Mesoporous functionalized carbon materials as advanced sorbents and electrode materials
Creators: name: Wu, Zhangxiong
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: The problems of environmental pollution and energy depletion have triggered enormous concerns across the world. There has been a vast abundance of global research development and demonstration efforts to alleviate these problems. However, it is obvious that these problems will continue in the 21st century, and thus advances in development and deployment of new technologies are in high demand. To approach these goals, fundamental research regarding creating novel materials carrying targeted functions and correlating their structure-property relations is required. This thesis, as a whole, aims to contribute to the fundamental research of material syntheses and their potential applications in environmental and energy areas. Ordered mesoporous carbon materials (OMCs) hold fascinating features including high specific surface areas, large pore volumes, uniform and tunable mesopores, good chemical and thermal stability and intrinsic electrical conductivity, etc. They possess wide potential in adsorption and separation, catalysis, sensing, delivery, energy storage and conversion. It is normally required to finely tune the properties by incorporating functionalities into these carbon materials so as to introduce unique properties and trigger new advances. However, it is still challenging to explore facile routes to introduce targeted functionalities into OMCs with well-controlled structure, porosity, desirable density and distribution of functionality, and other tailored properties such as optical, magnetic and electronic properties. To this end, this thesis centres on the synthesis of ordered mesoporous carbon-based materials carrying desirable surface and/ or framework functionalities or incorporating nanoparticles, aiming to develop advanced sorbents and electrode materials. As a whole, this thesis presents a systematic study over the following well-connected aspects of OMCs: modification of surface and/ or framework structure, graphitization of framework, incorporation of nanoparticles, and their usage in water treatment, CO2 capture and electrochemical energy storage and conversion. FDU-type OMCs obtained from organic-organic self assembly have attracted global interest and are feasible for large-scale production. However, their inert and hydrophobic nature is unfavorable in many cases. The work in Chapter 2 demonstrates a comprehensive study on the evolution of structure, porosity and surface oxides of mesoporous carbon FDU-15 under various wet oxidation conditions. Different oxidative solutions, including acidic ammonium persulfate, nitric acid and hydrogen peroxide solutions were adopted to oxidize FDU-15 under different experimental conditions, including concentrations, temperatures and durations. The entire oxidation process was studied and the mechanisms elucidated. FDU-15 possesses excellent mesostructural stability under strong oxidation conditions. The evolution trends of the structure and porosity with the increase of oxidative strength were carefully studied. It was found that the surface area and pore volume first decreased and then increased. A mechanism of micropore blocking and reopening with the increase of oxidative strength was proposed and correlated with the experimental results. Upon the oxidation, high densities of surface oxides, especially carboxylic groups, were generated on the surface of FDU-15. The contents and types of surface oxides, along with the surface hydrophilic and acidic properties were extensively characterized. The mesoporous oxidized carbon FDU-15 was found to be a highly efficient adsorbent for a variety of guest species, including ammonia gas, heavy metal ions, functional dyes and biomaterials, showing promising adsorption, separation and immobilization efficiency. While surface oxides are weakly acidic, surface nitrogen-containing groups act as basic sites. Nitrogen-enriched carbon materials are highly desirable in supercapacitors, fuel cells, catalysis, and adsorption. The work in Chapter 3 presents a simple but efficient post synthetic route to incorporate nitrogen-containing functionalities onto/ into the surface/ framework of FDU-type OMCs. Melamine molecules were loaded into a bimodal mesoporous carbon. It was found that the confined melamine molecules self-condensed into uniformly dispersed carbon nitrides under a heat treatment at ~ 500 °C. Subsequently, the carbon nitrides decomposed, leading to nitrogen-enriched carbons at 700 ~ 900 °C. The evolution of structure, porosity, elemental composition and the types of the nitrogen-containing functionalities with the increase of temperature were extensively studied. The ordered mesostructure was well retained after incorporation of nitrogen at high temperatures. The surface area was deteriorated considerably when a high content of carbon nitride was uniformly dispersed in the carbon, but then