New and revisited insights into the promotion of methanol synthesis catalysts by CO2

CO hydrogenation, CO2 hydrogenation, and water–gas shift (WGS) reactions have been simultaneously investigated over industry-like catalysts based on Cu–ZnO–Al2O3, under methanol synthesis conditions (513 K, 5.0 MPa). For this, a novel methodology has been applied: the concentration of carbon dioxide in the syngas feed was consecutively increased (R = CO2:(CO + CO2) = 0–100) resulting in a volcano-type plot of the rate of methanol formation and forming a hysteresis loop when decreasing the CO2 concentration again. H2O co-feeding experiments revealed that the enhancement of activity can be correlated with the WGS activity linking both hydrogenation paths of CO and CO2. On the other hand, excessive amounts of surface hydroxyls seem to inhibit methanol production, explaining the drop in activity at high CO2 concentrations. An investigation of the catalytic performance was accompanied by an extensive characterisation of the fresh and used catalytic materials by X-ray diffraction, temperature-programmed reduction by H2, N2O pulse chemisorption, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. It was shown that the copper surface area affects the CO2 hydrogenation; however, this parameter is unambiguously not the key descriptor for CO2-promoted methanol synthesis, which is a consequence of the synergistic interaction of zinc oxide and copper. This structural feature is further promoted by Al2O3 through stabilisation of the surface. The position of the activity maximum is determined by the surface ratio Cu : Zn. The hysteresis behaviour is a result of the continuous decrease of Cu dispersion and the fixation of copper species in its monovalent oxidation state, both detrimental for CO2 hydrogenation. CO hydrogenation is strongly affected by the Cu : Zn bulk ratio and thus the reducibility of the catalyst. These facts could be substantiated by the use of impregnated model catalysts.