Interaction of water with the (1x1) and (2x1) surfaces of α-Fe2O3(012)

The interaction of water with the (1x1) and (2x1) surfaces of alpha-Fe2O3(012) was examined with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS), low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS) in the temperature range between 100 and 950 K. The (1x1) surface is fully oxidized and has a bulk-like concentration and structure of cation and anion sites. After vacuum annealing at 950 K a (2 x 1) pattern is observed in LEED. Although the structure of the (2 x 1) surface is not fully understood, it possesses a greater surface concentration of cation sites than the (1 x 1) surface, some of which are probably reduced. H2O adsorbs dissociatively on both surfaces as evidenced by KREELS losses at 3625 and 960 cm(-1) due to the stretching and bending modes of terminal hydroxyl groups. These losses shift as expected for D2O. Bridging hydroxyls are also formed by proton transfer to bridging oxygen anion sites from dissociating water molecules, but have a poorly resolved O-H stretch at 3400 cm(-1) suggesting they are involved in hydrogen-bonding interactions. Further evidence for water dissociation on both surfaces comes from isotopic scrambling of oxygen between these hydroxyls and the O-18-enriched surfaces. Although both surfaces dissociate water, the structural differences between the (1 x 1) and (2x1) surfaces result in different ratios of molecular-to-dissociative water. Terminal hydroxyls occupy roughly 6x10(14) sites cm(-2) on the (1 x 1) surface, but only about 4.5 x 10(14) sites cm(-2) on the (2 x 1) surface. The balance of available cation sites bind molecular water that evolves in TPD below 300 K from either surface. This molecular water is readily detected in both HREELS and SSIMS. The surface structure also influences the hydroxyl recombinative desorption kinetics. Terminal and bridging hydroxyls recombine to liberate water in TPD at 350 K from the (1 x 1) surface and at 405 K from the (2 x 1) surface. The recombinative desorption state of water at 350 K from the (1 x 1) surface exhibits first-order desorption kinetics with an activation energy of about 120 kJ mol(-1) and a pre-exponential of 1 x 10(17) s(-1). The first-order behavior and the high pre-exponential suggests that recombinative desorption from the (1 x 1 ) surface involves pairing of bridging and terminal hydroxyl groups. In contrast, recombinative desorption from the (2 x 1) surface is pseudo-zeroth order in appearance suggesting that hydroxyls are bound in one-dimensional arrays with desorption occurring preferentially at the ends of each array. (C) 1998 Published by Elsevier Science B.V. All rights reserved.