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Journal of Molecular Catalysis A: Chemical 393 (2014) 182–190 Contents lists available at ScienceDirect Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata Possible sites of copper located on hydroxyapatite structure and the identification of active sites for formaldehyde oxidation Zhenping Qu , Yahui Sun, Dan Chen, Yi Wang Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China article info Article history: Received 31 January 2014 Received in revised form 4 June 2014 Accepted 6 June 2014 Available online 14 June 2014 Keywords: CuHAP Cu location Active sites HCHO oxidation Preparation method abstract Different methods including ion exchange, co-precipitation and impregnation were used for the prepara- tion of copper doped hydroxyapatite catalysts (CuHAP) to adjust the location of Cu on HAP and resulted in the difference of catalytic performance. Adjustments of synthesis conditions (ion exchange) were adopted for the further identification of active Cu sites for formaldehyde oxidation over CuHAP catalysts. Methods of characterization including XRD, H 2 -TPR, EPR and XPS were used for the identification of Cu species and its environment on the HAP. A hypothesis of five possible sites for Cu location was proposed based on the features of HAP structure and the experimental results. It has also been concluded that the highly- dispersed Cu(II) clusters was mainly responsible for HCHO oxidation. The best activity was achieved over CuHAP (1.4 wt.%) prepared by the ion exchange method with the complete conversion temperature of 180 C. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Formaldehyde (HCHO) is one of the most common volatile organic compounds which can be harmful to the health of human beings even exposed under low concentrations. It emits from build- ing and decorating materials, plastic cements, paintings and so forth. Exposure to formaldehyde can cause irritation, wheezing and coughing, fatigue, skin rash and severe allergic reactions at elevated concentrations. Therefore, the removal of HCHO is an issue of great concern that needs to be urgently solved and catalytic oxidation is a promising technique among all the current techniques under investigation [1,2]. Conventionally, the basic catalytic systems for HCHO oxidation being investigated are noble metal catalysts and transition metal oxides [2–8]. Further modification of the systems, such as mixed transition metal oxides and noble metal doped mixed transition metal oxides are also of great concern because of their potential to improve the catalytic activity [9–11]. Nobel metals mainly under investigation are Pt, Au and Ag. The complete conversion temperatures under 100 C are reported for some of these catalysts [10,12–14]. However, in spite of the rela- tively effective performance that has been achieved, the high price of noble metal may limit the wide application of these materials. Corresponding author. Tel.: +86 411 84708083; fax: +86 411 84708083. E-mail addresses: [email protected], [email protected] (Z. Qu). For the catalytic systems of transition metal oxides, major studied metal oxides are MnO x , CeO 2 and Co 3 O 4 [5,9,11,15–18], the com- plete conversion temperatures are over 100 C in most of the cases, even higher than 200 C under some circumstances. In recently studies, improved catalytic activity has been achieved on transition metal oxides with special structure or the addition of other metals [19–21]. Besides the non-ideal catalytic performance, the toxicity of some transition metal oxides may also hinder the practical appli- cation of these materials [22,23]. Therefore, it is necessary to find new materials that are economical, safe and non-toxic to substitute these traditional materials for HCHO oxidation. Hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 , is the main inorganic com- ponent of natural bone and teeth which means that it is safe and non-toxic. And its flexible and special structure results in the good ion exchange capacity and thermal stability. HAP crystallizes in the hexagonal system (P6 3 /m point group) and has two formula units per unit cell (Fig. 1). Calcium ions at site I(Ca I ) are coordinated by nine oxygen atoms, while calcium ions at site II (Ca II ) are coordi- nated by five oxygen atoms and one hydroxyl group. However, the coordination number of Ca II ion is 7, one of them being a weak bond to another oxygen [24]. The main channel of HAP which plays a sig- nificant role in the exchange property of HAP is oriented along the c-axis and is delimited by oxygen and six Ca II ions. The OH groups hosted by the channels balance the positive charge of the matrix and have great mobility [25–27]. In addition, some Ca 2+ ions (Ca I and Ca II ) exposed on the surface (S) are also easily to be substituted, and these Ca I and Ca II ions on the surface can be marked as Ca s . http://dx.doi.org/10.1016/j.molcata.2014.06.008 1381-1169/© 2014 Elsevier B.V. All rights reserved.

