Air and Surface Purification Using Heterogeneous Photocatalysis: Enhanced Indoor Sanitisation Through W₁₈O₄₉ and ZnO Catalyst Systems

Abstract

Indoor air quality management has become increasingly critical for public health, particularly after the global COVID-19 respiratory disease outbreaks that highlighted airborne pathogen transmission risks. This review investigates an advanced air and surface purification method that is used in devices utilising heterogeneous photocatalysis with tungsten oxide (W₁₈O₄₉) and zinc oxide (ZnO) catalyst systems to generate controlled concentrations of hydrogen peroxide for continuous indoor sanitisation. The photocatalytic system converts ambient water vapour into aerosolised hydrogen peroxide at concentrations of 0.04–0.05 ppm, significantly below established safety thresholds, while maintaining effective antimicrobial activity. The W₁₈O₄₉ catalyst demonstrates superior visible-light absorption compared to conventional titanium dioxide (TiO₂) systems, with ZnO serving as an effective cocatalyst to reduce electron–hole recombination and enhance reactive oxygen species generation. Safety analysis based on OSHA, WHO, and ACGIH guidelines confirms that continuous exposure to these low hydrogen peroxide concentrations poses no health risks to occupants. Real-world applications demonstrate up to 90% reduction in airborne pathogens and a 20–30% decrease in sick leave rates in office environments. The technology offers significant economic benefits through reduced healthcare costs and improved productivity while providing environmentally sustainable air purification without harmful residues. This photocatalytic approach represents a scientifically validated, safe, and economically viable solution for next-generation indoor air quality management across healthcare, educational, commercial, and residential sectors.

1. Introduction

Indoor air pollution has emerged as a critical public health concern, with the World Health Organisation (WHO) estimating that people spend approximately 80–90% of their time indoors, where air pollutant concentrations can be 2–5 times higher than outdoor levels [1,2]. Global warming will increase both the risk of pandemics and infections, and intense heat will increase, in many cases, the time people spend indoors [3,4]. This prolonged exposure to contaminated indoor air has been linked to increased rates of respiratory diseases, cardiovascular diseases, and lung cancer, with the WHO reporting that indoor air pollution is 5–10 times higher than outdoor pollution and causes approximately two million premature deaths annually [5]. In addition to these health impacts, a report by the Centres for Disease Control and Prevention (CDC) estimated that improving indoor air quality could reduce sick leaves by 20–30% in office environments [6].

Fine particulate matter (PM_2.5) has attracted particular attention due to its significant correlation with increased morbidity and negative health effects [7,8]. Epidemiological evidence has consistently shown that chronic exposure to indoor PM_2.5 is associated with increased hospital admissions and mortality, while about 75% of indoor PM_2.5 originates from outdoor sources in the absence of indoor pollution activities [2]. However, indoor pollution sources, including cigarettes, cooking activities, and incense burning, affect indoor PM_2.5 concentrations [9,10]. While PM_2.5 remains an important concern, existing filtration-based technologies already provide the partial mitigation of particulate matter. By contrast, other harmful indoor pollutants—such as volatile organic compounds (TVOCs) and airborne pathogens—are not effectively addressed by current systems, despite their substantial role in indoor exposure [11,12]. The microbiome in indoor air represents approximately 34% of air contamination, composed of bacteria, fungi, viruses, and their metabolites that exist in suspension or associate with inorganic particulates to form microbial aerosols [9,11,13].

Traditional air purification methods, including mechanical filtration and ultraviolet germicidal irradiation, have demonstrated significant limitations in addressing the complex nature of indoor air contamination. While high-efficiency particulate air (HEPA) filters effectively remove particles, they cannot neutralise gaseous pollutants or chemicals like formaldehyde, benzene, toluene, and similar or pathogens that may pass through or colonise the filter [12]. These filters need to be replaced frequently as they lose efficiency and have a significant impact on the air volume and electrical consumption of HVAC equipment. Current air purification technologies include filtration (accounting for 90% of air purifiers), electrostatic purification that electrifies and adsorbs particles on collector plates, and negative ion purification that condenses small particles into larger ones for easier removal [14]. These technologies are also challenged by elevated outdoor pollution events, such as haze, where infiltration may offset indoor purification gains [15,16]. The above technologies are classified as passive since pathogens and particles are only removed at the proximity or when passing near the air purification equipment, therefore having a reduced efficiency in eliminating pathogen transmission inside rooms, and they have no effect on pathogen or chemical compound removal from surfaces. Therefore, novel strategies are needed to achieve more comprehensive and active air sanitation.

