The Fabrication Of Bimetallic Catalysts Based On The Topological Transformation Of Layered Double Hydroxides

Bimetallic catalysts possess unique physical and chemical properties that are distinct from those of their parent metals, which therefore offer the opportunity to design new efficient catalysts. However, how to prepare bimetallic catalysts with uniform composition and controllable structure is still a challenging goal, due to the significant difference in properties between two metal components. In this dissertation, we explore the endogenous/exogenous construction method to fabricate bimetallic catalysts with tunable structure and improved performance, based on the topological transformation process of layered double hydroxides (LDHs) precursors. By virtue of the versatility of LDHs in chemical composition and morphology, we successfully synthetized several kinds of bimetallic catalysts with tunable composition, structure, morphology, particle size as well as improved dispersity and stability. Their catalytic performances toward hydrogen production reactions and selective hydrogenation reactions were studied. Experimental studies and DFT theoretical investigations were carried out to reveal the geometry and electronic effects on the adsorption/reaction behavior of reactants. This study made a beneficial exploration for the rational design and development of new bimetallic catalysts with excellent performace.The detailed contents are listed as follows:1. Bimetallic catalysts for hydrogen production reactionsBy using the endogenous construction method, the Cu-Co catalysts with tunable metal ratio were prepared through the topotactic transformation process of the ternary (CuχCoy,)2Al-LDHs precursor with different Cu/Co ratios, and the effect of metal composition on catalytic perfomances toward hydrogen production from ammonia borane was studied. The catalytic activity is determined by the Cu/Co ratio, and the 1/1 sample achieves the best performance with a hydrolysis completion time less than 4.0 min at a rate of ～1000mL/(min·gcat), which is comparable to the noble metal catalysts (e.g., Ru, Pt). The structural characterizations demonstrate the Cu-Co interaction facilitates the generation of highly active metallic Co phase during the catalytic reaction, accounting for the improved hydrogen production activity. Moreover, the formation of monolithic Cu-Co catalysts by in-situ growth of LDH precursor further improves the activity and recyclability for hydrogen production from ammonia borane.Based on the exogenous construction method, the Ni@(RhNi-alloy)/Al2O3 nanocomposites with core-shell structure were prepared via LDH precursor, and the structure effect of bimetallic Ni-Rh nanocrystal on the catalytic perfomance toward hydrogen production from hydrazine borane was investigated. Compared with other bimetallic Ni-Rh nanocomposites prepared by traditional routes, the Ni@(RhNi-alloy)/Al2O3 catalyst in this work exhibits the highest hydrogen production activity (5.74 ± 0.2 equiv. (H2+N2) per hydrazine borane) with good reusability and durability by magnetic separation. The structure measurements demonstrate the abundant defects existing at Rh-Ni alloy shell provide a large number of highly active sites, accounting for the satisfactory activity toward hydrazine borane decomposition. Moreover, the ultrathin RhNi-alloy shell (-2 nm) significantly improved the utilization of noble metal Rh with very low Rh/Ni ratio (1/15).2. Intermetallic catalysts for selective hydrogenation reactionsBy the introduction of the second main group elements via the endogenous/exogenous construction method, three kinds of intermetallic catalysts (Ni-In, Ni-Sn, Ni-Ga) with tunable composition and size were prepared through the topotactic transformation process of Ni-containing LDHs precursors, and their catalytic behavior toward selective hydrogenation of unsaturated aldehyde or phenylacetylene was studied thoroughly. During the LDHs topological transformation process, the Ni-promoted reduction of main group elements facilitates the formation of intermetallic catalysts, and the anchoring effect of matrix guarantees the high dispersity and stability of supported intermetallic nanoparticles. The catalytic evaluations demonstrate these intermetallic catalysts (Ni-In, Ni-Sn, Ni-Ga) dramatically improve the hydrogenation selectivity from unsaturated aldehyde/phenylacetylene to unsaturated alcohol/phenylacetylene.A furter comparative study for hydrogenation of various unsaturated aldehydes, aldehydes, alkenes and alkynes demonstrates that the Ni-based intermetallic catalysts weaken the hydrogenation activity of C=C group while that of C=O group remains unchanged, and their relative hydrogenation activity determines the product selectivity. Experimental studies and DFT theoretical investigations manifest the Ni-based intermetallics (Ni-In, Ni-Sn, Ni-Ga) have specific crystal structure and well-organized atomic arrangement, resulting in unique geometric and electronic features. The interspersion between two metals reduces the continuous Ni sites; the electron transfer occurs from the main group element to Ni through their chemical bonds. As demonstrated by in-situ infrared spectroscopy for the adsorption/reaction of probe molecules (e.g., butene and ethanal), the separated Ni sites depress the hydrogenation activity of C=C group via decreased di-o(CC) adsorption; while the electron interaction facilitates the hydrogenation of C=O group through enhanced di-a(CO) adsorption, accounting for the improved hydrogenation selectivity of unsaturated alcohol or phenylethylene. These new insights into the structure-property relationship over Ni-based intermetallic compounds would inspire the design of novel and efficient bimetallic catalysts toward selective hydrogenation reactions.