Characterization of Ni@Pt and Co@Pt overlayer catalysts using XAS studies

Abstract

The use of XAS studies on Co@Pt and Ni@Pt pseudomorphic overlayer catalysts along with a ethylene hydrogenation descriptor reaction have shown that the d-band of Pt can be effected by the parent metal that it is overlayed on. Pt bond distances were found to be shorter than pure Pt when atop the base Ni and Co indicating that the base metal was exerting compressive strain on the Pt and, for two reasons, causing the Pt d-band to broaden and therefore shift downwards in energy. First, the compressive strain alone has been shown to broaden the d-band of larger atoms atop of smaller ones (i.e. Pt atop Ni) and in the case of the Co@Pt sample Co has one less valence electron then Ni and Pt. This allows for more, than Ni and Pt, of a synergistic interaction between Co and Pt in the overlayer samples resulting in even larger Pt d-bandwidth broadening and shifting. This XAS data was confirmed via reactivity studies as the overlayer samples of both Ni and Co based catalysts had lower reactivity than the pure Pt samples.

Particle Size or Electronic Effect? An XAS Study of Re@Pd Overlayer Catalysts

“…The Re@Pd TD sample is slightly higher in Re white line intensity meaning its d-band has shifted less upward than the SD d-band. The same phenomenon with the parent metal while line intensity in the TD sample shown here is also reported by Morris et al for TD samples where the parent metal was Co or Ni instead of Re [30]. The TD sample is closer in absorption to the bulk Re parent and the Re in the Re-Pd bimetallic catalysts.…”

“…4, indicates that a bimetallic, non-overlayer, system of Re and Pd is present. Morris et al via identical data analysis techniques presented here, observed the same lack of organization and overlayer formation in their bimetallic Ni-Pt, and Co-Pt samples as well [30]. The Re@Pd SD catalysts clearly have more Re in the bulk phase and more closely align with the Re parent catalyst, indicating isolated Re clusters whereas the Re-Pd Bimetallics are more disperse.…”

“…The Ni and Co K-edge white lines published by Morris et al exhibit a similar upward shift to the Pd K-edge white line in Fig. 5 [30]. In the Ni and Co supported Pt overlayer systems studied by Morris et al the Pt d-band was shown to have moved downward via L IIIedge XAS, therefore the d-band of the Ni and Co must have shifted upward [29,30].…”

“…The Re d-band shifts upwards in energy as a result of the Pd d-band shifting downwards in energy. Morris et al saw the same interactions between Ni and Co with Pt overlayers [30]. The corollary is shown in Fig.…”

“…5 [30]. In the Ni and Co supported Pt overlayer systems studied by Morris et al the Pt d-band was shown to have moved downward via L IIIedge XAS, therefore the d-band of the Ni and Co must have shifted upward [29,30]. The upward shift of the Ni and Co d-band was noticeable in the K-edge XAS because the d and p bands are slightly correlated [10].…”