Investigation into Photocatalytic Degradation of Gaseous Ammonia in

Ind. Eng. Chem. Res. 2008, 47, 4363–4368

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Investigation into Photocatalytic Degradation of Gaseous Ammonia in CPCR Qijin Geng,†,‡ Qingjie Guo,*,† Changqing Cao,† Yunchen Zhang,‡ and Lintong Wang‡ Key Laboratory of Clean Chemical Process, College of Chemical Engineering, Qingdao UniVersity of Science and Technology, Shandong ProVince, 266042, People’s Republic of China, and Department of Chemistry and Chemical Engineering, Weifang UniVersity, 261061, People’s Republic of China

Addition of urea-based antifreeze admixtures during cement mixing in construction of buildings has led to increasing indoor air pollution because of continuous transformation and emission of urea to gaseous ammonia on indoor concrete walls. To control ammonia pollution from indoor concrete walls, a circulated photocatalytic reactor (CPCR) was designed to intensify the performance for the decomposition of gaseous ammonia in the present study and TiO2 film photocatalysts were prepared by the sol-gel method and coating onto the inner wall of this reactor using a bonder of polyacrylic ester emulsion, which was characterized by FTIR, TEM, and SEM. In particular, the influences of initial concentration of ammonia on the degradation conversion (Dp), apparent reaction rate constants (kr), initial degradation rate (r), deactivation, and regeneration of catalyst in CPCR were investigated. Furthermore, a designed equation of surface catalytic kinetics was developed for describing the decomposition of ammonia in CPCR. The total number of adsorption sites available for the gas molecules NT and the adsorption equilibrium constant Kads values were determined through a linearfitting method. Finally, undesirable NO2- and NO3- were detected as the intermediates in the process of photodegradation at different initial concentration of ammonia, which was detected by catalytic kinetic spectrophotometry. The results indicated that the degradation conversion (Dp), initial degradation rate (r), degraded products, and half-life (t1/2) were closely correlated to the initial concentration of ammonia. It was found that the reaction kinetics fixed the pseudofirst-order kinetic equation of photocatalytic degradation of gaseous ammonia in CPCR, and the kinetic results are discussed in terms of adsorption of ammonia and products degraded. 1. Introduction In recent years, antifreezes based on urea or ammonia compounds are being widely applied in construction of concrete buildings. The produced gaseous ammonia can be released into the indoor environment through slow diffusion in concrete walls, resulting in the increase of indoor air pollution. Additionally, animal production is a main source of ammonia emission into the environment; repeated exposure can cause eye irritation, acute and chronic skin diseases, and adverse effects on the respiratory system, and it can even affect the nervous system. Therefore, too high a concentration of indoor ammonia has attracted people’s close concern, and how to reduce the risk caused by ammonia in indoor air becomes a big issue in some countries, particularly China. Most of the best available techniques that could be used today are not very widely applied in the field because of costs, especially in existing livestock buildings. The traditional remediation techniques, such as adsorption, ventilation, and filtration, are not effective for such low concentration pollutants. Oxidation is currently considered to be the most effective technology to decompose the airborne pollutants. In particular, ultraviolet (UV) related technologies are very effective compared with other oxidation processes on removing the contaminants in air at considerably mild conditions. Among the UV-related processes, UV/TiO2 process has been considered to be a promising alternative for decomposition of various refractory contaminants in gaseous streams in the past decades. The photocatalytic oxidation of ammonia in UVirradiated TiO2 has been reported by Ismagilov1,2 and Son,3 and support materials including paper,4 nonwoven textile,5 and * To whom correspondence should be addressed. Tel.: +86-53284022506. Fax: +86-532-84022757. E-mail [email protected]. † Qingdao University of Science and Technology. ‡ Weifang University.

