The microscopic insight into how and why catalytically active nanoparticles change their shape during oxidation and reduction reactions is a pivotal challenge in the fundamental understanding of heterogeneous catalysis. We report an oxygen-induced shape transformation of rhodium nanoparticles on magnesium oxide (001) substrates that is lifted upon carbon monoxide exposure at 600 kelvin. A Wulff analysis of high-resolution in situ x-ray diffraction, combined with transmission electron microscopy, shows that this phenomenon is driven by the formation of a oxygen-rhodium-oxygen surface oxide at the rhodium nanofacets. This experimental access into the behavior of such nanoparticles during a catalytic cycle is useful for the development of improved heterogeneous catalysts.

We studied the interaction of oxygen with MgO(100) supported Pd nanoparticles at 10(-5) mbar oxygen pressure and a sample temperature of 570 K. We employed high-resolution X-ray reciprocal space mapping, which allows us to resolve the average particle shape from the quantitative analysis of intensity diffraction rods running perpendicular to corresponding facet surfaces. We identified the oxygen induced formation of nanosized (112) facets which is reversible in a CO atmosphere. Our results give direct evidence for the microscopic evolution of the nanoparticle shape under reactant exposure, which is essential for an atomistic understanding of catalytic reactions on nanoparticles.

No abstract

No abstract

We investigated the structure and formation of surface and bulk oxide on Pd(111) as a function of temperature and oxygen pressure from ultrahigh vacuum up to atmospheric pressures by means of in situ X-ray diffraction. Our X-ray diffraction data of the quasi 2-D surface oxide layer are compatible with the structural model of a Pd 5 O 4 layer proposed by density functional theory. In the temperature range from 650 to 950 K, the formation of the Pd 5 O 4 surface oxide layer can be described by a constant oxygen chemical potential, giving evidence that its formation takes place in local thermodynamical equilibrium with the surrounding oxygen gas phase. Above 950 K, the formation of the surface oxide layer is no longer observed, and a direct transition from chemisorbed oxygen to bulk oxide takes place, implying that the surface oxide layer is thermodynamically unstable under these conditions. Our results suggest that the oxygen chemical potential stability regime for the Pd 5 O 4 surface oxide layer is much smaller than recently predicted by density functional theory.

Noble metal nanoparticles supported by oxide carriers are widely employed in heterogeneous catalysis. In order to improve catalyst efficiency it is important to understand oxidation processes on the atomic scale. Here we studied oxygen-induced shape changes of Pt nanoparticles on MgO(001) by means of in-situ surface x-ray diffraction (SXRD) and x-ray reflectivity measurements (XRR). The x-ray results on the particle morphology were complemented by transmission electron microscopy (TEM) studies. The samples were prepared by means of physical vapor deposition and differed in average particle size and lateral particle size distribution. The oxygen-induced particle shape changes were found to be independent of particle size * To whom correspondence should be addressed † Max-Planck-Institut für Intelligente Systeme (former Max-Planck-Institut für Metallforschung), Heisenbergstr. and were characterized by the emergence of higher indexed facets. We propose a particle sizedependent oxidation behavior with a kinetically hindered bulk oxide formation. The formed bulk oxide structures were concluded to be (110)-oriented Pt 3 O 4 and a modified structure of (0001)-oriented α-PtO 2 . CO exposure of the oxidized particles did not lead to a shape change reversibility.

No abstract

No abstract