Catalytic Converters Decrease the Emissions of Newer Cars

As we slowly passed through the infamously congested intersection of the 60, 90 and 215 freeways in Riverside, Calif., we could see blue sky. Thirty years ago when we passed this same intersection, buildings along the road were barely visible, let alone the sky because of the smog.

Much of the credit for cleaning up the air here, and for the rest of country, goes to the catalytic converter, originally developed by Engelhard Minerals & Chemicals Corp. The Environmental Protection Agency (EPA) points out that compared to vehicles of the early 1970s, vehicles now sold in the U.S. emit 96 percent less carbon monoxide (CO), 98 percent less hydrocarbons (HC), and 90 percent less oxides of nitrogen (NOx).

Catalytic converters typically use a ceramic or stainless steel honeycombed monolith substrate containing precious metal catalysts. The core, or substrate, supports the catalyst. The catalyst — noble metals like platinum, palladium, and rhodium — provides faster chemical reactions at a lower temperature. The coated substrate is wrapped in mat that expands when heated, securing and insulating the substrate. All this is contained in a stainless steel shell usually located between the engine and muffler. As exhaust gases pass over the catalysts they are converted into harmless gases and water.

Between 1975 and 1981, two-way catalytic converters were installed. These converted carbon monoxide to carbon dioxide and unburned hydrocarbons — that is, unburned and partially-burned fuel — to carbon dioxide and water. Starting in 1981, the three-way converter was installed in order to meet more stringent emission standards that now required reductions in NOx (nitrogen oxides) emissions, as well. A third reaction converted nitrogen oxides to nitrogen and oxygen.

Since emission regulations only specify required reductions and not the methods in which they must be accomplished, catalytic converters are not mandated. However, currently they are the only practical means for spark ignition engines to meet today's emission requirements.

Unleaded gasoline is required because catalysts are poisoned by lead. Catalytic converters alone cannot be credited for cleaning the air. They were greatly aided by electronic engine control (EEC) systems that are required for converters to operate properly. EECs also brought more efficient and cleaner running engines.

As their name might imply, precious metals are rare and expensive. Platinum is about $1400 an ounce, palladium $350, and rhodium $6,400. Indeed, catalytic converters are frequently stolen off vehicles even though they contain quite small amounts of the precious metals, typically less than $50 worth. Also, catalytic converters have to compete for these precious materials with another rapidly growing application — fuel cells.

As the price of precious metals rise, automobile and catalytic converter manufacturers are researching ways to reduce the amount used in their catalytic converters. For example, Mazda has developed a method that uses nanotechnology to create a catalyst material structure requiring substantially less precious metals. This enables Mazda to reduce the amount of platinum and palladium used in automotive catalysts by 70 to 90 percent without any loss in performance or durability. Likewise, Nissan is using nanotechnology for more uniform application of the catalytic materials so less material is required.

Of course, precious materials are extensively recovered from used converters and recycled. Now, researchers at the Cardiff University in Wales are proposing recovering platinum from roadways, where road sweepers would recover minute amounts of platinum that are emitted from vehicle tailpipes.

Nanostellar, a spinoff from Stanford University, has developed an application of nanotechnology that would allow less expensive gold and palladium to be substituted for more expensive platinum used in catalytic converters. Nanostellar's technology also allows manufacturers to readily change material between gold, palladium, and platinum according to their market price.

Catalysts play other important emissions reduction roles, as well. Engelhard, now BASF Catalysts, developed its PremAir catalyst that destroys harmful ground-level ozone already in the air. Ozone is the main component of smog. When applied to heat-exchange surfaces such as car radiators, PremAir effectively turns them into "smog eaters." The catalyst reduces ozone in the air by converting ozone molecules that pass over coated surfaces into oxygen molecules upon contact. This destroys up to 80 percent of the ground level ozone that passes across the catalyst surfaces. Seven car companies have used PremAir catalysts and PremAir catalysts are at work in over 3 million vehicles in the U.S. It's just another example of better living — or breathing — through chemistry, as they say.