UHV Surface Science Investigations of Adsorption, Reaction and Diffusion in Ultra-thin Metal Organic Framework Films
Abstract
Chemical warfare agents (CWAs) are a group of extremely dangerous molecules that can be used against American soldiers in various theaters of war involving terrorist organizations (e.g., ISIS) as well as hostile nations. It is important to be able to effectively defuse these threats/CWAs via rapid and early detection, protective measures, and destruction/isolation in order to maintain the health and viability of US warfighters. Metal organic frameworks (MOFs) (metal ions or clusters coordinated to organic ligands creating materials with high porosity and tunable properties) are promising with regard to playing a role in the detection, absorption, and catalytic decomposition of molecules used as CWAs. MOFs can show a very high degree of molecular specificity regarding gas separations and adsorption. Because of the high degree of tunability and structural variability providing steric control and enhancing catalysis, the promise of MOFs with regard to CWAs was recognized early, and significant and important investigations have been performed, especially in the liquidÐphase destruction of CWAs. We propose investigations of the interactions of gas-phase molecules and CWA simulants with MOFs employing ultrahigh vacuum surface science methods that will yield fundamental insights regarding the use of MOFs and their applicability to CWA detection, absorption, and destruction. We will begin our studies using the ZIF-8 MOF (ZIF = Zeolitic Imidazolate Framework) with plans to study additional MOFs in the future, especially HKUST-1, UiO-66-NH2 and MIL-101. We initially propose to grow ultra-thin films of ZIF-8 to study adsorption/absorption, reactions and transport on and within the framework under ultrahigh vacuum (UHV) and near-atmospheric pressure conditions. Very little research utilizing surface science tools toward the study of MOFs has been performed previously and such studies could advance the field significantly. We will extensively employ molecular beams and thermal desorption mass spectrometry in our studies of ultra-thin in situ grown MOF films since these techniques provide exquisite precision in measurements of surface area and adsorption/absorption phenomena. It is difficult, to employ the standard surface area measurement techniques (e.g., BET isotherm) to ultra-thin films because typically the amount of absorbent material is so small. The use of a collimated molecular beam will limit gas exposure to the MOF without significant exposure to other surfaces. This coupled with the use of a quadrupole mass spectrometer for temperature programmed desorption in vacuum allows very accurate detection of the gas during desorption with excellent signal to noise. A comparison of the quantity of desorbing gas from a MOF to the quantity of desorbing gas from a flat surface allows an excellent estimation of the surface area of the MOF, even for a single monolayer of MOF thin film. To the best of our knowledge, such techniques have not been previously applied to the study of MOFs. Additional techniques we plan to employ include low energy electron diffraction, x-ray diffraction, scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and steady-state reactivity measurements at atmospheric pressure. As mentioned earlier once we have gained sufficient experience with measurements involving ZIF-8 we will begin investigations of HKUST-1, UiO-66-NH2 and MIL-101.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 15, 2018
- Source ID
- W911NF1710214
Entities
People
- Charles Mullins
Organizations
- Army Contracting Command
- United States Army
- University of Texas at Austin