Effect of preparation procedures on catalytic activity and selectivity of copper-based mixed oxides in selective catalytic oxidation of ammonia into nitrogen and water vapour

Effect of preparation procedures on catalytic activity and selectivity of copper-based mixed oxides in selective catalytic oxidation of ammonia into nitrogen and water vapour

Accepted Manuscript Title: Effect of preparation procedures on catalytic activity and selectivity of copper-based mixed oxides in selective catalytic ...

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Accepted Manuscript Title: Effect of preparation procedures on catalytic activity and selectivity of copper-based mixed oxides in selective catalytic oxidation of ammonia into nitrogen and water vapour Authors: Magdalena Jabło´nska, Marek Nocu´n, Kinga Goł˛abek, Regina Palkovits PII: DOI: Reference:

S0169-4332(17)31799-3 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.144 APSUSC 36344

To appear in:

APSUSC

Received date: Revised date: Accepted date:

16-2-2017 12-6-2017 13-6-2017

Please cite this article as: Magdalena Jabło´nska, Marek Nocu´n, Kinga Goł˛abek, Regina Palkovits, Effect of preparation procedures on catalytic activity and selectivity of copper-based mixed oxides in selective catalytic oxidation of ammonia into nitrogen and water vapour, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.144 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of preparation procedures on catalytic activity and selectivity of copper-based mixed oxides in selective catalytic oxidation of ammonia into nitrogen and water vapour

Magdalena Jabłońskaa,b, Marek Nocuńc, Kinga Gołąbekd, Regina Palkovitsa,b,*

a

Chair of Heterogeneous Catalysis and Chemical Technology, RWTH Aachen University, Worringerweg 2,

52074 Aachen, Germany b

Center for Automotive Catalytic Systems Aachen, RWTH Aachen University, Schinkelstr. 8, 52062 Aachen,

Germany c

Faculty of Material Science and Ceramics, AGH University of Science and Technology, Mickiewicza 30, 30-059

Kraków, Poland d

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland

*

Corresponding author. Tel: +49 241 80 26497; Fax: +49 241 80 22177. E-mail address: [email protected]

aachen.de (R. Palkovits)

Graphical abstract

1

Highlights   

Preparation techniques influenced catalysts activity and selectivity in NH3-SCO. Coprecipitation favoured formation of easily reducible highly dispersed CuOx species. Suitable amount of Al and Zr in catalysts enhanced activity and/or N2 selectivity.

Abstract The selective oxidation of ammonia into nitrogen and water vapour (NH3-SCO) was studied over Cu-Mg(Zn)Al-(Zr) mixed metal oxides, obtained by coprecipitation and their subsequent calcination. The effect of acidbase properties of Cu-Mg-Al-Ox on catalytic activity was investigated by changing the Mg/Al molar ratio. Other Cu-containing oxides were prepared by rehydration of calcined Mg-Al hydrotalcite-like compounds or thermal decomposition of metal nitrate precursors. XRD, BET, NH3-TPD, H2-TPR, XPS, FTIR with adsorption of pyridine and CO as well as TEM techniques were used for catalysts characterization. The results of catalytic tests revealed a crucial role of easily reducible highly dispersed copper oxide species to obtain enhanced activity and N2 selectivity in NH3-SCO. The selective catalytic reduction of NO by NH3 (NH3-SCR) and in situ DRIFT of NH3 sorption indicated that NH3-SCO proceeds according to the internal selective catalytic reduction mechanism (i-SCR).

Keywords: hydrotalcite-like compounds, mixed metal oxides, copper, ammonia oxidation, i-SCR mechanism

1. Introduction

The selective catalytic oxidation of ammonia into nitrogen and water vapour (NH3-SCO; 4NH3 + 3O2  2N2 + 6H2O) serves as one of the most efficient methods to abate ammonia from exhaust gases, e.g. after selective catalytic reduction of NO by NH3 (NH3-SCR, DeNOx; 4NH3 + 4NO + O2  4N2 + 6H2O). Recently published reviews provided a detailed overview on the activity, selectivity and stability of various catalytic systems in NH3SCO [1,2]. Among them, copper-based materials – deposited on inorganic oxides (e.g. Al2O3, TiO2), copper modified clays (e.g. hydrotalcite derived mixed metal oxides, porous clay heterostructures), or copper exchanged 2

zeolites (e.g. ZSM-5, Bea) – present the most efficient catalysts. Cu-Mg-Al-Ox mixed metal oxides derived from hydrotalcite-like compounds guarantee the highest activity and selectivity to nitrogen among copper-containing clays. The magnesium-aluminium oxide matrix appears to be an attractive host material for copper, as verified from various studies concerning N-containing by-product removal of exhaust gases (e.g. [3,4]). However, none of the mentioned studies elaborates on the effect of preparation procedure for the incorporation of copper within Mg-Al-Ox on copper dispersion, particle size and its state. Furthermore, the catalytic activity, selectivity and stability over modified clays has been less investigated than over oxide supports or zeolites (modified mainly with noble metals). Qu et al. [5] revealed that highly dispersed and small silver particles (5-7 nm) supported on Al2O3 (among SiO2, NaY and TiO2) facilitated a superior catalytic activity and N2 selectivity in NH3-SCO. GóraMarek et al. [6] came to similar conclusions focussing on silver loaded zeolites USY. Regarding hydrotalcite derived mixed metal oxides, Gao et al. [7,8] reported that the addition of suitable amounts of ZrO2 enhanced copper dispersion. Moreover, copper supported on ZrO2 proved to be the most efficient catalyst among copper supported on Al2O3 or TiO2 [9]. Therefore, by using a well-controlled coprecipitation procedure, Cu-Mg-Al-ZrOx could possibly enhance activity for NH3-SCO. The substitution of cations within brucite layers of hydrotalcitelike compounds provides also a facile and highly reproducible route to adjust their acid-base properties. Besides, adjusting the Mg/Al molar ratio, Bezen et al. [10] found that the acid side concentration of Mg-Zn-Al-Ox increased with higher zinc content due to its larger cation radius and the higher reducibility of ZnO than MgO. Another interesting property of hydrotalcite-like compounds is their regeneration ability after moderate heating, a so called memory effect [11]. Mixed metal oxides, often amorphous with high specific surface area, are able to recover the layered structure by contact with aqueous solution of desired metal precursors. Consequently, hydrotalcite-like compounds and their derivatives are widely investigated due to remarkable properties of the final catalysts. The mechanism of NH3-SCO over different catalysts, including hydrotalcite derived mixed metal oxides, is still unclear. Among three recognized reaction pathways: (i) imide (NH) mechanism, (ii) hydrazine (N2H4) mechanism, and (iii) internal (or in situ) selective catalytic reduction mechanism (i-SCR), the last one is accepted by the majority of researchers. Recently published reviews provide a more detailed description of mechanisms 3

of the ammonia oxidation over a broad range of catalytic systems [1,2]. Briefly, i-SCR mechanism involves the oxidation of a significant percentage of NH3 into NOx species (intermediate). Subsequently, NHx species reduce adsorbed NOx species with formation of N2 and/or N2O [12,13]. The oxidation of ammonia to NOx is considered as a rate-limiting step in the low temperature range [14]. However, the elementary surface reaction steps depend on the applied catalysts, pretreatment and reaction conditions, etc.; therefore, general conclusions could not be draw and studies in this field are required in order to facilitate a rational design of efficient catalysts. In the present work, we studied a series of Cu-Mg(Zn)-Al-(Zr) mixed metal oxides . The acid-base properties of a Cu-Mg-Al-Ox series were adjusted by changing the Mg/Al molar ratio, in order to enhance catalysts activity and N2 selectivity in NH3-SCO. Variation of the synthesis procedure from coprecipitation to rehydration and thermal decomposition enabled drawing conclusions on reactivity of copper oxide species of Cu-Mg-Al-Ox. Furthermore, we pointed out the correlation between results of catalytic tests for NH3-SCO and NH3-SCR, and finally, we discussed the possible species involved in the reaction mechanisms for these systems.

