Structures and acid-base properties of La/Al2O3 -: Role of La addition to enhance thermal stability of γ-Al2O3

Structures and acid-base properties of La/Al2O3 with different La loadings and calcination temperatures were examined by surface-area measurements, catalyses, IR, XRD, and La K-edge XAFS spectroscopic techniques. Novel concepts for sintering mechanism of gamma-Al2O3 and the role of La addition to enhance the thermal stability were proposed. La atoms on gamma-Al2O3 are present in a highly dispersed form when the loading amount is less than 0.2 mmol g(Al2O3)(-1), and form aggregates above this concentration. The isolated La species are stable without structural transformation below 1273 K, but aggregated species change their structure to LaAlO3 perovskite at 1073 K. The addition of La atom to gamma-Al2O3 poisons strong Lewis acid sites on gamma-Al2O3, and converts them to new Lewis acid sites with medium strength. The titration of strong Lewis acid sites on Al2O3 is the key point to enhance thermal stability of alumina-based catalyst.