Invited Paper SS1-MoA3
Dissociation of Water on Modified Transition Metal Surfaces

Monday, October 15, 2007, 2:40 pm, Room 608

Water dissociation on surfaces of late transition metals has been a topic of intense discussion in the last few years.^1-4 It is remarkable in this context that different members of the Pt group, which normally show very similar chemical behavior, cause water to react in very different ways. This could be caused by small differences in the surface lattices, affecting the (mis)match with ice-like water layers, or by electronic differences, affecting the balance between inter-molecular hydrogen bonds and water-substrate bonds. Atomic or molecular coadsorbates are convenient means of modifying both types of bonds and were used in a systematic study to address this question. We compare the reactivity of water on clean and O-modified Pd{111}, Ir{111}, Pt{111} and Ru{0001} using LEED, TPD, high resolution XPS and NEXAFS. All four surfaces have the same surface symmetry and very similar lattice constants. Except for Ru{0001}, no dissociation is observed on the clean surfaces, even after prolonged irradiation with X-rays. Small amounts of oxygen, however, cause water to dissociate, whereas high coverages (> 0.25ML) tend to leave water intact with more stable surface bonds than for the clean surfaces.⁵ The exception here is Pt{111}, where water dissociates for all oxygen coverages up to saturation. Molecular co-adsorbates, such as CO and methanol tend to inhibit water dissociation. Possible geometrical effects were investigated using Pt{111}, {110} and {531}, these surfaces represent a series with increasing atomic roughness and coordination numbers decreasing from 9 to 6. We could not find any increased reactivity of low-coordinated Pt atoms with respect to the dissociation of water only an increase in the water-substrate bond strength. Small amounts of oxygen promote water dissociation on all three surfaces; a high coverage (exceeding half the saturation coverage) prevents dissociation on Pt{531} and Pt{110}, but not on Pt{111}. These results indicate that dissociation of water on Pt group surfaces depends more sensitively on electronic modifications, as induced by oxygen coadsorption, than on the surface geometry.

¹ A. Thiel, T.E.Madey, Surf. Sci. Rep. 7 (1987) 211; M.A. Henderson, Surf. Sci. Rep. 46 (2002) 1.
² P.J. Feibelman, Science 295 (2002) 58.
³ J. Weissenrieder, et al. PRL 93 (2004) 196102;
⁴ K. Andersson, et al. PRL 93 (2004) 196101.
⁵ M.J. Gladys, et al. CPL 414 (2005) 311.