cotton woven fabrics6 have been developed for possible applications of TiO2 photocatalysis process. The kinetic analysis on photocatalytic degradation of gaseous ammonia over the nanosized porous TiO2 films has been reported by Sopyan.7 However, there is little information in the current literature concerning adsorption and degradation of high concentration of gaseous ammonia by titanium dioxide catalyst supported on latex painting film and discussion of half-life (t1/2) of ammonia in a circulated photocatalytic reactor (CPCR). In this paper, a nano-TiO2 coating film photocatalyst was prepared with nanosized TiO2 gel and bonded with the water-fast polyacrylic ester emulsion to decompose gaseous ammonia photocatalytically in the designed CPCR. In addition, a detailed discussion is done on the initial concentration of ammonia and degraded products of ammonia influencing the photocatalytic degradation of ammonia. Moreover, the kinetics parameters of the photocatalytic degradation of ammonia in terms of Langmuir-Hinshelwoods (LH) kinetic model are investigated. 2. Experimental Section 2.1. Preparation and Characterization of Catalyst. Anatase TiO2 was prepared by a sol-gel method at low temperature using titanium butoxide (Ti(OC4H9)4, AR, China) as a precursor. In a typical synthesis, 10 mL of titanium butoxide was disolved in 50 mL of anhydrous ethanol, and the pH value, adjusted to 2.5 with HNO3, was measured by pHS-3B (Shanghai Tianpu Analysis Equipment Co., Ltd., China). The butoxide solution was added dropwise into 100 mL of deionized water with vigorous stirring (JB90-S, Shanghai, China) at room temperature. The solution obtained was dispersed in an ultrasonic cleaner (SK-5200HP, 200 W, 59 kHz, Shanghai Kudos Precision

10.1021/ie800274g CCC: $40.75  2008 American Chemical Society Published on Web 06/05/2008

4364 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008

Figure 2. FTIR of TiO2 powder supported on latex film. Figure 1. Schematic diagram of CPCR.

Instruments Co., Ltd., China) for 1 h and aged at room temperature for 12 h. To produce a nano-TiO2 film photocatalyst with nano-TiO2 fixed to the inner wall of quartz glass tube, a typical preparation process is given by the following: (1) the supported film was coated on inner wall of quartz glass tube with the water-fast polyacrylic ester emulsion (PB-09, Qingzhou Baoda Chemical Co., Ltd., China) and dried at room temperature; (2) the pure TiO2 sol was coated over the supported film fixed on the inner wall of the quartz glass tube by the rotation coating method and dried at 125 °C in a vacuum oven (DZF-6020, Shanghai Boxun Industry & Commerce Co., Ltd., China). This process could be repeated several times. The crystalline structure of the TiO2 sol particles and coating film were analyzed by FTIR (FTIR-380, Thermo Nicolet Corp., USA), SEM (field emission scanning electron microscopy, JSM6700F, JEOL, Japan) and TEM (transmission electron microcopy, JEM2010, JEOL, Japan), respectively. 2.2. Design of CPCR. The circulated photocatalytic degradation reactor used in this research is composed of an annular quartz glass tube (inner dimension of tube, 90 mm; the length 500 mm; working volume 2.82 L), nano-TiO2 film (surface area 0.141 m2) coated on inner wall of quartz glass tube and an ultraviolet lamp (25 W, Shanghai Yaming Lighting CO., Ltd., China), with the emission wavelength ranging from 228 to 400 nm and the maximum emission intensity at 253.7 nm. The central axis of the lamps is at the center of quartz glass tube, and the diameter distance from the lamp surface to the inner wall of quartz glass tube is 35 mm. The schematic diagram of photodegradation system is shown in Figure 1. 2.3. Absorptive Property of Catalyst. Adsorption experiment under dark conditions in CPCR was conducted to study the isothermal adsorption of ammonia. First, the valve of the reactor and the air pump are opened and the gaseous pollutant in reactor is circulated from the reservoir to the reaction region at 0.05 L min-1 for 50 min. Second, on reaching adsorptiondesorption equilibrium, the ammonia samples are collected from the sampling pore by a syringe (5 mL), measured by ultraviolet-visible spectrometer (TU-1901, Beijing Purkinje General Instrument Co., Ltd., China): 5 mL of ammonia sample is dissolved into 10 mL of 0.5 mmol L-1 H2SO4 (AR) solution, where 0.5 mL of 5% salicylic acid (C6H4(OH)COOH, AR) solution, 0.1 mL of 1% sodium nitroprusside dihydrate (Na2Fe(CN)5NO · 2H2O, AR) solution, and 0.1 mL of 0.05 mol L-1 hypochlorous sodium (NaClO, AR) solution are fed into in turn; after it reacted completely, we measured the absorbance at the maximum absorption wavelength (λm) 697 nm in