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Journal of Molecular Catalysis A: Chemical 393 (2014) 182–190

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical

journa l homepage: www.e lsev ier .com/ locate /molcata

ossible sites of copper located on hydroxyapatite structure and thedentification of active sites for formaldehyde oxidation

henping Qu ∗, Yahui Sun, Dan Chen, Yi Wangey Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology,alian University of Technology, Linggong Road 2, Dalian 116024, China

r t i c l e i n f o

rticle history:eceived 31 January 2014eceived in revised form 4 June 2014ccepted 6 June 2014vailable online 14 June 2014

a b s t r a c t

Different methods including ion exchange, co-precipitation and impregnation were used for the prepara-tion of copper doped hydroxyapatite catalysts (CuHAP) to adjust the location of Cu on HAP and resulted inthe difference of catalytic performance. Adjustments of synthesis conditions (ion exchange) were adoptedfor the further identification of active Cu sites for formaldehyde oxidation over CuHAP catalysts. Methodsof characterization including XRD, H2-TPR, EPR and XPS were used for the identification of Cu species

eywords:uHAPu locationctive sitesCHO oxidation

and its environment on the HAP. A hypothesis of five possible sites for Cu location was proposed basedon the features of HAP structure and the experimental results. It has also been concluded that the highly-dispersed Cu(II) clusters was mainly responsible for HCHO oxidation. The best activity was achieved overCuHAP (1.4 wt.%) prepared by the ion exchange method with the complete conversion temperature of180 ◦C.

reparation method

. Introduction

Formaldehyde (HCHO) is one of the most common volatilerganic compounds which can be harmful to the health of humaneings even exposed under low concentrations. It emits from build-

ng and decorating materials, plastic cements, paintings and soorth. Exposure to formaldehyde can cause irritation, wheezing andoughing, fatigue, skin rash and severe allergic reactions at elevatedoncentrations. Therefore, the removal of HCHO is an issue of greatoncern that needs to be urgently solved and catalytic oxidations a promising technique among all the current techniques undernvestigation [1,2].

Conventionally, the basic catalytic systems for HCHO oxidationeing investigated are noble metal catalysts and transition metalxides [2–8]. Further modification of the systems, such as mixedransition metal oxides and noble metal doped mixed transition

etal oxides are also of great concern because of their potential tomprove the catalytic activity [9–11].

Nobel metals mainly under investigation are Pt, Au and Ag. Theomplete conversion temperatures under 100 ◦C are reported for

ome of these catalysts [10,12–14]. However, in spite of the rela-ively effective performance that has been achieved, the high pricef noble metal may limit the wide application of these materials.

∗ Corresponding author. Tel.: +86 411 84708083; fax: +86 411 84708083.E-mail addresses: [email protected], [email protected] (Z. Qu).

ttp://dx.doi.org/10.1016/j.molcata.2014.06.008381-1169/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

For the catalytic systems of transition metal oxides, major studiedmetal oxides are MnOx, CeO2 and Co3O4 [5,9,11,15–18], the com-plete conversion temperatures are over 100 ◦C in most of the cases,even higher than 200 ◦C under some circumstances. In recentlystudies, improved catalytic activity has been achieved on transitionmetal oxides with special structure or the addition of other metals[19–21]. Besides the non-ideal catalytic performance, the toxicityof some transition metal oxides may also hinder the practical appli-cation of these materials [22,23]. Therefore, it is necessary to findnew materials that are economical, safe and non-toxic to substitutethese traditional materials for HCHO oxidation.

Hydroxyapatite, Ca10(PO4)6(OH)2, is the main inorganic com-ponent of natural bone and teeth which means that it is safe andnon-toxic. And its flexible and special structure results in the goodion exchange capacity and thermal stability. HAP crystallizes in thehexagonal system (P63/m point group) and has two formula unitsper unit cell (Fig. 1). Calcium ions at site I(CaI) are coordinated bynine oxygen atoms, while calcium ions at site II (CaII) are coordi-nated by five oxygen atoms and one hydroxyl group. However, thecoordination number of CaII ion is 7, one of them being a weak bondto another oxygen [24]. The main channel of HAP which plays a sig-nificant role in the exchange property of HAP is oriented along thec-axis and is delimited by oxygen and six CaII ions. The OH groups

hosted by the channels balance the positive charge of the matrixand have great mobility [25–27]. In addition, some Ca2+ ions (CaIand CaII) exposed on the surface (S) are also easily to be substituted,and these CaI and CaII ions on the surface can be marked as Cas.
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Z. Qu et al. / Journal of Molecular Catalysi

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ig. 1. Crystal structure of HAP (I: CaI II:CaII S:Cas): (a) crystal lattice; (b) projectionn ((a) and (b)) plane.

HAP has long been used as biological active material and cat-lyst carriers with a wide application in the fields of biomedicalaterials and catalysis [28]. Traditionally, as catalytic material,AP is more often applied and studied for reactions such aslaisen–Schmidt condensation, Michael addition, oxidative dehy-rogenation of alkanes, selective oxidation of alcohols and so forth29–33]. In recent years, HAP has been reported as a promising cat-lytic material for the reactions of CO oxidation, selective catalyticeduction of NOx and HCHO oxidation [26,28,34–36]. Xu et al. [36]eported the catalytic potential of HAP for HCHO oxidation, and theomplete conversion was achieved at 250 ◦C. Nevertheless, the cat-lytic performance of pure HAP is still not good enough. In order tochieve the better catalytic activity, other components need to beoped in or loaded on HAP.

