Gold–Copper Nanoparticles: Nanostructural Evolution and Bifunctional Catalytic Sites

Abstract

Understanding of the atomic-scale structure is essential for exploiting the unique catalytic properties of any nanoalloy catalyst. This report describes novel findings of an investigation of the nanoscale alloying of gold−copper (AuCu) nanoparticles and its impact on the surface catalytic functions. Two pathways have been explored for the formation of AuCu nanoparticles of different compositions, including wet chemical synthesis from mixed Au-and Cu-precursor molecules, and nanoscale alloying via an evolution of mixed Au-and Cuprecursor nanoparticles near the nanoscale melting temperatures. For the evolution of mixed precursor nanoparticles, synchrotron X-ray-based in situ real-time XRD was used to monitor the structural changes, revealing nanoscale alloying and reshaping toward an fcc-type nanoalloy (particle or cube) via a partial melting−resolidification mechanism. The nanoalloys supported on carbon or silica were characterized by in situ high-energy XRD/atomic pair disributoin function (PDF) analyses, revealing an intriguing lattice "expanding−shrinking" phenomenon depending on whether the catalyst is thermochemically processed under an oxidative or reductive atmosphere. This type of controllable structural changes is found to play an important role in determining the catalytic activity of the catalysts for carbon monoxide oxidation reaction. The tunable catalytic activities of the nanoalloys under thermochemically oxidative and reductive atmospheres are also discussed in terms of the bifunctional sites and the surface oxygenated metal species for carbon monoxide and oxygen activation. ■ INTRODUCTIONUnderstanding of the atomic-scale structure is essential for exploiting the unique catalytic properties of nanoalloy based catalysts. This understanding can be achieved by obtaining knowledge of the alloy formation process and the evolution of surface sites under reactive conditions. While the study of nanoscale alloy formation has been largely focused on the bottom-up approach, that is, the synthesis from the mixed metal precursors, recently, the focus has shifted to revealing the structural evolution of preformed metal or alloy nanoparticles. 1−4 One of the fundamental differences of metal particles confined to nanoscale dimensions from their bulk counterparts is the molecule-like chemical reactivity and the solid-like melting behavior. 5,6 This type of "molecule-solid duality" concept is applicable to multicomponent nanoscale systems (e.g., binary, ternary, etc.) in terms of alloying characteristics. Moreover, since metallic nanoparticles melt at lower temperatures than their bulk counterparts, the surface melting could take place at even lower temperatures, leading to an increased propensity of the nanoparticle surface atoms to detach, diffuse, and reattach. The knowledge of the dependency of surface melting on the size and strain 7 has been so far largely limited to unary metal particles. For small clusters of Au, Pt, Ag, and Cu with noncrystallographic 5-fold symmetry and irregular interatomic ordering, it i...

Structure and Reactivity of Cu-doped Au(111) Surfaces

“…From the catalytic point of view, supported copper and supported gold nanoparticles are very active for oxidation reaction and Cu/Au nanoparticulates have been reported to be catalytically active for a variety of reactions, in particular for the oxidation of CO and of alcohols to produce biofuels [70][71][72]. Moreover, supported gold catalytic systems can also operate as highly selective catalysts for the hydrogenation of important feedstocks such as benzaldehyde [73] and nitrobenzene [74].…”

Nanoscale Alloying in Electrocatalysts

“…In comparison with other methods such as plasmatic cleaning or chemical cleaning [22], thermochemical processing strategy is not only effective in removing the encapsulation, but also in refining the nanostructural parameters. The combination of the molecular encapsulation based synthesis and thermochemical processing strategies typically involves a sequence of steps for the preparation of nanoalloy catalysts: (1) chemical synthesis of the metal nanocrystal cores capped with ligands, (2) assembly of the encapsulated nanoparticles on supporting materials (e.g., carbon powders, TiO2 or SiO2), and (3) thermal treatment of the supported nanoparticles [12][13][14][15][16][17]. The size and composition of the nanoparticles produced by thermochemical processing are controllable.…”

“…The size and composition of the nanoparticles produced by thermochemical processing are controllable. As shown for a series of binary and ternary alloy nanoparticle systems in Table 1 [12][13][14][15][16][17][23][24][25][26][27][28][29][30][31][32][33][34][35], the catalysts prepared by the molecularly-mediated synthesis and thermochemical processing methods have demonstrated enhanced catalytic and electrocatalytic properties for oxygen reduction reaction (ORR), methanol oxidation reaction (MOR), and ethanol oxidation reaction (EOR), etc. …”

“…Besides various nanostructural design parameters [11], the understanding of how these factors play an important role on catalytic properties is increasingly important. In this article, some of the recent findings in the exploration of nanoscale alloying degree for the preparation of the supported nanoalloy catalysts for catalytic and electrocatalytic reactions [12][13][14][15][16][17] will be highlighted. One important focus is the understanding of the structural correlation of nanoscale alloying properties with the electrocatalytic properties.…”

A General Perspective of the Characterization and Quantification of Nanoparticles: Imaging, Spectroscopic, and Separation Techniques

“…202 As the X-ray photon energy is known, it is possible to determine the electron binding energy and identify the atomic core-level where the electron was extracted. XPS is useful to characterize NPs surfaces and coatings (e.g., the evolution in the composition of Au/Cu NPs along the time in catalysis).…”