Plasma-catalytic decomposition of nitrous oxide over γ-alumina-supported metal oxides

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Plasma-catalytic decomposition of nitrous oxide over γ-alumina-supported metal oxides Jin-Oh Joa, Quang Hung Trinha, Seong H. Kimb, Young Sun Moka,b, a b

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Department of Chemical and Biological Engineering, Jeju National University, Jeju 690-756, Korea Department of Chemical Engineering, Pennsylvania State University,University Park, PA 16802, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Plasma-catalytic Nitrous oxide Decomposition Metal oxides

This work investigated the decomposition of dilute N2O from gas streams with various oxygen contents by using a plasma-catalytic process over metal oxide catalysts supported on γ-Al2O3. Among the metals explored (Ru, Co, Cu, V, etc.), Ru was found to be the best catalyst for the decomposition of N2O in a plasma-catalytic reactor, and most of the experiments were conducted with alumina-supported Ru. The effects of applied voltage, reaction temperature, O2 content, gas flow rate and initial N2O content on the decomposition efficiency and byproducts formation were examined. Compared to the catalyst-alone case, the presence of plasma enhanced the decomposition efficiency by 30–50%, depending on the operating condition. The increase in the oxygen content from 0 to 20% largely decreased the catalytic decomposition efficiency, whereas in the presence of plasma N2O could be successfully decomposed even at 20% O2 content. The decomposition efficiency was not a strong function of the initial N2O concentration in the range of 225–1800 ppm, exhibiting pseudo first-order reaction kinetics. Without O2, there were negligible byproducts, but in the presence of O2, NO and NO2 were formed mainly due to the plasma-induced reactions between background molecules such as N2 and O2. The results obtained in this work showed the feasibility of plasma-catalytic process for the abatement of N2O.

1. Introduction Nitrous oxide (N2O) emitted from various human activities including agriculture, industrial processes, fossil fuel combustion and wastewater management is one of the major non-CO2 greenhouse gases. It is estimated to have a global warming potential (GWP) ∼300 times that of CO2 for a 100-year time horizon [1]. The technological options to reduce N2O emissions are generally based on catalysis, even though thermal destruction techniques are also available in cases that N2O is highly concentrated (e.g., adipic acid plants). When the concentration of N2O is low in the off-gas stream as in a nitric acid plant, non-selective catalytic reduction (NSCR) can be a measure. While NSCR is known to be effective in reducing N2O emissions by ∼90%, additional fuel is required for this process, increasing the operation cost [2]. At present, it appears that there is no commercially available technology developed to reduce N2O emissions from stationary and mobile combustions, indicating that more research is needed to develop cost-effective and reliable technologies. Recently, it has been reported that catalytic processes coupled with non-thermal plasma can be promising in treating a variety of air contaminants such as volatile organic compounds (VOCs), nitrogen

⁎

oxides (NOx) and halogenated hydrocarbons [3–7]. The same principles of plasma-catalysis that works for the treatment of VOCs, NOx and halocarbons can also apply in the decomposition of N2O, but it has been relatively less investigated [8,9]. Schmidt-Szałowski et al. [8] applied the gliding arc plasma combined with a catalytic bed to N2O processing. They found that in the presence of oxygen N2O is either oxidized to NO or decomposed to O2 and N2 by gliding arc plasma and several aluminasupported metal oxide catalysts (CuO, Fe2O3, etc.) combined with plasma catalyze N2O conversion into O2 and N2. Lee and Kim [9] investigated the argon plasma coupled with Ru/alumina catalyst for the decomposition of N2O, and they reported that argon plasma could break the chemically stable N2O bonds to form NO, N and O radicals even at low temperatures and the plasma-generating species could be more reactive on the active sites of the catalyst than in the gas phase. Since their experiments were performed under no-oxygen condition, however, it was not explained how oxygen plays a role in the N2O decomposition. Moreover, the results obtained with N2O-Ar mixture may not be realistic because the emissions of N2O in real situations are not likely in argon atmosphere. Meanwhile, in traditional thermal catalysis, the catalytic activity largely depends on the type of metal oxide catalyst. According to Taniou [10], Ru, Pd and Rh oxides are

Corresponding author at: Department of Chemical and Biological Engineering, Jeju National University, Jeju 690-756, Korea. E-mail address: [email protected] (Y.S. Mok).

http://dx.doi.org/10.1016/j.cattod.2017.05.028 Received 10 January 2017; Received in revised form 29 March 2017; Accepted 5 May 2017 0920-5861/ © 2017 Published by Elsevier B.V.

