Copper loaded η-alumina catalysts with different copper contents have been prepared by impregnation/evaporation method. The catalysts were characterized by XRD, FTIR, BET, UV–vis, H2-TPR and evaluated in the selective catalytic reduction of NO by NH3 and in the selective catalytic oxidation of NH3. The characterization techniques showed that the impregnation/evaporation method permits to obtain highly dispersed copper oxide species on the η-alumina surface when low amount of copper is used (1wt. % and 2 wt.%). The wet impregnation method made it possible to reach a well dispersion of the copper species on the surface of the alumina for the low copper contents Cu(1)-Al2O3 and Cu(2)-Al2O3. The latter justifies the similar behavior of Cu(1)-Al2O3) and Cu(2)-Al2O3 in the selective catalytic oxidation of NH3 where these catalysts exhibit a conversion of NH3 to N2 of the order of 100% at T > 500°C.
Research Article
Catalytic Activity Of Cu/η-Al2O3 Catalysts Prepared From Aluminum Scraps In The NH3-SCO and in the NH3-SCR of NO
https://doi.org/10.21203/rs.3.rs-550091/v1
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The selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen is considered a mature technology for the removal of NO from stationary sources (Forzatti, 2001; Usberti et al. 2015). Moreover, this technology has been adapted for the modern diesel exhaust after treatment system using urea as the ammonia precursor (urea-SCR) (Goldbach et al., 2017; Jung et al., 2017; Kröcher, 2018; Nova and Tronkoni, 2014; Piumetti et al., 2015; Yuan et al., 2015). This system contains besides the urea delivery device, a catalyst for the selective catalytic reduction of NO with NH3 (NH3-SCR) combined with an ammonia slip catalyst (ASC) for the selective catalytic oxidation of ammonia (NH3-SCO) (Piumetti et al., 2015; Walker, 2016).
Vanadium-based catalysts (V-catalysts) V2O5-WO3/TiO2 or V2O5 − MoO3/TiO2 are widely employed in stationary applications since the 1970s (Lai and Wachs, 2018). However, the major drawbacks bounded to the toxicity of vanadium and its weaker activity at low temperature limit the utilization of V-catalysts in automotive applications. Besides, some country regulations such as USA forbid the use of V-catalysts for automotive applications. Therefore, Cu/Fe-exchanged zeolites have been reported as alternative to vanadium-based catalysts because they are active and N2 selective for the NH3-SCR (Boron et al., 2019; Villamaina et al., 2019; Xin et al., 2018) and for the NH3-SCO (Jablonska, 2020). Among all the zeolite-catalysts, Cu/Fe-ZSM-5 and Fe/Cu-BEA are the most extensively investigated in the past 30 years (Hamoud et al., 2019; Villamaina et al., 2019; Xin et al., 2018). Copper-based zeolites are usually more active in the low-temperature (< 350°C) range while iron-based zeolites are more active at higher temperatures (> 350°C). Small-pore zeolite Cu-SSZ-13 has received great attention due to its higher activity and selectivity at low temperatures and improved hydrothermal stability for diesel vehicles. (Gao and Szanyi, 2018; Lambert, 2019; Shibata et al., 2019)
On the other hand, noble metals (Pt, Pd, and Rh) supported on metal oxides have been studied for the diesel exhaust after treatment system. Common metal oxides such as Al2O3, SiO2, CeO2, TiO2, and ZrO2 are used as support materials for the diesel oxidation catalyst (Jablonska, 2015; Sun et al., 2019). Nevertheless, γ-Al2O3 due to its high surface area (100–200 m2/g) and its good thermal stability is preferred to all other metal oxides (Kong et al., 2020; Panahi and Delahaye, 2017). For instance, Pt-γ-Al2O3 is used in the diesel oxidation catalyst (DOC) and ammonia slip catalyst (ASC) (svintsitskiy et al., 2020). The DOC oxidizes CO, unburnt hydrocarbons and NO in the exhaust gas to CO2, H2O and NO2, respectively. The ASC removed the excess of NH3 by selective catalytic oxidation (SCO) with