Nitrogen oxides (NOx) emission which is resulted from the burning of fossil energy contributes heavily to the damage of environment and human beings, so it is a hot topic and concerned popularly  . Hydrotalcites with a general formula of not only have a high catalytic activity to NOx removal but adsorb acid gas such as SOx, COx and NOx, because they are typically positively charged layers of brucite-like (Mg(OH)2)    . The divalent or trivalent cations in the octahedral sites can be substituted with some catalytic activity cations and the ions in the interlayers can be substituted with some functional acid gas adsorbents, and so they received a great attention to be catalysts and adsorbents  .
In the system with HTs or metal oxides as catalysts, reductant such as CO or NH3 is absolutely necessary  . Ammonia has been employed as reductant for removing NOx in industrial boilers and vehicle exhaust for years and many efforts have been made for improving NOx and SOx removal efficiency using ammonium as reductants. For example, Xinyan Zhang and his co-workers researched the selective catalytic reduction mechanism of N2O by NH3 over an Fe-Mordenite catalyst  . Huazhen Chang et al. prepared the fresh and sulfated MnOx-CeO2 catalysts and studied the performance of them for selective catalytic reduction of NOx by NH3 in a low temperature (T < 300˚C)  . However, in the present technologies, NH3 is needed to be supplied by an independent equipment and lead to the cost increase. On the other hand, the additional pollution resulted from ammonium itself is another serious problem  . Recently, the selective catalytic reduction of NOx by urea (urea/SCR) has been widely accepted as the most efficient NOx removal technologies to control the NOx emissions in diesel engine. However, they still have serious drawbacks due to an additional urea tank to be refilled periodically for the urea/SCR  . In our previous research, Mg/Al/Cu/NH4+ HT was prepared and an important result was found by means of sXAS x-ray adsorption scattering and CHN elementary analysis, i.e. ammonium can be induced into HT by incorporating with Cu2+  .
In this work, we report an efficient NOx storage-decomposition hydrotalcite based on Mg/Al/Cu-NH3・H2O HT for low-temperature NOx removal activity and the microscopic structure and thermal stability were investigated through combined lab-based tools. Besides the adsorption and storage activity removing NOx is reported, and the temperature programmed desorption test of NOx adsorbed sample under 175˚C from room temperature to 400˚C is investigated and the coherent mechanism is discussed.
2. Experimental Section
2.1. Hydrotalcites (HTs) Preparation
The Mg/Al/Cu HT incorporated with NH3 (Mg/Al/Cu-NH3∙H2O HT) were prepared via co-precipitation. 0.01 M aluminum nitrate nonahydrate, 0.03 M magnesium nitrate hexahydrate, 0.01 M copper nitrate hexahydrate, were dissolved in DI water (Solution 1). 0.01 M sodium hydroxide and 0.02 M sodium carbonate were dissolved in DI water (Solution 2). Solution 1 and solution 2 were slowly added dropwise into a flask containing 0.03 mol ammonia and 50 ml of water during vigorous stirring. The pH was controlled at 9 - 10 by controlling their addition rate at 60˚C. The slurry was stirred for an additional 2 h and aged quiescently at 80˚C for 18 h. The obtained precipitate was filtered, washed with distilled water until pH was 7, and then vacuum dried at 80˚C for 12 h into Mg/Al/Cu-NH3・H2O HT powder. The chemical reagents (AR) above obtained from Shanghai Sinopharm Chemical Reagent Co., Ltd.
2.2. NOx Storage and Decomposition Tests
The experiments were designed to obtain information on the NOx storage and decomposition activity of the samples when atmospheric pressure N2 with a flow rate of 54 - 60 l/h was fed into the reactor as a carrier gas. The gas mixture consisted of 10% NO and 90% N2 with the flow rate of 2.6 mL/min. An approximately 300 mg sample was placed in the center of a quartz reactor tube (Figure 1) with mass space-velocity of 237 h−1.
The sample which has been placed for 24 h in the drier was held in a small instrument covered with quartz cloth, and a K-type grounded thermocouple for temperature measurements was placed in the center of the tube which is blown by nitrogen gas before and after every test. The reactor was heated via a furnace and the temperature was controlled by a thermocouple connected with a temperature control device from 30˚C - 600˚C at room temperature and 60% humidity   . Also, the reactor’s exit was connected to an Germany MRU MGA infrared gas analyzer, which was used to analyze the inlet and outlet NOx, NO and NO2 concentration. The measurements last for 20 minutes.
The following equation was used to calculate the decomposition and storage rate of NOx.
NOxi: the inlet NOx concentration; NOxo: the outlet NOx concentration.
The desorption performance of the samples after adsorbing NOx for 30 minutes was obtained by putting it into the reactor once again with transferring N2 at room temperature as a balanced gas to blow for 10 minutes at the flow rate of 900 ml/min. Rising and keeping the temperature according to the program with a flow rate of 900 ml/min N2 as carrier gas, and then check the outlet NO and NO2 concentration varies with temperature.
Figure 1. Catalytic test system.
