Science and Technology for the Built Environment
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Deactivation and ultraviolet C-induced regeneration of photocatalytic oxidation air filters Lexuan Zhong, Chang-Seo Lee, Fariborz Haghighat & Ali Bahloul To cite this article: Lexuan Zhong, Chang-Seo Lee, Fariborz Haghighat & Ali Bahloul (2016): Deactivation and ultraviolet C-induced regeneration of photocatalytic oxidation air filters, Science and Technology for the Built Environment, DOI: 10.1080/23744731.2016.1171629 To link to this article: http://dx.doi.org/10.1080/23744731.2016.1171629
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Date: 19 May 2016, At: 07:47
Science and Technology for the Built Environment (2016) 00, 1–10 C 2016 ASHRAE. Copyright ISSN: 2374-4731 print / 2374-474X online DOI: 10.1080/23744731.2016.1171629
Deactivation and ultraviolet C-induced regeneration of photocatalytic oxidation air filters LEXUAN ZHONG1, CHANG-SEO LEE1, FARIBORZ HAGHIGHAT1,∗, and ALI BAHLOUL2 1
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Department of Building, Civil, and Environmental Engineering, Concordia University, Montreal, Quebec H3G 1M8, Canada Institut de Recherch´e Robert-Sauv´e en Sant´e et en S´ecurit´e du Travail (IRSST), Montreal, Quebec, Canada
Ultra-violet photocatalytic oxidation has been regarded as one of the promising air purification technologies for improving indoor air quality. However, limited availability of experimental data in terms of photocatalyst deactivation and regeneration has hindered successful implementation of ultra-violet photocatalytic oxidation air cleaners in mechanical ventilation systems. The objective of this study is to obtain knowledge of the ultraviolet C-induced regeneration method, the simplest on-site approach, for the recovery of photocatalytic activity of photocatalytic oxidation filters after challenging ultra-violet photocatalytic oxidation systems with approximately 100 ppb of acetone or methyl ethyl ketone. Experimental observations of photocatalyst deactivation, and characterization of fresh and deactivated photocatalyst with the scanning electron microscope technique were presented. During the regeneration process, the production rates of formaldehyde, acetaldehyde, and acetone were hourly quantified under a short-term and a long-term ultraviolet C illumination. The regeneration performance was also examined and compared by testing the singlepass removal efficiency of regenerated photocatalytic oxidation filters by two methods: ultraviolet C illumination and O3 -included ultraviolet C illumination. The results indicate that the ultraviolet C-induced regeneration method, superior to O3 -assisted ultraviolet C method, plays a certain role in partial recovery of the photocatalytic activity. The degree of recovery would depend on the nature of contaminant gases previously processed in ultra-violet photocatalytic oxidation since different VOCs generate various types and amounts of surface adsorbed by-products, which resist regeneration at a different level. Graphical Abstract. UV-PCO represents a new generation technology for improving indoor air quality. However, little is known about photocatalyst deactivation and regeneration which is a major concern for the purpose of commercialization. The current objective is to explore the UVC-induced regeneration method, the simplest on-site approach, employed in an HVAC system. This work demonstrates a systematic evaluation of deactivation and UV-induced regeneration performance under the conditions relevant to the actual applications for two VOCs. In addition, the gaseous by-product generation rates and O3-assited UVC-induced recovery method were examined for the first time.
Introduction Received December 7, 2015; accepted March 2, 2016 Lexuan Zhong, PhD, is a Postdoc research fellow. Chang-Seo Lee, PhD, is a Research Associate. Fariborz Haghighat, PhD, Fellow ASHRAE, is a Professor. Ali Bahloul, PhD, is a Researcher. ∗ Corresponding author e-mail:
[email protected]. ca Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uhvc.
