The effect of boron on the state and dispersion of Co/Al2O3 catalysts

The influence of boron on the chemical state and dispersion of CoAl2O3 catalysts has been investigated by bulk and surface spectroscopic techniques. Three series of CoAl2O3 catalysts were studied: one containing a constant Co loading of 3 wt% and boron loadings of 0.3 to 3 wt%, and two series containing 0.7 to 10 wt% Co and constant boron loadings of either 0 or 3 wt%. In the absence of boron, Co dispersion decreased with increasing Co loading above 1.5 wt% Co. The presence of boron had little effect on Co dispersion at low Co loadings, while catalysts with Co loadings in excess of 1.5 wt% exhibited an increase in Co dispersion compared to boron-free catalysts. X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, and gravimetric analysis were used to quantify the Co species present on the catalysts as a function of both Co and boron loading. In the absence of boron, Co exists as tetrahedral and octahedral Co2+ at low Co loadings, while Co3O4 is the primary phase at higher Co loadings. In the presence of boron, Co3O4 formation is suppressed for catalysts with Co loadings less than 8 wt% in favor of octahedral Co2+ and minor amounts of Co borate. The formation of Co3O4 occurs at Co loadings greater than 6 wt% for boron-containing catalysts. A mechanism is described to account for the effect of boron on the dispersion and chemical state of Co.