Raymond J. Gorte - US grants

This is an award to assist in the purchase of equipment to simultaneously obtain temperature-programmed desorption and thermogravimetric analysis data. This equipment will be used to study industrially important zeolite catalysts.

A chamber for sample preparation and transfer is being added to an existing photoelectron spectrometer. This equipment will be used to study catalysts consisting of thin films on oxide substrates. Thin metal films on various oxides will be used to examine changes in the electronic levels of the metal following adsorption and to determine reasons for differences in the properties of metal catalysts on different supports. Thin oxide films on other oxides will be examined to determine the nature of intermediates formed on mixed-oxide, acid catalysts. These systems are simplified models of classes of catalysts widely used in both production chemicals and refining of petroleum. Better understanding of the energetics involved in their behavior is an important step toward designing new catalysts.

The adsorption of simple organic molecules which can undergo acid-catalyzed chemistry will be examined in high-silica zeolites using a variety of techniques, including simultaneous temperature-programmed desorption and thermal gravimetric analysis, infrared spectroscopy, and carbon-13 nuclear magnetic resonance. The purpose of these adsorption studies will be to study the interaction of intermediates with the zeolite lattice and ultimately to formulate structure/reactivity relationships which can give insight into the chemistry occurring at the acid sites. Zeolites with different crystal structures and with different trivalent cations incorporated into the lattice will be studied in order to determine how these properties affect the properties of the acid sites. The experiments to be conducted should provide a basis for a more detailed understanding of the acidity of high-silica zeolites.

An Atomic absorption spectrometer is acquired. The instrument will be used for research on catalysis, structures of zeolites and other molecular sieves, and novel oxides formed by soft chemistry.

Adsorption is under development worldwide as a chemical- engineering unit operation for the separation and purification of gaseous and liquid mixtures. In addition to separation of air into its components and removal of pollutants from air and water streams, adsorption is being considered for a host of new applications, including separation of proteins from bioreactor product streams, recovery of carbon dioxide from combustion of fossil fuels, recovery of uranium from sea water, methane storage, and ultrapurification of water and raw materials for the electronics industry. Current research using molecular simulation by grand canonical Monte Carlo and molecular dynamics is providing new insights into the physics of adsorption. Results from these fundamental investigations provide a solid foundation for the development of new theories to replace the classical thermodynamic methods. The objective of this research is to bridge the gap between adsorption theory and practice by developing a molecular theory capable of predicting preferential adsorption from fluid mixtures. Adsorption isotherms of single gases and their binary mixtures, and isosteric heats of adsorption of single gases, will be measured. Experimental data collected for four binary systems will serve to test theories for predicting mixed-gas adsorption equilibria from single component isotherms and heats of adsorption.

Abstract

Proposal Title: Thermodynamic Measurements of Redox Properties of Supported Oxide Catalysts

Proposal Number: CTS - 0625324

Principal Investigator: John A. Vohs

Institution: University of Pennsylvania

Analysis (rationale for decision):



This research program will develop a novel application of a Coulomb titration technique to directly measure the heats of oxidation and reduction of supported monolayer, metal oxide, and selective oxidation catalysts. This analysis technique will then initially be used to characterize supported molybdena catalysts, and potentially also supported vanadia and chromia catalysts.

This program will directly measure, for the first time, the enthalpies and entropies of oxidation of supported oxide catalysts using Coulomb titration measurements. The measurements will be performed on well-characterized, supported metal oxide catalysts as a function of catalyst loading and support composition in order to develop a fundamental understanding of how these parameters affect catalyst performance. The data will be correlated with results from the literature in order to understand how the thermodynamic properties relate to catalytic performance. These correlations will significantly improve the understanding of what makes these commercially important catalysts active and selective. One important outcome of this work will be the development of a new, generally applicable technique for measuring critical, but difficult to obtain, properties of oxidation catalysts. A second important outcome will be the generation of new insights into how the structure and composition of supported oxide catalysts affect the enthalpies and free energies of oxidation for these commercially important materials. Finally, important insights into the factors that make supported oxides active and selective catalysts will be obtained.

The educational impact of this work will also be significant. In addition to training of the PhD students that will carry out the majority of the research, education of both undergraduates and high school students will be included in the project. A portion of the research program will be performed by both undergraduate engineering students and by high-school students, during the school year and over the summers.

