Structural transformations during the synthesis of mayenite ( Ca 12 Al 14 O 33) were investigated. The samples were prepared by a solid–state reaction and the transformations were researched by means of XRD, Rietveld analysis, SEM, and Raman spectroscopy. The three key phases ( CaAl 2 O 4, Ca 3 Al 2 O 6, Ca 5 Al 6 O 14) were identified and their role in the mayenite formation was assigned. The optimal low temperature pathway of the mayenite synthesis involving Ca 5 Al 6 O 14 intermediate was proposed.
Catalytic high temperature decomposition (secondary abatement) of nitrous oxide over calcium aluminate 12CaO Á 7Al 2 O 3 (mayenite) was studied in the model laboratory tests (TPSR) and pilot units (steady-state) using the real feed. X-ray diffraction (XRD), scanning electron microscopy (SEM), N 2 -sorption (BET), electron paramagnetic resonance (EPR) and Raman spectroscopies were used to characterize the synthesized material. The catalyst exhibited high efficiency and selectivity in N 2 O removal, reaching practically 100% conversion at 1150 K without appreciable total losses of NO x . Owing to its high thermal stability and resistivity to sintering and low cost of production raw materials, mayenite was found to be a promising catalyst for economically appealing secondary abatement of nitrous oxide in nitric acid plants.
Continuous wave (CW) and pulse electron paramagnetic resonance in a variant of hyperfine sublevel correlation spectroscopy (HYSCORE) were used for obtaining structural information concerning speciation and local environment of alien Cu(2+) and native O(2)(-) ions encaged in copper doped nanoporous 12CaO.7Al(2)O(3) (mayenite). The samples were prepared by a solid-state reaction and characterized by means of XRD, SEM, and Raman techniques. X-Band CW-EPR spectra showed that three different Cu(2+) species together with paramagnetic extraframework O(2)(-) anions were present in the mayenite sample, whereas extraframework OH(-) anions were revealed by Raman spectroscopy. (27)Al HYSCORE provided evidence for the interaction of Cu(2+) ions with the mayenite framework. Superhyperfine interaction of the Cu(2+) ions with proximal (d(Cu-OH) = 2.4 A) and distal (d(Cu-OH) = 5.0 A) OH(-) anions, located in the same and the nearby cage, respectively, was resolved by means of (1)H HYSCORE spectra. A different situation held for the encaged O(2)(-) radicals found to be sitting on the Ca(2+) ions. They exhibited only a weak superhyperfine interaction of 1 MHz with the (27)Al(3+) framework ions.
The supported double-promoted cobalt spinel catalyst has been tested for N 2 O decomposition in the model and real gas streams (tail gas) from nitric acid plant. Activity tests carried out in the model gas stream showed a high activity of the developed supported catalyst. However, during catalytic investigations performed under real industrial conditions it was found that this catalyst is very sensitive to NO x poisoning. The negative influence of these oxides is visible even at their low concentration. However, the stability tests performed in real nitric acid plant gas streams showed that the developed catalyst preserves its initial activity for at least 40 days. Therefore, the authors propose to apply it in a dual-bed catalytic system consisting of deNO x and deN 2 O catalyst beds. At the first stage, tail gas is purified from NO x in the NH 3-SCR process in the presence of commercial catalyst and at the second stage, catalytic decomposition of N 2 O into N 2 and O 2 takes place over the developed supported cobalt spinel catalyst. The deNO x catalyst bed applied in the proposed dual-bed catalytic system allowed for the complete conversion of NO x from the tail gases. After installation of the supported cobalt spinel catalyst downstream of the deNO x catalyst bed, a significant increase in deN 2 O catalyst activity in the real tail gas stream was observed. Under optimal conditions, this system (F = 150 L/h; T = 425-450 °C) allowed for the complete reduction of N 2 O emission. It is shown that the proposed catalytic system is robust and allows to obtain high conversions of both NO x and N 2 O.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.