Water adsorption and dissociation on BeO(001) and (100) surfaces

Abstract

Plateaus in water adsorption isotherms on hydroxylated BeO surfaces suggest significant differences between the hydroxylated (1 0 0) and (0 0 1) surface structures and reactivities. Density functional theory structures and energies clarify these differences. Using relaxed surface energies, a Wulff construction yields a prism crystal shape exposing long (1 0 0) sides and much smaller (0 0 1) faces. This is consistent with the BeO prisms observed when beryllium metal is oxidized. A water oxygen atom binds to a single surface beryllium ion in the preferred adsorption geometry on either surface. The water oxygen/beryllium bonding is stronger on the surface with greater beryllium atom exposure, namely the less-stable (0 0 1) surface. Water/beryllium coordination facilitates water dissociation. On the (0 0 1) surface, the dissociation products are a hydroxide bridging two beryllium ions and a metal-coordinated hydride with some surface charge depletion. On the (1 0 0) surface, water dissociates into a hydroxide ligating a Be atom and a proton coordinated to a surface oxygen but the lowest energy water state on the (1 0 0) surface is the undissociated metal-coordinated water. The (1 0 0) fully hydroxylated surface structure has a hydrogen bonding network which facilitates rapid proton shuffling within the network. The corresponding (0 0 1) hydroxylated surface is fairly open and lacks internal hydrogen bonding. This supports previous experimental interpretations of the step in water adsorption isotherms. Further, when the (1 0 0) surface is heated to 1000 K, hydroxides and protons associate and water desorbs. The more open (0 0 1) hydroxylated surface is stable at 1000 K. This is consistent with the experimental disappearance of the isotherm step when heating to 973 K.