Through it's ability to be compiled into the ChemShell QM/MM modelling environment, GAMESS-UK has been used to model the Quantum Mechanical region in the research described below.
Application: Modelling NOx and N2O Decomposition in Zeolites
Norsk Hydro is a major producer of nitrogen based fertilizers, resulting in the emission of N2O. N2O contributes to the catalytical destruction of stratospheric ozone, in addition to being a greenhouse gas, corresponding to 310 CO2 equivalents. Thus, effective conversion into harmless products is an industrial process of increasing importance. Within the framework of QUASI, Norsk Hydro has studied the catalytical abatement of nitrogen oxides and in particular N2O using metal-exchanged zeolites. Cu exchanged ZSM-5 is among the most active materials for the ambient temperature treatment of the off-gas (230-250°) in nitric acid production. Therefore it was chosen as a good starting point to gain a more detailed understanding of the reaction mechanism which so far has been limited.
We have studied the adsorption energies for NO and N2O adsorption at different Cu ion sites including two- (I2) and three-coordinated (M7 and Z6) sites. The calculated adsorption energies indicate that the most stable forms of adsorbed NO and N2O bind through the nitrogen end in accordance with experimental findings. However, the O bound adsorption mode for N2O is believed to be more important in the decomposition pathway. Comparing the different Cu+ sites within the zeolite framework, we find lower adsorbtion energies for the ring sites (Z6, M7) as compared to the I2 site, in agreement with other theoretical studies. The effect can be traced to the geometry of the adsorbed complex. The structure of the I2 sites remains essentially unchanged upon coordination, whereas the ring coordinated Cu+ (Z6, M7) has to give up the extra coordination to non-AlO4 framework oxygens to bind NO or N2O. This reduces the total binding energy.
By using the QM/MM scheme we have been able to characterize the
energetics of the individual steps in the N2O decomposition
reaction. The catalytic cycle of N2O decomposition for the oxygen end
adsorption is given in Figure 1.
|Figure 1:The QM/MM methodology allows us to study the relative reactivity of different sites.|
Preliminary results show that there is a difference in activation barrier (18 kcal/mol) between the gas phase model and QM/MM model. The N-O bond distance shows the largest geometry difference. The QM/MM result gives an elongated and more activated adsorbate complex. This causes a higher activation barrier when using the QM/MM scheme.
By using the QM/MM methodology, we predict that there is a difference in adsorption energies among the different sites. The zeolite environment has an impact on the adsorbate structure compared to the bare gas phase cluster calculations. This also influences the transition states and further calculations are currently in progress.
1 The project combines software development work of three academic groups active in the area, CLRC Daresbury (UK), the group of Prof. Walter Thiel at the Max Planck Institute für Kohlenforschung, Mülheim (DE), and the group of Prof C.R.A. Catlow at the Royal Institution (UK) with demonstration and applications work from modelling teams within three major European chemical industries, Norsk Hydro(NO), BASF (DE) and ICI (UK).