Catalytic Decomposition of Nitrogen Oxides by Bimetallic Catalysts Synthesized by Dielectric Barrier Discharge Plasma Technology

Nitrous oxide (N 2 O) is a common greenhouse gas and urgent need to be contained. Direct catalytic decomposition of N 2 O by high activity catalyst into N 2 and O 2 is a low-cost and harmless method. Bimetallic catalysts show good catalytic activity in many classes of reactions, and plasma technologies, applied to prepare of catalyst, are considered to be a promising method. In our contribution, DBD cold plasma is applied to synthesize Rhodium and Cobalt bimetallic catalysts for catalytic N 2 O decomposition. The influence of cobalt and rhodium content on N 2 O decomposition activity shows that the optimal amount of metal is determined as 5wt. % cobalt and 0.5wt. % rhodium loaded on Al 2 O 3 . The best working voltage is determined as 18kV. The results indicated that the Rh/Al 2 O 3 catalysts prepared by atmospheric-pressure DBD cold plasma showed smaller size and high dispersion of Rh particles, so that the metal-support interaction and the catalytic activity are enhanced. Atmospheric-pressure DBD cold plasma is proved to be an environmentally friendly and efficient method for preparing high performance Rhodium and Cobalt bimetallic catalysts for catalytic N 2 O decomposition.


Introduction
Nitrous oxide (N 2 O) is a common greenhouse gas and caused mainly by fossil fuel combustion and industrial production. This gas can lead to plenty of environmental problems like acid rain, photochemical smog and ozonelayer depletion. The concentration of N 2 O has increased at an annual rate of 0.2-0.3% since the industrial revolution [1][2][3]. Thus, it is quite necessary to search for the way to remove N 2 O.
In recent years, several ways for N 2 O removal are applied such as thermal decomposition [2], selective catalytic reduction [2,4] and direct catalytic decomposition [5,6]. Thermal decomposition requires for excessive temperature and energy consumption. Selective catalytic reduction need consume reducing gas, resulting in the increase of treatment costs. By comparison, direct catalytic decomposition of N2O into N 2 and O 2 is superior as this process is low-cost and harmless [7]. The catalyst for direct catalytic decomposition of N 2 O mainly includes zeolite, metal oxide and noble metal catalyst. Among them, the zeolitebased catalyst is easy to be sintered, and the metal oxide catalyst has poor thermal stability and poor activity. Rh is an ideal catalyst for N 2 O direct catalytic decomposition. It is very promising to improve Rh catalyst by changing catalyst morphology such as reduce the size of the loaded metal particles to improve dispersion. Studies have found that the incorporation of transition metals in Rh metal improves catalytic N 2 O direct catalytic decomposition activity.
At present, plasma technologies applied to prepare of catalyst are considered to be a promising approach. In general, three main trends in preparing catalysts using plasma technologies are attracted the attention of academia [8]: (1) ultrafine particle catalysts synthesis by plasma chemical; (2) plasma assisted deposition of catalytically active compounds on various carriers, especially plasma spraying for the preparation of supported catalysts; (3) plasma modification or plasma enhanced preparation of catalysts. Compared to conventional catalyst preparation, there are several merits of using plasmas: (1) reduced energy requirements; (2) a highly distributed active species; (3) enhanced catalyst activation, selectivity, and lifetime; (4) shortened preparation time.
Dielectric barrier discharge (DBD) is a simple and easily operated environmentally friendly approach for generating atmospheric-pressure cold plasma, and it can be used for preparing supported noble metals [9][10][11]. In our contribution, Rh-Co bimetallic catalyst is synthesized by means of atmospheric-pressure DBD cold plasma to catalytic decomposition of N2O.

Catalyst preparation
RhCl 3 and activated aluminium oxide are purchased from Sinopharm Chemical Reagent Co., Ltd. Cobalt nitrate, ferric nitrate and nickel nitrate are purchased from Tianjin Fuchen Chemical Reagent Co., Ltd. The pretreatment of Al 2 O 3 is calcined at 550ºC for 4h. All of supported metal catalysts are synthesized by wetness incipient impregnation. And then the precursor is dried at 110ºC for 2h and finally reduced by hydrogen at 450ºC for 2h or at DBD reactor of 18kV for 6min.
The schematic DBD cold plasma device for preparing catalysts at atmospheric pressure is shown in Scheme 1. The electrodes are stainless steel plates (50 mm in diameter). A reaction cell made of quartz is placed between the electrodes. The reaction cell consisted of two parts. The upper part was a quartz plate (70 mm in diameter, 1-mm thickness), while the lower part is a tank (75 mm in diameter, 9-mm height). The discharge gap was 8 mm. The power source (CTP-2000 K, Nanjing Suman Electronic Co., Ltd) is capable of supplying a bipolar sine wave output with 0-50 kV peak-to-peak voltage (Up-p) at the frequency of 10 kHz. Scheme 1 Schematic diagram of the atmospheric-pressure DBD cold plasma device (1-electrode, 2 -quartz glass, 3sample, 4 -cold plasma,)

