Hierarchical Porous Structures with Aligned Carbon Nanotubes as Efficient Adsorbents and Metal-Catalyst Supports

The overall goal of this study is two-fold: synthesis of multiscale nanostructures by growing aligned carbon nanotubes on porous foam substrates and investigation of their applicability as adsorbents and catalyst supports for environmental remediation applications. High purity, vertically-aligned arrays of carbon nanotubes (CNT) are grown on open-cell interconnected porous carbon foams by pre-activating them with an oxide buffer layer followed by chemical vapor deposition (CVD). This type of hierarchical morphology provides the capability of increasing surface area by several orders of magnitude, while tuning its morphology for targeted applications. Analytical models are also proposed in this study for specific surface area calculations, those agree well with the experimental measurements. These hierarchical carbon materials are seen to be powerful adsorbents of aqueous pollutants such as methylene blue dye. Their monolayer adsorption capacities correlate very well with the total CNT surface area determined from analytical models and with BET measurements, indicating full utilization of the nanotube surfaces. The hierarchical structures can also serve as base supports for attachment of metal nanoparticle catalysts. The catalysts investigated in this study are metallic palladium (Pd), oxidized palladium (PdO), and silver-palladium (Ag-Pd) nanoparticles combination. These are suitable for a variety of industrial applications such as hydrocarbon conversion, hydrogen storage, fuel cell electrodes and pollutant degradation. The current architecture allows synthesis of highly active catalyst structures utilizing very small quantities of precious metal that make the catalyst component significantly lighter and more compact than conventional systems. Detailed characterization of structure and surface chemical states of these nano-catalysts have been performed and their catalytic activities are tested by measuring the degradation kinetics of organic contaminants via bench-scale experiments. Catalytic degradation of atrazine, an emerging problematic contaminant, was quantified using high-performance liquid chromatography. Among Pd, PdO, and Ag-Pd nanoparticles, PdO in the presence of hydrogen was seen to provide the most rapid reaction rate. These nanocatalysts also enable rapid degradation of chlorinated hydrocarbons such as trichloroethylene and trichloroethane quantified using head-space gas chromatography, with PdO providing the fastest kinetic route. Durability tests indicated that the nano-particles and nanotubes are robust, and remain attached to the base support after long periods of rapid rotation in water. These results imply that such materials can provide compact and powerful surface active materials in future applications such as adsorbents, catalysts, porous electrodes, and energy storage devices

Highly Active Porous Catalysts Fabricated by Attachment of Palladium Nanoparticles on Hierarchical Carbon Structures

The effectiveness of metal-based catalysts can be significantly enhanced by increasing the available surface area relative to the volume through the creation of hierarchical nanostructures. The catalyst demonstrated here is palladium, which is a widely recognized heterogeneous catalyst suitable for a variety of industrial applications such as water purification, hydrogen storage, and electrochemical devices. In this study, a novel multi-scale supporting material developed in this group, has been used as support. It consists of micro-porous graphitic carbon with strongly attached carbon nanotubes. This can increase the surface area by orders of magnitude without increasing the size or weight while still maintaining structural integrity. This allows miniaturization of palladium catalysts structures that are lighter, smaller and more compact than conventional ones. Fabrication issues of these structures have been successfully addressed. Detailed micro-structural as well as spectroscopic analysis of the nanoparticles have been performed. Variations of palladium nanoparticles distribution with processing conditions, and the possible ways of controlling this distribution will be presented. Surface spectroscopic analysis indicates that these are zero-valent metallic palladium and do not degrade with time. The catalytic activity of palladium nanoparticles has been tested via bench-scale experiments for reductive dechlorination of carbon tetrachloride. It is seen that palladium functionalized carbon nanotubes is highly effective in the degradation of carbon tetrachloride and similar organic pollutants found commonly in drinking water sources. It was also demonstrated that palladium functionalized carbon nanotubes can be used repeatedly as the valence state of palladium does not change, and thus can be cost-effective. Future scope of these results and their connection to future device applications will be discussed

Dechlorination of Environmental Contaminants Using a Hybrid Nanocatalyst: Palladium Nanoparticles Supported on Hierarchical Carbon Nanostructures

