hydrogen adsorption by late transition metal complexes confined in nanoporous carbon materials

Open Access
Author:
Andalibi, Mohammad Reza
Graduate Program:
Chemical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 12, 2014
Committee Members:
  • Henry C Foley, Thesis Advisor
Keywords:
  • Hydrogen adsorption
  • Kubas binding
  • Hydrogen Activation
  • Heat of adsorption
Abstract:
Solid-state hydrogen storage technologies are the most viable options for mobile vehicular applications mainly due to their compactness and safety as compared to compressed gas or liquid hydrogen tanks. However, each of the two groups classified as solid-state storage methods suffers from limitations. On the one hand, physisorption has extremely low storage capacity at near ambient temperature because of the low enthalpy of hydrogen sorption, but it is reversible. On the other hand, chemisorption has high heat of hydrogen binding ensuing significant uptake capacity associated with irreversibility at moderate temperatures, which necessitates providing heat to release absorbed hydrogen. Hence, developing intermediate technologies, integrating promising properties of both groups, has primary importance to realize hydrogen economy. Dihydrogen complexes are well known for their ability to reversibly interact with H2. Transition metal complexes with an unsaturated metal center are among compounds interacting with H2 through so called Kubas interaction. This kind of interaction involves side on donation of hydrogen σ bond electrons to an empty d-orbital of the metal center subsequently stabilized by back donation from the filled d-orbitals to the σ* orbital of H2. Such materials attract hydrogen with an enthalpy intermediate between that of physical and chemical sorption (ca. 60-85 kJ/mol for stable transition metal complexes and 15-40 kJ/mol for the weakly bound dihydrogen systems at high pressure). The major drawback of such systems is the redundant weight imposed by co-ligands which are necessary to arrest dihydrogen structure and prevent bond rupture. This, in turn, adds to system weight which reduces the overall uptake capacity. Dihydrogen complexation is the first step in H2 activation process by metal complexes. In other words, any hydrogenation catalyst potentially can form dihydrogen complex. Pioneering works on H2 activation by Halpern and coworkers introduced Cu(II) complexes (e.g. cupric acetate) as active catalysts able to heterolytically break dihydrogen via homogeneous mechanism. These complexes were able to reduce more reactive materials such as Cr2O72- or para-benzoquinone but not less reactive olefins. Halpern deduced in the latter case that the reverse reaction producing the initial complex and dihydrogen defeats hydrogen transfer from the hydride to the substrate. This was the incentive for the current study, that is these materials react with dihydrogen in a reversible manner and further they are lighter than conventional Kubas compounds. This entices seeking a way to stabilize dihydrogen complexes of such compounds at reasonable conditions for use as a storage medium. Recently, researchers have come up with a novel technique called nanoconfinement to alter thermodynamics and/or kinetics of chemical hydrides. Based on this method, reducing the size of storage medium to nm range via its confinement in porous scaffolds could alter its behavior. This is mainly due to increased surface energy induced by decreasing the particle size that renders molecules more reactive. Moreover, nanoconfinement could arrest dihydrogen structure by steric hindrance as it does not let proton and hydride separate enough upon H2 cleavage. Therefore, the goal of this study was to employ nanoporous carbon scaffolds as a host for nanosized, lightweight, under-coordinated Cu complexes in order to tailor their behavior so that they reversibly interact with H2 at ambient temperature. This novel class of storage media offers benefits from both ordinary chemisorption and physisorption systems at the same time.