Evidence of Nanoconfinement Effects in the Adsorption of Hydrogen

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Evidence of Nanoconfinement Effects in the Adsorption of Hydrogen on Coinage Metal Complexes Dispersed within Porous Carbon Mohammad Reza Andalibi, Ali Qajar, and Henry C Foley J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05173 • Publication Date (Web): 24 Aug 2015 Downloaded from http://pubs.acs.org on September 1, 2015

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The Journal of Physical Chemistry

Evidence of Nanoconfinement Effects in the Adsorption of Hydrogen on Coinage Metal Complexes Dispersed within Porous Carbon Mohammad R. Andalibi, Ali Qajar †, Henry C. Foley ‡,* Fenske Laboratory, Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802-440 USA.

1 ACS Paragon Plus Environment


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ABSTRACT. In this article we report the results of careful, room-temperature, high-pressure adsorption studies, in which simple complexes of copper and silver reversibly adsorb hydrogen, when dispersed within nanoporous carbon. Whereas, these complexes alone did not adsorb hydrogen, when they were nanoconfined within carbon, they adsorbed 1-3 moles H2 per metal center. As a figure of merit, nanoconfined cupric formate adsorbed ~ 2.6 H2/Cu at 100 bar and 20°C, much larger than any reported metal-containing adsorption medium. On carbon alone the heat of hydrogen adsorption decreased with an increase in adsorption extent, limiting to insufficient levels of uptake. By contrast, when these metal salts were dispersed within the carbon, the heats of adsorption increased markedly in a linear manner, meaning that the thermodynamics has moved in the right direction and are not self-limiting. Such a thermodynamic behavior is associated with side-on dihydrogen binding onto a metal center, the so-called Kubas binding. Such interaction, however, is generally observed with earlier transition metals and is therefore rather unexpected with coinage metals. Thus, we infer that there are cooperative interactions between hydrogen, the carbon pore and the metal center which may be exploited to enhance reversible hydrogen adsorption and to reach more practical levels.

KEYWORDS. Carbon nanoconfinement, Hydrogen storage, Isosteric heat of adsorption, Dihydrogen complexation, Undercoordinated metal complexes.


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1. INTRODUCTION 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.1 There are two groups of solid-state storage materials and each suffers from limitations. On the one hand, physisorptive materials have extremely low storage capacity at ambient temperatures because of the low enthalpy of hydrogen sorption, but the interaction with hydrogen molecules is reversible and the kinetics is fast. On the other hand, chemisorptive materials exhibit high heats of hydrogen binding ensuing significant uptake capacity associated with irreversibility and slow kinetics at moderate temperatures, which necessitates providing heat to release the absorbed hydrogen with an acceptable rate. Accordingly, developing materials with new properties and intermediate hydrogen interaction energetics (e.g., Kubas-type hydrogen storage materials, which are the most prominent examples in this category) will be necessary if we are ever to realize the eventual use of hydrogen as a vehicular fuel.2-5 Herein, we report on hydrogen storage materials in which coordinatively unsaturated copper(II) and silver(I) carboxylates confined in nanoporous carbon of pore size commensurate with the metal complex molecules have been used as media for dihydrogen complexation. A number of variations have been tested in order to elucidate the effect of ligands and metal center on complexation efficiency as well as isosteric heat of adsorption. For this purpose, Cu(OAc)2 (OAc = acetate), Cu(OFo)2 (OFo = formate), and Cu(OPr)2 (OPr = propionate) were chosen to study the effect of ligand electron donation capability. Furthermore, AgOAc was selected as a contrast to copper. The nanocomposites were then characterized using a number of common techniques in order to extract rational structure-property correlations. While low metal complex loading in our samples leads to gravimetric uptake capacities far below the Department of



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Energy’s targets,6 this “Proof of Principle” study is aimed at investigating the later transition metals (e.g., Ag(I) and Cu(II), here) as Kubas binding media and also the effect that the first coordination sphere would have on the heat of adsorption. This is of paramount importance considering the readily tunable nature of these nanocomposites by simply changing the metal center, the first coordination sphere (ligands), and the second coordination sphere (surface chemistry and pore structure of the support material). Future optimization of these factors would hopefully lead to a promissing hydrogen storage material for practical onboard applications.

