不同有機(jī)分子改性羥基磷灰石用于甲醛催化氧化

1、available at www.scie nce d journal hom e page: HYPERLINK /locate/chnjc www. /locate/chnjc Chinese Journal of Catalysis 35 (2014) 19271936催化學(xué)報(bào) 2014年 第35卷 第12期 | HYPERLINK / ArticleFormaldehyde catalytic oxidation over hydroxyapatite modified with various organic moleculesYahui Sun a, Zhenping Qu a,*

2、, Dan Chen a, Hui Wang a, Fan Zhang b, Qiang Fu ba Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian116024, Liaoning, Chinab State Key Laboratory of Catalysis, Dalian Institute of Chemical

3、 Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, ChinaA R T I C L E I N F OA B S T R A C T Article history:Received 13 April 2014Accepted 28 April 2014Published 20 December 2014Keywords:Modified hydroxyapatite Sodium citrateSpecific surface area Hydroxyl groupFormaldehyde catalytic ox

4、idationHydroxyapatite (HAP) was modified by adding various organic molecules, such as cetyltrime thylammonium bromide, sodium dodecyl sulfate, and sodium citrate, during the precipitation of HAP. Sodium citratemodified HAP displayed the best activity for formaldehyde oxidation, achieving complete co

5、nversion at 240 C. The influence of the organic modifiers on the structure of HAP was assessed by Xray diffraction, Fourier transform infrared spectroscopy, N2 adsorptiondesorption, scanning electron microscopy, and thermogravimetry/derivative thermogravimetry. The higher specific surface area and p

6、ore volume, and smaller pores, owing to modification with sodium citrate, favored adsorption, mass transfer, and interaction process during formaldehyde oxidation. Fur thermore, the higher hydroxyl group content observed in sodium citratemodified HAP enhanced interactions between formaldehyde and HA

7、P, thus resulting in higher catalytic activity. 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.Published by Elsevier B.V. All rights reserved.IntroductionAs one of the most common volatile organic compounds, formaldehyde (HCHO) is generating increasing attention as it poses

8、potential health risks to humans even at low concentra tions. Thus, the removal of HCHO has become an important issue 1,2. Numerous studies have been carried out for the abatement of HCHO; the main techniques being investigated are adsorption, plasma decomposition, biological/botanical filtration, a

9、nd catalytic oxidation 1. Among all these tech niques, catalytic oxidation is a promising method for HCHO removal because of its efficiency, convenience, and no second ary pollution. Commonly studied catalysts include noble metals (e.g., Pt, Au, Pd, and Ag) 36 and transition metal oxide cata lysts (

10、e.g., MnOx and CeO2) 79. Moreover, transition metaloxides are usually employed as substrate for loading noble metal catalysts 1013. Noble metalloaded catalysts show relatively better activities towards HCHO oxidation (complete conversion is generally achieved at around 100 C or below) 5,14. However,

11、 the high cost of noble metal limits the wide practical application of noble metal catalysts. For transition metal oxides, complete HCHO conversion temperatures are generally above 100 C, and even above 200 C under some circumstances 7,9,13. Besides poor performance, the toxicity of some commonly us

12、ed transition metal oxides (MnOx) limits the application of such catalyst systems 15,16. In recent years, many studies on HCHO catalytic oxidation have been conducted to improve the catalytic performance. However, the studied catalytic systems are still focused on noble metal catalysts and transitio

13、n metal oxides such as MnOx, Ag/CeO2, Co3O4, and* Corresponding author. Tel: +8615542663636 Fax: +8641184708083; Email: HYPERLINK mailto:quzhenping quzhenpingThis work was supported by the National Natural Science Foundation of China (21377016), the Fundamental Research Funds for the Central Univer

14、sities (DUT13LK27), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05).DOI: 10.1016/S18722067(14)601297 | HYPERLINK /science/journal/18722067 /science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 12, December 20141928Yahui Sun et al. / Chinese Journal

15、of Catalysis 35 (2014) 19271936Pt/CeO2 1722. The drawbacks of such catalytic systems are yet to be addressed. Thus, economical, safe, and nontoxic novel materials are required for HCHO catalytic oxidation.As the main inorganic component of natural bone and teeth, with a wide application in the field

16、 of biomedical materials as biological active materials, hydroxyapatite (HAP) is safe and nontoxic 23. Moreover, unlike noble metals, HAP is a cheaper alternative. In 2010, Xu et al. 24 reported HAP as a promising novel, nonprecious metal catalyst for HCHO combustion, whereby the hydroxyl groups bon

