Research Note pubs.acs.org/IECR
Effects of Steam on Ni/Al2O3 Catalysts for Ammonia Decomposition Ryosuke Atsumi,† Reiji Noda,‡ Hideyuki Takagi,§ Luigi Vecchione,¶ Andrea Di Carlo,¶ Zaccaria Del Prete,¶ and Koji Kuramoto*,§ †
Department of Chemical and Environmental Technology, Faculty of Engineering, Gunma University, 1-5-1 Tenjincho, Kiryu, Gunma 376-8515, Japan ‡ Division of Environmental Engineering Science, Faculty of Science and Engineering, Gunma University, 1-5-1 Tenjincho, Kiryu, Gunma 376-8515, Japan § Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ¶ Department of Mechanical and Aerospace Engineering, Sapienza University of Rome, 00184 Rome, Italy ABSTRACT: Decomposition of gaseous NH3 from ammonia (NH3)-containing wastewater was explored using Ni-loading Al2O3 catalysts. The thermochemical decomposition of an NH3/steam mixture (wet-NH3) with different steam contents at 873, 923, and 973 K using a fixed-bed reactor under ambient pressure. The present results indicated that the catalysts can be deactivated by the formation of NiAl2O4, which can be thermodynamically generated, and confirmed that the extent of deactivation was greatly affected by the partial pressure of the steam (PH2O). The catalytic activities at 873 K decreased with increasing PH2O, whereas the activity was constant above PH2O of 25 kPa. However, the NH3 conversion was almost independent of the NH3 flow rate and temperature, and ∼30% of the NH3 was decomposed at each temperature. This study indicated that, even in the presence of steam, this catalyst could decompose NH3 from NH4+-containing water. that contained high concentrations of NH4+ was injected into a catalyst bed at a high temperature to generate hydrogen. Ni catalysts have been well-studied for the decomposition of ammonia, because Ni is the most active non-noble metal for the pure NH3 (dry-NH3) decomposition reaction.7−9 Muroyama et al. reported that a Ni-loaded Al2O3 catalyst (designated as Ni/ Al 2 O 3 ) shows a higher catalytic activity for dry-NH 3 decomposition than other Ni-loaded ceramic supports (e.g., SiO2, ZrO2, and La2O3).5 However, the effects of the presence of steam on wet-NH 3 decomposition have not been investigated. Ni catalysts are potentially deactivated through the oxidation of Ni nanoparticles by steam.6 Various studies on the steam reforming of hydrocarbons indicate that the surface of Ni-loaded catalysts can often be oxidized by steam or deactivated by the adsorption of −OH groups onto the surface.6 We estimated the activity of Ni catalysts for the decomposition of wet-NH3 using a continuous-gas-flow fixedbed reactor to study the feasibility of thermochemical decomposition of NH3 from NH4+-containing water. NH3 decomposition tests were performed to clarify the deactivation behavior of the catalyst with time and the extent of NH3 conversion achieved against various partial pressures of steam.
1. INTRODUCTION The growing requirements for food accompanying the world population explosion has led to an increased concern about the discharge of inorganic nitrogen species (e.g., NO3− and NH4+) associated with livestock excrement into streamwater and fields.1−3 Therefore, we consider that it is important to reduce the ammonia nitrogen contained in animal waste via a denitrification processes of NH3, because the NO3− produced from NH4+ within soils can potentially cause the eutrophication of streamwater. If NH4+ can be converted to hydrogen, it was possible to prevent this environmental problem and apply drainage water with animal wastes as a hydrogen source.4 Botte et al. have investigated NH3 alkaline electrolytic cell for decomposition of NH3 in wastewater.10−13 This cell unit is operated at low temperature and ambient pressure. The electrolysis for NH3 was one of the promising techniques to produce renewable hydrogen from NH4+-containing wastewater. However, as reported by Miura et al., it is difficult to obtain hydrogen stably under a condition of high current density on the electrolysis of NH3, because adsorbed N atoms on the electrode generated in the electrolysis reaction can deactivate the electrode catalyst.14 Therefore, we have tried to explore an alternative technique for stable decomposition of NH3 from NH4+-containing wet waste. In the present work, we attempted the thermochemical catalytic decomposition of NH3 in an NH3/steam mixture (wet-NH3) with different steam contents at 873, 923, and 973 K using a fixed-bed reactor under ambient pressure. The Niloaded Al 2O 3 catalyst was applied to thermochemical decomposition of NH3 from simulated drainage water containing ammonia nitrogen (NH4+). In our concept, water © 2014 American Chemical Society
Received: Revised: Accepted: Published: 17849
August 29, 2014 October 10, 2014 October 29, 2014 October 29, 2014 dx.doi.org/10.1021/ie503411a | Ind. Eng. Chem. Res. 2014, 53, 17849−17853
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Figure 1. Illustration of the experimental setup.
