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Promising zirconia-mixed Al-based nitrogen carriers for chemical looping of NH3: Reduced NH3 decomposition and improved NH3 yield
Fuel 264 (2020) 116821
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Full Length Article
Promising zirconia-mixed Al-based nitrogen carriers for chemical looping of NH3: Reduced NH3 decomposition and improved NH3 yield
T
⁎
Ye Wua,b, , Yuan Gaoa,b, Quan Zhanga,b, Tianyi Caic, Xiaoping Chenc, Dong Liua,b, Maohong Fand a MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China b Advanced Combustion Laboratory, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China c Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy & Environment, Southeast University, Nanjing 210096, People's Republic of China d Departments of Chemical and Petroleum Engineering, University of Wyoming, University Avenue, Laramie WY 82071, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: NH3 Chemical looping ammonia generation (CLAG) N-desorption reaction ZrO2
The chemical looping ammonia generation (CLAG) of N-sorption/desorption reactions via Al-based N-carriers for producing ammonia (NH3) (a non-carbon fuel) is thought to be an important alternative to the conventional Haber–Bosh NH3 synthesis technology. However, the low yield of NH3, because of significant NH3 decomposition at the required N-desorption temperature, inhibits the development of CLAG. Accordingly, ZrO2 mixed AlN was designed in this study to investigate the NH3 generation characteristics of AlN during the N-desorption process in a stationary bed reactor. The results showed that ZrO2 can improve the formation of NH3 because the molecular adsorption of NH3 occurred on the ZrO2 surface, thus preventing NH3 from decomposing. The ZrO2 loadings and steam concentrations in the atmosphere positively affect the yield and generation efficiency of NH3, while increasing the reaction temperature increased the NH3 yield from 1.2 to 3.3 mmol, but reduced the NH3 generation efficiency from 91.7% to 60.3%.
1. Introduction
combustion reactions of H2 and NH3 are [9,10]
With the rapid development of the global economy and continuous advancement of industrialization, increasing amount of fossil fuels are consumed in large quantities, and the resulting large amount of CO2 and corresponding environmental problems are becoming serious [1,2]. To alleviate these problems, carbon-free approaches for the utilization of fossil fuels should be developed and actively promoted. Hydrogen (H2) is a promising clean energy source, and technology for hydrogen generation from fossil fuels is developing rapidly [3,4]. However, some limitations preclude the application of H2 as a clean gas fuel – H2, as a physical stability material, is difficult to store and transport by liquefaction [5]. Furthermore, the energy density of H2 is lower than those of traditional gas fuels [6]. Therefore, cleaner gas fuels and their corresponding generation technologies should be considered. Ammonia (NH3) is a promising clean gas fuel, and its combustion products are water and nitrogen [7,8], although NOx is also present in the products. Researchers are still working on reducing the NOx content that would allow using NH3 as a clean fuel in the future. The
H2 +
1 O2 → H2 O 2
NH3 +
3 3 1 O2 → H2 O + N2 4 2 2
θ ΔH298 = −242.5 kJ / mol
θ ΔH298 = −317.5 kJ / mol
(R1) (R2)
Compared with H2, the energy density of NH3 is considerably higher (approximately 1.5 times that of H2). Moreover, NH3 can be easily liquefied, which significantly reduces its storage and transportation costs [6]. Today, more than 90% of the world’s NH3 is synthesized using the Haber–Bosch synthesis (HBS) process, which uses H2 and N2 as raw materials; the synthesis occurs at high pressures and high temperatures [6,11]. The HBS reaction is
N2 + 3H2
Catalyst
⇔
2NH3
(R3)
However, owing to the limitation of the thermodynamic stability, the conversion efficiency of the HBS (R3) is only in the 25%–35% range [12]. Thus, better NH3 synthesis methods should be developed to
⁎ Corresponding author at: MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Power Engineering, Nanjing University of Science & Technology, Xiaolingwei road No. 200, Jiangsu 210094, People’s Republic of China. E-mail address: [email protected] (Y. Wu).
https://doi.org/10.1016/j.fuel.2019.116821 Received 18 April 2019; Received in revised form 20 November 2019; Accepted 4 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. XRD results for the prepared 40 wt% of ZAN sample.
still requires attention. ZrO2 is a special material with acid, basic, oxidizing, and reducing properties [26]; thus, it has been widely used as a catalyst or carrier material in various catalytic reactions, including the conventional HBS) process [27,28]. Furthermore, temperature programmed desorption test (NH3-TPD) showed that there are abundant Lewis acid sites on the surface of ZrO2 [29], which implies that NH3 could be molecularly adsorbed on the surface of ZrO2. This property may protect NH3 and inhibit the decomposition of NH3 to increase the NH3 yield in the Ndesorption reaction. Therefore, we tried to use ZrO2 as a protective agent to increase the NH3 yield in the N-desorption reaction. In this work, N-desorption performances of prepared zirconia-mixed nitrogen carriers (AlN/ZrO2) under different conditions, including ZrO2 loadings, temperature, and steam concentration, were studied in a stationary bed reactor. In addition, characterization methods, including X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the prepared/spent zirconia-loaded nitrogen carriers (AlN/ZrO2).
increase the NH3 yield. Chemical looping ammonia generation (CLAG) is one of the most promising methods for NH3 synthesis, and the N sorption/desorption reactions occur during the entire process, as shown below [13,14]. The N-sorption step:
3C + N2 + Al2 O3 → 2AlN + 3CO
θ ΔH298 = 708.1 kJ / mol
(R4)
θ ΔH298 = −274.1 kJ / mol
(R5)
The N-desorption step:
2AlN + 3H2 O → Al2 O3 + 2NH3
Compared with the HBS method, this method can not only produce NH3 more efficiently, but also can transfer energy from fossil fuels to NH3 and realize the carbon-free utilization of fossil fuels. Aluminum nitride (AlN) is an excellent ceramic material and semiconductor material [15,16]; therefore, many studies have addressed the use of R4 or carbothermal reduction nitridation for reducing the reaction temperature and increasing the AlN conversion. For example, Forslund et al. found that AlN can be converted from Al2O3 in a graphite furnace with flowing nitrogen gas between 1200 °C and 1500 °C, using a powder mixture with the Al2O3:C molar ratio of 1:3 [17,18]. Further, Tsuge et al. found that the reactivity of the Al-source decreased in the order of γ-Al2O3 > Al(OH)3 > α-Al2O3 [19], and the reactivity of the C-source decreased in the order of petcoke > active carbon > wood charcoal > carbon black [20]. To further improve the alumina conversion efficiency and reduce the activation energy of R4, several compounds, including CaF2, Y2O3, Yb2O3, and Cr2O3, were added to the C and Al2O3 mixture. Related research shows that the presence of calcium-containing catalysts, especially CaF2, has the best catalytic effect and can reduce the temperature of the carbothermal reduction reaction by 200 °C [21]. However, mainstream research on R5 has been focused on suppressing the hydrolysis of AlN for maintaining its stability as a ceramic material, which is opposite to our goal of promoting the hydrolysis of AlN to produce NH3. For example, Krnel et al. found that the hydrolysis of AlN was prevented at low pH values, but accelerated in an alkaline environment [22]. Bartel et al. found that the fracture energy of the AlN bonds is so high that it affects the speed of R5 directly, but an increase in the hydroxyl on the AlN surface can significantly decrease the activation energy of the N-desorption reaction or promote the hydrolysis of AlN [23]. Based on this, our previous studies found that Fe2O3 [24] and rutile-TiO2 [25] are ideal AlN hydrolysis catalysts, because H2O molecules are easily absorbed to form free –OH. However, the decomposition of NH3 at the N-desorption temperature is still the main reason for the low conversion efficiency of NH3. Therefore, the inhibition of the decomposition of NH3 to further increase the NH3 yield
2. Experimental 2.1. Sample preparation and characterization The AlN powder was obtained from Advanced Technology and Materials Co. Ltd. (N greater than 32.5%; partial size, 2 μm), and the zirconium dioxide (ZrO2) powder was obtained from Sinopharm Chemical Reagent Co., Ltd (purity greater than 99.0%). All ZrO2 samples were calcined under 900 °C to remove water and volatiles. Then, predetermined amounts of AlN powder and ZrO2 powder were grinded in a mortar for 30 min, for intensive mixing. The resulting powder was the ZrO2-loaded AlN sample (hereafter called “ZAN sample”). The target weight percentages were 0, 20, 40, 60, and 80 wt% of ZrO2 in the samples, and the corresponding names were 0 wt% of ZAN, 20 wt% of ZAN, 40 wt% of ZAN, 60 wt% of ZAN, and 80 wt% of ZAN, respectively. The XRD data were collected using a BrukerD8 Advanced X-ray diffractometer with Cu KR as a radiation source (λ = 0.15418 nm); the scanning rate was 8°/min in the 20°–80° range . The XRD data for the 40 wt% of ZAN are shown in Fig. 1. Only ZrO2 (o) and AlN (Δ) lattice were in the sample. A Hitachi S4800 field emission SEM was used to study the obtained ZAN samples before hydrolysis. The morphologies of the 20 wt% of ZAN and 80 wt% of ZAN samples are shown in Fig. 2(a) and (b), respectively. Both AlN and ZrO2 appeared as irregular particles in the ZAN samples, with the particle size of approximately 5 μm. After fully mixing and grinding, as shown in the element mappings in Fig. 2(a) and (b), the AlN and ZrO2 particles were fractured and distributed evenly in 2
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20 wt% of ZAN
Al
Zr
Al
Zr
(a)
80 wt% of ZAN
(b) Fig. 2. SEM micrographs and element mappings for different ZAN scenarios: (a) 20 wt% of ZAN; (b) 80 wt% of ZAN.
