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Chapter 7 157
adsorbed at different temperatures can be plotted. Subsequently, the Clausius−Clapeyron
equation,
−Q st = RT 2 ( ∂lnp
∂T ) n
was used to calculated the isosteric heats of adsorption(-Qst) as a function of the amount
of H2 adsorbed (n). -Qst is a value represent for the average binding energy of an adorbate
at specific surface coverage.
7.1.9 X-ray absorption near edge structure (XANES)
Data acquisition and analysis for XANES were done at beam line BL8 of the synchrotron
radiation facility DELTA by W. Zhang with the assistance of R. Wagner (and A.
Schneemann).
Simplified theory behind XANES
XANES, also known as near edge X-ray absorption fine structure (NEXAFS), is a kind of
absorption spectroscopy showing the features in the X-ray absorption spectra (XAS) of
condensed matter because of the photoabsorption cross section for electronic transitions
from an atomic core level to final states in the energy region of 50-100 eV above the
selected atomic core level ionization energy. X-rays are ionizing electromagnetic radiation
which afford sufficient energy to excite a core electron of an atom to an excited state (an
empty below the ionization threshold), or to the continuum which is above the ionization
threshold. A core hole is the space that a core electron occupied before it absorbs an X-ray
photon and ejected from its core shell. Subsequently, another atomic electron drops down
in to fill the core hole and release energies either through Auger electron ejection or X-ray
Fluorescence.
When the energy of X-ray radiation is scanning through the binding energy region of a
core shell, there will be a sudden appearance of absorption increase, corresponding to
absorption of the X-ray photon by a specific type of core electrons (eg. 1s, 2s, 2p electrons,
etc.). This gives rise to a so-called absorption edge (K, L, M edge, etc.) in the XAS spectrum
due to its vertical appearance. The binding energies of different core electrons are distinct.
Atoms with a higher oxidation state need more energetic X-ray to excite its core level due