Ca promoted Pd/SiO2 catalysts for the synthesis of methanol from CO:: the location of the promoter

A series of Ca promoted Pd/SiO2 catalysts with different Ca loadings has been prepared and tested for hydrogenation of CO at 280 degrees C and 25 bar. Addition of small quantities of Ca dramatically promotes the production of methanol and suppresses the formation of unwanted hydrocarbons, while addition of larger amounts has relatively Little further effect. From the chemisorption of H-2 and CO, transmission electron microscopy (TEM) and temperature programmed reduction (TPR) measurements it is concluded that (i) Ca locates itself preferentially on the Pd particles until a maximum coverage is reached and (ii) that Ca species located on (and perhaps also near) the Pd particles give rise to the catalytic activity for methanol. The affinity of the promoter for the noble metal particle is an important factor affecting its ability to promote the reaction; Cs/Pd/SiO2 and Mg/Pd/SiO2 are shown to be less effective catalysts, and this is at least partly due to a weaker affinity of the promoter for the noble metal. infrared (IR) spectra of adsorbed CO suggest that Ca is preferentially located on the (111) sites of Pd/SiO2 rather than the (100) sites. (C) 1999 Elsevier Science B.V. All rights reserved.