Page 173 - Principles of Catalyst Development

CATALYST CHARACTERIZATION 161
decays into the hole releasing enough energy to eject another electron. This
is the Auger electron, with a kinetic energy characteristic of its atomic or
molecular energy level, i.e., representative of the atom. All elements except
H and He are detectable to a depth depending on the surface and incident
energy. Probing from 0.5-1.0 nm is possible. Certainly, measurements of
up to ten layers can be controlled. Quantitative analysis and chemical
(environmental) information is not as reliable as with XPS (see below) and
the electron beam can induce damage. Nevertheless, it is a rapid method
for probing surfaces, which, when combined with Ar+ bombardment to
"scrape" surface layers, results in information on depth profilesY25)

7.4.1.6. X-Ray Photon Spectroscopy (XPS)
In x-ray photon spectroscopy (X PS), a soft x-ray photon (170 e V) ejects
any electron with less binding energy. When properly detected and counted,
the electrons give a spectrum characteristic of the element. Excellent sensitiv-
ity is possible with probing depths of 1-20 layers. X-ray photon spectroscopy
spectra are sensitive to the chemical environment, so that analysis of the
"chemical shift" leads to information on structure and differences in sur-
roundings are detected. The technique has poor lateral resolution, with
sample areas from 0.5 to tOO mm , and profiling with Ar+ is not as easy as
2
with AES. (226 )

7.4.1.'7. Ultraviolet Photon Spectroscopy (UPS)

Ultraviolet photon spectroscopy is identical to XPS, e:xcept that lower-
energy photons (30 eV) are used, limiting analysis to valence electrons. This
is useful in the analysis of adsorbed states but is no advantage over XPS
for surface characterization.

7.4.2. Structure

Identification of surface phase structure is best accomplished with
either SEM or TEM, which provide facilities for electron diffraction of
selected surface regions. An individual crystallite or surface region is selec-
ted, and its structure analyzed in the same way as bulk structure is determined
with XRD.
This is complementary to XPS and UPS spectra that reveal compound
types through chemical shifts, usually with "fingerprinting." Auger electron
spectroscopy (AES) is also used but interpretation is complicated.
An example is the study of sulfided CoMo! AI 20, hydrodesulfurization
catalysts, showing the presence of discrete crystallites of COyS g and MaS},