Please cite this article as: Jo, J.-O., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.05.028

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highly active for the N2O decomposition, while Fe, Ni, Cu and In oxides exhibit relatively poor activities. Likewise, the plasma-catalytic activity for the N2O decomposition can also be affected by the type of catalyst, which needs to be examined under the influence of plasma. As well, when a catalyst is used, the N2O decomposition efficiency can be very sensitive to O2 content [11,12]. In this work, the plasma-catalytic decomposition of N2O was investigated over γ-alumina-supported metal oxide catalysts, giving the first consideration to RuO2/γ-alumina through a screening test. A one-stage plasma-catalytic reactor in which the catalyst pellets were in direct contact with alternating-current (AC) driven plasma was employed for this work. The inhibitory effect of the oxygen content in N2O2-N2O mixture on the N2O decomposition was examined in the range up to 20% by volume. The process variables evaluated in this work included applied voltage, feed gas flow rate, inlet N2O concentration and reaction temperature.

catalytic reactor is for the reduction of NOx (NO and NO2). The amount of the Ag/γ-Al2O3 catalyst for NOx reduction was 7.5 g. The N2O concentration was analyzed by a Fourier transform infrared spectrophotometer (FTIR-7600, Lambda Scientific, Australia) equipped with a 16 cm long homemade gas cell. For the FTIR measurements, the resolution was set to 1.0 cm−1, and ten IR spectra were taken at a given experimental condition to take an average. The concentrations of NOx (NO and NO2) formed as byproducts were measured by a NOx analyzer (rbr-ecom KD, rbr-Computertechnik GmbH, Germany). The applied voltage across the plasma-catalytic reactor was measured with a digital oscilloscope (DPO3034, Tektronix, USA) and a 1000:1 high voltage probe (P6015, Tektronix, USA). The discharge power was determined by the so-called Lissajous charge-voltage figure [15]. The discharge power values at different reaction temperatures are presented as a function of applied voltage in Fig. 2.

2. Experimental section

3. Results and discussion

2.1. Catalyst preparation

3.1. Plasma-catalytic activities of metal oxides

The supported catalysts were prepared by impregnating γ-Al2O3 pellets with aqueous solutions of metal precursors. As an example, for the preparation of Ru/γ-Al2O3, a given amount of γ-Al2O3 pellets (length: 3.2–3.5 mm; diameter: 3.2 mm; Alfa Aesar, USA) was impregnated with an aqueous solution of RuCl3 (Sigma-Aldrich, USA). The Brunauer-Emmett- Teller (BET) specific surface area of the γ-Al2O3 pellets was measured to be 216.5 m2 g−1 by a surface area and pore size analyzer (Autosorb-1-mp, USA). After the impregnation, drying overnight at 110 °C and calcining at 550 °C for 6 h in air atmosphere were performed. A series of other supported catalysts (Ag/γ-Al2O3, Ce/γAl2O3, Co/γ-Al2O3, Cu/γ-Al2O3, Fe/γ-Al2O3, Ni/γ-Al2O3, V/γ-Al2O3) were also prepared in the same way by impregnating the γ-Al2O3 pellets with the aqueous solutions of AgNO3 (99.8%, Daejung, Korea), Ce(NO3) 3·6H2O (99%, Yakuri Pure Chemicals Co., Japan), CoN2O6·6H2O (98%, Sigma-Aldrich), Cu(NO3) 2·3H2O (99%, Daejung, Korea), Fe (NO3)3·6H2O (99%, Daejung, Korea), NiN2O6·6H2O (99%, Daejung, Korea) and NH4VO3 (99%, Junsei, Japan). When calcination is carried out at 550 °C for 6 h under the air atmosphere after the impregnation and drying, nitrate, ammonium and chlorine are all thermally decomposed and released [10,13,14]. The metal loading in the prepared catalysts was 2.0 wt%.