comparison with water as blank. Calibration plots relevant to the absorbance to the concentration of ammonia were established based on Beer-Lambert’s law. Thereafter, the adsorption efficiency of ammonia was calculated by eq 1. 2.4. Photocatalyses and Analyses. The photocatalytic activity of the film catalyst in CPCR was evaluated by measuring the change of ammonia concentration as a function of irradiation time. First, the valve of the reactor and the air pump were opened and the gaseous pollutant in reactor was circulated from the reservoir to the reaction region with the required speed 50 min to reach adsorption-desorption equilibrium; then the UV lamp was switched on to illuminate the nano-TiO2 film under the gas circulation at the speed of 0.05 L min-1. At regular time intervals of 30 min, 5 mL ammonia samples were taken to measure as mentioned before. Thereafter, the decomposition efficiency (Dp) of ammonia was calculated by eq 2. % adsorption efficiency )

C0 - Ct × 100 C0

% decomposition efficiency )

Ce - Ct × 100 Ce

(1) (2)

where C0 and Ce are the original and equilibrium ammonia concentrations, respectively, Ct is the remaining ammonia concentration at t minutes. 2.5. Detection of Decomposition Products. After washing the inner wall of reactor completely after one cycle of photocatalysis, half of the obtained solution was taken to measure residual ammonia by spectrophotometer, and the other half to detect photocatalytic decomposition products of ammonia, such as nitrate (NO2-) and nitrite (NO3-). The concentration of NO2- (NO3- can reduce to NO2- by zinc powder) was detected by catalytic kinetic spectrophotometry under wavelength at 520 nm.8 3. Results and Discussion 3.1. Characterization of TiO2 Film Catalyst. Figure 2 illustrates the FTIR spectra of the polyacrylic ester emulsion film supported TiO2 powder. From the spectra, we can infer that the peaks of 450-750,9 1731, 1485, and 2870-2980 cm-1 were the stretching vibration of Ti-O bond, CdO, COO-, and saturated C-H bond in polyacrylic ester, respectively. Figures 3 and 4 show the TEM picture and the SEM micrograph of powder supported on the film of the quartz glass tube, respectively. As shown in the TEM photographs, the TiO2 particulates were uniformly distributed on the surface of the emulsion film. The particulates were of spherical to subspherical shape and well dispersed. The deposition of TiO2 particles on

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Figure 3. TEM picture of TiO2 powders.

Figure 5. Adsorption, degradation, and regression results by eq 4 for the adsorption of ammonia in CPCR.

Figure 4. SEM micrograph of TiO2 powders.

Figure 6. Relationship between degraded products and initial concentration of ammonia.

quartz glass tube is uniform; most of them load evenly on the surface with diameters range from 5 to 10 nm. These particles act as small crystals with average diameters of ca. 5-10 nm which constitutes the polycrystalline films, apparently rather porous. The homogenous deposition of titania particles on quartz glass tube can be attributed to the method of depositing titania onto the support under vacuum conditions. Compared with the traditional dipping method, this new method has two virtues: first, the coating nanoparticles can deposit more uniformly onto the support and the nonuniformity of large-sized aggregation of the coating nanoparticles can be avoided; second, the coating nanoparticles can deposit more firmly onto the support because the residual air on the support can be removed thoroughly under vacuum in the depositing process and catalyst was bonded by polyacrylic ester emulsion. 3.2. Adsorption of ammonia on catalyst. The isothermal adsorption of ammonia on the film catalyst in the CPCR has been studied. The kinetics of adsorption in the dark was followed during 50 min under circulation at the speed of 0.05 L min-1 for the different initial concentrations of ammonia ranging from 3.68 to 38.10 mg L-1. The conventional Langmuir isotherm model with a surface coverage θ is given by θ)

