Ru is considered as an effective active species for N 2 O decomposition; however, there is disagreement about which ruthenium species is key for catalytic activity. In order to understand the role of Ru species in N 2 O decomposition, -air) catalysts with different ratios of metallic Ru were prepared and evaluated for their catalytic activities. Various characterizations, especially in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), were applied to investigate the

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... tionship between activity and different Ru species. The results indicate that the N 2 O conversion displayed a linear relationship with the amount of metallic Ru. The DRIFTS results of adsorption for N 2 O show that metallic Ru was the active site. The catalytic processes are put forward based on metallic Ru species. The deactivation with increasing times used is due to the decrease in the amount of metallic Ru and agglomerates of Ru particles on the surface of catalysts. Ru is an effective catalyst for N-O bond dissociation, so that it promotes N 2 O decomposition [24] [25] [26] [27] . Benco's [14] report confirmed the activation of the N 2 O molecule by calculation using DFT over Ru. It was found that the activation energy of Ru was much lower than that of Co and Fe. Kawi et al. [28] investigated Ru on the MCM-41, showing far high activity for N 2 O decomposition, and N 2 O conversion reached 100% at 400 • C. The catalytic activity of Ru metal was influenced by size, support, interaction and O 2 . Komvokis et al. [13] reported that Ru formed with sizes from 1-3 nm with high dispersion (70%), which showed much higher catalytic activity and stability for N 2 O decomposition. In the case of Rh catalysts [29] , Rh/MgO, Rh/SiO 2 with Rh sizes of 2.1-2.4 nm were more active than Rh/CeO 2 , Rh/Al 2 O 3 and Rh/TiO 2 with small particles of 1.0-1.4 nm for the N 2 O decomposition. For different supports [30] , the strength of interaction between Ru and support was concluded to be MgO > TiO 2 > Al 2 O 3 = SiO 2 . However, catalytic activity was as follows: Ru/TiO 2 > Ru/Al 2 O 3 > Ru/SiO 2 > Ru/MgO. Besides, the reducibility of the Ru metal particle was found to be a crucial factor. Komvokis reported that the catalytic activity of N 2 O decomposition was much higher by H 2 -pretreated catalyst (metallic Ru) because of the cyclic oxidation-reduction pathway of metallic Ru [31] . It was well known that desorption of O 2 represents the rate-determining step in N 2 O decomposition. The presence of O 2 has an inhibitory effect on the N 2 O decomposition and decreases reaction rates over supported noble metal catalysts [32] . Zhang et al. [30] reported that the sensitivity of noble metal to O 2 in feed gas was dependent on the reducibility of support material. It was concluded to be Ru/SiO 2 > Ru/Al 2 O 3 > Ru/TiO 2 > Ru/CeO 2 . In fact, the catalytic performance and deactivation of supported Ru catalysts is still a complex issue for N 2 O decomposition. In the present study, a series of Ru/Al 2 O 3 catalysts as model catalysts is prepared by using different pretreatment methods, and their catalytic activity and stability in the decomposition of N 2 O were investigated. Furthermore, the change of Ru species and the deactivation factor were suggested by combining various characterizations, including XPS, TEM, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Temperature-programmed desorption of N 2 O (N 2 O-TPD), etc. Meanwhile, we put forward the reaction process of N 2 O decomposition over Ru on the surface of Ru/Al 2 O 3 catalysts. Results Characterization of Prepared Samples The Ru/Al 2 O 3 , the five-times used Ru/Al 2 O 3 (after using Ru/Al 2 O 3 five-times) and Al 2 O 3 samples were studied by XRD in Figure 1 . All of the reflections of the Ru/Al 2 O 3 and five-times used Ru/Al 2 O 3 catalysts present in the diffractogram match well with the cubic spinel structure of γ-Al 2 O 3 (JCPDS PDF No. 10-0425) [33] . It was observed that neither metallic Ru nor RuO x species could be identified in the patterns of Ru/Al 2 O 3 and five-times used Ru/Al 2 O 3 catalysts, which indicated that ruthenium was represented by the mean of the high dispersion in these samples.