Exploring Shape Dependency of Catalysts for Chemical Reactions
Abstract
Recent advancements in the synthesis of nanoparticles, particularly one-dimensional (1D) nanoparticles, have significantly enhanced the catalytic activity of various processes by enabling precise control over their morphologies and exposing unique crystallographic facets. The shape of nanoparticles is closely linked to their chemical, physical, electrical, optical, magnetic, and catalytic properties. This dissertation investigates the impact of 1D nanoparticle catalysts on two distinct and well-known chemical reactions. In the first part of the study, we explore the catalytic activity and selectivity of silver catalysts in the ethylene epoxidation reaction, which are intrinsically linked to their faceting. Silver nanowire catalysts representing Ag(100) and silver spherical catalysts embodying Ag(111) were synthesized using modified polyol and wet impregnation methods, respectively. The modified polyol method is meticulously detailed to address synthesis challenges, enhancing reproducibility and scalability. To account for the size-dependent nature of catalysis, spherical particles of two distinct sizes were synthesized as comparative benchmarks for silver nanowire activity. Optimal cesium loading was applied to silver spherical catalysts, while varying cesium loadings were used for silver nanowires. This study explores their effectiveness in the ethylene epoxidation reaction, marking a novel inquiry in this field. Unpromoted silver nanowires exhibited superior ethylene oxide selectivity compared to both unpromoted and promoted spherical catalysts. Furthermore, cesium promotion enhanced ethylene oxide selectivity, achieving activity levels comparable to the best reported in the literature. Morphological analysis of spent silver nanowire catalysts revealed exceptional stability even after extended study periods. In the second part, we focus on the role of 1D nanowire supports for ruthenium-based catalysts in ammonia synthesis and decomposition reactions. Ammonia has become a crucial energy storage intermediate and carbon-free energy carrier. Using the hydrothermal method, praseodymium oxide nanowire catalysts were synthesized and demonstrated higher ammonia synthesis rates compared to nanosphere catalysts at lower ruthenium loadings, and higher ammonia conversion at all ruthenium loadings in decomposition reactions. The ammonia conversion reached 100% at 50°C lower than other catalysts for 5 wt% Ru/PrxOy nanowires. Despite similar oxygen vacancies and crystallinity between nanowires and nanospheres, CO DRIFTs analysis revealed significant differences, including the formation of small ruthenium nanoparticles with high dispersion on nanospheres.