Nads KadsCe ) NT 1 + KadsCe

(3)

which can be used to determine the total number of adsorption sites available for the gas molecules, NT, and the adsorption

Table 1. Langmuir Isotherm Parameters: Total Number of Adsorption Sites (NT) and Adsorption Constant (Kads) per m2 NT (mmol m-2)

Kads (mmol-1 m-2)

R

34.23

202.18

0.994

a

a

N

P

7

Eg), 3.2 eV for TiO2, produces excited high-energy states of electron and hole pairs (e-/h+). Part of these photogenerated carriers recombine in the bulk of the semiconductor, while the rest migrate to the surface of particles, where the holes act as powerful oxidants and the electrons act as powerful reductants and initiate a wide range of chemical redox reactions, which can lead to complete mineralization of pollutants. Nano-TiO2 film photocatalyst supported on the inner wall of the quartz glass tube in CPCR was expected to intensify the photodegradation of ammonia. Figure 5 showed that the degradation efficiency of ammonia was a function of the initial concentrations of ammonia, and this catalyst could have degraded ammonia obviously in the range of ammonia concentrations from 3.68 to 38.10 mg L-1. In addition, the degradation efficiency increased with the increase of ammonia concentration at very low concentration, and then the degradation efficiency of ammonia decreased with increase of ammonia concentration at high concentration. Similar results were observed and discussed for the decomposition of trichloroethylene5 and ethylene14 by UV/TiO2 process with saturated adsorption under the fixed active sites. The TiO2 film photocatalysts based on the inner wall of CPCR has high photocatalytic activity in terms of the high degradation efficiency (Dp) of ammonia in Figure 5. This may be owing to the generation of hydroxyl radicals which can attack organic substrates adsorbed on the surface of catalyst to lead to their degradation and mineralization. The further explanation is based on the generation of hydroxyl radicals on the surface of TiO2 particulates based on the latex film. In the case of TiO2 film, photocatalysts on latex film may provide with enough water to facilitate the production of hydroxyl radicals, resulting in the improved photocatalytic oxidation of ammonia. A previous study has reported that decomposition of ethanol by UV/TiO2 process was enhanced with an increase in the relative humility.14 In addition, this may be related to microstructure characteristics of the coating film. It is a fact that the latex film has relatively loose microstructure as a result of high amorphous regions and lower crystalline regions. Therefore, it is believed that the latex film may exhibit high adsorption affinity for ammonia by means of their loose microstructure. The relatively high adsorption of ammonia onto the latex film may accelerate photocatalytic oxidation of ammonia on the catalyst surface. On the other hand, the degraded products may have a great influence on the photocatalytic action, just because ammonia can be oxidized to NO2- and NO3- by •OH radical or superoxide radical anions (O2•-) which are generated on the surface of TiO2. From the experimental result of NO2- and

NO3- in Figure 6, it is shown that the total amount of NO2and NO3- increases with increasing initial ammonia concentration. Figure 6 also shows that the relationship between the concentration of ammonia ranging from 3.68 to 38.10 mg L-1 and degradation products is approximated to the beelines, respectively, and it is approximated to the functions given by Cp1 ) 1.176 + 0.745C0(3.68 - 18.10 mg L-1 ;

R ) 0.973) (5)

Cp2 ) 11.12 + 0.132C0(18.10 - 38.10 mg L-1 ;

R ) 0.997) (6)

where Cpi and C0 are the amount of degraded products and the initial concentration of ammonia, respectively; and the slopes of the linear regression expressions are k1 ) 0.745 and k2 ) 0.132, respectively. The results show that the variation of concentration of degraded products in the photocatalytic degradation process is in agreement with the change of degradation efficiency in Figure 5. The product amount increases dramatically on increasing the ammonia concentration from 3.68 to 18.10 mg L-1 (k1 ) 0.745), while it increases slowly with a further increase of ammonia concentration over 18.10 mg L-1 (k2 ) 0.132). 3.3.2. Influence of Initial Concentration on Degradation Kinetics. The reaction rate can be described by LangmuirHinshelwoods kinetic rate mechanism, the simplest representation for the reaction rate of ammonia is given by eq 7. r)-

