Low-concentration formaldehyde removal from air using a cycled storage–discharge (CSD) plasma catalytic process

Chemical Engineering Science 66 (2011) 3922–3929

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Low-concentration formaldehyde removal from air using a cycled storage–discharge (CSD) plasma catalytic process De-Zhi Zhao, Xiao-Song Li, Chuan Shi, Hong-Yu Fan, Ai-Min Zhu n Laboratory of Plasma Physical Chemistry, School of Physics and Optoelectronic Engineering, School of Chemistry, Dalian University of Technology, 116024 Dalian, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 February 2011 Received in revised form 13 May 2011 Accepted 13 May 2011 Available online 23 May 2011

Cycled storage–discharge (CSD) plasma catalytic process was applied to low-concentration formaldehyde removal from air for the first time. HZSM-5, Ag/HZSM-5 (Ag/HZ), Cu/HZSM-5 (Cu/HZ) and AgCu/HZSM-5 (AgCu/HZ) catalysts were used for the process and their performance for formaldehyde storage and plasma catalytic oxidation of stored formaldehyde towards carbon dioxide was investigated in details. AgCu/HZ exhibited the highest formaldehyde adsorption capacity among these catalysts due to the synergistic effect of Ag and Cu, and it was very selective for oxidizing stored formaldehyde into CO2 by either oxygen or air plasmas at the discharge stage. The CSD process for lowconcentration formaldehyde removal had extremely low energy cost, excellent humidity tolerance and almost no secondary pollution. At the discharge stage, in addition to pure oxygen, air can also be used as the discharge gas for complete oxidation of formaldehyde, and the nitrogen oxides produced can be negligible because the discharge period was much shorter than the storage period. The stability of the AgCu/HZ catalysts during the CSD process was also investigated. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Formaldehyde removal Plasma Adsorption Catalysis Catalyst selectivity Zeolite

1. Introduction Formaldehyde (HCHO) is one of the volatile organic compounds (VOCs), which is known to contribute to poor indoor air quality, and is suspected of being a main cause of the ‘‘sick building’’ syndrome. A number of studies have been made for removing formaldehyde from gas streams, including adsorption, catalytic and photocatalytic oxidation processes. For the adsorption process, adsorbents have limited adsorption capacity and thereby it is necessary either to regenerate the adsorptionsaturated adsorbents or to dispose of them. Moreover, in the adsorption process, formaldehyde is only transferred from the gas phase to a solid phase without being destructed, which can potentially result in secondary pollution (Domingo-Garcı´a et al., 1999; Tanada et al., 1999). For the catalytic oxidation process, formaldehyde can be removed at relatively high temperature (Wang and Li, 2009; Yang et al., 2005) using non-noble metal catalysts or at room temperature using noble metal catalysts (Tang et al., 2008; Zhang et al., 2005, 2006). For the photocatalytic removal technique, its efficiency is still not high enough (Ao et al., 2004; Chin et al., 2006; Di et al., 2009; Liu et al., 2005; Noguchi and Fujishima, 1998; Obee, 1996; Obee and Brown, 1995; Shiraishi et al., 2005a, 2005b). In the last two decades, atmospheric pressure non-thermal plasmas (NTP), which can produce

n

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0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.05.019

reactive species at low temperature, have been studied for formaldehyde removal (Blin-Simiand et al., 2009; Chang and Lee, 1995; Ding et al., 2005, 2006; Shimizu et al., 2008; Storch and Kushner, 1993). However, high energy cost (especially when formaldehyde exists at ppb level in indoor air), secondary pollutants (such as NOx and CO) and poor humidity tolerance make the NTP-based technique impractical. To solve these problems, a cycled storage–discharge (CSD) plasma catalytic process was proposed and employed to remove benzene (Fan et al., 2009; Kim et al., 2008; Ogata et al., 2001), toluene (Kim et al., 2008; Kuroki et al., 2007, 2009), xylene (Kuroki et al., 2010) and NOx (Okubo et al., 2002, 2005). Fig. 1 shows schematically the working principle of the CSD plasma catalytic process for VOCs removal and the difference in discharge power between the CSD and normal plasma catalytic processes. Briefly, in a cycle of the CSD plasma catalytic process, the low-concentration VOCs in air are first stored on catalysts at a storage stage (plasma off) and then the stored VOCs are oxidized to CO2 by plasma at a discharge stage (plasma on). To make a direct comparison between the CSD and normal plasma catalytic processes, assuming the flow rate of the polluted air is F1, their energy costs for purifying 1 m3 air (EC, kWh/m3) are defined as follows, respectively: EC CSD ¼

