Particle-Size-Dependent Interaction of NO₂ with Pd Nanoparticles Supported on Model NOx Storage Materials

Combining scanning tunneling microscopy, molecular beam methods, and time-resolved infrared reflection absorption spectroscopy, we investigate the structure and reactivity of Pd nanoparticles supported on single-crystal-based model NO x storage materials. The latter consist of barium aluminate-like nanoparticles supported on an Al 2 O 3 film on a NiAl(110) substrate. On these surfaces, Pd deposition under ultra-high-vacuum conditions gives rise to the growth of three-dimensional Pd particles, nucleating at the predeposited barium aluminate aggregates. The reactivity of these systems toward NO 2 is tested systematically as a function of NO 2 exposure and Pd particle size. At room temperature, NO, generated by dissociative adsorption of NO 2, sequentially covers the following sites on the Pd nanoparticles: (i) hollow sites on (111) facets, (ii) bridge sites at particles edges and particle defects, and, finally, (iii) on-top sites at particle corners. The occupation of different sites on the particles is monitored as a function of NO 2 exposure and Pd particle size. Characteristic differences in the site occupation and the coverage dependence are observed as a function of particle size. At elevated NO 2 exposures, all NO-related features disappear, indicating the onset of oxidation of the Pd particles. For the Pd-containing systems, several new vibrational bands are observed. These new features are assigned to surface nitrates adsorbed on oxidized Pd particles and, tentatively, to surface nitrates adsorbed on sites that arise from the interaction between barium aluminate and oxidized Pd particles. In the limit of high NO 2 exposure, these new surface nitrates coexist with surface nitrates formed on the uncovered fraction of the barium aluminate nanoparticles.