Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

In the past decade, the surface plasmon resonance of Ag and Au nanoparticles has been investigated to improve the efficiency of photocatalytic processes. The photocatalytic production of fuels is particularly interesting for its ability to store the sun's energy in chemical bonds that can be released later without producing harmful byproducts. This Feature Article reviews recent work demonstrating plasmon-enhanced photocatalytic water splitting, reduction of CO2 with H2O to form hydrocarbon fuels, and degradation of organic molecules. Focus is placed on several possible mechanisms that have been previously discussed in the literature. A particular emphasis is given to several aspects of these mechanisms that are not fully understood and will require further investigation.

A Review of Surface Plasmon Resonance‐Enhanced Photocatalysis

It is well known that urbanization and industrialization have resulted in the rapidly increasing emissions of volatile organic compounds (VOCs), which are a major contributor to the formation of secondary pollutants (e.g., tropospheric ozone, PAN (peroxyacetyl nitrate), and secondary organic aerosols) and photochemical smog. The emission of these pollutants has led to a large decline in air quality in numerous regions around the world, which has ultimately led to concerns regarding their impact on human health and general well-being. Catalytic oxidation is regarded as one of the most promising strategies for VOC removal from industrial waste streams. This Review systematically documents the progresses and developments made in the understanding and design of heterogeneous catalysts for VOC oxidation over the past two decades. It addresses in detail how catalytic performance is often drastically affected by the pollutant sources and reaction conditions. It also highlights the primary routes for catalyst deactivation and discusses protocols for their subsequent reactivation. Kinetic models and proposed oxidation mechanisms for representative VOCs are also provided. Typical catalytic reactors and oxidizers for industrial VOC destruction are further discussed. We believe that this Review will provide a great foundation and reference point for future design and development in this field.

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Recent Advances in the Catalytic Oxidation of Volatile Organic Compounds: A Review Based on Pollutant Sorts and Sources.

Catalytic oxidation of volatile organic compounds (VOCs) – A review

Transition metal oxides (TMOs) have attracted significant attention for energy storage applications such as supercapacitors due to their good electrical conductivity, high electrochemical response (by providing Faradaic reactions), low manufacturing costs, and easy processability. Despite exhibiting these attractive characteristics, the practical applications of TMOs for supercapacitors are still relatively limited. This is largely due to their continuous Faradaic reactions, which can lead to major changes or destruction of their structure as well phase changes (in some cases) during cycling, leading to the degradation in their capacitive performance over time. Hence, there is an immediate need to develop new synthesis methods, which will readily provide stable porous architectures, controlled phase, as well as useful control over dimensions (1-D, 2-D, and 3-D) of the metal oxides for improving their performance in supercapacitor applications. Since its discovery in late 1990s, metal–organic frameworks (M...

Metal-Organic Framework-Derived Nanoporous Metal Oxides toward Supercapacitor Applications: Progress and Prospects.

A novel template-free oxalate route was applied to synthesize mesoporous manganese oxides with high surface area (355 m2 g−1) and well-defined mesopores which can be obtained in large quantities. The physicochemical properties of the materials were characterized by means of TG, XRD, SEM, TEM, H2-TPR and XPS techniques. All catalysts were tested on catalytic deep oxidation of benzene, and the effects of calcination temperature on the features of catalyst structure and catalytic activity were investigated. Manganese oxides prepared by oxalate route exhibited better catalytic activities for complete oxidation of benzene, toluene and o-xylene as compared with related manganese oxides prepared by other different methods (NaOH route, NH4HCO3 route and nanocasting strategy), and especially the temperature for benzene conversion of 90% on the oxalate-derived manganese oxide catalysts was 209 °C, which is 132 °C lower than required for the catalyst prepared by NaOH route. The catalytic performance of manganese oxide is correlated with surface area, pore size, low-temperature reducibility and distribution of surface species. The mole ratio of Mn4+/Mn2+ on the samples which performed with better catalytic activity was close to 1.0. This is good for the redox process of Mn4+ ↔ Mn3+ ↔ Mn2+ which is the key factor in determining the activity on MnOx, further indicating that the oxalate route is good for keeping the distribution of manganese oxidation states at an appropriate degree. A possible process of VOCs’ complete oxidation on manganese oxide catalysts is discussed. In addition, the best catalyst was highly stable with prolonged time on stream and was resistant to water vapor.

Oxalate route for promoting activity of manganese oxide catalysts in total VOCs’ oxidation: effect of calcination temperature and preparation method

Core−shell Au@TiO2 nanoparticles with truncated wedge-shaped TiO2 morphology have been synthesized successfully by a simple and flexible hydrothermal route. Morphological evolution of TiO2 shells was investigated by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction technique. It has been revealed that the truncated wedge-shaped TiO2 shells experience an epitaxially segmented orientation growth. Also, the (101) crystal planes of TiO2 crystals grow preferentially on the surface of gold nanocrystals stabilizing the heterointerfaces, then faster [001] growth results in the “budding” process occurs, producing growth sites on the initial deposition TiO2 layers, where the TiO2 crystals grow up into truncated wedge-shaped morphologies. It is also found that morphological evolution of TiO2 shells is dependent on the produced F− ion concentration from hydrolyzed TiF4 precursors. The produced F− ions not only facilitate the formation of well-defin...

