(208b) CO Oxidation By Pt Single Atoms and PtnOx Clusters on Ceria

Authors: 
Goldsmith, B. R., University of Michigan
Wang, H., Tianjin University
Liu, J. X., University of Michigan
Allard, L., Oak Ridge National Laboratory
Lee, S., Argonne National Laboratory
Liu, J., Tufts University
Li, W., General Motors Global R&D
Flytzani-Stephanopoulos, M., Tufts University
Yang, M., General Motors
Li, H., Tufts University
Wang, J., Tianjin University
Wang, J., Tianjin University
Understanding heterogeneous catalysts for low-temperature CO oxidation is important for designing improved catalysts for purifying vehicle emissions during engine cold start and cold weather. In this talk, we discuss our efforts to probe CO oxidation by isolated Pt single atoms (Pt1) and small ~1 nm Pt oxide clusters (PtnOx) on ceria under oxygen-rich conditions. We show PtnOx/CeO2 catalysts have a turnover frequency (TOF) per platinum atom that is 100-1000x larger than Pt1/CeO2 for CO oxidation between 80 – 100 oC ([O2]/[CO] ~ 50). We predict the stable and active PtnOx/CeO2 structure and CO oxidation reaction mechanism under experimentally relevant conditions using a combined genetic algorithm/grand canonical Monte Carlo/density functional theory approach. Agreement of Pt chemical valence, Pt coordination environment, CO-Pt adsorption behavior, and reaction kinetics was shown between our model systems and experiment for both the PtnOx/CeO2 and Pt1/CeO2 based on experimental XPS, EXAFS, and CO-DRIFTS spectroscopy. The representative ~1 nm catalytic domain in the PtnOx/CeO2 catalysts is modeled as a one-layer thick Pt8O14 that distinguishes itself from either single atoms or 3-D nanoclusters. Each tetragonally coordinated Pt atom in this PtnOx cluster is separated but also bridged by oxygen atoms through a Pt-O-Pt unit. Both our theoretical and experimental results suggest that the exciting catalytic performance is attributed only to the Pt-O-Pt ensemble in PtnOx/CeO2 under these oxygen rich conditions (Figure 1, right)—the lattice oxygen from the ceria particle (~20 nm, dominated by CeO2(111) surfaces) does not participate in this unusually high catalytic activity at low temperatures. We envision that, if the cross-linked Pt-O-Pt can be stabilized through similar oxygen linkages, a similar catalytic species may be created on various support substrates other than ceria, such as alumina. These findings open new possibilities for designing catalysts with orders of magnitude higher activity by utilizing multiple but individually separated metal atoms and alternative oxygen intermediates.