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 18O2, 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 O2 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 FeO x . 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 FeO x 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 FeO x support after reductive pretreatment.

Nanoporous gold was functionalized with a photosensitizer, a zinc(II) phthalocyanine derivative. Such systems are active for the generation of reactive singlet oxygen which can be used for photocatalytic oxidation reactions. This study aims to demonstrate the versatility of such an approach, in terms of substrates and the employed solvent, only possible for a truly heterogeneous catalytic system. The activity of the hybrid system was studied for [4 + 2] cycloadditions of three different types of dienes and a total of eight substrates in two organic solvents and once in water. The highest activity was measured for 1,3-diphenylisobenzofuran, which is also highest in terms of sensitivity for the reaction with 1O2. Trends in conversion could be anticipated based on reported values for the rate constant for the reaction of 1O2. In almost all cases, an amplification of the conversion by immobilization of the sensitizer onto nanoporous gold was observed. The limiting case was ergosterol, which was the largest of all substrates with a van-der-Waals radius of about 2.1 nm. Additional factors such as the limited lifetime of 1O2 in different solvents as well as the hampered diffusion of the substrates were identified. Graphic abstract