Development of hybrid catalytic systems for CO2 reforming of methane

Abstract

Increasing global concern over the rise of human emissions of CO2 into Earth’s atmosphere has prompted extensive research into ways of utilizing this gas for better economical use. The reaction of CO2 with methane to produce more valuable compounds is one of the important subjects in catalysis, energy and environmental research, since it provides a pathway to reduce these greenhouse gases. This thesis focuses on the development of novel metal-oxide catalysts for the production of synthesis gas (syngas ─ CO and H2) via CO2 reforming of methane, using both the conventional fixed-bed and microwave reactor systems. Mono- and bimetallic catalysts with good microwave absorption properties will be synthesized, and their performances investigated under both catalytic systems. This thesis aims to study the impact of a clean, strong, metal-support interaction on syngas production via CO2 reforming of methane, and to determine the best possible approach to generate carbonaceous materials in a way that is not detrimental to the performance of the catalysts during the reforming reaction. Synthesis of mono-metallic catalysts via hydrothermal method was conducted then treated using conventional thermal calcination and microwave plasma treatment. Microwave plasma’s power and exposure time effects on metal-support interface generation and catalytic performance in the reforming reaction were investigated. Increase in the plasma power from 150 to 250 W generated nanosized nickel particles of 1.5−4 nm on ceria support, whose particle sizes range between 30−85 nm. Conventional thermal calcination generated nickel particle size of 3−30 nm dispersed within the CeO2 support. The catalytic performances of both the plasma treated and the thermally calcined materials were then tested in CO2 with methane using the conventional fixed-bed reactor. The plasma treated samples showed superior and more stable turnover frequencies (TOFs) with time than the thermally calcined samples, which can be ascribed to the strong metal-support interaction and formation of well-dispersed Ni particles generated as a result of concurrent treatment of both the uncalcined ceria support and the loaded metal precursor. Synthesis of bi-metallic catalysts used for syngas production via CO2 reforming of methane using both conventional fixed-bed and microwave reactor systems was then examined. Three different bi-metallic catalysts were synthesized via hydrothermal approach and treated using the conventional thermal calcination. The promoters were chosen based on their potential to either enhance microwave absorption properties or suppress active metal (nickel particle) II enlargement during the synthesis and reaction stages. The performances of these bi-metallic catalysts were investigated in reforming reaction using the fixed-bed reactor to optimize the production of syngas. Our main aims in using the microwave reactor system for the reforming reaction are (i) tuning the formation of the carbonaceous materials via selective heating mechanism, since microwave radiation absorption solely depends on the catalyst properties, (ii) producing carbonaceous materials that will not hinder syngas production but might assist catalytic reaction, and (iii) utilizing the produced carbon composites in electrochemical applications, such as fuel cells. Carbon is well known for its poisoning effect on catalysts during syngas production using the conventional fixed-bed reactor, but its impact on catalytic performance might differ under microwave irradiation, since some carbonaceous materials have excellent microwave absorption properties. Different nanocarbons were grown in-situ on the bi-metallic catalysts by a selective heating mechanism, tuning their structures by the catalytic metal ions which controlled microwave absorption. Superior catalytic performance was obtained on the graphene-containing composite which is attributed to the locally-heated nickel particles. Morphology and composition also determine electrochemical performance, with the highest specific capacitance of ~60 F/g obtained on the multi-walled carbon nanotube/graphitic nanofiber composite. This work opens up excellent way to utilize carbonaceous materials, generally regarded as “waste”, as electrode materials for energy storage devices. It may also have other applications, such as for biosensors, adsorption and batteries.