The Surface Chemistry of Methanol on Cu3Pd(111): Effects of Metal Alloying and Reaction with Hydrogen

Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) was used to study the adsorption and surface chemistry of methanol on a Cu3Pd(111) model surface. The composition and morphological properties of the pristine Cu3Pd(111) substrate were analyzed using a combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and low-energy ion scattering (LEIS). The results of ion scattering showed segregation of Pd toward the surface, with a Pd/Cu ratio close to 0.5, a larger value than the ratio of 0.33 expected for a Cu3Pd bimetallic structure. The surface of the alloy exhibited a good LEED pattern with 3-fold long-range periodicities. In STM, clusters of palladium with hexagonal arrays and Pd–Pd distances of 2.7–2.8 Å were detected. Bonding to copper perturbed the electronic properties of the atoms in the Pd clusters, shifting their 4d states toward higher binding energy with respect to the Fermi level. The valence band spectrum of Cu3Pd(111) exhibited a line shape that was very different from those displayed by Cu(111) or Pd(111). At low pressures, the adsorption of methanol on Cu3Pd(111) at 300 K mainly produced CH3O, CO, and CHx species. AP-XPS showed that most Pd atoms in the surface of the Cu3Pd(111) alloy interacted with the decomposition products of methanol. No significant changes were observed in the core levels of copper upon the adsorption and dissociation of methanol, suggesting that the molecule mainly interacted with Pd sites of the alloy. Reaction with hydrogen led to fast removal of CHx, C, and PdCx species from Cu3Pd(111) at moderate (<450 K) temperatures and prevented a CHx → C transformation. If the stability of adsorbed CH3O is used as a descriptor for the hydrogenation of CO2 to methanol, Cu3Pd should be a much better catalyst than monometallic palladium. This may be a consequence of electronic and ensemble effects in the alloy that moderate the reactivity of Pd sites.