Structural Effects and Interactions of Carbon Dioxide Molecules

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Structural Effects and Interactions of Carbon Dioxide Molecules Adsorbed on Ni, Zn, and Cd Nitroprussides R. Roque-Malherbe,*,† O.N.C. Uwakweh,§ C. Lozano,† R. Polanco,† A. Hernandez-Maldonado,‡ P. Fierro,|| F. Lugo,† and J. N. Primera-Pedrozo‡ †

)

Institute for Physical Chemical Applied Research, School of Science, University of Turabo, PO Box 3030, Gurabo, Puerto Rico, 00778-3030, USA ‡ Department of Chemical Engineering, University of Puerto Rico—Mayag€uez Campus, Mayag€uez, Puerto Rico 00681-9000, USA § Engineering Science and Materials Department, College of Engineering, University of Puerto Rico—Mayag€uez Campus, Mayaguez, Puerto Rico 00681-9044, USA Chemistry Department, University of Puerto Rico—Mayag€uez Campus, Mayag€uez, Puerto Rico 00681-9000, USA ABSTRACT: A thorough structural characterization of the synthesized Ni-, Zn-, and Cd-nitroprussides (NPs) with X-ray diffraction (XRD), thermogravimetric analysis, diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), M€ossbauer spectroscopy, and magnetic measurements was performed. However, the innovative part of the research was the study of the structural effects and the interactions of the CO2 molecule with the Ni-, Zn-, Cd-NP frameworks. DRIFTS of adsorbed molecules, high pressure adsorption, and in situ XRD adsorption experiments were performed. The DRIFTS spectra displayed peaks assigned to CO2 physical adsorption and the formation of adducts (M2+ 3 3 3 OdCdO). The fitting of the adsorption data to the DubininRadushkevich equation and a Langmuir-type equation for volume filling allowed the calculation of the micropore volume and the isosteric heats of adsorption. The calculated parameters indicated an unusual behavior of the adsorption process in Cd-NP at high pressure. This fact was caused by the interaction of CO2 molecules with the framework cations and the small adsorption space of Cd-NP. The Ni- and Zn-NPs behaved normally. The Pawley fitting of the XRD profiles of the dehydrated materials and under CO2 adsorption indicated that in both cases Ni-NP, Zn-NP, and Cd-NPs displayed the Fm3m, R3, and Pnma space groups, respectively. The dehydrated samples demonstrated a change in the cell parameters. However, only Cd-NP presented a noticeable variation of its cell parameters under CO2 adsorption. This fact was linked to the unusual behavior of the adsorption process in Cd-NP. Additionally, was shown that dehydrated Ni-, Zn-, and Cd-NPs can store 27, 22, and 15 wt % of CO2 at 298 K and 9 atm., respectively. Then, Ni- and Cd-NPs are excellent for CO2 storage, and Cd-NP is good for gas cleaning.

1. INTRODUCTION Prussian blue analogs14 and nitroprussides518 are transition metal cyanides displaying frameworks built with transition metals bridged through the linear cyanide ion. These structures possesses interesting properties such as, light-induced phenomena,2 magnetism,4,5,16 physical adsorption,13,1315 and other properties that make these coordination polymers an interesting class of materials. Pentacyanonitrosylferrates commonly named nitroprussides (NPs) are a group of metal cyanides, consisting of microporous frameworks that are assembled from [Fe(CN)5NO]2 units bridged though M2+ cations by means of the CN ligands.811 In this tridimensional porous structure O atoms at the end of the NO ligands stays free, and the M2+ cation coordinates to one or more water molecules, at the same time as the rest of the water molecules are hydrogen-bonded to the coordinated water molecules.811 In some of these materials, the M2+ cations are spin sites bridged by a spacer, i.e., [Fe(CN)5NO]2. Then, r 2011 American Chemical Society

superexchange interactions, coupling the spin sites5 are possible. In the case of Ni-NP, the spacer allows the control of the magnetic coupling between spin sites by light irradiation.6 Additionally, Mn-, Fe-, Co-, Ni-, and Cu-NPs follow the modified CurieWeiss law, a characteristic behavior of paramagnetic materials, and at low temperatures show ferromagnetic order.19 In porous NPs, after thermal dehydration, both the coordinated and the hydrogen-bonded or zeolitic water molecules are detached, leading to materials with an open channel system suitable for the adsorption of small molecules.1518 Adsorption2028 is an important property of materials, in particular in the case of NPs that also shows magnetic properties.5,6,19 A good probe to study adsorption on microporous materials is the carbon dioxide molecule.24 Carbon dioxide adsorption is an Received: May 31, 2011 Revised: June 27, 2011 Published: July 02, 2011 15555

dx.doi.org/10.1021/jp205104r | J. Phys. Chem. C 2011, 115, 15555–15569

The Journal of Physical Chemistry C excellent tool for the measurement of the micropore volume26,27 and adsorption interactions.29,30 Additionally, for the study of the physical and chemical properties of solid surfaces, the infrared spectra of adsorbed carbon dioxide are revealing, since this molecule is a small and weakly interacting probe adsorbate.3135 Also, adsorption is an excellent process for the separation, storage, and recovery of gases.2023 In particular, the separation, storage, and recovery of carbon dioxide are very important research topics, since it is a reactant in significant industrial processes and a greenhouse gas that contributes to global warming.24 Here we report a research of the effect on the structure and the interactions of the adsorbed CO2 molecule with the framework of Ni-, Zn-, and Cd-NPs. The tested NPs were very well characterized with X-ray diffraction (XRD), thermogravimetric analysis (TGA), diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), M€ossbauer spectroscopy (MS), and magnetic measurements with a vibrating sample magnetometer (VSM). This careful characterization is needed since small variations in the synthesis methodology could lead to materials with dissimilar properties, and we had made a change in the synthesis methodology of the tested NPs.18 Subsequently, a meticulous structural characterization to unambiguously ascertain the framework structure, the coordination of Fe2+ and M2+ cations, and the state of water in the tested materials was performed. Afterward, to investigate the structural effects and the interactions of the CO2 molecule with the Ni-, Zn-, and Cd-NP frameworks during adsorption, DRIFTS of adsorbed molecules, high-pressure volumetric adsorption of carbon dioxide, and in situ XRD investigations of CO2 adsorption experiments, for the first time in these materials, were performed. With this means, a novel quantitative research of the carbon dioxide interactions within the adsorption space of Ni-, Zn-, and CdNPs was carried out. In this regard, the volume of the available adsorption space, the intensity of the adsorption field within the adsorption space, the specific interactions of the carbon dioxide molecule with the nitroprusside frameworks, and the influence of the adsorption process in the structure of the NPs were studied.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthetic Procedure. All the consumed chemicals were analytical grade without additional purification. The water used in the synthesis process was bidistilled. The produced nitroprussides were synthesized by adding 0.025 mols of solid Na2[Fe(CN)5NO] 3 2H2O to a solution containing 0.025 mols of the corresponding Ni(II), Zn(II), and Cd(II) salt, that is, Ni(NO3)2, Zn(NO3)2, and Cd(NO3)2 in 250 mL of water under constant stirring.18 The formed precipitates were then separated, successively washed with distilled water, and dried at 70 °C during 24 h. The final concentrations of the reagents were 0.1 M. In previous reports 0.01 M solutions were normally used, and the Na2[Fe(CN)5NO] was added in water solution.17 2.2. Methods. XRD tests were carried out with a Bruker D8 Advance system in a BraggBrentano vertical goniometer configuration. The angular measurements were made with a (θ/2θ) of (0.0001 reproducibility, applying steps of 0.01° from 5 to 80° to get XRD profiles that could be accurately resolved by leastsquares methods. The X-ray radiation source was a ceramic XRD Cu anode tube type KFL C 2K of 2.2 kW with a long fine focus. A variable computer-controlled motor driven divergence slit with

