The following note relates to the validity and feasibility of measurement
of very small crystallites/domains in a noble-metal-on-carbon catalytic
system.
Crystallography is cubic or disordered cubic (tetragonal?). Unit cell edge
is about 4 Angstroms. Multiple phases often are present: a) the metal; b)
an alloy with slightly different unit cell size. Using Scherrer method,
"crystallite size" ranges from 30-150 Angstroms. At the low end, when
multiple phase are present there is severe(!) overlap of the peaks,
especially the [111] and [200]. Higher orders of reflections are both weak
and even more severely overlapped. Transmission electron microscopy
measurements (TEM) show "crystallites" of drastically different sizes. It
is hypothesized the volume-percent of noble metal in the largest particles
adversely affects the catalytic properties of the system.
Q1. Using powder XRD, can meaningful information be obtained about the
populations of sizes in a specimen comprised of "crystallites" ranging from
20-200 Angstroms? Note that in addition to the distribution of broadening
due to crystallite size variations, there is also the probability of
variations in unit cell size caused by the presence of a second (or more)
alloy component(s). The latter condition manifests itself by a peak
centroid shift.
Q2. Is the measurement of a 30 Angstrom "crystallite size" meaningful? It
is accepted by practitioners of surface science techniques (ESCA, etc.)
that a "surface" is about 30 Angstroms deep.
Q3. If practical, what would be required to obtain both the data and the
software to process the data? Obtaining synchotron data is not an option
in this case. Software may run on either PC or Macintosh.
Please respond both to this group and directly to me at the address below.
Thank you.
Frank L. May - Research Investigator
Department of Chemistry-Benton 315
University of Missouri-St. Louis
8001 Natural Bridge Rd.
St. Louis, MO 63121
voice: (314) 516-5098
fax: (314) 516-5342
[EMAIL PROTECTED]
