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01
迄今为止,已经发展了多种技术可将CO₂转化为碳氢化合物或高附加值化学品,主要包括热催化[1,2]、生物催化[3]、光电催化[4,5]、电催化[6,7]和光催化还原[8-10]等。
传统热催化还原CO₂要在高温(至少500℃)和高压(10 bar)条件下进行[11]。光催化CO₂还原过程模拟自然光合作用,利用太阳能和光催化剂将CO₂和H₂O进行催化转化(亦称人工光合作用),在常温、常压条件下便可实现太阳能燃料和高附加值化学品的生产,如:甲醇、乙醇、碳氢化合物等[12,13],如图1所示。因此,光催化CO₂还原也被认为是解决全球能源和环境问题的最有前途的方案之一。近年来,光催化CO₂还原的相关研究日渐增多。
相比于传统热催化方法,该反应具有如下四大优势[14]:
① 反应外部能量供应仅为太阳能,取之不尽用之不竭;
② 反应以H₂O和CO₂为反应原料,易于获取;
③ 反应条件温和,一般为常温、常压;
④ 反应无二次污染。
图1. A自然光合作用,B人工光合成示意图[12]
02
光催化CO₂还原反应是一个复杂的多步过程。一般情况下,该反应过程主要涉及如下三个步骤[14]:
① 半导体光催化剂受到能量大于其禁带宽度(Eg)的光激发;
② 光生电子和光生空穴的分离;
③ 光生电子迁移到光催化剂表面与CO₂和H⁺发生反应并形成还原产物,光生空穴与H₂O发生氧化反应产生O₂。
整个光催化CO₂还原反应过程可以在纯气相中发生,也可在溶液体系中发生[12]。
图2. 光催化CO₂还原示意图[13].
03
目前,光催化CO₂还原反应的产物主要包括:C1类产物(CO、CH₄、CH₃OH、HCOOH)和C2及C2+类产物(C₂H₄、C₂H₆、C₃H₆、C₂H₅OH等)。在化工领域中,光催化CO₂还原反应的产物分别具有不同作用[15,16]。
① CO主要可被用作费托合成反应的原料气,用于生产高碳类化学品;
② CH₄是天然气的主要成分,同时也可被用于CO₂的重整反应;
③ 液态产物CH₃OH和HCOOH主要可被用于燃料电池,CH₃OH也可作汽油的添加剂;
④ 乙烯主要用于聚乙烯和乙二醇的生产,乙烷用于制备乙烯。乙醇主要应用于化学溶剂、医疗和燃料中;
⑤ 乙二醇用于聚乙烯对苯二甲酸酯(涤纶的原料)的生产。
04
CO₂的C=O键能高达750 kJ·mol⁻¹,其线性对称分子结构使其不易被活化[12,17]。因此,在热力学上,CO₂的活化需要高能输入。受制于转化效率和选择性问题,目前的光催化CO₂还原研究仍处于实验室阶段。
现阶段光催化CO₂反应主要面临以下几方面挑战[18,12]:
① 催化剂有限的光吸收能力;
② 严重的光生载流子复合;
③ CO₂难于吸附活化;
④ 竞争反应(析氢反应)需被有效抑制;
⑤ 光催化剂的稳定性有待提升;
⑥ 待开发简便的催化剂合成工艺;
⑦ 缺乏大量反应机理研究,还原产物的选择性难于调控。
针对以上问题:
一方面可以通过设计合成高效催化剂提升光催化CO₂还原反应的转化效率和提高目标产物的选择性;
另一方面,泊菲莱科技期望与各位专家朋友们进行交流和深入合作,开发设计合理的反应器,通过优化反应工艺,积极推动光催化CO₂还原反应的相关研究。
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