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Architecturally Controlled Bimetallic Nanoparticles for Heterogeneous Catalysis
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This work develops synthetic methods for architecturally controlled AuPt and CuPt bimetallic nanomaterials. The AuPt heteroaggregate, AuPt alloy spherical nanoparticles, and AuPt alloy nanowires were prepared by a sequential or co-reduction method. The unique AuPt heteroaggregate nanostructures, synthesized by the sequential reduction method, contain Au cores with Pt tendrils extending from the Au surfaces. The AuPt alloy nanoparticles or nanowires were prepared by the rapid co-reduction method. This rapid co-reduction method prevents the phase separation and traps the metastable AuPt alloy phase. The AuPt heteroaggregate, alloy spherical nanoparticles, alloy nanowires, and the reported Au@Pt core-shell structures, constitute the rare example of a bimetallic system containing all reported architectures in the literature. Kinetically stabilized Cu@Pt core-shell nanoparticles were prepared by deposition of Pt onto Cu nanoparticles. The Cu@Pt particles exhibit high stability toward alloying upon annealing. In contrast, the Pt@Cu particles readily transform into alloy structures in the same conditions. This abnormal stability of the Cu@Pt particles is attributed to the Kirkendall mass transport effect, where the inherent diffusion direction from the Pt to Cu is hindered by a limited population of vacancies in the Cu cores. These architecturally controlled bimetallic nanomaterials were applied in CO tolerant hydrogen oxidation and de-NOx reactions with hydrogen. In the H2/CO/O2 fuel, the AuPt alloy nanoparticles are CO tolerant in hydrogen oxidation, and the AuPt heteroaggregate nanoparticles exhibit enhanced preferential CO oxidation in the presence of Fe promoters. In the NO/H2 reaction, the Cu@Pt nanoparticles maintain the high activity of pure Pt particles and have a higher selectivity for N2. Under 4/1 H2/NO conditions, the selectivity for N2 over the Cu@Pt catalyst is 45%. In contrast, under the same conditions, the pure Pt catalyst exhibits a selectivity of 22%. The Pt@Pd catalyst enhances activity as well as selectivity due to the near surface alloy effect.