Adsorption and Desorption of HD on the MetalâOrganic Framework

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Article

Adsorption and Desorption of HD on the Metal-Organic Framework Cu Zn (btc) Studied by 3-Pulse ESEEM Spectroscopy 2.97

0.03

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Mantas Šim#nas, Bettina Jee, Martin Hartmann, Juras Banys, and Andreas Pöppl J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11058 • Publication Date (Web): 04 Dec 2015 Downloaded from http://pubs.acs.org on December 5, 2015

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The Journal of Physical Chemistry

Adsorption and Desorption of HD on the Metal-Organic Framework Cu2.97Zn0.03(btc)2 Studied by 3-Pulse ESEEM Spectroscopy Mantas Šim˙enas,†,§ Bettina Jee,‡,§ Martin Hartmann,¶ J¯uras Banys,† and Andreas Pöppl∗,‡ Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania, Faculty of Physics and Earth Sciences, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany, and Erlangen Catalysis Research Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany E-mail: [email protected] Phone: +49 341 9732608. Fax: +49 341 9732649

∗

To whom correspondence should be addressed Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania ‡ Faculty of Physics and Earth Sciences, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany ¶ Erlangen Catalysis Research Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany § Contributed equally to this work †

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Abstract Cu2.97 Zn0.03 (btc)2 is a structural analogue of the well-known HKUST-1 metalorganic framework. In this compound 1% of the Cu2+ ions in the paddle-wheel units are substituted by Zn2+ resulting in the formation of Cu/Zn paddle-wheel units in low concentration. The paramagnetic Cu2+ ions of these mixed Cu/Zn pairs allow to perform pulsed electron paramagnetic resonance experiments at low temperatures. Here we report on the 3-pulse electron spin echo envelope modulation (3p ESEEM) study of the deuterated hydrogen gas HD adsorption and desorption in Cu2.97 Zn0.03 (btc)2 . The HD adsorption sites in this modified compound were identified by precisely simulating experimentally observed 3p ESEEM time domain pattern. To elucidate the HD desorption process the 3p ESEEM experiments were performed at different temperatures. Employing this method, the detachment of HD from the Cu2+ binding sites is found to already occur slightly above 6 K temperature. Hereby 3p ESEEM spectroscopy reveals to be a powerful method to study adsorption of small molecules in the local environment of Cu2+ ions.

Keywords Metal-organic frameworks, HKUST-1, Pulsed EPR spectroscopy, Electron spin echo envelope modulation, Adsorption

Introduction The adsorption and desorption of hydrogen gas in various porous materials is of high interest in terms of energy storage and transport. 1–3 In the so called metal-organic framework (MOF) compounds free binding sites at metal ions are considered to play a significant role for the hydrogen adsorption properties. 4–6 Cu3 (btc)2 (btc: 1,3,5 −benzenetricarboxylate, Figure 1),

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also known as HKUST-1, is a model MOF material which provides such coordinatively unsaturated sites at the Cu2+ ions of the Cu/Cu paddle-wheel units. 7 Its hydrogen adsorption and desorption properties had been intensively studied using various experimental techniques such as thermal desorption spectroscopy, 8 infrared spectroscopy, 9 neutron diffraction 10,11 or inelastic neutron scattering. 12,13 At low loadings of D2 (up to four molecules per copper site) neutron diffraction experiments indicated a progressive filling of six distinct D2 sites in Cu3 (btc)2 . 10

Cu O C

Figure 1: Structure of Cu3 (btc)2 along [001] and [111] directions. Hydrogen atoms are not shown. In addition to these methods, electron paramagnetic resonance (EPR) spectroscopy proved to be a powerful tool for investigating intrinsic and adsorbed-gas induced phenomena in Cu3 (btc)2 and similar MOFs. 14–16 Previous pulsed EPR studies demonstrated that paramagnetic Cu2+ ions (3d9 , S = 1/2) in the slightly modified compound Cu2.97 Zn0.03 (btc)2 are very sensitive probes to explore the adsorption of CH3 OH,

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CO2 and

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CO. 17,18 In addition,

former low temperature pulsed electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) low temperature study of the adsorbed HD and D2 gases on Cu2.97 Zn0.03 (btc)2 indicated a side-on coordination of the HD and D2 molecules at the Cu2+ site. 19 Here we employ 3-pulse electron spin echo envelope modulation (3p ESEEM) experiments to microscopically explore the adsorption and the temperature-induced desorption of the deuterated hydrogen HD molecules from the Cu2+ binding sites of mixed Cu/Zn paddle3



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wheel units of Cu2.97 Zn0.03 (btc)2 . 3p ESEEM allows us to study the temperature dependence of deuterium modulation providing straightforward access to the density of HD in the vicinity of Cu2+ ion. Such weak interactions between Cu2+ ions and HD molecules are usually not resolved in the continuous-wave EPR spectrum. Note that pulsed EPR spectroscopy cannot access adsorption sites on Cu/Cu paddle-wheels, since at low temperature such units are in the total electron spin S = 0 state. 14 However, the mixed Cu/Zn units are paramagnetic (S = 1/2) and thus their environment can be studied with these methods. The low concentration of such units is necessary to minimize the unwanted dipolar interactions. The reason for selecting deuterated gas as a loading material instead of H2 is that deuterium nuclei provide higher sensitivity in the 3p ESEEM experiments. 20 The HD gas was chosen instead of D2 to simplify the experimental data interpretation and simulations.

