NO direct catalytic decomposition over cobalt based mixed oxides modified by potassium
K. Pacultováa, K. Karásková a, T. Bílková a, K. Jirátováb, L. Obalováa
a Institute of Environmental Technology, CEET, VSB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava, Czech Republic
b Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Rozvojová 2/135, 165 02 Prague, Czech Republic
The main aim was to study potassium modified Co based mixed oxide catalysts with respect to the direct catalytic NO decomposition - understanding of reaction mechanism, active sites formation and catalyst stability. To achieve this, the K/Co-Mn-Al containing catalysts were prepared via different methods, characterized (AAS, ICP, XRD, XAS, N2 physisorption, TPR H2, TPD NO/O2/NO+O2, TPD CO2, SEM, TEM, XPS, TGA, WF, SR-TAD) and tested for NO catalytic decomposition (dynamic and steady state experiments) in inert and in O2, H2O or CO2 presence. The effect of active phase precursor (hydrotalcite, nitrate, carbonate), K promoter precursor (nitrate, carbonate, oxalate, acetate), the way of K deposition (impregnation, wet cake impregnation, coprecipitation), presence and amount of structural promoter (Mn, Zn, Mg, Pb), additional electronic promoter (Ce) and reaction mechanism were studied. Catalytic tests were focused on activity, stability and deactivation features.
It was found out that the parameters of catalyst preparation necessary for obtaining active materials are (i) activation in O2 rich atmosphere, (ii) calcination at 700 °C and (iii) modification by optimal potassium amount [1]. The catalytic activity depends on K amount that influences catalyst basicity and reducibility. Optimal residual K amount depends on the nominal K content and on the deposition method, reflecting in the change of catalyst surface area. The most effective way is coprecipitation of transition metal nitrates with solution of K2CO3/KOH, washing and calcination [2]. For K modified samples, the Co presence is essential for the activity, however, part of expensive and toxic cobalt can be replaced by Mg or Zn without any decrease in the activity [3]. The active sites are Co2+/Co3+ redox pairs in spinel structure interacting with K species, while presence of Mn and Al increases catalyst long-term high temperature stability. Transition metals in octahedral coordination represent sites for NO chemisorption, whereas K ensures NO oxidation to surface intermediates. O2 desorption is the slowest step. Langmuir-Hinshelwood mechanism is prevailing way of reaction [4]. The addition of O2 to the feed gas has detrimental effect on NO conversion; the effect is concentration dependent and reversible. The O2 inhibition is ascribed to oxidation of loosely bound nitrogen oxide species on the Co3+ octahedral sites to more stable surface nitrates [5].
Acknowledgements: The research was funded by ERDF project "Institute of Environmental Technology – Excellent Research" [No. CZ.02.1.01/0.0/0.0/16_019/0000853] and by Czech Science Foundation [No. 18-19519S]. Experimental results were accomplished by using Large Research Infrastructures ENREGAT and CATPRO supported by the Ministry of Education, Youth and Sports of the Czech Republic under projects No. LM2018098 and No. LM2015039.
References
[1] Pacultová, K., et al., Catalysts 2019, 9 (7), 26.
[2] Jirátová, K., et al., Catalysts 2019, 9 (7), 592.
[3] Karásková, K., et al., Catalysts 2020, 10 (8), 931.
[4] Bilková, T., et al., J. Taiwan Inst. Chem. Eng. 2021, 120, 257-266.
[5] Pacultová, K., et al., Molecular Catalysis 2021, 510, 111695.