A growing number of topics on ultra-violet photocatalytic oxidation (UV-PCO) technology for air treatment have been discussed in academia to study the feasibility of this technology for application in buildings mechanical ventilation systems. Most available reports focus on the UV-PCO performance testing of many kinds of photocatalysts for various challenge contaminants in different reactor designs and operational conditions (Aghighi et al. 2015; Chen and Zhang 2008; Farhanian
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2 et al. 2013; Mo et al. 2009; Yang et al. 2009; Zhong et al. 2013; Zhong and Haghighat 2015). Numerous experimental results prove the ability of this technology in oxidizing a wide range of chemical compounds to water and CO2 . However, limited research has been carried out on the direct analysis of photocatalysts lifetime. Even it has been taken for granted that PCO filters can be used forever based on the conventional catalytic mechanism. However, incomplete decomposition indeed exists in PCO devices, particularly under the condition of high airflow rates or low light intensities, leading to the less active surface sites on the photocatalyst surface as a result of chemisorption of intermediates, such as formic, oxalic, and acetic acids (Chang et al., 2005; Mo et al., 2013; Thevenet et al., 2014). The deactivation of photocatalytic based air cleaners inevitably leads to poor air cleaning performance. Periodical replacement of PCO filters with a certain unacceptable low efficiency should be considered as a part of the overall cost for PCO devices. Before PCO filters become fully deactivated, it is necessary to develop an effective regeneration method to recover the photocatalytic ability and thus to reduce the operation cost and improve its performance. Some strategies for extending the life cycle of PCO filters, including vacuum UV (VUV) illumination, calcination, H2 O2 under UV irradiance, as well as synthetic air under ultraviolet A illumination, have been proposed for specific applications (Miranda-Garcia 2014; Thevenet et al. 2014; Zhao 2014). These methods have been demonstrated their usefulness in mineralization of absorbed intermediates on the catalyst surfaces. However, they may not be feasible to be utilized in the field, such as the ozone problem from VUV illumination and high temperature (i.e., 400◦ ) required for calcination. In addition, if they are sent out to be regenerated, labor work, other related transportation, and treatment cost increase the application cost of UV-PCO air cleaners. In order to comply with green building design approaches, both energy and resource efficient ways to reactivate photocatalysts are needed to be developed. At this moment, the simplest way is to use the same condition as the UV-PCO units operate, but with clean air (absence of chemical compounds). In this method, treatment and maintenance of UV-PCO air cleaners can be carried out continuously, and more importantly, no extra cost is needed for sending the filters to the specific post-treatment facilities. Acetone and methyl ethyl ketone (MEK) were selected as challenge compounds: they belong to the same chemical family and exhibit various PCO performances by different PCO substrates (Zhong et al. 2015). This article reports the outcomes of UV-PCO air cleaners’ regeneration under the conditions relevant to the actual applications using a 4-parallel duct system. First, photocatalyst deactivation was observed with an emphasis on photocatalyst characterization of fresh and used PCO filters using the scanning electron microscope (SEM) technique. Then, the regeneration performance was evaluated by quantitative analysis of the production rate of gaseous by-products using high-performance liquid chromatography (HPLC). Finally, the UV-PCO performance after regenerated by UVC illumination and O3 -assisted UVC illumination was compared and assessed.
Science and Technology for the Built Environment Experimental UV-PCO reactor setup Regeneration experiments were implemented using a 4parallel duct system, which helped the study to be carried out in a time-efficient manner. The duct system was equipped with pleated prefilters at the air entrance so that the deactivation by the fouling effect from dust in air can be prevented. Two types of PCO filters, TiO2 -coated fiberglass filter, and TiO2 -coated carbon cloth filter, were installed in two PCO reactors and were evaluated simultaneously. The PCO reactor was set up with two filters as well as two UV lamps (Ster-L-Ray, Atlantic Ultraviolet Inc.): two UV lamps were sandwiched between two filters in a distance of 2 in (5.1 cm). Approximately 95% of the UV energy emitted from UV lamps is at the mercury resonance wavelength of 254 nm, which indicated that the relative irradiance of the other wavelengths of interest, such as 313 and 365 nm, is considerably small. The irradiance of 254 nm was continuously monitored by a UV radiometer (Steril-Aire), which was located at the corner of each duct. The airflow rate through each duct was in a range of 75–100 cfm (2.12–2.83 m3/min) and air velocity was 0.4–0.5 m/s. The detailed description of this testing system can be found in Zhong et al. (2013). Figure 1 shows the analytical system in this study. For the UV-PCO tests, the inlet volatile organic compounds (VOCs) concentration was monitored by an online photo-acoustic multi-gas monitor (INNOVA) equipped with an auto sampler (CBISS MK3) to ensure consistency of the challenge concentration and, consequently, to ensure the same deactivation level for regeneration tests. Meanwhile, the sampling was established by placing the sampling lines and sampling pumps at the upstream and downstream of each duct. The real-time data acquisition system was set up by mounting appropriate sensors in designated positions to monitor the airflow rate, temperature, and relative humidity (RH) in each duct. Gaseous byproducts were analyzed by an offline calibrated HPLC (Perkin Elmer). The ozone background concentration in each effluent stream was measured by a calibrated six-channel ozone analyzer (Model 465L) that was programmed to alternatively and continuously take sample from each duct with an accuracy of ±0.1% of reading.
