Direct Decomposition of NO on C-type Cubic Rare Earth Oxides Based on Y2O3

Abstract

C-type cubic (Y 0.99¹x Tb x Ba 0.01 ) 2 O 2.99+¤ (0¯x¯0.4) catalysts were prepared by coprecipitation. The NO decomposition activities at 1173 K were higher than that of (Gd 0.7 Y 0.26 -Ba 0.04 ) 2 O 2.96 reported previously. The highest catalytic activity was obtained for (Y 0.69 Tb 0.3 Ba 0.01 ) 2 O 2.99+¤ , and 100% NO conversion into N 2 and O 2 was realized at 1173 K on this catalyst. Furthermore, the conversion ratio was maintained as high as 70.8% even in the presence of 5 vol % oxygen.The emission of nitrogen oxides (NO x ) is strictly regulated, because they are not only harmful to human beings but also responsible for photochemical smog and acid rain. The NO x species in the exhaust gas emitted at high temperatures is principally thermodynamically stable NO and the amount of NO 2 is negligible. Accordingly, research should be focused on the NO decomposition in the case of catalytic NO x remediation.For the methods of NO removal, selective catalytic reduction processes using ammonia, urea, or hydrocarbons are the mainstream procedures.1 In these processes, the decomposition efficiency is sufficient, and the reaction process is stable at high temperatures. However, separate specialized equipment is necessary to supply the reducing agents and secure control is indispensable, because ammonia is both toxic and flammable.In contrast to the above methods that utilize selective catalytic reduction, direct decomposition of NO into N 2 and O 2 (2NO ¼ N 2 + O 2 ) is an appropriate route for NO removal, because this process is simple and reductants mentioned above are not required. A number of materials have been reported as active catalysts for direct NO decomposition, such as zeolites, 2 perovskites, 35 and other mixed or complex oxides. 614 However, the activity of conventional catalysts for NO decomposition decreases in the presence of oxygen, due to strong oxygen adsorption.In contrast, it was found that C-type cubic rare earth oxides based on Gd 2 O 3 , (Gd 1¹x¹y Y x Ba y ) 2 O 3¹y (0¯x¯0.26, 0¯y0.08), can exhibit higher activity for direct NO decomposition than that of the perovskite-type catalysts in the presence of oxygen, as reported in our previous study. 14,15 There are three types of crystal structure in rare earth oxide, namely, A-(hexagonal), B-(monoclinic), and C-type (cubic), according to the ionic size of the respective rare earth element. 16 The lattice volume of the rare earth oxides increases in the order of Atype < B-type < C-type, and for this reason the C-type structure possesses the largest interstitial open space suitable for NO decomposition among the three types.It has been generally accepted that oxide vacancies in the catalyst play an important role in the direct catalytic decomposition of NO. 37 Additional introduction of oxide anion vacancies by partial substitution of the trivalent rare earth cations with divalent Ba 2+ ions will enhance the catalytic activity. In some of conventional catalysts, it has been suggested that the removal of surface oxygen can be easily achieved when...

Synthesis of Highly Effective CeO x –MnO y –BaO Catalysts for Direct NO Decomposition

“…It should be noted that this temperature and the W/F value are much lower than those reported for other catalysts to obtain the same NO conversion [24][25][26][27][28] [28] catalysts were reported to exhibit *12%, *13%, \15%, *10%, and *5% NO conversion to N 2 , respectively, at 600°C under W/F of 3.0 g s mL -1 . On the other hand, ST-2(Ce 0.8 Mn 0.15 Ba 0.05 O x ) exhibited 45% NO conversion to N 2 under the same reaction conditions (Fig.…”

“…Catalysts, such as precious metals [12][13][14], single metal oxides [15,16], Cu-ZSM-5 [17][18][19], alkali-doped Co 3 O 4 [20,21], perovskites [22][23][24][25][26], C-type cubic rare earth oxides [27,28], and Ba 0.5 /BaY 2 O 4 [29], have been found to be effective for direct decomposition of NO. We found that Ba-Ce-Co mixed oxides prepared by a polymerized complex method showed high NO decomposition activities [30].…”

para‐Quinone Methide: a New Player in Asymmetric Catalysis

“…To address these issues, our research group has focused on the development of C‐type cubic rare earth sesquioxides, R 2 O 3 (R: rare earths), as novel catalysts for direct NO decomposition 7–10. At temperatures below approximately 2000 °C, three crystal polymorphs of rare earth sesquioxides are known to exist depending on the ionic size of the respective rare earth elements: A‐type hexagonal, B‐type monoclinic, and C‐type cubic (Figure 1).…”

Fundamental Aspects of Rare Earth Oxides Affecting Direct NO Decomposition Catalysis

“…[7][8][9][10] At temperatures below approximately 2000°C, three crystal polymorphs of rare earth sesquioxides are known to exist depending on the ionic size of the respective rare earth elements: A-type hexagonal, B-type monoclinic, and C-type cubic ( Figure 1). [11] Among these, the C-type represents a cubic bixbyite-type structure, which the coordination environments of the rare earth cations within each crystal lattice.…”

“…The lattice volume of the C-type structure is larger than those of A-type and B-type structures and thus it possesses large interstitial open spaces, which have been found to play an important role during direct NO decomposition in our previous studies. [7][8][9][10] In particular, a C-type cubic (Y 0.69 Tb 0.30 Ba 0.01 ) 2 O 2.99+δ catalyst, in which the Y 3+ sites are partially substituted with Tb 3+/4+ and Ba 2+ , can realize 100 % NO decomposition into N 2 and O 2 at 900°C. [7] To date, however, there have been no systematic studies concerning the fundamental factors associated with pure rare earth oxides (REOs) that exhibit direct NO decomposition activity, and the crucial role of the crystal structure with regard to NO decomposition remains unclear.…”