The optimized reaction conditions (gas space velocity: 1200 h−1, water flow: 0.08 mL/min, catalyst activation: calcined at 300 °C for 3 h and then reduced at 200 °C for 2 h) were obtained on Au/CeO2 catalyst. Many studies on Au/CeO2 catalyst have been reported [3,4,11–16] and our attentions will focus on uranium oxide-supported gold catalysts.
The activity of Au/UO3–T is as a function of calcination tempera- ture (T) of uranium oxide. The highest activity with 54.6% CO conver- sion at 350 °C was found in Au/UO3 catalyst, as shown by Fig. 8. However, gold catalyst with uranium oxide obtained at 500 °C or higher, i.e., Au/U3O8, shows very low activity (b5% CO conversion at 350 °C). Mckee [31] also found that U3O8 had lower activity than UO3 for oxidation of carbon. The activity order at reaction tempera- ture of 350 °C is summarized as follows: Au/UO3 N Au/UO3- 350 N Au/UO3-400 N Au/UO3-450 N Au/U3O8.
The activity order is easily correlated to the phase transition from UO3 to U3O8 phases as well as its textual property. Smaller pore size and specific surface area can make the transportation of the reactant molecules to active reaction sites more difficult. Larger gold particles could cause a blockage of the adsorption and spillover of the reactant molecules, resulting in the decreasing of catalytic activity. Wang et al. [38] demonstrated that the catalytic activity of porous U3O8 is distinctly