The effect of several γ-alumina-supported metal oxide catalysts on the plasma-catalytic decomposition of N2O is shown in Fig. 3. The decomposition was carried out in the presence of 10% O2 at applied voltages in the range of 13.2–25.6 kV. When high voltage was not applied, i.e., at 0 kV, the catalyst separately exerted action on the decomposition of N2O. Generally, the N2O decomposition efficiency increased with increasing the voltage for all the catalysts. This is because more reactive species were generated at a higher voltage. When plasma is created, a variety of reactive species are formed by processes such as excitation, ionization and dissociation, including excited N2 molecules, N radicals and N2 ions. These species decompose N2O as follows (see Refs. [16–19] for reactions (1)–(4) respectively):

N2 (A3Σ+u ) + N2 O→ 2N2 + O, k1 = 3.97 × 1012cm3mol−1s−1

(1)

N2 (a′1Σ−u ) + N2 O→ products, k2 = ×1014cm3mol−1s−1

(2)

N( 2D)

(3)

+ N2 O→ N2 + NO, k3 = 1.32 ×

1012cm3mol−1s−1

N+2 + N2 O→ N2 + N2 O+, k 4 = 3.61 × 1014cm3mol−1s−1

(4)

where the rate constants in reactions (1)–(4) are at 298 K. The N2O decomposition efficiency largely depended on the type of catalyst used. As seen, Ru, Cu and Ce supported on γ-alumina exhibited catalytic activity for the N2O decomposition, among which the decomposition efficiency with Ru/γ-alumina was the highest. At a voltage of 25.6 kV, the N2O decomposition efficiency with Ru/γ-alumina was enhanced by approximately 40% in comparison with the bare γ-alumina case. On the other hand, Ag, Co, Fe, Ni and V oxides supported on γ-alumina showed no or negative catalytic activity under plasma discharge condition, probably due to reversible adsorption and desorption of O2 onto the active sites [12]. According to the literature [20], Ru in a valence state more than 4 exhibits a tendency for tetrahedral coordination, and thus, Ru surface atoms, with a valence state ≥ 4 and tetrahedrally coordinated, can interact with N2O, changing their coordination. On Ru sites with high reactivity and unsaturated coordination, the following reaction can occur:

2.2. Experimental methods Fig. 1 shows the schematic of the present plasma-catalytic reactor comprising an alumina ceramic tube (inner diameter: 26 mm; thickness: 2 mm), catalyst pellets (26 g or 35 mL), a concentric threaded stainless steel rod (diameter: 6 mm) acting as the discharging electrode, and a ground electrode (aluminum foil; length: 80 mm) wrapping around the ceramic tube. The plasma-catalytic reactor was energized by an AC high voltage (400 Hz) in the range of 12–26 kV (peak value). As understood from Fig. 1, plasma was generated in the catalyst-packed bed, so that the plasma was in direct contact with the catalyst pellets. The feed gas was prepared by mixing O2, N2 and N2O whose flow rates were controlled using mass flow controllers (AFC 500, Atovac, Korea). The typical O2 and N2O contents in the feed gas were 10% by volume and 450 ppm (parts per million, volumetric). The effect of the oxygen content was examined in the range up to 20%, while that of the N2O concentration in the range of 225–1800 ppm. The flow rate of the feed gas was typically 1.0 L min−1, and varied from 0.5 to 2.0 L min−1. So as to investigate the effect of reaction temperature on the N2O decomposition, the plasma-catalytic reactor was installed in a temperature-controlled tube furnace. The reaction temperature was changed from 200 to 350 °C. Plasma discharge in N2-O2 mixture can generate NO and NO2. The catalyst bed (Ag/γ-Al2O3) right after the plasma-

RuO2 + N2O → RuO3 + N2

(5)

RuO3 and RuO2 are in equilibrium: 2RuO3 = 2RuO2 + O2

(6)

Further reaction to decompose N2O can occur by RuO3 as follows: RuO3 + N2O → RuO2 + N2 + O2

(7)

This reaction evolves oxygen, thereby regenerating the active sites. In 2

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Fig. 1. Schematic diagram of the experimental apparatus.

