Effect of Fe promotion on the surface reaction parameters of Pt/γ-Al2O3 for the selective oxidation of CO

Selective oxidation of CO in hydrogen is an important reaction for producing hydrogen from hydrocarbons suitable for use in fuel cells. Pt has been shown to be very active for this reaction. This article reports on the results of an investigation into the impact of Fe promotion on Pt/γ-Al2O3 using isotopic transient kinetic analysis (ITKA). In this study, Fe promotion was found to have an impact on activity, selectivity, and also time-on-stream behavior of surface reaction parameters. It increased activity and selectivity, as has been also noted by others. ITKA revealed that the higher activity of PtFe is due mainly to an increase in intrinsic site activity when compared with nonpromoted Pt. Fe promotion did not affect significantly the total concentration of active surface intermediates. In a previous study, Pt/γ-Al2O3 was found to exhibit steady activity for selective CO oxidation after an initial rapid partial deactivation. The PtFe catalyst also showed rapid initial partial deactivation similar to Pt. The activities of both catalysts decreased with time-on-stream about the same degree in reaching a pseudo-steady state. Unlike for Pt where initial partial deactivation was due primarily to a decrease in active intermediates, the initial rapid partial deactivation for PtFe was the result of both a decrease in the concentration of surface intermediates and a decrease in the average intrinsic site activity, but was due mainly to the decrease in the intrinsic site activity. The intrinsic site activity of PtFe approached that of Pt with time-on-stream. It would appear that carbon deposition causes the initial partial deactivation on Pt and may partially do so on PtFe. However, evidence suggests that reoxidation of Fe is likely a significant cause of the loss of activity of PtFe. As partial deactivation proceeds, the effect of Fe promotion of the Pt sites decreases.