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2.5° Soller slits were included to allow kept the irradiated area on the sample surface constant. A Ni filter was placed, prior to the detector, to eliminate CuKss radiation. A LynxEye one-dimensional detector was employed. This detector is based on a Bruker AXS compound silicon strip technology and increases measured intensities, without sacrificing resolution and peak shape. This together with the use of small scanning step resulted in highquality XRD profiles suitable for mathematical treatment. The gathering of the in situ XRD profiles of the dehydrated NP samples, at room temperature, was performed using an Anton Paar HTK-1200N stage. To be dehydrated the samples were treated at 100 °C for 2 h under a N2 (Praxair, 99.99%) flowing at a rate of 50 mL/min. This chamber was designed to be used in the range from room temperature up to 1200 °C. The sample was mounted on an alumina sample holder with temperature sensor located just below the sample.18 The same chamber was applied to collect the in situ XRD profiles of the NPs during carbon dioxide adsorption at 298 K and 1 atm. For the adsorption experiment, CO2 (Praxair, 99.99%) was allowed into the chamber at flow at a rate of 50 mL/min. The TGA testing process was carried out with a TA, Q-500 instrument.24 Samples were placed onto a ceramic sample holder suspended from an analytical balance. The sample and holder were heated according to a predetermined thermal cycle: the temperature was linearly scanned, from 25 to 300 °C, at a heating rate of 5 °C/min under a pure N2 flow of 100 mL/min. The instrument software all automatically controlled data collection, temperature control, heating rate, and gas flow. The TGA data was collected as a wt % vs T (°C) profile, where wt % = (Mt/M0) 100 and is the percent ratio of the sample mass during the thermal treatment, Mt, and the initial mass of the sample M0. DRIFTS were gathered using a Thermo Scientific Nicolet iS10 FTIR spectrometer. The data of the hydrated and dehydrated samples were collected at a resolution of 4 cm1 employing 100 scans per sample. A background, with pure KBr, provided by Nicolet, applying the same conditions was always gathered previous to sample collection. Both the hydrated and dehydrated samples spectra were obtained at room temperature under N2 (Praxair, 99.99%) flowing at a rate of 50 cc/min. Sample dehydration was performed at 100 °C for 2 h under a pure N2 flow of 50 mL/min in the IR high temperature cell. For gathering of the DRIFTS spectra for the NPs during carbon dioxide adsorption, the background was carefully measured using the dehydrated sample at room temperature. Afterward, CO2 (Praxair, 99.99%) was allowed into the sample chamber of the IR high temperature cell at flow at a rate of 50 mL/min rate for three minutes followed by purging under N2 (Praxair, 99.99%) at the same flow rate for one minute. Spectra of the carbon dioxide molecule adsorbed on the NP frameworks were then obtained at room temperature under N2 flow. The room-temperature MS measurements were carried out with a SEECo supplied spectrometer operating at constant acceleration mode with a 50 mCi 57Co γ-ray source in a Rh matrix made by Rietverc GmbH. The velocity scale was chosen to ensure a thorough scan of the materials to reveal all the features associated with the sample material. The 1024-point raw data were folded and analyzed using WMOSS, a public domain Mossbauer spectral analysis Program available at www.SEECo. us, which was formerly WEBRES Company. The calibration was made with reference to R-Fe metal. The spectral fittings of the well resolved doublets was carried out using WMoss 3.09 Program, based on a theoretical model known as the quadrupole-splitting 15556

dx.doi.org/10.1021/jp205104r |J. Phys. Chem. C 2011, 115, 15555–15569

The Journal of Physical Chemistry C

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Figure 1. XRD patterns of the Ni-, Zn-, and Cd-NP samples at 298 K.

Table 1. Space Group and Cell Parameters Calculated with the RPsolved powder Patterns Applying the Pawley WPPD Method sample

a [Å]

Ni-NP

10.19(1)

Zn-NP Cd-NP

19.345(1) 14.25(1)

b [Å]

c [Å]

SG

10.65(1)

R3 Pmna

Fm3m 17.701(1) 7.64(1)

distribution (QSD) and Voigt-based fitting (VBF). This model addresses the case of a sample that does not have a homogeneous Fe environment. The model implemented the VBF method of Rancourt and Ping.36 The magnetic measurements, i.e., the magnetization curves (M vs H) were carried out at room temperature (298 K) in the vibrating sample magnetometer (VSM), Lakeshore 7400 Series. The maximum magnetic field applied was 2.2 T. The powder sample was weighted, located in the sample holder, and then applied the ramp from 2.2 to 2.2 T and thereafter in backward direction. Carbon dioxide equilibrium adsorption at 273 and 298 K and pressures up to 9 atm. were obtained with a Micromeritics ASAP 2050 static volumetric adsorption system. The adsorption isotherms of CO2 (Praxair, 99.99%) at 273 and 298 K were obtained for samples degassed at 373 K for five hours in high vacuum (106 Torr). The backfilling process was carried out using helium (Praxair, 99.99%) in both cases.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of the Synthesized Ni-, Zn-, and Cd-NPs. For nearly two decades the structure of NPs

has been studied.818 Nevertheless, changes in the synthesis process could lead to materials with different properties, requiring a meticulous structural characterization. That is, the polymorphic nature of NPs15,18 induced us to carry out the present structural characterization effort to get a comprehensive understanding of the properties of the tested NPs. In the present case, where we are aiming to perform a thoughtful research of the

