Detailed Mechanism of Ethanol Transformation into Syngas on Catalysts Based on Mesoporous MgAl2O4 Support Loaded with Ru plus Ni/(PrCeZrO or MnCr2O4) Active Components

Mechanism of ethanol partial oxidation into syngas over catalysts based on mesoporous MgAl2O4 spinel loaded with fluorite PrCeZrO or spinel MnCr2O4 oxides and promoted by Ru + Ni was studied by in situ FTIRS and O-18 SSITKA. Surface species (ethoxy, adsorbed ethanol, acetaldehyde, acetate, etc.) were identified and their thermal stability and reactivity were estimated. Analysis of kinetics of the O-18 transfer into reaction products (CO, CO2, CH3CHO) allowed to estimate the rates of steps and present a scheme of the reaction mechanism including (1) fast CH3CHO formation on mixed metal oxide sites; (2) rate-limiting stage of surface oxygen species incorporation into acetaldehyde or ethoxy species with C-C bond rupture yielding CO and CO2 along with H-2 and H2O; (3) water gas shift reaction by redox mechanism affecting CO/CO2 ratio and their oxygen isotope fraction. Strong interaction of PrCeZrO or MnCr2O4 oxides with MgAl2O4 support results in decreasing constants of main reaction steps in comparison with those for catalysts based on bulk fluorite and spinel oxides, correlating with a higher surface oxygen bonding strength and its low coverage revealed by pulse microcalorimetry. DFT analysis confirmed a low energy barrier of the step of Ru-O oxygen incorporation into C-C bond of ethoxy species with its rupture explaining a higher syngas selectivity for Ru-doped catalysts.