CO_2对非负载型镍催化剂甲烷裂解性能的影响
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摘要
采用沉淀法分别以乙醇洗涤和以乙醇洗涤后再以水洗涤氢氧化镍,焙烧制备了不同形貌非负载型镍催化剂,对催化剂进行了X射线衍射(XRD)表征,分析了催化剂暴露的晶面,考察了CO_2对该催化剂CH4裂解性能的影响。结果表明:反应气氛中引入~8%(vol)的CO_2可显著提高Ni-et的甲烷裂解性能;同时也可小幅提高Ni-etwt的甲烷裂解性能;适量CO_2的存在可阻止活性较低的非负载型镍的活性位在反应过程中被产物碳包裹,从而有效提高其活性。活性较高的Ni-etwt催化剂不易因被产物碳包裹而失活,使CO_2对其裂解活性影响较小。当CO_2含量较高(~50%)时,CO_2的存在会使更多反应掉的CH4不能全部转化为碳纳米材料和氢气,降低甲烷裂解的选择性;因此,过高含量的CO_2不利于催化剂催化甲烷裂解制氢和碳纳米材料。
The non-supported nickel catalysts were prepared by calcination of the ethanol rinsed nickel hydroxide precipitates(Ni-et) and calcination of the nickel hydroxide precipitates rinsed first by ethanol and subsequently by deionized water(Ni-etwt).The exposed crystal plane was analysed by X-ray diffraction(XRD) characterization.The catalytic performance of Ni-et catalyst which exhibited a worse catalytic activity for pure methane decomposition was enhanced by the presence of ~ 8%(vol) CO_2 in volume remarkably,while the catalytic performance of Ni-etwt catalyst which exhibited a better catalytic activity was enhanced not so obviously as the Ni-et catalyst.It was concluded from the analysis of the exposed crystal plane according to the XRD results that an appropriate content of CO_2 in the reaction atmosphere could inhibit the covering of the active sites which were fewer for Ni-et catalyst,and the catalytic activity was enhanced effectively;while there were more active sites for Ni-etwt catalyst,and the catalyst could not deactivate easily by the encapsulation of the active site,so the effect of CO_2 on the catalytic performances of Ni-etwt catalyst was limited;when the content of CO_2 in the reaction atmosphere was increased to50%,there were more reacted CH4 that could not converted to carbon nanofiber and hydrogen completely,and the selectivity of methane decomposition was decreased.Therefore,high content of CO_2 was not beneficial to methane decomposition.
The non-supported nickel catalysts were prepared by calcination of the ethanol rinsed nickel hydroxide precipitates(Ni-et) and calcination of the nickel hydroxide precipitates rinsed first by ethanol and subsequently by deionized water(Ni-etwt).The exposed crystal plane was analysed by X-ray diffraction(XRD) characterization.The catalytic performance of Ni-et catalyst which exhibited a worse catalytic activity for pure methane decomposition was enhanced by the presence of ~ 8%(vol) CO_2 in volume remarkably,while the catalytic performance of Ni-etwt catalyst which exhibited a better catalytic activity was enhanced not so obviously as the Ni-et catalyst.It was concluded from the analysis of the exposed crystal plane according to the XRD results that an appropriate content of CO_2 in the reaction atmosphere could inhibit the covering of the active sites which were fewer for Ni-et catalyst,and the catalytic activity was enhanced effectively;while there were more active sites for Ni-etwt catalyst,and the catalyst could not deactivate easily by the encapsulation of the active site,so the effect of CO_2 on the catalytic performances of Ni-etwt catalyst was limited;when the content of CO_2 in the reaction atmosphere was increased to50%,there were more reacted CH4 that could not converted to carbon nanofiber and hydrogen completely,and the selectivity of methane decomposition was decreased.Therefore,high content of CO_2 was not beneficial to methane decomposition.
