The aqueous synthesis and characterization of bimetallic gold-copper nanoparticle catalysts for CO oxidation

Abstract

Bimetallic nanoparticles have attracted significant interest in the field of heterogeneous catalysis due to their unique and tunable properties, arising from synergistic effects. Of particular interest are gold-copper nanoparticles, which demonstrate potential in catalyzing the oxidation of carbon monoxide (CO). While gold can act as an active site for CO, copper can facilitate the dissociation of oxygen, thereby enabling an efficient conversion of CO. This is important for reducing the negative impact of CO on human health and the environment.

The conventional synthesis of gold-copper nanoparticles has primarily been limited to organic synthesis due to the instability of copper towards oxidation. Nonetheless, an aqueous synthesis of gold-copper nanoparticles would offer a more sustainable, non-toxic and inexpensive alternative. Therefore, the synthesis of gold-copper nanoparticles in aqueous solvents has been investigated in this work. The synthesis was based on colloidal strategies to gain a high degree of control over the morphology. The nanoparticles were characterized with state-of-the-art electron microscopy techniques, ultraviolet-visible spectroscopy, x-ray diffraction, Fourier transform-infrared spectroscopy, thermal gravimetric analysis and inductively coupled plasma atomic emission spectroscopy.

This thesis presented three colloidal, aqueous synthesis routes to obtain gold-copper nanoparticles. The routes were based on a novel approach of growing oxidized copper on gold nanoparticles in an aqueous solution, followed by thermal reduction. The first synthesis route utilized gold nanoparticles coated with a mesoporous silica shell. Although the silica shell provided stability, it restricted the copper content (<30%) in the obtained gold-copper nanoparticles. The second synthesis route eliminated the silica shell, thereby obtaining Au@Cu2O nanoparticles with higher copper contents up to approximately 87%. However, the nanoparticles clustered upon deposition on a silica support, leading to aggregation when subjected to thermal treatment. The synthesis third route, in which Cu2O was grown on supported gold nanoparticles, resulted in well-dispersed gold-copper nanoparticles, but also the formation of monometallic Cu2O particles on the support. Although further optimization would be beneficial, the third method showed promise for synthesizing gold-copper nanoparticles in aqueous solution.

The catalytic potential of supported gold-copper nanoparticles, synthesized via the third method, was evaluated for CO oxidation. The supported bimetallic gold-copper nanoparticles exhibited higher turnover frequency values compared to their monometallic gold counterparts. Furthermore, gold-copper nanoparticles with a higher copper content were found to be more active. Additionally, core-shell Au@CuxO nanoparticles were found to be significantly less active. Further research is necessary to obtain a more in-depth understanding of the catalytic performance of gold-copper nanoparticles in CO oxidation.