Selective Surfaces: Quaternary Co(Ni)MoS-Based Chalcogels with

Aug 23, 2012 - The Mo6+ species undergoes redox reactions during network assembly, giving rise to Mo4+/5+-containing species where the Mo is bound to ...
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Selective Surfaces: Quaternary Co(Ni)MoS-Based Chalcogels with Divalent (Pb2+, Cd2+, Pd2+) and Trivalent (Cr3+, Bi3+) Metals for Gas Separation Kyriaki Polychronopoulou, Christos D. Malliakas, Jiaqing He, and Mercouri G. Kanatzidis* Department of Chemistry, Northwestern University, 2145 North Sheridan Road, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Porous chalcogels with tunable compositions of CoxM1−xMoS4 and NixM1−xMoS4, where M = Pd2+, Pb2+, Cd2+, Bi3+, or Cr3+ and x = 0.3−0.7, were synthesized by metathesis reactions between the metal ions and MoS42−. Solvent exchange, counterion removal and CO2 supercritical drying led to the formation of aerogels. All chalcogels exhibited high surface areas (170−510 m2/g) and pore volumes in the 0.56−1.50 cm3/g range. Electron microscopy coupled with nitrogen adsorption measurements suggest the presence of both mesoporosity (2 nm < d < 50 nm) and macroporosity (d > 50 nm, where d is the average pore size). Pyridine adsorption corroborated for the acid character of the aerogels. We present X-ray photoelectron spectroscopic and X-ray scattering evidence that the [MoS4]2− unit does not stay intact when bound to the metals in the chalcogel structure. The Mo6+ species undergoes redox reactions during network assembly, giving rise to Mo4+/5+-containing species where the Mo is bound to sulfide and polysulfide ligands. The chalcogels exhibit high adsorption selectivities for CO2 and C2H6 over H2, N2, and CH4 whereas specific compositions exhibited among the highest CO2 enthalpy of adsorption reported so far for a porous material (up to 47 kJ/mol). The Co-Pb-MoS4 and Co-Cr-MoS4 chalcogels exhibited a 2-fold to 4-fold increase in CO2/H2 selectivity compared to ternary CoMoS4 chalcogels. KEYWORDS: chalcogenides, aerogels, porosity, gas adsorption

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INTRODUCTION The inherent high surface area and low density of aerogels makes them ideal for numerous applications, including catalysis,1−3 gas sensing,4 superinsulation, and environmental remediation.5 The extensive body of studies reported on aerogels, is mainly based on oxides, such as titania, zirconia, niobia, silica, alumina, and chromia.6 However, chalcogenidebased aerogels, generally referred to as chalcogels, are an emerging class of materials with intrinsic and unique characteristics, such as more polarizable surface atoms, high surface area, small/tunable band gaps, and the ability to be functionalized.7 These make them promising candidate materials for photocatalysis,8 photovoltaic cells,9 wastewater remediation,10−16 and possibly other functions. Previous preparations of metal chalcogenide aerogels included thiolysis,17,19,20 nanoparticle condensation,18,21 and metathesis reactions.11 The method we developed22 is based on the metathesis (or partner-switching) reaction, makes use of molecular chalcogenido anions and linking metal cations. The flexibility of this method gives the advantage of tuning the resulting material properties through appropriate selection of anions and cations. At the same time, a judicious match of the components constituting the gel is needed in order to force the building block and linker metal to engage in a controlled selfassembly process. Rapid precipitation23 or a permanent © 2012 American Chemical Society

solution with no gelation taking place should be avoided. Based on this approach, a new class of platinum-based chalcogels consisting of platinum, germanium, and chalcogens was realized.11,24 The same synthetic method was extended to tetrathiomolybdate-based chalcogels, where enhanced catalytic activity for hydrodesulfurization reactions was observed.22 The high surface areas and multifunctional nature make chalcogels especially promising for gas separation. In particular, different chalcogels were reported to exhibit high selectivity in CO2 and C2H6 over H2 and CH4 adsorption.15,22 The latter is relevant to the exit gas stream composition of water-gas-shift and steam-reforming reactions (reactions widely used for H2 production nowadays).25 For example, separation of gas pairs such as CO2/H2, CO2/CH4, and CO2/N2 are key steps in precombustion capture of CO2, natural gas sweetening and postcombustion capture of CO2 processes leading ultimately at upgrading of the raw gas. The aforementioned conditioning makes the gas suitable for applications in fuel cells,26 and higher efficiency power plants.27,28 In the present study, the repertoire of metathesis chemistry for the synthesis of chalcogels was extended to new materials. Received: May 10, 2012 Revised: August 2, 2012 Published: August 23, 2012 3380

dx.doi.org/10.1021/cm301444p | Chem. Mater. 2012, 24, 3380−3392

Chemistry of Materials

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Table 1. Synthesis Conditions for All of the Co-Based Titled Chalcogels Chalcogel 1

Co0.5Pb0.5MoS4

2

Co0.5Pb0.5MoS4 (Conc.)

3

Co0.5Pb0.5MoS4b

4

Co0.7Pb0.3MoS4

5

Co0.7Pb0.3·MoS4c

6

Co0.7Pb0.3MoS4

7

Co0.5Cd0.5MoS4

8

Co0.5Pd0.5MoS4

9

Co0.5Cr0.33MoS4

10

Co0.5Cr0.33MoS4b

11 12

Co0.7Cr0.2MoS4 Co0.3Cr0.46MoS4

13

Co0.5Bi0.33MoS4

14

Co0.7Bi0.2MoS4

12

1-Co-Cr-MoS4

13

2-Co-Pb-MoS4

14

3-Co-Bi-MoS4

M1 precursor (g) Co(NO3)2·6H2O 0.029 Co(NO3)2·6H2O 0.073 Co(NO3)2·6H2O 0.073 Co(NO3)2·6H2O 0.041 Co(NO3)2·6H2O 0.10 Co(NO3)2·6H2O 0.10 Co(NO3)2·6H2O 0.073 Co(NO3)2·6H2O 0.073 Co(NO3)2·6H2O 0.073 Co(NO3)2·6H2O 0.073 Co(NO3)2·6H2O Co(NO3)2·6H2O 0.043 Co(NO3)2·6H2O 0.073 Co(NO3)2·6H2O 0.10 Co(NO3)2·6H2O 3.34 Co(NO3)2·6H2O 3.34 Co(NO3)2·6H2O 3.34

M2 precursor (g)

Mo-prec. (NH4)2MoS4 (g)

Pb(CH3COO)2·3H2O 0.038 Pb(CH3COO)2·3H2O

M1:M2:Mo molar ratios/initial concentrations (mmol)

aging time (days)

yielda (%)

0.052

0.5:0.5:1/ (0.1:0.1:0.2)

10

84

0.13

(0.5:0.5:1/0.25:0.25:0.5)

7

50

(0.5:0.5:1/0.25:0.25:0.5)

7

50

0.7:0.3:1 (0.14:0.06:0.2)

8

91

0.7:0.3:1 (0.35:0.15:0.5)

1

50

0.7:0.3:1 (0.35:0.15:0.5)

8

50

0.5:0.5:1 (0.25:0.25:0.5)

7

52

0.5:0.5:1 (0.25:0.25:0.5)

7

52

0.5:0.33:1 (0.25:0.17:0.5)

7

59

0.5:0.33:1 (0.25:0.17:0.5)

7

59

0.7:0.2:1 (0.35:0.1:0.5) 0.3:0.46:1 (0.15:0.23:0.5)

