Theoretical and Experimental Analysis of Oxygen Separation from Air

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Theoretical and Experimental Analysis of Oxygen Separation from Air over Ni-Transition Metal Complexes Duane D. Miller,*,†,‡ Ranjani Siriwardane,‡ and Thomas Simonyi†,‡ † ‡

URS Corporation, 3610 Collins Ferry Road, Morgantown, West Virginia 26507-0880, United States U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, West Virginia 26507-0880, United States ABSTRACT: The separation of O2 from air over nickel-transition metal complexes has been studied using in situ infrared and Raman spectroscopy, thermogravimetric analysis, volumetric gas sorption, and quantum chemical simulation methods. Exposure of O2 to the solid Ni-transition metal complexes produces a reactive oxygen species at ambient temperatures. The infrared transient responses, during the absorption process, indicate that the ligand groups interact with oxygen, producing both weakly bound and strongly bound oxygen species. The results indicate that the reactive oxygen interacts weakly with the cyanide ligand groups, which are easily removed during the pressure swing absorption/desorption process at 298 K and 689.5 kPa. Temperature-programmed desorption revealed that the oxygen absorbed at the Ni center was bound stronger than the ligand-bound oxygen, evidenced by its removal at 393 K and the disappearance of a hydrogen-bonded species. The results obtained for the absorption/desorption process suggest that the persistence of the activated oxygen and reactivity with the transition metal ligands are an important factor for improving the absorption capacity of the organometallic sorbent. The in situ infrared spectroscopy study reveals the chemical structure of the ligand groups acting as adsorption sites for the reversible O2 uptake of the Ni-transition metal complex; the ligandO2 interaction is an important factor for air separation sorbent development using organometallic complexes.

1. INTRODUCTION The effective removal of CO2 from power plants is challenging because existing methods for separating CO2 from the gas mixture require a significant fraction of the power plant output. The separation task can be simplified by replacing the conventional air oxidant with pure oxygen so the combustion products for any hydrocarbon are carbon dioxide and water. The commercial air separation process for this “oxy-fuel” combustion technique requires energy-intensive cryogenic separation. To reduce the energy penalty, interest has been focused on other techniques for air separation. Considerable work has been devoted to the study of the reversible reactions of dioxygen with transition metal complexes because such complexes can be used to model biological systems,1
5 have the potential to be used as oxidation catalysts,6,7 and can be used as sorbents for separating dioxygen from gases or liquids.8
13 The absorption of O2 from air is an appealing approach as a more economical method for obtaining pure oxygen. The absorption of O2 on transition metal complexes has been proposed to proceed via the following steps:14 Ln M þ O2 f Ln M
O2 Ln M

Ln M
O2 þ Ln M f Ln M
O2
MLn L

Ln M
O2 f MLn OO

ð1Þ ð2Þ ð3Þ

O2 absorbs on the transition metal ion as a dioxygen species (Step 1) followed by the coordination with a second transition metal complex (Step 2) to form an oxo-dimetal complex. r 2011 American Chemical Society

Absorbed O2 can also interact with the transition metal ligands by the oxidative addition (Step 3). Studies on these proposed steps over transition metal complexes15
17 have revealed that 2
reductive activation of an O2 molecule (O2 f O
2 f O2 ) is affected by the oxidative addition to a transition metal center. The dioxygen complexes can be divided into two types according to the characteristics of the absorbed oxygen ligand,18,19 (i) the 2
super oxide (O
2 ) and (ii) the peroxide (O2 ) complexes. These complexes are further classified as to whether the oxygen is bound to one metal atom or two metal atoms. Synthetic chemistry has provided a chemical basis for the structures and physicochemical properties of various active-oxygen species Mn
O2 (M = Fe,3,20
22 Co,10
12,23,24 Cu,25 Cr,19 and n = 1 or 2). Among those metal complexes, nickel ions6,26
30 are capable of producing similar Mn
O2 species, and thus, nickel-based transition metal complexes may be candidates as an O2 sorbent in the air separation technology. Cobalt containing transition metal complexes supported on silica, prepared by the sol
gel process,31 on mesoporous materials,32 on zeolite materials,23 and coordination polymers12 have been studied for air separation applications. Studies have also shown that the electron-withdrawing substituent acting as Lewis acids11,12 can weaken the cobalt
oxygen interaction through the ligand group attached to the metal center allowing for reversible O2 sorption. The structure of the ligand groups and their reactivity with oxygen are important factors for the absorption and desorption processes over the transition metal complexes. The chemistry of the O2 sorption is Received: May 25, 2011 Revised: August 17, 2011 Published: August 17, 2011 4261

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Figure 2. Experimental apparatus. Figure 1. Molecular structures of the Ni-transition metal complexes, 1NiA, 2NiA, 3NiA, and 4NiA.

not fully understood where the ligand groups behave as potential absorption sites for increasing the concentration of the absorbed O2 species and promoting the reversible absorption and desorption of O2. The ligand reactivity and interaction with O2 can enhance air separation sorbent technology under the condition that ligand substitution by oxidative elimination reactions14 may be avoided during the absorption and desorption processes. In this work, we found that the ligand groups affect the absorbed O2 species producing both strongly and weakly bound O2. The nature of the absorbed species on the Ni-transition metal complexes during O2 absorption is investigated using in situ infrared and Raman spectroscopy, thermogravimetric analysis, volumetric gas sorption, and quantum chemical simulation methods. The absorbed O2 species interaction with the ligands, reactivity, and selectivity provide the basis for elucidation of the reaction pathway and the role of the ligands during the O2 absorption and desorption processes.

2. EXPERIMENTAL METHODS 2.1. Materials. Commercial Potassium-tetracyano-nickelate(II) (1NiA), N,N0 -Bis(salicylidene)-ethylenediamino nickel(II) (2NiA), Dichlorobis(triphenylphosphine) nickel(II) (3NiA), and 5,10,15,20-tetraphenyl-21H,23H-porphinenickel(II) (4NiA) were purchased from Sigma Aldrich and characterized by XRD analysis prior to use. The structures of these complexes are shown in Figure 1. The UHP-grade helium and oxygen used for the absorption experiments were obtained from Butler Gas Products Co. Inc. 2.2. Raman Spectroscopy Study of Absorbed O2. The Raman spectra were collected on a Thermo-Nicolet Nexus 670 FTIR equipped with a FT-RAMAN module and an indium gallium arsenide (InGaAs) detector. The Raman spectra resulted from 1064 nm excitation with a spectral resolution of 4 cm
1. The oxygenated solid samples

