Evaluation of gas / solid photocatalytic performance for the removal of VOCs at ppb and sub-ppb levels

Time-resolved spectroscopic investigation for the practical application of a photocatalytic air purifier

The photocatalytic efficiency for removing volatile organic compounds (VOCs) is significantly influenced by operational parameters like humidity and flow velocity, exhibiting notable and inconsistent fluctuations in both lab-scale and large-scale demonstrations. In this study, operando spectroscopy and isotope analysis were employed to investigate the correlation between humidity levels and degradation of gaseous acetaldehyde using TiO2 photocatalysts, aiming to demonstrate the scaling-up of photocatalytic air purifier. It was observed that rate constants for the mineralization of acetaldehyde rapidly decreased by 30% as relative humidity increased from 25% to 80% in the flow system (with an air velocity, v = 0.78 m/s). However, batch system showed smaller change with only a 10% reduction of the rate constant. Humidity fluctuations were more pronounced under high-speed conditions and were amplified in air purifier (v = 3.8 m/s). Time-resolved operando spectroscopy using an 13C isotope of acetaldehyde revealed that humidity's distinct role in dark adsorption and photocatalytic reactions. Water was found to inhibit the formation of crotonaldehyde during aldol condensation reaction in dark condition. Moreover, water suppressed photocatalytic mineralization by inhibiting acetate oxidation to formate. These findings provide valuable insights for improving realistic air purification processes, underscoring the importance of identifying key intermediates and controlling humidity to enhance the selectivity of gaseous pollutant oxidation reactions.

Infancy of peracetic acid activation by iron, a new Fenton-based process: A review

The exacerbated global water scarcity and stricter water directives are leading to an increment in the recycled water use, requiring the development of new cost-effective advanced water treatments to provide safe water to the population. In this sense, peracetic acid (PAA, CH3C(O)OOH) is an environmentally friendly disinfectant with the potential to challenge the dominance of chlorine in large wastewater treatment plants in the near future. PAA can be used as an alternative oxidant to H2O2 to carry out the Fenton reaction, and it has recently been proven as more effective than H2O2 towards emerging pollutants degradation at circumneutral pH values and in the presence of anions. PAA activation by homogeneous and heterogeneous iron-based materials generates - besides HO• and FeO2+ - more selective CH3C(O)O• and CH3C(O)OO• radicals, slightly scavenged by typical HO• quenchers (e.g., bicarbonates), which extends PAA use to complex water matrices. This is reflected in an exponential progress of iron-PAA publications during the last few years. Although some reviews of PAA general properties and uses in water treatment were recently published, there is no account on the research and environmental applications of PAA activation by Fe-based materials, in spite of its gratifying progress. In view of these statements, here we provide a holistic review of the types of iron-based PAA activation systems and analyse the diverse iron compounds employed to date (e.g., ferrous and ferric salts, ferrate(VI), spinel ferrites), the use of external ferric reducing/chelating agents (e.g., picolinic acid, l-cysteine, boron) and of UV-visible irradiation systems, analysing the mechanisms involved in each case. Comparison of PAA activation by iron vs. other transition metals (particularly cobalt) is also discussed. This work aims at providing a thorough understanding of the Fe/PAA-based processes, facilitating useful insights into its advantages and limitations, overlooked issues, and prospects, leading to its popularisation and know-how increment.

More than One Century of History for Photocatalysis, from Past, Present and Future Perspectives

In this review, we analyzed the history and the past and present trends in photocatalysis research, trying to outline possible scenarios for the future in the medium term. The in-depth analysis of the literature reported here—from a mere bibliometric point of view—allowed us to divide the history of photocatalysis into four different periods characterized by different maturity of the topic and different bibliometric features. The turn of the 20th century saw an explosion in scientific production, which is still continuing now and has reached more than 15,000 papers published each year. Research interest is still growing significantly, and the analysis of different keywords suggests that such growth is substantial and not merely due to “publish or perish” behavior. The analysis of the most-investigated topics in the field of photocatalysis highlighted that, during its history, the focus of the research moved from inorganic oxides to carbon and hybrid materials. Concomitantly, the investigation of the “geography” of photocatalysis allowed us to underline its evolution over the years, with the repositioning of its center of mass from the Atlantic Ocean (USA and Europe) to Asia (China and India). Photocatalysis is active as never before but still awaiting major breakthroughs, which would allow a much broader technological and commercial output. Without such breakthroughs in this decade, the growth in scientific interest could level out or even decrease.