Due to the good economical efficiency, as well as the promisingatalytic potential, Cu has been used as the main active componentf catalysts in various studies [37–41], including the applicationor HCHO oxidation [6,37,38]. However, the studies of Cu for HCHOxidation is pretty rare so far, and the catalysts with Cu as the activeomponent do not show satisfactory activity towards HCHO oxida-ion in the reported works. It is still worthy of detailed study to geteep understanding because it is a promising alternative of nobleetals, and further find avenues to improve its performance.In the present work, HAP was chosen as the catalyst support, and

small amount of copper (Cu), instead of noble metals, was dopedn HAP to improve the catalytic activity. Besides the economicalfficiency of Cu, Cu doped HAP is also usually used as antibacterialaterials because of its environmental safety and stability [42].Because of the existence of different Ca2+ sites (CaI, CaII and Cas)

nd the skeleton of HAP, there are various sites that are possible for

u species to locate on. Different locations and species of Cu might

ead to the difference in the material structure and compositionnd further result in the different activity for HCHO oxidation.herefore, different methods including ion exchange, co-

s A: Chemical 393 (2014) 182–190 183

precipitation and impregnation were used for the preparation ofCuHAP to adjust the location of Cu, and the active sites for HCHOoxidation were identified through the adjustment of Cu locationsand the portion of certain species. Various characterization tech-niques, XRD, H2-TPR, XPS and ESR were used to identify Cu speciesand its environment.

2. Experimental methods

2.1. Preparation of catalysts

2.1.1. Ion exchange and impregnation methodsThe first step for both methods was the preparation of the

HAP support. The HAP powder was prepared through an aque-ous precipitation method using (NH4)2HPO4 and Ca(NO3)2·4H2O asprecursors. Ammonia (NH3) solution was used for pH adjustmentduring the precipitation process. A solution of 0.2 M Ca(NO3)2·4H2O(4.72 g in 100 mL deionized water) was stirred and the temperaturewas maintained at 40 ◦C. A solution of 0.3 M (NH4)2HPO4 (1.58 g in40 mL deionized water) was added dropwise to the Ca(NO3)2·4H2Osolution. Then the pH of the suspension was adjusted by ammoniasolution (35%) to 10, followed by 8 h of reaction under stirring. Afterthat, the reaction solution was transferred into a Teflon-lined auto-clave and heated at 100 ◦C for 8 h. Finally, the resultant powderswere washed and dried at 100 ◦C overnight and then calcined at700 ◦C for 2 h.

As for the ion exchange method, 1 g of HAP powderwas introduced into 100 mL 0.01 M copper nitrate (0.242 gCu(NO3)2·3H2O) solution under stirring. The temperature wasmaintained at 40 ◦C during the ion exchange process, which was2 h. Then the suspension was filtered. After being washed and fil-tered for several times, the recovered solid was dried at 100 ◦C for24 h. The catalyst was marked as CuHAP-IE when different meth-ods were being compared or CuHAP-0.01-1-40 to illustrate thesynthesis conditions in detail. Further adjustment of the synthe-sis conditions included the ion exchange temperature (70 ◦C), thesolution concentration (0.005 M) and the exchange times (2 times).The catalysts obtained were marked as CuHAP-0.01-1-70, CuHAP-0.005-2-40, CuHAP-0.01-2-40.

The impregnated sample was prepared by soaking 1 g HAPpowder in 2 mL copper nitrate solution (0.242 g Cu(NO3)2·3H2O)overnight at room temperature. Then the loaded catalyst was driedat 100 ◦C. The catalyst was marked as CuHAP-IM.

2.1.2. Co-precipitation methodThe first step of this method was the preparation of 100 mL solu-

tion containing 0.018 mol Ca(NO3)2·4H2O (4.248 g) and 0.002 molCu(NO3)2·3H2O (0.484 g). The remaining steps were as the sameas the process preparing HAP powder described above. And theobtained sample was marked as CuHAP-CP.

2.2. Characterization techniques

The samples were pre-treated at 400 ◦C for 1 h in the flow ofO2/Ar before all the characterication tests.

2.2.1. XRDThe crystallinity of the catalysts and some Cu species were

established and identified by X-ray diffraction (Rigaku D/max-�bX-ray diffractometer) on operating with a Cu-K� radiation in the10◦ ≤ 2� ≤ 80◦ range at RT.

