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

技术说明：氧化还原反应是地球表面砷、铬和硒的移动和地球化学（和非地球化学）循环的驱动力。这些反应的动力学是已知的硫化物和氧化物矿物表面催化的基础上，从宏观实验的经验推导出的速率定律的发展，但实际的反应机制（发生在纳米级）是远远没有被理解。主要研究人员以前的研究已经证明了邻近效应，自旋跃迁和氧解离等过程对控制氧化还原反应速率的重要性。这个项目将试图在已知速率定律的背景下量化这些原子尺度的过程，从而将宏观尺度的可观测量与可能限制速率的纳米尺度过程联系起来。分枝和达特茅斯学院的方法是他们独特的分子模拟（量子力学来解释电荷和电子自旋转移），电化学方法和表面探针技术的组合，以描述表面介导的氧化还原机制和解决个别的速率决定步骤。分子模拟将预测表面上的共吸附物之间可能发生的活化状态，并深入了解反应机制本身，特别是可能限制速率的步骤，如自旋转移，配体重组，键断裂或传质（仅举几例）。传统上没有被应用于研究环境反应的微电极技术将用于快速评估多变量空间中的表面特异性机制和动力学。最后，氧化还原依赖的扫描探针显微镜将允许原位观察这些过程。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“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复杂有机分子形成模板的作用（即，更广泛的意义和重要性。探讨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，并且在这方面可能最重要的是，为今后对微生物-矿物界面的研究提供了必要的基础。这项研究的结果将是重要的工程师开发可渗透的反应性屏障，有毒元素或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。对技术开发的潜在影响是广泛的，包括非均相催化剂、化学传感器、防腐蚀工艺和光伏等。

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.