SEM images

61

of the band under 1200 cm^-1, with respect to the at not activated sample. This fact is due to the interactions between clusters and support.

4000 3500 3000 2500 2000 1500 1000 500

Absorbance

Wavenumbers (cm^-1)

FIG. 6.20: FT-IR spectra for FT04

62

FIG. 6.22 SEM image for FT02

The SEM image of FT03 catalyst is shown in FIG 6.23. It shows a good covering of the surface but the EDX data give different values from the estimated one (18% instead 11% for the Cobalt). This fact can be explained also in this case by the dishomogeneity of surface covering.

FIG. 6.23 SEM image for FT03

The SEM image and the EDX data of FT04 catalyst (FIG 6.24) show also in this case a surface not very homogeneous but EDX data confirm the presence of Cobalt and Iron.

FIG. 6.24: SEM image for FT04

63 6.4 Catalytic tests

FT01 was tested in a fixed bed reactor. 5 gram of catalyst were charged into the reactor and activated under hydrogen flow at 400°C. After activation, a mixture of H2 and CO, with molar ratio 2:1 was fed in the reactor. The catalyst was tested under pressure of 20 bar at different temperatures. The results (Graph 6.1) indicate a low conversion, with the maximum around 30%

at 210°C; at higher temperatures we can observe a decrease of the conversion due to the deactivation of the catalyst.

Graph 6.1: Conversion vs temperature for FT01

A test to evaluate the behaviour during the time was carried out at 220°C. This test shows a rapid deactivation, hypothesized also in the previous test, and high formation of methane (over 30%), that remains stable during the test, as well as other products not desiderable like CO2 (6-8%) and light hydrocarbons (8-10%).

0 20 40 60 80 100

180 190 200 210 220 230

Conversion (%)

Temperature (°C)

FT01

64

Graph 6.2: Conversion vs time and selectivity of lighter products for FT01

The condensed products after this test were analyzed to obtain the ASF distribution (GRAPH 6.3).

From this analysis, the value of α results to be 0.65.

Graph 6.3: linearized distribution of condensed products for FT01

5 gram of catalyst FT02 was charged into a fixed bed reactor and activated under hydrogen flow at 400°C. For the reaction a mixture of H2 and CO, with molar ratio 2:1 was fed to the reactor. The catalyst was tested at 20 bar of pressure between 200°C and 220°C. The results (Graph 6.4) show a good conversion value, around 60%, at 220°C.

0 10 20 30 40 50 60

0 1 2 3 4

Conversion/Selectivity (%)

Days of test

FT01

Conversion

Methane selectivity CO2 selectivity C2-C4 Selectivity

-16 -14 -12 -10 -8 -6 -4 -2 0

0 10 20 30 40

ln(Wn/n)

n° of carbon

FT01

65

Graph 6.4: Conversion vs temperature for FT02

The test of the catalytic activity during the time was carried out at 220°C. Initially, the conversion was good, around 50%, but it decreased rapidly under 30%. This decrease of activity is due to the formation of waxes that remain on the active sites. The selectivity to methane is about 20%. The selectivity to CO2 is under 5% and the selectivity to lighter hydrocarbons is lower than 10%. After the test, 10 g of waxes were extracted from the catalyst. The deactivation can be explained with the recovering of active sites from waxes. This fact is also supported by the low selectivity to lighter hydrocarbons.

Graph 6.5: Conversion vs time for FT02

From the analysis of condensed products for FT02, the value of α in ASF distribution was 0.79.

0 20 40 60 80 100

190 200 210 220 230

Conversion (%)

Temperature (°C)

FT02

0 10 20 30 40 50 60

0 1 2 3 4 5

Conversion/Selectivity (%)

Days of test

FT02

Conversion

Methane selectivity CO2 Selectivity C2-C4 selectivity

66

Graph 6.6: linearized distribution of condensed products for FT02

Also in this case, 5 gram of FT03 catalyst were charged into a fixed bed reactor and activated under hydrogen flow at 400°C. After that, the mixture of H2/CO with 2/1 molar ratio was fed to the reactor. The catalyst was tested at 20 bar of pressure between 290°C and 340°C. The results (Graph 6.7) show that the conversion increases at increasing temperatures until the 80% of conversion is reached at 340°C.

Graph 6.7: Conversion vs temperature for FT03

The test of the catalytic activity during the time was carried out at 300°C. Initially the conversion was good, around 50%, but it decreased rapidly under 30%. This decrease of activity is due to the formation of waxes that remain on the active sites. The selectivity to methane is about 20%. The selectivity to CO2 is under 5% and the selectivity to lighter hydrocarbons is lower than 10%. After the test, 10 g of waxes were extracted from the catalyst. The deactivation can be explained with

-12 -11 -10 -9 -8 -7 -6

0 10 20 30 40

ln(Wn/n)

n° of carbons

FT02

0 10 20 30 40 50 60 70 80 90 100

280 290 300 310 320 330 340 350

Conversion (%)

Temperature (°C)

FT03

67

the covering of the active sites from waxes. This fact is also supported by the low selectivity to lighter hydrocarbons. In this case the catalyst did not produce heavy hydrocarbons and it was not possible to calculate the α value.

Graph 6.8: Conversion vs time for FT03

5 gram of sample FT04 were charged into a fixed bed reactor and activated under hydrogen flow at 400°C. A mixture of H2 and CO, 2/1 molar ratio, was fed to the reactor. The catalyst was tested at 20 bar of pressure between 225°C and 265°C. The results (Graph 6.9) show that the conversion is around 40% and grows very slowly at increasing temperatures; at 265°C the conversion is about 50%.

Graph 6.9: Conversion vs temperature for FT04

0 10 20 30 40 50 60 70 80 90

0 1 2 3 4 5

Conversion/Selectivity (%)

Days of test

FT03

Conversion

Methane selectivity CO2 selectivity C2-C4 selectivity

0 20 40 60 80 100

220 230 240 250 260 270

Conversion (%)

Temperature (°C)

FT04

68

A productivity test (Graph 6.10) was carried on at 265°C. The catalyst shows a rapid decrease of conversion during the test, as well as the increasing of methane production with the progress of the test. CO2 and lighter products show a slight growth and they are around 20%. Also in this case, as for FT03 sample, the catalyst does not produce heavy hydrocarbons.

Graph 6.10: Conversion vs time for FT04