2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. The Ni/Al2O3 catalyst was synthesized by a wet impregnation method using a nominal Ni loading of 10 wt %. Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, Wako Pure Chemical Industries, Ltd.) was used as a precursor. The Ni precursor and Al2O3 (basic γ-Al2O3, 75−150 μm of the particle size, Wako Pure Chemical Industries, Ltd.) were dissolved in ion-exchanged water, and then this slurry was stirred for 24 h. This sample was dried using a rotary evaporator at 383 K under reduced pressure. The resulting Ni/Al2O3 was calcinated at 973 K for 1 h under an argon atmosphere and then reduced at 973 K for 2 h under a H2 atmosphere. 2.2. Dry- or Wet-NH3 Decomposition Test. The catalytic activity of Ni/Al2O3 was estimated using a continuous-gas-flow fixed-bed reactor at ambient pressure. Figure 1 shows the schematic illustration of the experimental setup. This equipment consists of float flow meters, a syringe pump, an impinger bottle for NH3 trapping, a soap-film meter, and two stainless steel tubes as a vaporizer and the main reactor. The vaporizer provides steam to accompany NH3 into the catalyst bed. To enhance the vaporization of water, alumina particles (1 mm diameter) were packed in the middle of the vaporizer tube, which was maintained at 413 K using an electric heater. Ion-exchanged water was injected into the alumina bed to generate steam. Ni/Al2O3 particles (0.2 g) were packed in the middle of the main reactor as the catalyst bed. The temperature of the catalyst bed was maintained at 873, 923, or 973 K using an electric furnace. Wet-NH3 was fed into the Ni/ Al2O3 catalyst bed. The flow rates of the provided gases (Ar, H2, and NH3) were controlled by float meters, whereas the flow rate of the steam was determined by the flow rate of water controlled with a syringe pump. Unreacted NH3 and steam in the outlet gases were trapped using a 10 wt % H2SO4 solution, and the flow rates of gases which consisted of only H2 and N2 generated from the decomposition of NH3 were measured using a soap-film flow meter to estimate the NH3 conversion.
Figure 2. Change in ammonia conversion with time for dry- or wetNH3 decomposition over Ni/Al2O3 catalyt. The temperature was 873 K and the partial pressure of steam (PH2O) was varied over a range of 10−50 kPa.
conversion over the Ni/Al2O3 catalyst at 873 K against the steam partial pressure (10−50 kPa) tested with a flow rate of 750 mL min−1 gcat−1 for both dry- and wet-NH3. Ni/Al2O3 under flowing dry-NH3 was found to maintain NH3 conversion at ca. 65% for 1 h. When the gas was switched from dry-NH3 to wet-NH3, the conversions at each steam partial pressure decreased for the first ca. 20 min, but the catalytic activities were not completely lost. The conversions under flowing wet-NH3 were normalized by the initial conversion value to obtain the fractional conversion. Figure 3 shows the fractional conversion versus PH2O at 873 K. The deactivation behavior for the conversion of NH3 above 25 kPa steam were found to be almost the same with the catalytic activity for wet-NH3 decreased to ca. one-half of the initial conversion value. We had speculated that the fractional conversions decreased monotonically against steam partial pressure.
