A critical review on TiO2 based photocatalytic CO2 reduction system: Strategies to improve efficiency

Halide Perovskite Nanocrystal Photocatalysts for CO2 Reduction: Successes and Challenges

In current research, halide perovskite nanocrystals have emerged as one of the potential materials for light-harvesting and photovoltaic applications. However, because of phase sensitivity, their exploration as photocatalysts in polar mediums is limited. It has been recently reported that these nanocrystals are capable of driving solar-to-chemical production through CO₂ reduction. Using bare nanocrystals and also coupling in different supports, several reports on CO₂ reduction in low polar mediums were reported, and the mechanism of involved redox processes was also proposed. Considering the importance of this upcoming catalytic activity of perovskites, in this Perspective, details of the developments in the field established to date and supported by several established facts are reported. In addition, some unestablished stories or unsolved pathways surrounding the redox process and the importance of using a polar solvent which confused the understanding of the exclusive roles of perovskite nanocrystals in catalysis are also discussed. Further, the future prospects of these materials that face challenges in dispersing in polar solvents, a key process in redox catalysis for CO₂ reduction, are also discussed.

CuMoxW(1-x)O4 Solid Solution Display Visible Light Photoreduction of CO2 to CH3OH Coupling with Oxidation of Amine to Imine

The photoreduction of carbon dioxide (CO₂) to valuable fuels is a promising strategy for the prevention of rising atmospheric levels of CO₂ and the depletion of fossil fuel reserves. However, most reported photocatalysts are only active in the ultraviolet region, which necessitates co-catalysts and sacrificial agents in the reaction systems, leading to an unsatisfied economy of the process in energy and atoms. In this research, a CuMo_xW_(1-x)O₄ solid solution was synthesized, characterized, and tested for the photocatalytic reduction of CO₂ in the presence of amines. The results revealed that the yield of CH₃OH from CO₂ was 1017.7 μmol/g under 24 h visible light irradiation using CuW_0.7Mo_0.3O₄ (x = 0.7) as the catalyst. This was associated with the maximum conversion (82.1%) of benzylamine to N-benzylidene benzylamine with high selectivity (>99%). These results give new insight into the photocatalytic reduction of CO₂ for valuable chemical products in an economic way.

Recent Advances in Solar-Driven Carbon Dioxide Conversion: Expectations versus Reality

Solar-driven carbon dioxide (CO₂) conversion to fuels and high-value chemicals can contribute to the better utilization of renewable energy sources. Photosynthetic (PS), photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic plus electrochemical (PV+EC) approaches are intensively studied strategies. We aimed to compare the performance of these approaches using unified metrics and to highlight representative studies with outstanding performance in a given aspect. Most importantly, a statistical analysis was carried out to compare the differences in activity, selectivity, and durability of the various approaches, and the underlying causes are discussed in detail. Several interesting trends were found: (i) Only the minority of the studies present comprehensive metrics. (ii) The CO₂ reduction products and their relative amount vary across the different approaches. (iii) Only the PV+EC approach is likely to lead to industrial technologies in the midterm future. Last, a brief perspective on new directions is given to stimulate discussion and future research activity.

Cu2O Nanoparticles with Both {100} and {111} Facets for Enhancing the Selectivity and Activity of CO2 Electroreduction to Ethylene

Cu2O nanoparticles (NPs) enclosed with different crystal facets, namely, c-Cu2O NPs with {100} facets, o-Cu2O NPs with {111} facets, and t-Cu2O NPs with both {111} and {100} facets, are prepared and their electrocatalytic properties for the reduction of CO2 to C2H4 are evaluated. It is shown that the selectivity and activity of the C2H4 production depend strongly on the crystal facets exposed in Cu2O NPs. The selectivities for the C2H4 production increases in the order, c-Cu2O < o-Cu2O < t-Cu2O, (with FEC2H4 = 38%, 45%, and 59%, respectively). This study suggests that Cu2O NPs are more likely responsible for the selectivity and activity for the C2H4 production than the metallic Cu NPs produced on the surface of Cu2O NPs. This work provides a new route for enhancing the selectivity of the electrocatalytic CO2 reduction by crystal facet engineering.

