The XRD patterns of iron-based catalysts modified by different additives were shown in Fig. . All the catalysts exhibited the characteristic peaks associated with the Fe3O4 phase (PDF#19-0629). The diffraction peaks attributed to CuO were also observed for Fe100Cu20K10 catalyst. The characteristic diffraction peaks attributed to Zn or Mn species were not observed in the other catalysts, indicating that Zn or Mn species were highly dispersed in the catalyst. Calculations from the Scherrer formula showed that the Mn modification significantly reduced the size of the Fe3O4 crystals compared to Zn and Cu, with an average particle size of around 11.3 nm for the Fe100Mn20K5 catalyst. At the relative pressure (P/P0) range of 0.5-1.0, all the three catalysts, Fe120K10, Fe100Mn20K10 and Fe100Mn20K5, exhibited typical mesoporous structures with the average pore size located in the range of 2-10 nm (Fig. ). Moreover, the incorporation of both K and Mn promoters benefit in the increase of the surface area, pore volume and average pore size of these Fe-based catalysts (Table ). The SEM images of the iron-based catalysts modified with different additives are shown in Fig. . All the catalysts consisted of closely spaced nanoparticles with a large number of pores located between the particles. Besides, the morphology of the Fe100Mn20K5 catalyst was investigated by TEM (Fig. ), where the samples were arranged as nano-Fe3O4 particle form presenting the lattice spacing of 2.41 Å and 2.52 Å corresponding to the ( ) and (331) plane of Fe3O4, respectively. The EDS mapping (Fig. ) also confirmed that the elements Mn and K were highly dispersed in Fe100Mn20K5.