This paper demonstrates the effectiveness of a new type of hybrid nanocatalyst material that combines the high surface area of nanoparticles and nanotubes with the structural robustness and ease of handling larger supports. The hybrid material is made by fabricating palladium nanoparticles on two types of carbon supports: as-received microcellular foam (Foam) and foam with carbon nanotubes anchored on the pore walls (CNT/Foam). Catalytic reductive dechlorination of carbon tetrachloride with these materials has been investigated using gas chromatography. It is seen that while both palladium-functionalized carbon supports are highly effective in the degradation of carbon tetrachloride, the rate of degradation is significantly increased with palladium on CNT/Foam. However, there is scope to increase this rate further if the wettability of these structures can be enhanced in the future. Microstructural and spectroscopic analyses of the fresh and used catalysts have been compared which indicates that there is no change in density or surface chemical states of the catalyst after prolonged use in dechlorination test. This implies that these materials can be used repeatedly and hence provide a simple, powerful, and cost-effective approach for dechlorination of water

Palladium Nanoparticles on Hierarchical Carbon Surfaces: A New Architecture for Robust Nano-Catalysts

Surface activity of heterogeneous catalysts can be enhanced if their sizes are reduced to nanometers. However, loose nanomaterials pose potential health and environmental risks. This issue has been addressed by attachment of palladium nanoparticles on multi-scale hierarchical carbon supports that have exceptionally high surface area per volume. The supports consist of porous carbon foam whose surface has been either chemically functionalized, or morphologically altered by grafting of carbon-nanotubes. It is seen that whereas chemical functionalization does provide some increase in nano-catalyst loading, morphological modification is significantly more powerful. It has the potential to create orders of magnitude increase in catalytic activity within the same overall volume. The synthesis techniques have been investigated in sufficient detail to provide significant control over the density and size of nanoparticles. Abundant distribution of nanoparticles is observed even within the deeper pores of the microcellular foam. The nanoparticles are seen to be metallic Pd having face centered cubic structure. Additionally, the nano-particles and nanotubes are durable, and remain attached to the base support after long periods of rapid rotation in water. These robust hybrid structures show promise in future applications such as sensors, water purification systems, fuel cell electrodes and hydrogen storage sponges

Dechlorination of Environmental Contaminants Using a Hybrid Nanocatalyst: Palladium Nanoparticles Supported on Hierarchical Carbon Nanostructures

This paper demonstrates the effectiveness of a new type of hybrid nanocatalyst material that combines the high surface area of nanoparticles and nanotubes with the structural robustness and ease of handling larger supports. The hybrid material is made by fabricating palladium nanoparticles on two types of carbon supports: as-received microcellular foam (Foam) and foam with carbon nanotubes anchored on the pore walls (CNT/Foam). Catalytic reductive dechlorination of carbon tetrachloride with these materials has been investigated using gas chromatography. It is seen that while both palladium-functionalized carbon supports are highly effective in the degradation of carbon tetrachloride, the rate of degradation is significantly increased with palladium on CNT/Foam. However, there is scope to increase this rate further if the wettability of these structures can be enhanced in the future. Microstructural and spectroscopic analyses of the fresh and used catalysts have been compared which indicates that there is no change in density or surface chemical states of the catalyst after prolonged use in dechlorination test. This implies that these materials can be used repeatedly and hence provide a simple, powerful, and cost-effective approach for dechlorination of water

Palladium Nanoparticles on Hierarchical Carbon Surfaces: A New Architecture for Robust Nano-Catalysts

Surface activity of heterogeneous catalysts can be enhanced if their sizes are reduced to nanometers. However, loose nanomaterials pose potential health and environmental risks. This issue has been addressed by attachment of palladium nanoparticles on multi-scale hierarchical carbon supports that have exceptionally high surface area per volume. The supports consist of porous carbon foam whose surface has been either chemically functionalized, or morphologically altered by grafting of carbon-nanotubes. It is seen that whereas chemical functionalization does provide some increase in nano-catalyst loading, morphological modification is significantly more powerful. It has the potential to create orders of magnitude increase in catalytic activity within the same overall volume. The synthesis techniques have been investigated in sufficient detail to provide significant control over the density and size of nanoparticles. Abundant distribution of nanoparticles is observed even within the deeper pores of the microcellular foam. The nanoparticles are seen to be metallic Pd having face centered cubic structure. Additionally, the nano-particles and nanotubes are durable, and remain attached to the base support after long periods of rapid rotation in water. These robust hybrid structures show promise in future applications such as sensors, water purification systems, fuel cell electrodes and hydrogen storage sponges

Dechlorination of Environmental Contaminants Using a Hybrid Nanocatalyst: Palladium Nanoparticles Supported on Hierarchical Carbon Nanostructures