Heterogeneous photocatalysis represents a sophisticated approach to air purification that harnesses light energy to generate reactive oxygen species (ROS) capable of degrading organic pollutants and inactivating microorganisms. Titanium dioxide (TiO₂) has been the most extensively studied photocatalyst due to its chemical stability, non-toxicity, and strong oxidising power [17]. However, TiO₂’s primary limitation lies in its wide bandgap (3.2 eV), which restricts its activation to ultraviolet light wavelengths below 387 nm and achieving maximum efficiency specially near the UVC bandwidth near 254 nm; these bandwidths are harmful, and therefore, these devices need to be hidden from exposure to humans or animals, significantly limiting their efficiency under typical indoor lighting conditions.

In response to this limitation, next-generation photocatalysts have been designed to operate efficiently under visible light. Among these, tungsten oxide sub-stoichiometric compounds, especially W₁₈O₄₉, have gained attention for their narrower bandgap and strong visible-light responsiveness. When combined with zinc oxide (ZnO), which acts as a cocatalyst to suppress electron–hole recombination, these materials exhibit enhanced ROS production and improved disinfection performance. This synergy allows for continuous, light-driven sanitisation in indoor settings without requiring direct UV exposure.

This article presents a comprehensive investigation aimed at evaluating the scientific principles, safety profile, and practical effectiveness of an advanced air and surface purification device that employs heterogeneous photocatalysis using W₁₈O₄₉ and ZnO catalysts. While TiO₂-based systems have been widely studied, their reliance on UVC light restricts indoor use, and other alternatives such as graphene remain costly and difficult to scale. This highlights a research gap in developing catalyst systems that are both economically viable and effective under visible-light conditions for safe operation in occupied indoor spaces. The objective of this article is therefore to present a synthesis of existing knowledge and a novel viewpoint on W₁₈O₄₉/ZnO photocatalytic systems, rather than to report new experimental results, with the aim of clarifying their potential role in next-generation indoor air and surface sanitisation.

2. Photocatalytic System Design

Photocatalytic air-sanitisation technologies are increasingly recognised as a safer, more energy-efficient, and sustainable alternative to conventional chemical disinfectants and UV-based sterilisation systems [18,19,20]. While TiO₂ remains the benchmark photocatalyst since its first demonstration [21], its activity is largely restricted to the UV range and thus performs poorly under typical indoor lighting conditions [19]. To overcome this limitation, recent studies have advanced plasmonic and heterojunction strategies—such as Ag/TiO₂ or WO₃/ZnO or W₁₈O₄₉/ZnO composites, which extend light absorption into the visible spectrum and improve charge separation efficiency [22,23,24]. As illustrated in Figure 1, the performance of different catalysts under visible-light conditions highlights these improvements, with composite systems like W₁₈O₄₉/ZnO clearly outperforming pure TiO₂. The combination of W₁₈O₄₉ with ZnO as a cocatalyst shows the highest efficiency due to plasmonic electron transfer and effective charge separation. Adding ZnO as a cocatalyst significantly improves both the performance of WO₃ and W₁₈O₄₉ since it increases the energy absorption at lower frequencies and forms a classic Type-II (staggered) heterojunction. Building on these insights, the present system adopts a tailored multi-component catalyst architecture optimised for indoor disinfection. The system employs a multi-component catalyst, primarily based on W₁₈O₄₉, with ZnO incorporated as a cocatalyst to enhance the efficiency and stability of photocatalytic reactions [25]. This design leverages visible-light-responsive materials to ensure disinfection efficacy even under low-UV indoor conditions, where traditional TiO₂-based systems typically underperform [17,26].

2.1. Catalyst Characterisation

W₁₈O₄₉ serves as the primary photocatalyst due to its narrower bandgap and economic viability compared to conventional TiO₂ [24,27,28], enabling effective absorption in the visible-light spectrum [26]. ZnO is incorporated as a cocatalyst to suppress electron–hole recombination, thereby enhancing overall photocatalytic activity [23,25]. This multi-component system enables the utilisation of a broader range of light frequencies and improves photocatalytic efficiency, operational stability, and adaptability to varied indoor environments [29]. Furthermore, the synergistic interaction between W₁₈O₄₉ and ZnO facilitates improved charge separation and reactive oxygen species (ROS) generation, which are critical for the oxidative inactivation of airborne and surface pathogens under ambient illumination [25,30]. Beyond their optical and structural advantages, the W₁₈O₄₉/ZnO systems function by promoting the formation of •OH radicals and H₂O₂, both of which are well-documented antimicrobial agents [30,31,32]. Their complementary behaviour, short-lived, highly reactive radicals acting near the catalyst, and longer-lived H₂O₂ diffusion further create a spatially distributed sanitisation effect [25,33].

