Zeroing In On Active Catalytic Sites

Figuring out precisely where the active sites on a solid catalyst are located is a tough job. Some analytical methods can provide surface structure maps with atomic resolution. But because many of those techniques lack chemical specificity, they fall short when it comes to elucidating site-specific chemical activity.

Good news: The task of uncovering the chemical role played by specific sites on catalyst surfaces may have just gotten a little easier. Researchers at the University of California, Riverside, have developed a procedure that identifies the nature of the sites to which specific reagent molecules bond and the selectivity of that process-key steps in understanding reaction mechanisms. The technique combines titration procedures with surface spectroscopy.

To demonstrate the new method, chemistry professor Francisco Zaera and postdoc Hansheng Guo focused on a nickel crystal that had been treated with various amounts of oxygen. The crystal, which is characterized by fragmented Ni-O rows, serves as a model catalyst for transition-metal-mediated oxidation reactions, the researchers explain.

First the team exposed the metal to a reactant gas-carbon monoxide in one experiment and ammonia in another-and then dosed it with xenon, which functions as a selective probe to help determine which sites were occupied by the reactant molecules. To uncover that information, the group probed the manner in which xenon adsorbed (bonded) to the nickel surface through a combination of photoelectron spectroscopy and temperature-programmed-desorption spectroscopy studies.

On the basis of those measurements, the team reports that ammonia bonds selectively to unsaturated oxygen atoms located at the ends of Ni-O rows. In contrast, carbon monoxide bonds indiscriminately across the surface (Nat. Mater. 2006, 5, 489). Zaera and Guo add that the oxygen atoms at the ends of the rows are particularly active for hydrogen abstraction and other steps in hydrocarbon conversion reactions.

Understanding the nature of the active sites that drive certain chemical transformations is an important but often elusive goal in catalytic studies, says Daniel R. Strongin, a chemistry professor at Temple University, Philadelphia, and a specialist in surface science.

Zaera's group has developed "an elegant methodology," Strongin comments, using xenon as a probe to elucidate the chemical nature of the sites that exhibit selective binding toward catalytically relevant molecules. He adds that the methodology is general enough to have the potential to be applicable to many chemically important systems.

Strongin expects that the new technique will provide information that's complementary to results obtained via microscopy methods that are useful for imaging surface sites but often lack the ability to probe the chemical activity of those sites.