Recent Advances in TiO2-Based Photocatalysts for Reduction of CO2 to Fuels

Titanium dioxide (TiO₂) has attracted increasing attention as a candidate for the photocatalytic reduction of carbon dioxide (CO₂) to convert anthropogenic CO₂ gas into fuels combined with storage of intermittent and renewable solar energy in forms of chemical bonds for closing the carbon cycle. However, pristine TiO₂ possesses a large band gap (3.2 eV), fast recombination of electrons and holes, and low selectivity for the photoreduction of CO₂. Recently, considerable progress has been made in the improvement of the performance of TiO₂ photocatalysts for CO₂ reduction. In this review, we first discuss the fundamentals of and challenges in CO₂ photoreduction on TiO₂-based catalysts. Next, the recently emerging progress and advances in TiO₂ nanostructured and hybrid materials for overcoming the mentioned obstacles to achieve high light-harvesting capability, improved adsorption and activation of CO₂, excellent photocatalytic activity, the ability to impede the recombination of electrons-holes pairs, and efficient suppression of hydrogen evolution are discussed. In addition, approaches and strategies for improvements in TiO₂-based photocatalysts and their working mechanisms are thoroughly summarized and analyzed. Lastly, the current challenges and prospects of CO₂ photocatalytic reactions on TiO₂-based catalysts are also presented.

Photocatalytic Reduction of CO2 Over Me (Pt, Pd, Ni, Cu)/TiO2 Catalysts

A series of TiO2 photocatalysts loaded with various metals (Pt, Pd, Ni, and Cu) were prepared by using the wet impregnation method. Their physicochemical properties were studied by using XRD, BET, TPR-H2, FTIR and TPD-NH3/CO2 techniques. The photocatalytic activity of samples was investigated in the gas-phase reduction of carbon dioxide under continuous flow operation mode. Among all investigated catalysts, the Pt and Ni were the most active in terms of the formation rate of methanol. In general, the photocatalytic activity of modified TiO2 decreased with increasing metal loading and reaction time. The reversible deactivation of photocatalysts was associated with the covering of TiO2 surface by the reaction products.

Electrochemical CO2 Reduction to CO Catalyzed by 2D Nanostructures

Electrochemical CO2 reduction towards value-added chemical feedstocks has been extensively studied in recent years to resolve the energy and environmental problems. The practical application of electrochemical CO2 reduction technology requires a cost-effective, highly efficient, and robust catalyst. To date, vigorous research have been carried out to increase the proficiency of electrocatalysts. In recent years, two-dimensional (2D) graphene and transition metal chalcogenides (TMCs) have displayed excellent activity towards CO2 reduction. This review focuses on the recent progress of 2D graphene and TMCs for selective electrochemical CO2 reduction into CO.

Tungsten oxide-based visible light-driven photocatalysts: crystal and electronic structures and strategies for photocatalytic efficiency enhancement

Photocatalysis (PC) technology has received global attention due to its high potential of addressing both environmental and energy issues using only solar light as energy input.

Industrial carbon dioxide capture and utilization: state of the art and future challenges

This review covers the sustainable development of advanced improvements in CO₂ capture and utilization.

Carbon Nanotubes Incorporated Z-Scheme Assembly of AgBr/TiO2 for Photocatalytic Hydrogen Production under Visible Light Irradiations

Photocatalytic H₂ production is a promising strategy toward green energy and alternative to carbon-based fuels which are the root cause of global warming and pollution. In this study, carbon nanotubes (CNTs) incorporated Z-scheme assembly of AgBr/TiO₂ was developed for photocatalytic H₂ production under visible light irradiations. Synthesized photocatalysts were characterized through transmission electron microscope (TEM), X-ray photoelectron spectra (XPS), X-ray diffractometer (XRD), Fourier transform infrared (FTIR), photoluminescence spectra (PL), Brunauer Emmet-Teller(BET), and UV-vis spectroscopy analysis techniques. The composite photocatalysts exhibited a H₂ production of 477 ppm which was three-folds higher than that produced by TiO₂. The good performance was attributed to the strong interaction of three components and the reduced charge recombination, which was 89 and 56.3 times lower than the TiO₂ and AgBr/TiO₂. Furthermore, the role of surface acidic and basic groups was assessed and the photocatalytic results demonstrated the importance of surface functional groups. In addition, the composites exhibited stability and reusability for five consecutive cycles of reaction. Thus, improved performance of the photocatalyst was credited to the CNTs as an electron mediator, surface functional groups, higher surface area, enhanced charge separation and extended visible light absorption edge. This work provides new development of Z-scheme photocatalysts for sustainable H₂ production.

