Transient HCO/HCOO- species formation during Fischer-Tropsch over an Fe-Co spinel using low Ribblet ratio syngas: a combined operando IR and kinetic study

The comprehensive kinetics of simultaneous CO consumption in a Fischer-Tropsch and water-gas shift reaction network was systematically investigated over a supported FeCo2O4 spinel catalyst. An operando FTIR-MS study reveals hydrogen-assisted cleavage of the C-O bond through intermediate formyl (HCO) species formation over the spinel catalyst surface as the prominent route for hydrocarbon chain initiation in the Fischer-Tropsch process. In accordance, the Langmuir-Hinshelwood-Hougen-Watson model confirms that the first reaction of adsorbed hydrogen (CO* + H* HCO* + *) is governed by equilibrium, while its second reaction (HCO* + H* C* + H2O*) is kinetically controlled. Six reaction mechanisms comprising Fischer-Tropsch and water-gas shift rate expressions were developed. These models were initially discriminated based on their accuracy to fit the random experimental data collected using a laboratory scale high pressure plug flow reactor with negligible heat and mass transfer limitations, and realistic kinetic parameters were estimated using the Levenberg-Marquardt algorithm. The relatively lower percentage of deviation in the comprehensive model than in other literature-reported models ensures the preciseness of the predicted data. Our mechanistic model FT-III3/WGS-I postulates that the atomic hydrogen-assisted decomposition of surface intermediate HCO initiates the hydrocarbon chain on Fischer-Tropsch active sites, whereas in water-gas shift active sites, the reaction between adsorbed CO and associative H2O to form intermediate formate (HCOO*) species controls water-gas shift activity when syngas of a low Ribblet ratio is used as a feedstock under conventional Fischer-Tropsch process conditions.

Automated summary

This study systematically investigated the kinetics of CO consumption in a Fischer-Tropsch and water-gas shift reaction network over a supported FeCo2O4 spinel catalyst. The results revealed the hydrogen-assisted cleavage of the C-O bond through the formation of formyl (HCO) species as the main route for hydrocarbon chain initiation in the Fischer-Tropsch process. Six reaction mechanisms comprising Fischer-Tropsch and water-gas shift rate expressions were developed and validated. The precision of the predicted data in the comprehensive model was higher compared to other literature-reported models, indicating its reliability.