  Water dissociation represents one of the most important reactions in
catalysis, essential to the surface and nano sciences [e.g., Hass et al.,
Science, 1998, 282, 265-268; Brown et al., Science 2001, 294, 67-69; Bikondoa
et al., Nature 2005, 5, 189-192]. However, the dissociation mechanism on most
oxide surfaces is not well understood due to the experimental challenges of
preparing surface structures and characterizing reaction pathways. To remedy
this problem, we propose the metal organic framework MOF-74 as an ideal model
system to study water reactions. Its crystalline structure is well
characterized; the metal oxide node mimics surfaces with exposed cations; and
it degrades in water. Combining in situ IR spectroscopy and first-principles
calculations, we explored the MOF-74/water interaction as a function of vapor
pressure and temperature. Here, we show that, while adsorption is reversible
below the water condensation pressure (~19.7 Torr) at room temperature, a
reaction takes place at ~150 centigrades even at low water vapor pressures.
This important finding is unambiguously demonstrated by a clear spectroscopic
signature for the direct reaction using D2O, which is not present using H2O due
to strong phonon coupling. Specifically, a sharp absorption band appears at 970
cm-1 when D2O is introduced at above 150 centigrades, which we attribute to an
O-D bending vibration on the phenolate linker. Although H2O undergoes a similar
dissociation reaction, the corresponding O-H mode is too strongly coupled to
MOF vibrations to detect. In contrast, the O-D mode falls in the phonon gap of
the MOF and remains localized.
