(688a) A Reaction Mechanism for the Nitrous Oxide Decomposition on Binuclear Oxygen Bridged Iron Sites in Fe-Zsm-5

The increasing concentration of greenhouse gases in the atmosphere and its consequences on global warming is an important environmental problem. The world's nitric acid plants represent the single greatest industrial source of nitrous oxide (N₂O) which is the third most important greenhouse gas following carbon dioxide (CO₂) and methane (CH₄). The catalytic decomposition of nitrous oxide from industrial tail gas streams is an effective and cost efficient way of decreasing greenhouse gas emissions. Iron zeolites have been proposed as possible catalysts for the stoichiometric decomposition of nitrous oxide into nitrogen and oxygen under industrial tail gas conditions.

During the last few years a tremendous effort has been spent, both experimentally and theoretically, to elucidate the reaction mechanism and the nature of the active site in Fe-ZSM-5. However, there is still an ongoing debate in the literature whether single iron sites of type Z^-[FeO]⁺ or bi-/oligonuclear oxygen bridged iron sites of type Z^2-[-OFeOFeO-]²⁺ are the main active species for the nitrous oxide decomposition (Z represents the zeolite lattice). Important progress was made recently by two comprehensive density functional (DFT) studies in combination with kinetic modelling investigating the N₂O decomposition on single iron sites in the absence [1,2] and presence of NO [3,4]. These investigations resulted in reaction mechanisms that are able to explain a variety of experimental results but fail to predict a low temperature O₂ desorption peak measured by some authors [5,6].

It is the aim of the present study to elucidate a reaction mechanism of the N₂O decomposition on binuclear oxygen bridged iron sites in Fe-ZSM-5 on a molecular level using DFT. For a reaction network consisting of about 50 elementary reactions, the geometries and energies of potential energy minima as well as transition states are determined on different spin surfaces. Transition states are localized using a combination of interpolation and local methods. Approximate saddle points are obtained with the growing string method [7], while for the subsequent refinement a modified version of the dimer method [8] is employed. Transition state theory is then used to calculate all relevant reaction rate constants.

The results show that if two charge exchange sites in the zeolite are in close proximity, two isolated hydroxylated single iron sites of type Z^-[Fe(OH)₂]⁺ readily form an oxygen bridged iron site of type Z^2-[HOFeOFeOH]²⁺ while releasing water. On these hydroxylated oxygen bridged iron sites various mechanisms for O₂ formation were found. At higher temperatures water desorbs from active bi-iron sites. Then, N₂O decomposes on Z^2-[FeOFe]²⁺ sites by loading an oxygen atom on each iron atom. It was found that this mechanism with an oxygen recombination step afterwards and subsequent desorption is favoured over two decomposition steps on the same iron atom. The highest reaction barrier along the reaction path is a N₂O decomposition step which leads to a first order kinetic in the N₂O partial pressure. The barrier heights are comparable to the ones calculated on single iron sites [1,2]. Therefore, the macroscopic behavior of the catalytic cycle at high temperatures is expected to be similar to the one on single iron sites.

Literature

[1] A. Heyden, B. Peters, A. T. Bell, F. J. Keil, J. Phys. Chem. B 109, 1857 (2005)

[2] A. Heyden, A. T. Bell, F. J. Keil, J. Catal. 233, 26 (2005)

[3] A. Heyden, N. Hansen, A. T. Bell, F. J. Keil, in preparation

[4] A. Heyden, N. Hansen, A. T. Bell, F. J. Keil, in preparation

[5] T. V. Voskoboinikov, H.-Y. Chen, W. M. H. Sachtler, Appl. Catal. B 19, 279 (1998)

[6] B. R. Wood, J. A. Reimer, A. T. Bell, M. T. Janicke, K. C. Ott, J. Catal. 224, 148 (2004)

[7] B. Peters, A. Heyden, A. T. Bell, A. Chakraborty, J. Chem. Phys. 120, 7877 (2004)

[8] A. Heyden, A. T. Bell, F. J. Keil, J. Chem. Phys., 123, 224101 (2005)