Size- and Shape-Selected Metal Nanoparticle Catalysts And Their Evolution Under Operando Conditions

Beatriz Roldan Cuenya

The rational design of the next-generation of catalysts requires detailed knowledge of the correlation between their structure, chemical composition, and reactivity. Even though Pt is among the most industrially relevant and widely investigated nanoparticle (NP) catalysts, its complex interaction with common reactants such as oxygen or water 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 NPs have been synthesized by means of diblock copolymer encapsulation.

The influence of the NP size on the binding of oxygen and hydrogen, onset of oxide formation, and maximum saturation coverages will be discussed using as example geometrically well-defined Pt NPs supported on SiO2 and -Al2O3. Furthermore, the role of the chemical environment on the stability of these NPs against sintering, dissolution and volatilization will be illustrated. Nanoparticle-support interactions will be discussed using as model system Pt/TiO2(110). Patterning of this oxide substrate into nanostripes mediated by thermally stable micelle-synthesized NPs will be demonstrated. Additionally, the epitaxial relation between the substrate and the NPs will be shown to affect the shape of the most thermodynamically stable NPs.

The effect of the nanoparticle shape on the reactivity of Pt NP catalysts 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 (<140°C), while the complete oxidation (>140°C) was catalyzed by oxygen-covered metallic Pt NPs. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidative catalytic reactions and the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.