8 9

58 60

0.5:0.33:1 (0.25:0.17:0.5)

7

36

0.7:0.2:1 (0.35:0.1:0.5)

7

34

0.7:0.2:1/ (11.6:3.3:16.5)

12

62

0.7:0.3:1/ (11.6:4.95:16.5)

12

74

0.7:0.2:1/ (11.6:3.3:16.5)

14

42

0.094 Pb(CH3COO)2·3H2O 0.13 0.094 Pb(CH3COO)2·3H2O 0.052 0.022 Pb(CH3COO)2·3H2O 0.13 0.057 Pb(acac)2·3H2O 0.13 0.061 Cd(acac)2 0.13 0.078 Pd(acac)2 0.13 0.076 Cr(NO3)3·9H2O 0.13 0.068 Cr(NO3)3·9H2O 0.13 0.068 Cr(NO3)3·9H2O 0.13 Cr(NO3)3·9H2O 0.13 0.092 Bi(CH3COO)3 0.13 0.065 Bi(CH3COO)3 0.13 0.038 Scale-Up Chalcogels Cr(NO3)3·9H2O 4.29 1.32 Pb(CH3COO)2·3H2O 4.29 1.87 Bi(CH3COO)2 4.29 1.27

a The product yield has been calculated based on the limiting reagent. bReverse Addition of Precursors (RAP), where M2 precursor is added first to the (NH4)2MoS4 solution and then M1 precursor is added to the M1 precursor + (NH4)2MoS4 solution. cThis chalcogel has been synthesized in the high concentration regime.

Table 2. Synthesis Conditions for All of the Ni-Based Titled Chalcogels chalcogel

M1 precursor (g)

M2 precursor (g)

1

Ni0.5Pb0.5MoS4

2

Ni0.5Pb0.5MoS4b

3

Ni0.7Pb0.3MoS4

4

Ni0.7Cr0.2MoS4

5

Ni0.5Pd0.5MoS4

Ni(NO3)2·6H2O 0.073 Ni(NO3)2·6H2O 0.073 Ni(NO3)2·6H2O 0.040 Ni(NO3)2·6H2O 0.10 Ni(NO3)2·6H2O 0.073

Pb(NO3)2·3H2O 0.082 Pb(NO3)2·3H2O 0.082 Pb(ac)2·3H2O 0.023 Cr(NO3)3 0.040 Pd(acac)2 0.076

Mo prec. (NH4)2MoS4 (g)

M1:M2:Mo molar ratios/initial concentrations (mmol)

aging time (days)

yielda (%)

0.13

0.5:0.5:1/ (0.25:0.25:0.5)

10

45

0.13

0.5:0.5:1/ (0.25:0.25:0.5)

10

47

0.052

0.7:0.3:1/ (0.14:0.06:0.2)

26

90

0.13

0.7:0.2:1/ (0.35:0.1:0.5)

10

59

0.13

0.5:0.5:1/ (0.25:0.25:0.5)

9

52

The product yield has been calculated based upon the limiting reagent. bReverse Addition of Precursors (RAP), where M2 precursor is added first to the (NH4)2MoS4 solution and then M1 precursor is added to the M1 precursor + (NH4)2MoS4 solution. a

Bi3+, Cr3+. The effects of chemical composition on the structure, morphology, acidity, chemical environment, thermal stability, and chemical stability (oxidation, hydrolysis) are discussed. The new materials exhibit enhanced CO2 adsorption

In particular, we show that the addition of two different metal linkers simultaneously (divalent−divalent and divalent−trivalent combinations) leads to a novel family of Co-M2+/3+-MoS4 and Ni-M2+/3+-MoS4 chalcogels, where M = Pb2+, Cd2+, Pd2+, 3381

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H2O and EtOH) was provided every 12 h. The monolithic gel in EtOH (alcogel) was subjected to drying in supercritical CO2. Supercritical Drying of Alcogels. A critical-point drying apparatus (Tousimis Autosamdri 815B series) was used to remove the solvent from the alcogels. The wet alcogel was placed into the supercritical point dryer chamber. Liquid carbon dioxide (LCO2) was then introduced into the chamber to exchange the remnant ethanol solvent over the period of 8 h, and fresh CO2 was introduced into the chamber every 20 min over the course of 8 h. The supercritical drying was performed at a temperature of 41 °C and a pressure of 1400 psi for 4 min. The drying step was followed by the bleeding step, where gaseous CO2 was slowly bled out of the chamber. The black-colored chalcogels were subsequently placed into a nitrogen atmosphere glovebox in order to avoid air oxidation. In the case of the Co-Bi-MoS4 scale-up chalcogel (∼4 g), it was noticed that, when exposed to air, it ignites (the upper layers mostly) and the material color changes to an ash gray. Nitrogen Adsorption Measurements. A Micromeritics ASAP 2020 system was used to measure the surface area and pore size distribution of the synthesized chalcogels through nitrogen adsorption/desorption porosimetry at 77.15 K. The samples were degassed at 343 K under vacuum for 8 h before analysis to ensure the removal of adsorbed impurities. Brunauer−Emmett−Teller (BET) and Barrett− Joyner−Halenda (BJH) models30 were adopted for measurement of surface area and pore size distribution, respectively. X-ray Diffraction (XRD) and Pair Distribution Function (PDF) Analysis. Diffraction experiments for PDF analysis were performed at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL), in Argonne, IL, using high-energy X-rays with the powder samples packed in a 1-mm Kapton capillary. For data collection, an Xray energy of 58 keV (λ = 0.2128 Å) was used to record diffraction patterns to high values of momentum transfer while eliminating the fluorescence phenomenon from the sample. Using Fit 2D software, the two-dimensional (2D) images were integrated to one-dimensional (1D) powder diffraction pattern, masking areas obscured by the beam stop arm. The PDFs,

capacity, along with exceptional CO2/H2, CO2/CH4, C2H6/H2, and C2H6/CH4 selectivities that are higher than other chalcogels and other mesoporous materials. This underscores their potential as unique separation media for a wide range of gases.

