Vehicle Emissions

Nanocatalytic Converters

Presentation

Nanofabrication consists of two approaches: bottom-up and top-down. The top-down method achieves a higher resolution, and bottom-up growth achieves a finer manipulation. Top-down methods, such as electron beam lithography, etch material subtractively, and are limited to 10 nm resolution. Interest has shifted to bottom-up processes, as they manipulate on the scale of the atom. However, there are issues in controllability (i.e. the creation of uniform structures), and the limitation of materials and structures that can be created.1,2 The result of nanofabrication in catalytic converters is cost savings and health benefits, but progress is slow due to process development and material selection.

The first catalytic converter in the US was invented in 1974 by John J. Mooney and Carl D. Keith of Engelhard Corporation. In 1981, two-way catalytic converters, which oxidized CO and HC, were replaced by three-way catalytic converters, which could also reduce NOx. These main pollutants are the result of the combustion of fossil fuel in an internal combustion engine. They are emitted from the tailpipe as primary pollutants and become secondary pollutants, such as ground-level ozone (O3) in sunlight.3 To decrease these three most harmful emissions, HC and CO are oxidized to CO2 and water by reacting with platinum and palladium, and NOx is reduced to N2 and O2 by reacting with platinum and rhodium.3 NOx is a problem at high engine temperature (> 2,700 C), not at engine start-up.3

A catalytic converter consists of a monolith substrate, washcoat, and noble metals. The substrate consists of ceramics (i.e. cordierite) or metals (i.e. Al2O3 or Kanthal (FeCrAl)) and has a honeycomb structure with parallel channels of 0.5 – 100 mm in diameter (Patel et al. 2012). The precious metals are embedded in the functionalized substrate. These catalyst particles are suspended in the washcoat solution (i.e. catalyst carrier) and deposited on the catalyst support and form the catalyst bed. The washcoat consists of alumina, silica, titania, or Ce and Zr.3

One of the pressing issues facing society is the burning of fossil fuels and its creation of anthropogenic pollution. Air pollution can be decreased by pre-treatment of fuel and post-treatment of exhaust gases. There will always be incomplete combustion, resulting in unburned hydrocarbons (UHCs), fuel evaporation, volatile organic compounds (VOCs), and imperfect post-treatment of exhaust gases. Transport pollution accounts for 28% of total air pollution.3 This may not seem significant, but most vehicles are driven in densely populated areas, which is hazardous to human health.

Post-treatment of exhaust gases reduces acid rain, ozone depletion, CO and particulate matter, or any of the 189 hazardous air pollutants defined in the 1990 Clean Air Act.3 Using precious metal catalyst nanoparticles reduces the material and pollution generated. Precious metals such as platinum, palladium, and rhodium on a ceramic base act as catalysts. Nanoparticles have a higher surface area to volume ratio, and therefore fewer precious metals are needed to achieve the same catalysis. 70% to 90% less material is used by depositing 5 nm particles, which significantly reduces costs.5 Dynamic light scattering ensures the particles are monodispersed.

Catalytic converters have high cold-start emissions, high material costs, and both direct and indirect health risks. Cold-start emissions can be passed through a separate absorber, such as a zeolite or molecular sieve substance, in which 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 the manufacture of catalytic converters and when they break off over time and flow out of the tailpipe. Research suggests emissions of platinum-group metals (PGMs) from catalytic converters along US roads could be the root cause of an alarming rise in allergies and asthma.5 Costs and health effects are abated by using fewer precious metal nanoparticles as the catalyst.

Nanoparticles, however, pose performance challenges. When exposed to high-temperature exhaust gases, they may sinter together. This causes nanoparticles to agglomerate out of the nanosized regime, and negates the benefits of the large surface area to volume ratio. CSI, now part of CDTi, is a company that uses a mixed-phase catalyst (MPC) of oxide particles and 5-10 nm PGMs to avoid sintering.

