Collaborative research: A multi-method approach to determine the role of semiconducting oxide and sulfide surfaces in catalyzing As, Cr, and Se redox reactions

氧化还原反应是地球表面As、Cr和Se的动员和地球化学(和生物地球化学)循环背后的驱动力。已知这些反应的动力学是由硫化物和氧化物矿物表面催化的，这是基于从宏观实验中经验得出的速率定律，但实际的反应机理(发生在纳米级)还远未被理解。该原理研究人员之前的研究已经证明了邻近效应、自旋转变和氧解离等过程对控制氧化还原反应速度的重要性。这个项目将试图在已知的速率定律的背景下量化这些原子尺度的过程，从而将宏观可观察到的过程与可能受到速率限制的纳米尺度的过程联系起来。M.大学和达特茅斯学院方法的优点是他们独特地结合了分子模拟(解释电荷和电子自旋转移的量子力学)、电化学方法和表面探针技术来描述表面介导的氧化还原机制并解决单独的速率决定步骤。分子模拟将预测表面共吸附之间可能出现的活化状态，并深入了解反应机理本身，特别是可能是速率限制的步骤，如自旋转移、配体重组、键断裂或质量传输(仅举几例)。微电极技术传统上不被应用于环境反应的研究，它将被用来在多变量空间中快速评估表面特定的机理和动力学。最后，依赖氧化还原的扫描探针显微镜将允许在现场观察这些过程。这项研究是未来开发更全面的动力学氧化还原模型的必要的过渡状态，该模型将包括细菌介导的氧化还原过程、生物矿化以及硫化物和氧化物矿物表面作为形成复杂有机分子(即生命起源的前体)的模板的作用。更广泛的意义和重要性。回答上述问题对于理解有毒元素的地球化学循环具有深远的意义，可能最重要的是，在这方面，为未来研究微生物-矿物界面提供必要的基础。这项研究的结果将对开发可渗透的反应屏障以固定有毒元素的工程师或对地球科学家开发预测模型来描述这些物种在近地表环境中的流动性具有重要意义。对技术发展的潜在影响是广泛的，包括多相催化剂、化学传感器、防腐工艺和光伏，仅举几例。

Technical description.Redox reactions are the driving force behind the mobilization and geochemical (and biogeochemical) cycling of As, Cr, and Se at the Earth's surface. The kinetics of these reactions are known to be catalyzed by sulfide and oxide mineral surfaces based on the development of empirically derived rate laws from macroscale experiments, but the actual reaction mechanisms (occurring at the nanoscale) are far from being understood. Previous studies by the Principle Investigators have demonstrated the importance of processes such as proximity effects, spintransitions, and oxygen dissociation to control the rate of redox reactions. This project will attempt to quantify these atomic scale processes in the context of the known rate laws, thereby linking the macroscale observables with the nanoscale processes that may be rate limiting.The strength of the U. of M. and Dartmouth College approach is their unique combination of molecular simulations (quantum mechanical to account for charge and electron spin transfer), electrochemical methods, and surface probe techniques to describe surface-mediated redox mechanisms and to resolve individual rate-determining steps. Molecular simulations will predict possible activated states that can occur between co-adsorbates on the surface and give insight into the reaction mechanism itself, especially steps that may be rate limiting, such as spin transfer, ligand reorganization, bond breaking, or mass transport (to name a few). Microelectrode techniques that have not traditionally been applied to study environmental reactions will be used to rapidly evaluate surface-specific mechanisms and kinetics in multivariable space. Finally, redox-dependent scanning probe microscopy will allow to observe these processes in situ.This study is a necessary "transition state" for the development of more comprehensive kinetic redox models in the future that will include bacterially-mediated redox processes, biomineralization, and the role of sulfide and oxide mineral surfaces as templates for the formation of complex organic molecules (i.e., precursors for origin of life).Broader significance and importance.Answering the aforementioned questions has profound implications for understanding the geochemical cycling of toxic elements and, maybe most importantly in this regard, provides a necessary foundation for future investigations on microbe-mineral interfaces. The results of this study will be important to the engineer developing permeable reactive barriers to immobilize toxic elements or to the geoscientist developing predictive models to describe the mobility of these species in the near-surface environment. The potential implications for technology development are widespread, including heterogeneous catalysts, chemical sensors, anti-corrosion processes, and photovoltaics, to name a few.