Photocatalytic filters One commercial photocatalyst, titanium dioxide (TiO2 ) coated-fiberglass (Saint-Gobain), and one prepared TiO2 coated-carbon cloth were used in this study. Fibrous activated carbon filters were prepared by dip coating a 5% by weight suspension of the TiO2 (particle size of 21 nm, Sigma-Aldrich Co.) in deionized water using an ultrasonic bath for 20 min on each side, and then were taken out in a vertical position and naturally dried in the ambient indoor conditions. The detailed morphology of tested PCO filters by the SEM technology and energy dispersive spectroscopy (EDS) elemental analysis is described elsewhere (Lee et al. 2015).
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DNPH tube MEK or Acetone
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Fig. 1. Schematic of the testing system.
Experimental procedure UV-PCO treatment test The objective of UV-PCO treatment test is to prepare PCO filters to be deactivated in the same manner and to evaluate the UVC-induced regeneration performance. UV-PCO treatment test was designed to be one workday test. First, the fans were turned on and airflow rates were checked using an airflow meter (DIFF automatic, Observator instruments); the multigas analyzer and the O3 monitor were used to measure the background concentration for 30 min. When the experimental conditions became stable, injection of the challenge VOC at a given injection rate was stated in order to have a stable inlet concentration of around 100 ppb in each duct. Following an injection lasting 10 min, UVC lamps were switched on to get a stable UV output for 5 min, and then dinitrophenylhydrazine (DNPH) samples were taken at a sampling rate of 1.0 L/min for 1 h to explore the decomposition of the challenge VOC and the generation of by-products. Sixty liters of air was sampled by 2, 4-DNPH cartridges (Supelco LpDNPH S10L) from upstream and downstream of each duct (shown in Figure 1) to convert trapped VOCs to the hydrazone derivatives. The derivatives were eluted from the cartridge in acetonitrile and were separated and analyzed by the HPLC. Upon the first DNPH sampling completion, the second DNPH sampling was set up to run and so on. A total of four sets of DNPH samples were consecutively taken during the UV-PCO treatment test. At the end of experiment, the injection was stopped and the UVC lamps were turned off. Before turning off the fans, the airflow rates were checked once again by the flow meter for airflow assurance. The whole duration of a UV-PCO treatment test, including the waiting time for environmental stability and the UV-PCO running time, lasted approximate 5 h. UVC-induced regeneration test The first step of regeneration test was the same as that of UV-PCO treatment test: setting up the airflow to be the expected rates for each duct; turning on the on-line multi-gas analyzer, the ozone monitor, and other data acquisition systems to record testing data. Then UVC lamps were switched on, and at the same time, DNPH samples were taken at proper
sampling ports. The sets of DNPH samples were flexible, depending on the test intention. Gas phase characterization Air sampling and analysis method of this study follows the EPA TO-11A method (1999). The HPLC apparatus was calibrated with six component carbonyl-DNPH mixtures (formaldehyde, acetaldehyde, acrolein, acetone, propionaldehyde, and crotonaldehyde) with each analytical concentration of 15 μg/mL and MEK-DNPH of 100 μg/mL. The calibration curve was performed by a series of calibration points covering the concentration range of interest for the purpose of quantitative results with HPLC. A single point calibration was run periodically as an unknown to check the integrity of the calibration. Air sample eluate was separated and analyzed by the HPLC with UV detection (360 nm) equipped with a C18 Brownlee validated micro-bore column (250 mm × 4.6 mm ID, 5 μm film thickness). Seventy-two percent acetonitrile and 28% distilled water were used as mobile phase with a flow rate of 1.0 mL/min. Each vial was analyzed for 10 min with an injection volume of 20 μL. Evaluation method Photocatalyst activity is calculated from the ratio of experimentally measured decrement of a challenge VOC to its inlet concentration, which is quantified by Eq. (1). By-products yield (ppb) represents the net amount of compound produced during the PCO reaction and it can be obtained by Eq. (2). Photocatalyst regeneration process is defined as the by-product production rate which is the mass of gaseous byproducts produced during a given period of time (1 h) per unit area of PCO filters and is calculated by Eq. (3). Qair Cup,t − Cdown,t Cup,t − Cdown,t × 100 = × 100 Qair Cup,t Cup,t (1) mdown−bp,t − mup−bp,t (2) = t × Qsampli ng × b
nt =
Cbp,t
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rbp,t =
Science and Technology for the Built Environment (mdown−bp,t − mup−bp,t ) × t×A
Qair Qsampli ng
(3)
where ηt (%) is the single-pass efficiency of a VOC; Cup,t (mg/m3) is the upstream VOC concentration as a function of time; Cdown,t (mg/m3) is the downstream VOC concentration as a function of time; Qair (m3/min) is the airflow rate; Cbp,t ppb) is the net by-product yield through PCO filters; mup−bp,t − mdown−bp,t μg) is the net mass of generated by-products using 1 h DNPH sampling analyzed by HPLC; Qsampling (L/min) is the sampling airflow rate through a 2, 4DNPH-silica cartridge; t is the sampling duration (60 min), b is the conversion constant from μg/m3 to ppb for a specific by-product; A is the total cross-section area of PCO filters exposed to UV lights (m2); and rbp,t mg/h/m2) is the by-product generation rate measured as the mass per unit of surface area of PCO filters per hour.
Results and discussion Observed deactivation The UV-PCO treatment test was designed to deactivate PCO filters under identical experimental conditions. Figure 2 shows the example of the PCO performance tested by 100.2 ± 0.8 ppb acetone as a function of time. It evidences that the PCO performance is not constant with irradiance time. Because of the observation of decreasing trends of the single-pass removal efficiency for two PCO filters, the generated refractory intermediates, such as organic acids that were investigated by other researchers, were reasonably assumed to be the reason for the deactivation of the photocatalyst by occupying active photocatalytic sites on the photocatalyst surface (Aghighi et al. 2015; Thevenet et al. 2014). Generation of gaseous byproducts implies that the mineralization is not complete. The concentrations of acetaldehyde at the downstream slightly increased during the whole 4-h UV-PCO test, while the concentrations of formaldehyde gradually decreased with the lapse of time for both filters. It further indicates that the conversion from acetaldehyde to formaldehyde was hindered by some other competitive transient by-products, probably adsorbed organic acids, with the irradiance exposure. Table S1 in the Appendix shows the physical properties of challenge VOCs and potential by-products, which indicate acids with low vapor pressure and miscible solubility tend to be adsorbed onto hydrophilic TiO2 surfaces. The deduced oxidation pathway of PCO of acetone is presented in Figure 3. In an effort to further examine the deactivation phenomenon of PCO filters deployed in ambient air, a series of consecutive-weekday without regeneration tests was conducted using a single VOC of n-hexane of 500 ppb–2 ppm, ethanol of 500 ppb–2 ppm, 2-butanol of 100 ppb–800 ppb, and ethylene of 8.6 ppm. All chemicals were reagent grade colorless liquid or gas. Upon the completion of deactivation tests, the yellowish color on TiO2 -fiberglass was apparently observed (shown in Figure S1 in Appendix).
