Strong oxide-oxide interactions in silica-supported magnetite catalysts. IV. Catalytic consequences of the interaction in water-gas shift

Water-gas shift was studied at 663 K and 0.1 MPa over a series of silica-supported magnetite (Fe₃O₄) catalysts having magnetite particles from ca. 10 to 160 nm in size. The surface sites on these catalysts were titrated using NO adsorption at 273 K and adsorption from a CO₂ CO gas mixture (CO₂:CO = 85: 15) at 663 K. For these silica-supported catalysts, the water-gas shift turnover frequency, based on NO adsorption for site titration, decreased with decreasing particle size. In contrast, unsupported magnetite did not show this particle size dependence. For the smallest silica-supported magnetite particles, the turnover frequency was three orders of magnitude lower than over unsupported magnetite. The extent of CO and CO₂ adsorption, per surface site as titrated by NO, was also observed to decrease with decreasing particle size; and, the water-gas shift turnover frequency, based on CO₂ CO adsorption for site titration, is independent of particle size. Thus, while NO adsorption can be used to measure the magnetite surface area of silica-supported samples, the CO₂ CO adsorption uptake is proportional to the number of active sites on the magnetite surface. Compared to unsupported magnetite, the origin of the low catalytic activity of silica-supported magnetite (based on NO adsorption for site titration) is interpreted as being due to the effects of Si⁴⁺ substitution into the surface of magnetite, which causes the iron cations at the surface to become electron deficient and coordinatively more saturated with oxygen anions.