oxygen to N2 and H2O. Pt-γ-Al2O3 catalyst is considered to be the most active for ammonia oxidation below 300°C than the other noble metals (Pd, and Rh).However,Pt-γ-Al2O3 catalyst has high selectivity towards N2O and NO over 300°C (Hansen et al.,2017). The drawbacks of noble-metal catalysts motivate the vehicle manufacturers to reduce their content or substitute them with cheaper Mn/Cu-based oxides catalysts (Damma et al., 2019). Indeed, CuO/γ-Al2O3 catalyst has been proposed to substitute the noble metal-based emission control catalysts in the NH3-SCR (Jeong et al., 1999; Kwak et al., 2012; Xie et al., 2004) and NH3-SCO (Ghosh et al., 2020; Jablonska et al., 2018; Jablonska, 2015; Strom et al., 2018). Major challenges are associated with the design of a suitable downstream catalyst: (i) the catalyst should exhibit high activity at relatively low temperatures (< 400 ◦C) in order to avoid the need for additional heating of exhaust gases, (ii) the material has to possess sufficient stability in the presence of high concentrations of water vapour or other components of waste gases (COx, SOx) and (iii) should selectively convert NH3 into N2 (Jablonska et al., 2016). In NH3-SCO, almost 100% conversion of ammonia is required in order to eliminate ammonia odour. Altogether, a design of oxidation catalysts of high efficiency, selectivity to N2 and stability remains challenging. A promising class of catalysts are Cu based systems which will be discussed more comprehensively in the following (Jablonska and Regina, 2016). In the order of Il’chenko and Ivanovna (1976), copper oxide was found as one of the most efficient catalysts in selective ammonia oxidation into nitrogen and water vapour. Further studies over preoxidised polycrystalline copper foil proved copper oxide as active phase for NH3-SCO. However, due to unsatisfying selectivity to N2, further studies concerning ammonia oxidation over copper oxide supported e.g. on -Al2O3 (Strom et al., 2018) were carried out.
In our previous work (Jraba et al., 2018), we reported the preparation of γ-Al2O3 and η-Al2O3 with high surface areas using aluminum chips collected from metal manufacturing industry as starting materials. η-Al2O3 and γ-Al2O3 were obtained by calcination at 500°C of bayerite (α-Al(OH)3) and pseudo-boehmite, respectively. γ-Al2O3 and η-Al2O3 are considered to be the most important among other alumina’s due to their high specific surface area (200–500 m2/g) and acid-base properties. Particularly, γ-Al2O3 counts for the most important industrial applications as adsorbents, catalysts and catalyst supports. To our best knowledge, η-Al2O3 has never been employed as support for the preparation of CuO/ Al2O3 catalysts for the NH3-SCR of NO and for the NH3-SCO. Thus, this work is devoted to the preparation, characterization and catalytic activity of copper loaded η-alumina catalysts in the reactions, in presence of water vapour, of NH3 –SCR of NO and of NH3-SCO. The prepared catalysts were characterized by XRD, SEM, TEM, NH3-TPD, H2-TPR, UV-vis and N2 adsorption-desorption techniques.
2.1 Preparation of the catalysts
Five copper loaded η-alumina catalysts Cu(x)-Al2O3, with x theoretical copper loadings were prepared by wet impregnation/evaporation technique. In a flask containing 100 mL of distilled water, the desired amount of copper acetate Cu(CO2CH3)2.H2O was added to obtain the copper contents of 1 wt.%, 2 wt.%, 3wt.%, 5 wt.% and 7.5 wt%. After the total dissolution of the copper acetate, a mass of 1.5 g of alumina η-Al2O3 was added. The flask is then mounted on a rotary evaporator and the suspension is stirred for 4 hours at 80 ° C. After this step, the water was evaporated under reduced pressure for about 1 hour. Once dry, the solid was placed in an oven at 80°C overnight and finally calcined at 500°C under an air stream (2°C/min) for 10 hours. In Fig. 1 are reported the photographs of the prepared catalysts.
