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Metal-catalyzed graphitic nanostructures as sorbents for vapor-phaseammonia†

Jeffrey W. Long,*a Matthew Laskoski,b Gregory W. Peterson,c Teddy M. Keller,b Katherine A. Pettigrewa

and Bryan J. Schindlerd

Received 21st September 2010, Accepted 15th December 2010

DOI: 10.1039/c0jm03167d

Activated carbons have long been used as substrates for the filtration of vapor-phase molecules, often

with metal salts or oxides added to improve their sorption capacities for specific agents, but their real-

world performance and applicability may be hindered by such factors as long-term stability and complex

processing. En route to a new class of carbon-based sorbents, we have developed solid-state synthetic

methods to produce bulk carbonaceous solids based on the pyrolysis of thermoset solids containing low

concentrations (<1 wt%) of metal precursors (based on either Ni, Fe, or Co) that decompose in situ to

catalyze the formation of a complex graphitic nanostructure. Selective combustion of residual

amorphous carbon from the pyrolyzed solid generates a mesoporous network that facilitates diffusional

transport of gas-phase molecules to the interior surfaces of the solid, and also converts the entrained

metals to their respective metal oxide forms. We examine the ammonia-sorption properties of a series of

these graphitic nanostructured compositions, and demonstrate that ammonia uptake is primarily

determined by the type of residual metal oxide, with the Co-containing carbonaceous solid providing the

best ammonia-sorption capacity (1.76 mol kg�1). Thermal reduction of the Co-containing material

drastically decreases its ammonia-sorption capacity, showing that the oxide form (in this case Co3O4) of

the entrained metal nanoparticles is most active for ammonia filtration. The effects of the carbon–oxygen

functionalities on the nanostructured graphitic surfaces for ammonia sorption are also discussed.

Introduction

Nanostructured carbons are often used as sorbents for toxic

small molecules, such as ammonia, which presents particular

challenges for filtration because of its high vapor pressure.1,2 In

the case of carbonaceous materials, oxidation of the carbon

surface improves the adsorption of ammonia by the addition of

functionalities (carboxylic, hydroxyl), whereby hydrogen

bonding, complexation or acid–base chemistry can occur.3–7 For

example, Seredych and Bandosz oxidized graphite using fuming

nitric acid and found enhanced reactivity towards ammonia from

the resultant carboxylic functional groups;8 their results

aCode 6170, Surface Chemistry Branch, Naval Research Laboratory,Washington, DC, 20375, USA. E-mail: jeffrey.long@nrl.navy.milbCode 6120, Materials Chemistry Branch, Naval Research Laboratory,Washington, DC, 20375, USAcUS Army Research, Development and Engineering Command EdgewoodChemical Biological Center Research & Technology Directorate, CBRFiltration Team, Aberdeen Proving Ground, MD, 21010, USAdSAIC Gunpowder Branch, PO Box 68, Aberdeen Proving Ground, MD,21010, USA

† Electronic supplementary information (ESI) available: additionalX-ray diffraction, supplementary pore-size distribution plots,water-sorption isotherms, expanded versions of ammonia-breakthroughplots, and additional information on ammonia-breakthrough testingapparatus. See DOI: 10.1039/c0jm03167d

This journal is ª The Royal Society of Chemistry 2011

indicated the formation of ammonium ions through Brønsted

activity and amides through bond-breaking chemistry. The

addition of metal salts or oxides to carbon sorbents also

improves their uptake for ammonia via complexation, as in the

case of metal chloride salts,9 or by acid–base interactions, as

shown for molybdenum and tungsten oxides.10 Although effec-

tive for improving filtration performance, the methods used to

incorporate these active metal components add complexity and

cost to the production of the final product, and the spatial

distribution and long-term stability of the metal impregnants

may also be difficult to control.

Over the past several years, we have developed a general solid-

state synthetic approach for the production in large quantity,

high yield, and moldable forms of multi-walled carbon nano-

tubes (MWNTs), segmented or bamboo-type nanotubes, and/or

graphitic nanofibers/nanoribbons.11–16 This general method is

based on in situ generation of low concentrations of metal

nanoparticles (typically <2 weight%) via the thermal decompo-

sition of melt-processable organometallic compounds and/or

metal salts in the presence of an excess amount of a polymer-

izable aromatic carbon precursor. For pyrolysis temperatures

exceeding 700 �C, the resulting carbonaceous solids are typically

composites of interpenetrating graphitic and amorphous carbon

phases, in which the ratio of crystalline-to-amorphous carbon is

determined by such factors as the type of transition-metal

J. Mater. Chem., 2011, 21, 3477–3484 | 3477

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catalyst (Fe, Ni, or Co), the concentration of the catalyst, the

chemical structure of the polymeric precursor, and the carbon-

ization temperature/time profile.

