Research On The Preparation Of Non-stoichiometric Magnesium Aluminum Spinel Ultra-fine Active Powders And Their Performance

Magnesium-aluminum spinel powder is a functional material with excellent stability,especially for the utilization on the special ceramics and supports for industrial catalysts.The initial state of the powder plays an important role in the influence on both applications of special ceramics and supports for industrial catalysts.Among them,the effect of active spinel powder with high specific surface energy and high internal distortion energy is particularly significant.So far,the reports for this aspect is still rare.In addition,there are few investigations on the systematic study of the preparation of spinel powder in combination with its sintering properties and catalytic properties.In this treatise,we mainly focused on the issue of controlling the synthesis of the ultrafine spinel powder.Based on the synthetic approach of controlled hydrolysis on bimetallic alkoxide,the synthetic parameters,including magnesium-aluminum ratio and others,which affect the active structure of magnesium aluminate spinel powder and the activity index,were systematically studied.Furthermore,the effect of active magnesium-aluminum spinel powder on the high density and sintering properties of sintered ceramic was further studied.Moreover,the dispersion and stability of the active components of PtNi alloys were investigated based on the active spinel composition and lattice parameters.The relationship between the structure of the catalyst and the catalytic performance was discussed on the catalytic performance for the oxidation of benzene.The main results of the researches are as follows:(1)Synthesis of ultra-fine magnesium aluminum spinelInitially,a bimetal alkoxide was fabricated by the reaction of magnesium and aluminum with monohydric alcohol.Then,the hydrolysis process of the bimetal alkoxide is controlled by tuning the synthetic parameters(typically refers to the type of organic alcohol,ammonia concentration,the dispersant,hydrolysis time and temperature,alkoxide concentration,drying process,calcination temperature,co-solvent,etc.).As a result,non-stoichiometric ratio of magnesium-aluminum spinel of MgO·nAl2O3(1.0?n?2.0)with single-phase spinel structure can be prepared at the calcinated temperature(1200?).The powder size is measured to be 12±5?m.The phase-forming temperature is reduced by 300? compared to that(1500?)for the powder prepared with conventional non-metering ratio.This result is closely related to the high activity of the powder and has important significance in researches and applications.(2)Sintering of high-density magnesium-aluminum spinel for ceramicsBy a combination of dry-pressing and vacuum-sintering techniques and using non-stoichiometric ratio of MgO·nAl2O3 spinel as raw material,several parameters,namely the powder particle size,sintering atmosphere,co-solvent,sintering temperature and holding time,were considered on the effect of the compactness of magnesium aluminate spinel for ceramics.As investigated,the ceramic sample was obtained by dry-pressing MgO·1.5Al2O3 powder(particle size of 4.81?m)especially at the state of 18 MPa in a one-way dry pressure and further calcinated at 1600? in 1×10-3Pa for 3 hours.Such acquired ceramic sample displays the highest degree of densification with the relative density of 98.12%,which is similar to the the cold isostatic pressing process.The results show that the sintering activity of MgO·1.5Al2O3 powder can be still well obtained under dry pressing conditions.It opens up a new avenue for dry pressing sintered spinel polycrystalline transparent ceramics.(3)Study on highly efficient catalysts of spinels supported platinum-nickel over catalytic performanceA series of PtxNiy/MgO·nAl2O3 catalysts were prepared by using various spinels with magnesium-aluminum ratios for catalytic performance of benzene oxidation.As a consequence,it demonstrates that the T50%of the PtNi1.5/MgO·1.5Al2O3 catalyst is 162?,which is?40? lower than that of the PtNi1.5/MgO·Al2O3 catalyst.The theoretical calculation of Pt2Ni4/MgAl2nO3n+1(110)model indicates that the active center of Pt2Ni4is dependent on the spinel lattice parameter of MgAl2nO3n+1.Compared with other models,the structure of Pt2Ni4 on the MgAl3O5.5(110)surface,of which the adsorption strength is moderate.Both the adsorption of benzene molecules and the desorption of products are associated with the highly efficient activity.The catalytic temperature is reduced from 204? to 162? of the conventional spinel carrier,with a drop of about 40?.The deep insights provides important reference value for the design and synthesis of highly efficient bimetallic alloy catalysts.