NaOH-Modified Ceramic Honeycomb with Enhanced Formaldehyde

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NaOH-modified Ceramic Honeycomb with Enhanced Formaldehyde adsorption and removal Performance Jiaguo Yu, Xinyang Li, zhihua xu, and Wei Xiao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es4019892 • Publication Date (Web): 29 Jul 2013 Downloaded from http://pubs.acs.org on August 5, 2013

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Environmental Science & Technology

NaOH-modified Ceramic Honeycomb with Enhanced Formaldehyde adsorption and removal Performance

Jiaguo Yu*a, Xinyang Lia, Zhihua Xua, Wei Xiao*b

a

State Key Laboratory of Advanced Technology for Material Synthesis and Processing,

Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, P.R. China Tel: 0086-27-87871029, Fax：0086-27-87879468，E-mail: [email protected] b

School of Resource & Environmental Sciences, Wuhan University, Wuhan, 430072, P.R.

China Tel/Fax: 86-27-68775799, Email: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT NaOH-modified ceramic honeycombs (Na-CH) were simply prepared by impregnating ceramic honeycombs (CH) into NaOH aqueous solution. It was clearly shown that the surface modification incurs higher specific surface area and smaller grain sizes of the CH without destruction of their integrity. Moreover, the introduced surface NaOH can trigger Cannizzaro disproportionation of surface-absorbed formaldehyde (HCHO) on Na-CH, resulting in catalytic transformation of HCHO into less-toxic formate and methoxy salts. The NaOH concentration during impregnating treatment has a great influence on HCHO adsorption and removal efficiency, while the impregnation time and temperature have little influence on the efficiency. When the CH was impregnated in 1 M NaOH aqueous solution for 0.5 h at room temperature, the HCHO removal efficiency at ambient temperature can reach about 80% with an initial HCHO concentration of 250 ppm. Moreover, the used Na-CH can be facilely regenerated via 1-min blow using a common electric hair dryer, with the generation of less toxic HCOOH and CH3OH, and recovery of NaOH. Using such a mild, fast and practical regeneration method, the regenerated NaCH showed slight degradation in adsorption and removal capability towards HCHO. The enhanced performance of Na-CH obtained was attributed to the presence of NaOH, and increase of specific surface area and surface hydroxyl groups. Considering no demand of noble-metal for HCHO removal at ambient temperature and practical reusable capability of Na-CH under mild conditions, this work may provide some new insights into the design and fabrication of advanced catalysts for indoor air purification. Keywords: Ceramic

honeycomb, NaOH

modification,


Formaldehyde

removal

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INTRODUCTION Indoor air purification is crucial for human health because people generally spend more than 80% of their time in houses, offices and cars. Volatile organic compounds (VOCs) are among the most important indoor air pollutants. Formaldehyde (HCHO) is regarded as one of the dominating VOCs emitted from building and finishing materials including paints, wood-based panels, laminate floors and smoking fumes.1 High-concentration formaldehyde has far-reaching impact on human respiratory, nervous and immune systems. And long-term exposure to indoor air even containing a few ppm of formaldehyde may cause adverse effects on human health. Therefore, it is urgent to further develop more efficient and environmentally friendly approaches for removal of formaldehyde. Several approaches have been studied and developed for formaldehyde removal to satisfy the stringent environmental regulations. In general, removal of HCHO mainly includes physical adsorption,2-5 chemisorption,6-7 photocatalytic oxidization,8 thermal catalytic oxidization,9-13 plasma technology,14-16 and biological/botanical filtration.17-18 However, the removal of indoor formaldehyde is still a challenging problem due to the limited adsorption capacity, high energy consumption, high temperature required, byproduct formation and low efficiency of the above mentioned methods. Alternatively, a combination of physical adsorption and chemical reaction is believed to be effective for eliminating HCHO emission in a certain short period.19-21 Moreover, the widespread commercial use of almost all the adsorbents and catalysts in the form of powders and particles has been significantly retarded due to the inevitable aggregation of particles and difficulties on handling/regeneration of powders. To address the forenamed drawbacks of powder materials, monolith-type materials come into considerations. The monolith-type materials (e.g. CH) have merits on facile operation, 3 ACS Paragon Plus Environment


