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Hydroxyapatite supported cobalt catalysts for hydrogen generation

 Justyn Wayne Jaworski a,b, Sunghwa Cho b, Yeoungyong Kim b, Jong Hwa Jung b, Hyo Sang Jeon c,d,Byoung Koun Min c,d,⇑, Ki-Young Kwon b,⇑

a Department of Chemical Engineering, Hanyang University, Seoul 133-791, South Koreab Department of Chemistry, Gyeongsang National University and RINS, Jinju 660-701, South Koreac Clean Energy Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, South Koread University of Science and Technology, 176 Gajung-dong, 217 Yuseong-gu, Daejeon 305-350, South Korea

a r t i c l e i n f o

 Article history:

Received 31 July 2012Accepted 15 November 2012Available online 3 December 2012

Keywords:

Hydroxyapatite supportHydrogen generationCalcium deficientCobaltIon exchange

a b s t r a c t

The controlled generation of H2 from storage materials by using an efficient catalytic support is a highlysought after technology; however, the majority of successes utilize expensive materials consideredunfeasible. In our report on the creation of a novel, durable, and inexpensive catalytic support materialfor hydrogen generation, we examine a critical surface modification of hydroxyapatite (HAP) with cobaltions to provide the necessary catalytic transition metal for the fast hydrolysis of the hydrogen storagematerial, sodium borohydride (NaBH4). By altering the morphology and composition of the HAP crystalsupports, we revealed novel methods for enhancing the hydrogen generation rates. Particularly, loweringthe Ca composition during synthesis of the HAP crystals afforded a Ca deficient HAP capable of exhibitinga higher surface coverage of cobalt, thereby eliciting faster hydrolysis reaction rates in comparison withthe amorphous HAP control having the characteristic Ca content for HAP. A more significant increase inhydrogen generation was observed when using single crystal HAP in comparison with amorphous andcalcium deficient HAP supports. Despite the smaller surface area of the hydrothermally prepared singlecrystal HAP, it provided significantly faster hydrogen generation. Each of the HAP supports exhibitrepeatability withcatalytic efficiency decreasing by approximately 25% over3 weeks upon repeateddaily

exposure to solutions of the hydrogen storage material NaBH4. Through these experiments, we provedthat altering the composition and morphology of cobalt ion exchanged HAP supports can offers a usefulmeans for increasing the rate of controlled hydrogen generation.

Crown Copyright    2012 Published by Elsevier Inc. All rights reserved.

1. Introduction

Unlike fossil fuels, hydrogen provides a green, sustainable fuel,which has been considered an ideal future energy reservoir  [1,2] .The use of hydrogen fuel is environmentally benign, since wateris the only by-product of the combustion process [3–5] . Moreover,hydrogen is readily available from water and a variety of othersources, thereby removing the need for competition over energyresources [6] . Researchers have made an incredible effort to gener-ate hydrogen in an environmentally clean and cost-effective man-ner [7,8] . Recent works in Co containing synthetic ligands haveshed new light on some of the catalytic mechanism of molecularhydrogen evolution [9] . In pursuing this goal, water splitting intohydrogen and oxygen using homo/heterogeneous catalysts hasbeen the most comprehensively studied method for acquiringhydrogen  [2,10,11] . Once hydrogen is efficiently generated, an-other challenge arises for its effective, safe, and convenient storage

for practical hydrogen fuel usage [12,13] . Hydrogen storage meth-ods have been proposed using high pressure, cryogenics, and vari-ous physisorbing compounds for reversible uptake and release of hydrogen. Because of its high flammability, material safety underambient conditions is an important consideration. For this reason,chemical hydrides have received well-deserved interest due totheir hydrogen generation capabilities and safe transportability[4,14,15].

NaBH4ðaqÞ þ 2H2OðlÞ  !catalyst

NaBO2ðaqÞ þ 4H2ð g Þ ð1Þ

One chemical hydride, sodium borohydride (NaBH4), has beenexplored as a model material for the effective storage of hydrogen,due to its safety for transport and high hydrogen storage capacity(2.4 L of H2 per gram of NaBH 4) [16,17] . In addition to being a highenergy density hydrogen carrier, NaBH4 has proven to have severalother advantages including the following: stability in air, ability togenerate hydrogen at low temperatures, non-hazardous reactionproducts, and the ability to control the hydrogen generation rate[18,19]. Control of the hydrogen generation rate must be done un-der alkaline solution for stability, where a solution of 1–10% NaOH

0021-9797/$ - see front matter Crown Copyright    2012 Published by Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcis.2012.11.036

⇑ Corresponding authors. Fax: +82 55 761 0244.

