镍纳米粒子耦合氧空位高效催化转化棕榈酸制备烷烃

Ni nanoparticle coupled surface oxygen vacancies for efficient synergistic conversion of palmitic acid into alkanes

随着能源需求的增加和生态环境的恶化,可再生资源的开发与利用越来越受到人们的重视.其中,生物质能源分布广泛,储量丰富,是化石燃料的理想替代品.然而生物质具有高含氧量、高粘度和低热值等特性,开发高效的加氢脱氧催化剂对生物质资源的开发和利用具有重要的应用价值.近年来,研究者们对生物质(脂肪酸及其衍生物)加氢脱氧催化体系进行了大量研究,发现Ni/CeO_(2)基催化剂能够有效地催化生物质转化并获得较高的生物油产率,然而CeO_(2)载体的氧空位含量与Ni纳米颗粒尺寸、催化剂脱氧性能之间的关系仍然不明晰.本文采用水热合成法和沉淀法分别制备了H-CeO_(2)和P-CeO_(2)载体(商用CeO_(2)标记为C-CeO_(2)),通过浸渍法制备了Ni/H-CeO_(2),Ni/P-CeO_(2)和Ni/C-CeO_(2)催化剂,同时采用无氧空位的SiO_(2)做载体制备了Ni/SiO_(2)催化剂,研究了CeO_(2)氧空位含量对Ni纳米粒子尺寸的影响及其与加氢脱氧催化性能之间的关系.EPR结果表明,不同载体上氧空位含量的顺序为H-CeO_(2)>P-CeO_(2)>C-CeO_(2).XRD结果表明,在H-CeO_(2)载体上生成的Ni纳米粒子尺寸最小,分散度最高;在C-CeO_(2)载体上生成的Ni纳米粒子尺寸最大,分散度最差;而在无氧空位的SiO_(2)载体上生成的Ni纳米粒子尺寸明显大于H-CeO_(2)和P-CeO_(2)载体上生成的Ni纳米粒子.XPS结果显示,不同载体上的O表面/(O表面+O晶格)的比值顺序为H-CeO_(2)>P-CeO_(2)>C-CeO_(2),表明H-CeO_(2)载体上具有最高的氧空位含量,C-CeO_(2)载体上氧空位含量最低,与EPR结果一致;Ni/H-CeO_(2),Ni/P-CeO_(2)和Ni/C-CeO_(2)催化剂上氧空位含量顺序与载体一致,但其氧空位含量比纯载体高,说明Ni有利于氧空位的生成.Raman结果表明,Ni/H-CeO_(2)催化剂上具有的氧空位含量最高,Ni/P-CeO_(2)次之,Ni/C-CeO_(2)最差,与XPS结果一致.H2-TPR结果表明,氧空位的存在增强了Ni与CeO_(2)载体的相互作用,有利于Ni的分散.SEM和TEM实验结果也表明了在氧空位含量最高的H-CeO_(2)载体上Ni分散度最高.DFT结果进一步证实了高的氧空位含量增强了金属-载体相互作用,促进了Ni的分散.电荷分析表明,Ni纳米颗粒和CeO_(2)载体之间存在电子转移,在具有丰富氧空位的表面上从Ni转移到CeO_(2)的电子数少于规则表面的电子数,Ni原子将电子转移到载体上,降低了Ni原子的氧化程度.活性结果表明,三种纯载体催化转化棕榈酸的效果均不佳,烷烃选择性均低于5.0%;负载活性组分Ni后,催化性能明显提升.在Ni/C-CeO_(2)催化剂上,棕榈酸的转化率为72.1%,十五烷的选择性为21.9%;Ni/P-CeO_(2)催化剂对棕榈酸的转化率达到99.6%,十五烷的选择性为77.6%;Ni/H-CeO_(2)催化剂具有最佳的催化活性,其催化性能高于大部分文献报道的贵金属催化剂,棕榈酸转化率为100%,十五烷选择性达到94.8%;而在无氧空位的Ni/SiO_(2)催化剂上性能最差,棕榈酸转化率仅为52.9%,得到少量十五烷,也未检测到十六烷,这表明丰富的氧空位在棕榈酸转化过程中发挥了重要作用.结合文献及活性结果分析,丰富的氧空位有利于吸附更多的棕榈酸,从而有利于其转化.通过反应后的XRD及TG测试结果发现,Ni纳米颗粒的尺寸在反应后没有显著变化,表明氧空位也能有效地稳定Ni纳米颗粒尺寸.不同尺寸Ni纳米颗粒的催化剂活性结果表明,在较小尺寸的Ni纳米颗粒催化剂上可以获得更高的十五烷选择性,表明十五烷选择性与Ni纳米颗粒尺寸相关联.综上,Ni/H-CeO_(2)的优异催化性能归因于高氧空位含量和小尺寸Ni纳米颗粒的协同耦合作用.

The catalytic transformation of renewable biomass oil(mainly comprising fatty acids and triglycerides)into high‐value alkanes is a versatile technique,and Ni‐based catalysts are considered to be the most suitable substitutes for precious metals.Ni nanoparticles supported on CeO_(2)carriers,prepared by hydrothermal synthesis(Ni/H‐CeO_(2))with abundant oxygen vacancies,exhibited superior catalytic activity compared to precious metal catalysts.For the hydrodeoxygenation of palmitic acid,the Ni/H‐CeO_(2)catalyst converted palmitic acid into pentadecane with a 94.8%selectivity under mild reaction conditions.The outstanding catalytic performance of Ni/H‐CeO_(2)can be attributed to the synergistic effect between the Ni nanoparticles for activating hydrogen and the abundant oxygen vacancies for adsorbing oxygen from palmitic acid.The abundant oxygen vacancies of Ni/H‐CeO_(2)improved the interaction between the Ni metal and CeO_(2)support,as confirmed by density functional theory calculations.Therefore,the abundant oxygen vacancies were more conducive to the dispersion of Ni,resulting in the formation of Ni nanoparticles,which enhanced the potential for hydrogen activation due to the increased number of exposed Ni and electronic effects.The high pentadecane selectivity was governed by small Ni nanoparticles.This study provides a novel strategy to obtain an efficient hydrodeoxygenation catalyst for converting biomass oil into biofuel.