Abstract
Nanoparticle sintering during catalytic reactions is a major cause for catalyst deactivation. Understanding its atomic-scale processes and finding strategies to reduce it is of paramount scientific and economic interest. Here, we report on the composition-dependent three-dimensional restructuring of epitaxial platinum–rhodium alloy nanoparticles on alumina during carbon monoxide oxidation at 550 K and near-atmospheric pressures employing in situ high-energy grazing incidence x-ray diffraction, online mass spectrometry and a combinatorial sample design. For platinum-rich particles our results disclose a dramatic reaction-induced height increase, accompanied by a corresponding reduction of the total particle surface coverage. We find this restructuring to be progressively reduced for particles with increasing rhodium composition. We explain our observations by a carbon monoxide oxidation promoted non-classical Ostwald ripening process during which smaller particles are destabilized by the heat of reaction. Its driving force lies in the initial particle shape which features for platinum-rich particles a kinetically stabilized, low aspect ratio.
Highlights
Nanoparticle sintering during catalytic reactions is a major cause for catalyst deactivation
A major challenge are the harsh conditions during catalytic reactions since they often lead to nanoparticle sintering, which results in catalyst deactivation primarily due to the loss of active surface area[1,2,3,4]
We report a systematic study of the compositiondependent sintering, shape and size changes of epitaxial Pt–Rh alloy nanoparticles on Al2O3(0001) under catalytic CO oxidation reaction conditions close to atmospheric pressures
Summary
To systematically unravel structural changes during CO oxidation we measured for each Pt–Rh composition and reactivity condition a two-dimensional (2D) reciprocal space map centred around the particle ð311Þ Bragg peak (Fig. 1). As key observation we note that the finite height fringes for Pt-rich particles progressively shift towards the Bragg peak position, when increasing the oxygen pressure and switching to higher catalytic activity (Fig. 2a). This implies that their height increases significantly when CO oxidation sets in because the L-spacing of their minima is inversely proportional to the particle height. Pt–Rh compositions ((a) Pt; (b) Pt0.85Rh0.15; (c) Pt0.7Rh0.3; (d) Pt0.5Rh0.5; (e) Rh) and various conditions (green: i; blue: iii; and red: vi). (f) compositiondependent particle heights and aspect ratios
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