EXAFS as a tool for catalyst characterization: a review of the ... - INT
EXAFS as a tool for catalyst characterization: a review of the ... - INT 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.
Figure 4: Amplitude of the Radial Distribution Function at the Pd K-edge for palladium foil (solid line) and Pd/G (dotted line) and Pd 16 Co 84 /G (dashed line). Table 1: EXAFS parameters at the Pd K-edge Pd - Pd pair Pd - Co pair Sample n 1 R 1 (Å) σ 2 (Å 2 ) n 2 R 2 (Å) σ 2 (Å 2 ) Q (%) n 1 +n 2 Pd foil 12.0 2.75 - - - - - - Pd/G 9.1 2.74 0.0000 - - - 5 - Pd 16 Co 84 /G 6.7 2.73 0.0310 4.2 2.61 0.0050 8 10.9 The RDF of the bimetallic catalyst is not so different from the RDF of both Co/G catalyst and reference (Figure 5). This suggests that the environment of cobalt atoms in the Pd 16 Co 84 /G and in the reference sample is similar. However, since a fairly good fit cannot be obtained only with Co atoms in the first coordination sphere of cobalt, it was
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Figure 4: Amplitude <strong>of</strong> <strong>the</strong> Radial Distribution Function at <strong>the</strong> Pd K-edge <strong>for</strong><br />
palladium foil (solid line) and Pd/G (dotted line) and Pd 16 Co 84 /G (d<strong>as</strong>hed line).<br />
Table 1: <strong>EXAFS</strong> parameters at <strong>the</strong> Pd K-edge<br />
Pd - Pd pair<br />
Pd - Co pair<br />
Sample n 1 R 1 (Å) σ 2 (Å 2 ) n 2 R 2 (Å) σ 2 (Å 2 ) Q (%) n 1 +n 2<br />
Pd foil 12.0 2.75 - - - - - -<br />
Pd/G 9.1 2.74 0.0000 - - - 5 -<br />
Pd 16 Co 84 /G 6.7 2.73 0.0310 4.2 2.61 0.0050 8 10.9<br />
The RDF <strong>of</strong> <strong>the</strong> bimetallic <strong>catalyst</strong> is not so different from <strong>the</strong> RDF <strong>of</strong><br />
both Co/G <strong>catalyst</strong> and reference (Figure 5). This suggests that <strong>the</strong><br />
environment <strong>of</strong> cobalt atoms in <strong>the</strong> Pd 16 Co 84 /G and in <strong>the</strong> reference<br />
sample is similar. However, since a fairly good fit cannot be obtained<br />
only with Co atoms in <strong>the</strong> first coordination sphere <strong>of</strong> cobalt, it w<strong>as</strong>
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