CSD Pdischarge t2

EC normal ¼

F 1 t1 normal Pdischarge

F1

ð1Þ

ð2Þ

D.-Z. Zhao et al. / Chemical Engineering Science 66 (2011) 3922–3929 CSD where, t1 is the storage period of the CSD process, and Pdischarge and t2 are the discharge power and the discharge period at a discharge normal stage of the CSD process, respectively. Pdischarge is the discharge power of a normal plasma catalytic process. From Eq. (2), it is clear that the energy cost of a normal plasma catalytic process is characterized by its energy density. In the CSD process, the flow rate at a discharge stage (F2) is far CSD normal is lower than Pdischarge . In lower than F1, and accordingly Pdischarge

this case, if t2 5t1, then ECCSD 5ECnormal. Therefore, both long storage period and short discharge period are necessary for achieving a far lower energy cost in the CSD process than that in the normal process. However, because of the competitive adsorption of moisture in air, it becomes quite difficult to achieve a very long storage period for formaldehyde removal, especially

Polluted Air (VOCs) Catalyst

CO22, H22O O

Cycled operation

Clean Air Storage stage (F1, t1)

Discharge stage (F2, t2)

normal

Discharge power

normal plasma process

Pdischarge

CSD CSD plasma catalytic process P discharge

0

t1

t2 Time

Fig. 1. (a) The working principle of the CSD plasma catalytic process for VOCs removal and (b) the difference in discharge power between the CSD and normal plasma catalytic processes.

N2

3923

for low-concentration formaldehyde removal. As a result, the CSD process has not been used for formaldehyde removal so far. Herein, low-concentration formaldehyde removal from air using the CSD process with AgCu/HZSM-5 (AgCu/HZ) catalysts was reported for the first time. The performances of formaldehyde storage and plasma catalytic oxidation of stored formaldehyde on AgCu/HZ catalysts were investigated in details. The stability of the AgCu/HZ catalysts during the CSD process was investigated preliminarily.

2. Experimental Fig. 2 shows the schematic diagram of the experimental setup designed for low-concentration formaldehyde removal with the CSD plasma catalytic process. The catalyst-packed dielectric barrier discharge (DBD) reactor is composed of a quartz tube (i.d. 8 mm), a stainless steel rod (3 mm in diameter) placed along its axis as the high-voltage electrode and catalyst pellets (20–40 mesh, 1.5 ml in stack volume) filled between the quartz tube and the central rod. A stainless steel wire mesh was wound on the outside surface of the quartz tube as the grounded electrode. High voltage with an AC frequency of 2 kHz was applied to the high-voltage electrode. The discharge power was measured via the area of the voltage–charge Lissajous figure (Rosenthal and Davis, 1975). The Ag/HZSM-5 (Ag/HZ), Cu/HZSM-5 (Cu/HZ) and AgCu/HZSM-5 (AgCu/HZ) catalysts were prepared by a conventional incipient wetness impregnation method using HZSM-5 (HZ, SiO2/Al2O3 ¼ 360) as the support and metal nitrates as precursors. The impregnated samples were aged at room temperature for 12 h, dried at 393 K for 2 h and calcined in air at 873 K for 3 h. The metal loading of the catalysts was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Optima 2000DV, USA). The silver loading of Ag/HZ catalysts was 5.2 wt% and the copper loading of Cu/HZ catalysts was 5.1 wt%. The silver and copper loadings of AgCu/HZ catalysts were 3.6 and 2.1 wt%, respectively. The BET surface areas of Ag/HZ, Cu/HZ and AgCu/HZ catalysts were 310, 314 and 299 m2/g, respectively, as measured by N2 adsorption–desorption isotherms at 77 K (Autosorb-1-MP instrument, USA). The chemical binding energy of the Ag and Cu atoms on the catalysts was examined by X-ray photoelectron spectroscopy (XPS, ESCALAB250 Thermo VG, USA) using an AlKa X-ray source (1486.6 eV) operated at 15 kV and 300 W.

