Na-promoted Pt catalysts in core-shell structure for the low-temperature Water-Gas-Shift reaction

摘要

The water-gas-shift (WGS) reaction is a key step in high-purity hydrogen production. Platinum-based catalysts are active over a wide temperature range and can be stabilized on certain supports, such as ceria and zirconia during WGS operation cycles. Thus, Pt is a good albeit much costlier alternative to Cu-ZnO catalysts for the demanding application to fuel cell systems. The activity of Pt on reducible ceria for low-temperature applications has been attributed to the interaction of Pt with the support and nanoscale ceria with a lot of surface oxygen defects is superior to high-temperature annealed ceria of large crystal size and low defect concentration . Addition of alkali was recently reported to promote the WGS activity of Pt on reducible oxide supports , and very recently our group reported that alkali promotion of Pt is possible even on an inert oxide surface such as silica . Moreover, a very small amount of oxidized Pt, atomically dispersed and stabilized by the alkali, Na-O-Pt(OH)_x, appeared to contain the active sites. From the practical viewpoint, this is important as it allows the choice of a low-cost support, e.g. silica or alumina for low-temperature shift applications. It also allows one to design a catalyst with potentially trace amounts of the costly Pt metal. From a fundamental viewpoint, understanding the promotion would shed light on the mechanistic aspects and allow rational designs of new WGS catalysts using very small amounts of Pt. In this work, we investigated the water-gas shift reaction on a differently designed catalyst, comprising a core-shell structure, Pt@SiO_2 and evaluated its activity and stability and compared it to Pt prepared by impregnation on silica surfaces. The core-shell structured catalyst Pt@SiO_2 was synthesized by the reverse microemulsion (ME) method . Alkali ions were added during the preparation or in a second step by impregnation. In addition to the Pt core, we found that platinum is atomically dispersed throughout the silica shell and sodium ions stabilize the atomically dispersed platinum species in the support. Using XPS, we found that the binding energy of Pt shifts to higher values, and oxidized Pt species were found on the Pt-