Copper-Based Nanocrystals and Their Use as Catalysts for the Electrochemical Reduction of Carbon Dioxide

By Lyu, Zhiheng

Georgia Institute of Technology

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Abstract

Advisors: Younan Xia, John Zhang, Angus P. Wilkinson, Henry S. La Pierre, Zhiqun Lin

Benefiting from the high abundance, low price, and fascinating properties of copper (Cu), Cu and Cu-based nanocrystals have found wide-spread use in many applications. In recent years, they received increasing attention as catalysts due to their uniqueness in generating substantial amounts of hydrocarbons and oxygenates in electrochemical CO2 reduction. Both experimental and computational studies suggest that the selectivity and activity of Cu-based catalysts are highly dependent on their surface structures, emphasizing the importance of shape-controlled synthesis of Cu nanocrystals. In this dissertation, I will introduce different strategies of synthesizing Cu nanocrystals with well-defined shapes, together with their catalytic performance in electrochemical CO2 reduction. By leveraging the reduction potential difference, a trace amount of Pd(II) was introduced to the Cu(II) precursor to induce the formation of Pd seeds, onto which Cu atoms deposited and grew into a right bipyramidal shape enclosed by {100} facets and twin boundaries. The coordination of hexadecylamine to metal ions was also revealed, which significantly slowed down their reduction rates and contributed to the generation of multiple parallel planar defects in a seed. Switching from one-pot synthesis to seed-mediated growth, Au@Cu core-shell nanocubes with sizes below 30 nm were produced, taking advantage of the small size of 5-nm Au seeds. Due to the large lattice mismatch (12%) between Au and Cu, an island growth mode was observed for Cu, resulting in a random location of Au core inside the Cu shell. When applying Pd icosahedra with a relatively larger size (13 nm) as seeds, Pd-Cu Janus nanocrystals with different shapes and twinned structures were obtained. By varying the reduction rate of Cu(II) precursor from slow to fast, Cu atoms selectively grew from either the vertex or the edge of an icosahedral seed, leading to the formation of penta-twinned or singly-twinned nanostructures. The presence of twin boundaries, exposure of Pd as a CO generator, and phase-segregated morphology of Pd and Cu all made Pd-Cu nanocrystals promising catalysts for electrochemical CO2 reduction. In addition to manipulating the twin defects and composition, I also demonstrated that both C2+ selectivity and catalytic stability of Cu nanocrystals could be improved by introducing surface oxidation and controlling the oxidation pathway. Compared to Cu nanowires oxidized by O2, which possessed a rough surface and non-uniform oxide layer, those oxidized by H2O2 showed a much smoother surface covered by oxide sheath with even thickness. The uniformity of the sheath greatly mitigated the fragmentation of nanowires, contributing to their superior stability in CO2 reduction.

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Researchers should cite this work as follows:

  • Lyu, Zhiheng (2021), "Copper-Based Nanocrystals and Their Use as Catalysts for the Electrochemical Reduction of Carbon Dioxide," https://matin.gatech.edu/resources/4049.

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