For Dischage Stage

O2

Filter For Storage Stage

N2

FTIR

2

Filter 4

O2

O3 Analyzer 9

1

6

5

11

3

10

Water Addition

N2 5

7

13

12

COx/N2O Analyzer

8

HCHO Generation Unit

Catalyst-packed DBD Reactor VOC-to-CO2 Converter Plasma Power Supply

Fig. 2. Schematic diagram of the experimental setup: (1) mass flow controller; (2) valve; (3) three-way valve; (4) four-way valve; (5) water bath; (6) water; (7) trioxymethylene; (8) oven; (9) quartz tube; (10) grounded electrode; (11) high-voltage electrode; (12) CuOMnO2/g-Al2O3 catalysts and (13) AgCu/HZ catalysts.

D.-Z. Zhao et al. / Chemical Engineering Science 66 (2011) 3922–3929

All the feed gases used in this work were of high-purity grade ( 499.99%). The gas flow rates were adjusted and controlled by mass flow controllers (MFC) (SevenStar Co., China). Before performing the experiment, the catalysts were pretreated in a gas mixture composed of 80% N2 and 20% O2 at 873 K for 2 h, and then were cooled down to room temperature in this gas stream. During the storage stage, a total flow rate of 300 ml/min, which corresponded to a gas hourly space velocity (GHSV) of 12,000 h  1, was employed. The formaldehyde was produced via catalytic depolymerization of trioxymethylene vapor in a N2-diluted gas stream at 433 K over glass pellets coated with 85% phosphoric acid. The trioxymethylene vapor was generated from solid trioxymethylene in a water bath (Di et al., 2009). Water was carried into the gas stream by passing N2 or O2 through a bubbler in a water bath at room temperature. The amount of water in the simulated air (80% N2 þ20% O2), which was expressed as relative humidity (RH) at 298 K, was controlled via adjusting the flow rate of N2 and/or O2. During the discharge stage, 60 ml/min of oxygen (or 80% N2 þ20% O2) was switched into the reactor. All the experiments at the storage and discharge stages were carried out at room temperature and atmospheric pressure. The concentrations of CO, CO2 and N2O were measured online using an infrared absorption spectrometer (SICK-MAIHAK-S710, Germany). In this work, HCHO concentration cannot be measured directly by a Fourier transform-infrared (FT-IR) spectrometer due to the disturbance of moisture. Thereby, HCHO was measured by its conversion to CO2 in a homemade VOC-to-CO2 converter (CuO–MnO2/g-Al2O3 catalysts) at 673 K (Fan et al., 2009). Colket et al. (1974) also pointed out that an accurate measurement of HCHO concentration should be obtained by first oxidizing HCHO to CO2. The O3 concentration at the discharge stage was monitored by an ozone analyzer (Mini-HiCon, USA). An on-line FT-IR spectrometer (Nicolet-Antaris IGS Analyzer, USA) equipped with a gas cell of 2-m optical path length and an MCT-A detector was used to determine any possible by-products at the discharge stage. The FT-IR spectra were averaged by 64 scans at a resolution of 2 cm  1. The definitions of carbon balance (BC, %), CO2 selectivity (SCO2 , %), CO selectivity (SCO, %), stored-HCHO conversion to CO2 (XðHCHOÞs -CO2 , %) and energy cost (EC, kWh/m3) have been reported in our previous paper (Fan et al., 2009).

3. Results and discussion 3.1. Storage performance of AgCu/HZ catalysts at a storage stage 3.1.1. Synergistic effect of Ag and Cu loaded on HZ catalysts for HCHO storage Fig. 3 compares the breakthrough capacity of HCHO on HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts in simulated air streams containing HCHO with 50% RH and at a flow rate of 300 ml/min. The breakthrough capacity for HCHO storage (nb) was calculated using HCHO concentration (CHCHO), total flow rate (F1), breakthrough time (tb) and volume of catalysts (Vcat.): nb ¼

CHCHO F1 tb Vcat:

ð3Þ

HCHO concentrations in the simulated air streams were 24.4, 29.7, 29.6 and 26.2 ppm, respectively, for the cases of HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts. Therefore, the breakthrough capacities of HCHO on HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts were 8.5, 18.5, 11.8 and 38.9 mmol/ml-cat., respectively. This means that AgCu/HZ catalysts possessed much higher breakthrough capacity for HCHO than HZ, Ag/HZ or Cu/HZ catalysts, which can be attributed to a synergistic effect of Ag and Cu.