2. Experimental

2.1 Catalyst preparation

Three Cu-containing materials were prepared by coprecipitation (cop.), rehydration of calcined hydrotalcite-like compounds (reh.) and thermal decomposition of metal nitrates (decom.). Table S1 (Supplementary Information) summarizes detailed compositions of all prepared materials. The series of Cu-Mg(Zn)-Al-(Zr) precursors with intended molar ratios were synthesized by coprecipitation using 1 M aqueous solutions of the following metal nitrates: Cu(NO3)23H2O (Sigma-Aldrich), Mg(NO3)26H2O (Sigma-Aldrich), Zn(NO3)2·6H2O (Sigma-Aldrich), Al(NO3)39H2O (Sigma-Aldrich) and ZrO(NO3)2·xH2O (Sigma-Aldrich). A solution of NaOH (Chemsolute) was used as a precipitating agent. Metal nitrate solutions were added to a vigorously stirred solution containing a slight over-stoichiometric excess of Na2CO3 (Sigma-

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Aldrich). The pH was maintained at pH = 10.0  0.2 by dropwise addition of 1 M NaOH solution. The obtained slurry was aged at 60°C for 0.5 h, filtered, washed with distilled water and dried at room temperature. The sample with an intended Cu content of 10 wt.%, was prepared by rehydration of Mg-Al (Mg/Al = 79/21, mol.%) mixed metal oxides in 0.2 M aqueous solution of the metal nitrate: Cu(NO3)23H2O (Sigma-Aldrich). The mixture: 2.5 g of freshly calcined (static air at 600C for 6 h) Mg-Al hydrotalcite-like compounds per 19.5 cm3 of solution, was stirred at room temperature for 0.5 h, filtered, washed with distilled water and dried at room temperature. The material obtained by thermal decomposition of mixtures of metal nitrate precursors was obtained by dissolving the respective amounts of Mg(NO3)26H2O (Sigma-Aldrich), Al(NO3)39H2O (Sigma-Aldrich) and Cu(NO3)23H2O (Sigma-Aldrich) (Cu/Mg/Al = 8/63/29, mol.%) in distilled H2O. Subsequently, water was evaporated at 80C and the resulting solid was pestled in a mortar. All the obtained samples were calcined in static air at 600C for 6 h. Mixed metal oxides obtained from hydrotalcite-like compounds were kept in a desiccator in order to avoid reconstruction of the hydroxide-like structure. For catalytic experiments, a fraction of particle size in the range of 0.250-0.500 mm was used.

2.2 Catalyst characterization

2.2.1 Structural and textural parameters

The X-ray diffraction (XRD) patterns of the samples were recorded with a D5000 Siemens diffractometer using Cu Kα radiation (λ = 1.54056 Å, 45 kV, 40 mA). The cell parameters a and c were determined by analysis of (1 1 0) reflection and basal (0 0 3) and (0 0 6) reflections, respectively. The crystal sizes were calculated using Scherrer equation.

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The specific surface area (SBET) of the samples was determined by low-temperature (-196C) N2 physisorption using Quantachrome Quadrasorb SI. Prior to nitrogen adsorption, the samples were outgassed at 250C for 12 h using a Quantachrome Flovac degasser. Transmission electron microscopy (TEM) measurements were performed using a FEI Tecnai G220 X-TWIN electron microscope operating at 200 kV and providing 0.25 nm resolution. X-ray fluorescence (XRF) spectroscopy was used to determine the chemical composition of the samples. Spectra were recorded under vacuum on an Eagle II Röntgenanalytik Systeme spectrometer.

2.2.2 Temperature-programed studies

Temperature-programmed reduction (H2-TPR) experiments of the materials (30 mg) were performed using a Quantachrome ChemBET Pulsar TPR/TPD instrument. H2-TPR runs were carried out starting from room temperature to 1000°C, with a linear heating rate of 10°C min-1 and in a flow (25 cm3 min-1) of 5 vol.% H2 diluted in Ar. Water vapour was removed from effluent gas by means of a cold trap. H2 consumption was detected and recorded by a TCD detector. The temperature programmed desorption of ammonia (NH3-TPD) of the samples was performed in a flow microreactor system equipped with QMS MKS, Cirrus 2 detector. Prior to ammonia sorption, the sample (100 mg) was outgassed in a flow of pure Ar at 600ºC for 1 h. Subsequently, the microreactor was cooled down to 70ºC and the sample was saturated in a flow (20 cm3 min-1) of gas mixture containing 1.0 vol.% of NH3 diluted in Ar for about 2 h. Then, the sample was purged in a flow of pure Ar until a constant baseline level was attained (about 2 h). In the next step, the temperature of the reactor was raised in the range of 70-600C with a linear heating rate of 5C min-1 in a flow of pure Ar (20 cm3 min-1).

2.2.3 IR spectroscopy studies with probe molecules

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Prior to FTIR studies all samples were pressed into the form of self-supporting wafers (ca. 5-10 mg cm-2) and pretreated in situ in a homemade quartz IR cell at 400C under vacuum conditions for 1 h. IR spectra were recorded with a Tensor 27 Bruker spectrometer equipped with a MCT detector. The spectral resolution was 2 cm−1. Sorption of CO as probe molecule was performed at room temperature. The concentrations of chemisorbed CO were calculated from the maximum intensities of the Cu0-CO bands and the corresponding value of the extinction coefficient (0.094 cm2 μmol-1) [15]. Pyridine (Py) was adsorbed at room temperature, next the physisorbed molecules were removed in 0.5 h by evacuation. The concentration of Brönsted and Lewis acid sites was determined in quantitative IR studies of pyridine adsorption [16]. Values of 0.10 cm2 μmol-1 and 0.07 cm2 μmol-1 were obtained for the 1440 cm-1 band of pyridine coordinatively bonded to Lewis sites (PyL) and for the 1545 cm-1 band of pyridinium ion (PyH+), respectively. Next, the Py contacted sample was heated to 250C, kept at this temperature for 2 min and cooled down to room temperature, while collecting the spectrum. The amount of copper sites interacting with CO and Py was estimated using the values of the extinction coefficient reported for zeolites. Since the C=C bond vibrations of Py in Py-Lewis (the band at 1440 cm-1) adducts and PyH+ ions (the band at 1545 cm-1) are not very sensitive for the type of cation and acidic property of protonic sites, respectively, the extinction coefficients of the PyL and PyH+ bands are effectively applied to other systems. Indeed, in our case the frequencies of PyL and PyH+ bands were the same as for zeolitc systems (1450-1440 cm1

and 1545 cm-1); therefore it was supposed that the extinction coefficients of these bands were also comparable.

