Removal of nitrogen oxides from car exhaust fumes continues to be a challenge for environmental scientists. Now chemists in Poland might be able to help.
Car exhaust fumes are a major source of nitrogen oxides (collectively known as NOx), which contribute to acid rain and ground-level ozone, the major ingredient in smog. Catalytic converters are widely-used used to remove NOx from exhaust emissions, using a process called selective catalytic reduction (SCR). But despite the widespread use of SCR, little is known about the active sites in the catalytic process, in particular the oxidation state of the metal centres in the catalyst.
Piotr Pietrzyk and Zbigniew Sojka from Jagiellonian University, Krakow, Poland, have now discovered a missing piece of the mechanism, which may provide crucial information in understanding this catalytic process.
The catalytic cycle in catalytic converters is still not well understood
SCR works by reducing NOx to water vapour and nitrogen gas. NOx is mixed with a carrier gas (usually hydrogen or a hydrocarbon gas) and then absorbed onto a metal-containing catalyst. This allows NOx to be reduced to harmless by-products at mild temperatures.
Pietrzyk says that, until now, the role of the metal centre in the mechanism had been overlooked. 'Lots of papers dealing with spectroscopic studies of the SCR of NOx systems are limited to IR (infra-red) measurements... that give only the ligands' point of view, ignoring the state of the metal site,' he said.
Pietrzyk and Sojka used electron paramagnetic resonance (EPR), a method that provides information about the oxidation state of a metal centre, to look at the catalytic process. They found that the metal centres, in this case cobalt, are reduced from a plus two oxidation state to a zero oxidation state during the reaction. The Polish scientists believe that these reduced cobalt centres are more likely to react with the carrier gas containing the NOx, explaining the unusual effectiveness of cobalt catalysts in NOx reduction.