DRIFTS/MS Studies during Chemical Transients and SSITKA of the CO/H2 Reaction over Co-MgO Catalysts

Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) has been employed along with chemical and isotope transients to study the catalytic CO hydrogenation over Co/MgO catalysts in a single fixed-bed reactor at T = 523 K and ambient pressure conditions (H-2/CO = 3). According to the operando DRIFTS measurements, the catalyst surface contains hydroxyl groups, adsorbed CO, formate, and methylene groups in the steady-state of the reaction. Transient experiments following fast changes in the feed (chemical transient kinetics, CTK) or isotope composition (steady-state isotopic transient kinetic analysis, SSITKA) have been carried out during DRIFTS and demonstrate that the formate/methylene seen by DRIFTS plays no role as imminent intermediates of the ambient pressure Fischer-Tropsch (1717) reaction. The SSITKA experiments (replacing (CO)-C-12 by (CO)-C-13) show that the exchange rate of formate/methylene is significantly lower than that of ethane, which is one of the main reaction products of CO hydrogenation (followed by mass spectrometry). Formate is most probably bound as bidentate mu(2)-species to MgO or at the Co/MgO interface, while methylene stands for skeleton CH2 in either hydrocarbon or carboxylate.