August 10, 2012
A Cheaper and Cleaner Catalyst for Burning Methane
April Flowers for redOrbit.com - Your Universe Online
Natural gas has become an increasingly important energy source as the world's accessible oil reserves are depleted.
The primary component of natural gas is methane, which releases less carbon dioxide than many other hydrocarbon fuels. However, because the methane molecule is so stable, the energy within can't always be accessed, and escaping unburned methane is a greenhouse gas twenty times more powerful than carbon dioxide. This makes current methods of using methane as an energy source a Pandora's Box of good and bad.
Researchers from the University of Pennsylvania, along with international colleagues from Italy and Spain, have created a new material that catalyzes the burning of methane 30 times better than currently available catalysts.
Published in the journal Science, this discovery offers a way to more completely exploit energy from methane, potentially reducing emissions of this powerful greenhouse gas from vehicles that run on natural gas. It may also offer a cleaner and cheaper way of generating energy from catalytic combustion in gas turbines.
"It's hard to come up with materials that are active enough and stable enough to withstand the harsh conditions of methane combustion," said Raymond J. Gorte, the Russell Pearce and Elizabeth Crimian Heuer Professor in Penn's Department of Chemical and Biomolecular Engineering. "Our materials look promising for some important applications."
Catalysts are materials that make a chemical transformation quicker, easier, more energy-efficient and often safer. Catalysts that are currently available to burn methane, however, do not do so completely. The unburned methane escapes into the atmosphere and contributes to climate change.
Additionally, these conventional catalysts can require high temperatures of 600 — 700 degrees Celsius to encourage reactions to move along. This is problematic, as many of the catalysts themselves lose efficiency or deactivate when exposed to the high temperatures generated by methane combustion.
Even more environmental harm may result when methane is used to produce energy in a gas turbine. Methane in this process is burned at over 800 degrees Celsius, and when the temperatures reach around 1,300 degrees or higher, harmful byproducts such as nitrogen oxides, sulfur oxides and carbon monoxide are produced.
Metal nanoparticles, in particular palladium (Pd), deposited on oxides such as cerium oxide (CeO2), are used as conventional catalysts. The research team tweaked that approach, instead using a method that relies on self-assembly of nanoparticles.
They built palladium particle at just 1.8 nanometers in diameter and then surrounded them with a porous shell of cerium oxide. Because small particles like this tend to clump when heated, reducing the efficiency of a catalyst, the team deposited them on a hydrophobic surface composed of aluminum oxide to ensure even distribution.
"These techniques are common in the nanotechnology community, but I think it's a novel approach in making catalyst materials," Gorte said.
Testing showed that the core-shell nanostructure performed 30 times better than the best methane catalyst currently available, using the same amount of metal. It completely burned methane at 400 degrees Celsius.
The team intends further study on the new catalyst to better understand why it works so well. They also want to try the same methods on other materials.
"We can use this assembly method to test different types of metals and oxides," Cargnello said. "That will allow us to prepare a whole library of materials, some of which might be very good at catalyzing reactions besides methane combustion."