2. EXPERIMENTAL METHODS 2.1. RAW MATERIALS AND NANOCOMPOSITE PREPARATION The support material used in this study was a commercial nanoporous carbon material provided by ACS Material Co. (called ACS hereafter) with a surface area ~ 2000 m2/g and pore size 2.0-2.2 nm. Copper(II) acetate monohydrate (98%) was purchased from Sigma Aldrich. Copper(II) formate tetrahydrate (98%) and silver acetate anhydrous (99%) were also purchased from Alfa Aesar. Finally, copper(II) propionate monohydrate was purchased from Pfaltz&Bauer Company. All of the chemicals were used as received. Distilled water was used as the solvent in all the experiments since the above mentioned metal carboxylates are barely soluble in other solvents. We used wet impregnation followed by filtration and drying as an efficient technique for nanocomposite preparation. This method gives greater dispersion compared to incipient wetness impregnation.7 Upon wet impregnation of the porous support with the desired solution of metal complex at 60°C for 24 h (copper(II) formate solution was contacted with the support at room temperature), the solid was filtered off and it was washed with 250 mL distilled water to remove unadsorbed molecules. Then it was dried slowly at 100°C to prevent aggregation of metal


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complex molecules due to solvent evaporation (copper(II) formate nanocomposite was dried at 50°C to prevent its evaporation as its bulk boiling point is 50°C).8 It is worth mentioning that impregnation step was done only once to prevent any possible aggregation that may occur as a result of multiple metal complex loading. This has the drawback of low metal complex loading and hence, low hydrogen uptake capacities. Nonetheless, high dispersion is vital in our investigation that revolves around the nature of Kubas binding in these systems. Finally, simple mixture of carbon with cupric acetate prepared in a mortar and pestle and ball-milled metal complex were also tested as control samples.

2.2. NANOCOMPOSITE TESTING AND CHARACTERIZATION Hydrogen adsorption experiments were done on a high pressure setup working based on differential pressure between an empty reference leg and a sample leg.9 All the samples— excluding cupric formate—were outgassed at 100°C before each experiment until complete dehydration and/or until no appreciable weight change was detectable. Cupric formate was outgassed at 50°C since degassing at 100°C led to its partial evaporation. The results are presented as Gibbs surface excesses measured at specific temperature and up to the desired pressure. For each data point, sufficient equilibration time was given to the system although adsorption kinetics were very fast and equilibration was reached in less than 1 minute. Adsorption reversibility was checked over 5 cycles of adsorption-desorption and all the samples showed excellent cycling behavior with no loss in their uptake capacity. The surface excess data points from the first set of experiments were fitted to an exponential function in the form of Equation (1) using Sigma Plot 12.0 software: = (1 − )

(1)



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where is hydrogen uptake in mmol/g and is equilibrium pressure in bar. Constants and were determined using the internal functions included in Sigma Plot 12.0 library. Constant was used to estimate the saturation uptake value at high pressure for each sample. Beside the former exponential fit, Equation (2) was used for fitting pressure data as a function of uptake: = . .

(2)

where, and are constants calculated in Sigma Plot 12.0 program. All the fits, either by Equation (1) or (2) had ≥ 0.98. The application of Equation (2) was for the sake of consistency with previous reports on Kubas compounds prepared by Antonelli’s group.10 Fitted data from Equation (2) were used to calculate the isosteric heat of adsorption according to modified Clausius–Clapeyron equation: = −∆"#$% =

&' & (( ) & − &' '

(3)

where and ' are equilibrium pressures at temperatures & and &' , respectively, along the isosteres and is the universal gas constant.11 Adsorption data at 20°C and 30°C where used for the preceding calculations. By directly substituting Equation (2) at two different temperatures in Equation (3): = −∆"#$% =

&' & )( * + + ( − ' )& − &' '

(4)

Nitrogen physisorption was done utilizing a Micromeritics ASAP 2020 adsorption instrument. Samples were outgassed prior to characterization under a dynamic vacuum at 100°C (50°C for cupric formate) until no appreciable weight loss was observed. Micropore volume was calculated using t-plot method. This value corresponds to pore sizes smaller than 2 nm which are classified as micropores based on IUPAC classification. Moreover, the total uptake at / / = 0.99 was used to calculate the total pore volume.12-13 BET surface area analysis was done in order to


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extract specific surface area information based on nitrogen physisorption data.12-13 Finally, pore size distributions were derived using Density Functional Theory with nitrogen adsorption data at 77 K with slit pore shaped carbon as the model. The method was non-negative regularization and no smoothing was applied.13-14 Thermogravimetric analysis coupled to mass spectrometry (TGA-MS) data were collected on a TGA Q50 with a Pfeiffer Vacuum Mass Spectrometer. Helium gas was used as the carrier gas. The data were used to assess the thermal stability of nanocomposite and the temperature range in which different fragments desorb. X-ray photoelectron spectroscopy (XPS) data were collected on a Kratos Analytical Axis Ultra. High resolution Cu2p3/2 and O1s spectra were used to assess the nature of surface metal compounds before and after hydrogen adsorption. All the data were interpreted after charge referencing of C1s binding energy to 285 eV.15 All the x-ray diffraction patterns were collected on a PANalytical Xpert Pro MPD instrument using a CuKα X-ray source. The step size used for the measurements was 0.03°. Measurements were done for 5°

Evidence of Nanoconfinement Effects in the Adsorption of Hydrogen

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