Catalytic Decomposition of N₂O over Cu–Zn/ZnAl₂O₄ Catalysts

Abstract

The catalytic decomposition of N₂O was investigated over Cu-Zn/ZnAl₂O₄ catalysts in the temperature range of 400–650 °C Catalytic samples have been prepared by wet impregnation method. Prepared catalysts were characterized using several techniques like BET surface area, X-ray diffraction (XRD), and Scanning electron microscopy (SEM). The Cu-Zn/ZnAl₂O₄ showed higher catalytic performance along with long term stability during N₂O decomposition. The Cu-Zn/ZnAl₂O₄ catalysts yielded 100% N₂O conversion at 650 °C. The Cu-Zn/ZnAl₂O₄ catalysts are promising for decrease this strong greenhouse gas in the chemical industry.

1. Introduction

Global warming is focus of intensive concern worldwide. Nitrous oxide (N₂O) is an invisible, non-poisonous gas that has been identified as a potential contributor for ozone destruction in the stratosphere over the past decade and recognized as a rather strong greenhouse gas [1]. N₂O has a lifetime of 114–130 years under atmospheric conditions, and its global warming potential (GWP) is approximately 310 times higher than that of carbon dioxide (CO₂) [2]. Now, the concentration of N₂O in the atmosphere is still rising. The emission of N₂O comes from both natural sources and human contributions. Natural emissions include terrestrial, marine, and atmospheric sources. The anthropogenic sources include mainly the biological transformation of fertilizer’s nitrogen into N₂O (agriculture), biomass burning, combustion of fossil fuels, industrial activities, wastewater treatment, aquaculture and the use of solvents. The continuous increase of N₂O concentration in the atmosphere is mainly due to human activities such as adipic acid production, nitric acid production, fuel and biomass combustion, etc. [3,4]. For these reasons, researchers have paid a great deal of attention to N₂O because of its possible environmental impacts. There are many methods that can be used to reduce the emission concentration of N₂O, such as selective catalytic reduction with hydrocarbons (HC-SCR), thermal decomposition, selective adsorption, and direct catalytic N₂O decomposition into N₂ and O₂ [5]. Among various types of abatement technologies, direct catalytic decomposition is economical and does not produce CO₂, and becomes one of the most attracting methods. The catalytic decomposition of N₂O (up to 450 °C) provides an attractive solution for reducing N₂O emissions in tail gas from chemical processes. It is the most qualified method to reduce N2O emissions because it does not require the addition of reducing agents, and it does not produce harmful by-products.

A large number of catalysts, such as pure oxides, noble metals, mixed oxides, and iron zeolites have been evaluated for N₂O decomposition reaction [6,7,8,9,10]. Among them, noble-metal catalysts exhibit satisfactory activity at intermediate temperatures [11,12]. However, their high cost represents an important obstacle towards practical applications. Most zeolite-based catalysts can be largely deactivated in the presence of water vapor and excess oxygen and are very poor in practical applications. Considering lower economic cost and excellent catalytic performance, copper based materials has a great potential for industrial applications. It was reported that Cs-doped CuO, YBa₂Cu₃O₇, Nd₂CuO₄ and Cu-containing zeolites were active for N₂O decomposition [13,14,15,16]. Zinc aluminate (ZnAl₂O₄) has high mechanical resistance, high thermal stability, or inertness to water vapor, making it an attractive material for the catalyst and a carrier for active metals instead of the more traditional systems. High temperature calcination of Al₂O₃-supported metal nitrates is a convenient method to form spinel phase [17,18].

In this study, Cu-Zn/ZnAl₂O₄ catalysts were prepared. Due to the use of cheap materials and simple preparation process, the manufacturing cost of Cu-Zn/ZnAl₂O₄ catalysts is low. Their activity and stability for decomposition of N₂O are tested in the presence of oxygen. The expected results will be beneficial to industrial researchers in studying the catalytic decomposition of N₂O.

2. Results and Discussion

2.1. Effect of Mixed Acid Ratio on N₂O Decomposition

Figure 1 shows the effects of the different mixed acid ratio on the activity of Cu-Zn/ZnAl₂O₄ in N₂O decomposition. It can be found that the use of inorganic and organic mixed acid solution as a solvent had certain effect on improving the catalytic activity of the catalyst. When the ratio of nitric acid to acetic acid was 1:2, the catalyst had a relatively high catalytic activity. Thus, the optimal ratio of nitric acid and acetic acid was 1:2 for preparing Cu-Zn/ZnAl₂O₄.

