Low-Temperature Adsorption of H2 and D2 on Dehydrated and Water

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Low-Temperature Adsorption of H and D on Dehydrated and Water Precovered CPO-27-Ni Nikola Drenchev, Mihail Yordanov Mihaylov, Pascal D. C. Dietzel, Alberto Albinati, Peter A Georgiev, and Konstantin Ivanov Hadjiivanov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08722 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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The Journal of Physical Chemistry

Low-Temperature Adsorption of H2 and D2 on Dehydrated and Water Precovered CPO-27-Ni

Nikola Drenchev,a Mihail Mihaylov,a Pascal D. C. Dietzel,b Alberto Albinati,c Peter A. Georgievc* and Konstantin Hadjiivanova*

a

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113,

Bulgaria b

Department of Chemistry, University of Bergen, P.O. Box 7803, N-5020 Bergen, Norway.

c

Department of Chemistry, University of Milan, via C. Golgi 19, Milano 20133, Italy.

Abstract Metal-organic frameworks (MOF) possessing open metal sites (e.g. from the CPO-27 series) are a promising class of materials for hydrogen storage. However, there is still no consensus on the vibrational signatures of H2 adsorbed on different sites. In this work we report results of a combined FTIR and DFT study on H2 adsorption on dehydrated and D2O precovered CPO-27-Ni. For unambiguous interpretation of the results adsorption of CO was also studied. Low-temperature CO adsorption on dehydrated CPO-27-Ni results in formation of Ni2+−CO adducts stabilized by π-back donation and characterized by a CO stretching frequency at 2182 cm-1 (low coverage). With D2O-precovered sample CO replaces part of the preadsorbed D2O molecules and in this case the ν(CO) is significantly lowered (2172 cm-1). When H2 is adsorbed at 100 K on dehydrated sample, complexes involving Ni2+ sites are formed and characterized by a ν(H−H) band at 4031 cm-1. A satellite band (Qtrans mode) is detected at 4249 cm-1. Similar results

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2 were obtained after D2 adsorption. However, D2 was more strongly adsorbed than H2 and the Qtrans mode was of lower relative intensity. Only at high H2/D2 equilibrium pressures (≥ 50 mbar) occupation of secondary sites was clearly detected by a ν(H−H) band at 4118 cm-1 or ν(D−D) band at 2964 cm-1. No significant differences in the strength of adsorption of H2 and D2 were detected in this case suggesting the mode of adsorption is different from that realized for the complexes involving Ni2+ sites. Progressive filling of the Ni2+ sites by D2O leads to a strong decrease in intensity of the H2/D2 bands associated with Ni2+ sites and red shift of their frequencies indicating decrease of the interaction strength. On the contrary, the bands due to H2/D2 interaction with secondary sites appear with enhanced intensities and are blue shifted which suggest increase of the interaction strength. These results were confirmed by DFT calculations showing an increase of the H2 adsorption enthalpy on secondary sites (identified as framework oxygen atoms) in presence of water due to attractive interaction of H2 with the nearby water molecule. Spectral evidences of direct interaction of adsorbed H2/D2 with adsorbed water were also found. Our results allow confirming that H2 adsorbed on open Ni2+ sites is characterized by a stretching frequency lower than 4100 cm-1 and some proposed values of Men+−H2 adducts above 4100 cm-1 are in fact due to H2 interacting with secondary sites affected by H2O located in vicinity.

1. Introduction The interest in H2 adsorption on MOF materials arises from the efforts to develop efficient hydrogen storage materials.1,2 Although the adsorption capacity of these materials is rather high, hydrogen is only weakly adsorbed, because of the predominant electrostatic and dispersive


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3 interactions involved. Consequently, the resultant values for the adsorption enthalpies are too low to provide significant population of the adsorption sites at ambient conditions. For that reason the interest is being focused to materials able to provide some higher adsorption energies. The highest-to-date isosteric heat of H2 adsorption on MOFs is reported for materials possessing open metal sites, and in particular CPO-27-Ni (Ni–MOF-74), namely 13.5 - 15 kJ mol-1.3-5 Although this value still remains low for practical applications, the details on the host-guest interaction and related guest dynamics are of essential importance with respect to the general understanding of the process and design of novel materials with improved binding capabilities. In order to get a mechanistic insight in the adsorption process and discriminate between the varieties of sites in a given material, in situ vibrational spectroscopy techniques are usually utilized. Dihydrogen exists in two forms, ortho-H2 and para-H2 differing in the relative orientation of the nuclear spins in the two species, being parallel in the ortho-H2 and anti-parallel in the para- H2. In the free state and at room temperature the molar ratio between the ortho- and paraforms is 3 : 1, entirely determined by the nuclear spin multiplicities. At temperatures near the Normal Boiling Point (NBP), in thermodynamic equilibrium, which is very much facilitated by the presence of magnetic field gradients due, for instance, to paramagnetic species, the only present is para-H2 and the measured spectra are dominated by the para-to-ortho neutron induced spin flip transitions in inelastic neutron scattering. Vibrational spectroscopy, on the other hand, provides direct access to the internal dihydrogen molecular vibrational mode. The Raman observed H–H stretching modes of the ortho- and para-forms in the free state are at 4155.2 and 4161.1 cm-1, respectively.6 The purely vibrational bands of H2 are also denoted by Q(0) and Q(1) for para- and ortho-H2, respectively. As a homonuclear diatomic molecule H2 is IR inactive, but in the adsorbed state it possess small induced dipole moment and the ν(H–H) modes become IR



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4 active, bearing valuable information about the interaction strengths at the nature of the adsorption sites. In more details, ν(H–H) are shifted to lower frequencies. Because diatomic molecules in “on-top” configurations are expected to increase their vibrational frequency when placed in an electrostatic field, the shift direction suggests that adsorbed H2 molecules are in “side-on” configuration.7 Consequently, the symmetry reduction is relatively small which results in a low intensity of the bands. It is generally recognized that the stronger the interaction of H2 with the surface, the larger the shift of the ν(H–H) modes and the higher the IR molar absorptivity coefficient are. Although this trend is valid in most cases, there are many well documented examples of some deviations which are due to the complexity of the interaction and multiple bonding. Also, when H2 forms equally strong bonds with anionic or cationic sites, the shift is larger in the latter case.3 When H2 is adsorbed, the Q(0) and Q(1) bands usually appear in the region between 4150 and 4000 cm-1, but can reach significantly lower wavenumbers when the interaction is strong. More specifically, the ν(H–H) values are in the range 4120-4090 cm-1 when H2 interacts with acidic hydroxyls in zeolites.8 When H2 interacts with cations, ν(H–H) is observed below 4112 cm-1 and can be shifted down to 3080 cm-1.9 In addition, adsorbed H2 is characterized by a set of other bands, often with remarkable intensity: •

the vibrational-rotational components due to quadrupolar induction: S(0) around 4500 cm-1 for para-H2 and S(1) at 4715 cm-1 for ortho-H2;

•

changes in the center-of-mass translational quantum number, usually observed in the 4300-4150 cm-1 region (Qtransl bands).