2.2. Photocatalytic Mechanism

For the photocatalytic disinfection and generation of H₂O₂ indoors, two key requirements must be satisfied: exposure of the catalyst to an adequate light source and sufficient availability of water vapor molecules [19,21,22]. To achieve the target concentration, it is necessary to balance the energy intensity from the light with the effective surface area of the catalyst, which we optimised by testing different light sources, distances, and catalyst geometries [19,21,22]. Since the water molecule is the reactant, ambient humidity and airflow are also critical. The device requires at least 12% relative humidity to ensure adequate H₂O supply and airflow between 2 and 10 m/s to maintain efficient operation. The catalyst is oriented at a small angle (5–10°) to promote air–catalyst interactions, increasing collisions and the adsorption of water molecules. Under these conditions, calculations show that the system can generate ~0.05 ppm H₂O₂ per m³ of indoor air, accounting for standard ventilation losses, with adjustments required for buildings under negative pressure or high air-exchange rates. As an example, in order to achieve the desired concentration for a space of approximately 100 m² (350 m³), we are using an approximate 20 W light source, placed at approximately 1–2 cm of the catalyst, and the catalyst is composed of a double layer of 50 mm × 200 mm × 12 mm panels, with an impregnated honeycomb aluminium structure, and the whole structure (catalyst + energy source) is placed in an air flow.

The schematic diagram for the photocatalytic mechanism is shown in Figure 2. The generation of H₂O₂ has also been demonstrated in earlier TiO₂ and graphene-based photocatalysts [21,27], though challenges remain regarding efficiency, wavelength requirements, and cost. The W₁₈O₄₉/ZnO system described here addresses these by extending absorption into the visible spectrum [22,23,24].

Upon light irradiation (UV or visible), the photocatalyst is activated as follows:

Catalyst (W₁₈O₄₉/ZnO) + hν → e⁻ (conduction band) + h⁺ (valence band)

This fundamental photoexcitation event involves the absorption of photon energy (hν), which promotes electrons from the valence band to the conduction band, generating electron–hole pairs that initiate surface redox reactions.

Subsequent redox reactions include the following:

Reduction of Oxygen: O₂ + e⁻ → •O₂⁻

The photogenerated electrons reduce molecular oxygen (O₂) adsorbed on the catalyst surface to form superoxide radicals (•O₂⁻), which are moderately reactive ROS capable of damaging microbial cell walls and membranes.

Oxidation of Water: H₂O + h⁺ → •OH + H⁺

Simultaneously, photogenerated holes oxidise water molecules to produce hydroxyl radicals (•OH), which are among the most potent oxidants in aqueous systems, known to degrade organic pollutants and inactivate microorganisms.

Formation of hydrogen peroxide:

•OH + •OH → H₂O₂

Two hydroxyl radicals can recombine to form H₂O₂, providing a sustained oxidative environment without requiring direct radical attack.

•O₂⁻ + 2H⁺ + e⁻ → H₂O₂

Alternatively, superoxide radicals can react with protons and electrons to form hydrogen peroxide via a secondary redox pathway, further contributing to the antimicrobial functionality of the system. This cascade of radical-based reactions allows for the steady, low-level in situ generation of H₂O₂, maintaining a microbicidal environment without the need for external chemical input.

Some pilot case studies have been performed employing this method against key pathogens and viruses in 10 m³ chambers under controlled conditions. Results showed >90% elimination of Clostridioides difficile, Escherichia coli, and Staphylococcus aureus within 30–120 min and the similarly high inactivation of SARS-CoV-2 and Influenza A (H1N1) within 60–120 min. Similar results using tungsten oxides is reported for bacteria and viruses by a number of studies in various environmental conditions with an efficiency of ~85% [34,35]. Comparable studies have also demonstrated the broad antimicrobial activity of hydrogen-peroxide-based systems. For instance, hydrogen peroxide vapor (HPV) has been shown to markedly reduce C. difficile contamination in healthcare environments [36], while the dry fogging of H₂O₂ effectively inactivates both SARS-CoV-2 and Influenza A in a concentration–time-dependent manner [37]. Accordingly, although the system achieved consistent pathogen inactivation under test conditions, notable limitations remain for the real-world deployment for various indoor conditions [19]. Specifically, relative humidity must be maintained above 12% (optimally > 20%) to generate H₂O₂ effectively, airflow should be between 2 m/s and 10 m/s, and system efficiency can be compromised in spaces with negative pressure (e.g., hospitals) or high extraction volumes (e.g., kitchens), requiring design adaptation to compensate for accelerated air exchange [36].

