Electrocatalytic reduction of oxygen on metal nanoparticles in the absence and presence of interactions with metal-oxide supports

Abstract

A combined experimental and theoretical approach is reported with a goal to develop a deeper understanding of strong metal support interactions (SMSI) and particularly their role in the oxygen reduction reaction (ORR). This was accomplished by developing a well-defined experimental system that can almost perfectly be modeled using first principles theory. First, an electrocatalytic model was constructed based on NP-mediated electron transfer (eT). That is, thin films of Al₂O₃ (2.5-5.7 nm) were deposited onto pyrolyzed photoresist film (PPF) electrodes resulting in passivation of faradaic current. Next, previously passivated eT was recovered by immobilizing dendrimer-encapsulated PtNPs (Pt DENs) (1.3 nm) onto the Al₂O₃ surface. The resulting PPF/Al₂O₃/Pt DENs electrodes were stable under various electrochemical conditions and showed an activity for the ORR. When DENs are immobilized onto solid supports, the dendrimers prevent direct contact between the encapsulated NPs and underlying supports. Consequently, in such systems, SMSI are not observed. Therefore, in the next step of the study we developed an ultraviolet/ozone (UV/O₃)-based procedure that allows for the removal of dendrimers, as confirmed by X-ray photoelectron spectroscopy (XPS), without affecting shape, size, or composition of the encapsulated NPs. Third, electrocatalytic activity of a PPF/Al₂O₃/G6-OH(Pt₅₅) electrode was studied before and after the UV/O₃ treatment using a novel microelectrochemical flow cell. The results indicated that direct interactions between Al₂O₃ and PtNPs do not affect the reaction pathway for the ORR. This was indeed anticipated because Al₂O₃ is a non-reducible oxide and will be used in the future studies as a control metal-oxide support. Finally, we switched to a theory-first approach in which density function theory (DFT) was used to predict catalysts having desired properties based on the previously discussed experimental model. Based on the results of the DFT calculations, a PPF/SnO [subscript x] (x = 1.9 or 2.0)/Au₁₄₇ DEN system was studied before and after the removal of dendrimers. Experimental results indicated that improvements in activity for the ORR were observed when Au₁₄₇ NPs interacted directly with SnO [subscript x] supports. Moreover, XPS studies showed that the observed catalytic enhancements were due to eT from surface oxygen vacancies in SnO [subscript x] to Au₁₄₇ NPs. These experimental results agreed with the theoretical predictions.