Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2017, 121, 27369−27378
Influence of Metal−Organic Framework Porosity on Hydrogen Generation from Nanoconfined Ammonia Borane Jing-Yang Chung,† Chi-Wei Liao,† Yi-Wei Chang,‡ Bor Kae Chang,‡ Hao Wang,§ Jing Li,§ and Cheng-Yu Wang*,∥ †
Department Department § Department ∥ Department ‡
of of of of
Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan Chemical and Materials Engineering, National Central University, Taoyuan 32001, Taiwan Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, United States Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
S Supporting Information *
ABSTRACT: Hydrogen released from chemical hydride ammonia borane (AB, NH3BH3) can be greatly improved when AB is confined in metal−organic frameworks (MOFs), showing reduced decomposition temperature and suppressed unwanted byproducts. However, it is still debatable whether the mechanism of improved AB dehydrogenation is due to catalysis or nanosize. In this research, selected MOFs (IRMOF-1, IRMOF-10, UiO-66, UiO-67, and MIL-53(Al)) were chosen to explore both catalytic effect of the metal clusters and the manipulation of pore size for nanoconfinement by variations in ligand length. When AB particle size was restricted by the controlled micropores of MOFs, we observed that the decomposition temperature was not correlated to the MOF catalytic environment, but inversely proportional to the reciprocal of the particle size. The results correspond well with the derived thermodynamic model for AB decomposition considering surface tension of nanoparticles.
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INTRODUCTION The high chemical energy density of hydrogen (142 MJ/kg; carbon fuels 47 MJ/kg) makes it a strong candidate as a renewable energy carrier,1 in order to reduce the dependence on fossil fuels with limited reserve.2,3 Despite the merits, hydrogen storage is one of the key hindrances for energy application.4,5 Among several materials proposed, such as sorbent,6 metal,3,7 and chemical hydride,8,9 ammonia borane (AB, NH3BH3) has shown possible superiority in solid-state hydrogen storage, for the reasons of high hydrogen loading (19.6 wt %), low molecular weight, and stability at room temperature. However, high thermolysis temperatures of AB with three equivalent H2 with about 6.5 wt % released at each step are observed at ∼100 , ∼130, and over 500 °C. Detrimental byproducts such as ammonia, diborane, and borazine are also generated during AB decomposition.10,11 High dehydrogenation temperature and byproducts impede the utilization of AB in hydrogen economy. In order to reduce the hydride decomposition temperatures, several methods are developed, such as catalytic hydrolysis,12−17 catalyzed thermal decomposition,18−20 nanoconfinement, etc. In particular, nanoconfinement of AB in porous materials, including porous carbon,21−24 zeolite,25 and metal organic frameworks (MOFs),26−34 is an effective means to decrease decomposition temperature (Td). Though nanoconfinement empirically shows effectiveness on hydrogen © 2017 American Chemical Society
generation temperature reduction, the theoretical mechanism remains debatable. One possible theory is the nanosize of hydride. Incorporation of AB in MOF-5 reduced hydrogen generation temperature at 84 °C relative to 114 °C of unmodified AB.34 Yang et al.24 dispersed AB in microporous carbon from templated zeolite via AB dissolution/recrystallization in methanol solution. Hydrogen was released from the size-reduced AB at 86 °C, lower than the first stage desorption temperature by 30 °C. Reduction in Td was also reported in mesoporous carbon with confined AB.21 Sepehri et al.35 attributed the reduced Td and improved kinetics to the smaller size of AB controlled by mesoporous carbon scaffold. High surface to volume ratio of confined AB indicated significant elevated surface energy, which reduced activation energy and Td. Linear relationship was reported with no catalyst addition: Td of nanoconfined AB is inversely correlated to the pore size of scaffold. In addition, decomposition of confined AB is analogized to the particles size effect over melting/freezing of metal36 or ceramic37 considering surface tension in the literature. The melting point difference (ΔTm) can be described in the Gibbs− Thomson eq (eq 1; σ: surface tension, ρ: solid density, ΔH: Received: October 24, 2017 Revised: November 28, 2017 Published: November 29, 2017 27369
DOI: 10.1021/acs.jpcc.7b10526 J. Phys. Chem. C 2017, 121, 27369−27378
Article
The Journal of Physical Chemistry C
Figure 1. Structure of (A) IRMOF-1, (B) IRMOF-10, (C) UiO-66, (D) UiO-67, (E) MIL-53(Al)np. Color code: Zn, purple; Zr, cyan; Al, pink; C, gray; O, red; H, white.
