Contrast in Metal−Ligand Effects on Pt_nM Electrocatalysts with M Equal Ru vs Mo and Sn As Exhibited by in Situ XANES and EXAFS Measurements in Methanol

Abstract

In situ X-ray absorption spectroscopy (XAS) measurements, at the Pt L3 edge (XANES and EXAFS), were carried out on carbon-supported Pt n Mo and PtSn electrocatalysts in an electrochemical cell in 1 M HClO4 with 0.3 M methanol. The CO, OH, and H relative adsorbate coverages on Pt are determined as a function of the applied potential via the ΔXANES technique and compared with comparable data previously reported for Pt n Ru. The more reactive Sn and Mo atoms on the Pt surface form the oxide over the potentials of interest, while Ru has variable oxide content depending on Ru island size and potential. The strength of the electronic ligand effect appears to increase in the order Ru < MoO n < SnO n < RuO n , where the Pt−CO bond strength is found to decrease and the Pt−OH bond strength increase with ligand effect. In the Sn and Mo bimetallics, the ligand effect is found to be sufficiently strong to allow CO replacement by H2 at low potentials. These widely different ligand effects may provide a straightforward explanation for the previously observed anode behavior in fuel cells: Pt n Mo better in reformate but Pt n Ru better in methanol.

Operando XAS Techniques: Past, Present, and Future

“…Hence the resulting Dm line shapes reflect the chemistry, site symmetry, and magnitude of adsorbed species on the surface. It should be further noted that contributions in m coming from surface sites not available for adsorption (e.g., those in contact with the support or with neighboring particles, those covered by surface islands or just not in the region of the required electrochemical three-phase boundary) will cancel out when taking the difference, so the Dm technique can even give an indication of the accessible surface Pt sites as shown previously [63,64].…”

“…Assuming that both the Pt and M atom XAS data were measured, one can then obtain four coordination numbers, two at each edge, namely, the Pt-Pt, Pt-M, M-Pt, and M-M coordination numbers. These are the critical parameters that have been used to propose an average nanoparticle morphology [64,68] and given below for one case.…”

“…However, the sensitivity of the Dm XANES technique is sufficient to distinguish adsorbed atop OH species from n-fold O, which is critical in fuel cell work. Figure 17.7 also shows how the amplitude of the Dm signature increases with increasing O coverage, and for the most part we assume that the Dm magnitude is representative of relative adsorbate coverage, although we have discussed deviations from this previously [64,68].…”

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

“…The presence of adsorbates such as hydrogen or CO on metals has also been observed to shift the XANES edge location and intensity [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Hydrogen has been found consistently to broaden the L 3 edge and shift it to higher energy [27].…”

Improved electrocatalytic stability in ethanol oxidation by microwave-assisted selective deposition of SnO2 and Pt onto carbon

“…These changes in the electronic structure can lead to a more efficient oxidation of the intermediates or to weaker Pt-CO interactions with a similar outcome. 2,15,38 On the other hand, the increase of the CO-tolerance of Pt through the bi-functional mechanism arises from the ability of tin oxide to dissociate water molecules at lower potentials than platinum, forming (OH) ad species that provide the oxygen required for the oxidation of CO to CO 2 at the metal surface and freeing the active sites. 11,37 Furthermore, it has also been suggested that saturation of SnO 2 sites by H 2 O/OH weakens the interaction of water with Pt, making the metal active sites available for ethanol oxidation.…”