed Research and Development Program at PNNL, a multi program national laboratory operated by Battelle for the U.S. Department of Energy.