(312e) Alkali-Promoted Atomic Pt-Ohx Species on “Active” or “Inert” Supports Catalyze the Low-Temperature Water-Gas Shift Reaction

The water-gas shift (WGS) is an important reaction for hydrogen upgrading during fuel gas processing. Atomically dispersed Pt-O_x-support sites have been found to catalyze the low temperature WGS reaction on “active” supports, such as CeO_x [1]. However, on “inert” supports, such as alumina and silica, precious metal atoms are unstable and easily agglomerate into spectator nanoparticles. For such surfaces, we have recently shown that alkali metal oxides can be used as promoters of the platinum on silica [2], alumina [2], and even on carbon nanotubes [3], such that the high activity of the atomically dispersed Pt-OH_x- species is preserved. Here we have extended this work to other “active” and “inert” supports, including anatase, K type L-zeolites (KL), and mesoporous MCM-41. Through the use of alkali hydroxides, active and stable (up to 673 K) platinum catalysts were developed in this work. While the catalysts show large differences in activity without sodium promotion (Pt/Anatase> Pt/KL> Pt/MCM-41 ≈0 below 573 K), the sodium modified samples show the same, considerably improved activity in both product-free and full gas conditions for the same metal loading (0.5 wt.% Pt) on any of these supports. Modifying only the supports with alkali metals does not improve the catalytic activity if the Pt-O-alkali interaction is not established before calcination. The metal atom-centric Pt-OH_x-O_y-Na_zcluster is the active site, with the same intrinsic activity on any support. XPS analysis shows that platinum in these structures is close to Pt(II), same as for the alkali-promoted platinum on silica and alumina [2]. The addition of alkali metals does not change the apparent activation energy for this reaction. The presentation will include characterization results as well as stability evaluation under various operating conditions.

Acknowledgement

            The financial support by the DOE/BES under Grant # DE-FG02-05ER15730 is gratefully acknowledged.

References

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