2.2.4 X-ray photoelectron spectra measurements

X-ray photoelectron spectra (XPS) were measured on a VSW spectrometer equipped with a hemispherical analyser. The photoelectron spectra were measured using a MgKα source (E = 1253.6 eV). The base pressure in the analysis chamber during the measurements was 310−6 Pa and the spectra were calibrated on the main carbon C 1s peak at 284.6 eV. The composition and chemical surrounding of the sample surface were investigated based on the areas and binding energies of Cu 2p, Mg 2p, Al 2p, C 1s and O 1s photoelectron peaks. Mathematical

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analyses of the XPS spectra were carried out using the XPSpeak 4.1 computer software (RWM. Kwok, The Chinese University of Hong Kong).

2.3 Catalytic tests

The catalytic experiments of selective ammonia oxidation (NH3-SCO) were performed under atmospheric pressure in a fixed-bed flow microreactor (i.d., 6 mm; l., 320 mm). The reactant concentrations (NH3 (m/z 16), N2 (m/z 28), NO (m/z 30), N2O (m/z 44), NO2 (m/z 46), H2O (m/z 18)) were continuously monitored using a QMS MKS, Cirrus 2 detector directly connected to the reactor outlet using a heated capillary. Prior to the test, the catalyst sample (100 mg) was outgassed at 600C for 1 h in a flow of pure Ar (20 cm3 min-1). The composition of the gas mixture at the reactor inlet consisted of [NH3] = 0.5 vol.%, [O2] = 2.5 vol.%, [Ar] = 97.0 vol.%. The total flow rate of the reaction mixture was 40 cm3 min-1, while the weight hourly space velocity (WHSV) was about 24,000 cm3 h-1 g-1. Studies were performed in the temperature range of 100-600°C with a linear heating rate of 5°C min-1. For selected samples additional catalytic tests were carried out including (i) polythermal and (ii) isothermal NH3-SCO in the presence of water vapour with the following composition of the gas mixture: [NH3] = 0.5 vol.%, [O2] = 2.5 vol.%, [H2O] = 3.2 vol.%, [Ar] = 93.8 vol.%, or (iii) NH3-SCR with the following composition of the gas mixture: [NO] = [NH3] = 0.25 vol.%, [O2] = 2.5 vol.%, [Ar] = 97.25 vol.%. The signal of the argon line served as the internal standard to compensate small fluctuations of the operating pressure. The sensitivity factors of the analysed lines were calibrated using commercial mixtures of gases.

2.4 Fourier transform infrared spectroscopy studies

The in situ DRIFT spectra were recorded with a Vertex 70-FTIR Bruker spectrometer equipped with a MCT detector. Prior to the FTIR study, the sample was pretreated in situ at 400C in a flow of pure N2 for 1 h and then cooled to 50C. The sample was saturated in a flow (5 cm3 min-1) of gas mixture containing 1.0 vol.% of NH3 diluted in Ar for about 0.5 h. Subsequently, physisorbed molecules were removed in 15 min by evacuation. Next, 8

the sample contacted with NH3 was heated in a flow of: (i) pure N2 or (ii) a gas mixture containing 5.0 vol.% of O2 diluted in Ar up to 100-350C, kept at a particular temperature for 10 min and cooled down to 50C, while the spectrum was collected. All spectra were recorded at a resolution of 4 cm-1 with 128 accumulated scans. The background spectrum was subtracted from the sample spectrum.

3. Results and discussion

3.1 Structural properties of Cu-Mg(Zn)-Al-(Zr) precursors

The synthesis of catalyst’s precursors was carried out at a range of M2+/M3+ mole ratio from 1.27-2.45. XRD analyses confirmed that the as-synthesized materials exhibited the typical reflections characteristic of crystalline hydrotalcite-like compounds with rhombohedral symmetry (space group R3m, 3R1 polytype) [17]. The diffraction patterns possess sharp and symmetrical reflections for (0 0 3), (0 0 6), (1 1 0) and (1 1 3), and broad and asymmetrical for (0 1 2), (0 1 5) and (0 1 8), characteristic of a well-crystallized hydrotalcite-like compounds in carbonate form (Fig. S1). An exception was the Cu-Mg-Al-Zr precursors (Cu/Mg/Al/Zr = 5/66/29/50-100, mol.%), for which the hydrotalcite-like phase was not formed. . Table 1 presents the cell parameters and crystallite sizes of the (Cu)-Mg(Zn)-Al-(Zr) precursors determined by the XRD measurements. The cell parameter a for Mg71Al29 was typical for hydrotalcite-like compounds with M2+/M3+ ratio of about 2.10-2.50 (e.g. [18]). The cation-cation distance of transition metal substituted Mg-Al hydrotalcite-like compounds varied in accordance to the ionic radii of Cu2+ (0.073 nm), Zn2+ (0.074 nm), Al3+ (0.053 nm), and Zr4+ (0.072 nm) cations [19]. XRD analyses demonstrated that the intensity of the hydrotalcite-like phase significantly decreased for precursor with 25 mol.% of Zr and did not appear for the materials with higher Zr content (50-100 mol.%). Thus, possibly the interaction of large distortions in the brucite-like layers appeared as a result of incorporation of Zr4+. The parameter c of about 2.3 nm for studied materials is typical for hydrotalcite-like compounds containing CO32- as the interlayer anions [20]. The variations in c parameter between various hydrotalcite-like compounds resulted probably from different coulombic attractive forces between the positively charged brucite-like layers and the 9

anions located in the interlayer space [21]. Introducing cation with higher electronegativity into the Mg-Al brucite-like structure possibly increased the strength of electrostatic interactions and consequently reduces the interlayer distance. Zn-based hydrotalcite-like compounds revealed bigger crystallite sizes than Mg-containing materials. Similar results were presented over a series with different compositions (e.g. [22,23]), and could be possibly explained by the faster crystal grain-growth process compared to nucleation in case of Zn-containing materials [24]. Consequently, their particles tended to grow larger.