We recently described a simple thermal oxidation procedure to

selectively combust and remove the entrained amorphous carbon

phase, thereby revealing a highly porous solid comprising

graphitic nanostructures and residual metal oxides from the

graphitization catalyst.17 We further demonstrated that the

interior surfaces of the graphitic nanostructure are highly

accessible to gas-phase molecules such as ammonia. Using the

design flexibility afforded by the solid-state synthetic approach

and subsequent selective-combustion purification, we are

exploring and optimizing these carbonaceous materials for

a variety of applications, including electrochemical energy

storage, hydrogen storage, and as sorbents for the filtration of

vapor-phase toxins. In many such cases, the incorporated metals

used to catalyze the formation of the graphitic nanostructure

during the initial synthesis may also serve as functional compo-

nents in the final product, particularly for applications that

involve the sorption or catalysis of small molecules.

In the present report, we examine the ammonia sorption

properties of a particular series of graphitic nanostructured

compositions developed at the NRL.13,17 Among the composi-

tions containing Ni, Fe, or Co nanoparticulate graphitization

catalysts, the Co-containing composition exhibits the highest

ammonia-sorption capacity. We further show that the oxide

form of the Co-nanoparticle graphitization catalyst that is

generated during the selective-combustion purification process

promotes more effective ammonia adsorption compared to the

same composition in which the metal oxide (Co3O4) has been

thermally reduced back to Co metal. The effects of the surface

carbon–oxygen functionalities present in the calcined form of

these carbonaceous materials are also discussed with respect to

their impact on ammonia sorption.

Experimental

Synthetic procedures

The synthesis and characterization of carbonaceous solids

prepared by the carbonization of thermoset resins based on

nickel(cyclooctadiene) (Ni(COD)2), dicobalt octacarbonyl

(Co2(CO)8) and diiron nonocarbonyl (Fe2(CO)9) and 1,2,4,5-

tetrakis(phenylethynyl)benzene (TPEB, 1) have been described in

previous publications.13 Briefly, the metal catalyst precursors

(Ni(COD)2, (Co2(CO)8) or (Fe2(CO)9) and 1 were dissolved in a

1 to 20 molar ratio in a common solvent such as methylene

chloride. The resulting solutions were stirred for 1 h and the

solvent was removed to yield dark solids. A circular Al planchet

was treated with Teflon mold release and the individual dark

products were melted and heated at 220 �C until the polymeri-

zation reaction was complete, which typically occurred within

1 h. The resulting polymeric solids were cooled, removed from

the mold, placed in a tube furnace, heated at 0.3 �C min�1 from

room temperature to 1000 �C under an argon atmosphere, and

cooled to room temperature at a rate of 0.5 �C min�1 to yield the

nanostructured carbonaceous solid.

Following the carbonization step, the resulting solids were

heated in a muffle furnace under static air to selectively combust

3478 | J. Mater. Chem., 2011, 21, 3477–3484

or remove any entrained amorphous carbon, using calcination

conditions (set temperature and time held at the temperature)

determined by separate thermal analysis experiments. The

calcination conditions varied according to the graphitization

catalyst used: Ni-catalyzed, 480 �C for 20 h; Fe-catalyzed, 460 �C

for 16 h; Co-catalyzed, 420 �C for 16 h. For the purposes of this

manuscript, the calcined versions of these materials are desig-

nated as ‘‘Ni-TPEB-calc’’, ‘‘Fe-TPEB-calc’’, and ‘‘Co-TPEB-calc,

respectively. Portions of the Co-TPEB-calc material were sub-

jected to an additional thermal treatment in a Lindberg tube

furnace under flowing 10% H2/90% argon (total flow rate ¼220 mL min�1), ramping at 5 �C min�1 to set points of either

300 �C or 700 �C, and holding at the set points for 2 h.

