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

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 (Pt_nO_x) on ceria under oxygen-rich conditions. We show Pt_nO_x/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 Pt_nO_x/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 Pt_nO_x/CeO₂ and Pt₁/CeO₂ based on experimental XPS, EXAFS, and CO-DRIFTS spectroscopy. The representative ~1 nm catalytic domain in the Pt_nO_x/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 Pt_nO_x 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 Pt_nO_x/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.