Chemical engineers have long known that the majority of catalytic reactions occur at the nanoscale, where materials have a larger relative surface area and greater reactivity.
Now, new evidence shows that some metal-oxide catalysts achieve an additional and dramatic increase in reactivity at the extreme low end of the nanoscale, in quantities measuring 1-2 nanometers or less.
Israel Wachs, the G. Whitney Snyder Professor of chemical engineering, made this discovery while anchoring nanoparticle catalysts of titania on a silica substrate. He succeeded in developing a “multi-level substrate,” with the titania serving as a nanoscaffold that is able to anchor metal-oxide catalytic active sites and control their reactivity.
One of those catalysts, tungsten oxide, is a solid acid whose applications include increasing the octane content of gasoline.
Wachs and his group were able to attach catalytic active sites of tungsten oxide to nanoparticles of titania ranging in size from 5 nm to less than 1 nm.
“Tungsten oxide’s behavior is a function of what it is attached to,” says Wachs. “If you’re seeking greater acidity in tungsten oxide, it is better for the tungsten oxide to be attached to titania than to silica, and it is better for it to be attached to smaller rather than to larger particles of titania.
“We achieved 100 times as much acidic reactivity with tungsten oxide attached to nanoparticles of titania measuring 1 nm or less, as we did with tungsten oxide attached to titania measuring roughly 5 nm,” says Wachs.
“This is sub-nanoscale work.”
Wachs characterized the substrates and nanocatalysts with Lehigh’s aberration-corrected transmission electron microscope and with Raman, IR and UV-visible spectroscopy.
The higher surface-free energy of the titania nanoparticles (in comparison to the silica) enables it to preferentially anchor redox agents, acid sites and other types of catalytic active sites, says Wachs. The relatively inert silica does not attract these catalytic active sites, but it prevents the titania nanoparticles from migrating and agglomerating within and it enabled Wachs to control the size of the titania nanoparticles.
“What we were able to bring about is a doubly supported catalyst,” says Wachs, who directs Lehigh’s Operando
Molecular Spectroscopy and Catalysis Research Lab. “The silica allows us to maintain control of the size of the titania nanoparticles. We can then selectively attach any catalytic active site we want onto the titania nanoparticles.”
Using a “delicate” preparation technique, Wachs says his group was able to engineer various sizes of titania nanoparticles and to correlate these sizes systematically with the catalytic rate of the tungsten oxide acidic reaction.
By manipulating the dimensions of the titania nanoparticles, Wachs’s group was also able to control another critical variable—the size of the titania bandgap, which is the energy difference between a material’s electronic non-conductive and conductive states.
At the nanoscale, Wachs says, titania is essentially a “nanoligand that controls the flow of electrons to catalytic active sites.”
“Tungsten oxide, as a catalytically acid site, does not want electrons; therefore, smaller nanoparticles [of titania] and a higher bandgap environment are better. On the other hand, vanadium oxide, which is used in redox reactions, needs electrons; therefore, it prefers larger particles of titania and a lower titania bandgap.”
Potential applications of his group’s discovery, says Wachs, include improved environmental catalysts for catalytic converters in automobiles as well as for reduction of industrial emissions.
Wachs’s project is supported by a $1-million grant from NSF’s Nanoscale Interdisciplinary Research Team (NIRT) program. He collaborates with researchers from Rice University (M.S. Wong) and the University of Virginia (M. Neurock), and with Chris Kiely, director of the Nanocharacterization Laboratory in Lehigh’s Center for Advanced Materials and Nanotechnology.