structural effects and interactions of an adsorbed test molecule, such as CO2, with the frameworks of the studied NPs, this is especially important. Consequently, this is one of the aims of the present paper. 3.1.1. XRD Study. In Figure 1 are shown the XRD profiles of the Ni-, Zn-, and Cd-NPs at 298 K. To resolve the powder patterns into separate Bragg components without any suggestion of a concrete structural model was applied the powder pattern decomposition (WPPD) method,37 utilizing the WPPD method proposed by Pawley.38 This method can refine the unit cell parameters and decompose the whole powder pattern into individual reflections. The concrete computer program applied to carry out the calculations was the Bruker DIFFRACplus TOPAS software. This is a graphics-based, nonlinear leastsquares profile analysis program that integrates between other features the WPPD Pawley method. In Table 1 are reported the cell parameters and the space groups (SPs). These results indicated that the Ni-NP crystallizes in the cubic Fm3m SP; the Zn-NPs shows a rhombohedral R3 SP framework, and the Cd-NPs crystallize in the orthorhombic Pnma SP. The measured parameters and the proposed space groups are coincident with the structural data reported for these materials in literature.6,8,9,11,17 3.1.2. TGA. In Figure 2 are reported the TGA profiles of the Ni-, Zn-, and Cd-NP samples and the derivative of these TG profiles. The TG profile and as well the derivative of the profile of the NiNP are complex when compared to the TG profiles of the Znand Cd-NPs. In the Ni-NP TG profile many inflections of the curve are observed. These variations indicated that the evolution of the water molecules took place in different steps, in which hydrogen-bonded water and coordinated water were released. The derivative of the TG profile, of the Zn-NP, and also its TG profile are relatively complex as well. In the Zn-NP TG profile are also evidenced modulations of the profile. These inflections denote that the water release process took place in steps where hydrogen-bonded water and coordinated water, respectively, were liberated. The analysis of the TG profiles of the Cd-NP sample, shown in Figure 2, indicated that at relatively low temperatures that is below 100 °C, takes place the release of water, in two steps. These peaks can be related, sequentially, to the liberation of hydrogen-bonded water and coordinated water, respectively.15,18 The total weight lost (WtL) during water elimination in the Ni-NP sample was, WtLNi-NP = 0.29 g/g. For the Zn-NP sample, the total weight lost during water removal was, WtLZn-NP = 0.18 g/g, and for Cd-NP it was WtLCd-NP = 0.09 g/g. Meanwhile, the three NPs appear to be stable from 100 to 250 °C as demonstrated by the absence of significant changes in that zone of the TGA profile. Finally, the weight lost observed after at 250 °C was associated to the decomposition of the NPs by the liberation of the structural NO and CN groups contained in the framework.9,15 The released amount of water indicated that the formula unit of the as-synthesized Ni-, Zn-, and Cd-NP samples must be Ni[Fe(CN)5NO] 3 4H2O, Zn[Fe(CN)5NO] 3 3H2O, and Cd[Fe(CN)5NO] 3 2H2O, respectively.8,11,16 The reported TG analysis is in reasonable agreement with the TG data presented in literature.6,15 3.1.3. DRIFTS. DRIFT spectra of the hydrated (as-synthesized) and dehydrated Ni-, Zn-, and Cd-NP samples in the range between 13002700 cm1 and 27003800 cm1 are shown in Figure 3. In general IR data in the range between 14002700 cm1, in inorganic 15557



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Figure 2. TG profiles of the Ni-, Zn-, and Cd-NPs (wt % vs T graphic) and derivative of the TG profiles of these materials.

Figure 3. DRIFTS spectra of the hydrated (as-synthesized) and dehydrated Ni-, Zn-, and Cd-NP samples at 298 K.

compounds, includes information concerning the framework vibrations, and those in the range 27003800 cm1 contain information related basically with the hydration water.39 Figure 3 shows the vibrations observed in the range between: 13002700 cm1, which are related to the v(CN) stretching vibration at around 1610 cm1, a 1950-cm1 band corresponding to v(NO) stretching vibration, and a 2200-cm1 band due to δ(Fe—CtN) vibration.40 All these vibrations, distinctive of NPs,40 are maintained after dehydration, with minor shifts, excluding the v(CN) stretching vibration corresponding to the unlinked cyanide, which disappears in the Cd-NP. This band, possibly, vanishes since all the CNs are linked in the dehydrated state,15 as is the case for Cu-NP.18 Vibrations related to the water molecules contained in the NP porous framework are usually present in the 30003800-cm1

range. These contributions in NPs are characterized by one broad band in the 32503400-cm1 range corresponding to hydrogen bonded or zeolitic water and narrow peaks at around 3650 cm1 related to linked water.8,9,15,18 Both bands disappear after dehydration. In general DRIFTS results correlate well with the TGA data. That is, IR absorption bands in the 32003800-cm1 range are related to hydrogen-bonded water and coordinated water, respectively. 3.1.4. MS Effect Study. The MS reported in Figure 4 consist of only one quadrupole doublet. This fact was undoubtedly demonstrated by fitting the experimental spectra.36 The three spectra were well-resolved doublets, and the fitting exercise was carried out without constraining the parameters. The reproducibility of the data implied high confidence on the quoted results as shown in Table 2. Accordingly, they were fitted with one sole Fe site, as the χ2 values reflected the convergence of the data to satisfactory 15558



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Figure 4. MS of the as-synthesized Ni-, Zn-, and Cd-NPs at 298 K.

Table 2. M€ ossbauer Hyperfine Parameters Dataa sample

fwhm (mm/s)

QDS (mm/s)

IS (mm/s)

h+/h

Zn-NP

0.234

1.864

0.661

0.995

Ni-NP

0.253

1.892

0.664

0.971

Cd-NP

0.240

1.860

0.664

0.973

a

Key: fwhm,: full width at half maximum; QDS (Δ), center of the quadrupole splitting distribution; IS (δ), isomer shift; h+/h, doublet asymmetric parameter (i.e., the difference between the doublet peaks heights).

levels, when the theoretical fit is compared to the experimental data points. The parameters characterizing the doublets, that is, δ [mm/s], the isomer shift, and Δ [mm/s], the quadrupole splitting, are reported in Table 2. The reported δ and Δ M€ossbauer parameters are almost unaffected by the M2+ (Ni2+, Zn2+, and Cd2+) outer cation. Meanwhile, the value reported for the isomer shift is characteristic of Fe2+ cations in a low spin electronic (LS) configuration. Since, Fe is surrounded by strongly bonded ligands; therefore, a strong crystal field that produces the LS configuration in octahedral coordination is generated.42,43 Additionally, the value measured for the quadrupole splitting is considerable. This amount is due to a significant charge asymmetry produced in the region surrounding the Fe2+ cations by the NO group.15 3.1.5. Magnetochemistry. With the term magnetochemistry4448 is designed the study of the magnetic properties of materials. In this regard, a parameter that allows an assessment of the magnetic properties of a paramagnetic material, as Ni-NP, is the effective magnetic moment, since it is independent of temperature and the external field strength. The effective magnetic moment (μeff), measured in Bohr magnetrons (μB) considering only the spin of the unpaired electrons of the magnetic centers present in the material under study, is given by44,45 μef f ¼ 2:0023ð