Atomic-level insight into effect of substrate concentration and relative humidity on photocatalytic degradation mechanism of gaseous styrene

Substrate concentration and relative humidity (RH) impact the photocatalytic efficiency of industrial aromatic hydrocarbons, but how they influence intermediate formation and degradation pathway remains unclear. With the help of oxygen isotope tracing method, the effects of these two environmental parameters on degradation mechanism of styrene were revealed at atomic level. Increasing styrene concentration favored product formation, which was however inhibited by RH elevation. Gaseous products were not directly formed in gaseous phase, but originated from desorption of interfacial intermediates. The volatile aldehydes and furans further exchanged their ¹⁶O with ¹⁸O in H₂¹⁸O. Increase of RH showed higher enhancement on ¹⁸O distribution in all products and pathways than that of substrate concentration. Low RH preferred high generation of ¹⁶O₂^•- and ⁽¹⁶⁾¹O₂, dominating reaction to form 1-phenyl-1,2-ethandiol, 2-hydroxy-1-phenyl-ethanon and phenylglyoxal monohydrate in sequence. Successive production of benzyl alcohol, benzaldehyde and benzoic acid through the reaction of styrene with promoted ^•18OH by increasing RH became predominant. Hydration was firstly observed and confirmed as an important gaseous transformation step of aldehyde and furan products. Our findings provide a deep insight into photocatalytic degradation mechanism of aromatic hydrocarbons regulated by environmental parameters to further improve their industrial purification efficiency, and are helpful predicting environmental geochemistry fate of organics and preventing their negative impact on natural environment.

Competitive adsorption of multicomponent volatile organic compounds on biochar

Crude oil contaminated lands are recognised to have significant contributions to airborne volatile organic compounds (VOCs) with adverse effects on human health and tropospheric ozone. Soil capping systems for controlling harmful emissions are critical engineering solutions where advanced soil remediation techniques are neither available nor feasible. Studies on the adsorption of single VOC species in biochar have shown promising results as a potential capping material; however, current understanding of mixed gas system and multi-component adsorption of VOCs on biochar which would represent more realistic in situ conditions is very limited. We present, for the first time, the results of a study on competitive adsorption of mixed VOCs, including aromatic and non-aromatic VOCs commonly emitted from crude oil contaminated sites on two types of biochar pyrolysed at 500°C from wheat straw and bagasse as feedstock. The kinetics of sorption of multicomponent VOCs including acetone, hexane, toluene and p-xylene in biochar are studied based on the results of an extensive experimental investigation using a bespoke laboratory setup. Both biochar types used in this study presented a high sorption capacity for VOC compounds when tested individually (51-110 mg/g). For the multicomponent mixture, the competition for occupying sorption sites on biochar surface resulted in a lower absolute sorption capacity for each species, however, the overall sorption capacity of biochar remained more or less similar to that observed in the single gas experiments (50-109 mg/g). The chemical interactions via hydrogen bonds, electrostatic attraction, and pore-filling were found to be the main mechanisms of adsorption of VOC in the biochar studied. The efficiency of biochar regeneration was assessed through five cycles of adsorption-desorption tests and was found to be between 88% and 96%. The incomplete desorption observed confirm the formation of likely permanent bonds and heel build-ups during the sorption process.

Recent advances in photocatalytic decomposition of water and pollutants for sustainable application

Photoinduced reduction and oxidation, the important processes in photocatalytic water splitting and organic degradation, have generated increasing interest to address the energy and environmental issues. In this review, the recent developments in bandgap and interfacial engineering for enhanced light absorption, efficient charge separation and interfacial reaction are focused toward the applications in photocatalytic water splitting and organic degradation. In photoinduced reduction for hydrogen evolution, three major strategies are discussed: cocatalysts, sacrificial agents and heterojunctions. In photoinduced oxidation for organic degradation, three types of emerging pollutants of current concerns are highlighted: organic dyes, pharmaceuticals and volatile organic compounds. The key challenges of promising photocatalysts are discussed for future development and practical application.