Characterization of lanthanum-modified γ-alumina by X-ray photoelectron spectroscopy and carbon dioxide absorption

Abstract

The chemical interaction of dispersed lanthanum with γ-alumina was investigated by combining X-ray photoelectron spectroscopy (XPS) and carbon dioxide absorption. In order to establish the saturation concentration of lanthanum dispersed in the alumina surface layer, a series of materials was formulated at lanthanum concentrations corresponding to La/Al atomic ratios in the range from 0.05 to 2.0. The samples were prepared by impregnation of γ-alumina with aqueous lanthanum nitrate solutions, followed by calcination in air at 925 °C. Upon exposure to carbon dioxide, particles of La 2O 3 reacted to form bulk La 2 (CO 3) 3, while lanthanum interacting with alumina did not. The reacted carbon dioxide was quantified by observing the carbonate in the C 1s spectra. The La/Al atomic ratio observed by XPS at low loadings from 0.05 to 0.15 was in good agreement with the bulk ratios. The carbon dioxide uptake remained constant at these loadings, while the La 3d 5/2 spectral envelope appeared approximately 1.8 eV higher than that expected for La 2O 3. This shift to higher binding energy and the agreement of theoretical and XPS measurements is consistent with a monolayer dispersion previously reported. As the La loading increased above La/Al =0.15, the ratio measured by XPS increased first slightly, and then substantially over the bulk La/Al ratio, while the amount of carbon dioxide absorbed increased at the same time. In addition, the O 1s, Al 2p and La 3d 5/2 spectral envelopes gradually shifted to lower binding energies. As the La/Al bulk ratio approached 2.0, the La 3d 5/2 and O 1s spectral envelopes were shifted to what is expected for La 2O 3, while the Al 2p peak shifted to a binding energy observed for LaAlO 3. The differences observed spectrally and quantitatively at low and high loadings imply a change in the coordination of lanthanum.