Long-lifetime water-washable ceramic catalyst filter for air ...

Rational design concept of CCF

The design of CCF is based on a novel concept, wherein PMs are collected in the inlet channel of the ceramic filter, and VOC gases are decomposed by a photocatalyst coated in the outlet channel, under ambient conditions. This design concept of CCF can provide a definite advantage of being able to remove various air pollutants in the space of itself. The CF possesses an inlet channel with plugs and a porous inner wall, through which the air flows toward the outlet channel after penetrating the wall (Fig. 1). First, we selected a commercial CF designed with a conjugated plug to allow wall-flow through the porous wall for in-flowing air18,19. The filtration mechanism of CF to remove PM is a combination of deep-bed and cake filtrations, as shown in Fig. 17. The greatest advantage of CF as PM removal filter is that the surface area over volume of the filter porous wall (m2/m3) is much larger than that of other filters prepared by polymers and fibres, which enables it to be used for longer durations19. Concerning regeneration, ceramics fired at high temperatures exhibit strong heat- and water-resistant properties and are also used as water-treatment membranes20. In addition to PM removal, we used it as a support for photocatalysts to oxidise VOCs to CO2 under ambient conditions without thermal sources, as shown in Fig. 1. The photocatalysts can remove VOC by the PCO reaction using ROS such as hydroxyl radical (&#;OH) and superoxide anion (&#;O2&#;) generated from the photocatalyst under UV light irradiation at room temperature. Our novel design concept entails that, along sequential air flow, PM are first captured by the ceramic porous wall at the inlet channel with initial deep-bed and following cake filtration mechanism, and then VOCs exiting the porous wall are decomposed using the PCO reaction by the UV&#;photocatalyst system at the outlet channel. Finally, to realise this concept, we also attempted an uncommon method of catalyst coating over the outlet channel surface for preventing catalyst deactivation by PM.

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The illustration shows a rational design concept for CCF preparation. Two main mechanisms, namely wall-flow filtration of PM by porous ceramic filter with two types of filtration mechanism, namely deep-bed and cake filtration, and photocatalytic perfect oxidation reaction for VOC removal by the developed Cu2O/TiO2 catalyst were applied to simultaneously remove the PM and VOC.

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For the rational design of CCF, we prepared commercial ceramic filters with 100, 200, and 300 CPSI, where CPSI implies cells per square inch of the filter. To determine the optimal length of CCF, we investigated the pressure drop of the filters along the CF length using a pressure drop model developed by Masoudi et al7. and Konstandopoulos & Johnson21 (Supplementary Fig. 1). The flow characteristics such as distributions of velocity and pressure of CFs inside each CF channel were investigated using computational fluid dynamics (CFD) simulations (Supplementary Fig. 2a). Using this developed model, the porous-wall lengths were optimised to 60&#;120&#;mm, which indicate the ranges of minimum pressure drop (Supplementary Fig. 2b). The predicted pressure drop was estimated to be 71&#;Pa at an optimal porous-wall length of 105&#;mm (Supplementary Table 1). Based on these results, commercial CFs with a filter length of 115&#;mm with each plug of 5&#;mm were procured from Corning Co. with 200 CPSI and 8.0&#;mil wall thickness (0.203&#;mm). Thus, the CF having an optimal length was carefully employed to prepare the CCF.