HCHO breakthrough capacity (µmol/ml-cat.)

3924

40

30

20

10

0 HZ

Ag/HZ

Cu/HZ

AgCu/HZ

Fig. 3. HCHO breakthrough capacity of HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts. Conditions: simulated air at 300 ml/min, 50% RH, GHSV¼ 12,000 h  1, CHCHO ¼ 24.4 ppm (HZ), 29.7 ppm (Ag/HZ), 29.6 ppm (Cu/HZ) and 26.2 ppm (AgCu/HZ).

Silvain et al. (2000) reported that silver atoms can incorporate with copper atoms, leading to the occupation of some Cu sites by Ag. To verify the chemical binding states of the Ag and Cu atoms, Ag/HZ, Cu/HZ and AgCu/HZ catalysts were studied by XPS and the spectra were compared in Fig. 4. The Cu 2p3/2 peak at 933.2 eV of Cu/HZ catalysts was attributed to Cu2 þ (Janas et al., 2009; Zhou et al., 2004) and the Ag 3d5/2 peak at 368.4 eV of Ag/HZ catalysts was assigned to metallic silver (Richter et al., 2002). For AgCu/HZ catalysts, the position of Cu 2p3/2 and Ag 3d5/2 peaks shifted to  934.0 eV (Fig. 4a) and 368.7 eV (Fig. 4b), respectively. The shift toward higher energy indicates the increase in the amount of Cu3 þ ions and the formation of Ag–Cu species on AgCu/HZ catalysts (Silvain et al., 2000), which is very likely to cause the increase of HCHO breakthrough capacity on AgCu/HZ catalysts (Cotton and Wilkinson, 1966; Shen et al., 1999). 3.1.2. Excellent humidity tolerance of AgCu/HZ catalysts A good catalyst for the CSD plasma catalytic process should have a very wide range of humidity tolerance. Therefore, at various humidity conditions (dry gas and RH ¼20%, 50%, 80% and 93%), HCHO breakthrough capacity over AgCu/HZ catalysts in simulated air streams was investigated. From Fig. 5, it can be seen that HCHO breakthrough capacity in humid gas streams decreased to a small extent compared with that in a dry gas stream. But across a wide range of RH (from 20% to 93%), HCHO breakthrough capacity over AgCu/HZ catalysts kept almost constant. Even if at 93% of RH, HCHO breakthrough capacity was still as high as 40.8 mmol/ml-cat.. The excellent humidity tolerance of AgCu/HZ catalysts for HCHO adsorption derives from the hydrophobic property of high-silica zeolite (Baek et al., 2004; Fan et al., 2009) as well as the selective adsorption of AgCu/HZ catalysts towards HCHO. 3.1.3. Long breakthrough time for low-concentration HCHO in humid air streams According to Eq. (1), the longer the storage period (t1) of the CSD plasma catalytic process, the lower its energy cost would be. However, the storage period is limited by the breakthrough time tb (t1 otb) and thus a long breakthrough time is necessary to achieve a low energy cost for the CSD plasma catalytic process. Generally, in a dry gas stream, breakthrough time (tb) of an adsorbate is inversely proportional to its concentration. But how does the breakthrough time vary with its concentration in a

D.-Z. Zhao et al. / Chemical Engineering Science 66 (2011) 3922–3929

3925

Cu/HZ Cu 2p3/2

AgCu/HZ

tb = 163 min

20

20

950

945 940 935 Binding energy (eV)

930

925

10

0.5

300

350 Time (min)

10

Intensity (a.u.)

100

200

tb = 690 min

1.0 tb

0.5

0.0 690 700 Time (min)

300 400 Time (min)

500

710

600

700

Fig. 6. Storage curves of HCHO on AgCu/HZ catalysts at various HCHO concentrations. Conditions: 300 ml/min flow rate of simulated air, 50% RH, GHSV¼ 12,000 h  1, CHCHO ¼ 26.2 ppm (’), 10.5 ppm (K) and 6.3 ppm (m).

