last updated 9/22/10

The mechanism of high temperature water gas shift over ferrochrome catalysts has been debated in the past, but few, if any, mechanistic kinetics studies have been reported. In this project, density functional theory is being used to create models for the active sites involved in water-gas shift over iron oxide-based catalysts. In addition, mechanistic kinetic models are being fit to experimental water-gas shift rate data. The goals are to find consistent site and kinetic models that accurately describe the kinetics and to determine whether those models can be extended to explain the effect of substituting copper cations into the iron oxide. The nature of copper as a high temperature shift promoter is unclear. Some feel that under reaction conditions, small particles of metallic copper form and that these are responsible for the promotional effect. Others hold that copper substitutes for iron at cation sites within the oxide, and this is the source of catalyst promotion. We speculate that in the former situation (metallic copper as the active site), the mechanism of the reaction would change appreciably whereas in the latter case the mechanism might be the same for unpromoted and copper-promoted catalysts.

We have created DFT models for the active site on {100}, {110} and {111} surfaces of magnetite. The best agreement with experimental data was found for the {111} surface site. We have also used DFT to estimate the effect of copper substitution on the surface and in the surface. The DFT models suggest that when copper substitutes in the surface at iron cation sites, changes in the heat of adsorption of water-gas shift intermediates would be of the order of 10-20 kJ/mol, and when copper substitutes on the surface the changes would be of the order of 60-80 kJ/mol.

We have fit a four-step mechanistic redox model, and separately a four-step mechanistic formate model, to water-gas shift kinetic data for a ferrochrome catalyst. We have done the same for a copper-promoted ferrochrome catalyst. In the case of the ferrochrome catalyst, each of the two mechanistic models fit the data well, and statistically the two fits were comparable. This is not surprising; it has not been possible in the past to discriminate between these two classes of mechanisms on the basis of kinetics. The species appearing in the mechanistic kinetic models are the same ones that we studied via computational chemistry, as described above.

The same two mechanistic kinetic models also fit the data for a copper-promoted ferrochrome catalyst well, and again, the two fits were statistically comparable. In both cases, the values of the heats of adsorption of the reaction intermediates derived from the kinetic modeling changed by 65 kJ mol^-1 or less. The DFT calculations predicted changes in the heats of adsorption of this order of magnitude when copper cations substituted for iron cations within the oxide lattice.

We observed that the degree of inhibition of water-gas shift by CO₂ decreased as the copper loading increased up to a level of ca. 5 to 10 wt%. That is, the power-law reaction order with respect to CO₂ became less negative, going from -0.67 to -0.26 at 5 wt% loading. Via x-ray diffraction, we did detect the presence of metallic copper particles in the samples with higher copper loadings after use in the reactor. Metallic copper may also have been present in used catalysts with lower copper loadings, but it could not be detected by XRD.

While these results are not conclusive, they tend to favor the view that the promotional effect of copper upon the kinetics of high temperature shift is due to the substitution of copper cations for iron cations within the iron oxide lattice, and not due to the formation of catalytically active metallic copper particles. Specifically, if metallic copper particles were responsible for the activity of the promoted catalysts, one would expect the mechanism to change since the power-law kinetics are reported to be different for promoted and unpromoted catalysts. Put differently, it would be quite surprising if the kinetics of water-gas shift over copper particles proceeded via the same mechanism it as that for iron oxide catalysts and that the thermodynamic heats of adsorption of the species participating in the mechanism were nearly the same (i.e. within 10 to 65 kJ mol^-1, depending on the particular species) on copper particles as they were on ferrochrome.

This project was most recently supported by a grant from the Petroleum Research Fund.