PM removal by surface treatment

Figure 2a illustrates the internal cross-section morphology along the filter length describing a specific feature of the CCF. According to the filtration mechanism of the CF, an air flow with PMs and gaseous air pollutants that entered into the filter would penetrate through the porous wall, where only PMs can be captured on and/or in the pores of the wall. The core of the CCF that removes PM is obtained by an additional surface coating treatment of metal oxide, known as the &#;membrane&#;. The morphology of the porous surface with microstructures before and after the membrane coating on the bare-CF can be seen in the scanning electron microscope (SEM) images (Fig. 2b, Supplementary Fig. 3). The membrane with a net-type shape was uniformly spread over the wall surface in the filter channel. The vertical cross-section of the CF shows well-coated membrane layers on the inner wall surface (Fig. 2c), and the characteristic membrane component (Bi) is observed along the wall, which is mainly composed of Mg in cordierite (Fig. 2d). The mean pore size after membrane coating was reduced from 11.6 to 8.2&#;μm while maintaining the porosity of CF (Supplementary Fig. 3c). It is well known that the pore size reduction could affect permeability related to filter efficiency. In particular, the control of mean (average) pore size plays a critical role in determining the quality factor, including filter efficiency (FE) and pressure drop22,23. We evaluated the filter efficiency (FE, %) and pressure drop (Δp, Pa) under a single-pass test condition of an air flow of 1&#;m/s by using a customised aerodynamic equipment. Compared to the bare-CF, the membrane coated filter achieved an enhanced PM10 (particulate matter, size below 10&#;μm) FE of 98% and PM2.5 (particulate matter, size below 2.5&#;μm) FE of 97.7%, respectively (Table 1). It plays an important role in successfully removing PMs by cake filtration mechanism beyond the initial deep-bed filtration through the CF7. Here, we improved the filtration mechanism by membrane coating, which resulted in a significant increase in the FE of PM1 (particulate matter, size below 1&#;μm) as detailed in Table 1.

a Inner channels of CF and illustration of PM removal by wall-flow filtration in the CF. b SEM images before and after inorganic coating to fabricate the membrane on the CF cell surfaces. c SEM image of cross-sectional sliced CF by moulding and polishing: void of wall (black), membrane layer (white), and wall comprising cordierite (grey). d Distribution of main elements of the wall (Mg) and membrane (Bi) by SEM-EPMA element mapping.

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VOC removal by coating of synthesized photocatalyst

To remove VOC by the CCF, we developed a Cu2O-TiO2 catalyst based on a TiO2 (anatase) photocatalyst. Cu2O having Cu1+ can act as a co-catalyst in the conduction band as an electron acceptor, further producing ROS such as superoxide anion (&#;O2&#;) (Fig. 1)24,25. The oxidation state of Cu on the surface of TiO2 was confirmed to be Cu1+ by X-ray photoelectron spectroscopy (XPS) analysis and Raman spectroscopy (Supplementary Fig. 4a, b). The basic characterisations, such as UV&#;visible absorbance and electrochemical impedance spectroscopy (EIS), were measured (Supplementary Fig. 4c, d). The PCO reaction activity of Cu2O/TiO2 catalyst can be improved because Cu2O behaves as an electron acceptor, leading to easy separation of hole&#;electron pairs and reduction of hole&#;electron recombination26. To verify this mechanism, we evaluated the electrochemical characteristics by measuring the photocurrent density, Mott&#;Schottky plot, and photoluminescence (PL) (Fig. 3a&#;c). With these enhanced characteristics of the Cu2O/TiO2 catalyst, we measured the intrinsic formaldehyde decomposition activity of powder catalysts (TiO2, CuO/TiO2, and Cu2O/TiO2) under relative humidity (RH) 0% and RH 50% conditions, resulting in 93% @ Cu2O/TiO2&#;>&#;84% @ CuO/TiO2&#;>&#;78% @ TiO2 at RH 50%, which was higher than that at RH 0% owing to the formation of hydroxyl radical (&#;OH) by the oxidation reaction with water (Fig. 3d)12,14. Furthermore, we assessed the removal efficiencies with five representative VOCs, namely formaldehyde, ammonia, acetaldehyde, acetic acid, and toluene gas, over 1&#;g of Cu2O/TiO2 catalyst coated on honeycomb (Fig. 3e). The average removal efficiency was 82.4%, with the highest being 93% for acetic acid and the lowest being 62% for toluene. Therefore, we confirmed the development of a new TiO2-based catalyst via Cu2O as co-catalyst, which improves the photocatalytic activity through facile charge separation and high charge carrier density. For the Cu2O/TiO2 catalyst, we coated similar amounts of catalysts for fabricating CCFs (38&#;40&#;g of Cu2O/TiO2 or TiO2 catalysts per apparent volume of the filter (L)), where we initially used the CF without the membrane to confirm the catalyst performance. Enhanced reaction efficiencies (REs: VOC removal and CO2 production) of nearly 90% using the Cu2O/TiO2 catalyst were obtained with CO2 as the product gas, while those using TiO2 were 76% (Fig. 3f). In particular, we extensively monitored the feasible side gas-phased products of PCO reaction, including CO by Fourier transform infrared spectroscopy (FT-IR) along the reaction time, but these were not detected (Fig. 3g). This implies that CO2 was the only product resulting from the PCO reaction. Thus, we concluded that all the reacted HCHO gases were perfectly decomposed into CO2 gas by the PCO reactions over the Cu2O/TiO2 catalyst. We used formaldehyde (HCHO), which is a well-known as highly carcinogenic gas2, as a representative VOC for the removal test (see Methods). The light intensity test and ray tracing simulation (Zemax) were conducted to optimise the UVA-LED system (Supplementary Figs. 5 and 6). We set up the UV-activated system optimally with a light source using a 2&#;×&#;2 UVA LED array with a suitable light intensity of 38.1 (centre) and 40.8&#;mW/cm2 (side), and a maximum light propagation distance of approximately 30&#;mm inside the CCF.