Materials characterization

Nitrogen-sorption porosimetry was performed with a Micro-

meritics ASAP2010 Accelerated Surface Area and Porosity

Analyzer; all samples were degassed under vacuum at 150 �C for

24 h prior to measurements. Temperature-programmed reduction

(TPR) experiments were performed using a Micromeritics

Autochem II Chemisorption Unit coupled with a Pfeiffer Vacuum

ThermoStar� Residual Gas Analyzer (RGA) attachment (with

internal quadrupole mass spectrometer). Samples for TPR

measurements were first degassed at 150 �C in UHP He (99.999%)

for 2 h, then ramped at 10 �C min�1 in 10% H2/90% argon to

700 �C, while the composition of the eluting gas stream was

monitored by a thermal conductivity detector in the chemisorp-

tion unit and at selected mass-to-charge (m/z) channels with the

RGA.

X-Ray diffraction analysis was performed with a Bruker D8

powder diffractometer, using CuKa radiation from a rotating

anode X-ray source. Transmission electron microscopy (TEM)

studies were performed on a JEOL 2200FS equipped with

a Gatan CCD camera; a Noran System Six EDS was used to

evaluate the polyhedral carbon structures with visible lattice

fringes. Powders of the nanostructured carbonaceous materials

were analyzed by brushing a small amount onto holey-carbon

film supports. Multiple areas were imaged to ensure that the

results obtained were representative of the whole sample. Scan-

ning electron microscopy (SEM) studies were performed on

a Zeiss Model Supra 55 Electron Microscope. X-Ray photo-

electron spectroscopy measurements were performed using

a Thermo Scientific K-Alpha X-ray photoelectron spectrometer

with a monochromatic Al-Ka X-ray source. Samples were

analyzed as powders that were filled into the wells of a Cu

mounting plate (no signals originating from the sample stage

were observed). A flood gun was used in all experiments to

minimize any charging effects with the powdered samples.

Breakthrough testing

Ammonia breakthrough experiments were conducted on the

carbonaceous powders using a micro-breakthrough push system;

for a full description, see the work by Peterson et al.18 Briefly,

a ballast was charged with a mixture of ammonia and air; this

pressurized stream was then mixed with a dry or humid diluent

stream at a rate necessary to achieve an ammonia concentration

of 1000 mg m�3 (�1430 ppm). The respective powders were

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packed in a 4 mm ID glass fritted tube to a height of 4 mm. All

samples (�25 mg) were dried at 150 �C for 1 h and humid

samples were then pre-humidified for 2 h. The effluent stream

was constantly monitored using a Hewlett Packard 5890 Series II

gas chromatograph equipped with a photoionization detector

and a 10.6 eV lamp. Test conditions are specified in Table S1

(ESI†). Expanded versions of the breakthrough curves, focusing

on the initial ammonia breakthrough of the respective sample

beds, are also shown in the ESI†.

Results and discussion

Structural characteristics of calcined metal–TPEB compositions

The microscale and nanoscale structures of the metal–TPEB

compositions examined in this study are determined mainly by

the morphology of graphitic networks that develop during the

solid-state carbonization process, as catalyzed by the low

concentrations of incorporated metal nanoparticles that form

in situ by decomposition of the respective organometallic

precursors. As described previously for Ni-TPEB-based carbo-

naceous solids,17 selective-combustion of the amorphous carbon

component reveals the underlying graphitic nanostructure, and

results in a porous solid that remains monolithic. Qualitatively

similar structures are observed for the related Fe-TPEB-calc and

Fig. 1 (a and b) SEM characterization for the Co-TPEB samples (low

and high resolution); (c and d) TEM characterization for the Co-TPEB

samples (low and high resolution); (e and f) TEM characterization for the

Fe-TPEB samples (low and high resolution).

This journal is ª The Royal Society of Chemistry 2011

Co-TPEB-calc materials. When observed by SEM (Fig. 1a and

b), the calcined solids exhibit interpenetrating, aperiodic

networks of solid and disordered voids with feature sizes ranging

from �10 to 200 nm. The mesopore/macropore network that

forms via combustion of the amorphous carbon serves a critical

function in providing a facile pathway for the infiltration of gas-

phase molecules to the interior graphitic surfaces, as demon-

strated previously for the dynamic sorption of ammonia on the

Ni-TPEB-calc solid.17 A closer inspection of the solid domains

using TEM (Fig. 1c–f) reveals a complex graphitic nanostructure

comprising cockleshelled,19 spherical, tubular, and ribbon-like

morphologies, with feature sizes ranging from several nano-

metres to �20 nm with different curvatures and lengths, for both

the Fe-TPEB-calc and Co-TPEB-calc materials. The areas of

higher, sharper contrast that are observed dispersed throughout

the graphitic nanostructure (Fig. 1c and e) are assigned to the

metal nanoparticles (in this case Co or Fe) that form in situ during

the carbonization process, and exhibit a wide range of particle

sizes, from a few nm to�100 nm. Due to the embedded nature of

the metal nanoparticles within the carbonaceous matrix, it was

difficult to obtain a more quantitative particle-size analysis.