Ag 3d3/2

40

humid gas stream because of the competitive adsorption of moisture? Especially, in indoor air, H2O concentration is 104 times more than HCHO concentration, which is typically at ppb level. Therefore, the effect of HCHO concentration on the breakthrough time was examined in the simulated air stream at 300 ml/min with 50% RH. As shown in Fig. 6, when HCHO concentration decreased from 26.2 to 6.3 ppm, the breakthrough time increased from 163 to 690 min. That is, the breakthrough time increased approximately in inverse proportion to HCHO concentration, thanks to the selective adsorption towards HCHO and the hydrophobic property of the AgCu/HZ catalysts. This experimental result confirmed the breakthrough time for lowconcentration HCHO in humid air streams becomes very long. For hundreds of ppb of HCHO in indoor air, it is predicted that the breakthrough time will extend to thousands of minutes. From the viewpoint of HCHO storage, the AgCu/HZ catalysts exhibited excellent humidity tolerance and very long breakthrough time for low-concentration HCHO in humid air streams, which made itself a highly suitable catalyst for the CSD plasma catalytic process.

30

3.2. Performance of plasma catalytic oxidation over AgCu/HZ catalysts at a discharge stage

378

376

374

372 370 368 Binding energy (eV)

366

364

Fig. 4. XPS spectra of Cu/HZ, Ag/HZ and AgCu/HZ catalysts: (a) Cu 2p and (b) Ag 3d peaks.

HCHO breakthrough capacity (µmol/ml-cat)

190

400

680

Ag 3d5/2

170 180 Time (min)

0.0

0 30

0

AgCu/HZ

0.0

tb

0

Ag/HZ

tb

0.5

tb = 320 min

1.0

20 955

1.0

160

0 30

HCHO concentration (ppm)

Intensity (a.u.)

Cu 2p1/2

HCHO concentration (ppm)

HCHO concentration (ppm)

10

HCHO concentration (ppm)

30

50

20

10

0 Dry gas

20%

50%

80%

93%

Relative humidity Fig. 5. Effect of relative humidity (RH) on HCHO breakthrough capacity of AgCu/HZ catalysts. Conditions: simulated air at 300 ml/min, GHSV ¼12,000 h  1, CHCHO ¼ 25.8 ppm (dry gas), 26.2 ppm (RH ¼20%), 26.2 ppm (RH¼ 50%), 26.6 ppm (RH ¼ 80%) and 26.6 ppm (RH ¼ 93%).

3.2.1. Synergistic effect of Ag and Cu loaded on HZ catalysts for complete oxidation of HCHO At a discharge stage of the CSD plasma catalytic process, the stored HCHO on the catalysts should be totally oxidized to CO2. Therefore, the plasma catalytic oxidation of stored HCHO on HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts was investigated. Firstly, at the storage stage, a simulated air stream with 50% RH at 300 ml/min was flowed through the catalyst beds for 40 min (CHCHO ¼24.4, 27.2, 26.7 and 26.6 ppm for the cases of HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts, respectively) and about 14 mmol of HCHO was stored on the catalysts. Then, at the discharge stage, the stored HCHO on the CSD catalysts was oxidized in oxygen plasma (Pdischarge ¼ 2:3 W and 60 ml/min O2). Fig. 7a shows the evolution curves of CO and CO2 as a function of discharge time in plasma oxidation of formaldehyde

3926

D.-Z. Zhao et al. / Chemical Engineering Science 66 (2011) 3922–3929

CO2 evolution (µmol/min)

12

HCHO adsorption mainly occurs on the metal sites (Cotton and Wilkinson, 1966; Shen et al., 1999) on Ag/HZ or AgCu/HZ catalysts, while it happens on the zeolites over HZ or Cu/HZ catalysts (Go´raMarek and Datka, 2008). On Ag/HZ and AgCu/HZ catalysts, there was almost no CO produced and about 100% of CO2 selectivity was achieved (Fig. 7b). In contrast, on the HZ and Cu/HZ catalysts, CO was produced with 12.9% and 4.1% selectivities, respectively. In terms of carbon balance (Fig. 7b), the Ag/HZ catalyst was the lowest, which suggests there were still some residual carbon species on Ag/HZ catalysts. The carbon balance of the HZ, Cu/HZ and AgCu/HZ catalysts reached  100%, suggesting that there were no other byproducts derived from stored HCHO. Therefore, AgCu/HZ catalysts exhibited almost complete oxidation of stored HCHO to CO2 at the discharge stage, as well as excellent performance for HCHO storage in humid air at the storage stage.