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EXPERIMENTAL SECTION

For synthesis, the following starting materials were used: Co(NO3)2·6H2O (Aldrich, 99.9%), Pb(NO3)2·3H2O (Aldrich, 99%), Pb(acac)2 (where acac = acetylacetonate, Aldrich, 99%), Pb(ac)2·3H2O (Aldrich, 99%), Cd(NO3)2.4H2O (Aldrich 98%), Bi(CH3COO)3 (Aldrich, 99.99%), Cr(NO3)2.9H2O (Aldrich, 99%), Cd(acac)2 (Aldrich, 99.9%), Ni(NO3)2.6H2O (Aldrich, 98%), Pd(acac)2 (Aldrich, 99%). Representative examples of chalcogels’ synthesis are presented below, along with the scale-up synthesis. All of the syntheses’ conditions are summarized in Tables 1 and 2. Synthesis of (NH4)2MoS4. An amount of 40 g of MoO3 was dissolved in 500 mL of concentrated ammonium hydroxide by applying gentle heating and stirring, according to the process described elsewhere.29 When the solution became clear, H2S was bubbled through it for 4 h. Gradually, the solution turned into orange and then deep red. A red crystalline product, (NH4)2MoS4, appeared, which was filtered, washed with ethanol and ether, and dried under vacuum. The yield of the reaction was 85%, based on MoO3. The product was stored in an N2 atmosphere glovebox. Synthesis of Co0.5Pb0.5MoS4 Chalcogel. An amount of 0.25 mmol (0.073 g) of Co(NO3)2·6H2O (Sigma−Aldrich, 99.9%) was dissolved in 2 mL of formamide. In a separate vial, 0.25 mmol of Pb(CH3COO)2·3H2O (or Pb(NO3)2·3H2O) also was dissolved in 2 mL of formamide. Ammonium tetrathiomolybdate (0.5 mmol, 0.13 g) was dissolved in 2 mL of formamide. The Co precursor solution was added into the tetrathiomolybdate solution, which was followed by the addition of the Pb precursor solution. No stirring or heating was applied. As the polymerization proceeded, the viscosity of the solution increased, which eventually solidified to a monolithic black gel. Synthesis of Co 0.5Cr 0.33MoS 4 Chalcogel with Reverse Addition of Precursors (RAP). An amount of 0.25 mmol (0.073 g) of Co(NO3)2·6H2O (Sigma−Aldrich, 99.9%) was dissolved in 2 mL of formamide. In a separate vial, 0.17 mmol of Cr(NO3)3·9H2O (trivalent linker metals) (0.068 g) also was dissolved in 2 mL of formamide. Ammonium tetrathiomolybdate (0.5 mmol, 0.13 g) was dissolved in 2 mL of formamide. The Cr precursor solution was added into the tetrathiomolybdate solution, which was followed by the addition of the Co precursor solution. No stirring or heating was applied. As the polymerization occurred, the viscosity of the solution increased, which eventually solidified to a monolithic black gel. It took 1−10 days for the viscous solution to transform to a rigid gel for the Co-based chalcogels. Scale-Up Synthesis of Co0.7Cr0.2MoS4 Chalcogel. An amount of 11.6 mmol (3.34 g) of Co(NO3)2·6H2O (Sigma−Aldrich, 99.9%) was dissolved in 66 mL of formamide. In a separate vial, 3.3 mmol (1.32 g) of Cr(NO3)3·9H2O (Aldrich 99%) also was dissolved in 66 mL of formamide. Ammonium tetrathiomolybdate was used as the Mo source in these syntheses. In this case, 16.5 mmol (4.29 g) were dissolved in 66 mL of formamide. The Co precursor solution (pink color) was slowly added into the tetrathiomolybdate solution (deep red color), which was followed by the addition of the Cr solution (blue color). Upon the mixing of the above solutions a black solution was formed with an observable increase of solution viscosity. It took 12−14 days for the transformation of the viscous solution to a rigid gel. The same procedure was followed in the case of Co0.7Pb0.3MoS4 and Co0.7Bi0.2MoS4 chalcogels. Solvent Exchange. After the aging period where the reaction was kept undisturbed for 7−14 days (up to 1 mo. in the case of Ni-based chalcogels), the rigid black chalcogel was obtained. The remaining formamide was decanted and the rigid gel was soaked in an EtOH/ H2O (4:1) solution for 3 d and then in 100% EtOH for 7 d, to remove byproducts and impurities. A fresh supply of soaking solvent (EtOH/

G(r ) = 4πr[ρ(r ) − ρ0 ] where ρ(r) and ρ0 are the instantaneous and average densities, were extracted using PDFgetX2 software,31 subtracting the contributions from the sample environment and background to the measured diffraction intensities. Corrections for multiple scattering, X-ray polarization, sample absorption, and Compton scattering were then applied to obtain the structure function, S(Q). Direct Fourier transform of the reduced structure function F(Q) = Q[S(Q) − 1], yielded G(r), the PDF as previously described.32 X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron studies were performed using an Omicron ESCA system equipped with a monochromatic Al Kα X-ray source (1486.6 eV) and operated at 300 W. Samples were analyzed under vacuum (P < 10−8 Torr), whereas survey scans and high-resolution scans were collected using pass energies of 50 and 25 eV, respectively. Binding energies were referred to the C 1s binding energy at 284.6 eV. The samples’ exposure to air was minimized in order to avoid any oxidation. A low-energy electron flood gun was employed for charge neutralization. Prior to XPS measurements, the powders were mounted on stubs and put into the entry-load chamber to pump overnight. Scanning and Transmission Electron Microscopy. Scanning electron microscopy (SEM) images of the chalcogel samples were taken using a Hitachi Model S3400 system. Pulverized chalcogel samples were gently deposited on carbon tape and placed in the SEM chamber for image capture. Preparation for the transmission electron microscopy (TEM) imaging was done by suspending the chalcogel samples in ether and then casting on a holey carbon-coated Cu grid. High-resolution transmission electron microscopy (HRTEM) micrographs were obtained using a field-emission-type instrument (JEOL, Model 2100F) operating at 200 kV. Thermogravimetric Analysis (TGA). The thermal stability of samples was measured using a Shimadzu Model TGA-50 system that had a platinum foil basket. Thermal profile of the chalcogels was 3382

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Table 3. Elemental Composition and Pore Network Properties of Co-Based Chalcogels chalcogel

BET surface area (m2/g)a

Co0.5Pb0.5MoS4 Co0.5Pb0.5MoS4 (Conc.) Co0.5Pb0.5MoS4b (RAP) Co0.7Pb0.3MoS4c Co0.7Pb0.3MoS4 Co0.5Cd0.5MoS4 Co0.5Pd0.5MoS4 Co0.5Cr0.33MoS4 Co0.5Cr0.33MoS4b (RAP) Co0.3Cr0.46MoS4 Co0.7Bi0.2MoS4

174 272 205 417 242 322 317 335 352 198 240

1-Co-Cr-MoS4 2-Co-Pb-MoS4 3-Co-Bi-MoS4

441 390 511

SiO2 equivalent

rel. atom. ratio (EDS)

total pore volume (cm3/g)

ave. pore diameter (nm)

1.10 1.08 0.76 1.47 0.63 0.80 0.87 1.08 0.65 0.56 0.76

7.1−9.1 7.1−7.7 6.4−7.2 6.4−7.3 5.6−6.3 5.4−6.7 7.6−8.7 5.1−6.2 4.4−5.8 5.3−6.6 6.1−6.5

1.14 1.14 1.14

6.0−6.8 5.9−6.6 5.3−5.9

1327 Co0.8Pb0.7MoS5.3 2082 Co0.8Pb0.6MoS6 1245 Co0.6Pb0.4MoS4.7 2736 Co0.8Pb0.7MoS3.3 1587 Co0.8Pb0.7MoS3.2 1808 Co0.5Cd0.6MoS4.5 1619 1620 Co0.5Cr0.4MoS4.5 1484 Co0.5Cr0.3MoS3.5 888 Co0.4Cr0.6MoS3.7 1318 Co0.7Bi0.2MoS4.7 Scale-Up Chalcogels 2056 2123 2787

Area values are reported to within ±6%. bReverse addition of precursors (RAP) synthesis. cThis chalcogel has been synthesized at high conc. regime, using Pb(acac)2 as the precursor.

a

Table 4. Elemental Composition and Pore Network Properties of Ni-Based Chalcogels

a

chalcogel

BET surface area (m2/g)a

SiO2 equivalent (m2/g)

rel. atom. ratio (EDS)

total pore volume (cm3/g)

ave. pore diameter (nm)

Ni0.5Pb0.5MoS4 Ni0.5Pb0.5MoS4b Ni0.7Pb0.3MoS4 Ni0.7Cr0.2MoS4 Ni0.5Pd0.5MoS4

264 240 280 347 326

2321 1177 1975 1813

Ni0.5Pb1.0MoS6.1 Ni0.6Pb0.2MoS3.8 Ni1.7Pb0.23MoS5.6 Ni0.8Cr0.2MoS5

0.83 0.78 0.84 1.21 0.85

12.2−13.4 10.8−12.9 12.0−13.3 5.8−14.4 7.3−8.1

Area values are reported to within ±6%. bReverse addition of precursors (RAP) synthesis.

