With emissions stories dominating headlines over the past year, such as VW’s international diesel scandal and the domestic scuffle over the EPA’s new woodstove regulations, rarely has this much public attention been directed toward catalytic converters, which are designed to reduce harmful emissions. But catalysis offers more than just tailpipe emissions control.
For hundreds of years, catalysts have steered our society toward sustainability goals by reducing energy consumption and allowing the cleaner production of plastics, as well as commodity and fine chemicals.
The catalytic converter in gasoline cars works by lessening the energy input required to convert carbon monoxide and unburned hydrocarbons into more benign CO2. These exhaust gases flow over a precious metal surface, where they combine with oxygen in their conversion to CO2. The catalyst remains unchanged, allowing thousands upon millions of reactions over its lifetime.
In addition to carbon monoxide and unburned hydrocarbons, under certain operating conditions, diesel engines contend with nitrogen oxides (NO and NO2, generically referenced as NOx). As a major contributor to smog in urban areas — not as big of a concern here in rural Maine — NOx emissions rightfully receive tight scrutiny and require their own special catalytic converter, where NOx reacts on a metal surface with other nitrogen-rich compounds such as urea or ammonia to produce the benign products nitrogen gas, water and potentially CO2.
While catalytic conversion of gaseous small molecules in tailpipe emissions currently draws significant public attention, catalytic gas conversion has been an important industrial process for years. For over a century, we have been converting atmospheric nitrogen gas into other nitrogen-rich compounds used for fertilizers, cleaners, refrigerants, explosives and much more. At the turn of the 20th century, we used an extremely energy-intensive non-catalytic process for converting nitrogen and oxygen gases to nitric acid, which eventually was replaced by an alternative, catalytic process.
Here in Maine, we’re lucky to have local suppliers of high-quality composted fertilizers who rely on inputs of nitrogen-rich waste-streams such as lobster shells, food scraps and manure. Nevertheless, the global importance of nitrogenous feedstocks provided by these synthetic processes cannot be overstated, and we have catalysis to thank for roughly a hundred years of energy savings on a massive industrial scale.
The 2010 Nobel committee’s recognition of Akira Suzuki, Ei-ichi Negishi and Richard F. Heck for their work in developing catalytic techniques to stitch together larger molecules from smaller individual pieces demonstrates the importance of catalysis in our society beyond the conversion of gases over a solid surface. Without the processes they pioneered, the synthetic route to many materials and pharmaceuticals would have either never been realized or would require a greater energy and material input to arrive at the desired product.
To forge a more sustainable path forward, we need to think not only about the sourcing of raw materials and the properties of our products but also on how we process these raw materials into products. Catalysts can help us achieve these goals. They are more than emission control systems on your automobile; they drive down energy demands and often the material inputs and waste outputs for chemical processes.
Reuben Hudson is a postdoctoral fellow in chemistry at Colby College in Waterville.