HZ Ag/HZ Cu/HZ AgCu/HZ

8

4

0 0

2

4 6 Discharge time (min)

8

10

CO evolution (µmol/min)

3 HZ Ag/HZ Cu/HZ AgCu/HZ

2

1

0 2

0

4

6

8

10

12

Discharge time (min) 120 Carbon balance

Sco2

t2 = 10 min

Percentage (%)

100 80 60 40 20 0 HZ

Ag/HZ

Cu/HZ

AgCu/HZ

Fig. 7. (a) CO2 and (b) CO evolutions with discharge time and (c) carbon balance and CO2 selectivity in plasma catalytic oxidation of stored HCHO over HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts (storage stage: simulated air at 300 ml/min, 50% RH, GHSV¼ 12,000 h  1, CHCHO ¼24.4 ppm (HZ), 27.2 ppm (Ag/HZ), 26.7 ppm (Cu/HZ) CSD and 26.6 ppm (AgCu/HZ), t1 ¼ 40 min; discharge stage: O2 at 60 ml/min, Pdischarge ¼ 2:3 W).

stored on HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts. It illustrates that the CO2 evolution on the AgCu/HZ catalysts was slower than that on the HZ or Cu/HZ catalysts, but faster than that on the Ag/HZ catalysts. It may be caused by the difference in adsorption sites towards HCHO among the HZ, Ag/HZ, Cu/HZ and AgCu/HZ catalysts.

3.2.2. Effect of discharge power on plasma catalytic oxidation of stored HCHO The effect of discharge power on plasma catalytic oxidation of stored HCHO was examined with AgCu/HZ catalysts. Firstly, at the storage stage, about 15 mmol HCHO was adsorbed on the AgCu/HZ catalysts by flowing a simulated air stream with 50% RH at 300 ml/min through the AgCu/HZ catalysts beds for 40 min CSD (CHCHO ¼30.7, 27.2 and 27.3 ppm for the cases of Pdischarge ¼ 1:4, 2:3 and 3.1 W, respectively) . Then, at the discharge stage, the stored HCHO on the AgCu/HZ catalysts was oxidized in oxygen plasma CSD (O2 at 60 ml/min). As shown in Fig. 8, at Pdischarge ¼ 1:4 W, the rates of CO2 evolution and stored HCHO oxidation into CO2 were the lowest, and only 72% of stored HCHO was oxidized into CO2 even at a long discharge time. It should be pointed out that only CO2 was produced through the whole discharge period with the AgCu/HZ catalysts. Some residual HCHO still stayed on the catalysts. As the discharge power increased to 2.3 W, the oxidation rates increased dramatically and almost 100% of stored HCHO was oxidized into CO2 within 10 min. But when the discharge power was increased to 3.1 W, the CO2 evolution rate increased further relative to that at 2.3 W, but the change was not significant. This indicated that there was an optimum discharge power, at which high oxidation rates as well as low energy cost can be achieved. 3.2.3. Very short discharge period and extremely low energy cost for the CSD plasma catalytic process According to Eq. (1), the storage period (t1) and the discharge period (t2) are the two important parameters for the energy cost of the CSD plasma catalytic process. The AgCu/HZ catalyst has exhibited long breakthrough time for low-concentration HCHO (see Section 3.1.3), which suggests long storage period can be employed to achieve low energy cost. However, how about the variation of the discharge period with the storage period? To answer this question, the discharge periods required at various storage periods were investigated. At the storage stage, a simulated air stream with 50% RH at 300 ml/min was controlled to pass through the AgCu/HZ catalyst bed for 100 min (CHCHO ¼ 6.5 ppm), 300 min (CHCHO ¼6.8 ppm) and 690 min (CHCHO ¼ 6.3 ppm), respectively. At the discharge stage, the stored HCHO on the AgCu/HZ catalysts was oxidized in oxygen plasma CSD ¼ 2:3 W and O2 flow at 60 ml/min). From Fig. 9, it (Pdischarge appears that the discharge period was almost irrelevant to the storage period and a 10-min discharge period was required for the three cases to achieve  100% conversion of HCHO to CO2. Thereby, it is concluded that the energy cost is in inverse proportion to the storage period because of the irrelevance of the discharge period to the storage period. Taking the case of CSD t1 ¼690 min (F1 ¼300 ml/min, Pdischarge ¼ 2:3W and t2 ¼10 min) as an example, the energy cost of the CSD plasma catalytic process

D.-Z. Zhao et al. / Chemical Engineering Science 66 (2011) 3922–3929

3927

8

1.4 W 2.3 W 3.1W

6

CO2 evolution (µmol/min)