Table 1

The heat treatment of precursors at 600C destroyed their structure since no characteristic reflections were present in XRD patterns of the corresponding mixed metal oxides. The formation of different phases depended on the starting composition of the precursors . Fig. 1 shows the XRD patterns of the samples after calcination. The XRD patterns of Cu-Mg-Al-Ox contained only reflections typical of poorly crystallized MgO with reflections at 36, 43 and 63 2, belonging to the space group Fm3m [25]. These reflections decreased with decreasing magnesium concentration in the samples. The same reflections were recorded for the sample obtained by rehydration of mixed metal oxides – Cu/Mg-Al-Ox reh., indicating that transition metal oxide species were of a small crystallite size and/or of amorphous nature. However, formation of CuO at 2 of 35 and 39, could not be excluded. For the series of Cu-Mg-Al-Zr, the increase of the amount of zirconium in the samples caused, that the reflections characteristic of distorted MgO-type oxide (2 of 34, 43 and 63) disappeared and new reflections appeared at 31, 35, 52 and 61 2. The diffractions of tetragonal ZrO2 appeared at 2 of 31 (superposition of reflections characteristic of ZrO2 and MgAl2O4 and/or CuAl2O4) [7,26], while reflections characteristic of CuO appeared at 35, 52 and 61 2 [22]. Additionally, the presence of monoclinic ZrO2 (m-ZrO2, 2 of 32) [7], could not be excluded. The poorly crystallized ZnO with 2 of 32, 34, 36, 48, 57, 63 and 69 [27] appeared in both Zncontaining samples (Fig. S2). For the sample obtained by thermal decomposition of metal nitrates, besides reflections characteristic for MgO (2 of 36, 43, 63 and 79), other reflections characteristic for CuO appeared at

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35, 39, 49, 58, 66 and 68 2, respectively. Additionally, possible spinel phases – MgAl2O4 and/or CuAl2O4 – were located at 36 (superposition of reflections characteristic of MgO and MgAl2O4 and/or CuAl2O4), 45, 59 and 74 2.

Fig. 1.

3.2 Structural and textural properties of mixed metal oxides

The specific surface areas (SBET) of the obtained mixed metal oxides depended on their chemical composition. Table 2 presents the results of SBET of the obtained mixed metal oxides. The specific surface areas decreased slightly with increasing Mg/Al molar ratio in the series of Cu-Mg-Al-Ox. SBET for Zr-containing samples were lower than those for Zr-free samples. Furthermore, Mg-containing materials revealed higher values of SBET than corresponding Zn-containing samples, which was in accordance with previous studies of Jabłońska et al. [22]. The incorporation of copper by both coprecipitation and rehydration techniques reduced the specific surface area of Mg-Al-Ox. The sample obtained by thermal decomposition revealed lower SBET than for corresponding hydrotalcite derived mixed metal oxides, possibly due to formation of spinels possessing low specific surface areas.

Table 2

3.3 Redox properties of mixed metal oxides

The redox properties of the catalysts were studied by temperature-programmed reduction (H2-TPR). Fig. 2 presents the H2-TPR profiles of the obtained mixed metal oxides. The profiles found for Cu-Mg-Al-Ox with varying Mg/Al molar ratio, consisted of a sharp peak, which corresponded to the reduction of Cu2+ in highly dispersed CuOx to metallic copper [9,22]. The presence of alumina intensified interactions among different Cu11

Mg-Al-Ox mixed metal oxides, since it retarded the reduction of copper oxide species. Similarly, Ge et al. [28] reported that the presence of aluminium cations in Mg-Fe-Al-Ox retarded the reduction of iron oxide species. The incorporation of suitable amount of zirconium to Cu-Mg-Al-Ox (5/66/29, mol.%) resulted in a shift of the peak to lower temperature compared to 340C for the Zr-free sample being located at 315 and 328C for samples with compositions of 5/66/29/25 and 5/66/29/50, mol.%, respectively. Thus, the increase in Zr content of 25-50 mol.% facilitated reduction of copper oxide species in Cu-Mg-Al-Zr-Ox. While further increase of Zr content ( 75 mol.%) caused a gradual decrease of the peak corresponding to the reduction of easily reducible highly dispersed CuOx. Gao et al. [7] found that Zr excess in the mixed metal oxides caused a strong interaction of some Cu2+ and/or Cu+ species and t-ZrO2 and/or m-ZrO2, which led to lower reducibility of copper oxide species in Cu-ZnAl-Zr-Ox. Thus, the broad reduction peaks at high temperatures (700-900C) in our H2-TPR profiles of Cu-MgAl-Zr-Ox appeared due to the reduction of bulk CuOx and copper in CuAl2O4. Interestingly, Cu-Zn-Al-Ox and Cu-Zn-Al-Zr-Ox (5/66/29/(25), mol.%) possessed similar profiles with sharp peaks centred at 336 and 333C corresponding to the reduction of highly dispersed CuOx (Fig. S3). The broad reduction peak located at about 687 and 665C (for Zr-free and -containing samples, respectively) corresponded to the reduction of ZnO to Zn0 [29]. Finally, the sublimation of metallic zinc represented by the roughness of reduction plots appeared above 800C, in line with results presented earlier by Jabłońska et al. [22] over Cu-ZnAl-Ox (0.6/1.4/1.0, mol.%). Again, the incorporation of suitable amount of zirconium (25 mol.%) within the CuZn-Al-Ox structure enhanced the reducibility of copper oxide species, indicating that the shift in reduction peaks could be ascribed to the dispersion of copper oxide particles. Consequently, the reducibility of copper oxide species depended on the Al and/or Zr content in the samples. Comparison of the results obtained for copper-containing mixed oxides obtained by three different techniques revealed similar H2-TPR profiles of the samples prepared by rehydration (Cu/Mg-Al-Ox) and thermal decomposition (Cu-Mg-Al-Ox decom.). Two types of copper oxide species appeared in the profiles of these samples represented by the shoulders at low temperatures and a sharp peaks at higher temperatures. XRD analyses revealed CuO and CuAl2O4 x as crystalline phases for Cu-Mg-Al-Ox decom., and CuO as minor species for Cu/Mg-Al-Ox. However, combining XRD analysis with the result of XPS (see below), different copper oxide 12

species (CuO and CuAl2O4) were identified on the surface of the sample obtained by rehydration. Thus, the low temperature shoulders in the H2-TPR profiles of Cu/Mg-Al-Ox and Cu-Mg-Al-Ox decom. represented the reduction of highly dispersed CuOx, while the peak at higher temperatures could be attributed to the reduction of bulk CuOx and copper in CuAl2O4 [30,31]. The amounts of hydrogen consumed during the measurements (H2 uptakes) reached 1.41 and 1.05 mmol g-1 for the samples obtained by rehydration and thermal decomposition, respectively. While the real copper content was very similar to the its intended loading. The sample obtained by coprecipitation (Cu-Mg-Al-Ox cop.) revealed H2 uptake of 0.88 mmol g-1 and a similar profile as the series of Cu-Mg-Al-Ox. A higher amount of copper in the sample – 8/63/29 versus 5/66/29, mol.%, shifted the peak of the reduction of CuOx species to lower temperatures, which is in line with the results reported by Chmielarz et al. [32] over Cu-Mg-Al-Ox with different copper loading (5, 10, 20, mol.%). xx Consequently, the highly dispersed copper oxide species of Cu-Mg-Al-Ox (5/66/29, mol.%) revealed significantly lower reducibility that of the sample with higher copper loading (8/63/29, mol.%).

Fig. 2.