17、ded to the Ca2+ channels may act as active sites.To date, the examination of HAP as a catalytic material for HCHO oxidation remains rare. However, as a novel, nonpre cious metal catalyst with demonstrated activity towards HCHO oxidation, HAP is worth exploring further. It is well known that the cata

18、lyst performance is closely related to its structure. Based on the reported study 1, HCHO adsorption and its in teraction with the support are related to the HCHO oxidation process. In that case, larger adsorption areas and abundant interaction between the reaction gas and catalyst will be im portan

19、t for activity enhancement. Organic modifiers have been widely used in morphology and sizecontrolled synthesis of nanosized metals and inorganic materials, as well as for the generation of pores and vacancies in the structure 2530. To this effect, in this study, organic modifiers (i.e., cetyltri met

20、hylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and sodium citrate (SC) were employed during the synthesis of HAP to modify its structure. Various characteri zation techniques and subsequent activity tests were condu cted to elucidate the structureperformance relationship.ExperimentalPrepa

21、ration of catalystsHAP powder was prepared through an aqueous precipita tionhydrothermal method using (NH4)2HPO4 (AR, Kemiou Chemical Reagent Co., Ltd, Tianjin) and Ca(NO3)24H2O (AR, Damao Chemical Reagent Factory, Tianjin) as precursors. Am monia (NH3) solution (AR, Sinopharm Chemical Reagent Co.,

22、Ltd, Shanghai) was used for pH adjustments during the precip itation process. A solution of 0.2 mol/L Ca(NO3)24H2O (4.72 g in 100 mL deionized water) containing an organic modifier (5 wt%, CTAB (AR, Sinopharm Chemical Reagent Co., Ltd, Shang hai), SDS (AR, Kemiou Chemical Reagent Co., Ltd, Tianjin),

23、 or SC (AR, Reagent No. 1 Factory of Shanghai Chemical Reagent Co., Ltd., Shanghai) was stirred under a constant temperature of 40 C. A solution of 0.3 mol/L (NH4)2HPO4 (1.58 g in 40 mL deionized water) was added dropwise to the Ca(NO3)24H2O solution. Then, the pH of the suspension was adjusted to 1

24、0 with ammonia solution (35%), followed by 8 h of reaction un der stirring. Subsequently, the reaction solution was trans ferred to a Teflonlined autoclave and heated at 100 C for 12 h. Finally, the resulting powders were centrifuged and washed multiple times and dried at 100 C overnight and then ca

25、lcined at 700 C for 2 h. The obtained HAP samples were denoted as HAPCTAB, HAPSDS, and HAPSC. Nonmodified HAP was also synthesized using the same procedure for comparison studies and denoted as HAPBLANK.Characterization of catalystsThe crystallinity of the catalysts was established and identi fied b

26、y Xray diffraction (XRD, Rigaku D/maxb Xray diffrac tometer, Japan) using Cu K radiation in the 2 range of 1080 at room temperature. Fourier transform infrared spectroscopy (FTIR) was carried out on specimens that were prepared into pellets containing the HAP samples and KBr. FTIR spectra were recor

27、ded on a Shimadzu IRPrestige21 spectrophotometer (Japan) in the range of 4000400 cm1. A background spectrum of KBr was subtracted from each sample spectrum. Scanning electron microscopy (SEM) analysis was performed on a JEOL JSM6360 scanning electron microscope (USA) operating at an acceleration vol

28、tage of 2030 kV. N2 ad sorptiondesorption measurements were carried out on a Quantachrome Quadrasorb S1 (USA). Prior to analysis, each sample was heated at 200 C for 4 h under vacuum. Surface areas were calculated using the BET method. The pore size was calculated using the BJH model. Thermogravimet

29、ry/derivative thermogravimetry analysis (TG/DTG) was performed on a WCT1C thermobalance (Beijing) in thetemperature range of 20900 C.Catalytic activity testsHCHO oxidation activity tests were carried out in a fixedbed flow reactor under atmospheric pressure. Typically,0.2 g catalyst was loaded in a

30、quartz tube reactor for activity test. The catalyst was calcined at 400 C for 1 h in an O2/Ar flow before the reaction. During the reaction, gaseous HCHO was generated by flowing He over trioxymethylene (99.5%, Acros Organics) in an incubator placed in an icewater mixture. The feeding stream consist

