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

In situ X-ray absorption spectroscopy (XAS) measurements, at the Pt L 3 edge (XANES and EXAFS), were carried out on carbon-supported Pt n Mo and PtSn electrocatalysts in an electrochemical cell in 1 M HClO 4 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 H 2 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.