The characteristics such as powder X Ray diffraction (XRD) patterns for Mg/Al/Cu-NH3・ H2O HT at room temperature were performed using a Brukers X-Ray Diffractometer equipped with Cu Kα radiation (λ = 0.15406 nm, 40 kW, 4 mA). The patterns were acquired over a 2θ range of 5˚ - 80˚ within increment of 0.02˚ and scan speed of 0.5 s  . Fourier transform infrared (FT-IR) spectra were recorded at room temperature from 4000 to 350 cm−1 using an iS10 Thermo Fisher spectrometer with a total reflection measuring head. 16 scans were collected with a resolution of 2 cm−1. Thermogravimetric (TG) and DTG were carried out on a SII Nano TG-DTG 6300 instrument. Analysis was done from 50˚C to 700˚C at a heating rate of 10˚C∙min−1 under nitrogen (100 mL∙min−1)  .
3. Results and Discussion
3.1. XRD Characterization
Powder XRD of Mg/Al/Cu-NH3・ H2O HT is in agreement with the standard hydrotalcite peaks. The diffraction peaks which 2θ located at 11.63˚, 23.12˚, 35.6˚, 38.78˚, 46.46˚ and 61.83˚ are assigned to characteristic Mg/Al HT layered structure  , apparently, 2θ located at 29.37˚, 48.93˚ and etc attributed to another new crystallinity resulted from the corporation of Cu2+ with NH3・H2O. Layer-layer d-spacing which is calculated from (003) peaks is about 0.75nm correlating with the reported hydrotalcite  (Figure 2).
3.2. NOx Storage Performance
The previous research has provided direct evidence that ammonium incorporated
Figure 2. XRD spectra.
into Mg/Al/Cu/NH4+ HT could release under 156˚C from HT  , and play an important role for NOx removal. Herein, NO and NOx removal performances of Mg/Al/Cu-NH3∙H2O HT at 110˚C (the concentration of O2 is zero), 156˚C (the concentration of O2 is 1.2% - 1.7%), 175˚C (the concentration of O2 is zero) and no catalyst (the concentration of O2 is 1.2% - 1.7%) are focused on as shown in Figure 3(a) and Figure 3(b).
Figure 3. NO emission concentration with removal time (a) and NOx emission concentration with removal time (b).
Figure 3(a) and Figure 3(b) clearly shows that the changes of NO and NOx emission concentration with the removal time when simulation gas NOx flows through Mg/Al/Cu-NH3・H2O HT at different temperatures, separately. The results of graph (a) indicate the NO removal rates of HT with 45.6%, 63.6% and 34.7%, separately at 110˚C, 156˚C and 175˚C are higher than that of “No catalyst” within 290s, which agrees with the physical adsorption action of HTs  . The results registered from 290s to 1200s show the NO emission concentration at 156˚C under 1.2% - 1.7% O2 existence is lower than that of “no catalyst” can be attributed to the consumption of some NO being oxidized into NO2, which reacts with the ammonium incorporated into Mg/Al/Cu-NH3∙H2O HT released from HT at this temperature. In contrast, the results of NO emission concentration at 110˚C and 175˚C are higher than that of “No catalyst” reveal that not only NO can’t react with ammonium efficiently, but NO adsorbed in the structure of HTs will release in that period as well. Graph (b) reflects that Mg/Al/Cu-NH3∙H2O HT is effective for NOx removal within 500 s at different temperature with or without O2 existence, and the optimum NOx removal condition is 156˚C with a little O2 existence. The amount of NO and NOx storage conversion with about 0.13 mmol/g and 0.10 mmol/g, separately proves Mg/Al/Cu-NH3∙H2O HT has higher storage conversion amount at the calculation of the data at 1200 s. The amount of NO storage conversion with 0.60 mmol/g is three times higher than that of NiMgAl hydrotalcite with 0.18 mmol/g at the calculation of the data at 5400 s  . It can be concluded the Mg/Al/Cu-NH3∙H2O HT can realize physical and chemical adsorption for NOx at the same time with O2 existence at 156˚C, NOx can be stored in HT in the form of nitrates, while, only physical adsorption for NOx without O2 existence.
3.3. NO Desorption Test
NOx temperature-programmed desorption of sample 175˚C was carried out in a conventional flow system equipped with a temperature controller under N2 as a carrier gas at a flow rate of 0.9 l/min. 300 mg sample was loaded in a quartz tube reactor, which was heated at 5˚C/min from 100˚C to 400˚C. The NO, NO2 and NOx emission concentration is the averaged value of multiple measurements in the span of 60 minutes. It can be seen from Figure 4, the sample begins to release NO at about 160˚C under heating and reaches the highest releasing speed at 265˚C - 270˚C. There are two peaks on the decomposition curves, the first one is assigned to the decomposition of NH4NO2, and the second one at 365˚C - 370˚C is assigned to the decomposition of metal nitrates such as magnesium nitrate. The thermal decomposition reaction equation of magnesium nitrate is as below  :
The sample begins to release NO2 at about 250˚C and reaches the highest releasing speed at 310˚C - 315˚C, which is attributed to the decomposition of NH4NO3. In the view of the releasing of NO and NO2, the optimum thermal decomposition temperature range is 270˚C - 310˚C.
