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Bottom-up Nano-fabrication Method to Improve Cost & Performance in Catalytic Converters

In general, nano-fabrication consists of two major approaches: bottom-up and top-down. The top-down method tends to achieve higher resolution whereas bottom-up growth achieves finer scale manipulation. Top-down methods, such as electron beam lithography, which subtractively etches material away, are limited to just 10 nm resolution. Interest has shifted to bottom-up processes for its ability to manipulate on the scale of atom and molecular sizes, so-called nano-manipulation. However, the drawbacks are issues in controllability (i.e. fabricating uniform structures), as well as being limited in the materials and types of structures that can be created.1,2 A nanofabrication technique should be flexible in the type of materials it can deposit and types of structures it can produce and have high speed and resolution.2 The role of nanofabrication in catalytic converters is bound to be impactful and lasting due to its cost savings and health benefits. However, the technical hurdles, such as process development and material selection, have slowed its progress. A nano-catalytic converter should be reliable, durable, and inexpensive, which is most easily achieved by manufacturing with a large-scale technique. 

The first catalytic converter in the US was invented in 1974 by John J. Mooney and Carl D. Keith at Engelhard Corporation. In 1981, two-way catalytic converters were replaced with three-way catalytic converters (TWC’s) which could reduce NOx in addition to oxidizing CO and HC. These main pollutants are the result of burning gasoline in an internal combustion engine. They are emitted from the tailpipe as primary pollutants and become ground-level ozone (O3), a secondary pollutant, in the presence of sunlight.3 To lessen these three most harmful emissions, HC and CO are oxidized to form CO2 and water by reacting with platinum and palladium and NOx undergoes a reduction reaction with platinum and rhodium to become N2 and O2.3 NOx is an issue at high engine temperatures (> 2,700 ºC), not during engine start-up.3

The current catalytic converter consists of a substrate/support, waistcoat, and noble metals. The substrate is made of either ceramic (i.e. cordierite) or metal (i.e. Al2O3 or Kanthal (FeCrAl)) and has a honeycomb structure with parallel channels of 0.5 – 100 mm diameter (Patel et al. 2012). The precious metals are embedded into the functionalized substrate. These catalyst particles are suspended in the washcoat solution, which is deposited on the monolith forming the catalyst bed. The particles are decorated on the geometrically rough and porous washcoat surface via solution synthesis. The washcoat, which acts as a heterogeneous catalyst/catalyst carrier, is typically alumina, silica or titania and can also be composed of Ce and Zr.3

One of the pressing issues facing society, especially as more countries become industrialized, is the burning of coal and fossil fuels and its creation of excessive anthropogenic pollution. Air pollution can be reduced by both pre-treating fuel and post-treating the exhaust. There will always be incomplete combustion resulting in unburned hydrocarbons (UHCs), fuel evaporation creating volatile organic carbons (VOCs), and less than perfect post-treatment of exhaust gases. The pollution from the transportation sector accounts for 28% of all air pollution.3 This may not seem significant to the total, but the majority of the vehicles operate in areas with high populations which is problematic due to the hazardous-to-human-health emissions.

Post-treatment of exhaust reduces pollution that causes acid rain, ozone depletion, CO and particulate matter, or any of the 189 hazardous air pollutants defined by the 1990 Clean Air Act.3 Substantial reduction in pollution and decrease in the precious metals used, which decreases the cost and the potentially harmful effects of the precious metals on people and the environment, is achieved by leveraging nanoparticle noble metal catalysts. Catalysts speed up chemical reactions and consist of the precious metals’ platinum, palladium, and rhodium over a ceramic base. The appropriately named precious metals are expensive which is why it is advantageous to use nanosized precious metal particles. The nanoparticles have a higher surface area to volume ratio and therefore less amount of precious metals are required to achieve the equivalent amount of catalysis of larger-sized particles. By depositing particles of 5 nm in size the amount of precious metals is decreased by 70% – 90% which significantly reduces the cost.5When ensuring the particles are monodispersed, the technique of dynamic light scattering is employed.

