1、Chinese Journal of Catalysis 35 (2014) 19271936 催 化 学 报 2014年 第35 卷 第12 期 | ArticleFormaldehyde catalytic oxidation over hydroxyapatite modified with various organic moleculesYahui Sun a, Zhenping Qu a,*, Dan Chen a, Hui Wang a, Fan Zhang b, Qiang Fu ba Key Laboratory of Industrial Ecology and Envir
2、onmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian116024, Liaoning, Chinab State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, ChinaA R T I C L E I N F O A B S
3、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 oxidationHydroxyapatite (HAP) was modified by adding various organic molecules, such as cetyltrime
4、 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 conversion at 240 C. The influence of the organic modifiers on the structure of HAP was assessed b
5、y Xray diffraction, Fourier transform infrared spectroscopy, N2 adsorptiondesorption, scanning electron microscopy, and thermogravimetry/derivative thermogravimetry. The higher specific surface area and pore volume, and smaller pores, owing to modification with sodium citrate, favored adsorption, ma
6、ss transfer, and interaction process during formaldehyde oxidation. Fur thermore, the higher hydroxyl group content observed in sodium citratemodified HAP enhanced interactions between formaldehyde and HAP, thus resulting in higher catalytic activity. 2014, Dalian Institute of Chemical Physics, Chin
7、ese Academy of Sciences.Published by Elsevier B.V. All rights reserved.1. IntroductionAs one of the most common volatile organic compounds, formaldehyde (HCHO) is generating increasing attention as it poses potential health risks to humans even at low concentra tions. Thus, the removal of HCHO has b
8、ecome 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, and catalytic oxidation 1. Among all these tech niques, catalytic oxidation is a promising met
9、hod 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 (e.g., MnOx and CeO2) 79. Moreover, transition metaloxides are usually employed as substrate f
10、or 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, the high cost of noble metal limits the wide practical application of noble metal catalysts.
11、 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 used transition metal oxides (MnOx) limits the application of such catalyst systems 15,16. In r
12、ecent 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 transition metal oxides such as MnOx, Ag/CeO2, Co3O4, and* Corresponding author. Tel: +8615542663636 F
13、ax: +8641184708083; Email: This work was supported by the National Natural Science Foundation of China (21377016), the Fundamental Research Funds for the Central Univer sities (DUT13LK27), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05).DOI: 10.1016/S18
14、722067(14)601297 | http:/ | Chin. J. Catal., Vol. 35, No. 12, December 2014available at www.scie nce d journal hom e page: www. /locate/chnjc 1928 Yahui Sun et al. / Chinese Journal of Catalysis 35 (2014) 19271936Pt/CeO2 1722. The drawbacks of such catalytic systems are yet to be addressed. Thus,
15、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 of biomedical materials as biological active materials, hydroxyapatite (HAP) is safe and nontoxic 23. Moreover, unl
16、ike 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 bonded to the Ca2+ channels may act as active sites.To date, the examination of HAP as a catalytic material for HCHO ox
17、idation 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 catalyst performance is closely related to its structure. Based on the reported study 1, HCHO adsorption and its in tera
18、ction 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 portant for activity enhancement. Organic modifiers have been widely used in morphology and sizecontrolled synthesis of na
19、nosized 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 methylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and sodium citrate (SC) were employed during the synthesis
20、 of HAP to modify its structure. Various characteri zation techniques and subsequent activity tests were condu cted to elucidate the structureperformance relationship.2. Experimental2.1. Preparation of catalystsHAP powder was prepared through an aqueous precipita tionhydrothermal method using (NH4)2
21、HPO4 (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., Ltd, Shanghai) was used for pH adjustments during the precip itation process. A solution of 0.2 mol/L Ca(NO3
22、)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), or SC (AR, Reagent No. 1 Factory of Shanghai Chemical Reagent Co., Ltd., Shanghai) was stirred under a cons
23、tant 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 10 with ammonia solution (35%), followed by 8 h of reaction un der stirring. Subsequently, the reaction solut
24、ion 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 calcined at 700 C for 2 h. The obtained HAP samples were denoted as HAPCTAB, HAPSDS, and HAPSC. Nonmodified HA