(1NiA, 2NiA, 3NiA, and 4NiA, 100 mg) were transferred to sealed NMR tubes (∼0.5 mL per tube), and Raman spectra were obtained at room temperature. The samples were then heated under vacuum to 120 °C (ramp rate of 10 °C/min) to remove any absorbed oxygen species and transferred to sealed NMR tubes under Ar flow at 298 K for Raman analysis. 2.3. In Situ IR O2 Absorption/Desorption Study. The in situ IR experimental apparatus, shown in Figure 2, consists of (i) a reactant metering system (Brooks Instrument 5850 mass flow controllers), (ii) a gas sampling section including a 4-port valve, and (iii) a diffuse reflectance infrared Fourier transform spectroscopy accessory (DRIFTS, Thermo Nicolet) inside a Fourier Transform Infrared Spectrometer (Nexus 670, Thermo-Nicolet). The 4-port valve allows switching of the inlet flow from 100% He to 100% O2 while maintaining a total flow rate of 40 cm3/min over the sorbents at 689.5 kPa. The changes in the concentration of IR-active species were monitored by FTIR. The IR spectra collected by DRIFTS reported in absorbance units (A) were obtained by A =
log(I0/I),33 where Io is the background IR single beam spectrum (32 coadded scans and resolution 4 cm
1) under 100% He flow and I is the IR single beam spectrum during the absorption study. 2.4. Thermogravimetric Analysis. High pressure Thermogravimetric analysis (HP-TGA) was conducted in a thermogravimetric analyzer (Cahn Thermax 500) to investigate the oxygen absorption capacity from air of the Ni-based transition metal complexes. Approximately 700-mg samples were placed in a 19-mm
deep, 17-mm-diameter crucible. The sample was first purged in 150 cm3/min Ar flow at 2068.4 kPa for 30 min and then heated to 393 K at a heating rate of 10 K/min to remove any absorbed oxygen species from the nickel-based sorbent. The oxygen sorption cycle was then performed utilizing 150 cm3/min air (21% O2, 79% N2) at 298 K and 2068.4 kPa for 90 min. 2.5. Volumetric Gas Sorption. The oxygen and nitrogen isotherm measurements were performed using a custom-built volumetric gas analyzer equipped with Baratron (MKS 121A-11737, 0
1000 Torr) and Setra 3100 (0
3447.4 kPa) pressure transducers. The samples were 4262

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Table 1. Binding Energies of the Ni Complexes with O2 and H2O at the Optimized State (B3LYP/LAN2DZ level, 298 K, 1 atm) sorbents 1NiA

O2 binding energy (kJ/mol)
188.21

2NiA


51.52

3NiA


124.64

4NiA


174.65

evacuated to a pressure of 10
4
10
5 Torr and heated to 120 °C for 15 min prior to the sorption measurements. The pressure measurements were taken from 0 to 2064.4 kPa in 344.7 kPa increments allowing for pressure equilibrium to be reached (∼15 min) marked by a pressure differential of (0.1% on subsequent pressure measurements. 2.6. DFT Calculations. The DFT calculations were performed using the Spartan ’08 (Wave function Inc., USA) software package. The optimized molecular geometries were calculated using the Becke’s threeparameter exchange functional and gradient-corrected functional of Lee, Yang, and Parr (B3LYP).34,35 In the computation, the inner electrons of the nickel atoms were frozen and replaced using the effective core potential (ECP) of Hay and Wadt and the concomitant LANL2DZ basis set.36 The open shells were calculated using the unrestricted DFT calculation UB3LYP/LANL2DZ. The binding energies for the Ni complexes with O2 were obtained by eq 4: Eb ¼ E½Ni_complex þ O2 
E½Ni_complex
E½O2 

ð4Þ

Where E[Ni_complex + O2] is the total energy of the Ni complex with O2, E[Ni_complex] is the total energy of the Ni complex, and E[O2] is the total energy of the O2 molecule.

3. RESULTS 3.1. Interactions between Oxygen and the Ni Complexes. In order to evaluate the O2 absorption ability of the Ni complexes, the O2
Ni-complex systems were first investigated using the DFT B3LYP with an effective core potential basis set LANL2DZ. The geometry optimizations of the four sorbents led to an arrangement in which the oxygen coordinates with the Ni center as an end-on species for the 1NiA, 2NiA, 3NiA, and 4NiA sorbents. The optimized structures show the oxygen OdO bond distances (d) in the O2
Ni-complex are d = 1.44 Å for the 1NiA sorbent, d = 1.25 Å for the 2NiA sorbent, d = 1.26 Å for the 3NiA sorbent, and d = 1.32 Å for the 4NiA sorbent. The OdO bond lengths for the absorbed O2 are longer than those of gaseous O2 (1.21 Å). The lengthening of the OdO bond is indicative of the weakening of the oxygen double bond resulting from the absorption of the oxygen molecule on the Ni complex. Table 1 lists the O2 binding energies for the four Ni complexes. The theoretical O2 binding energies were determined to be
188.21 kJ/mol for 1NiA,
51.52 kJ/mol for 2NiA,
124.64 kJ/mol for 3NiA, and
174.65 kJ/mol for 4NiA. Negative values of binding energies indicate an increase in stability of the optimized O2
Ni complex structure with respect to the free O2 and free Ni complex molecules. A comparison of the binding energies with the adsorbed oxygen bond lengths reveals a trend in the optimized geometries. The lengthening of the OdO bond indicates that the weakening of the double bond is consistent with the calculated binding energies. The more negative the binding energy, the stronger the O2
Ni bond leading to the increase in OdO bond length. The strength of the binding energies for the Ni-complexes indicates that these transition

Figure 3. O2/N2 molar ratio during volumetric gas absorption over 1NiA, 2NiA, 3NiA, 4NiA, bentonite, Zeolite 5A, and Zeolite 13X sorbents at 298 K.

metal complexes are potential materials that are capable of effectively absorbing O2 from air. Additionally, the strength of the binding energy will contribute to the regeneration energies required for the thermal release of O2 from the sorbent. The theoretical O2 binding energies over the Ni complexes led to the evaluation of these sorbents using volumetric sorption and thermogravimetric analysis. 3.2. Oxygen Uptake Study. The O2/N2 molar ratio of the volumetric O2 and N2 absorption isotherms, obtained at 298 K and 2068.4 kPa, for the bentonite, zeolites 5A and 13X and the Ni complexes, are shown in Figure 3. The bentonite powder was analyzed in powder form. The 1NiA (15 mesh, 1.19 mm), 2NiA, 3NiA, and 4NiA sorbents were sieved at 30 mesh (0.595 mm). The zeolite 5A and 13X pellets were crushed to powder with mortar and pestle to 30 mesh (0.595 mm) particle size. The molar ratio indicates the selectivity of the sorbent for O2 absorption. The isotherms having a molar ratio of 1.0 indicate a 50% uptake of O2. The isotherms below 1.0 are selective for N2 absorption, and those above 1.0 are selective for O2 absorption. The zeolite 5A and 13X were tested to obtain baseline data. The absorption isotherms indicate that the amount of O2 absorbed increased rapidly with increasing pressure between 10 and 100 kPa. Increasing the pressure from 100 to 2068 kPa resulted in a gradual increase in both O2 and N2 absorption for the bentonite, zeolite, and the Ni-transition metal complexes indicated by the horizontal trend in the O2/N2 molar ratio shown in Figure 3. The O2 saturation limit was not observed within this pressure range. The O2 absorption capacity for bentonite was 0.41 mol/kg and for the zeolite materials was determined to be 1.06 mol/kg for zeolite 5A and 0.65 mol/kg for zeolite 13X. These amounts are consistent with those found by others in previous testing.37 The 2NiA sorbent exhibited the largest O2 absorption capacity, at 1.56 mol/kg with 50% selectivity. A reasonable amount of O2 was also absorbed on the 3NiA (1.13 mol/kg) and 4NiA (1.18 mol/kg) sorbents, and the 1NiA sorbent exhibited the lowest absorption capacity of 0.71 mol/kg. The 1NiA, 3NiA, and 4NiA sorbents absorbed a lower amount of O2 than that of 2NiA and show greater selectivity for O2 absorption. The O2 absorption isotherm for the 2NiA sorbent yielded the highest capacity corresponding to the lowest theoretical O2 binding energy of
51.52 kJ/mol, whereas the 1NiA sorbent showing the lowest absorption capacity had a binding energy of
188.21 kJ/mol. The volumetric sorption results, coupled with the theoretical analysis indicate that the strength of the O2 4263

