(Invited) De-Alloying of CuxAu(1-x) Alloys at Different Length Scales for the Development of Active Nanoporous Au Catalysts

Abstract

The development of highly active, durable, and cost-effective catalysts is of paramount importance for the fuel cell industry. Most catalysts for relevant applications contain Pt that is known as one of the most expensive metals.1 To date, the best way to reduce the total catalyst cost is associated with the use of an inert support coated by a small amount of Pt-based catalyst. A good candidate for the support is nanoporous Au (NPG).1-3 As highly conductive material, NPG has an open, 3D interconnected porous framework with surface area that even in ultrathin NPG layer configuration could be tens to hundreds of times larger than the area of planar Au. Once developed on a C-based support, the NPG can be terminated with Pt or Pt-containing alloys by surface-limited redox replacement (SLRR), and then applied as the catalyst in the key catalytic reactions.4 In our previous work, we established an all-electrochemical synthetic approach for a NPG-based catalyst.4 The fabrication procedure entails AgxAu(1−x) alloy electrodeposition followed by selective dissolution of Ag (de-alloying) ultimately generating the desired NPG structure with a thickness of less than 20 nm. To further study the NPG applicability in catalysis, in this report we switch our focus to a CuxAu(1−x) precursor alloy synthesized for this work in different size and shape through various routes. Generally, the precursor is subjected to de-alloying and the resulting NPG is structurally and morphologically characterized and assessed for performance and durability in formic acid oxidation tests after platinization with ultra thin catalytically active layer. More specifically, CuxAu(1−x) bulk alloys, electrodeposited alloys, and chemically synthesized Cu3Au nanorods (intermetallic and random alloy)5-7 are subjected to a comparative de-alloying study with emphasis on their parting behavior and resulting structures. The de-alloying curves exhibit quite unique behavior owing to size and structural differences. Overall, the de-alloying curves for the system of interest illustrate multiple peaks that indirectly suggest phase coexistence dependent upon the synthetic route of the precursor alloy (melting, electrodeposition nanoparticle synthesis). The NPG resulting from the de-alloying process has been studied for surface area development by Pb underpotential deposition (Pb UPD). XRD is used to study the potential presence of different phases. SEM and TEM images display the morphology before and after de-alloying. Finally, basic activity and durability tests of the catalytic properties of various Pt-coated de-alloyed structures during formic acid oxidation are presented and discussed as well. In these tests the assessment of accordingly prepared catalysts in standard HCOOH oxidation experiments attests to their high catalytic activity which in some aspects exceeds the performance of recently studied nanoparticle based catalysts.