XPS and XAFS Pt L_2,3-Edge Studies of Dispersed Metallic Pt and PtSn Clusters on SiO₂ Obtained by Organometallic Synthesis:  Structural and Electronic Characteristics

Abstract

In this work, well-defined silica-supported Pt and PtSn catalysts were prepared by surface organometallic reactions and were characterized by EXAFS-XANES at the Pt L2,3 edge and XPS of the Pt 4f and Sn 3d levels. These catalysts were tested in the catalytic hydrogenation of cinnamaldehyde in the liquid phase. XPS results showed that in PtSn−OM* (not having butyl groups anchored on the surface), tin was present as Sn(0) adatoms “decorating” the metallic surface, isolating Pt atoms; in the PtSn−OM catalyst (Bu groups remain grafted on the surface), tin was found in the form of Sn(0) and Sn(II, IV) in similar proportions. EXAFS experiments do not provide evidence of the existence of PtSn alloys in any of these systems. In the bimetallic PtSn−BM systems, it was possible to observe by XPS that tin was found in the form of Sn(0) and Sn(II, IV). EXAFS experiments, in this case, allowed us to demonstrate the existence of a PtSn alloy diluting metallic Pt atoms. In tin-modified systems, when a fraction of ionic tin is present, the activation of the CO group of the cinnamaldehyde is favored, increasing the selectivity to UOL. An increase in the number of Pt 5d holes measured by Pt-L2,3 XANES could be indicating as a d → s, p rehybridization process in the PtSn 3D small nanoclusters present in PtSn−BM catalysts.

Transition metal-based bimetallic catalysts for the chemoselective hydrogenation of furfuraldehyde

“…43 The presence of metallic tin in bimetallic catalysts is an indication of the existence of bimetallic PtSn phases (even alloys), as has been readily assessed by EXAFS analysis performed on analogous systems to the ones employed here. 44 The PtSn 0.8 catalyst showed a higher Sn 0 /Pt ratio than the PtSn 0.2 system, a result that could be assigned to the following effects: (i) dilution of platinum by metallic tin, due to the formation of SnPt phases, (ii) coverage of superficial metal particles by oxidized tin species. The catalytic surface may be depicted by the coexistence of SnPt phases (maybe alloys), ionic tin located at the platinum-support interface and some segregated metallic platinum.…”

“…43 The presence of metallic tin in bimetallic catalysts is an indication of the existence of bimetallic PtSn phases (even alloys), as has been readily assessed by EXAFS analysis performed on analogous systems to the ones employed here. 44 The PtSn 0.8 catalyst showed a higher Sn 0 /Pt ratio than the PtSn 0.2 system, a result that could be assigned to the following effects: (i) dilution of platinum by metallic tin, due to the formation Vol. 21, No.…”

Adsorbate-Induced Structural Changes in 1–3 nm Platinum Nanoparticles

“…[45][46] Adsorbates, such as CO and H 2 , would increase the coordination of Pt atoms thereby relaxing the contraction. This hypothesis is indeed supported by our PDF results.…”

Carbon-supported PtSn Catalysts: Characterization and Catalytic Properties

“…The data obtained for the PtSn/Al2O3(CI) catalyst are also According to several authors 49) 52) , the Pt _ Pt bond contributes to the larger distance range of the FT-EXAFS profile (at about 2.77 Å) whereas the contribution of the Pt _ Sn bond appears at slightly lower distances (depending on the formed PtSn species). Ramallo et al 16) also report that the Pt _ Sn distance is slightly lower than the Pt _ Pt distance (2.68 versus 2.71 Å). However, in the work of Caballero et al 20) a peak at 2.68 Å was assigned to the Pt _ Pt bond and a peak at 2.93 Å to the Pt _ Sn bond.…”

“…There is also disagreement about the assignation of the peak at lower distances (< 2.10 Å). This peak has been assigned to the interaction of platinum clusters with the support via Pt _ O _ Sn 2+ species 20) , not clearly defined 49) , explained as an interference phenomenon between the Pt _ Pt and Pt _ Sn bonds 21), 52) , and as a Pt _ O interaction at 1.99 Å related to interaction with the SiO2 support 16) . These discrepancies reveal the difficulty in accurate quantification of the EXAFS oscillations and show that the use of appropriate references and complementary techniques is necessary for the correct assessment of bond distances and co-ordination numbers.…”