3.4 Status of copper oxide species in mixed metal oxides

Furthermore, we focused our attention on identifying the dispersion, distribution and state of copper oxide species in materials obtained by different techniques. . Fig. 3 shows a comparison of the spectra after CO adsorption of mixed metal oxides. The samples obtained by coprecipitation (Cu-Mg-Al-Ox cop.) and rehydration (Cu/Mg-AlOx reh.) revealed similar profiles with the main CO band at around 2108 cm-1. Additionally, a small shoulder appeared at higher frequencies at approximately 2152 cm-1. A similar scarcely discernible shoulder was recorded in the spectrum of a material obtained by thermal decomposition (Cu-Mg-Al-Ox decom.), while the main band appears at 2099 cm-1. The two bands at 2099 or 2109 cm-1 correspond to Cu0-CO species, while the shoulder at 2152 cm-1 appeared due to the presence of Cu+-CO [33]. Thermal treatment of the samples in vacuum was associated to the autoreduction process when the majority of Cu2+ species was reduced to Cu+ ions. Non-reduced 13

Cu2+ cations could exist in the form of an oxide. However in our case no bands associated with CO interacting with Cu2+ were detected in the samples pointing to the pretty negligible concentration of Cu2+ sites. H2-TPR measurements showed that the higher dispersion of CuOx led to a lower reduction temperature. Therefore, the amount of easily reducible highly dispersed copper oxide species on the surface of the material obtained by coprecipitation was significantly higher than that for materials obtained by rehydration and thermal decomposition, as summarized in Table 3.

Table 3

Fig. 4 presents the Cu 2p peaks of the XPS spectra of samples obtained by coprecipitation (Cu-Mg-Al-Ox cop.) and rehydration (Cu/Mg-Al-Ox reh.), while Table 4 summarizes the values of the Cu 2p BEs spectra as well as ratios of Cu/(Al+Mg) and Isat/Imp (the intensity of Cu2+ satellite peak to the intensity of the main photoelectron line). For Cu-Mg-Al-Ox cop., XPS peak appeared at around 934.2 eV (BE) with the shake-up peaks, which indicated that the copper oxide species were present as Cu2+. For sample obtained by rehydration, the peaks representing Cu2+ appeared at around 933.2 2+ and 935.7 eV 2+x24. The Cu/(Al+Mg) molar ratio for Cu/Mg-AlOx was higher than for Cu-Mg-Al-Ox cop. (1.35 versus 1.09), indicating a higher concentration of copper oxide species on its surface. Furthermore, the higher value of Isat/Imp for Cu/Mg-Al-Ox compared to Cu-Mg-Al-Ox cop. (0.61 and 0.49, respectively), confirmed the coexistence of CuO and CuAl2O4 in the sample obtained by rehydration [34,35]. The identification of different copper oxide species by XPS was in line with the obtained H2-TPR profile of Cu/Mg-Al-Ox, where Cu2+ species with different reducibility were identified. Fig. 5 shows the TEM images of the samples obtained by coprecipitation and rehydration. The copper oxide particles appeared as dark spheres. The dispersion of copper oxide species of the samples varied depending on the preparation method. Rather aggregated CuOx existed in the sample obtained from rehydration, while coprecipitation produced homogenously distributed copper oxide species.

Fig. 4. and Fig. 5 14

Table 4

3.5 Acidic properties of mixed metal oxides

The surface acidity of the selected samples was measured by temperature programmed desorption of ammonia (NH3-TPD). In the series of Cu-Mg-Al-Ox with different Mg/Al molar ratio, acidity changed only in the narrow range – 251 mol g-1 for sample with composition of 5/66/29, mol.% to 291 mol g-1 of 5/51/44, mol.%. Fig. 6 presents the ammonia desorption curves for samples obtained by three different techniques. The ammonia desorption profiles were spread in the temperature range from 70 to 350C with a maximum centred at 150160C. As summarized in Table 3, the highest concentration of total acidity – 263 mol g-1 revealed the sample obtained by coprecipitation. Further measurements of pyridine sorption possessed only bands indicative of pyridine adsorbed on Lewis acid sites in all FTIR spectra recorded after heating to 250C; however, the intensities of Lewis acid sites were different (Fig. 7). As shown in Table 3, the sample obtained by thermal decomposition (Cu-Mg-Al-Ox decom.) – containing spinel phases – revealed the highest amount of Lewis acidic sites, followed by the sample obtained by coprecipitation (Cu-Mg-Al-Ox cop.) and finally the sample obtained by rehydration (Cu/Mg-Al-Ox reh.). These results were in agreement with the previous report by Yuan et al. [36], where MgAl2O4 showed more unsaturated sites than MgO and -Al2O3, and thus a higher concentration of Lewis acid sites. Moreover, the higher concentration of Lewis acid sites over the sample obtained by coprecipitation than rehydration resulted from the higher amount of surface electron-acceptor centres originating from copper oxide species. The differences in the acidity values resulting from NH3-TPD and FTIR of pyridine sorption appeared possibly due to not exactly identical experimental conditions. However, pyridine as a stronger base than ammonia probed a higher amount of acid sites with various strengths. Fig. 6. and Fig. 7.

3.6 Catalytic tests over mixed metal oxides

15

All obtained materials were tested as catalysts in the selective oxidation of ammonia into nitrogen and water vapour. N2 was the desired product of the reaction, while NO and N2O were the only detected undesired byproducts. The catalytic test in the absence of catalyst showed that the ammonia conversion started at temperatures as high as 475C, and reached below 20% at 600C. N2 was the main product of gas phase reaction (Fig. S4). Fig. 8 and Table 5 present the results of catalytic tests over mixed metal oxides. The Cu-Mg-Al-Ox hydrotalcite derived mixed metal oxides facilitated the enhanced activity and N2 selectivity in NH3-SCO among all studied materials. The catalysts with varying Mg/Al molar ratio revealed nearly the same catalytic results (Fig. 8A). The ammonia oxidation over the series of Cu-Mg-Al-Ox started at about 150C and reached full conversion at 500C. N2 was the main reaction product in the studied temperature range. A slightly higher selectivity to N2 could be achieved at temperatures above 550C for samples with higher surface acidity, i.e. higher content of aluminium. Thus, mainly the presence of copper oxide species was an essential factor determining activity and selectivity in the ammonia oxidation. The enhanced activities of Zr-containing catalysts were attributed to the easily reducible copper oxide species in these catalysts, as evidenced by H2-TPR studies. However, both Zr-containing samples revealed similar profiles of catalytic results (Fig. 8B). Only a slightly higher selectivity to N2 occurred over CuMg-Al-Zr-Ox. Thus, possibly the higher overall reducibility of copper oxide species in Cu-Zn-Al-Zr-Ox decreased the selectivity to N2. Only, a suitable amount of zirconium was favourable for the enhanced activities of Zr-containing catalysts derived from Cu-Mg(Zn)-Al-Zr precursors. The presence of easily reducible copper oxide species was an essential factor determining high activity in the low temperature range, but also caused a decrease of N2 selectivity at higher temperatures. Moreover, it is likely that the activity and selectivity in NH3SCO is mainly determined by materials’ redox properties (e.g. [9,37]). Consequently, the differences in the H2TPR profiles reflected the results of catalytic tests over Cu-Mg-Al-Ox obtained by different techniques (Fig. 8C,D). The copper oxide species in the sample obtained by rehydration (Cu/Mg-Al-Ox reh.) were more catalytically active in NH3-SCO than in sample obtained by thermal decomposition (Cu-Mg-Al-Ox decom.), due to a higher amount of easily reducible CuOx as verified by H2-TPR studies. These results were supported by FTIR data of the CO-preadsorbed samples, which showed that aggregated copper oxide species were characterized by higher electron-donor properties (the copper carbonyl bands of lower frequencies were observed for highly 16