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi SðS þ 1ÞÞμB

ð1Þ

where S = 1/2 for one unpaired electron, S = 1 for two unpaired electrons, and so on. In the special circumstance in which S = 1, μ = 2.0023((2)1/2)μB = 2.83 μB. The magnetization curve of the Ni-NP measured with the help of the VSM is reported in Figure 5. This data allows the determination of the effective magnetic moment (μeff) for Ni-NP, yielding μeff = 2.9 μB, a value that is to some extent larger than the spin-only value 2.83 μB. Given that the electronic configuration of Ni2+ is [Ar] 3d8, it is located in octahedral sites and exhibit a high-spin electronic configuration. Therefore, the magnetic measurements are consistent with high-spin Ni2+ located in octahedral sites. The other two Me2+ (Zn2+ and Cd2+) are, as well, located in octahedral coordination surrounded by NC and H2O. Nevertheless, since these cations show the following electronic configurations, Zn2+ [Ar] 3d10 and Cd2+ [Kr] 4d10, they do not have unpaired electrons; consequently the measured magnetization curves show a diamagnetic behavior. 3.2. Adsorption Interactions of CO2 with the Tested NP Frameworks and Morphological Investigation of the Adsorption Space of Ni-, Zn-, and Cd-NPs. For our purpose, in the frame of the research reported here, the worthy of note conclusions of the previous section are that the interface between the framework and the adsorption space of the NPs is covered by oxygen, linked in this manner, FeNO, to the NP framework. Meanwhile, unsaturated M bonds, produced when the H2O molecules are released during heating, are also part of the interface between the framework and the adsorption space of the dehydrated NPs. Besides, Fe is not openly exposed to the adsorption space as long as it is totally surrounded by ligands. It is as possible to state that we are dealing with micropororous crystalline materials with pore diameters in the range of the carbon dioxide molecular diameter,15 a molecule with an ellipsoidal form 5.4 Å long and a diameter of 3.4 Å.49 Then, CO2 should be adsorbed at 298 K in the tested NPs. 3.2.1. DRIFTS Research of the Adsorption Interactions of CO2 with Ni-, Zn-, and Cd-NPs. A DRIFTS adsorption investigation 15559



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Figure 5. Magnetization curve of Ni-NP at 298 K.

Figure 6. DRIFTS spectra of CO2 adsorbed on Ni-, Zn-, and Cd-NPs at 298 K.

that had not been previously reported for the tested NPs will be now described. In this regard, the DRIFTS spectra of the tested NPs at 298 K during CO2 adsorption were collected as was previously explained. The spectra fitted with Lorentz functions are shown in Figure 6. The fitting process was performed with the peak separation and analysis software PeakFit (Seasolve Software Inc., Framingham, Massachusetts) based on the least-squares procedure. The calculated, as the best fitting parameters, peak positions, xci , and amplitudes, Ai, are reported in Table 3. The free carbon dioxide molecule belongs to the D∞h point group symmetry, showing four fundamental vibration modes,

that is, the symmetric stretching, v1 (1338 cm1), the doubly degenerate bending vibration, v2a and v2b (667 cm1), and the asymmetric stretching vibration v3 (2349 cm1).33 The v2 and v3 modes are infrared active, whereas v1 is only Raman active, in the free molecule. However, when a carbon dioxide molecule interacts with a surface, it is no longer a free molecule, and its symmetry is lowered; as a result the v1 mode becomes infrared active, and a small band is observed, whereas the other modes undergo moderate changes in wavenumber.35 The spectra (Figure 6) and the calculated parameters (Table 3) show that the carbon dioxide molecule has direct 15560



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Table 3. Resolved Peak Positions and Amplitudes of the DRIFTS Spectra of Carbon Dioxide Adsorbed on the Tested NPs x1c [cm1]

A1 [%]

2327

3.0

2338

2.3

2359

4.3

Zn-NP

2271

1.3

2326

12.3

2340

10.7

2366

Cd-NP

2291

5.2

2323

3.4

2336

8.9

2368

sample

x2c [cm1]

Ni-NP

A2 [%]

x3c [cm1]

2 Table 4. Micropore Volume (WCO DR ) and Characteristic EnCO2 ergy of Adsorption (EDR ) Calculated by Fitting the Experimental CO2 Adsorption Data with the Dubinin Radushkevich (DR) Isotherm Equation and Micropore 2 O Volume (WH TGA ) Estimated with the TGA Data

sample

T

O 2 WH TGA

2 WCO DR

2 ECO DR

qDR iso (0.37)

[K]

3

3

[cm /g]

[kJ/mol]

[kJ/mol]

[cm /g]

Ni-NP

273

0.275

9

11

Ni-NP Ni-NP

298

0.287

9

11

Zn-NP

273

0.242

12

14

Zn-NP

298

0.251

12

14

0.403

Zn-NP

0.224

Cd-NP

273

0.127

21

25

Cd-NP

298

0.143

22

26

Cd-NP

0.100

interactions with the NP frameworks, as evidenced by the presence of peaks related with two adsorption states of the CO2 molecule. To be precise, the asymmetric stretching vibration, v3, of the adsorbed carbon dioxide molecule was observed at about 2338 cm1. This band corresponds to carbon dioxide physically adsorbed,24,34 in the NP adsorption space. That is, this band is the result of the attachment of the carbon dioxide molecule by dispersive and electrostatic forces to the adsorption space defined by the NP micropores.24 The adsorption process produces the confinement of the carbon dioxide molecule and thereafter the frequency shift18 to 2338 cm1. This state of the carbon dioxide molecule is equivalent to zeolitic water. In addition, there is another band observed at 2362 cm1, which should correspond to adsorption of carbon dioxide on an electron accepting Lewis acid site24,3133,50 forming the following adducts: M2+ 3 3 3 OdCdO. This state of the carbon dioxide molecule is equivalent to that of coordinated water. It is necessary to state now that the bands located around 2290, 2321, and 2421 cm1 are assigned to combination bands.3133,35,51 As was previously mentioned, the information obtained during the characterization guarantee that the adsorption space surface in dehydrated NPs is covered by oxygen anions and unsaturated M bonds. Thereafter, the finding made with the DRIFTS data are consistent with this information, since it indicated that CO2 molecules were adsorbed on two adsorption states, one of them a state where the CO2 molecule is physically adsorbed, fundamentally over the oxygen, and another adsorption state where the CO2 molecule interacts with the M2+ (Ni2+, Zn2+, and Cd2+) cations located in the framework, which act as electron-accepting Lewis acid sites.24,52 Lewis acidbase reactions result in more or less stable adducts whose steadiness can be explained by the model of the hard and soft acids and bases scheme, i.e., an acidbase adduct is produced by the charge transfer from the highest-occupied molecular orbital of the base to the lowest-unoccupied molecular orbital of the acid.53

A3 [%]

x4c [cm1]

x5c [cm1]

A5 [%]

6.4

2421

1.9

13.6

2420

0.1

A4 [%]

In our case, the importance of these findings is related to the fact that the M 2+ cations are located in the framework of the corresponding NP, in a site accessible to small adsorbed molecules, as shown in the present section. Therefore, it is possible to assume that an acidbase interaction between the M 2+ cation and small basic molecules should produce a charge transfer that could be detected, for example, by a change in the effective magnetic moment (μ eff) of the M-NP. This interaction will be possibly detected in magnetic measurements. 3.2.2. Morphological Study of the NPs Adsorption Space. 3.2.2.1. TGA Data. This method can be applied as a novel means for the study of the morphology of the adsorption space in porous materials, since in the hydrated state the TG profile of these materials, from 30 to 150 °C, is merely a water thermo-desorption profile. In this regard, it is possible to O 2 calculate the TG micropore volume, WH TGA , with the help of the TG data and the so-called Gurvich rule.21,22 This rule is expressed as follows: Vi = VLna, where Vi is the volume of the adsorption space, na is the amount adsorbed in this volume, and VL is the molar volume of the liquid phase that conforms the adsorbed phase. To calculate with our data the micropore volume, the following relation was applied H2 O ¼ WTGA