N2O decomposition
The decomposition of N 2 O with the action of only catalysts is tested in a transparent quartz tube (6mm inner diameter) as the fixed-bed reactor. 0.3g catalyst is packed in the middle position of the tube and sealed by quartz wool both sides. A gas stream of 10% N 2 O-He flowed through the tube at a rate of 30mL/min. The gas from the outlet is analyzed by a gas chromatograph (GC, Fuli 9790) that can separate He, N 2 , O 2 and N 2 O. The reaction begins after a one-hour gas flow process at room temperature and each reaction temperature will be kept for 30min to measure the conversation. The conversion of N 2 O was calculated according to the equation (1): where X is the conversion of N 2 O, [N 2 O] initial is the peak area of N 2 O at room temperature and [N 2 O] reaction is the peak area of N 2 O at reaction temperature.

Characterization
XRD patterns are recorded on a SHIMADZU-6100 X-Ray Diffractometer using CuKα radiation at a scanning speed of 10ºC/min. TEM images are recorded on a FEI-G2F30.

Bimetallic catalysts activity investigation
Catalysts with different cobalt contents were studied. 1wt.%Co-0.5wt.%Rh/Al 2 O 3, 3wt.%Co-0.5wt.%Rh/Al 2 O 3, 5wt.%Co-0.5wt.%Rh/Al 2 O 3, catalysts were prepared by hydrogen reduction at 450ºC for 2h and ready for the decomposition reaction. The results of decomposition of N 2 O reaction over four different catalysts are shown in Fig. 2. Fig. 2 shows that the incorporation of Co increases the catalytic decomposition activity of the catalyst. As the amount of Co incorporation increases, the catalytic activity increases. As a result, 5wt.% is determined the superior cobalt content.

Bimetallic catalysts prepared by atmospheric-pressure DBD cold plasma activity investigation
In order to investigate the catalytic decomposition ability of the catalysts reducted by atmospheric-pressure DBD plamsa at different voltage conditions, 0.5wt.%Rh/Al 2 O 3 were prepared in different voltages. 9kV, 12kV , 15kV, 18kV were investigated, and the results of the N 2 O decomposition conversion are shown in Fig. 3. It indicate that the higher the reduction voltage, the better catalytic activity of the N 2 O decomposition. The catalyst prepared at 18kV shows the best activity, therefore, 18kV is selected as the ideal reduction condition. In the last set of experiment two catalysts preparing methods of high-temperature hydrogen reduction and atmospheric-pressure DBD cold plamsa hydrogen reduction were compared. 0.5wt.%Rh/Al 2 O 3 and 5wt.%Co-0.5wt.%Rh/Al 2 O 3 prepared by atmosphericpressure DBD cold plasma hydrogen reduction at 18kV (named 0.5wt.%Rh/Al 2 O 3 -P and 5wt.%Co-0.5wt.%Rh/Al 2 O 3 -P) were compared with 0.5wt.%Rh/Al 2 O 3 prepared by high-temperature hydrogen reduction (0.5wt.%Rh/Al 2 O 3 -C). The result shows in Fig.4.   TEM images of 0.5wt.%Rh/Al 2 O 3 -C are shown in A and B, and 0.5wt.%Rh/Al 2 O 3 -P are shown in C and D. The mean particle size of Rh in 0.5wt.%Rh/Al 2 O 3 -P was 3-5 nm, which was much smaller than that in 0.5wt.%Rh/Al 2 O 3 -C (about 10 nm). This confirmed that Rh particles were better dispersion for Rh/Al 2 O 3 prepared by atmospheric-pressure DBD cold plasma.
In summary, Rh-Co bimetallic catalyst can get the better catalytic activity than any other one of them, and atmospheric-pressure DBD cold plasma is proved to be an environmentally friendly and efficient method for preparing the catalysts of N 2 O decomposition.

Conclusion
High performance Rhodium and Cobalt bimetallic catalysts were prepared by DBD cold plasma at atmospheric pressure using the mixture of H2 and Ar as working gas to catalytic N 2 O decomposition. The influence of cobalt and rhodium content on N 2 O decomposition activity showed that the optimal amount of metal was 5wt.% cobalt and 0.5% rhodium loaded on Al 2 O 3 . The best working voltage was determined as 18kV. The results indicated that the Rh/Al 2 O 3 catalysts prepared by atmospheric-pressure DBD cold plasma showed smaller size and high dispersion of Rh particles, so that the metal-support interaction and the catalytic activity were enhanced.
Atmospheric-pressure DBD cold plasma was proved to be an environmentally friendly and efficient method for preparing high performance Rhodium and Cobalt bimetallic catalysts for catalytic N2O decomposition.