Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST) , 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Yoshida, Sakyo-ku, Kyoto 606-8501, Japan.
Acc Chem Res. 2017 Jun 20;50(6):1449-1458. doi: 10.1021/acs.accounts.7b00132. Epub 2017 May 19.
To meet the ever-increasing energy demand, the development of effective, renewable, and environmentally friendly sources of alternative energy is imperative. Hydrogen (H) is a renewable, clean energy carrier, which exhibits a threefold energy density compared to gasoline; H is considered one of the most promising alternative energy carriers for enabling a secure, clean energy future. However, the realization of a hydrogen economy is restricted by several unresolved issues. Particularly, one of the most difficult challenges is the development of a safe, efficient hydrogen storage and delivery system. To this end, hydrogen storage techniques based on liquid-phase chemical hydrogen storage materials have become an attractive choice. Formic acid (FA) with a high volumetric capacity of 53 g H/L demonstrates promise as a safe, convenient liquid hydrogen carrier. However, generating H from FA in a controlled manner at ambient temperature is still challenging, which primarily depends on the catalyst used. Hence, for practical purposes, it is imperative to develop high-performance heterogeneous catalysts for the dehydrogenation of FA. Ultrasmall metal NPs with a high surface-to-volume ratio and "clean" surface, and hence a high density of active sites exposed to reactants, are of significance for heterogeneous catalysis. However, the size of these "clean" ultrasmall metal NPs inevitably increase, and these particles undergo aggregation during synthesis and catalysis because of their high surface energy. The immobilization of metal NPs into appropriate support materials affords considerable advantages for catalytic applications, which not only offers spatial confinement to control the nucleation and growth of particles, but also prevents them from aggregation; hence, catalytic performance is significantly enhanced. In addition, the functionalization of the support with electron-rich groups is beneficial to the formation of intermediates for FA dehydrogenation, which in turn promotes the catalytic performance. In this Account, studies of hydrogen generation from FA using heterogeneous catalysts were reviewed, mainly focusing on the results reported by our group. By varying support materials (metal-organic frameworks, silica, graphene, and porous carbons) and synthetic strategies, a wide range of highly active metal NP catalysts for efficient H generation from FA under mild conditions were developed. In addition, the design and synthetic strategies were described, by which the size and composition of the NPs, as well as the well-defined NPs-support interactions, can be controlled for the enhancement of catalytic performance for the FA dehydrogenation. Furthermore, the performance of the prepared catalysts for the effective release of H from FA for the purpose of liquid-phase chemical hydrogen storage was discussed. Finally, the challenges, expected improvements, and future opportunities in this research area were summarized.
为满足不断增长的能源需求，开发高效、可再生且环境友好的替代能源势在必行。氢气（H）是一种可再生的清洁能源载体，其能量密度是汽油的三倍；H 被认为是最有前途的替代能源载体之一，可实现安全、清洁的能源未来。然而，氢气经济的实现受到了几个未解决的问题的限制。特别是，最困难的挑战之一是开发安全、高效的氢气储存和输送系统。为此，基于液相化学储氢材料的氢气储存技术已成为一种极具吸引力的选择。甲酸（FA）具有 53 g H/L 的高体积容量，有望成为一种安全、方便的液态氢气载体。然而，在环境温度下以受控方式从 FA 中产生 H 仍然具有挑战性，这主要取决于所使用的催化剂。因此，对于实际应用，必须开发用于 FA 脱氢的高性能多相催化剂。具有高表面积与体积比和“清洁”表面的超小金属纳米粒子，以及由此产生的暴露于反应物的高密度活性位，对于多相催化具有重要意义。然而，这些“清洁”的超小金属纳米粒子的尺寸不可避免地会增大，并且由于其高表面能，这些颗粒在合成和催化过程中会发生聚集。将金属纳米粒子固定在适当的载体材料中可为催化应用带来诸多优势，不仅为颗粒的成核和生长提供空间限制，还防止其聚集；因此，催化性能得到显著增强。此外，载体的功能化有利于 FA 脱氢中间产物的形成，从而促进了催化性能。本综述主要聚焦于我们小组的研究成果，讨论了使用多相催化剂从 FA 中制取氢气的研究，综述了利用不同的载体材料（金属有机骨架、二氧化硅、石墨烯和多孔碳）和合成策略，开发出了一系列在温和条件下高效制取 H 的、具有高活性的金属纳米粒子催化剂。此外，还描述了设计和合成策略，通过该策略可以控制 NPs 的尺寸和组成以及 NPs-载体相互作用的明确性，从而提高 FA 脱氢的催化性能。此外，还讨论了为液相化学储氢目的从 FA 中有效释放 H 而制备的催化剂的性能。最后，总结了该研究领域的挑战、预期改进和未来机遇。
