ARTICLE pubs.acs.org/JPCC
Hydrogenation of CuBTC Framework with the Introduction of a PtC Hydrogen Spillover Catalyst Xiao Ming Liu,† Sami-ullah Rather,†,^ Qixiu Li,†,‡ Angela Lueking,*,†,‡,§ Yonggang Zhao,|| and Jing Li|| Materials Research Laboratory, ‡Department of Energy & Mineral Engineering, and §Department of Chemical Engineering, EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, United States
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bS Supporting Information ABSTRACT: Hydrogen uptake of a microporous metal organic framework, CuBTC, is increased 3.5-fold at 298 K and 20 bar upon the addition of a hydrogen spillover catalyst, from 0.17 to 0.61 wt %. Structural integrity upon mixing with the catalyst is important to achieve this level of uptake. Increasing the adsorption temperature to 323 K significantly reduces the rate of uptake, but 0.55 wt % uptake is observed when the experimental equilibration time is extended. The slow, pressureindependent uptake at 323 K, along with the desorption behavior is suggestive of a hydrogenation process of the CuBTC substrate. PXRD analysis suggests the hydrogenated sample remains intact and FTIR demonstrates hydrogenation of the carboxylate group of the BTC ligand but finds no evidence for hydrogenation of the carbons of the BTC ligand. Although hydrogenation of the CuBTC does not lead to readily desorbable H2, the results shed light on a possible mechanism of the hydrogen spillover process.
1. INTRODUCTION A considerable challenge for hydrogen storage for mobile applications has been to engineer solid-state storage materials with intermediate binding energies to enable adsorption and desorption near ambient temperature conditions.1 Moderate temperature operation provides several logistical advantages, including a significant decrease in auxiliary equipment required to maintain the storage tank, simplicity of operation, and heat management requirements upon adsorption and desorption. Great strides have been made in tailoring the structure and ligand surface chemistry in microporous metal organic frameworks (MMOFs) for physisorption of hydrogen,213 yet the operative adsorption temperatures of MMOFs continue to be low, as the physical interaction between molecular hydrogen and a surface is governed by weak interactions. The weak interactions typically lead to hydrogen desorption at low temperatures, with pore/channel size being a dominant factor in binding energy and desorption temperature.14 A strategy to increase adsorption temperature is to incorporate a catalytic entity that dissociates the hydrogen, and then the hydrogen “spills over” to the support.1533 Reviews of hydrogen spillover34,35 and increasing hydrogen storage via spillover36,37 are provided elsewhere. Focusing on the studies relevant to MMOFs, the hydrogen uptake of IRMOF1 and IRMOF8 at 298 K was significantly increased upon introduction of a PtC catalyst known to dissociate H2: from 0.4 to 1.5 wt % for IRMOF1 and from 0.5 to 1.7 wt % for IRMOF8 at 10 MPa.38,39 The hydrogen uptake was further increased when the catalyst-MMOF contact was enhanced r 2011 American Chemical Society
via carbonization of a sugar to form a “bridge” (br) to enhance interfacial diffusion from the catalyst to the MMOF receptor, leading to 4 wt % excess adsorption at 100 bar and 298 K for brPtC/IRMOF8.39 Tsao et al. reproduced this level of uptake in two independent laboratories using gravimetric methods, achieving up to 4.2 wt % at 6.9 MPa after extended equilibration for brPtC/IRMOF8.28 This “extended equilibration” time is a feature often associated with hydrogen spillover and may be a unique aspect of spillover, as physisorption at 298 K is rapid. Similarly, mixing MOF177 with PtC increased the hydrogen uptake from 0.62 to 0.8 wt % at 298 K and 100 bar.40 Mixing MIL-101 with PtC increased the hydrogen uptake from 0.54 to 0.75 wt % at 293 K and 50 bar, with a further increase to 1.14 wt % after the bridging treatment.41 Direct incorporation of metal catalysts into MMOFs has also been reported to increase the uptake, at both low temperature (77 K)42 and ambient temperature (298 K).43,44 Direct incorporation of metals into MMOFs tends to have a drastic effect on surface area, channel size, or porosity as the metals may locate within the MMOF structure. For example, incorporation of 2.5 nm Pd nanoparticles into MIL-100(Al) reduced surface area from 1200 to 380 m2/g and micro pore volume from 0.40 to 0.12 cc/g, even as hydrogen uptake at 298 K and 4 MPa was increased by almost a factor of 2.43 Elsewhere, direct Pt-doping of IRMOF8 Received: August 3, 2011 Revised: October 11, 2011 Published: December 09, 2011 3477
dx.doi.org/10.1021/jp207411b | J. Phys. Chem. C 2012, 116, 3477–3485
The Journal of Physical Chemistry C
ARTICLE
2. EXPERIMENTAL METHODS
Figure 1. PXRD patterns of as prepared Cu-BTC and PtC/CuBTC composite: (a) as prepared CuBTC; PtC/CuBTC prepared (b and b0 ) by hand grinding in a low oxygen glovebox (two separate preparations) and (c and c0 ) hand grinding in air; (d) ball milling in air; (e) sample b after H2 adsorption measurements. The main peaks in the XRD pattern of lines a, b, d, and e can be indexed to face-centered cubic (FCC) crystal lattice of Fm3m symmetry of CuBTC. The intensity of line a has been reduced by a factor of 3 relative to the others for scaling. All samples are measured in PXRD with solvent except line e. All data (except line c) have been normalized to the intensity of the feature at 11.6.
via thermal deposition of organo-metallic precursors led to 0.8 wt % at 100 bar and 298 K for optimal metal particle size.45 Small catalyst particle size (
