Steps in the NO to NO2 oxidation reaction, catalyzed by Pt nanoparticles. Particles tended to flatten, or redisperse, with PtO forming on their surfaces.
Stricter environmental regulations enacted in the last few years are putting a squeeze on emissions from car engines, including nitrogen oxide. While modern "lean-burn" gasoline and diesel vehicles use less fuel, they also require more oxygen. As a result, traditional three-way catalytic converters – which scrub out the nitrogen oxide, unburned hydrocarbons, and carbon monoxide – don't adequately reduce the rate of nitrogen oxide emissions.
These circumstances are spurring researchers to investigate how to optimize the catalytic reactions that govern the storage and reduction of nitrogen oxides (NO and NO2, collectively denoted NOx) in a vehicle's exhaust stream. These updated processes require a catalyst that performs two jobs: a storage function, often using a barium-oxide-based material as the storage medium; and an oxidation-reduction function, typically based on platinum (Pt), which converts NO to NO2 using oxygen in the air.
In this work, researchers from the University of Central Florida, Brookhaven National Laboratory (BNL) and Ruhr University Bochum in Germany studied a model catalyst system of Pt nanoparticles supported on a surface of aluminum oxide (Al2O3). Using x-rays at Brookhaven's National Synchrotron Light Source, the group followed the evolution of the particles' structure and chemical state during the conversion of NO to NO2, which is a crucial step in the full NOx storage and reduction reaction (of which the final products are nitrogen and oxygen). The information they learned will support additional studies aimed at finding optimal nanoscale Pt catalysts for modern lean-burn engines.
The nanoparticle samples used in this investigation were roughly the same diameter, 0.6 nanometers, but different shapes: two types were relatively flat, the third was more spherical. They were studied under "lean operation conditions, " which means that there is an excess of air as opposed to an excess of fuel. At NSLS beamline X18B, the researchers "watched" the reaction using two x-ray absorption techniques that are sensitive to the local electronic structure of the molecules in the sample. The techniques yielded specific information on the nature of the Pt-O bonds as the reaction took place.