Modulation Of Double Active Sites In Cu-Based Catalysts And Corresponding Cooperative Catalysis Mechanism

Cu-based catalysts are widely used in a wide range of industrial catalysis, especially in coal chemical industry field and high value-added fine chemicals synthesis. It has been reported that the double active sites (i.e., Cu+/Cu0, bimetallic active site) and their synergetic effect play a key role in determining the catalytic performance of Cu-based catalysts. However, some problems remain unsolved in the design, preparation and application of the Cu-based catalysts:the fine control over double active sites is rather difficult to obtain optimized structure and desired activity; the understanding on structure of double active sites and cooperative catalysis mechanism is lacking, which is a bottleneck for the enhancement of catalytic performances. Therefore, in this dissertation, several kinds of highly-efficient Cu-based catalysts were prepared via the topological transformation method of layered double hydroxides (LDHs) precursors, and their catalytic performances toward catalytic conversion of synthesis gas and glycerol hydrogenolysis were investigated. A precise modulation over double active sites (i.e., Cu+/Cu0, bimetallic active site) of Cu-based catalysts was achieved;an experiment-calculation combination study was carried out to reveal the structure-property correlation and understand the cooperative catalysis mechanism of double active sites. This study made a beneficial exploration for the development of novel and effective Cu-based catalysts, which show promising catalytic behavior in industrial applications.The detailed contents are listed as follows:1. Modulation on Cu0/Cu+ active site in Cu-based heterogeneous catalysts toward glycerol hydrogenolysis to 1,2-propanediolOn the purpose of enhancing the catalytic activity and stability of Cu-based catalysts, CeOx was introduced onto the surface of supported Cu nanoparticles with high dispersion derived from CuMgAl-LDHs precursor. The resulting heterogeneous catalyst Cu@CeOx is highly-active for glycerol hydrogenolysis to 1,2-propanediol (conversion:100%, selectivity:99.8%). In situ characterization techniques(in-situ XAFS and in-situ FTIR) were carried out under reaction conditions, and the results demonstrate that the unique structure of Cu@CeOx effectively imposes interfacial cooperative catalysis between Cu and CeOx. The concentration of Cu0 increases and that of Cu+ decreases significantly; and vacancy of CeOx plays the active site which strengthens the binding of the intermediates and increases the activity of the catalyst. In addition, this loading of CeOx as shell stabilizes the Cu nanoparticles and suppresses the agglomeration during the reaction process.In order to regulate the catalytic selectivity of Cu-based nanocatalysts for glycerol hydrogenolysis, Ni was introduced into the CuMgAl-LDHs precursor. A series of CuxNiy alloy catalysts with different Cu/Ni ratios were fabricated by changing the metal ratio and topotactic transformation condition of LDHs precursor. On the basis of detailed characterizations, it is found that a stable and balanced Cu0 and Cu+ active species is effectively maintained via controlling the Cu/Ni ratio in CuxNiy alloy catalysts. The correlation between product selectivity (1,2-propanediol of 1,3-propanediol) and the Cu0/Cu+ active species was studied, and a high selectivity for 1,3-propanediol (45.9%) is obtained for Cu-based catalysts for the first time. In additional, DFT theoretical investigations indicate that the introduction of Ni regulates the fine structure of Cu0/Cu+ active species, which induces different adsorbed states of intermediates and thus influences the catalytic selectivity.2. Modulation on bimetallic active site in Cu-based catalysts toward synthesis of mixed alcohols from syngasCuxFey bimetallic nanocatalysts have been prepared via the in situ topotactic transformation process of CuFeMg-LDHs precursors. By tuning the metal ratio and topotactic transformation conditions of LDHs precursor, a series of CuxFey bimetallic catalysts were fabricated. The CuxFey bimetallic catalysts shows high CO conversion (56.89%) and the mixed alcohols (C1-C5 alcohols: 49.07%), as a result of the high dispersion of Cu and Fe species as well as the strong synergistic effect between these double active sites, which superior to the traditional coprecipitation CuxFey bimetallic catalysts. Therefore, this work provides a facile and effective method for the preparation of Cu-based bimetallic catalysts with high catalytic activity, which can be potentially used in syngas conversion to mixed alcohols.Further, we demonstrate the fabrication of core-shell Cu@(CuCo-alloy) nanoparticles (NPs) embedded on a Al2O3 matrix via an in situ growth of CuCoAl-LDH nanoplatelets on Al substrates followed by a calcination-reduction process, and they serve as efficient catalysts toward CO hydrogenation to produce higher alcohols (C6+OH). The composition, particle size and morphology can be tuned by changing the Cu/Co molar ratio in the CuCoAl-LDH precursors, and the best catalytic behavior was obtained over the Cu1Co2 catalyst with a CO conversion of 21.5% and a excellent selectivity of C6+slate 1-alcohols (48.9%). The structure measurements revealed that the modulation of double active sites in CuxCoy catalysts optimize a balanced electronic and geometric interaction between Cu and Co as well as the strong synergistic effect between these double active sites, which contributes to the significantly enhanced catalytic performances. In addition, the 3D hierarchical structure of the CuxCoy bimetallic catalyst facilitates mass diffusion/transportation as well as prevents hotspot formation, accounting for its stability and recyclability. The CuxCoy bimetallic catalyst with significantly improved catalytic behavior can be potentially used in CO hydrogenation to produce higher alcohols.