X-Ray diffraction measurements confirm that these metal

nanoparticles are converted to their respective metal oxide

forms—NiO,17 a-Fe2O3 (ESI†), and Co3O4—during the calci-

nation process. The nanoparticulate metals and corresponding

metal oxides are incorporated at relatively low loadings. For

example, prior analysis of the Ni-TPEB-calc material revealed

a residual NiO content of 2.0 wt%.17 In the present case,

elemental analysis was also performed on the Co-TPEB-calc and

Fe-TPEB-calc materials, with the Co-TPEB-calc analog having

a Co content of 3.0 wt%, corresponding to 4.1 wt% Co3O4, while

the Fe-TPEB-calc material contained 1.3 wt% Fe and 1.9 wt%

Fe2O3. The metal/metal oxide content of the final product is

ultimately a result of both the initial metal content of precursor

mixture and the subsequent mass loss that occurs during

carbonization and calcination. The adventitious formation of

highly dispersed metal oxide nanoparticles in the purified

graphitic nanomaterials produced by our process may ultimately

be beneficial for the filtration of small molecules such as

ammonia, via acid–base interactions10 or potentially through

catalytic mechanisms that proceed at the metal oxide surfaces. In

the following sections, we examine how the oxidation state of the

metal in a series of Co-TPEB carbon nanomaterials impacts their

performance for ammonia filtration.

The effectiveness of a sorbent depends not only on surface

functionality, but also on the nature of the pore–solid architec-

ture. Nitrogen-sorption porosimetry was performed on the series

of calcined solids to assess such characteristics as specific surface

area, pore volume, and pore-size distribution. A summary of N2-

sorption metrics for the Ni-, Fe-, and Co-TPEB-calc materials is

shown in Table 1. The Ni-TPEB-calc material exhibits relatively

modest specific surface area (140 m2 g�1), consistent with the

larger feature size of the graphitic nanostructure in that mate-

rial,17 relative to the Fe-TPEB-calc and Co-TPEB-calc versions

(see Fig. 1), which exhibit total specific surface areas of 360 and

500 m2 g�1, respectively. The Co-TPEB-calc material is further

distinguished by significant surface area and pore volume located

in micropores (<2 nm), which may also enhance its adsorption

for small-molecule gases.20 The origin of the microporosity in the

J. Mater. Chem., 2011, 21, 3477–3484 | 3479

Table 1 Metrics derived from nitrogen-sorption porosimetry

Carbonsample

BET surfacearea/m2 g�1

Microporeareaa/m2 g�1

Cumulative porevolumeb/cm3 g�1

Microporevolumea/cm3

g�1

Ni-TPEB-calc

140 18 0.37 0.01

Fe-TPEB-calc

360 50 0.86 0.02

Co-TPEB-calc

500 340 0.60 0.16

Co-TPEB-calc-300-red

470 320 0.68 0.15

Co-TPEB-calc-700-red

530 360 0.71 0.17

a Calculated using the t-plot method. b Calculated using the Barrett–Joyner–Halenda model.

Fig. 3 Ammonia-breakthrough curves for various carbon sorbents

under (a) dry and (b) humid conditions.

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Co-TPEB-calc material, which is not observed in the Ni-TPEB-

calc or Fe-TPEB-calc analogs, is not presently understood, but

may arise from some open tubular structures, which are difficult

to observe by TEM in this complex graphitic nanostructure.

Pore-size distribution plots were also generated from the N2-

sorption data (see Fig. 2). All materials exhibited a broad range

of pore sizes from �2 to 40 nm, but with much of the pore

volume concentrated in pores <10 nm, a size range that should

still facilitate diffusional transport of gas-phase molecules to the

interior surfaces of these materials. Low-pressure isotherm data

for the Co-TPEB-calc material were also fitted using the MP

method to model the structure of the micropores (see ESI†).