acquired under N2 atmosphere with a flow rate of 40 mL/min, whereas thermal analysis was conducted in the 25−600 °C range with a heating rate of 10 °C/min. Annealing Studies. An amount of 100 mg of the chalcogels (Ni0.5Pb0.5MoS4 and Co0.7Pb0.3MoS4) were placed on a boat and the boat was put and sealed in a stainless steel custom-made autoclave. The autoclave was placed inside a quartz tube under continuous flow of nitrogen. The quartz tube with the autoclave was placed in a tube furnace and heated up to the target temperature (200, 400, or 600 °C) for 2 h. Chemical Stability Studies. Chalcogel samples were exposed in air for a period of 4 months in order to assess their oxidation tendency. For the hydrolysis stability test, the CoMoS4 chalcogel was soaked in 20 mL of water and the suspension was subjected to stirring for 2 h. Elemental Analysis. Quantitative microprobe analysis was performed on a Hitachi Model S-3400N VP-SEM equipped with a Noran EDS detector. Data acquisition was performed several times in different areas of flat surfaces of chalcogel samples using an accelerating voltage of 20 kV and an accumulation time of 120 s. C,H,N Analysis. Carbon−hydrogen−nitrogen (C,H,N) analysis was performed over some of the supercritically dried chalcogels, by Midwest MicroLab LLC. Infrared (IR) Spectroscopy. Infrared spectra of chalcogels were acquired using a Thermo Nicolet 6700 FTIR spectrometer in the midIR (4000−500 cm−1) and far-IR (600−200 cm−1) regions. Spectra were collected on fine powders in DRIFT mode under nitrogen atmosphere and averaging 60 and 120 interferograms for the mid-IR and far-IR regions, respectively, with a resolution of 2 cm−1. Probing the Chalcogels’ Surface Acidity. Pyridine anhydrous (Aldrich, 99 wt %) was used as a probe molecule with basic character. Pyridine adsorption was performed by equilibrating the powder chalcogels with pyridine vapors for 60 min at 45 °C. Infrared spectra (4000−400 cm−1) were acquired immediately after the 60-min interaction with pyridine vapors and following evacuation under different conditions in terms of duration and temperature of evacuation. In particular, degassing for 1 h and 3 h at room

temperature, 90 min at 100 °C, and 90 min at 200 °C (four different evacuation combinations) was performed. Gas Adsorption Studies. A Micromeritics ASAP 2020 apparatus was used for physisorption measurements of hydrogen, methane, ethane, carbon dioxide, and nitrogen at 273 K. A cryogenic water bath (50:50 vol % water:ethylene glycol mixture) was used in a NESLAB RTE10 Digital Plus (Thermo Electron Corporation) chiller system to keep the temperature constant during the experiments. Gas Selectivity Using the Ideal Adsorbed Solution Theory (IAST). This approach has been described previously33 and in the Supporting Information (SI). The IAST model was used to predict the selectivity in binary gas mixtures (CO2/H2, CO2/CH4, C2H6/CH4, C2H6/H2), based on fitting of the single-component adsorption isotherms.

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RESULTS AND DISCUSSION Synthesis and Chalcogel Formation. The metathesis reaction producing the chalcogels is highly dependent on a number of parameters, such as metal linker precursor, building block, temperature of synthesis, basicity, solvent, counteranions, etc. In this study, Co2+ and Ni2+ were used as linkers, together with co-linkers such as Pb2+, Pd2+, Cd2+ and Bi3+, Cr3+. We also investigated the monodentate or bidentate nature of ligands (e.g., nitrate, acetate, acetylacetonate), and the effect of the initial concentration of [MoS4]2− (0.2 mmol vs 0.5 mmol) on the ability to form gels. Also, the order of addition of the metal linkers was investigated, as will be described below. In the following discussion, the 0.2 and 0.5 mmol amounts of the “MoS4” precursor will be referenced as the low and high concentration regimes, respectively. The result was 19 different rigid chalcogels, the properties of which are summarized in Tables 3 and 4. The general reaction frameworks in the cases of divalent−divalent and divalent−trivalent metal linkers are given by the two equations below: 3383

dx.doi.org/10.1021/cm301444p | Chem. Mater. 2012, 24, 3380−3392

Chemistry of Materials

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0.5Co2 + + 0.5Pb2 + + [MoS4 ]2 − → Co0.5Pb0.5MoS4 0.5Co2 + + 0.33Cr 3 + + [MoS4 ]2 − → Co0.5Cr0.33MoS4

The elemental compositions of all the chalcogels synthesized in this work were studied using EDS. For example, for the CoCr-MoS4 chalcogel, the Co:Mo, Co:Cr, and Mo:S metal atomic ratios were found to be 0.5:1, 0.5:0.4, and 1:4.5, respectively, close to the nominal ones (0.5:1, 0.5:0.33, and 1:4.0). However, in some cases (e.g., Co0.5Pb0.5MoS4, Ni0.5Pb0.5MoS4), differences were observed between the nominal and the actual stoichiometries and this could be due to variation in the release rate of each metal ion from the precursor environment, thus causing differences in the gelation rates. In the case of Pbcontaining chalcogels, the PbS formation could be a reason for the abnormalities in the EDS ratios. The Mo:S stoichiometry deviation from the 1:4 value could be due to polysulfide formation, as will be discussed later in the XPS section. The XPS technique was also employed in order to study the surface atomic composition for some representative compositions. The metal ratios found using XPS were in good agreement with the nominal and EDS compositions. From C,H,N analysis on representative compositions of Co-Pb-MoS4, Co-Cr-MoS4, and Ni-Pb-MoS4, elemental C, H, and N were found in the ranges of 1.3%−3.12%, 0.9%−1.6%, and 0.5%−3.5%, respectively. The C,H,N analysis indicates the presence of remnant formamide and acetate or acetylacetonate (precursors) in the inorganic framework. Effect of Concentration of Starting Materials (Precursor Molecule and Metal Linker). In order to optimize the gelation process for the Co0.5Pb0.5MoS4 chalcogel, two different initial concentrations of the constituents [MoS4]2−:Co2+:Pb2+ in molar ratios of 2:1:1 were used. Namely, the 0.2:0.1:0.1 mmol ratio (referred to as the low concentration regime) and 0.5:0.25:0.25 mmol ratio (referred to as the high concentration regime), respectively, were applied. In the second case, the higher concentration of precursor molecules leads to a more rigid material that implies a more extensively “polymerized” inorganic framework. For this case, a higher “mechanical” stability and 56% higher surface area (from 174 m2/g to 272 m2/g) was observed, and this could be indicative of a more cross-linked polymeric network. In the case of CoxPb1−xMoS4 (x = 0.3, 0.5), a more prolonged gelation period was needed when starting from low concentrations. In the case of the Pbpoor system CoxPb1−xMoS4 (x = 0.7), no gelation was achieved, keeping the 0.2:0.14:0.06 mmol ratio, respectively, when lead nitrate was used. For the Pb-poor CoxPb1−xMoS4 (x = 0.7) system, it is noteworthy that when starting from a more concentrated solution, rigid gels formed within one day (see Figure 1). In some cases, such as Co-Cd-MoS 4 , the concentration of precursors proved to be crucial for gelation to occur, since no gel was formed at the low concentration regime, regardless of the Cd2+ ion precursor (e.g., acetate or acetylacetonate complexes). The successful gelation (see Tables 1 and 3) was achieved by starting with a high concentration of “MoS4” precursor and linker salts (MoS4:Co:Cd ratio of 0.5:0.25:0.25 mmol leading to a MoS4:Co:Cd ratio of 1:0.5:0.5). In the case of Ni0.5Pb0.5MoS4, the low concentration (MoS4:Ni:Pb) ratio did not yield gels either with lead nitrate or with lead acetate (Ni(NO3)2 was used in all cases), whereas the high concentration ratio led to rigid gels. The case of Ni0.7Pb0.3MoS4 successfully led to gel formation when a low

