Theoretical investigations on structural, electronic, adsorptive and catalytic properties of BiPdn (n = 2 - 20) bimetallic nanoclusters

Through electrochemical reduction, toxic air pollutants (such as nitric oxide (NO) and carbon monoxide (CO)) can be converted into less harmful gases N ₂ O and CO ₂. In this article, the geometric structural, electronic, adsorptive and catalytic properties of BiPd _n (n = 2-20) clusters are investigated by density functional theory (DFT) method so as to explore potential stable and efficient small-size catalysts. In general, the cluster configurations are cage structures with one Bi atom as the apex. BiPd _n (n = 6, 12 and 17) with the higher symmetry (C _5V) would be more difficult for electronic transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in view of their relatively larger HOMO-LUMO energy gaps E _g. The largest electronegativity (≈ 5.285 eV) and Mulliken charge population (≈ 0.097 e) of BiPd ₁₂ cluster may indicate the strongest electronic interaction among the clusters. The lowest adsorption energies E _ads of NO (≈ −0.378 eV) and CO (≈ −0.899 eV) molecules on BiPd ₁₂ and BiPd ₁₇ clusters, respectively, show the feasibility of both clusters for adsorption of NO or CO. The catalytic properties of oxidation of NO and CO molecules on BiPd ₁₂ are respectively calculated so as to evaluate the catalytic activities of this cluster for the corresponding reactions. NO molecule on BiPd ₁₂ could not be directly decomposed into O _ads and N _ads due to the high reaction energy barrier (≈ 1.207 eV). Unlikely, (NO) ₂ dimmer can act with Pd to form a five-membered ring through the dime mechanism, with the unstable (NO) ₂ on BiPd ₁₂ easily decomposed into a N ₂ O molecule and leaving an oxygen atom at the active site. This reaction needs to overcome the relatively low activation energy of 0.655 eV, and the exothermic reaction also means that this process can easily occur at room temperature. Then the remaining O _ads on BiPd ₁₂ can combine with CO to form CO ₂, with the reaction energy barrier of only 0.268 eV. It seems that BiPd ₁₂ cluster has the higher symmetry and stability as well as potentially high catalytic activity for electrochemical reduction of NO.