引文
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11 Zhang W,Ge Q J,Xu H Y.Influences of reaction conditions on methane decomposition over non-supported Ni catalyst.Journal of Energy Chemistry,2011;20:339-344
12 Li Z R,Xu Y,Ma X D.CO2enhanced carbon nanotube synthesis from pyrolysis of hydrocarbons.Chem Commun,2008;3260-3262
13 Asai K,Takane K,Nagayasu Y,et al.Decomposition of methane in the presence of carbon dioxide over Ni catalysts.Chemical Engineering Science,2008;63:5083-5088
14 Hu L H,Peng Q,Li Y D.Selective synthesis of Co3O4nanocrystal with different shape and crystal plane effect on catalytic property for methane combustion.J Am Chem Soc,2008;130(48):16136-16137
15 Xie X W,Li Y,Liu Z Q,et al.Low-temperature oxidation of COcatalysed by Co3O4nanorods.Nature,2009;458:746-749
16 Reshetenko T V,Avdeeva L B,Ismagilov Z R.Carbon capacious Ni-Cu-Al2O3catalysts for high-temperature methane decomposition.Applied Catalysis A:General,2003;247:51-63
2 Niu Q J,Guo J X,Chen B L,et al.Bimetal-organic frameworks/polymer core-shell nanofibers derived heteroatom-doped carbon materials as electrocatalysts for oxygen reduction reaction.Carbon,2017;114:250-260
3 Ju J G,Kang W M,Deng N P,et al.Preparation and characterization of PVA-based carbon nanofibers with honeycomb-like porous structure via electro-blown spinning method.Microporous and Mesoporous Materials,2017;239:416-425
4 Al-Enizi A M,Elzatahry A A,Abdullah A M,et al.High electrocatalytic performance of nitrogen-doped carbon nanofiber-supported nickel oxide nanocomposite for methanol oxidation in alkaline medium.Applied Surface Science,2017;401:306-313
5 Ashok J,Subrahmanyam M,Venugopal A.Development of methane decomposition catalysts for COxfree hydrogen.Catal Surv Asia,2008;(12):229-237
6 Cunha A F,rf2o J J M,Figueiredo J L.Catalytic decomposition of methane on Raney-type catalysts.Applied Catalysis A:General,2008;348:103-112
7 Zhang W,Ge Q J,Xu H Y.Influences of precipitate rinsing solvents on Ni catalyst for methane decomposition to COx-free hydrogen.JPhys Chem A,2010;114:3818-3823
8 Li Y,Zhang B C,Xie X W,et al.Novel Ni catalysts for methane decomposition to hydrogen and carbon nanofibers.J Catal,2006;238:412-424
9张微,葛庆杰,徐恒泳.镍前驱体对非负载型镍催化剂上甲烷分解活性的影响.催化学报,2010;31(11):1358-1362Zhang W,Ge Q J,Xu H Y.Influence of nickel precursors on catalytic activity of non-supported Ni for methane decomposition.Chin JCatal,2010;31(11):1358-1362
10张微,葛庆杰,徐恒泳.阶跃升温分解法对非负载型镍催化甲烷分解活性的影响.催化学报,2010;31(9):1162-1166Zhang W,Ge Q J,Xu H Y.Influence of step temperature elevating decomposition on the catalytic activity of non-supported Ni for methane decomposition.Chin J Catal,2010;31(9):1162-1166
11 Zhang W,Ge Q J,Xu H Y.Influences of reaction conditions on methane decomposition over non-supported Ni catalyst.Journal of Energy Chemistry,2011;20:339-344
12 Li Z R,Xu Y,Ma X D.CO2enhanced carbon nanotube synthesis from pyrolysis of hydrocarbons.Chem Commun,2008;3260-3262
13 Asai K,Takane K,Nagayasu Y,et al.Decomposition of methane in the presence of carbon dioxide over Ni catalysts.Chemical Engineering Science,2008;63:5083-5088
14 Hu L H,Peng Q,Li Y D.Selective synthesis of Co3O4nanocrystal with different shape and crystal plane effect on catalytic property for methane combustion.J Am Chem Soc,2008;130(48):16136-16137
15 Xie X W,Li Y,Liu Z Q,et al.Low-temperature oxidation of COcatalysed by Co3O4nanorods.Nature,2009;458:746-749
16 Reshetenko T V,Avdeeva L B,Ismagilov Z R.Carbon capacious Ni-Cu-Al2O3catalysts for high-temperature methane decomposition.Applied Catalysis A:General,2003;247:51-63
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