Ammonia-sorption on calcined metal–TPEB compositions as

a function of their residual metal nanoparticles

Ammonia breakthrough curves for the Co-TPEB-calc, Fe-

TPEB-calc, and Ni-TPEB-calc compositions are illustrated in

Fig. 3, corresponding to dry and humid challenge conditions,

respectively. Testing was conducted on equivalent volumes of

each sample, and the breakthrough curves are plotted on a rela-

tive time scale to account for differences in mass used for each

Fig. 2 Pore-size distribution plots for the Ni-TPEB-calc, Fe-TPEB-calc,

and Co-TPEB-calc compositions, derived using a DFT model and

assuming a slit-shaped pore geometry.

3480 | J. Mater. Chem., 2011, 21, 3477–3484

test. Under dry conditions ammonia begins breaking through on

both the Fe-TPEB-calc and Ni-TPEB-calc analogs almost

immediately, and exhibits very sharp curves to saturation indi-

cating very limited adsorption/reaction capacities (see Fig. 3a).

The Co-TPEB-calc analog, on the other hand, has a delay in

ammonia breakthrough and also shows a somewhat elongated

curve, which may indicate significant interaction between

ammonia and the metal oxide nanoparticles and/or graphitic

surfaces. The pore structure may also play a role in ammonia

removal, with the Co-TPEB-calc composition having signifi-

cantly higher pore volumes in the micropore range. In all cases,

much of the adsorbed ammonia eventually elutes after the

challenge is terminated and the sample is purged with clean air,

indicating that the ammonia sorption is most likely due to

a specific physisorption interaction that is not yet understood.

Among the series of materials investigated, the Co-TPEB-calc

composition provides the best ammonia removal by far, with

a calculated capacity of 1.76 mol kg�1 as compared to 0.51 and

0.19 mol kg�1 for the Fe-TPEB-calc and Ni-TPEB-calc materials,

respectively (see Table 2). By comparison, the dynamic

ammonia-sorption capacities of typical off-the-shelf activated

carbons (with specific surface areas >1000 m2 g�1) are on the

order of 0.1 mol kg�1,6,18 while boiling activated carbons in

concentrated acid to introduce acidic carbon–oxygen surface

functionalities can extend their sorption capacities to 2.4 mol

kg�1.6 Petit and Bandosz investigated activated carbons

impregnated with acidic tungsten and molybdenum oxides, and

reported dynamic ammonia-sorption capacities as high as

1.5 mol kg�1.for carbons containing 10.7 wt% Mo.10

This journal is ª The Royal Society of Chemistry 2011

Table 2 Summary of ammonia-breakthrough results

SampleRelativehumidity Mass/mg

Loading to saturation

(mol kg�1) (mg g�1)

Ni-TPEB-calc 0% 22.5 0.19 3.280% 19.5 0.30 5.1

Fe-TPEB-calc 0% 15.3 0.51 8.780% 13.6 0.62 10.6

Co-TPEB-calc 0% 9.4 1.76 30.080% 10.2 0.81 13.8

Co-TPEB-calc-300-red 0% 11.8 0.83 14.180% 10.8 1.48 25.2

Co-TPEB-calc-700-red 0% 4.6 0.19 3.280% 10.0 0.69 11.8

Fig. 4 Temperature-programmed reduction of the Co-TPEB-calc

material heated at 10 �C min�1 under a 10% H2/90% Ar gas flow. The

response of the TCD is shown on the left y-axis and detector currents

from the various RGA channels are shown on the right y-axis.

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In the present case, the significantly higher ammonia-sorption