2.2 Characterization of Cu(x)-Al2O3 catalysts
X-ray powder diffraction patterns were obtained using a D8 ADVANCE BRUKER 40 Kv 40 mA Detector Lynx eye Geometrie Bragg Brenterno (ICGM MAES) using Cu Kα (λ = 0.15418 nm) incident radiation. The diffractograms were recorded at room temperature (RT) between 4° and 70 ° counted in 2θ at a scan speed of 0.02°/s.
The textural properties, surface area and porosity of the support and the catalysts were determined from nitrogen adsorption–desorption isotherms measured at -196°C using the “micrometrics Tristar Surface Area and Porosity analyzer”. The sample (approximately 100 mg) was weighed exactly in a glass tube lined with an "insert" to reduce the void volume. Before all measurements, the samples were treated under high vacuum overnight at 150°C.
H2-TPR profiles were carried out with an automated Micromeritics Autochem 2910 analyzer. Before H2-TPR measurements, samples (50 mg) were pretreated in a quartz U-tube reactor under 5%O2/He flow (30 cm3/min) at 550°C (10°C/min) for 30 min and then cooled under helium to 60°C. The samples were then reduced from 60°C to 800°C (5°C/min) under 3% H2/Ar atmosphere (30 cm3/min). The reduction gas H2/Ar, was passed after the reactor through a freezing trap (propan-2-ol + liquid nitrogen) kept at -80°C to remove the formed water. Hydrogen consumption was monitored continuously by a thermal conductivity detector.
The ammonia desorption programmed as a function of the temperature (NH3-TPD) was carried out using the same H2-TPR equipment. A mass of 30 mg of catalyst is pretreated at 450°C for 30 min, under air flow (30 cm3/min), then saturated with ammonia at 100°C and purged with helium for 45 min. Following this adsorption, the physisorbed ammonia is removed by leaving the sample for 2 h at 100°C, under a helium flow rate of 30 cm3/min. Finally, the temperature was raised to 550°C (10°C/min), under a helium flow rate of 30 cm3/min.
The Selective Catalytic Reduction of NO by NH3 was carried out in a fixed-bed quartz flow reactor operating at atmospheric pressure. The catalyst (24 mg) was activated in-situ at 550°C for 1 hour under a flow of O2/He (20/80, v/v) and then cooled to 180°C. A feed mixture of 1000 ppm NO, 1000 ppm NH3, 8% O2 in He and 3.5% H2O was then passed through the catalyst at a flow rate of 100 cm3/min (VVH = 250000 cm3/g. h). The NH3-SCR was carried out on programmed temperature from 180°C to 500°C with the heating rate of 5°C/min.
For the Selective Catalytic Oxidation of ammonia (NH3-SCO), the test was carried out on the catalysts already tested in the NH3-SCR of NO. At 550°C, the NO flow is cut off and the NH3-SCO experiments were carried out adjusting He flow and by decreasing the temperature from 500°C to 180°C with the heating rate of 5°C/ min.
The reactants and products were analysed by a quadruple mass spectrometer (Pfeiffer Omnistar) equipped with Channeltron and Faraday detectors (0–200 amu) following these characteristic masses: NO (30), N2 (14, 28), N2O (28, 30, 44), NH3 (15, 17, 18), O2 (16, 32) and H2O (17, 18).
The percentages of NO (XNO) and NH3 (XNH3) conversions were calculated on the basis of the differences in their concentrations measured before and after the catalyst bed.
