Lin-Lin Wang (Ames Laboratory)

Computational Modeling of Transition-Metal Alloyed Nanoparticles in Working Condition

Transition-metal alloyed nanoparticles (NPs) are key components in current and emerging energy technologies because they are found to improve catalytic activity and selectivity for many energy-conversion processes. However, the difficulty of characterizing and describing the structural and compositional changes in alloyed NPs under reactive conditions remains a significant challenge. To address this challenge, several examples will be presented to show the effects of adsorbate, support and alloying on the properties of alloyed NPs. Hydrogen-passivation was predicted to inhibit the shearing instability in small Pt NPs and stabilize the ordered bulk-like structure. Recently, this prediction on the effect of adsorbate on NP structure has been verified by experiments and the effect of different supports has also been included. For the effect of alloying, two dominating factors and their interplay have been identified to determine the core-shell preference in alloyed NPs. Take a step further to address the challenge to describe the compositional profile of alloyed NPs under reactive condition, the cluster expansion (CE) method have been extended to treat both alloyed NPs and adsorbates on the same footing. The ability to evaluate the energetics over a huge number of configurations from the optimal CE Hamiltonian at the accuracy of the input first-principles calculations has been used to study the configurational thermodynamics of bimetallic NPs and adsorbates in detail. Ag-Au NPs has been found to prefer multi-shell configurations. The composition of Pd-Pt core-shell NPs can be tuned to give the optimal adsorption energy for hydrogen evolution reaction. The voltammetry for Pt(111)/H has been simulated to reveal the role of a small population of hydrogen adsorbed on non-fcc sites. With these examples, computational modeling is shown to provide an accurate description of the compositional profile for alloyed NPs and enable rational design of alloyed NP catalysts.

References:

L.-L. Wang, T. L. Tan, and D. D. Johnson, Nanoalloy composition-temperature phase diagram for catalyst design: Case study of Ag-Au, PRB 86, 035438 (2012)

T. L. Tan, L.-L. Wang, et al. A comprehensive search for stable Pt-Pd nanoalloy configurations and their use as tunable catalysts, Nano Lett. 12, 4875-4880 (2012)

L. Li, L.-L. Wang, et al. Noncrystalline-to-crystalline transformations in Pt nanoparticles, JACS 135, 13062-13072 (2013) L.-L. Wang and D. D. Johnson, Shear instabilities in metallic nanoparticles: hydrogen-stabilized structure of Pt37 on carbon, JACS 129, 3658-3664 (2007)