中国科学院过程工程研究所机构知识库

将二氧化碳(CO2)转化为可利用的能源可以同时解决能源短缺和全球变暖两大问题。其中，利用电化学的方法将CO2高效催化还原为可存储的化学能，例如一氧化碳(CO)、甲酸(HCOOH)等，可以实现自然界中“碳循环”以减少碳在大气中的积累，同时这还将是一种新型储能方式，因而极具应用潜力。而这一转换过程的可行性主要取决于电极材料的催化性能，因此，设计并构建一种高效催化剂是实现该技术的关键。针对CO2电催化还原中法拉第效率低、催化材料合成复杂、活性不高等问题，本论文通过定向合成目标结构、简化合成工艺流程等方式来制备高选择性的催化剂。其中，以金属有机框架(MOFs)为基体，利用其孔道结构“捕捉”和分散单核金属前驱体，经一步煅烧后形成金属掺杂的多孔碳基催化材料，开发了一种简单、高效的催化剂合成方法，制备了一系列不同过度金属掺杂的单原子或纳米催化材料；此外，通过一步还原法简化了金属铋纳米催化材料的制备流程，并研究了这些单原子或纳米材料在CO2电化学转换中的催化性能。本论文主要研究内容如下：(1) Ni单原子催化材料：以Ni为掺杂金属元素，沸石咪唑骨架(ZIF-8)为碳骨架前驱体，二氰二胺(DCD)为配位体，经一步煅烧合成以配位不饱和的Ni-N键为活性中心的单原子催化剂，并探究其在CO2转换中的催化性能及反应机理。研究表明，该催化剂中的Ni以原子形式分散在碳纳米管上。相比于Ni金属片或纳米颗粒，Ni单原子在催化还原CO2过程中表现出极强的催化活性和选择性，解决了其它金属电极由于析氢反应剧烈而造成的选择性不高问题。(2) Fe/Ni纳米合金碳基催化材料：在ZIF-8中同时引入Fe和Ni两种金属前驱体，经高温煅烧后形成的Fe/Ni合金纳米颗粒均匀的分散在碳骨架中，形貌均匀，结构稳定。电化学测试表明该材料能高效地将CO2选择性还原为CO。由于催化剂中的部分电子从Fe/Ni纳米合金转移至其与掺氮碳骨架的交界，因而使催化剂表面呈富电子状态，促进了CO2获得电子而还原的过程。同时，这种三维碳骨架在高温下出现破损，这极大的增加了其比表面积，大幅度的提高了材料的催化活性。(3) Bi纳米催化材料：采用一步还原法制备Bi纳米材料，该纳米结构拥有较大的电化学活性面积，暴露更多边、角、缺陷及特定指数晶面，有效的提升了还原CO2的催化性能。实验表明，Bi纳米材料能高效地将CO2催化还原为甲酸，且性能稳定。密度泛函理论(DFT)计算表明，Bi纳米材料暴露的特定指数晶面，尤其是Bi(012)，对形成*OCHO反应中间物有着较小的能量壁垒，因而表现出对甲酸的高选择性。同时，其简单的制备方式也使得该催化材料极具产业化潜力。;Converting carbon dioxide (CO2) into renewable energy plays an important role in our global efforts to mitigate both the energy shortage and climate change. In particular, electrochemical reduction of CO2 to value-added chemicals such as carbon monoxide (CO) and formate (HCOO-) can realize carbon-neutral cycle so as to reduce the accumulation of carbon in the atmosphere. In the sense, this conversion can also be regarded as a new type of energy storage, which has great potential for practical application. However, the efficiency of this conversion depends heavily on the catalytic performance of electrode. Therefore, designing and constructing a catalyst with high efficiency is crucial to realize this technology.Herein, a series of highly selective catalysts are elaborately constructed with simplified synthesis strategy to address the problems of low Faradaic efficiency for CO2 electroreduction and complicated synthesis route of electrode. The metal organic frameworks (MOFs) are employed as substrates, utilizing its pore structure to uniformly adsorb the single metal precursor. After a simple process of pyrolysis, a metal-doped carbon framework catalyst can be fabricated. A series of single-atom/nano materials with different metal doped are prepared through this simple and effective strategy. Moreover, the preparation of bismuth (Bi) electrocatalyst is simplified by one-step reduction method. The electrocatalytic performance of these single-atom/nano catalysts was then investigated. The main achievements are as follows:(1) Nickel-single-atom catalyst: Zeolitic imidazolate frameworks (ZIF-8), doping with Ni and dicyandiamide, are used as precursor to fabricate coordinatively unsaturated Ni-N sites through one-pot pyrolysis. Afterward, its catalytic performance for CO2 electroreduction and corresponding reaction mechanism were investigated. Ni in the catalyst is atomically dispersed on the surface of the carbon nanotubes. Different from Ni plate or nanoparticles, nickel-single-atom catalyst shows remarkable catalytic activity and selectivity for CO2 electroreduction, which solves the problem of low Faradaic efficiency caused by favorable hydrogen evolution reaction on most metal electrodes.(2) Fe/Ni nano-alloy catalyst: A uniformly dispersed and stable catalyst of Fe/Ni nano-alloy was fabricated by pyrolyzing Fe and Ni salts modified ZIF-8. Electrochemical measurements verified its remarkable intrinsic catalytic activity for electroreduction CO2 to CO. Due to the electrons transferred from Fe/Ni nano-alloy, the nitrogen-doped carbon framework would be in an electron-rich state, promoting the reduction of CO2 on graphene shell. Worth noting that this particular 3D carbon frameworks offer an enlarged specific surface area, which greatly improves its electrocatalytic performance.(3) Bismush nanostructure: A facile and one-pot synthesis strategy of Bi nanostructure was reported by using Sodium borohydrlde to reduce Bi3+. Thanks to the enhanced electrochemical active area, which exposed more edges, steps, defect and high-index planes, remarkable catalytic performance with high activity and selectivity was achieved. Experimental results show that CO2 can be efficiently reduced to formate on Bi nanostructure. Density functional theory (DFT) calculation shows that the formation of *OCHO has much lower energy barrier on Bi high-index planes, particularly on Bi (012), leading to the generation of formate in the end. Most importantly, its simplified synthesis strategy gives the catalyst great potential in practical application.