easy regeneration, less secondary pollution over their powder counterparts. In particular, Monolith honeycombs consist of a large number of parallel channels capable of providing good contact between the monolith and gas streams. Such unique structures are beneficial to the application of monolith honeycombs as adsorbents or supports for catalysts when large gas volumes are treated, in which monolith honeycombs offer lower pressure drop, shorter diffusion lengths and less obstruction than those of particulate matters.22 The first important application of CH monoliths was in the automobile industry as support for catalysts used in purification of exhaust gases. Since then, these CH monoliths have been introduced into diesel particulate filter, stationary emission control, molten metal filter, natural gas storage, indoor air purification, chemical process catalyst support, water filtration and so on.23-24 The robust integrity of monolith honeycombs facilitates their surface modification for further improvement on pollutant removal capabilities. It was demonstrated that the catalytic degradation of HCHO can be enhanced by surface modification with NaOH. Recently, Zhang et al reported a novel alkali-metal-promoted Pt/TiO2 catalyst after surface modification with NaOH for the ambient degradation of HCHO,25 with emphasis on specifying the roles of introduced Na ions. Actually, the present NaOH can trigger the Cannizzaro disproportionation of HCHO with the formation of formate ions and methoxy groups.26-27 The introduced NaOH in Na-CH shall also introduce extra functions into CH in addition to its pristine properties. Therefore enhanced HCHO removal capability can be expected in the Na-CH. In this paper, we propose and demonstrate a simple and economical method to treat CH with alkali for efficient removal of formaldehyde. With


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the elucidation of interplays between Na-CH and HCHO, the HCHO removal capability of Na-CH is systematically investigated and discussed in terms of treatment parameters. We hope give some hints on designing cost-affordable and recyclable materials for efficient removal of HCHO at ambient conditions and without utilization of precious materials.

EXPERIMENTAL Preparation. All chemicals used were of analytical-grade and were used as received without further purification. Ceramic honeycombs used were commercially available from Wuhan University of Technology. Deionized water was used in all experiments. CH was impregnated in NaOH aqueous solution (0.1-6.0 M) for 0.5-6 h. The temperature of NaOH aqueous solutions changed from 25 to 100 oC. After impregnation, the CH was dried at 80 oC for 2 h in air without any rinse. Hereinafter, CH, Na-CH and Na-CH-F represent original ceramic honeycomb, ceramic honeycomb after being impregnated in 1 M NaOH aqueous for 0.5 h at room temperature and ceramic honeycomb after being impregnated in 1 M NaOH aqueous for 0.5 h at room temperature with following exposure to gas-phase formaldehyde for 1 h, respectively.

Characterization. Powder X-ray diffraction (XRD) patterns were obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation source ( λ = 1.5418 Å) at a scan rate (2θ) of 0.05° s-1. The accelerating voltage and the applied current were 40 kV and 80 mA,



respectively. Morphology observation was performed on an S-4800 field emission scanning electron microscope (SEM, Hitachi, Japan). The Fourier transform infrared spectra (FT-IR) of the samples were recorded on IRAffinity-1 FT-IR spectrometer. X-Ray photoelectron spectroscopy (XPS) measurement was performed in an ultrahigh vacuum with a VG ESCALAB 210 electron spectrometer equipped with a multichannel detector. The spectra were excited using Mg Kα (1253.6 eV) radiation (operated at 200 W) of a twin anode in the constant analyzer energy mode with a pass energy of 30 eV.