CO2 evolution (µmol/min)

30

4

2

t1 = 100 min t1 = 300 min

20

t1 = 690 min 15 10 5 0

0 0

2

4

10 6 8 Discharge time (min)

12

14

0

16

2

4 6 8 Discharge time (min)

10

12

120

120

carbon banlance

100

100

80

80

Percentage (%)

Stored-HCHO conversion to CO2 (%)

25

60 40

1.4 W 2.3 W 3.1 W

20

Sco2 t2 = 10 min

60

40

20

0 0

2

4

6 8 10 Discharge time (min)

12

14

16

Fig. 8. (a) CO2 evolution and (b) stored-HCHO conversion to CO2 as a function of discharge time at various discharge powers over AgCu/HZ catalysts (storage stage: simulated air at 300 ml/min, 50% RH, GHSV ¼12,000 h  1, CHCHO ¼ 30.7 ppm (&), 27.2 ppm (J) and 27.3 ppm (W), t1 ¼ 40 min; discharge stage: O2 at 60 ml/min).

was 1.9  10  3 kWh/m3-air for purifying air containing 6.3 ppm of HCHO with 50% RH. Based upon the results in Section 3.1.3, the storage period can further extend for purifying indoor air containing formaldehyde at even lower concentration (e.g., several hundred ppb), and accordingly the energy cost should decrease to 10  5–10  4 kWh/m3-air. This extremely low energy cost undoubtedly provides a significant boost for the environmental applications of the plasma catalytic technique.

3.2.4. Air as a possible discharge gas for complete oxidation of HCHO As a discharge gas for the CSD plasma catalytic process, air is easier to obtain than oxygen. To investigate if air is a suitable discharge gas for the oxidation of HCHO, both simulated air (80% N2 þ 20% O2) and oxygen were tried as the discharge gas for plasma catalytic oxidation of stored HCHO on AgCu/HZ catalysts. As shown in Fig. 10, the rate of CO2 evolution in air plasma was lower than that in oxygen plasma, and accordingly the discharge period in air plasma extended to some extent. However, carbon balance and CO2 selectivity through a discharge period in air plasma are the same as those in oxygen plasma. Due to consumption and decomposition of O3 over AgCu/HZ catalysts, no O3 was detected at the outlet of the catalyst-packed DBD reactor using oxygen or air as the discharge gas. Because nitrogen

0

t1 = 100 min

t1 = 300 min

t1 = 690 min

Fig. 9. (a) CO2 evolution with discharge time and (b) carbon balance and CO2 selectivity in plasma catalytic oxidation of stored HCHO over AgCu/HZ catalysts at various storage periods (storage stage: simulated air at 300 ml/min, 50% RH, CHCHO ¼6.5 ppm (t1 ¼100 min), 6.8 ppm (t1 ¼300 min) and 6.3 ppm (t1 ¼690 min); CSD discharge stage: O2 at 60 ml/min, Pdischarge ¼ 2:3 W).

oxides (such as NO, NO2 and N2O) are possible by-products when air is used as the discharge gas (Blin-Simiand et al., 2008; Harling et al., 2008; Sun et al., 2003; Van Durme et al., 2007), on-line FT-IR measurements for the gaseous products at the discharge stage were performed. For oxygen plasma, only CO2 was detected. For the air plasma, besides CO2 product, N2O and NO2 were produced in small amount, as shown in Fig. 11. NO2 was not detected until majority of stored HCHO was consumed after 4-min discharge time, which implies that NOx production is suppressed significantly in the presence of stored HCHO. At 10-min discharge time, about 70 ppm NO2 was produced in the discharge gas of 60 ml/min air. As the discharge time t2 is much shorter than the storage time t1 in the CSD plasma catalytic process, the nitrogen oxides produced can be extremely low when air is used as the discharge gas. Actually, because the storage period can be further extended for purifying indoor air with much lower formaldehyde concentration, the secondary pollution from air plasma is negligible. 3.3. Stability of AgCu/HZ catalysts during CSD plasma catalytic process To investigate the stability of the AgCu/HZ catalysts during the CSD plasma catalytic process, five ‘‘storage–discharge’’ cycles were performed. The conditions at the storage stage are as

D.-Z. Zhao et al. / Chemical Engineering Science 66 (2011) 3922–3929

Discharge Stage

Storage Stage Cycle 1

25

air plasma

CO2 evolution (µmol/min)

