The burning of fossil fuels is one of the most pressing issues facing society, as it results in the creation of anthropogenic pollution. Measures to reduce air pollution, including pre-treatment of fuel and post-treatment of exhaust gases. However, despite these efforts, there will always be some incomplete combustion, resulting in unburned hydrocarbons (UHCs), fuel evaporation, volatile organic compounds (VOCs), and imperfect post-treatment of exhaust gases. Notably, transport pollution alone accounts for 28% of total air pollution, which may not seem significant, but most vehicles are driven in densely populated areas.3 This poses a significant risk to human health, especially in areas with high traffic density.
Post-treatment of exhaust gases is vital for mitigating the negative impacts of fossil fuel combustion. This process mitigates acid rain, ozone depletion, particulate matter, and any of the 189 hazardous air pollutants defined in the 1990 Clean Air Act.3 Using precious metal catalyst nanoparticles, such as platinum, palladium, and rhodium on a ceramic base, significantly reduces both the material and pollution generated. These nanoparticles have a higher surface area to volume ratio, requiring fewer precious metals for the same catalysis. By depositing 5 nm particles, up to 70-90% less material is used, leading to significant cost savings.4 To ensure the nanoparticles are monodispersed, dynamic light scattering is used.
In 1974, John J. Mooney and Carl D. Keith of Engelhard Corporation invented the first two-way catalytic converter in the US. In 1981, three-way catalytic converters were introduced, which could also reduce NOx in addition to oxidizing CO and HC. These three pollutants result from the combustion of fossil fuel in an internal combustion engine. They are primary pollutants emitted from the tailpipe and can become secondary pollutants such as ground-level ozone (O3) when exposed to sunlight. To reduce these harmful emissions, platinum and palladium are used to oxidize HC and CO into CO2 and water, while platinum and rhodium are used to reduce NOx into N2 and O2. Note that NOx is problematic at high engine temperatures (> 2,700°C) but not during engine start-up.
A catalytic converter is made up of three main components: a monolithic substrate, a washcoat, and noble metals. The substrate has a honeycomb structure with parallel channels ranging from 0.5 to 100 mm in diameter, and it can be made of ceramics (such as cordierite) or metals (like Al2O3 or Kanthal (FeCrAl)). The precious metals are embedded in the substrate, and these catalyst particles are suspended in the washcoat solution, acting as the catalyst carrier. The washcoat is typically made of alumina, silica, titania, or Ce and Zr, deposited on the catalyst support to form the catalyst bed.3
Catalytic converters present drawbacks, including high cold-start emissions, high material costs, and both direct and indirect health risks. To mitigate cold-start emissions, an absorber like a zeolite or molecular sieve captures emissions for later release into the exhaust stream when the engine has reached its light-off temperature. However, the use of precious metals in catalytic converters can have deleterious effects on human health, both during manufacturing and as they break off and flow out of the tailpipe. Research suggests that emissions of platinum-group metals (PGMs) from catalytic converters along US roads could be contributing to an alarming rise in allergies and asthma.4 To reduce costs and health risks, using fewer precious metal nanoparticles as catalysts is recommended.
Nanofabrication involves building atom-by-atom or molecule-by-molecule, nanomachining involves removing material, and nanoprinting involves depositing material onto a surface in a specific pattern. Methods such as nanomachining or nanoprinting tend to be cost-prohibitive.8 Nanofabrication can be achieved through two approaches: top-down and bottom-up. Top-down methods, like electron beam lithography, subtractively etch materials and can achieve resolutions up to 10 nm. On the other hand, bottom-up methods manipulate at the atomic level and offer finer control. Despite its potential, the bottom-up approach faces challenges with controllability and the selection of materials and structures. In catalytic converters, nanofabrication has the potential to deliver cost savings and health benefits, but progress is slow due to the need for process development and material selection.
Although nanoparticles are effective catalysts, they can pose performance challenges when exposed to high-temperature exhaust gases. This is because they may sinter together, causing agglomeration and loss of nanosized properties. To avoid this issue, some companies use a mixed-phase catalyst (MPC) consisting of oxide particles and 5 to 10 nm PGMs. CSI, now part of CDTi, is utilizing this approach. By using oxide particles in addition to PGMs, the MPC can prevent sintering and maintain the nanosized properties.
An approach to improve the catalytic converter efficiency involves embedding precious metals with a diameter of less than 5 nm in fixed positions on the surface of ceramic beads that are 100 nm in diameter.5 In October 2007, a Japanese company claimed to have achieved this configuration, but encountered sintering issues at that time.5 The Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) in Dresden, Germany conducted a study on how to prevent sintering, while Mazda worked on this technology between 2003 and 2007.5 Previously, the migration of catalyst particles over the ceramic beads caused them to agglomerate into larger particles of 100 nm due to exhaust heat. However, coating platinum particles with a porous silica layer significantly improves thermostability. Nissan reported a 50% reduction in the use of precious metals and planned to share this technology with its French partner, Renault. They also intended to launch a new vehicle with nanoparticle catalysts in late 2008 or early 2009.5
Currently, only 10% of platinum particles in a catalytic converter are active in catalyzing the conversion of harmful exhaust gases, but the aim is to enhance this catalytic activity by exploring different shapes for precious metals and the washcoat.6 One promising shape for increased catalysis is nanotetrapods, which resemble a tetrahedral shape. The structures, with a bond extending off the washcoat, provide a sturdy three-leg base and a dangling bond or radical, which is a subgroup of atoms. The radical can be modified by imparting an electronic charge, creating a free covalent bond that enables a strong chemisorption bond with the exhaust gases. It is also crucial to explore different materials that are more cost-effective, such as using a cobalt catalyst instead of PGMs.