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Figure 4. Gravimetric O2/N2 uptake curve for sorbent 5A, 13X, and 2NiA and the average uptake % by weight.

binding energy affects the O2 absorption capacity but does not affect the O2 selectivity. The lower binding energy of the 2NiA sorbent allows for a more efficient regeneration cycle for O2 removal. However, the 2NiA sorbent exhibits O2/N2 molar ratio near 1.0 indicating that the sorbent is ineffective for separating these two gases as compared to the 1NiA, 3NiA, and 4NiA sorbents. The increased absorption capacity also depends on the number of available O2 sorption sites per kg of materials which may be higher for the 2NiA sorbent though these sites may have low binding energy. The results indicate that the Ni-complexes absorb a larger amount of O2 than those of the 13X and 5A zeolites. In addition to this, the large particle size 1NiA (1.19 mm) limiting the number of metal sites available for O2/N2 absorption did not significantly affect the selectivity and O2 capacity as compared to the 2NiA, 3NiA, and 4NiA sorbents. The Brunauer
Emmett
Teller (BET) surface area was determined to be 0.179 m2/g for 1NiA, 2.741 m2/g for 2NiA, 0.194 m2/g for 3NiA, and 0.229 m2/g for 4NiA sorbent. The 4NiA sorbent exhibited larger O2 uptake capacity (Figure 3) than that of the 2NiA sorbent, though the BET surface area is lower for the 4NiA sorbent. The low O2 uptake capacities are related to the low BET surface areas of the Ni-complexes. The increase in selectivity for absorbed O2 over the 1NiA, 3NiA, and 4NiA complexes suggest that the ligand groups may interact with O2 participating in the absorption and influencing both the absorption capacity and selectivity of the sorbents. Oxygen absorption tests were conducted by thermo gravimetric analysis with a gas mixture containing O2 to evaluate the competitive gas absorption behavior of the Ni-complexes. Figure 4 shows the HP-TGA curve during one air/Ar cycle over the 2NiA sorbent at 298 K and 2068.4 kPa. While performing the HP-TGA study, switching the inlet flow from Ar to air shows an increase in weight which arises from a change in the gas density. As a result, a blank TGA experiment consisting of glass beads was performed prior to each sorbent absorption experiment. The blank profile (baseline curve) was subtracted from the sorbent curve in order to determine the real weight change of the Ni complexes due to oxygen uptake. On introduction of air to the 2NiA sorbent, initially held under Ar flow, a very slow uptake of 0.31 mmol/g O2 corresponding to 0.31 mmol of O2 per mmole of complex was observed over a 90-min period. Flushing the

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Figure 5. Raman spectra of sorbents (a) 1NiA and (b) 2NiA in Air and in Ar following the removal of O2 under vacuum at 395 K. * Structural features of the transition metal complex.

balance chamber with Ar, the removal of gaseous O2 and the weakly absorbed O2 was observed by the rapid decrease in weight indicated in Figure 4. Similarly, analysis of the thermograms (not shown) for oxygen uptake and removal indicate 0.27 mmol/g O2 on 1NiA, 0.21 mmol/g O2 on 3NiA, 0.20 mmol/g O2 on 4NiA, and 0.39 mmol/g O2 on zeolite 13X, and 0.87 mmol/g O2 on zeolite 5A. The TGA results indicate that the Zeolite 5A and 13X exhibit the highest O2/N2 uptake per weight basis whereas, for the organometallic sorbents, the 2NiA sorbent showed the highest O2/N2 uptake per weight basis. Though the zeolite and 2NiA sorbents exhibit the higher O2/N2 uptake capacity, their low O2/N2 selectivity based on the volumetric gas sorption data excludes their practical use for high purity O2 separation processes. On a mole basis, the HP-TGA analysis reveals that the 4NiA sorbent has a higher O2 uptake capacity (molar ratio 0.42) than those of the organometallic sorbents 1NiA, 2NiA, and 3NiA. The higher molar ratio corresponds to a higher absorption capacity of the sorbent. The sorbent 1NiA also exhibits the lowest absorption capacity (molar ratio 0.20) consistent with the volumetric sorption study. At 414 kPa (20% of 2068 kPa), the isotherm data show 0.25, 0.5, 0.37, and 0.37 mol/kg of O2 uptake, which are slightly higher than the TGA data. On the basis of the TGA results, the organometallic sorbents exhibit slower kinetics for O2/N2 absorption than those of the zeolite materials. The TGA results do not indicate the competitive gas sorption since both O2/N2 sorption will contribute to weight gain. Even though the rate of total gas adsorption is high for zeolite 13X and 5A, the competitive gas absorption ratio calculated using the volumetric gas sorption data indicates that the selectivity for O2 sorption over N2 at 298 K was greater for the Ni-complexes than that with the zeolites. 3.3. Raman Spectral Studies on O2 Absorption. Exposure of the 1NiA sorbent to O2 at 298 K and 101.3 kPa, shown in Figure 5a, produced Raman bands at 618 and 983 cm
1. Previous studies have shown that the superoxide species (O
2 ) has stretching vibrations at 1128 cm
1 on CaO-Al2O3,38 and at 1160
1015 cm
1 on MgO
CoO.39,40 The stretching vibrational frequencies for peroxide species (O2
2 ) have generally been found in the range of 640
970 cm
1.9,41
44 Therefore, the increase in Raman intensity centered at 618 and 983 cm
1 during O2 adsorption, are assigned to the ν(Ni
O)27,45 and ν(O
O)45 vibrations, respectively, for the absorbed oxygen. 4264

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Figure 6. Exposure of O2 to the 1NiA sorbent at 298 K and 689.5 kPa, (a) IR spectra and (b) IR intensity profiles of the species formed.

The absence of these Raman bands prior to O2 exposure indicates these bands are related to the oxygen absorbed on the transition metal ion. The Raman intensity at 403 cm
1, as identified by previous studies with solid K2[Ni(CN)4] 3 H2O,46 is for the Ni
C vibration and at 300 cm
1 is for the Ni
CN bending vibration (Figure 5a). The Raman spectra recorded for the 2NiA sorbent, in Figure 5b, show the C
O vibration at 1025 cm
1, C
H bending vibration at 801 cm
1, the in-plane bending vibration of the aromatic ring at 598 cm
1, the Ni
O stretching vibration at 430 cm
1, and the bending vibration of the aromatic ring at 381 and 347 cm
1.47,48 Exposure of the 2NiA sorbent to oxygen caused an increase in Raman intensity at 630 cm
1 that is assigned to the ν(Ni
O) vibration of absorbed oxygen. Oxygen absorption studies on the 3NiA and 4NiA samples encountered sample fluorescence that prevented collection of Raman spectra for these sorbents. 3.4. FTIR Study on O2 Absorption/Desorption over the 1NiA Sorbent. Figure 6 shows the IR absorbance spectra and IR intensity profiles during exposure of O2 to the 1NiA sorbent at 298 K and 689.5 kPa. The background IR spectra were collected during the He flow prior to exposing the 1NiA sorbent to O2. Flowing O2 over the 1NiA sorbent, shown in Figure 6a, led to the formation of a CdN
O2 species indicated by the N
O vibration at 1355 cm
1.49,50 A decrease in IR intensity at 2124 cm
1 for the CtN ligand51 also followed the increase in IR intensity for the formation of the CdN
O2 species, CtN at 1668 cm
1 and the CdN+
(O2)2 species51 at 1548 cm
1, indicating that O2 is interacting with the CtN ligand groups. The formation of these species suggests the absorption of O2 led to the formation of a reactive oxygen species.52,53 Exposure of the transition metal complexes to atmosphere during loading of the sorbent resulted in the presence of absorbed H2O (H2Oad). The formation of the reactive oxygen species, in the presence of H2Oab, led to an increase in IR intensity at 3187 cm
1 for the formation of a 52,54 and at 3400 cm
1 due to the Ni
O
2 3 3 3 H2Oab species formation of a CdN
O2 3 3 3 H2O hydrogen-bonded species.55,56 The broadness of the IR bands in the 3500
3000 cm
1 region is consistent with the formation of these hydrogen-bonded species.

