Heterogeneous catalysis, being the focus of attention in the realm of catalysis, plays a vital role in modern chemical and energy industries. A prototype of heterogeneous catalyst consists of metal nanoparticles dispersed and supported on a substrate. Transition metal oxide is one of the key components of heterogeneous catalyst and is frequently used as catalyst support for noble metal nanoparticle catalysts due to low cost. As a result of the high cost of noble metal elements, it is particularly favorable to design and develop transition metal oxide-based nanocatalysts mainly made of earth-abundant elements with no or less noble metal with comparable or better catalytic performance than noble metal-based nanocatalysts in a catalytic reaction.In some cases, surface chemistry and structure of nanocatalysts are not invariable during catalysis. They evolve in terms of surface restructuring or phase change, which contributes to the complexity of catalyst surface under different catalytic conditions. Transition metal oxides, especially reducible transition metal oxides, have multiple cationic valence states and crystallographic structures. New catalytic active phases or sites could be formed upon surface restructuring under certain catalytic conditions while they may not be preserved if exposed to ambient conditions. Thus, it is essential to characterize catalyst surface under reaction conditions so that chemistry and structure of catalyst surface could be correlated with the corresponding catalytic performance. It also suggests a new route to design nanocatalysts through restructuring catalyst precursor under certain catalytic conditions tracked with in-situ analytical techniques.Catalysis occurs on catalyst surface. For noble metal nanoparticle catalysts, only atoms exposed on surface participate in catalytic processes, while atoms in bulk do not. In order to make full use of noble metal atoms, it is crucial to maximize the dispersion. A configuration of noble metal atoms singly dispersed on oxide support offers a way to fully use each and every noble metal atom in a catalytic reaction and reach cost-effective utilization. More importantly, restructuring such catalysts with atomic dispersion under certain conditions gives rise to the formation of singly dispersed bimetallic sites by virtue of bonding between singly anchored metal atoms and the metal atoms of the catalyst support. Compared with bimetallic nanoparticle catalysts, singly dispersed bimetallic sites are typically positively charged and exhibit a different chemisorption to a reactant and/or an intermediate, thus facilitating its dissociation. In addition, the isolation of bimetallic sites minimizes the choices of potential binding configurations of a reactant molecule in contrast to the potential multiple choices on surface of a bimetallic nanoparticle with continuously packed bimetallic sites. Thus, a catalyst of singly dispersed bimetallic sites could exhibit high catalytic selectivity for an ideal product. Following the strategies of catalyst design, cobalt oxide catalysts and singly dispersed bimetallic sites supported on cobalt oxide catalysts were developed and studied in both energy transformation (water-gas shift) and environmental remediation (reduction of nitric oxide to nitrogen). In-situ techniques such as ambient-pressure photoelectron spectroscopy (AP-XPS) and extended x-ray absorption fine structure spectroscopy (EXAFS) were used to track the evolution of catalyst surface chemistry and structure under reaction conditions. A correlation between catalyst surface chemistries and structures and the corresponding catalytic performances was established. Combined with theoretical calculations, a mechanistic understanding at a molecular level was achieved. These studies have provided deep insights for further design and development of nanocatalysts.