Figure 2 SEM images of the prepared supports.
As shown in Figure 2S (see Supporting Information), the isotherms of the prepared supports and catalysts were of type IV in shape with typical hysteresis loop. The supports of S440, S1100, S2100, S6000 and the corresponding cobalt catalysts(Co/S440, Co/S1100, Co/S2100, Co/S6000) displayed similar type H1 hysteresis loop, which is representative of an adsorbent with a narrow distribution of relatively uniform mesopores53. However, for the samples of S50, S150, S280 and the corresponding catalysts, the hysteresis loop shifted to a higher p/p0, which means the existence of larger pores34. Therefore, the nitrogen physisorption should be combined with mercury intrusion porosimetry to obtain the reliable macropore distribution information 53. The detailed textural properties of the supports and catalysts are listed in Table 2. The large specific surface (~500 m2/g), beneficial for active phase dispersion, was mainly attributed to the smaller pores (~8 nm) produced by ammonia etching. The large pore volume of the supports, beneficial for mass transfer, was due to the presence of macropores. The significant decrease in the specific surface and pore volume of the cobalt-loaded catalysts compared to the supports could be attributed to the partial pore blocking by cobalt crystallites. In summary, except for S0 and Co/S0 which were monodispersed mesopore structure, the other supports and catalyst samples all exhibited meso-macropore structure.
The wide-angle XRD patterns of the prepared catalysts are presented in Figure 1S(b) (see Supporting Information). For all the catalyst samples, peaks appearing in 31.4°, 36.9°, 44.8°, 59.4°, and 65.2° corresponded to the diffractions of Co3O4 phase. The average crystallite sizes of Co3O4, calculated from the Scherrer equation at the most intense diffraction (2θ = 36.9°) are listed in Table 2. Almost all catalysts had similar crystallites size, except Co/S50 and Co/S150 presenting a slightly larger crystallites size, which can be attributed that small macropore diameter of Co/S50 and Co/S150 affects the nucleation and growth of cobalt particle. The typical TEM images of the catalyst samples, as presented in Figure 4S (see Supporting Information), indicated that all catalysts exhibited a worm-hole-like pore structure, which could be attributed to the mesopores of 7.76~9.60 nm and the active species were uniformly distributed.
Table 2 The physical properties of the catalyst samples.