Evaluation of Formaldehyde Adsorption and Removal Activity. Test of HCHO adsorption and removal capability was performed in an organic glass box covered by a layer of aluminum foil paper on its inner wall at ambient temperature. The ceramic honeycomb was placed on the bottom of a glass petri dish with a diameter of 14 cm. After placing the sample-contained dish in the bottom of reactor with a glass slide cover, 20 µL of condensed HCHO (38 wt %) was injected into the reactor and a 5-watt fan was placed in the bottom of reactor during the whole adsorption process. After 2~3 h, the HCHO solution was completely volatilized and the concentration of HCHO became stabilized. HCHO, CO2 and water vapor were on-line analyzed with a Photoacoustic IR Multigas Monitor (INNOVA air Tech Instruments Model 1412). The HCHO vapor was allowed to reach adsorption/desorption equilibrium within the reactor prior to sampling. The initial concentration of HCHO after adsorption/desorption equilibrium was controlled at ca. 250 ppm, which remained constant until the glass slide cover on the petri dish was removed to trigger HCHO adsorption and removal. The data were obtained after 1 h HCHO adsorption and removal when steady states reached. Prior to the recycle


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experiments, the used Na-CH were regenerated via 1-min blow using a common electric hair dryer without any rinse treatment. To elucidate the regeneration mechanism, a mimic regeneration condition, i.e. irradiation under an infrared lamp (220 V, 285 W) of the used CH that was placed in a sealed bottle was employed. After irradiation (10 min, ca. 80 oC), the upper volatilized gas was sampled and analyzed using gas chromatography (GC2014C, Shimadzu, Japan, FID, nitrogen as a carrier gas and capillary column). The removal efficiency of HCHO was calculated as follows: Removal efficiency =

[ HCHO ]initial − [ HCHO ] final [ HCHO ]initial

× 100%

where [HCHO]initial is the adsorption/desorption equilibrium concentration of HCHO before the test, and [HCHO]final is the HCHO concentration after the termination of the test.

RESULTS AND DISCUSSION Phase Structures and Morphology. The photo of CH used in this study is shown in Figure 1. The height and diameter of CH are 1.2 and 4.75 cm, respectively. As can be seen, many triangular-shaped channels (300 meshes) were patterned in the CH, facilitating to supply more active contact sites for HCHO. It is shown that the color of CH changed from white to pale-yellow after NaOH modification. XRD patterns of the CH before and after NaOH modification are shown in Figure 2. The XRD patterns of both CH and Na-CH are well indexed to alpha-alumina (JCPDS no. 43-1484), suggesting the presence of alumina in the CH. The XRD pattern of Na-CH is just the same as that of CH. 7 ACS Paragon Plus Environment


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The external morphology and microstructures of Na-CH were investigated by SEM. Figure 3 shows the low- and high- magnification SEM images of CH and Na-CH, respectively. The SEM images of CH (Figures. 3a and b) exhibit a bulk structure consisting of layers with smooth surface. And the grain boundaries of alumina crystal can be clearly seen. Compared with CH, the SEM images of Na-CH (Figures. 3c and d) obtained by impregnating CH in 1 M NaOH aqueous solution for 0.5 h at room temperature show an obvious rough surface composed of hierarchical spindles. Considering the amphoteric properties of alumina, it can be inferred that alumina reacted with NaOH, resulting in the formation of sodium metaaluminates (NaAlO2). Afterward, the formed NaAlO2 reacted with CO2 and H2O in air with the generation of sodium carbonates and aluminum hydroxides. Based on HSC Chemistry 6.1, both the above reactions are thermodynamically spontaneous at 25 oC or higher temperatures. These reaction equations are as following: 2NaOH + Al2O3 = 2NaAlO2 + H2O, ∆G = -34.3 kJ mol-1 at 25 oC 2NaAlO2 + CO2 + 3H2O = 2Al(OH)3 + Na2CO3, ∆G = -81.4 kJ mol-1 at 25 oC

(1) (2)

The unchanged crystal structures between CH and Na-CH (as shown in Figure 2) indicate that the reaction between alumina and NaOH mainly occurred on the surface of CH or such a surface modification performs with the formation of lowcrystallinity/amorphous products (e.g. Al(OH)3, Na2CO3 and/or NaAlO2). In addition, the low contents of above intermediates or products derived from only a surface modification contribute to the absence of diffraction peaks in the XRD pattern. It is clearly shown in Figures. 1 and 3 that the surface modification incurs higher specific surface area and smaller grain sizes of the Na-CH without destruction of their integrity. Therefore, a


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higher adsorption capacity, enhanced reaction kinetics and unchanged stability can be expected in the Na-CH.

