1234 Catal. Sci. Technol., 2013, 3, 1234--1239 This journal is c The Royal Society of Chemistry 2013

Cite this: Catal. Sci. Technol.,2013,3, 1234

Nickel nanoparticles catalyse reversible hydration ofcarbon dioxide for mineralization carbon captureand storage

Gaurav A. Bhaduri and Lidija Siller*

The separation and storage of CO2 in geological form as mineral carbonates has been seen as a viable method

to reduce the concentration of CO2 from the atmosphere. Mineralization of CO2 to mineral salts like calcium

carbonate provides a stable storage of CO2. Reversible hydration of CO2 to carbonic acid is the rate limiting step

in the mineralization process. We report catalysis of the reversible hydration of CO2 using nickel nanoparticles

(NiNPs) at room temperature and atmospheric pressure. The catalytic activity of the NiNPs is pH independent

and as they are water insoluble and magnetic they can be magnetically separated for reuse. The reaction steps

were characterized using X-ray photoemission spectroscopy and a possible reaction mechanism is described.

Introduction

Since the identification of carbon dioxide as an anthropogenic greenhouse gas, there has been extensive research devoted to carbondioxide capture and storage (CCS) which has been extensivelyreviewed.1–10 Of the various propositions for storage of carbondioxide, that which has gained the greatest interest of governmentsand industries is storage of carbon dioxide in geological form.2,4,5,10

CCS is generally divided into two different forms: (a) geologicalstorage in saline aquifers or enhanced oil recovery (terrestrial oroceanic);10 or (b) mineral sequestration4,11 either as an in situ12 orex situ13 process. The former approach requires there to be acontinuous monitoring of the oil well for possible leaks5 (especiallyin oceanic storage) while the latter provides a more reliable ‘main-tenance-free’ solution to the problem of carbon dioxide storage.

Calcium carbonate is an abundant thermodynamically stablematerial.1 The current calcium carbonate present on Earth acts as acarbon reservoir estimated to be equivalent to 1.5� 1017 metric tonsof carbon dioxide.14 Therefore conversion of carbon dioxide tomineral carbonates has been proven environmentally and geologi-cally safe for long-term storage of carbon dioxide.1,11,14 The twomajor steps in the mineralization of carbon dioxide to calciumcarbonate are conversion of carbon dioxide to carbonic acid followedby neutralization of the acid; the rate limiting step being thehydration of carbon dioxide to carbonic acid. At present theCarbonic Anhydrase (CA) enzyme is the most promising candidatefor this process as it catalyses the reversible hydration of carbon

dioxide at mild pH values (pH between 7 and 5), the fastest ratebeing that of human CA II.14 CA is water soluble and there has beenintensive research for the use of CA immobilized on varioussupports.15–22 The limitations with the use of enzyme are cost ofextraction and specific operating parameters i.e. pH (pH between 7and 10) and temperature (4–30 1C).14–18 CA catalyses the hydrationreaction at a pH > 7 and the dehydration of the bicarbonate ion ata pH o 7.12 Thus it is important to maintain the pH of thesolution above 7 at all times and all the research is focused at pHabove 7.14–18 There have been few reports on other organic catalystsfor the reversible hydration of carbon dioxide.23–25 Kiese andHastings26 and Caplow27 have reported the catalysis of the hydrationof carbon dioxide using halogens whereas Guo et al.28 and Theeet al.29 reported the use of borate.

In the present work we report the catalysis of the reversiblehydration of carbon dioxide by an inorganic metal catalyst, nickelnanoparticles [NiNPs] for application in ex situ mineralization ofcarbon dioxide. NiNPs are water insoluble and magnetic, hence canbe magnetically separated and reused in the process. The catalyticactivity of the nanoparticles is pH independent and there is norequirement for any additional reagents for the process [such asbuffers required for the use of CA]. Moreover, the catalyst is active atroom temperature [RT] and atmospheric pressure.

ExperimentalMaterials

The nickel nanoparticles were purchased from Nano Technologies(Korea) and 99% pure CO2 from BOC (UK). Sodium hydroxide andhydrochloric acid (0.1 M) were bought from Sigma Aldrich (UK)

School of Chemical Engineering and Advanced Material, Newcastle University,

Newcastle upon Tyne, NE1 7RU, UK. E-mail: lidija.siller@newcastle.ac.uk

Received 15th November 2012,Accepted 16th January 2013

DOI: 10.1039/c3cy20791a

www.rsc.org/catalysis

CatalysisScience & Technology

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This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol., 2013, 3, 1234--1239 1235

and used as-received without further purification. Water used inthe process and chemical preparation was distilled and filteredand had a low conductivity of 2 mS cm�1. 0.1 M NaOH solution wasprepared by dissolving NaOH in DI water and was standardisedusing 0.1 M HCl solution. The NiNP suspensions were prepared byadding the required amount of NiNPs in distilled water and ultra-sonicated for 5 min in an Ultra-sonicator (Hilsonic).