Nanotetrapods can be created using several methods, including CVD, PVD, VLS-growth, heteroepitaxy, and solution synthesis.8 Solution synthesis is an advantageous method for creating nanotetrapods due to its ease of use, flexibility in substrate choice, mild reaction conditions, and low scalability costs.8 This technique is ideal for producing complex 0D catalyst structures such as nuclei, polyhedrons, cubes, or spheres.8 Other methods such as templating, photolithography, and micropatterning have also been used to create catalysts with specific structural features and functionalities.8
A novel morphology is achieved through selective dissolution to produce nanotetrapods within a hollow nano-tetrapod structure.8 The process involves creating CdS nanotetrapods and coating them with silica shells, followed by an HF etching step to remove the inner shell while preserving the outer shell due to its stronger Si-O cross-linking.8 Platinum nanoparticles are then added to the tetrapods, which bind to the outer shell and nucleate as a result of the etching process that removes the silica inner shell.8 Additionally, the CdS nanotetrapods inside the SiO2 interiors are subjected to Ag+ or Pd2+ solution for one hour, inducing further cation exchange and generating Ag2S or PdS nanotetrapods within the SiO2 interiors.8
There exist other nanostructures besides tetrapods, such as branched growth from 1D structures like tubes, rods, and wires.8 An SEM image of an array of carbon nanotubes (CNT’s)-ZnO is shown in Figure 6a.8 The vertical array of CNT’s on a Tantalum plate was coated with a thin film of ZnO using radio-frequency sputtering.8 The structure was then subjected to hydrothermal reactions in a solution saturated with Zn(OH)42-, where the ZnO film acted as seeds to form dense arrays of ZnO nanobranches.8 Secondary nanobranches have been formed using nanowires of ZnO, TiO2, SnO2, Fe2O3, and WOx as backbones, and two-dimensional structures like disks and sheets could also support nanobranch growth.8
After confirming the feasibility of certain structures in a given material, the subsequent stage involves arranging these structures in a controlled manner with a specific size and spacing.8 One possible technique is template-less self-assembly, which can produce novel catalysts and functionalities by leveraging weak chemical reactions such as van der Waals and capillary forces, pi-pi interactions, and hydrogen bonds.8 Additionally, traditional hard templates, such as mesoporous silica templates with uniform pores, can be used to create interconnected nanowire networks.8 For example, by utilizing a polycarbonate membrane as a hard template and undergoing a multi-step process involving gold and uranium atom irradiation, followed by etching with an aqueous NaOH solution, a 3D network of interconnecting nanochannels can be formed.8 These channels can then be filled with CdTe or other metal nanowires through electrodeposition, and the resulting 3D structure can be made free-standing by removing the polymer matrix.8
An alternative configuration for catalysts involves single-walled nanohorns (SWNHs), derived from single-walled nanotubes (SWNTs). With a tubule length of 40-50 nm, diameter of 2-3 nm, and cone opening angle of 20 degrees, thousands of nanohorns combine to form a “dahlia-like” or “bud-like” structure measuring 80-100 nm in diameter. Instead of the typical carbon material, TiO2 substrate material could be explored, followed by dispersion of platinum particles onto the substrate through a nanoporous silica solution. Synthesizing carbon SWNHs involves high purity CO2 laser ablation or arc discharge without a metal catalyst, where process parameters like temperature, pressure, voltage, and current can change the size and purity of the SWNHs. Functionalizing carbon nanohorns can occur through covalent bonding, pi-pi stacking, supramolecular assembly, or deposition of metal nanoparticles.
To treat exhaust gases, particularly CO2 exhaust gas, a configuration like Metal-Organic Framework (MOF) could be effective. Exposed metal cation sites (Mg2) in MOF can capture CO2, which is further adsorbed with diamines attached to open coordination sites. Metal cations or coordination compounds are linked through ligand anions (linkers or complexing agents) to form a coordination network. A proposed design would use the structure and materials depicted in Figure 4. Initially, the surface of the 1D TiO2 nanowire would be functionalized. Next, the SiO2 washcoat would be deposited onto the fibers through solution synthesis. The washcoat would act as the catalyst carrier, containing suspended nanotetrapod platinum particles. Finally, platinum particles could be functionalized to enhance their catalytic activity and ensure strong chemisorption of exhaust gases on the particles.