II International Symposium on Carbon for Catalysis ABSTRACTS

OP-I-1
A
B
4.00
Activity (10 -5 mol CO*g Co -1 s -1 )
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0.0 5.0 10.0 15.0 20.0 25.0
Cobalt particle size (nm)
Figure 1: (A) TEM of Co/CNF and (B) CTY at 1 bar versus cobalt particle size.
The drop of activity with particle size beyond 6 nm is in line with a constant TOF of about
0.01 s -1 . The strong drop of activity below 6 nm points to a particle size effect. Using in situ
XANES and EXAFS (Co K-edge), i.e. at 1 bar synthesis gas and 220 °C we could exclude
formation of cobalt oxide and of cobalt carbide as possible causes of the particle size effect.
EXAFS showed a drop of the coordination number upon exposure of Co/CNF (4 nm Co
particles) to synthesis gas pointing to a reconstruction of the cobalt particles possibly due to a
transformation of hemi-spherical to flat particles. Extensive reconstruction is in line with
observations on single crystal cobalt surfaces [5].
A
B
Selectivity (wt%)
50
40
30
20
10
0
% C1
% C5+
TOF
CoH Co431Mn Co95Mn Co39Mn Co19Mn Co11Mn
20
18
16
14
12
10
8
6
4
2
0
TOF (10 -3 s -1 )
Co Mn
Figure 2: ( A) Cobalt and manganese map from STEM-EELS and (B) Catalysis data for FT at
1 bar and 220 °C, CoxxMn with xx= Co/Mn atomic ratio.
The structure and effects of manganese oxide promotion on FT at 1 bar are shown in
Figure 2. From STEM-EELS experiments [6] we have found that MnO was always associated
with Co on the carbon nanofibers (Fig. 2A). The methane selectivity as found to decrease
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