Fig. 2. Dependence of discharge power on the applied voltage at different temperatures.

Fig. 3. Effect of the type of metal oxide catalysts on the plasma-catalytic decomposition of N2O (O2 content: 10%; flow rate: 1 L min−1; inlet N2O: 450 ppm; temperature: 300 °C).

6+

this manner, N2O acts as a reducing agent for Ru sites and as an oxidizing agent for Ru4+. The results in Fig. 3 suggest that Ru/γalumina is the best catalyst for the plasma-catalytic decomposition of N2O, and further experiments were conducted with this catalyst.

oxygen content was changed from 0 to 20%. Similarly, the plasmacatalytic decomposition efficiency decreased with increasing the O2 content, but the extent of decrease was not as prominent as in the catalyst-alone case. The decomposition of N2O over metal oxide catalysts can be expressed as a Langmuir–Hinshelwood mechanism [10,12]:

3.2. Effect of operating variables Fig. 4 shows the effect of O2 content on the N2O decomposition, where the results obtained in the absence of plasma (catalyst alone) are compared with those in the presence of plasma (voltage: 25.6 kV). In both cases, i.e., with and without plasma, the N2O decomposition efficiency tended to decrease with increasing the O2 content. This result reveals that N2O molecules compete with O2 molecules for the same active sites of the catalyst and the adsorption of O2 onto the active sites could inhibit the decomposition of N2O. In the absence of plasma, the decomposition efficiency drastically decreased from 59 to 9% as the

N2 O+ (*) → 2N2 + *O

(8)

1 O2 + (*) 2

(9)

* O→

where * symbol stands for an active site of the catalyst. In this mechanism, the adsorbed surface oxygen (*O) migrates from one active site to another to form O2 by recombination, which is known to be the rate-determining step [10]. In order for the catalyst to recover its 3

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Fig. 4. Effect of the O2 content on the N2O decomposition (Voltage: 25.6 kV; flow rate: 1 L min−1; inlet N2O: 450 ppm; temperature: 300 °C).

Fig. 6. Dependence of the N2O decomposition efficiency on the reaction temperatures (O2 content: 10%; flow rate: 1 L min−1; inlet N2O: 450 ppm).

activity for the N2O decomposition, the adsorbed surface oxygen (*O) should desorb as molecular O2 into gas phase. Conversely, it is natural that the presence of O2 in the gas phase should decrease the N2O decomposition. The inlet N2O concentration can be one of the most influencing factors in the plasma-catalytic process. The effect of the inlet N2O concentration on the decomposition efficiency is presented in Fig. 5. The inlet N2O concentration was changed from 225 to 1800 ppm with the other parameters kept constant. As can be seen, the decomposition efficiency did not largely depend on the inlet N2O concentration, even though it was varied in a wide range. According to Li et al. [21], the decomposition of N2O over Ru/Al2O3 catalyst can be expressed as a first order kinetics with respect to N2O concentration, i.e., the decomposition efficiency is not a function of initial concentration, which agrees well with the result observed in Fig. 5 on the whole. One interesting observation in Fig. 5 is that the N2O decomposition efficiencies without plasma (0 kV) slightly decreased with increasing the inlet concentra-

tion, whereas those with plasma at voltages higher than 21.4 kV were reversed. As in reactions (1) and (3) above, the decomposition of N2O by plasma produces O radical and NO, and these species may serve as scavengers of the adsorbed surface oxygen (*O), regenerating the active sites of the catalyst. N2O itself can also remove the adsorbed surface oxygen. The relevant reactions are as follows (see Refs. [22] and [10] for reactions. (11) and (12) respectively):

