Untersuchung und Optimierung cobaltit-basierter Spinellkatalysatoren für die Zersetzung von N$_{2}$O aus Abgasströmen der HNO$_{3}$-Synthese

• Investigation and optimization of cobaltit-based spinels as catalysts for the decomposition of N$_{2}$O from exhaust gases of HNO$_{3}$-production

Franken, Tanja; Palkovits, Regina (Thesis advisor); Liauw, Marcel (Thesis advisor)

Aachen (2016, 2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

In this work spinels on the basis of cobalt were elaborated and optimized in therms of their activity for the decomposition of N$_{2}$O into N$_{2}$ and O$_{2}$. Via selective exchange of Co-atoms by other metal ions in the form of M$_{x}$Co$_{3-x}$O$_{4}$ (M = Mg, Cr, Zn, Cu) an optimal spinel composition was investigated. Herein, the use of M = Cu was well suited in order to investigate suitable properties for high N$_{2}$O decomposition activities. The partial exchange of Cu in Cu$_{x}$Co$_{3-x}$O$_{4}$ (x = 0, 0.125, 0.25, 0.5, 0.75, 1) showed a strong dependency of activity on the degree of exchange. The highest activity was obtained with x = 0.25 pointing to an synergetic effect between Co and Cu atoms. Under real reaction conditions this spinel composition showed the highest activity, as well, even though all catalysts showed inhibition due to additional gases in the reactant mixture. All catalysts were characterized with elemental analysis via ICP-OES in terms of their compositions and via XRD, N$_{2}$-Physisorption in terms of their physico-chemical-properties. These properties were correlated with their catalytic activities. Herein, high specific surface areas of the catalysts lead to increased catalytic activities. But high surface area catalysts prepared via nanocasting approach tend strongly to sinter under reaction conditions and show therefore a decrease in long term activity. As further optimization strategy the supporting of the active spinel phase on several oxidic carrier (SiO$_{2}$, H-ZSM-5, MgO, TiO$_{2}$, ZrO$_{2}$ and CeO$_{2}$) was investigated. With this approach the amount of active phase can be strongly decreased to 5 wt% without significant loss in catalytic performance by using CeO$_{2}$ or ZrO$_{2}$ as support. The use of TEM-EDX-elemental mapping revealed an increasing dispersion of the active species on the surface of the carrier due to decrease of loading. These catalysts showed high long term activity for at least 60h under real reaction conditions.As a third optimization approach doping of spinels with alkaline (earth) elements was applied. Potassium was the best dopant for the full catalysts with a molar ratio of 0.01, due to its ability to promote desorption of oxygen from the surface of the catalyst and thus facilitating the rate determining step. Furthermore, it could be shown by single wise addition of gases present under real reaction conditions (O$_{2}$, NO, H$_{2}$O), that NO-oxidation takes place as a concurring site reaction leading to the observed inhibition for N$_{2}$O-decomposition under real reaction conditions. In the last part of this thesis in depth in situ (XRD, XANES, DRIFTS) combined with ex situ (TPD-O$_{2}$, TPR) analysis were performed in order to obtain insights into the catalytic reaction mechanism. Surprisingly, reduction of the catalyst under reaction conditions was observed via XANES even though oxidation would have been expected according to the literature known reaction mechanism and the way N$_{2}$O adsorbs on surfaces. From this result and underlined by TPR and TPD analysis release of lattice oxygen atoms during catalysis was concluded. Based on this a catalytic mechanism is proposed in which the in situ activation of Co species in octahedral positions is the crucial step for the observed increased activity of Cu$_{0.25}$Co$_{2.75}$O$_{4}$ compared to the other investigated cobaltite based spinels. In brief, this thesis provides besides possible optimization strategies of Co-based spinels in depth insights into the inevitable properties for catalysts with high deN$_{2}$O activity.