Modulation On Active Site Structure Of Ni-based Catalysts And Their Catalytic Performance Toward Alcohols Conversion

Supported Ni-based catalysts have been widely used in many important industrial catalytic processes, especially in energy and chemical industry field and high-value chemical synthesis. At present, with the refinement of product production process and the high requirement of industrial products, the development of efficient supported Ni-based catalysts to achieve target product activity and selectivity has attracted considerable attention from both fundamental study and industrial research. It is well known that the catalytic performances (activity and selectivity) of supported metal catalysts largely depend on the structure of active sites (geometry and electronic structure).Nevertheless, some key scientific problems are still needed to be resolved on the basis of determination and regulation of active structure, and catalytic mechanism of supported Ni-based catalysts is high necessary for the design of new catalysts. Hence, in this dissertation, two kinds of supported Ni-based catalysts were prepared on the basis of a topological transformation method of the NiAl-hydrotalcite precursor, by regulating metal Ni surface defects and the structure of support acid-base sites, respectively, and their catalytic performances toward catalytic conversion of alcohols (ethanol steam reforming(ESR) and oxidant-free dehydrogenation of alcohols) were studied and investigated in detail. Furthermore, various research methods (i.e., in situ characterization techniques, theoretical calculations) were performed to determine the active sites of these two supported Ni-based catalysts qualitatively and quantitatively, so as to understand the modulation of active sites structure (i.e., Ni surface defects, metal Ni and acid-base sites of support),and to enhance the catalytic performances. This study provides new ideas for the structural design and controllable preparation of supported Ni-based catalysts, which can be potentially applied in catalytic conversion of alcohols.The detailed contents and results are listed as follows:1. Modulation of Ni surface defects structure in Ni-based heterogeneous catalysts toward ethanol steam reformingIn order to modulate Ni surface defects structure in Ni-based catalysts,supported Ni nanoparticles with a high concentration of surface defects were firstly prepared on the basis of the NiAl-hydrotalcite precursor. Subsequently, a Ru-Ni bimetallic catalyst (denoted as Ru-Ni/Al2O3) was obtained by employing the surface defects of Ni nanoparticles as nucleation sites to induce the growth of Ru clusters. The resulting Ru-Ni bimetallic catalyst exhibits a high activity and a significantly enhanced H2 yield toward ESR (4.2 molH2/molEtOH, superior to the previously reported ESR catalysts working at medium-low temperature). HRTEM, XAFS and positron annihilation spectroscopy (PAS) results demonstrate that anchoring Ru clusters onto the defect sites of Ni nanoparticles leads to the formation of the Ru-Ni interface structure with a high catalytic activity for ESR. In addition, both the experimental studies and DFT calculations verify that the Ru-Ni interface with a moderate d-band center position facilitates the C-C bond cleavage and the resulting WGSR. Meanwhile, anchoring Ru clusters onto the defect sites of Ni nanoparticles effectively inhibits C-O bond rupture and the subsequent side reaction of methanation. Therefore, this study develops a method to effectively modulate Ni surface defects structure of Ni-based nanocatalysts, reveals the surface defect-induced reaction mechanism and related bond-breaking selectivity in a complex reaction, which would be extended to the design and preparation of high performance Ni-based catalyst.2. Synergic catalysis between metal Ni and acid-base sites toward oxidant-free dehydrogenation of alcohols and corresponding catalysis mechanismOn the purpose of enhancing the synergic catalysis between metal Ni and supports acid-base sites, by using the "in situ one-step preparation method", an acid-base promoted Ni nanocatalyst supported on NiAl-mixed metal oxide(denoted as Ni/NiAl-MMO) was successfully prepared based on a precise control over the in situ structural topotactic transformation of NiAl-LDH precursor (e.g., reduction temperature, heating rate, reduction time and reducing atmosphere, etc.). The resulting Ni/NiAl-MMO catalyst shows largely enhanced catalytic performance (2-octanone yield of nearly 100% and product formation rate of 78.5 mmol g-1h-1) for oxidant-free dehydrogenation of 2-octanol,superior to the conventional Ni/Al2O3 catalyst. In situ studies including XRD,Raman and EXAFS verify that the cubic NiO-like phase (Al3+-doped NiO species) are well-distributed in the amorphous Al2O3 to form NiAl-MMO support. Furthermore, CO2-TPD and NH3-TPD results demonstrate the NiO-like phase intensively affect the acid-base structure of NiAl-MMO catalysts (e.g., strength and concentration). Studies on the structure-property correlation based on operando time-resolved EXAFS spectra and kinetic isotope effect (KIE) measurements reveal that such an excellent catalytic performance is attributed to the optimized synergic catalysis between NiO and medium-strong acid-base sites of support, which accelerates the bond cleavage of kinetically key steps: a-C-H and O-H, respectively. Therefore, this work demonstrates an effective strategy to largely improve catalytic performance via enhancing the metal-support synergic catalysis, which provides a new method for the design and preparation of high performance Ni-based catalyst.