Controlled Synthesis Of Single-atom Catalysts And Their Applications In Heterogeneous Catalysis

Currently, the global energy shortage and environmental pollution has received ever-increasing attention from the society and government. As the biggest developing country, China has met unprecedented energy and environmental crisis. An efficient strategy to solve the energy and environmental problem is to develop highly active and selective catalysts. There is still a long way to go to improve the widely used catalysts. Because of the unique electronic and geometric structure, single-atom catalysts as a novel catalyst have exhibited remarkable activity, selectivity, and stability in a variety of vital reactions such as the transformation of small molecules,organic reactions, and electrochemical reactions. To this end, great attention has been focused on the application of single-atom catalysts and the investigation of their catalytic mechanism. Herein, we reported a facile, convenient method to synthesize single-atom catalysts at the large scale. Moreover, we extended the concept of single-atom catalysts to biological enzyme and conventional catalysts for important reactions. In addition, we investigated the catalytic mechanism at the atomic-level and demonstrated the key factors that governed the catalytic properties in order to provide a guideline for developing efficient and cheap catalytic systems. Related research works are summarized as follows:1. Extending the concept of single-atom catalysts to biological enzyme by fabricating CaHPO4-a-amylase hybrid nanobiocatalytic system. Owing to allosteric effect, Ca2+was considered as the active center in the nanobiocatalytic system. We acquired such new nanobiocatalytic systems with three different morphologies, i.e., nanoflowers,nanoplates, and parallel hexahedrons. Catalytic tests indicated that the enzymatic activity was influenced by allosteric regulation and morphology of the as-synthesized nanostructures.2. Fabricating Rh single atoms supported on CoO nanosheets ?Rh1/CoO?. During the hydroformylation reaction, Rh1/CoO revealed higher catalytic activity and selectivity for linear products compared with Rh nanocluster and nanoparticle counterparts.In-situ spectroscopy studies and theoretical calculations revealed that a structural reconstruction of Rh single atoms in Rh/CoO occurred in the atmosphere of CO and H2, facilitating the adsorption and activation of reactants. The unique configuration and coordination of Rh single atoms benefits the selectivity for linear products.3. Constructing Rh single atoms supported on VO₂ nanorods ?Rh1/VO₂?. VO₂ is a special material which undergoes a metal-insulator phase transition. During the ammonia-borane hydrolysis, the activation energy changed with temperature.Mechanistic studies indicate that the catalytic performance depended directly on the highest occupied state of the single Rh atoms, which was determined by the band structure of the substrates. Based on this understanding, we rationally designed non-noble metal catalysts that exhibited significant catalytic activity towards ammonia-borane hydrolysis.4. Extending the concept of single-atom catalysts to nonmetal element centers and conventional catalysts by preparing Co4N nanosheets for CO₂ hydrogenation. Co4N nanosheets were synthesized via the treatment of Co nanosheets with NH3 at high temperature. The catalytic activity of Co4N nanosheets was an order of magnitude higher than that of Co nanosheets. In-situ spectroscopy studies indicate that Co4NHx was the actual active phase. The amido-hydrogen atoms in Co4NHx directly added to CO₂ to form HCOO* as the intermediates. The unique reaction path significantly lowered the temperature for the activation of CO₂ over Co4N nanosheets, thereby boosting the catalytic activity. Nonmetal element, N, was regarded as a component of the special catalytic active center.