Control and Impact of the Nanoscale Distribution of Supported Cobalt Particles Used in Fischer–Tropsch Catalysis

Abstract

The proximity of nanoparticles may affect the performance, in particular the stability, of supported metal catalysts. Short interparticle distances often arise during catalyst preparation by formation of aggregates. The cause of aggregation of cobalt nanoparticles during the synthesis of highly loaded silica-supported catalysts was found to originate from the drying process after impregnation of the silica grains with an aqueous cobalt nitrate precursor. Maximal spacing of the Co3O4 nanoparticles was obtained by fluid-bed drying at 100 °C in a N2 flow. Below this temperature, redistribution of liquid occurred before and during precipitation of a solid phase, leading to aggregation of the cobalt particles. At higher temperatures, nucleation and growth of Co3O4 occurred during the drying process also giving rise to aggregation. Fischer-Tropsch catalysis performed under industrially relevant conditions for unpromoted and Pt-promoted cobalt catalysts revealed that the size of aggregates (13-80 nm) of Co particles (size ~9 nm) had little effect on activity. Large aggregates exhibited higher selectivities to long chain alkanes, possibly related to higher olefin formation with subsequent readsorption and secondary chain growth. Most importantly, larger aggregates of Co particles gave rise to extensive migration of cobalt (up to 75%) to the external surface of the macroscopic catalyst grains (38-75 μm). Although particle size did not increase inside the silica support grains, migration of cobalt to the external surface partly led to particle growth, thus causing a loss of activity. This cobalt migration over macroscopic length scales was suppressed by maximizing the distance between nanoparticles over the support. Clearly, the nanoscale distribution of particles is an important design parameter of supported catalysts in particular and functional nanomaterials in general.

Stabilized ordered-mesoporous Co3O4 structures using Al pillar for the superior CO hydrogenation activity to hydrocarbons

“…The ordered regular pores with a diameter of around 6-8 nm were clearly visible, and the mesoporous structures remained even after the metal oxide pillaring step. However, somewhat larger pore diameters of around 8 nm were observed on the Al/meso-Co 3 O 4 , Mn/meso- (1) the oxygen titration method used for calculating the particle size of Co 3 O 4 can underestimate the degree of reduction due to simultaneous formation of the CoO phase, which is more stable than Co 3 O 4 below 400 • C [42][43][44], (2) Even though the particle sizes of Co 3 O 4 calculated from XRD are generally well matched with the metallic size of cobalt particles analyzed by chemisorption, the particle size difference between Co 3 O 4 and metallic Co generally increase with the increase of the pore diameter of the support as a result of large cobalt oxide particles breaking into smaller particles during the drying or reduction step [45]. These may be the reasons for the frequent observation that the particle sizes of Co 3 O 4 inside pores are larger than the pore diameter of the support [46].…”

“…8. XPS analysis of Co 2p 3/2 on the fresh and used metal oxide pillared mesoCo3O4 catalysts.particles can be partially encapsulated[35,45] with the Mn or Si oxides by dominantly distributing on the outer surfaces of mesoCo 3 O 4 , and by suppressing the transport of syngas to the active sites resulted in showing lower catalytic activities. Based on the observed values of I M /I Co , we suggest that a lot of the impregnated Al 2 O 3 in the Al/meso-Co 3 O 4 may be incorporated to the framework of meso-Co 3 O 4 , which is responsible for a highly enhanced stability of mesopore structures with a stable catalytic activity.The possible coke formation for catalyst deactivation was also verified by characterizing C1s peak through XPS and TGA analysis of the used metal-oxide pillared meso-Co 3 O 4 .…”

Role of Metal–Support Interactions, Particle Size, and Metal–Metal Synergy in CuNi Nanocatalysts for H2 Generation

“…This could be achieved through the choice of a suitable support, the selection of proper metal precursors and the preparation method. [24][25][26][27][28] On the other hand, the safe and efficient storage of hydrogen is recognized as one of the major technological barriers preventing the widespread hydrogen on-board application. [29][30][31] Catalyst-assisted hydrogen generation by decomposition of hydrogen storage molecules is one of the most studied and desired approaches towards a hydrogen-powered society.…”

Effect of support surface treatment on the synthesis, structure, and performance of Co/CNT Fischer–Tropsch catalysts

“…The materials were characterized by means of nitrogen physisorption, transmission electron microscopy, Raman spectroscopy, and acid-base titrations. Supported cobalt catalysts were prepared on these CNT materials using incipient wetness impregnation with different solvents and a drying and heat treatment protocol that has been proven to lead to good distributions of the supported metal particles over the support surface [45,46]. The impact of the chosen materials and methods on dispersion and distribution of cobalt was studied by electron microscopy and X-ray diffraction.…”