3.1 Characterization of the catalysts
The XRD patterns of the support η-Al2O3, CuO (Sigma Aldrich, ACS reagent ≥ 99.0 %) and the prepared catalysts Cu(x)-Al2O3 are shown in Fig. 2. The characteristic peaks at angles in 2θ 19.5°, 37.5°, 39.7°, 45.8°, 60.8° and 67.2° correspond to η-Al2O3 phase having spinel lattice (JCPDS, No. 04-0875). The introduction of copper leads to the destruction of the structure of η-Al2O3 for the catalysts with higher copper contents Cu(3)-Al2O3, Cu(5)-Al2O3 and Cu (7.5) -Al2O3. These catalysts showed the characteristic peaks of CuO (JCPDS, No. 80 − 0076) at the angles in 2θ 32.6°, 35.6°, 38.8° and 48.8°, 58.3° and 61.5° (Liang et al., 2012). On the other hand, for the Cu(1)-Al2O3 and Cu(2)-Al2O3 catalysts, there is a slight decrease in the intensity of the peaks of the support and particularly the peak at 19.5 ° and the absence of the diffraction peaks of CuO. It appears that the copper species present in the Cu(1)-Al2O3 and Cu (2)-Al2O3 catalysts are small and well dispersed on the surface of the support. Friedman et al. (Friedman et al., 1978) showed that the saturation of the CuO/ γ-Al2O3 catalyst surface a CuO monolayer occurs for a Cu content of about 4–5% by weight for every 100 m2/g of alumina. Beyond this threshold, crystalline CuO was observed.
The SEM micrographs of the η-Al2O3 and the catalysts Cu(2)-Al2O3 and Cu(3)-Al2O3 are illustrated in Fig. 3. The SEM micrograph of alumina η-Al2O3 is made up small agglomerate particles. The introduction of copper leads to a change in the morphology of η-Al2O3 particles. For example, the Cu(2)-Al2O3 catalyst presents a sponge-like morphology, which reveals a high level of porosity. On the other hand, for the Cu(3)-Al2O3 catalyst, one can see two phases. The first one is relative to sintered alumina particles and the second is related to CuO particles. It appears that high levels of copper favor the sintering of alumina at lower temperatures than usual. Sintering leads to the drop of the specific surface and the deterioration of the dispersion state of the copper species on the surface of the support.
The TEM images of the Cu(1)-Al2O3, Cu(2)-Al2O3 and Cu(3)-Al2O3 catalysts are reported in Figs. 4. For (Cu(1)-Al2O3 catalyst, we note that the copper particles are very small and well dispersed on the support η-Al2O3. The increase of the amount of copper leads to the increase of copper species size. For Cu(2)-Al2O3 catalyst, copper particles have size about 5–10 nm. Whereas for the Cu(3)-Al2O3 catalyst, we note the presence of black spherical particles exceeding 70 nm attributed to copper oxide CuO as shown by XRD.
Textural properties of the support η-Al2O3 and the prepared catalysts Cu(x)-Al2O3 are presented in Table 1 and Fig. 5. It is noted that the SBET of the catalysts decrease after the wet impregnation/evaporation with copper acetate. For example, the specific surface area of the support (SBET = 417 m2/g) decreases by 18% when 1% of copper was added (SBET = 343 m2/g) and 60% with the higher content of copper 7.5% (SBET = 169. m2/ g). Indeed, the XRD have shown that Cu(3)-Al2O3, Cu(5)-Al2O3 and Cu(7.5)-Al2O3 catalysts contain large CuO particles which block the porous structure of η-Al2O3. On the other hand, for Cu(1)-Al2O3 and Cu (2)-Al2O3 catalysts the copper species are well dispersed on the surface of the support and the decrease of SBET was moderate (only 16% for Cu(2)-Al2O3 catalyst). On the other hand, we notice an increase in the pore volume up to a copper quantity of 2% wt. and then a decrease beyond this value. Actually, the pore volume of the support which was Vp = 0.295 cm3/g increases by about 30% (Vp = 0.387 cm3/g) for 1% Cu and 40% (Vp = 0.411cm3/ g) for 2% Cu. This result could explain the morphology of η-Al2O3 and the formation of macropores as shown by SEM technique. On the other hand, increasing the copper content from 3–7.5% induces a reduction in the pore volumes of the catalysts due to the sintering of η-Al2O3 particles.