2.2. Effect of Total Acid and Pseudo-Boehmite Ratio for N₂O Decomposition

Figure 2 shows the N₂O decomposition performance over the different total acid and pseudo-boehmite ratio catalysts. It was clearly seen that under the same acetic acid and nitric acid ratio, the total acid and pseudo-boehmite ratio had little effect on the catalytic decomposition activity of the catalyst N₂O. Taking into account the economic reasons, the ratio of total acid and pseudo-boehmite was 0.06 that seemed appropriate for producing Cu-Zn/ZnAl₂O₄.

2.3. Effect of Cu-Zn Loading Amount on N₂O Decomposition

To investigate the effect of Cu-Zn loading amount on N₂O decomposition, several Cu-Zn/ZnAl₂O₄ catalysts with different Cu-Zn loading amount were prepared. Figure 3 shows the catalytic performance results of N₂O decomposition over the Cu-Zn/ZnAl₂O₄ with different Cu-Zn loading amount. As showed in Figure 3 below 450 °C, with the increase of Cu-Zn loading amount, the activity of the catalyst was gradually increased in the range of 30–45 wt %. Above 450 °C, addition of Cu-Zn loading amount increased the catalytic activity and then the activity decreased when Cu-Zn loading amount was increased to 45 wt %. The Cu-Zn loading amount has a more significant effect on catalytic activity above 450 °C. The result may suggest that suitable Cu-Zn loading amount is 35–40 wt %. It was recommended to use amount, which can save raw materials and reduce costs. Hence, this 35 wt % Cu-Zn loading amount was chosen as optimum for N₂O decomposition, and further studies were carried out using this Cu-Zn loading amount.

2.4. Characterization of the Catalyst

The XRD patterns of the fresh Cu-Zn/ZnAl₂O₄ and used Cu-Zn/ZnAl₂O₄ catalysts are shown in Figure 4 and Figure 5. Individual phases found in the samples were marked in the graph. The fresh and used catalyst exhibited a crystalline nature with dominating ZnAl₂O₄ phase. In the XRD pattern of the fresh catalyst, ZnO phase (JCPDS Card No. 36-1451) was observed. The absence of any peaks belonging to the copper phase can be explained by considering the fact that the concentration of CuO particles was low and well dispersed on the surface of the catalyst. In the XRD pattern of the used catalyst, no diffraction peaks of ZnO were observed, most likely reason was the low concentration of ZnO after N₂O decomposition reaction.

The BET specific surface area values of the fresh and used catalyst are reported in

Table 1. BET area and pore diameter of the fresh and used catalyst.

Catalyst	BET Area (m2·g−1)	Pore Diameter (nm)
fresh	81.77	77.0
used	94.20	75.4

. As observed, the mean pore diameter of the used catalysts was lower than that of the fresh catalysts. However, the specific surface area of the used catalysts was larger that of the fresh catalysts. The main cause of this change was that the porosity was developed after catalytic reaction. Furthermore, the surface areas (81.77–94.20 m²·g^–1) obtained before and after the catalytic tests were not notably changed, indicating that the catalyst had good structural stability.

Figure 6 shows the surface morphology of the fresh and used catalyst. The fresh catalyst showed rough surface morphology with inhomogeneous small particles. The used catalyst had a low surface roughness, which was possibly due to gas flushing during the catalytic reaction.

2.5. Catalytic Performance

The catalytic activity result for the Cu-Zn/ZnAl₂O₄ catalysts is presented in Figure 7. As shown in Figure 7, no obvious reaction was observed over the Cu-Zn/ZnAl₂O₄ catalysts until about 400 °C, and they started to show some activities above 450 °C. The temperature needs to be over 610 °C in order to reach 90% N₂O conversion (T₉₀).

The stability of catalysts is important in determining their practical usefulness. The catalytic stability of the Cu-Zn/ZnAl₂O₄ catalyst for N₂O decomposition is shown in Figure 8. These tests were carried out at T = 650 °C. In model atmosphere (8.1 vol % N₂O, 10.2 vol % O₂, N₂ balance), N₂O conversion over the Cu-Zn/ZnAl₂O₄ catalyst after reaction at T = 650 °C for 500 h keeps at 99%, indicating that the catalyst has high activity and stability.

