(594e) The Synergy between Fe and Ru in No-Assisted N2o Decomposition over Fe/Ru-Fer Catalysts: a Mechanistic Explanation

Abstract: Fe-Ru-FER has a very high activity for N₂O decomposition in the presence of NO. A synergy between Fe and Ru was identified. Based on activity data and the results of in operando spectroscopy we can attribute the synergy between Fe and Ru to a combination of kinetic cooperation and the fact that Fe acts as a trap for NO and thereby reduces the inhibition of the Ru component by NO. The redox properties of Fe and Ru are not dramatically changed in the bimetallic catalyst.

Keywords: N₂O decomposition, synergy, iron, ruthenium, XAS, IR

 

Introduction

The development of improved catalysts that convert N₂O at moderate temperatures (below 773 K) is important for decomposition of N₂O in the tail gases of nitric acid plants. Noble metal zeolite catalysts have very high intrinsic activities for N₂O decomposition, but they are strongly deactivated by NO and O₂ [1]. Iron zeolites show the opposite behavior: Their N₂O decomposition activity is strongly enhanced by the presence of NO. NO acts as a catalyst for N₂O decomposition via the following mechanism:

N₂O + NO → NO₂ + N₂                                 (1)

N₂O + NO₂ → NO + N₂ + O₂                                 (2)

If Fe is combined with a noble metal, like Ru, a very high activity for the decomposition of N₂O in the presence of NO and O₂ can be obtained [2]. Interestingly, the catalytic activity of the bimetallic catalyst is even higher than the sum of the two single components. There is a synergy between Fe and Ru. A purely kinetic cooperation may explain the synergy: The Fe catalyst is very active for reaction (1), but the formation of O₂ via (2)  is relatively slow. The Ru catalyst offers a second channel for the decomposition of NO₂ to NO and O₂ via the reaction NO₂ → NO + ½ O₂. This reaction is in equilibrium over the Ru catalyst. However, the adsorption and redox properties of the bimetallic catalyst might change compared to the single components. Here, we combine operando IR, XAS and kinetic results to explain the synergy.

Results and discussion

The IR experiments were carried out with Fe-FER, Fe-Ru-FER and Ru-FER samples in a flow of 3000 ppm N₂O and 800 ppm NO in He, between 300 to 400°C. Fig.1 shows the IR spectra of the adsorbed surface species at 300°C. Ru-FER exhibits a high concentration of stable surface nitrates (band at 1640 cm^-1), which are responsible for the inhibition of N₂O decomposition activity. Fe-FER contains much less nitrate species. Interestingly, the spectrum of the bimetallic catalysts Fe-Ru-FER resembles closely that of Fe-FER, i.e. Fe prevents the formation of inhibiting nitrate species on Ru by trapping NO. In operando XANES measurements were performed in fluorescence mode at the Dubble beamline, ESRF Grenoble. Fig. 2 shows the Fe K-edge XANES spectra of Fe-FER measured in H₂ (fully reduced to Fe²⁺), in N₂O (fully oxidized to Fe³⁺) and in the reaction mixture with NO + N₂O (+ O₂) at 400°C. In the reaction mixture, about 40% of the iron sites is in oxidation state +II. For the bimetallic catalyst this fraction is slightly lower, i.e. the presence of Ru does not have a positive influence on the redox properties of the Fe component. On the Ru K-edge, the spectra always resembled that of RuO₂. This indicates that only a minority of the Ru atoms takes part in the catalytic cycle. No difference between Fe-FER and Fe-Ru-FER could be detected. 

Conclusions

Based on activity data and the results of in operando spectroscopy we can attribute the synergy between Fe and Ru to a combination of kinetic cooperation (see above) and the fact that Fe acts as a trap for NO and thereby reduces the inhibition of the Ru component by NO. The redox properties of Fe and Ru are not dramatically changed in the bimetallic catalyst.

Fig. 1. IR spectra of Fe-, Fe-Ru- and Ru-FER in 3000 ppm N2O and 800 ppm NO at 300°C.

Fig. 2.  XANES spectra of Fe-FER in different reaction mixtures at 400°C

References

[1] G.Centi, A. Galli, B. Montanari, S. Perathoner, A. Vaccari, Catal. Today 35 (1997) 113.

[2]J. A. Z. Pieterse, G. Mul, I. Melian-Cabrera, R.W. van den Brink, Catal. Lett. 99 (2005) 41.

[3] G. D. Pirngruber, M. Luechinger, P. K. Roy, A. Cecchetto, P. Smirniotis, J. Catal. 224 (2004) 429.