a Photocurrent density by chronoamperometry: using 1 LED with a power density of 50&#;mW/cm2 (365&#;nm) at 0.5&#;V (vs. Ag/AgCl). b Charge carrier density (cm&#;3) analysis by Mott&#;Schottky plot: 3.06&#;×&#;&#;cm&#;3 for Cu2O/TiO2, 2.81&#;×&#;&#;cm&#;3 for CuO/TiO2, and 1.73&#;×&#;&#;cm&#;3 for TiO2 at flat band potential (EFB) of &#;0.4&#;V. c Photoluminescence: 300-nm excitation by UV and response emission of 310 to 590&#;nm on the powder samples. d VOC (HCHO) decomposition efficiencies of Cu2O/TiO2, CuO/TiO2, and TiO2 catalysts. Note that the efficiencies were calculated by ratio of CO2 produced by the PCO reaction to a given concentration of HCHO gas. The detailed reaction conditions are available in the supplementary materials. e VOC removal efficiencies of five VOCs over the honeycomb (100 CPSI) coated by the Cu2O/TiO2 catalyst. The 100&#;L chamber test sequentially proceeded with each VOC at 10 ppm in an air balance for 30&#;min. This test was conducted three times for each VOC gas. The final average value indicated the arithmetic mean of the efficiencies. f VOC reaction efficiencies (HCHO removal and CO2 production) of CCF coated with Cu2O/TiO2 and TiO2 catalysts. Detailed reaction conditions are available in the supplementary materials. g FT-IR spectra at different stages of the reaction corresponding to the five points shown in Fig. 3f. Note that the FT-IR spectrum of by-pass, and gaseous FT-IR spectra of HCHO, CO, and CO2 gases are indicated as per the references.

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Fabrication of CCF

Figure 4a shows a snapshot of the dip-coating of the catalyst with membrane-coated CF for CCF fabrication. When the membrane coated CF was dipped in the catalyst coating slurry, the slurry is not absorbed into the cell, implying no penetration beyond the wall. It is an important design rule that the catalyst should be coated on the outlet channel surface against air flow to prevent deactivation by PM. To observe the inside of the cell (channel), we conducted SEM analysis along with Electron Probe X-ray Microanalysis (EPMA) to confirm the spatial distribution of the main elements in the cell. Figure 4b shows the SEM image of cross-sectional CCF and the element mapping of Ti, which indicates a well-coated catalyst layer on the inner channel surface of CCF. The SEM image shows the wall centre of CCF, which indicates the inside pores (black), coated catalysts (grey), and membranes (white). The mapping image of Ti demonstrates that the catalyst was coated on a single side including the inside wall, implying that no penetration of the coating occurred to the other side (membrane coating zone). Although the inability of the slurry to penetrate the membrane was not investigated in this study, it may be caused by the micro/nano structures generated by the surface treatment (membrane coating) as per Cassie&#;Baxter27. For practical CCF fabrication, we confirmed the relationship among the pressure drop, FE, and RE (or removal efficiency) along the catalyst coating lengths (Fig. 4c, d). The pressure drop increased exponentially along the coating length; however, it was stable up to the 50&#;mm coating zone. FE decreased gradually as the coating length increased. However, RE achieved an optimum value at the 50&#;mm coating zone. Thus, we obtained the optimum specifications to fabricate the CCF for practical applications from systematic testing along the catalyst coating length (Fig. 4d, Supplementary Table 2). In addition, to better understand the characterisation along the coating zone length, we investigated the wall-normal velocity along the CF length using CFD simulations (Fig. 4e). The wall-normal velocity passing through the inner wall of the CF increases from above 30% of the total CF length to the maximum at the outlet channel, which implies the photocatalytic reaction might be the most effective within 70% of the total CF length from the outlet.

