191 research outputs found
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
Spectrums of Black Hole in de Sitter Spacetime with Highly Damped Quasinormal Modes: High Overtone Case
Motivated by recent physical interpretation on quasinormal modes presented by
Maggiore, the adiabatic quantity method given by Kunstatter is used to
calculate the spectrums of a non-extremal Schwarzschild de Sitter black hole in
this paper, as well as electrically charged case. According to highly damped
Konoplya and Zhidenko's numerical observational results for high overtone
modes\cite{Konoplya}, we found that the asymptotic non-flat spacetime structure
leads two interesting facts as followings: (i) near inner event horizon, the
area and entropy spectrums, which are given by ,
, are equally spaced accurately. (ii) However, near
outer cosmological horizon the spectrums, which are in the form of , , are not
markedly equidistant. Finally, we also discuss the electrically charged case
and find the black holes in de Sitter spacetime have similar quantization
behavior no matter with or without charge.Comment: 12 pages, 2 firures, published versio
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