Insight on Reaction Pathways of Photocatalytic CO2 Conversion

Photocatalytic CO2 conversion to value-added chemicals is a promising solution to mitigate the current energy and environmental issues but is a challenging process. The main obstacles include the inertness of CO2 molecule, the sluggish multi-electron process, the unfavorable thermodynamics, and the selectivity control to preferable products. Furthermore, the lack of fundamental understanding of the reaction pathways accounts for the very moderate performance in the field. Therefore, in this Perspective, we attempt to discuss the possible reaction mechanisms toward all C1 and C2 value-added products, taking into account the experimental evidence and theoretical calculation on the surface adsorption, proton and electron transfer, and products desorption. Finally, the remaining challenges in the field, including mechanistic understanding, reactor design, economic consideration, and potential solutions, are critically discussed by us

Methane transformation by photocatalysis

Methane hydrate and shale gas are predicted to have substantial reserves, far beyond the sum of other fossil fuels. Using methane instead of crude oil as a building block is, thus, a very attractive strategy for synthesizing valuable chemicals. Because methane is so inert, its direct conversion needs a high activation energy and typically requires harsh reaction conditions or strong oxidants. Photocatalysis, which employs photons operated under very mild conditions, is a promising technology to reduce the thermodynamic barrier in direct methane conversion and to avoid the common issues of overoxidation and catalyst deactivation. In this Review, we cover the development of photocatalysts and co-catalysts, including the use of inorganic materials and polymeric semiconductors, and explain how the use of batch or flow reaction systems affects the reaction kinetics and product selectivity. We also discuss efforts to understand the underlying reaction mechanisms from both a photophysical and a chemical perspective. Finally, we present our view of the challenges facing this field and suggest potential solutions

Bimetallic FeOₓ–MOₓ Loaded TiO₂ (M = Cu, Co) Nanocomposite Photocatalysts for Complete Mineralization of Herbicides

A series of monometallic and bimetallic cocatalyst(s), comprising FeOx, CuOx, CoOx, FeOx–CuOx, and FeOx–CoOx loaded TiO2 catalysts prepared by the surface impregnation method, were investigated for the photocatalytic mineralization of the widely used four herbicides: 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), 2,4-dichlorophenoxyacetic acid (2,4-D), and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). It was found that FeOx–CoOx/TiO2 showed the highest photocatalytic efficiency toward mineralization of selected herbicides. FeOx–CoOx/TiO2 achieves 92% TOC removal in 180 min, representing nearly three time activity of the benchmark PC50 TiO2. From XPS analysis, FeOOH, CuO, and CoO were determined to be loaded onto the TiO2 surface. The outstanding photocatalytic performance of the optimized FeOx–CoOx/TiO2 sample for herbicides mineralization is due to an increased charge separation and enhanced hydroxyl radicals production monitored by diverse spectroscopies. Based on the proposed charge transfer mechanism, FeOx–CoOx cocatalyst species accelerate the transfer of photogenerated holes on TiO2, thus facilitating hydroxyl radicals production

Charge carrier dynamics and reaction intermediates in heterogeneous photocatalysis by time-resolved spectroscopies

Sunlight as the most abundant renewable energy holds the promise to make our society sustainable. However, due to its low power density and intermittence, efficient conversion and storage of solar energy as a clean fuel are crucial. Apart from solar fuel synthesis, sunlight can also be used to drive other reactions including organic conversion and air/water purification. Given such potential of photocatalysis, the past few decades have seen a surge in the discovery of photocatalysts. However, the current photocatalytic efficiency is still very moderate. To address this challenge, it is important to understand fundamental factors that dominate the efficiency of a photocatalytic process to enable the rational design and development of photocatalytic systems. Many recent studies highlighted transient absorption spectroscopy (TAS) and time-resolved infrared (TRIR) spectroscopy as powerful approaches to characterise charge carrier dynamics and reaction pathways to elucidate the reasons behind low photocatalytic efficiencies, and to rationalise photocatalytic activities exhibited by closely related materials. Accordingly, as a fast-moving area, the past decade has witnessed an explosion in reports on charge carrier dynamics and reaction mechanisms on a wide range of photocatalytic materials. This critical review will discuss the application of TAS and TRIR in a wide range of heterogeneous photocatalytic systems, demonstrating the variety of ways in which these techniques can be used to understand the correlation between materials design, charge carrier behaviour, and photocatalytic activity. Finally, it provides a comprehensive outlook for potential developments in the area of time-resolved spectroscopies with an aim to provide design strategies for photocatalysts