FT-IR Analysis. FT-IR measurements were used to further investigate the influence of NaOH modification on the surface properties of the CH and Na-CH samples, and the interaction between the Na-CH sample and HCHO. The corresponding spectra are shown in Figure 4. The broad bands at ca. 3450 cm-1 in all the four samples are attributed to the stretching vibration of hydroxyl group.28-29 However, the bands of Na-CH and Na-CH-F are more broad and intense than that of CH, indicating the presence of more amounts of hydroxyl groups in the two surface-modified samples. It suggests that the NaOH-modification process grafts extra surface OH- groups. The weak peak centered at ca. 2374 cm-1 in all the samples was due to the stretching vibration of CO2. The sharp band at ca. 1431 cm-1 in Na-CH and NaCH-F can be assigned to carbonate ions.30 This band appears very minor in the CH sample due to its very low NaOH content (see Figure 4a). Such a band becomes intensified in the NaOH-modified sample (Na-CH), understandable with the fact of accelerated formation of carbonates in the presence of large amount of surface-modified NaOH. The band at ca. 1448 cm-1 in Na-CH-F (NaOH-modified ceramic honeycombs after treatment of HCHO) can be assigned to the presence of methoxy groups.26-27 Compared to the CH3O absorption peak at 1455 cm-1 reported in reference,26 the red shift can be attributed to the co-existence and interfere of carbonates. A new band at ca. 1640 cm-1 occurs in Na-CH-F, which can be assigned to the formation of formate groups.26-27,30 The detected methoxy and formate groups indicate that a Cannizzaro reaction of HCHO



occurs on the surface of Na-CH.26-27

XPS Analysis. The reaction mechanism between Na-CH and HCHO was further studied and elucidated by XPS measurements. XPS survey spectra of CH and Na-CH are shown in Figure 5a. The presence of Al (74.2 eV), O (531 eV), Na (1073 eV) and Si (153 eV) is observed, further confirming that the CH is mainly composed of Al2O3. It can be observed that the peak of Na 1s for Na-CH is much stronger than that of CH, mainly resulted from the surface-modified NaOH in CH after impregnation in NaOH solutions. The high-resolution C 1s spectra of CH, Na-CH and Na-CH-F are shown in Figure 5b. The peak locating around 284.8 eV (C-C bond) can be assigned to the adventitious carbon. Compared to CH, a new peak appears in the spectra of Na-CH and Na-CH-F. For Na-CH, the new peak locating at 289.3 eV is due to CO32-, mainly resulting from the reactions 1-2. However, for Na-CH-F, the new peak can be deconvolved to two peaks at 289.3 eV and 289.6 eV, corresponding to CO32- and HCOO-, respectively.31 The presence of HCOO- further indicates the oxidation of HCHO over the Na-CH at ambient temperature via the Cannizzaro reaction. Based on the FT-IR and XPS data, the adsorption mechanism of HCHO on Na-CH can be summarized as follows. Volatilized HCHO is first physically adsorbed onto the surface of Na-CH. As schematically illustrated in Figure 6, the present surface NaOH then triggers a nucleophilic-addition transformation from HCHO to dioxymethylene.27 The formed dioxymethylene then reacts with another HCHO molecule, in which Na and H species from dioxymethylene transfer to the HCHO molecule, with the formation of formate and methoxy salts. HCHO is first


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adsorbed and then disproportionates into nontoxic formate and methoxy salts (e.g. HCOONa and CH3ONa) on the Na-CH surface, promising the application of surfaceNaOH-modified ceramic honeycombs on HCHO removal.

