(130d) Effect of Iron and Titanium Oxide Promoters On Platinum Model Catalysts for the Water-Gas Shift Reaction

Iron oxide and titania enhance the water-gas shift (WGS) reaction rate per surface area of metal on palladium and platinum catalysts. To study the effect of these promoters, we prepared samples on ceria and non-porous model spherical alumina supports by atomic layer deposition. Employing promoters deposited as monolayers (as opposed to studying the promoter as a bulk support) is advantageous as the signal from the promoter can come only from the surface.

One set of samples is 3.6 wt% Pt/Al₂O₃ sample promoted with 10 wt% TiO₂ and another 0.5 wt% Pt/Al₂O₃ promoted with 11 wt% Fe. The WGS reaction rate for the titania promoted sample is 1.3×10^-2 and that for Fe promoted sample is 4.5×10^-2 mole H₂ mole Pt^-1 s^-1 under 7% CO, 8.5% CO₂, 22% H₂O, 37% H₂ at 1 atm total pressure at 250 ^oC, a promotion of a factor of 17 and 60, respectively when compared with the un-promoted Pt/Al₂O₃ catalyst.

The catalysts were analyzed in situ under 0.5 mbar pressure using ambient pressure synchrotron-based Photoemission Spectroscopy (PES). High-resolution core-level spectra were obtained both under reducing atmosphere of H₂ at 300 ^oC and under the WGS reaction mixture of 7% CO, 2% H₂O, 37% H₂, 9% CO₂ at 250 ^oC at a total pressure of 0.5 mbar. The probed volume depends on the escape depth of photoelectrons, which is a function of photoelectron kinetic energy. To obtain the depth profile, the photon energy of the exciting radiation was varied so as to obtain the kinetic energy of photoelectrons ranging from 120 to 420 eV. The oxidation state of metals under different atmospheres and change in composition as a function of depth were investigated. Based on the findings, models describing the structure of catalysts were developed and compared to images obtained by Environmental High-Resolution Transmission Electron Microscopy (HR-TEM).