Researchers have discovered two pieces of a scientific puzzle in the most widely used process for removing polluting sulfur compounds from crude oil. The discovery could help chemists tailor the catalytic process, known as hydrodesulfurization, for maximum efficiency and economy, according to Paul S. Weiss, professor of chemistry at Penn State, who conducted the research along with Penn State graduate student James G. Kushmerick.
A paper describing the research is published in the current issue of Journal of Physical Chemistry B, published on December 10.
The chemists made their discovery with one of the most powerful and stable microscopes in the world--an instrument they designed and built themselves. "It is so stable that the position we examine drifts no more than one atomic site per day, so we are able to observe individual molecules for days at a time and to measure the electronic structures that control chemical reactions on surfaces," says Weiss.
The chemists observed a cluster of three nickel atoms and its interaction with a molybdenum disulfide surface. Tiny molybdenum disulfide crystallites on an oxide base--with nickel or cobalt added as a reaction promoter--are used by refineries worldwide as catalysts for removing sulfur-containing thiophene compounds from crude oil.
"We were amazed to discover how mobile the nickel atoms were even well below room temperature," says Kushmerick, who had to cool his specimen down to just 4 (degrees) Kelvin above absolute zero and keep it sealed inside his microscope's vacuum chambers to keep the nickel atoms from skimming around the surface in a blur. "Because the bonds of the molybdenum disulfide are already saturated, there is nothing really to hold the nickel firmly in place," Weiss explains. The chemists found that, at 4K, the nickel atoms were so loosely bound they were easy to move around with the fine tip of their microscope's needle.
Before this research, it was known that nickel somehow made molybdenum disulfide more effective at bonding with the sulfur in thiophenes, and researchers believed that these reactions could occur only on the edges of the thin catalytic sheets but not on their broad, flat planes. "Experiments had shown that catalysis was enhanced if the molybdenum disulfide sheets were spread out rather than stacked on top of one another when they were exposed to incoming thiophenes," Weiss explains. Whether the sheets were spread out or stacked did not affect how much reactive edge area was exposed to the thiophenes, but it greatly affected how much of the unreactive plane area was exposed. "It was a mystery why the planes would be important when the reactants do not even stick to them," Weiss says, but we think we may have answered part of that question."
Kushmerick and Weiss suspect that nickel's surprising ability to glide around on molybdenum disulfide is one of the keys to how it promotes a more effective catalytic reaction. "After we got the nickel atoms to stay still on the molybdenum disulfide surface we were able to measure their electronic properties, and we found the cluster's empty electron orbitals were greatly enhanced," Weiss says. "Occupancy of electron orbitals at specific energies is the major factor that dominates interactions among molecules."
"The nickel cluster had created an excellent place for sulfur in thiophenes to bind because sulfur is nucleophilic--it likes to donate its electrons to empty orbitals," Weiss says. When the chemists paired this new finding with their discovery of nickel's propensity for skating around on the surface, they realized the nickel atoms could act as a kind of sticky ballbearing--it could both capture thiophenes from the oil and help them move around to find edge sites, where catalytic reactions can lock them in place so they can be separated and chemically removed from the oil. "If you have reactive nickel atoms roaming around, they are more likely to capture the thiophenes, which would not otherwise stick to the basal plane of the catalytic crystallites," Weiss says.
"We can now think about designing better catalysts by finding promoters that would be most effective at capturing the reactants and allowing them to move around the surface to the best catalytic binding sites," Weiss says. This discovery also gives chemists a new way to think about how to set up catalytic reactions, in general, by designing systems to enhance surface mobility for reacting molecules. "Every little bit of efficiency you can get out of these catalytic processes translates into a big effect in terms of results."
This research project received financial support from the National Science Foundation, the Office of Naval Research, the Petroleum Research Fund administered by the American Chemical Society, the Exxon Education Foundation, and Air Products. Paul Weiss receives support as a Fellow of the Alfred P. Sloan Foundation and the John Simon Guggenheim Memorial Foundation.
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