The effect of Fe and other promoters on the activity of Pd/ceria for the water-gas shift reaction

Abstract

The effect of adding oxide promoters, at monolayer coverages, to Pd/ceria catalysts was studied for the watergas-shift reaction in 25 torr each of H 2 O and CO under differential conditions. Oxides of Fe, Tb, Gd, Y, Sn, Sm, Pr, Eu, Bi, Cr, V, Pb, and Mo were added to ceria by wet impregnation of aqueous salts and calcined to 873 K prior to addition of 1 wt% Pd. While most of the oxides had a negligible effect on the water-gas-shift rates, rates on the Fe-promoted sample were 8 times higher than on the unpromoted Pd/ceria at 473 K, while rates on the Mo-and Pb-promoted cerias were significantly lower than on pure Pd/ceria. By varying the Fe loading, it was found that the maximum catalytic activity was achieved at the monolayer coverage. Addition of Fe in bulk ceria through sol-gel and preparation of Fe 2 O 3 -CeO 2 physical mixtures had no effect on the catalytic activity. AbstractThe effect of adding oxide promoters, at monolayer coverages, to Pd/ceria catalysts was studied for the water-gas-shift reaction in 25 torr each of H 2 O and CO under differential conditions. Oxides of Fe, Tb, Gd, Y, Sn, Sm, Pr, Eu, Bi, Cr, V, Pb, and Mo were added to ceria by wet impregnation of aqueous salts and calcined to 873 K prior to addition of 1 wt% Pd. While most of the oxides had a negligible effect on the water-gas-shift rates, rates on the Fe-promoted sample were 8 times higher than on the unpromoted Pd/ceria at 473 K, while rates on the Moand Pb-promoted cerias were significantly lower than on pure Pd/ceria. By varying the Fe loading, it was found that the maximum catalytic activity was achieved at the monolayer coverage. Addition of Fe in bulk ceria through sol-gel and preparation of Fe 2 O 3 -CeO 2 physical mixtures had no effect on the catalytic activity.

The Activity of Fe–Pd Alloys for the Water–Gas Shift Reaction

“…Pd/Fe 2 O 3 showed rates that were comparable to Pd/ceria, but both of these catalysts were much less active than the catalysts containing all three components. In agreement with an earlier study [7], the activation energy of the Fe-containing catalysts was somewhat higher than that found on Pd/ceria. Finally, it should be noted that the activity of 2.2 wt% Fe 2 O 3 on ceria, extrapolated to 453 K, is expected to be only 0.0012x10 18 molecules/s .…”

“…In order to maintain differential conversions, it was necessary to use a higher temperature range for the less active catalysts, so that the most active catalysts are those for which the data is on upper right of the plots. In agreement with the earlier study of Pd on Fedoped ceria [7], the activities of the Pd catalysts prepared with Fe-doped ceria were higher than that with undoped ceria; and there is an optimal Fe content for maximizing the rates. However, the optimum Fe content was different for the two Pd loadings.…”

“…In recent work from our laboratory [7], the effect of a large number of promoters was investigated on Pd/ceria catalysts. The majority of these "promoters" (Tb, Gd, Y, Sn, Sm, Pr, Eu, Bi, and Cr) had minimal effect on the catalytic activity.…”

Deactivation of the water–gas-shift activity of Pd/ceria by Mo

“…No similar exchange was observed with Pd. More recently, Wang and Gorte reported large enhancements in the WGS activity of Pd/ceria catalysts that had Fe incorporated into the surface [16]. Surprisingly, we find that the addition of Fe causes no enhancement for Pt-based catalysts [28].…”

“…In a recent study of water-gas-shift (WGS) catalysis over Pd/ceria [16], it was shown that the addition of only 5 mol% of MoO x to the surface of ceria decreased the reaction rates at 473 K by an order of magnitude without changing the activation energy for the reaction. This result is interesting for a number of reasons.…”