First-principles study towards the reactivity of the Pd(111) surface with low Zn deposition

Methanol steam reforming (MSR) is an important means to produce hydrogen. While metal Pd shows no selectivity to MSR, PdZn alloy exhibits both high selectivity and activity towards this process. Recently a high temperature desorption peak of formaldehyde is observed when methanol is dosed onto Pd(111) surfaces on which 0.03-0.06 monolayer Zn is deposited. Strikingly such surface which is predominated by Pd atoms was suspected to be active for MSR. To determine the structure on which the high desorption peak is observed and its performance to MSR, we studied adsorption and dehydrogenation of formaldehyde on various models. It is demonstrated that the high desorption peak of CH2O may originate from the supported surface clusters. The calculated energy barriers of CH2O dehydrogenation show that while formaldehyde can decompose easily into formyl on the supported PdZn and Pd-2 clusters, this process is kinetically difficult on the surface Zn-3 clusters. It is further revealed that formation of dioxymethylene, the proposed precursor for CO2 production, from formaldehyde and oxygen is feasible on the surface Zn cluster. Based on these calculations we predict that compared with 1:1 PdZn alloy, the activity of the Zn clusters to MSR is lower, though its selectivity may be higher. (C) 2011 American Institute of Physics. [doi:10.1063/1.3587136]