A research team from the University of Pittsburgh has overcome a major obstacle that has so far limited the development of nanomaterials, from which they produce effective catalysts for producing hydrogen and reducing the toxicity of vehicle emissions.
Professor Götz Veser from the School of Engineering, and his research team, prepared metal alloy particles with a thickness of four nanometers that are able to withstand temperatures of more than eight hundred and fifty degrees Celsius - at least two hundred and fifty degrees higher than typical metallic nanoparticles. Due to their special design, from the catalytic metals platinum and rhodium, these particles become very active by removing the heat-sensitive components within them as the temperature increases, similar to a gecko shedding its tail for self-defense.
"The internal instability of this type of particles has been an obstacle to the development of many applications, from sensors to fuel production," explains the researcher. "The amazing ability of nanoparticles to give birth to completely new fields and allow the development of more efficient processes in a dramatic way has been demonstrated in laboratory experiments, but little of this has been practically translated into everyday life due to this sensitivity to heat. For us, in order to fully exploit the benefits of nanoparticles, they must withstand harsh conditions that exist in practical applications."
The researchers present an original approach to the stabilization of metallic catalysts smaller than five nanometers in size. Materials within this size range present a larger surface area and thus enable the utilization of the particles for more efficient reactions. However, they also pile together at a temperature of six hundred degrees Celsius - a temperature lower than most of the temperatures at which normal catalytic reactions occur - and become too large a mass of particles.
Attempts to stabilize the metals have included putting them into heat-resistant nanostructures, but the most promising methods have only succeeded for a size range of 15-10 nanometers, the lead researcher wrote. Researcher Veser designed oxide-based nanostructures that stabilize particles as small as ten nanometers.
In their study, the scientists combined platinum with rhodium, which has a high melting point. They tested the alloy by a combustion reaction in methane and found that the composite structure was not only a very active catalyst, but that its particles maintained an average size of 4.3 nanometers, even when exposed to heat of 850 degrees Celsius. In fact, small amounts of particles as small as four nanometers remained even when the temperature reached 950 degrees Celsius, although most particles swelled to eight times that size.
The researchers were surprised to find that the alloy did not simply withstand the enormous heat. Instead, she "sacrificed" the more heat-sensitive metal, platinum, and built a new array of rhodium-rich catalyst at the end of the reaction. At a temperature of about 700 degrees Celsius, the alloy began to melt. The platinum was slowly removed from the structure forming larger particles along with other stray platinum atoms, leaving the more stable alloy particles to anneal. The researchers predicted that this self-stabilization process would occur in all metal catalysts composed of an alloy containing another, more stable metal.