The changes in IR intensity of absorbed species as a function of time were plotted in Figure 6b. The variation in IR intensities results from changes in the number of absorption sites, the absorption equilibrium, and rate constants. The increase in IR intensity at 1355 cm
1 for CdN
O2 and corresponding decrease in IR intensity at 2124 cm
1 (CtN) indicates that the O2 is interacting with the CtN ligands according to Reaction Step (5) as shown below. O2

Ln NiII
C  N f Ln NiII
C ¼ N
O2 ð1668, 1355cm-1 Þ

Ln NiII ðH2 Oad Þ

f Ln NiII
O2 3 3 3 H2 Oab Ln NiII
O2 s -1 ð3187cm Þ

O2

Ln NiII
C ¼ N
O2 f Ln NiII
C ¼ Nþ
ðO2 Þ2 ð1548cm-1 Þ

ð5Þ

ð6Þ ð7Þ

Ln NiII ðH2 Oad Þ

Ln NiII
C ¼ N
O2 s f Ln NiII
C ¼ N
O2 3 3 3 H2 Oab
1 ð3400cm Þ

ð8Þ The rapid formation of CdN
O2 species at 1355 cm
1 (Reaction Step 5) indicates that activation of the O2 molecule, according to Reaction Step (1), for absorption at the CtN ligand is not rate limiting. This is consistent with previous studies that have shown, using stop flow UV
vis spectroscopy, that the 57,58 occurs within approxiactivation of oxygen as an O
2 species mately 1.00 ms and precedes that of the formation of an oxospecies. After 1.1 min O2 exposure, the CdN
O2 formation rate decreases, indicated by a slower rate of increase in IR intensity on longer O2 exposure time. The decrease in the rate of formation suggests a secondary absorption of oxygen is occurring according to Reaction Step (7) for the CdN+
(O2)2 species. The lead-lag behavior of the IR intensities shown in Figure 6b for the CN
O2 (1355 cm
1) and CdN+
(O2)2 (1548 cm
1) indicate that the CtN absorption site reacts first with the activated oxygen 4265

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Figure 7. Infrared spectra of absorbed species on 1NiA sorbent during (a) step switch to He at 298 K and (b) temperature-programmed desorption from 298-to-393 K and 689.5 kPa.

species and is then followed by the absorption of another reactive oxygen. The lagging behavior of the IR intensity at 3400 cm
1 for the CdN
O2 3 3 3 H2O hydrogen bonded species (Reaction Step 8) also indicates the presence of a secondary absorption process for the CdN
O2 species. To further understand the nature of the absorbing species, we investigated the desorption process while switching the inlet flow from O2 to He. Switching the reactor flow from O2 to He, after saturation of the 1NiA sorbent at 298 K and 689.5 kPa, shown in Figure 7a, resulted in a decrease in IR intensities at 1668, 1579, and 1323 cm
1 corresponding to the removal of the CdN+
(O2)2 and CdN
O2 species, respectively. The decrease in IR intensities indicates that the change in partial pressure of O2 removes a weakly absorbed O2 from the sorbent. The decrease in IR intensity at 3497 cm
1 also corresponds to the removal of the weakly absorbed CdN
O2 3 3 3 H2O hydrogen-bonded species, indicating that the CdN
O2 formation may be partially stabilized by hydrogen bonding with H2Oad. The removal of the weakly absorbed CdN
O2 species also resulted in an increase in IR intensity at 2142 cm
1 for the CtN vibration. The appearance of the CtN vibration is consistent with the reversible O2 absorption at the CtN ligand according to Reaction Step (5). Increasing the temperature from 298 to 393 K caused the remaining strongly absorbed O2 to be removed from the sorbent, shown in Figure 7b. The background spectrum for the TPD study is obtained after purging the sorbent under He flow, following the O2 absorption and prior to heating. The decrease in IR intensity at 1348 cm
1 (CdN
O2), indicates that the activated oxygen at the nickel center interacting with the CtN ligand (Reaction Step 9) is being removed. Ln NiII
C  N þ Ln NiII
O2 f Ln NiII
C ¼ N
O2
NiII Ln ð1348cm
1 Þ

ð9Þ

Figure 8. Infrared spectra of absorbed species during (a) O2 absorption over 2NiA and (b) temperature-programmed desorption from 298-to393 K at 689.5 kPa.

Additionally, the decrease in IR intensity at 3187 cm
1 (Ni
O
2 3 3 3 H2Oab) indicates the removal of the activated O2 absorbed at the Ni center. The absence of the δHOH scissoring mode (1640 cm
1) for H2Oab, during heating to 393 K, indicates that the decrease in IR intensity at 3187 cm
1 is related to the hydrogen-bonded Ni
O
2 3 3 3 H2Oab and not for the removal of H2Oab from the Ni-complex. The thermal removal of the absorbing O2 species also provides evidence for the room temperature stability of the absorbed O2. A comparison of the IR spectra in Figure 7a with that of Figure 7b reveals that the weakly absorbed O2 species are those on the CtN ligand groups and the strongly absorbed O2 species are those adsorbed at the Ni metal center. 3.5. FTIR Study on O2 Absorption/Desorption over the 2NiA Sorbent. Figure 8a shows that the absorption of O2 at 298 K and 689.5 kPa led to an increase in IR intensities at 3656 and 3432 cm
1 and a decrease in IR intensity at 2932 cm
1 for the ν(C
H) vibration. The decrease in IR intensity at 2932 cm
1 for ν(C
H), under prolonged exposure to gaseous O2, is an indication of the reactivity of the activated oxygen with the aromatic rings of the 2NiA sorbent during the absorption process. Ethane absorption studies on zeolites have shown59 that the IR ν(C
H) bands may be affected by donor
acceptor bonding with positive or negative charging of the absorbed molecules and polarization of the active sites. The IR intensity (I) is proportional to the square of the derivative of the dipole moment I µ (dμ/dq)2 60 created by the stretching of chemical bonds via their vibrations over the normal coordinate, the ν(C
H) intensity is thus directly related to the polarizability of the chemical bond and is closely related to the reactivity of the absorbed oxygen species. Prolonged exposure of the 2NiA sorbent to O2 also resulted in an increase in IR intensity at 3656 and 3432 cm
1. In-situ IR studies 52,53 have on the interaction of superoxide (O
2 ) with H2O allowed the assignment of these spectral features. The increase in IR intensity at 3432 cm
1 is assigned to the ionic hydrogen
1 results bonded Ni
O
2 3 3 3 H2Oab, and the band at 3656 cm 4266

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Energy & Fuels from the strong interwater OH hydrogen bonding. The increase in IR intensity at 3656 and 3432 cm
1 and decrease in IR intensity at 2932 cm
1 are indicative of the existence of an activated oxygen species on the 2NiA sorbent. Removal of the oxygen species during TPD at 393 K and 689.5 kPa under He flow (see Figure 8b) resulted in the removal of the hydrogen bonding species and the concomitant decrease in IR intensity at 3656 and 3432 cm
1. The IR spectra for the 3NiA and 4NiA sorbents were also collected during O2 absorption and desorption. The ligand absorption for the 3NiA and 4NiA sorbents (i.e., aromatic rings) are somewhat less reactive than those of the CtN ligand groups, making it difficult to interpret the association kinetics of the absorbing and desorbing species. Due to the complexity of the ligand molecules, the IR spectra for the 3NiA and 4NiA are not presented in this work.