O+ * O→ O2 + (*)

(10)

NO + *O → NO2 + (*)

(11)

N2 O + *O → N2 + *O2

(12)

Reactions (10)–(12) may elucidate why higher inlet N2O concentrations led to higher decomposition efficiencies. Fig. 6 shows the dependence of the N2O decomposition efficiency on the reaction temperatures ranging from 200 to 350 °C. In all cases, the plasma-catalysis resulted in better N2O decomposition efficiencies than the thermal catalysis (0 kV), because in the combined plasma-catalytic process both plasma and catalysis contributed to the decomposition of N2O. Moreover, the combination of plasma with catalysis often results in a synergistic effect [3]. For example, at a temperature of 300 °C, the catalyst-alone and plasma-alone (at 25.6 kV) decomposition efficiencies were 22% and 37%, respectively, which adds up to 59%, while the plasma-catalytic decomposition efficiency at the same voltage was 75%, revealing that there was a synergy of plasma and Ru/Al2O3 catalyst in the decomposition of N2O. Meanwhile, the differences between the plasma-catalytic and thermal catalytic decomposition efficiencies were getting decreased with increasing the reaction temperature, which indicates that the plasma-catalytic process has its effectiveness at low temperatures where the catalytic activity is necessarily low. Generally, feed gas flow rate or residence time plays an important role in chemical reactions. In this work, the effect of residence time was examined by varying the feed gas flow rate from 0.5 to 2.0 L min−1, and the results are presented in Fig. 7. It can be noticed that at lower voltages the increase in the gas flow rate largely decreased the N2O decomposition efficiency obviously due to decreased reaction time, but at higher voltages the N2O decomposition efficiency was less sensitive to the changes in the gas flow rate. This result is because the contribution of plasma to the N2O decomposition was getting influential as the voltage was increased. Note that the rates of plasma-induced reactions depend mainly on the electrical power delivered to the

Fig. 5. Effect of the inlet N2O concentration on the decomposition efficiency (O2 content: 10%; flow rate: 1 L min−1; temperature: 300 °C).

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Fig. 9. Equilibrium conversion of N2O decomposition reaction (O2 content: 10%; initial N2O: 200 ppm).

Fig. 7. Effect of the feed gas flow rate (O2 content: 10%; inlet N2O: 450 ppm; temperature: 300 °C).

reactor. The highest decomposition efficiency was observed at 0.5 L min−1, where the decomposition efficiency was 87% at 25.6 kV. When the gas flow rate was 4 times increased to 2.0 L min−1, the decomposition efficiency obtained at the same voltage was 63%. 3.3. Byproducts formation Fig. 8 shows the FTIR spectra of the effluents processed at different plasma strengths (applied voltages). As can be seen, the absorption bands related to NO and NO2 grew as the voltage was increased from 13.2 to 25.6 kV. In these spectra, no N-containing byproducts other than NO and NO2 were observed. It is believed that these NO and NO2 mostly resulted from plasma-induced reactions between N2 and O2, rather than from the plasma-catalytic decomposition of N2O. The formations of NO and NO2 by plasma are initiated by electron-molecule collisions [23–25]:

e+ O2 → e+ O+ O

(13)

e+ N2 → e+ N+ N

(14)

O+ N2 → NO + N

(15)

N+ O2 → NO + O

(16)

NO + oxidative secies(O, O3) → NO2

(17)

where e stands for a high-energy electron created by plasma. As will be shown below, thermodynamic calculations suggest that N2O is mainly decomposed into N2, not NO. Fig. 9 presents the equilibrium conversion of N2O decomposition reaction. The thermodynamic data for the calculation of equilibrium constant such as standard enthalpy of formation, Gibbs free energy of formation and heat capacity are tabulated in Table 1 [26,27]. Two different decomposition reactions can be considered:

N2 O= N2 + 0.5O2

(18)

N2 O+ 0.5O2 = 2NO

(19)