Figure 4. Desorption curves.
3.4. Thermal Analysis
Thermogravimetric analysis (TGA) and (DTG) traces of Mg/Al/Cu-NH3・ H2O HT without adsorbing NOx (0 sample) and after NOx adsorption for 1200 seconds at 110˚C (110˚C sample) and 156 (156˚C sample), separately are obtained under nitrogen atmosphere. Figure 5(a) and Figure 5(b) describe the degradation process of the three samples. The samples present three stages of weight loss at TGA. The first stage, which occurs at a temperature below 159˚C, is associated with the removal of the small molecules such as water and ammonium weakly adsorbed in the interlayer, the second stage of the thermal decomposition is observed between 159˚C and 325˚C, and there is an apparent thermal decomposition over 325˚C with 156˚C sample. Mg/Al/Cu-NH3∙H2O HT without adsorbing NOx displays a peak in DTG trace below 159˚C attributed to the removal of ammonium or water which has been proved in the former research  , however, the samples after adsorbing NOx at 110 and 159˚C don’t exhibit any peaks which reveals that the small molecules has released from HTs when they are heated. It can be seen from the second stage, there is only one peak at 244˚C for every sample, which is assigned to the decomposition and removal of hydroxyl groups in the brucite-like layers, as well as the , and other interlayer anions decomposition occurs over 200˚C, 156˚C sample showing weaker peak than the other two can be assigned to part of groups such as OH− or released from HT with the temperature increases. Meanwhile, in the case of the 156˚C sample, a strong peak at about 368˚C resulted from the decomposition of nitrates stored in HT is incoherent with the result of NOx storage test.
3.5. FT-IR Analysis
The infrared adsorption spectra in Figure 6 display the characteristic bands for
Figure 5. (a) TGA and (b) DTG trace.
the OH and CO bonds that are abundantly present in the mineral. Both Mg/Al/Cu-NH3・H2O HT and Mg/Al/Cu-NH3・H2O HT after adsorbing NOx at 156˚C present the similar spectrum with the typical hydrotalcites  . Didier Tichit and his co-workers investigated the FT-IR spectra of Mg/Al HT, and found the bands between 3500 and 3700 cm−1 can be attributed to hydroxyl groups stretching vibration, and the broad bands at 3400 cm−1 is assigned to the ν (NH)  . So 3325, 3408 and 3463 cm−1 at Figure 6 are visible to be attributed
Figure 6. FT-IR spectra.
to the stretching vibrations of the amino group, which proves ammonium is incorporated by bonding with Cu2+. Compared to Mg/Al/Cu-NH3・ H2O HT, the amino group band spectra of Mg/Al/Cu-NH3・ H2O HT after adsorbing NOx at 156˚C nearly disappeared, which indicates ammonium bounded with metal ions released when heating. In regard to the weak band at 1634 - 1642 cm−1 assigned to interlaminar water molecules deformation mode doesn’t appear in this specie, it can be ascribed the amount of interlayer water is little, which proves the mass loss at TGA below 159˚C mainly result from the release of ammonium further more and the new phase in XRD should be resulted from ammonia  . The peak around 1402 cm−1 is caused by the asymmetric stretching vibration of the C-O bond of group. Compared with the group in free state (1415 cm−1), this peak noticeably shifts to a lower wave number, which reveals that the inserted between layers are not truly free ions, however, due to the hydrogen bonds with amino group isn’t as strong as interlaminar water molecules, herein, the peak of group shifts to 1402 cm−1, not to 1380 cm−1 as reported. So, it can be concluded that and ammonium is easier to induce into the structure than water molecules, and this is very important to prepare ammonium storage HTs. The weak band at 1358 cm−1 on the sample-156˚C should be assigned to the surface nitrite or nitrate transferred by the reaction between ammonium and NOx  . The other absorption bands below 800 cm−1 are associated with the stretching and bending modes of metal-oxygen bonds.
The Mg/Al/Cu-NH3・H2O HT is prepared by co-precipitation, and the NOx storage activity is discussed at 110˚C, 156˚C and 175˚C with or without oxygen. Meanwhile, NOx temperature-programmed desorption test of the NOx adsorbed sample at 175˚C is also investigated. Based on XRD result, we determine that a new phase resulted from the corporation of Cu2+ with NH3・H2O existed in HT. NOx storage performance test proved the optimum NOx removal condition is 156˚C with a little O2 existence. TGA and DTG analysis indicate the nitrates which are formed by ammonium released from HT before 156˚C reacting with NOx will decompose over 350˚C. The FT-IR results agree with TGA and DTG analysis further. From NOx temperature-programmed desorption test of the NOx adsorbed sample at 175˚C, we found the adsorbed NO will release over 160˚C and NO2 will release over 240˚C continuously, so it is proved Mg/Al/Cu-NH3・H2O HT has the significant storage activity under 156˚C.
We acknowledge the financial support received from university students’ research and innovation projects of Shanghai (Nos. cs1501011, cs1604006) and the Science and Technology Funds from Shanghai Automotive Industry Corp. (No.1532).