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Developing Scaling Relationships for Molecular Electrocatalysis through Studies of Fe-Porphyrin-Catalyzed O2 Reduction.
Accounts of Chemical Research ( IF 18.3 ) Pub Date : 2020-04-13 , DOI: 10.1021/acs.accounts.0c00044
Daniel J Martin 1 , Catherine F Wise 1 , Michael L Pegis 2 , James M Mayer 1
Affiliation  

ConspectusThe oxygen reduction reaction (ORR) is a multiproton/multielectron transformation in which dioxygen (O2) is reduced to water or hydrogen peroxide and serves as the cathode reaction in most fuel cells. The ORR (O2 + 4e- + 4H+ → 2H2O) involves up to nine substrates and thus requires navigating a complicated reaction landscape, typically with several high-energy intermediates. Many catalysts can perform this reaction, though few operate with fast rates and at low overpotentials (close to the thermodynamic potential). Attempts to optimize these parameters, both in homogeneous and heterogeneous electrocatalytic systems, have focused on modifying catalyst design and understanding kinetic/thermodynamic relationships between catalytic intermediates. One such method for analyzing and predicting catalyst reactivity and efficiency has been the development of "molecular scaling relationships". Here, we share our experience deriving and utilizing molecular scaling relationships for soluble, iron-porphyrin-catalyzed O2 reduction in organic solvents. These relationships correlate turnover frequencies (TOFmax) and effective overpotentials (ηeff), properties uniquely defined for homogeneous catalysts. Following a general introduction of scaling relationships for both homogeneous and heterogeneous electrocatalysis, we describe the components of such scaling relationships: (i) the overall thermochemistry of the reaction and (ii) the rate and rate law of the catalyzed reaction. We then show how connecting these thermodynamic and kinetic parameters reveals multiple molecular scaling relationships for iron-porphyrin-catalyzed O2 reduction. For example, the log(TOFmax) responds steeply to changes in ηeff that result from different catalyst reduction potentials (18.5 decades in TOFmax/V in ηeff) but much less dramatically to changes in ηeff that arise from varying the pKa of the acid buffer (5.1 decades in TOFmax/V in ηeff). Thus, a single scaling relationship is not always sufficient for describing molecular electrocatalysis. This is particularly evident when the catalyst identity and reaction conditions are coupled. Using these multiple scaling relationships, we demonstrate that the metrics of turnover frequency and effective overpotential can be predictably tuned to achieve faster rates at lowered overpotentials. This Account uses a collection of related stories describing our research on soluble iron-porphyrin-catalyzed ORR to show how molecular scaling relationships can be derived and used for any electrocatalytic reaction. Such scaling relationships are powerful tools that connect the thermochemistry, mechanism, and rate law for a catalytic system. We hope that this collection shows the utility and simplicity of the molecular scaling approach for understanding catalysis, for enabling direct comparisons between catalyst systems, and for optimizing catalytic processes.

中文翻译:

通过研究铁卟啉催化的O2还原,发展分子电催化的比例关系。

结论氧还原反应(ORR)是一种多质子/多电子转化,其中双氧(O2)被还原为水或过氧化氢,并且在大多数燃料电池中用作阴极反应。ORR(O2 + 4e- + 4H +→2H2O)涉及多达9个底物,因此需要导航复杂的反应态势,通常需要使用几种高能中间体。许多催化剂可以进行该反应,尽管很少有催化剂能以高速率和低过电势(接近热力学势)运行。在均相和非均相电催化系统中试图优化这些参数的尝试都集中在改进催化剂设计和理解催化中间体之间的动力学/热力学关系上。一种用于分析和预测催化剂反应性和效率的方法是“分子比例关系”的发展。在这里,我们分享了我们的经验,这些经验推导并利用了分子比例关系来实现有机溶剂中可溶的,铁卟啉催化的氧气还原。这些关系将周转频率(TOFmax)和有效超电势(ηeff)关联起来,这是均相催化剂的唯一定义。在对均相和非均相电催化的比例关系进行了一般性介绍之后,我们描述了这种比例关系的组成部分:(i)反应的整体热化学和(ii)催化反应的速率和速率定律。然后,我们说明如何将这些热力学和动力学参数联系起来揭示铁卟啉催化的O2还原的多个分子比例关系。例如,log(TOFmax)对由不同的催化剂还原电位(TOFmax / V在ηeff中为18.5十年)引起的ηeff变化的响应很陡,但对因改变酸缓冲液的pKa而引起的ηeff的变化的响应要小得多( TOFmax / V以ηeff表示5.1年。因此,单一的比例关系并不总是足以描述分子的电催化作用。当催化剂特性和反应条件耦合时,这尤其明显。使用这些多重比例关系,我们证明可以预测调整周转频率和有效超电势的指标,以在降低的超电势下获得更快的速度。该帐户使用了一系列相关故事,这些故事描述了我们对可溶性铁卟啉催化的ORR的研究,以显示如何得出分子比例关系并用于任何电催化反应。这种比例关系是连接催化系统的热化学,机理和速率定律的强大工具。我们希望这个集合展示了分子缩放方法在理解催化,实现催化剂系统之间的直接比较以及优化催化过程方面的实用性和简单性。催化系统的机理和速率定律。我们希望这个集合展示了分子缩放方法在理解催化,实现催化剂系统之间的直接比较以及优化催化过程方面的实用性和简单性。催化系统的机理和速率定律。我们希望这个集合展示了分子缩放方法在理解催化,实现催化剂系统之间的直接比较以及优化催化过程方面的实用性和简单性。
更新日期:2020-04-13
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