Silica-encapsulated platinum catalysts for the low-temperature water-gas shift reaction
Encapsulated platinum in the form of core–shell Pt@SiO2 prepared by reverse microemulsion, can be rendered catalytically active for the low-temperature water-gas shift (WGS) reaction by the addition of alkali ions during the synthesis steps or by impregnation of the dried (uncalcined) core–shell materials. We report that positively charged platinum species, embedded throughout the silica shell and stabilized by the alkali ions are the active sites for the WGS reaction. Water dissociation and hydroxyl regeneration take place on the Na-promoted Pt–Ox sites. It is shown by cyclic CO-TPR experiments with intermittent ambient rehydration that the activity remains constant, and the onset temperature of the reaction is the same, ∼150 °C, with cycling. The apparent activation energies for the WGS reaction in realistic fuel gas mixtures over the various encapsulated Pt-based catalysts evaluated here and those on other supports are all in the same range, 70 ± 5 kJ/mol. Hence any difference in the reaction rates between the open- and encapsulated-Pt catalysts is attributed to their different number of active sites accessible to the reactants. The Pt-Na@SiO2 catalyst structures show remarkable stability with time-on-stream at 350 °C.
Graphical abstract.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Encapsulated platinum in the form of core–shell Pt@SiO2 was prepared by reverse microemulsion method. ► With alkali ion addition, Pt-Na@SiO2 or Na(IMP)-Pt@SiO2 are active catalysts for the low-temperature water-gas shift (WGS) reaction. ► The active Pt–Ox(OH)–Na species in the silica shell stabilize both the Pt and Na ions; dissociate water and regenerate surface hydroxyl groups. ► The apparent activation energy for the WGS reaction over Pt-based catalysts is independent of the support structure and type.
Journal: Applied Catalysis B: Environmental - Volume 127, 30 October 2012, Pages 342–350