(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.