Effects of thermal treatment on the microstructures of sol-gel synthesized La3+ + PEG doped alumina

Abstract

Active porous alumina was prepared via a sol-gel method and subjected to thermal treatment in the temperature range 500 - 1200°C. The effect of calcinations temperature, polyethylen glycol combined with La3+, added to the boehmite sol, on activated alumina surface area, pore size, pore distribution and surface fractal dimension was studied. The N2 adsorption/desorption analyses of samples was performed to determine the adsorption isotherms, hysteresis and the textural properties. The adding of La3+ to boehmite sol inhibits the surface area loss of produced aluminas during its calcinations at high temperatures. Thermal treatment of La3+-doped alumina samples at temperature from 500 to 1200°C causes a considerable decrease of its specific surface area from 280 to 65 m2g-1, pore volume from 0,44 to 0,27 cm3g-1, increase of its pore size data from 3 to 6 nm and increase of its surface fractal dimension form 2,068 to 2,192. From behavior of surface fractal dimension of doped alumina samples can be concluded that there is no phase transformation to α-Al2O3 in all temperature range, i.e. addition of lanthanum to boehmite sol in atomic ratio of 0,03 La3+/Al3+ can raise phase transformation temperature to α-Al2O3 above 1200°C. .

Aktivni porozni aluminijumoksid, dobijen sol - gel metodom, je termički obrađen na povišenim temperaturama u intervalu od 500 do 1200°C. Ispitivan je uticaj temperature termičke obrade i dodatka polietilen glikola (PEG) i La3+-jona na specifičnu površinu, zapreminu pora i prečnik pora, kao i fraktalnu dimenziju površine aktiviranog aluminijum-oksida. Na osnovu podataka iz adsorpciono-desorpcionih izotermi azota određen je tip pora i izračunate su vrednosti osnovnih parametara teksturalnih svojstava uzoraka. Porast temperature termičke obrade prouzrokovao je smanjenje specifične površine sa 280 na 65 m2g-1, zapremine pora sa 0,45 na 0,27 cm3g-1, porast preovlađujućeg prečnika pora od 3 do 6 nm i porast fraktalne dimenzije površine od 2,068 do 2,192. Na osnovu dobijenih rezultata rendgenostrukturne analize može se zaključiti da u ispitivanom temperaturnom intervalu ne dolazi do fazne transformacije u α-Al2O3 i da dodatak lantana u odnosu 0,03 molLa3+/ molAl3+ pomera faznu transformaciju aktiviranog aluminijum-oksida u α-Al2O3 iznad 1200°C. .