Selective catalytic oxidation (SCO) of ammonia into nitrogen and water vapour over hydrotalcite originated mixed metal oxides ? a short review
Magdalena JABŁOŃSKA, Lucjan CHMIELARZ, Agnieszka WĘGRZYN ? Faculty of Chemistry, Jagiellonian University in Krakow, Poland
Please cite as: CHEMIK 2013, 67, 8, 701?710
Selective catalytic oxidation of ammonia into nitrogen and water vapour is currently regarded as one of the most promising technologies for abatement of ammonia emissions from waste gases. This paper gives an overview of SCO technology, including the catalyst development and possible mechanisms of SCO reaction. The use of mixed metal oxides obtained from synthetic hydrotalcite-like materials in the role of catalysts for the SCO reaction has been reviewed. Obtained results clearly show huge potential of tested materials as industrial catalysts. Finally, future research directions in the area of NH3 oxidation based on mixed oxide catalysts derived from hydrotalcite-like materials are proposed.
Keywords: hydrotalcite-like materials, oxide catalysts, selective catalytic oxidation of ammonia
Ammonia is a colorless, toxic gas with a characteristic pungent smell . Ambient NH3 is emitted by both biogenic and anthropogenic sources . The problem of NH3 emission into atmosphere is becoming an increasingly important issue due to the numerous processes in which ammonia is used as reactant or produced as byproduct. Industrial processes that are plagued with NH3 slip problem, including urea manufacture , nitrogen fertilizer production [3, 4], ammonium phosphate manufacture , biomass, coal gasification [5÷7] and petroleum refining or refrigeration . Additionally, an increasing amount of ammonia slip affected urea SCR technology used to diminish nitrogen oxides (NOx) emissions from power plants and other stationary sources of NOx emissions , as well as Diesel cars equipped with AdBlue systems . Other sources of NH3 emissions, such as beet sugar production, froth flotation, mineral wool production, metals processing or other miscellaneous sources, were also investigated [4, 10]. To control NH3 emission several technologies such as catalytic ammonia decomposition , condensation , membrane separation , adsorption by activated carbon or other adsorbent material [3, 13, 14], have been developed. In this work, the SCO technology, including catalysts based on mixed metal oxides obtained from hydrotalcite-like compounds and SCO mechanisms are reviewed. Besides, major conclusions and some research directions are presented.
1. Selective catalytic oxidation (SCO)
To meet the more and more stringent NH3 emissions the treatment technologies of ammonia emissions from waste gases based on physical and/or chemical processes have been developed. Among them the low-temperature selective catalytic oxidation (SCO) of ammonia into nitrogen is potentially considered as one of the most efficient technology for ammonia removal from oxygen-containing waste gases [7, 15, 16]. The catalytic oxidation of ammonia, depending on operating conditions and the type of catalysts used, can proceed in the three principal reactions (equations 1÷3) :
Reaction (1) constitutes the basis of the industrial manufacture of nitric acid, so called Ostwald process. For this reaction high temperature (800?900°C) is required and Pt/Rh gauze is used as catalyst . The N2O, which is formed in reaction (2) is used as a oxidizing agent for partial oxidation of hydrocarbons (e.g. oxidation of benzene to phenol) [18÷20]. Selective oxidation of NH3 into N2O needs significantly lower temperatures (>500°C). Several catalytic systems were reported to convert NH3 into N2O with high selectivity (e.g. Mn-Bi oxides systems) [18, 21]. On the other hand, the process described by reaction (3) is of great importance due to conversion of toxic ammonia into environmental friendly nitrogen  and is considered as one of the potentially approaches for the abatement of NH3 pollution. The SCO process should be selective to N2 and should prevent oxidation into considered above by-products (NO, N2O) . The effectiveness of the SCO process can be improved by high-performance catalysts, which will be able to decrease the reaction time (space time) or temperature of total NH3 oxidation . In particular, catalysts for the SCO process should be active, selective, stable, as well as resistant for all typical components of combustion gases (H2O, CO2, SO2). Additionally, catalysts for this process should be active at relatively low temperatures (250?400°C) to operate at temperatures of flue gases, without necessity for their heating.