Similar findings were reported on deactivation of photocatalysts (Fresno et al. 2008; Lewandowski and Ollis 2003; Li et al. 2011). Hay et al. (2010) found that silica layer formed by PCO of silicon-containing volatile and semi-volatile organic compounds which are frequently found in ambient air, led to rapid catalyst deactivation. Catalyst characterization The crystalline phases of the two photocatalysts were determined by powder x-ray diffraction (XRD, PANalytical X’pert Pro) with a copper Kα radiation at 45 kV and 40 mA. The XRD spectrum was acquired from 20◦ to 90◦ (2θ ) with a 0.02 step size and 2 s of a scan time per step. The analysis of the x-ray patterns was carried out using X’Pert HighScore Plus Rietveld analysis software in combination with Pearson’s crystal database. Figure 4 shows the XRD patterns, indicating 100% anatase phase TiO2 on fiberglass filters and 80.8% anatase/19.2% rutile phase TiO2 on carbon cloth filters. The crystallite sizes estimated by Scherrer equation assuming the shape factor of 0.9 were 9.9, 19, and 29 nm for 100% anatase phase, 80.8% anatase phase, and 19.2% rutile phase TiO2 , respectively, which is in good agreement with that the diffraction pattern peak intensity increases with increasing crystallite size. It is worth mentioning that pure TiO2 was tested by XRD for carbon cloth filters so it exhibits sharp distinct peaks, whereas XRD results of crystalline TiO2 containing amorphous impurity (silica fibers) for fiberglass filters show large wide humps instead of high intensity narrower peaks. In order to collect more evidence of deactivated phenomena existing in used filters, a SEM analysis was performed on deactivated TiO2 -fiberglass filters (yellowish one) and fresh one, shown in Figure 5. The analysis indicates the fibers in the yellowish filter were scattered more disordered than those in the fresh one. The magnified images clearly demonstrate the bumpy and caved-in regions distributed on some parts of surfaces of deactivated fibers, and raised points on the fresh fibers. This finding can be partially attributed to the loss of TiO2 nanoparticles after the exposure to high air flow rates, and be partially attributed to the poisoning by strong chemisorbed intermediates (Argyle and Bartholomew 2015). The SEM images of TiO2 particle agglomeration between fibers also show that the surface of agglomeration was changed from flat to irregular through the PCO interactions. UVC-induced regeneration Acetone test After the fact of catalyst deactivation was confirmed by the UV-PCO performance evaluation and surface analysis, the regeneration tests were carried out for the same duration as the UV-PCO running time at a treatment test (4 h) to check the photoactivity recovery using the UVC-induced method. Figure S2, in the Appendix, presents the profiles of the production rates of formaldehyde, acetaldehyde, and acetone with irradiance time for two filters after they were deacti-
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Fig. 2. Single-pass removal efficiency and gaseous by-product yield for; (a) TiO2 -fiberglass filters. (b) TiO2 -carbon cloth filters treated by 110.2 ± 1.7 ppb acetone at 17.3% ± 0.4% RH.
vated by 104.6 ± 0.9 ppb acetone at 13.9% ± 0.6% RH for 4 h. The acetone production rate decreased with time due to the desorption mechanism combined with PCO degradation. The production rate of acetaldehyde was constant within the 4-h test for both filters, which were similar as observed in the previously mentioned UV-PCO treatment tests. Nevertheless, formaldehyde exhibited an increasing trend, indicating that surface adsorbed acetone and other intermediates took PCO reactions to be converted to formaldehyde. This evidenced the regeneration occurring on the surface of PCO filters using the UVC-induced method. Due to the high output of formaldehyde in 4-h regeneration test, it is necessary to examine the production rates of by-products for a longer time. Figure 6 presents the production rates of by-products versus time for a period of 27-h irradiance exposure. Here, it is worth mentioning that the humidity in the first 6-h UV illumination was 12.4% ± 0.5% RH and then it was increased to 32.0% ± 0.8% RH afterward until the end. Both profiles demonstrate yields of formaldehyde and acetaldehyde increased with time at the first 4 h and then sharply went down because of the regeneration tests were stopped and re-continued at second day under the same conditions for 2 h. After that, output of formaldehyde and acetaldehyde sharply reduced again and kept a constant at 32.0% ± 0.8% RH. This phenomenon could be attributed to the faster regeneration process as a result of more hydroxyl radicals introduced in the presence of a high humidity. Ameen and Raupp (1999) verified that regeneration of deactivated catalyst needed much
Acetone O
Acetaldehyde O
C H3C
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MEK test In order to examine the feasibility of UVC induced regeneration method for PCO filters contaminated with other VOCs, regeneration tests after PCO treatment of 95.8 ± 0.6 ppb MEK were continuously performed twice in the same duration. The PCO performance of MEK using two filters was demonstrated in Figure S3 (see Appendix). Figure 7 shows the production rates of formaldehyde, acetaldehyde, and acetone at various treatment times (0.6, 1.6, 4.1, and 5.1 h) for repeated tests. The second test was conducted using the filter regenerated from the first test. RH conditions were 28.4% ± 0.3% and 32.5% ± 0.6%, respectively. It clearly demonstrates that byproduct production rates from the second test, particularly formaldehyde, were lower than those from the first test. This finding implies PCO filters were partially reactivated under UVC illumination, leading to continuous conversion of ad-
Acetic acid O
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shorter treatment time at 80% RH than that of 20% RH. The adsorbed acetone on the carbon cloth was quickly released to the air at beginning of period, and then acetone at downstream slightly increased for both PCO filters. After introduction of a higher humidity, the acetone production rate gradually decreased, which verifies again that a high humidity helps to accelerate the regeneration rate for two substrates. Moreover, a decrease of adsorption capacity of acetone with humidity is another reason to expel acetone from PCO filters (Zhong et al. 2012).
