The direct epoxidation of propylene to propylene oxide (PO) using molecular oxygen is an attractive alternative to current production methods using chlorohydrin or hydroperoxide-mediated processes, which are environmentally harmful and expensive. Although direct ethylene epoxidation using Ag-based catalysts has been practiced industrially for decades, due to the presence of allylic hydrogen in propylene the selectivity toward epoxide is generally much lower for propylene than for ethylene. Mechanistic understanding on well-characterized surfaces of model catalysts can potentially provide guidance to effectively alter the electronic properties of the catalyst in order to increase PO selectivity. This review summarizes both experimental and theoretical studies on model catalysts for propylene epoxidation and their contributions to elucidating the reaction mechanism, intermediates, and active sites. We first show examples of experimental studies on Cu, Ag, and Au surfaces, and compare the reaction pathways and intermediates on these surfaces. Novel approaches including plasmon-mediated catalysis and utilization of shape-controlled crystal facets that open new opportunities for improving PO selectivity will also be discussed. We then describe how density functional theory (DFT) calculations have provided important insights into the reaction mechanism and active sites on Cu, Ag, and Au surfaces and clusters. Propylene oxidation pathways on other relevant metal surfaces will also be discussed. The combined experimental and computational studies elucidate the nature of surface oxygen species and the role of the oxametallacycle intermediate. We conclude by highlighting design principles and insights for guiding further development of active and selective propylene epoxidation catalysts.

使用分子氧将丙烯直接环氧化为环氧丙烷（PO）是目前使用氯醇或氢过氧化物介导的方法的有吸引力的替代方法，这些方法对环境有害且昂贵。尽管使用基于Ag的催化剂的直接乙烯环氧化已经在工业上实践了几十年，但是由于丙烯中存在烯丙基氢，丙烯对环氧化物的选择性通常比乙烯低得多。对模型催化剂特征充分的表面的机械理解可潜在地提供指导，以有效地改变催化剂的电子性能，从而增加PO的选择性。这篇综述总结了用于丙烯环氧化的模型催化剂的实验和理论研究，以及它们对阐明反应机理，中间体和活性位点的贡献。我们首先显示在Cu，Ag和Au表面上进行实验研究的示例，并比较这些表面上的反应途径和中间体。还将讨论新颖的方法，包括等离激元介导的催化和形状控制的晶面的利用，这些方法为提高PO选择性提供了新的机会。然后，我们将描述密度泛函理论（DFT）的计算方法如何提供重要的见解，以了解Cu，Ag和Au的表面和簇上的反应机理以及活性部位。还将讨论其他相关金属表面上的丙烯氧化途径。实验和计算研究相结合，阐明了表面氧物种的性质以及氧杂金属环中间体的作用。最后，我们通过重点介绍设计原理和见解来指导活性和选择性丙烯环氧化催化剂的进一步开发。