Insights into Spectator-Directed Catalysis: CO Adsorption on Amine-Capped Platinum Nanoparticles on Oxide Supports

Introducing spectator molecules to the surface of supported noble metal nanoparticles is an innovative approach to improve the selectivity of heterogeneous catalysts. Colloidal synthesis of the nanoparticles allows researchers to select the spectator and the nanoparticle size, as well as the subsequent particle loading on different supports under well-defined conditions. However, understanding the interplay of the various effects that spectators can have on the catalytic properties of metal surfaces still requires further development. In this work, dodecylamine (DDA) is used to develop insights into the influence of spectator species on the chemical properties of 1.4-3.7 nm colloidal Pt nanoparticles on different supports (powders of Al₂O₃, ZnO, and TiO₂). DDA deposition results in two chemically distinct spectator species on the Pt surface depending on temperature, as evidenced from X-ray photoelectron spectroscopy (XPS). DDA selectively blocks terrace sites on the Pt nanoparticles at room temperature, as apparent from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as a surface-sensitive probe molecule. The electron donor effect of the amine group in DDA influences the electron densities of the accessible lower coordinated, reactive Pt adsorption sites as indicated from spectral shifts in DRIFTS and XPS. Furthermore, DDA suppresses CO-induced surface reconstruction of the Pt surface and metal-support interactions, although these effects depend on temperature and support composition. Therefore, spectators may be used to adjust the nature of metal nanoparticle-oxidic support interactions. The experimental findings and mechanistic explanations are supported by density functional theory calculations. These results may build a platform in understanding the fundamental properties of amine spectators in Pt-based catalysis, activating specific sites, enhancing site selectivity, acting as sensors, and directing the metal-support interaction.