Active sites are the individual reactors at the molecular scale distributed on the heterogeneous catalyst surface. To a large extent, they determine the catalytic performances and the reaction pathway of a reaction. Therefore, understanding the nature and structure of the actives sites is crucial to improve and develop novel, robust and practical catalysts. The wide application of state-of-the-art characterization techniques these years makes it possible to obtain crucial information about the

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... ctive sites for some catalysts. The Cu-based catalysts are widely used for water gas shift (WGS) and methanol synthesis from syngas (CO + H 2 ). Although having some technical issues in the direct conversion of CO 2 into value-added products, they are still promising for this reaction to mitigate CO 2 concentration in the atmosphere. In the last several years, intensive efforts have been made to study Cu-based catalysts, and substantial progress has been achieved in understanding their active sites and the reaction mechanism. This review discusses the structure and nature of active sites of Cu-based catalysts for CO 2 valorization in thermo-, photo-, and electro-catalysis. We present the characterization results of different types of Cu-based catalysts applied in these processes, unravel their active sites and structures, and figure out the most important and critical factors that drive the reactions on the sites. The principle and applications of various characterization techniques are also briefly analyzed and compared. It is expected to provide fundamental insights and perspectives for designing highly active and efficient catalysts for CO 2 conversion.