Limitations of catalytic converters include its high amount of cold-start emissions, its high material costs, and both direct and indirect health hazards. The cold-start emissions can be run through a separate absorber, such as a zeolite or a molecular sieve-type substance, where unacceptable emissions are captured and later released back into the exhaust stream when the engine has reached its “light-off” temperature. The precious metals have deleterious effects on humans both during catalytic converter manufacturing and when they mechanically break off over time and flow out of the tailpipe. Ongoing research suggests that emissions of platinum-group metals (PGM’s) from catalytic converters along US highways might be a root cause of an alarming rise in allergies and asthma.5 Costs and health effects are abated by using less precious metal material with nanoparticles as the catalyst. However, performance challenges exist with nanoparticles for example when exposed to the high-temperature exhaust gases they may sinter together. This causes the nanoparticles to agglomerate, growing out of the nanosized regime, and negate the benefits of the large surface area to volume ratio offered by particles of nano size. CSI now part of CDTi is a company that uses a mixed-phase catalyst (MPC) of oxide particles and 5-10 nm PGM’s to avoid sintering of the PGM particles.

One approach is embedding precious metals (< 5 nm diameter) into fixed positions in the surface of ceramic beads that are 100 nm across.5 A Japanese company as of Oct. 2007, claimed to have achieved this configuration yet at the time they still had sintering issues.The Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) in Dresden, Germany has been studying how to prevent sintering and Mazda also has been working on the technology from 2003 – 2007.5 Previously exhaust heat would cause the particles to migrate over the ceramic beads and agglomerate into larger particles (~100 nm). By coating the platinum particles with a porous silica layer, the thermostability is improved substantially. Nissan reported cutting precious metal usage by 50% and had planned to share its technology with French partner Renault. They also planned to launch a new vehicle using nanoparticle catalysts in late 2008 or early 2009.5

At the moment only about 10% of the platinum particles in a catalytic converter are active.7 Considering this low percentage, the goal is to grow different shapes for the precious metals and washcoat thereby making the surfaces more catalytically active.4 Furthermore, using different materials that are both more chemically active and less expensive is paramount. An inexpensive alternative to the PGM’s is the use of a less expensive cobalt catalyst. An example of a different shape that would increase catalysis is nano-tetrapods which can grab hold of passing gas molecules. Tetrapods are similar in crystal structure to a tetrahedral shape and, therefore, can have a bond reaching off the washcoat. This shape provides a sturdy three-leg base and a dangling bond or radical (a subgroup of atoms). The radical can be functionalized by giving it an electronic charge. This free covalent bond allows a strong chemisorption bond with the exhaust gases to take place.

The focus of my experimental design will be on nano-tetrapods. Nano and microscale patterning methods with varying degrees of accuracy exist. Possible methods of forming nano-tetrapods or just a simple multi-pod (0D – 3D, with only a few branches) shaped catalyst include CVD, PVD, VLS-growth, heteroepitaxy, solution synthesis, etc..9 Solution synthesis offers the advantages of easy operation, wider choices of substrates possible due to mild reaction conditions, and easy production scale-up at low cost.9 Preparing these structurally complex (0D core- nuclei, polyhedrons, cubes, or spheres) catalysts at low-cost solution synthesis is preferred.9 However, other methods such as templating, photolithography, micropatterning, etc. have been used to prepare catalysts with desired structural features and functionalities.9 Methods such as nano-machining or nano-printing also exist but are cost-prohibitive.9 An interesting morphology was created by using a selective dissolution to create nano-tetrapods in a hollow nano-tetrapod.The CdS nano-tetrapods were created and then encapsulated with silica shells.9 Next, an HF etching was performed which can selectively remove the inner shell while the outer shell remained intact due to a higher degree of Si-O cross-linking.9 The tetrapods are then decorated with Pt nanoparticles. Pt can stick to the outer shell and nucleate due to the silica inner shell being etched away.9 Further the CdS nano-tetrapods placed within the hollow SiO2 interiors can be exposed to Ag+ or Pd2+ solution for 1 hour allowing them to undergo further cation exchange resulting in Ag2S or PdS nano-tetrapods in the SiO2 interiors.9

