Photocatalytic hydrogen production via water splitting using a noble metal on a TiO₂ is a technology that has developed rapidly over the past few years. Specifically, palladium doped TiO₂ irradiated with near-UV or alternatively with visible light has shown promising results. With this end in mind, strategically designed experiments were developed in the Photo-CREC Water-II (PCW-II) Reactor using a 0.25 wt.% Pd-TiO₂ under near-UV and visible light, and ethanol as an organic scavenger. Acetaldehyde, carbon monoxide, carbon dioxide, methane, ethane, ethylene, and hydrogen peroxide together with hydrogen were the main chemical species observed. A Langmuir adsorption isotherm was also established for hydrogen peroxide. On this basis, it is shown that pH variations, hydrogen peroxide formation/adsorption, and the production of various redox chemical species provide an excellent carbon element balance, as well as OH^• and H^• radicals balances. Under near-UV irradiation, 113 cm³ STP of H₂ is produced after 6 h, reaching an 99.8% elemental carbon balance and 99.2% OH^• and H^• and radical balance. It is also proven that a similar reaction network can be considered adequate for the photoreduced Pd-TiO₂ photocatalyst yielding 29 cm³ STP of H₂ with 95.4% carbon and the 97.5% OH^•–H^• radical balance closures. It is shown on this basis that a proposed “series-parallel” reaction network describes the water splitting reaction using the mesoporous Pd-TiO₂ and ethanol as organic scavenger.

Photocatalysis for “green” hydrogen production is a technology of increasing importance that has been studied using both TiO₂–based and heterojunction composite-based semiconductors. Different irradiation sources and reactor units can be considered for the enhancement of photocatalysis. Current approaches also consider the use of electron/hole scavengers, organic species, such as ethanol, that are “available” in agricultural waste, in communities around the world. Alternatively, organic pollutants present in wastewaters can be used as organic scavengers, reducing health and environmental concerns for plants, animals, and humans. Thus, photocatalysis may help reduce the carbon footprint of energy production by generating H₂, a friendly energy carrier, and by minimizing water contamination. This review discusses the most up-to-date and important information on photocatalysis for hydrogen production, providing a critical evaluation of: (1) The synthesis and characterization of semiconductor materials; (2) The design of photocatalytic reactors; (3) The reaction engineering of photocatalysis; (4) Photocatalysis energy efficiencies; and (5) The future opportunities for photocatalysis using artificial intelligence. Overall, this review describes the state-of-the-art of TiO₂–based and heterojunction composite-based semiconductors that produce H₂ from aqueous systems, demonstrating the viability of photocatalysis for “green” hydrogen production.

Photocatalysis for air treatment or photocatalytic oxidation (PCO) is a relatively new technology which requires titanium dioxide (TiO₂) and a source of light (Visible or near-UV) to degrade pollutants contained in air streams. Present approaches for the photodegradation of indoor pollutants in air streams aim to eliminate volatile organic compounds (VOCs) and viruses, which are both toxic and harmful to human health. Photocatalysis for air treatment is an inexpensive and innovative green process. Additionally, it is a technology with a reduced environmental footprint when compared to other conventional air treatments which demand significant energy, require the disposal of used materials, and release CO₂ and other greenhouse gases to the environment. This review discusses the most current and relevant information on photocatalysis for air treatment. This article also provides a critical review of (1) the most commonly used TiO₂-based semiconductors, (2) the experimental syntheses and the various photocatalytic organic species degradation conversions, (3) the developed kinetics and computational fluid dynamics (CFD) and (4) the proposed Quantum Yields (QYs) and Photocatalytic Thermodynamic Efficiency Factors (PTEFs). Furthermore, this article contains important information on significant factors affecting the photocatalytic degradation of organic pollutants, such as reactor designs and type of photoreactor irradiation. Overall, this review describes state-of-the-art photocatalysis for air treatment to eliminate harmful indoor organic molecules, reviewing as well the potential applications for the inactivation of SARS-CoV2 (COVID-19) viruses.

Photoreduction with visible light can enhance the photocatalytic activity of TiO₂ for the production of hydrogen. In this article, we present a strategy to photoreduce a palladium-doped TiO₂ photocatalyst by using near-UV light prior to its utilization. A sol-gel methodology was employed to prepare the photocatalysts with different metal loadings (0.25–5.00 wt% Pd). The structural and morphological characteristics of the synthesized Pd-TiO₂ were analyzed by using X-ray Diffraction (XRD), BET Surface Area (S_BET), TemperatureProgrammed Reduction (TPR), Chemisorption and X-ray Photoelectron Spectroscopy (XPS). Hydrogen was produced by water splitting under visible light irradiation using ethanol as an organic scavenger. Experiments were developed in the Photo-CREC Water-II (PCW-II) Reactor designed at the CREC-UWO (Chemical Reactor Engineering Centre). It was shown that the mesoporous 0.25 wt% Pd-TiO₂ with 2.5 1eV band gap exhibits, under visible light, the best hydrogen production performance, with a 1.58% Quantum Yield being achieved.

Several TiO₂ based semiconductors with different Pt loadings are prepared using incipient impregnation, wet impregnation and the sol-gel method. These photocatalysts are evaluated in the Photo-CREC-Water II Photoreactor for hydrogen production via water dissociation, using an organic renewable scavenger (ethanol). Results obtained show the influence of the photocatalyst preparation in the production of hydrogen and in the observed quantum yields. Furthermore, it is established that the reaction networks leading to hydrogen production, using various photocatalysts, share common features. This analysis is developed by both identifying and quantifying different chemical species and their changes with irradiation time. Key species in this oxidation–reduction network are hydrogen, hydrogen peroxide, ethanol, methane, ethane, acetaldehyde and carbon dioxide. On this basis, it is shown that under an inert gas atmosphere, ethanol consumption is sub-stoichiometric. This points towards simultaneous ethanol consumption and the formation of the ethanol scavenger.