2018 AIChE Annual Meeting

(504f) Balancing Reactivity and Stability in Metal Nanoparticle and Alumina Support Systems Via Redox Reactions: A Multiscale Computational and Experimental Approach

Authors

Matthew Curnan - Presenter, Pohang University of Science and Technology (POSTECH)
Wissam A. Saidi, University of Pittsburgh
Judith C. Yang, University of Pittsburgh
Henry Ayoola, University of Pittsburgh
Matthew McCann, University of Pittsburgh
Pt nanoparticles (NPs) supported on γ-Al2O3 (Pt/γ-Al2O3) serve in catalytic applications ranging from methanol reformation to CO oxidation, benefiting from the large surface area and strong metal-support interaction of γ-Al2O3 capable of stabilizing NPs. Additionally, Pt/γ-Al2O3 catalyst systems benefit from the high catalytic reactivity of small, nanoscale metal Pt NPs, the high surface-to-volume ratios of which enable many available sites for catalytic reactions and bonding to supports. However, under the reaction conditions – such as high (>600 °C) temperatures or highly oxidizing environments – frequently employed in industrial catalytic applications, Pt NPs are affected by issues such as poor long-term thermal stability and catalytic deactivation from Pt oxide formation. Solutions attempting to address these issues, such as the introduction of a reducing H2 environment to remove Pt oxides, can change Pt NP morphology and reduce the effective surface area of the Pt/γ-Al2O3 interface, weakening the metal-support interaction and further destabilizing the catalyst system. In contrast, the addition of lower pressure oxidative pretreatments of O2 or H2O to Pt/γ-Al2O3 systems can further stabilize Pt/γ-Al2O3 via Pt-O and Pt-OH interfacial bonding. Thus, for Pt/γ-Al2O3 systems, catalysts can be rationally designed to be both stable and reactive by balancing the presence of reducing and oxidizing environments.

Though both O vacancies on oxide supports and metal-metal or metal-oxide bonding can stabilize metal-support catalyst systems, comparisons between experimental and computational Energy Electron Loss Spectroscopy (EELS) results show only metal-metal and metal-oxide bonding are found to be stable at the Pt/γ-Al2O3 interface. Therefore, computational models will seek to find temperature dependent conditions under which O2 and H2 environments can stabilize Pt NPs without forming metal oxide layers on them, as well as determining whether differently composed Pd and Rh NPs observe the same redox driven trade-off. In this study, energetic and structural data achieved using Density Functional Theory (DFT) modeling of M/γ-Al2O3 (M = Pt, Pd, Rh) systems will be applied to a multiscale Reactive Force Field (RFF) optimization approach, which will parameterize RFFs capable of performing Molecular Dynamics (MD) simulations. RFF MD simulations will then be completed under different temperatures, H2/O2 coverages, metal NP sizes, and other considerations to determine the redox reaction conditions under which the smallest possible metal NPs can retain catalytic activity while staying stabilized on γ-Al2O3 supports.