Effect of pre-treatment on physico-chemical properties and stability of carbon nanotubes supported iron Fischer-Tropsch catalysts

This paper presents the effects of acid treatment on the activity, product selectivity and life span of iron Fischer-Tropsch (Fr) catalysts supported on carbon nanotubes (CNTs). Two different types of CNTs with low surface area (similar to 25 m(2)/g) and high surface area (similar to 170 m(2)/g) were prepared and treated with 35 wt% HNO3 at 25 degrees C and 110 degrees C for 16 h. Fe/CNTs were prepared using incipient wetness impregnation method with iron loading of 10 wt%. The CNT supports and catalysts were characterized by nitrogen adsorption, ICP, TPR, XRD, SEM, TEM and Raman spectroscopy. The acid treatments at 25 degrees C and 110 degrees C increased the BET surface area by 18% and 31%. The results of Raman analysis revealed that the acid treatment increased the number of defects which are considered as anchoring site for metal particles. TEM analysis showed that the major parts of the iron particles were homogenously distributed inside the acid treated nanotubes. According to the XRD analysis, the acid treatment on both families of CNTs resulted in decrease in metal particle sizes. The FT synthesis was carried out in a fixed-bed microreactor (275 degrees C. 2 MPa, CO/H-2 = 2) for 120 h. Among the catalysts studied, Fe catalyst supported on pre-treated CNTs at 110 degrees C was stable and active while, the other catalysts experienced rapid deactivations. The Fe catalyst supported on CNTs low surface area and larger diameter showed much lower CH4 and higher C-5(+) selectivities. (C) 2008 Elsevier B.V. All rights reserved.