2. Ammonia oxidation catalysts
The catalyst used for the oxidation of ammonia are classified in three main groups: i) metallic catalysts, ii) mixed oxide catalysts and iii) ion-exchanged zeolites. Noble metals (e.g. Pt [15, 25], Pd [15, 26], Ir , Ru , Ag [22, 27]) are characterized by high activity in the low temperature region (<200°C). Unfortunately, the studied catalysts present relatively poor selectivity to N2. Second group of catalysts consist of transition metals or their oxides (e.g. Co3O4, MnO2 or V2O5 , Ni, Fe and Mn supported on g-Al2O3 [7, 28], CuO/Al2O3 [29, 30] or Fe2O3-Al2O3, Fe2O3-TiO2, Fe2O3-ZrO2 ). This group of the catalysts show higher selectivity to N2 but operates at temperatures significantly higher than noble metals (<400°C). Zeolites, such as zeolite ZSM-5, Y, mordenite, beta, ferrierite, chabazite, all modified with transition metal ions [32, 33], have been found to be another group of materials tested as potential catalysts for the selective ammonia oxidation. The studies of zeolite materials have not resulted in development of an efficient catalyst fulfilling all parameters needed for its commercialization. It was caused mainly by difficulties in obtaining active and selective catalysts operating within the low-temperatures range (<400°C). Recently, mixed metal oxides derived form hydrotalcite-like materials are considered as promising catalysts for selective catalytic oxidation of ammonia into N2.
3. SCO on catalysts derived from hydrotalcite-like materials
3.1. Features of hydrotalcite-like compounds
Hydrotalcite-like compounds (HTs), are a group of naturally occurring anionic clays [34, 35]. The structure of these materials can be represented by starting from a brucite network [Mg(OH)2], which is presented in Figure 1. In particular, the structure of hydrotalcitelike materials is created by replacing a fraction of divalent Mg2+ cations in the brucite lattice by trivalent Al3+ cations, conferring a positive layer charge. This charge is electrically balanced by the incorporation of anions and water molecules into the interlayer region. It is possible to synthesize hydrotalcite-like materials with broad spectrum of divalent (e.g. Cu2+, Co2+, Ni2+) and trivalent (e.g. Fe3+, Cr3+, Mn3+) cations. Thermal decomposition of these materials at moderate temperatures, results in homogenously dispersed mixed oxides of metals, exhibiting high surface area (>200 m2/g) and good thermal stability that is usually required for heterogeneous catalysts [35, 36].
3.2. Catalytic tests on mixed metal oxide catalysts derived from hydrotalcite-like compounds
The catalytic performance of calcined hydrotalcite-like compounds in the selective catalytic oxidation of ammonia (SCO) had been studied under atmospheric pressure in a fixed-bed flow reactor. The composition of the gas mixture at the reactor inlet was [NH3]=0.5 vol.%, [O2]=2.5 vol.%. Helium was used as a balancing gas at a total flow rate of 40 cm3/min, while a space velocity was about 15,400h-1. In order to develop active and selective to N2 catalysts based on HTs materials, the influence of incorporating of di- and trivalent transition metal cations into the mixed oxides from corresponding HTs precursors, were investigated. The first examination was focused on a Mg/Al-HTs series of hydrotalcite derived mixed metal oxides containing additionally in the structure Cu, Co, Ni or Fe and with different molar ratio of the transition metals (5-20 mol.%) . The obtained results showed that both kind and content of transition metal introduced into Mg(Al)O matrix strongly influence the activity and selectivity to N2 of the catalysts of the SCO process. The studies allowed ranking transition metals in terms of the decreasing activity in the following order: Cu>Co>Ni>Fe. Ammonia oxidation over Cu(5 mol.%)/Mg/Al (Cu/Mg/Al = 5.0:66.0:29.0) catalyst started at about 150°C and at 400°C the total conversion of NH3 was reached. An increase in copper loading to 10 and 20 mol.%, only slightly influenced activity of the studied catalytic materials. An opposite order (with an exception of copper) was found for the selectivity to N2 measured for the catalysts containing 10 mol.% of transition metal: Fe>Ni>Cu>Co. Concluding, catalysts containing copper or iron were found to the most interesting systems for the SCO process.