Formaldehyde O
Fig. 3. Photocatalytic oxidation pathway of acetone.
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Fig. 4. XRD patterns of two types of TiO2 catalysts (top: TiO2 -fiberglass; down: TiO2 -carbon cloth).
sorbed by-products remained from the first regeneration test to gaseous by-products. It should be noted that higher amounts of gaseous byproducts were generated from TiO2 -fiberglass filters than those from TiO2 -carbon cloth filters for both acetone and
MEK regeneration tests (shown in Figures 6 and further proves the previous finding that substrate erty plays a certain role on chemical adsorption, conversion, and regeneration performance (Zhong 2012).
Fig. 5. SEM images for; (a) deactivated TiO2 -fiberglass filters. (b) fresh TiO2 -fiberglass filters.
7). It propPCO et al.
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It is found that the O3 -involved UV-PCO is superior to the no O3 -involved UV-PCO for the VOCs treatment due to the formation of more radicals and the enhancement of electron capture (Zhong et al. 2013). Hence, it is worthy of examining the regeneration behavior by comparing O3 and no O3 involved UVC illumination. O3 of 1285.9 ± 10.4 ppb was generated from two VUV lamps, which were placed at the upstream of the PCO reactor under a flow rate of 105 ± 2 cfm. Figure 8a shows the temporal evolution of MEK conversion rate as a function of irradiance time for fiberglassTiO2 filters. Two Test-1 were simultaneously performed at two ducts under the same environmental conditions (106 ± 2 cfm, 49.6% ± 1.5% RH, 21.3 ± 0.3◦ C) with average single-pass removal efficiency of 18.0% and 20.8%, respectively, which confirms repeatability. Then two Test2 were carried out after 5 h UVC-induced regeneration and 5 h UVC plus O3 exposure regeneration, respectively. The average conversion rates of MEK for Test-2 were 8.7% (no O3 ) and 8.9% (with O3 ), respectively, which recovered 48.3% and 42.5% of Test-1 PCO activity. Therefore, the O3 -assisted UVC method did not improve or even slightly decrease the regeneration performance by the UVC method.
TiO2-fiberglass
25 20 15
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Figure 8b shows comparison of by-product concentrations generated by two methods. O3 -assisted UVC method helps to convert adsorbed residual compounds and generated surface adsorbed intermediates to gaseous by-products. Consequently, the amounts of formaldehyde, acetaldehyde, and acetone by the O3 -assisted UVC method were analyzed to be higher than those by the UVC method. The more complexity of radicals introduced by O3 is attributed to the observed phenomenon. However, more amounts of gaseous by-products do not mean more complete conversion of adsorbed products. Meanwhile, more adsorbed carboxylic acids, such as formic acid and acetic acid, could be produced at the present of O3 . They attached and blocked on the surface of PCO filters, which is a possible reason for a decline of partial recovery by the O3 involved UVC method. In order to collect sufficient data for estimation of the operation cost of PCO air cleaners integrated into HVAC systems, this study focused on examining the feasibility of the on-site UVC-induced regeneration method toward photocatalyst reactivation. The single-pass mode of a UVPCO duct system for the treatment of ketones was investigated using a new PCO treatment method. First, the gradual decrease of acetone conversion rate and generation of formaldehyde and acetaldehyde as a function of irradiance time were
By-product generation rate (mg/h/m2)
UVC regeneration versus UVC + O3 regeneration
By-product generation rate (mg/h/m2)
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Fig. 6. Gaseous by-product production rates as a function of irradiance time for two filters which were previously deactivated by 111.6 ± 1.1 ppb acetone at 14.1% ± 0.3% RH.