This paper demonstrates the effectiveness of a new type of hybrid nanocatalyst material that combines the high surface area of nanoparticles and nanotubes with the structural robustness and ease of handling larger supports. The hybrid material is made by fabricating palladium nanoparticles on two types of carbon supports: as-received microcellular foam (Foam) and foam with carbon nanotubes anchored on the pore walls (CNT/Foam). Catalytic reductive dechlorination of carbon tetrachloride with these materials has been investigated using gas chromatography. It is seen that while both palladium-functionalized carbon supports are highly effective in the degradation of carbon tetrachloride, the rate of degradation is significantly increased with palladium on CNT/Foam. However, there is scope to increase this rate further if the wettability of these structures can be enhanced in the future. Microstructural and spectroscopic analyses of the fresh and used catalysts have been compared which indicates that there is no change in density or surface chemical states of the catalyst after prolonged use in dechlorination test. This implies that these materials can be used repeatedly and hence provide a simple, powerful, and cost-effective approach for dechlorination of water

Electrochemical Charge Storage in Hierarchical Carbon Manifolds

The use of hierarchical assemblies constituted from macroporous structures (e.g.,reticulated vitreous carbon, RVC) where the internal pore area is covered with closely spaced nanostructures (e.g., carbon nanotubes, CNT) is proposed for substantially enhancing the energy density of electrochemical capacitors, while maintaining large charge/discharge rates. While the macroscale pores enable storage of substantial electrolyte volumes that would contribute through redox reactions to the energy density, the closely spaced nanostructures provide a large geometric area and capacitance in addition to enabling rate independent Faradaic charge storage via thin layer electrochemistry (TLE). A fifty fold increase in the double layer capacitance, in addition to increased Faradaic charge density – with potential for orders of magnitude improvement, was observed for the RVC-CNT electrodes, in comparison to the bare RVC foam electrode. It was seen that the hierarchical assembly enables the contribution from ∼94% of the net volume of the wetted RVC-CNT electrode for active Faradaic charge storage

Electrochemical Charge Storage in Hierarchical Carbon Manifolds

The use of hierarchical assemblies constituted from macroporous structures (e.g.,reticulated vitreous carbon, RVC) where the internal pore area is covered with closely spaced nanostructures (e.g., carbon nanotubes, CNT) is proposed for substantially enhancing the energy density of electrochemical capacitors, while maintaining large charge/discharge rates. While the macroscale pores enable storage of substantial electrolyte volumes that would contribute through redox reactions to the energy density, the closely spaced nanostructures provide a large geometric area and capacitance in addition to enabling rate independent Faradaic charge storage via thin layer electrochemistry (TLE). A fifty fold increase in the double layer capacitance, in addition to increased Faradaic charge density – with potential for orders of magnitude improvement, was observed for the RVC-CNT electrodes, in comparison to the bare RVC foam electrode. It was seen that the hierarchical assembly enables the contribution from ∼94% of the net volume of the wetted RVC-CNT electrode for active Faradaic charge storage

Hierarchical Hybrid Carbon Nano-structures as Robust and Reusable Adsorbents: Kinetic Studies with Model Dye Compound

This study demonstrates the potential of using multi-scale hierarchical carbon structures as robust, reusable solids suitable for removal of aqueous pollutants such as dye molecules from wastewater. Carpet-like vertical arrays of carbon nanotubes (CNT) were attached on surfaces of porous carbon foams by pre-coating with silica buffer layer followed by chemical vapor deposition (CVD). The surface morphology and specific surface areas were varied by controlling the buffer layer thickness and CVD deposition times. Surface characteristics have been correlated with adsorption thermodynamics and kinetics by investigating the removal rates of methylene blue (MB) dye in simulated water. The results show that MB adsorption capacity correlates well with total CNT surface area in the carpet, indicating full utilization of the nanotube surfaces. Adsorption rates fit well with pseudo second order kinetics model. Maximum MB adsorption capacity of the CNT surfaces in this structure was estimated by extrapolating equilibrium adsorption amounts at different dye concentrations using the Langmuir isotherm. This was found to be about 43.5 mg/g, which compares favorably with adsorption capacity of isolated nanotubes. It must be noted that whereas isolated nanotubes can disperse in the liquid and pose environmental threats, the hierarchical solids as demonstrated in this study can be repeatedly agitated in water without the loss of CNT or performance deterioration. These results indicate promising application potential for these types of hybrid materials in environmental purification application