WtLMe-NP 1 WtLMe-NP

ð2Þ

We will now explain how eq 2 was obtained. Since, the mass of degassed adsorbent is ms, subsequently, by definition, the amount adsorbed (in mol/g,) is given by54 na ≈

na ms

ð3Þ

where na ≈ nσ and nσ is the surface excess amount. Thereafter, for the TG data, ms = 1 WtLMe-NP and na = WtLMe-NP/MH2O, where MH2O = 18 g/mol is the molecular weight of water. Since, VH2OL = 18 cm3/mol, it follows that H2 O ¼ VHL 2 O na ¼ VHL 2 O ðWtLMe-NP =MH2 O ms Þ WTGA

¼

WtLMe-NP ðcm3 =gÞ 1 WtLMe-NP

ð4Þ

2O In Table 4 are reported the calculated values of WH TGA for the three tested NPs. 2O The amounts reported for WH TGA have the tendency to be similar or a bit higher than those measured with the help of nitrogen adsorption at 77 K16 and H2O adsorption at 300 K.15 Our own data (Table 4) is a testimony of this assertion. Afterward, taking into account the tendency of the parameters calculated with the help of adsorption measurements reveals that they have a relative error of about 2025%.22,55 It is then possible to assert that the method proposed in the current section shows an acceptable precision.

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Figure 7. Adsorption isotherms (na vs ln P plot) of CO2 on Ni-, Zn-, and Cd-NPs at 273 and 298 K up to 9 atm.

Figure 8. DR plots of the adsorption data of CO2 on Ni-, Zn-, and Cd-NPs at 273 and 298 K.

3.2.2.1. CO2 Morphological Adsorption Test and Storage Capacity. To get the carbon dioxide adsorption data the tested materials were preactivated at 100 °C, a temperature high enough to release all the water present in the micropores as indicated by the TGA data. In Figure 7 are reported the semilogarthmic plots of the obtained adsorption isotherms of carbon dioxide at 273 and 298 K up to 9 atm. on the tested Ni-, Zn-, and Cd-NPs, i.e., the na vs ln P plot. It is necessary to state now that, as far as we know, this is the first high-pressure CO2 adsorption study of these materials, so far.

The data reported in Figure 7 allows us to affirm that the NiNP can store 27 wt % of CO2 at 298 K and 9 atm, the Zn-NP can store 22 wt % of CO2 at 298 K and 9 atm, and the Cd-NP can store 15 wt % of CO2 at 298 K and 9 atm. The amount reported for Ni-NP is considerable. To calculate the micropore volume, the DR adsorption isotherm equation was applied, since, a vast amount of data indicates that the adsorption process in the micropore range is very well described by it.2124,28 This adsorption isotherm 15562


The Journal of Physical Chemistry C equation can be represented in a loglog scale, as follows28,56 2 2 RT P0 lnðna Þ ¼ lnðNa Þ ln ð5Þ E P where na is the amount adsorbed, P0/P is the inverse of the relative pressure, E is a parameter named the characteristic energy of adsorption and, Na is the maximum amount adsorbed in the micropore volume. The linear fitting of the adsorption data by the DR isotherm equation shown in Figure 8 evidenced that the equilibrium adsorption process is typical of a microporous adsorbent. The values of the micropore volume were calculated 3 2 2 CO with the help of WCO MP = V LNa, where VL = 41.3 cm /g and Na is one of the best-fitting parameters calculated by the linear fitting of eq 5 to the adsorption data in the pressure range, 0.00004 < P/P0 < 2 0.1 (Figure 8). The WCO MP calculated values are reported in Table 4. We must note now that the Cd-NP isotherms show an irregular behavior, i.e., the isotherms reverse their order at high pressures. This abnormal behavior of the adsorption process in the Cd-NP, at high pressure (Figure 7), could be related to the interaction of CO2 molecules with framework cations and the small adsorption space of the framework of this NP. These effects creates an intense mutual interaction between the adsorbate and the adsorbent.57,58 This interaction increases the volume of the adsorption space (Table 4), causing the anomalous conduct, explicitly, the high-temperature isotherm is below the low temperature one at high pressures. 3.2.3. Research of the Potential Fields within the Adsorption Space. We know that, in general, if a molecule contacts the surface of a solid adsorbent, it becomes subjected to diverse interaction fields, such as the dispersion energy, ϕD, repulsion energy, ϕR, polarization energy, ϕP, field dipole energy, ϕEμ, field gradient quadrupole energy, ϕEQ, as well some specific interactions, such as the acidbase interaction, ϕAB.2023 Normally, it is also necessary to take into account the adsorbateadsorbate interaction energy, ϕAA. Dispersion and repulsion are the fundamental forces present during physical adsorption in all adsorbents. To be precise, dispersion and repulsion interactions are present in all adsorption gassolid systems since they are nonspecific interactions.2224 However, the electrostatic interactions between the adsorbed molecule and the adsorbent framework, that is, ϕP, ϕEμ, and, ϕEQ, depend on the structure and composition of the adsorbed molecule and the adsorbent itself.2023 In the case of a molecule like the carbon dioxide molecule, which shows a noticeable quadrupolar moment, i.e.,59 QCO2 = 4.3 1042 C 2, it gives rise to specific interactions, where the combination of the dispersive and electrostatic attractive interactions are normally stronger than merely the dispersion interactions. In the case of interest here it is accepted that carbon dioxide interacts with surfaces through its dispersive and quadrupole moment interactions.2024,29 However, we have found in the present case another interaction, i.e., the CO2 molecule interact with the M2+ cation located in the framework. The parameters calculated with the DR adsorption isotherm equation not only allow us to evaluate the micropore volume of the sample but also the adsorption interaction between the adsorbate and the adsorbent. To evaluate this interaction will 2 be used the characteristic energy of adsorption, ECO DR . This parameter is the other best fitting parameter calculated by the linear fitting of eq 5 to the adsorption data (Figure 7 and Table 4).

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The quantitative evaluation of the interaction between carbon dioxide and the tested NPs will be carried out with the help of the isosteric heat of adsorption, qiso.2124 In this regard, we will show now that it is possible to calculate qiso using only one isotherm,24 as follows56 qiso ðθÞ ¼ ΔGðθÞ þ EFðT, θÞ

ð6Þ

In which:

P ΔGðθÞ ¼ RT ln P0

ð7Þ

and 1=n 1 RT 1 ln FðT, θÞ ¼ 2 θ

ð8Þ

where θ = na/Na, R = d ln Na/dT, and E and n are the parameters of the Dubinin adsorption equation. It is also possible to assert that56 E = ΔG(1/e), where θ = 1/e, in which e ≈ 2.71828183 is the base of the Napierian logarithm system. Now with the help of eqs 68, for θ = 1/e ≈ 0.37, it is possible to get the following equation qiso ð0:37Þ ¼ ΔGð0:37Þ þ EFðT, 0:37Þ ¼ E½FðT, 0:37Þ 1