Consequently, the materials containing both transition metals (Cu and Fe) in the structure were prepared, and examined in the SCO process . The molar ratio of copper varied in the range 0?10 mol.%, whereas the concentration of iron was constant (10 mol.%). The catalytic tests showed that presence of copper activated catalysts for the low-temperature ammonia oxidation. Taking into account both high activity and selectivity to N2, the optimum results was found for Cu(5 mol.%)/Mg/Fe (Cu/Mg/Fe = 5.0:20.0:10.0) mixed metal oxides. For this catalyst complete ammonia oxidation in the reaction mixture was obtained at 400°C with a selectivity to N2 of about 88%. Further modification of the catalysts composition, by substitution of Mg2+ by Zn2+ in the Cu(6 mol.%)/Mg/Fe (Cu/Mg/Fe = 6.0:14.0:10.0) catalyst and an influence of such changes for the catalytic activity and selectivity to N2, was examined . The Cu/Zn/Fe catalyst was found to be less active than the Cu/Mg/Fe counterpart, especially in the low temperature range. Additionally, this catalyst showed very poor selectivity to N2, which dropped to 17% at 425°C. More significantly, influence of calcination temperature on catalytic properties was investigated. Comparison of the results of catalytic tests for the sample calcined at 600°C and 900°C revealed that the lower temperature of calcination lead to more active and selective catalytic materials. For example, for the Cu/Mg/Fe sample calcined at 600°C revealed the complete NH3 conversion in the reaction mixture at 375°C. Selectivity to N2 did not drop below 76% in the studied temperature range. An increase in calcination temperature to 900°C of the Cu/Mg/Fe sample decreased both activity as well as its selectivity to N2. Total oxidation of NH3 was achieved at 500°C with 57% of the selectivity to N2. This effect could be explained by significant decrease in surface area of the sample calcined at higher temperature (from 42 to 2 m2/g) and formation of more aggregated CuOx species with lower reducibility, which catalytically operate at higher temperatures. In sum, calcination temperature of 600°C was found to guarantee optimum activity and selectivity to N2 for the studied catalysts. Finally, the Cu(5 mol.%)/Mg/Al (Cu/Mg/Al = 5.0:66.0:29.0) catalyst was modified with small amounts (0.2 wt.%) of noble metals, such as Pd, Rh and Pt . The studies allowed ranking the noble metals deposited on the Cu/Mg/Al sample in the following activity order: Pt > Pd > Rh. The highest activity was observed for the sample modified with platinum. The Cu/Mg/Al-Pt catalyst was able to totally oxidize NH3 at 350°C. In case of the selectivity to N2, the order was found to be opposite to the order of activity. The highest selectivity to N2, above 85% in the studied temperature range, was achieved over the Rh-containing catalyst. Catalytic results are presented in Table 1.
Concluding, Cu-containing mixed metal oxides obtained from hydrotalcite-like materials were found to be active and selective catalysts for low-temperature NH3 oxidation into N2 and water vapour. The Cu/Mg/Al catalysts appear to be potential candidate for the NH3 slip catalysts.
3.3. Mechanisms of SCO reaction
Three major mechanisms of the SCO process were proposed in scientific literature: i) hydrazine mechanism, ii) imide mechanism and iii) internal selective catalytic reduction mechanism.
Hydrazine (N2H4) mechanism includes oxidation of NH3 by molecular oxygen (O) with formation of amide (NH2) species. In the next step, coupling of amide species results in the formation of hydrazinium-type intermediate and its subsequent oxidation by O2 to N2 and/or N2O [41, 42]. This mechanism was suggested for the SCO process in the presence of catalysts based on transition metal oxides (e.g. CuO/Al2O3, CuO/TiO2 or Fe2O3/TiO2) [41, 42]. The second mechanism, called imide (NH) mechanism, was proposed by Zawadzki . In the first step NH3 is oxidized with formation of imide. Which in the next step reacts with atomic oxygen (O) to form nitrosyl (HNO) intermediate. N2 and H2O are formed in the reaction between nitrosyl and imide. Other final products such as NO, N2O and H2O are formed as a result of stages involving intermediate compounds: HNO, NH and O2. This mechanism was proposed for Pt and transition metal oxide catalysts (e.g. CuO, MnO2 and Fe2O3) . According to above mentioned mechanism, formation of active atomic oxygen (O) is the crucial reaction step . The internal selective catalytic reduction (i-SCR) mechanism was proposed among others for the process of NH3 oxidation over hydrotalcite-like derived mixed metal oxides catalytic systems containing copper and/or iron. This mechanism involves two steps: (i) part of NH3 is oxidized to NO in low-temperature range (equation 4):
If the SCO process proceeds according to the i-SCR mechanism, the active and selective catalysts of NH3 oxidation should be also active in the process of the selective catalytic reduction of NO with ammonia (SCR, DeNOx). Results of the catalytic studies of the SCR process for the Cu-containing samples (e.g. Cu/Mg/Fe = 5.0:20.0:10.0, Cu/Mg/Al = 5.0:66.0:29.0) revealed high activity in the low-temperature range. N2 and N2O were found to be the only N-containing reaction products. Moreover, it was found that the SCR process started at lower temperatures than SCO reaction. In Figure 2 the results of the catalytic studies of the SCO and SCR process for the Cu(5 mol.%)/Mg/Fe (Cu/Mg/Fe = 5.0:20.0:10.0) sample are presented. It was observed that ammonia oxidation in the SCO process over this catalyst started at about 275?C, whereas conversion of NO in the SCR process was noticed at lower temperatures, at about 175?C. Therefore, it could be concluded that oxidation of NH3 into NO (eq.4.) is a rate determining step in the low-temperature range. Consequently, the idea of the studies of bifunctional catalysts appeared . The temperature of the SCO process decreased by introduction of noble metals (Pd, Rh, Pt), active in oxidation of ammonia into NO, while transition metals (Cu, Fe) accelerated the process of reduction of NO with NH3. Concerning the results of catalytic tests for the SCR process performed for those catalysts, it was noticed that the oxidation of NH3 to N2 obeyed i-SCR mechanism.
