Elucidating Electric Field-Induced Rate Promotion of Brønsted Acid-Catalyzed Alcohol Dehydration

Applied potentials have been demonstrated as a powerful tool to promote heterogeneous Brønsted acid catalysis by orders of magnitude, leveraging interfacial electric fields to stabilize protonated intermediates. However, the use of flat two-dimensional electrodes with inherently low active site densities limits the application of conventional thermochemical characterization techniques that can probe the nature of catalytic active sites. Here, we use kinetic analyses with an electrostatics-based model to elucidate the intricacies of potential-induced rate promotion, employing liquid-phase dehydration of 1-methylcyclopentanol catalyzed by carboxylic acid groups on carbon nanotubes as a probe system. Using a basket electrode to directly polarize catalyst powder, we demonstrate that thermocatalytic reaction rates can be promoted by 100,000-fold, exhibiting a log–linear dependence on applied potential with rate-potential scalings as high as 125 ± 4 mV per 10-fold rate increase. In agreement with model predictions, we show that lower ionic strengths attenuate potential sensitivity, resulting from a weakening of the interfacial electric field that interacts with the acidic proton. Furthermore, we experimentally confirm the model-predicted “isokinetic potential” (at ∼0.6 V vs Ag/AgCl)the potential at which all rate scaling lines at various ionic strengths intersect, making the rate independent of ionic strength. Base titrations reveal that only ∼8% of the carboxylic acid sites are catalytically active, yet these same active sites are operational at the highest and lowest potentials. Collectively, our results provide a key methodology for modeling catalytic effects of electric fields, quantifying active sites under applied potential, and demonstrating fundamental principles of electric field-induced rate promotion.

外加电势已被证明是一种可将多相布朗斯特酸（Brønsted acid）催化活性提升数个数量级的强效手段，其通过界面电场稳定质子化中间体来实现催化促进。然而，本征活性位点密度较低的平面二维电极，限制了可用于表征催化活性位点本质的常规热化学表征技术的应用。本研究采用基于静电学的模型结合动力学分析，以碳纳米管表面羧基催化的1-甲基环戊醇液相脱水反应作为探针体系，阐明电势诱导的速率提升的复杂机制。本研究通过篮式电极直接极化催化剂粉体，证明热催化反应速率可被提升10万倍，且反应速率与外加电势呈现对数线性依赖关系，速率-电势标度系数最高可达125±4 mV/每10倍速率提升。与模型预测一致，本研究发现较低的离子强度会减弱电势响应敏感性，这源于与酸性质子相互作用的界面电场被削弱。此外，本研究通过实验验证了模型预测的“等动力学电势”（相对于Ag/AgCl参比电极约为0.6 V）——即在该电势下，不同离子强度下的所有速率标度曲线相交，使得反应速率与离子强度无关。碱滴定实验表明，仅约8%的羧基位点具有催化活性，但这些活性位点在最高和最低外加电势下均可发挥催化作用。综上，本研究结果为电场催化效应的建模、外加电势下活性位点的定量分析，以及电场诱导速率提升的基本原理验证提供了关键方法学支撑。