Experimental Methods 3p ESEEM (pulse sequence: π/2 −τ −π/2 −T −π/2 −τ −echo) experiments were performed on a Bruker ELEXYS E580 spectrometer operating at X-band frequency (9.75 GHz). For the experiment nonselective microwave pulses (t π2 = 16 ns) were used. In most of the experiments the delay between the first and the second pulses τ was set to 224 ns. The delay T between the second and third pulse was incremented in steps of 14 ns starting from 80 ns. The obtained 3p ESEEM time domain (TD) pattern was corrected by subtracting a doubleexponential decay and then Fourier transformed to the frequency domain (FD) spectrum. Simulations of the TD signals were performed using EasySpin 5.0.12 software. 21,22 Adsorption of the deuterated hydrogen HD was performed on activated Cu2.97 Zn0.03 (btc)2 sample. Synthesis and the activation procedure was described earlier. 17 The HD loading in our samples corresponds to two HD molecules per each Cu2+ ion in the framework.

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Results and Discussion The 3p ESEEM experiments were carried out at 340.9 mT corresponding to the gxx,yy position in the Cu2+ EPR powder spectrum (Figure 2a). In such experiment the obtained amplitude of the stimulated electron echo is modulated by nuclei which are surrounding the Cu2+ ion and have a nuclear spin I 6= 0. 23,24 Thus, 3p ESEEM probes the local environment of the paramagnetic center. The TD patterns recorded at 6 K and τ = 224 ns of the activated and HD loaded Cu2.97 Zn0.03 (btc)2 are shown in Figure 2b. Note that our experiments were performed with samples having a loading of two HD molecules per metal ion. For the activated sample the typical modulation of 1H nuclei (IH = 1/2) with a period of about 69 ns is observed at 6 K. After Fourier transformation this modulation results in a single peak at the 1H Larmor frequency νLH = 14.51 MHz in the FD spectrum (Figure 2c). In the absence of adsorbed molecules this peak is assigned to the weakly coupled protons from the aromatic rings of the btc linker. After adsorption of HD a modulation with a longer period of about 448 ns is superposing the proton modulation (Figure 2b). The FD spectrum reveals three well resolved peaks around the deuterium (ID = 1) Larmor frequency νLD = 2.23 MHz (see Figure 2c). The central peak (marked by the asterisk) corresponds to a weak interaction between HD and the Cu2+ center, while outer peaks (open circles) originate from the splitting due to relatively strong coupling. The interactions involving a single deuterium nucleus that are in principle accessible by the 3p ESEEM spectroscopy are summarized in the following ligand spin Hamiltonian 20

H D = SAD I D − βn gD B D I D + I D QD I D .

(1)

Here the first term describes the hyperfine (hf) interaction between unpaired electron from Cu2+ center and deuterium nucleus. S and I D are electron and deuterium nuclear spin operators, respectively, and AD is the deuterium hf tensor. The second term is the nu5



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D

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Figure 2: (a) EPR spectra of activated Cu2.97 Zn0.03 (btc)2 and loaded with HD. Asterisks indicate the field positions (340.9 mT) where the 3p ESEEM patterns were recorded. 3p ESEEM (b) TD and (c) FD signals of activated Cu2.97 Zn0.03 (btc)2 and after adsorption of HD. The weakly and strongly coupled deuterium signals in the FD are marked by the asterisk and open circles, respectively. The experiments were performed at 6 K and τ = 224 ns.

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clear Zeeman interaction between nuclear spin and external magnetic field B. βn and gD are nuclear magneton and gyromagnetic ratio of the deuterium nucleus, respectively. The last term describes the nuclear quadrupole (nq) interaction characterized by the deuterium nuclear quadrupole tensor QD . Namely the hf interaction is responsible for the observed splitting in the FD spectrum (Figure 2c). To investigate spatial distribution of HD molecules in the vicinity of Cu2+ ions, we performed careful simulations of the 3p ESEEM TD trace recorded at 6 K. In addition to the ligand Hamiltonian H D , we had to include the following core spin Hamiltonian of Cu2+ paramagnetic center in the simulations