To prove the hypothesis about the i-SCR mechanism of the process of NH3 oxidation over the hydrotalcite derived catalysts, additional studies were carried out. The results of the catalytic tests for the Cu(5 mol.%)/Mg/Fe (Cu/Mg/Fe = 5.0:20.0:10.0) mixed metal oxides performed with various contact times are presented in Table 2. Studies on effect of space velocity (SV) on activity of the studied catalyst, revealed that an increase in SV (shorter contact time) shifted the NH3 conversion curves into higher temperatures. Selectivity to N2 decreased and simultaneously increased selectivity to NO. Selectivity to N2O was only slightly modified. Concluding, the obtained results revealed that the process of NH3 oxidation proceeds according to the sequence of the reactions presented above (equations 4÷6). Further studies using temperature programmed methods are necessary for a detailed clarification of the mechanism over the mixed metal oxides.
4. Concluding remarks
From the presented above studies focused on the development of SCO catalysts based on hydrotalcite-like compounds, it could be concluded that activity of these catalysts depended on their chemical composition and phase composition (dependent on calcination temperature). It was found that calcined HTs with an addition of copper are active and selective catalyst for the NH3 oxidation process. The internal SCR (i-SCR) mechanism was proposed for the reaction performed over the studied catalysts. Therefore, it could be concluded that active and selective catalysts need a good combination of noble metals, such as Pd, Rh, Pt, responsible for oxidation of ammonia into NOx as well as transition metals such as Cu or Fe, active in the SCR process. Presented solution gives good example of such design.
The research was partially carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08).
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Magdalena JABŁOŃSKA ? M.Sc., graduated from the Faculty of Chemistry of the Jagiellonian University (2010), Eng., graduated from Faculty of Energy and Fuel of the University of Science and Technology (2013). Currently she is a Ph.D. student in the Department of Chemical Technology of JU. She is the author of several papers related to the application of the catalytic methods for purification of waste gases. Internship abroad: Univeristy of Warwick (2011), Universitat Politechnica de Valencia (2012). e-mail: email@example.com
Lucjan CHMIELARZ ? D.Sc., Prof. JU, Graduated from the Jagiellonian University in Krakow (1992). He received his Ph.D. and D.Sc. degrees (1997 and 2007, respectively) in chemistry from the Jagiellonian University. Since 2010 he is a professor of the Jagiellonian University. He is a Vice Dean for student affairs of Faculty of Chemistry JU (since 2012) and head of Group of Chemical Environmental Technologies (since 2008). He specializes in synthesis and characteristics of micro- and mesoporous materials with controlled porous structure for catalysis and adsorption as well as catalytic processes for purification of flue gases.
Agnieszka WĘGRZYN ? Ph.D., graduated from the Faculty of Chemistry of the Jagiellonian University (2005). Ph.D. thesis was awarded the prize for outstanding dissertation. Scholarships and foreign contracts: University of Leipzig (2004/2005), University of Warwick (2007), Ecole Nationale Supérieure des Mines de Saint-Étienne (assistant professor, 2010). She works currently at the Faculty of Chemistry of the Jagiellonian University in the Environmental Technology Group. Research interests are related to heterogeneous catalysis, synthesis of advanced materials, mainly adsorbents and catalysts as well as technologies of environmental protection.