Indoor gas phase photoactivity of yttrium modified titanate films for fast acetaldehyde oxidation
Introduction
Aldehydes have been identified as the main compounds responsible for the loss of air quality in megacities. The main sources of acetaldehyde emissions are adhesives, wood materials for buildings, glues and coatings, inks, deodorants, indoor smoking, decomposition of volatile organic compounds (VOCs) from vehicle exhaust gases, fireplaces, and outdoor wood preservation products; in addition to the anthropogenic release from terrestrial plants, root fermentation reactions and so on (NTP National Toxicology Program, 2016; Spinazzè et al., 2020). The environmental-friendly resolutions for decontamination are focusing on the emission reduction of gas-phase pollutants from cleaning substances, home pesticides, printers, paints, etc., using exhaust-device catalysts and indoor filters together with adequate ventilation. Alternatively, processes and devices are being developed to adapt to alternative energy sources such as sulfur-free fuels, alcohols, natural gas, biofuels from plants, which are more eco-friendly, and carbon-free hydrogen electric and fuel cells. However, the acetaldehyde emissions from vehicles that use bioethanol turn out to be higher than those from traditional gasoline engines due to organic molecules and road conditions (Spinazzè et al., 2020; Cao et al., 2020). Acetaldehyde emitting from incomplete combustion of hydrocarbons and biofuels stands as one of major public concerns, and clear challenges need to be conducted for the atmospheric pollution-cleaning strategies.
On the other hand, respiratory effects of aldehyde exposure go from nasal congestion to lung effects and eczema; liver damage has also been reported. Aldehyde has been identified as a human carcinogen with chronic inhalation (Zhou et al., 1400; World Health’s Organizati, 1281; NTP National Toxicology Program, 2016) for a current emission regulation ranging from 3 to 79 μg m−3. In this sense, some events in megacities, like the Beijing Olympic Games in summer 2018, were monitored for studying the emission of aldehydes, where the acetaldehyde (ACH) concentration reached 27.1 ± 15.7 μg m−3, remained stable and slowly decreased after the event. ACH is associated with primary emission sources (Altemose et al., 2015), where schools, libraries and coffee shops are places with considerable ACH exposure. Nowadays, this pandemic situation is causing indoor concentration risk, exceeding outdoor levels due to higher personal exposure, (0.5 μg m−3 EPA/600/8-86-015A), which is growing steadily with more homework/home office activities and the concomitant frequent use of spray disinfectant products. Reports on ACH emissions from photocopiers, 0.174 μg m−3 (Spinazzè et al., 2020), showed that they exceeded the outdoor concentration (0.071 μg m−3), where emissions were evaluated in more than 140 office rooms in the framework of the European OFFICAIR research project. Therefore, high risk by ACH exposure become great concerns and new technologies for mitigation this ACH compound are in urgent need.
In this context, TiO2-based materials have been used for gas-phase photooxidation of ACH. Mesoporous TiO2 films fabricated by spin-coating achieved ACH (100 μmol) decomposition within 17 h, where the photoactivity was related to the surface area and anatase crystalline phase (Fan et al., 2007). Pt/reduced TiO2 loaded on carbonized filter paper under UV and visible irradiation showed higher activity than the bare P25 TiO2 material, both in a closed-circulation and continuous flow setup (Kim et al., 2017). It has been found that a highly porous carbon support and Pt loading are key factors for the photocatalytic oxidation of ACH (80 ppmv) with good stability, however, the ACH adsorption and optimum flow rate still remain under discussion. The use of other noble metals such as Ag and Pd also helps the ACH photo-decomposition at room temperature. Nevertheless, Pt, which has been widely used as thermocatalyst, has achieved the mineralization of ACH together with UV irradiation. As an example, ACH (100 ppm) was mineralized in a tubular reactor with photocatalyst coating (Sano et al., 2003). Under humid conditions (50%), Ag and Pd decreased their activity, but Pt allowed the formation of O2− due to UV absorption. On the other hand, highly ordered TiO2 nanotube arrays are very effective structures for ACH degradation according to the length of the arrays (Liu et al., 2008). The morphology of the arrays seems to be crucial for the mineralization of 110–100 ppmv of ACH, for the structure gives stability and diffusion, which improve mineralization with respect to TiO2–P25 films. Also, a continuous flow reactor system was used at ambient temperature to test the activity of silver nanowires supported on TiO2; the composites were coated onto glass slides (Zeng et al., 2018). In this case, one-dimensional-core–shell structures allow the generation of O2− radicals that enhance the photocatalytic performance and stability of films under a 260-W fluorescence lamp. Six cyclic experiments confirmed the stability of the structures in the oxidation of 500 ppm of ACH for 900 min.
In this research work, the use of high surface area nanostructures for gas phase acetaldehyde decomposition is studied. The hydrothermal microwave assisted synthesis of titanium titanates with yttrium incorporation produced the photoactive nanostructures described above. Average humidity conditions of 50% for room temperature acetaldehyde degradation were set in order to get insights into the effects exerted by water, fiber morphology and yttrium load on the gas-phase ACH degradation.
Section snippets
Hydrothermal synthesis of yttrium nanostructures
The synthesis of nanotubular-like titanates was carried out by means of a microwave-assisted hydrothermal method (Mw-HyM) using an Eyela MWO-1000 Wave Magic microwave apparatus. Firstly, 10 M NaOH (80 mL), (J.T. Baker, ACS grade) and Y(NO3)3·6H2O (10 mL, Aldrich 99.8%) aqueous solutions were prepared using double-distilled water and stirred separately for 10 min. The corresponding yttrium nitrate amounts to get 1, 3, and 5 wt% of yttrium with respect to titanate, 2 g of doped nanostructure,
Material characterizations
The XRD spectra of the Nt-Y materials show hydrogen titanium oxide hydrate as a monoclinic system and main crystalline phase, based on JCPDS (00-047-0124), weak peaks at 10, 24, 28 and 48, and sharply intense peaks at 25.35, 27.39, 36, 37.75 and 48.26 with anatase crystalline phase referred to JCPDS (21–1272) and tetragonal system as secondary phase. The difference in intensities is related to the size range, diameter, and interlayer defects of titanate structures, which leads to the broadening
Conclusions
The doping of yttrium cations into the titanate structure using a one-step hydrothermal method produces photoactive films for acetaldehyde oxidation as a model of VOC degradation; specifically, 1 wt% of Y boosted the photoactive sites. High concentration of acetaldehyde (200 ppm and 50% of humidity) was quickly degraded in 25 min by the nanofibrillar-yttrium-titanate-nanostructured films, keeping stability for 2 consecutive cycles. Humidity and defects of the as-synthesized nanofiber structures
Sample credit author statement
M. S. and J. I., contributed for development of methodology, materials analysis, S.K contributed for conceptualization, writing-reviewing and editing and materials characterization. N.S., C.T., and A.F. contributed in support for materials characterizations and data interpretation. V.R.-G. contributed for funding acquisition conceptualization, supervision of experiments, and writing-original-draft preparation and writing-reviewing and editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
V.R.-G. received support from CONACyT-Mexico for the Visiting Scientists and Scientists on Sabbatical Leave Fellowship Program 2018–2019. We thank B. Rivera-Escoto, A. I. Peña-Maldonado and Hector Silva-Pereira of LINAN-IPICYT for the characterization of the materials studied in this work (XRD-SEM-TEM). S. K. appreciates the postdoctoral position from Japan Society for the Promotion of Science (JSPS)-P18337.
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