(654e) Steady State Isotopic Transient Kinetic Analysis of Supported Pt and Au Nanoparticles for the Water Gas Shift (WGS) Reaction Using a Novel Operando Transmission Fourier Transform Infrared (FTIR) Reactor

An operando FTIR reactor was developed for simultaneous collection of kinetic data and monitoring of adsorbed species under reaction conditions. The reactor is made of stainless steel and can be easily loaded and unloaded. It can be operated up to 450 ^oC and pressure range from 1 to 5 atm. Due to a low dead volume, a 95% signal decay time of less than 2 s for inert gas switching was achieved at a flow rate of 50 ml/min. The apparatus enables steady state isotopic transient kinetic analysis to be performed by monitoring the effluent gaseous species by mass spectroscopy while the adsorbed species are monitored simultaneously by time-resolved transmission FTIR. The reaction rates and kinetics of the low temperature water gas shift (WGS) reaction on Pt and Au catalysts measured by this reactor were comparable to the ones measured in a regular plug flow reactor (PFR).

Details of Pt and Au nanoparticle catalysis of WGS reactions were investigated by Operando FTIR, isotopic transient experiments.  For a Pt/CeO₂ catalyst, the total amount of surface intermediates, determined by isotopic transient experiments, was found to be approximately 85% of the surface Pt, which is comparable to the CO surface coverage on Pt during WGS reaction [1]. Simultaneous time resolved IR data showed that the normalized IR peak intensities of ¹³CO adsorbed on Pt best correlate with the normalized MS response of ¹³CO₂,while the normalized IR peak intensities of ¹³CH, from formates, do not. These results suggest that metallic surface Pt atoms are the dominant active sites for WGS reaction on Pt/CeO₂ catalysts.

Bromine poisoning effect, Operando FTIR, and isotopic transient experiments were also used to investigate the active sites of Au/TiO₂ catalysts for the low temperature WGS reaction. With addition of only 4 mol% Br to Au, the reaction rate decreased about 50%; while adding 8mol% Br to Au led to an undetectable reaction rate at 200 ^oC. TEM and kinetics results indicate that the poisoning effect of Br on Au/ TiO₂ is primarily due to Br blocking the active sites. Operando IR data show that normalized peak area of CO adsorbed on Au⁰ at 2100 cm^-1 best correlates with the WGS reaction rate on thesecatalysts. The fraction of the total moles of Au found to be the active sites on Au/TiO₂, Au(4mol%Br)/TiO₂ catalysts, is 0.60±0.04% and 0.30±0.02% respectively. The turnover rate (TOR) calculated based on the number of active sites from the isotopic transient experiment for Au(4mol%Br)//TiO₂ is 1.7±0.1 s^-1, which is similar to that for Au/TiO₂, 1.5±0.1 s^-1. These results suggest that metallic corner Au atoms are the dominant active sites for the WGS reaction on Au/TiO₂ catalysts.

References

1.        Guo, N., Fingland, B.R., Williams, W.D., Kispersky, V.F.,  Jelic J., Delgass, W.N., Ribeiro, F.H., Meyer, R.J., and Miller J.T., Phys. Chem. Chem. Phys, 12, 5678, (2010).