4. DISCUSSION Vibrational spectroscopy has provided evidence for the absorption of O2 over transition metal complexes in solution9,15,19,29,41,61,62 as well as in the solid state.18,21,22,62 Studies have shown that oxygen absorbs on the transition metal center and the Ni
O2 complex is a thermodynamically stable species.2,63 Raman spectroscopy revealed an absorbed oxygen species on the 1NiA sorbent (983 and 618 cm
1) and on the 2NiA sorbent (630 cm
1) at 298 K and 101.3 kPa, shown in Figure 3. The presence of this species indicates that the solid Ni-complexes are capable of forming the Ni
O2 complex at room temperature in the solid state. Studies have shown that the oxygen coordination geometry, chemistry, and stability are dependent upon the ligand environment of the absorption site,64
74 where oxygen fixation is essential to pick up gaseous O2 molecules present at low concentrations. From the viewpoint of sorbent development, the process of hydrolysis and oxygenation leading to the reductive activation of O2 is not desirable, but rather, oxygen fixation is preferred. Oxygen absorption is accomplished by the cooperative forward and backward donation between the metal centers and the O2 ligand. When O2 is coordinated to the Ni ion, the sp2 lone pair on O2 is donated to the dz2 orbital on the metal to form a σ-bond, accompanied by the formation of a π-bond by π-back-bonding into the empty π*-orbitals of O2.5,13 The Mulliken population analysis of the optimized structures at the B3LYP//LAN2DZ level show an electron occupancy that is consistent with this bonding scheme on the Ni-complexes. The stability of the O2 complex is dependent upon the electron-donating ability of the Ni d-orbitals to π*-orbitals of O2 (i.e., the greater the electron density of the Ni ion, the more stable the formation of the Ni
O2 and faster O2 binding rate of the complex). The calculated O2 binding energy shows that the electron donating CtN groups increase the electron density of the Ni ion and thus the 1NiA sorbent exhibits the larger binding energy (
188.21 kJ/mol) than those of the other Ni-complexes in Table 1. The O2 volumetric gas sorption experiments are consistent with the theoretical analysis and the electron-donating effect of the CtN groups. The 2NiA complex having the lower binding energy (
51.52 kJ/mol) exhibited the highest absorption capacity among the Ni-complexes during the volumetric sorption experiments. The observed experimental and theoretical results suggest that the O2 binding energy affects the efficiency of the regeneration cycle but does not affect the selectivity for the

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absorption of O2 during the volumetric gas study. The surface area of the materials ranged from 0.179 to 2.741 m2/g. Since the surface areas were very low, the differences in performances cannot be mainly due to the effect on surface area. The 2NiA sorbent with the highest BET surface area (2.741 m2/g) from Ni based sorbents may have more sorption sites than that with the other materials. The in situ FTIR study indicates the presence of weak and strong absorption sites; the presence of weak absorption sites is an important factor for increasing the O2 uptake capacity during the competitive adsorption of O2. The free O2 is not believed to be involved in the formation of the CdN
O2 and CdN+
(O2)2 species because the triplet state of O2 is unreactive; thus, these oxygen species must first proceed by O2 activation at the Ni center followed by its interaction with the ligand groups. The requirement for O2 activation and interaction with the ligand groups illustrates the importance of Reaction Step (1) and the formation of the Ni
O
2 species on the absorption of O2 over the Ni-complexes for the catalytic separation of air. The IR wavenumbers for the ligand interaction with the activated oxygen are those for the N
O type species on the 1NiA sorbent. The overlapping of these bands as well as the nature of the activated oxygen make it difficult to identify the dynamic behavior of these species which have similar structures to CdN
O2 and CdN+
(O2)2. The presence of these IR active species, in Figure 6a, and the reversible absorption
desorption suggest that the conversion of the oxygen species to the peroxo M
O2
M bridging species did not occur as is reported for solution phase reactions that show O2 binding to metal ions via a 1,2-μ-peroxo bridge binding mode.6,15,26 The absence of the gaseous N2O species also indicates that the activated oxygen species did not cause the dissociative absorption and oxidation of the cyanide ligands. In + addition to this, the absorbed Ni
O
2 , CdN
O2, and CdN
(O2)2 species were stabilized by hydrogen bonding with H2Oad evidenced by the increase in IR intensity at 3400 and 3187 cm
1 shown in Figure 6a. These hydrogen bonded species allowed for the elucidation of the absorbing O2 as weakly and strongly absorbed O2 species. The desorption study revealed the nature of the absorbing O2 as weakly and strongly absorbed O2. The change in partial pressure of O2 led to the removal of the weakly bonded CdN
O2 and CdN+
(O2)2 species (Figure 7a) as well as the hydrogen-bonded complex evidenced by the decrease in IR intensities. The TPD study, in Figure 7b, revealed that the active O2 species remains strongly bonded to the Ni metal center as evidenced by the IR intensity at 3187 cm
1 that is believed to be 52 the spectral peak for a Ni
O
2 3 3 3 H2Oad.complex. The weakly + adsorbed O2 is the CdN
O2 and CdN
(O2)2 species whereas the strongly adsorbed oxygen is the Ni
O
2 species. The reactivity of the activated O2 molecule toward the CtN ligands, along with studies showing the high reactivity of organicbased peroxyl molecules, suggest that the CdN
O2 and CdN+
(O2)2 species are excellent candidates to exhibit a persistent radical effect. The in situ IR results for the 1NiA sorbent indicate that the activation of molecular oxygen and subsequent formation of the CdN
O2 and CdN+
(O2)2 species led to the formation of a persistent oxygen in the solid state, thus, the cyanide ligand groups lead to the efficient trapping of a transient activated oxygen species. The solid
gas absorption prevents the bimolecular self-reactions of the activated O2 species; however, the stabilization of the Ni
O
2 species is evidenced by its interaction with the H2Oad. 4267

dx.doi.org/10.1021/ef200779m |Energy Fuels 2011, 25, 4261–4270

Energy & Fuels Isotope studies75 for the aliphatic C
H bond functionalization with a Ni
O
2 active species show the decomposition of a [Ni2(Me-tpa-CH2OO)]2+ complex in the presence of H218O under N2 gave a mixture of 18O
16O oxygenated species. The formation of the Me
tpa
COO
and Me-tpa-CH2OH in the presence of H218O, indicates that the oxygen atoms of the Me
tpa
COO
interact with water, suggesting that oxygen intermediates are capable of reacting with absorbed water. The leading behavior on the formation of the oxygenated ligands occurring first followed by its reaction with H218O appears to occur similarly on the Ni-complexes for the CdN
O2 and CdN+
(O2)2 species. The simplest explanation that is consistent with the experimental data is a stepwise oxygen transfer process proceeding from an activated oxygen intermediate that interacts with the ligand groups and then later with the H2Oab. The most significant result of this study is that the Ni ion activates the coordinated O2 but does not promote oxygen insertion into the ligand groups which are also coordinated at the Ni center. The presence of both strongly and weakly absorbed O2 species would also be consistent with the stepwise transfer of an O2 species according to Reaction Step (3). Unlike the dimetal sites for 2-electron oxidation of the substrate, the activated O2 species on the solid Ni complex shows it is reactive with the ligand groups but does not cleave the bonds of the ligands, 70
74,76 which is advantageous for the reversible sorption required for oxygen separation.