ð9Þ

The parameter F(T,0.37) was previously calculated24 yielding the following value, F(T,0.37) ≈ 2.16. Thereafter, was obtained the relation qiso(0.37) = 1.16E, expression that allowed the calculation of the isosteric heat of adsorption. The results reported in Table 4 indicate that the isosteric heat of adsorption for θ = 0.37 (Table 4) depends on the size of the adsorption space, and the bigger value is that reported for Cd-NP. Consequently, the carbon dioxide molecules are more confined in Cd-NP. Statistical physics20,22 had been applied to mathematically model the equilibrium adsorption process in microporous materials and deduce adsorption isotherm equations.20,22,23,60,61 In the case of zeolites these methods were combined6062 with the Dubinin theory of volume filling.28 To develop the model of interest here, the adsorption space was considered energetically homogeneous and the adsorption process was considered as a 3-D volumetric occupation of the adsorption space56 and not as a 2-D surface covering process, such as the one described by traditional Langmuir processes. The isotherm equations, for immobile and mobile adsorption with lateral interactions, deduced by means of the Grand Canonical Ensemble,22 following the conditions previously imposed, has the following form60,61 θ¼

na KFGT P ¼ 1 þ KFGT P Na

KFGT

! g ½ðE0 Ea0 Þ kθ ¼ K0 exp exp RT RT

ð10aÞ

ð10bÞ

where K0 is a constant for T = const, different for the mobile and immobile cases, Eg0 is the reference energy state for the gas molecule, Ea0 is the reference energy state for the adsorbed molecule in the homogeneous adsorption field inside the cavity or channel, k is a constant characterizing the lateral interactions, and θ = na/Na is the adsorption space volume fractional occupation or loading, where na, is the amount adsorbed and Na is the maximum amount adsorbed in the adsorption space volume. 15563



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Figure 9. Fitting of the CO2 adsorption data with a LT isotherm equation.

Equation 10a is a FowlerGuggenheim Type (FGT) adsorption isotherm equation, but in this case, describing a volume filling rather than a surface coverage.22 For k ≈ 0, eq 10a is reduced to a Langmuir-type (LT) adsorption isotherm equation61 θ¼

na KLT P ¼ 1 þ KLT P Na

ð11aÞ

Table 5. Na and K Calculated by Fitting the Experimental CO2 Adsorption Data with the LT Isotherm Equation and 2 Micropore Volume (WCO LT ) and Isosteric Heat of Adsorption Calculated According to eq 10a with the FGT Plot sample T [K] Na [mmol/g]

KLT

3 LT 2 WCO LT [cm /g] qiso (0) [kJ/mol]

Ni-NP Ni-NP

273 298

6.90 6.93

0.00136 0.00057

0.285 0.286

23 ( 1

ð11bÞ

Zn-NP

273

4.52

0.00312

0.187

24 ( 1

Zn-NP

298

4.67

0.00126

0.193

also describing a volume filling instead of a surface coverage. In Figure 9 are reported the nonlinear fitting of the experimental adsorption data to the LT adsorption isotherm, eq 11a for θ < 0.5, that is, for relatively low fractional occupations, where k ≈ 0. In Table 5 are reported Na, the maximum amount adsorbed in the micropore volume, and KLT, both calculated with the help of the best fitting parameters. The fitting process was as well executed with the peak separation and analysis software PeakFit. Now, with the help of the following expression20,61

Cd-NP

273

2.21

0.04610

0.091

Cd-NP

298

2.42

0.02120

0.100

g

KLT ¼ K0 exp

½ðE0 Ea0 Þ RT

K ¼ K0 eqiso ð0Þ=RT

!

ð12Þ

We can estimate the isosteric heat of adsorption for zero fractional occupation or loading, qLT iso (0), with the help of the values calculated for KLT at 273 and 298 K (Table 5). The previously calculated adsorption data indicates that the interaction of the carbon dioxide molecule with the NPs is not

25 ( 1

very strong, since the measured isosteric heats of adsorption are below 26 kJ/mol. Thereafter, we are in the presence of physical adsorption processes. In physical adsorption processes it is admitted that the carbon dioxide molecules are attracted to the framework by the influence of dispersive and quadrupole interactions.2024 In the present case is also included the specific interaction, which was previously detected. The surface of the adsorption space in dehydrated NPs is covered by oxygen anions and the unsaturated M bonds. In this regard, the contribution of the dispersion forces to the isosteric heat of adsorption of carbon dioxide in a cylindrical pore of 0.31 nm covered with oxygen anions, was previously calculated,24 yielding qiso(0) = 12.8 kJ/mol. The contribution of the attraction of the quadrupole interaction to the isosteric heat of adsorption at zero loading is smaller than 15 kJ/mol63 15564



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Figure 10. XRD patterns of the dehydrated Ni-, Zn-, and Cd-NP samples and during CO2 adsorption at 298 K.

Figure 11. Detail of the XRD profiles of the dehydrated Ni-, Zn-, and Cd-NP samples and during CO2 adsorption at 298 K.

for silica and is normally bigger than those contribution provided by the dispersion forces.64 Thereafter, in the present case, the

contribution of the attraction of the quadrupole interaction to the isosteric heat of adsorption at zero loading should be qiso(0) ≈ 15565



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Table 6. Space Group and Cell Parameters Calculated with the Resolved Powder Patterns Applying the Pawley WPPD Method sample

a [Å]

b [Å]

c [Å]

SG

Ni-NP-CO2-adsorbed

10.152 (1)

Fm3m

Ni-NP-dehydrated

10.136(1)

Fm3m Fm3m

Ni-NP-as-synthesized

10.19(1)

Zn-NP-CO2-adsorbed

19.274(3)

17.717(3)

R3

Zn-NP-dehydrated Zn-NP-as-synthesized

19.280(1) 19.345(1)

17.723(1) 17.701(1)

R3 R3

Cd-NP-CO2-adsorbed

13.063(1)

7.432(1)

10.505(1)

Pmna

Cd-NP-dehydrated

13.031(2)

7.393(1)

10.471(2)

Pmna

Cd-NP-as-synthesized

14.25(1)

7.64(1)

10.65(1)

Pmna

1213 kJ/mol. Therefore, the contribution of the specific interactions of the CO2 molecule with the M2+ cations to the isosteric heat of adsorption at zero loading, should be qiso(0) ≈ 1213 kJ/mol. These estimations are made using the previously reported isosteric heat of CO2 adsorption on a surface, such as silica, that is covered by oxygen data24,63,64 and the data reported here. The previous analysis explains the higher amount of CO2 adsorption on Cd-NP at low pressures (Figure 9) in relation to the amounts reported for Ni- and Zn-NPs. This fact together with the high amount of CO2 adsorption on Ni- and Zn-NPs, at high pressures, indicates that Ni- and Zn-NPs are excellent for CO2 storage and Cd-NP is good for gas cleaning. We have previously commented the anomalous conduct of the Cd-NP during carbon dioxide adsorption. The explanation given was the following as long as the pore size of the channels formed in the orthorhombic framework of the Cd-NP are close to the kinetic diameter of the CO2 molecule; this fact causes an effect named confinement or surface interaction potential overlapping.57,58 This effect favors relatively strong attraction and repulsion interactions between the framework of the adsorbent and the adsorbed molecule, and also between the adsorbate molecules that affect the framework of the Cd-NP. To test this hypothesis, the XRD profile of the studied NPs were gathered, as far as we know for the first time in the research of the tested NPs for dehydrated samples and for samples during carbon dioxide adsorption at 298 K and 1 atm. 3.3. XRD Research of the Effect of Dehydration and CO2 Adsorption in the Structure of the Framework of the Tested Ni-, Zn-, and Cd-NPs. So far, in our analysis we have only considered the adsorbateadsorbent and adsorbateadsorbate interactions, that is, that the solid provides, merely, an invariable adsorption space and adsorption field. This is one of the fundamental hypotheses for the development of the theory of physical adsorption, i.e., the inertness of the adsorbent during the adsorption process.2023 It is our understanding that the abnormal behavior of Cd-NP at relatively high pressures, as was previously recognized, is fundamentally due to the fact that this adsorbent is affected by the adsorbate in the gas phase during the adsorption process. To substantiate this consideration we collected in situ XRD profiles at 298 K of the dehydrated NPs and during CO2 adsorption. In Figures 10 and 11 are shown the XRD profiles of the NP samples dehydrated at 373 K for 2 h. in N2 flow (50 mL/min) and collected at 298 K also in N2 flow inside the high temperature cell. Also in Figures 10 and 11 are displayed the XRD profiles gathered during CO2 adsorption at 298 K and 1 atm on the tested NPs. For the collection of the XRD

