Reactivity and Characterization of Intermetallic Alloy Catalysts for Alkane Dehydrogenation
As the United States works towards energy independence shale gas has become an attractive domestic recourse for use as a feedstock to produce fuels. One potential approach to utilize shale gas is to first convert the C 2 and C3 paraffinic components into olefins, valuable chemical building blocks, by catalytic dehydrogenation. The goal of this dissertation is to study how the geometric and electronic changes to a metal upon alloying influence its selectivity for light alkane dehydrogenation. In the first three projects bimetallic catalysts comprising of either Pd or Pt and a post-transition metal known to promote olefin selectivity were investigated. In all the systems studied the bimetallic catalysts were found to be more selective for ethane dehydrogenation than the monometallic analogue. In situ characterizations revealed the formation of intermetallic compounds (IMC) which contained either small ensembles of or completely isolated active atoms in the bimetallic catalysts. It is believed that these geometric changes to the active metal are the dominant factor leading to improved dehydrogenation selectivities. From a study performed on Pd-In catalysts it was proposed that IMC structures similar to the active metal are preferentially formed. In a separate study, two distinct IMC structures were formed in Pt-In catalysts with different In:Pt atomic ratios and the two phases were found to have different turnover rates (TOR) and apparent activation energies. These results showed that the catalytic properties of metals could be altered by forming different IMC structures. Lastly, a study on Pt-Zn catalysts revealed changes in energy of the 5d states of Pt upon IMC formation. The observed energy change is believed to be responsible for increases in dehydrogenation TOR. In the fourth project Pt-Fe bimetallic catalysts were investigated as an extrapolation of the findings of the first set of studies. Pt and Fe were found to form three IMC structures as the Fe:Pt atomic ratio was varied. All three structures contained Pt atoms with local geometries identical to the catalysts selective for ethane dehydrogenation. When tested for propane dehydrogenation the IMC catalysts were found to be highly selective for propylene. Although Pt and Fe are both catalytic, the much higher activity of the former results in the latter behaving as an inert diluent. This results in the small ensembles of and isolated Pt atoms in the IMC structures being highly selective for dehydrogenation. Electronic structure measurements and calculations showed small changes in the average energies of the 5d states of Pt as the Fe content of the IMC changed. Associated with the valance energy shifts were changes in metal-adsorbate bond strengths which were believed to be the cause of increased dehydrogenation TOR. These results demonstrated that it is possible to change the electronic structure of metals by forming IMCs with different promoters or stoichiometries. While electronic effects play a secondary role in alkane dehydrogenation, this insight could provide useful for other catalytic chemistries.
Miller, Purdue University.
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