Methanation of CO2 over alkali-promoted Ru/TiO2 catalysts: II. Effect of alkali additives on the reaction pathway

Abstract

The effect of alkali (Li, Na, K, Cs) promotion of TiO2 on the CO2 hydrogenation pathway has been investigated over 0.5%Ru/TiO2 and 5%Ru/TiO2 catalysts, employing in situ FTIR spectroscopy (DRIFTS) and transient-mass spectrometry techniques. In the case of 0.5%Ru/TiO2 catalysts, characterized by well-dispersed Ru particles, specific activity and selectivity toward methane increases significantly with the addition of suitable amount of alkalis. The nature and population of Ru-bonded carbonyl species, formed on the surface of 0.5%Ru/TiO2 catalyst under CO2 hydrogenation conditions, varies significantly upon alkali promotion. The population of reactive Rux-CO species, which has been previously found to be direct methane precursors, increases in the presence of alkalis following a trend similar to that of specific activity. Results of DRIFT and mass spectrometry experiments provide evidences that alkali promotion enhances the dissociative adsorption of CO and hydrogenation of the so formed surface carbon to methane, which seems not to be operable over the unpromoted 0.5%Ru/TiO2 catalyst. In the case of 5%Ru/TiO2 catalysts, characterized by large Ru particles, both CO2 methanation activity and the nature/population of intermediate surface species seems not to be significantly affected by the presence of sodium. It is suggested that the structure sensitivity of the reaction predominates to the effect of alkali promotion and therefore, CO2 methanation activity cannot be further improved.