capacity of the Co-TPEB-calc composition cannot solely be

attributed to its modestly higher specific surface area (500 m2 g�1

vs. 360 m2 g�1 for the Fe-TPEB-calc material). Although there are

variations in the total metal/metal oxide content among this

series, the Co-TPEB-calc composition still exhibits the highest

capacity beyond what would be expected for its modestly higher

metal oxide content. We have observed similarly enhanced

ammonia-sorption capacities with other Co-containing graphitic

nanostructures, but derived from alternative carbon precursors

such as Novolac resins.21 Cobalt oxides (specifically Co3O4) have

been reported as effective ammonia-oxidation catalysts at

elevated temperatures (>700 �C),22 but it is unclear whether such

mechanisms are operational at room temperature. The ammonia

capacities under dry conditions for this series do generally follow

the trend of isoelectric points for the respective metal oxides,

ranging from �10 for NiO to 9 for a-Fe2O3 to 7.5 for Co3O423–25

which may indicate that the acid–base character of the metal

oxide is a factor for ammonia sorption. Regardless, the enhanced

capacity of the Co-TPEB-calc material does not derive simply

from ammonia sorption at the surfaces of the incorporated metal

oxide nanoparticle. Normalizing the molar capacity of ammonia

to the known Co3O4 molar concentration would yield a NH3-to-

Co3O4 molar ratio of 3.4, far beyond a theoretical monolayer

adsorption, particularly considering the particle size and dis-

persity (�5 to 20 nm) of the Co3O4. Further work is underway to

identify the specific adsorption mechanisms for the Co-TPEB-

calc composition and related nanostructured graphitic materials.

Under humid conditions (Fig. 3b), the Co-TPEB-calc material

again outperforms the other two analogs in breakthrough

capacity. The Fe-TPEB-calc composition actually has the best

initial rate of ammonia removal, yet the Co-containing analog has

substantial capacity as seen by the shape of the curve at higher

C/C0 values, indicative of specific interaction of the ammonia

either with the cobalt oxide, as observed under dry conditions, or

through hydrogen-bonding with adsorbed moisture. The Co-

TPEB-calc material adsorbs the most moisture (�12 wt%) at 80%

relative humidity compared to the Fe-TPEB-calc (�9 wt%) and

Ni-TPEB-calc (�2 wt%) analogs (water-sorption isotherms for all

three carbon materials are shown in the ESI†). The higher mois-

ture content in the Co-TPEB-calc sorbent should facilitate better

ammonia removal via dissolution in the adsorbed water layer. All

samples show significant desorption after feed termination, indi-

cating that the strength of ammonia sorption is modest.

This journal is ª The Royal Society of Chemistry 2011

While the Co-TPEB-calc material provides the best ammonia

removal of the samples tested, it is also the only sample for which

ammonia capacity decreases with increasing humidity. Typically,

ammonia removal capacity increases with increasing relative

humidity on activated carbons due to the relatively high solu-

bility of ammonia in water. In the case of the Co-TPEB-calc

sorbent, competitive adsorption between water and ammonia at

the active cobalt oxide sites may result in somewhat lower

ammonia capacities in humid conditions, overshadowing the

expected solubility effects. The ammonia-sorption behavior

under humid conditions may also be affected by the formation of

ammonium ions in the adsorbed water layer when acidic surface

sites are present, as noted in the work by Le Leuch and

Bandosz.26

Tuning the oxygen functionalities of metal and carbon

components for the Co-containing solids

With the Co-TPEB-calc composition identified as the most effec-

tive sorbent among those studied for ammonia filtration, we

selected this material for further investigation, with the goal of

elucidating the origins of its superior ammonia-sorption proper-

ties. The Co-TPEB-calc solid possesses a complex structure in

which multiple factors, such as specific adsorption or catalytic

interactions of the incorporated Co3O4 nanoparticles or the pres-

ence of significantly greater micropore volume may distinguish this

material from the Fe-TPEB-calc and Ni-TPEB-calc analogs.

Carbon–oxygen functionalities on the surface of these graphitic

nanostructures may also play a role in the ammonia-sorption

properties of these materials via acid–base interactions, as

described previously for activated carbon6 and graphite oxide.7,27,28

In order to deconvolute these various substrate effects with

respect to the filtration of ammonia, we applied a series of

thermal-reduction treatments to the Co-TPEB-calc material with

the goal of selectively reducing Co-oxide and/or carbon–oxygen

functionalities, while otherwise maintaining a consistent pore–

solid architecture. Temperature-programmed reduction (TPR)

measurements were first performed on the Co-TPEB-calc

powder (see Fig. 4) to determine the appropriate treatment

conditions. The composition of the evolving gases during the

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TPR experiment was monitored using both a non-selective

thermal conductivity detector (TCD), and a residual gas analyzer

(RGA) that was tuned to track evolved H2O, CO, and CO2.

