Lim Chern-Hooi, Holder Aaron M, Hynes James T, Musgrave Charles B
National Renewable Energy Laboratory , Golden, Colorado 80401, United States.
Chemistry Department, Ecole Normale Supérieure-PSL Research University, Sorbonne Universités-UPMC University Paris 06, CNRS UMR 8640 Pasteur , 24 rue Lhomond, 75005 Paris, France.
J Phys Chem Lett. 2015 Dec 17;6(24):5078-92. doi: 10.1021/acs.jpclett.5b01827. Epub 2015 Dec 10.
Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH2 obtained by 1H(+)/1e(-)/1H(+)/1e(-) reductions of pyridine. The Py/PyH2 redox couple is analogous to NADP(+)/NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e(-) reduction. A HT-PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H(+) such as Pt and Pd produce H2 and create a high interfacial pH, both obstructing dihydropyridine production and formate protonation and thus ultimately CO2 reduction by HTPTs. The role of water molecule proton relays is discussed. Finally, we suggest future CO2 reduction strategies by organic (photo)catalysts.
二氢吡啶是用于将二氧化碳催化还原为甲醇的可再生有机氢化物还原剂。在此，我们讨论这一重要还原反应的各个方面。一个能说明各种一般原理的核心内容是我们提出的理论催化机制，即通过从吡啶经1H(+)/1e(-)/1H(+)/1e(-)还原得到的二氢吡啶PyH2进行连续的氢化物转移（HTs）和质子转移（PTs）来实现二氧化碳还原。Py/PyH2氧化还原对类似于NADP(+)/NADPH，因为两者都是通过再芳构化来驱动氢化物转移。由于氢化物转移涉及2e(-)还原，避免了高能自由基中间体及其相关的高势垒/过电位。氢化物转移 - 质子转移序列决定了还原中间体在进一步还原以最终生成甲醇之前要先质子化；这些质子化过程借助偏置阴极得以实现，偏置阴极能显著降低局部pH值。相比之下，像Pt和Pd这样能有效还原H(+)的阴极会产生氢气并导致界面pH值升高，这两者都会阻碍二氢吡啶的生成以及甲酸盐的质子化，从而最终阻碍通过氢化物转移 - 质子转移实现的二氧化碳还原。文中还讨论了水分子质子传递的作用。最后，我们提出了未来有机（光）催化剂用于二氧化碳还原的策略。
