Pore diameter effects on thermal stability of lanthanum modified γ-Al2O3

To investigate the impact of pore diameter on the thermal stability of La-doped transition aluminas, we examined three γ-Al 2 O 3 materials with distinct pore diameters, both with and without La doping. Via material characterizations including N 2 adsorption-desorption, X-ray diffraction (XRD), thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS), 27 Al and 1 H nuclear magnetic resonance (NMR), we discover that in the absence of La doping, the thermal stability of Al 2 O 3 decreases significantly with decreasing pore diameter. For La-doped Al 2 O 3 , the influence of pore diameter on thermal stability is mitigated at 1100 °C but re-emerges at 1200 °C. We propose that the La species that provide structure protection are highly, or even atomically dispersed on the alumina surface. La 2 O 3 or LaAlO 3 aggregates, if even present, are not the main La moieties that provide structure protection. The highly dispersed La species occupy defective sites on Al 2 O 3 , effectively slowing down phase transformation to α-Al 2 O 3 . At 1200 °C, La diffuses into the Al 2 O 3 bulk and as such, loses its protective function.