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Environ Eng Res > Volume 21(3); 2016 > Article
Mendoza, Lee, and Kang: Photocatalytic removal of NOx using TiO2-coated zeolite

Abstract

Application of photocatalytic nanoparticles has been recently gaining an increased attention as air purifying material for sustainable urban development. The present work reports the photocatalytic removal of gaseous phase nitrogen oxides (NOx) using TiO2–coated zeolite to be applied as a filter media for the urban green infrastructure such as raingardens. The TiO2–coated zeolite was synthesized by simple wet chemistry method and tested in a continuous-flow photo-reactor for its removal efficiency of NOx under different conditions of the weight percentage of TiO2 coated on the zeolite, and gas retention time. The removal efficiency of NOx in general increased as the weight percentage of TiO2 coated on the zeolite increased up to 15–20%. Greater than 90% of NOx was removed at a retention time of one minute using the TiO2–coated zeolite (TiO2 weight percentage = 20%). Overall, TiO2–coated zeolite showed greater efficiency of NOx removal compared to TiO2 powder probably by providing additional reaction sites from the porous structure of zeolite. It was presumed that the degradation of NOx is attributed to both the physical adsorption and photocatalytic oxidation that could simultaneously occur at the catalyst surface.

1. Introduction

Nitrogen monoxide (NO) and dioxide (NO2), which are collectively called nitrogen oxides(NOx) are harmful and poisonous gases that are emitted mainly from anthropogenic sources such as industrial power plants and automobile engines [12]. Air pollution caused by photochemical oxidants, such as ozone and NOx, is one of the serious problems faced by urban areas [3]. The annual mean concentrations of NOx in urban areas globally are in the range of 20–90 ppb but hourly averages can often exceed 1000 ppb on heavy vehicular traffic conditions [4]. NOx has significant impacts on the environment, human and animal health, and plant vegetation. Adverse effects of NOx include acid rain, photochemical smog, ozone layer depletion, greenhouse effect, and ecological toxification [5]. Furthermore, NOx has been also a recurring problem of worsening indoor air quality (IAQ) in most building structures, and diseases related to lower IAQ have been reported [6]. Recent study done by Shakerkhatibi, et al. (2015) [7] showed that gaseous air pollutants of NO2, NO, and CO were associated with the hospital admissions for chronic obstructive pulmonary disease.
The needs for mitigating the negative effects of NOx have increased over the past few decades. Photocatalytic oxidation (PCO) has been suggested as an efficient and cost effective approach to control airborne pollutants such as NOx [8]. PCO relies on photocatalysts which utilize ultraviolet (UV) light radiation from sunlight or artificial light assisting in oxidizing various pollutants [9]. Among the photocatalysts used in PCO processes, titanium dioxide (TiO2) has been most widely used because of its chemical stability, non-toxicity, and relatively low cost [1011]. In addition, TiO2 nanoparticles are able to provide more active sites than standard TiO2 powder [12], making the degradation of pollutants more effective. Upon irradiation of UV light, TiO2 generates electrons and holes in the conduction and valence bands, respectively which could participate in the oxidation-reduction (redox) reactions for pollutant degradation [1315]. The proposed mechanism of NOx photocatalytic oxidation consist of three stages: NO initially reacts with the OH- radical formed from the TiO2 surface reaction of H2O and oxygen, resulting in HNO2 (first stage) before subsequently becoming HNO3 (second stage) that would be desorbed at the TiO2 surface, and thus regenerating the catalyst (third stage). The mechanisms were described in more detail by various researchers [1618].
Since the first discovery of super-hydrophilicity of TiO2 by Fujishima, et al. (2000) [19], TiO2 has been applied to building materials with the aim of air cleaning, self-cleaning, and anti-fogging functions [2023]. Recent applications of TiO2 has been widen to the outdoor building materials such as pavements and concrete surfaces to control urban airborne pollutants such as NOx [18, 2328]. The present study is a preliminary study to eventually propose the application of photocatalytic nanoparticles to the surface media layer (i.e. zeolite) of urban green infrastructure practices such as rain gardens, providing multiple functions of controlling air and water quality in urban settling. Rain garden, or also called as bio-retention area, is one of the urban stormwater management practices recommended by the United States’ Environmental Protection Agency (US EPA) since 2000 [29]. Rain gardens are low depressions in the landscape that are planted with trees and/or shrubs, and covered with a bark mulch layer or ground cover allowing for the infiltration of storm water to recharge aquifers, and reduce surface runoffs. Various designs have been developed for rain garden systems [30] but researches on the use of nanoparticles on stormwater management practices are limited in number [2426, 3132].
The scope of the present research is directed on identifying the effectiveness of applying TiO2 nanoparticles on natural zeolites, which will be added within the rain garden system, for adding function of NOx removal from the urban atmosphere. The photocatalytic removal of NOx in air was investigated thru a lab-scale continuous flow reactor with TiO2 coated onto natural zeolite as a preliminary study for its applicability to actual rain garden systems.