Figure 1. Precursor solutions ((A) Cr(NO3)3, (B) (NH4)2MoS4, (C) Co(NO3)2, and (D) Ni(NO3)2) in formamide. (E) Inverted vial of a rigid wet gel of Co-Cr-MoS4.

concentration of precursors was used. For the Ni0.5Bi0.33MoS4 and Ni0.5Cr0.33MoS4 systems, no gelation was noticed for either low or high concentrations of the starting solutions. This is in contrast to CoxBi1−xMoS4 (x = 0.5−0.7) and CoxCr1−xMoS4 (x = 0.3−0.7) systems where rigid gels successfully formed for the entire range of compositions. For Pd2+ as a colinker, in the Ni0.5Pd0.5MoS4 and Co0.5Pd0.5MoS4 systems, the gelation was successful, with Pd(acac)2 as the precursor. The times to achieve gelation for the systems discussed above were different for the Ni- and Co-based chalcogels, implying that different kinetics were followed in each of these cases. The gelation time was 1−10 d for the Co-based chalcogels (12−14 d for the scale up chalcogels), whereas in the case of Ni-based chalcogels, the gelation time was up to 1 month at room temperature (composition-dependent). The typical amount of final product received after drying was 60−80 mg and the yield was varied in the 34%−90% range (see Tables 1 and 2). The short gelation times of the Co-based systems allowed us to successfully scale up the synthesis of some chalcogels. In particular, the Co0.7Pb0.3MoS4, Co0.7Cr0.2MoS4, and Co0.7Bi0.2MoS4 chalcogels were successfully scaled up to 4g batches. The order of addition of the metal linker precursors in the synthesis of CoxCr2/3(1−x)MoS4, CoxPb1−xMoS4, and NixPb1−xMoS4 (x = 0.5) did not make a significant difference as these successfully formed gels in a similar time frame. Special caution was needed during the handling of Co−Bi−MoS4 (scale-up quantity) since exposure in air led to ignition, presumably due to the large surface area, favoring rapid oxidation. X-ray Diffraction and Pair Distribution Function (PDF) Analysis. In-house PXRD was used to study the structural characteristics of all chalcogels prepared in this study. Almost all Ni- and Co-based chalcogels were found to be amorphous, as indicated by the predominant broad features in the PXRD patterns (see Figure 2A), except for the case of Pb-containing chalcogels, where a nanocrystalline PbS phase was also present, depending on the Pb precursor used, to be discussed below (see Figures S1A−C in the SI). These findings are also supported by diffraction experiments, using high-energy synchrotron X-ray radiation (see Figures S3−S6 in the SI). In particular, the diffraction patterns for Co0.5Pb0.5MoS4 and Ni0.5Pb0.5MoS4 chalcogels showed Bragg rings corresponding to crystalline PbS (see Figures S3A, S3C, S6C in the SI). Some diffraction features were also observed in the case of Co0.5Bi0.33MoS4 chalcogel corresponding to Co9S8 and MoS2 phases (see Figures S2 and S5A in the SI). Although the Copoor (Co0.3Bi0.46MoS4) chalcogel was amorphous (see Figure S5B in the SI). The Co−Cr−MoS4, Co−Pd−MoS4, Ni−Cr− 3384

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The second significant correlation vector in the PDF occurs at 4.0 Å (Co-Pb-MoS4, Figure 2D) and may be due to Mo···M distances in the chalcogel. This length decreases to 3.87 Å in the case of Co−Bi−MoS4 (Figure 2B) and a further decrease was observed (3.66 Å) for Co−Cr−MoS4 (Figure 2B). This suggests a difference in the local structure of the Mo/M/S (M: Bi3+, Cr3+) environment and, consequently, variations in the acidity and basicity of the sulfide surfaces, as will be discussed later.36 As mentioned above, almost all chalcogels were amorphous, except those containing Pb, where crystalline PbS could be detected (see blue line in Figure 2D). The same conclusion can be drawn from Figures S3A, S3C, and S6C in the SI. The presence of PbS depended on the Pb precursor used (see Figure 2D and Figure S1A in the SI). Specifically, when lead acetate was used in the synthesis, PbS was detected in the PXRD pattern (see Figure S1A in the SI); however, no PbS was observed with lead acac. We hypothesize that the chelating ligand acac slows down the availability of Pb ions in the assembly reaction allowing network formation, rather than the side reaction forming PbS. The PDF analysis (Figure 2D) confirms this showing strong correlation vectors at high distances in the case of the lead acetate precursor (consistent with PbS phase) and no strong correlation vectors above ∼6 Å for the case of lead acac precursor consistent with the absence of any crystalline phase. This dramatic difference attests to the importance of controlling the linking-ion release/availability from the chelating environment to the solution and it is in agreement with previous findings on Zn/Sb/S-based chalcogels.16 Surface Area. The surface areas of the title chalcogels were measured using the Brunauer−Emmett−Teller (BET)37 model and N2 physisorption at 77.15 K. The values of the BET surface areas for Co- and Ni-based chalcogels are given in Tables 3 and 4, respectively. In the case of the Co0.5Pb0.5MoS4 chalcogel, the surface area increased from 174 m2/g to 272 m2/g with increasing concentration of starting precursors in solution during synthesis. The N2 adsorption−desorption isotherms for all chalcogels were very similar in shape (isotherm Type II; see Figure 3), that is, minor adsorption observed at the low

Figure 2. (A) Powder X-ray diffraction (PXRD) pattern using a Cu Kα (λ = 1.541 Å) beam source on CoxCr2/3(1−x)MoS4 (x = 0.3−0.7). Also shown are pair distribution functions (G(r)), based on highenergy XRD (58 keV, λ = 0.2128 Å) showing various correlation vectors corresponding to Mo−S, Mo−Mo, S−S, Co−S bonds/ distances ((B) Co-chalcogels with different second metal linker, (C) Ni-chalcogels containing different second metal linkers, and (D) CoPb-MoS4 chalcogel being synthesized using a different coordination environment of Pb).

