Ever since the work carried out by people such as Robert C. Stempel in the 1970’s, automotive catalytic convertors have used precious metals such as platinum, palladium and rhodium to promote the reaction of exhaust gases; by so doing converting nitrous oxides, carbon monoxide and unburned hydrocarbons into nitrogen gas, carbon dioxide and water.
Initially when regulations only mandated reductions in hydrocarbon and carbon monoxide emissions platinum and palladium were used as catalysts. Later, when NOx standards were added to the regulations, rhodium was incorporated into the catalyst mix.
Forty years later these metals still dominate the catalytic landscape; but this domination is being threatened by several factors.
According to Johnson Matthey, a leading refiner of precious metals, industrial uses excluding catalytic converters make up about 30% of total platinum demand. Catalytic converters for the auto industry use 33% of the world’s platinum while jewelry accounts for 29%. The remaining 8% is made up of platinum coins, bars and ingots used for investment purposes.
Strikes and other labour unrest by miners in South Africa and higher taxation by the government have caused the largest platinum miner, Anglo American Platinum (AGPPY), to close some of its South African mines.
In neighbouring Zimbabwe, the government has banned the export of platinum ore and has given miners two years to build platinum refineries in an effort to boost the value added to platinum exports. In a further development, the government recently expropriated land owned by Zimplats, the biggest platinum miner in Zimbabwe, curtailing the company’s expansion plans.
Faced with high costs and these unsettling conditions automotive manufacturers and research teams have been working on solutions to either use the metals more effectively or even replace them entirely.
Catalytic material developments to reduce the use of platinum group metals
As far back as 2008 the Japanese Mitsui Mining and Smelting Company developed a catalyst that replaced platinum with silver in converters used on diesel engines.
Despite initial results showing potential, especially in cost savings, the technology is yet to be seen in production.
As Mitsui Mining discovered, laboratory techniques and processes are often difficult to replicate in production. One company that has been able to convert research into a viable product is Honda.
The new development allows palladium to speed up the process of absorption and desorption of oxygen, thereby enabling reduced use of rhodium in the purification of exhaust emissions while reducing overall use of precious metals by 22% (including a 50% reduction in rhodium). Moreover, the development of the new catalyst has reduced the cost by 37%.
However, with diesel catalytic convertors being the primary consumers of platinum, a team at Nanostellar in Redwood, California, has developed a mineral catalyst that outperforms platinum at a fraction of the cost.
Dr. Kyeongjae “K.J.” Cho, professor of materials science and engineering and physics at UT Dallas and co-founder of Nanostellar relates that computer modelling showed that, although rare in nature, mullite was a cost-effective substitute. Notwithstanding the cost savings, laboratory tests indicate that converters using mullite would have 45 percent lower emissions than those using platinum.
Even though the compositions they studied are not the aluminosilicate mullite familiar to most materials scientists the atomic structure of the compound is the same,
Mn2O5, and referred to as “Mn-mullite” which is further doped with cerium and strontium.
Dr. Cho claims that the catalyst, called Noxicat has achieved the goal of replacing precious metals with oxides that are commonly found in the environment.
Confirming the findings on oxides, a team led by Wei Li at General Motors research and development labs in Warren, Michigan, earlier showed that oxides of the common mineral ore perovskite — fortified with the metals strontium and palladium — perform better than platinum at cleaning up the exhaust emissions of ‘lean-burn’ combustion engines. This also applies to diesel engines.
Key to the performance of the catalyst is the use of strontium, which helps the oxygen atoms to bind to the catalyst during the crucial oxidation step.
In a discussion on platinum replacement in automotive catalytic convertors, David Jollie of Johnson Matthey, claims the real challenge will be to get the technology from the laboratory to the vehicle.
With several technologies and materials showing promising results there can be little doubt that platinum group metals’ days of dominance are numbered. Factor in electric vehicles and hybrids, and a drop in demand by the automotive industry can be predicted – at least for use in catalytic converters.