Peculiarities of a Solid-Phase Method for the Production of Al-Fe/SiO_2 and Al-Co/SiO_2 Powder Catalysts

摘要

We studied the possibility of producing powder catalysts Al-Fe/SiO_2 and Al-Co/SiO_2 by mixing precursor powders of Al, Fe, Co, and SiO_2 in a planetary mill in gross weight ratios corresponding to the domain of existence of intermetallic compounds Al_3Fe and Al_3Co and annealing in vacuum at 900°C for 30 min. Annealing in air at lower temperatures (580-600°C) leads to the formation of corundum Al_2O_3, mullite Al_6Si_2O_(13), and silicides CoSi_2, CoSi, Co_2Si, FeSi_2, and Fe_3Si in the composite structure. The synthesized composite powders 13Al/4Co/6.5SiO_2 and 13Al/4Fe/6.5SiO_2 contain nonequilibrium phases. Powders without sintered masses with good flow and a fractional composition of less than 100 (im are obtained after synthesis in vacuum at 900°C for 30 min. The fractional composition of powder Fe-Al/SiO_2 is characterized by a distribution of less than 50 μm (27.1%), 50-63 μm (15.3%), and 63-100 μm (57.4%); the fractional composition of powder Co-Al/SiO_2 is 37.5%, 16.2%, and 46.3%, respectively. According to X-ray phase analysis, the powders synthesized at 900°C contain the phases of Fe_3Al, Fe_(0.5)Al_(0.5), Fe_(14)Al_(86), Co_2Al_5, and Co_(27)Al_(73); they do not contain silicides and mullites of the type Al_6Si_2O_(13). It is experimentally established that the mass fractions of the precursors Al, Fe, and Co are (Al + Fe): SiO_2 = 31 : 69 and (Al + Co): SiO_2 = 58 : 42 as a result of cladding of powder SiO_2. It is concluded that the vacuum annealing mode of 900°C for 30 min does not provide the formation of intermetallic structures of the type Al_(13)Fe_4/Al_3Fe and Al_(13)Co_4/Al_3Co during the synthesis. We suggest optimizing the synthesis mode by using precursors of finer fractions and increasing the time of their grinding, mixing, and annealing.