Effect of surface modification of Fe/g-C3N4 catalyst on the product distribution in CO hydrogenation
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摘要: 采用尿素热缩合法制备了氮化碳（g-C3N4），经H2O2、NH3·H2O处理、浸渍法负载Fe制得改性Fe/g-C3N4，对比研究了改性前后催化剂的CO加氢性能。结合XRD、SEM、FT-IR、CO2-TPD、CO-TPD、H2-TPR、接触角测试和N2物理吸附-脱附等系列表征，探究了表面预处理对Fe/g-C3N4催化剂织构性质以及CO加氢产物分布的影响。结果表明，不同改性方法对催化剂的织构性质和CO加氢性能影响显著。尿素热缩合法制备的g-C3N4具有典型蜂窝状结构，Fe与g-C3N4相互作用较强，且高度分散；改性前后样品均呈亲水性，且H2O2、NH3·H2O处理后亲水性增强，H2O2处理增强了表面羟基，NH3·H2O处理增加了表面氨基，促进了CO吸附，促使Fe(NCN)物相生成；预处理后的催化剂表面碱性增强。在CO加氢反应中，两步改性后的Fe/AM-g-C3N4催化剂，CO2选择性降至11.61%；Fe/AM-g-C3N4表面碱性增强，抑制了烯烃二次加氢，烯烃选择性较高，
${\rm{C}}_2^=-{\rm{C}}_4^= $ 达32.37%，O/P值3.23。-
关键词:
- CO加氢 /
- 表面改性 /
- Fe/g-C3N4催化剂 /
- 产物分布
Abstract: Carbon nitride (g-C3N4) prepared using thermal condensation of urea was pretreated by H2O2/NH3·H2O and used as support to obtain Fe/g-C3N4 catalyst via impregnation method. The catalytic performance of the catalysts both before and after modification was investigated in CO hydrogenation. Combining detailed characterizations, such as XRD, SEM, TEM, FT-IR, TG, CO2-TPD, CO-TPD, H2-TPR, contact angle measurement, and N2 physical adsorption and desorption, we investigated the effects of surface pretreatment on the texture properties of Fe/g-C3N4 catalysts and the product distribution of CO hydrogenation. The results demonstrate that various pretreatment techniques have significant influences on the textural properties and catalytic performance of the catalysts. The prepared g-C3N4 with a typical honeycomb structure has strong interaction with highly dispersed Fe. Both before and after modification, the materials are hydrophilic, and the hydrophilicity is increased after treatment with H2O2 and NH3·H2O. Treatment with H2O2 enhances surface hydroxyl groups. NH3·H2O treatment improves surface amino groups, promotes CO adsorption, and facilitates the formation of Fe(NCN) phase. The surface basicity of all pretreated catalysts is enhanced. The water gas shift (WGS) reaction activity of the two-step modified catalyst Fe/AM-g-C3N4 was lower, and the CO2 selectivity in CO hydrogenation was reduced to 11.61%. Due to the enhanced basicity of Fe/AM-g-C3N4, the secondary hydrogenation ability of olefins was inhibited to obtain higher olefin selectivity with${\rm{C}}_2^=-{\rm{C}}_4^=$ of 32.37% and an O/P value of 3.23.-
Key words:
- CO hydrogenation /
- surface modification /
- Fe/g-C3N4 catalyst /
- product distribution
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图 1 不同预处理g-C3N4（a）和反应前（b）反应后（c）催化剂的XRD谱图
Figure 1 XRD patterns of (a) g-C3N4 (b) fresh and (c) spent catalysts
图 2 催化剂的SEM照片
Figure 2 SEM images of fresh (a) g-C3N4, (b) OH-g-C3N4, (c) NH-g-C3N4, (d) AM-g-C3N4, (e) Fe/g-C3N4, (f) Fe/OH-g-C3N4, (g) Fe/NH-g-C3N4 and (h) Fe/AM-g-C3N4 samples
图 3 催化剂的TEM照片和Fe/AM-g-C3N4 EDX分析
Figure 3 TEM images of fresh (a) g-C3N4, (b) OH-g-C3N4, (c) NH-g-C3N4, (d) AM-g-C3N4, (e) Fe/g-C3N4, (f) Fe/OH-g-C3N4, (g) Fe/NH-g-C3N4 and (h) Fe/AM-g-C3N4 and EDX analysis of Fe/AM-g-C3N4 sample
图 4 样品的N2吸附-脱附曲线(a)与孔径分布(b)
Figure 4 N2 adsorption and desorption isotherms (a) and pore diameter distribution (b) of samples
图 5 不同预处理g-C3N4（a）和负载Fe后（b）样品的FT-IR谱图
Figure 5 FT-IR spectra of pretreated g-C3N4 (a) and Fe/x-g-C3N4 (b) samples
图 10 (a) g-C3N4、(b) OH-g-C3N4、(c) NH-g-C3N4、(d)AM-g-C3N4的接触角
Figure 10 Contact angles of (a) g-C3N4, (b) OH-g-C3N4, (c) NH-g-C3N4, (d)AM-g-C3N4 samples
图 12 （a）表面处理对CO加氢产物分布的影响，（b）CO转化率随时间的变化
反应条件：H2/CO=2, 1000 h−1, 280 ℃, 2 MPa, TOS = 48 h
Figure 12 Product distribution (a) and CO conversion with time on stream (b) for the catalysts
Reaction conditions: H2/CO=2, 1000 h−1, 280 ℃, 2 MPa, TOS = 48 h.
表 1 催化剂的织构性质
Table 1 Textural property of samples
Sample BET surface
area A / (m2·g−1)Pore volume
va / (cm3·g−1)Average pore
size
db /nmg-C3N4 26.82 0.090 23.00 OH-g-C3N4 19.06 0.064 27.79 NH-g-C3N4 13.46 0.083 38.90 AM-g-C3N4 8.27 0.045 42.28 Fe/g-C3N4 46.49 0.233 49.08 Fe/OH-g-C3N4 49.92 0.258 35.51 Fe/NH-g-C3N4 16.88 0.088 35.21 Fe/AM-g-C3N4 23.88 0.133 34.44 a-BJH adsorption pore volume; b-BJH adsorption average pore size. 
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