Invited Paper SS+NS-TuA7
Structure, Chemical State, and Reactivity Investigations of Size- and Shape-Selected Nanocatalysts under Operando Conditions

Tuesday, October 30, 2012, 4:00 pm, Room 21

The rational design of the next-generation of catalysts requires detailed knowledge of the correlation between structure, chemical composition, and reactivity. Even though Pt and Pd are among the most industrially relevant and widely investigated nanocatalysts, their complex interaction with common reactants such as oxygen still provides many challenges to the scientific community. In this work, the relation between the structure and reactivity of nanocatalysts “at work” was obtained via X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, and mass spectrometry. Homogeneous size- and shape-selected metal nanoparticles (NPs) have been synthesized by means of diblock copolymer encapsulation.

The influence of the nanoparticle shape on the reactivity of Pt nanocatalysts on γ-Al₂O₃ will be described. Nanoparticles with similar size distributions (~0.8-1 nm) but with different shapes were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the NP surface and the onset reaction temperature was observed. Furthermore, platinum oxides were found to be the active species for the partial oxidation of 2-propanol, while the complete oxidation was catalyzed by oxygen-covered metallic Pt NPs.

The evolution of the structure and oxidation state of ZrO₂-supported Pd nanocatalysts during the in situ reduction of NO with H₂ will also be discussed. Prior to the onset of the reaction, NO-induced redispersion of the Pd NPs over the ZrO₂ support was observed, and Pd^δ+ species detected. This process parallels the high production of N₂O observed at the onset of the reaction (>120°C), while at higher temperatures (≥ 150°C) the selectivity shifts toward N₂. Interestingly, concomitant with the onset of N₂ production, the Pd atoms re-aggregate into large metallic Pd NPs, which were found to constitute the active phase for the H₂-reduction of NO. The evolution of the oxidation state of Pd and Pt NPs during the oxidation of NO and the role of the NP size will also be presented.

Our findings highlight the decisive role of the nanoparticle structure and chemical state in catalytic reactions and the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.