E-mail addresses:  bkmin@kist.re.kr  (B.K. Min),  kykwon@gnu.ac.kr  (K.-Y. Kwon).

 Journal of Colloid and Interface Science 394 (2013) 401–408

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is typically used to stabilize the NaBH4  to prevent its uncontrolledself-hydrolysis  [20] . By using an effective catalyst such as that de-scribed in this work, the base-stabilized NaBH4   solution can becontrollably hydrolyzed to activate the production of hydrogen ina directed manner as indicated in Eq. (1)  [19] .

The generation of hydrogen by a catalyst requires a stable cata-lyst-support structure to provide controlled starting and stopping

of the reaction with high efficiency. Previously, our group as wellas others have shown that support structures bearing catalystswith high price noble metals (including Ru, Pt, and Rh) performwith highreusability[21–23]; however, a more abundant, low-costalternative is necessary. In order to be practical, researchers haveattempted alloying with inactive metals such as Cu, Ag, or Pd,though the results revealed decreased performance [24] . To solvethis problem, Ni and Co based catalysts have been explored dueto their low cost and high performance in hydrolysis of NaBH4

[4,25–28]. In the past, Co powders have had a tendency to travelwith bubbles produced from the hydrolysis reactionand have beenfound to clog the reactors of hydrogen generation systems [24,26] .For the purpose of real-world applications, a catalyst-supportstructure can improve catalyst dispersion. Recent works havefound supported Co catalysts to provide high activity, long-life-time, and low cost for supply of hydrogen from the hydrolysis of sodium borohydride [26,29] .

An often noted drawback of many supported Co systems forhydrogen generation is the lack of reusability in NaBH4  hydrolysis[30]. Whilethe exact mechanism as to how the surface is becomingdeactivated remains unclear, the formation of borate by-producthas often been observed to block the catalytic surface and further-more has shown to be removable by washing to afford a fullyrecovered system [31] . If given high activity, reusability, and eco-nomic feasibility, a supported Co catalyst may provide a viableprospect for controlled hydrogen generation. In the followingstudy, we demonstrate improved reusability in a supported cata-lysts produced by Co ion exchange on the surface of hydroxyapa-tite (HAP). While the development of an improved catalyst

material is of key importance, system level designs which reduceby-product and regenerate used NaBH4  are critical for the futureof NaBH4 based hydrogen storagesystems[32]. A range of catalystsfor sodium borohydride hydrolysis including Ru/anion-exchangeresins, Pt/LiCoO2, Ru/carbon, PtRu/LiCoO2  among many others thathave been proposed and approach 100% hydrolysis, typically gen-erating hydrogen at a rate of 0.1–2.8 L/(min g of catalyst)  [33] .Having performed an in-depth look at a variety of other catalyticsupport structures, including nanoclusters, polymeric supports,and even foams [4,18,34,35] , we can appreciate that certain mate-rial characteristics can be determining factors for the efficiency of the support. Specifically, the support structure should not agglom-erate, should maintain a high active surface area, minimize deacti-vation due to borate by-product formation, and should not

deteriorate due to surface stress resulting from rapid hydrogenbubble generation. Aside from supported catalysts systems, ap-proaches for mobile hydrogen storage systems have also success-fully demonstrated the use of aqueous solutions of cobalt as partof a dual-solid-fuel system known to have advantages in terms of hydrogen storage density and fuel conversion [36] . In such a sys-tem, Co2B particulates are formed which are catalytically activebut also subject to rapid deactivation. Creating a supported Co cat-alyst with stability and reusability is thereby seen as a technicalchallenge for hydrogen generation.

