Abstract
The dynamical structure of a catalyst determines the availability of active sites on its surface. However, how nanoparticle (NP) catalysts re-structure under reaction conditions and how these changes associate with catalytic activity remains poorly understood. Using operando transmission electron microscopy, we show that Pd NPs exhibit reversible structural and activity changes during heating and cooling in mixed gas environments containing O2 and CO. Below 400 °C, the NPs form flat low index facets and are inactive towards CO oxidation. Above 400 °C, the NPs become rounder, and conversion of CO to CO2 increases significantly. This behavior reverses when the temperature is later reduced. Pt and Rh NPs under similar conditions do not exhibit such reversible transformations. We propose that adsorbed CO molecules suppress the activity of Pd NPs at lower temperatures by stabilizing low index facets and reducing the number of active sites. This hypothesis is supported by thermodynamic calculations.
Highlights
The dynamical structure of a catalyst determines the availability of active sites on its surface
Using density functional (DFT) calculations and Wulff constructions, we show that when in contact with a binary carbon monoxide (CO)/O2 mixture, the number of under-coordinated edge sites on a Pd NP abruptly increases at temperatures above 300 °C, whereas the number of edge sites on a Pt NP remains relatively constant with temperature
To mitigate the signal-to-noise ratio issue, the image sequences were captured using the electron counting mode of a direct electron detection camera, an imaging mode used in cryo-electron microscopy of biological specimens[27], and the imaging parameters were optimized to maintain lattice resolution
Summary
The dynamical structure of a catalyst determines the availability of active sites on its surface. It is known that the surfaces of noble metal crystals change in oxygen (O2)-rich environments, but the effect of these changes on catalytic activity is still being debated[2,3] To answer these questions, we have to study these materials under reaction conditions because the dynamical working structures may be different from those characterized under vacuum. These results provide direct evidence of how the morphology change due to CO adsorption can suppress the number of active sites on a Pd NP’s surface at lower temperatures The reversibility of these surface facet transformations in Pd NPs with temperature implies that the active morphology of such catalysts cannot be observed outside of reaction conditions
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