CH4 and CH2O Oxidation in Lean Gas Engine Exhaust Using Fe2O3 Catalysts

Abstract

AbstractA systematic series of commercial α-Fe2O3 catalysts was investigated with respect to the after-treatment of the lean exhaust emissions of gas engines. The samples were physico-chemically characterized by X-ray diffraction, Laser Raman spectroscopy, N2 physisorption, temperature-programmed reduction with CO and the temperature-programmed desorption of CO2, whereas the catalytic efficiency was evaluated using a model exhaust gas. Structure–activity correlations showed that for the oxidation of CH4 the number of active Fe sites and the availability of surface and subsurface oxygen are crucial properties of the catalysts. By contrast, the conversion of CH2O is driven by the CO2 adsorption capacity and the amount of OH surface species, in line with the mechanistic understanding gained by step function experiments and diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). The in-situ DRIFTS studies suggested that the CH2O oxidation follows a Cannizzaro-type mechanism including the reaction of two CH2O molecules with a surface OH site to form CH3OH and formate species. The subsequent conversion of the formate moieties with H2O results in the reconstruction of the OH groups and the release of formic acid. The latter is assumed to decompose into CO2 and H2 which finally oxidizes to H2O. The best iron oxide catalyst was upscaled to the level of a real catalytic converter, which was tested in the lean exhaust gas of a 600 kW biomethane engine. As a result, the catalyst demonstrated high activity with regard to CH2O and CO removal above 300 and 500 °C, respectively, while only negligible conversion of CH4 occurred.