In this work, we examine several morphological and composi-tional forms of hydroxyapatite (HAP) supported Co catalysts to ad-dress the challenging issues in the use of NaBH4   for controlledhydrogen generation. Aside from being inexpensive, durable, easy

to manufacture, environmentally benign, and stable in basic solu-tion, we have previously found that HAP is an effective material

for the immobilization of metal ions onto the surface   [21] . Re-cently, through the cation exchange of protons of the P-OH groupswith Co2+ ions, researchers have determined that Co is capable of being quickly immobilized onto the HAP surface with equilibriumoccurring after only 5 min [37] . Moreover, the amount of Co con-tent on HAP can be easily controlled with capacities of up to0.13–0.30 mmole of Co per gram of HAP, depending on the variety

of HAP support [38] . In this study, we created single crystal HAP(s_HAP), amorphous HAP (a_HAP), and calcium deficient amor-phous HAP (cd_HAP) catalytic supports to examine the effect of composition and morphology on the generation of hydrogen. Ouranalysis reveals the amount of Co immobilization depends on thedegree of calcium composition of the HAP support, wherein cal-cium deficient HAP could provide a higher content of surface Coand hence provide a more effective catalytic support for hydrolysisof sodium borohydride. Interestingly, we find that the single crys-tal HAP, despite having a lower surface area, results in a greaterrate of hydrogen generation per weight of catalytic support (Coand HAP). Examination of the long-term stability by measurementof total hydrogen generation demonstrated that each of the threecatalytic support structures were reusable over the course of 20 days. These main results and supporting experiments are re-ported and discussed herein.

2. Materials and methods

 2.1. Synthesis of calcium deficient HAP 

Puriss grade reagents were obtained from Sigma Aldrich. Prep-aration of Ca deficient HAP was carried out by adjusting the molarratio of Ca/P to 1. 5.59 g of (NH4)2HPO4   dissolved in 420 mL of water is added dropwise into 10 g of Ca(NO3)24H2O dissolved in420 mL of water while stirring under ambient temperature andpressure. After the solution of pH is adjusted to 10 using concen-trated ammonia solution, the solution is refluxed for 2 h under a1atmN

2 condition while stirring. After cooling to room tempera-

ture, the mixture was filtered and washed three times withapproximately 200 mL distilled water (the final filtrate solutionhas a pH  6.7). Finally, the white precipitate is dried overnightin a 70 C oven under static air.

 2.2. Synthesis of amorphous HAP 

Preparation method of amorphous HAP is same to that of cal-cium deficient HAP except for a Ca/P molar ratio of 1.67 was used.

 2.3. Synthesis of single crystal HAP 

Puriss grade reagents were obtained from Sigma Aldrich. To the

30 mL solution of Ca(NO3)2

4H2O (7.79 g), 50 mL solution of Na2-HPO42H2O (3.56 g) is added dropwise. The solution is adjustedto pH 10 by adding 2 M NaOH solution. The solution is transferredto a Teflon lined autoclave reactor and is aged at 200 C for 24 hwithout stirring (while heating, the pressure reached approxi-mately 10 bar). After cooling the solution mixture to room temper-ature, white precipitates were obtained. The precipitate is washedthree times using approximately 200 mL of DI water and filtered.Finally, the white precipitate is dried overnight in a 70 C oven un-der static air.

 2.4. Hydroxyapatite characterization

The crystalline phases of the Ca deficient HAP, single crystal,

and amorphous HAP were evaluated by XRD (3 kW Cu X-ray Dif-fractometer, D8 Advance, Bruker AXS Germany) before and after

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Co ion exchange. The morphologies of the samples were observedusing a transmission electron microscope (Jeol 200 kV JEM-2010).

 2.5. Ion exchange of cobalt on hydroxyapatite

ACS grade cobalt(II) nitrate was obtained from Sigma Aldrich.Cobalt(II) nitrate, Co(NO3)2, was dissolved in 20 mL of distilled

water to concentrations of 0.01, 0.05, 0.1, 0.14, 0.18, 0.22, and0.25 mM. Thesesolutions were then exposed to 2 g of singlecrystalHAP, amorphous HAP, or Ca deficient HAP. Initially, the sampleswere shaken vigorously and then sonicated for 10 min. The sam-ples were then mixed moderately for another 20 min on a rockingplatform. After ion exchange, the solution was filtered and thenwashed twice with 200 mL of distilled water. The samples weredried overnight in a 70 C oven under static air for 24 h beforeundergoing elemental analysis and hydrogen generationexperiments.