The equilibrium constants for the above reactions are determined from the thermodynamic data given in Table 1 [28]. As in Fig. 9, the equilibrium conversion for reaction (18) is nearly 100% in the entire temperature range. Reaction (19) is an endothermic reaction that is not thermodynamically favored. When reaction (19) is considered, the equilibrium conversion is calculated to be negligible below 500 K and Table 1 o o Standard enthalpy of formation (ΔH 298 ), Gibbs free energy of formation (ΔG298 ) and heat capacity (CP) of relevant components [26,27]. CP (J/mok·K) o ΔH 298 (kJ/mol)

o ΔG298 (kJ/mol)

N2

0

0

O2

0

0

NO

90.4

86.6

N2O

81.6

103.8

28.99 + 1.85T-9.65T2 + 16.64T3

31.32 − 20.24T + 57.87T2 − 36.51T3 23.83 + 12.59T-1.14T2 − 1.50T3 + 0.21/T2 27.68 + 51.15T-30.64T2 + 6.85T3 − 0.16/T2 Fig. 8. FTIR spectra of the effluents processed at different plasma strengths (O2 content: 10%; flow rate: 1 L min−1; inlet N2O: 450 ppm; temperature: 300 °C).

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not substantial even at higher temperatures, indicating that the formation of NO is primarily caused by the aforementioned plasmainduced reactions between N2 and O2, not from the decomposition of N2O. The concentrations of NO and NO2 formed in the plasma-catalytic reactor are shown in Fig. 10. As seen, more NO and NO2 were formed at higher voltages and higher oxygen content, because the plasma-induced reactions between N2 and O2 took place more actively. Selective catalytic reduction (SCR) systems are typical NOx (NO and NO2) removal method. For SCR of NOx, ammonia, urea and hydrocarbons can be used as a reducing agent. Hydrocarbon-SCR (HC-SCR) uses hydrocarbon as the reducing agent [29]. Fig. 11 shows the NOx reduction results obtained using n-heptane as the reducing agent. The n-heptane/NOx ratio was adjusted to 0.86 (C/N ratio = 6:1). The Ag/γAl2O3 is one of the most common catalysts used in the HC-SCR reaction [30–32]. As shown above (Fig. 10), different voltages produced different concentrations of NO and NO2. In Fig. 11, the horizontal axis values are the voltages applied to the plasma-catalytic reactor. As seen, all the NOx formed in the plasma-catalytic reactor was totally removed by the SCR at 300 °C in the presence of n-heptane reducing agent. 4. Conclusions The abatement of N2O has been performed by a plasma-catalysis combined system over several metal oxide catalysts supported on γAl2O3. Different parameters such as applied voltage, type of catalyst, oxygen content, inlet N2O concentration, gas flow rate, reaction temperature have been examined. The main conclusions are as follows. The N2O decomposition efficiency increased with increasing the applied voltage because of enhanced generation of reactive species. Without plasma, the catalytic N2O decomposition efficiency greatly decreased with increasing the O2 content, but in the presence of plasma the effect of oxygen content was not as significant as in the catalystalone case. The plasma-catalytic N2O decomposition efficiency was not a strong function of inlet N2O concentration. Increasing the gas flow rate had a great influence on the N2O decomposition rate due to decreased reaction time, but as the applied voltage was increased, the N2O decomposition became less sensitive to the changes in the gas flow rate. In the presence of O2, the N2O decomposition produced NOx because of the plasma-induced reactions between N2 and O2. The present plasma-catalytic reactor followed by a NOx reduction catalyst was able to totally remove the byproducts such as NO and NO2 formed.

Fig. 10. Concentrations of NO and NO2 formed in the plasma-catalytic reactor at different O2 contents (flow rate: 1 L min−1; inlet N2O: 450 ppm; temperature: 300 °C).

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Fig. 11. NOx concentrations before and after the selective catalytic reduction over Ag/γAl2O3 (O2 content: 10%; flow rate: 1 L min−1; inlet N2O: 450 ppm; temperature: 300 °C).

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