Methanation from synthesis gas: In-situ characterization of catalysts
The implementation of sustainable energy supply has become a major issue. The production of Synthetic Natural Gas (SNG) from biomass can make an important contribution to achieve the goal. SNG can be produced via gasification of wood, followed by gas-cleaning and catalytic conversion of the producer gas in a methanation reactor. The producer gas upstream of the methanation step consists mainly of H2, CO, CH4, CO2, H2O, and hydrocarbons. Especially olefins can deactivate the nickel catalyst if applied in a fixed-bed reactor by formation of carbon species on the surface [I. Czekaj, F. Loviat, F. Raimondi, J. Wambach, S. Biollaz, A. Wokaun; Characterization of surface processes at the Ni-based catalyst during the methanation of biomass-derived synthesis gas: X-ray photoelectron spectroscopy (XPS); Applied Catalysis A: General 329 (2007) 68–78]
Comparison of XP spectra (Ni 2p3/2 region) showing the surface changes during a methanation run on a Ni/Al2O3 catalyst and suggested mechanism of processes during methanation at Ni/Al2O3 surface
HR-TEM image showing carbon whiskers. The black dots were identified to be Ni clusters.e
On the contrary, in fluidised bed methanation, catalyst particles are moving through different areas of the reactor. This enables internal regeneration of the catalyst by converting or decomposing the carbon species on the catalyst surface in the upper zones which have low CO and olefin concentration, but are relatively rich in steam and hydrogen.
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was applied. DRIFT enables the in situ investigation of gaseous and adsorbed species inside a reaction cell, while the catalyst is working. In order to obtain a deeper insight into reaction mechanism of olefin decomposition during CO-methanation, Modulation Excitation Spectroscopy is applied. This technique facilitates to distinguish between active and spectator species and additional sensitivity
enhancement is achieved. For further elucidation of reaction mechanisms, hydrogen was partially exchanged by deuterium.
Phase resolved DRIFTS spectra during CO-methanation with deuterium and periodical addition of ethylene over nickel catalyst at 330°C.
In the phase-resolved domain of the DRIFTS spectra (see Figure below), the formation of CHD3 and CD4 could be proved as soon as C2H4 was fed over nickel catalyst. The phase domain enables a clear assignment of CD4 and CHD3 as product of C2H4 decomposition since only contributions of the periodical changes are visible while the H2/CO ratio and thus their reaction products remained constant. It can therefore be derived that under the condition of the experiment, atomic carbon is an important intermediate on the catalyst surface.