Nanoparticle Catalysts

Group Members

Liang Zhang, Samuel T. Chill, Zhiyao Duan, Wenjie Tang and Chun-Yaung (Albert) Lu

Overview


The science and technology of catalysis is particularly important at this time due to the energy and environmental challenges facing society. Research in the 1990s, showing surprising activity of Au nanoparticles, has largely motivated a search for new catalytic materials on the nanoscale with properties that are different from their bulk counterparts. Another significant factor in the development of new catalysts has been the growth of computing power and improvements in theoretical methods to help understand experiments at the atomic scale and to provide guidelines for catalyst design. Experiments, demonstrating the high activity of nanoparticle catalysts, have inspired the development of theoretical methods for calculating reaction mechanisms and screening for new catalysts. Iterating between theory and experiment is a promising strategy for understanding nanoparticle catalysis and reducing the cost of the development cycle for new catalysts.


Our group tightly collaborate with experiment group lead by Prof. Richard Crooks at UT Austin, where dendrimer-encapsulated nanoparticles (DENs), as a model system are synthesized and characterized at atomic level for direct comparison with theory. Summarized by the figure above, our collaboration leads to refinement of the theory and the prediction of better nanoparticle electrocatalyst candidates. This in turn leads to the next generation of more active electrocatalysts that can be used for a wide variety of applications. Detailed review of our work has been presented in the following two papers:

1. R. M. Anderson, D. F. Yancey, L. Zhang, S. T. Chill, G. Henkelman, and R. M. Crooks, A Theoretical and Experimental Approach for Correlating Nanoparticle Structure and Electrocatalytic Activity, Acc. Chem. Res. 48 1351-1357 (2015). DOI

2. L. Zhang, R. M. Anderson, R. M. Crooks, and G. Henkelman, Correlating Structure and Function of Metal Nanoparticles for Catalysis, Surf. Sci. 640 65-72 (2015). DOI

Determining Nanoparticles Structure

In order to model the properties of DENs it is necessary to characterize their structure. Determining the overall shape of 1-2 nm particles using only experimental methods is challenging. Validity of common characterization techniques, such as XRD and EXASF, on such small scale is still under estimation. On the other hand, in principle, it is possible to take a purely theoretical approach to determine the particle's geometry by locating the structure that corresponds to the global minimum potential energy as calculated by DFT. However the system is much larger than just the particle(interactions with the solvent and the dendrimer should be considered), greatly increasing the computational cost. Thus there is also a need for experimental structural information input on the theory side. We are developing new structural analysis tools, which intergrates the DFT simulation with experiment measured electro-chemical voltammogram, TEM image, XRD and EXAFS spectrum. This will allow an unprecedented level of feedback of experimental data to support the leading computational models.

  • DFT meets XRD-PDF


  • DFT meets Underpotential Deposition Voltammogram


Correlating nanoparticle Strcuture and Function

Alloy nanoparticles have a broad range of applications for many catalytic processes. In this research, we focus on the oxygen reduction reaction (ORR). The ORR is the cathode reaction of proton-exchange membrane fuel cells. It involves at least two elementary processes, O2 dissociation and the subsequent removal of the dissociated species by further reduction to H2O. ORR activity can be measured experimentally using electrochemical techniques, including cyclic voltammetry (CV) and rotating disk voltammetry (RDV). Theoretically, the ORR activity of a surface is understood to correlate with the O binding energy. Pioneering work by Nørskov and co-workers have established a volcano-shaped correlation between the O binding energy and ORR activity. On the strong-binding side of the volcano, oxygen species over-bind to the catalyst surface and limit the kinetics of product formation. On the weak-binding side of the volcano the reaction is limited by oxygen dissociation and adsorption. Pt offers the optimal tradeoff of the pure metals and the peak of the volcano corresponds to an oxygen binding energy which is slightly weaker than on a Pt(111) surface. The underlying reason that oxygen binding is a good reactivity descriptor is that there is a Brønsted-Evans-Polanyi (BEP) correlation between the binding and transition state energies, so that a single descriptor can describe both limits.In the following, we consider several nanoparticle alloys strucutures and seek to find structures with oxygen binding energies, and hence ORR activities, that approach or exceed that of Pt.

  • Core@Shell Nanoparticles


  • Random Alloy Nanopartcles


  • Alloy-Core@Shell Nanoparticles