Effect of Particle Size and Adsorbates on the L3, L2 and L1 X-ray Absorption Near Edge Structure of Supported Pt Nanoparticles

Abstract

Pt nano-particles from about 1 to 10 nm have been prepared on silica, alkali-silica, alumina, silica-alumina, carbon and SBA-15 supports. EXAFS spectra of the reduced catalysts in He show a contraction of the Pt-Pt bond distance as particle size is decreased below 3 nm. The bond length decreased as much as 0.13 Å for 1 nm Pt particles. Adsorption of CO and H 2 lead to a increase in PtPt bond distance to that near Pt foil, e.g., 2.77 Å . In addition to changes in the Pt bond distance with size, as the particle size decreases below about 5 nm there is a shift in the XANES to higher energy at the L 3 edge, a decrease in intensity near the edge and an increase in intensity beyond the edge. We suggest these features correspond to effects of coordination (the decrease at the edge) and lattice contraction (the increase beyond the edge). At the L 2 edge, there are only small shifts to higher energy at the edge. However, beyond the edge, there are large increases in intensity with decreasing particle size. At the L 1 edge there are no changes in position or shape of the XANES spectra. Adsorption of CO and H 2 also lead to changes in the L 3 and L 2 edges, however, no changes are observed at the L 1 edge. Density Functional Theory and XANES calculations show that the trends in the experimental XANES can be explained in terms of the states available near the edge. Both CO and H 2 adsorption result in a depletion of states at the Fermi level but the creation of anti-bonding states above the Fermi level which give rise to intensity increases beyond the edge.Keywords Pt nanoparticles Á Bond length contraction Á Particle size effect in XANES spectra Á Particle size effect in Pt bond length Á Pt XANES Á EXAFS

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

“…As observed in previous studies, adsorption of H 2 and CO results in changes in the shape of 7 the Pt L 3 XANES spectra. 15,45 Figure 7 shows the Pt L 3 edge for 1-3 nm Pt/Al 2 O 3 with adsorbed H 2 and CO. The L 3 edge shifts to higher energy and the peaks broaden with adsorbed H 2 and CO.…”

“…13 A similar trend was also found 2 for supported precious metal nanoparticles. [14][15] A 5% contraction was observed for supported, 1 nm Au and Pt nanoparticles independent of the supports.…”

“…Our previous X-ray absorption near edge structure (XANES) study on Pt nanoparticles supported on different surfaces showed that the support effects with respect to lattice constants were minimal at constant Pt nanoparticle size. 15 Thus, in this work, we focused on 1-3 nm Pt nanoparticles supported on spherical alumina (Al 2 O 3 , Alfa-Aesar NanoDur™). This spherical alumina support facilitates transmission electron microscopy (TEM) measurements.…”

Bimetallic Pt–Pd/silica–alumina hydrotreating catalysts – Part I: Physicochemical characterization

“…The interatomic distances for the Pt-Pt, Pt-Pd, Pd-Pd, and Pd-Pt pairs showed generally a slight contraction (max. 1.5%) typical for small metal particles [34]. Regardless of the composition of the support, the total coordination of Pt, that is, sum of coordination numbers (CN Pt-Pt + CN Pt-Pd ), is around 11, whereas the total coordination of Pd (CN Pd-Pd + CN Pd-Pt ) is nearly 6.…”

Single-Atom Alloy Pd–Ag Catalyst for Selective Hydrogenation of Acrolein

“…An empirical relationship between the coordination number and particle size previously obtained from a series of platinum samples was used since both silver and platinum are fcc (face centered cubic) metals. 40 However, since there is no specific rule to choose the Debye-Waller factor(DWF), some error is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 10 introduced in our size estimation. This error in the fits was determined by fixing the difference in the DWF between the reference and the samples at 0.001 Å 2 .…”