 2.6. Analysis of elemental composition

ACS grade nitric acid was obtained from Sigma Aldrich. In orderto monitor the elemental composition of the ion exchanged cata-lytic supports, a small amount of each support (5 mg) was sepa-rately dissolved in 1 mL of 70% nitric acid followed by theaddition of 30 mL of distilled water. The samples underwent ele-mental analysis using an Inductively Coupled Plasma (ICP) spec-trometer (OPTIMA 4300DV/5300DV, Perkin Elmer) to observe theelemental composition of cobalt, calcium, and phosphorous.

 2.7. Catalytic generation of hydrogen

In order to measure the generation of hydrogen by the sup-ported catalyst systems, the volume of water displaced from an in-verted graduated cylinder was measured from a closed systemwith the catalyst and substrate in a connected round bottom flaskat 22 C and 1 atm. Using 0.1 g of HAP supported catalyst (s_Co-

HAP, a_CoHAP, or cd_CoHAP having particle sizes as indicated byTEM) placed in the round bottom flask, 0.2 g of sodium borohy-dride dissolved in 20 mL 1% NaOH (0.25 M) was added to the flaskwith stirring at 1000 rpm using a Teflon-coatedstir bar and a mag-netic stirrer. (See Supplementary Fig. 4  for analysis of the effects of stirring speed on the hydrogen generation rate). The closed systemwas created after addition of the sodium borohydride by replace-ment of the stopcock. The volume of water displaced from the cyl-inder, and the time was simultaneouslyrecorded until the reactionwas complete. Blank reactions were carried out with only sodiumborohydride presentin the sodiumhydroxide solutionrevealing nohydrogen generation (negative controls). Other controls whichyielded no hydrogen generation were control samples of 0.1 gs_HAP, a_HAP, or cd_HAP without undergoing cobalt ion exchange

but tested in the same manner as above. Additional control exper-iments using 0.03 mmole of pure Co(NO3)2  without any HAP havealso been conducted using the methods above to obtain the rate of hydrolysis using standalone cobalt in an amount equivalent to thatpresent on some of the supported CoHAP catalysts. It should benoted that due to particulate formation when using pure Co(NO3)2,a proportion of the cobalt remains unavailable for reaction therebyrendering them as inactive Co sites. (See Supplementary Fig. 3  fordata and additional analysis).

 2.8. Catalyst stability experiments

In these experiments, s_CoHAP, cd_CoHAP, or a_CoHAP catalystsupports were generated by a 0.1 M Co(NO3)2  ion exchange reac-

tion as described in Section  2.5 . Using the previously describedexperimental setup for the catalytic generation of hydrogen (Sec-

tion  2.7 ), the volume of hydrogen generated from a controlledamount (0.2 g in 20 mL basic solution of 1% NaOH) of sodiumboro-hydride exposure was measured. The same 0.1 g of CoHAP catalystsupports wasrepeatedly exposedto sodium borohydride daily overa 3 week period, and there generation rates were measured to as-sess the stability. At   24 h intervals, measurements were taken,and the solution was replaced with a new 20 mL solution contain-

ing 0.2 g sodium borohydride in 1% NaOH. The catalytic supportsolution was not removed or washed during this process. Controlexperiment using 0.03 mmole of pure Co(NO3)2  without any HAPwas also conducted in this manner to observe the reusability of standalone cobalt.

 2.9. Surface characterization of catalytic supports before and after 

 prolonged hydrogen generation

Samples of the prepared s_CoHAP, a_CoHAP, and cd_CoHAPwere used for repeated daily hydrolysis of sodium borohydridefor 3 weeks as explained in Section 2.8 . The resulting reacted sam-ples were washed three times with approximately 200mL distilledwater and then subjected to FTIR as were also the unreacted sam-

ples of s_CoHAP, cd_CoHAP, and a_CoHAP. IR spectra of the CoHAPsamples were obtained as KBr pellets, using 32 scans in the rangeof 400–4000cm1, with a Shimadzu FTIR 8400S instrument. Sam-ples of s_CoHAP before and after sodium borohydride exposurewere subjected to X-ray photoelectron spectroscopy (XPS). TheXPS analysis was conducted with a PHI 5000 VersaProbe (Ulvac-PHI) spectrometer using monochromatic Al Ka   lines of Al(1486.6eV) as an X-ray source.