Other nanostructures besides tetrapods exist such as branched growth off of 1D structures (i.e. tubes, rods, and wires).9 Figure 6a shows an SEM image of an array of carbon nanotubes (CNT’s)-ZnO.9 Radio-frequency sputtering was used to coat a vertical array of CNT’s (on a Ta plate) with a thin film of ZnO.9  It was then immersed in a solution saturated with Zn(OH)42- where hydrothermal reactions took place.9 The ZnO film acted as seeds to assist in the formation of dense arrays of ZnO nano-branches.9 So far nanowires of ZnO, TiO2, SnO2, Fe2O3, and WOx have been used as backbones for secondary nano-branch formation.9 Additionally, 2D structures (i.e. disks and sheets) could also support the growth of nano-branches.9 From a real application point of view, the next step after proving certain structures of a given material are possible is to create an ordered array of these structures with controlled size and spacing.9 Another option is template-less self-assembly which can be used to develop new catalysts and add functionalities.9 This method could entail weak chemical reactions involving van der Waals and capillary forces, π-π interactions and hydrogen bonds.9 Lastly, interconnecting nanowire networks can be created using conventional hard templates (i.e. mesoporous silica templates with uniform pores).9  Using a polycarbonate membrane as the hard template and irradiating with Au and U atoms in many steps followed by etching with an aqueous NaOH solution results in a 3D network of interconnecting nano-channels.CdTe nanowires, or another type of metal nano-wires, can fill up the nanochannels through electrodeposition.9 Finally, by removing the polymer matrix, a free-standing 3D structure is created.9

Another configuration for catalysts is single-walled nano-horns (SWNH’s). SWNH’s are derived from single-walled nanotubes (SWNT’s). The nano-horns have a tubule length of 40-50 nm, a diameter of 2-3 nm, and a cone opening angle of 20º. Thousands of the nano-horns are combined to form a ‘dahlia-like’ or ‘bud-like’ structure with a diameter of 80-100 nm. Instead of the current route of making the tubes out of carbon material the use of TiO2 as the substrate material should be tested. Next, Pt particles can be dispersed onto the substrate via a nanoporous silica solution. The two procedures for synthesizing the carbon SWNH’s are high purity CO2 laser ablation or arc discharge without a metal catalyst. The size and purity of the SWNH’s can be changed by varying the process parameters such as temperature, pressure, voltage, and current. Methods to functionalize carbon nano-horns include covalent bonding, π-π stacking, supramolecular assembly, and deposition of metal nanoparticles.

Metal-Organic Framework (MOF) is a configuration effective at treating exhaust gases, specifically COexhaust gas. MOF with exposed metal cation sites (Mg2) grabs onto CO2. CO2 is more strongly adsorbed by appending diamines to the open coordination sites. The metal cations, or coordination compounds, are connected via a ligand anion (linker or complexing agent) to make a coordination network.

My design would be similar to the structure and materials used in figure 4. Initially, creating the 1D TiO2 nanowire and functionalizing its surface. Then, through solution synthesis, depositing the SiO2 washcoat onto the fibers; the washcoat would be the catalyst carrier and contain suspended nano-tetrapod platinum particles. Lastly, the platinum particles could be functionalized to increase their catalytic activity and ensure strong chemisorption of exhaust gases to the particles.

References

  1. Tseng, A. A. Nanofabrication : Fundamentals And Applications. Singapore: World Scientific, 2008. eBook Collection (EBSCOhost).
  2. Yamazaki, Kenji, and Hiroshi Yamaguchi. “Universal Three-Dimensional Nanofabrication for Hard Materials.” Journal of Vacuum Science & Technology: Part B-Nanotechnology & MicroelectronicsComputers & Applied Sciences Complete. 2013.
  3. Shah, Rishabh Urvesh. “Automotive Air Pollution and its Control by Catalytic Converters.” The University of Illinois at Urbana-Champaign. 2013. https://www.academia.edu/5646250/White_Paper_Automotive_Air_Pollution_and_its_Control_by_Catalytic_Converters
  4. Markus, Frank. “Replacing precious metals with fake rubies Technologue.” Motor Trend. 2015.
  5. Stafford, Ned. “Catalytic Converters go Nano.” Royal Society of Chemistry. Chemistry World. 2007. https://www.chemistryworld.com/news/catalytic-converters-go-nano/3000781.article
  6. Thole, Julie. “Nanotechnology promises better catalytic converter.” 2010. http://phys.org/news204827696.html.
  7. Tilley, Richard. “Catalytic Converters and Platinum Nanoparticles.” Science Learning Hub. The University of Waikato. 2008. http://www.sciencelearn.org.nz/Contexts/Nanoscience/Sci-Media/Video/Catalytic-converters-and-platinum-nanoparticles.
  8. Serp, Philippe, Philippot, Karin. “Nanomaterials in Catalysis”. 2009. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.200805273
  9. Suib, Steven L.. “New and Future Developments in Catalysis: Catalysis by Nanoparticles”. Elsevier. 2013.
  10. “Metal-Organic Frameworks: CO2 Capture”. Long Group.

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