The initial TPR peak that is observed at �260 �C in both the

TCD response and in the RGA channel for H2O (m/z ¼ 18) is

consistent with the reduction of Co3O4, which may proceed

through a CoO intermediate between 300 and 400 �C, en route to

forming Co metal nanoparticles.29,30 Additional reduction is

noted between 400 and 700 �C in the form of a large convoluted

peak shape in the TCD response that is in part coincident with

a large RGA peak for CO (m/z ¼ 28) and a smaller peak for CO2

(m/z¼ 44). The detection of CO and CO2 at these temperatures is

consistent with the desorption of surface-sited carbon–oxygen

functionalities, as has been previously noted for activated

carbon.31–33 The temperature range of the prominent peak in this

region, at �600 �C, and the preferential evolution of CO relative

to CO2 are consistent with decomposition of phenolic and/or

carbonyl functionalities from the graphitic surface.

On the basis of the results of the TPR analysis, we chose two

temperatures to thermally reduce portions of the Co-TPEB-calc

carbon: 300 �C to reduce the Co3O4 phase under 10% H2/90%

argon gas flow; and 700 �C to desorb surface-sited carbon–

oxygen functionalities. The resulting samples are denoted at ‘‘Co-

TPEB-calc-300-red’’ and ‘‘Co-TPEB-calc-700-red’’, respectively.

Powder X-ray diffraction measurements confirmed that the

crystalline phase of the incorporated cobalt species in the initial

Co-TPEB-calc form is Co3O4, as evidenced by the diffraction

peaks at 32 and 37� 2 � q (see Fig. 5). These peaks are greatly

reduced after thermal reduction at 300 �C, suggesting that the

Co3O4 converts to another phase (one that does not strongly

diffract), but clear diffraction evidence for Co metal is only

observed for the sample reduced at 700 �C. The persistence of the

graphitic peak with comparable peak width confirms that the

Fig. 5 Powder X-ray diffraction for a series of Co-TPEB carbon samples.

Also shown are reference patterns for graphite, Co3O4, and Co metal.

3482 | J. Mater. Chem., 2011, 21, 3477–3484

underlying graphitic nanostructure is not markedly altered by

these thermal reduction treatments.

The TPR results suggest that carbon–oxygen functionalities

are ultimately removed from these materials, particularly for the

Co-TPEB-calc-700-red material. X-Ray photoelectron spec-

troscopy, which is a well-established method for characterizing

the surface chemistry of carbonaceous materials,34 was used to

monitor changes in surface carbon–oxygen surface functionality

of the Co-TPEB series as a function of thermal treatment.

Deconvolution of the C1s spectrum for the initial Co-TPEB-calc

material reveals a wide range of carbon–oxygen functionalities,

including carboxylate and carbonyl groups, but with phenol/

hydroxyl moieties (C1s peak at 285.8 eV) being the most prom-

inent (see Fig. 6).

Because the TPEB precursor to these materials contains no

oxygen, we posit that the oxygen functionalities observed for the

Co-TPEB-calc material are generated during the calcination/

purification step, which involves heating in air at 420 �C,

conditions that may partially oxidize the graphitic surfaces. The

presence of carbon–oxygen functionalities, particularly those

that are either weakly acidic (phenol) or strongly acidic

(carboxylic acid) should promote the adsorption of

ammonia,3,6,22 although we note that the quantity of adventitious

carbon–oxygen functionalities in the present materials is far less

than those previously reported carbon-based sorbents that are

intentionally oxidized. Considering only the total integration of

Fig. 6 X-Ray photoelectron spectra in the C1s region for the series of

Co-TPEB materials. Deconvoluted peaks are shown with dashed lines,

which are also labeled according with their respective assignments to

particular carbon functionalities.

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Fig. 7 X-Ray photoelectron spectra and corresponding peak fits in the

O1s region for the series of Co-TPEB materials.

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C1s and O1s peaks (see Fig. 7), the relative carbon and oxygen

contents of the Co-TPEB-calc material were 89.9 and 10.1 atom

percent, respectively. The estimated oxygen contribution from

the entrained Co3O4 nanoparticles is less than one atom percent,

and therefore no attempt was made to deconvolute O1s peaks

that would be specifically attributed to Co3O4 (expected between

531 and 532 eV).35

Thermal reduction of Co-TPEB-calc composition at 300 �C

does not markedly alter the shape or relative peak areas of the

C1s or O1s spectra, with the Co-TPEB-calc-300-red sample

showing XPS-derived carbon and oxygen contents of 90.7 and

9.3 atom percent, respectively. After reduction at 700 �C, the

total oxygen content decreases to 4.5 atom percent, which can be

primarily ascribed to the loss of carbonyl-type functionalities, as

indicated by the decrease in peak intensity at �531.2 eV in the

O1s region (see Fig. 7). The changing carbon–oxygen surface

functionality is also reflected in the water-sorption isotherms for

this series, with the Co-TPEB-calc and Co-TPEB-calc-300-red

materials showing similar isotherms, while the 700 �C reduced

analog adsorbs approximately half as much water as possible,

consistent with a much less hydrophilic carbon surface after

thermal desorption of the carbonyl functionalities.

