Influence of pH of the Impregnation Solution on the Catalytic Properties of Co/γ-Alumina for Fischer−Tropsch Synthesis

Abstract

The Co/γ-Al2O3 catalysts were prepared by the slurry impregnation of an aqueous solution of cobalt(II) nitrate precursor. Nitric acid or ammonium hydroxide was added to the cobalt nitrate solution, during impregnation, to give an acidic or basic environment. The changes in the particle size of cobalt species were estimated by X-ray diffraction (XRD) and hydrogen chemisorption. The reduction degree of cobalt oxides was measured by temperature-programmed reduction (TPR). The catalysts prepared under acidic conditions showed a higher reduction degree compared to those prepared at higher pH because of the reduced salt−support interaction. During the Fischer−Tropsch synthesis at 220 °C, employing the catalysts prepared at a different pH (0.80, 4.94, 9.96, and 11.12), a considerable difference in the initial activity was observed, depending upon the cobalt metal surface area. However, after stabilization, all of the catalysts attained a similar level of conversion, possibly because of the active-site rearrangement, deactivation, and wax formation on the catalyst surface. At a higher reaction temperature of 240 °C, the catalysts prepared at lower solution pH exhibited higher conversion than those prepared at higher solution pH. The cobalt species on the catalysts prepared under acidic conditions had a heterogeneous particle size distribution, showing higher steady-state activity, because of the reduced interaction with the support. The product distribution revealed a higher selectivity to C1 and C8+ on the catalyst prepared with a higher solution pH.

Fischer–Tropsch synthesis over alumina supported bimetallic Co–Ni catalyst: Effect of impregnation sequence and solution

“…Nowadays, elements other than noble metal, such as Zr [22], alkali earth elements [23], rare earth elements [24], and others [25][26][27], are examined as the reduction promoter. Other methods, such as improvement in the preparation method of Co/Al 2 O 3 catalysts [28][29][30][31][32][33] and structural control of Al 2 O 3 support [34][35][36][37][38][39], are also studied in order to improve the Co reducibility of Co/Al 2 O 3 catalysts.…”

Effect of cobalt precursors on the dispersion, reduction, and CO oxidation of CoO /γ-Al2O3 catalysts calcined in N2

“…It is suggested that the catalytic performances of CoO x /c-Al 2 O 3 catalysts in these reactions are mainly related to the dispersion, valence and interaction of surface cobalt species with c-Al 2 O 3 , which are dependent on the preparation conditions including cobalt precursors, calcination temperature and atmosphere, etc. [16][17][18][19][20][21]. Among these factors, cobalt precursors could greatly influence the type and dispersion of cobalt species.…”

Cobalt supported on alumina as green catalyst for Biginelli reaction in mild conditions: effect of catalyst preparation method

“…Moreover, it was observed that the reagent nature, operation conditions (pH…) and synthesis method used can influence the distribution and the active species nature [53]. A pH modification can alter the particle size [57]. The citric acid use in the case of the IMP method favors a "fully" dispersed active species [58].…”