MoS4, and Ni−Pd−MoS4 chalcogels were amorphous, showing only diffuse diffraction patterns. PDF analysis can give valuable information of local order. Exposure of the samples to high-energy synchrotron X-ray beam (58 keV = 0.2128 Å) for 4 min led to PDFs that revealed correlation vectors that correspond to Mo−S, Mo−Mo, S−S, and Co−S bonds and Mo···M distances (see Figures 2B−D). For other correlation lengths (e.g., Bi−Bi, Bi−S, Pb−S, Co− Co, etc.), more information is provided in Table S1 in the SI.34 In particular, the correlation vector at 2.31−2.40 Å corresponds to Mo−S, Mo−Mo, S−S, and Ni−S distances.34 For comparison, the PDF of the precursor (NH4)2MoS4 (compound of Mo6+) shows the nearest-neighbor correlation peak of Mo−S at 2.18 Å.22 Therefore, the observed elongation of the Mo−S bond indicates a reduction of the Mo6+ centers in the chalcogels.35 This is in agreement with the XPS results that also support a reduction of the Mo in the chalcogels (see below). In addition, a variation of correlation vector at 2.37 Å is observed as a function of the second metal linker (e.g., Pb2+, Bi3+, Cr3+), for Co−Cr−MoS4, Co−Bi−MoS4 and Co−Pb−MoS4 (see Figures 2B and 2D). In the case of Ni-based chalcogels, this correlation vector is at 2.38 Å (see Figure 2C). This arises for the various nearestneighbor distances such as Mo−S, Mo−Mo, S−S, and Ni−S. This variation proves how the different size (1.03 Å for Bi3+, 0.63 Å for Cr3+, 1.49 Å for Pb2+, 0.72 Å for Co2+) or valence of the metal ions affects the short-range interaction in the chalcogel (bond strength). The variation of the bonds/ distances could tune the chalcogel−gas interaction, as well as their behavior toward thermal degradation, air oxidation, and hydrolysis. Signature correlation vectors were traced, such as the one at 2.88 Å in the Ni−Cr−MoS4 chalcogel, which corresponds to the Cr−Mo distance, which was not observed in the Ni−Pd−MoS4 chalcogel (see Figure 2C).

Figure 3. Nitrogen adsorption and desorption isotherms of representative (A) Co-Cr-MoS4 and (B) Ni-Cr-MoS4 chalcogels.

pressure regime (P/P0 < 0.05) and major adsorption at the higher pressures (0.7 < P/P0 < 1.0). A small hysteresis loop was observed in all isotherms, because the desorption rate is lower than the adsorption rate of nitrogen (attributed to percolation effects on porous media).30 The N2 isotherms of different compositions of chalcogels and the three scaled-up chalcogels, namely, Co-Cr-MoS4, Co-Pb-MoS4, and Co-Bi-MoS4, were all similar and are presented in the Supporting Information (Figures S7, S8, S9, and S10, respectively). 3385

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demonstrated that the particles have a rather random polymeric interconnected network of pores and nanoparticles in the 5−17 nm range (Figure 5). The absence of electron diffraction Bragg

The surface areas observed for the title chalcogels correspond to silica surface areas in the 1000−2800 m2/g range, as shown in Table 3. The porosity of the title chalcogels is higher than those of most chalcogenide gels reported previously.38,39 The average pore diameter, assuming a cylindrical pore shape and using the 4 V/A method, calculated based on the BET surface area and the total pore volume was found to be in the 4.4−14.4 nm range with Ni-based chalcogels being on the higher side of this range (see Table 4). The total pore volume of these chalcogels is higher, compared to that of Sb- and Pt/Sb/Scontaining chalcogels.16 The chalcogels synthesized using metal acac salts tend to possess higher surface areas (e.g., 317 m2/g for Co-Pd-MoS4, 326 m2/g for Ni-Pd-MoS4, 417 m2/g for Co-Pb-MoS4, and 322 m2/g for Co-Cd-MoS4). The scaled-up chalcogels presented similar surface areas to their small-scale counterparts (Table 3) only in the case of Co-Pb-MoS4 chalcogel. In the case of CoCr-MoS4 and Co-Bi-MoS4 chalcogels, the synthesis parameters such as precursor feed rate toward reaching a critical concentration, and mixing even in this scale-up range affected the final product. By comparison, the analogous precipitated systems (no gelation) exhibited much lower BET surface areas. For example, for CoxPb1−xMoS4 (x = 0.3−0.7) precipitates, the BET surface areas were 58 m2/g (x = 0.3), 76 m2/g (x = 0.5), and 90 m2/g (x = 0.7), respectively, whereas for the Ni-based precipitates, the surface areas were ∼19−55 m2/g. Scanning Electron Microscopy (SEM)/Transmission Electron Microscopy (TEM). Figure 4 presents typical SEM

Figure 5. TEM micrographs of (A, B) Co-Pb-MoS4, (C) Co-Cr-MoS4, and (D) Ni-Cr-MoS4 chalcogels. The chalcogels do not exhibit Bragg diffraction ring patterns (except for the case of Pb), confirming the lack of crystalline character in the chalcogel network.

rings (Pb case excluded) is in agreement with the X-ray diffraction (XRD) results and support the amorphous character of the chalcogels. In Figure 5B, the dashed circled area indicates the PbS crystalline phase. The SAED insert in Figure 5B was acquired from amorphous areas of the chalcogel. Chemical Bonding Environment (XPS Studies). X-ray photoelectron spectroscopy (XPS) was used to study the surface atomic composition and the chemical bonding environment in some representative chalcogels. Figure 6 presents the Mo 3d and S 2p region of the spectrum for the Ni-Pb-MoS4 (Figures 6A and 6B) and Co-Pb-MoS4 (Figures 6C and 6D) chalcogels, respectively. For Mo 3d, the shoulder at 226 eV is due to the S 2s photoelectrons and is the signature

Figure 4. Typical SEM micrographs of (A, C) Co-Pb-MoS4, (B, D) Ni-Pb-MoS4, and (E) Co-Cr-MoS4 synthesized at different concentration regimes, namely, low (panels A and B) and high (panels C, D, and E) of metal linkers and building blocks. (F) Typical EDS elemental analysis results of Co-Cr-MoS4.

micrographs of Co-Pb-MoS4, Co-Cr-MoS4 and Ni-Pb-MoS4 synthesized at different concentration regimes, namely low (Figures 4A and 4B) and high concentration (Figures 4C and 4D) of metal linkers and [MoS4]2− precursor molecule. It can be observed that, by synthesizing the chalcogel systems in the high concentration regime, a more spongy morphology (Figures 4C−E) of the final chalcogel is obtained. Representative EDS analysis results are presented in Figure 4F. The microstructure of the chalcogels was also investigated using TEM. Representative images from different regions along with selected-area electron diffraction (SAED) patterns

Figure 6. Deconvoluted X-ray photoelectron core level spectra of NiPb-MoS4 ((A) Mo 3d region and (B) S 2p region) and Co-Pb-MoS4 ((C) Mo 3d region and (D) S 2p region). 3386