3. Experimental

3.1. Supports Preparation

The supports used in this study were prepared by the kneading method. Pseudo-boehmite powder and SiO₂ powder were thoroughly mixed with the inorganic binder, sesbania powder. Next, a mixed solution of nitric acid and acetic acid was added dropwise to the mixture in order to make slurry. The mixture was kneaded to best plastic state, then extruded, calcined at 550 °C for 4 h and shaped.

3.2. Preparation of Cu-Zn/ZnAl₂O₄ Catalysts

The supported Cu-Zn catalysts were prepared using an incipient wetness impregnation method. In a typical impregnation process, Zn (NO₃)₂ 6H₂O, Cu(NO₃)₂ 3H₂O, Ni(NO₃)₂ 6H₂O and additives were dissolved in distilled water. The catalyst carrier was wetted with a small amount of water, and then the carrier was impregnated with the same volume in a mixed solution and kept at 60 °C for 24 h. After impregnation, each catalyst was dried in an oven at 100 °C for 10 h. The catalysts were then calcined at 800 °C for 6 h in air to obtain the Cu-Zn/ZnAl₂O₄ catalysts. A photograph of prepared catalyst pellets is given in Figure 9. The catalyst is cylindrical granule with diameter of 5 mm and length of 5–10 mm.

3.3. Catalysts Characterization

BET surface area, pore diameter and pore volume of the catalysts were determined by N₂ adsorption at −196 °C using a Micrometrics ASAP 2020 instrument (ASAP2020, Micromeritics, Norcross, GA, USA). XRD patterns of the catalysts were recorded using an X-ray diffractometer (XRD-7000, Shimadzu, Kyoto, Japan) in with Cu Kα radiation. The 2θ range was kept between 10 and 80 degrees with a scan speed of 6°·min⁻¹. Scanning electron microscope (SEM) (Quanta 400F, FEI Company, Hillsboro, OR, USA) was used to analyze the surface morphology of the catalyst.

3.4. Activity Tests

The evaluation of catalysts for N₂O catalytic decomposition was carried out using a fixed-bed stainless steel reactor with an inner diameter of 58 mm and 850 mm in length. The catalyst bed contained 60 mL of catalyst. The GHSV (gas hourly space velocity) was determined by dividing the standard gas flow rate by the catalyst bed volume. The reactant gas mixture (8.1 vol % N₂O, 10.2 vol % O₂, N₂ balance) was fed to the reactor. The composition simulates high concentration of N₂O in the industrial process. The total flow rate was 1200 mL·min⁻¹, which could be converted to a gas hourly space velocity (GHSV) of 1200 h⁻¹. The reaction products were analyzed by using an on-line gas SP-3420 gas chromatograph (GC) equipped with a thermal conductivity detector (TCD).

The N₂O conversion was calculated according to the following equation:

$${\mathrm{X}}_{N_{2}O}(\%)=\frac{(C_{N_2{O}_{in}}-C_{N_2{O}_{out}})}{C_{N_2{O}_{in}}}\times 100$$

(1)

where X_N2O was the percent conversion of N₂O; C_N2Oin and C_N2Oout were concentrations of N₂O (ppm) in the inlet and outlet, respectively.

A long-term stability test was was done at 650 °C and GHSV = 1200 h⁻¹.

4. Conclusions

In this work, Cu-Zn/ZnAl₂O₄ catalysts was prepared by high temperature calcination of Al₂O₃-supported metal nitrates and characterized by using XRD, SEM and N₂ adsorption-desorption. The Cu-Zn/ZnAl₂O₄ catalysts were used to reduce N₂O emission of simulated sources based on their activity and stability tests. Prepared catalysts were tested for high concentration N₂O catalytic decomposition. The results illustrate that Cu-Zn/ZnAl₂O₄ catalyst is quite effective for the catalytic decomposition of N₂O under model atmosphere. N₂O can be completely decomposed at 650 °C in oxygen atmosphere. No deactivation and good performance of the catalyst were confirmed by 500 h stability tests.