(aq), qOH = 0.2 × 10−3mol/ mL ) was used to titrate the collected solution, to calculate the amount of the generated NH3. Then, the solid products were collected to calculate the conversion amount of AlN. Each experiment was performed at least thrice for reducing the error, and all the reported results are averages. The calculation processes are explained below.
the ZAN samples, and the distribution of AlN was denser in the 20 wt% of ZAN sample and more dispersed in the 80 wt% of ZAN sample. 2.2. Experiments 2.2.1. Tests conducted using the stationary bed reactor All experimental data in this study were collected using the stationary bed reactor. The structure of the stationary bed reactor is shown in Fig. 3. The studied samples were placed in the tube furnace (8), and the reaction temperature was increased to the desired temperature, at the heating rate of 15 °C/min. During the heating, Ar was passed into the furnace to protect the ZAN samples. After the temperature reached the target value, the reaction gas (containing 40 vol% to 80 vol% H2O, Ar balanced) was flown into the furnace at 500 mL/min, and was allowed to react with the ZAN sample for 1 h. The vapor was controlled and generated by the syringe pump (1), and then heated at 200 °C in the steam generator (3). All the pipes were covered with a heater band to prevent the vapor condensation during the experiments. During the 1-h reaction, all generated gases were passed through the NH3 absorption bottle (9), which contained an excess dilute sulfuric solution (H2SO4(aq), qH ≈ 1.8 × 10−5mol/ mL , VH = 150 mL ). After all the NH3 gas was fully absorbed, a standard sodium hydroxide solution (NaOH
1) Yield of NH3 Here, we used acid-base neutralization titration to calculate the yield of NH3 [30]. After NH3 was absorbed by the diluted H2SO4, three drops of methyl red were placed into the NH3 absorption bottle. Owing to the residual H+ in the solution, the solution’s color became pink. Then, NaOH (aq) was used to slowly neutralize the residual H2SO4, until the solution’s color changed from pink to light red, which indicated the end of the titration process. The volume of the NaOH solution (ΔVOH ) used in the titration tests was recorded to calculate the NH3 yield as follows:
nN = 2 × qH × VH − qOH×ΔVOH
(E1)
where nN is the total yield of NH3 under the desired condition (in mol); qH is the concentration of the diluted H2SO4 (aq), 1.8 × 10−5 mol/mL; VH is the volume of the diluted H2SO4 (aq), 150 mL; qOH is the concentration of NaOH (aq), 0.2 × 10−3 mol/mL; and ΔVOH is the volume of NaOH (aq) consumed during the titration (in mL). 2) Conversion of AlN Here, we used the amount of AlN converted in 1 h to compare the rates of the N-desorption reaction for different scenarios. The mass of the ZAN sample before and after the reaction was registered to calculate the amount of converted AlN, as follows:
nA = 2 ×
m2 − m1 ΔM
(E2)
where nA is the amount of converted AlN, in mol; ΔM represents the mole mass change of the N-desorption reaction, 20 g/mol; m1 and m2 represent the mass of the ZAN sample (in grams) before and after the reaction, respectively.
Fig. 3. Structure of stationary bed reactor (1: syringe pump; 2: temperature controller; 3: steam generator; 4: valve 1; 5: valve 2; 6: Ar cylinder; 7: mass flow controller; 8: tube furnace; 9: NH3 absorption bottle; 10: titrator). 3
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3) NH3 generation efficiency from the amount of converted AlN It is evident that 1 mol of AlN could theoretically yield 1 mol of NH3, according to R5. However, NH3 is easily decomposed into N2 and H2; therefore, here we used η to evaluate the NH3 generation efficiency. The value of η can be calculated as follows:
η=
nN × 100% nA
(E3)
where η represents the NH3 generation efficiency from the amount of converted AlN, in %. 2.2.2. NH3 decomposition tests To validate the enhancement effect of ZrO2 on the NH3 generation, we investigated the decomposition characteristics of NH3 for four different conditions (room temperature and 900 °C, with and without ZrO2), using the stationary bed reactor (Fig. 3). All the decomposition tests were conducted at the atmospheric pressure. The temperature in the furnace was either 25 °C or 900 °C·NH3 at 50 mL/min and Ar-balanced gas at 200 mL/min were flown through the tube of the furnace (8). The exit gases were passed into the absorption bottle containing a diluted sulfuric acid (9). For clarification, because some NH3 decomposes into N2 and H2 in the furnace, the remaining NH3 was absorbed by the diluted H2SO4. Three drops of methyl red were dropped into the absorption bottle, to serve as an indicator. After the color of the absorption solution in the absorption bottle (9) changed from pink into light red, the gas supply to the furnace was stopped and the overall time from the start to the color change was registered. The longer it took to change the color, the more NH3 was decomposed. Each experiment was conducted thrice, and averages were taken for reducing the error.
Fig. 4. Yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η), for different ZrO2 loadings (AlN mass: 0.4 g; H2O/Ar flow rate: 500 mL/min; steam concentration: 80 vol%; reaction temperature: 900 °C).
reaction was studied using the stationary bed reactor. The results are described below. 3.1.1. ZrO2 loading ZrO2 loadings in the ZAN samples played an important role in the Ndesorption reaction. The yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η) for different ZrO2 loadings are shown in Fig. 4. These results show that ZrO2 did not have any effect on the AlN hydrolysis. In fact, previously published reports suggest that H2O is also adsorbed and dissociates on the surface of ZrO2 [37,39], but this adsorption disappears above 400 °C [40]; thus, ZrO2 does not catalyze the hydrolysis of AlN at 900 °C. In contrast, as R6 shows, NH3 decomposes into N2 and H2 at high temperatures, owing to the thermodynamic instability of NH3.
2.3. DFT calculations The common ZrO2 has three crystal types, monoclinic (m-ZrO2), tetragonal (t-ZrO2), and cubic (c-ZrO2). At temperatures under 1200 °C, ZrO2 is in the monoclinic phase [31]. All of our experiments were conducted at temperatures under 1000 °C; therefore, here we chose mZrO2 for density functional theory (DFT) calculations. The XRD curves in Fig. 1 also show that ZrO2 in our experiments was m-ZrO2. All of the DFT calculations were performed using the CASTEP code [32]. The generalized gradient approximation (GGA) [33] and the Perdew–Burke–Ernzerhof (PBE) [34] scheme were used to describe the exchange correlation energy. The ultrasoft pseudopotential [35] was used for describing ionic cores. The plane wave cutoff energy was 400 eV for ZrO2(0 0 1), and the k-point grid determined using the Monkhorst–Pack method [36] sampled the first Brillouin zone of the surface unit cell with the division in the reciprocal space of less than 0.04 Å−1. For all the calculations, the convergence criteria were 2.0 × 10-6 eV/atom for SCF, 0.01 eV Å−1 for maximal force, 5.0 × 10-6 eV/atom for energy, and 0.01 eV/Å for maximal displacement. The NH3 molecule was modeled as an isolated molecule in a periodic box with the dimensions of 20 × 20 × 20 Å3. The optimized bulk structure of ZrO2 and the calculated lattice are shown in Fig. S1 and Table S1. The ZrO2(0 0 1) surface has been shown to be the most stable and most naturally occurring in grown crystals [37–39]. Therefore, the ZrO2 (0 0 1) surface was used for calculating the adsorption of NH3. The model establishment and selection of parameter values are shown as Fig. S2 in the supplementary material.