5. CONCLUSIONS The in situ IR study shows that oxygen absorption produces the adsorbed species CdN
O2, CdN+
(O2)2, and CN
O
2 3 3 3 H2O and provides evidence for the presence of activated oxygen
(Ni
O
2 ) by the formation of the Ni
O2 3 3 3 H2Oab species over sorbent 1NiA. The oxygen that is present as the Ni
O
2 species is characterized as a strongly absorbed oxygen species. The thermal stability of the Ni
O
2 species is evidenced by the presence of Ni
O
2 3 3 3 H2Oab hydrogen-bonded species that is removed at 393 K. The CdN
O2, CdN+
(O2)2, and CN
O
2 3 3 3 H2O species are easily removed from the sorbent by the pressure swing desorption process and are characterized as a weakly absorbed species. The results obtained during the absorption and desorption process provide important details on the relationship between the formation and interaction of the reactive oxygen species over the Ni-transition metal complexes during the absorption process. The reversible absorption of O2 at the ligand sites that do not cleave the ligand bonds is advantageous for the separation of air and for improving the stability of the sorbent to repeated air/argon cycling. The Chemical structure of the ligand groups acting as weak absorption sites is an important factor for sorbent development on these Ni-transition metal complexes. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research in CO2 Capture under the RES Contract DE-FE0004000.

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’ REFERENCES (1) Hornícek, J.; Dvoriakova, H.; Bour, P. Intramolecular Proton Transfer in Calixphyrin Derivatives. J. Phys. Chem. A 2010, 114, 3649. (2) Lee, Y.; Park, G. Y.; Lucas, H. R.; Vajda, P. L.; Kamaraj, K.; Vance, M. A.; Milligan, A. E.; Woertink, J. S.; Siegler, M. A.; Narducci Sarjeant, A. A.; Zakharov, L. N.; Rheingold, A. L.; Solomon, E. I.; Karlin, K. D. Copper(I)/O2 Chemistry with Imidazole Containing Tripodal Tetradentate Ligands Leading to μ-1,2-Peroxo
Dicopper(II) Species. Inorg. Chem. 2009, 48, 11297. (3) Tiago de Oliveira, F.; Chanda, A.; Banerjee, D.; Shan, X.; Mondal, S.; Que, L., Jr.; Bominaar, E. L.; Muenck, E.; Collins, T. J. Chemical and spectroscopic evidence for an FeV-Oxo Complex. Science (Washington, D.C., U. S.) 2007, 315, 835. (4) Martinho, M.; Banse, F.; Bartoli, J.-F.; Mattioli Tony, A.; Battioni, P.; Horner, O.; Bourcier, S.; Girerd, J.-J. New example of a non-heme mononuclear iron(IV) oxo complex. Spectroscopic data and oxidation activity. Inorg. Chem. 2005, 44, 9592. (5) Jones, R. D.; Summerville, D. A.; Basolo, F. Synthetic oxygen carriers related to biological systems. Chem. Rev. 1979, 79, 139. (6) Cho, J.; Furutachi, H.; Fujinami, S.; Tosha, T.; Ohtsu, H.; Ikeda, O.; Suzuki, A.; Nomura, M.; Uruga, T.; Tanida, H.; Kawai, T.; Tanaka, K.; Kitagawa, T.; Suzuki, M. Sequential reaction intermediates in aliphatic C
H bond functionalization initiated by a bis(μ-oxo)dinickel(III) complex. Inorg. Chem. 2006, 45, 2873. (7) Shiren, K.; Ogo, S.; Fujinami, S.; Hayashi, H.; Suzuki, M.; Uehara, A.; Watanabe, Y.; Moro-oka, Y. Synthesis, structures, and properties of bis(μ-oxo)nickel(III) and bis(μ-superoxo)nickel(II) complexes: An unusual conversion of a NiIII2(μ-O)2 core into a NiII2(μOO)2 core by H2O2 and oxygenation of ligand. J. Am. Chem. Soc. 1999, 122, 254. (8) Bakac, A. Oxygen Activation with Transition-Metal Complexes in Aqueous Solution. Inorg. Chem. 2010, 49, 3584. (9) Ghiladi, R. A.; Chuf
an, E. E.; del Río, D.; Solomon, E. I.; Krebs, C.; Huynh, B. H.; Huang, H.-w.; Mo€enne-Loccoz, P.; Kaderli, S.; Honecker, M.; Zuberb€uhler, A. D.; Marzilli, L.; Cotter, R. J.; Karlin, K. D. Further Insights into the Spectroscopic Properties, Electronic Structure, and Kinetics of Formation of the Heme
Peroxo
Copper Complex [(F8TPP)FeIII
(O22
)
CuII(TMPA)]+. Inorg. Chem. 2007, 46, 3889. (10) Yang, J.; Huang, P. A study of cobalt(II) porphyrins on their oxygen-binding behaviors and oxygen-facilitated transport properties in polymeric membranes. Chem. Mater. 2000, 12, 2693. (11) Meier, I. K.; Pearlstein, R. M.; Ramprasad, D.; Pez, G. P. Tetrabutylammonium tetracyanocobaltate(II) dioxygen carriers. Inorg. Chem. 1997, 36, 1707. (12) Ramprasad, D.; Pez, G. P.; Toby, B. H.; Markley, T. J.; Pearlstein, R. M. Solid state lithium cyanocobaltates with a high capacity for reversible dioxygen binding: synthesis, reactivity, and structures. J. Am. Chem. Soc. 1995, 117, 10694. (13) Li, G. Q.; Govind, R. Separation of oxygen from air using coordination complexes: A review. Ind. Eng. Chem. Res. 1994, 33, 755. (14) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; John Wiley & Sons, Inc.: New York, 1999. (15) Kieber-Emmons, M. T.; Riordan, C. G. Dioxygen activation at monovalent nickel. Acc. Chem. Res. 2007, 40, 618. (16) Weschler, C. J.; Hoffman, B. M.; Basolo, F. Synthetic oxygen carrier. Dioxygen adduct of a manganese porphyrin. J. Am. Chem. Soc. 1975, 97, 5278. (17) Vaska, L. Dioxygen-metal complexes: toward a unified view. Acc. Chem. Res. 1976, 9, 175. (18) Suzuki, M.; Ishiguro, T.; Kozuka, M.; Nakamoto, K. Resonance Raman spectra, excitation profiles, and infrared spectra of [N,N0 ethylenebis(salicylideniminato)]cobalt(II) in the solid state. Inorg. Chem. 1981, 20, 1993. (19) Bakac, A.; Scott, S. L.; Espenson, J. H.; Rodgers, K. R. Interaction of chromium(II) complexes with molecular oxygen. Spectroscopic and 4268

