Platinum/Apatite Water-Gas Shift Catalysts

Water-gas shift (WGS) micro and membrane reactors are interesting components for compact H2 production and purification devices, but they require catalysts with very high activity for optimum efficiency to minimize catalyst bed thickness and mass transfer limitations. On the other hand, activation of H2O is known to be more challenging than CO in this reaction. Catalysts comprising ca. 2 nm large Pt particles on hydrophilic apatites are found to have very high WGS activity, with specific reaction rates exceeding those of a highly active Pt/CeO2 catalyst by up to 50% at 573 K. These apatite-supported catalysts exhibit stable CO conversions at 673 K without showing any CH4 formation tendencies up to 723 K. WGS activity increases with Ca/P ratio in the apatite, leveling off around Ca/P ≈ 1.75, and formate has been identified as the main reaction intermediate. The outstanding WGS performance is attributed to the superior activation of H2O on these ionic oxides due to coordination of H2O to Lewis acidic Ca2+ ions and H bonding to basic O atoms of PO43– units. This renders H2O molecules highly polarized and thus reactive on apatite surfaces with the ensuing formate-like intermediates being well stabilized through bonding to multiple Ca2+ ions, as well. Thus, apatites provide an intriguing alternative to increasingly expensive rare-earth oxides in high-performance noble-metal WGS catalysts not only for micro and membrane reactors.