3. Results

 3.1. Characterization

The crystallite shape of Ca deficient and amorphous HAP pre-

pared by precipitation resulted in crystals exhibiting a plate-likestructure,  Fig. 1 , consistent with that found in the literature forHAP synthesized by wet-chemical precipitation [39] . The averagecrystal size distribution of the Ca deficient and amorphous HAPsamples was determined to be approximately 78 nm in length.The aspect ratio of the crystals did not appear homogeneous, andthe shape of the major plane was consistently asymmetric. In con-trast, the single crystal HAP samples were significantly larger witha length of approximately 155 nm and possessing an elongatedhexagonal rod shape. From the TEM images, we can also see thatno morphological changes can be observed due to ion exchangein Co solution for a_HAP, cd_HAP, and s_HAP.

By XRD qualitative analysis (Fig. 2), we verified that the synthe-sis produced hydroxyapatite structures, without the presence of 

impurity phases such as CaO, CaHPO4, or   b-Ca(PO4)2, and theXRD data were found to be consistent with the XRD patternsknown in literature [40–42] . The sharp peaks of the XRD patternsfor s_HAP and s_CoHAP reveal that the single crystal was not al-teredin itsbulk crystal structure due to ionexchange. For the otherHAP samples as well, ion exchange was found to not affect the bulkcrystal structure as determined by comparison of the XRD datapatterns. This indicates the ion exchange is taking placeon the out-most several layers of the crystals. Fromchemical analysis, Ca defi-cient hydroxyapatite was found to have a Ca:P ratio of 1.448, andthe hexagonal crystal system of the Ca deficient hydroxyapatite,Ca10 x(HPO4)(PO4)6 x(OH)2 x, was determined to have 0.3036vacancies per 10 calcium ions. In addition to these comparisons,the amorphous and Ca deficient HAP were found through nitrogen

adsorption experiments to have a similar BET surface area of approximately 56.4± 3 m2/g, while the BET surface area of single

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crystal HAP was found to be 6.7± 0.2 m2/g. 1. We also find that thesurface area was not affected by ion exchange as is consistent withour TEM images revealing no morphological changes after ionexchange.

 3.2. Effect of hydroxyapatite calcium composition on Co ion exchange

Ionexchange of Co(NO3)2 in the presence ofCadeficient HAP andamorphous HAP revealed the amount of cobalt that couldbe incor-

porated onto the supports as related to Ca composition. In order toidentifythe Co content of the catalyticsupports after ion exchange,

ICPelemental analysiswasperformon bothCadeficient aswell asonamorphous HAP support structures for various ion exchange reac-tion concentrations of Co(NO3)2. We identified that Co saturationon the surface of the HAP supports occurs at a 0.1 M concentrationof Co(NO3)2  in solution( Fig. 3a). Under these saturation conditions,theamorphousHAP supports contained approximately 0.2 mmolesof Co per gram of CoHAP support. A slightly higher Co content of 0.3 mmoles per gram of cd_CoHAP is observed from Fig. 3b. Bylow-ering the Ca composition during synthesis of the HAP crystals, it is

evident that the Cadeficient HAP support was capable of exhibitingapproximately 50% higher surface coverage of cobalt.

Fig. 1. TEM images of (A) amorphous HAP (a_HAP), (B) Co ion exchanged amorphous HAP (a_CoHAP), (C) Ca deficient HAP (cd_HAP), (D) Co ion exchanged Ca deficient HAP(cd_CoHAP), (E) single crystal HAP (s_HAP), (F) Co ion exchanged single crystal HAP (s_CoHAP). All scale bars in the figure represent 100 nm.