2NH3
temperature
→
N2 + 3H2
(R6)
Therefore, not all AlN was effectively converted to NH3. However, the yield of NH3 increased rapidly (from 40% for pure AlN to 80% for 80 wt% of ZAN) with ZrO2 loading, which may be attributed to the molecular adsorption of NH3 on the ZrO2 surface. The XRD test results for the products of R5 are shown in Fig. 5. In addition to the unreacted AlN (Δ), α-Al2O3 (#) is the only crystal lattice in the production of pure AlN. Moreover, as shown in Fig. 5(b), ZrO2 (o) remains among the products of the N-desorption reaction with ZAN. Therefore, ZrO2 serves as a catalyst or protective agent during the NH3
3. Results and discussion 3.1. Factors affecting the NH3 generation Fig. 5. XRD analysis results for the products of the N-desorption reaction (a: products for the N-desorption of pure AlN; b: products for the N-desorption of catalyzed AlN with ZrO2).
To study the NH3 generation and the N-desorption reaction in the prepared ZAN samples, the effect of several parameters, including ZrO2 loading, temperature, and steam concentration, on the N-desorption 4
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Fig. 6. Yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η), for different steam concentrations (AlN mass: 0.4 g; pure AlN and 80 wt% of ZAN; H2O/Ar flow rate: 500 mL/min; reaction temperature: 900 °C).
Fig. 8. Equilibrium constant of the NH3 decomposition (R6).
temperature, but the generation efficiency of NH3 (η) decreased. The amount of converted AlN (nA ) in 1 h of reaction was very small for reaction temperatures under 900 °C. When the temperature increased from 900 °C to 1000 °C, the amount of converted AlN in 1 h of reaction increased rapidly. As is shown by E4, the amount of converted AlN (nA ) increases with increasing reaction temperature.
generation process. 3.1.2. Steam concentration Fig. 6 shows the effect of the steam concentration on the yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η), during the N-desorption reaction. Clearly, more AlN is involved in the hydrolysis as the steam concentration increases from 40 vol% to 80 vol%. Therefore, the corresponding yield of NH3 increases. Meanwhile, the NH3 generation efficiency also increased slightly with increasing steam concentration. This phenomenon can be explained in two ways. First, with an increase in the steam concentration, more NH3 is dissolved in the steam and carried out, resulting in less decomposition of NH3. Second, compared with gases, the specific heat capacity of the steam is very high. As the steam passes through the furnace, a significant amount of heat is carried out by the steam. Therefore, the actual temperature in the furnace decreases slightly, resulting in a slight decrease in the decomposed NH3, and the NH3 generation efficiency slightly increases.
nA ∝ kR5 = AR5 e−
Ea, R5 RT
(E4)
Here, kR5 is the reaction rate coefficient of the N-desorption reaction (R5); Ea,R5 is the activation energy of R5, 238.4 kJ/mol; AR5 is the frequency factor of R5; R is the universal molar gas constant, 8.314 J/ (mol K); and T is the reaction temperature (in Kelvins). However, the generation efficiency of NH3 (η) decreased rapidly when the reaction temperature increased from 800 °C to 1000 °C. This was owing to the thermodynamic instability of NH3, as shown in Fig. 8. In addition, by combining the Gibbs–Helmholtz equation (E5) and the relationship between the Gibbs free energy change and the reaction equilibrium constant K (E6) [24]:
3.1.3. Temperature Temperature is another important parameter that affects the Ndesorption reaction of ZAN. As Fig. 7 shows, both the yield of NH3 (nN ) and the amount of converted AlN (nA ) increased with increasing
⎛∂ (ΔG0/ T ) ⎞ = − ΔH2 ∂T ⎠ p T2 ⎝
(E5)
ΔG0 = −RTlnK
(E6)
we have
∂lnK ΔH2 = > 0 or T ↑ ∝ K ↑ ∂T RT 2
(E7)
where ΔG0 (≈ 34.3 kJ/mol ) [41] is the Gibbs free energy change of the decomposition of NH3 (R6); K is the equilibrium constant of R6; and ΔH (≈ 91.82 kJ/mol ) [41] represents the enthalpy constant of R6. The value of K increases as T increases, which means that more NH3 is decomposed as temperature increases. However, compared with pure AlN, the decomposition of NH3 at higher temperatures is not so apparent when AlN is loaded on ZrO2. Therefore, ZrO2 plays a significant role in inhibiting the NH3 decomposition. 3.2. DFT analysis and inhibition mechanism To study the inhibition mechanism of ZrO2 in the NH3 decomposition, the CASTEP software based on the DFT framework was used to calculate the mechanism. It was reported that the top O atoms have the strongest adsorption [39]; therefore, two types of adsorption structures, considered to have the strongest adsorption sites, were used in our calculations. The initial and optimized configurations are shown in Fig. 9 and in Table 1. In the two optimized configurations, the
Fig. 7. Yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η), for different temperatures (AlN mass: 0.4 g; samples: pure AlN and 80 wt% of ZAN; H2O/Ar flow rate: 500 mL/min; steam concentration: 80 vol%). 5
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106.774 H3 H2 106.914 106.687 H1
adsorption. This adsorption mode is reversible; therefore, NH3 can still be completely desorbed on the ZrO2 surface without chemical changes. The projected densities of state (PDOS) for the relative atoms in configurations A and B are shown in Fig. 10. It can be seen that H and O atoms in the both configurations exhibit significant resonances around −15 eV and −6 eV, indicating electron transfer between the H atom in NH3 and the top O atom in ZrO2(0 0 1). Therefore, the PDOS curves validate the interaction between the H and O atoms in the calculation results, and NH3 molecules can be adsorbed on the ZrO2 surface. The results of the DFT calculations suggest that NH3 can be physically adsorbed on a perfect ZrO2 surface, with a relatively high adsorption energy, −1.296 eV (125.04 kJ/mol). However, the simulation conditions are still far too idealized compared with the realistic experimental ones. For example, the temperature factor, surface defects, and surface groups such as hydroxyl groups were not considered in our simulations. To further validate the possible inhibiting effect of ZrO2 on NH3 decomposition, NH3 decomposition experiments were carried out under room temperature or high temperature with γ-Al2O3/TiO2/ ZrO2, to validate the effect of ZrO2 on NH3. The experimental results are shown in Fig. 11. For clarification, the shorter the color change time of the indicator, the less NH3 is decomposed. The color change time (approximately 500 s) was the same at room temperature, with or without ZrO2, because NH3 hardly decomposed at room temperature. When the temperature increased to 900 °C, the required time for the sample without the catalyst increased by more than 140 s (to approximately 640 s). This was because a higher temperature (900 °C) accelerated the decomposition of NH3. In addition, the presence of TiO2 and γ-Al2O3 seemed not to affect the decomposition of NH3 at 900 °C, because the time required for the color to change was still approximately 640 s. However, the time required for the indicator to change its color was approximately 579 s at 900 °C when ZrO2 was present. Previous studies indicate that NH3 could be molecularly adsorbed by the Lewis acid sites, which abound in many materials such as TiO2, γ-Al2O3, and ZrO2 [42–44]. However, the sorption performances were different. For example, the NH3 adsorption temperature was above 450 °C for the ZrO2 surface [42], but those for γ-Al2O3 and TiO2 (approximately 250 °C and 300 °C) were under that for ZrO2 [43,44]. Thus, the acidities of γ-Al2O3, TiO2, and ZrO2 were in the order ZrO2 > TiO2 > γ-Al2O3, and ZrO2 was surmised to show the best acidity at 900 °C. Therefore, the molecular adsorption of NH3 on the ZrO2 surface is a possible mechanism. The possible inhibition mechanism is described in Fig. 12. The generated NH3 from the N-desorption reaction decomposes rapidly in the furnace at high temperatures, without ZrO2 loading. However, in the presence of ZrO2, many free NH3 molecules are adsorbed and transferred to the ZrO2 surface; thus, less NH3 decomposition in the furnace enhances the NH3 yield.
