Catalytic Oxidation of CO over Ordered Mesoporous Platinum
Journal article, 2008
Hexagonal phase mesoporous (H-1-Pt) was recently reported to have different catalytic properties compared to conventional platinum catalysts. To further investigate this observation the catalytic activity of H-1-Pt/Al2O3 for CO oxidation was compared with the activity of a corresponding catalyst prepared from Pt-black (Pt-black/Al2O3). The H-1-Pt/Al2O3 catalyst showed ignition at lower temperatures but extinction at higher temperatures compared to Pt-black/Al2O3. These observations were further supported by oxygen step-response experiments at constant temperature, where the H-1-Pt/Al2O3 catalyst showed ignition at lower oxygen concentrations when starting from a CO poisoned surface and extinction at higher O-2 concentrations when starting from the high-reactive state. Furthermore, adsorption of CO on the catalysts was studied in situ using infrared spectroscopy in absence and presence of oxygen after pre-oxidation and pre-reduction, respectively. At 150 degrees C the H-1-Pt/Al2O3 sample showed activity for CO oxidation in the presence of oxygen regardless of pretreatment, whereas Pt-black/Al2O3 was inactive due to CO self-poisoning. The differences observed in the low reactive state are suggested to be due to structural differences of the platinum surface in the catalysts resulting in a lower sensitivity of the H-1-Pt/Al2O3 catalyst towards CO self-poisoning and a higher capacity to activate oxygen, and thus a higher activity for CO oxidation. During the high reactive state, the observed higher sensitivity to the concentration ratio between CO and oxygen, and to the temperature is likely due to less optimal ratio between the sticking coefficients of the reactants on the H-1-Pt catalyst and to higher mass-transport limitations in its narrower pores during the initial stage of the extinction.