H Cu = βe Bg Cu S + SACu I Cu ,

(2)

where the first term describes the electron Zeeman interaction characterized by the g-tensor, and βe is the Bohr magneton. The second term takes into account the hf interaction between unpaired electron and copper nucleus (ICu = 3/2). Note that the tensors g Cu and ACu were previously determined using continuous-wave EPR spectroscopy. 17 First of all, let us consider the simplest case of HD distribution within the pores that corresponds to HD coordination only at the axial sites of Cu/Cu or Cu/Zn paddle-wheel units (HD(1) site, see Figure 3 for HD site labeling). Previous neutron diffraction study performed at 5 K had shown that at a loading of two D2 molecules per Cu all such sites in Cu3 (btc)2 were fully occupied. 10 To properly simulate TD traces, one has to specify AD and QD tensors of deuterium nuclei which interact with the chosen Cu2+ center. For the strongest coupled (closest to Cu2+ ) deuterium these tensors were previously determined with high precision by HYSCORE and pulsed ENDOR experiments 19 . In addition to the site occupancy factors, the positions of D2 molecules within the Cu3 (btc)2 framework were also reported by Peterson et al. 10 allowing us to estimate AD (magnitude and orientation) of distant (weakly coupled) deuterium nuclei. For such nuclei the isotropic hf contribution is negligible and thus the hf

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tensor is AD = (−T, −T, 2T ), where T is the dipolar hf interaction. T can be calculated using the point-dipole approximation 20

T =

µ0 gCu gD βe βn . 4π R3

(3)

Here gCu is the average value of the g Cu -tensor components and R is the distance from Cu2+ to HD. Note that we only take into account HD with R < 10 Å in the simulation. Also we ignored the nq interaction for distant nuclei, since it had only a minor effect on the simulations. HD(1)

HD(2)

HD(3)

HD(4)

HD(5)

HD(6)

Cu/Zn O C HD

Figure 3: Possible HD adsorption sites in Cu3 (btc)2 as determined from neutron powder diffraction study. 10 HD(1) represents the coordination of HD molecules at the unsaturated axial sites of the paddle-wheel units. HD(2) and HD(5) are within the 5 Å pores, while HD(4) and HD(6) reside within the large pores of 9 Å diameter. HD(3) site is situated in the windows accessing 5 Å pores. Results of the TD simulation together with the corresponding FD spectrum are presented in Figure 4a and b, respectively. It is obvious that the simulation which includes solely HD(1) sites does not agree with the experimental data obtained at 6 K. The FD spectrum of simulated TD reveals that HD(1) species alone cannot reproduce the experimentally observed central peak which corresponds to weakly coupled deuterium nuclei. This indicates that 8


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besides the HD molecules bound at the Cu2+ ions, indirectly coordinated HD molecules are present in the pores. This result is expected, since in the simulation we only took into account roughly half of the loaded HD molecules. However, neutron diffraction study revealed that at a loading of two molecules per copper ion, the HD(3) site (Figure 3) was also fully populated, while the HD(2) site was found to be slightly filled (∼20%). 10 Thus, in the subsequent simulation we included HD molecules at sites HD(1), HD(2) and HD(3). Note that to mimic a partial HD(2) site occupancy, we averaged 200 simulations with different random HD arrangements corresponding to the observed occupancy of this site. The match between the simulated and measured TD traces improved significantly after considering occupation of HD(2) and HD(3) sites, but it remains unsatisfactory. As can be seen from the corresponding FD spectrum, the intensity of the central line increased greatly, but not sufficiently. Therefore, we decided to introduce a possibility of a minor occupation (up to 20%) of other plausible HD sites: HD(4), HD(5) and HD(6). These sites were found to be partially filled at higher level of D2 loading. 10 Also we reduced the HD(1) and HD(3) site occupation to 80% to model a non-uniform distribution of the HD molecules across the pore system. The obtained TD trace (average of 200 simulations) is also presented in Figure 4 revealing perfect agreement with the experiment. It was unnecessary to sample more than 200 realizations, since the result would change only slightly as indicated in Figure S1 (Supporting Information, SI). Note that we verified the proposed simulation procedure by simulating the 3p ESEEM TD traces recorded at different τ and magnetic field values (presented in the SI). The need to consider HD(4), HD(5) and HD(6) site occupancy might suggest that not all of the pores are equally accessible to the HD gas and thus some of them have a local loading of more than 2 HD molecules per Cu2+ ion. It could be that residual solvent molecules and structural internal or surface defects of the crystallites prevent access to some of the pores. Indeed, in the previous EPR studies structural defects were found in Cu3 (btc)2 14 and other MOFs containing Cu/Cu pairs. 15,16 The origin of such defects was determined to be the