2.3. Safety Assessment Methodology

A safety evaluation can be conducted based on established guidelines from the Occupational Safety and Health Administration (OSHA) [38], WHO [39], and American Conference of Governmental Industrial Hygienists (ACGIH) [40]. Hydrogen peroxide concentrations are to be compared against established permissible exposure limits and threshold limit values. In addition, the degradation kinetics of H₂O₂ via human endogenous enzymes (e.g., catalase) were reviewed to assess its biological compatibility. The photochemical system was also benchmarked against ozone-generating devices, with findings indicating a substantially lower cytotoxicity risk for H₂O₂-based approaches.

3. Effectiveness and Safety of Low Concentrations of Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) technology represents an innovative approach to indoor air purification, generating aerosolised H₂O₂ at low concentrations (approximately 0.05 ppm) that disperses evenly throughout indoor environments to neutralise pathogens and degrade pollutants. This concentration is significantly below established safety thresholds set by regulatory agencies including OSHA’s Permissible Exposure Limit of 1 ppm (8 h TWA), ACGIH’s Threshold Limit Value of 1 ppm, and WHO guidelines confirming safety below 1 ppm for indoor environments [38,39,40]. Long-term-exposure studies demonstrate that hydrogen peroxide at concentrations below 1 ppm does not cause irritation, respiratory issues, or adverse health effects, with the Journal of Occupational and Environmental Medicine confirming safety at 0.05 ppm levels [41,42]. As a naturally occurring compound in human metabolism, low-concentration H₂O₂ is rapidly broken down by enzymes such as catalase and peroxidase, minimising potential harm. The antimicrobial efficacy of low-concentration hydrogen peroxide has been extensively validated through laboratory and real-world applications. Research demonstrates up to a 90% reduction in airborne pathogen concentrations in indoor environments, with particular effectiveness against respiratory viruses (Influenza, SARS-CoV-2) and bacteria (Staphylococcus aureus, Mycobacterium tuberculosis) [31,32]. Surface disinfection capabilities include biofilm degradation and the elimination of bacteria, viruses, and fungi, critical for reducing fomite-based transmission. Healthcare facility studies show reduced surface contamination and lower hospital-acquired infection rates when hydrogen-peroxide-based systems are implemented [33]. Technology’s dual-action approach addresses both airborne and surface pathogens while maintaining safe exposure levels, offering significant potential for improving indoor air quality and reducing infection transmission rates.

4. Enhanced Photocatalytic Performance and Safety of W₁₈O₄₉/ZnO Systems

The W₁₈O₄₉ and ZnO catalyst systems offer substantial advantages over conventional TiO₂-based photocatalytic systems [25,29]. The enhanced visible light absorption and reduced electron–hole recombination led to the more efficient generation of reactive oxygen species, enabling effective sanitisation at lower energy inputs and under typical indoor lighting conditions. The safety profile of the generated hydrogen peroxide concentrations is particularly noteworthy. At 0.05 ppm, the concentration is not only well below regulatory safety thresholds but also represents a level naturally occurring in biological systems, where hydrogen peroxide is produced as part of normal cellular metabolism and rapidly neutralised by endogenous enzymes such as catalase and peroxidase. The economic implications of this technology extend beyond direct healthcare cost savings. The demonstrated reduction in workplace absenteeism translates to improved productivity and a reduced burden on healthcare systems. The environmental benefits of hydrogen peroxide decomposition into water and oxygen, leaving no harmful residues, align with growing demands for sustainable sanitisation solutions. However, several considerations warrant ongoing investigation. Long-term stability of the catalyst system under varying environmental conditions requires continued monitoring. Additionally, the optimisation of the system design for different indoor environments and pollutant loads remains an active area of research.

Low-Level H₂O₂ for Indoor Air Sanitisation

The studies summarised in

Table 1. Evidence supporting the efficacy and safety of low-dose H₂O₂ and visible-light photocatalysts.