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5 •

Overtones, appearing at wavenumbers slightly lower than the double of the fundamental vibration. It was reported that overtones are remarkably intense when the interaction of H2 with the surface is strong.10,11

For precise assignment of the bands, adsorption of isotopologues (HD, D2) is often helpful. However, it should be emphasized that D2 has some specific peculiarities. The reference Raman D−D band in the gas phase is observed at 2993.5 cm-1.6 The spectral difference between the ortho- and para-D2 is very small, ca. 2 cm-1 6,12 and consequently the two forms are usually not resolved in the adsorbed state. A maximum 1/3 of the D2 molecules can be in the para-form at room temperature, and this value decreases with temperature lowering.13 Because the ν(D–D) modes are observed at significantly lower frequencies as compared to ν(H– H), the spectra of adsorbed deuterium are normally less noisy and of better quality. However, they are often obscured by some background MOF bands. The theoretical isotopic shift factor, ν(H−H)/ν(D−D), is 1.4137. Due to the anharmonicity, the experimentally observed value by Raman measurements is slightly lower, i.e., 1.3900.6 Values of 1.393 – 1.350 can be calculated from data reported for adsorbed H2/D2, usually the stronger interaction leads to a smaller shift.9,12,14 It has to be pointed out that CPO-27-M materials are isostructural to the original MOF74−M compound reported by Yaghi et al.15 Adsorption of hydrogen on CPO-27-M has been studied by various experimental techniques and theoretical simulations. In particular, the interactions of H2 Zn-containing material has been studied by means of neutron diffraction which revealed the preferential occupation of the adsorption sites near the Zn2+ ions at low loadings of H2.2



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6 The dynamics of the adsorbed hydrogen at the Ni2+ centers in CPO-27-Ni was characterized using Inelastic Neutron Scattering (INS) techniques which showed that although stronger than in any other MOF material studied so far, the H2 : Ni2+ interactions still fall in the typical range of electrostatic and dispersive forces.4 This was confirmed by consequent neutron diffraction experiments16 of adsorbed deuterium on open Ni2+ sites, at a distance at c.a. 2.2 Å, much longer that for H2 on Cu+ ions (c.a. 1.7 Å) where orbital interactions clearly dominate leading to the formation of a typical η2-H2 complex.17 The strong preference of H2 towards the Ni2+ sites over secondary O-ligand environments and the phenyl linker sites was also confirmed. Coincidentally, using a combination of sorption, INS and quantum chemical methods, the weakest H2-M2+ center interactions and correspondingly the longest H2-to-M2+ distance in the CPO-27-Me framework was found for Cu2+, just 6 kJ mol-1 H2.18 In this case the adsorption onto the secondary sites, i.e. oxygen ligands, appeared competitive to the metal sites. Coming back to the vibrational spectroscopy of the H−H stretching mode, a careful IR study of the low temperature (down to 20 K) adsorption of H2 on CPO-27-Ni was performed by Bordiga et al.3 No hydrogen adsorption was detected at ambient temperature even in the presence of 50 mbar of H2. However, when the temperature was gradually lowered, two bands at 4035 and 4028 cm-1 appeared at 180 K and further grew in concert. These two bands were assigned to Ni2+−H2 species. Because the two bands were of similar intensity and the intensity ratio was not affected by temperature, the authors preferred to associate them with two different complexes having different geometries and also different oxygen atoms in the first coordination sphere of the Ni2+ sites to which H2 was bound. Note that the temperature of appearance of adsorbed H2 (Tonset = 180 K) is considerably higher as compared to other MOFs. The relatively strong


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7 adsorption has been considered as a suggestion that charge transfer from the H2 to the metal occurred. The adsorption enthalpy for the system was calculated to be 13 - 13.53,4 or 15 kJ mol-1.5 Further lowering of the temperature results in development of bands at 4150-4110 cm-1 associated with H2 adsorbed on the organic linkers and second adsorption layer. Initially, a doublet at 4132 and 4121 cm-1 was observed. Then, a band at 4138 cm-1 grew and two new components at 4146 and 4135 cm-1 appeared. Simultaneously, the initially observed bands at 4138 cm-1 and at 4121 cm-1 were eroded. At the same time complex changes occurred with the Ni2+−H2 bands. The final spectrum was characterized by two main bands at 4035 and 4020 cm-1 and shoulders at 4033 and 4012 cm-1, respectively. The observed red shift was interpreted in terms of the gradual modification of the surrounding. Desorption experiments indicated that the Ni2+−H2 species were stable at 20 K and their destruction started at 50 K. The spectra registered during desorption were mirror to the spectra recorded at cooling and the intensity ratio between the bands was almost preserved. However, it was noted that the band at 4035 cm-1 was slightly more intense than the band at 4028 cm-1. This was interpreted as a restricted conversion of ortho- to para-H2 and shows that the authors have not totally ruled out the possibility the two bands to arise from the H2 isomers, as also proposed in another work.5 Similar results were obtained by other authors using diffuse reflectance IR spectroscopy.10 They reported that low-temperature (35 K) H2 adsorption on the CPO-27-Ni leads to appearance of H2 adsorbed on Ni2+ sites, as evidenced by a composite band with maximum at 4036 cm-1. A high-frequency shoulder is also evident in the published spectra. In addition, rather intense Qtrans bands were detected around 4250 cm-1. When HD and D2 were adsorbed, the main band was detected at 3518 and 2889 cm-1, respectively. At higher coverage secondary sites are filled (bands



