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.

To elucidate the role of oxygen vacancy defects, various Mn-based oxides with oxygen vacancy defects are employed to the toluene oxidation, which are synthesized by adjusting solvent and double-complexation routes. The MnOx-ET catalyst shows the highest catalytic activity (T90 = 225 °C) for toluene oxidation. Compared with other Mn-based oxides, the as-prepared MnOx-ET catalyst has more surficial oxygen vacancies and good oxygen storage capacity (OSC), which is the reason on its remarkable activity for toluene oxidation. In addition, in situ DRIFTS study reveals that both lattice oxygen and adsorbed oxygen species can participate in the activation-oxidation process of toluene, which results in two reaction routes for the toluene oxidation. The rich oxygen-vacancy concentration of catalysts accelerates the key steps for the activation and generation of oxidized products. Quasi-in situ XPS results further confirm that enrich adsorbed-oxygen species as active oxygen and increasing Mn4+ concentration enhance the superior activity for toluene oxidation.

Highly efficient mesoporous MnO2 catalysts for the total toluene oxidation: Oxygen-Vacancy defect engineering and involved intermediates using in situ DRIFTS

Volatile organic compounds (VOCs) with the properties of volatility, toxicity and diffusivity pose a serious threat to human health and eco-environment. Catalytic oxidation technology has been considered as a highly efficient option for the treatment of VOCs. This review systematically summarizes the recent processes and advances on catalytic elimination of VOCs over nanoparticles catalysts. Firstly, catalytic performances of catalysts for VOC degradation are evaluated and compared on the basis of unified performance metrics. Secondly, catalytic mechanisms of VOC oxidation, based on experimental and theoretical studies, are systematically introduced. Then, catalytic reactors employed in VOC elimination processes are summarized. In particular, photothermocatalysis by integrating (thermo)catalysis with photocatalysis is also elucidated. Lastly, the perspectives to the scientific issues and challenges faced, as well as the future outlooks are proposed. Collectively, this review will provide theoretical and experimental foundation for rational fabrication and application of nanoparticles catalysts toward VOC elimination in future.

Recent advances in VOC elimination by catalytic oxidation technology onto various nanoparticles catalysts: a critical review

As a promising approach for carbon dioxide capture, chemical looping combustion has been extensively investigated for more than two decades. However, the chemical looping strategy can be and has been extended well beyond carbon capture. In fact, significant impacts on emission reduction, energy conservation, and value-creation can be anticipated from chemical looping beyond combustion (CLBC). This article aims to demonstrate the versatility and transformational benefits of CLBC. Specifically, we focus on the use of oxygen carriers or redox catalysts for chemical production – a $4 trillion industry that consumes 40.9 quadrillion BTU of energy. Compared to state-of-the-art chemical production technologies, we illustrate that chemical looping offers significant opportunities for process intensification and exergy loss minimization. In many cases, an order of magnitude reduction in energy consumption and CO2 emission can be realized without the needs for carbon dioxide capture. In addition to providing various CLBC examples, this article elaborates on generalized design principles for CLBC, potential benefits and pitfalls, as well as redox catalyst selection, design, optimization, and redox reaction mechanism.

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Chemical looping beyond combustion – a perspective

The design of high-efficiency non-noble bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is paramount for water splitting technologies and associated renewable energy systems. Spinel-structured oxides with rich redox properties can serve as alternative low-cost OER electrocatalysts but with poor HER performance. Here, zirconium regulation in 3D CoFe2 O4 (CoFeZr oxides) nanosheets on nickel foam, as a novel strategy inducing bifunctionality toward OER and HER for overall water splitting, is reported. It is found that the incorporation of Zr into CoFe2 O4 can tune the nanosheet morphology and electronic structure around the Co and Fe sites for optimizing adsorption energies, thus effectively enhancing the intrinsic activity of active sites. The as-synthesized 3D CoFeZr oxide nanosheet exhibits high OER activity with small overpotential, low Tafel slope, and good stability. Moreover, it shows unprecedented HER activity with a small overpotential of 104 mV at 10 mA cm-2 in alkaline media, which is better than ever reported counterparts. When employing the CoFeZr oxides nanosheets as both anode and cathode catalysts for overall water splitting, a current density of 10 mA cm-2 is achieved at the cell voltage of 1.63 V in 1.0 m KOH.