CO2 uptake

Determination of the concentration of CO2 was performed in a20 ml jacketed vessel purchased from Soham Scientific. CO2

gas (at 1 atm, flow rate 1.69 mM min�1) was bubbled in 10 ml ofDI water or NiNPs suspension for 30 min and then titrated with0.1 M NaOH solution. There was a CO2 environment over thesurface of the water/NiNPs suspension. The temperature of thereaction chamber was kept constant at 20 1C, by circulatingwater through the jacket using a constant temperature waterbath (BS5, Fisher Scientific).

Reaction kinetics

CO2 absorption rate experiments were undertaken with a fixedvolume flask of water (200 ml) in a 250 ml glass reactor,(LS Industries) and CO2 was sparged at 1 atm (0.01 MPa, flowrate 1.69 mM min�1) pressure using a sinter. The pH andconductivity were measured using a pH 209 bench top pH meter(Hanna Instruments) and pIONneer30 (Radiometer analytical).The temperature was maintained by immersing the reactor in aconstant temperature water bath (BS5, Fisher Scientific). Theconductivity of the aqueous suspension of NiNPs was measuredafter ultrasonication for 10 minutes, using the pIONneer30(Radiometer analytical).

Characterization techniques

Specimens for HRTEM measurements were prepared on Cugrids with lacey carbon films (300 mesh, Agar Scientific). Thesize distribution of the NiNPs was analyzed by HRTEM using aJEOL 2100F field emission gun instrument operating at 200 keVlocated in Durham University, UK. The samples for XPS analy-sis were prepared by separating the Ni nanoparticles from thesolution after they had precipitated. The nanoparticles wereremoved from the solution using a micropipette (Eppendorf),dropped on a Si wafer and dried for a day before analysis. TheXPS analysis was carried out in an X-ray Photoemission Spectro-meter (Kartos Axis Ultra 165) equipped with a monochromatic

Al Ka X-ray source. The pass energy used was 20 eV. The XPSresults were fitted using a Shirley background30 and Gaussian–Lorentzian (mixed) singlet peak shapes. For calibration the firstcomponent of the C 1s line was aligned to 284.8 eV corres-ponding to the binding energy of amorphous carbon.31 All theother peaks were fitted using mixed singlets.32 For the Ni spectraa Shirley background was subtracted from the original data andthe background subtracted data fitted using mixed singlets.

Results and discussion

Commercially purchased NiNPs [NanoTechnolgy, Korea] werecharacterized using high resolution transmission electronmicroscopy [HRTEM] to determine their size distribution[Fig. 1]. The majority of the particles have characteristic lengthsbelow 100 nm. The presence of nickel was confirmed usingenergy dispersive X-ray spectroscopy (EDX) [Fig. 2]. The crystalplanes of the nanoparticles can be seen in the Selected Area

Fig. 1 HRTEM images of the Ni nanoparticles.

Fig. 3 Selected area electron diffraction from Ni nanoparticles.

Fig. 2 Energy dispersive X-ray spectroscopy from Ni nanoparticles.

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Electron Diffraction [SEAD] pattern [Fig. 3] and correspond tothe [220], [222], [311], [400], [422] and [531] lattice planesrespectively.33

Fig. 4 shows the enhancement of CO2 solution concentration(all species of CO2, i.e. CO2(aq), H2CO3 and H+ and HCO3

�,present in water) as a function of the NiNP concentration. Theconcentration of dissolved CO2 was determined by titrating theCO2 solution with 0.1 M NaOH solution. The amount of CO2

dissolved in water (without the NiNPs) was similar to thatreported in the literature34 (lit.,34 B39 mM). A maximum isobserved at 30 ppm [three times the capacity of de-ionizedwater], as compared to that of water without NiNPs. By furtherincreasing the particle concentration a decrease in the CO2

dissolution was observed, and it can be attributed to the

Brownian motion of the particles that might change the equili-brium dissolution state. Based on this result all further analysiswere performed with a NiNP concentration of 30 ppm.

