Gas-phase incorporation of palladium onto ceria-doped silica aerogel for water-gas shift catalysis

The Water-Gas Shift (WGS) reaction is a means of producing hydrogen from coal-derived syngas. Palladium-promoted ceria has been investigated recently and has shown potential for low-temperature WGS catalysis. One limitation of traditional ceria is an inherent low surface area. While specialty cerias with surface areas as high as 300 m2/g have recently been prepared, they do not show structural stability at relevant temperatures. Supporting ceria on the surface of silica aerogel can take advantage of the very high surface area of the silica aerogel as well as the structural integrity of the support. Starting with a silica aerogel having an approximate surface area of 700 m²/g, ceria can be incorporated onto the surface with nominal loadings up to 40% (w/w), yielding final surface areas as high as 600 m²/g and generally above 450 m²/g. Palladium was added via the gas-phase incorporation (GPI) of a volatile organometallic complex, (η²-allyl) (η⁵-cyclopentadienyl)palladium, which is air-stable and prepared by a well-established procedure. Comparisons between GPI and conventional aqueous phase incorporation indicate increased activity for GPI-derived catalysts. For example, GPI can increase the WGS activity in excess of 150% for otherwise identical catalysts. This is presumably due to a higher dispersion of the Pd derived from GPI. A XRD analysis of catalysts derived from the GPI of Pd failed to show well-defined peaks indicative of agglomerated Pd particles, which agrees with the interpretation of high dispersion.