One approach is to embed precious metals (< 5 nm diameter) in fixed positions on the surface of ceramic beads (100 nm diameter).5 A Japanese company claimed to have achieved this configuration in Oct. 2007, but at that time they still had sintering issues.The Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) in Dresden, Germany studied how to prevent sintering, and Mazda has also worked on the technology from 2003 – 2007.5 Previously exhaust heat would cause the catalyst particles to migrate over the ceramic beads and agglomerate into larger particles (100 nm). By coating platinum particles with a porous silica layer, thermostability is significantly improved. Nissan reported a 50% reduction in precious metal use and planned to share its technology with French partner Renault. They also planned 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.7 Given this low percentage, the goal is to grow different shapes for the precious metals and washcoat, making the surfaces more catalytically active.4 Moreover, using different materials that are both more chemically active and less expensive is paramount. An inexpensive alternative to PGMs is using a less expensive cobalt catalyst. An example of a different shape that would increase catalysis is nanotetrapods, which can grab the passing gas molecules. Tetrapods are similar in crystal structure to a tetrahedral shape and 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 for a strong chemisorption bond with the exhaust gases.

My experimental design will focus on nanotetrapods. There are nanoscale and microscale pattern methods with varying accuracy. Possible methods of forming nanotetrapods or 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 simple operation, a wider selection of substrates due to mild reaction conditions, and a simple scale-up of production 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 nanomachining or nanoprinting also exist, but are cost-prohibitive.9 An interesting morphology was created by selective dissolution to create nanotetrapods in a hollow nano-tetrapod.The CdS nanotetrapods were created and then encapsulated with silica shells.9 Next, an HF etching was performed that can selectively remove the inner shell, while the outer shell remains intact from a higher degree of Si-O cross-linking.9 The tetrapods are then decorated with platinum nanoparticles. Platinum can stick to the outer shell and nucleate because the silica inner shell is etched away.9 Further, the CdS nanotetrapods placed within the hollow SiO2 interiors can be exposed to Ag+ or Pd2+ solution for one hour, allowing them to undergo further cation exchange, resulting in Ag2S or PdS nanotetrapods in the SiO2 interiors.9

Other nanostructures, in addition to tetrapods, exist, such as branched growth from 1D structures (e.g. 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 Tantalum 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 form dense arrays of ZnO nanobranches.9 So far, nanowires of ZnO, TiO2, SnO2, Fe2O3, and WOx have been used as backbones for secondary nanobranch formation.9 Additionally, 2D structures (i.e. disks and sheets) could also support the growth of nanobranches.9

From a real-world application, the next step, after proving that 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, pi-pi 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 gold and uranium atoms in many steps, followed by etching with an aqueous NaOH solution, results in a 3D network of interconnecting nanochannels.CdTe nanowires or other metal nanowires can fill the nanochannels through electrodeposition.9 Finally, a free-standing 3D structure is created by removing the polymer matrix.9

Another configuration for catalysts is single-walled nanohorns (SWNHs), which are derived from single-walled nanotubes (SWNTs). The nanohorns have a tubule length of 40-50 nm, a diameter of 2-3 nm, and a cone opening angle of 20 degrees. Thousands of nanohorns 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, TiO2 as substrate material should be tested. Next, platinum particles can be dispersed onto the substrate by a nanoporous silica solution. The two methods for synthesizing the carbon SWNHs are high purity CO2 laser ablation or arc discharge without a metal catalyst. The size and purity of the SWNHs can be changed by varying process parameters, such as temperature, pressure, voltage, and current. Methods for functionalizing carbon nanohorns include covalent bonding, pi-pi 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 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 form a coordination network.

My design would be like the structure and materials used in figure 4. Initially, creating the 1D TiO2 nanowire and functionalizing its surface. Then, through solution synthesis, deposit the SiO2 washcoat on the fibers. The washcoat would be the catalyst carrier and contain suspended nanotetrapod platinum particles. Lastly, platinum particles could be functionalized to increase their catalytic activity and ensure strong chemisorption of exhaust gases on 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
  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
  6. Thole, Julie. “Nanotechnology promises better catalytic converter.” 2010
  7. Tilley, Richard. “Catalytic Converters and Platinum Nanoparticles.” Science Learning Hub. The University of Waikato. 2008
  8. Serp, Philippe, Philippot, Karin. “Nanomaterials in Catalysis”. 2009
  9. Suib, Steven L. “New and Future Developments in Catalysis: Catalysis by Nanoparticles”. Elsevier. 2013.
  10. “Metal-Organic Frameworks: CO2 Capture”. Long Group.