31、ed of 500 ppm HCHO, 20 vol% O2, and balanced He; the total flow rate throughout the reactor was maintained at 30 mL/min by a mass flow meter. The effluents from the reactor were analyzed by an online gas chromato graph (GC 7890II, Techcomp, China) equipped with a flame ionization detector (FID). To

32、determine the exact concentration of produced CO2, a nickel catalyst converter was positioned in front of the FID to convert CO2 quantitatively into methane in the presence of hydrogen. Generally, the reaction data were collected until reaction balance was reached. No other car boncontaining compoun

33、ds except for CO2 in the products were detected for all the tested catalysts. Thus, the HCHO conversion was calculated as: HCHO conversion = CO2/CO2* 100%, where CO2* and CO2 represent respective concentrations of CO2 detected in the effluent when HCHO is completely oxidized and at a given reaction

34、temperature, respectively.Results and discussionCatalytic activity for HCHO oxidationThe catalytic activities of HAPBLANK and modified HAP samYahui Sun et al. / Chinese Journal of Catalysis 35 (2014) 192719361929HAPBLANK HAPCTAB HAPSDS HAPSCHAPSC-usedHAPBLANKPO43OHHAPCTABHAPSDS HAPSCHAPSC-used10080C

35、onversion (%)Transmittance6040200160180200220240260280300Temperature (oC)Fig. 1. HCHO conversion over HAPBLANK and modified HAP samples.400036003200 1200800400Wavenumber (cm1)Fig. 3. FTIR spectra of HAPBLANK and modified HAP samples.ples towards the oxidation of HCHO are shown in Fig. 1. As noted, t

36、he HAPSC sample displayed the best activity with 100% HCHO conversion at 240 C. In contrast, the remaining three samples exhibited HCHO conversion levels of less than 50% at the same temperature. Moreover, the addition of either CTAB or SDS during sample preparation resulted in catalysts with lower

37、activities when compared with that of the blank sample. Complete conversion was not observed even at 300 C for the HAPCTAB and HAPSDS samples. Further activity tests were per formed on the used HAPSC catalyst to determine the stability of the sample. The results are also shown in Fig. 1. As observed

38、, the activity of HAPSCused only decreased very slightly when compared with that of HAPSCfresh, indicating the good stability of the catalyst.Catalyst characterizationXRD analysisIntensityXRD patterns of the prepared HAP samples are shown in Fig. 2. Characteristic peaks of HAP (PDF No. 090432) 31 we

39、re clearly observed in all patterns, indicating successful formation of the HAP structure. Besides the good agreement with the standard HAP pattern, no impurities or distinct dif ferences were observed in the XRD patterns, thereby implyingHAPBLANK HAPCTAB HAPSDS HAPSC20253035404550556065702/(o)Fig.

40、2. XRD patterns of HAPBLANK and modified HAP samples.that the addition of different organic molecules does not insti gate any differences in the crystallinity for samples calcined at the same temperature. Moreover, the good agreement of the XRD patterns of the samples (calcined at 700 C) with charac

41、 teristic HAP peaks reflects the good thermal stability of the catalysts.FTIR spectroscopyFigure 3 presents the FTIR spectra of the HAP samples. No organic modifier remained on the surface of HAP after calcina tion at 700 C. The presence of PO4 and OH functional groups was confirmed by various chara

42、cteristic bands in the FTIR spectra. The peaks at around 1035 and 1091 cm1 were as signed to asymmetrical stretching modes of PO4 groups, the asymmetrical bending modes of which were detected at around 565 and 602 cm1 32. The symmetrical stretching vibrations of PO4 groups were reflected by bands at

43、 around 470 and 962 cm1 32. The peaks observed at 631 and 3580 cm1 were at tributed to the bending and stretching modes of OH groups in the hydroxyapatite structure, respectively 33. Although no residual modifiers were detected in the obtained samples, the peak at 3580 cm1 that corresponds to OH sho

44、ws distinct dif ferent intensities among the samples. Because the intensity of the peaks is influenced by the amount of samples tested, the OH content of the samples cannot be directly determined by the intensity of the OH characteristic peaks. Therefore, the relative peak intensity of the OH groups

45、 to the PO4 groups was used to estimate the OH content in each sample. The calculated areas of the peak at 3580 cm1 that was assigned to the stretching mode of OH groups and the peak at 962 cm1 that was ascribed to the stretching mode of PO4 groups are shown in Table 1; OH/PO4 ratios were also calcu