a Image of a single-side slurry coating with no through-wall penetration. b SEM image of CCF fabricated by coating the membrane and catalyst on different surfaces and distributing the main element of the catalyst (Ti) by SEM-EPMA element mapping. c Initial pressure drops of CCFs along the catalyst coating zone with lengths of 0 to 115&#;mm. d Filter and VOC removal efficiencies of CCFs along the catalyst coating zone with lengths of 0 to 115&#;mm. e CFD simulation of ceramic filter along the CF length. Wall-normal velocity through the wall under various inlet velocities from 0.12 to 1.2&#;m/s. Top inset indicates the wall position inside the cell, where the length of the arrow indicated the strength of the wall-normal velocity.

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Regeneration performances of CCF

Among the various regeneration methods such as water washing by water flow and sonication, we found that simple water washing in the direction against PM capture in CF or CCF is the most effective way to clean dust and regenerate the initial pressure drop of the filter (Supplementary Fig. 7a, b). The maximum dust loading capacity of CF up to final pressure drop (~250&#;Pa) was evaluated to be approximately 20&#;30&#;g/L, while the normal disposable HEPA or medium filters (MFs) for 6 months have a maximum capacity of 5&#;g/L (Supplementary Fig. 7c)28. For ten regenerations by water washing, the CF achieved an accumulated dust loading capacity exceeding 216&#;g/L, while maintaining an efficiency loss of barely 9% compared to the high initial efficiency of 98% at a high linear velocity of 1&#;m/s (Fig. 5a). However, as the regeneration cycle progressed, the commercial HEPA filter rapidly decreased in its dust loading capacity (48.4&#;g/L for ten regenerations) and could no longer be operated (Fig. 5b). The CF may possibly indicate a usage of 2 years without regeneration exceeding the maximum dust loading capacity four times that of the normal HEPA filters. Thus, it implies that we can use the CF for 20 years through ten regenerations of simple water washing. Finally, we simultaneously achieved PM and HCHO removal using CCF (Fig. 5c). For the CCF with Cu2O/TiO2 catalyst, the PM10 FE, HCHO removal efficiency, and pressure drop initially were about 95%, 82%, and 20&#;Pa at 10&#;L/min (linear velocity of 0.12&#;m/s), respectively. Unprecedentedly, these high performances are maintained even after ten regenerations with simple water washing. This result is firstly demonstrated by the CCF as a new class of filter realised using our main concept suggested in this study. Thus, with simple water washing, we achieved the facile regeneration of the CCF.

a Regeneration performances of PM removal efficiency and pressure drop of CF for ten regenerations by water washing. b Dust loading amount profile of the CF (Blue: total dust loading amount, 215.8&#;g/L, average 21.6&#;g/L) and the HEPA filter (Black: total dust loading amount, 48.4&#;g/L, average 4.8&#;g/L) according to regeneration cycles with water washing. c Simultaneous PM FE and VOC RE of CCF for ten regenerations by water washing.

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Application for air purification system

Prior to using the CCF as a prototype for various applications, we first applied the CF panel system to purify the air entering a building to replace current commercial MF (Ultra Low Pressure Drop (ULPD) in heating, ventilation, and air conditioning (HVAC) systems (Fig. 6a). Through this, we confirmed practically that PM2.5 FE remains higher than 98% for 30 months without replacement and regeneration of the CF panel system, while the MF showed low FE (62%) and required replacement every 3&#;6 months (Fig. 6b). In addition, we developed a free-standing air purification system applying CF as another type of proto system to purify the air in the underground parking lot (Fig. 6c). Note that installation of UV light system for CCF performance is still in progress. The FEs of PM10 and PM2.5 of the system are still observed above 90% for about 12 months for a flow rate of &#;m3/h (Fig. 6d). Note that all the evaluations are in-operation by now. Furthermore, we conducted a CFD simulation to predict the air flow and PM concentrations in huge underground parking lot (volume of 96,000&#;m3 for 600 cars). After 21 CCF proto systems were located, air flow was more uniformly distributed without a quiescent zone, and the overall volume averaged PM10 concentration showed a surprising reduction of 32% (Supplementary Fig. 8). To confirm the prediction result of the CFD simulations, we practically test the spatial efficiency using 21 installed CCF proto systems.

