EXAFS as a tool for catalyst characterization: a review of the ... - INT

where s(κ i ) is the standard deviation given by
(19)
The quality of the fit can also be expressed by a merit factor defined
as (Faudon et at., 1993)
(20)
Evidence of Alloy Formation of Graphite-Supported Palladium-
Cobalt Catalysts
It has been reported that the addition of a noble metal to a supported
cobalt catalyst creates important changes in the selectivity of the CO
+ H 2 reaction (Noronha et al., 1996; Idriss et al., 1992). In general,
these results are attributed to alloy formation. However, no clear
evidence of alloying is presented.
Recently, EXAFS was used to obtain both further evidence of Pd-Co
interaction and the mean composition of bimetallic particles
(Noronha, 1994). Pd/G (2.26 wt.%), Co/G (3.61 wt.%) and
Pd 16 Co 84 /G (3.37 Pd wt.% and 9.96 Co wt.%) catalysts were
prepared by the incipient wetness impregnation of the graphite.
As shown in Figure 4, the radial distribution function (RDF) of the
bimetallic catalyst is different from the RDF of both Pd/G catalyst and
reference sample (Pd foil). The first coordination shell consists of
palladium atoms alone in the Pd/G and in the reference. On the other
hand, data analysis revealed the presence of both Pd and Co atoms in
the first coordination shell of Pd (Table 1). The local atomic
concentration of palladium, obtained by the n 1 /n 1 +n 2 ratio, was
61.5% suggesting that the first shell around Pd atoms is clearly
palladium-enriched.