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(b) Fig. 8. Comparison of UV-PCO performance and by-product generation after UVC-induced regeneration and UVC + O3 regeneration for TiO2 -fiberglass filters: 118.6 ± 0.9 ppb MEK, 52.5% ± 2.1%RH, 1285.9 ± 10.4 ppb O3 .
observed. Then, the yellowish photocatalyst throughout the long-term exposure to the UV-PCO of various VOCs was perceived with a naked eye, and caved-in surfaces on the glassfibers and agglomeration were identified with the SEM technique. A series of regeneration tests was performed to identify and quantify gaseous by-products as well as their production rates using chromatographic techniques. UV-PCO recovery performance and by-product generation were compared and accessed by the UVC method and O3 -involved UVC illumination method. More detailed findings are as follows: 1. It was evidenced that the photocatalytic regeneration of the PCO filters can be partially achieved under UVC illumination with continuous airflow. 2. Regeneration performance by the proposed method was correlated to the property of substrates and the nature of VOCs previously treated, which led to various types and amounts of surface adsorbed by-products, major components resistance to regeneration.
3. High humid condition can shorten the regeneration duration by providing more hydroxyl radicals for conversion and mineralisation of surface adsorbed by-products. 4. UVC illumination is better than O3 -involved UVC illumination approach as a short-time regeneration method for partial recovery of PCO activity. In short, the on-site UVC-induced regeneration method is relatively mild treatment that partial removes catalyst poisons and restores part of the catalytic activity. Partial recovery of photocatalytic activity in a treatment/regeneration sequence in one system can extend the useful life of catalysts and benefit UV-PCO as a sustainable technology to be applied in HVAC systems for indoor air improvement. This study also points out that the identification and quantification of by-products adsorbed on the photocatalyst surfaces is an alternative approach to obtain in-depth knowledge on photocatalyst deactivation, which will be included in the future study. Depending upon the reversibility of the deactivation process, other regeneration methods, which might have the ability to selectively
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Acknowledgment The authors would like to thank Circul-Aire, Inc. for providing the test rig.
Funding
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The authors gratefully acknowledge the financial support of the Institut de Recherch´e Robert-Sauv´e en Sant´e et en S´ecurit´e du Travail (IRSST).
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Appendix Table S1. Physical properties of challenge VOCs and potential by-products.
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Property
Ketones
Aldehydes
VOCs class
Acetone
MEK
Molecular fformula Molar mass (g/mol) Density (g/mL) Boiling point (◦ c) Vapor pressure (mmHg) Solubility (at 20–25◦ c)
C3 H6 O 58.1 0.7925 56.1 180 miscible
C4 H8 O 72.1 0.8050 79.4 78 28%
Acids
Acetaldehyde
Formaldehyde
Acetic acid
Formic acid
C2 H4 O 44.1 0.785 20.6 740 Miscible
CH2 O 30.0 1.09 –21.1 >760 Miscible
C2 H4 O2 60.1 1.049 117.8 11 Miscible
CH2 O2 46.0 1.220 106.7 35 Miscible
Fig. S1. Observed color difference for TiO2-fiberglass filters (1 month continuously used and no regenerated TiO2-fiberglass filter versus fresh one).
Fig. S3. PCO performance of 95.8 ± 1.1 ppb MEK using two PCO filters under conditions of 75.8 ± 0.9 cfm and 24.6 ± 1.0% RH.
Fig. S2. Gaseous by-product production rates as a function of irradiance time for two filters which were previously deactivated by 104.6 ppb acetone at 13.9% RH.