a CF panel system installed at the HVAC facility of the building. b PM2.5 FE of CF and MF measured for 30 months in the building. c Free-standing proto-type CF system. d FE of PM10 and PM2.5 of the proto-type CF system measured for approximately 12 months. The efficiency was measured using a single-pass test method, which assessed the concentration of inlet and outlet PMs using an isokinetic sampling probe, as shown in the inset.

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Title: Do Water Filters Expire? Understanding the Lifespan of Water Filters

Water filters are essential for ensuring clean and safe drinking water by removing impurities and contaminants. However, it is important to understand whether water filters have an expiration date and how their effectiveness may change over time. In this blog post, we will delve into the topic of water filter lifespan and explore the factors that can affect their efficiency. By gaining insights into the longevity of water filters, you can make informed decisions about their replacement and maintain a continuous supply of filtered water.

How Water Filters Work

Water filters employ various mechanisms to eliminate contaminants and improve water quality. These mechanisms include physical filtration, chemical filtration, and biological filtration.

Physical filtration involves using a physical barrier to trap particles, sediments, and larger impurities present in the water. This process effectively removes visible debris and sediment.

Chemical filtration employs materials like activated carbon to adsorb impurities. The activated carbon has a large surface area that attracts and binds to contaminants, including chlorine, volatile organic compounds (VOCs), and certain chemicals.

Biological filtration utilizes specialized filters to eliminate bacteria, parasites, and other microorganisms from the water. These filters are designed to target and remove harmful biological contaminants, ensuring the safety of the drinking water.

Water filters consist of several components, including filter media, filter housing, and replaceable cartridges or membranes. The filter media is responsible for capturing and removing contaminants, while the filter housing holds the media in place and directs the flow of water. Cartridges or membranes contain the filter media and are designed for periodic replacement.

II. Factors Affecting Water Filter Lifespan The lifespan of water filters can be influenced by various factors, including filter type, water quality, usage, and maintenance.

A. Filter Type: Different types of filters have varying lifespans and effectiveness in removing specific contaminants. Common filter types include carbon filters, reverse osmosis filters, ceramic filters, and UV filters.

Carbon filters, such as activated carbon or charcoal filters, are widely used and effective in removing chlorine, unpleasant odors, and organic compounds. The lifespan of carbon filters typically ranges from 2 to 6 months, depending on the quality of the water being treated and the amount of water passing through the filter.

Reverse osmosis (RO) filters employ a semipermeable membrane to remove a wide range of contaminants, including dissolved solids, heavy metals, and certain chemicals. RO filters require replacement every 1 to 3 years, depending on usage and water quality.

Ceramic filters use porous ceramic material to trap sediments, bacteria, and other contaminants. These filters can be cleaned and reused, but they may require replacement after a few years to maintain optimal performance.

UV filters use ultraviolet light to deactivate or destroy bacteria, viruses, and other microorganisms. The UV lamp in these filters typically needs replacement annually to ensure consistent disinfection.

B. Water Quality: The quality of the water being filtered can impact the lifespan of water filters. Water with higher levels of contaminants or sediment may cause filters to clog more quickly, reducing their effectiveness and requiring more frequent replacements.

C. Usage and Water Consumption: The frequency and volume of water usage affect the lifespan of water filters. Filters used in high-demand environments or households with larger water consumption may need to be replaced more frequently. It is essential to follow the manufacturer's guidelines regarding filter replacement intervals based on usage.

D. Maintenance: Regular maintenance of water filters can help extend their lifespan. This includes proper cleaning and rinsing of filter components, as well as following any specific maintenance instructions provided by the manufacturer. Neglecting maintenance can lead to reduced filter performance and the need for.  

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