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8 in the 4050-4010 cm-1 region). Because similar bands were also recorded with other CPO-27-M materials (M = Zn, Mg, Mn and Co), it was concluded that the position of the bands hardly depended on the metal nature. With the filling of the secondary sites the band of the H2 adsorbed on Ni2+ sites was slightly shifted to lower frequencies which was attributed to weak H2···H2 interactions. Very different results were reported by Chabal et al.19 These authors studied the adsorption of H2 at 300 K and under high equilibrium pressures (up to 60 bar) on a series of CPO-27-M samples (M = Zn, Mg, Co and Ni). According to them, the stretching frequency of H2 adsorbed on open metal sites is shifted only by about 30 cm-1 with respect to the gas phase Raman frequency. In particular, the frequency found with CPO-27-Ni was at 4121 cm-1. The authors speculated that the larger shift previously found by other authors can be measured only when a next available oxygen site is occupied and the high shift value is due to H2-H2 interaction on neighboring sites of the same pore. These ideas were developed in a more recent study devoted to H2 adsorption on CPO-27-Co.20 It was reported that at ambient temperature and high H2 pressure only a band at 4125 cm-1 (attributed to H2 interacting with Co2+ sites) was formed. When the experiments were performed at 77 K the dominant band at low coverage was at 4041 cm-1. This band was attributed to H2 on cobalt site interacting with H2 adsorbed on oxygen site in vicinity. However, although H2 at oxygen site was expected at ~ 4120 cm-1, among the spectra presented there were some displaying only the band at 4041 cm-1 which impeaches the proposed assignment. The above findings were seriously criticized.10 It was reported that H2 adsorbed at ambient temperature and in the range between 20 and 100 bar on a fully dehydrated CPO-27-Co sample gave rise to a band at 4050 cm-1. However, only when the sample was pre-exposed to air, the main band was detected around 4125 cm-1. Similar observations were reported with CPO-27-


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9 Zn. It was concluded that contamination of the primary site increased the intensity of the bands associated with the secondary sites, likely due to an increase in the induced dipole moment via water-H2 interactions. These findings strongly suggest that adsorbed water is associated with the detection of ν(H−H) bands above 4100 cm-1. Indeed, the use of very high equilibrium pressures of H2 makes principally possible water contaminants, even at very low concentration, to be accumulated on the sample. Also, it is known that water adsorbed on c.u.s. metal sites is characterized by enhanced acidity10 and thus the new adsorption form could be due to H2 interacting with water hydroxyls. Note also that the possibility the effect to be due to other air components, such as O2 or CO2, remains open. In this work we comparatively study the adsorption of H2 and D2 on activated CPO-27-Ni sample and on CPO-27-Ni precovered with different amounts of H2O or D2O. The choice of D2O was determined by the possibility to follow more accurately the changes in the ν(OD) region where the level of noise is lower as compared to the ν(OH) region. The “dry” and “wet” samples were tested by low-temperature CO adsorption and then adsorption of H2 and D2 was comparatively studied. The results allowed us to conclude that bands above 4100 cm-1 appearing after H2 adsorption of “wet” CPO-27-Ni samples are associated with presence of water and the conclusions were supported by DFT calculations.

2. Experimental 2.1. Samples and Methods The CPO-27-Ni sample was prepared according to published recipes21 but under inert atmosphere.4



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10 The IR spectra were recorded on Nicolet Avatar 360 and Nicolet 6700 FTIR spectrometers accumulating up to 128 scans at a spectral resolution of 2 cm-1 and accuracy of 0.01 cm-1. A specially designed IR cell allowing measurements at 100 K and at ambient temperature was used. The cells were directly connected to a vacuum-adsorption apparatus with a residual pressure lower than 10-3 Pa. To obtain the IR spectra the sample was spread onto a KBr pellet. This technique ensured optimal intensity of the IR bands. Parallel experiments were performed with a self-supporting pellet in order to verify the lack of interaction with KBr. To obtain the spectra of adsorbed H2 we used as backgrounds the spectra of adsorbed D2 at the same conditions, the same is valid for the spectra of adsorbed D2. Hydrogen (99.999% purity) and deuterium (99.7% purity) were supplied by Messer. D2O (purity 99.9%) originated from Cambridge Isotope Laboratories, Inc. Carbon monoxide (>99.5% purity) was supplied by Merck. Before use, H2, D2 and CO were additionally purified by passing through a liquid nitrogen trap. 2.2. DFT Calculations The periodic structure of CPO-27-Ni was first relaxed by full geometry optimization of the 54 atom primitive rombohedral version of the previously published crystal structure of the compound,21 within the PBE parametrization of GGA, until forces were reduced to below 10 meV/Å. The projector augmented pseudopotential method22 as implemented in the Abinit v.7.8 23,24

was used to model the electronic wave function and electron-ion interactions, with an energy

cut off on the plane wave part of 820 eV. Additionally, a Hubbard U correction on the energy of Ni 3d electrons was applied with a magnitude of 8 eV and a correlation part J of 0.95 eV, as suggested by AZA25 for Ni2+ ions, with the double counting term calculated using the atomic limit method26 Shifted 2x2x2 k-point grid sampling of the Brillouin zone was applied. The


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11 resultant lattice parameters, 26.070 and 6.845 Å are by just 1.1 % and 1.4 %, respectively, larger than the parameters derived from low temperature neutron diffraction.16 To model the interactions between the Ni-centers and adsorbates, such as H2, H2O, and CO, at low loadings as well as adsorbate-adsorbate interactions, a 92-atom cluster was created out of a supercell of the hexagonal representation of the relaxed CPO-27-Ni structure. This contains 5 square pyramid units – 3 Ni - and two Li-ones terminating the helical chain (see Fig. S1 in the Supporting Information). Six phenyl rings were retained from the original dhtp ligand-formed framework, with all terminal carbon atoms saturated by hydrogens. On a first instance, the coordinates of all protons and the Li-atoms were optimized. Then these were kept fixed while relaxing only the coordinates of the three Ni atoms and the oxygen atoms forming their first coordination sphere. Only these Ni and O atoms were again optimized along with those of each adsorbate molecule. All cluster electronic structure calculations were performed with the GAUSSIAN 09, Rev. D.01, software package.27 The long range corrected hybrid functional ωB97X-D,28 which incorporates also an optimized version of Grimme’s D3 dispersion correction,29 was used for these studies, supplemented with following atomic basis sets: 6-31 on terminal H atoms and the Li -atoms, 631** on phenyl ring carbon atoms, 6-311** on the O-atoms of the host fragment, Def2TZVPPD30 on the Ni-centers and 6-311++G(d,2pd) for all guest molecule atoms. All calculations were started from and converged to a nearly pure septet spin state, S=3.00, corresponding to the high spin S=1 state for the Ni2+ ions. Basis Set Superposition Errors (BSSE) were computed at the optimized equilibrium geometries, via the counterpoise method.31 Tight geometry optimization and ultrafine integration criteria were applied for accurately computing the guestguest interactions in the mixed H2O, H2 adsorbate structure. Except two negative vibrational modes in non-relaxed outermost parts of the cluster (CO,OH-groups), frequency calculations