Fig. 2.  XRD pattern of amorphous HAP (a_HAP), Co ion exchanged amorphous HAP (a_CoHAP), Ca deficient HAP (cd_HAP), Co ion exchanged Ca deficient HAP (cd_CoHAP),single crystal HAP (s_HAP), and Co ion exchanged single crystal HAP (s_CoHAP),

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 3.3. Influence of Co content on hydrogen generation

As mentioned in the previous section, catalytic cobalt wasincorporated onto the support structure of Ca deficient HAP and

amorphous HAP samples by ion exchange under several differentCo(NO3)2  concentration conditions. Each resulting CoHAP catalyticsupport was tested to determine the hydrogen generation capabil-ities. We measured the hydrogen generation profiles from 0.2 g of sodium borohydride in 20 mL aqueous solution of 1% NaOH ex-posed to either a 0.1 g sample of Ca deficient CoHAP catalyst(cd_CoHAP) or amorphous CoHAP catalyst (a_CoHAP). Catalystsobtained from ion exchange reactions under higher Co(NO3)2  con-centrations exhibited increased catalytic activity for the case of both a_CoHAP (Fig. 4a) and cd_CoHAP (Fig. 4b). We believe the ob-served induction periods to merely represent the   30 mL dead-volume present in the system, which must be filled before the gen-erated hydrogen could be measured. From Fig. 4 , higher Co(NO3)2

concentrations in the ion exchange reaction resulted in catalytic

supports that exhibited faster hydrogen generationrates as a resultof higher loading of Co active sites on the surface. For a_CoHAPwith Co(NO3)2   ion exchange concentrations of 0.01M, 0.05 M,and 0.1 M (Fig. 4a), we measured, by observing the linear reactionrate over three samples, average hydrogen generation rates of 0.71 mL min1, 1.25mL min1, and 1.73 mL min1, respectively,per gram of catalytic support, that is the weight of cobalt andHAP combined. This rate represents the average linear region,excluding the induction period or reduced rate phases in the begin-ning and end of the reaction, respectively. By providing the rates of hydrolysis per weight of the entire catalytic support (Co and HAP),it is clear to see the effect of the calcium deficient HAP catalyticsupport as it offers a proportionally higher amount of Co contentas compared to a_HAP. Examining Ca deficient CoHAP (cd_CoHAP)

catalysts from ion exchange concentrations of 0.01 M, 0.05 M, and0.1M Co(NO3)2  ( Fig. 4b), we found average hydrogen generation

rates of 1.0 mL min1, 1.45 mL min1, and 2.54mL min1, respec-tively, per gram of catalytic support (weight of Co and HAP). Ascan be expected, due to the higher Co content capacity achievablefor the Ca deficient HAP support, the cd_CoHAP samples exhibitedfaster hydrogen generation rates in comparison to a_CoHAP.

 3.4. Influence of morphology on hydrogen generation

Using the three HAP samples synthesized in this work, a_HAP,cd_HAP, and s_HAP, Co ion exchange was performed at 0.1 M Coconcentration resulting in a_CoHAP, cd_CoHAP, and s_CoHAP forassessment of the morphological dependence on hydrogen gener-ation. We measured the hydrogen generation profiles from 0.2 gof sodium borohydride in 20 mL aqueous solution of 1% NaOH ex-posedto either a 0.1 g sample of Ca deficient CoHAP catalyst,amor-phous CoHAP catalyst, or the larger single crystal CoHAP catalyst.Ascan beobservedin (Fig. 5), a significant increaseis evident whenusing Co immobilized single crystal HAP supports as compared toamorphous and calcium deficient HAP supports. Given that thesame weight amount of 0.1 g of catalytic support was used in eachcase, the larger sized single crystal HAP supports will have repre-

senteda samplewith less surface area. Despite this, the singlecrys-tal HAP provided significantly higher hydrogen generation. This isreflected from the hydrogenrefl moree tonditions. a_C-323.6(morAP)]TJ0-1Tf1.4199-2.6247TD[(U

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tion results remains that the HAP possessing a lower Ca content al-

lowed for increased uptake of Co during ion exchange.By comparison of the amorphous HAP and Ca deficient HAP as

