Cu-Mg-Al hydrotalcite-like materials as precursors of effective catalysts for selective oxidation of ammonia to dinitrogen — The influence of Mg/Al ratio and calcination temperature Sylwia Basąg a , Zofia Piwowarska a , Andrzej Kowalczyk a , Agnieszka Węgrzyn a , Rafał Baran b , Barbara Gil a , Marek Michalik c , Lucjan Chmielarz a, ⁎ a Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Kraków, Poland b AGH University of Science and Technology, Faculty of Energy and Fuels, Mickiewicza 30, 30-059 Kraków, Poland c Jagiellonian University, Institute of Geological Sciences, Oleandry 2a, 30-063 Kraków, Poland abstract article info Article history: Received 4 March 2016 Received in revised form 23 April 2016 Accepted 18 May 2016 Available online xxxx Hydrotalcite originated Cu-Mg-Al mixed metal oxides were studied as catalysts for selective oxidation of ammo- nia to dinitrogen. Cu-Mg-Al hydrotalcite-like materials with copper content of 5 mol% and various molar Mg/Al ratios were synthetized by coprecipitation method and then calcined at 600, 700 and 800 °C. It was shown that both Mg/Al ratio as well as calcination temperature are very important synthesis parameters determining selec- tivity of the studied catalysts in ammonia oxidation process. The catalysts with lower Mg/Al ratio, so higher Al content, were more selective to dinitrogen. Moreover, it was shown that an increase in calcination temperature also resulted in the catalysts with a significantly improved selectivity to dinitrogen. These interesting effects were related to the formation of the Cu-containing spinel phases, which possibly are responsible for high selectivity to dinitrogen in the high temperature range. © 2016 Elsevier B.V. All rights reserved. Keywords: Hydrotalcite-like materials Mixed metal oxides Copper NH 3 -SCO 1. Introduction Hydrotalcite is a mineral characterized by the brucite-like structure of Mg(OH) 2 , in which part of the Mg 2+ octahedra (six-coordinated to OH – ) forming double layers, is substituted by trivalent aluminium cat- ions. The presence of Al 3+ in the brucite-like layers results in their pos- itive charging. This positive charge is compensated by anions, typically carbonates, which together with water molecules are located in the in- terlayer space of hydrotalcite. It is possibly to synthetize hydrotalcite as well as hydrotalcite-like materials, in which Mg 2+ as well as Al 3+ ions are partially or completely replaced by di- (e.g. Cu 2+ ) and/or trivalent (e.g. Fe 3+ ) cations, respectively (Cavani et al., 1991). The range of metal cations that can be incorporated into the brucite-like layers is rel- atively broad and is determined by their size, which should be similar to that of Mg 2+ in case of divalent cations and to Al 3+ in case of trivalent cations (Cavani et al., 1991). Calcination of hydrotalcite-like materials results in their thermal decomposition and formation of mixed metal oxides characterized by a relatively high specific surface area and ho- mogenous distribution of metal cations. Moreover, depending on calci- nation conditions the samples with various phase compositions can be obtained. Taking into account these properties and additionally a large number of various metals, which can be successfully incorporated into the brucite-like layers, hydrotalcites-like materials are very promising for the possible applications in catalysis (Nishida et al., 2009; Zhang et al., 2010; Chmielarz et al., 2012; Xu et al., 2015), including also the pro- cess of selective catalytic oxidation of ammonia to dinitrogen - NH 3 -SCO (Chmielarz et al., 2005, 2011, 2013; Jabłońska et al., 2013). Global emission of gaseous ammonia into the atmosphere was esti- mated to be about 44 million tons per year (without the oceanic sources) and about 75% of this emission is related to agriculture activi- ties, mainly volatilization from livestock wastes as well as losses from agriculture crops, especially when nitrogen fertilizers are used (FAO, 2001). The remaining 25% of the ammonia emission is poorly docu- mented, however it is suggested that about 13% is a result of biomass burning, about 0.2% is related to fossil fuel combustion (including trans- portation), while about 0.5% to ammonia emission from industrial pro- cesses (FAO, 2001). The last two sources of ammonia emission apparently seem to have only a negligible role in the global ammonia emission, however it is anticipated that contribution of these sources will increase in the future, mainly due to common use of cars with the systems of flue gases purification (Zhao and Wang, 1994). Average am- monia emission from gasoline European car not equipped with catalytic three-way converter is 2.2 mg km -1 , while for such car with catalytic three-way converter increases to 85 mg km -1 (Sutton et al., 2000). In case of diesel cars even higher increase in ammonia emission is Applied Clay Science xxx (2016) xxx–xxx ⁎ Corresponding author. E-mail address: chmielar@chemia.uj.edu.pl (L. Chmielarz). CLAY-03869; No of Pages 9 http://dx.doi.org/10.1016/j.clay.2016.05.019 0169-1317/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay Please cite this article as: Basąg, S., et al., Cu-Mg-Al hydrotalcite-like materials as precursors of effective catalysts for selective oxidation of ammonia to dinitrogen — The..., Appl. Clay Sci. (2016), http://dx.doi.org/10.1016/j.clay.2016.05.019