H3 113.303 H2 114.712 114.452 H1
3.00
1.776
(a1)
(a2)
H3 106.914 106.774 H1 H2 106.687 2.938 3.000
113.436
H3
115.199 H1 H2 2.542 110.567 1.840
(b2)
(b1)
Fig. 9. (a1) and (b1) Initial and (a2) and (b2) optimized structures of NH3 adsorbed on the ZrO2(0 0 1) surface (The red, gray, blue, and white spheres represent the O, Zr, N, and H atoms, respectively). Table 1 Initial and optimized structure parameters, adsorption energies, and bond populations for NH3 on ZrO2. Adsorbate structure Atomic distance (Å) Bond distance within adsorbate (Å) Bond angle of adsorbate (o)
Eads Bond population
a1 d H1-O16 d H2-O14 d N-H1 d N-H2 d N-H3 ∠H1NH2 ∠H1NH3 ∠H2NH3
a2
3 1.776 – – 1.03 1.043 1.03 1.024 1.03 1.024 106.69 114.45 106.91 114.71 106.77 113.3 −1.261 0.07(H1-O16)
b1
b1
2.938 2.542 3 1.84 1.03 1.028 1.029 1.04 1.03 1.025 106.687 110.567 106.914 113.436 106.774 115.199 −1.296 0.05(H2-O14)
4. Conclusions A stationary bed reactor was used for studying the N-desorption performance of desired ZrO2 loadings onto AlN (ZAN), under different conditions, including ZrO2 loading, steam concentration, and temperature. The results show that ZrO2 in the studied ZAN samples reduced the NH3 decomposition in the N-desorption reaction, with higher ZrO2 yielding better decomposition reduction. Increasing the temperature and steam concentration enhanced the hydrolysis of AlN and the corresponding NH3 generation; however, the NH3 generation efficiency clearly decreased with increasing temperature. Although the presence of ZrO2 significantly increased the NH3 yield in the N-desorption reaction and effectively improved the NH3 generation efficiency in CLAG, the CLAG technology can be improved further. For example, ZrO2 is expected to be organically combined with previous catalysts (TiO2 or Fe2O3) to further improve the utilization efficiency of raw materials. Furthermore, it is unclear whether ZrO2 has any adverse effects on the entire CLAG reaction. In addition, the cycling performance of the CLAG technology should be urgently studied.
calculated distance between H1 and surface O16 was 1.776 Å in configuration (a), and the distance between H2 and O14 was 1.840 Å in configuration (b). Both are shorter than the initial value of 3 Å. The adsorption energy (Eads) of the adsorbed NH3 was approximately −1.3 eV, and the bond populations between the H and O atoms were both positive (0.07 of H1-O16 for structure (a) and 0.05 of H2-O14 for structure (b)). This implies that the ZrO2(0 0 1) surface has a strong adsorption effect with respect to NH3, which is consistent with the properties of acid sites on ZrO2. However, by comparing the structural parameters of NH3 molecules across the configurations before and after the optimization, it can be seen that the adsorption process does not affect the structure of these molecules. The maximal change in the N–H bond length was only 1.3%. Furthermore, only the H atom in the NH3 molecule was adsorbed at the top O position on the ZrO2(0 0 1) surface, which implies that NH3 is not torn. Therefore, NH3 is adsorbed on the ZrO2(0 0 1) surface by molecular adsorption rather than by dissociated 6
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2.5
Fig. 10. Density of states for the NH3 adsorption on the ZrO2(0 0 1) surface.
CRediT authorship contribution statement Ye Wu: Conceptualization, Validation, Formal analysis, Investigation, Resources, Writing - original draft, Writing - review & editing. Yuan Gao: Formal analysis, Investigation, Writing - review & editing. Quan Zhang: Writing - review & editing. Tianyi Cai: Writing review & editing. Xiaoping Chen: Supervision. Dong Liu: Supervision. Maohong Fan: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We acknowledge the financial support of this work from the Fundamental Research Funds for the Central Universities, China (No. 30919011238), the Natural Science Foundation, China (51506095, 51476030, and 51576100), and the peak of the Six Talents Program, China (XNY004).
Fig. 11. Results of NH3 decomposition experiments for the stationary bed reactor (NH3 flow rate: 100 mL/min).
(a): NH3 release and decomposed in high temperature
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116821.
More NH3 decomposed Less NH3 decomposed
References
NH3
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[25] Gao Y, Wu Y, Zhang Q, Chen X, Jiang G, Liu D. N-desorption or NH3 generation of TiO2-loaded Al-based nitrogen carrier during chemical looping ammonia generation technology. Int J Hydrogen Energy 2018;43(34):16589–97. [26] Aramendia MA, Borau Victoriano, Jimenez C, Marinas JM, Porras A, Urbano FJ. Synthesis and characterization of ZrO2 as an acid–base catalyst Dehydration–dehydrogenation of propan-2-ol. J Chem Soc 1997;93(7):1431–8. [27] Wang Z, Ma Y, Lin J. Ruthenium catalyst supported on high-surface-area basic ZrO2 for ammonia synthesis. J Mol Catal A: Chem 2013;378:307–13. [28] Zhang Y, Zhan Y, Cao Y, Chen C, Lin X, Zheng Q. Low-Temperature Water-Gas Shift Reaction over Au/ZrO2 Catalysts Using Hydrothermally Synthesized Zirconia as Supports. Chin J Catal 2012;33(2–3):230–6. [29] Boqing X, Tsutomu Y, Kozo T, Juan L, Lubin Z. Characterization of Acid-base property of ZrO2 by Thermal Desorption: I. TPD Studies of Acid and Base probes. Acta Phys Chim Sin 1994;10:107–13. [30] Li Y, Pan C, Han W, Chai H, Liu H. An efficient route for the preparation of activated carbon supported ruthenium catalysts with high performance for ammonia synthesis. Catal Today 2011;174(1):97–105. [31] Haines J, Leger JM, Atouf A. Crystal Structure and Equation of State of CotunniteType Zirconia. J Am Ceram Soc 1995;78(2):445–8. [32] Clark SJ, Segall MD, PickardI CJ, Hasnip PJ, Probert MIJ, Refson K. First principles methods using CASTEP. Zeitschrift für Kristallographie - Crystalline Materials 2005;220:567–70. [33] Perdew JP, Burke K, Ernzerhof M. Generalized Gradient Approximation Made Simple. Phys Rev Lett 1996;77(18):3865–8. [34] Perdew JP, Burke K, Wang Y. Generalized gradient approximation for the exchangecorrelation hole of a many-electron system. Phys Rev B: Condens Matter 1996;54(23):16533–9. [35] Vanderbilt D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys Rev B 1990;41(11):7892–5. [36] Monkhorst HJ, Pack JD. Special points for Brillonin-zone integrations. Phys Rev B 1976;13(12):5188–92. [37] Iskandarov IM, Knizhnik AA, Rykova EA, Bagatur’yant AA, Potapkin BV, Korkin AA. First-principle investigation of the hydroxylation of zirconia and hafnia surfaces. Microelectronic Eng 2003;69(2):587–93. [38] Aarik J, Aidla A, MaKndar H, Sammelselg V, Uustare T. Texture development in nanocrystalline hafnium dioxide thin films grown by atomic layer deposition. J Cryst Growth 2000;220(1):105–13. [39] Wu Y, Cai T, Zhao W, Chen X, Liu H, Wang Y, et al. First-principles and experimental studies of [ZrO(OH)]+ or ZrO(OH)2 for enhancing CO2 desorption kinetics – imperative for significant reduction of CO2 capture energy consumption. J Mater Chem A 2018;6:17671–81. [40] Zhai L, Shi T, Wang H. Preparation and Formation Mechanism of Fibrous Hydrous Zireonia at Low pH valuse. J Inorganic Mater 2007;22(2):223–6. [41] Barin I. Thermochemical data of pure substances. Beijing: Science press; 2003. [42] Xu B, Tsutomu Y, Liang J. Characterization of Acid-base Property of ZrO2 by Thermal Desorption: I.TPD Studies of Acid and Base Probes. Acta Phys Chim Sin 1994;10(2):107–13. [43] Zhou Y, Chen Z, Gong H, Chen L, Wu W. Characteristics of dehydration during rice husk pyrolysis and catalytic mechanism of dehydration reaction with NiO/γ- Al2O3 as catalyst. Fuel 2019;245:131–8. [44] Pérez-López G, Ramírez-López R, Viveros T. Acidic Properties of Si- and AlPromoted TiO2 catalysts: effect on 2-propanol Dehydration Activity. Catal Today 2017;305:S0920586117304881.
