Density Functional Investigations of Methanol Dehydrogenation on Pd-Zn Surface Alloy

Methanol dehydrogenation on Pd (1 1 1) and various Pd-Zn surface alloy films supported on Pd (1 1 1) have been investigated using density functional method in combination with periodic slab models. Calculations show that compared to Pd(1 1 1) the interaction between CH3O and the films is enhanced, whereas that for CH2O and CHO is weakened. Zn in top layer facilitates the CH3O stability. At variance, the subsurface Zn reduces the interaction of CH2O and CHO with the substrate significantly. Addition of Zn promotes the O-H breaking of CH3OH and the dehydrogenation of CHO but hinders the dehydrogenation of CH3O and CH2O. Comparison shows that the third-layer Zn atoms have essentially no effect on the reactions. Our calculations demonstrate that the experimentally observed 360 K desorption peak cannot be originated from CH2O adsorbed at flat Pd-Zn alloy surfaces, and it is very likely that CH2O combines preferentially with some species before decomposing, into CHO during methanol steam reforming if CH2O is an intermediate. Finally, we show that the newly proposed relationship between the energy of the initial states and transition states exhibits better correlation than the classical BEP relation.