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

The influence of boron on the chemical state and dispersion of Co Al 2 O 3 catalysts has been investigated by bulk and surface spectroscopic techniques. Three series of Co Al 2 O 3 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.