γ-Alumina: The Essential and Unexpected Role of Water for the Structure, Stability, and Reactivity of Defect Sites

Combining experiments and DFT calculations, we show that tricoordinate Al-III Lewis acid sites, which are present as metastable species exclusively on the major (110) termination of gamma- and delta-Al2O3 particles, correspond to the defect sites, which are held responsible for the unique properties of activated (thermally pretreated) alumina. These defects are, in fact, largely responsible for the adsorption of N-2, and the splitting of CH4 and H-2. In contrast, five-coordinate Al surface sites of the minor (100) termination cannot account for the observed reactivity. The Al-III sites, which are formed upon partial dehydroxylation of the surface (the optimal pretreatment temperature being 700 degrees C for all probes), can coordinate N-2 selectively. In combination with specific O atoms, they form extremely reactive Al,O Lewis acid-base pairs that trigger the low-temperature heterolytic splitting of CH4 and H-2 to yield Al-CH3 and Al-H species, respectively. H2 is found overall more reactive than CH4 because of its higher acidity, hence it also reacts on four-coordinate sites of the (110) termination. Water has the dual role of stabilizing the (110) termination and modifying (often increasing) both the Lewis acidity of the aluminum and the basicity of nearby oxygens, hence the high reactivity of partially dehyxdroxylated alumina surfaces. In addition, we demonstrate that the presence of water enhances the acidity of certain four-coordinate Al atoms, which leads to strong coordination of the CO molecule with a spectroscopic signature similar to that on Al-III sites, thus showing the limits of this widely used probe for the acidity of oxides. Overall, the dual role of water translates into optimal water coverage, and this probably explains why in many catalyst preparations, optimal pretreatment temperatures are typically observed in the activation step of alumina.