Since the pH drop of the solution is a function of theformation of carbonic acid, the rate of pH change can berelated to the rate of the overall reaction [rA]14 i.e. reactions(1–3). Similar approaches have been reported for the study ofthe catalytic activity of CA.14 The rate of change of pH andconductivity are shown in Fig. 5. Two sets of experiments wereperformed at different initial pH values to test the catalyticactivity of Ni nanoparticles at pH values above and below 6. CAis not stable at low pH values [below pH of 5]14,35 and thusstudy of the catalytic activity of NiNPs becomes important atlow pH values. It can be seen from Fig. 5a and c that the changein pH in the presence of the catalyst [filled circles] is signifi-cantly more rapid than that without the catalyst [filled squares]for the two different initial pH values [at pH 6.2 and 5.5].

The reactions associated with this process are20

CO2(gas) ) CO2(aq) (1)

CO2(aq) + H2O ) H2CO3 (2)

H2CO3 3 H+ + HCO3� (3)

As there are no additional ions generated in the reaction thechange in conductivity of the solution is a measure of theformation of bicarbonate ions from CO2. CO2(aq) being neutralin charge would not be responsible for the increase in conduc-tivity of the solution. Thus the increase in the conductivity of the

Fig. 4 Increase in the amount of carbon dioxide absorbed in aqueous solutionof Ni nanoparticles as a function of particle concentration, at RT and at atmo-spheric pressure.

Fig. 5 pH and conductivity changes during bubbling of carbon dioxide through DI water and aqueous Ni nanoparticle suspension. (a) pH change starting frompH above 6, (b) conductivity change corresponding to pH change from above 6, (c) pH change starting at pH value below 6, (d) conductivity change corresponding topH change from below 6. Experiments are performed at RT and atmospheric pressure.

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solution is due to the generation of carbonic acid, providingproof of the catalytic activity. It can be observed from Fig. 5band d that the initial rate of increase in the conductivity of thesolution is higher in the presence of the NiNPs than in theirabsence. Thus the NiNPs act as a catalyst until the solution issaturated with bicarbonate ions and the surface of the NiNPsadsorbs some of those bicarbonates [see XPS analysis below]and/or CO2 gas.

In order to confirm that the increase in ion conductivity isnot purely due to the ions leached into the water from theNiNPs themselves, changes in conductivity of solutions whichonly contains NiNPs were analysed (Fig. 6a and b). It couldbe seen that this contribution to the ionic conductivity isnegligible. Therefore, we can conclude that the increase inconductivity of the solution is purely due to an increase inthe amount of bicarbonate ions alone.

It was also observed that there was an initial immediateincrease in the pH of DI water by 0.4–0.5 (Fig. 5a and b, observethe pH drop at 0 min) due to the addition of NiNPs and wesuggest on the basis of our X-ray photoemission spectroscopy(XPS) analysis (see below) that this is likely due to the dissocia-tion of water and formation of OH groups on the NiNP surface.This assertion is supported by the observation of OH specieson the [111]36 and [110]37 surfaces of single crystal Ni whenexposed to H2O at 300 K.36,37

In order to have an insight into the reaction mechanismand the species present on the nanoparticle surface, X-ray

photoelectron spectroscopy [XPS] was performed on the Ninanoparticles before [Fig. 7] and after carbon dioxide bubbling[Fig. 7]. All errors in energy position are �0.1 eV.

An insight into the chemical state of the NiNPs afterdissolution in DI water without carbon dioxide exposure canbe obtained by examination of the Ni 2p3/2 and O 1s core lines(Fig. 7). The Ni 2p3/2 line can be decomposed into peaks locatedat binding energies of 852.6 eV, 854.0 eV, 855.7 eV and aplasmon peak at 861.0 eV.31,38 The O 1s line was fitted by threepeaks located at 529.8 eV, 531.3 eV and 532.2 eV. The Ni 2p3/2

peak at 852.6 eV binding energy is associated with Ni0 (ref. 39)while that at 855.7 eV corresponds to nickel in the Ni2+

oxidation state and has a binding energy corresponding tothat of Ni(OH)2.31,40 Moreover, the O 1s peak at 531.3 eVcorresponds to oxygen in the hydroxyl (–OH) group associatedwith Ni(OH)2.31 The binding energy of the O 1s in multilayers ofH2O (532.4 eV)36 is at a higher binding energy than that of the�OH group (530.9 eV)36 thus the peak observed at 532.1 eV canbe assigned to water adsorbed at the surface of the Ni nano-particles.36,37 The Ni 2p3/2 peak at 854.0 eV corresponds toNiO which is confirmed by the presence of the O 1s peakat 529.8 eV.31,38

XPS of the NiNPs after carbon dioxide bubbling is shown inFig. 8. The Ni 2p3/2 is again fitted using three peaks, which arelocated at binding energies of 852.9 eV, 854.4 eV, and 855.9 eV,respectively, and a plasmon peak at 861.5 eV.31,38 The Ni peakat 852.9 eV corresponds to Ni0 (ref. 38) whereas the peaks at

Fig. 6 Conductivity change of the Ni nanoparticle suspension compared to that of the blank and Ni suspension when bubbled with CO2 for (a) pH above 6 and(b) pH below 6.