2.2.2. BETNitrogen adsorption measurements were carried out with a

Quantachrom quadrasorb S1. Before analysis, each sample was

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184 Z. Qu et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 182–190

t meth

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Fig. 2. XRD patterns of CuHAP catalysts synthesized by differen

eated for 4 h at 300 ◦C under vacuum. Surface areas were calcu-ated by the Brunauer–Emmett–Teller (BET) method. The pore size

as calculated using the Barrett–Joyner–Hatenda (BJH) model.

.2.3. H2-TPRThe nature, reducibility and amount of Cu species were iden-

ified and estimated by temperature programmed reduction withydrogen (H2-TPR). The experiments were carried out on a Quan-achrom Automated Chemisorption Analyzer. During the H2-TPRest, the catalyst was exposed to a flow of 30 mL/min H2/Ar (10 vol.%2) mixture. The temperature was heated to 800 ◦C at a ramping

ate of 10 ◦C/min. The H2 consumption was monitored by a thermalonduction detector (TCD).

.2.4. EPRElectron paramagnetic resonance (EPR) was used to get the sup-

ortive information of Cu species and environment. Measurementsere performed at room temperature on a Bruker (A200-9.5/12)

perating at the X band (∼9.8 GHz). The magnetic field was mod-lated at 100 kHz and the g value was determined from preciserequency and magnetic field values.

.2.5. ICP and XPSInductively coupled plasma (ICP) and X-ray photoelectron (XPS)

ere carried out to examine the distribution of Cu species asell as its content. ICP was performed on an OPTIMA 2000, whilePS was measured using an X-ray photoelectron spectrometer

ESCALAB250, Thermo) with a monochromatic X-ray source of Al� (1486.6 eV) under ultra-high vacuum. The binding energiesere calibrated internally by the carbon deposit C1s binding energy

BE) at 284.6 eV. Further information about Cu species on the sur-ace was also shown in the XPS results.

.3. Catalytic activity tests

HCHO oxidation activity tests were carried out in a fixed-bedow reactor under atmospheric pressure. Typically, 0.2 g catalystas loaded in a quartz tube reactor for activity test. The catalyst was

alcined at 400 C for 1 h in the flow of O2/Ar before the reaction. Dur-ng the reaction, gaseous HCHO was generated by flowing He overrioxymethylene (99.5%, Acros Organics) in an incubator kept ince water mixture. The feeding stream consisted of 500 ppm HCHO,

0 vol.% oxygen and balanced He and the total flow rate passinghrough the reactor was maintained at 30 mL/min by a mass-flow

eter. The effluents from the reactor were analyzed by on-line gashromatograph (GC 7890II, Techcomp, China) equipped with FID

ods: (a) full XRD spectra; (b) XRD spectra between 2� = 30–40◦ .

detectors. To determine the exact concentration of produced car-bon dioxide, a nickel catalyst converter was placed before the FIDdetector to convert CO2 quantitatively into methane in the pres-ence of hydrogen. Generally, the reaction data was collected untilreaction balance was reached. No other carbon containing com-pounds except CO2 in the products were detected for all the testedcatalysts. Thus, HCHO conversion was calculated as follows:

HCHOconversion(%) = [CO2]/[CO2] ∗ × 100%

where [CO2]* and [CO2] stand for the concentration of CO2 detectedin the effluent when HCHO is completely oxidized and at a givenreaction temperature, respectively.

3. Results and discussion

3.1. Possible sites of copper and relation of Cu species withactivity

Possible sites of copper on HAP were deduced based on the char-acterization of CuHAP synthesized by different methods and theknown features of HAP structure. Moreover, the relation betweenCu species and catalytic activity was studied by relating the char-acterization results with the activity figures.

3.1.1. Possible sites of copperFig. 2 shows the XRD patterns of CuHAP prepared by differ-

ent methods as well as that of pure HAP. Characteristic peaks ofHAP (PDF Ref. 09-0432) [43] are clearly displayed on patterns of allsamples, indicating that the structure of HAP has been successfullyformed.

However, different changes are observed for samples synthe-sized by different methods. The XRD pattern of CuHAP-IE is inthe best accordance with HAP without any impure peaks. Never-theless, as shown in Fig. 2b, relatively obvious peak shift can beobserved for CuHAP-IE, which implies that Cu species have beendoped into the HAP lattice for the ion exchanged sample. All thesamples show a characteristic peak at ∼35 ◦C. Because both HAPand CuO have characteristic peak at ∼35◦ [43,44], the relative peakintensity ratio of ∼31.5◦/∼35◦ has been calculated for all the sam-ples to identify the peak emerged at ∼35◦. The calculated resultsare shown in Table 1. As can be seen clearly, the ratio for CuHAP-IE(16.91) and HAP (16.75) are almost the same while that for CuHAP-

IM and CuHAP-CP are much smaller, 7.3 and 4.5, respectively. Thistrend can also be seen clearly in Fig. 2b. The results indicate that themuch stronger peak intensity at ∼35◦ for CuHAP-IM and CuHAP-CP is resulted from the peak overlap of HAP and CuO, and the peak
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Z. Qu et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 182–190 185

Table 1Chemical analysis of the samples: Cu content in the whole particle (Xw), Cu content in the surface phase (Xs), textural properties and H2/Cu ratio.