Co catalyst-support structures, we prove the importance in con-trolling the composition of the crystal support in order to createsufficient active sites for the Co catalyzed hydrolysis of sodiumborohydride. As mentioned above, XRD data in conjunction withTEM images reveal the Ca deficient HAP and amorphous HAP tobe structurallysimilar, while ICP data providea significantly higherdegree of Co incorporation on the Ca deficient HAP support. Fromhydrogen generation experiment, we can conclude that this addi-tional Co present for the case of cd_CoHAP is present on the surfaceof the catalytic support, thereby allowing the higher degree of cat-alytic activity. After close analysis of various Co ion exchange con-ditions, we identified the extent of Co incorporation was higher for

the calcium deficient HAP catalytic support until a point of satura-tion was reached in which the final Co loading capacity was in the

range of 0.2–0.35 mmoles of Co per gramof HAP, which is in agree-

ment with existing Co ion exchange studies [38,46] . This distinc-tion in saturation levels provided a 50% higher number of catalytic Co sites on the Ca deficient HAP in contrast to the amor-phous HAP allowing its higher catalytic activity. The key point thatthe cd_HAP supports allowed a higher amount of Co loading makesthis catalytic support formulation a significant improvement as isevident by its higher hydrogen generation rates, which are attrib-uted to the higher loading capacity of Co as compared to thea_HAP.

Since examining the hydrogen generation capabilities for theHAP supports as a function of their Co loading conditions revealsa clear increase in the catalytic activity for increased Co contenton the support, we can conclude that the increase in the hydrogengeneration rate is a direct result of increasing the number of cata-

lytic Co sites available on the surface of the support for sodiumborohydride hydrolysis. This result correlates well with an existing

Fig. 4.  Hydrogen generation measurements showing a dependence on Co content for (a) amorphous HAP (a_CoHAP) ion exchanged with Co(NO 3)2  concentrations of (solid)0.1 M, (triangle) 0.05 M, and (circle) 0.01M, as well as for (b) Calcium deficient HAP (cd_CoHAP) with ion exchange concentrations of (solid) 0.1 M, (triangle) 0.05 M, and(circle) 0.01 M Co(NO3)2. Average values (N  = 3) are provided without error bars for the sake of clarity. Experiments were conducted as outlined in Section  2.7 .

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dehydrogenation study that used Co2+ loaded hydroxyapatite as aheterogeneous catalyst   [47] . An interesting assessment of HAPsupported catalyst from our prior study using Ru ion exchangedHAP reveals a clear advantage for using Ru over Co with respect

to the hydrogen generation ratefrom hydrolysis of sodiumborohy-dride. Ru supported HAP reveals a factor of 4 increase in the hydro-gen generation rate when normalized by the amount of catalyst(337 mL/(min  mmole of Ru)) in comparison with Co ( 80 mL/(min  mmole of Co))  [21] .

Looking into the morphological differences among our synthe-sized HAP supports, the larger sized single crystal HAP provides a

catalytic support with an almost 10 times smaller surface area pergram as compared to the cd_HAP and a_HAP. It is worth noting thation exchange did not result in any noticeablechange in surface areaas determined by BET measurements and no observable change inmorphology from TEM. While the morphologies of a_HAP andcd_HAP may be indistinguishable, the single crystal HAP, owing tothe different synthesis method, possesses a unique morphology of hexagonal rods elongated along the [001] direction, which resultsin six equivalent (100)-like surfaces being exposed to provide themajority of the crystal with a relatively flat surface [48,49] .

Despite the larger size and correspondingly lower surface areaby weight, the single crystal CoHAP catalysts offered significantlyhigher hydrogen generation rates over the other supports. It is stillunclear if thispredominant (10 0) surfaceprovides anypreferentialsites for higher density Co loading or improved arrangement of ac-tive catalyst sites as compared to the exposed planes of the amor-phous or calcium deficient HAP surface. Recent studies have shownthat, by providing a sufficiently short distance between active sites,reaction rates of heterogeneous catalysts can be altered by mor-phological control of the exposed reactive crystal planes with pref-erential transition-metal active sites [21,50,51] . While we cannotassume the (100) s_CoHAP surface is more suitable for hydrogenformation, we can conclude that having a particular crystal mor-phology as well as composition control can provide a successfulmeans for increasing the hydrogen generation rates among CoHAPcatalytic supports.

