Structural effects of Na promotion for high water gas shift activity on Pt–Na/TiO2

Sodium (1–10 wt.%)-promoted 1 wt.% Pt/TiO2 catalysts were prepared by a co-impregnation method and tested for the water gas shift (WGS) reaction under differential reaction conditions. Significantly higher intrinsic activity is achieved with 2–4 wt.% Na addition, when 2–4 layers of NaOx are deposited on the support and the Pt particles are partially covered by NaOx, with moderate surface basicity and exposed Pt surface areas. In addition to the physical coverage, XPS data suggest that interactions between Pt and NaOx may occur by Pt electron donation to O in NaOx through Pt–O–Na. The strong metal–promoter interactions provide highly active sites for the WGS reaction at the periphery of the Pt–NaOx interface and also inhibit Pt particles from sintering. Optimal 4 wt.% Na (Na/Pt = 34) improves the intrinsic reaction rate and turnover frequency of Pt/TiO2 at 300 °C by 8 and 11 times, respectively, and appears to be significantly higher than reported in the literature. The catalysts were characterized by X-ray diffraction, N2 adsorption, CO chemisorption, transmission electron microscopy, H2 temperature-programmed reduction, CO2 temperature-programmed desorption, and X-ray photoelectron spectroscopy.