In Fig. 5 are reported the N2 adsorption-desorption isotherms of η-Al2O3 and Cu(x)-Al2O3 catalysts. All adsorption isotherms are of type IV having hysteresis loops characteristics for mesoporous solids (Petitto et al.,2013). Nevertheless, we note that the addition of copper to the η-Al2O3 changes the hysteresis loop from H3 to H2(b) type. This behavior could reflect a change in the pore shape and distribution with the introduction of copper. Indeed, H3 type hysteresis loop indicates the presence of narrow slit-like pores particles with internal voids of irregular shape and broad size distribution but the H2(b) hysteresis loop type shows a narrow distribution of pore shape with a wide neck size distribution (Cychosz and Thommes, 2018). Likewise, when the amount of copper increases there is a decrease in the adsorbed volume at low relative pressure (P/P°), indicating the decrease in microporosity and the increase in mesoporosity.
|
Sample |
SBET (m2/g) |
BJH Pore volume (cm3/g) |
BJH pore diameter (nm) |
|---|---|---|---|
|
η-Al2O3 |
417 |
0.295 |
4.50 |
|
Cu(1)-Al2O3 |
343 |
0.387 |
4.79 |
|
Cu(2)-Al2O3 |
351 |
0.411 |
4.85 |
|
Cu(3)-Al2O3 |
226 |
0.319 |
4.27 |
|
Cu(5)-Al2O3 |
229 |
0.229 |
4.67 |
|
Cu(7.5)-Al2O3 |
169 |
0.165 |
4.69 |
H2-TPR profiles of the studied samples are shown in Fig. 6. It is observed that Cu(1)-Al2O3 and Cu(2)-Al2O3 catalysts have similar reduction profiles (Fig. 7). With the increase of copper amount there is an increase of the intensity of the peaks. For Cu(2)-Al2O3 catalyst, the first peak around 130°C was attributed according to Yan et al., 1996 to the reduction of well dispersed CuO clusters on the surface of the support. The second peak extending from 300°C to 500°C corresponds to the reduction of highly dispersed Cu2+ cations in the structure of the alumina forming a surface spinel CuAl2O4 type (Aguila et al., 2008; Il’chenko et al., 1976). On the other hand, when the copper content was increased above 3 wt%, there are changes in the catalyst reduction profiles. Indeed, Cu(3)-Al2O3, Cu(5)-Al2O3 and Cu(7.5)-Al2O3 profiles show single reduction peaks centered in a temperature range of 140–380°C (Fig. 8). The extent of the peaks could indicate the existence of different CuO species with different sizes and environments. Fierro et al., 1994 reported that the supported CuO particle reduction temperature range extends from 200 to 300°C depending on the type of support. For the Cu(3)-Al2O3 catalyst, the peak ranges from 170 to 375°C and the temperature where the reduction rate is maximum is around 242°C.
The deconvolution of the reduction profile of the Cu (3) -Al2O3 catalyst is shown in Fig. 9. The results of deconvolution of the H2-TPR profiles of the Cu(3)-Al2O3,Cu(5)-Al2O3 and Cu(7.5)-Al2O3 catalysts are reported in Table 2.
