hydrogen storage in select metal-organic frameworks via hydrogen spillover

Open Access
Author:
Wang, Cheng-yu
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 24, 2014
Committee Members:
  • Angela Lueking, Dissertation Advisor
  • Chunshan Song, Committee Chair
  • Randy Lee Vander Wal, Committee Member
  • Robert Martin Rioux Jr., Committee Member
Keywords:
  • adsorption
  • catalysis
  • hydrogen spillover
  • metal-organic framework
Abstract:
Hydrogen storage is one key challenge to achieve hydrogen economy. Hydrogen spillover in metal-organic frameworks (MOFs) has been proposed to reach the Department of Energy (DOE) hydrogen storage goals 5.5 wt% (2015) and 7.5 wt% (ultimate). The adjustable surface, porosity, and chemical functionality are beneficial to the hydrogen spillover research, in order to observe the evidence of atomic hydrogen dissociated from transition metals, to understand possible mechanism of hydrogen spillover, and ultimately to manipulate the adsorbent structure to get high uptakes at ambient environments. In this study, copper type MOFs Cu-BTC and Cu-TDPAT, as well as zinc type MOF IRMOF-8, were studied. Different transition metal (Pt) doping methods were examined to determine the best way to synthesize catalyst-MOF composites, with no loss in structural integrity and with high hydrogen storage at room temperature. Pre-bridge (PB) technique was adopted to prepare platinum on activated carbon (Pt/AC) with copper type MOF Cu-TDPAT (T), which showed good structural stability and the highest hydrogen adsorption at 300 K 1 bar (4.9 cc/g STP, 0.045 wt%) among other catalyst-MOFs, and among the uptakes reported in the literature. Spectroscopic evidence in XPS and FTIR support the discovery of atomic hydrogen (1) in between the Cu-O-C bond that connects the TDPAT ligand to the copper paddlewheel (Cu PDW) in MOFs, (2) the sp2 N aromatic heterocycles in the center ring and (3) the secondary-amine type NH in the branches of TDPAT. However, hydrogen uptake of PB-T was 0.8 wt% at 70 bar, with limited slow kinetics. Furthermore, in addition to catalysts, addition of the activated carbon (AC) to MOF Cu-TDPAT also improved hydrogen storage. This is possibly due to the introduction of the defects, or partial charging of the MOF, supported by increased external surface area, the copper oxidation state measured by XPS, and density functional theory showing charged ligands are prone to hydrogenation. To further study the role of defects, carbon monoxide (CO) adsorption in FTIR was used to probe Cu-TDPAT and Cu-BTC. Strong CO-Cu+ adsorption is observed at 150 K ~5 psi, and in TPD-FTIR, CO desorption from Cu+ occurred over 300 K, relatively to CO-Cu2+ adsorption at over 200 K. No CO adsorption in Cu-TDPAT was found, regardless of pretreatment conditions, sample preparations, and adsorption temperatures. Pore size effect and electron donation/withdraw are consistent of lack of CO-Cu adsorption in Cu-TDPAT. Overall, the hydrogen uptake of catalyst Pt/AC on MOF Cu-TDPAT did not reach the DOE target 5.5 wt% at the pressure 70 bar, with the limitation of slow kinetics. The major limitations to catalyzed Cu-TDPAT (as well as other catalyzed MOFs) to meet the gravimetric targets at 300 K are high molecular weight of copper, high diffusion energy barrier, and material instability.