摘要
乙醇氧化重整将乙醇蒸汽重整和部分氧化相耦合,具有启动快、产氢效率高、可实现自供热等显著优点,是一种应用前景广阔的制氢技术。镍整体型催化剂催化的反应可在大空速下自热进行,而且催化剂空隙率大,具有压力降小,生产能力高等优点。在本课题实验工作中,镍金属基整体型催化剂用于乙醇氧化重整制氢反应。为了提高该催化剂的H2选择性和抗积碳能力,对催化剂进行助剂氧化物负载,取得了比较理想的结果。
在本论文工作中,以Ce(NO3)3和/或Zr(NO3)4溶液浸渍泡沫镍,再进行干燥和焙烧,制备了一系列负载CeO_2和CeO_2-ZrO_2的镍金属整体型催化剂。并采用XRD,SEM,BET和ICP等分析测试手段对催化剂进行了表征。同时考察了反应温度,空速,碳氧比和醇水比对催化剂反应活性和选择性的影响。结果表明:(1)在所有反应条件下,乙醇转化率接近100%,并且不受反应条件变化的影响;(2)负载CeO_2能够促进WGS反应的发生,提高H2选择性,降低副产物的选择性;CeO_2的最佳负载量是7.33 wt.%,以此负载量负载的催化剂Ce0.8Ni在温度为750°C,空速0.7×105 h-1,nC/nO为0.7,nC2H5OH/nH2O为0.25的条件下反应170min,H2,CH4和CO的平均选择性为98.3%,2.2%和31.2%,只检测到微量的C2H6,没有C2H4生成;(3)Ce/Zr最佳负载摩尔比为3.26,以此比例负载的催化剂Ce0.15Zr0.05Ni具有最高的H2选择性和最好的稳定性。XRD检测结果表明CeO_2和ZrO_2之间有较强的相互作用,说明形成了CeO_2-ZrO_2固溶体。正是由于固溶体的形成增强了固溶体中CeO_2的氧化还原性和热稳定性,从而改善了催化剂的催化性能和稳定性;(4)Ce0.8Ni和Ce0.15Zr0.05Ni氧化重整产物的选择性随着反应条件变化的趋势是相同的:H2选择性随着温度的升高或C2H5OH/H2O的减小而增大,CH_4,C_2H_4和C_2H_6的选择性随着温度的升高或C2H5OH/H2O的减小而减小;H2选择性随着GHSV或C/O比的增大而减小,CH_4,C_2H_4和C_2H_6的选择性随着GHSV或C/O比的增大而增大。
Oxidative steam reforming (OSR) of ethanol, which is a combination of steam reforming and partial oxidation reactions, is proposed as the most effective process for the conversion of ethanol to hydrogen. OSR also possesses the advantages of rapid start-up and possibility of thermally neutral operation. In this work, Ni monolith based catalysts were used for the oxidative steam reforming of ethanol. The oxide promoters, CeO_2 and CeO_2-ZrO_2, were added to the Ni monolith to promote its catalytic capability and stability.
A series of CeO_2, ZrO_2 or CeO_2-ZrO_2 promoted monolithic Ni catalysts were prepared by impregnating nickel sponge with Ce(NO3)3 and/or Zr(NO3)4 solutions. The catalysts were characterized with XRD, SEM, BET and ICP. The influences of several parameters on the catalytic activity and selectivity were examined including reaction temperature, GHSV, C/O and C2H5OH/H2O ratios. The results revealed that (1) in all the experiments of this work, the conversion of ethanol was complete and was not affected by the changes of the parameters investigated; (2) addition of CeO_2 promoted the WGS reaction, increased the selectivity to H2 while reduced the selectivities to undesirable products CH4, C2H4 and C2H6; the catalyst with the optimal CeO_2 loading of 7.33wt.% showed the selectivities to H2, CH4 and CO of 98.3%, 2.2% and 31.2%, respectively; trace amount of C2H6 and no C2H4 was observed during 170 min testing; (3) the optimal Ce/Zr loading molar ratio was 3.26; the catalyst Ce0.15Zr0.05Ni with the optimal loading ratio exhibited the highest selectivity to H2 and the best stability among the prepared series of CeZrNi catalysts; the XRD patterns revealed the existence of Ce-Zr interaction as a sign of the formation of the solid solution between CeO_2 and ZrO_2; it is supposed that the solid solution formation attributes to the enhancement of the redox property and thermal stability of the CeO_2-ZrO_2 and thus the improvement of its catalytic capability and stability; (4) the catalytic properties of Ce0.8Ni and Ce0.15Zr0.05Ni showed similar trends with the changes of reaction temperature, GHSV, C/O and C2H5OH/H2O ratios in the oxidative steam reforming: selectivity to H2 increased, while selectivities to CH4, C2H4 and C2H6 decreased with the increase of reaction temperature or the decrease of C2H5OH/H2O ratio; selectivity to H2 decreased, while selectivities to CH4, C_2H_4 and C_2H_6 increased with the increase of GHSV or C/O ratio.
引文
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