recovered upon the decomposition of carbon nitrides at 700 ~ 900 °C. The mesoporous nitrogen-enriched carbon materials are highly efficient for phenol removal. It was found that phenol molecules not only can be separated by physisorption with fast adsorption kinetics and large capacities, but also can be removed upon a photo-degradation by the nitrogen-containing functionalities with remarkable catalytic activity. The mesoporous nitrogen-enriched carbons were also tested as sorbents for CO2 capture. They delivered promising properties with high adsorption capacities and greatly enhanced heats of adsorption. Apart from surface modification, incorporation of nanoparticles into mesoporous carbons can also significantly extend their applications. However, it is very challenging to load highly concentrated and uniformly dispersed nanoparticles into mesoporous carbons without aggregating and blocking the mesopores. The work in Chapter 4 demonstrates a simple but general ammonia-atmosphere pre-hydrolysis post-synthetic route for synthesizing a series of novel mesoporous metal oxide@carbon nanocomposites. As a case study, a high content of iron nitrate precursors was loaded into a mesoporous surface-oxidized carbon matrix (FDU-type) with bimodal mesopores at 5.6 and 2.3 nm. The precursors were then converted in situ to iron hydroxides by hydrolysis under an ammonia atmosphere, and finally converted to magnetic iron oxides by calcination. The structure and porosity of the nanocomposites, the content and the location of the iron oxide nanoparticles, as well as the crystalline and magnetic properties were comprehensively studied. It was found that the mesostructure was well retained. High contents (&gt; 40 wt%) of iron oxide nanoparticles were selectively and exclusively confined in the primary mesochannels (5.6 nm) without aggregation. The surface area and pore volume decreased, but the empty connected mesopores retained an open mesopore network with a high mesoporosity to make the nanoparticles accessible. The mesoporous magnetic iron oxide@carbon nanocomposites were adopted as sorbents for arsenic removal. They show excellent performance with high adsorption capacities, fast uptake rate, ready magnetic separation and a long cyclic stability. CO2 capture has attracted global interest. Mesoporous carbon materials can be directly used for its uptake through physisorption. Alternatively, high-temperature CO2 sorbents were also reported but their cyclic performance was not satisfactory. The work in Chapter 5 demonstrates an efficient CO2 sorbent that can capture CO2 through both physi- and chemisorptions mechanisms. Calcium nitrate, phenolic resin and a triblock copolymer first co-assembled into ordered mesostructures through the evaporation-induced self-assembly pathway. After carbonation, calcium oxide (CaO) nanoparticles were incorporated into the mesoporous FDU-type carbon materials with high surface areas. The structure, porosity and the particle size evolution of the composites as a function of calcium content or carbonization temperature were extensively studied and well correlated with their CO2 sorption properties. It was found the surface areas of the composites can be up to ~ 1060 m2/g, and the calcium content can be up to ~ 20 wt% without loss of mesostructural regularity. The CaO nanoparticles were found highly dispersed. The composites were found of significance for CO2 physisorption with very high capacities (up to ~ 7 mmol/g at 273 K) and good selectivity over N2. Meanwhile, the nanocrystalline CaO particles were found to be highly active for CO2 chemisorption with large capacities (up to 3.0 mmol/g) at 200 ~ 500 °C, fast reaction kinetics at 450 °C (100 % of conversion). The cyclic stability of the CO2 chemisorption was also considerably improved a lot due to the confinement effect. Mesoporous carbons with ordered structures, high porosities and large mesopores are ubiquitous in many areas. However, converting their amorphous pore walls to graphitic, while retaining high surface areas and large pore volumes, remains a considerable challenge. The work in Chapter 6 shows the synthesis of ordered mesoporous graphitized pyrolytic carbons through direct chemical vapor deposition (CVD) from methane at 900 °C with mesoporous silicas as the hard templates. The synthesis procedure is quite simple without use of solvent, catalyst or carrying gas, but very efficient for producing mesoporous graphitized carbon materials. A comprehensive study regarding the synthesis process, control over the structure, pore architecture, porosity and graphitization was presented. It was found that the resultant carbon materials possessed controllable mesostructures, variable mesopore architectures, large pore volumes (up to 2.3 cm3/g), high surface areas (up to 750 m2/g) and highly graphitized pore walls with preferred (002) plane orientation (parallel to the carbon nanowires). These carbon materials were adopted as excellent supports for Pt as electrocatalysts for oxygen reduction, (...)