On the Origin of the Cobalt Particle Size Effects in Fischer−Tropsch Catalysis

Abstract

The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 degrees C and H(2)/CO = 10 v/v, it appeared that the surface residence times of reversibly bonded CH(x) and OH(x) intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quantitatively and the CH(4) selectivity qualitatively as a function of Co particle size for the FT reaction (220 degrees C, H(2)/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.

Self-healing catalysts: Co3O4 nanorods for Fischer–Tropsch synthesis

“…[28][29][30] In this range, the turnover frequency of the Co catalyst does not change with particle size. 31 Both the as-synthesized Co nanorods and the nanoparticles have the Co 3 O 4 spinel structure (PDF 41-1467), as determined by powder X-ray diffraction, see insets in The crystal facets exposed on the nanorod and nanoparticle surfaces were characterized with high resolution TEM, see 20 In contrast, the Co nanoparticles exhibit {001} and {111} facets at their surfaces (Fig. 1b), which is consistent with literature.…”

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

“…For FTS, the use of SSITKA has resulted in an improved understanding on the effect of catalyst properties (promoters [72][73][74][75][76][77][78][79][80][81][82][83], support [84][85][86][87][88][89][90], particle size etc. [91][92][93][94]) and the reaction conditions [95][96][97][98][99][100][101] on the performance. Concerning the mechanism, SSITKA gives the coverage of CO and surface carbon intermediates and the site coverage of H can be estimated.…”

“…Recently, the increase in demand for transportation fuels and the large reserves of natural gas have made FTS an attractive industrial process. In the FTS process, cobaltbased catalysts, owing to low water-gas shift activity and remarkable stability, is considered to be the favorable catalyst for the synthesis of long-chain hydrocarbons from synthesis gas with a H 2 /CO ratio about 2 [5][6][7].…”

Recent developments in the application of nanomaterials to understanding molecular level processes in cobalt catalysed Fischer–Tropsch synthesis

“…This is especially problematic with spectroscopic tools like EXAFS, which take a volume not a surface average. A number of routes have now been developed that allow for better control of particle size:  carbon nanofibres (CNFs) with oxide defects to anchor / control particle growth during incipient wetness preparation; 39 49 As shown in Table 2, the first four of these methods have yielded the clear result that small particles (below around 7 nm or less) are intrinsically less active (and where looked at also less selective towards higher molecular weight products). For CNF anchored nanoparticles at both 1 and 35 bar a significant decrease in site time yield was observed for samples with nanoparticles smaller than 6 nm.…”

“…For CNF anchored nanoparticles at both 1 and 35 bar a significant decrease in site time yield was observed for samples with nanoparticles smaller than 6 nm. 39,40 CNF supports were selected to eliminate the possibility of oxide supports oxidising the cobalt. Using SSITKA the authors showed small nanoparticles had a greater fraction of sites occupied by irreversibly adsorbed CO.…”