ZIRCONIUM

NANCY B. JACKSON, SANDIA NATIONAL LABORATORIES

You always remember your first love; isn't that what they say? Zirconia, the oxide form of zirconium, was my constant companion for so many years. Before going on an airplane, I would have to empty my coat pockets of the many vials of white powder that had accumulated during my trips to the X-ray diffraction machine. Somehow, I didn't think that even in the 1980s airport security would shrug off small vials of white powder. What fascinated me was zirconium's relationship with oxygen. Although the chemistry between oxygen and zirconium has led to its use as an oxygen sensor and an oxygen conductor, it is the surface zirconium-oxygen bond that filled my days and nights for many years. Zirconia is an interesting catalyst, and it is the Zr–O bond that makes it interesting.

In John Ekerdt's lab at the University of Texas, Austin, we were studying the formation of methanol from synthesis gas, CO and H₂, over a zirconia solid catalyst. The postdoc who worked on the problem before me had identified many surface species that form on zirconia in the presence of synthesis gas. The intermediates that played a role in the route from synthesis gas to methanol were formate and methoxide. The methoxide was a methyl group attached to an oxygen that was attached to the surface through a zirconium. The formate was attached to surface zirconium through two oxygens. In our temperature-programmed desorption work--the method in which we studied this reaction--methanol was made only when a small amount of water was added to the synthesis gas. Without water, the CO and H₂ made only methane.

The key to a better understanding of the methanol-formation mechanism would come from knowing where the oxygen in the methanol came from. Was it the water? It certainly seemed the obvious choice. With my vial of H₂¹⁸O, I was thrilled that for my first independent experiment in graduate school, I was going to be just like Melvin Calvin. I was going to find out where the oxygen came from. Okay, so this wasn't photosynthesis, and I was not likely to win the Nobel Prize. Nonetheless, I somehow felt a connection with science like I never had before. I was going to find out what happened to tiny atoms on surfaces--an unbelievable feat for this once-political-science major!

I marched into my lab early on a Saturday morning, knowing that, by the end of the day, I would know where the oxygen came from. The working hypothesis was that since methanol formed only when water was present, the oxygen in the methanol came from the water. This would be demonstrated by the formation of all labeled CH₃¹⁸OH. A second, much less likely possibility, was that the water was incorporated in the intermediate formate. In that case, the methanol would be 50% CH₃¹⁶OH and 50% CH₃¹⁸OH.

I was dumbfounded by the results. They were unequivocal and unexpected: It turned out there was not a drop of ¹⁸O in the methanol. The water appeared to break the methoxide bond between the O–Zr to give methanol. When water was not around, the bond between the C–O of the methoxide broke, leading to methane. It was almost like a substitution reaction was occurring between the water and the methoxide. Later experimentation confirmed and further defined the synthesis gas mechanisms over zirconia and the pivotal role the O–Zr bond plays.

I walked into my adviser's office Monday morning and proudly announced that I had been like Melvin Calvin during the weekend: I had figured out where the oxygen came from. But more than learning the source of oxygen, that weekend I had been hooked by research. I had learned the secrets of things I could not see, I had been surprised by nature, and figuring it all out was better than any puzzle I had ever done. Eventually, my plans for a master's degree turned into a Ph.D. To this day, when I'm asked why I got a Ph.D., I think back to that first weekend I spent with zirconia.

As my long relationship with zirconia was winding up, I wondered if it would be appropriate to ask my fiancé for a fabulous engagement ring of cubic zirconia stones of many colors. He said his family would never understand if he gave his wife-to-be a cubic zirconia ring. (Not everyone understands the love between a researcher and her first catalyst.) I settled for a sapphire. A diamond would have been just a cheap imitation.

Nancy B. Jackson is manager of the Chemical & Biological Sensing, Imaging & Analysis Department at Sandia National Laboratories. She is still learning the secrets of things she cannot see and is still often surprised by nature.

Top

Chemical & Engineering News
Copyright © 2003 American Chemical Society