Synthesis, Characterization, And Hydrogenation Properties Of Transition Metal Silicide Catalysts

Interstitial silicides of early transition metals have specific physical and chemical properties. Unfortunately, conventional preparation methods, such as molten salt method, co-reduction route, inherited from the microelectronic industry resulted in a low surface area and tough experimental conditions, which restricted their applications in catalysis. Based on this, we have reasonably designed silicide-modified nickel, nickel silicides, nickel-cobalt silicide solid solution, and Ni-Si/SiO2catalysts. Their catalytic activities have been detected by hydrogenation of alkyne and cinnamaldehyde, CO methanation, and hydrodesulfurization reaction.Nanoscale silicide-modified nickel catalysts with significant catalytic activity and high selectivity for phenylacetylene selective hydrogenation have been successfully synthesized by using the carbon template for oxides and further SiH4/H2silicification to silicides. Nickel silicide formation involves the following sequence as a function of increasing temperature:Ni (cubic)â†'Ni2Si (orthorhombic)â†'NiSi (orthorhombic)â†'NiSi2(cubic). The insertion of Si atoms into the interstitial sites between Ni atoms resulted in a significant change in the unit cell lattice of nickel. All of the silicide-modified nickel materials were ferromagnetic at room temperature, and saturation magnetization values drastically decreased when Si was present. The as-prepared bulk silicide-modified nickel showed above92%styrene selectivity in the hydrogenation of phenylacetylene under0.41MPa H2and at50℃for5h. In addition, only low conversions were obtained for styrene hydrogenation under the same hydrogen pressure and temperature for50min. These results indicated that these novel silicide-modified nickels are promising catalysts for the selective hydrogenation of unsaturated hydrocarbons.Raney Ni-Si catalysts were synthesized by treating Raney Ni with silane in fluidized bed reactor and tested in the selective hydrogenation of2-butyne-1.4-diol (BYD) in high concentration. Raney Ni-Si catalysts were composed of a Ni core surrounded by nickel silicide intermetallic compounds, which transformed form Ni-rich silicide (Ni2Si) to Si-rich silicide (NiSi2) with the increasing silicification temperature from250℃to450℃. The insertion of Si atoms into Raney Ni catalysts decreased the catalytic activity, but significantly improved the selectivity to2-butene-1,4-diol (BED). The beneficial effect of Si on the selective hydrogenation of BYD may be caused by the presence of Si at Ni-defect sites and the formation of nickel silicide surface intermetallic compounds, which suppress the hydrogenation of BED. Compared with the traditional Lindlar-type catalysts, such Raney Ni-Si materials can be used extensively in organic synthesis for selective hydrogenation of alkynes, avoiding the associated hazards of toxic additives.Nickel silicides with outstanding physical and chemical properties, were synthesized by the reaction of nickel oxide with silane at relatively low temperature and atmospheric pressure. Electrochemical measurements revealed that the nickel silicides exhibited remarkably like-platinum property and lower electric resistivity, indicating that the nickel silicides are potential and efficient materials for CMOS and electrocatalysis. Furthermore, nickel silicides particles presented much higher selectivity to the intermediate product (hydrocinnamaldehyde) than monometallic nickel catalyst, which may be attributed to the repulsive force between the electronegative silicon atoms in the nickel silicides and oxygen atoms in the C=O bond of cinnamaldehyde. In addition, nickel silicides showed excellent selectivity for the hydrogenation of phenylacetylene to styrene (ca.93%) due to the strong modification of the electronic structure derived from the interaction of nickel and silicon.Bulk nickel silicides (Ni2Si, NiSi, and NiSi2) have been designed as promising hydrodesulfurization catalysts with high activity toward hydrodesulfurization (HDS) and good sulfur tolerance, which is due to the formation of an electron-deficient Ni by the Ni-Si interaction in the nickel silicides. These compounds might also serve as highly sulfur tolerant substitutes for Ni catalysts in industrial applications. For improving the HDS activity, we report on the synthesis and characterization of ferromagnetic nickel-cobalt silicide (Ni1-x CoxSi2) solid solution catalysts having large surface area by the reaction of nickel cobalt oxide solid solutions with SiH4. The results showed that the saturation magnetization of the Ni1-x CoxSi2solid solutions with fluorite structure can be controlled by changing the molar ratio of Ni to Co. The nickel-rich Ni0.75Co0.25Si2catalyst was much more active than that of monometallic silicides (NiSi2and CoSi2) and significantly improved the hydrogenation property (31.5%HYD selectivity), proving the synergistic effect between the components. The valence electron concentration of the Ni increased with increasing the Co substitution, enhancing the metal-silicon and metal-metal interactions. In addition, the Si sites in the silicides alter the metal coordination, which engendered a high activity for the HDS of DBT and weakened the metal-sulfur bonds, improving the sulfur tolerance.Single phase Ni2Si nanoparticles (NPs) have been successfully synthesized by using the Rochow reverse reaction, in which organosilanes ((CH3)nSiCl4-n) were used as silicon source. The formation mechanism of Ni2Si NPs has been proposed, which involved reaction deposition and subsequently diffusion of Si atoms. Magnetism performance tests indicated that the saturation magnetization and coercive field of Ni2Si NPs depend greatly on the environmental temperature and particle size. The blocking temperature (TB) of the materials was found to strongly depend on selecting the organosilanes precursor:in the case of the Ni2Si-0(148K) and Ni2Si-2(336K). This novel methodology opened a route to prepare other metal silicides with single phase and stoichiometry.Silicon-nickel intermetallic compounds (IMCs) supported on silica (Ni-Si/SiO2), as a highly efficient catalyst for CO methanation, had been prepared by direct silicification of Ni/SiO2with silane at relatively low temperature in a fluidized bed reactor. The results indicate that uniform NiSix nanoparticles with about3-4nm were evenly dispersed on silica. The combined in situ FTIR and TPR-MS results suggest that the Ni-Si/SiO2catalysts afforded high activity in CO methanation, promoting the formation of CH4at ca.240℃.The catalytic hydrogenation of CO on the Ni-Si/SiO2was investigated in a fixed-bed reactor at GHSVs48,000mL·h-1·g-1under1atm in the temperature interval200-600℃. In the higher temperature reaction region (500-600℃), it is notable that the Ni-Si/SiO2catalysts present high activity for CO methanation as compared to the Ni/SiO2catalyst. More importantly, the Ni-Si/SiO2-350catalyst containing thermally stable Ni-Si IMCs shows significantly higher resistance to the sintering of Ni particles. Raman characterization of the spent materials qualitatively shows that carbon deposition observed on the conversional Ni/SiO2catalyst is much higher than that of the used Ni-Si/SiO2-350. It is proposed that small amounts of silicon interacting with Ni atoms selectively prevent the adsorption of resilient carbon species.