3. RESULTS AND DISCUSSION 3.1. Effects of Steam Partial Pressure on the Catalytic Activity of Ni/Al2O3. Figure 2 shows the plot of NH3 17850
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and the temperature is 873 K. Note that the NiAl2O4 ratio was calculated using the mole fraction ratio of NiAl2O4 against Ni, i.e., [NiAl2O4]/([Ni] + [NiAl2O4]). As shown in this figure, these calculations indicated that NiAl2O4 could form below ca. 40% NH3 conversion. One possible deactivation mechanism is the oxidation of the nickel surface by steam, because NiO is probably inactive for NH3 decomposition.6 Another possible mechanism is the formation of NiAl2O4; however, with the present data, it is currently not possible to determine the exact deactivation mechanism. This calculation indicated that NiO was not generated by steam oxidation, but NiAl2O4 could potentially be formed, as shown in the following equation: Ni(s) + H 2O(g ) + Al 2O3(s) → NiAl 2O4 (s) + H 2(g ) (1)
In the case of NiAl2O4 formation on the support surface, Ni atoms on the support surface diffuse into the Al2O3 with O atoms derived from H2O. Although the thermodynamic equilibrium calculation indicates the formation of NiAl2O4, XRD patterns (not shown) after catalysts tests were almost same as before tests and did not show the formation of NiO and NiAl2O4. We considered that the amount of NiAl2O4 was too small to detect by XRD. To observe the change of Ni and Al2O4 clearly by XRD, it is important to increase the amount of loaded nickel. Furthermore, the effects of absorbed −OH group could not estimate in the present study. Thus, we will try to undertake detailed analysis of the surface and the crystal structure of Ni catalysts as our future work to determine the mechanisms of catalytic deactivation. 3.2. Effects of Flow Rate and Temperature. The NH3 decomposition reaction (eq 2) is an endothermic reaction:
Figure 3. Fractional conversion of NH3 against the partial pressure of steam (PH2O) at 873 K.
The catalyst deactivation can occur due to the sintering of Ni-loaded particles. Therefore, the size of Ni crystallite (DNi) was analyzed by X-ray diffractometer (XRD), and then DNi was calculated using the Scherrer equation. DNi for the prepared catalyst was 26.7 nm, while DNi for the catalyst after the decomposition test (873 K, PH2O = 80 kPa) was 27.9 nm. Thus, we considered that the sintering of Ni particles did not cause the catalyst deactivation. To obtain further understanding of the deactivation mechanisms, we calculated the thermodynamic equilibrium of nickel on alumina under a wet-NH3 atmosphere, using the thermodynamic equilibrium calculation software FactSage (Ver. 5.2). Figure 4 shows the gas composition and
NH3 → 0.5N2 + 1.5H 2
(2)
ΔH ° = 46 kJ mol−1
Therefore, in principle, a higher temperature should result in a higher decomposition rate of NH3 over a Ni surface. Furthermore, the residence time of NH3 in the catalyst bed is also a key factor in a fixed-bed reactor. Thus, it is important to clarify the effects of the reactor operating conditions, e.g., flow rate and temperature, on the conversion of NH3. Figure 5 shows the NH3 conversion against dry- or wet-NH3 flow rates (0 or 80 kPa of steam partial pressure, respectively) at 873, 923, and 973 K. The value of this partial pressure corresponds to the vaporization of 20 vol % of an NH3 solution that is wellconcentrated, e.g., by membrane separation or NH3 stripping technique. The solid and dashed lines in this figure show the change in the conversion of NH3 with the NH3 flow rate [mL min−1 gcat−1] for dry-NH3 and wet-NH3, respectively, at each temperature. The results indicate that NH3 conversion under dry-NH3 increases with an increase in the temperature, and decreases as the flow rate increases. However, except in the case of 873 K, the conversions under wet-NH3 did not decrease monotonically as theflow rates increased. Although we cannot explain this behavior, it is possible that deactivation kinetics of catalysts against each flow rate were varied at 923 and 973 K. We will try to undertake the kinetic study of oxidation of Ni surface or formation of NiAl2O4 as our future work. The activities of the Ni/Al2O3 catalysts did not decrease change under flowing wet-NH3 with temperature or flow rate, whereas the fractional conversions at higher temperature were
Figure 4. Gas composition and NiAl2O4 ratio against NH3 conversion for wet-NH3.
