Combinatorial high-energy x-ray microbeam study of the size-dependent oxidation of Pd nanoparticles on MgO(100)

Abstract

We have studied the oxidation of MgO(100) supported Pd nanoparticles in a size range from $4\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}24\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at $570\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and near atmospheric pressures. Our experimental setup using high-energy x rays allows us to study in situ the oxidation behavior for different particle sizes under identical experimental conditions. We can identify three size regimes with a distinct oxidation mechanism. For particles smaller than $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter, kinetic barriers for bulk oxide formation are strongly reduced and complete transformation to PdO bulk oxide takes place, epitaxial to the MgO substrate. Particles with a diameter $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $<D<9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ undergo continuous shrinkage during oxidation accompanied by the formation of epitaxial PdO on MgO(100). For larger particles with $D>9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, we observe a complete passivation of the particle surface by a thin polycrystalline PdO layer, which prevents further oxidation. Our experiments demonstrate that particles with a few nanometers in size exhibit a much higher reactivity with oxygen, which is important for the performance of Pd nanoparticles as a CO oxidation catalyst.