Effect of the promoters on oxidation behavior of Fe-based Fischer-Tropsch catalyst: Deciphering the role of H2O
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摘要: 以纯Fe催化剂为研究对象，采用XRD、Raman和TPH等手段考察了催化剂的碳化程度、还原程度对H2O氧化过程的影响，获得了H2O氧化过程与催化剂中碳物种转变之间的相互影响规律；系统考察了典型的费托合成助剂K和SiO2存在时对催化剂物化性质以及H2O氧化行为的影响，发现催化剂的碳化程度越高，碳化铁的抗H2O氧化能力越强，氧化过程使得碳物种的石墨化程度增加。适量K助剂可促进碳化铁和催化剂表面石墨碳的形成，提高了碳化铁在H2O氧化过程中的稳定性；SiO2助剂的加入显著抑制了催化剂的碳化，但可有效提高碳化铁以及碳物种的稳定性。Abstract: The effect of carburization and reduction degree on H2O oxidation behaviour for the iron carbides in Fe-based FTS catalyst were firstly investigated using a combination method including X-ray diffraction (XRD), Raman and temperature-programmed-hydrogenation (TPH). The relationship between carbon species transformation and H2O oxidation behaviour of iron carbides was investigated simultaneously. Based on these observations, the influence of typical promoters like K and SiO2 on the structure and H2O oxidation behaviour of Fe-based FTS catalysts was further studied. The results indicated that, for the iron catalyst, the stability of iron carbides against H2O oxidation was increased with the increase of iron carbides content, and the H2O oxidation process led to the formation of more graphitic carbon. The carburization ability was effectively enhanced when certain amount of K promoter was incorporated. Addition of K into Fe-based FTS catalyst increased the number of graphitic carbons, which increased the stability of iron carbides toward H2O oxidation ultimately. It was also found that promotion of SiO2 greatly degraded the carburization degree of the catalyst, while their tendency to be oxidized to form Fe3O4 in the H2O atmosphere was obviously hindered.
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Key words:
- Fischer-Tropsch synthesis /
- iron-based catalysts /
- H2O oxidation /
- stability of iron carbide /
- promoter
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图 1 纯Fe催化剂不同时间碳化及水氧化后的XRD谱图
Figure 1 XRD patterns of Fe catalysts after carbonization (solid line) and oxidation (dotted line) for different times
图 2 纯Fe催化剂不同时间H2还原及水氧化后的XRD谱图
Figure 2 XRD patterns of Fe catalysts after reduction (solid line) and oxidization (dotted line) for different times
图 3 纯Fe催化剂不同时间碳化及水氧化后的Raman谱图
Figure 3 Raman spectra of Fe catalysts after carbonization and oxidization for different times
图 4 纯Fe催化剂碳化及不同时间带水氧化后的TPH谱图(formation rate of CH4(a.u.))
Figure 4 TPH profiles of Fe catalysts after carbonization and oxidization for different times
图 5 FeK系列催化剂碳化及水氧化后的XRD谱图
Figure 5 XRD patterns of FeK catalysts after carburization (solid line) and oxidization (dot line)
图 6 FeK系列催化剂碳化及水氧化后的Raman谱图
Figure 6 Raman spectra of FeK catalysts after carbonization and oxidation
图 7 FeK系列催化剂碳化及水氧化后的TPH谱图(formation rate of CH4(a.u.))
Figure 7 TPH profiles of FeK catalysts after carbonization and oxidization for different times
图 8 FeSi系列催化剂碳化及水氧化后的XRD谱图
Figure 8 XRD patterns of FeSi catalysts after carbonization (solid line) and oxidization (dot line)
图 9 FeSi系列催化剂碳化及水氧化后的Raman谱图
Figure 9 Raman spectra of FeSi catalysts after carbonization and oxidization
图 10 FeSi系列催化剂碳化及水氧化后的TPH谱图(formation rate of CH4(a.u.))