Zirconium-Regulation-Induced Bifunctionality in 3D Cobalt–Iron Oxide Nanosheets for Overall Water Splitting

Co3O4 spinel has been widely investigated as a promising catalyst for the oxidation of volatile organic compounds (VOCs). However, the roles of tetrahedrally coordinated Co2+ sites (Co2+Td) and octahedrally coordinated Co3+ sites (Co3+Oh) still remain elusive, because their oxidation states are strongly influenced by the local geometric and electronic structures of the cobalt ion. In this work, we separately studied the geometrical-site-dependent catalytic activity of Co2+ and Co3+ in VOC oxidation on the basis of a metal ion substitution strategy, by substituting Co2+ and Co3+ with inactive or low-active Zn2+(d0), Al3+(d0), and Fe3+(d5), respectively. Raman spectroscopy, X-ray absorption fine structure (XAFS), and in situ DRIFTS spectra were thoroughly applied to elucidate the active sites of a Co-based spinel catalyst. The results demonstrate that octahedrally coordinated Co2+ sites (Co2+Oh) are more easily oxidized to Co3+ species in comparison to Co2+Td, and Co3+ are responsible for the oxidative brea...

Geometrical-Site-Dependent Catalytic Activity of Ordered Mesoporous Co-Based Spinel for Benzene Oxidation: In Situ DRIFTS Study Coupled with Raman and XAFS Spectroscopy

Insights into the high performance of Mn-Co oxides derived from metal-organic frameworks for total toluene oxidation.

MnO2 nanoparticles encapsuled in spheres of Ce-Mn solid solution: Efficient catalyst and good water tolerance for low-temperature toluene oxidation

CuMn2O4 and CoMn2O4 spinels were facilely synthesized by oxidation–precipitation and subsequent heat treatment at relatively low temperature. Selective catalytic reduction (SCR) of NOx demonstrates that NOx conversions in CuMn2O4 with (111) plane (CuMn2O4-C) and in CuMn2O4-C with (311) plane (CuMn2O4-T) are more than 90% at 200 and 300 °C, respectively, which are superior to those in CoMn2O4-C and CoMn2O4-T. CuMn2O4-C and CoMn2O4-C exhibit higher absorption amounts of NH3/NO and more oxygen vacancies than CuMn2O4-T and CoMn2O4-T, respectively. In addition, CuMn2O4-C displays high catalytic activity and good stability in NH3-SCR in the presence of 100 ppm of SO2 and 10 vol % H2O. In situ diffuse reflection infrared Fourier transform spectroscopy results indicate the coexistence of Eley–Rideal and Langmuir–Hinshelwood mechanisms in CuMn2O4-C, and the Eley–Rideal mechanism is predominant.

Kai Zhang

Papers

Geometrical-Site-Dependent Catalytic Activity of Ordered Mesoporous Co-Based Spinel for Benzene Oxidation: In Situ DRIFTS Study Coupled with Raman and XAFS Spectroscopy

Insights into the high performance of Mn-Co oxides derived from metal-organic frameworks for total toluene oxidation.

MnO2 nanoparticles encapsuled in spheres of Ce-Mn solid solution: Efficient catalyst and good water tolerance for low-temperature toluene oxidation

Structure–Activity Relationships of AMn2O4 (A = Cu and Co) Spinels in Selective Catalytic Reduction of NOx: Experimental and Theoretical Study

Total oxidation of benzene over ACo 2 O 4 (A = Cu, Ni and Mn) catalysts: In situ DRIFTS account for understanding the reaction mechanism