Catalytic activity and selectivities of metal oxides and Pt/Al 2 O 3 were evaluated for the combustion of NH 3 , which is considered a carbon-free energy source. Catalytic NH 3 combustion using 3d transition metal oxides for the stoichiometric NH 3 O 2 reaction showed the following order of activity: MnO 2 > Co 3 O 4 > CuO > Fe 2 O 3 μ V 2 O 5 > NiO. This order implies that the activity increases with decreasing oxygen bond energy of the metal oxides. CuO exhibited higher N 2 selectivity than the other transition metal oxides.The investigation of more efficient and cleaner alternative fuel sources is imperative because of the excessive consumption of finite fossil fuel resources and the risks associated with the use of nuclear power. Recently, NH 3 has been considered a renewable and carbon-free energy source due to its high energy density (3160 W h L ¹1) and negligible thermal NO x emission. 13 However, in comparison with fossil fuels, NH 3 as a fuel poses the following problems: (1) high ignition temperature, (2) low combustion rate, and (3) N 2 O and fuel NO x production. Therefore, the development of new NH 3 combustion systems is necessary. One possible candidate is catalytic combustion, which is a promising technique for decreasing emissions from hydrocarbon-based (CH 4 , C 3 H 6 , etc.) fuels and has been actively studied for use in gas turbines, boilers, and jet engines in the 1980s. 4,5 Catalytic combustion has many advantages over conventional non-catalytic combustion, as NO x emission is greatly diminished by the low operating temperatures, and high efficiency can be attained through stable combustion. By applying a novel catalytic combustion system to NH 3 fuel, it is expected that low-temperature ignition as well as negligible emission of NO x can be achieved. To the best of our knowledge, there are no published studies regarding the catalytic combustion of NH 3 . In this study, to understand the basis of the catalytic NH 3 combustion properties of metal oxides and Pt/Al 2 O 3 , their combustion activity and product selectivities were evaluated. In addition, our study was extended to demonstrate the relationship between the NH 3 combustion activity of metal oxides and the oxygen bond energy.Commercially available metal oxides (Table S1, Supporting Information (SI)) were used for the catalytic NH 3 combustion. As a reference, 1.0 wt % Pt/γ-Al 2 O 3 was prepared by a conventional wet-impregnation method using an aqueous solution of [Pt(NH 3 ) 2 (NO 2 ) 2 ] (Tanaka Kikinzoku Kogyo), and subsequent calcination in air at 600°C for 3 h. Metal oxides and as-prepared Pt/Al 2 O 3 were characterized by X-ray diffraction (XRD), X-ray fluorescence, gas adsorption (BrunauerEmmettTeller (BET) surface area, and NH 3 -temperature-programmed desorption (TPD)). Catalytic NH 3 combustion was conducted in a flow reactor at atmospheric pressure (10°C min ¹1 , 1.0% NH 3 , 018% O 2 , He balance, 100 cm 3 min ¹1 and W/F = 5.0 © 10 ¹4 g min cm ¹3 ). The dependence of NH 3 combustion activity and product selectivities on the...

The catalytic NH 3 combustion properties and local structures of copper oxides (CuO x ) supported on aluminum oxide borates (Al 20 B 4 O 36 , 10Al 2 O 3 •2B 2 O 3 : 10A2B) were studied by means of high-angle annular dark-field scanning transmission electron microscopy, energy dispersive X-ray mapping, X-ray absorption fine structure, X-ray photoelectron spectroscopy, gas adsorption techniques, etc. Among the CuO x supported on various metal oxide materials, CuO x / 10A2B exhibited high catalytic NH 3 combustion activity, highest N 2 (lowest N 2 O•NO) selectivity, and high thermal stability. Because the combustion activity is closely associated with the reducibility and dispersion of CuO x , highly dispersed CuO x nanoparticles on supports are considered to play a key role in the low temperature light-off of NH 3 . For NO and N 2 O selectivities, the oxidation state of CuO x and the dissociative species of adsorbed NH 3 are suggested to be important catalytic combustion properties, respectively. On the basis of these discussions, the reaction mechanism of catalytic NH 3 combustion over CuO x /10A2B is described.

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The local structures and catalytic NH 3 combustion properties of copper oxides (CuO x ) and silver (Ag) catalysts supported on aluminum oxides (Al 2 O 3 ) were studied. In order to achieve high catalytic NH 3 combustion activity and high N 2 (low N 2 O/NO) selectivity, the preparation conditions for impregnated binary catalysts were optimized. In comparison with the single CuO x /Al 2 O 3 and Ag/Al 2 O 3 , binary CuO x −Ag supported on Al 2 O 3 showed high performance for catalytic NH 3 combustion. Among the binary catalysts, sequentially impregnated CuO x /Ag/Al 2 O 3 exhibited the highest activity and N 2 selectivity. Because the combustion activity is closely associated with the Ag−Ag coordination number estimated from Ag K-edge XAFS, highly dispersed Ag nanoparticles supported on Al 2 O 3 are considered to play a key role in the lowtemperature light-off of NH 3 . CuO x /Ag/Al 2 O 3 also showed higher N 2 (lower NO) selectivity for temperatures at which NH 3 conversion reached approximately 100%, indicating that the N 2 is directly produced from the NH 3 combustion reaction over CuO x /Ag/Al 2 O 3 . Based on several analyses, a reaction mechanism for catalytic NH 3 combustion over CuO x /Ag/Al 2 O 3 was finally suggested.

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