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 PO₄^3– 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

Structural and Permeation Kinetic Correlations in PdCuAg Membranes

Addition of Ag is a promising way to enhance the H₂ permeability of sulfur-tolerant PdCu membranes for cleanup of coal-derived hydrogen. We investigated a series of PdCuAg membranes with at least 70 atom % Pd to elucidate the interdependence between alloy structure and H₂ permeability. Membranes were prepared via sequential electroless plating of Pd, Ag, and Cu onto ceramic microfiltration membranes and subsequent alloying at elevated temperatures. Alloy formation was complicated by a wide miscibility gap in the PdCuAg phase diagram at the practically feasible operation temperatures. X-ray diffraction showed that the lattice constants of the fully alloyed ternary alloys obey Vegard’s law closely. In general, H₂ permeation rates increased with increasing Ag and decreasing Cu content of the membranes in the investigated temperature range. Detailed examination of the permeation kinetics revealed compensation between activation energy and pre-exponential factor of the corresponding H₂ permeation laws. The origin of this effect is discussed. Further analysis showed that the activation energy for H₂ permeation decreases overall with increasing lattice constant of the ternary alloy. The combination of these correlations results in a structure–function relationship that will facilitate rational design of PdCuAg membranes