production by coal gasification. Int J Hydrogen Energy 2017;42(4):2592–600. [5] Wang P, Chang F, Gao W, Guo J, Wu G, He T, et al. Breaking scaling relations to achieve low-temperature ammonia synthesis through LiH-mediated nitrogen transfer and hydrogenation. Nat Chem 2017;9(1):64–70. [6] Bartels JR. A feasibility study of implementing an Ammonia Economy. Doctor. Iowa State University; 2008. [7] Verkamp FJ, Hardin MC, Williams JR. Ammonia combustion properties and performance in gas-turbine burners. 1967. [8] Otomo J, Koshi M, Mitsumori T, Iwasaki H, Yamada K. Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion. Int J Hydrogen Energy 2018;43(5). [9] Hosseini SE, Wahid MA. Hydrogen production from renewable and sustainable energy resources: promising green energy carrier for clean development. Renew Sustain Energy Rev 2016;57:850–66. [10] Verkamp FJ, Hardin MC, Williams JR. Ammonia Combustion Properties And Performance In Gas-Turbine Burners. Symp (Int) Combust 1967;11:985–92. [11] Michalsky R, Pfromm PH. An Ionicity Rationale to Design Solid phase Metal Nitride Reactants for Solar Ammonia Production. J Phys Chem C 2012;116(44):23243–51. [12] Penkuhn M, Tsatsaronis G. Comparison of different ammonia synthesis loop configurations with the aid of advanced exergy analysis. Energy 2017;137:854–64. [13] Gálvez ME, Frei A, Halmann M, Steinfeld A. Ammonia Production via a Two-Step Al2O3 and AlN Thermochemical Cycle. 2. Kinetic Analysis. Ind Eng Chem Res 2007;46:2047-53. [14] Gálvez ME, Halmann M, Steinfeld A. Ammonia Production via a Two-Step Al2O3/ AlN Thermochemical Cycle. 1. Thermodynamic, Environmental, and Economic Analyses. Ind Eng Chem Res 2007;46:2042-6. [15] Guo L, Yang J, Feng Y, Qiu T, Chen B, Wan W. Preparation and properties of AlN ceramic suspension for non-aqueous gel casting. Ceram Int 2016;42(7):8066–71. [16] O’Leary SK, Foutz BE, Shur MS, Shur MS. Steady-State and Transient Electron Transport Within the III -V Nitride Semiconductors, GaN, AlN, and InN: A Review. J Mater Sci: Mater Electron 2006;17(2):87–126. [17] Forslund B, Zheng J. Carbothermal synthesis of aluminium nitride at elevated nitrogen pressures,1. Effect of process parameters on conversion rate. J Mater Sci 1993;28:3125–31. [18] Forslund B, Zheng J. Carbothermal synthesis of aluminium nitride at elevated nitrogen pressures, 2. Effect of process parameters on particle size and morphology. J Mater Sci 1993;28:3132–6. [19] Tsuge A, Inoue H, Kasori M, Shinozaki K. Raw material effect on AIN powder synthesis from Al203 carbothermal reduction. J Mater Sci 1990;25:2359–61. [20] Gálvez ME, Hischier I, Frei A, Steinfeld A. Ammonia Production via a Two-Step Al2O3 and AlN Thermochemical Cycle. 3. Influence of the Carbon Reducing Agent and Cyclability. Ind Eng Chem Res 2008;47:2231-7. [21] Molisani AL, Yoshimura HN. Low-temperature synthesis of AlN powder with multicomponent additive systems by carbothermal reduction–nitridation method. Mater Res Bull 2010;45(6):733–8. [22] Krnel K, Dražič G, Kosmač T. Degradation of AlN Powder in Aqueous Environments. J Mater Res 2011;19(04):1157–63. [23] Bartel CJ, Muhich CL, Weimer AW, Musgrave CB. Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping. ACS Appl Mater Interfaces 2016;8(28):18550–9. [24] Wu Y, Jiang G, Zhang H, Sun Z, Gao Y, Chen X, et al. Fe2O3, a cost effective and environmentally friendly catalyst for the generation of NH3 - a future fuel - using a new Al2O3-looping based technology. Chem Commun 2017;53(77):10664–7.
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Full Length Article
Promising zirconia-mixed Al-based nitrogen carriers for chemical looping of NH3: Reduced NH3 decomposition and improved NH3 yield
T
⁎
Ye Wua,b, , Yuan Gaoa,b, Quan Zhanga,b, Tianyi Caic, Xiaoping Chenc, Dong Liua,b, Maohong Fand a MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China b Advanced Combustion Laboratory, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China c Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy & Environment, Southeast University, Nanjing 210096, People's Republic of China d Departments of Chemical and Petroleum Engineering, University of Wyoming, University Avenue, Laramie WY 82071, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: NH3 Chemical looping ammonia generation (CLAG) N-desorption reaction ZrO2
The chemical looping ammonia generation (CLAG) of N-sorption/desorption reactions via Al-based N-carriers for producing ammonia (NH3) (a non-carbon fuel) is thought to be an important alternative to the conventional Haber–Bosh NH3 synthesis technology. However, the low yield of NH3, because of significant NH3 decomposition at the required N-desorption temperature, inhibits the development of CLAG. Accordingly, ZrO2 mixed AlN was designed in this study to investigate the NH3 generation characteristics of AlN during the N-desorption process in a stationary bed reactor. The results showed that ZrO2 can improve the formation of NH3 because the molecular adsorption of NH3 occurred on the ZrO2 surface, thus preventing NH3 from decomposing. The ZrO2 loadings and steam concentrations in the atmosphere positively affect the yield and generation efficiency of NH3, while increasing the reaction temperature increased the NH3 yield from 1.2 to 3.3 mmol, but reduced the NH3 generation efficiency from 91.7% to 60.3%.
1. Introduction
combustion reactions of H2 and NH3 are [9,10]
With the rapid development of the global economy and continuous advancement of industrialization, increasing amount of fossil fuels are consumed in large quantities, and the resulting large amount of CO2 and corresponding environmental problems are becoming serious [1,2]. To alleviate these problems, carbon-free approaches for the utilization of fossil fuels should be developed and actively promoted. Hydrogen (H2) is a promising clean energy source, and technology for hydrogen generation from fossil fuels is developing rapidly [3,4]. However, some limitations preclude the application of H2 as a clean gas fuel – H2, as a physical stability material, is difficult to store and transport by liquefaction [5]. Furthermore, the energy density of H2 is lower than those of traditional gas fuels [6]. Therefore, cleaner gas fuels and their corresponding generation technologies should be considered. Ammonia (NH3) is a promising clean gas fuel, and its combustion products are water and nitrogen [7,8], although NOx is also present in the products. Researchers are still working on reducing the NOx content that would allow using NH3 as a clean fuel in the future. The
H2 +
1 O2 → H2 O 2
NH3 +
3 3 1 O2 → H2 O + N2 4 2 2
θ ΔH298 = −242.5 kJ / mol
θ ΔH298 = −317.5 kJ / mol
(R1) (R2)
Compared with H2, the energy density of NH3 is considerably higher (approximately 1.5 times that of H2). Moreover, NH3 can be easily liquefied, which significantly reduces its storage and transportation costs [6]. Today, more than 90% of the world’s NH3 is synthesized using the Haber–Bosch synthesis (HBS) process, which uses H2 and N2 as raw materials; the synthesis occurs at high pressures and high temperatures [6,11]. The HBS reaction is
N2 + 3H2
Catalyst
⇔
2NH3
(R3)
However, owing to the limitation of the thermodynamic stability, the conversion efficiency of the HBS (R3) is only in the 25%–35% range [12]. Thus, better NH3 synthesis methods should be developed to
⁎ Corresponding author at: MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Power Engineering, Nanjing University of Science & Technology, Xiaolingwei road No. 200, Jiangsu 210094, People’s Republic of China. E-mail address: [email protected] (Y. Wu).
https://doi.org/10.1016/j.fuel.2019.116821 Received 18 April 2019; Received in revised form 20 November 2019; Accepted 4 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. XRD results for the prepared 40 wt% of ZAN sample.
