Quantum Theory and Measured Turnover Rates: Perovskite Chemical Transistors for Non-Faradaic Alkane Isomerization
Abstract
This STIR proposal aims to demonstrate the concept of perovskite proton conductors as base current conductors for chemical transistors that amplify organic transformations at the transistor collector-emitter surface. The source of the protonic base current is oxidation of hydrogen at an anode at near open circuit. The perovskite proton conductors selected for the program include yttrium doped BaCeO3. Substitution of Ce4+ with lower valent Y3+ generates oxygen vacancies that are filled with OH- when the perovskite is exposed to water at elevated temperature. The protons migrate along the corner sharing octahedra (oxygens) of the perovskite. The cathode (collector-emitter) component of the transistor is exposed to the organic reactant. Proton spillover, at the collector-emitter side of the transistor, provides acid catalysts for generation of carbocation intermediates. Spillover promoted isomerization of olefins at polymer electrolyte reactor cathodes has been demonstrated. The protonic current was amplified by isomerization at the cathode with transistor beta values (olefin/H+) as high as 38. This work aims to demonstrate the chemical transistor effect on the more challenging isomerization of fully saturated alkanes at temperatures of 250 - 300 ¡C, in particular, the isomerization of methylcyclohexane (MCH). Four carbocations intermediates were proposed by ExxonMobil in a study of MCH isomerization on acidic zeolites. The carbocation intermediates varied in stability depending on the distance of the cationic site from the methyl group. These intermediates give rise to 5 possible products, whereby the product distribution depends on the catalyst surface acidity. Products resulting from less stable carbocations increased with surface acidity. ExxonMobil used product distribution measurements to gauge the zeolite acidity. We seek to tune the acidity of the collector-emitter surface by adjustment of transistor protonic base currents. Higher base currents are expected to increase spillover proton coverage at the cathode (i.e., greater acidity). This is expected to shift the product distribution to be more inclusive of products from the less stable carbocation intermediates. In the case of zeolite catalysts, the structures of proposed non-classical carbocations are not well defined, and totally undeveloped for analogous spillover proton generated carbocations. This effort will elucidate carbocation structures by visualization of electron density topographies generated by gradients and Laplacians of DFT calculated electron densities to highlight minima, maxima and saddle points corresponding to bond critical points. Briefly, QTAIM shows topographical critical points corresponding to first and second derivatives of DFT calculated electron densities. The project objectives are: 1: Assemble a chemical transistor using doped perovskite proton conductors as the transistor base. Either bare perovskite, or Pd catalyzed perovskite surfaces will be used as collector-emitter surfaces. The all-inorganic structures enable elevated temperature operation for alkane isomerization. 2: Use of Quantum Theory of Atoms in Molecules (QTAIM) for visualization of surface processes at chemical transistor collector-emitter surfaces. 3: Correlate operando IR and Raman spectroscopy of collector-emitter surface processes to normal mode analysis obtained from the same DFT calculations used for QTAIM analyses. Significance of proposed activity: Lower temperature processes for alkane isomerization are of great interest to the petroleum industry. Of even broader importance is the tunability of surface acidity. Non-faradaic chemical transformations at electrode surfaces are at an infancy of understanding. This work strives to bring the concept of tunable acidity deeper into the mainstream.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 20, 2019
- Source ID
- W911NF1910318
Entities
People
- Eugene S Smotkin
Organizations
- Army Contracting Command
- Northeastern University
- United States Army