clustered species). Thus, NH3 adsorption and its activation on such centres was less efficient than for finely dispersed CuOx species. For both samples, an increase in the reaction temperature up to 500C resulted in the gradual decrease of N2 selectivity. Above 500C, the contribution to N2 increased, possibly due to an increasing role of aggregated copper oxide species with lower reducibility (Fig. 8C). Therefore, the oxidation of ammonia to NO over such species was much slower; thus, the selectivity to N2 increased. The copper-containing hydrotalcite derived catalysts were much more active and selective to N2 in the studied temperature range, than sample obtained by rehydration of calcined hydrotalcite-like compounds. Thus, the easily reducible highly dispersed copper oxide species was a key factor for high-performance in the ammonia oxidation. Furthermore, the increase in copper loading in Cu-Mg-Al-Ox from 5 to 8 mol.% shifted the temperature for total ammonia conversion to about 100C lower temperature; however, the selectivity to N2 dropped, especially at temperatures above 450C. Furthermore, decreasing N2 selectivity was observed with increasing copper loading from 10 up to 20 mol.% in Cu-Mg-Al-Ox [32]. The introduction of water vapour into the reaction mixture during NH3-SCO only slightly decreased ammonia conversion and N2 selectivity over Cu-Mg-Al-Ox (8/63/29, mol.%) obtained by coprecipitation (Fig. 8E). Furthermore, no significant depletion in conversion was observed in the long-term stability test in the presence of water vapour (Fig. 8F). After 400 min the conversion reached a stable level of about 89%, while N2 selectivity increased to 95%. The increase in N2 selectivity in the presence of H2O and over copper-based catalysts occurs due to aggregation of easily reducible CuOx species, as reported earlier by Lenihan and Curtin [38] over (3.0 wt.%)Cu/Beta. Ammonia oxidation during NH3-SCO over Cu-Mg-Al-Ox (8/63/29, mol.%) obtained by coprecipitation showed that N2 and N2O were the only products detected in the low temperature range, while NO appeared at above 400C. Furthermore, the studied catalyst was active and selective to N2 in the low-temperature NH3-SCR (Fig. 8D). NO conversion started at about 150C and reached a maximum of about 87% in the range of 250-300C, while the selectivity to N2 remained close to 100%. Above 300C, ammonia oxidation decreased the efficiency of NO reduction. The activities and N2 selectivities for samples obtained by rehydration (Cu/Mg-Al-Ox reh.) and thermal decomposition (Cu-Mg-Al-Ox decom.) were consequently lower, in line with the results for NH3-SCO. 17

Consequently, presented performances of the materials in NH3-SCR indicated that the ammonia oxidation could proceed an i-SCR mechanism.

Fig. 8. Table 5.

3.6 FTIR spectroscopy studies of NH3 transformation

In situ DRIFT studies were carried out in order to determine the possible species involved in the reaction mechanisms. Fig. 9A presents DRIFT spectra of adsorbed species at different temperatures due to contact of the sample obtained by coprecipitation (Cu-Mg-Al-Ox cop.) with NH3 and sucessive puring with N2. Several bands appeared at 1675, 1486, 1390, 1231, 1166 cm-1 after sorption of NH3 at 50C. The bands at 1486 and 1166 cm-1 corresponded to the deformation modes of NH4+ formed by the interaction of NH3 with Brönsted sites. The bands at 1675 and 1231 cm-1 appeared due to asymmetric and symmetric bending vibrations of the N-H bonds in NH3 coordinatively linked to Lewis acid sites [39–41]. In the NH stretching region, bands were found at 3375, 3335, 3267 and 3225 cm-1 [42,43]. The two negative bands at about 3750 and 3710 cm-1 appeared due to the hydroxyl consumption through interaction with NH3 to form NH4+ [13]. The band at 1390 cm-1 corresponded to NH4+ adspecies on -Al2O3 [44,45], which could exist in the amorphous phase of Cu-Mg-Al hydrotalcite derived mixed metal oxides. The intensity of the bands of NH3 coordinated on Brönsted acid sites decreased gradually with increasing temperature. As proven by pyridine sorption, around 250C, only NH3 coordinated on Lewis acid sites existed, which also decreased progressively upon heating. The drop in N2 selectivity at high-temperatures could be caused by the absence of ammonia chemisorbed on Brönsted acid sites, and consequently, the oxidation of ammonia to NO. At about 250C three new bands increased at 1640, 1584 and 1318 cm-1. The bands at 1640 and 1318 cm-1 corresponded to the amide (-NH2) scissorings and (-NH2) waggings, while the band at 1584 cm-1 represented -NH deformation modes [43,45,46]. These results suggested the subsequent dehydrogenation of ammonia upon heating: NH3 → NH2 → NH in absence of oxygen. The band at 1640 cm-1 could be also attributed 18

to bidentate nitrate. Furthermore, a new weak band was visible at 1224 cm-1 at 350C, which was assigned to nitrite species [36]. No chemisrbed ammonia existed at this temperature as evidenced by NH3-TPD measurments. Thus, adsorbed NHx could be oxidized to NO by lattice oxygen of the catalyst and adsorbed as NO2- and/or NO3on the catalyst surface. Fig. 9B shows the in situ DRIFT spectra of O2 passing over an NH3-pretreated Cu-MgAl-Ox. Similar bands at 1675, 1486, 1390, 1231, 1161 cm-1 occured after sorption of NH3 at 50C. The band intenisty of -NH2 or bidentate nitrate (1646 cm-1) and NO2- (1222 cm-1) increased around 250 and 350C, respctively. Also the intensity of the peaks arising due to the presence of nitrites and/or nitrates, was favored in the presence of oxygen. Chmielarz et al. [32] reported the decomposition of thermally stable nitrites/nitrates around 350C over Cu-Mg-Al-Ox (10/61/29, mol.%) by applying a temperature programmed surface reaction (TPSR). Additionally, for the sample obtained by thermal decomposition, i.e. higher amount of Lewis acid sites, the amount of nitric species gradually increased at 1651 and 1221 cm-1 (with successive purging with N2), and 1596, 1538, 1455, 1325 and 1217 cm-1 (with successive purging with O2) (results not shown). Consequently, NHx could react with in situ formed NOx- (NO3- and/or NO2-) species, and thus possibly follow an i-SCR mechanism. The detailed recognition of an i-SCR pathway needs additional studies, e.g. in situ DRIFT studies of interaction of NH3 with primarily formed NO [12,13], as well as in situ DRIFT coupled with transient techniques (TAP, SSITKA). However, comprehensive experiments concerning detailed reaction mechanism will be investigated separately.