For Cu(3)-Al2O3, the central peak was deconvolved into three peaks. The first is located around 209°C with a relative surface area of around 11%, the second around 232°C (15%) and the last around 274°C (74%). It is noted that the majority of CuO particles are reduced at high temperature because of their large size.
|
catalysts |
Maximum reduction temperature Tm (°C) |
||
|---|---|---|---|
|
Pic I |
Pic II |
Pic III |
|
|
Cu(3)-Al2O3 |
209 (11%) |
232 (15%) |
274 (74%) |
|
Cu(5)-Al2O3 |
- |
220 (46%) |
282 (54%) |
|
Cu(7.5)-Al2O3 |
179 (11%) |
222 (41%) |
268 (48%) |
The nature and environment of copper species present in the prepared catalysts have been studied by UV-vis spectroscopy. The UV-vis spectra of the carrier η-Al2O3 and the Cu(x)-Al2O3 catalysts are shown in Fig. 10. Generally, alumina is transparent in the UV-visible range. Nevertheless, the absorption band around 370 nm of support that could be attributed to impurities. The spectra of the Cu(1)-Al2O3 and Cu(2)-Al2O3 catalysts have the same profile characterized by a broad absorption band that extends from 350 to 650 nm and centered around 490 nm. This band could be attributed according to the literature (Chaudhary et al., 2018; Buvaneswari, 2015) to CuO or surface spinel type CuAl2O4 species. The catalysts Cu(3)-Al2O3, Cu(5)-Al2O3 and Cu(7.5)-Al2O3 exhibit characteristic spectra of well-crystallized CuO.
The NH3-TPD profiles of η-Al2O3 and the prepared Cu(x)-Al2O3 catalysts are reported in Fig. 11. The support η-Al2O3 has broad ammonia desorption peak which extends from 110°C to 475°C with a maximum at around 180°C. The catalysts Cu(1)-Al2O3 and Cu (2)-Al2O3 show similar desorption profiles to the support but with a higher intensity of the peaks. For the other catalysts Cu(3)-Al2O3, Cu(5)-Al2O3 and Cu(7.5)-Al2O3, the appearance of a peak around 300°C is observed which increases in intensity with the increase of the copper amounts. Generally, the temperature of desorbed ammonia is related to the strength of acidic sites in the samples. So, according to the maximum desorption temperature of ammonia (Td) (Carre et al., 2010), there are three types of acidic sites: i) weak acidic sites (150 ≤ Td (°C) ≤ 250), ii) average acidic sites (250 < Td (°C) ≤ 350) iii) strong acidic sites Td (°C) > 350. The NH3 desorption at T ≤ 150°C could be attributed to the NH3 molecules weakly bound to the surface of the support which have not been evacuated at 100°C.
3.2 Evaluation of the catalytic activity of Cu(x)-Al2O3 catalysts
3.2.1 Selective Catalytic Reduction of NO by NH3
The prepared catalysts were tested in the NH3-SCR of NO in the presence of an excess of oxygen and of water vapor according to reaction (I):
4NO + 4NH 3 + O2→ 4 N2 + 6H2O (I)
In Figs. 12 and 13 are reported the NO conversion and NH3 conversion of the prepared Cu(x)-Al2O3 catalysts in the NH3-SCR of NO. The NO conversion increased initially with increasing temperature, then reached a maximum and decreased. The evolution of the NO conversion passing through a maximum reflects the existence of a competition between two reactions; the first concerning the reduction of NO and the second the oxidation of NH3 by the oxygen present in the gas mixture. The competition between the two reactions is in favor of the oxidation of NH3 at high temperatures which explains the decline in NO conversion. The decrease in the NO conversion is also accompanied with some formation of N2O according to the two non-selective reactions II and III:
2NH 3 + 2O2 → 2N2O + 3H2O (II)
4NH 3 + 4NO + 3O2→ 4N2O + 6H2O (III)