Papers Resulting from this Project

“Copper Promotion of High Temperature Shift,” J. S. Coleman, M. Zhang, R. M. Van Natter and C. R. F. Lund, Catalysis Today, 160, 191-197 (2011). [more info]

“A DFT Study of the Effect of Copper Promotion upon Iron Oxide Surface Species,” R. M. Van Natter, J. S. Coleman and C. R. F. Lund, J. Molecular Cat. A, 311, 17 (2009). [more info]

“DFT Models for Active Sites on High Temperature Water-Gas Shift Catalysts,” R. M. Van Natter, J. S. Coleman and C. R. F. Lund, J. Molecular Cat. A, 292 76 (2008). [more info]

Presentations Resulting from this Project

“Role of Copper as a High Temperature Shift Promoter,” AIChE Annual Meeting, Nashville TN, November 8-13, 2009.

“Fe₃O₄-based High and Low Temperature Shift Catalysts,” Pittsburgh-Cleveland Catalysis Society, Pittsburgh, PA, October 9, 2009.

“Comparison of Unpromoted Ferrochrome WGS Catalysts to Those Promoted with Au or Cu.” AIChE Annual Meeting, Salt Lake City, November 2007.

“A Computational Study of Fe₃O₄ (100) Surface Species Related to Water-Gas Shift.” AIChE Annual Meeting, San Francisco, November 2006.

“Role of copper and other high temperature water-gas shift promoters,” 231^st ACS National Meeting, Atlanta, GA, March 2006.

“Promotion of ferrochrome water-gas shift catalysts with copper,” Pittsburgh-Cleveland Catalysis Society Meeting, Pittsburgh, PA, December, 2005.

“Microkinetic Modeling of High-Temperature Shift for Development of Catalysts for Membrane Reactors,” Engelhard Corporation, Iselin, NJ, July 29, 2004.

“Water-Gas Shift Kinetics over Iron Oxide Catalysts at Membrane Reactor Conditions,” 18^th North American Catalysis Society Meeting, Cancun, June 2003.

“High Temperature Water-Gas Shift at Membrane Reactor Conditions,” AIChE Annual Meeting, Indianapolis, IN, November 2002.

“Water Gas Shift Kinetics at Membrane Reactor Conditions,” University Coal Contractors Review Meeting, Pittsburgh, PA, June 4, 2002.

“Water-Gas Shift Kinetics over Modified Iron Oxide Catalysts at Membrane Reactor Conditions,” Pittsburgh-Cleveland Catalysis Society Meeting, Monroeville, PA, May 10, 2002.

“Water-Gas Shift over Promoted Iron Oxide,” AIChE Annual Meeting, Reno, NV, November 2001.

“Water-Gas Shift at Membrane Reactor Conditions,” University Coal Contractors Review Meeting, Pittsburgh, PA, June 5, 2001.

“Microkinetic Models for Water-Gas Shift at Membrane Reactor Conditions,” Chemical Engineering Department Seminar, Tufts University, March 26, 2001.

“Water Gas Shift at Membrane Reactor Conditions,” University Coal Research Contractors Review Meeting, Pittsburgh, PA, June 6-7, 2000.

Dissertations Resulting from this Project

Rainee M. VanNatter, “Active Site Models for High Temperature Water Gas Shift Reaction Over Iron Oxide Catalysts,” Ph. D. Dissertation, University at Buffalo, SUNY, Dept. of Chemical Engineering (2011). [more info]

John S. Coleman, “Mechanistic Modeling of the High Temperature Water Gas Shift Reaction on Ferrochrome,” Ph. D. Dissertation, University at Buffalo, SUNY, Dept. of Chemical Engineering (2008). [more info]

Mang Zhang, “An Experimental Investigation of Promoted Iron Based Oxide for High Temperature Water Gas Shift Reaction Catalysts at Membrane Reactor Conditions and an Experimental and Computational Study of Solvent Effects in Toluene Chlorination,” Ph. D. Dissertation, University at Buffalo, SUNY, Dept. of Chemical Engineering (2003). [more info]

Theses Resulting from this Project

Donghao Ma, “Kinetic Study of WGSR over Iron-Based Catalysts in a Membrane Reactor,” M. S. Thesis, University at Buffalo, SUNY, Dept. of Chemical Engineering (2001). [more info]