still requires attention. ZrO2 is a special material with acid, basic, oxidizing, and reducing properties [26]; thus, it has been widely used as a catalyst or carrier material in various catalytic reactions, including the conventional HBS) process [27,28]. Furthermore, temperature programmed desorption test (NH3-TPD) showed that there are abundant Lewis acid sites on the surface of ZrO2 [29], which implies that NH3 could be molecularly adsorbed on the surface of ZrO2. This property may protect NH3 and inhibit the decomposition of NH3 to increase the NH3 yield in the Ndesorption reaction. Therefore, we tried to use ZrO2 as a protective agent to increase the NH3 yield in the N-desorption reaction. In this work, N-desorption performances of prepared zirconia-mixed nitrogen carriers (AlN/ZrO2) under different conditions, including ZrO2 loadings, temperature, and steam concentration, were studied in a stationary bed reactor. In addition, characterization methods, including X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the prepared/spent zirconia-loaded nitrogen carriers (AlN/ZrO2).
increase the NH3 yield. Chemical looping ammonia generation (CLAG) is one of the most promising methods for NH3 synthesis, and the N sorption/desorption reactions occur during the entire process, as shown below [13,14]. The N-sorption step:
3C + N2 + Al2 O3 → 2AlN + 3CO
θ ΔH298 = 708.1 kJ / mol
(R4)
θ ΔH298 = −274.1 kJ / mol
(R5)
The N-desorption step:
2AlN + 3H2 O → Al2 O3 + 2NH3
Compared with the HBS method, this method can not only produce NH3 more efficiently, but also can transfer energy from fossil fuels to NH3 and realize the carbon-free utilization of fossil fuels. Aluminum nitride (AlN) is an excellent ceramic material and semiconductor material [15,16]; therefore, many studies have addressed the use of R4 or carbothermal reduction nitridation for reducing the reaction temperature and increasing the AlN conversion. For example, Forslund et al. found that AlN can be converted from Al2O3 in a graphite furnace with flowing nitrogen gas between 1200 °C and 1500 °C, using a powder mixture with the Al2O3:C molar ratio of 1:3 [17,18]. Further, Tsuge et al. found that the reactivity of the Al-source decreased in the order of γ-Al2O3 > Al(OH)3 > α-Al2O3 [19], and the reactivity of the C-source decreased in the order of petcoke > active carbon > wood charcoal > carbon black [20]. To further improve the alumina conversion efficiency and reduce the activation energy of R4, several compounds, including CaF2, Y2O3, Yb2O3, and Cr2O3, were added to the C and Al2O3 mixture. Related research shows that the presence of calcium-containing catalysts, especially CaF2, has the best catalytic effect and can reduce the temperature of the carbothermal reduction reaction by 200 °C [21]. However, mainstream research on R5 has been focused on suppressing the hydrolysis of AlN for maintaining its stability as a ceramic material, which is opposite to our goal of promoting the hydrolysis of AlN to produce NH3. For example, Krnel et al. found that the hydrolysis of AlN was prevented at low pH values, but accelerated in an alkaline environment [22]. Bartel et al. found that the fracture energy of the AlN bonds is so high that it affects the speed of R5 directly, but an increase in the hydroxyl on the AlN surface can significantly decrease the activation energy of the N-desorption reaction or promote the hydrolysis of AlN [23]. Based on this, our previous studies found that Fe2O3 [24] and rutile-TiO2 [25] are ideal AlN hydrolysis catalysts, because H2O molecules are easily absorbed to form free –OH. However, the decomposition of NH3 at the N-desorption temperature is still the main reason for the low conversion efficiency of NH3. Therefore, the inhibition of the decomposition of NH3 to further increase the NH3 yield
2. Experimental 2.1. Sample preparation and characterization The AlN powder was obtained from Advanced Technology and Materials Co. Ltd. (N greater than 32.5%; partial size, 2 μm), and the zirconium dioxide (ZrO2) powder was obtained from Sinopharm Chemical Reagent Co., Ltd (purity greater than 99.0%). All ZrO2 samples were calcined under 900 °C to remove water and volatiles. Then, predetermined amounts of AlN powder and ZrO2 powder were grinded in a mortar for 30 min, for intensive mixing. The resulting powder was the ZrO2-loaded AlN sample (hereafter called “ZAN sample”). The target weight percentages were 0, 20, 40, 60, and 80 wt% of ZrO2 in the samples, and the corresponding names were 0 wt% of ZAN, 20 wt% of ZAN, 40 wt% of ZAN, 60 wt% of ZAN, and 80 wt% of ZAN, respectively. The XRD data were collected using a BrukerD8 Advanced X-ray diffractometer with Cu KR as a radiation source (λ = 0.15418 nm); the scanning rate was 8°/min in the 20°–80° range . The XRD data for the 40 wt% of ZAN are shown in Fig. 1. Only ZrO2 (o) and AlN (Δ) lattice were in the sample. A Hitachi S4800 field emission SEM was used to study the obtained ZAN samples before hydrolysis. The morphologies of the 20 wt% of ZAN and 80 wt% of ZAN samples are shown in Fig. 2(a) and (b), respectively. Both AlN and ZrO2 appeared as irregular particles in the ZAN samples, with the particle size of approximately 5 μm. After fully mixing and grinding, as shown in the element mappings in Fig. 2(a) and (b), the AlN and ZrO2 particles were fractured and distributed evenly in 2
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20 wt% of ZAN
Al
Zr
Al
Zr
(a)
80 wt% of ZAN
(b) Fig. 2. SEM micrographs and element mappings for different ZAN scenarios: (a) 20 wt% of ZAN; (b) 80 wt% of ZAN.
(aq), qOH = 0.2 × 10−3mol/ mL ) was used to titrate the collected solution, to calculate the amount of the generated NH3. Then, the solid products were collected to calculate the conversion amount of AlN. Each experiment was performed at least thrice for reducing the error, and all the reported results are averages. The calculation processes are explained below.
the ZAN samples, and the distribution of AlN was denser in the 20 wt% of ZAN sample and more dispersed in the 80 wt% of ZAN sample. 2.2. Experiments 2.2.1. Tests conducted using the stationary bed reactor All experimental data in this study were collected using the stationary bed reactor. The structure of the stationary bed reactor is shown in Fig. 3. The studied samples were placed in the tube furnace (8), and the reaction temperature was increased to the desired temperature, at the heating rate of 15 °C/min. During the heating, Ar was passed into the furnace to protect the ZAN samples. After the temperature reached the target value, the reaction gas (containing 40 vol% to 80 vol% H2O, Ar balanced) was flown into the furnace at 500 mL/min, and was allowed to react with the ZAN sample for 1 h. The vapor was controlled and generated by the syringe pump (1), and then heated at 200 °C in the steam generator (3). All the pipes were covered with a heater band to prevent the vapor condensation during the experiments. During the 1-h reaction, all generated gases were passed through the NH3 absorption bottle (9), which contained an excess dilute sulfuric solution (H2SO4(aq), qH ≈ 1.8 × 10−5mol/ mL , VH = 150 mL ). After all the NH3 gas was fully absorbed, a standard sodium hydroxide solution (NaOH
1) Yield of NH3 Here, we used acid-base neutralization titration to calculate the yield of NH3 [30]. After NH3 was absorbed by the diluted H2SO4, three drops of methyl red were placed into the NH3 absorption bottle. Owing to the residual H+ in the solution, the solution’s color became pink. Then, NaOH (aq) was used to slowly neutralize the residual H2SO4, until the solution’s color changed from pink to light red, which indicated the end of the titration process. The volume of the NaOH solution (ΔVOH ) used in the titration tests was recorded to calculate the NH3 yield as follows:
nN = 2 × qH × VH − qOH×ΔVOH
(E1)
where nN is the total yield of NH3 under the desired condition (in mol); qH is the concentration of the diluted H2SO4 (aq), 1.8 × 10−5 mol/mL; VH is the volume of the diluted H2SO4 (aq), 150 mL; qOH is the concentration of NaOH (aq), 0.2 × 10−3 mol/mL; and ΔVOH is the volume of NaOH (aq) consumed during the titration (in mL). 2) Conversion of AlN Here, we used the amount of AlN converted in 1 h to compare the rates of the N-desorption reaction for different scenarios. The mass of the ZAN sample before and after the reaction was registered to calculate the amount of converted AlN, as follows:
nA = 2 ×
m2 − m1 ΔM
(E2)
where nA is the amount of converted AlN, in mol; ΔM represents the mole mass change of the N-desorption reaction, 20 g/mol; m1 and m2 represent the mass of the ZAN sample (in grams) before and after the reaction, respectively.