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12 showed that true minima have been found in all optimized structures, with respect to the main part of the host and the relatively weakly bound adsorbates.

3. Experimental Results 3.1. Initial Characterization and Background IR Spectra The phase purity of the CPO-27-Ni sample was established from the powder X-ray diffraction pattern and permanence of porosity from nitrogen adsorption (see Figs. S2 and S3 in the Supporting Information). Fig. 1 shows the evolution of the background spectrum of CPO-27-Ni in the OH stretching region during evacuation at increasing temperatures. The as-prepared sample is characterized by a very intense and out-of-scale band in the 3600-3100 cm-1 region (Fig. 1, spectrum a). This band markedly decreases in intensity after evacuation at ambient temperature and three bands become well distinguished in the region: two sharp bands at 3655 and 3557 cm-1 and a broad feature at 3408 cm-1 with a low-frequency shoulder (Fig. 1, spectrum b). The bands at 3655 and 3557 cm-1 are assigned to the symmetric and antisymmetric modes, respectively, of isolated water molecules likely coordinated to Ni2+ sites. The broad lower frequency band characterized H-bonded water. In addition, two weaker C−H bands (arising from the benzene ring) are seen at 3104 and 3057 cm-1.


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13 3500

2500

0.5 - 3104 3057

3655

- 3557

A

3000

- 3408

4000

b c

d e

a

0.02

5180 -

B

- 5235

Absorbance, a.u.

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a

d 5200

Wavenumber, cm

4800 -1

Figure 1. FTIR spectra of CPO-27-Ni. As prepared sample (a) and after 30 min evacuation at 293 (b), 393 (c),453 (d), and 523 K (e). Panel A shows the ν(OH) region and Panel B, the combination δ(OH) + ν(OH) region.

A good impression of the amount of adsorbed water can be achieved from the spectra around 5200 cm-1 where the (ν1 + ν3) combination modes of water appear8 (Fig. 1, panel B). An advantage of this region is the lack of contribution of free hydroxyl groups. The latter can be selectively monitored between 4700 – 4600 cm-1 (νOH + δOH modes). No bands were detected in this region which indicates that free hydroxyl groups, if any, are in a negligible concentration.



14 All of the described OH bands decrease in intensity and ultimately disappear with increase of the evacuation temperature. However, a broad feature of very low intensity extending from 3600 to 3000 cm-1 is still discernible even after evacuation at 453 K. Similar features can be seen in the published spectra of CPO-27-Ni and characterize strongly H-bonded hydroxyls probably originating from small amounts of residual acid. The spectra in the lower frequency region (Fig. S4 in the Supporting Information) are hardly affected by evacuation and show bands originating from carboxylates (1500-1400 cm-1), benzene ring (1630 - 1560 cm-1), adsorbed water (ca. 1640 cm-1) and v(C−O) vibration of the deprotonated species derived from the hydroxyl group (1280

- 2709 - 2606 - 2490

cm-1).19

2643 -

c

3657 3555 -

- 3588

- 3425

0.2 Absorbance, a.u.

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4000

a

3500

3000

b

2500

Wavenumber, cm

2000

-1

Figure 2. FTIR spectra of CPO-27-Ni evacuated at 393 K (a), after adsorption of small amount of D2O (sample CPO-27-Ni_D1) (b), and after adsorption of 1 mbar D2O followed by evacuation at 323 K (sample CPO-27-Ni_D2) (c).


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15 To achieve the aim of the study we designed some of the adsorption experiments to be performed on water precovered samples. In fact, we used three samples precovered with different amounts of D2O because the ν(OD) region is less noisy than ν(OH). One of the samples (CPO27-Ni_D1) was obtained by dosing small amounts of D2O on activated CPO-27-Ni thus mimicking incident entrance of small amounts of water into the system. The second sample (CPO-27-Ni_D2) was produced by adsorption of D2O (1 mbar) followed by evacuation at 323 K. In this case we expected practically all of the c.u.s. Ni2+ sites to be occupied by D2O molecules. The IR spectra registered after these treatments were used as backgrounds to obtain the spectra of adsorbed molecules and are presented on Fig. 2. Finally, we used a sample with a similar H2O/D2O content (CPO-27-Ni_D3) but in this case the H/D isotopic exchange degree was ca. 50 %. This allowed simultaneous monitoring of the adsorbate-induced changes in the ν(OH) and ν(OD) regions at identical conditions. The following peculiarities should be noted: •

The deuteration degree of the CPO-27-Ni_D1 sample is slightly lower than 50 %. This is due to the fact that the introduced doses of D2O are partly H-exchanged by the system walls during their pathway to the pellet.

•

With the CPO-27-Ni_D2 sample bands at 2709, 2606 and 2490 cm-1 are detected. They correspond to the OH bands at 3655, 3557 and 3408 cm-1, respectively, registered with the as-prepared sample evacuated at ambient temperature or at 323 K. Another band, at 2643 cm-1, without analogue on the D-free sample, is due to the OD modes of HOD molecules. The respective OH modes are detected at 3588 cm-1 (note that at high D/H ratio most of the H-containing water molecules are in the HOD form).



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16 •

The bands associated with HOD molecules (2643 and 3588 cm-1) were most intense with the CPO-27-Ni_D3 sample, consistent with the expectation that ca. 50 % of the water is in the HOD from.