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of the “sulfide” sulfur. Peak fitting was performed in order to study Mo oxidation states. The obtained data suggest that Mo exists in three different oxidation states, namely, Mo4+ (228.8 eV), Mo5+ (229.98 eV), and Moδ+ (6 > δ > 5) (231.05 eV). By comparing the Mo 3d spectra of Ni-MoS4 and Co-MoS422 with those of Ni-Pb-MoS4 (present study), we observe broadening and shifting to lower binding energy for the latter. The broadening is attributed to the presence of more than one oxidation state. The energy shift may indicate that the average oxidation state of the Mo center is lower than +6.40 It is interesting to note that the surface concentration of both the Mo4+ and Mo5+ species is ∼82.5% in the case of Ni-Pb-MoS4, whereas in the case of Co-Pb-MoS4 (Co-analogous system), it is up to ∼92%. In the case of Co-Pb-MoS4 synthesized using lead acac instead of lead acetate (previous cases discussed above), the population of Mo in Mo4+ and Mo5+ oxidation states is ∼75%. For the latter case, the Mo 3d core level spectrum is presented in Figure S11C in the SI. These results demonstrate that the [MoS4]2− unit undergoes a redox transformation to other Mo/S species. Regarding the S 2p core level spectra (Figures 6B and 6D), the following remarks are appropriate: the doublet at 161.2 and 162.5 eV corresponds to S2− species, the peak at 169.4 eV corresponds to sulfates (due to partial surface oxidation), whereas the peak at 162.9−164.4 eV corresponds to polysulfides. The S 2p at 165 eV corresponds to sulfides.41 The different sulfur species could contribute to a variation in surface basicity and charge distribution, which is likely associated with the gas adsorption performance of the chalcogels as will be discussed later. Mo-polysulfide species are well-known in molecular complexes.35,42 The presence of polysulfide species is consistent with the reduction of Mo6+ in [MoS4]2−, as discussed previously.35 The Co 2p doublet with the binding energies of 779.2 and 794.1 eV corresponds to Co atoms in a sulfidic environment interacting with Mo species (Co-Mo-S species) (see Figure S11D in the SI).43 Based on the XPS results, in the case of CoPb-MoS4, the Co:Pb, Co:Mo, and Mo:S ratios were found to be 0.51:1, 1:2, and 1:3.9 in a good agreement with the nominal values. By fitting the C 1s core level spectra obtained from the NiPb-MoS4 chalcogel (Figure S11A in the SI), two C 1s peaks at 284.8 and 288.2 eV are ascribed to the carbon atoms in the aliphatic chain (C−C) and the carboxylate (−COO−) moiety, respectively, whereas the peak at 286.4 eV corresponds to C− OH moieties.44 Fitting the O 1s core level spectra (see Figure S11B in the SI), the different oxygen species were found to correspond with the peaks at 530, 531.8, 533.2, and 534.8 eV. These species mainly originate from acac, nitrates, and hydroxyl groups.44 These results are in agreement with vibrational spectroscopy (IR) and C,H,N analysis data, which support the existence of solvent and precursor ligand residue in the chalcogels. Infrared Spectroscopy. Figure 7 presents the vibrational spectra obtained from quaternary Co- and Ni-containing chalcogels, along with that of the CoMoS4 parental chalcogel. Peaks in the region CO2 > CH4 > H 2

where the values are 4.5, 2.9, 2.6, and 0.9 (in units of 10−24 cm3), respectively. Because C2H6 and CO2 are more polarizable than the other two gases, they have stronger interaction with the highly polarizable sulfidic surfaces predominantly through dispersion forces.55,56 As shown in Figure 11A, the Co0.5Cr0.33MoS4 chalcogel presents practically no adsorption for N2, minor adsorption for H2 and CH4, whereas for C2H6 and CO2 adsorption is up to six times higher. From a technological point of view, these results (Figure 11A) are relevant in the context of gas separation. In particular, the WGS reaction (CO + H2O ↔ CO2 + H2, WGSR) leads to a mixture of H2 and CO257 at the reactor exit. Furthermore, at the exit gas stream of steam reforming reactions, CO2, H2, C2H6, CH4 can be found, depending on the hydrocarbon being reformed.57 Using the single-component isotherms of H2, CO2, CH4, C2H6, and N2, and by adopting the Ideal Adsorbed Solution Theory (IAST) model,33 the CO2/H2, CO2/CH4, C2H6/CH4, and C2H6/H2 selectivities were calculated (see Figure 11B and Figure S16 in the SI). The results of this study are useful since many gases are employed and different pairs of selectivities can be estimated (e.g., CO2/ CH4, CO2/H2, CO2/C2H6, etc.). These gas pairs are relevant in a number of different processes such as pre-combustion and post-combustion (in CO2 sequestration), natural gas sweetening, where gas separation is a key factor. Synthesis Concentration Regime and Gas Selectivity Interrelationship. The Co0.5Pb0.5MoS4 chalcogel synthesized at the high concentration regime (∼272 m2/g), presented a pressure (loading)-dependent CO2/H2 selectivity reaching ∼30 at the highest pressure (close to atmospheric) (Figure 11B), whereas the Co 0.5 Pb 0.5 MoS 4 chalcogel, synthesized at the low concentration regime (174 m2/g), exhibited a rather weak pressure dependence in their selectivity and CO2 preference over H2 of lower than 10. Order of Precursor Addition during Synthesis and Gas Selectivity Inter-relationship. In the case of Co0.5Cr0.33MoS4 chalcogels, both synthesized at the high concentration regime, similar BET values were obtained (335 and 352 m2/g). Performing the synthesis following the regular pathway, a weak loading dependence was observed. When the sequence of precursor addition was reversed, (RAP), the CO2/H2 selectivity approached a value of 75 at the highest pressure (close to atmospheric). Switch from a Co-Based Chalcogel to a Ni-Based Chalcogel and Gas Selectivity. The CO2/H2 selectivity exhibited by Co0.5Pb0.5MoS4 and Ni0.5Pb0.5MoS4 chalcogels (both synthesized at the low concentration regime) was comparable. Based on the above results, it can be stated that the high concentration regime synthesis affects the CO2/H2 selectivity, possibly because of the formation of a more interwoven network. Changing from a Co-Pb-MoS4 composition to a CoCr-MoS4 composition, a significant CO2/H2 selectivity increase was observed (keeping the rest of parameters the same), in contrast to the absence of such a selectivity change in the NiPb-MoS 4 composition. It is likely that the increased concentration and the metal ions valence variation (+2, +2; +2, +3) creates changes in surface charge and acidity

Figure 10. Mid-IR spectra over Ni0.5Pd0.5MoS4 (denoted as “Ni−Pd”) and Co0.5Pb0.5MoS4 (denoted as “Co−Pb”) following pyridine adsorption and after applying different evacuation conditions for (A, B) “Ni−Pd” and (C, D) “Co−Pb”. The reference spectrum of pyridine is provided (black curve) in all cases.

great importance in the hydrodesulfurization (HDS) reaction,54 for which chalcogels of CoMoS4 composition exhibited promising performance.22 Gas Adsorption and Selectivity. Pure component adsorption equilibrium isotherms of the Co0.5Cr0.33MoS4 chalcogel for CH4, CO2, H2, N2, and C2H6 are presented in Figure 11A. The gas adsorption studies showed that

Figure 11. CO2, CH4, C2H6, N2, and H2 adsorption isotherms for (A) Co-Cr-MoS4 at 273 K and (B) calculated CO2/H2 selectivity, according to the IAST model. (C) Isosteric heat of CO2 adsorption over Co- and Ni-based chalcogels. (D) Isosteric heat of H2 adsorption over Co- and Ni-based chalcogels.