Fig. 3. Structure of stationary bed reactor (1: syringe pump; 2: temperature controller; 3: steam generator; 4: valve 1; 5: valve 2; 6: Ar cylinder; 7: mass flow controller; 8: tube furnace; 9: NH3 absorption bottle; 10: titrator). 3
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3) NH3 generation efficiency from the amount of converted AlN It is evident that 1 mol of AlN could theoretically yield 1 mol of NH3, according to R5. However, NH3 is easily decomposed into N2 and H2; therefore, here we used η to evaluate the NH3 generation efficiency. The value of η can be calculated as follows:
η=
nN × 100% nA
(E3)
where η represents the NH3 generation efficiency from the amount of converted AlN, in %. 2.2.2. NH3 decomposition tests To validate the enhancement effect of ZrO2 on the NH3 generation, we investigated the decomposition characteristics of NH3 for four different conditions (room temperature and 900 °C, with and without ZrO2), using the stationary bed reactor (Fig. 3). All the decomposition tests were conducted at the atmospheric pressure. The temperature in the furnace was either 25 °C or 900 °C·NH3 at 50 mL/min and Ar-balanced gas at 200 mL/min were flown through the tube of the furnace (8). The exit gases were passed into the absorption bottle containing a diluted sulfuric acid (9). For clarification, because some NH3 decomposes into N2 and H2 in the furnace, the remaining NH3 was absorbed by the diluted H2SO4. Three drops of methyl red were dropped into the absorption bottle, to serve as an indicator. After the color of the absorption solution in the absorption bottle (9) changed from pink into light red, the gas supply to the furnace was stopped and the overall time from the start to the color change was registered. The longer it took to change the color, the more NH3 was decomposed. Each experiment was conducted thrice, and averages were taken for reducing the error.
Fig. 4. Yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η), for different ZrO2 loadings (AlN mass: 0.4 g; H2O/Ar flow rate: 500 mL/min; steam concentration: 80 vol%; reaction temperature: 900 °C).
reaction was studied using the stationary bed reactor. The results are described below. 3.1.1. ZrO2 loading ZrO2 loadings in the ZAN samples played an important role in the Ndesorption reaction. The yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η) for different ZrO2 loadings are shown in Fig. 4. These results show that ZrO2 did not have any effect on the AlN hydrolysis. In fact, previously published reports suggest that H2O is also adsorbed and dissociates on the surface of ZrO2 [37,39], but this adsorption disappears above 400 °C [40]; thus, ZrO2 does not catalyze the hydrolysis of AlN at 900 °C. In contrast, as R6 shows, NH3 decomposes into N2 and H2 at high temperatures, owing to the thermodynamic instability of NH3.
2.3. DFT calculations The common ZrO2 has three crystal types, monoclinic (m-ZrO2), tetragonal (t-ZrO2), and cubic (c-ZrO2). At temperatures under 1200 °C, ZrO2 is in the monoclinic phase [31]. All of our experiments were conducted at temperatures under 1000 °C; therefore, here we chose mZrO2 for density functional theory (DFT) calculations. The XRD curves in Fig. 1 also show that ZrO2 in our experiments was m-ZrO2. All of the DFT calculations were performed using the CASTEP code [32]. The generalized gradient approximation (GGA) [33] and the Perdew–Burke–Ernzerhof (PBE) [34] scheme were used to describe the exchange correlation energy. The ultrasoft pseudopotential [35] was used for describing ionic cores. The plane wave cutoff energy was 400 eV for ZrO2(0 0 1), and the k-point grid determined using the Monkhorst–Pack method [36] sampled the first Brillouin zone of the surface unit cell with the division in the reciprocal space of less than 0.04 Å−1. For all the calculations, the convergence criteria were 2.0 × 10-6 eV/atom for SCF, 0.01 eV Å−1 for maximal force, 5.0 × 10-6 eV/atom for energy, and 0.01 eV/Å for maximal displacement. The NH3 molecule was modeled as an isolated molecule in a periodic box with the dimensions of 20 × 20 × 20 Å3. The optimized bulk structure of ZrO2 and the calculated lattice are shown in Fig. S1 and Table S1. The ZrO2(0 0 1) surface has been shown to be the most stable and most naturally occurring in grown crystals [37–39]. Therefore, the ZrO2 (0 0 1) surface was used for calculating the adsorption of NH3. The model establishment and selection of parameter values are shown as Fig. S2 in the supplementary material.
2NH3
temperature
→
N2 + 3H2
(R6)
Therefore, not all AlN was effectively converted to NH3. However, the yield of NH3 increased rapidly (from 40% for pure AlN to 80% for 80 wt% of ZAN) with ZrO2 loading, which may be attributed to the molecular adsorption of NH3 on the ZrO2 surface. The XRD test results for the products of R5 are shown in Fig. 5. In addition to the unreacted AlN (Δ), α-Al2O3 (#) is the only crystal lattice in the production of pure AlN. Moreover, as shown in Fig. 5(b), ZrO2 (o) remains among the products of the N-desorption reaction with ZAN. Therefore, ZrO2 serves as a catalyst or protective agent during the NH3
3. Results and discussion 3.1. Factors affecting the NH3 generation Fig. 5. XRD analysis results for the products of the N-desorption reaction (a: products for the N-desorption of pure AlN; b: products for the N-desorption of catalyzed AlN with ZrO2).
To study the NH3 generation and the N-desorption reaction in the prepared ZAN samples, the effect of several parameters, including ZrO2 loading, temperature, and steam concentration, on the N-desorption 4
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Fig. 6. Yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η), for different steam concentrations (AlN mass: 0.4 g; pure AlN and 80 wt% of ZAN; H2O/Ar flow rate: 500 mL/min; reaction temperature: 900 °C).
Fig. 8. Equilibrium constant of the NH3 decomposition (R6).
temperature, but the generation efficiency of NH3 (η) decreased. The amount of converted AlN (nA ) in 1 h of reaction was very small for reaction temperatures under 900 °C. When the temperature increased from 900 °C to 1000 °C, the amount of converted AlN in 1 h of reaction increased rapidly. As is shown by E4, the amount of converted AlN (nA ) increases with increasing reaction temperature.
generation process. 3.1.2. Steam concentration Fig. 6 shows the effect of the steam concentration on the yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η), during the N-desorption reaction. Clearly, more AlN is involved in the hydrolysis as the steam concentration increases from 40 vol% to 80 vol%. Therefore, the corresponding yield of NH3 increases. Meanwhile, the NH3 generation efficiency also increased slightly with increasing steam concentration. This phenomenon can be explained in two ways. First, with an increase in the steam concentration, more NH3 is dissolved in the steam and carried out, resulting in less decomposition of NH3. Second, compared with gases, the specific heat capacity of the steam is very high. As the steam passes through the furnace, a significant amount of heat is carried out by the steam. Therefore, the actual temperature in the furnace decreases slightly, resulting in a slight decrease in the decomposed NH3, and the NH3 generation efficiency slightly increases.
