Here we report that density functional theory (DFT) can be used to accurately predict how Au nanoparticle (NP) catalysts cooperate with SnO x ( x = 1.9 or 2.0) supports to carry out the oxygen reduction reaction (ORR). Specifically, dendrimers were used to encapsulate AuNPs and prevent their interactions with the underlying SnO x supports. After removal of the dendrimers, however, the AuNPs are brought into direct contact with the support and hence feel its effect. The results show that both SnO1.9 and SnO2.0 supports strongly enhance the electrocatalytic properties of AuNPs for the ORR. In the case of AuNP interaction with a SnO1.9 support, the number of electrons involved in the ORR ( neff) increases from 2.1 ± 0.2 to 2.9 ± 0.1 following removal of the dendrimers, indicating an increased preference for the desired four-electron product (water), while the overpotential decreases by 0.32 V. Similarly, direct interactions between AuNPs and a SnO2.0 support result in an increase in neff from 2.2 ± 0.1 to 3.1 ± 0.1 and a reduction of the overpotential by 0.28 V. These experimental results are in excellent agreement with the theoretically predicted onset potential shift of 0.30 V. According to the DFT calculations, the observed activity enhancements are attributed to the existence of anionic Au resulting from electron transfer from surface oxygen vacancies within the SnO x supports to the AuNPs. This theoretical finding was confirmed experimentally using X-ray photoelectron spectroscopy. Importantly, the calculations reported here were performed prior to the experiments. In other words, this study represents an unusual case of theory accurately predicting the electrocatalytic manifestation of strong metal support interactions.

Here, we report a new kind of microelectrochemical flow system that is well suited for studying electrode modifications, like thin films prepared by atomic layer deposition (ALD), that require substrates to have a two-dimensional form factor. The design provides a means for electrodes to be modified ex situ and then incorporated directly into the flow cell. The electrodes can be removed after testing and further modified or tested before being reincorporated into the flow cell. Using this cell, mass-transfer coefficients up to 0.011 cm/s and collection efficiencies up to 57 ± 10% have been achieved. Electrodes modified with an ultrathin layer of ALD Al2O3 and an overlayer of Pt dendrimer-encapsulated nanoparticles (DENs) have been incorporated into the flow cell and their electrocatalytic properties evaluated. Subsequently, the dendrimer was removed from the Pt DENs using a UV/O3 treatment, and this provided direct contact between the Al2O3 layer and the NPs. Finally, the product distribution for the oxygen reduction reaction (water vs H2O2) was evaluated in the presence and absence of Pt-Al2O3 support interactions.

We report that ultraviolet/ozone (UV/O3) treatment can be used to remove sixth-generation, hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers from dendrimer-encapsulated Pt nanoparticles (Pt DENs) previously immobilized onto a pyrolyzed photoresist film (PPF) electrode. Results from X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and electrochemical experiments indicate that removal of the dendrimer proceeds without changes to the size, shape, or electrocatalytic properties of the encapsulated nanoparticles. The UV/O3 treatment did not damage the PPF electrode. The electrocatalytic properties of the DENs before and after removal of the dendrimer were nearly identical.

Electrocatalytic oxygen reduction at carbon electrodes fully passivated by Al2O3 is reported. Specifically, pyrolyzed polymer film (PPF) electrodes were prepared and then coated with pinhole-free Al2O3 layers ranging in thickness from 2.5 to 5.7 nm. All of these ultrathin oxide film thicknesses completely passivated the PPF electrodes, resulting in no faradaic current for either inner-sphere or outer-sphere electrochemical reactions. The electrodes could, however, be reactivated by immobilizing Pt dendrimer-encapsulated nanoparticles (DENs), containing an average of 55 atoms each, on the oxide surface. These PPF/Al2O3/Pt DEN electrodes were completely stable under a variety of electrochemical and solution conditions, and they are active for simple electron-transfer reactions and for more complex electrocatalytic processes. This approach for preparing well-defined oxide electrodes opens the door to a better understanding of the effect of oxide supports on reactions electrocatalyzed by metal nanoparticles.