Catalysts Xw (wt.%) ICP Xs (wt.%) XPS Surface area (m2/g) Pore volume (m3/g) Pore size (nm) XRD peakintensity ratioa(∼31.5◦/∼35◦)

H2/Cub

BET

CuHAP-IE 1.4 6.7 27.89 0.2742 39.32 16.91 0.47CuHAP-CP 4.7 3.9 30.39 0.3024 39.81 4.5 0.40CuHAP-IM 4.6 3.3 33.15 0.2776 33.49 7.3 0.43HAP 26.42 0.3004 45.48 16.75

D patlts.

aaptCCCho3odttdt

aptIpfipir

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a The relative peak intensity ratio(∼31.5◦/∼35◦) of the samples obtained from XRb The relative molar ratio of H2/Cu calculated from the TPR patterns and ICP resu

t ∼35◦ for CuHAP-IE is the characteristic peak of HAP. Moreover,nother CuO characteristic peak at 38.64◦ is also observed on theatterns of CuHAP-CP and CuHAP-IM while it is also not shown onhe pattern of CuHAP-IE. The presence of characteristic peaks ofuO at 2� = 35.48◦, 38.64◦ [44] on XRD patterns of CuHAP-IM anduHAP-CP indicates the existence of bulk CuO. And the absence ofuO characteristic peaks on the pattern of CuHAP-IE implies theigh dispersion of Cu species on HAP. In addition, another obvi-us characteristic peaks of Ca19Cu2(PO4)14 at 2� = 13.68◦, 27.89◦,4.50◦ [45] are observed on CuHAP-CP sample and the crystallinityf HAP is decreased. The formation of Ca19Cu2(PO4)14 is possiblyue to the doping of Cu2+ into the sites where Ca2+ should be forhe stabilized normal structure of HAP. Thus, it can be deduced fromhe formation of Ca19Cu2(PO4)14 and the weak crystallinity that theope of Cu by co-precipitation method may have interfered withhe structure of HAP to some extent.

The textual properties of the samples analyzed by nitrogendsorption measurements are shown in Table 1. The specific area,ore volume and pore size do not show much difference among thehree samples. Cu contents in the three samples are determined byCP and the results are also shown in Table 1. During the synthesisrocess, the theriotical doping amount of Cu has been kept the sameor all the three samples. However, according to the results shownn Table 1, the actual contents of Cu show a difference. CuHAP-IEossesses the least amount of Cu(1.4 wt.%) while the Cu contents

n CuHAP-CP and CuHAP-IM are much larger, 4.7 wt.% and 4.6 wt.%,

espectively.

H2-TPR measurements were performed to get the informationf copper locations according to the reducibility of Cu species.he results are shown in Fig. 3. As shown in the graph, no H2

Fig. 3. H2-TPR profiles of CuHAP catalysts synthesized by different methods.

terns.

consumption is observed for HAP. Thus, all the H2 consumptionpeaks are resulted from the reduction of Cu species. Obviously,the total peak area of CuHAP-IE is much smaller, this is becauseof the less Cu content in the sample as shown by ICP. The caculatedH2/Cu ratios for the three samples are shown in Table 1. The ratiosare lower than 1, and the deviation from the theoretical value (1)implies that Cu(II) species are not completely reduced to Cu0 andindicates the presence of Cu(I) species after H2-TPR tests [46].

However, we can still get information about Cu location andspecies according to the reducibility reflected by the H2 consump-tion temperature. The whole test temperature range can be dividedinto two regions: low temperature region (LT, T < 350 ◦C) and hightemperature region (HT, T > 350 ◦C). The LT profiles are assignedto Cu species located on the surface, while the HT profiles areattributed to Cu species located in the channels or doped into thelattice, which make it harder for Cu species to be reduced [46].

As can be seen from the graph, only one peak is observed forCuHAP-IE in LT region at ∼230 ◦C that can be assigned to the reduc-tion of surface highly-dispersed Cu(II) clusters [39,47]. Besides thesimilar peaks at the same temperature (∼230 ◦C), much strongerpeaks at around 300 ◦C are detected for CuHAP-IM and CuHAP-CP samples, implying the formation of bulk CuO on the surface[39,47,48], in accordance with the XRD results.