3.2. Adsorption of CO on CPO-27-Ni with Different Amounts of Preadsorbed Water Carbon monoxide is the most used IR probe molecule to assess the oxidation and coordination state of c.u.s. metal cations.32 When coordinated to isolated Ni2+ ions in zeolites, CO gives rise to bands around 2220-2000 cm-1 and it is considered that the π-back bonding is negligible.33,34 With Ni2+ sites of lower electrophilicity CO forms weaker bonds and, consequently, the CO stretching frequency is observed at lower wavenumbers. For instance, carbonyls of Ni2+ cations grafted on silica are detected around 2195-2185 cm-1 and only at low temperature. The IR spectra of CO adsorbed on CPO-27-Ni activated at 393 K are presented in Fig. 3A. At low coverage a band at 2182 cm-1 is detected and is gradually shifted to 2177 cm-1 with coverage increase. The relatively high stability and low CO stretching frequency indicate stabilization of the carbonyls formed by back π-donation, as already supposed.3 In presence of more CO the intensity of the band is slightly affected and the maximum is slightly blue shifted. Simultaneously, bands at 2137 and 2134 cm-1, characterizing physically adsorbed CO, develop. The two-direction shift with coverage increase is unexpected. The shift at high coverages occurs in the presence of physically adsorbed CO and could be related to interaction between coordinated and weakly adsorbed CO molecules. The blue shift with coverage decrease is often reported with oxide systems and usually interpreted as due to static interaction between the adsorbed molecules.32


Page 17 of 37

17

- 2137 - 2134

2177

A

a Absorbance, a.u.

2182 f 2176

B

0.5

g 2181

p

C 2175 - 2149

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


q x 2172 2200

2180

2160

Wavenumber, cm

2140

2120

-1

Figure 3. FTIR spectra of CO adsorbed at 100 K on CPO-27-Ni. Panel A, sample activated at 393 K; panel B, sample CPO-27-Ni_D1 (small amount of pre-adsorbed D2O, for details see text) and panel C, sample CPO-27-Ni_D2 (large amount of pre-adsorbed D2O). The intensity scale is equal for all panels and the spectra are background and gas-phase corrected. Panel A: Initial CO equilibrium pressure of 1 mbar, followed by evacuation at 100 K (a-c) and at elevated temperatures (d-f). Panel B. Initial CO equilibrium pressure of 1 mbar, followed by evacuation at 100 K (g-k) and at elevated temperatures (l-p). Panel C. Initial CO equilibrium pressure of 1 mbar, followed by evacuation at 100 K (q-v) and at elevated temperatures (w-x).



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Page 18 of 37

18 Similar spectra were obtained with the CPO-27-Ni_D1 sample where the amount of preadsorbed D2O was low (Fig. 3B). In this case the frequencies of the adsorbed CO are slightly red shifted and a new band at 2149 cm-1 was detected. The latter develops after the filling of the Ni2+ sites. This band is assigned to CO forming H-bonded complexes with OH/OD groups from adsorbed water molecules. The relatively low value of ν(CO) indicates weak interaction. The respective changes in the OH/OD regions are complex and difficult to follow because of the low intensity of the bands. They will be discussed in details with the CPO-27-Ni_D2 sample. Surprisingly, CO adsorption on the CPO-27-Ni_D2 sample leads to formation of an intense Ni2+-CO band (Fig. 3C). An interesting peculiarity is that this band is continuously blue shifted with coverage, contrary to the cases with the previously studied samples. After saturation of the band, the OH−CO band (2149 cm-1) starts to develop. Analysis of the spectra in the OD region shows some interesting phenomena (Fig. S5 in the Supporting Information). In particular, negative bands due to the symmetric and antisymmetric OD modes of adsorbed D2O were observed (2709 and 2602 cm-1) which suggested replacement of D2O by CO. However, because some uncertainties arose, we repeated the experiments by successive adsorption of small doses of CO at low temperature. In thus way we avoided any temperature and memory effects. For these experiments we used the CPO-27-Ni_D3 sample (with analogous total water content) in order to be able to monitor simultaneously the changes in the OH and OD regions.


Page 19 of 37

- 2711 2672 -

19

- 2149

2431 -

e g

3600

3200

3606

g-f

0.05

g

g e -2373

e

2489 -

2800

2173

- 2641

d

a 2603 -

- 2732

d

2709

3600

3200

0.1

d

0.05 2600

0.1

2176

g-f

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3376

2400

a

a 2200 2160 2120

Wavenumber, cm

-1

Figure 4. Difference FTIR spectra registered after successively adsorption of small CO doses at 100 K on a CPO-27-Ni sample precovered with D2O (sample CPO-27-Ni_D3). Spectra a-g correspond to increasing coverage. The difference (g-f) spectrum is also shown. The top panels correspond to high coverage, while the bottom panels, to medium and low coverage.

The spectral changes induced by successive adsorption of small CO doses on CPO-27Ni_D3 are presented on Fig. 4 and clearly show two types of successive occurring interactions (lower and upper part of the Figure). Even the first CO dose leads to development of a carbonyl band at 2172 cm-1 (spectrum not shown). This band develops and is slightly blue shifted with the increase of the amount of CO introduced to the system (Fig. 4, spectra a-d). Simultaneously,



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Page 20 of 37

20 negative ν(OD) bands at 2709, 2641 and 2603 cm-1 are formed. A careful analysis indicates weak negative feature also at 2732 and 2667 cm-1. At the same time a broad band at lower frequencies with resolved maxima at 2489, 2431 and 2373 cm-1 develop. Similar changes are detected in the OH region (see the inset in Fig. 4). The three principal negative bands are at 3638, 3624 and 3582 cm-1 with weak additional features at 3700 and 3654 cm-1. The appearing broad band also consists of three components (3376, 3293 and 3217 cm-1) but they are much less resolved as compared to the respective band in the OD region. This is attributed to affecting of the shifted OH and OD bands by Fermi resonance in different way. The results indicate that CO replaces water molecules from the Ni2+ sites in order to form carbonyls. Additional important factor favoring the process is the energy gained by the adsorption of released water molecules on other H2O molecules in order to form a second adsorption layer. Therefore, our assumption that practically all of the Ni2+ sites were initially occupied by water (or D2O) was correct. It was also supported by the H2 adsorption experiments (see below). There are reports that CO can replace water from some cations, especially those forming a strong π-back bond with CO.35,36 However, this behavior is unprecedented for Ni2+ sites. It confirms the above made supposition that, with the Ni2+ ions in CPO-27-Ni, CO forms carbonyls stabilized by π-back donation. At high coverages, when the HOH−CO species are formed (band at 2149 cm-1), the changes in the spectra are restricted to a shift of the OD band at 2711 cm-1 to 2672 cm-1 and the OH band at 3666 to 3606 cm-1, ∆ν(OH) ≅ -60 cm-1 (see the difference spectrum g-f in Fig. 4). These results indicate a measurable (although not high) acidity of OH groups of water presumably coordinated to Ni2+ sites. For instance, the silanol hydroxyls (proven to interact with