NiAl2O4 ratio against NH3 conversion for wet-NH3 with a steam partial pressure of 80 kPa at 873 K. Note that the steam with partial pressures for 80 kPa does not cause the decrease in the equilibrium conversion of NH3. In other words, almost perfect decomposition is potentially achieved, even under wetNH3 decomposition. The partial pressure of wet-NH3 is 80 kPa, 17851
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in Al2O3 are required. We will attempt to explore this point in our future work.
4. CONCLUSION NH3 decomposition over Ni/Al2O3 was conducted by providing wet-NH3 into the catalyst bed at 873, 923, and 973 K at ambient pressure. Although Ni/Al2O3 was initially deactivated by steam for ca. 20 min, the catalyst subsequently showed almost the same activity. At 873 K, the catalytic activity was decreased to half the initial conversion value in the presence of steam with a partial pressure of 10−80 kPa. The conversion of wet-NH3 showed little dependency on the NH3 flow rate (750−1500 mL min−1 gcat−1) or temperature, whereas the conversion for dry-NH3 decreased as the flow rate increased. From the results of thermodynamic equilibrium calculations, we considered that the deactivation of Ni was due to the formation of NiAl2O4. We also evaluated the catalytic activities against temperature and flow rate under wet-NH3. The conversions at 923 and 973 K did not decreased monotonically as flow rates increased or temperature decreased. However, we succeeded in verifying the partial but stable decomposition of NH3 from NH4+-containing water using a Niloaded catalyst. However, we were not able to achieve complete decomposition of NH3 in wet-NH3 using Ni/Al2O3.
Figure 5. Ammonia conversion against flow rate for dry- or wet-NH3 decomposition. The temperatures are 873, 923, and 973 K, and PH2O = 0 or 80 kPa.
lower than that at 873 K. Figure 6 shows the fractional conversion of wet-NH3 against temperature for flow rates of
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 6. Fractional conversion against temperature for ammonia flow rates of 750, 1000, and 1500 mL min−1 g−1. The temperature was maintained at 873, 923, or 973 K, and the fractional conversion indicates the normalized conversion after providing wet-NH3 for 1 h.
750, 1000, and 1500 mL min−1 gcat−1. The fractional conversion is shown to decrease with temperature and increase at each flow rate. Thermodynamic calculations (not shown) indicated that the decomposition reaction of NiAl2O4, which is described as the reverse reaction of eq 1, progressed as the temperature increased. Even though less NiAl2O4 is generated at higher temperatures, the fractional conversion decreased as the temperature increased (Figure 6). We cannot explain this behavior; however, it is possible that an increase in the diffusion distance of Ni and O atoms with the increasing temperature resulted in an enhancement of NiAl2O4 formation, and therefore decreased the observed fractional conversion. To determine the effects of temperature on the rate of NiAl2O4 formation, the correct diffusion coefficients of Ni and O atoms 17852
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(12) Vite, F.; Cooper, M.; Botte, G. G. On the use of ammonia electrolysis for hydrogen production. J. Power Sources 2005, 142, 18− 26. (13) Boggs, B. K.; Botte, G. G. On-board hydrogen storage and production: An application of ammonia electrolysis. J. Power Sources 2009, 192, 573−581. (14) Endo, K.; Nakamura, K.; Katayama, Y.; Miura, T. Pt-Me (Me = Ir, Ru, Ni) binary alloys as an ammonia oxidation anode. Electrochem. Acta 2004, 49, 2503−2509.
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