46、lated based on the calculated areas. HAPSC featured higher OH/PO4 ratios, indicating that higher amounts of OH groups were formed on the surface of HAP after modifi cation with SC. It has been proposed that OH groups play an important role in the adsorption/activation of HCHO 24. Therefore, the high

47、 content of OH groups in HAPSC was respon sible for its significantly improved activity.The FTIR spectrum of HAPSCused is also shown in Fig. 3. As observed, the shape of the spectrum of HAPSCused was not1930Yahui Sun et al. / Chinese Journal of Catalysis 35 (2014) 19271936Table 1SampleOH peak area(3

48、580 cm1)PO4 peak area(962 cm1)OH/PO4HAPBLANK 4661982.35HAPCTAB 6172732.26HAPSDS 5802152.69HAPSC 8282203.76HAPSCused 6371753.64OH and PO4 peak areas of different samples detected by FTIR and asso ciated relative OH/PO4 ratios.700HAPSC(a) HAPBLANKHAPCTAB HAPSDS600Volume (mL/g)500400300distinctively di

49、fferent from that of fresh HAPSC, further indicat ing the good stability of the sample. Moreover, the intensity of the OH groups did not change considerably. For better com parison, the same calculation was adopted to evaluate the OH content in the two samples (Table 1). Based on the calculated resu

50、lts, the OH/PO4 ratio only decreased slightly from 3.76 to3.64. Therefore, the content of the OH groups can be consid ered to be stable before and after the reaction, thereby ex plaining the similarity in the activities of fresh HAPSC and used HAPSC.3.2.3. SEM and BET analysesThe SEM images of the f

51、our samples (Fig. 4) display the fea tures and structures of the HAP samples. HAPBLANK and HAPSC possessed a uniform sheet stacking structure with a fluffy sur face. The addition of sodium citrate during the synthesis pro cess did not change the integrity or uniformity of HAP struc ture. In contrast

52、, the addition of CTAB (HAPCTAB) and SDS (HAPSDS) destroyed the uniformity of the samples and featured a relatively compact surface.Differences in the structure can be evaluated from the N2 adsorptiondesorption studies. As shown in Fig. 5(a), all HAP samples featured a type IV adsorptiondesorption i

53、sotherm with an H3 hysteresis loop according to the IUPAC classifica tion 29,32. The H3 hysteresis loop indicates the presence of a20010000.00.81.0(b)HAPSCHAPCTABHAPBLANK HAPSDSp/p00.010dV/dD (cm3 g nm )10.00810.0060.0040.0020.000020406080100120D/nmFig. 5. N2 adsorptiondesorption isotherms

54、(a) and pore size distribu tion (b) of HAPBLANK and modified HAP samples.sheet stacking structure in the samples, which is in accordance with the SEM analysis. In comparison with the isotherms of the three other samples, HAPSC featured a different desorption branch at the higher p/p0 values that is

55、indicative of the pres ence of relatively uniform and small pores, owing to the exist ing resistance during the desorption process. This trend can be observed in Fig. 5(b) that shows the pore size distribution of all four HAP samples; accordingly, the pore size distribution of HAPSC shifts to the lo

56、wer pore size region.The textural properties, including the specific surface area, pore volume, and average pore diameter, of the studied sam ples are listed in Table 2. The specific surface areas of the four samples were in the range of 2640 m2/g. The average pore size was around 40 nm. As observed

57、, the sample modified with SC, which presented the best catalytic activity, possessed the highest specific surface area (40.36 m2/g) and pore volume (0.3692 m3/g), and the smallest average pore diameter (36.58 nm). In contrast, the SDSmodified HAP sample showed oppoHAPBLANKHAPSCHAPCTABHAPSDSTable 2T

58、extural properties of samples analyzed by N2 adsorptiondesorption method.SampleBET surface area (m2/g)Pore volume (m3/g)Average diameter (nm)Fig. 4. SEM images of HAPBLANK and modified HAP samples.HAPBLANK26.420.300445.48HAPCTAB27.520.304944.32HAPSDS23.130.280148.44HAPSC40.360.369236.58 Yahui Sun et

59、 al. / Chinese Journal of Catalysis 35 (2014) 192719361931site trends. The higher specific surface area and pore channel of HAPSC provide additional sites for HCHO adsorption, whereas the smaller pores contribute to longer retention times and fa vor reactions between HCHO and oxygen on HAP.3.3. Effe

60、ct of organic modifiers and structureperformance relationshipBased on the characterization results discussed above, it can be seen that the addition of organic modifiers did not cause considerable differences in the crystallinity of the resulting samples, and no organic modifiers were retained in th