Page 21 of 37

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21 H2) are characterized by a shift ∆ν(OH) ≅ -56 cm-1. These findings suggest that a very weak interaction between H2 and water adsorbed on CPO-27-Ni is not excluded. Note also that we have no data on the acidity of the 2732 cm-1 hydroxyls because they preferentially interact with H2O molecules replaced by CO, which suggests a relatively high acidity. 3.3. Adsorption of H2 and D2 on Activated CPO-27-Ni Sample For this set of experiments we used the sample evacuated at 393 K but essentially the same results were obtained with the 453 K evacuated specimen. Adsorption of H2 (50 mbar equilibrium pressure) at 100 K on the activated sample leads to the appearance of two main bands in the ν(H−H) region, at 4031 and 4249 cm-1 (Fig. 5A, spectrum a). A weak feature at 4118 cm-1 is also discernible. In agreement with several previous reports3,5,10 we assign the band at 4031 cm-1 to H2 on moiety containing a bare Ni2+ site. The band at 4249 cm-1 arises from the Qtrans mode while the weak band at 4118 cm-1 is attributed to H2 adsorbed on a secondary site not involving Ni2+ cations. Decrease of the equilibrium pressure (Fig. 5A, spectra b-d) leads to a fast disappearance of the band at 4118 cm-1 confirming the much weaker adsorption enthalpy of this adsorption form. The bands at 4031 and 4249 cm-1 decrease in concert but much slower. At equilibrium pressure of 1 mbar they still keep ca. 50 % of their initial intensities and the maximum of the 4031 cm-1 band shifted to 4026.5 cm-1. At very low coverages the value reached 4019 cm-1 (Fig. S6 in the Supporting Information). Two ν(H−H) bands below 4100 cm-1, at 4035 and 4028 cm-1 were reported in a previous work.3 These bands were associated with two families of Ni2+ sites, although the possibility to arise from ortho- and para-H2 was not ruled out.3,5 In our case no two bands were resolved, although we were able to distinguish between the two H2 isomers adsorbed on zeolitic



22 hydroxyls.14 In any case the band is broader as compared to the corresponding D2 band (see below) which indicates contribution of ortho- and para-H2 species. The observation of a single band is consistent with the known structure of the empty CPO-27-Ni which has one crystallographically unique open metal site.

4400

4200

4000

3800

- 4031

A

d

- 4118

- 4249

0.01

a

B

- 2895

0.01

h 3100

- 2964

- 3056

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

e 3000

2900

Wavenumber, cm

2800 -1

Figure 5. FTIR spectra of H2 (panel A) and D2 (panel B) adsorbed on CPO-27-Ni activated at 393 K. Equilibrium H2/D2 pressure of 50 (a, e), 10 (b, f), 5 (c, g) and 1 mbar (d, h). The spectra are background corrected. For the correction of the spectra in panel B, spectra registered after adsorption of H2 at similar conditions were used.


Page 23 of 37

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23 The spectra registered after adsorption of D2 are shown in Fig. 5B. The ν(D−D) band associated with Ni2+ sites was detected at 2895 cm-1. The isotopic shift factor thus appears to be 1.392 which is consistent with Raman observations for the gas phase (1.390) and with a huge amount of published experimental data on H2/D2 adsorption on OH groups and alkali metal cation in zeolites (1.389 – 1.393). A lower value of the isotopic shift factor has been reported for very strongly adsorbed hydrogen, e.g. on Cu+ ions in zeolites.9 Note that the ν(D−D) band at 2895 cm-1 is less sensitive to the equilibrium pressure as compared to the ν(H−H) band at 4031 cm-1. This shows that D2 is more strongly adsorbed, in agreement with earlier reports with zeolites.14 Here again the maximum is slightly blue shifted with coverage increase (2893 cm-1 under 1 mbar equilibrium pressure and 2882 cm-1 at very low coverage, see Fig. S4). We also compared the FWHM of the H2 and D2 bands associated with Ni2+ sites. The FWHM of the band at 4030 cm-1 is 26 cm-1. For the D−D band we found a FWHM value of 17 cm-1, which, corrected with the isotopic shift factor, corresponds to a value for the H−H modes of 23.5 cm-1. Thus, the D−D band appears to be narrower from the H−H band. This phenomenon is due to the fact that the spectral difference between the ortho- and para-D2 is only 2 cm-1, a value definitely lower that the difference between the ortho- and para-H2 (ca. 6 cm-1) which result to a smaller FWHM of the D2 band even after correction with the isotopic shift factor. Interestingly, the relative intensity of the Qtrans mode of adsorbed D2 is much lower as compared to the same H2 band. This is attributed to the greater mass of D2 which hinders the translation modes. Finally, the band assigned to H2 adsorbed on a secondary site appeared at 2964 cm-1 after adsorption of D2. In this case again, it easily disappeared with the decrease of the equilibrium pressure.



24 3.4. Adsorption of H2 and D2 on Wet CPO-27-Ni Sample First we will discuss the spectra obtained after H2 and D2 adsorption on the CPO-27Ni_D2 sample (Fig. 6). Due to the low intensity of the bands, the spectra in the ν(H−H) region are noisy while the spectra of adsorbed D2 are of higher quality.

4150

4100

A

4050

- 4115

0.002

- 4052

a

f

B

- 2959

0.002

g

- 2910

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

l 3000

2950

Wavenumber, cm

2900 -1

Figure 6. FTIR spectra of H2 (panel A) and D2 (panel B) adsorbed on CPO-27-Ni_D2 (for details see text) at 100 K. Equilibrium H2/D2 pressure of 50 (a, g), 40 (b, h), 20 (c, i), 10 (d, j), 5 (e, k) and 2 mbar (f, l). The spectra are background corrected.