Author (Year)	Focus Area	Key Finding	Implication	Exposure Duration	Light Condition
Sanguinet and Edmiston [43]	Air and Surface Disinfection	Dry H2O2 reduced microbial loads by >90% in clinical settings, including resistant organisms.	Supports effectiveness of 0.05 ppm H2O2 for continuous disinfection.	Continuous (multi-hour)	Not specified (ambient/room light assumed)
Wright et al. [45]	Surface Disinfection and Human Safety	Low-dose aerosol H2O2 significantly reduced S. aureus and C. difficile without health risks.	Demonstrates efficacy and safety of low-level H2O2.	Continuous (≥24 h/day for several weeks)	Not specified; ambient clinical lighting
Graham et al. [44]	Viral Inactivation	98% reduction in SARS-CoV-2 titres within 2 h of H2O2 exposure.	Confirms antiviral potential of low-dose H2O2.	2–4 h exposure per cycle	Not specified (ambient/room light assumed)
Rawal et al. [26]	Photocatalytic Performance (W18O49)	W18O49/TiO2 catalyst had >2× efficiency of WO3/TiO2 under ≥422 nm light.	W18O49 has superior visible-light activity for indoor use.	2 h exposure	Visible-light (λ ≥ 422 nm)
Chang et al. [30]	ROS Generation and Bactericidal Activity	W18O49 hybrids generated abundant ROS under visible light.	Validates W18O49’s indoor bactericidal effectiveness.	30 min irradiation + 24 h incubation	Full-spectrum light and Dark
Watt et al. [46]	Endogenous H2O2 Metabolism	Mitochondrial H2O2 is degraded by catalase and peroxidases.	Explains why low-dose H2O2 is safe in biological systems.	Minutes to hours (e.g., symptoms within minutes, pulmonary oedema up to 24–72 h post-exposure)	Not applicable (no light-based mechanism discussed)
Ernstgård et al. [41]	Inhalation Safety	H2O2 up to 0.5 ppm caused no lasting harm or inflammation.	Confirms indoor safety threshold for H2O2 exposure.	30 min to 2 h	Visible light (λ ≥ 420 nm, 300 W Xe lamp)

reinforce the article’s core claims and contextualise its conclusions within broader empirical evidence. This study asserts that around 0.05 ppm H₂O₂ is both effective and safe. Some research strengthens this by showing consistent outcomes across diverse settings, from pathogen inactivation [43,44] to occupational safety thresholds [41,45]. Notably, the enzymatic degradability of H₂O₂, often mentioned but rarely quantified in applied studies, is grounded here by mechanistic insights from [46], offering biochemical justification for its low toxicity. The comparison with ozone is particularly valuable, positioning H₂O₂ not just as “safe enough” but as preferable in disinfection strategies. Similarly, the article’s advocacy for W₁₈O₄₉ is supported by both performance data and ROS-based mechanisms [26,30], though further real-world validation in indoor environments could strengthen its practical relevance.

5. Conclusions

This article validates the scientific principles, safety profile, and practical effectiveness of air- and surface-purification-devices utilising W₁₈O₄₉ and ZnO photocatalytic systems. The technology successfully generates controlled concentrations of hydrogen peroxide for continuous indoor sanitisation while maintaining safety levels significantly below established regulatory thresholds.

The superior performance of W₁₈O₄₉ compared to conventional TiO₂ systems, particularly in visible light applications, combined with the synergistic effects of ZnO cocatalysts, represents a significant advancement in photocatalytic air purification technology. The demonstrated pathogen-reduction efficacy, coupled with substantial economic benefits through reduced healthcare costs and improved productivity, supports the technology’s potential for widespread implementation across various indoor environments.

Based on comprehensive toxicological evidence and a safety analysis, the installation of these photocatalytic air purification devices is recommended for improving indoor air quality and occupant health in healthcare facilities, educational institutions, commercial buildings, and residential spaces. By reducing the concentration of pathogens in the air and on surfaces, hydrogen-peroxide-based systems can lower the incidence of infections among occupants. This is particularly important in shared spaces like offices, schools, and healthcare facilities. A study conducted in office environments found that the use of air sanitisation technologies reduced the incidence of respiratory illnesses by up to 50%, leading to fewer sick leaves.

Additional applications in food preservation—such as in refrigerated warehouses or transport containers—are being studied. While the effectiveness of low-concentration H₂O₂ against all bacterial types requires further analysis, it could significantly reduce food degradation during storage and transport, preventing spoilage and weight loss caused by microbial activity.