When iron is ion-exchanged into the zeolite ZSM-5, it can assume different oxidation states, and it can be reversibly cycled between different oxidation states. Not surprisingly, Fe/ZSM-5 is an active catalyst for a number of reduction/oxidation (redox) reactions. We have studied catalysis of redox reactions by Fe/ZSM-5, with particular emphasis on the decomposition of nitrous oxide (N₂O). This is a redox reaction where it is generally accepted that N₂O first oxidizes a catalyst site, generating N₂, and then another N₂O subsequently reduces the oxidized site, generating another N₂ and O₂.

When the group first began working in the area, it was known that nitric oxide (NO) could participate stoichiometrically in the reaction. Effectively, the NO replaces N₂O in the reduction half of the redox cycle, thereby generating NO₂. At the same time, the rate of reaction increases substantially. Our group discovered that even a very small, sub-stoichiometric, amount of added NO would cause a substantial increase in reaction rate and, importantly, that the increase in rate was greater than could be accounted for by stoichiometric participation of NO.

To explain this effect, we postulated that the active site undergoing redox cycling might be a surface nitrate/nitrite couple. The role of small amounts of NO was proposed to involve generation of additional surface nitrites, which could then undergo many turnovers. We subsequently conducted experimental and computational chemistry studies probing this explanation. Our results, and those of others, suggest that nitrite/nitrate redox cycles are not significant participants in the redox catalysis. One alternative hypothesis that was advanced suggested that NO adsorbed on a site adjacent to the active site where it served as a temporary location for storage of an oxygen atom. An experimental study in our group found that Fe/ZSM-5 wherein the iron sites are isolated from other sites also experiences the enhancement in activity with addition of trace NO. This suggests that a nearby site for NO adsorption is not necessary.

In order to better understand the redox processes involved, we included a number of additional redox reactions in our experimental studies. Simultaneously, computational chemistry was used to study surface intermediates and activated complexes, involving both isolated sites and paired sites. We have found that a broad range of redox reactions can be consistently described using a much smaller set of mechanistic reaction steps, and there is good agreement between the kinetic parameters derived from mechanistic modeling and those derived from computational chemistry.

“Effects of Zeolite Channel Walls and Cation Migration on N₂O Decomposition Energies in Fe/ZSM-5,” C. R. F. Lund, J. Catal., 243, 438-441 (2006). [more info]

“The Effect of NO upon N₂O Decomposition over Fe/ZSM-5 with Low Iron Loading,” C. Sang, B. H. Kim and C. R. F. Lund, J. Phys. Chem. B, 109 (6), 2295-2301 (2005). [more info]

“Possible Role of Nitrite/Nitrate Redox Cycles in N₂O Decomposition and Light-Off over Fe-ZSM-5,” C. Sang and C. R. F. Lund, Catal. Lett., 73 (1), 73-77 (2001). [more info]

“Isothermal “Light-Off” during Catalytic N₂O Decomposition over Fe/ZSM-5,” C. Sang and C. R. F. Lund, Catal. Lett., 70, 165-173 (2000). [more info]

“Effects of Cation Migration upon Energetics of Fe/ZSM-5 Catalyzed Reactions,” AIChE Annual Meeting, Cincinnati, OH, November 2005.

“Computational Investigation of Cation Migration and its Effect Upon the Thermochemistry of N₂O Decomposition over Fe/ZSM-5,” 19^th North American Meeting of the Catalysis Society, Philadelphia, PA, May 2005.

“A Unifying Mechanistic Model for Redox Reactions over Fe/ZSM-5,” AIChE Annual Meeting, Austin, TX, November 2004.

“The Effect of Sodium Cations upon Redox Cycles involving Fe/ZSM-5,” AIChE Annual Meeting, San Francisco, CA, November 2003.

“Redox Cycles over Fe/ZSM-5,” 18^th North American Catalysis Society Meeting, Cancun, June 2003.

“Redox Cycles over Fe/ZSM-5,” AIChE Annual Meeting, Indianapolis, IN, November 2002.

“Surface Intermediates in Redox Cycles on Fe/ZSM-5,” Pittsburgh-Cleveland Catalysis Society Meeting, Cranberry PA, December 7, 2001.

“An Experimental and Computational Investigation of Redox Cycles during N₂O Decomposition,” Chemical Engineering Department Seminar, Worchester Polytechnic Institute, October 11, 2001.

“A Nitrite/Nitrate Redox Cycle for N₂O Decomposition over Fe/ZSM-5,” 17^th North American Catalysis Society Meeting, Toronto, June 2001.

“An Experimental and Computational Study of N₂O Decomposition on Fe/ZSM-5,” Center for Computational Research Colloquium Series, University at Buffalo, February 23, 2001.

“N₂O Decomposition over Fe-ZSM-5,” AIChE Annual Meeting, Los Angeles, CA, November 2000.

“Ab-initio modeling of iron dimers in ZSM-5 and other iron clusters,” AIChE Annual Meeting, Los Angeles, CA, November 2000.

“N₂O Decomposition over Fe-ZSM-5: Two Levels of Catalytic Activity,” Catalysis Gordon Conference, Colby-Sawyer College, New London, NH, June 26-30, 2000.

“Activation of Oxygen on Fe/ZSM-5,” AIChE Annual Meeting, Dallas, TX, November 1999.

Bin Chen, “A Mechanistic Study of Redox Pathways in Fe/ZSM-5,” Ph. D. Dissertation, University at Buffalo, SUNY, Dept. of Chemical Engineering (2006). [more info]

Chimin Sang, “Investigations on membrane application to catalytic methylamines synthesis and kinetics of nitrous oxide decomposition on FeZSM-5,” PhD Dissertation, University at Buffalo, SUNY, Dept. of Chemical Engineering (2001). [more info]