25、P was also synthesized using the same procedure for comparison studies and denoted as HAPBLANK.2.2. Characterization of catalystsThe crystallinity of the catalysts was established and identi fied by Xray diffraction (XRD, Rigaku D/maxb Xray diffrac tometer, Japan) using Cu K radiation in the 2 range
26、 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 recorded on a Shimadzu IRPrestige21 spectrophotometer (Japan) in the range of 4000400 cm1. A background spec
27、trum 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 voltage of 2030 kV. N2 ad sorptiondesorption measurements were carried out on a Quantachrome Quadrasorb S1
28、 (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. Thermogravimetry/derivative thermogravimetry analysis (TG/DTG) was performed on a WCT1C thermobalance (Beijing) in th
29、etemperature range of 20900 C.2.3. 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 quartz tube reactor for activity test. The catalyst was calcined at 400 C for 1 h in an O2/Ar flow
30、 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 consisted of 500 ppm HCHO, 20 vol% O2, and balanced He; the total flow rate throughout the reactor was ma
31、intained 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 determine the exact concentration of produced CO2, a nickel catalyst converter was positioned in f
32、ront 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 compounds except for CO2 in the products were detected for all the tested catalysts. Thus, the HCHO conve
33、rsion 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 temperature, respectively.3. Results and discussion3.1. Catalytic activity for HCHO oxidationThe c
34、atalytic activities of HAPBLANK and modified HAP samHAPBLANK PO43OHHAPCTABHAPSDS HAPSCHAPSC-usedYahui Sun et al. / Chinese Journal of Catalysis 35 (2014) 19271936 1929100806040200160 180 200 220 240 260 280 300Temperature (oC)Fig. 1. HCHO conversion over HAPBLANK and modified HAP samples.4000 3600 3
35、200 1200 800 400Wavenumber (cm1)Fig. 3. FTIR spectra of HAPBLANK and modified HAP samples.ples towards the oxidation of HCHO are shown in Fig. 1. As noted, the HAPSC sample displayed the best activity with 100% HCHO conversion at 240 C. In contrast, the remaining three samples exhibited HCHO convers
36、ion 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 activities when compared with that of the blank sample. Complete conversion was not observed even at 300 C for the HAPCTAB and HAPSDS samples.
37、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, the activity of HAPSCused only decreased very slightly when compared with that of HAPSCfresh, indicating the good stability of the catalyst.3
38、.2. Catalyst characterization3.2.1. XRD analysisXRD patterns of the prepared HAP samples are shown in Fig. 2. Characteristic peaks of HAP (PDF No. 090432) 31 were clearly observed in all patterns, indicating successful formation of the HAP structure. Besides the good agreement with the standard HAP
39、pattern, no impurities or distinct dif ferences were observed in the XRD patterns, thereby implyingHAPBLANK HAPCTAB HAPSDS HAPSC20 25 30 35 40 45 50 55 60 65 702/(o)Fig. 2. XRD patterns of HAPBLANK and modified HAP samples.that the addition of different organic molecules does not insti gate any diff
40、erences 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 teristic HAP peaks reflects the good thermal stability of the catalysts.3.2.2. FTIR spectroscopyFigure 3 presents the FTIR spectr
41、a 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 characteristic bands in the FTIR spectra. The peaks at around 1035 and 1091 cm1 were as signed to asymmetrical stretching modes
42、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 around 470 and 962 cm1 32. The peaks observed at 631 and 3580 cm1 were at tributed to the bending and stretching modes of
43、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 shows distinct dif ferent intensities among the samples. Because the intensity of the peaks is influenced by the amount of sam
44、ples 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 to the PO4 groups was used to estimate the OH content in each sample. The calculated areas of the peak at 3580 cm1 that wa
45、s 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 calculated based on the calculated areas. HAPSC featured higher OH/PO4 ratios, indicating that higher amounts of OH groups were
46、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 content of OH groups in HAPSC was respon sible for its significantly improved activity.The FTIR spectrum of HAPSCused is a
47、lso shown in Fig. 3. As observed, the shape of the spectrum of HAPSCused was notHAPBLANK HAPCTAB HAPSDS HAPSCHAPSC-usedConversion(%)IntensityTransmittance1930 Yahui Sun et al. / Chinese Journal of Catalysis 35 (2014) 19271936Table 1OH and PO4 peak areas of different samples detected by FTIR and asso
48、 ciated relative OH/PO4 ratios.700600500400300distinctively different 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 cont
49、ent in the two samples (Table 1). Based on the calculated results, 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