Cu(1)-Al2O3 and Cu(2)-Al2O3 catalysts have similar NO and NH3 conversion profiles up to 475°C. Beyond this temperature, the NO conversion decreases for the Cu(2)-Al2O3 catalyst, whereas it continues to increase for Cu(1)-Al2O3 up to 500°C where a maximum NO conversion is about 91%. If we look to the conversion of NO to N2, we notice that these two catalysts are almost selective towards N2 (Fig. 14), due to the fact that the oxidation of NH3 is less favored due to the better dispersion of copper. The others catalysts, (Cu(3)-Al2O3, Cu(5)-Al2O3 and Cu(7.5)-Al2O3, display a volcano-shape curve of NO conversion as temperature increases while NH3 conversion continue to increase with the temperature. For example (Fig. 12), the catalyst Cu(7.5)-Al2O3 has a maximum NO conversion of the order of 40% at 350°C which is accompanied with small N2O production. Therefore, the large drop in NO conversion on this catalyst above 350°C is due to the NH3 oxidation into NO. Moreover, above 425°C, no NO reduction by ammonia occurs since NO concentration in the outlet gas is superior at the NO concentration in the inlet gas. The behavior of these catalysts could be related to the presence of large CuO particles. One can conclude that the two catalysts Cu(1)-Al2O3 and Cu (2)-Al2O3 are the most efficient in the reduction of NO by NH3 in the presence of 3.5% of water vapor. Nevertheless, the Cu(1)-Al2O3 has a slightly better NO reduction behaviour at high temperature. It has been found that the high NO conversion of these two catalysts can be related to the presence of small CuO clusters deposited on the surface of η-Al2O3 alumina and to CuAl2O4 surface spinel. We believe that small CuO clusters deposited on the surface and easily reduced at low temperatures are responsible for high temperature N2 selectivity. In fact, according to the H2-TPR results, the quantity of these copper species is greater in the case of Cu(2)-Al2O3 the and Cu(1)-Al2O3 catalysts.
Kwak et al., 2012 investigated the NH3-SCR of NO reaction under lean conditions on CuO-γ-Al2O3 catalysts. They showed that on 10 wt % CuO/γ-Al2O3, the NOx conversion is about 30% at 350°C and NH3 reacts primarily with oxygen to produce NOx. However, on a 0.5 wt CuO/γAl2O3 catalyst, NH3 reacts with NO to form N2 and the NOx conversion to N2 was almost 80% at 450°C.
3.2.2 Selective catalytic Oxidation of NH3
The oxidation profiles of NH3 in the presence of 3.5% water vapor of the prepared catalysts Cu(x)-Al2O3 are presented in Fig. 15. The studied reaction is as follows (IV):
4NH 3 + 3O2 → 2N2 + 6H2O (IV)
For all catalysts, it is noted that ammonia oxidation increases with the increase in temperature. For the Cu(3)-Al2O3, Cu (5)-Al2O3 and Cu(7.5)-Al2O3 catalysts, a gradual increase in NH3 oxidation from 200°C to 400°C was recorded. But above 400°C, the oxidation of NH3 decreases slightly. On the other hand, for Cu(1)-Al2O3 and Cu(2)-Al2O3 a gradual slower increase of the NH3 conversion was recorded from 250 ° C to 550 ° C. These two catalysts are much less active towards ammonia oxidation as we have already seen previously in NH3-SCR of NO. The catalytic activity of the catalyst Cu(1)-Al2O3 and Cu(2)-Al2O3 in the oxidation of NH3 could be related to the highly dispersed CuO on the support which are reduced at low temperature T = 130°C. According to Gang et al., 2000; 1999 complete oxidation of NH3 was obtained at 350°C on Cu(10%)-γAl2O3 catalyst with N2 selectivity of 90%. Liang et al., 2012 obtained similar results for Cu(10%)-γAl2O3 catalysts prepared by different copper precursors (nitrate, acetate and sulfate) and calcined at 500°C and 600°C. They showed that a mixture of CuO and CuAl2O4 species is formed on the various Cu(10%)-γAl2O3 catalysts. On the other hand, the dispersion and the nature of the copper species have a significant influence on the activity of the catalysts. Indeed, the highly dispersed CuO nanoparticles on the support are responsible for the high activity of Cu(10%)-γAl2O3 catalysts. Lenihan and Curtin (2009) using lower levels of copper (Cu(3.4%)/γ-Al2O3) found conversions of the order of 100% in NH3.
