Structure–Performance Relationships on Co-Based Fischer–Tropsch Synthesis Catalysts: The More Defect-Free, the Better

Abstract

Understanding and utilizing structureperformance relationships in catalytic nanomaterials is the epitome of catalysis science. Knowledge at the atomic level can potentially allow rational design of more selective and energy efficient catalytic materials. Fischer-Tropsch synthesis on cobalt is an example of a complicated system that operates in a narrow process regime, and the nature of the reaction product is governed by numerous parameters. On an industrial model catalyst, we have simplified the structure of the active, metallic nanoparticles into predominantly hexagonal close packed structure via the use of a Co 2 C precursor. By varying the final reduction temperature, we could mildly modify catalyst microstructural properties at the nanoparticle (NP) level. Catalytic materials, although with minimal structural differences, showed significantly different performance. Evidently there is a narrow window for complete utilization of the hexagonal close packed Co crystallites that lays between removal of lattice carbon, that remains from the Co 2 C precursor, and the initiation of stacking disorder, due to transition to the face centered cubic Co structure. Fischer-Tropsch synthesis performance indicators show that Co NPs with minimum number of crystal defects outperform catalysts with lattice defects, either due to the existence of lattice carbon or stacking faults. Therefore, catalyst preparation and activation procedures probably should be designed targeting defect free Co crystallites.

α-alumina as catalyst support in Co Fischer-Tropsch synthesis and the effect of added water; encompassing transient effects

“…6 A structural model was proposed recently by Tsakoumis et al in an investigation on the hcp and fcc phases of cobalt in FTS. 7 It was shown that crystals with less defects, possibly due to absence of stacking faults, are beneficial for high activity and selectivity. The conclusions were based on examination of catalysts on both γ-alumina and α-alumina supports by XRD and XANES.…”

Significance of C3 Olefin to Paraffin Ratio in Cobalt Fischer–Tropsch Synthesis

“…First, fewer cobalt sites will become available for hydrogenation. More important is probably that larger and more regular metal crystallites (more ideal stacking, fewer strains and distortions) favor formation of CH x polymerization monomers at the expense of hydrogen [16]. GHSV is reduced in period B to achieve the targeted 45% CO conversion.…”

“…Additional work includes analysis of defects and hcp/fcc phases in Co crystallites. Tsakoumis et al found that Co nano-particles with minimum number of crystal defects, either due to lattice carbon or stacking faults, have high TOF (turn-over-frequency) and chain-growth probability [16]. These observations were coupled to enhanced surface population of CHx* and high o/p values (C3: 3.31 at standard conditions); explained by low relative concentration of hydrogen.…”

“…The authors claim that the increase in chain growth, and decrease in o/p ratio, is due to the enhanced secondary reaction (re-adsorption and re-initiation) of α-olefins at prolonged bed residence Co/Re on γ-alumina supports with varying pore characteristics [10]; II: Co and Co/Re on γ-alumina, silica and titania supports [11]; III: as II, but initial data at low conversion; IV: Co and Co/Re on γ-alumina with different pore sizes [12]; V: Co/Re on γ-alumina powder and washcoats on cordierite, alumina, and steel monoliths [13]; VI: Co/Re on γ-alumina washcoats on alumina monolith; VII: Co with common crystallite size on different alumina phases [14]; VIII [15] Additional work includes analysis of defects and hcp/fcc phases in Co crystallites. Tsakoumis et al found that Co nano-particles with minimum number of crystal defects, either due to lattice carbon or stacking faults, have high TOF (turn-over-frequency) and chain-growth probability [16]. These observations were coupled to enhanced surface population of CH x * and high o/p values (C 3 : 3.31 at standard conditions); explained by low relative concentration of hydrogen.…”

The effect of heteroatoms on the formation of magnetic ceramic nanocomposites in pyrolysis of organometallic precursors: similar molecular structure, but totally different morphology, composition and properties

“…Very importantly, in the classic cobalt-catalyzed Fischer-Tropsch reaction, great achievements have demonstrated that hcp-Co has better catalytic activity than fcc-Co [57,58]. This is mainly due to the special surface structure of the hcp crystal phase, which has higher intrinsic activity and density of active sites.…”