Author ORCID Identifier

http://orcid.org/0000-0002-4277-0324

Degree Year

2017

Document Type

Thesis

Degree Name

Bachelor of Arts

Department

Physics and Astronomy

Advisor(s)

Stephen FitzGerald

Keywords

MOFs, Metal-organic frameworks, Porous materials, H2-D2 separation, Hydrogen separation, Deuterium separation, Temperature-programmed desorption, Thermal desorption spectroscopy, Infrared spectroscopy, Trapped hydrogen, Quantum sieving

Abstract

In this thesis we provide an introduction to the use of Metal-Organic Frameworks (MOFs) for hydrogen storage and for the separation of hydrogen isotopologues, H2 and D2. MOFs are a class of materials comprised of `building-block’ metal-oxide clusters connected by organic ligands, which have the capacity to adsorb molecules such as hydrogen through weak, physisorptive mechanisms. We provide some background on the quantum mechanical structure of hydrogen isotopologues, the structure of a few state-of-the-art MOFs, the quantum mechanics of infrared spectroscopy, and the desorption dynamics of adsorbates generally. We provide a description of the experimental apparatus and procedure used in this work to acquire thermal desorption (TD) and simultaneous, in situ infrared (IR) spectra. Notably, this apparatus makes use of a pressure gauge to record TD spectra—to the best of the author’s knowledge, this is the first time such an apparatus has been created and shown to produce reproducible, physically-informative TD spectra. We demonstrate the potential of this novel spectroscopic technique on three MOFs, as we report their respective TDS and IR signatures. The agreement between our TDS and IR techniques is remarkable, as is the amount of information apparent in the TD spectra, and the agreement of our TD spectra with those in the literature. With our simple technique we are able to clearly distinguish the TD spectra of H2 and D2, allowing for the evaluation of MOFs with respect to their isotopologue separating ability.

In addition to a proof of concept as to the proficiency of the experimental apparatus, this work presents two main findings: that the desorption of hydrogen isotopologues from MOFs does not follow the coverage-independent Polanyi-Wigner equation, and that stronger binding MOFs exhibit diminishing returns with respect to their ability to separate hydrogen isotopologues via temperature programming.

As we argue on several occasions in this thesis, the TD spectra of hydrogen desorbing from the MOFs examined with our technique do not obey the coverage-independent Polanyi-Wigner equation. This is foremost demonstrated by the poor ab initio fits of our spectra to the equation. This result is also corroborated by the coverage dependence of the TD spectra of Co-MOF-74 (dobdc), however, and further by the ramp rate dependence of these spectra. In demonstrating this result, we advise against the use of the coverage-independent Polanyi-Wigner equation—and analysis techniques based off of it—when considering the desorption of hydrogen from MOFs. As these techniques have begun to feature prominently in the literature, this result proves exceedingly pertinent.

We arrive at the latter conclusion by examining the MOFs reported on as a group, and examining the separation of H2 and D2 TD peaks as a function of MOF binding energy. We conclude through experimental as well as through computational techniques that the prospect of temperature-programmed separation through total desorption of H2 and total adsorption of D2 is exceedingly bleak. This surprising result rules out the most straightforward use of MOFs for hydrogen isotopologue separation, what we name Zero Point Energy Separation (ZPES) at a single site. As the field surrounding MOFs tacitly assumes this as a promising possibility, again this result proves exceedingly pertinent. The prospect of more imaginative uses of MOFs for temperature-programmed isotopologue separation remains open, as does the possibility of isotopologue separation through other mechanisms involving MOFs.

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