ABSTRACT
In this contribution, the steady-state isotopic transient kinetic analysis-diffuse reflectance Fourier transform spectroscopy (SSITKA-DRIFTS) method provides further support to the conclusion that not only are infrared active formates likely intermediates in the water-gas shift (WGS) reaction, in agreement with the mechanism proposed by Shido and Iwasawa for Rh/ceria, but designing catalysts based on formate C-H bond weakening can lead to significantly higher
19.1 Introduction ............................................................................................. 366 19.2 Experimental ........................................................................................... 369
19.2.1 Catalyst Preparation ..................................................................... 369 19.2.2 BET Surface Area ........................................................................ 370 19.2.3 SSITKA-DRIFTS Method .......................................................... 370
19.2.3.1 System Dynamics ......................................................... 370 19.3 Results and Discussion ............................................................................ 372 19.4 Conclusions .............................................................................................. 390 Acknowledgments ............................................................................................. 391 References ......................................................................................................... 391
catalytic activity. This is in agreement with their proposal that the rate-limiting step involves cleaving the formate C-H bond. In situ DRIFTS experiments demonstrate that doping Pt/zirconia water-gas shift catalysts with alkali cations such as Na significantly weakens the formate C-H bond, such that the C-H stretching band position moves to lower wavenumbers, from 2,880 cm-1 for the Pt/ZrO2 catalyst to the range of 2,804 to 2,845 cm-1 for the Na-doped Pt/ZrO2 catalyst. Relative to undoped Pt/ZrO2, the formate coverage during steady-state watergas shift was very low at 225°C, since the formates were reacting too rapidly to accurately assess. However, by lowering the temperature to 185°C, the formate decomposition rate was slowed such that the coverage increased enough to monitor the formate reactive exchange from the 12C to the 13C label during 12CO to 13CO switching. In all tests, the formate C-H band reactive exchange rate was virtually the same as the product CO2 exchange rate. Even at 185°C, the reactive exchange time of formate for the alkali-doped catalyst was shorter than that of the undoped Pt/zirconia catalyst at the higher temperature condition.