Co0.5Cr0.33MoS4 and other chalcogels (not all results are shown) selectively adsorb CO2 and C2H6 over H2 and CH4 at 273 K and atmospheric pressure (Figure 11A). The major challenge for the chalcogels, in terms of evaluating their gas adsorption selectivity, is the minor size differences among the gases investigated. For example, the kinetic diameter of C2H6, CO2, H2, and N2 is 4.4, 3.30, 2.89, and 3.64 Å, respectively27 3389

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catalysts that have Mo as a promoter are used for methane reforming in the presence of CO2 (dry reforming),64 and Pd is among the most active catalytic metal for CO2 fixation.65 Considering that the CO2, H2 adsorption sites are different, and based on the much higher quadrupole moment of CO2 over H2, the Co(Ni)-Pd-MoS4 systems may present more suitable Pdbased sites for CO2 interaction. The Pd-containing chalcogel’s isosteric heat of adsorption for H2, measured at 77 and 87 K, showed more typical values of ∼5.2 and ∼5.7 kJ/mol, respectively, corresponding to hydrogen (H2) loadings of 17 and 24 cm3 per g of sorbent (Figure 11D). Finally, the present chalcogels exhibit similar or higher tendency to adsorb H2 than the very effective PtGeTe chalcogel,11,26 especially close to atmospheric pressure (see Figure S17 in the SI). The chalcogel−gas (adsorbent−adsorbate) interaction is derived from the enthalpy trends data and single-component adsorption data. The single-component adsorption isotherms of CO2, H2, and C2H6 over Co0.5Pb0.5MoS4 and Co0.5Cr0.33MoS4 chalcogels were fitted using Langmuir 67 and Fowler− Guggeinheim models67 (see Figures S18−S20 in the SI). Both models are valid (plots parallel to the x-axis of coverage) in describing the chalcogel−CO 2 and chalcogel−C 2 H 6 interaction, depending on the pair system of interest. The above models were less accurate for the chalcogel−H2 interaction, whereas the Freundlich model67 seems to be more acceptable for a certain pressure range (see Figure S21 in the SI). This model is inefficient with regard to describing the surface−H2 interaction. This could be due to the low interaction of H2 with the surface, as indicated by the low heat of adsorption and the subsequently low amounts adsorbed at near-ambient temperatures.

(unsaturated metal ions act as Lewis acid sites), which ultimately favor the CO2 adsorption. In the case of the Co-Pb-MoS4 chalcogel, the presence of PbS nanoparticles (XRD/synchrotron studies) resembles the case of supported or dispersed nanoparticles in an amorphous porous matrix. The high surface area of the chalcogel porous matrix facilitates the dispersion of the nanoparticles. The nanoparticles contribute their own surface, which may have different adsorptive properties, compared to the porous matrix itself, thus altering the CO2 adsorptive behavior. Based on the quadrupole moments of CO2 (4.3 × 1026 esu cm2) and H2 (0.662 × 1026 esu cm2), the electrostatic interaction between CO2 and the atoms in the inorganic framework should be greater than H2 at lower pressures.58 The quaternary chalcogels reported here exhibit significantly enhanced CO2/H2 selectivity by factors of 2 and 4, compared to the ternary CoMoS4 or NiMoS4 reported previously.22 These values are lower than those of the CoMo3S13 chalcogel, where CO2/H2 selectivities over 100 were reported.15 The selectivity enhancement for the CO2/H2 pair observed in this study is associated with their high CO2 enthalpy of adsorption, compared to CoMoS4 or NiMoS4, as will be discussed below. The CO2/N2 selectivity is more difficult to assess since the IAST model becomes less accurate for mixtures where one component is much more favorably adsorbed.59 Another selectivity of interest to consider is for the pair CO2/CH4 (see Figure S16A in the SI). A pressure-dependent selectivity profile was obtained for both the Co-Pb-MoS4 and Co-Cr-MoS4 chalcogels with CO2/CH4 selectivity values reaching 35:1 in both cases at high pressures. Another important separation efficiency to report and discuss is that of C2H6/CH4, because these hydrocarbons are components of natural gas (see Figure S16B in the SI). As can be seen from the single-component adsorption isotherms, C2H6 is adsorbed in a considerably higher amount than CH4. Co-Pb-MoS4 has much higher C2H6/CH4 selectivity, compared to Co-Cr-MoS4 in the entire pressure range. In addition, the C2H6/CH4 selectivity in the case of Co-Pb-MoS4 is much more pressure-dependent than for Co-Cr-MoS4. The adsorption preference of Co-PbMoS4 for C2H6 over CH4 could be associated with the more polarizable nature of C2H6 (see Table S2 in the SI). For Co-PbMoS4 and Co-Cr-MoS4 chalcogels, the C2H6/H2 selectivity presented a pressure-dependent behavior (see Figure S16C in the SI). There is a clear preference for C2H6 over H2 with IAST selectivities being in the range from 30:1 to 60:1. The significance of the above results may be in providing guidelines for selecting synthesis parameters to create optimized gas separation media. CO2 and H2 Isosteric Heat of Adsorption. Analysis of the CO2 adsorption isotherms of NiMoS4,22 Ni-Pd-MoS4 and CoPd-MoS4 (this work) at two different temperatures (263 and 273 K) allow us to calculate the isosteric heat of CO2 adsorption. This was estimated at ∼25, ∼41, and ∼47 kJ/ mol, respectively (see Figure 11C). The last two are the highest adsorption enthalpies calculated for a chalcogel or any other type of porous material. For example, the CO2 adsorption enthalpy of the chalcogels materials (CoMo3S13 chalcogel (13 kJ/mol),15 NiCo-chalcogel (28 kJ/mol),22) and other porous materials (such as silicate (28 kJ/mol)60 and MOFs (29−34 kJ/ mol)) is much lower.61−63 The Pd-containing chalcogels of this study exhibited higher CO2 enthalpy of adsorption than mesoporous germanium chalcogenido frameworks.33 Coincidentally, Ni−Pd bimetallic

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CONCLUSIONS Metathesis reactions of [MoS4]2− with Co2+ and Ni2+ and a wide range of metal co-linkers (e.g., Pd2+, Pb2+, Cd2+, Bi3+, and Cr3+) lead to high-surface-area (up to 511 m2/g) quaternary chalcogels. By tuning the ternary chalcogel composition, it is possible to change the strength of their acidic active sites (metal centers acting as Lewis acid sites), as well as their thermal stability. Co-based chalcogels were resistant to thermal degradation, hydrolysis, and air exposure. High concentrations of the starting precursors increased the surface area and produced more spongy chalcogels. Using chelated metal salts as precursors was beneficial in achieving gelation. The [MoS4]2− unit does not stay intact in the chalcogel structure undergoing redox reactions, giving rise to other Mo/Sx species where the Mo oxidation state is 4+/5+. Various sulfur species, including sulfides and polysulfides, are part of the chalcogel structure. The expansion of the ternary compositions of CoMoS4 and NiMoS4 to quaternary ones through the addition of the second linker metals has a marked effect on the interaction of the chalcogels with gases. Namely, a CO2/H2 selectivity of 70 and 30 was obtained for Co-Cr-MoS4 and Co-Pb-MoS4 chalcogels, compared to ∼17 for CoMoS4.22 Selectivity values for other gas molecule pairs, e.g., C2H6/CH4, suggest the possible applicability of these materials in separating exhaust gas streams containing hydrocarbons. By nature, these materials should be resistant to sulfur poisoning and are promising candidates for use in industrial gas stream separations, where sulfur-containing molecules are usually present.66 3390

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ASSOCIATED CONTENT

S Supporting Information *

IAST model, N2 porosimetry, far-IR studies, IAST selectivities for C2H6/CH4, C2H6/H2, CO2/CH4 pairs of gases, XPS, TGA thermal analysis, high-energy synchrotron diffraction data, conventional XRD data, complementary H2 adsorption data, Langmuir Model, Fowler−Guggenheim Model, and Freundlich Model for chalcogel−gas interaction. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail:[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS These studies were supported in part by National Science Foundation (DMR-1104965). This work made use of the J. B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-0520513) at the Materials Research Center of Northwestern University. SEM, TEM, and XPS work were performed in the EPIC and KECK facility, respectively, of the NUANCE at Northwestern University. The NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, the Keck Foundation, the State of Illinois, and Northwestern University. Work performed at Argonne National Laboratory and use of the Advanced Photon Source for PDF and PXRD were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (under Contract No. DE-AC02-06CH11357).

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