The nitrogen adsorption–desorption isotherms of Au/UO3 and Au/U3O8 were shown in Fig. 3. According to the original IUPAC clas- sification, both isotherms are similar to III type isotherm in shape. Therefore, the interaction between the sorption agent and the cata- lyst surface is probably weak. The Au/U3O8 does not show a distinct pore distribution of larger than 1 nm, corresponding to a micropo- rous structure as shown in Fig. 4. However, the Au/UO3 shows both a small pore of 2 nm and a large pore of 5–7 nm, which respectively corresponds to a microporous and a mesoporous structure.
As Table 1 lists, raising calcination temperature from 300 to 800 °C can cause a decrease of the total pore volume from 0.04 to 0.01 cm3/g, the average pore radius from 1.9 to 1.7 nm, and the specific surface area from 6.0 to 2.8 m2/g. Kumar et al. reported that U3O8 on the outer surface has fewer pores than UO3 [33]. Similar results were also report- ed [20,22,23,30]. So we believe that heat treatment of uranium oxide at