Structural and electronic properties of a surface alloy of Pd and Cu on Cu{001}

The Catalytic Reactivity of Alloys; Ethanol and Formic Acid Decomposition on Cu-Pd(110)

The effect of alloying Cu and Pd on the reactivity pattern for formic acid and for ethanol has been examined. The electronic structure of the material is strongly affected by the alloying, with the d-band lowered in energy and filled, compared with Pd alone. Hence the reactivity would be expected to be strongly affected by the alloying. This appears to be the case for formic acid decomposition, whose decomposition temperature in temperature-programmed desorption is shifted by alloying and is between the temperatures for the individual components (at 350 K, compared with 250 and 470 K for Pd and Cu, respectively). However, when a different molecule is chosen as the probe of surface reactivity, namely, ethanol, we come to a very different conclusion. Here the individual reactivity patterns for the two elemental components of the alloy are seen, namely, dehydrogenation on the Cu (to produce acetaldehyde) and decarbonylation on Pd (to methane and CO). There are effects of alloying on destabilizing the former pathway and stabilizing the latter, but the major conclusion from this work is that it is not average electronic structure that dictates reactivity but the individual atomic nature of the surface components. Only monodentate adsorbates truly probe this behavior.

Diffusion kinetics in the Pd/Cu(001) surface alloy

We use atom-tracking scanning tunneling microscopy to study the diffusion of Pd in the Pd/Cu(001) surface alloy as a function of temperature. By following the motion of individual Pd atoms incorporated in the surface, we show that Pd diffuses by a vacancy-exchange mechanism. We measure an activation energy for the diffusion of incorporated Pd atoms of 0.88 eV, which is in good agreement with our ab initio calculated energy of 0.94 eV.