HCHO Adsorption and Removal Activity The HCHO removal capability of Na-CH was then systematically investigated. Figure 7 shows the effect of NaOH concentration during impregnation process on the HCHO removal efficiency over Na-CH. The CH without NaOH modification shows a negligible activity towards HCHO removal. In contrast, the Na-CH samples exhibit an enhanced activity towards HCHO removal. As can be seen from Figure 7, the NaOH concentration has a great influence on the HCHO removal efficiency. In the range of the NaOH concentration studied, the HCHO removal efficiency increases with the NaOH concentration increasing from 0.1 to 2 M. Such an enhancement tendency becomes negligible with further increasing the NaOH concentration to 6 M, indicating the saturation of NaOH upon impregnation treatment in 2 M NaOH. The Na-CH sample prepared by impregnation treatment in 2 M NaOH shows the maximum HCHO removal efficiency of 83%. NaOH modification can improve the activity of CH towards HCHO removal by introducing surface NaOH. It can also be observed that the HCHO removal efficiency over the Na-CH modified by 1 M NaOH is close to that modified by 2 M NaOH, so, we choose the Na-CH modified by 1 M NaOH for the later tests. The effect of impregnation time on the HCHO removal efficiency of Na-CH is shown in Figure 8. All the CH samples were impregnated in 1 M NaOH aqueous solution. The HCHO removal efficiency over Na-CH slightly increases with increasing



impregnating time, but the increment was not obvious. It suggests that the surface of CH is almost modified at the first 0.5 h. So, 0.5 h was chosen as the impregnation time in following study. Figure 9 shows the effect of impregnation temperature on the HCHO removal efficiency. All the CH samples were impregnated in 1 M NaOH aqueous solution for 0.5 h with varying impregnating temperatures. It can be observed that the HCHO removal efficiencies over these Na-CH samples were similar, implying that the impregnating temperature has little effect on the HCHO removal efficiency. Therefore, the ceramic honeycombs can be NaOH-modified at room temperature for the energy conservation.

Regeneration. The reusable capability of catalysts is relevant to their practical applications in terms of economic and environmental viability. In order to investigate the stability of Na-CH towards the HCHO removal, a recycled experiment was performed with the Na-CH impregnated in 1 M NaOH aqueous solution for 0.5 h at room temperature. The used NaCH samples were regenerated via 1-min blow using a common electric hair dryer without any rinse treatment. As shown in Figure 10, the Na-CH sample exhibited a high and stable HCHO removal efficiency, indicating that Na-CH can be efficiently reused. Such a mild-condition but fast regeneration process is quite similar with direct sunshine irradiation, promising its application in practical regeneration. To elucidate the regeneration mechanism, a mimic regeneration condition, i.e. irradiation under an infrared lamp of the used Na-CH that was placed in a sealed bottle was employed. After irradiation, the upper volatilized gas was sampled and analyzed using gas


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chromatography. It is shown that the volatilized gas consists of HCOOH and CH3OH. Such results suggest that the regeneration of the used Na-CH sample can be fulfilled under mild conditions via the following two reactions: HCOONa + H2O = HCOOH + NaOH

(3)

CH3ONa + H2O = CH3OH + NaOH

(4)

As depicted as reactions 3 and 4, NaOH is recovered on the surface of CH with the formation of less toxic HCOOH and CH3OH. The above regeneration was also confirmed by the FT-IR spectrum of generated Na-CH via 1-min blow using a common electric hair dryer without any rinse treatment. As shown in Figure 4, the regenerated Na-CH sample shows a similar spectrum with that of Na-CH. The occurrence of the broad band at 3450 cm-1 clearly proves the recovery of NaOH in the surface. Such recovered NaOH leads to the appearance of carbonate bands (at 1431 cm-1) in the regenerated sample. The bands ascribed to formate ions (at 1641 cm-1) and methoxy salts (at 1448 cm-1) present in the Na-CH-F sample become absent in the regenerated Na-CH sample, agreeing well with the reactions 3 and 4. It is understandable that HCOOH and CH3OH are less toxic. In potential practical application, the NaOH modified CH can be used as an effective, affordable and recyclable catalyst to capture and immobilize highly-toxic HCHO in indoor air with the formation/enrichment of non-toxic HCOONa and CH3ONa on surface of Na-CH. Then the used Na-CH can be treated under mild condition (e.g. sunshine irradiation) for regeneration. During the regeneration process, the formed HCOOH and CH3OH can be easily collected for further centralized disposal and/or reuse. Therefore, a closed loop of HCHO removal can be fulfilled with an excellent economic and environmental viability.



