Infrared spectroscopy on size-controlled synthesized Pt-based nano-catalysts

Pt, Ru and Pt/Ru nano-particles, synthesized in ethylene glycol solutions, are studied using infrared (IR) spectroscopy and high resolution transmission electron microscopy (HRTEM). The synthesis method allows the control of the mono- and bi-metallic catalyst particle sizes between 1 and 5.5 nm. The IR spectra of CO adsorbed (COads) on the Pt, Ru and bi-metallic Pt/Ru colloids are recorded as a function of the particle size. The stretching frequency of COads depends on the particle size and composition. Strong IR bands due to the stretching vibration of COads are observed between 2010 and 2050 cm−1 for the Pt nano-particles, while two IR bands between 2030 and 2060 cm−1 for linear bonded COads, and at lower wavenumbers between 1950 and 1980 cm−1 for bridged bonded COads, are found for the Ru particles. The IR spectra for the Pt/Ru nano-sized catalyst particles show complex behaviour. For the larger particles (>2 ± 0.5 nm), two IR bands representative of COads on Ru and Pt-Ru alloy phases, are observed in the range of 1970-2050 cm−1. A decrease in the particle size results in the appearance of a third band at ∼2020 cm−1, indicative of COads on Pt. The relative intensity of the band for COads on the Pt-Ru alloy vs. the Pt phase decreases with decreasing particle size. These results suggest that Ru is partially dissolved in the Pt lattice for the larger Pt/Ru nano-particles and that a separate Ru phase is also present. A Pt-Ru alloy and Ru phase is observed for all Pt/Ru particles prepared in this work. However, a decrease in particle size results in a decrease of the number of Pt and Ru atoms in the Pt-Ru alloy phase, as they are increasingly present as single Pt and Ru phases.