2. Materials and Methods

2.1. Synthesis of the Nanoparticle-coated Zeolite

Natural zeolite (DAEJUNG Chemicals, Korea) with a mean diameter of 3 mm was calcined at 600°C for 2 h to remove any organic impurities present, and subsequently cooled at room temperature. Then, the prepared zeolite was added to a flask with an aqueous suspension of TiO2 (DAEJUNG Chemicals, Korea) nanoparticles (mean diameter was 300 nm), and was shaken at 150 rpm for 2 h to ensure homogenous coating of the TiO2 nanoparticles on the zeolite surface. Finally the mixed solution was oven dried at 105oC for 24 h followed by calcination at 600oC for 2 h, producing the TiO2-coated zeolite. Scanning electron microscope (SEM; JEOL-7800F, JEOL, Japan) with Energy Dispersive X-ray Spectrometer (EDS) and X-ray diffractometer (XRD; Ultima IV, Rigaku, Japan) were conducted to characterize the morphology, composition, and crystallinity of the TiO2-coated zeolite. Surface area measurements were carried out using the BET analysis method (Autosorb-iQ 2ST/MP, Quantachrome, U.S.A.).

2.2. Photocatalytic NOx Degradation Experiments

A continuous-flow photo-reactor was used to measure the performance of the TiO2-coated zeolite in removing NOx in the air. Fig. 1 shows the laboratory setup consisting of NOx source (5 ppm NO in N2, DONG-AH Gas, Korea), purified air source (< 1% hydrocarbon impurities, DONG-AH Gas, Korea), flow controllers (DWYER, USA), box-type photo-reactor (made of acryl), UV-A lamp (20 W, SANKYO DENKI, Japan) and a chemiluminiscent NOx analyzer (ECOTECH, SERINUS 40, Australia). Either of the two photo-reactors with different dimensions (Reactor 1: L = 310 mm, W = 110 mm, H = 55, Reactor 2: L = 600 mm; W = 220 mm; H = 100 mm) was used when appropriate for the convenience of adjusting the gas retention time of the reactor to a required value during the test. A total of 6 UV-A lamps (20 W each with light intensity of 0.38 mW/cm2) were simultaneously used to provide sufficient light energy for the photocatalytic reaction.
An appropriate amount of the TiO2-coated zeolite was loaded into the reactor and then the reactor was carefully sealed. Afterwards, the NO containing nitrogen gas and the purified air were allowed to flow into the reactor at flow rates of 0.2 L/min and 2 L/min, respectively, until equilibrium NOx concentration in the inflow has achieved. The purified air was passed through a humidifier before being mixed with NO containing nitrogen gas in order to achieve a required level of relative humidity (40–60%) in the reactor. The gas flow was continued for 30 minutes before the light source was turned on. Afterwards, the TiO2-coated zeolite was irradiated for 60 minutes during which the NOx concentration of the outflow gas was recorded at 1 min intervals by the NOx analyzer. After the one hour of irradiation, the light source was turned off and then the gas valves were closed. All experiments were conducted at ambient temperature (18–20°C). The detailed experimental procedure can be referred to published literatures [2527, 3132] and the ISO 22197-1:2007 standard for air purification performance of semiconducting photocalytic materials [33].

3. Results and Discussion

3.1. Characterization of the Synthesized Coated Zeolite

The scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS) images of the TiO2-coated zeolite were presented in Fig. 2. The TiO2 nanoparticles are fairly distributed on the zeolite surface but particle agglomerations were often observed on the zeolite surface probably due to the sintering of TiO2 particles at high temperature calcination [33, 36]. However, it should be mentioned that the controlled distribution of guest particles (TiO2) with minimal particle agglomeration on the host particle surface (zeolite) is still an active research in the nano-technology field [8, 1213, 1819]. The EDS scan confirmed that TiO2 particles were present on the zeolite surface based on the detected elements. The major peak of Ti at ~4.5 keV seen on the EDS spectrum represents the binding energy for the Ti4+ oxidation present in most TiO2 phases (such as anatase, rutile, and brookite). The crystal structure of the synthesized coated zeolite was examined using XRD. The diffraction peaks matched with that of tetragonal anatase TiO2 (a = 3.7892 Å, c = 9.5370 Å; Crystallography Open Database, No. 5000223) indicating that TiO2 was successfully loaded on the zeolite surface as shown in Fig. 3.
The BET surface listed in Table 1 exhibits a decrease in surface area when TiO2 was coated in zeolite. This could be due to the decrease of zeolite mesopore sites where N2 gas was adsorbed, which is also an indication of a successful coating of the zeolite surface with the TiO2 nanoparticles. However, the decreased in BET surface area (i.e., mesopores) had negligible effects to the pollutant removal efficiency of the coated zeolite.