dx.doi.org/10.1021/ef200779m |Energy Fuels 2011, 25, 4261–4270

Energy & Fuels kinetic evidence for β.1-superoxo complex formation. J. Am. Chem. Soc. 1995, 117, 6483. (20) Komatsu, T.; Furubayashi, Y.; Nishide, H.; Tsuchida, E. 5,10,15,20-Tetrakis(a,a,a,a-o-pivalamidophenyl)porphinatoiron(II) bearing a covalently linked axial imidazole via m-aminobenzoic acid. Synthesis and influence of imidazole basicity on O2-binding affinity. Inorg. Chim. Acta 1999, 295, 234. (21) Proniewicz, L. M.; Isobe, T.; Nakamoto, K. Infrared spectra of dioxygen adducts of iron(II)(salen): observation of two O---O stretching bands. Inorg. Chim. Acta 1989, 155, 91. (22) Watanabe, T.; Ama, T.; Nakamoto, K. Matrix-isolation infrared spectra of dioxygen adducts of iron(II) porphyrins and related compounds. J. Phys. Chem. 1984, 88, 440. (23) Hutson, N. D.; Yang, R. T. Synthesis and characterization of the sorption properties of oxygen-binding cobalt complexes immobilized in nanoporous materials. Ind. Eng. Chem. Res. 2000, 39, 2252. (24) Hikichi, S.; Akita, M.; Moro-oka, Y. New aspects of the cobaltdioxygen complex chemistry opened by hydrotris(pyrazoly)borate ligands (TpR): unique properties of TpRCo-dioxygen complexes. Coord. Chem. Rev. 2000, 198, 61. (25) Smirnov, V. V.; Roth, J. P. Evidence for Cu
O2 intermediates in superoxide oxidations by biomimetic copper(II) complexes. J. Am. Chem. Soc. 2006, 128, 3683. (26) Suzuki, M. Ligand effects on dioxygen activation by copper and nickel complexes: Reactivity and intermediates. Acc. Chem. Res. 2007, 40, 609. (27) Itoh, S.; Bandoh, H.; Nakagawa, M.; Nagatomo, S.; Kitagawa, T.; Karlin, K. D.; Fukuzumi, S. Formation, characterization, and reactivity of bis(μ-oxo)dinickel(III) complexes supported by A series of bis[2-(2-pyridyl)ethyl]amine ligands. J. Am. Chem. Soc. 2001, 123, 11168. (28) Hikichi, S.; Yoshizawa, M.; Sasakura, Y.; Komatsuzaki, H.; Moro-Oka, Y.; Akita, M. Structural characterization and intramolecular aliphatic C
H oxidation ability of MIII(micro -O)2MIII complexes of Ni and Co with the hydrotris-(3,5-dialkyl-4-X-pyrazolyl)borate ligands TpMe2,X (X = Me, H, Br) and TpiPr2. Chem.—Eur. J. 2001, 7, 5011. (29) Mandimutsira, B. S.; Yamarik, J. L.; Brunold, T. C.; Gu, W.; Cramer, S. P.; Riordan, C. G. Dioxygen activation by a nickel thioether complex: Characterization of a NiIII2(μ-O)2 Core. J. Am. Chem. Soc. 2001, 123, 9194. (30) Itoh, S.; Bandoh, H.; Nagatomo, S.; Kitagawa, T.; Fukuzumi, S. Aliphatic hydroxylation by a bis(μ-oxo)dinickel(III) complex. J. Am. Chem. Soc. 1999, 121, 8945. (31) Corriu, R. J. P.; Lancelle-Beltran, E.; Mehdi, A.; Reye, C.; Brandes, S.; Guilard, R. Hybrid materials containing metal(II) Schiff base complex covalently linked to the silica matrix by two Si
C Bonds: Reaction with Dioxygen. Chem. Mater. 2003, 15, 3152. (32) Corriu, R. J. P.; Lancelle-Beltran, E.; Mehdi, A.; Rey
e, C.; Brandes, S.; Guilard, R. Ordered mesoporous hybrid materials containing cobalt(II) Schiff base complex. J. Mater. Chem. 2002, 12, 1355. (33) Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed.; Thomson Brooks/Cole: Belmont, 2007. (34) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: Atom. Mol. Opt. Phys. 1988, 38, 3098. (35) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648. (36) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms scandium to mercury. J. Chem. Phys. 1985, 82, 270. (37) Mofarahi, M.; Towfighi, J.; Fathi, L. Oxygen separation from air by four-bed pressure swing adsorption. Ind. Eng. Chem. Res. 2009, 48, 5439. (38) Hayashi, K.; Matsuishi, S.; Ueda, N.; Hirano, M.; Hosono, H. Maximum incorporation of oxygen radicals, O- and O2
, into 12CaO 3 7Al2O3 with a Nanoporous Structure. Chem. Mater. 2003, 15, 1851. (39) Giamello, E.; Sojka, Z.; Che, M.; Zecchina, A. Spectroscopic study of superoxide species formed by low-temperature adsorption of

ARTICLE

oxygen onto cobalt oxide (CoO)-magnesium oxide solid solutions: An example of synthetic heterogeneous oxygen carriers. J. Phys. Chem. 1986, 90, 6084. (40) Zecchina, A.; Spoto, G.; Coluccia, S. Surface dioxygen adducts on magnesium oxide-cobalt(II) oxide solid solutions: analogy with cobalt-based homogeneous oxygen carriers. J. Mol. Catal. 1982, 14, 351. (41) Lee, Y.; Park, Ga, Y.; Lucas Heather, R.; Vajda Peter, L.; Kamaraj, K.; Vance Michael, A.; Milligan Ashley, E.; Woertink Julia, S.; Siegler Maxime, A.; Narducci Sarjeant Amy, A.; Zakharov Lev, N.; Rheingold Arnold, L.; Solomon Edward, I.; Karlin Kenneth, D. Copper(I)/ O2 chemistry with imidazole containing tripodal tetradentate ligands leading to m-1,2-peroxo-dicopper(II) species. Inorg. Chem. 2009, 48, 11297. (42) Baldwin, M. J.; Root, D. E.; Pate, J. E.; Fujisawa, K.; Kitajima, N.; Solomon, E. I. Spectroscopic studies of side-on peroxide-bridged binuclear copper(II) model complexes of relevance to oxyhemocyanin and oxytyrosinase. J. Am. Chem. Soc. 1992, 114, 10421. (43) Gland, J. L.; Sexton, B. A.; Fisher, G. B. Oxygen interactions with the Pt(111) surface. Surf. Sci. 1980, 95, 587. (44) Howe, R. F.; Liddy, J. P.; Metcalfe, A. Infrared studies of oxygen adsorbed on evaporated germanium films. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1595. (45) Davydov, A. Molecular Spectroscopy of Oxide Catalyst Surfaces; John Wiley & Sons Ltd.: West Sussex, 2003. (46) J
oszai, R.; Beszeda, I.; B
enyei, A. C.; Fischer, A.; Kov
acs, M.; Maliarik, M.; Nagy, P.; Shchukarev, A.; T
oth, I. Metal
metal bond or isolated metal centers? Interaction of Hg(CN)2 with square planar transition metal cyanides. Inorg. Chem. 2005, 44, 9643. (47) Datta, M.; Brown, D. H.; Smith, W. E. The resonance Raman profile of a nickel Schiff base complex at 10 K. Spectrochim. Acta Part A: Mol. Spectrosc. 1983, 39, 37. (48) Colthup, M. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy, 2nd ed.; Academic Press: New York, 1975. (49) Wiley, R. H.; Wakefield, B. J. Infrared spectra of the nitrile N-oxides: some new furoxans. J. Org. Chem. 1960, 25, 546. (50) Yamakawa, M.; Kubota, T.; Akazawa, H. Electronic spectra of nitrile N-oxides and the solvent effect on them. Bull. Chem. Soc. Jpn. 1967, 40, 1600. (51) Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of Organic Compounds, 7th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005. (52) Weber, J. M.; Kelley, J. A.; Nielsen, S. B.; Ayotte, P.; Johnson, M. A. Isolating the spectroscopic signature of a hydration shell with the use of clusters: Superoxide tetrahydrate. Science (Washington, D. C.) 2000, 287, 2461. (53) Weber, J. M.; Kelley, J. A.; Robertson, W. H.; Johnson, M. A. Hydration of a structured excess charge distribution: Infrared spectroscopy of the O2-(H2O)n, (1 < n < 5) clusters. J. Chem. Phys. 2001, 114, 2698. (54) Antonchenko, V. Y.; Kryachko, E. S. Interaction of superoxide with water hexamer clusters. Chem. Phys. 2006, 327, 485. (55) Pei-Dong, L.; Feng, W.; Li-Jun, Z.; Wen-Xue, L.; Xiao-Hong, L.; Jin-Ling, D.; Yun-Hong, Z.; Gao-Qing, L. Molecular events in deliquescence and efflorescence phase transitions of sodium nitrate particles studied by Fourier transform infrared attenuated total reflection spectroscopy. J. Chem. Phys. 2008, 129, 104509. (56) Szanyi, J.; Kwak, J. H.; Peden, C. H. F. The effect of water on the adsorption of NO2 in Na
and Ba
Y, FAU zeolites: A combined FTIR and TPD investigation. J. Phys. Chem. B 2004, 108, 3746. (57) Ghiladi, R. A.; Kretzer, R. M.; Guzei, I.; Rheingold, A. L.; Neuhold, Y.-M.; Hatwell, K. R.; Zuberb€uhler, A. D.; Karlin, K. D. (F8TPP)FeII/O2 Reactivity studies {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2
)}: Spectroscopic (UV
visible and NMR) and kinetic study of solvent-dependent (Fe/O2 = 1:1 or 2:1) reversible O2-reduction and ferryl formation. Inorg. Chem. 2001, 40, 5754. (58) Kopf, M.-A.; Neuhold, Y.-M.; Zuberbuhler, A. D.; Karlin, K. D. Oxo- and hydroxo-bridged heme-copper assemblies formed from acid
base or metal
dioxygen chemistry. Inorg. Chem. 1999, 38, 3093. 4269

