Mass spectrometry and in situ infrared diffuse reflectance analysis of the decomposition of HCOOH adsorbed on Ln2O3-promoted Rh/Al2O3 catalysts

Abstract

In order to ascertain the role of formate ion in CO_x methanation, the decomposition of adsorbed formate on a series of lanthanide-promoted Rh/Al₂O₃ catalysts (Ln = La, Ce, Sm, Yb, Lu) is reported. On the supports the decomposition occurs through a dehydration mechanism. The decomposition process is drastically modified on the rhodium catalysts, in these cases the decomposition proceeds mainly through a dehydrogenation reaction. Moreover, the decomposition temperature is decreased by 100 K with respect to the one observed on the supports alone. The presence of small amounts of gas-phase CO together with the appearance of adsorbed CO at the dehydrogenation temperatures leads us to propose that the HCOO^– generated during CO_x methanation cannot be considered as a non-reactive species. The CO seems to proceed from CO₂ dissociation, its yield being a function of the metal dispersion which is determined by the promoter. The thermal stability of adsorbed CO is also determined by the promoter and is related to the activity of the catalyst in the CO_x methanation.