KadsC0 dC ) kr dt 1 + KadsC0

(7)

First, when the concentration of ammonia is very small, the value of KadsC0 is far smaller than 1, so eq 7 can be abbreviated as eq 8. dC ) krKadsC0 dt

r)-

(8)

where r is the reaction rate for the degradation of ammonia and C is the concentration of ammonia at the reaction time t minutes. The integrated form is given by -ln

C ) krKadst C0

(9)

From the above formula, the following conclusion can be reached: the relationship between -ln(C/C0) and reaction time t is linear. In this case, the reaction follows the ideal first-order expression, and the half-life can be given by eq 10. t1/2 )

ln 2 Kadskr

(10)

Second, the integrated form of eq 7 is C0 C0 - C C + t) Kadskr kr ln

(11)

where C0 is the initial concentration of ammonia and t is the reaction time. When C/C0 ) 0.5, without consideration of / adsorption of products and intermediates, t1/2 can be obtained: t/1/2 )

ln 2 0.5 + C Kadskr kr 0

(12)

From the comparison eq 10 to eq 12, the difference between / t1/2 and t1/2 is not only related to ln 2/Kadskr, but also to 0.5/

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Figure 7. Relationship between t1/2 and initial concentration of ammonia.

krC0, and t/1/2 extends with the increase of the initial concentration of ammonia, and therefore this reaction follows a pseudo-firstorder expression. The experimental results in Figure 7 show that t1/2 increases from 0.97 to 12.38 h with the concentration of ammonia ranging from 3.68 to 38.10 mg L-1. It is obvious that the half-life increases with the increase of initial ammonia concentration, because it is important for adsorption of reactant on catalyst surface in photocatalysis, but the adsorption of reactant on catalyst surface increases with the increase of initial ammonia concentration without competition by degradation products. Finally, ammonia can be degraded and turned into NO2- and NO3- which can adsorb on catalyst surface. If we consider it, eq 5 may be modified as eq 13. r)-

KadskrC0 dC ) dt 1 + KC0 + KpCp

(13)

where Kp is the adsorption constant of the degraded product and Cp is the content of the products. On the assumption of Cp ) C0 - C, the integration of eq 10 yields C0 C0 + Kp(C0 - C) KpC0 ln C C C 0 C t) + + Kadskr kr Kadskr ln

(14)

// When C ) 0.5C0, t1/2 is given by

t// 1/2 )

ln2 + 0.5Kp Kp ln2 + 0.5Kads + C0 Kadskr Kadskr

(15)

// t1/2 is the half-life with considering of the competition byproduct, so the competition between ammonia and its products should not neglected. In comparing eq 12 with eq 15, the conclusion can be obtained that the extending of t1/2 is not only related to the increasing of initial ammonia concentration, but also to the adsorption of degraded products with increase of initial concentration of ammonia as well. These results may be related to the following reasons: (1) the degraded products, NO2- or NO3-, adsorbed on the surface of catalyst and inhibited the ammonia molecular from adsorbing into the reactive sites of catalyst; (2) the degraded products, as scavengers of holes generated in photocatalytic process, were detrimental to the photocatalytic action; (3) the process of photocatalytic degradation of ammonia, acting as a radical mechanism, in which ammonia was transited into NO3- by the mediate product NO2-, may be retarded by the transition of NO2- to NO3-. 3.4. Deactivation and Regeneration of Catalyst. The deactivation of photocatalyst in the process of degradation of

Figure 8. Deactivation and regeneration of catalyst (catalyst 31.2 mmol; illumination time 3 h; gas flow rate 0.05 L min-1.)