The principal ν(H−H) band is located at 4115 cm-1. The respective ν(D−D) band was detected at 2959 cm-1 and was narrower than the 4115 cm-1 band due to restricted ortho-para


Page 25 of 37

25 forms broadening. Interestingly, no difference between the strength of adsorption of H2 and D2 is evident in this case which suggest a different mode of adsorption, as compared to the form associated with the Ni2+ sites. An important observation is that the adsorption form is less sensitive to the equilibrium pressure, as compared to the secondary sites on the dry sample and

a b c

3000

2960 -

0.01

2950

2900 -

2895 -

the respective bands are more intense under 40 mbar equilibrium pressure (see also Fig. 7).

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2900

Wavenumber, cm

2850 -1

Figure 7. FTIR spectra of D2 (40 mbar) adsorbed on CPO-27-Ni samples. Sample activated at 393 K (a), sample CPO-27-Ni_D1 (b) and sample CPO-27-Ni_D2 (c). For details see text.

In addition, bands of weak intensity were detected at 4052 cm-1 (H2 adsorption) and 2910 cm-1 (D2 adsorption). These bands are assigned to H2/D2 interacting with Ni2+ sites. The higher frequencies, as compared with the dry sample, correspond to smaller shifts with respect to the gas phase modes and indicate a weaker interaction with the Ni2+ ions as compared to the activated sample. This is consistent with the CO adsorption experiments which revealed the lower acidity of the Ni2+ sites on the D2O precovered sample. Note also that the intensity of these bands was



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Page 26 of 37

26 negligible as compared to the situation with the activated samples (see Fig. 7) which shows that only a negligible part of the Ni2+ sites were not occupied by water. Difference spectra show that adsorption of H2/D2 is accompanied by some changes of the spectra in the OD region (Fig. 8). A band at 2728 cm-1 is eroded and shifted to 2701 cm-1 and this process correlates well with the intensities of the ν(H−H) and ν(D−D) bands. At high coverage other OD bands (e.g. 2600 cm-1) seem to be affected. This behavior is similar to the spectral changes observed at high CO coverages, but the observed shift is smaller. Therefore, these results indicate that adsorbed water plays role in the adsorption of H2 and D2. Finally, we will briefly consider the spectra of H2/D2 adsorbed on CPO-27-Ni_D1. It can be also estimated that about 2/3 of the Ni2+ sites are occupied by H2O/D2O molecules. The remaining free sites affect less strongly the adsorbed D2 as seen by the lower, by 5 cm-1, D−D frequency. It is also clear from Fig. 8 that the band of D2 adsorbed on secondary sites appears with intermediate intensity, between those found with activated and wet samples, but the wavenumber is rather close to that observed with the wet sample. As a conclusion, our IR results indicate that the nature of the secondary sites for H2 adsorption on water precovered CPO-27-Ni is different from those detected with the activated sample. Strong evidence of involving water in the process was also found. However, from our IR results we cannot make conclusions whether the active sites are totally new or water just modifies the already existing secondary adsorption sites. To answer this question we performed DFT calculations.


Page 27 of 37

27 4150 4100 4050

A

B

- 2701

0.002

4115 -

a

- 2600

- 2728

0.005

b c d

C

D

- 2701

0.005 e

0.002

2960 -

- 2728

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


f g h

2800

2700

2600

3000 2960 2920

Wavenumber, cm

-1

Figure 8. Difference FTIR spectra of H2 and D2 adsorbed at 100 K on the CPO-27-Ni_D2 sample (for details see text). Panels A and B, adsorption of H2; panels C and D, adsorption of D2. Panels A and C, ν(O-D) region, panel B, ν(H-H) region, and panel C, ν(D-D) region. The spectra reflects the changes during dilution of: H2 from 40 to 20 mbar (a), from 20 to 10 mbar (b), from 10 to 5 mbar (c), and from 5 to 1 mbar (d); D2 from 40 to 20 mbar (e), from 20 to 10 mbar (f), from 10 to 5 mbar (g), and from 5 to 1 mbar (h).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

28

4. Computational Results and Discussion The computed binding energies at different adsorption sites for H2, H2O, and H2/H2O are provided in Table 1. Our values agree closely with the previously published experimental and computational results, where available.3,15 The overall adsorption thermodynamics of the studied species and combinations suggests that, as previously observed by IR3 and Neutron Scattering experiments,2,4,16 H2 is preferentially adsorbed above the Ni2+ centers, at a computed distance of 2.19 Å (Fig. 9a) and a corresponding red shift of the stretching H−H mode of about -116 cm-1, in a very good agreement with the FTIR result, Fig. 5 as well. The secondary adsorption site near a group of O-ligands is occupied then, being weaker, computed binding energy of ca. -6 kJ mol-1 H2, with a smaller magnitude of the corresponding frequency red shift of -36 cm-1, agreeing very well to the FTIR feature observed at 4118 cm-1 in Fig. 5. Recall that in the free state this frequency is 4162 cm-1 for para- and 4155 cm-1 for the ortho-hydrogen species. Notably, when water is adsorbed at the Ni2+ site, the binding of H2 at the nearby O-site appears markedly increased, by more than 50%, Table 1. This is due to the fact that, when H2O is coordinated to the Ni2+ site, the oxygen ligand representing the secondary site is pulled towards the water proton and weak interaction with the water molecule, as also evidenced by IR, occurs. This explains the high intensity of the IR band at 4115 cm-1 (Fig. 6) originating from H2 molecules adsorbed at the secondary O-sites, even at relatively higher-than-expected temperatures, when the neighbouring Ni2+ site is occupied by H2O. The computed frequency shift of H2 molecules near H2O : Ni2+ adducts is expectedly slightly higher than that for H2 on the same sites, near an H2 : Ni2+ adduct, Table 1.


