Catalytic ammonia combustion properties and operando characterization of copper oxides supported on aluminum silicates and silicon oxides

Abstract

Although NH3 has recently been regarded as a renewable and carbon-free energy source, NH3 fuel has a high ignition temperature and its use results in the production of N2O/NOx. To overcome these issues, in this work, we focused on a novel catalytic combustion system and copper oxides (CuOx) catalysts supported on aluminum silicates (Al6O13Si2, 3Al2O3·2SiO2, 3A2S) and silicon oxides (SiO2). The preparation methods for 3A2S as a support material were optimized to achieve high catalytic NH3 combustion activity and high N2 (low N2O/NO) selectivity. Because the CuOx supported on 3A2S prepared by an alkoxide method and subsequent calcination at 1200 °C for 5 h in air exhibited high catalytic performance for NH3 combustion, the properties of the catalyst in addition to CuOx/SiO2 thermally aged at 900 °C for 100 h in air were also evaluated using high-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray mapping, operando X-ray absorption fine structure analysis, X-ray photoelectron spectroscopy, and gas adsorption techniques. Our findings suggest that the catalytic NH3 combustion activity, NO selectivity, and N2O selectivity are closely associated with the reducibility (dispersion) of CuOx, local structures around Cu, fraction of the oxidation state (Cu2+), and adsorption species of NH3 (NH, imide). Finally, we propose a reaction mechanism for catalytic NH3 combustion over not only CuOx/3A2S but also CuOx/SiO2.