enthalpy change, d: particle size),10 in which the fusion enthalpy change ΔHf (eq 2) is composed of bulk (ΔHbulk) and surface (ΔHsurf) parts,37 especially when small particles are considered. Combination of eq 1 and eq 2 makes the Td of AB a complex function of particle size d, instead of linear correlation. ΔTm = Tm − Tm(d) =
ΔHf =
4σslTm dΔHf ρs
6 ΔHsurf + ΔHbulk d
Td reduction. Similarly, Td was lowered when AB was added to thulium (Tm)-based MOF Tm(BTC) via either impregnation (Td = 77 °C) or hand-milling (Td = 79 °C). Even AB physically ground with nonporous Tm2 O 3 catalyst demonstrated decomposition improvement (Td = 85 °C).33 Hence, the catalytic ability of the supports is believed to contribute to AB dehydrogenation promotion, rather than nanoconfinement effect. Some studies showed that active or catalytic sites served as sorbents for byproduct removal.28 Ammonia can only be reduced when lithium ions (Li+) was added to mesoporous carbon framework CMK-3.21 In addition to the temperature reduction observed in confined AB in MOFs, metal clusters as active sites for ammonia trap were observed with Y in JUC-32Y,26 Mg and Zn in MOF-74,27,30 Fe in MIL-53(Fe),31 etc.32,33 All in all, the mechanism of AB dehydrogenation is under debate as to whether the Td reduction is due to catalytic effect or because of nanoconfinement. Hence, in this research we explored the possible criteria of hydrogen production temperature from nanoconfined AB in
(1)
(2)
On the contrary, it was mentioned that active or catalytic sites play a critical role in the promotion of AB dehydrogenation, instead of nanoconfinement effect. Lower Td was reported in the cases of AB physically mixed with carbon nanotubes (CNTs) with decorated Pt,18 zeolite with active Zn,25 and MOF with metal clusters,33 though in this case AB molecules were hardly confined in porous structure. Moreover, AB milled with nonporous zinc chloride (ZnCl2) also showed 27370
DOI: 10.1021/acs.jpcc.7b10526 J. Phys. Chem. C 2017, 121, 27369−27378
Article
The Journal of Physical Chemistry C
in the channels of MOFs and thus shows closed gate structure with narrow pores (2.6 × 13.6 Å2, Figure 1E). AB@MOFs. In order to be certain that infiltrated AB molecules were successfully confined in microporous MOFs, a fourfold ratio of total pore volume of MOFs obtained by textural property analysis (described in the following section) to the theoretical volume of 50 mg of AB (∼64.1 μL based on the density 0.78 g/mL) was used, instead of AB to metal molar ratio 1:1 that was broadly adopted in the literature. In this study, 50 mg of AB was dissolved in 5 mL of methanol followed by the addition of MOFs with preselected dose (Supporting Information, Table S1). The suspension was stirred for 2 h for better mixing, and methanol was evacuated in vacuum to collect fully dried AB@MOFs without filtration or further flushing in order to avoid possible AB removal. Characterizations. The powder X-ray diffraction (PXRD) patterns were obtained by Bruker D8 multipurpose X-ray diffractometer with 2θ from 5° to 70°, 40 kV 100 mA, step size 0.04°, scan speed 4°/min, and Cu Kα (λ = 1.543 Å) X-ray source. Full-width half-maximum (fwhm) of the diffraction peaks is applied to estimate the crystallite size of neat AB with the Scherrer equation. Textural properties of MOFs were determined by nitrogen adsorption isotherms at 77 K up to 1 bar (relative pressure P/Po = 1) via Micromeritics ASAP 2020 volumetric gas adsorption analyzer. Specific surface area (SSA) was estimated from BET with P/Po data points below 0.04 applied for MOFs.44,45 Mentioned in the following Methodology section, we revisited the Horváth−Kawazoe (H−K) model corrected by Yang et al.46 to obtain proper parameters for MOF microporous pore size distribution (PSD). Mesopore sizes were evaluated via BJH method when applicable.47 Total pore volume was determined at P/Po = 0.95. The textural property analysis was carried out after a pretreatment at high vacuum (
