Kinetics Studies Of CO Oxidation Over Pt-and Cu-based Catalysts

Catalytic CO oxidation has attracted extensive attention due to its important practical applications and academic value. Therefore, different catalyst systems have been investigated for this this reaction, in order to understand some essential questions in catalysis, such as structure-sensitivity (particle size effect), active sites and reaction mechanism. In the thesis, a series of Pt-and Cu based catalysts were prepared at tested for CO oxidation. The catalysts were characterized by BET, ICP, XRD, TEM, H2-TPR, CO-TPR, O2-MS, XPS and in-situ DRIFTS techniques, and incorrelation with their reaction behaviors, particle size effect was clarified. In addition, kinetic investigations were conducted to deduce reaction pathway and reaction mechanism.The main contents of the thesis are as follows:1. A series of Pt/TiO2catalysts were prepared using an incipient wetness method and tested for CO oxidation. Crystallite sizes of Pt species were evaluated by means of CO-TPR, TEM and XRD methods. It was found that the crystallite sizes of Pt species increased with increasing Pt loading, but the reactivities for CO oxidation first increased and then decreased. A2.0Pt/TiO2catalyst was found to be much more active than a2.0Pt/SiO2catalyst, indicating the important role of TiO2in the reaction. Kinetic results showed that the rate expressions of CO oxidation over the2.0Pt/TiO2catalysts were r=1.98x10-7Pco0.29Po20.19(40℃), r=3.43x10-7Pco0.21Po20.28(60℃), r=2.30x10-6Pco-0.07Po20.56(80℃), r=5.15x10-6Pco-0.11Po20.52(100℃). This reaction takes place at the Pt-TiO2interface, with CO chemisorbed on surface Pt atoms and O2dissociatively chemisorbed on TiO2. The Pt atoms located at the Pt-TiO2interface were the active sites for this reaction.2. A series of Cu catalysts supported on SiO2, TiO2and CeO2were prepared using a hydrolysis chemical adsorption method and tested for CO oxidation. Crystallite sizes of CuO were evaluated by means of N2O chemisorption, TEM and XRD. It was found that the crystallite sizes of CuO increased with increasing loading. For the CuO/SiO2catalysts, the activities of CO oxidation increased with increasing CuO loading in the catalyst; while for the CuO/TiO2and CuO/CeO2catalysts, the activities first increased and then decreased with CuO loadings. Compared with the CuO/SiO2catalyst, CuO/TiO2and CuO/CeO2catalysts were more active for CO oxidation, due to the enhanced redox proerties of these catalysts via metal-support interaction. The effects of CuO particle size in the CuO/TiO2and CuO/CeO2catalysts on the reaction was also investigated. A detailed calculation of turnover frequency (TOF) based on the active sites located on the periphery of the CuO-TiO2(CeO2) interface showed that the TOF value was much higher on a single active site over large CuO crystallite than over the small one, suggesting that the CO oxidation on CuO-TiO2(CeO2) catalyst is structure sensitive. Detailed kinetic studies were performed over5.0CuO/SiO2,5.0CuO/TiO2and5.0CuO/CeO2catalysts. The reaction rate equations of the5.0CuO/SiO2and5.0CuO/TiO2catalysts were r=5.87x10-7Pco0.85Po20.22(240℃) and r=5.06x10-7Pco0.65Po20.19(90℃), respectively. Langmuir-Hinshelwood (L-H) type reaction models were proposed to describe the catalytic behaviors of these two catalysts. The activation energies of CO oxidation over these catalysts were calculated to be75.3kJ/mol (5.0CuO/SiO2) and67.4kJ/mol (5.0CuO/TiO2). The reaction rate equation of5.0CuO/CeO2catalytic system at70℃was r=4.51x10-7Pco0.7Po2-0.08, implying the reaction rate was essentially dependent on CO partial pressure but independent of O2partial pressure. A Mars-van Krevelen type reaction model was proposed to describe the catalytic behavior of the catalyst. Meanwhile, the activation energy of this system was53.4kJ/mol.