ACKNOWLEDGEMENTS This work was partially supported by the 863 Program (2012AA062701), 973 Program (2013CB632402), NSFC (51072154, 21177100 and 51272199) and Fundamental Research Funds for the Central Universities (2013-VII-030) and Self-determined and Innovative Research Funds of SKLWUT (2013-ZD-1). Also, this work was financially supported by PSFC (2012M521482).

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P.E., Bomben, K.D., Chastain, J., Eds. Pekin-Elmer Inc. Physical Electronics Division: Eden Prairie, MN, 1992.



Figure Captions Figure 1. Photos of CH and Na-CH. Figure 2. XRD patterns of CH and Na-CH. Figure 3. SEM images of CH (a,b) and Na-CH (c,d). Figure 4. FTIR spectra of CH, Na-CH, Na-CH-F and regenerated Na-CH. Figure 5. XPS survey spectra (a) of CH and Na-CH and high-resolution C1s XPS spectra (b) of CH, Na-CH and Na-CH-F. Figure 6. Reaction mechanism of HCHO in NaOH-modified ceramic honeycombs. Figure 7. Effect of NaOH concentration on the HCHO removal efficiency of CH impregnated in NaOH aqueous solution for 0.5 h at room temperature. Figure 8. Effect of impregnating time on HCHO removal efficiency of CH impregnated in 1 M NaOH aqueous solution at room temperature. Figure 9. Effect of impregnating temperatures on HCHO removal efficiency of CH impregnated in 1 M aqueous solution for 0.5 h. Figure 10 Effects of recycle times on formaldehyde removal efficiency of CH impregnated in 1 M NaOH aqueous solution for 0.5 h at room temperature.


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Figure 1. Photos of CH and Na-CH.


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(214) (300)

(116) (024)

(113)

(104) (110)

Intensity (a.u.)

(012)


Na-CH

CH 10

20

30 40 50 60 2 Theta (degrees)

70

Figure 2. XRD patterns of CH and Na-CH.


80

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Figure 3. SEM images of CH (a,b) and Na-CH (c,d).


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1641

3450

Transmission (a.u.)

2374


CH Na-CH

regenerated Na-CH 4000

3500

3000

2500

2000

1431

1448

Na-CH-F

1500

1000

-1

Wavenumbers (cm ) Figure 4. FTIR spectra of CH, Na-CH, Na-CH-F and regenerated Na-CH.



Si2p Al2p Na2s

C1s

Na-CH

Si2s Al2s

O KLL

Na KLL

Na1s

Intensity (a.u.)

(a)

O1s

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CH

1200

1000

800

600

400

200

0

Binding energy (eV)

(b) Intensity (a.u.)

CH

C-C

Na2CO3

Na-CH

HCOONa

Na-CH-F 282

284

286

288

290

292

Binding energy (eV) Figure 5. XPS survey spectra (a) of CH and Na-CH and high-resolution C1s XPS spectra (b) of CH, Na-CH and Na-CH-F.



Figure 6. Reaction mechanism of HCHO in NaOH-modified ceramic honeycombs.


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Page 25 of 29


100 2M 6M

80

Efficiency (%)

1M 60 40

0.5 M

20

0.1 M 0M

0 0

10

20

30

40

Time (min)

50

60

Figure 7. Effect of NaOH concentration on the HCHO removal efficiency of CH

impregnated in NaOH aqueous solution for 0.5 h at room temperature.



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100

Efficiency (%)

80

60

40

20

0 0.5

1

2

6

Time (h) Figure 8. Effect of impregnating time on HCHO removal efficiency of CH

impregnated in 1 M NaOH aqueous solution at room temperature.


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100

Effeciency (%)

80

60

40

20

0 20

40

60

80

Temperature (oC)

100

Figure 9. Effect of impregnating temperatures on HCHO removal efficiency of CH

impregnated in 1 M aqueous solution for 0.5 h.



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80

Efficiency (%)

70 60 50 40 30 20 10 0 1

2

3

4

5

Recycle time Figure 10 Effects of recycle times on formaldehyde removal efficiency of CH

impregnated in 1 M NaOH aqueous solution for 0.5 h at room temperature.


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TOC


NaOH-Modified Ceramic Honeycomb with Enhanced Formaldehyde

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