CO Oxidation and Site Speciation for Alloyed Palladium-Platinum Model Catalysts Studied by in Situ FTIR Spectroscopy

In situ Fourier transform infrared spectroscopy was used to study transient CO oxidation over a series of bimetallic Pd-Pt catalysts with different Pd:Pt molar ratios. The catalysts were found to contain both alloyed PdPt nanoparticles (particle sizes 25-35 nm) and monometallic Pd nanoparticles (sizes below 10 nm). For oxygen-free conditions, CO reduces the surface while simultaneously function as a chemical probe molecule whereby the CO adsorption sites can be characterized. Under these conditions, it is shown that adsorbed carbonyl species form both on the Pd and Pt. On platinum, CO adsorbs predominantly linearly on top, whereas on palladium it adsorbes in bridged configurations. This behavior is used for site speciation of the catalysts. The spectra from the bimetallic Pd-Pt catalysts are more complicated than a direct superposition of the spectra for the monometallic catalysts as a consequence of alloy formation and enrichment of Pd at the surface of the reduced catalysts. The temperature-programmed CO oxidation results show that the addition of Pd to the Pt catalyst supported on alumina shifts the CO-poisoned state to lower temperatures, therefore increasing the temperature range for the CO oxidation at low temperatures.