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broken Cu/Cu paddle-wheel units. 25 In addition, the comparison between experimental and simulated TD traces with recovered decaying baseline (Figure 4c) reveals that the modulation depth parameter kmod 26,27 (see inset in Figure 5 for the definition) is lower in the experimental spectrum, although the decay corrected traces agree perfectly. This again implies that the HD sites in the real material are non uniformly occupied across the whole sample. Consequently, there exist an appreciable number of pores in the Cu2.97 Zn0.03 (btc)2 which are restricted to HD molecules. In these pores the occupation of the HD sites would be substantially lower or even zero resulting in reduction of the experimentally observed modulation depth. To investigate the HD desorption, the temperature dependent 3p ESEEM measurements (340.9 mT, τ = 224 ns) were performed at temperatures between 6 K and 35 K. The obtained TD (normalized) and FD signals are presented in Figure 5a and 5b, respectively. With rising temperature the decay of the TD trace becomes more rapid as a result of increased relaxation rate. This complicates the FD analysis, since spectral peaks above 25 K become too broad to reliably distinguish between strongly and weakly coupled deuterium nuclei. Below this temperature we approximated these peaks using three Gaussian spectral lines and determined the corresponding intensities. The temperature dependence of the signal intensity ratio IW /IS of weakly to strongly coupled deuterium is shown in Figure 6a. Here IS mainly represents the strongest coupled HD at the HD(1) site as determined by the simulation (first curve in Figure 4b), while all distant deuterium nuclei contribute to IW . Note that distant HD molecules at other HD(1) sites also add to IW , but the influence is very weak. A clear increase of IW /IS with rising temperature is observed indicating that HD molecules gradually desorb from HD(1) sites, but tend to stay within the pores in the close proximity to Cu2+ centers. The most likely scenario is that HD molecules also desorb from other sites, since the adsorption enthalpy at the HD(1) site is believed to be the highest. 12 In addition, the temperature dependence of the modulation depth parameter kmod measured at 420 ns (Figure 6b) also reflects the HD desorption process. The TD simulation

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Figure 5: (a) TD and (b) FD of 3p ESEEM experiments recorded at different temperatures. The weakly and strongly coupled deuterium signals in the FD are indicated by the asterisk and open circles, respectively. The inset in (a) explains the definition of the modulation depth parameter kmod . demonstrates that kmod at this position is mainly determined by the deuterium at the HD(1) site and therefore it can be taken as a measure of the relative amount of adsorbed HD species at the Cu2+ sites. We observe a decrease in the modulation depth towards higher temperatures reflecting the local HD desorption process from the Cu2+ sites which already starts below 12 K. Thermal desorption experiments of H2 in Cu3 (btc)2 revealed that desorption takes place mainly at temperatures between 27 K and 80 K. 8 However, our local study of the Cu2+ coordination environment indicates that the HD desorption already starts at temperatures lower than 12 K, yet the desorbed molecules remain inside the pores. The desorbed HD molecules are expected to diffuse through the nanoporous channels where they are constantly adsorbed at the sites for a limited period of time. Such desorption/adsorption phenomenon are considered to be a dynamic process. As the temperature increases, the time a molecule spends on the HD(1) site decreases and it is effectively detected as weakly coupled species. Thus the intensity ratio IW /IS increases with rising temperature. Only at higher temperatures 12


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Figure 6: Temperature dependence of (a) the intensity ratio between the weakly and strongly coupled deuterium nuclei FD signals and (b) modulation depth parameter kmod extracted from the TD traces at T = 420 ns. the molecules are released from the material and become detectable in thermal desorption experiments. Unfortunately, at such temperatures HD molecules are not accessible to the 3p ESEEM experiments due to fast relaxation of the electron spin. We note that a similar gas desorption with detaching from adsorption sites and afterwards remaining in the pores was also observed in many porous zeolites with different Lewis acid sites (see Ref. 28,29 ).

Conclusions In summary, using 3p ESEEM spectroscopy we have investigated the adsorption and desorption of HD in the HKUST-1 analog metal-organic framework Cu2.97 Zn0.03 (btc)2 . We observed the electron spin echo amplitude modulation by deuterium nuclei in the vicinity of Cu2+ ion. Sophisticated simulations allowed us to distinguish the contribution of different HD adsorption sites to the observed time domain trace. Temperature dependent 3p ESEEM measurements allowed us to locally probe the desorption of HD from Cu2+ ions. We ob-

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served an initial detachment of the HD molecules from the adsorption sites already at very low temperatures. In our experiments we also showed that the desorbed molecules remain close to these sites, since they still can be detected by their smaller hf coupling.

Acknowledgement The authors thank the German Research Foundation (DFG) for financial support of this project within the priority program SPP 1362. M.Š. thanks German Academic Exchange Service (DAAD) for the financial support.

Supporting Information Available Additional details of simulations. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Adsorption and Desorption of HD on the MetalâOrganic Framework

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Adsorption and Desorption of HD on the MetalâOrganic Framework