Synthesis of Core−Shell Au@TiO2 Nanoparticles with Truncated Wedge-Shaped Morphology and Their Photocatalytic Properties

Polymer/graphene composites have attracted much attention due to their unique organic–inorganic hybrid structure and exceptional properties. In this paper, we report the synthesis of polystyrene/reduced graphene oxide (PS/r-GO) composites by a two-step in situ reduction technique, which consists of a hydrazine hydrate reduction and a subsequent thermal reduction at 200 °C for 12 h. The structure and micromorphology of PS/r-GO composites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. The results show that the GO can be efficiently reduced by the two-step in situ reduction method, and the r-GO sheets are well dispersed and ultimately form a continuous network structure in the polymer matrix. PS/r-GO composite films (5 wt% GO) are prepared by the hot press molding method, possessing a conductivity as high as 22.68 S m−1. The superior conductivity arises from the high reduction degree of GO and its high dispersion and the formation of a network structure in the polymer matrix. These polymer/r-GO composites are expected to be applied in multiple electric devices. The techniques for preparing polymer/r-GO composite films could be further extended to other similar systems.

Synthesis of network reduced graphene oxide in polystyrene matrix by a two-step reduction method for superior conductivity of the composite

The design and synthesis of nanostructured ZnO with high chemical sensing properties, especially towards ppb or sub-ppm level VOC gases is still highly desired and challengeable. Herein, the hierarchical hollow ZnO nanocages were synthesized by a facile strategy through the simple and direct pyrolysis of Zn-based metal-organic framework. The as-synthesized hollow ZnO products present the typical hierarchical structures with hollow interiors enveloped by interpenetrated ZnO nanoparticles as porous shell, providing structurally combined meso-/macro-porous channels for facilitating the diffusion and surface reaction of gas molecules. The gas-sensing experiments demonstrate that, in contrast with singular ZnO nanoparticles, the ZnO nanocages show significantly enhanced chemical sensing sensitivity and selectivity towards low-concentration volatile organic compounds, typically, acetone and benzene. Furthermore, the ZnO hollow nanocages perform sub-ppm level sensitivity with 2.3 ppm(-1) towards 0.1 ppm benzene, and ppb level sensitivity with 15.3 ppm(-1) towards 50 ppb acetone, respectively. The enhanced sensing performance of the MOF-derived ZnO nanocages is ascribed to the unique hierarchical structure with high specific surface area and abundant exposed active sites with surface-adsorbed oxygen. (C) 2015 Elsevier B.V. All rights reserved.

MOF-derived hierarchical hollow ZnO nanocages with enhanced low-concentration VOCs gas-sensing performance

Zinc oxide nanoparticles are prepated by calcining zinc hydrocarbonate precursors at 300-700 degrees C (ZnO300-700). and corresponding gas sensing property are tested at 300 degrees C by using formaldehyde as the probe Although the nanoparticle sizes are found to gradually increase with calcination temperature, the sensor measurements reveal the size-independent behavior that ZnO500 and ZnO300 have the highest and lowest responses, respectively Spectroscopic characterization further reveals nonstoichiometric compositions of ZnO nanoparticles ZnO300 has the largest excess oxygen (oxygen interstitial, O(1)). whereas ZnO500 has the largest excess zinc (oxygen vacancy, V(o) and/or zinc interstitial, Zn(1)) Accordingly, a new sensing mechanism is proposed for ZnO nanoparticle sensois Excess zinc favors chemisorption of oxygen onto the nanoparticle surface, leading to reacting with mot e formaldehyde molecules to get a high signal On the contrary, excess oxygen inhibits free oxygen to be chemisorbed onto the nanoparticle surface, and thus decreases the gas response Finally, this new sensing mechanism is verified by testing gas response of ZnO500 nanoparticles annealed at different atmospheres (C) 2010 Elsevier B V All lights reserved

Xiaofeng Wu

Papers

Oxalate route for promoting activity of manganese oxide catalysts in total VOCs’ oxidation: effect of calcination temperature and preparation method

Synthesis of Core−Shell Au@TiO2 Nanoparticles with Truncated Wedge-Shaped Morphology and Their Photocatalytic Properties

Synthesis of network reduced graphene oxide in polystyrene matrix by a two-step reduction method for superior conductivity of the composite

MOF-derived hierarchical hollow ZnO nanocages with enhanced low-concentration VOCs gas-sensing performance

Counterintuitive sensing mechanism of ZnO nanoparticle based gas sensors