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Fundamental understandings of surface chemistry and catalysis of solid catalysts are of great importance for the developments of efficient catalysts and corresponding catalytic processes, but have been remaining as a challenge due to the complex nature of heterogeneous catalysis. Model catalysts approach based on catalytic materials with uniform and well-defined surface structures is an effective strategy. Single crystals-based model catalysts have been successfully used for surface chemistry studies of solid catalysts, but encounter the so-called "materials gap" and "pressure gap" when applied for catalysis studies of solid catalysts. Recently catalytic nanocrystals with uniform and well-defined surface structures have emerged as a novel type of model catalysts whose surface chemistry and catalysis can be studied under the same operational reaction condition as working powder catalysts, and they are recognized as a novel type of model catalysts that can bridge the "materials gap" and "pressure gap" between single crystals-based model catalysts and powder catalysts. Herein we review recent progress of surface chemistry and catalysis of important oxide catalysts including CeO 2 , TiO 2 and Cu 2 O acquired by model catalysts from single crystals to nanocrystals with an aim at summarizing the commonalities and discussing the differences among model catalysts with complexities at different levels. Firstly, the complex nature of surface chemistry and catalysis of solid catalysts is briefly introduced. In the following sections, the model catalysts approach is described and surface chemistry and catalysis of CeO 2 , TiO 2 and Cu 2 O single crystal and nanocrystal model catalysts are reviewed. Finally, concluding remarks and future prospects are given on a comprehensive approach of model catalysts from single crystals to nanocrystals for the investigations of surface chemistry and catalysis of powder catalysts approaching the working conditions as closely as possible. Image 1 [ABSTRACT FROM AUTHOR]

• Morphology-dependent CeO 2 catalysis is established in C 2 H 2 semihydrogenation reaction. • Catalytic performance follows an order of r-CeO 2 -500 ≫ c-CeO 2 > r-CeO 2 -700. • Ability to chemisorb C 2 H 2 (a) follows an order of r-CeO 2 -500 ≫ c-CeO 2 ≈ r-CeO 2 -700. • Ability to dissociate H 2 follows an order of c-CeO 2 > r-CeO 2 -500 ≫ r-CeO 2 -700. We herein investigated the catalytic performances and reaction mechanisms of CeO 2 cubes (denoted as c-CeO 2), CeO 2 rods calcined at 500 °C (denoted as r-CeO 2 -500), and CeO 2 rods calcined at 700 °C (denoted as r-CeO 2 -700) in the acetylene semihydrogenation reaction. r-CeO 2 -500 is most catalytically active while r-CeO 2 -700 is least. The acetylene semihydrogenation reaction catalyzed by various CeO 2 nanocrystals was observed to follow the same reaction mechanism. C 2 H 2 adsorbs at both O and Ce sites on CeO 2 surfaces to form C 2 H 2 (a) species that facilely hydrogenates stepwise to C 2 H 3 (a) and C 2 H 4 (a). The surface hydrogenation reactions proceed more facilely at the O site than at the Ce site. The desorption of C 2 H 4 (a) proceeds slowly and is the rate-limiting step. c-CeO 2 , r-CeO 2 -500 and r-CeO 2 -700 do not exhibit obvious differences among these elementary surface reactions, but show much different abilities to dissociate H 2 and chemisorb C 2 H 2. The C 2 H 2 (a) coverage follows an order of r-CeO 2 -500 ≫ c-CeO 2 ≈ r-CeO 2 -700. The ability to dissociate H 2 follows an order of c-CeO 2 > r-CeO 2 -500 ≫ r-CeO 2 -700. The observed morphology-dependent behaviors of various CeO 2 nanocrystals were correlated to their defects and exposed facets. These results successfully demonstrate morphology-dependent CeO 2 catalysis in acetylene semihydrogenation reaction and elucidate the underlying mechanisms. [ABSTRACT FROM AUTHOR]