In the HT region, the broad peak for CuHAP-IM at around 440 ◦Cis attributed to the reduction of CuO particles that are located insidethe channels of HAP [46]. For CuHAP-CP and CuHAP-IE, the higherpeaks over 500 ◦C might be caused by Cu2+ doped into the lattice.Considering the structure of HAP (CaI and CaII) and the process ofthe two synthesis methods, CaII ions in the c-axis oriented channelare more easily to be substituted during the ion exchange process.Therefore, the broad peak at around 550 ◦C should be attributedto Cu2+ located at CaII sites. The peak at 650 ◦C which is observedonly for CuHAP-CP is assigned to Cu2+ ions at CaI sites during theformation of Ca19Cu2(PO4)14 which is only observed for CuHAP-CPaccording to XRD patterns.

The state of Cu species on the surface of all the three samples wasfurther investigated by XPS (Fig. 4). The binding energy of the Cu 2ptransition peaks marked in the graph provides the information of Cuspecies on the surface. Obvious shake-up satellite peaks (CuHAP-IM: 945.5 eV, 965.6 eV; CuHAP-CP: 944.4 eV, 964.5 eV; CuHAP-IE:946.7 eV, 966.8 eV) are observed at higher apparent binding energyfor all the three samples. Since these satellites are not observed forCu+ or Cu0, it is a characteristic feature of all Cu(II) compounds andis the best indicator of the presence of oxidation state [49,50]. Thespectra are resolved and fitted to get detailed further informationof surface Cu species on HAP. The peaks at around 936.4 eV for thethree samples are assigned to dispersed small Cu(II) clusters on thesurface [51], corresponding to the peak at ∼230 ◦C observed in the

TPR profiles for all the three samples. The significant shift to highbinding energy compared to bulk CuO(933.6 eV) indicates the gooddispersion of Cu oxide species [51]. For CuHAP-IM and CuHAP-CPsamples, the peaks at relatively lower binding energy (∼934 eV)
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186 Z. Qu et al. / Journal of Molecular Catalysi

F

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ig. 4. Cu 2p XPS spectra for CuHAP catalysts synthesized by different methods.

eflect the presence of aggregate CuO species, which has also beeneflected by the peaks at around 300 ◦C in the TPR profiles for thewo samples. And the small peak located at higher binding energy∼938 eV) for CuHAP-IE is attributed to surface Cu(II) clusters inifferent coordination environment [52,53]. The binding energyosition of the same Cu species depends on the chemical compo-ition of the sample and particularly on the near environment ofhe Cu2+ cations, and it also relies on the crystalline structure of the

aterials and the content of substituant cation [52,53].The distribution of Cu in CuHAP is also reflected by XPS, asso-

iated with ICP. As shown in Table 1, it is obvious that moreu has been doped in or loaded on HAP for samples synthesizedhrough impregnation or co-precipitation method (4.6% and 4.7%).he amount of Cu in CuHAP-IE sample is much less, with the Xw

f 1.4%. However, according to the calculation of XPS results, theass ratio of Cu in the surface phase of CuHAP-IE is 6.7%, indicat-

ng that Cu is surface enriched on CuHAP-IE, in accordance withhe TPR pattern. The Xs for CuHAP-IM and CuHAP-CP is 3.3% and.9%, less than Xw determined by ICP. However, according to TPResults, large amount of Cu is located on the surface for CuHAP-M and CuHAP-CP. This contradiction of TPR and XPS is becausehat the CuO particle size on the surface of CuHAP-IM and CuHAP-

P(∼28 nm and ∼26 nm, respectively, calculated by XRD) exceedshe XPS penetration depth and results in the less amount of surfacepecies caculated by XPS. Therefore, it can be concluded from ICPnd XPS that highly dispersed Cu(II) clusters are enriched on the

Fig. 5. EPR spectra of CuHAP catalysts synthesized by different method

s A: Chemical 393 (2014) 182–190

surface of CuHAP-IE, while the amount of this kind of Cu species ismuch less for CuHAP-IM and CuHAP-CP in spite of the larger overallamount of Cu.

The EPR spectra of CuHAP-IE, CuHAP-CP and Cu-HAP-IM are dis-played in Fig. 5. All the three samples show axially symmetricalsignals which are generally attribuated to Cu(II) species interac-ting with the support [54]. Four-line hyperfine splitting signal withg// = 2.39, A// = 120 G is obviously observed for CuHAP-IM (Fig. 5b).The signal is ascribed to isolated Cu2+ in octahedral environment[54–56]. Considering the synthesis process and features of CuHAP-IM, the isolated Cu2+ may be formed from the weak substitution ofCaS that happens along with the impregnation. The axial symmet-ric signal of CuHAP-IE becomes broader and its intensity decreasescompared with CuHAP-IM. The broad isotropic signal can probablybe attributed to the dipolar and spin–spin exchange interactionsamong Cu(II) ions of clusters [54]. Together with other character-ization results, this further proved that the main phase of Cu onCuHAP-IE is surface highly dispersed small Cu(II) clusters. The dom-inating strong signal observed for CuHAP-CP may arise from theaggregated entities which might show some ferromagnetic inter-action as suggested by the high spectral intensity [57]. A strongerinteraction between Cu and the structure of HAP existes on CuHAP-CP sample.

