Preparation Of High Surface Specific Area Silicon Carbide And Its Application In Ammonia Synthesis

Silicon carbide (SIC) has excellent mechanical strength, thermal stability and good thermal conductivity. Therefore, it has been widely used as a good material in the ceramic and abrasive industry. SiC with high surface specific area is also considered to be a promising material for using as catalyst support, which was used to many catalytic reactions and showed its superiority. So, it has important significant to synthesis high surface area silicon carbide. Tetraethoxysilane (TEOS) was used as silicon source and saccharose was used as carbon source for preparing the carbonaceous silicon xerogels. Then the xerogels were treated by the carbothermal reaction at high temperature under inert gas or vaccum condition. XRD, IR, SEM, TEM and N₂ physisorption were carried out to characterize the obtain samples. A kind of high surface specific area silicon carbide suit for ammonia synthesis was chosed, and it was used as support to prepare ruthenium catalyst. The ammonia synthesis activities for a series of rutheniun catalyst were measured at a high tempererature and under a high pressure. The results were showed as follows:1. The time of gel would be shorted by the suitable mole ratio of water/TEOS or the addition of nickel nitrate.2. When the silicon carbide precursors were treated under inert gas, the carbothermal reduction temperature for preparing silicon carbide from the precursors without Ni need to be 1600℃. The obtained silicon carbide has a high surface specific area above 140m²/g, and its pore size distribution is quite centralization; the cabothermal reduction temperature would be decreased when the Ni was employed to the precursors, but the surface specific area of the obtained sample was decreased at the same time, and the pore size distributions go to diversification. When the Ni/Si mole ratio was achieved 0.007, the carbothermal reduction temperature would no longer decline obviously, while the size of the silicon carbide increased, and the surface specific area decreased.3. When the silicon carbide precursors were treated under vacuum, we have proposed a new method for direct determination of the lowest carbothermal reduction for preparing SiC from the thermobarograms. This method has been successfully applied to study the effect of Ni catalyst in the SiC precursors on carbothermal reduction temperature for preparing SiC. And the lowest carbothermal reduction temperature for preparing SiC has been successfully determined from the corresponding thermobarogram. The results showed that the carbothermal reduction temperature for preparing SiC under dynamic vacuum condition was lower than that under inert gases for 450℃, and the obtained surface specific area of silicon carbide was improved. What was more, the carbothermal reduction temperature would drop to 850℃, when the mole ratio of Fe/Si=0.06 was employed in the SiC precursor, but the surface specific area of the obtained sample was also decreased at the same time. Just as like above condition when the Ni/Si mole ratio was continued increased above 0.06, the carbothermal reduction temperature would no longer decline obviously.4. The high surface specific area silicon carbide was used as catalyst support for ammonia synthesis. The results showed that the kind of silicon carbide suit for ammonia synthesis was the one prepared from Ni/Si mole ratio is 0.005 which was treated under inert gases. When the amount of ruthenium is 1～4ï¼…, the active for ammonia synthesis would be quickly improved with the increament of the amount of ruthenium; But when the amount of ruthenium is 4～8ï¼…, the active for ammonia synthesis would be slowly improved with the increament of the amount of ruthenium. The adjustment of Ba-promoter and K-promoter for double-promoted the catalyst would benefit for the high active. The prepared SiC-supported ruthenium catalyst has a relative high active(11.35ï¼…) under Ru=4ï¼…, Ba=4ï¼…, K=8ï¼…, 475℃, 10.0Mpa and 10 000h^-1, and the active would maintain when the catalyst was tested for the heat resistance at this temperature for 20h, which showed the stability performance of the prepared SiC-supported catalyst. It indicated that the prepared high surface area silicon carbide would be a new support for ammonia synthesis.