4. Conclusions

The series of Cu-Mg(Zn)-Al-(Zr) mixed metal oxides with different molar ratios was successfully obtained by coprecipitation, followed by their thermal decomposition. Other copper-based oxides were prepared by rehydration of Mg-Al hydrotalcite derived mixed metal oxides and their subsequent calcination, or thermal decomposition of metal nitrates. Coprecipitation favoured formation of mixed metal oxides, while mixed metal oxides together with spinels resulted from rehydration and thermal decomposition techniques. 5-8 mol.% of copper was optimal for Cu-Mg-Al-Ox dedicated for NH3-SCO and NH3-SCR. Based on H2-TPR, FTIR with 19

adsorption of CO, XPS and TEM measurements, the amount of finely dispersed copper oxide species in Cu-MgAl-Ox obtained by coprecipitation was significantly higher than that for materials obtained by applying other methods. Such CuOx species contributed to the high activity, N2 selectivity and a relatively high stability under wet reaction conditions in NH3-SCO. FTIR studies of adsorbed pyridine revealed that only the NH3 coordinated on Lewis acid sites contributed to the catalytic activity and selectivity over Cu-Mg-Al-Ox. NH3 was first oxidized to NOx, which was reduced by NHx intermediates or unreacted NH3 into N2 and/or N2O. Easily reducible highly dispersed copper oxide species were favourable for ammonia adsorption and its selective oxidation; thus, further studies are underway on hydrotalcite-like compounds obtained using a microemulsion method in order to control the size of CuO clusters.

Acknowledgement Funded by the Excellence Initiative of the German federal and state governments in the frame of the Center for Automotive Catalytic Systems Aachen (ACA) at RWTH Aachen University. We thank B. Hermanns (RWTH Aachen University, IAC) for XRF measurements.

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24

500

Cu-Mg-Al-Ox cop.

A P

P

5/51/44

Intensity [a.u]

400 5/56/39 300

200

5/61/34

100 5/66/29 0 10

20

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50

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Cu-Mg-Al-Zr-Ox cop.

Z+S T

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Intensity [a.u]

40

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Cu-Mg-Al-Ox 8/63/29 decom.

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Intensity [a.u]

60

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T

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300

ST

TS

T

S

P

Cu-Mg-Al-Ox 8/63/29 cop. P

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100

(10 wt.%)Cu/Mg-Al-Ox 71/29 reh. P

T PT

0 10

20

30

40

50

60

70

80

0

2

Fig. 1. X-ray diffraction patterns of Cu-Mg-Al-(Zr) and Cu/Mg-Al mixed metal oxides; P – periclase (MgO), T – tenorite (CuO), and S – magnesium aluminate (MgAl2O4) and/or copper aluminate (CuAl2O4), Z – tetragonal ZrO2 (t-ZrO2).

25

350

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Cu-Mg-Al-Ox cop.

366

300

5/51/44

H2 consumption [a.u.]

363

250 5/56/39

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H2 consumption [a.u.]

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Cu-Mg-Al-Ox 8/63/29 decom.

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(10 wt.%)Cu/Mg-Al-Ox 71/29 reh.

0 100

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300

400

500

600

700

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900

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Fig. 2. Temperature programmed reduction profiles of Cu-Mg-Al-(Zr) and Cu/Mg-Al mixed metal oxides.

26

0.15 2099

Absorbance [a.u.]

Cu-Mg-Al-Ox 8/63/29 decom. 2108

0.10

0.05

Cu-Mg-Al-Ox 8/63/29 cop.

2152

0.00 2300

2200

(10 wt.%)Cu/Mg-Al-Ox 71/29 reh. 2100

2000

1900

1800

-1

Wavenumber [cm ]

Fig. 3. FTIR spectra of CO adsorption at room temperature of Cu-Mg-Al and Cu/Mg-Al mixed metal oxides.

27

Cu-Mg-Al-Ox 8/63/29 cop.

A

(10 wt.%)Cu/Mg-Al-Ox 71/29 reh.

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Cu 2p

980

970

960

950

940

Binding energy [eV]

Cu 2p

930

920

980

970

960

950

940

930

920

Binding energy [eV]

Fig. 4. X-ray photoelectron spectra of Cu-Mg-Al and Cu/Mg-Al mixed metal oxides.

28

A (10 wt.%)Cu/Mg-Al-Ox 71/29 reh.

B Cu-Mg-Al-Ox 8/63/29 cop.

Fig. 5. TEM images of Cu-Mg-Al and Cu/Mg-Al mixed metal oxides.

29

152

Cu-Mg-Al-Ox 8/63/29 decom.

Intensity [a.u.]

0.0010

157

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Cu-Mg-Al-Ox 8/63/29 cop. 148

(10 wt.%)Cu/Mg-Al-Ox 71/29 reh.

0.0000 100

200

300

400

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600

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Temperature [ C]

Fig. 6. Temperature programmed desorption profiles of Cu-Mg-Al and Cu/Mg-Al mixed metal oxides.

30

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1441

Cu-Mg-Al-Ox 8/63/29 decom.

Absorbance [a.u.]

1600 1575

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Cu-Mg-Al-Ox 8/63/29 cop. 1443

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0.00 1700

1443

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1600

1550

1500

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Fig. 7. FTIR spectra of pyridine desorption on Cu-Mg-Al and Cu/Mg-Al mixed metal oxides.

31

100

100

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Cu-Mg-Al-Zr-Ox cop.

5/66/29 5/61/34 5/56/39 5/51/44

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Cu-Mg-Al-Ox cop.

N2 selectivity

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Cu-Mg-Al-Ox 8/63/29 decom. Cu-Mg-Al-Ox 8/63/29 cop. (10 wt.%)Cu/Mg-Al-Ox 71/29 reh.

NO conversion/Selectivity [%]

Conversion/Selectivity [%]

100

75

0 100

N2 selectivity 200

300

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D see Fig. 8C

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0

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F

N2 selectivity

Cu-Mg-Al-Ox 8/63/29 cop.

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Conversion/Selectivity [%]

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NO selectivity N2O selectivity

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Temperature [ C]

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Temperature [ C] 100

400

NH3 conversion Cu-Mg-Al-Ox 8/63/29 cop.

80

0

T = 375 C

15

N2O selectivity

NO selectivity

0 200

300

400 0

Temperature [ C]

500

600

0

100

200

300

400

Time-on-stream [min]

32

Fig. 8. Results of catalytic tests for NH3-SCO (A-C, E,F) or NH3-SCR (D) performed over Cu-Mg(Zn)-Al-(Zr) and Cu/Mg-Al mixed metal oxides; representations of symbols from Fig. 8C, D are equal.

33

A

Cu-Mg-Al-Ox 8/63/29 cop.

Kubelka-Munk function [a.u.]