ammonia may be explained on the basis of the following reasons: (1) The degraded products, NO2- and NO3-, adsorbed on the surface of catalyst which inhibited the ammonia molecular to adsorb into the reactive sites of catalyst, just as Wang15 reported that deactivation of catalyst was observed and attributed to the adsorption of reaction intermediates on TiO2 surface. (2) The degraded products or NH4+, as scavenger of holes or electrons, were detrimental to the photocatalytic action.7 The regeneration was very important for the practical application of photocatalyst. In this section, the regeneration of the film photocatalyst is investigated. At low (13.50 mg L-1) and high (23.40 mg L-1) concentration of ammonia, respectively, comparing the removal efficiencies of ammonia in the first run with the second one of photocatalysis in CPCR, they decreased obviously in the second run, particularly at higher concentration of ammonia; this is because the active center of TiO2 adsorbed more degraded products at high concentration of ammonia than at the low one, which inhibited photodegradation of ammonia. However, the reactivity of catalyst can be regenerated by washing completely and drying again as shown in the runs 3, 4, and 5 of Figure 8. From the results we can conclude that only a little of the TiO2 particles coated on quartz glass tube releases or dissolves into the washing solution because if the titania released or dissolved into the washing solution, by repetitive use of the catalyst, the photocatalytic activity would decrease sharply. The feasibility of the regeneration method can be explained by the following: (1) the bonder between the catalyst particles and the inner wall of the quartz glass tube is water-tight polyacrylic ester emulsion, so the TiO2 film immobilized at the inner surface of the reactor is very stable against washing damage; (2) the degraded compounds adsorbed on the catalyst surface, such as NO2- or NO3-, can be removed easily by washing because they are physically adsorbed on the catalyst surface and dissolved in water. 4. Conclusions The following conclusions can be derived from the present study.

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1. The preparation of titanium dioxide film photocatalyst supported on polyacrylic ester emulsion and the successful utilization of it in CPCR for the photocatalytic degradation of gaseous ammonia ranging from 3.68 to 38.10 mg L-1 in the presence of UV light was investigated; as a result, CPCR was an efficient photocatalytic reactor, in particular, the separation postreaction and regeneration of catalyst without major technical or economical problems. 2. High adsorption efficiency was achieved by TiO2 film catalyst in CPCR; in particular, we discussed the mechanism of adsorption between gaseous ammonia molecules and sites of catalyst or the microstructure of polyacrylic ester emulsion film; a synergistic effect was observed leading to an enhancement the adsorption process. 3. Analyzing the variation of t1/2 at low and high concentration of ammonia, we have developed the kinetic model of photocatalytic degradation of ammonia, which is used to explained the progress of photocatalysis. Acknowledgment This investigation was supported by National Natural Science Foundation of China (Contract No. 20676064), Construction Project of Taishan Scholar of Shandong Province (JS200510036), Awarding Foundation of Young Scientist of Shandong Province (2006BS08002), and Program for New Century Excellent Talents in University (NCET-07-0473). Nomenclature Ce (mg L-1) ) equilibrium concentration Ct (mg L-1) ) remaining concentration at t minute C0 (mg L-1) ) initial concentration Cp (mg L-1) ) degraded product concentration Dads (%) ) adsorption efficiency Dp (%) ) degradation efficiency Eg (eV) ) band gap energy hγ (eV) ) light energy Kads (mmol-1 m-2) ) adsorption equilibrium constant of ammonia Kp (mmol-1 m-2) ) adsorption equilibrium constant of ammonia kr (mmol m2 s-1) ) reaction rate constants k1, k2 ) slope of linear regression expressions Nads (mmol m-2) ) number of adsorbed gaseous molecules NT (mmol m-2) ) total number of adsorption sites r (mg L-1 min-1) ) initial degradation rate θ (%) ) surface coverage

t (min) ) time / // t1/2, t1/2 , t1/2 (min) ) half-life

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ReceiVed for reView February 18, 2008 ReVised manuscript receiVed April 14, 2008 Accepted April 29, 2008 IE800274G