Characterization of Supported Platinum Nanocrystallites with Different Sizes and These Effects on a Model Reaction

Abstract

Different nanosized platinum crystallites dispersed on a silica support have been extensively characterized with the help of probe molecule adsorption and instrumental measurements, and their reactivity to the reduction of N₂O by H₂ as a model reaction has been studied. All measurements using different techniques, i.e., H₂–N₂O titration, H₂ chemisorption, X-ray diffraction and high-resolution transmission electron microscopy, disclosed that the platinum system consisted of nanocrystallites with average sizes of ca. 1.1–22.2 nm, depending on the thermal excursion. In situ diffuse reflectance infrared Fourier transform spectra of CO adsorbed on nanodispersed platinum particles with the indicated range of their sizes after adsorptive dissociation of N₂O at 90 °C showed absorption bands near 2188, 2075, and 2088 cm⁻¹ even on the biggest platinum nanocrystallites and these had also some low coordination sites giving the 2188 cm⁻¹ vibration, proposing that all the platinum nanoparticles could exist in three different coordination environments. The reduction of N₂O by H₂ at 110 °C yielded no significant difference in turnover frequency between different platinum nanocrystallite sizes, disclosing that this reaction was structure-insensitive. The turnover frequency in the model reaction at 125 °C over the finest platinum size sample gave no principal distinction between flowing mixtures containing N₂O and H₂ whose absolute concentrations varied. All these low temperature reactions could be, to a good approximation, explained by an overall stoichiometry of N₂O:H₂ = 1:1.