Pore curvature and support composition effects on the electronic properties of supported Pt catalysts: An infrared spectroscopy study with CO as probe molecule

Supported 1 wt% Pt-based catalyst materials have been used as model systems to study pore curvature and support composition effects on the electronic properties of supported Pt nanoparticles. For this purpose, Pt nanoparticles have been loaded onto microporous (ITQ-1), mesoporous (Si-MCM-41, Si-MCM-48 and Si-SBA-15), macroporous (SiO2) all-silica supports, as well as onto a macroporous SiO2 support, impregnated with monovalent (Na+, Cs+) and divalent (Mg2+, Ba2+) cations. Time- and temperature-dependent infrared spectroscopy with CO as probe molecule has been used to investigate the adsorption and desorption properties of CO from these supported Pt nanoparticles. At 350 K, a narrow and smooth linear Pt-coordinated CO vibration band at 2070 cm−1 was observed for the all-silica catalysts. The IR-CO-TPD results revealed a slightly higher desorption rate for the micro- and mesoporous supports, probably due to larger non-bonding electrostatic interactions between CO and the pore walls. A systematic shift from linear (L) to bridge (B) bonded CO upon a decrease of the radius of curvature, which would indicate an increasing electron charge on the supported Pt nanoparticles, is however not observed. In contrast, a relationship between the L:B band intensity ratio and the Lewis acidity of the monovalent and divalent cations, as expressed by the Kamlet-Taft parameter α, was observed for the Pt/SiO2 catalysts. This effect is less pronounced than for zeolite-supported Pt nanoparticles (J. Phys. Chem. B 2005, 109, 3822–3831), but the results demonstrate that the correlation can be easily transferred from one support type to another, thus providing further guidelines for the design of improved automotive catalysts.