Role of alkali metal in a highly active Pd/alkali/Fe₂O ₃ catalyst for water gas shift reaction

Pd/K/Fe₂O₃ catalyst shows high activity for water gas shift (WGS) reaction. The K/Pd molar ratio on the catalyst has a remarkable effect on WGS activity. The optimum K/Pd molar ratio is about 2. In this study, to clarify the role of alkali metal (potassium), WGS activities of Pd/alkali/Fe₂O₃ (alkali: Li, Na, K or Cs) catalysts were investigated and the reaction mechanism over the Pd/K/Fe₂O ₃ catalyst was examined with transient response analyses using a concentration jump method with a quadrupole mass spectrometer (Q-Mass). Results showed that the addition of alkali metals such as Na, K and Cs drastically enhanced WGS activity, except for Li-loaded catalyst. WGS reaction over the highly active Pd/K/Fe₂O₃ catalyst was found to proceed via reduction-oxidation (redox) using lattice oxygen. Thereby, CO was oxidized by lattice oxygen in Fe₂O₃. The consumed lattice oxygen was regenerated by H₂O. The results of CO-TPR revealed that oxidation of CO by lattice oxygen were enhanced by K, Cs or Na addition. H₂-TPR results showed that reduction of Fe₂O₃ was suppressed by an increment of loading amount of potassium, which brought a high stability for the WGS reaction. The result of CO/H₂O-TPR showed that steam can promote the regeneration of consumed lattice oxygen. The synergistic effect brought a high release ability of lattice oxygen for CO oxidation and also high regenerating ability of consumed lattice oxygen by H₂O to produce hydrogen, to the Pd/K/Fe₂O₃ catalyst. It enabled high catalytic performance of the Pd/K/Fe₂O₃ catalyst with K/Pd molar ratio of 2.