Journal
ACS CATALYSIS
Volume 7, Issue 10, Pages 6843-6857Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.7b01600
Keywords
catalysis; mechanisms of reactions; in situ spectroscopy; ambient pressure XPS; nanotechnology; DFT; ceria
Categories
Funding
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-98CH10886]
- Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy [DE-AC02-05CH11231]
- Office of Science of the U.S. Department of Energy [DE-AC02-05CH11231]
- Philip Morris International
- National Science Foundation [DGE-1122374]
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Herein we investigate the reaction intermediates formed during CO oxidation on copper-substituted ceria nanoparticles (Cu0.1Ce0.9O2-x) by means of in situ spectroscopic techniques and identify an activity descriptor that rationalizes a trend with other metal substitutes (M0.1Ce0.9O2-x M = Mn, Fe, Co, Ni). In situ X-ray absorption spectroscopy (XAS) performed under catalytic conditions demonstrates that O2- transfer occurs at dispersed copper centers, which are redox active during catalysis. In situ XAS reveals a dramatic reduction at the copper centers that is fully reversible under catalytic conditions, which rationalizes the high catalytic activity of Cu0.1Ce0.9O2-x Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) show that CO can be oxidized to CO32- in the absence of O-2. We find that CO32- desorbs as CO2 only under oxygen-rich conditions when the oxygen vacancy is filled by the dissociative adsorption of O-2. These data, along with kinetic analyses, lend support to a mechanism in which the breaking of copper oxygen bonds is rate-determining under oxygen-rich conditions, while refilling the resulting oxygen vacancy is rate determining under oxygen-lean conditions. On the basis of these observations and density functional calculations, we introduce the computed oxygen vacancy formation energy (E-vac) as an activity descriptor for substituted ceria materials and demonstrate that Evac successfully rationalizes the trend in the activities of M0.1Ce0.9O2-x catalysts that spans three orders of magnitude. The applicability of Evac as a useful design descriptor is demonstrated by the catalytic performance of the ternary oxide Cu0.1La0.1,Ce0.8O2-x, which has an apparent activation energy rivaling those of state-of-the-art Au/TiO2 materials. Thus, we suggest that cost-effective catalysts for CO oxidation can be rationally designed by judicious choice of substituting metal through the computational screening of E-vac.
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