3267 3375 3335 3225

1675 1640

1224

0

50 C

1231 1584 1390 1486 1318

1166

0

100 C 0

150 C 0

200 C 0

250 C 0

300 C 0 350 C

4000

3500

B

2000

1500

Kubelka-Munk function [a.u.]

3375 3267 3335 3225

1675 1646

1222

0

50 C

1231 1486

1390

1161

0

100 C 0

150 C 0

200 C 0

250 C 0

300 C 0 350 C

4000

3500

2000

1500

-1

Wavenumber [cm ]

Fig. 9. FTIR spectra of the adsorbed species arising from contact with NH3 with Cu-Mg-Al mixed metal oxides at 50C and successive purging with N2 (A), or 5.0 vol.% of O2 diluted in Ar (B) at various temperatures.

34

Table 1 Structural parameters of (Cu)-Mg(Zn)-Al-(Zr) precursors. Sample codes Molar ratio

Cell Crystallit Cell Crystallit paramete e size paramete e size r r a [nm]

Da [nm]

c [nm]

Dc [nm]

Mg-Al 71/29 cop.

Mg/Al = 71/29

0.3053

25

2.3376

18

Cu-Mg-Al 5/51/44 cop.

Cu/Mg/Al = 5/51/44

0.3028

16

2.2875

8

Cu-Mg-Al 5/56/39 cop.

Cu/Mg/Al = 5/56/39

0.3028

18

2.2875

8

Cu-Mg-Al 5/61/34 cop.

Cu/Mg/Al = 5/61/34

0.3040

17

2.2806

14

Cu-Mg-Al 5/66/29 cop.

Cu/Mg/Al = 5/66/29

0.3053

16

2.2944

20

Cu-Mg-Al 8/63/29 cop.

Cu/Mg/Al = 8/63/29

0.3058

31

2.2323

17

Cu-Mg-Al-Zr 5/66/29/25 cop. Cu/Mg/Al/Zr = 5/66/29/25 Cu-Mg-Al-Zr 5/66/29/50 cop. Cu/Mg/Al/Zr = 5/66/29/50 Cu-Mg-Al-Zr 5/66/29/75 cop. Cu/Mg/Al/Zr = 5/66/29/75 Cu-Mg-Al-Zr 5/66/29/100 Cu/Mg/Al/Zr = cop. 5/66/29/100 Cu-Zn-Al 5/66/29/ cop. Cu/Zn/Al = 5/66/29

0.3058

15

2.3715

4

-

-

-

-

-

-

-

-

-

-

-

-

0.3084

38

2.2806

25

Cu-Zn-Al-Zr 5/66/29/25 cop.

0.3072

31

2.2635

14

Cu/Zn/Al/Zr = 5/66/29/25

35

Table 2 Specific surface areas (SBET) of (Cu)-Mg(Zn)-Al-(Zr) and Cu/Mg-Al mixed metal oxides. Sample codes SBET [m2 g-1] Mg-Al-Ox 71/29 cop. Cu-Mg-Al-Ox 5/51/44 cop.

140 139

Cu-Mg-Al-Ox 5/56/39 cop. Cu-Mg-Al-Ox 5/61/34 cop.

137 134

Cu-Mg-Al-Ox 5/66/29 cop. Cu-Mg-Al-Ox 8/63/29 cop. Cu-Mg-Al-Zr-Ox 5/66/29/25 cop.

132 131 85

Cu-Mg-Al-Zr-Ox 5/66/29/50 cop.

41

Cu-Mg-Al-Zr-Ox 5/66/29/75 cop.

32

Cu-Mg-Al-Zr-Ox 5/66/29/100 cop. Cu-Zn-Al-Ox 5/66/29/ cop.

15 94

Cu-Zn-Al-Zr-Ox 5/66/29/25 cop. (10 wt.%)Cu/Mg-Al-Ox 71/29 reh. Cu-Mg-Al-Ox 8/63/29 decom.

67 108 85

36

Table 3 Copper contents, concentrations of chemisorbed CO (at room temperature), NH3 and pyridine (at 250C), as well as amounts of H2 consumed during TPR measurements (H2 uptake) of Cu-Mg-Al and Cu/Mg-Al mixed metal oxides. Sample codes Cu content Concentrati Concentrati Concentrati H2 uptake [mol.%]a on of on of on of [mmol g-1]b chemisorbe chemisorbe Lewis acid d CO d NH3 sites -1 -1 [mol g ] [mol g ] [mol g-1]

Cu-Mg-Al-Ox 8/63/29 cop.

9.9 5

210

263

350

0.88

(10 wt.%)Cu/Mg-Al-Ox 71/29 reh.

7.2 9

134

171

245

1.05

Cu-Mg-Al-Ox 8/63/29 decom.

8.3 8

78

188

595

1.41

a

Determined with XRF analysis. Calculated by equation: Y = 9E-09X + 2E-07, R2 = 0.9996, and X, Y referred to the area of each reduction peak and the H2 consumption, respectively. b

37

Table 4 XPS results of Cu-Mg-Al and Cu/Mg-Al mixed metal oxides. Sample codes Peak Positiona FWHMb

Peak area [a.u.]

Cu/(Al+ Mg)c

Isat/Impc

1.09

0.49

1.35

0.61

[eV] Cu-Mg-Al-Ox 8/63/29 cop.

(10 wt.%)Cu/Mg-Al-Ox 71/29 reh.

Cu 2p

Cu 2p

961.7

6.0

1218.5

943.7

5.9

1669.9

934.2

3.7

5952.2

960.8

6.0

1779.5

942.5

5.8

1554.9

935.7

2.6

420.3

933.2

3.3

5459.2

a

Position of binding energy, bFull width at half-maximum, cEstimated from the integrated areas of the respective XPS peaks.

38

Table 5 Comparison of the results of catalytic tests (T100 temperature needed for 100% of NH3 conversion) over CuMg(Zn)-Al-(Zr) and Cu/Mg-Al mixed metal oxides. Sample codes T100 N2 selectivity NO selectivity N2O selectivity at T100 [%] [C] Cu-Mg-Al-Ox 5/51/44 cop. 500 87 7 6 Cu-Mg-Al-Ox 5/56/39 cop. 500 86 9 5 Cu-Mg-Al-Ox 5/61/34 cop. 500 86 10 4 Cu-Mg-Al-Ox 5/66/29 cop. 500 86 11 3 Cu-Mg-Al-Ox 8/63/29 cop. 400 91 2 7 with 3.2% H2O 425 88 4 8 Cu-Mg-Al-Zr-Ox 5/66/29/25 cop. 450 87 8 5 Cu-Mg-Al-Zr-Ox 5/66/29/50 cop. 450 79 16 5 Cu-Mg-Al-Zr-Ox 5/66/29/75 cop. 500 72 23 5 Cu-Mg-Al-Zr-Ox 5/66/29/100 cop. 500 50 44 6 Cu-Zn-Al-Zr-Ox 5/66/29/25 cop. 450 83 13 4 (10 wt.%)Cu/Mg-Al-Ox 71/29 reh. 400 86 8 6 Cu-Mg-Al-Ox 8/63/29 decom. 450 47 47 6

39