Synthesis of Plasmonic Photocatalysts for Water Splitting

Production of H2, O2, and some useful chemicals by solar water splitting is widely expected to be one of the ultimate technologies in solving energy and environmental problems worldwide. Plasmonic enhancement of photocatalytic water splitting is attracting much attention. However, the enhancement factors reported so far are not as high as expected. Hence, further investigation of the plasmonic photocatalysts for water splitting is now needed. In this paper, recent work demonstrating plasmonic photocatalytic water splitting is reviewed. Particular emphasis is given to the fabrication process and the morphological features of the plasmonic photocatalysts.

The Response of UV/Blue Light and Ozone Sensing Using Ag-TiO2 Planar Nanocomposite Thin Film

We successfully fabricated a planar nanocomposite film that uses a composite of silver nanoparticles and titanium dioxide film (Ag-TiO₂) for ultraviolet (UV) and blue light detection and application in ozone gas sensor. Ultraviolet-visible spectra revealed that silver nanoparticles have a strong surface plasmon resonance (SPR) effect. A strong redshift of the plasmonic peak when the silver nanoparticles covered the TiO₂ thin film was observed. The value of conductivity change for the Ag-TiO₂ composite is 4–8 times greater than that of TiO₂ film under UV and blue light irradiation. The Ag-TiO₂ nanocomposite film successfully sensed 100 ppb ozone. The gas response of the composite film increased by roughly six and four times under UV and blue light irradiation, respectively. We demonstrated that a Ag-TiO₂ composite gas sensor can be used with visible light (blue). The planar composite significantly enhances photo catalysis. The composite films have practical application potential for wearable devices.

Design of core-shell titania-heteropolyacid-metal nanocomposites for photocatalytic reduction of CO2 to CO at ambient temperature

The photocatalytic conversion of CO₂ not only reduces the greenhouse effect, but also provides value-added solar fuels and chemicals. Herein, we report the design of new efficient core-shell nanocomposites for selective photocatalytic CO₂ to CO conversion, which occurs at ambient temperature. A combination of characterization techniques (TEM, STEM-EDX, XPS, XRD, FTIR photoluminescence) indicates that the CO₂ reduction occurs over zinc species highly dispersed on the heteropolyacid/titania core-shell nanocomposites. These core-shell structures create a semiconductor heterojunction, which increases charge separation and the lifetime of charge carriers' and leads to higher electron flux. In situ FTIR investigation of the reaction mechanism revealed that the reaction involved surface zinc bicarbonates as key reaction intermediates. In a series of catalysts containing noble and transition metals, zinc phosphotungstic acid-titania nanocomposites exhibit high activity reaching 50 μmol CO g^-1 h^-1 and selectivity (73%) in the CO₂ photocatalytic reduction to CO at ambient temperature. The competitive water splitting reaction has been significantly suppressed over the Zn sites in the presence of CO₂.

A C-Doped TiO2/Fe3O4 Nanocomposite for Photocatalytic Dye Degradation under Natural Sunlight Irradiation

Magnetically recyclable C-doped TiO2/Fe3O4 (C-TiO2/Fe3O4) nanocomposite was successfully synthesized via a sol–gel method. The synthesized samples were characterized using SEM, energy-dispersive X-ray spectroscopy (EDS), FTIR, and UV-VIS diffuse reflectance spectroscopy (DRS) techniques. The results clearly showed that a C-TiO2/Fe3O4 nanocomposite was produced. The photocatalytic activities of the prepared pristine (TiO2), C-doped TiO2 (C-TiO2) and C-TiO2/Fe3O4 were evaluated by the photodegradation of methyl orange (MO) under natural sunlight. The effect of catalyst loading and MO concentration were studied and optimized. The C-TiO2/Fe3O4 nanocomposite exhibited an excellent photocatalytic activity (99.68%) that was higher than the TiO2 (55.41%) and C-TiO2 (70%) photocatalysts within 150 min. The magnetic nanocomposite could be easily recovered from the treated solution by applying external magnetic field. The C-TiO2/Fe3O4 composite showed excellent photocatalytic performance for four consecutive photocatalytic reactions. Thus, this work could provide a simple method for the mass production of highly photoactive and stable C-TiO2/Fe3O4 photocatalyst for environmental remediation.