dx.doi.org/10.1021/ef200779m |Energy Fuels 2011, 25, 4261–4270

Energy & Fuels

ARTICLE

(59) Kazansky, V. B.; Subbotina, I. R.; Pronin, A. A.; Schl€ogl, R.; Jentoft, F. C. Unusual infrared spectrum of ethane adsorbed by gallium oxide. J. Phys. Chem. B 2006, 110, 7975. (60) Herzberg, G. H. Molecular Spectra and Molecular Structure II. Infrared and Raman Spectra of Polyatomic Molecules; D. Van Nostrand Co. Inc.: New York, London, 1964. (61) Lee, E. L.; Wachs, I. E. In situ raman spectroscopy of SiO2supported transition metal oxide catalysts: An isotopic 18O
16O exchange study. J. Phys. Chem. C 2008, 112, 6487. (62) Johnson, C.; Long, B.; Nguyen, J. G.; Day, V. W.; Borovik, A. S.; Subramaniam, B.; Guzman, J. correlation between active center structure and enhanced dioxygen binding in Co(salen) nanoparticles: Characterization by in situ infrared, raman, and X-ray absorption spectroscopies. J. Phys. Chem. C 2008, 112, 12272. (63) Jazdzewski, B. A.; Reynolds, A. M.; Holland, P. L.; Young, V. G., Jr.; Kaderli, S.; Zuberbuhler, A. D.; Tolman, W. B. Copper(I)-phenolate complexes as models of the reduced active site of galactose oxidase: synthesis, characterization, and O2 reactivity. J. Biol. Inorg. Chem. 2003, 8, 381. (64) Lippert, C. A.; Arnstein, S. A.; Sherrill, C. D.; Soper, J. D. Redox-Active Ligands Facilitate Bimetallic O2 Homolysis at FiveCoordinate Oxorhenium(V) Centers. J. Am. Chem. Soc. 2010, 132, 3879. (65) Astolfi, P.; Marini, M.; Stipa, P. Radical trapping properties of 3-aryl-2H-benzo[1,4]oxazin-4-oxides. J. Org. Chem. 2007, 72, 8677. (66) Allouch, A.; Roubaud, V.; Lauricella, R.; Bouteiller, J.-C.; Tuccio, B. Spin trapping of superoxide by diester-nitrones. Org. Biomol. Chem. 2005, 3, 2458. (67) Liang, H.-C.; Zhang, C. X.; Henson, M. J.; Sommer, R. D.; Hatwell, K. R.; Kaderli, S.; Zuberb€uhler, A. D.; Rheingold, A. L.; Solomon, E. I.; Karlin, K. D. Contrasting copper
dioxygen chemistry arising from alike tridentate alkyltriamine copper(I) complexes. J. Am. Chem. Soc. 2002, 124, 4170. (68) Karoui, H.; Nsanzumuhire, C.; Le Moigne, F.; Tordo, P. Synthesis of a new spin trap: 2-(Diethoxyphosphoryl)-2-phenyl-3,4dihydro-2H-pyrrole 1-oxide. J. Org. Chem. 1999, 64, 1471. (69) Frejaville, C.; Karoui, H.; Tuccio, B.; Moigne, F. L.; Culcasi, M.; Pietri, S.; Lauricella, R.; Tordo, P. 5-(Diethoxyphosphoryl)-5-methyl-1pyrroline N-oxide: A new efficient phosphorylated nitrone for the in vitro and in vivo spin trapping of oxygen-centered radicals. J. Med. Chem. 1995, 38, 258. (70) Chen, D.; Motekaitis, R. J.; Martell, A. E. Dioxygen adducts of nickel(II) and cobalt(II) dioxopentaazamacrocyclic complexes: Kinetics, stabilities, and hydroxylation of the ligands in the nickel dioxygen complexes. Inorg. Chem. 1991, 30, 1396. (71) Kimura, E.; Anan, H.; Koike, T.; Shiro, M. A convenient synthesis of a macrocyclic dioxo pentaamine and X-ray crystal structure of its monohydrazoic acid salt. J. Org. Chem. 1989, 54, 3998. (72) Machida, R.; Kimura, E.; Kushi, Y. Coordination of pentadentate macrocycles, doubly deprotonated 1,4,7,10,13-pentaazacyclohexadecane-14,16-dione to high-spin nickel(II) and low-spin nickel(III). X-ray study of a novel redox system. Inorg. Chem. 1986, 25, 3461. (73) Kimura, E.; Machida, R.; Kodama, M. Macrocyclic dioxopentaamines: Novel ligands for 1:1 nickel(II)-oxygen (Ni(II)-O2) adduct formation. J. Am. Chem. Soc. 1984, 106, 5497. (74) Kimura, E.; Sakonaka, A.; Machida, R.; Kodama, M. Novel nickel(II) complexes with doubly deprotonated dioxopentaamine macrocyclic ligands for uptake and activation of molecular oxygen. J. Am. Chem. Soc. 1982, 104, 4255. (75) Cho, J.; Furutachi, H.; Fujinami, S.; Suzuki, M. A bis(mualkylperoxo)dinickel(II) complex as a reaction intermediate for the oxidation of the methyl groups of the Me2-tpa ligand to carboxylate and alkoxide ligands. Angew. Chem., Int. Ed. 2004, 43, 3300. (76) Kimura, E.; Machida, R. A mono-ozygenase model for selective aromatic hydroxylation with nickel(II)-macrocyclic polyamines. J. Chem. Soc., Chem. Commun. 1984, 499.

4270

dx.doi.org/10.1021/ef200779m |Energy Fuels 2011, 25, 4261–4270