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首页> 外文会议>Pacific Rim Meeting on Electrochemical and Solid-State Science >Origin of the Maximum in the Activity Volcano Correlations for MN4 Molecular Catalysts Compared to That for Metallic Electrodes
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Origin of the Maximum in the Activity Volcano Correlations for MN4 Molecular Catalysts Compared to That for Metallic Electrodes

机译：与金属电极相比，Mn4分子催化剂活性火山相关的最大值的来源

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A classical reactivity descriptor in electrocatalysis is the binding energy of key intermediates to the active sites. This is well documented for the catalytic activity of metal electrodes, alloys and metal oxides and less studied for molecular catalysts. The activity at constant potential plotted versus these descriptors has the shape of a volcano. This is explained by the classical Sabatier principle, which states that there is an optimum "bond strength" (not too strong, not too weak) establishing the best catalyst for a given reaction. Essentially one of the key intermediates is the binding of the reacting molecule "R" to the active sites at the rate determining step as: [MN4]_(ad) + R_((aq)) ←→ [RMN4~+]_(ad) + e~- for oxidations and [MN4]_(ad) + R_((aq)) + e~- ←→ [RMN4~-]_(ad) for reductions where MN4 is surface confined macrocyclic complex. If the reaction involves the transfer of several electrons, several adsorbed intermediates can be involved. If the controlling step is the first step, it is expected that for the binding step when Gad = 0 a maximum activity should be observed and the partial coverage of adsorbed intermediate is = 0.5. This implies an equilibrium constant equal to one for the first step and for the best catalyst. In this work we have tested this hypothesis by studying hydrazine eiectrooxidation and glutathione in alkaline media using several iron porphyrins and iron phthalocyanines as catalysts immobilized on graphite and carbon nanotubes. Figure A shows the trends in reactivity versus the Fe(III)/(II) redox potential for the oxidation of glutathione describing a typical volcano correlation. However if the currents are divided by the θ_(Fe(II)), the fraction of active sites calculated at E=-0.3 V vs SCE using the Nernst equation, log(i/θ_(Fe(II)),)E vs. E°_(Fe(III)/(II)) gives a straight line of slope -0.140V/decade. The maximum corresponds to θ_(Fe(II)) = 0-5 and occurs at a potential equal to that used for comparing the activities. A similar behaviour is observed for the oxidation of hydrazine but the continuation of the straight line is observed at potentials more positive than that of the maximum. Again the maximum is observed at the potential chosen for comparison. Those catalysts having Eo' Fe(III)/(II) θ_(Fe(II))) than -0.56V are in the Fe(III) state as predicted by the Nernst equation. That oxidation state Fe(III) is inactive for the reaction as OH- ions are strongly bound to Fe(III), especially in alkaline media. So the falling of the activities in the strong binding side of the volcano can be attributed preferentially to a gradual decline in the number of Fe(II) active sites and not to gradual decrease of the fraction (1-θ) of empty or available active sites due to occupancy by intermediates. This phenomena has also been observed for ORR and for thiols oxidation and seems to be a unique feature of molecular catalysts compared to metal catalysts even though Schmickler and Santos have shown that some volcano correlations for HER on metals that are in an oxidized form, and then inactive for this reason.
机译：电催化的经典反应性描述符是活性位点的关键中间体的结合能量。这对金属电极，合金和金属氧化物的催化活性有很好的记录，并且对分子催化剂的研究较少。恒定电位的活动与这些描述符相比具有火山的形状。这是由经典的皂苔原理解释的，这使得具有最佳的“粘合强度”（不太强，而不是过于弱），为给定反应建立最佳催化剂。基本上，关键中间体之一是反应分子“R”在速率确定步骤中的活性位点的结合，如：[MN4] _（AD）+ R _（（AQ））←→[RMN4〜+] _（ AD）+ E〜 - 用于氧化和[MN4] _（AD）+ R _（（aq））+ e〜←→[RMN4〜 - ] _（AD），其中MN4是表面限制宏型复合物。如果反应涉及几种电子的转移，则可以涉及几种吸附的中间体。如果控制步骤是第一步，则预期对于结合步骤，当GAD = 0时，应观察到最大活性，并且吸附中间体的部分覆盖率是= 0.5。这意味着一个用于第一步和最佳催化剂的平衡常数等于一个。在这项工作中，我们通过使用几种铁卟啉和铁酞菁在碱性介质中研究肼酰基氧化和谷胱甘肽作为固定在石墨和碳纳米管上的催化剂来测试这一假设。图A显示了反应性的趋势与Fe（III）/（II）氧化谷胱甘肽氧化的氧化还原电位，所述谷胱甘肽描述典型的火山相关性。然而，如果电流除以θ_（fe（ii）），则使用nernst方程，log（i /θ_（fe（ii）），）e vs 。E°_（Fe（iii）/（ii））提供直线斜率-0.140V /十年。最大值对应于θ_（fe（ii））= 0-5，并且发生在等于用于比较活动的电位。对于肼的氧化观察到类似的行为，但是在潜在的潜力比最大值的潜力下观察到直线的延续。再次在选择以进行比较的电位下观察到最大值。那些具有EO'FE（III）/（II）θ_（Fe（II）））的催化剂在NERNST方程预测的FE（III）状态下。氧化态Fe（III）对于反应无活性，因为OH-离子强烈结合到Fe（III），特别是碱性培养基。因此，火山的强绑定侧的活动的下降可以优先归因于Fe（ii）活性位点的数量逐渐下降，而不是逐渐减少空或可用活性的馏分（1-θ）由于中间体占用的网站。对于ORR和硫醇氧化也已经观察到这种现象，并且似乎是与金属催化剂相比的分子催化剂的独特特征，即使Schmickler和Santos已经表明她在氧化形式的金属上的一些火山相关性，那么出于这个原因不活动。

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