Figure 10 TPH profiles of FeSi catalysts after carbonization and oxidization for different times
表 1 纯Fe催化剂碳化及氧化后的XRD & Raman表征
Table 1 XRD and Raman results of Fe catalysts after carbonization and oxidization
Sample Content
wmol/%FeCx
phase shift
wmol/%Oxidation
degreea
D/%Particle
size d/nmID/IG Peak area of
carbon deposition
via Raman
/(×104 a.u.)Increase ratio
of carbon deposition
after oxidation
by H2Ob /%Fe5C2 Fe3O4 Fe5C2 Fe3O4 Fe-C1 36.16 63.85 30.92 85.51 7.65 21.23 1.66 4.42 130.77 Fe-C1W2 5.24 94.75 2.75 25.83 0.87 10.18 Fe-C3 68.02 31.98 41.13 60.47 9.04 20.82 1.37 5.23 57.17 Fe-C3W2 26.89 73.11 4.26 26.97 0.96 8.22 Fe-C5 84.30 15.70 49.31 58.49 10.48 17.74 1.41 5.52 7.25 Fe-C5W2 34.99 65.02 7.40 23.31 1.08 5.92 a: calculated as: (FeCx content before oxidation -FeCx content after oxidation)/ FeCx content before oxidation ×100%;
b: calculated from (area of deposited carbon after oxidation-area of deposited carbon before oxidation)/ area of deposited carbon before oxidation ×100%
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表 2 纯Fe催化剂H2还原和氧化后的XRD拟合
Table 2 XRD results of the Fe catalysts after reduction and oxidization
Sample Content w/% Particle size d/nm Fe3O4 Fe Fe3O4 Fe Fe-H1 90.18 9.82 26.02 16.82 Fe-H1W2 100.00 0.00 29.41 - Fe-H3 68.60 31.40 26.12 20.29 Fe-H3W2 100.00 0.00 35.70 - Fe-H5 50.35 49.65 29.81 22.58 Fe-H5W2 100.00 0.00 43.48 -
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表 3 纯Fe催化剂的TPH拟合
Table 3 TPH results of the Fe catalysts
Sample α atomic
carbon
/(×10-6 a.u.)β polymeric,
amorphous
aggregates
/(×10-5 a.u.)γ iron
carbides
/(×10-5 a.u.)δ graphitic
(crystalline)
films
/(×10-6 a.u.)Total δ
graphitic
/(×10-6 a.u.)aTotal
carbon
/(×10-4 a.u.)bFe-C5 2.24 16.42 11.85 9.09 0.08 0.84 - 0.84 3.77 Fe-C5W0.5 - - 8.35 3.12 2.77 2.80 0.63 3.43 1.46 Fe-C5W1 - - 2.80 9.05 3.35 6.69 1.82 8.51 1.60 Fe-C5W2 - 4.21 3.93 5.91 1.13 2.94 3.48 6.42 1.58 a: sum of two different kind of δ-carbon; b: sum of all carbon species
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表 4 FeK系列催化剂的XRD & Raman表征
Table 4 XRD and Raman results of FeK catalysts after carbonization and oxidization
Sample Content wmol/% FeCx
phase shift
wmol/%Oxidation
degreea D/%Particle size d/nm ID/IG Peak area of
carbon deposition via
Raman/(×104 a.u.)FeCx Fe3O4 Fe5C2 Fe3O4 Fe-C1 36.16 63.85 30.92 85.51 7.65 21.23 1.66 4.19 Fe-C1W2 5.24 94.75 2.75 25.83 0.87 9.65 Fe1K-C1 64.11 35.89 45.51 70.99 5.85 20.66 1.47 4.11 Fe1K-C1W2 18.59 81.41 5.34 22.16 0.9 9.34 Fe3K-C1 61.38 38.62 22.47 36.61 6.14 18.52 1.41 3.93 Fe3K-C1W2 38.91 61.09 3.30 22.06 1.10 5.13 Fe5K-C1 33.85 66.15 15.95 47.12 5.32 21.31 1.45 3.63 Fe5K-C1W2 17.90 82.10 3.97 25.88 0.92 3.71 a: calculated as: (FeCx content before oxidation -FeCx content after oxidation)/ FeCx content before oxidation ×100%