Based on the above characterization results, five possible sitesfor Cu locating on HAP during the synthesis processes can be sum-marized as marked in Fig. 6a: Site 1: CaS (dispersed Cu(II) clusters);Site 2: attached on the surface(bulk CuO); Site 3: CaII; Site 4: CaI(Ca19Cu2(PO4)14); Site 5: in the c-axis oriented channel(CuO par-ticles). And Cu locations and species on the three CuHAP samplessynthesized by different methods are shown in Fig. 6b.

The main Cu species for CuHAP-IE samples is the dispersed Cu(II)clusters, which is resulted from the subsititution of CaS. The highdispersion of surface Cu(II) clusters is probably due to the high dis-persion of CaS on the original HAP structure. Besides, there is alsosmall amount of Cu2+ doped into the lattice by substituting CaII.

For the CuHAP-CP sample, besides the relatively small amountof dispersed Cu(II) clusters, more amount of Cu species on the sur-face is in the form of bulk CuO, In addition, a specific Cu species,Ca19Cu2(PO4)14, is observed on CuHAP-CP. Considering the synthe-sis process of co-precipitation, the formation of HAP and doping ofCu happen at the same time. It is reasonable to assume that Cu2+

have occupied some of the sites where Ca2+ ions are supposed tobe, leading to the formation of Ca19Cu2(PO4)14.

For CuHAP-IM sample, the dominant Cu species on the surfaceof CuHAP-IM is the bulk CuO. A small amount of dispersed Cu(II)species also present on the surface of CuHAP-IM. Moreover, someCuO particles are located in the channels. The large amount of bulk

s: (a) full EPR spectra; (b) EPR spectra between 2000 and 3600 G.

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Z. Qu et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 182–190 187

Fig. 6. Cu locations on HAP structure: (a) a summary of possible sites of Cu species on HAP. (b) Locations of Cu on three CuHAP samples.

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188 Z. Qu et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 182–190

F

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Fig. 8. HCHO conversions over different CuHAP-IE catalys.

TC

ig. 7. HCHO conversions over CuHAP catalysts synthesized by different methods.

uO and channel CuO particles are resulted from the decompositionf Cu(NO3)2 attached on the surface or in the channel of HAP.

.1.2. Relationship of Cu species with the activity for HCHOxidation

The activity figures of the catalysts for HCHO oxidation arehown in Fig. 7. The complete oxidation of HCHO is achievedor all the three catalysts at 180 ◦C compared with the completeonversion temperature at 240 ◦C for HAP. The doping of Cu has sig-ificantly improved the catalytic performance of HAP. It is obvioushat CuHAP-IE shows the best activity during the whole detectedemperature range. A conversion of 86% is achieved at 150 ◦Cver CuHAP-IE, while the conversion over CuHAP-CP and Cu-HAP-M at this temperature point is only 63% and 33%, respectively.ssociating the activity test results with Cu locations and speciesiscussed above, it can be deduced that the better activity achievedver CuHAP-IE might be due to the surface enriched highly dis-ersed small Cu(II) clusters. While the amount of this Cu speciesecreases for CuHAP-CP and CuHAP-IM, the activity of the catalystsecreases along with it.

Several recycle activity tests are performed for the catalysts, andhe results turn out to be quite close, which indicates the goodeproducibility and stability of the catalysts for HCHO oxidation.oreover, it has been found that certain amount of water could

mprove the catalytic activity. However, the relationship betweenater ratio and the activity still needs more further experiements

nd will be investigated in the near future. And in order to study theffects of Cu species, all the activity tests are performed in water-ree system to emilinate H2O influence. Also, considering the factorshat affect the activity of CuHAP may be complex due to the differ-nt preparation methods, it is necessary to take one further step forhe identification of active copper sites for HCHO oxidation.

.2. Identification of active copper site for HCHO oxidation

It has been already known that the ion exchange conditions,uch as temperature, exchange times and concentration, all haveffects on the ion exchange process. Thus, different conditions weredopted to adjust the location of Cu.

able 2u content in the whole particle of CuHAP-IE catalysts determined by ICP.

Catalysts CuHAP-0.01-1–40 CuHAP-0.01-1–70Xw (wt.%) 1.42 7.49

Fig. 9. H2-TPR profiles of different CuHAP-IE catalysts.

