Atomic Ensemble and Electronic Effects in Ag-Rich AgPd Nanoalloy Catalysts for Oxygen Reduction in Alkaline Media

Abstract

The ability to design and characterize uniform, bimetallic alloy nanoparticles, where the less active metal enhances the activity of the more active metal, would be of broad interest in catalysis. Herein, we demonstrate that simultaneous reduction of Ag and Pd precursors provides uniform, Ag-rich AgPd alloy nanoparticles (~5 nm) with high activities for the oxygen reduction reaction (ORR) in alkaline media. The particles are crystalline and uniformly alloyed, as shown by X-ray diffraction and probe corrected scanning transmission electron microscopy. The ORR mass activity per total metal was 60% higher for the AgPd2 alloy relative to pure Pd. The mass activities were 2.7 and 3.2 times higher for Ag9Pd (340 mA/mgmetal) and Ag4Pd (598 mA/mgmetal), respectively, than those expected for a linear combination of mass activities of Ag (60 mA/mgAg) and Pd (799 mA/mgPd) particles, based on rotating disk voltammetry. Moreover, these synergy factors reached 5-fold on a Pd mass basis. For silver-rich alloys (Ag≥4Pd), the particle surface is shown to contain single Pd atoms surrounded by Ag from cyclic voltammetry and CO stripping measurements. This morphology is favorable for the high activity through a combination of modified electronic structure, as shown by XPS, and ensemble effects, which facilitate the steps of oxygen bond breaking and desorption for the ORR. This concept of tuning the heteroatomic interactions on the surface of small nanoparticles with low concentrations of precious metals for high synergy in catalytic activity may be expected to be applicable to a wide variety of nanoalloys.

Porous AgPd–Pd Composite Nanotubes as Highly Efficient Electrocatalysts for Lithium–Oxygen Batteries

“…[ 40 ] The AgPd alloy sheaths act as a highly effi cient catalyst during the reversible absorption and desorption of hydrogen, owing to the combination of geometric and synergistic effects. [38][39][40][41] To the best of our knowledge, however, there is still no report on using porous AgPd NTs as electrocatalyst for Li-O 2 batteries.…”

“…[28][29][30][31][32][33][34][35] Because of a combination of ligand, geometric, and/or ensemble effects, bimetallic catalysts strongly enhance the kinetics of the ORR and OER. [ 8,21,28,32,[36][37][38][39] Therefore, some bimetallic composites with NT structure have been further designed by controlling the reaction process. For example, Ag nanowires coated with AgPd alloy sheaths were synthesized and used for reversible and Pd, as indexed in the XRD Rietveld refi nement results in Figure 1 a-d.…”

“…Compared with Ag NWs, AgPd-3 NTs also show an improvement by a factor of nearly 19 at ≈0.85 V. This result is also consistent with the conclusion reached by Stevenson's and Xia's groups that AgPd alloy could enhance the catalytic activity by several times compared to pure monometallic systems. [ 13,38,40 ] Cyclic voltammograms (CVs) in O 2 -saturated 0.1 M KOH electrolyte were also used to reveal the ORR activity of the as-prepared samples in Figure 3 c. In the CV for pure Ag NWs, characteristic oxidation peaks appear between 1.067 and 1.467 V, corresponding to Ag 2 O monolayer, AgOH bulk, and Ag 2 O bulk formation, respectively. [ 44,45 ] The reduction of silver oxide is observed to be reversible on the negative scan, at 1.087 V. In the case of the AgPd-1 NTs, the oxide reduction peak of Ag is shifted left, which may be due to the alloy layer on the surface.…”

Engineering Metallic Nanoparticles for Enhancing and Probing Catalytic Reactions

“…[8] Deposition of a thin (~ 3 monolayer) Pt shell produced fctFePt@Pt NPs, which showed higher activity compared to their fcc-analogues due to release of analysis to microscopy and spectroscopy based characterization and was used to evidence ensemble effects as the origin of enhanced activity observed in Ag-rich AgPd alloy NPs for ORR. [9] Colloidal AgPd alloy NPs with tuneable composition were prepared by simultaneous reduction of metal salts. CV and CO stripping were used to probe the local environment of the surface atoms as a function of composition.…”

Mesoporous TiN as a Noncarbon Support of Ag‐Rich PtAg Nanoalloy Catalysts for Oxygen Reduction Reaction in Alkaline Media

“…Bimetallic catalysts allow more flexible design and often exhibit superior activity for many chemical transformations as compared to pure monometallic systems. [10] PtAg bimetallic catalysts are of interest for the ORR in alkaline media because both of them are active. Silver has been the subject of many studies due to its reasonably high activity, good long-term stability, and relatively low price (at least 30 fold less expensive than Pt).…”