(583a) Performance Enhancement of Low Temperature Water Gas Shift Catalysts by Palladium-Iron and Platinum-Iron Interactions

We report on the effect of iron addition to Pd and Pt on three supports (Al₂O₃, ZrO₂ and CeO₂) as catalysts for the water gas shift reaction. Compared to a 2.3%Pd/Al₂O₃ catalyst, Pd/Fe samples with similar Pd loadings showed a promotion of the WGS rate per mole of Pd in the range of 35 to 235 times. On Pt/Al₂O₃ catalysts an increase in rate per mole of Pt (up to 25 times) was measured with the increase in Fe wt% loading from 0.6 to 11%. Interestingly, the addition of Fe to Pd supported on Al₂O₃, CeO₂ and ZrO₂ or to Pt supported on Al₂O₃ and CeO₂ erases the support and metal effects, i.e. although Pt has a turnover rate about 10 times higher than Pd on the three supports, the addition of Fe makes the rate similar on Pt and Pd catalysts on the three supports.

Extended X-ray absorption fine structure (EXAFS) and in situ diffuse reflectance infrared spectroscopy data showed that Pd and Pt particles were partially covered by a shell of Fe. In addition, for Al₂O₃ supported catalysts with Fe wt% loading between 1.8 and 13.5, the rate per gram of catalyst correlated with the amount of hydrogen consumed by titration of surface oxygen after the sample was exposed to O₂ at 100°C. These results suggest that reducible Fe species are active sites. X-ray photoelectron spectroscopy (XPS) data measured after reaction showed that some of the Fe is reduced to the zero valent form. We also found that the amount of Fe⁰ correlates with the rate per gram. In line with these results, we propose that with the addition of iron the reaction takes place over new redox sites, mainly reduced Fe on top of the noble metal. As no Fe-M (M = Pd, Pt) alloy formation was observed by EXAFS, we believe that the role of Pd or Pt is the initial reduction of nearby Fe by H₂ spillover, and then the stabilization of a surface layer of reducible Fe. The promotion effect is therefore explained by the creation of more active redox sites as compared to those on Pd or Pt on Al₂O₃. This hypothesis is consistent with our kinetic observations on Pd and Pt on reducible supports like CeO₂ and ZrO₂, where the addition of iron showed minimal promotion mainly because these materials already provide enough redox sites for the reaction.