Support Functionalization To Retard Ostwald Ripening in Copper Methanol Synthesis Catalysts

Abstract

A main reason for catalyst deactivation in supported catalysts for methanol synthesis is copper particle growth. We have functionalized the support surface in order to suppress the formation and/or transport of mobile copper species and thereby catalyst deactivation. A Stober silica support was functionalized by treatment with aminopropyltriethoxysilane, which introduces aminopropyl groups on the surface. Copper was deposited on both unfunctionalized and functionalized Stober silica via incipient wetness impregnation with aqueous copper nitrate solutions followed by drying and calcination. Similar particle size distributions (1-5 nm) were obtained for both supports by changing the flow of N-2 to a flow of 2% NO/N-2 during calcination of the unfunctionalized and amine-functionalized silica, respectively. The effect of support functionalization with aminopropyl groups was an increased stability in the methanol synthesis reaction (40 bar, 260 degrees C, 23% CO/7% CO2/60% H-2/10% Ar, 3% COx conversion) due to more limited copper particle growth as determined by transmission electron microscopy (TEM). Changing the interparticle distance did not have an influence on the deactivation rate, while the addition of few very large copper particles did, indicating that Ostwald ripening was most probably the dominant particle growth mechanism for these samples. In situ TEM images showed the contact angle between the reduced copper particles and the support. As shape and size was similar on silica as on amine-functionalized silica, the thermodynamic stability of the copper particles was unaltered. The driving force for copper particle growth was thus unchanged upon functionalization. We therefore suggest that Ostwald ripening in methanol synthesis catalysts was retarded by inhibiting the transport of copper species over the support surface. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed a decrease in the number of surface groups (hydroxyl, methoxy, and aminopropyl) upon functionalization because aminopropyltriethoxysilane reacted with multiple hydroxyl groups. Because of that, the distance between neighboring functional groups was increased, suppressing the mobility of Ostwald ripening species from one copper particle to another.