Figure 12. Isosteric heat of CO2 adsorption vs loading plot.

profiles for the three NPs during adsorption, CO2 (Praxair, 99.99%) at 1 atm. was allowed into the high temperature XRD cell at flow at a rate of 50 mL/min rate at 298 K. Then, the XRD profiles were gathered at room temperature in the 1 atm. CO2 environment. To resolve the powder patterns reported in Figure 10 was also applied the Pawley WPPD method.38 With this methodology were refined the unit cell parameters. In Table 6 are reported the cell parameters and the space groups corresponding to the six XRD profiles. These results indicated that the Ni-NP shows a cubic Fm3m space group framework (Table 6) in the dehydrated status and throughout the CO2 adsorption process. Additionally, the Ni-NP exhibits a lower cell parameter after dehydration, but during CO2 adsorption is not observed a noticeable change of the cell parameters in relation to the material in the dehydrated state (Table 6 and Figure 11). Meanwhile, the Zn-NP sample displays a rhombohedral R3 space group framework (Table 6) in the dehydrated state and as well during adsorption. This NP shows a change of the cell parameters after dehydration; however, during CO2 adsorption is not observed a perceptible change of the cell parameters (Table 6 and Figure 11). The Cd-NP sample shows an orthorhombic Pnma space group framework (Table 6) in the dehydrated state and during adsorption. This NP sample shows a significant decrease of the cell volume after dehydration; besides, during CO2 adsorption is observed a small increase of the cell parameters (Table 6 and Figure 11). The lowering of the cell parameters during dehydration can be explained by a surface effect. That is, when the framework atoms linked to the crystallization water are released, during dehydration of the NP, the “surface energy” is higher. Thereafter, a tendency to reduce the surface is translated to a decrease of the cell parameters reported in Table 6. When carbon dioxide is adsorbed the reverse effect should be noted. In this regard, a small increase in the cell parameters during carbon dioxide adsorption, for the Cd-NP, is noted (Table 6 and Figure 11). However, for the Ni- and Zn-NPs the parameters and the XRD profile are practically identical (Table 6 and Figure 11). Consequently, the unusual behavior of the adsorption process in the Cd-NP, at high pressure (Figure 7), previously explained by the increase in the volume of the adsorption space (Table 4) 15566


The Journal of Physical Chemistry C produced by the interactions between the CO2 molecules and the NP framework is now clearly evidenced by the XRD data. The fact that the other NPs behaves normally is also explained. 3.4. Dependence of the Isosteric Heat of CO2 Adsorption in the Tested Ni-, Zn-, and Cd-NPs with the Fractional Occupation. Two isotherms in semilogarithmic plot (Figure 7) can as well be employed for the calculation of the isosteric heat of adsorption, qiso, applying the ClausiusClapeyron equation21 ln P2 ln P1 2 d ln P qiso ≈RT ≈RT1 T2 ð12Þ dT na T2 T1 na where na is the amount adsorbed in mmol/g, Ti the temperature, and Pi the equilibrium adsorbate pressure at constant loading. In Figure 12 are reported the isosteric heat of adsorption calculated according with eq 12 for the Ni- and Zn-NPs, plotted vs loading. The dependence of the isosteric heat of CO2 adsorption in the CdNPs with the fractional occupation was not reported; since, the inertness of the adsorbent is one of the fundamental hypotheses2023 for the application of eq 12; subsequently, since the Cd-NP is not inert during CO2 adsorption the application of this equation to the Cd-NP is not justifiable. In fact, when applied, it yields values smaller than those calculated with the help of the DR and the LT plots. The data reported in Figure 12, indicates that the adsorption process in the Ni- and Zn-NPs are energetically heterogeneous.65 However, in the range of loading, 0.1 < θ < 0.5, both NPs shows an energetically homogeneous adsorption process. This fact demands a clarification. During adsorption in micropores the adsorption process at very low relative pressure is dominated practically completely by the interactions of the adsorbate and the pore wall, after that, at higher pressures, lateral interactions between adsorbate molecules (AA interactions) are also present.2024 In general, adsorption is normally energetically heterogeneous as long as the adsorption field inside the micropore is heterogeneous and depends upon the position of the adsorbate inside the adsorption space. The isoteric heat of adsorption should be a decreasing function of the micropore volume recovery, θ.65 Nevertheless, in some cases the energetically heterogeneous character of the process is masked by the presence of lateral interactions, which provoke the “homogenization” of the adsorption field. That is the adsorption energy corresponding to wall and lateral interactions are mutually compensated to produce an apparently homogeneous adsorption energy. This is the case here for, 0.1 < θ < 0.5. In this regard, this homogeneity of the adsorption field is the cause of the good fitting of our data by the LT adsorption isotherm equation for θ < 0.5 (Figure 9). LT In Tables 4 and 5, qDR iso (0.37) and qiso (0), respectively, are reported. These magnitudes are average values since are calculated with a plot of the DR and LT isotherms in the range 0.00004 < P/P0 < 0.1. These data also evidenced the heterogeneity of the adsorption process for the Ni- and Zn-NPs since qLT iso (θ = 0) > qDR iso (θ = 0.37). However, given that these magnitudes are average values do not coincide with the values displayed in Figure 12. DR The difference between qLT iso (θ = 0) and qiso (θ = 0.37) signifies that the adsorption process for the Ni- and Zn-NPs is composed of two principal types of interactions, i.e., at small loadings an interaction with the framework where the adsorbateadsorbate (AA) interaction do not play an important role and a second type in which the adsorbateadsorbate (AA) interaction perform a significant role.

ARTICLE LT In Tables 4 and 5, qDR iso (0.37) and qiso (0) are also reported for LT the Cd-NP. In this case, qiso (θ = 0) ≈ qDR iso (θ = 0.37); this fact means that the adsorption process for the Cd-NP is fundamentally an interaction with the framework where the adsorbate adsorbate (AA) interactions do not play an important role because of the small adsorption space of this NP.