nA ∝ kR5 = AR5 e−
Ea, R5 RT
(E4)
Here, kR5 is the reaction rate coefficient of the N-desorption reaction (R5); Ea,R5 is the activation energy of R5, 238.4 kJ/mol; AR5 is the frequency factor of R5; R is the universal molar gas constant, 8.314 J/ (mol K); and T is the reaction temperature (in Kelvins). However, the generation efficiency of NH3 (η) decreased rapidly when the reaction temperature increased from 800 °C to 1000 °C. This was owing to the thermodynamic instability of NH3, as shown in Fig. 8. In addition, by combining the Gibbs–Helmholtz equation (E5) and the relationship between the Gibbs free energy change and the reaction equilibrium constant K (E6) [24]:
3.1.3. Temperature Temperature is another important parameter that affects the Ndesorption reaction of ZAN. As Fig. 7 shows, both the yield of NH3 (nN ) and the amount of converted AlN (nA ) increased with increasing
⎛∂ (ΔG0/ T ) ⎞ = − ΔH2 ∂T ⎠ p T2 ⎝
(E5)
ΔG0 = −RTlnK
(E6)
we have
∂lnK ΔH2 = > 0 or T ↑ ∝ K ↑ ∂T RT 2
(E7)
where ΔG0 (≈ 34.3 kJ/mol ) [41] is the Gibbs free energy change of the decomposition of NH3 (R6); K is the equilibrium constant of R6; and ΔH (≈ 91.82 kJ/mol ) [41] represents the enthalpy constant of R6. The value of K increases as T increases, which means that more NH3 is decomposed as temperature increases. However, compared with pure AlN, the decomposition of NH3 at higher temperatures is not so apparent when AlN is loaded on ZrO2. Therefore, ZrO2 plays a significant role in inhibiting the NH3 decomposition. 3.2. DFT analysis and inhibition mechanism To study the inhibition mechanism of ZrO2 in the NH3 decomposition, the CASTEP software based on the DFT framework was used to calculate the mechanism. It was reported that the top O atoms have the strongest adsorption [39]; therefore, two types of adsorption structures, considered to have the strongest adsorption sites, were used in our calculations. The initial and optimized configurations are shown in Fig. 9 and in Table 1. In the two optimized configurations, the
Fig. 7. Yield of NH3 (nN ), amount of converted AlN (nA ), and corresponding NH3 generation efficiency (η), for different temperatures (AlN mass: 0.4 g; samples: pure AlN and 80 wt% of ZAN; H2O/Ar flow rate: 500 mL/min; steam concentration: 80 vol%). 5
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106.774 H3 H2 106.914 106.687 H1
adsorption. This adsorption mode is reversible; therefore, NH3 can still be completely desorbed on the ZrO2 surface without chemical changes. The projected densities of state (PDOS) for the relative atoms in configurations A and B are shown in Fig. 10. It can be seen that H and O atoms in the both configurations exhibit significant resonances around −15 eV and −6 eV, indicating electron transfer between the H atom in NH3 and the top O atom in ZrO2(0 0 1). Therefore, the PDOS curves validate the interaction between the H and O atoms in the calculation results, and NH3 molecules can be adsorbed on the ZrO2 surface. The results of the DFT calculations suggest that NH3 can be physically adsorbed on a perfect ZrO2 surface, with a relatively high adsorption energy, −1.296 eV (125.04 kJ/mol). However, the simulation conditions are still far too idealized compared with the realistic experimental ones. For example, the temperature factor, surface defects, and surface groups such as hydroxyl groups were not considered in our simulations. To further validate the possible inhibiting effect of ZrO2 on NH3 decomposition, NH3 decomposition experiments were carried out under room temperature or high temperature with γ-Al2O3/TiO2/ ZrO2, to validate the effect of ZrO2 on NH3. The experimental results are shown in Fig. 11. For clarification, the shorter the color change time of the indicator, the less NH3 is decomposed. The color change time (approximately 500 s) was the same at room temperature, with or without ZrO2, because NH3 hardly decomposed at room temperature. When the temperature increased to 900 °C, the required time for the sample without the catalyst increased by more than 140 s (to approximately 640 s). This was because a higher temperature (900 °C) accelerated the decomposition of NH3. In addition, the presence of TiO2 and γ-Al2O3 seemed not to affect the decomposition of NH3 at 900 °C, because the time required for the color to change was still approximately 640 s. However, the time required for the indicator to change its color was approximately 579 s at 900 °C when ZrO2 was present. Previous studies indicate that NH3 could be molecularly adsorbed by the Lewis acid sites, which abound in many materials such as TiO2, γ-Al2O3, and ZrO2 [42–44]. However, the sorption performances were different. For example, the NH3 adsorption temperature was above 450 °C for the ZrO2 surface [42], but those for γ-Al2O3 and TiO2 (approximately 250 °C and 300 °C) were under that for ZrO2 [43,44]. Thus, the acidities of γ-Al2O3, TiO2, and ZrO2 were in the order ZrO2 > TiO2 > γ-Al2O3, and ZrO2 was surmised to show the best acidity at 900 °C. Therefore, the molecular adsorption of NH3 on the ZrO2 surface is a possible mechanism. The possible inhibition mechanism is described in Fig. 12. The generated NH3 from the N-desorption reaction decomposes rapidly in the furnace at high temperatures, without ZrO2 loading. However, in the presence of ZrO2, many free NH3 molecules are adsorbed and transferred to the ZrO2 surface; thus, less NH3 decomposition in the furnace enhances the NH3 yield.
H3 113.303 H2 114.712 114.452 H1
3.00
1.776
(a1)
(a2)
H3 106.914 106.774 H1 H2 106.687 2.938 3.000
113.436
H3
115.199 H1 H2 2.542 110.567 1.840
(b2)
(b1)
Fig. 9. (a1) and (b1) Initial and (a2) and (b2) optimized structures of NH3 adsorbed on the ZrO2(0 0 1) surface (The red, gray, blue, and white spheres represent the O, Zr, N, and H atoms, respectively). Table 1 Initial and optimized structure parameters, adsorption energies, and bond populations for NH3 on ZrO2. Adsorbate structure Atomic distance (Å) Bond distance within adsorbate (Å) Bond angle of adsorbate (o)
Eads Bond population
a1 d H1-O16 d H2-O14 d N-H1 d N-H2 d N-H3 ∠H1NH2 ∠H1NH3 ∠H2NH3
a2
3 1.776 – – 1.03 1.043 1.03 1.024 1.03 1.024 106.69 114.45 106.91 114.71 106.77 113.3 −1.261 0.07(H1-O16)
b1
b1
2.938 2.542 3 1.84 1.03 1.028 1.029 1.04 1.03 1.025 106.687 110.567 106.914 113.436 106.774 115.199 −1.296 0.05(H2-O14)
4. Conclusions A stationary bed reactor was used for studying the N-desorption performance of desired ZrO2 loadings onto AlN (ZAN), under different conditions, including ZrO2 loading, steam concentration, and temperature. The results show that ZrO2 in the studied ZAN samples reduced the NH3 decomposition in the N-desorption reaction, with higher ZrO2 yielding better decomposition reduction. Increasing the temperature and steam concentration enhanced the hydrolysis of AlN and the corresponding NH3 generation; however, the NH3 generation efficiency clearly decreased with increasing temperature. Although the presence of ZrO2 significantly increased the NH3 yield in the N-desorption reaction and effectively improved the NH3 generation efficiency in CLAG, the CLAG technology can be improved further. For example, ZrO2 is expected to be organically combined with previous catalysts (TiO2 or Fe2O3) to further improve the utilization efficiency of raw materials. Furthermore, it is unclear whether ZrO2 has any adverse effects on the entire CLAG reaction. In addition, the cycling performance of the CLAG technology should be urgently studied.
calculated distance between H1 and surface O16 was 1.776 Å in configuration (a), and the distance between H2 and O14 was 1.840 Å in configuration (b). Both are shorter than the initial value of 3 Å. The adsorption energy (Eads) of the adsorbed NH3 was approximately −1.3 eV, and the bond populations between the H and O atoms were both positive (0.07 of H1-O16 for structure (a) and 0.05 of H2-O14 for structure (b)). This implies that the ZrO2(0 0 1) surface has a strong adsorption effect with respect to NH3, which is consistent with the properties of acid sites on ZrO2. However, by comparing the structural parameters of NH3 molecules across the configurations before and after the optimization, it can be seen that the adsorption process does not affect the structure of these molecules. The maximal change in the N–H bond length was only 1.3%. Furthermore, only the H atom in the NH3 molecule was adsorbed at the top O position on the ZrO2(0 0 1) surface, which implies that NH3 is not torn. Therefore, NH3 is adsorbed on the ZrO2(0 0 1) surface by molecular adsorption rather than by dissociated 6
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2.5
Fig. 10. Density of states for the NH3 adsorption on the ZrO2(0 0 1) surface.
CRediT authorship contribution statement Ye Wu: Conceptualization, Validation, Formal analysis, Investigation, Resources, Writing - original draft, Writing - review & editing. Yuan Gao: Formal analysis, Investigation, Writing - review & editing. Quan Zhang: Writing - review & editing. Tianyi Cai: Writing review & editing. Xiaoping Chen: Supervision. Dong Liu: Supervision. Maohong Fan: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We acknowledge the financial support of this work from the Fundamental Research Funds for the Central Universities, China (No. 30919011238), the Natural Science Foundation, China (51506095, 51476030, and 51576100), and the peak of the Six Talents Program, China (XNY004).
Fig. 11. Results of NH3 decomposition experiments for the stationary bed reactor (NH3 flow rate: 100 mL/min).
(a): NH3 release and decomposed in high temperature
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116821.
More NH3 decomposed Less NH3 decomposed
References
NH3
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