CO2 evolution (µmol/min)

30

O2 plasma

20 15 10 5

Cycle 2

Cycle 3

Cycle 4

Cycle 5

6

15

4

10

2

5

HCHO stored (µmol)

3928

0 0

2

4 6 10 8 Discharge time (min)

12

0

50

100

200

250

120

Sco2

t2 = 10 min

t2 = 10 min

t2 = 14 min

150

Time (min)

carbon banlance

120

0

0

14

100

Percentage (%)

Percentage (%)

100 80 60 40

80 Carbon balance Sco2 60

20

40 1

0 air plasma

O2 plasma

3 N2O

2 1

NO2 N2O

Absorbance

4

CO2

0 2 4 6 8 10 4000

3000

2000 Wavenumbers (cm-1)

D

ge

tim

e

3

4

5

Cycle number

Fig. 10. (a) CO2 evolution with discharge time and (b) carbon balance and CO2 selectivity in plasma catalytic oxidation of stored HCHO using simulated air or O2 as discharge gas (storage stage: simulated air at 300 ml/min, 50% RH, GHSV¼ 12,000 h  1, CHCHO ¼5.3 ppm (air plasma), and 6.3 ppm (O2 plasma), t1 ¼780 min (air plasma), and 690 min (O2 plasma); discharge stage: F2 ¼ 60 ml/min, CSD Pdischarge ¼ 2:3 W).

CO2

2

(m

)

in

ar

h isc

1000

Fig. 11. FT-IR spectra of the plasma catalytic oxidation of HCHO over AgCu/HZ catalysts in air plasma at various discharge times (the other conditions are given in Fig. 10).

Fig. 12. Comparison of (a) CO2 evolution and HCHO storage and (b) carbon balance and CO2 selectivity in five ‘‘storage–discharge’’ cycles (storage stage: 300 ml/min flow rate of simulated air, 50% RH, GHSV¼ 12,000 h  1, CHCHO ¼ 28.7 ppm (cycle 1), 29.4 ppm (cycle 2), 29.4 ppm (cycle 3), 29.3 ppm (cycle 4) and 28.4 ppm (cycle 5), CSD t1 ¼ 40 min; discharge stage: 60 ml/min O2, Pdischarge ¼ 2:3 W).

follows: HCHO concentration¼28.7 ppm (cycle 1), 29.4 ppm (cycle 2), 29.4 ppm (cycle 3), 29.3 ppm (cycle 4) and 28.4 ppm (cycle 5), RH¼ 50%, flow rate of simulated air (F1)¼300 ml/min and storage period (t1)¼40 min. The conditions at the discharge stage are as follows: flow rate of O2 (F2)¼60 ml/min, discharge CSD power ðPdischarge Þ ¼ 2:3 W and discharge period (t2)¼ 10 min. As shown in Fig. 12, the profiles of CO2 evolution were similar, and the carbon balance and CO2 selectivity were kept at 100%. These preliminary results suggested that AgCu/HZ catalyst was very stable during the CSD plasma catalytic process.

4. Conclusions Low-concentration HCHO removal from air using a CSD plasma catalytic process over AgCu/HZ catalysts is reported for the first time. This process exhibited extremely low energy cost, excellent humidity tolerance and almost no secondary pollution. For HCHO storage, AgCu/HZ catalysts possessed much higher HCHO breakthrough capacity than HZ, Ag/HZ or Cu/HZ catalysts. HCHO breakthrough capacity of AgCu/HZ catalysts kept almost constant over a wide range of RH (from 20% to 93% at 298 K).

D.-Z. Zhao et al. / Chemical Engineering Science 66 (2011) 3922–3929

The breakthrough time of AgCu/HZ catalysts increased approximately in inverse proportion to HCHO concentration even in humid gas streams, which indicated that the breakthrough time would be greatly extended for indoor air containing HCHO only at ppb level. At the discharge stage, Ag and Cu in AgCu/HZ catalysts exhibited positive synergistic effect towards complete oxidation of HCHO. The discharge period was almost irrelevant to the storage period, so the energy cost could be lowered with long storage period. Air can be used as a discharge gas for complete oxidation of HCHO and the nitrogen oxides produced can be negligible. AgCu/HZ catalysts also exhibited good stability during the CSD plasma catalytic process.

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