NON-TECHNICAL SUMMARY: Solid oxide fuel cells (SOFC) are highly efficient devices for the conversion of the chemical energy stored in a range of fuels directly into electrical energy. Their inherent high efficiency results in less fossil fuel consumption and green house gas emissions compared to other energy conversion technologies. This Materials World Network project focuses on the development of a novel class of materials for use in the electrodes in SOFC that will increase both their efficiency and long-term durability, which will in turn help to hasten their commercialization. The specific materials that are being investigated, transition metal doped titanates and vanadates, undergo structural transformations upon exposure to reducing conditions that result in the precipitation of catalytically active metal nanoparticles on their surfaces. These metal nanoparticles catalyze the chemical reactions that take place in the fuel cell electrodes, thereby improving the device performance. The mechanism of this process is being determined and this insight will be used to design electrode compositions and microstructures with optimal properties. Researchers at the University of Pennsylvania in the US and the University of St. Andrews in the United Kingdom will collaborate on the project. This collaboration will include student exchanges at both graduate and undergraduate levels that will enhance the students' education and better prepare them to be the technological leaders of the future.

TECHNICAL SUMMARY: Solid oxide fuel cells (SOFC) have potential as highly efficient devices for the conversion of the chemical energy stored in a range of fuels directly into electrical energy. Electrocatalytic materials that are stable under SOFC operating conditions are needed, however, for this potential to be realized. In this Materials World Networ project, exsolution/dissolution of catalytically active transition metals out of and into an electronically conducting host oxide is being investigated as a means to tailor the catalytic properties of SOFC anodes and to regenerate activity that is lost due to sintering or adsorption of poisons. The specific materials systems under investigation include transition metal doped conducting titanates and vanadates which have the perovskite structure. The mechanism of the exsolution of transition metals, such as Ni, Pt, or Pd, from these host oxides under reducing conditions, its dependence on the oxide composition and defect chemistry, and the relationships between microstructure and electrochemical performance is being determined. The interaction of the exsolved metal nanoparticles with the oxide surface will also be characterized and the insight obtained in these studies will be used to design materials systems for which the exsolved metal nanoparticles are highly stable and resistant to coarsening via Ostwald ripening. The use of dissolution/exsolution cycles as an in situ means to regenerate catalytic activity in working SOFCs will also be investigated.

This project is supported by the Ceramics Program and Office of Special Programs, Division of Materials Research.

Ammonia synthesis is essential for production of fertilizers used in food production, along with many other applications. The present method for producing ammonia, the 100-year-old Haber-Bosch process, is based on natural gas and consumes 1-2% of total global energy. It is a high-pressure, high-temperature process that is only economical if used for large-scale production. There would be significant advantages to producing ammonia directly from renewable forms of electricity at the location where it is needed. The goal of this project is to demonstrate that this can be accomplished with high efficiency.

This project will seek to develop an electrochemical method for production of ammonia from hydrogen and nitrogen using a proton-conducting, ceramic, solid-oxide electrochemical cell. The central hypothesis is that atmospheric-pressure, ammonia synthesis can be realized by electrochemically driving hydrogen onto catalytic surfaces that are normally limited by high nitride coverage. The project will seek to develop electrode catalysts that are able to dissociate molecular nitrogen, the rate-limiting step in conventional Haber-Bosch synthesis, while simultaneously showing low activity for hydrogen recombination so as to achieve a high hydrogen fugacity at the electrode surface. The project will take advantage of the infiltration methods previously developed for electrode synthesis in Solid Oxide Fuel Cells which allows a wide range of materials to be used for the electrodes. The project will explore mixed electronic-protonic conductors that can be added to the electrode to enhance the three-phase boundary where the electrochemical reaction can occur. The choice of electrocatalysts will be guided by complementary theoretical studies. Small-scale demonstration cells will be produced. If successful, the project will have a dramatic impact on the energy demand and carbon dioxide emissions from ammonia synthesis. The project will support both graduate and undergraduate students in conducting the research, and the PIs will develop faculty-led peer mentorship programs that aim to increase retention of underrepresented undergraduate students in STEM.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.