4. SUMMARY Careful research with XRD, TGA, DRIFTS, MS, magnetic measurements, and CO2 adsorption was carried out. This is not an original topic; however it is unavoidable, because of the polymorphic nature of NPs. The Pawley fitting of the XRD profiles of the as-synthesized NPs indicated that Ni-NP, Zn-NP, and Cd-NP crystallize in the Fm3m, R3, and Pnma space groups, respectively. The TG data allowed us to measure the total weight lost (WtL) during water elimination yielding the following results WtLNi-NP = 0.29 g/g, WtLZn-NP = 0.18 g/g, and WtLCdNP = 0.09 g/g. The determination with TG of the released water demonstrated that Ni[Fe(CN)5NO] 3 4H2O, Zn[Fe(CN)5NO] 3 3H2O, and Cd[Fe(CN)5NO] 3 2H2O were obtained. The reported δ and Δ M€ossbauer parameters are practically unaltered by the M2+ (Ni2+, Zn2+, and Cd2+) outer cation. The Fe2+ cations are located in a low-spin electronic configuration, in octahedral coordination. Meanwhile, the quadrupole splitting show a relatively big value due to a significant charge asymmetry produced in the region surrounding the Fe2+ cations by the NO group. The magnetic measurements are consistent with high-spin Ni2+ in octahedral sites. The other two Me2+ cations, i.e., Zn2+ and Cd2+, are also located in octahedral coordination, but both show diamagnetic behavior. A DRIFTS test of adsorbed CO2 displayed peaks assigned to contributions due to physical adsorption and the formation of adducts. The importance of this finding is related to the fact that it is feasible a charge transfer that could be detected for example by a change in the effective magnetic moment (μeff) of the Me-NP. The TGA study as well allowed the measurement of the 2 O micropore volumes, yielding the following values: WH TGA (Ni) = H2 O CO2 3 3 0.403 cm /g, WTGA (Zn) = 0.224 cm /g, and WAd (Cd) = 0.100 cm3/g. The fitting of the adsorption data to the DR equation and a LT equation for volume filling allowed the calculation of the micropore volume and the isosteric heats of adsorption. The unusual behavior of the adsorption process in Cd-NP, at high pressure, was explained as an effect caused by the small volume of the adsorption space of this NP. The other two NPs behaved normally. The Pawley fitting of the XRD profiles of the dehydrated and under CO2 adsorption materials indicated that for both states NiNP, Zn-NP, and Cd-NP display the Fm3m, R3, and Pnma space groups, respectively. The dehydrated samples demonstrated a change in the cell parameters. However, only Cd-NP shows a variation of its cell parameters under CO2 adsorption. This fact was linked to the unusual behavior of the adsorption process in Cd-NP. Additionally, was shown that the dehydrated Ni-, Zn-, and Cd-NPs can store 27, 22, and 15 wt % of CO2 at 298 K and 9 atm., respectively. The amount reported for the Ni-NP is noteworthy. The analysis made explains the higher amount of CO2 adsorption on Cd-NP at low pressures (Figure 9) in relation to the amounts reported for Ni- and Zn-NPs. This fact together with the high amount CO2 adsorption on Ni- and Zn-NPs at high pressure indicate that the former NPs are excellent for CO2 storage and Cd-NP is good for gas cleaning. 15567



’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] and [email protected]. Phone: 787-743-7979 ext. 4260. Fax: 787-743-7979 ext. 4114.

’ ACKNOWLEDGMENT The authors R.R.M., C.L., R.P., and F.L. acknowledge the financial support provided by the US Department of Energy through the Massey Chair project at the University of Turabo and recognize the support of the National Science Foundation under the project CHE-0959334. Finally, we express our gratitude to Emmanuel Pi~ neiro for the help in the materials characterization process and Jari R. Cabarcas Bolivar and Ermides Chavez Baldovino for its help during the M€ossbauer study of the three tested samples. ’ REFERENCES (1) Natesakhawat, S.; Culp, J. T.; Matranga, C.; Bockrath, B. J. Phys. Chem. C 2007, 111, 1055. (2) Kaye, S. S. R.; Choi, H. J.; Long, J. R. J. Am. Chem. Soc. 2008, 130, 16921. (3) Kaye, S. S. R.; Choi, H. J.; Long, J. R. J. Am. Chem. Soc. 2005, 127, 6506. (4) Zhang, J.; Lachgar, A. J. Am. Chem. Soc. 2007, 129, 250. (5) Gu, Z.-Z.; Sato, O.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Phys. Chem. 1996, 100, 18289. (6) Gu, Z.-Z.; Sato, O.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Chem. Mater. 1997, 9, 1092. (7) Gentil, L. A.; Baran, E. J.; Aymonino, P. J. Inorg. Chim. Acta 1976, 20, 251. (8) Mullica, D. F.; Sappenfield, E. L.; Tippin, D. B.; Leschnitzer, D. H. Inorg. Chim. Acta 1989, 164, 99. (9) Mullica, D. F.; Tippin, D. B.; Sappenfield, E. L. J. Coord. Chem. 1991, 24, 83. (10) Benavente, A.; de Moran, J. A.; Aymonino, P. J. J. Chem. Crystallogr. 1997, 27, 343. (11) Reguera, E.; Dago, A.; Gomez, A.; Bertran, J. F. Polyhedron 1996, 15, 3139. (12) Funck, K. E.; Hilfiger, M. G.; Berlinguette, C. P.; Shatruk, M.; Wernsdorfer, W.; Dunbar, K. R. Inorg. Chem. 2009, 48, 3438. (13) Gomez, A.; Reguera, E.; Cranswick, L. M. D. Polyhedron 2001, 20, 165. (14) Boxhoorn, G.; Moolhuysen, J.; Coolegem, J. P. G.; van Santen, R. A. J. Chem. Soc., Chem. Commun. 1985, 1305. (15) Balmaseda, J.; Reguera, E.; Gomez, A.; Roque, J.; Vazquez, C.; M. Autie, M. J. Phys. Chem. B 2003, 107, 11360. (16) Culp, J. T.; Matranga, C.; Smith, M.; Bittner, E. W.; Bockrath, B. J. Phys. Chem. B 2006, 110, 8325. (17) Reguera, L.; Balmaseda, J.; Krap, C. P.; Reguera, E. J. Phys. Chem. C 2008, 112, 10490. (18) Roque-Malherbe, R.; Lozano, C.; Polanco, R.; Marquez, F.; Lugo, F.; Hernandez-Maldonado, A.; Primera-Pedrozo, J. N. J. Solid State Chem. 2011, 184, 1236. (19) Zentkova, M.; Mihalik, M.; Toth, I.; Mitroova, Z.; Zentko, A.; Sendek, M.; Kovac, J.; Lukacova, M. M.; Marysko, M.; Miglierini, M. J. Magn. Magn. Mat. 2004, 272276, E753. (20) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley and Sons: New York, 1984. (21) Rouquerol, F.; Rouquerol, J.; Sing, K. S. W. Adsorption by Powder Porous Solids: Academic Press: New York, 1999. (22) Roque-Malherbe, R. Adsorption and Diffusion in Nanoporous Materials; CRC Press: Boca Raton, FL, 2007. (23) Roque-Malherbe, R. The Physical Chemistry of Materials: Energy and Environmental Applications; CRC Press: Boca Raton, FL, 2010.

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