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表 5 FeK催化剂的TPH拟合
Table 5 TPH results of FeK catalysts
Sample
no.α atomic
carbon
/(×10-8 a.u.)β polymeric,
amorphous
aggregates
/(×10-7 a.u.)γ iron
carbides
/(×10-6 a.u.)δ graphitic
(crystalline) films
/(×10-7 a.u.)Total δ
graphitic
/(×10-7 a.u.)aTotal
carbon
/(×10-6 a.u.)bFe-C1 4.05 - 33.04 0.93 0.31 0.13 - 0.13 4.59 Fe-C1W2 - 6.83 10.96 0.44 0.19 0.22 - 0.22 2.44 Fe1K-C1 9.75 - 24.42 0.42 0.87 - 2.74 2.74 4.11 Fe1K-C1W2 - 2.37 - 1.48 0.59 2.05 2.31 4.36 2.74 Fe3K-C1 5.37 - 2.50 1.78 1.94 2.36 0.79 3.15 4.34 Fe3K-C1W2 - - 5.13 2.27 0.94 3.40 - 3.40 4.06 Fe5K-C1 3.80 - 1.19 1.67 2.02 - 2.84 2.84 4.13 Fe5K-C1W2 - - 0.37 2.22 1.53 - 2.97 2.97 4.08 a: sum of two different kind of δ-carbon; b: sum of all carbon species
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表 6 FeSi系列催化剂的XRD & Rama表征
Table 6 XRD and Raman results of FeSi catalysts after carbonization and oxidization
Sample Content
wmol/%FeCx
phase
shift wmol/%Oxidation
degreea
D/%Particle size d/nm ID/IG Peak area of
carbon deposition via
Raman/(×104 a.u.)FeCx Fe3O4 Fe5C2 Fe3O4 Fe-C5 84.30 15.70 49.31 58.49 10.48 17.74 1.28 4.92 Fe-C5W2 34.99 65.02 7.40 23.31 0.94 7.87 Fe5Si-C5 66.29 33.71 24.25 36.58 6.47 12.99 1.67 2.68 Fe5Si-C5W2 42.04 57.96 5.38 18.92 1.11 2.13 Fe10Si-C5 58.31 41.69 18.71 32.09 6.72 4.44 1.43 1.44 Fe10Si-C5W2 39.60 60.40 5.58 16.24 1.48 1.29 Fe20Si-C5 42.50 57.50 9.64 22.68 5.53 2.02 1.07 1.02 Fe20Si-C5W2 32.86 67.14 4.66 8.77 1.04 0.54 a: calculated as: (FeCx content before oxidation -FeCx content after oxidation)/ FeCx content before oxidation ×100%
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表 7 FeSi催化剂的TPH拟合
Table 7 TPH results of FeSi catalysts
Sample α atomic
carbon
/(×10-7 a.u.)β polymeric,
amorphous
aggregates
/(×10-6 a.u.)γ iron
carbides
/(×10-6 a.u.)δ graphitic
(crystalline)
films
/(×10-7 a.u.)Total δ
graphitic
/(×10-7 a.u.)aTotal
carbon
/(×10-5 a.u.)bFe-C5 22.36 164.20 118.52 90.93 0.77 8.43 - 8.43 37.75 Fe-C5W2 - 42.12 39.27 59.07 11.27 29.39 34.60 63.99 15.81 Fe5Si-C5 - - 2.72 3.40 7.91 51.70 - 51.70 1.92 Fe5Si-C5W2 - - 1.08 1.63 2.56 0.39 - 0.39 0.53 Fe10Si-C5 6.38 1.19 2.46 1.17 3.44 - 30.78 30.78 1.20 Fe10Si-C5W2 3.28 1.83 1.99 1.09 1.38 - 27.39 27.39 0.94 Fe20Si-C5 0.97 0.25 0.68 0.54 2.27 - - 0 0.38 Fe20Si-C5W2 1.08 0.19 1.38 1.26 1.53 - - 0 0.45 a: sum of two different kind of δ-carbon; b: sum of all carbon species
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