As shown in Table 2, the Cu content in CuHAP-IE increaseswith higher exchange temperature of 70 ◦C (7.49%), more exchangetimes of 2 (2.65%) and lower concentration of 0.005 M (3.75%).Interestingly, the performance of these catalysts for HCHO oxida-tion do not get better along with the increase of Cu content. (Fig. 8)Therefore, the activity of CuHAP catalysts does not relate directlyto the total amount of Cu. CuHAP-0.01-1-40 (CuHAP-IE) still showsthe best activity with the complete conversion temperature at180 ◦C. The conversion at 180 ◦C decreases to 92%, 76% and 73%as the Cu content increases for CuHAP-0.01-2-40, CuHAP-0.005-2-40 and CuHAP-0.01-1-70. This phenomenon must be caused by thedecrease of active sites as a result of the adjustment of ion exchangeconditions.

H2-TPR was performed to obtain the information of Cu loca-

tion and species on different catalysts. The results are shownin Fig. 9. It is a clear trend that the H2 consumption peak at200–250 ◦C assigned as the reduction of the dispersed small Cu(II)clusters on the surface decreases in the same sequence as the

CuHAP-0.01-2–40 CuHAP-0.005-2–402.65 3.57

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Z. Qu et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 182–190 189

a; (b)

dfss[o2opitXpwsbp

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Fig. 10. XRD patterns of different CuHAP-IE catalysts: (a) full XRD spectr

ecrease of activity. A new peak at 350–450 ◦C has emergedor CuHAP-0.01-2-40, CuHAP-0.005-2-40 and CuHAP-0.01-1-70amples. It is assigned to Cu2(OH)PO4 formed through the dis-olution/precipitation mechanism on the surface of HAP crystal48]. The presence of Cu2(OH)PO4 is further confirmed by the obvi-us characteristic peaks observed at 2� = 15.292◦, 18.398◦, 18.685◦,3.919◦, 30.764◦ on the XRD patterns (Fig. 10b) [58]. The increasef ion exchange temperature to 70 ◦C might have accelerated therocess of dissolution/precipitation on the surface, which resulted

n the more amount and badly disperse of Cu2(OH)PO4 reflected byhe much more apparent characteristic peaks of Cu2(OH)PO4 in theRD pattern (Fig. 10a). The broad peak at around 550 ◦C which wasreviouslyattributed to Cu located at CaII has also become strongerith the adjustment of synthesis conditions. No change of the HAP

tructure after the substitute of Ca2+ at CaII by Cu is again provedy the agreement with HAP with good crystallinity but discernedeak shifts (Fig. 10a and c) [43].

Both the increase of ion exchange temperature and decreasef solution concentration facilitate the substitution of CaII ionsy Cu. And increasing the times of ion exchange also results inore Cu located in CaII sites. Moreover, the process of dissolu-

ion/precipitation can be accelerated under higher temperature andith more exchange times. These processes leading to more Cu

ocated at CaII and Cu2(OH)PO4 on the surface can interfere withhe formation of dispersed small Cu(II) clusters. Thus it is sug-ested that the decrease of the activity of the catalysts is due to

he decrease of the amount of small Cu(II) clusters on the surface,hich further confirm that the active sites of Cu located on HAP

or HCHO oxidation is the dispersed small Cu(II) clusters formed onhe surface.

XRD spectra between 2� = 10–32◦; (c) XRD spectra between 2� = 25–35◦ .

4. Conclusions

Based on the discussion of the characterization results and thealready known structure of HAP, five possible sites for Cu locatedon the HAP structure were proposed (Fig. 6): (1) substitution ofCas (small Cu(II) clusters); (2) on the surface of HAP (CuO orCu2(OH)PO4); (3) substitution of CaII; (4) substitution of CaI duringthe formation (Ca19Cu2(PO4)14); (5) in the c-axis oiented channel(CuO). And the Cu location and species on HAP can be adjusted bydifferent preparation methods.

The doping of Cu significantly improved the catalytic per-formance of HAP, and CuHAP-IE (CuHAP-0.01-1-40) (1.4 wt.%),which was surface enriched with small Cu(II) clusters, showed thebest activity among all the catalysts detected with the completeconversion temperature at 180 ◦C. Associating the characterizationdiscussion with the activity results, a unified view of the active sitesfor HCHO oxidation over CuHAP was derived: the dispersed smallCu(II) clusters formed from the substitution of Ca2+ on the surface(Cas) mainly catalyzed the oxidation of HCHO.

It is reasonable to suggest that the avenue for further activityimprovement of CuHAP for HCHO oxidation should be to increasethe amount of surface dispersed small Cu(II) clusters, which can beachieved by the modification of the surface structure of HAP and theregulation of Cu location through adjustment of synthesis methodsand conditions.

Acknowledgements

We gratefully acknowledge the financial support of theNational Natural Science Foundation of China (no. 21377016), the

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undamental Research Funds for the Central UniversitiesDUT13LK27) and Program for Changjiang Scholars and Innovativeesearch Team in University (IRT 13R05).

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