3.2. Effect of TiO2 Particles in the Photocatalytic NOx Degradation

Preliminary experiments were performed using TiO2 particles only as shown in Fig. 4. After the UV lamp was turned on, an immediate decrease in the NO and NOx concentration was observed. NO concentration remained constant throughout the irradiation period of one hour while NOx (NO + NO2) concentration gradually increased due to the increased concentration of NO2 created from the oxidation of NO. The NOx removal efficiency (R.E.) was calculated using Eq. (1).
(1)
%R.E.=(Co-CAve)Co×100%
where Cave is the average concentration of NOx in the outflow during the one hour of irradiation, and C0 is the NOx concentration of the inflow. Before the light source was turned on, no significant change in the pollutant gas was observed, indicating that the chemical transformation of NOx can be attributed to the photocatalytic mechanism [10, 1314]. The calculated NOx and NO removal efficiencies based on the concentration profile from Fig. 4 were 48% and 64%, respectively, which are similar to most of the previously reported values in the literature [18, 20, 2628, 3132]. When the light source was turned off after the one hour of irradiation, NOx concentration immediately returned to its initial value, indicating no permanent physical adsorption of NOx on the photocatalyst particles.

3.3. Role of Zeolite in the Photocatalytic Degradation

The removal efficiency generally improved when TiO2 was coated onto natural zeolite as shown in Fig. 5. The improved efficiency can be due to the capability of zeolite to act as adsorbent for the nitrate (NO3) evolved from the photocatalytic oxidation of NOx. Zeolite can also provide for increased active sites for NO gas to react with TiO2 because of its porous structure [35, 36]. That is zeolite can assist TiO2 nanoparticles in capturing the target pollutants for the subsequent photocatalytic reactions [3739].
A control test was performed; the uncoated natural zeolite was placed inside the photo-reactor and subjected to the same irradiation procedure, and no significant change was observed in the NOx concentration, indicating that there is a synergistic effect with the addition of TiO2 for the efficient removal of NOx.

3.4. Effect of Varying TiO2 Mass Loadings in Zeolite Media

Three TiO2-coated zeolite with different TiO2 weight percentages (i.e. 10 wt.%, 15 wt.%, and 20 wt.%) were compared in terms of the NOx removal efficiency. Overall, as the weight percentage of TiO2 with respect to zeolite increased, the removal efficiency also increased, which might be due to the increased active sites for photocatalytic reactions in the media. However, variances were observed in the removal efficiency among different replicate samples of the coated zeolite, which could be due the uneven distribution of the TiO2 particles over the zeolite aggregates during the coating process. The variation in NOx removal efficiency among different replicate samples decreased as the mass percentages of TiO2 increased due probably to the increased probability of the TiO2 particles to be well distributed over the zeolite aggregates. Therefore, the TiO2-coated zeolite with 20 wt.% TiO2 was used for the subsequent photocatalytic experiments.

3.5. Effect of Gas Retention Time in the Photocatalytic Degradation

Fig. 6 shows the removal efficiencies of NOx using the coated TiO2-zeolite at different gas retention times. The removal efficiency increased as the gas retention time increased. Lower retention time values have lesser time for the pollutant gas to come into contact with the TiO2 catalyst and thereby reducing its efficiency. Greater than 90% of NOx was removed at a retention time of one minute using the TiO2-coated zeolite (TiO2 weight percentage = 20%).

4. Conclusions

The effectiveness of coating TiO2 particles into natural zeolite has been demonstrated in this study. Generally, the removal efficiency of NOx increased as the weight percentage of TiO2 coated on the zeolite but variances in the NOx removal were observed among different replicate samples of the coated zeolite, which could be due to the uneven TiO2 distribution on the zeolite aggregates during the coating process. Overall, TiO2-coated zeolite showed greater efficiency compared to TiO2 powder probably because zeolite can provide additional reaction sites from its porous structure. The degradation of NOx have been attributed to both the physical adsorption and photocatalytic mechanisms simultaneously happening on the catalyst surface. A proportional relationship was also seen between the removal efficiency and retention time as well. Sufficient contact time with the particles would be required to ensure adsorption and degradation of the pollutant. More than 90% of NOx was removed at a retention time of one minute using the TiO2-coated zeolite (with TiO2 weight percentage at 20%).

Acknowledgments

This research was supported by a grant (14CTAP-C086804-01) from the Technology Advancement Research Program funded by the Ministry of Land, Infrastructure and Transport of the Korean government.

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Fig. 1
Schematic of the laboratory setup for the NOx removal experiment.
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Fig. 2
Scanning electron microscope (SEM) and the energy dispersive X-ray spectrometer images for the TiO2 coated zeolite.
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Fig. 3
X-ray diffraction pattern for the TiO2-coated zeolite.
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Fig. 4
Example of the NOx concentration profile with respect to time during photocatalytic experiments.
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Fig. 5
Comparison of the NOx degradation performance between TiO2 nanoparticles and TiO2 coated zeolite. Notes: T1 = TiO2 powders under UV-A irradiation; TCZ-1 = TiO2 (10 wt.%) coated zeolite; TCZ-2 = TiO2 (15 wt.%) coated zeolite; TCZ-3 = TiO2 (20 wt.%) coated zeolite. Error bars indicate standard deviations of four replicate experiments.
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Fig. 6
Effect of retention time on the removal efficiency for (20 wt.%) TiO2 coated zeolite. Error bars indicate standard deviations of four replicate experiments
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Table 1
BET Surface Area of the Tio2 Coated Zeolite and Bare Zeolite
Catalyst BET Surface Area, m2/g
TiO2 coated zeolite 80.593
Zeolite only 208.782
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