Fig. 7 XPS spectra of NiNPs before bubbling carbon dioxide; (a) O 1s line and (b) Ni 2p line.

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854.4 eV and 855.9 eV correspond to the Ni2+ oxidation state.38

The carbon C 1s line is also fitted with three peaks located at284.8 eV, 286.0 eV, and 288.1 eV. The C 1s component at 284.8 eVbinding energy is assigned to adventitious carbon.31 The C 1speak at 286.0 eV corresponds to carbon in alcohol groups andthat at 288.1 eV to carbon in ester groups.31 The O 1s line is fittedwith four peaks located at 530.1 eV, 531.7 eV, 532.5 eV and 533.2eV. The peak for O 1s at 530.0 eV and the Ni 2p3/2 peak at 854.4eV are assigned to NiO on NiNPs.38 The O 1s peaks at 531.7 eVand 533.2 eV correspond to the two different oxygen sites in theester (–COO–) group (the first corresponding to the oxygendouble bonded to carbon and the second to the oxygen singlebonded to carbon) and the O 1s peak at 532.5 eV corresponds tothe alcohol group (–C–OH).31,41 Bicarbonate molecules containboth an ester and an alcoholic carbon, therefore we interpret thepresence as a signature of bicarbonate species present on thenickel surface. We suggest that the Ni 2p3/2 peak at 855.9 eVcorresponds to Ni(HCO3)X adsorbed at the Ni surface.

Based on the interpretation of the XPS results we can derivea possible reaction mechanism which is presented in Fig. 9. Inthe aqueous environment there is the generation of hydroxylgroups on the surface of the Ni nanoparticles. These hydroxylgroups are then attacked by the carbon dioxide molecule to formbicarbonate ions on the Ni surface which are then displaced bywater molecules, which then lose hydrogen ions and regeneratethe hydroxyl ions on the Ni surface. The absence of the –OHgroup on the surface of NiNPs [Fig. 8] in the XPS results suggestsa possible conversion of –OH groups to –HCO3 groups when CO2

is bubbled in the NiNPs aqueous suspension. There were nohydroxyl groups observed in the XPS results of the Ni

nanoparticles after CO2 bubbling, indicating the conversion ofthe hydroxyl groups to bicarbonate groups in the reaction.

It can be expected that other nano-forms of nickel and itsalloys alone or on the support will also be catalytically active forthe hydration reaction of CO2 and therefore finding the optimalinorganic catalyst for mineral storage of CO2 is worthy of futurestudies. It should also be noted that NiNPs are toxic and therehave been recent studies that have been reported on theirtoxicity.42–45 Thus NiNPs cannot be used for undergroundinjection of CO2, but should be retailed and reused in thereactor (or process) with minimal exposure to the environment.Considering 99% recovery of the catalyst (NiNP) in the above

Fig. 8 XPS spectra of NiNPs after bubbling with carbon dioxide; (A) O 1s, (B) Ni 2p3/2 and (C) C 1s.

Fig. 9 Schematic of the reaction mechanism of hydrogenation of CO2 by NiNPs.

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mentioned process the cost incurred per ton capture of CO2

would be B7.9 (USD), considering the current price of Ni metalbeing 16605 USD pre ton.46

Conclusion

Here we report the catalytic activity of the NiNPs for the reversiblehydration of carbon dioxide. The catalyst showed activity indepen-dent of the pH of the solution, at room temperature and atmo-spheric pressure. A threefold enhancement in the dissolution of CO2

in water was observed in the presence of NiNPs (30 ppm). There wasalso an initial increase in the pH of water by addition of NiNPs,which was due to the formation of Ni(OH)x on the NiNP surface asseen from the XPS analysis. The XPS analysis provided the reactionsteps on the basis of which the reaction mechanism was suggested.

Acknowledgements

GAB would like to thank Newcastle University for a TeachingScholarship. We would like to thank Dr B. G. Mendis for the HRTEManalysis and Dr M. R. C. Hunt for critical reading of the manuscript.We thank the EPSRC (SECURE, EP/K004689/1) for funding.

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