Fig. 8 Ammonia-breakthrough curves for various Co-TPEB-based

sorbents under (a) dry and (b) humid conditions.

Effect of oxygen functionalities of metal and carbon components

on ammonia sorption

The Co-TPEB-based materials described above provide an

instructive series of compositions with which to deconvolute the

role of the various metal/metal-oxide nanoparticles and carbon–

oxygen functionalities contained therein with respect to

ammonia sorption, while eliminating effects that might otherwise

be ascribed to the physical pore structure, which is essentially

invariant for this series (see N2-porosimetry results in Table 1

and pore-size distribution plots in the ESI†). For ammonia

breakthrough in a dry environment, the parent Co-TPEB-calc

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material clearly outperforms the thermally reduced analogs (see

Fig. 8a). The Co-TPEB-calc-700-red material shows rapid

breakthrough of the ammonia, while the Co-TPEB-calc-300-red

material provides a minimum amount of delay. Both of the

reduced materials show a very sharp curve to saturation, while

the Co-TPEB-calc material has an elongated curve at higher

ammonia concentrations.

The differences between the Co-TPEB-calc and the Co-TPEB-

calc-300-red material demonstrate the active role of the Co3O4

nanoparticles for removal of ammonia. Further reduction of the

material to Co-TPEB-calc-700-red also changes the carbon

surface by decreasing the amount of carbon–oxygen function-

alities groups and completing the reduction of the cobalt oxide to

its metallic form. Both of these changes ultimately result in very

limited capacity for ammonia. The Co-TPEB-calc material has

a calculated capacity of 1.76 mol kg�1, whereas the Co-TPEB-

calc-300-red and Co-TPEB-calc-700-red materials provide

capacities of only 0.83 mol kg�1 and 0.19 mol kg�1, respectively

(see Table 2).

The ammonia-breakthrough behavior is more complicated

under humid conditions (see Fig. 8b and Table 2), with the Co-

TPEB-calc-300-red material actually exhibiting the highest

capacity (1.48 mol kg�1), followed by the Co-TPEB-calc (0.81

mol kg�1) and Co-TPEB-calc-700-red (0.69 mol kg�1) materials.

While the greater capacity of the Co-TPEB-calc-300-red material

relative to the parent Co-TPEB-calc under humid conditions is

not yet understood, the interaction of the intermediate Co phase

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formed at 300 �C and ammonia dissolved in the adsorbed water

layer may play a role. The decrease in the capacity upon further

reduction to Co-TPEB-calc-700-red material can be attributed to

the loss of both the acidic carbon–oxygen sites on the graphitic

surfaces and also cobalt oxide being reduced to its metallic state.

Conclusions

Nanostructured graphitic solids produced via solid-state

methods exhibit promising performance for the adsorption of

gas-phase ammonia. Our preliminary studies demonstrate that

the Co-catalyzed carbonaceous solid provides the highest

ammonia-sorption capacities, facilitated by the residual Co3O4

nanoparticles dispersed throughout the graphitic nanostructure,

while acidic carbon–oxygen surface functionalities play a smaller

role in the ammonia-uptake mechanism. Future efforts will focus

on improving the strength of ammonia sorption on these and

related materials, and on developing a more thorough under-

standing of the relevant adsorption mechanisms. The findings of

this preliminary investigation will be used to translate the

advantages of our solid-state synthetic approach toward the

production of large-scale, moldable, solid forms of nano-

structured graphitic compositions in new materials that are

specifically designed for the filtration of vapor-phase toxins, such

as ammonia.

Acknowledgements

Financial support for this project was provided by the Defense

Threat Reduction Agency and the Office of Naval Research. The

authors thank Dr Jean M. Wallace (Nova Research, Inc.,

Alexandria, VA) for assistance with the TPR experiments, and

Dr Debra R. Rolison (Naval Research Laboratory) for helpful

discussions in the preparation of this manuscript.

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