The impact of nanoparticle and support synergy on water-gas shift catalyst design

Wilce Damion Williams, Purdue University

Abstract

The low temperature water-gas shift reaction (CO + H2O ↔ CO2 + H2) is important in the production of hydrogen from carbonaceous materials. Higher conversions are attainable at lower temperatures, but lower temperatures result in lower reaction rates, so high performance catalysts are desired to speed up the reaction. This research is focused on designing high turnover rate (TOR) catalysts using an approach called Discovery Informatics where high-throughput experimentation, catalyst characterization techniques, and kinetic modeling are combined to develop design strategies based on the trends observed for different catalyst systems. The effect of the metal and the support have been examined by studying the effect of Mo as a promoter for Pt based catalysts, and also by studying the effect of Au particle size on different supports. The similarity of the PtMo / Al2O3 and PtMo / SiO2 TOR advances the hypothesis that reducible Mo-oxide in close proximity to the Pt enhances the rate in a manner similar to the support effect. The rate per mole of Au varies in a distinctly different manner than that for Pt. Thus, a study of Au / rutile TiO2 catalysts was conducted to better understand the trends observed for Au based catalysts. It was concluded that metallic corner atoms with low Au-Au coordination are the dominant active site for Au / rutile catalysts. These experimental results show that the WGS reaction is bi-functional in nature because the catalyst rates scale with the amount of CO adsorbed on the metallic atoms while the support or metal oxide promoters can increase the rate for Pd, Pt, and Au catalysts. Microkinetic modeling was performed to describe and predict experimental data in order to understand what kinetic parameters are important in the WGS reaction. Bayesian parameter estimation was used to fit a microkinetic model with experimental data collected on a Pt / Al2O3 catalyst. The microkinetic model was capable of describing the data with the use of coverage dependent rate constants and a stronger binding energy of OH. The results of the model imply that a relatively empty surface which dissociates the carboxyl intermediate easily will be a good WGS catalyst. As a follow up to that study, a scaling model was created based on the carboxyl mechanism. The results of the model are discussed in regards to implications for bi-functional water-gas shift catalysts. To achieve maximum turnover frequencies, a proposed catalyst design strategy is to combine metals that adsorb O weakly, such as Au clusters or Pt nanoparticles, with supports that exhibit strong interactions with oxygen and are capable of easily dissociating water.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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