Bimetallic Pd-Cu catalysts for selective CO₂ hydrogenation to methanol

This paper reports on novel Pd-Cu bimetallic catalysts for selective CO₂ hydrogenation to methanol. Strong synergistic effect on promoting methanol formation was observed over amorphous silica supported Pd-Cu bimetallic catalysts when the Pd/(Pd+Cu) atomic ratios lied in the range of 0.25-0.34. The methanol formation rate over Pd(0.25)-Cu/SiO₂ was two times higher than the simple sum of those over monometallic Cu and Pd catalysts. The Pd-Cu bimetallic catalysts were characterized by X-ray diffraction, transmission electron microscopy, scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and hydrogen temperature-programmed desorption. Detailed characterization results demonstrated the importance of two well-dispersed Pd-Cu alloy particles (PdCu and PdCu₃) for the observed methanol promotion over Pd-Cu bimetallic catalysts. Similar bimetallic promotion was also observed for Pd-Cu catalysts supported on uniform mesoporous MCM-41, SBA-15 and MSU-F. Conversion-selectivity profile of the Pd-Cu/SiO₂ catalyst suggested that CO₂ was a primary carbon source for methanol synthesis at lower CO₂ conversion, and byproduct CO contributed at higher CO₂ conversion within the conversion range examined.