Molecular adsorption and dissociation of CO2 on TiO2 anatase (001) activated by oxygen vacancies

A review on TiO2-x-based materials for photocatalytic CO2 reduction

Photocatalytic CO₂ reduction technology has a broad potential for dealing with the issues of energy shortage and global warming. As a widely studied material used in the photocatalytic process, titanium dioxide (TiO₂) has been continuously modified and tailored for more desirable application. Recently, the defective/reduced titanium dioxide (TiO_2-x) catalyst has attracted broad attention due to its excellent photocatalytic performance for CO₂ reduction. In this perspective review, we comprehensively present the recent progress in TiO_2-x-based materials for photocatalytic CO₂ reduction. In detail, the review starts with the fundamentals of CO₂ photocatalytic reduction. Then, the synthesis of a defective TiO₂ structure is introduced for the regulation of its photocatalytic performance, especially its optical properties and dissociative adsorption properties. In addition, the current application of TiO_2-x-based photocatalysts for CO₂ reduction is also highlighted, such as metal-TiO_2-x, oxide-TiO_2-x and TiO_2-x-carbon-based photocatalysts. Finally, the existing challenges and possible scope of photocatalytic CO₂ reduction over TiO_2-x-based materials are discussed. We hope that this review can provide an effective reference for the development of more efficient and reasonable photocatalysts based on TiO_2-x.

DFT Modeling of CO2 Adsorption and HCOO• Group Conversion in Anatase Au-TiO2-Based Photocatalysis

Due to the merits of carbon circulation and hydrocarbon production, solar-assisted photocatalysis has been regarded as an ideal option for securing a sustainable future of energy and environment. In the photocatalytic carbon cycle process, surface reactions including the adsorption of CO₂ and the conversion of CO₂ into CH₄, CH₃OH, etc. are crucial to be examined ascribed to their significant influence on the performance of the photocatalysis. Because the conversion reaction starts from the formation of HCOO^•, the density functional theory (DFT) model was established in this study to investigate the micromechanism of CO₂ adsorption and the conversion of CO₂ to HCOO^• group in the anatase Au-TiO₂ photocatalytic system. The CO₂ adsorption bonding in six configurations was simulated, on which basis the effects of the proportion of water molecules and the lattice temperature increase due to the local surface plasmon resonance (LSPR) on the photocatalytic CO₂ adsorption and conversion were specifically analyzed. The results show that the experimental conditions that water molecules are released before CO₂ are favorable for the formation of the adsorption configuration in which HCOO^• tends to be produced without the need of reaction activation energy. This is reasonable since the intermediate C atoms do not participate in bonding under these conditions. Moreover, Au clusters have an insignificant influence on the adsorption behaviors of CO₂ including the adsorption sites and configurations on TiO₂ surfaces. As a result, the reaction rate is reduced due to the temperature increase caused by the LSPR effect. Nevertheless, the reaction maintains a very high rate. Interestingly, configurations that require activation energy are also possible to be resulted, which exerts a positive influence of temperature on the conversion rate of CO₂. It is found that the rate of the reaction can be improved by approximately 1-10 times with a temperature rise of 50 K above the ambient.

Understanding rich oxygen vacant hollow CeO2@MoSe2 heterojunction for accelerating photocatalytic CO2 reduction

Solar-driven CO₂ reduction into gas fuels is desirable for a sustainable carbon cycle. To improve the photocatalytic activity of CO₂RR, the unique rich oxygen (Vo) vacant hollow CeO₂@MoSe₂ is designed. The introduction of Vo is conducive to the capture of electrons by CO₂ and promotes the process of photocatalytic CO₂RR. The heterojunction formed by introducing the narrow band gap semiconductor MoSe₂ increases the absorption range of visible light and improves the separation efficiency of photogenerated carriers. The hollow structure improves the CO₂ adsorption capacity and improves the use of light effect. Therefore, the prepared H-Vo-CeO₂@49.7 wt% MoSe₂ exhibits enhanced photocatalytic activity for CO₂RR. The yield of CH₄ and CO is 10.2 µmol and 33.2 µmol in 4 h. Moreover, the photocatalytic mechanism is discussed through DFT and in-situ DRIFTS. This work provides new insights to the role of Vo in active CO₂ photoconversion and exploits an important application of CO₂ reduction.

Investigation on the Suitability of Engelhard Titanium Silicate as a Support for Ni-Catalysts in the Methanation Reaction

Methanation reaction of carbon dioxide is currently envisaged as a facile solution for the storage and transportation of low-grade energies, contributing at the same time to the mitigation of CO2 emissions. In this work, a nickel catalyst impregnated onto a new support, Engelhard Titanium Silicates (ETS), is proposed, and its catalytic performance was tested toward the CO2 methanation reaction. Two types of ETS material were investigated, ETS-4 and ETS-10, that differ from each other in the titanium content, with Si/Ti around 2 and 3% by weight, respectively. Catalysts, loaded with 5% of nickel, were tested in the CO2 methanation reaction in the temperature range of 300–500 °C and were characterized by XRD, SEM–EDX, N2 adsorption–desorption and H2-TPR. Results showed an interesting catalytic activity of the Ni/ETS catalysts. Particularly, the best catalytic performances are showed by Ni/ETS-10: 68% CO2 conversion and 98% CH4 selectivity at T = 400 °C. The comparison of catalytic performance of Ni/ETS-10 with those obtained by other Ni-zeolites catalysts confirms that Ni/ETS-10 catalyst is a promising one for the CO2 methanation reaction.

A CeO2 Semiconductor as a Photocatalytic and Photoelectrocatalytic Material for the Remediation of Pollutants in Industrial Wastewater: A Review

The direct discharge of industrial wastewater into the environment results in serious contamination. Photocatalytic treatment with the application of sunlight and its enhancement by coupling with electrocatalytic degradation offers an inexpensive and green technology enabling the total removal of refractory pollutants such as surfactants, pharmaceuticals, pesticides, textile dyes, and heavy metals, from industrial wastewater. Among metal oxide—semiconductors, cerium dioxide (CeO2) is one of the photocatalysts most commonly applied in pollutant degradation. CeO2 exhibits promising photocatalytic activity. Nonetheless, the position of conduction bands (CB) and valence bands (VB) in CeO2 limits its application as an efficient photocatalyst utilizing solar energy. Its photocatalytic activity in wastewater treatment can be improved by various modification techniques, including changes in morphology, doping with metal cation dopants and non-metal dopants, coupling with other semiconductors, and combining it with carbon supporting materials. This paper presents a general overview of CeO2 application as a single or composite photocatalyst in the treatment of various pollutants. The photocatalytic characteristics of CeO2 and its composites are described. The main photocatalytic reactions with the participation of CeO2 under UV and VIS irradiation are presented. This review summarizes the existing knowledge, with a particular focus on the main experimental conditions employed in the photocatalytic and photoelectrocatalytic degradation of various pollutants with the application of CeO2 as a single and composite photocatalyst.