From our long-term stability measurements, we observeda pat-tern of gradual deactivation of the catalyst over repeated daily use,falling to nearly 75% over a 3 week period. Each of the three cata-

lytic support structures exhibited similar deactivation over thecourse of the 20 day experiment. To attempt to determine thecause of the deactivation, we observed the samples before andafter prolonged sodium borohydride exposure using a variety of 

Fig. 5.  Two successive hydrogen generation measurements (a and b) showing adependence of HAP morphology on the generation rate. a_CoHAP: Amorphous HAPpreviously ion exchanged with Co(NO3)2   concentrations of 0.1 M (squares),cd_CoHAP: calcium deficient HAP ion exchanged with Co(NO3)2  concentrations of 0.1 M (circles), s_CoHAP: single crystalline HAP ion exchanged with Co(NO3)2

concentrations of 0.1 M (triangles). Data are provided for individual samples.Experiments were conducted as outlined in Section   2.7   with time point zeropositioned as the onset of measurable hydrogen production.

Fig. 6. Stabilityexperiment of 0.1 g of amorphous CoHAP,Ca deficient CoHAP, andsinglecrystal CoHAP catalyst (previously ion exchanged with 0.1 M Co(NO 3)2  and using the

experimental conditions in Section 2.8 ). The percentage completion of the hydrolysis reaction from repeated exposures to 20 mL of 1% sodium borohydride in 1% NaOHdecreased over time during the 3 week experiment. Data are provided for individual samples.

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analytical tools. BET measurements revealed no significant changein the surface area, indicating that surface erosion did not play arole. XPS analysis revealed that the Co(II) oxidation remained un-changed after deactivation with no relevant differences in thespectra before and after hydrogen generation, see SupplementaryFig. 2 for full details and analysis. Wedidhoweverfind a noticeabledifference in the FTIR spectra when comparing each of the catalytic

supports (before and after 3 weeks of sodium borohydride hydro-lysis). Specifically, a characteristic B–O stretching band was ob-served after hydrogen generation, as seen in   SupplementaryFig. 1. Since boron oxide could not be observed by XPS or SEM,the exact cause of the gradual deactivation remains unclear.Although given the FTIR results, the formation of borate by-prod-uct may be plausible, as it is often known to block the catalytic sur-face in sodium borohydride hydrolysis reactions [31] .

5. Conclusion

Since cobalt appears naturally, due to mining and coal combus-tion, it is readily available in large amounts. This attribute favorsits use in the catalytic generation of hydrogen over expensive andrare noble metal based catalysts. By immobilizing Co onto ahydroxyapatite support, we proved that the CoHAP provides aneffective catalytic support for thehydrolysis of sodiumborohydrideresulting intheusefulgenerationof hydrogen.Byfurther examiningvariousforms of hydroxyapatite based on Ca composition andmor-phology, we revealed beneficial strategies for increasing the cata-lytic activity of these support structures for the generation of hydrogen. Specifically, we found that Ca deficient HAP provides a50%higherloadingcapacityof Coundersaturatedion exchangecon-ditions. Through this, wereveal a higher number of catalytically ac-tive surface sites useful for increasing the hydrogen generationrates. In addition,wefindthat thesinglecrystalHAPmorphologyaf-fords increased hydrogen generation rates. Accordingly, improve-ments in the HAP support structures can be realized throughaltering the synthesis conditions of Ca composition in hydroxyapa-

tite as well as the synthesis method to alter the morphology, whichdramatically affects the activity of a CoHAP catalytic support.Through our 3 week course of repeatability studies, we found thateach form of HAP possessed similar catalytic stabilities to repeatedexposure to sodium borohydride solutions decreasing to approxi-mately 75% of the original activity. Future endeavors to increasethe reusability of a CoHAP based hydrogen generation system mayprove valuable in creating a commercially applicable catalytic sup-port giventhe high catalytic activity,the ease of fabrication, and theabundance of starting materials.

 Acknowledgments

This work was supported by the National Research Foundation

of Korea (NRF-2010-0006157) and by the research fund of Hany-ang University (HY-2011-N). This work was also supported bythe Priority Research Centers Program through the National Re-search Foundation of Korea (NRF) funded by the Ministry of Educa-tion, Science and Technology (2012R1A6A1029029).

 Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at  http://dx.doi.org/10.1016/j.jcis.2012.11.036 .

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