Article,
The effect of gold particle size on AuAu bond length and reactivity toward oxygen in supported catalysts
J. Catal., 240 (2): 222 - 234 (2006)
DOI: DOI: 10.1016/j.jcat.2006.04.004
Abstract
Au catalysts with different metallic particle sizes and supported
on silica, alumina, titania, zirconia, ceria, and niobia were prepared,
and the reduced catalysts were characterized by EXAFS spectroscopy.
As the AuAu coordination number decreased, the interatomic bond length
decreased. The AuAu bond length contraction appears to be independent
of the support type. A correlation between the dispersion of Pt catalysts
determined by hydrogen chemisorption and the EXAFS PtPt coordination
number was established and used to determine the dispersion of fully
reduced Au catalysts. In addition, the Au particle size was estimated
using a literature correlation of the EXAFS coordination number.
For particles larger than about 40 �, there was little change in
the metallic bond length, whereas in catalysts with gold particles
smaller than 30 �, the AuAu distance decreased with decreasing particle
size, with a maximum contraction of about 0.15 �. Decreasing particle
size also brought a decrease in the intensity of the white line of
the XANES spectrum. Both the decrease in bond distance and white
line intensity were consistent with an increase in the d-electron
density of Au atoms in very small particles. Au particles smaller
than about 30 � were also reactive to air, leading to oxidation of
up to 15% of the atoms of the gold particles, depending on the size;
larger particles were not oxidized. These oxidized Au atoms in small
particles are suggested to be active for CO oxidation.