The nature of the copper precursor and the method of preparation were found to be determinants in the formation of active copper species in the NH3-SCO. For example, a sulphate precursor leads to the formation of CuAl2O4, whereas CuO of higher crystallinity is formed using an acetate compared to a nitrate precursor (Jung et al., 2017). However, the nature of the active species in NH3- SCO has not been fully verified yet. Gang et al., 2000 claimed that the surface CuAl2O4 spinel phase is responsible for the higher catalytic activity relative to CuO. A study conducted by Liang et al., 2012 has shown that a mixture of CuO and CuAl2O4 phases is formed on the various Cu (10%) - γAl2O3 catalysts. On the other hand, the dispersion and the nature of the copper species have a significant influence on the activity of the catalysts. Indeed, CuO nanoparticles highly dispersed on the support and easily reduced at low temperature are responsible for the high conversion of NH3.
Figure 16, 17 and Fig. 18 presents the selectivity profiles towards NO, N2O and N2 respectively obtained in the NH3-SCO reaction. N2 is the desired gas product, while NO and N2O are undesired by-products. The Cu(1)-Al2O3 and Cu(2)-Al2O3 catalyst has N2 selectivity close to 95% over the temperature range of 200–400°C. Additionally, the transition metal oxides have been widely studied in the scientific literature (Jablonska and Palkovits, 2016; Sazonova et al., 1996). This type of catalysts showed higher selectivity to N2, however, they need significantly higher operation temperatures as high as 300–500 ◦C than noble metal catalysts. For the three others Cu(x)-Al2O3, selectivities towards N2 are much lower. At 550℃ (Fig. 16), 48%, 40%, 30% of selectivity towards NO were measured for (Cu(7.5)-Al2O3, Cu(5)-Al2O3, Cu(3)-Al2O3) respectively.
The NH3-SCO method is an effective method for oxidizing NH3 into N2. The overall selectivity into N2 was close to 100% for Cu(1)-Al2O3 and above 95% for Cu(2)-Al2O3 over all the temperature while for the other three catalysts, N2 selectivity remains as high as 95% only at temperature below 350°C, as reported by Jabłońska et al.,2017; 2018. Highly dispersed CuOx favor moderate activity but N2 selectivity up to 550°C in NH3-SCO (Chmielarz et al., 2005). In a similar work, Dong et al. (2013) showed that nitrogen gas was primarily formed by the direct dissociation of the NO produced by the oxidation of the adsorbed NH3 (Dong et al., 2014).
Copper-supported η-Al2O3 catalysts have prepared and tested in the selective catalytic reduction of NO by NH3 and in the selective catalytic oxidation of NH3.The impregnation/evaporation method has successfully dispersed the copper species on the surface of the alumina for the low copper contents; Cu(1)-Al2O3 and Cu(2)-Al2O3 catalysts. The XRD showed that the introduction of an additional amount of copper leads to the destruction of the alumina structure when copper content exceeded 3% wt.%. The Cu(3)-Al2O3, Cu(5)-Al2O3 and Cu (7.5) -Al2O3 catalysts contain mainly large CuO particles. Cu(2)-Al2O3 and Cu(1)-Al2O3 catalysts have interesting NO conversion to N2 in the NH3-SCR of NO. This activity could be related essentially to the small CuO clusters deposited on the alumina surface and CuAl2O4 species. For NH3-SCO, the catalyst Cu(1)-Al2O3 and Cu(2)-Al2O3 exhibit similar behavior resulting in the conversion of NH3 to N2 of about 100% at T > 500 ° C. This conversion could be attributed to CuO nanoparticles highly dispersed on the support and easily reduced at low temperature.
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Authors' contributions
Nawel Jraba: analyzed the data and write the complete paper.
HassibTounsi, Thabet Makhlouf and Gerard Delahay: gave this idea of work.
All authors read and approved the final manuscript.
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