Synthesis and Characterization of Highly Durable Nanocatalysts by Protecting Active Sites

Abstract

At present, the practical application of nanocatalysts is hindered by their low durability. To overcome this limitation, in this thesis, I synthesized highly durable nanocatalysts by protecting the catalyst active sites and then characterized the products. By immobilizing the acid and base catalysts on the nanomaterial surface, I secured the activity of each acid or base catalyst. In the case of the oxygen reduction reaction (ORR) catalyst, the nanoparticles (NPs) are protected from dissolution and agglomeration under the harsh fuel cell operating conditions by coating the nanocatalyst surface with a thin N-doped carbon shell. Following the introduction chapter, Chapter 2 describes the successful synthesis of a magnetically separable mesoporous site-isolated acid-base catalyst using a one-pot reaction. The catalyst showed excellent performance with very high yield and selectivity for the conversion of benzaldehyde dimethyl acetal to 1-nitro-2-phenylethylene via benzaldehyde using tandem acid-catalyzed deacetalization and base-catalyzed Henry reaction. The catalyst could be easily recovered using a magnet and dispersed in subsequent reaction mixtures, enabling recycling of the catalyst for up to five uses without loss of catalytic activity. Furthermore, comparative studies reveal that the larger-pore materials exhibited higher catalytic activity than the smaller-pore materials. In Chapter 3, I present the synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe NPs coated with a dual-purpose N-doped carbon shell. Ordered fct-PtFe NPs with a size of only a few nanometers were obtained by the thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell formed in situ from the dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution under the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, I achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. The mass activity and specific activity of the ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell are 11.4 and 10.5 times higher, respectively, than those of a commercial Pt/C catalyst. Moreover, a membrane electrode assembly (MEA) fabricated using this catalyst exhibited long-term stability for 100 h without significant activity loss. In situ X-ray absorption near edge structure (XANES) and energy-dispersive X-ray spectroscopy (EDS) studies confirmed that the ordered fct-PtFe structure is critical for the long-term stability of this nanocatalyst. The strategy developed herein, namely, utilizing an N-doped carbon shell to obtain small ordered-fct PtFe nanocatalysts and protect the catalyst during fuel cell cycling, is expected to provide a simple and effective route for the commercialization of fuel cells.