Improvement of thermal stability of Pd/γ -Al_2O_3 membranes by the dopant of Lanthana

The thermal stability of unsupported Pd/γ - Al2O3 membranes was studied by determining the changes of the pore structure (including average pore size,pore volume and BET surface area) of alumina gels, prepared by sol - gel method, after sintered at different temperatures ranging from 400 ℃ to 1 200 ℃ . The average pore size of the pure alumina membrane increased sharply after sintering at temperatures higher than 1 000℃. Addition of 3 % Lanthana can considerably stabilize the pore structure of the unsupported alumina membrane. The pore diameter for the La - doped membrane was stabilized within 7. 2 nm after sintering at 1 100℃ for 5 h. The substantial increase in the pore size for the Pd/γ - Al2O3 membrane at the sintering temperature of 1 000 ℃ to 1 200 ℃ was accompanied by the phase transformation from γ - Al2O3 to a - Al2O3. The addition of Lanthana can raise this phase transformation by more than 100 ℃ .