Astudy On The Essential Properties Of Pt/SiO₂ Catalysts

As an interdisciplinary of catalysis and nanoscience, nanocatalysis undertakes the responsibility of enhancing the activity and selectivity of the catalyst in the 21th century. The most obvious difference between nanomaterials and industrial catalysts concentrates on the small size and the regular morphology of nanoparticles (NPs). Many researchers have paid attention to studying the relationship between size and activity and/or selectivity. However, the study on morphological effect is far from sufficiency. Furthermore, the gap between the single crystal catalysts and supported catalysts, originated from the extreme research conditions in surface study makes the morphological effect important. From this aspect, the ideal theory can connect with the actual catalytic activity so as to advance the application of catalysis. In this paper, Pt nanoparticles with various shapes were prepared and supported NPs catalysts were obtained. CO oxidation was chosen as probe to clarify the relationship between NPs morphology and activity of the supported catalysts. Based on Transmission Electron Microscopy, Scanning Electron Micorscopy, UV-Vis Spectroscopy, X-ray Diffraction, X-ray Fluorescence, and so on, some of the physical and chemical properties were characterized. In-situ Diffraction Reflection Infrared Fourier Translation Spectroscopy was also used to investigate the catalytic mechanism over the supported catalysts for CO oxidation.Based on the method of polyol reduction and controllable effect of Ag, Pt NPs with three different shapes were prepared. Researches indicated that they were cubic Pt surrounded by {100} facets, octahedral Pt by {111} facets and cuboctahedral Pt as a combination of {100} and {111} facets. After precipitation and purification, Pt NPs were redispersed into ethanol and deposited onto the support. The characterization by TEM, SEM and XRD showed the NPs were with a diameter of about 10 nm, well crystallized and highly dispersed on the support. Besides, the experimental results by UV-Vis attested the existence and controlling mechanism of Ag42+ cluster. Through characterizing the supernatant, it could also be seen that most of the residual Ag were removed through purification. Therefore, the addition of Ag had no essential influence on the catalysts.CO oxidation activities of the three kinds of catalysts exhibited distinct difference. Cubic catalyst performed the best catalytic activity while octahedral was the worst. The properties of adsorbed CO bands of the supported catalysts were studies by in-situ DRIFTS. It was proved that the difference of activities resulted from the variant of crystal facets surrounding the surface of NPs. The adsorbed CO of cubic catalyst was easy to desorb and react with O2, which would lower the CO self-poisoning effect. This performance is similar to those of Pt (100) single crystal. Langmuir-Hinshelwood mechanism was also examed, testifying it was the adsorbed CO that reacted with O2 to produce CO₂.In summary, the impact of NPs morphology on activity of Pt/SiO₂ catalysts was well studied in this paper. It demonstrated that, in order to increase the activity of the catalysts, cubic NPs were available because of weak CO self-poisoning effect.