Effect of Alumina Modification on the Reducibility of Co3O4 Crystallites Studied on Inverse-Model Catalysts

Cobalt-Based Fischer–Tropsch Synthesis: A Kinetic Evaluation of Metal–Support Interactions Using an Inverse Model System

“…Metallic cobalt is the catalytic active phase in the Fischer-Tropsch synthesis [4], and the precursor needs to be reduced prior to the FT synthesis. Temperature programmed reduction profiles of the calcined material showed a significant broadening of the reduction peak upon modification with aluminum, which was accompanied by a shift of the reduction peaks to higher reduction temperatures [18]. This was ascribed to a blockage of defect sites by alumina, thereby hindering hydrogen activation and the nucleation of reduced cobalt phases.…”

“…These materials were reduced prior to the Fischer-Tropsch synthesis isothermally at 350 • C for 10 h. The degree of reduction (DoR) decreased gradually from 96% to 86% upon increasing alumina loading from 0 to 2.5 wt.% (see Table 1). This further implies that the modification of alumina impedes the reduction of Co 3 O 4 as observed in the profiles obtained from temperature-programmed reduction (TPR) [18]. The catalysts studied in this work were essentially bulk cobalt catalysts, and these materials were expected to sinter extensively during catalyst activation, despite the low reduction temperature (the chosen reduction temperature (350 • C) is above the Hüttig temperature of metallic cobalt (253 • C), implying the significant mobility of cobalt atoms at defect sites [20], which may lead to sintering if cobalt particles are in contact with each other).…”

“…Cobalt oxide (Co 3 O 4 ) with an average crystallite size of 14 nm was prepared by the calcination of a cobalt carbonate precursor at 300 • C [18], which was impregnated with aluminum sec-butoxide in dry n-hexane to achieve a weight loading of 0.1, 0.5 or 2.5 wt.% Al [18]. The formation of a separate, alumina phase could not be observed using X-ray diffraction (XRD), although analysis of transmission electron microscopy (TEM) images showed the presence of small alumina islands (<1 nm) on the surface of Co 3 O 4 [18].…”

“…Interestingly, the introduction of even a small amount of aluminum to Co 3 O 4 can drastically reduce the extent of catalyst sintering, whilst still achieving a high degree of reduction. The alumina islands on the surface of cobalt oxide [18] may act here as a spacer preventing direct contact between cobalt particles and thereby reducing the extent of sintering. This also indicates that contact between alumina and the cobalt phase remains intact throughout the reduction of Co 3 O 4 to metallic cobalt.…”

“…Inverse models [15][16][17][18][19], in which the support (or its precursor) is deposited on the catalytically active material (or its precursor), may be used to distinguish between interface effects on the one hand and changes in morphology and degree of reduction on the other hand. We have shown that the deposition of small amounts of the precursor of a support compound in the form of an alkoxide on Co 3 O 4 (as the precursor to metallic cobalt) results in the formation of small islands of support-like structures on the surface of the precursor of the catalytically active material [18,19]. The controlled presence of these nano-sized islands (with a typical size of less than 1 nm) on the catalytically active phase may mimic the metal-support interface and may thus give insights in the role of the metal-support interactions on the activity of the catalytic active phase.…”