CO/H2 Adsorption on a Ru/Al2O3 model catalyst for Fischer Trospch: effect of water concentration on the surface species

by E. Jiménez-Barrera, P. Bazin, C. Lopez-Cartes, F. Romero-Sarria, M. Daturi, J.A. Odriozola

doi : 10.1016/j.apcatb.2018.06.053

Abstract

Water presence and concentration strongly influence CO conversion and C₅₊ selectivity in the Fischer Tropsch reaction. In this work, the influence of the water concentration was investigated using a model Ru/Al₂O₃ (5 wt.%) catalyst. The surface species formed after CO and H₂ adsorption in dry and wet (different water concentrations) conditions were analyzed by FTIR. Firstly, water adsorption was carried out up to complete filling of the pores and then CO was put in contact with the catalyst. The absence of adsorbed CO species in these conditions evidences that CO diffusion in water controls the access of the gas to the active sites and explains the negative effect of high water concentrations reported by some authors. Moreover, the adsorption of a mixture of CO+H₂+H₂O, being the water concentration close to that needed to have a monolayer, and a dry mixture of CO+H₂ were carried out and compared. Results evidence that water in this low concentration, is able to gasify the surface carbon species formed by CO dissociation on the metallic sites. This cleaning effect is related to the positive effect of water on CO conversion detected by some authors.

Highlights

• Surface species formed after adsorption of a wet mixture (CO+H₂) strongly depends on the water concentration in this mixture.

• Low water concentrations (close to monolayer) provoke the surface carbon gasification, increasing the amount of adsorbed CO on the metal sites.

• Water in high concentrations (~13% in this work) influences the CO diffusion into the pores resulting in a low amount of adsorbed CO.