Effect of Sodium on the Catalytic Properties of VOx/CeO2 Catalysts for Oxidative Dehydrogenation of Methanol

A series of VOx/CeO2 catalysts with various sodium loadings (Na/V ratio from 0 to 1) have been studied for oxidative dehydrogenation (ODH) of methanol. The effect of sodium on the surface structure, redox properties, and surface acidity/basicity of VOx/CeO2 was investigated using hydrogen temperature-programmed reduction (H2-TPR), Raman spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The experimental results indicate that the effect of sodium on VOx/CeO2 is highly dependent on the Na/V ratio. At a low Na/V ratio (Na/V < 0.25), sodium addition only slightly decreases the redox ability of VOx/CeO2 and has little effect on its activity and selectivity to formaldehyde, even though the Brönsted acidity is almost completely eliminated at a Na/V ratio of 0.25. At a high Na/V ratio (Na/V > 0.25), sodium addition greatly alters the nature of the active sites by V–O–Ce bond cleavage and V–O–Na bond formation, leading to significantly reduced activity of the VOx/CeO2 catalysts. At Na/V > 0.25, the selectivity to formaldehyde also decreases with increasing Na/V ratio due to (1) the suppressed reducibility of VOx and (2) increased basicity leading to increased CO2.