Page 29 of 37

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29

3.95 Å

2.20 Å

2.16 Å

2.60 Å

2.70 Å

2.37 Å

a

b

Figure 9. First coordination sphere of Ni2+ sites when H2 is adsorbed on: (a) primary and secondary sites of CPO-27-Ni and (b) Ni-CPO-27 with a water molecule coordinated to a Ni2+ site. The complete clusters used in the corresponding calculations are presented in the Supporting Information. Color scheme: blue – nickel; red – oxygen, grey – carbon, and white – hydrogen. The extra attractive interactions between H2O and H2 in (b) with respect to the H2-H2 in (a) reflect in shorter distances between the O atom from the Ni2+ basal plane and the nearby proton of the H2 molecule, 2.37Å vs. 2.60 Å, and the much shorter adsorbate-adsorbate distance, 2.70 vs. 3.95 Å.

Table 1. BSSE Corrected Binding Energies, the Magnitude of the BSSE Correction, and the Frequency Shifts of the Adsorbed Species with Respect to the Corresponding Free States. Adsorbed Species

Eb, kJ mol-1

BSSE, kJ mol-1 ∆ν(H−H), cm-1

H2O at Ni2+

-71

9.1

-

H2 at Ni2+

-16

4.3

-116.3

H2 at Ni2+

-17.1

4.2

-118.5

H2 at O site

-5.7

4.2

-35.5

H2 at O site in presence of H2O

-9.8

4.7

-68



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Page 30 of 37

30 As a further reference check for the validity of the above computational results we also calculated the binding energies of pure CO and H2O adsorbates. The CO binding energy was computed both in the primitive periodic structure of CPO-27-Ni, for 1 CO molecule per Ni2+ center, that is 6 CO molecules in the primitive unit cell, as well as for the cluster. The two values came out practically identical: -50 kJ mol-1 CO in the periodic model and -47 kJ mol-1 CO in the cluster closely matching previous theoretical and experimental results37 which suggested substantial π-back bonding of adsorbed CO. We were not able to find accurate experimental or theoretical published H2O binding energy data. Based on the fact that water is replacing NO adsorbed on the Ni-sites in the same material38 it was concluded that the binding of water must be of similar strength to that of NO for which the differential heat, measured over a range of concentrations up to 0.2 of the capacity of the Ni-site, was found to be -90 ÷ -92 kJ mol-1 NO, dropping to -69 kJ mol-1 at the Ni-sites capacity completion. Our value of -71 kJ mol-1 H2O seems reasonable then, suggesting that when present in excess, water should replace NO. Similarly, with a binding energy about 50 kJ mol-1, excess amounts of CO should be expected to replace H2O from the Ni2+ sites as suggested by the FTIR results presented in Fig. 4. As already discussed, the formation of a second water adsorption layer corroborates the process. Notably, the same secondary site seems to be only weakly populated by H2 at the temperatures of the experiment, c.a.100 K, as seen by the strong pressure dependence of the band intensity (Fig. S7 in the Supporting Information). This clearly suggests a significant attractive interaction between the H2O and H2 adsorbates, enhancing the net adsorption strength at the secondary set of adsorption sites in the framework.


Page 31 of 37

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31

5. Conclusions We have performed in situ FTIR studies of H2 and CO adsorbed on dehydrated and water precovered CPO-27-Ni coordination polymer material. The IR observed spectral features of H2 confirm some of the previously accepted site population scenarios especially the fact that H2 adsorbed on open metal sites is characterized by ν(H−H) lower than 4100 cm-1. We find a rather simple shape for the vibrational modes of CO, H2 and D2 adsorbed on open metal sites, indicating one family of equivalent Ni2+ sites. Formation of Ni2+–H2 complex is completely blocked by preadsorbed H2O. Quantum chemical calculations show that in this case dihydrogen is readily adsorbed on the secondary adsorption sites due to attractive interaction with the nearby water molecule which substantially enhance the adsorptive power at the secondary set of adsorption sites. Computed binding energies for H2, H2O, CO, and the investigated mixtures, as well the corresponding internal vibrational dynamics, agree well to the experimental observations. It is worth pointing out that the CPO-27 structure provides a rare example of Ni2+ sites where CO is able to replace pre-adsorbed water pointing to some possible CO-getter applications in a humid environment.

Corresponding authors Email: [email protected] (Konstantin Hadjiivanov). Tel: +35 9 2 979 35 98 Email: [email protected] (Peter A. Gergiev). Tel: +39 02 5031 4454

Acknowledgments The research leading to these results has received funding from the National Science Fund of Bulgaria (Contract no. DFNI T02/20) and from the Research Council of Norway through the ISP-



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Page 32 of 37

32 KJEMI program (Grant no. 209339). PAG and AA acknowledge the access to the CINECA HPC facilities, provided by the University of Milano.

Supporting Information Complete clusters used in the calculations, powder X-ray diffraction pattern, nitrogen adsorption isotherms, background FTIR spectra, FTIR spectra of adsorbed CO, H2 and D2, and full References no 23, 27 and 37. This information is available free of charge via the Internet at http://pubs.acs.org.

References (1) Murray, L. J.; Dinca, M.; Long, J. R. Hydrogen Storage in Metal–Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1294–1314. (2) Liu, Y.; Kabbour, H., Brown, C. M., Neumann, D. A., Ahn, C. C. Increasing the Density of Adsorbed Hydrogen with Coordinatively Unsaturated Metal Centers in Metal-Organic Frameworks. Langmuir, 2008, 24, 4772–4777. (3) Vitillo, J. G.; Regli, L.; Chavan, S.; Ricchiardi, G.; Spoto, G.; Dietzel, P. D. C.; Bordiga, S.; Zecchina, A. Role of Exposed Metal Sites in Hydrogen Storage in MOFs. J. Am. Chem. Soc., 2008, 130, 8386–8396. (4) Dietzel, P. D. C., Georgiev, P. A., Eckert, J., Blom, R., Strassle, T., Unruh, T. Interaction of Hydrogen with Accessible Metal Sites in the Metal–Organic Frameworks M2(dhtp) (CPO-27M; M = Ni, Co, Mg). Chem. Commun. 2010, 46, 4962–4964. (5) Chavan, S. M.; Zavorotynska, O.; Lamberti, C.; Bordiga, S. H2 Interaction with Divalent Cations in Isostructural MOFs: a Key Study for Variable Temperature Infrared Spectroscopy. Dalton Trans. 2013, 42, 12586–12595.


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Low-Temperature Adsorption of H2 and D2 on Dehydrated and Water

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