Photocatalytic Methane Conversion to C1 Oxygenates over Palladium and Oxygen Vacancies Co-Decorated TiO_{2}

Direct methane conversion to value-added chemicals through photocatalysis is promising but still has great challenges in both efficient activation of C–H bonds and suppression of over-oxidation. Herein, palladium nanoparticles and oxygen vacancies (OVs) co-modified TiO_{2} photocatalysts are prepared and employed for photocatalytic CH_{4} conversion at room temperature. Under optimized conditions with O_{2} and water as the oxidants, a high yield of liquid oxygenates, e.g., 54 693 μmol g^{−1} h^{−1} with a nearly 100% selectivity has been achieved. Mechanism investigations reveal that Pd and OVs synergistically promote charge separation, with Pd and OVs acting as hole and electron acceptors, respectively. Isotopic experiments elucidate that both H_{2}O and O_{2} are oxygen sources for oxygenate production, where O_{2} is the predominant one

Review—Origin and Promotional Effects of Plasmonics in Photocatalysis

Plasmonic effects including near-field coupling, light scattering, guided mode through surface plasmon polaritons (SPPs), Förster resonant energy transfer (FRET), and thermoplasmonics are extensively used for harnessing inexhaustible solar energy for photovoltaics and photocatalysis. Recently, plasmonic hot carrier-driven photocatalysis has received additional attention thanks to its specific selectivity in the catalytic conversion of gas molecules and organic compounds, resulting from the direct injection of hot carriers into the lowest unoccupied molecular orbital of the adsorbate molecule. The excellent light trapping property and high efficiency of hot charge-carrier generation through electromagnetic surface plasmon decay have been identified as the dominant mechanisms that promote energy-intensive chemical reactions at room temperature and atmospheric pressure. However, understanding the electromagnetic effects of plasmonics and distinguishing them from chemical effects in photocatalysis is challenging. While there exist several reviews underlining the experimental observations of plasmonic effects, this critical review addresses the physical origin of the various plasmon-related phenomena and how they can promote photocatalysis. The conditions under which each plasmonic effect dominates and how to distinguish one from another is also discussed. Finally, future research directions are proposed with the aim to accelerate progress in this field at the interface between chemistry and physics

On-demand continuous H-2 release by methanol dehydrogenation and reforming via photocatalysis in a membrane reactor

A robust flow membrane reactor is established and utilised for efficient green H2 production in photocatalytic methanol dehydrogenation and reforming, benefiting from its enhanced mass transfer and effective charge separation in the catalysts

Experimental and computational investigation of heat transfer in a microwave-assisted flow system

Microwave technology is gaining popularity as a tool for chemical process intensification and an alternative to conventional heating. However, in flow systems non-uniform temperature profiles are commonly encountered and hence methods to characterise and improve them are required. In this work, we studied the effects of various operational parameters-microwave power, inlet flow rate, tube orientation and pressure-on the electric field and temperature profiles of water flowing in a PTFE tube (2.4 mm internal diameter), placed in a commercial single-mode microwave applicator. A finite element model was developed to estimate the longitudinal temperature profiles and the absorbed microwave power, while in situ temperature monitoring was performed by a fibre optic probe placed at multiple locations inside the tube. The water temperature inside the tube increased by increasing the microwave power input and temperature profiles stabilised beyond 20 W, while the percentage absorbed microwave power showed the inverse trend. When changing the tube orientation or decreasing the inlet flow rate, microwave absorption decreased significantly. When the pressure was increased to 2.3 bara, water temperature increased by ~ 20 o C. Results from this study provide valuable insights on achievable temperature profiles and energy efficiency of microwave-assisted flow synthesis systems.