Effects of Particle Size on Strong Metal-Support Interactions Using Colloidal "Surfactant-Free" Pt Nanoparticles Supported on Fe3O4

Colloidal "surfactant-free" Pt nanoparticles (NPs) within the size range 1-4 nm supported on Fe3O4 were synthesized and applied as model systems to systematically study the role of size effects for strong metal-support interactions (SMSIs) with CO oxidation as a model reaction. Kinetic studies, isotopic labeling experiments with O-18(2), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were applied to explore the reaction mechanism and the surface of the catalyst before and after reductive pretreatments. It was found that pure iron oxide was catalytically active in CO oxidation, and experimental evidence for a Mars van Krevelen mechanism between CO and lattice O was found. The turnover frequencies (TOFs) for small Pt NPs (<= 3 nm) supported on iron oxide and normalized to the number of Pt atoms located at the periphery of the Pt-support interface were similar under reaction conditions, indicating that the reaction mainly proceeds at the interface. However, with increasing particle size, the contribution of a Langmuir-Hinshelwood mechanism of chemisorbed CO and O-2 in addition to the Mars van Krevelen mechanism increases. After reductive pretreatment, the activity of the catalyst decreased significantly, which could be related to partial encapsulation of the monometallic Pt NPs with FeOx. To study whether also on the nanoscale an interaction between Pt and iron oxide similar to the beneficial or detrimental SMSI effect observed for the Pt particles supported on iron oxide can also be achieved, bimetallic Fe-Pt NPs of a mean size of 3-4 nm were deposited on inert Al2O3. It could be shown that surface segregation of Fe and formation of FeOx after reduction and exposure to oxygen took place. As a result, the activity of bimetallic NPs decreased due to loss of active Pt surface, revealing an effect similar to the detrimental SMSI detected for Pt NPs on FeOx support after reductive pretreatment.