Single-component Metal Oxides Research Articles

Core–Shell Electrode Material Co9S8@Ni(OH)2 for High-Performance Supercapacitors

Developing electrodes with hybrid architectures holds significant importance in enhancing the efficiency of energy storage and conversion processes. In general, single-component metal oxides tend to exhibit poor operational stability and sluggish ionic-electron mobility. To address these issues, we have successfully synthesized a hybrid-structured material, Co8S9@Ni(OH)2, leveraging the easily modifiable characteristics of spinel-structured Co8S9. This hybrid material was prepared through a hydrothermal synthesis method. The resulting Co8S9@Ni(OH)2 hybrid structure demonstrates an impressive specific capacitance of 1029 C g−1 at 1 A g−1, accompanied by outstanding stability. In addition, the as-prepared samples shows a capacitance retention of 81.09% initial capacitance, indicating that the as-prepared sample possess an excellent cycle stability.

Interface engineering of hybrid ZnCo2O4@Ni2.5Mo6S6.7 structures for flexible energy storage and alkaline water splitting

Designing electrodes with hybrid structures is significant for improving energy storage and conversion efficiency. Overall, single-component metal oxides suffer from poor working stability and slow ionic electron mobility. In order to solve these problems, based on the easy-to-modify properties of spinel-structured ZnCo2O4, hybrid-structured ZnCo2O4@Ni2.5Mo6S6.7 was successfully grown on nickel foam through hydrothermal synthesis. The hybrid-structured ZnCo2O4@Ni2.5Mo6S6.7 exhibits high specific capacitance of 848 C g−1 at 1 A g−1, and excellent stability. The assembled supercapacitor device reaches a high energy density of 177.9 W kg−1 at 2700 Wh kg−1, excellent cycle performance, and mechanical flexibility. In alkaline water splitting, this hybrid structure presents low overpotentials, low water splitting cell voltage, and excellent stability. Density functional theory calculations confirm that the construction of Ni2.5Mo6S6.7 sulfide shell can reduce hydrogen adsorption energy, increase charge carrier density on the catalyst surface, and achieve a small Gibbs free energy difference in the fundamental rate-determining step, suggesting the improved electrocatalytic performance.

Dexterous synthesis hybrid MnO2/PPy nanosheets grown on ultrafine NiO nanoarrays for high performance flexible hybrid capacitors

Developing flexible device with high energy density and long cycle stability remains still a challenge for energy storage. At the same time, single-component metal oxides suffer from poor working stability and slow ionic electron mobility. In order to address these issues and meet fast increasing demand for alternative energy, high efficient energy storage devices are required. Herein, ultrafine NiO/MnO2/PPy nanoarrays are designed and prepared through a simple chemical reaction. The as-prepared samples are utilized as electrode materials and exhibit an excellent electrochemical performance. In addition, Due to their unique structural characteristics, it shows high specific capacitance (1100 C g−1 at 1 A g−1) and capacitance retention of 82.79 % after 10,000 cycles. Furthermore, quasi-solid-state asymmetric device is assembled and delivers an energy density of 49.8 Wh kg−1 at a power density of 856 W kg−1. This work provides the design method of ultrafine nanoarrays with an excellent electrochemical property for next energy storage fields.

Mesoporous Mn-based multi-component metal oxide for fast chemical warfare agent degradation

Protection against harmful chemical compounds is a major social concern, especially chemical warfare agents (CWAs) that are easy to synthesize and can cause mass casualties. The metal oxides with large surface area, porous structures, and a large number of active sites have shown outstanding performance for the adsorption and decontamination of CWAs. However, single-component metal oxides show a lower degradation rate for chemical warfare agents. Here, we demonstrate a convenient homogeneous hydrolysis method for the synthesis of mesoporous manganese-based multi-component metal oxides. Our strategy enables Ce or/and Zr to be doped into the δ-MnO2 structure during the forming process, remarkably enlarging the surface area and providing rich active sites for CWA catalysis. The as-synthesized multi-component metal oxides exhibit excellent performance for the CWA degradation. It is surprising to find that the best degree of removal of mustard gas (HD), soman (GD), and the VX nerve agent (VX) is 90.60%, 87.32%, and 100%, respectively, reaction with multi-component samples in 4 h, 2.92-fold increase to HD, 8.28-fold increase to GD, and 1.67-fold increase to VX contrast with undoped δ-MnO2. This work may provide an avenue for developing new generation chemical warfare agent decontamination materials and other catalysts.

Regulating Crystal Structure and Atomic Arrangement in Single-Component Metal Oxides through Electrochemical Conversion for Efficient Overall Water Splitting.

Single-component transition-metal oxide (TMO: FeOx, NiOx, or CoOx) nanosheets grown on nickel foam (NF) were electrochemically optimized with Li ion (Na ion)-induced conversion reaction for bifunctional electrocatalysis. The optimum FeOx/NF-Li electrocatalyst exhibits low overpotentials of 239 mV for hydrogen evolution reaction and 276 mV for oxygen evolution reaction at a current density of 100 mA cm-2. A two-electrode water splitting cell using FeOx/NF-Li as both anode and cathode requires only 1.60 V to achieve a current density of 10 mA cm-2. The impressive water splitting performance of the FeOx/NF-Li electrode is revealed to be attributed to Li-induced electrochemical conversion, which alters the crystal structure, creating more active sites for electrocatalytic reactions, as well as introduces O vacancies increasing the electron density and the intrinsic conductivity. More importantly, the atomic arrangement is regulated from tetrahedral Fe(Td) to octahedral Fe(Oh) coordination, which acts as catalytically active sites with reduced Gibbs free energy for the rate-determining steps. This electrochemical conversion reaction can be extended to other TMOs (i.e., NiOx/NF and CoOx/NF) for promoted electrocatalytic water splitting performances. This study provides an in-depth understanding on the nature of atomic and electronic structure evolution to promote the electrocatalytic activity.

Novel Two-Dimensional WO3/Bi2W2O9 Nanocomposites for Rapid H2S Detection at Low Temperatures.

Compared with single-component metal oxides, multicomponent metal oxides show good gas sensing performance in the field of gas sensing, but they still need to be further improved in terms of rapid response. In this paper, a two-dimensional flaky WO3/Bi2W2O9 composite material with a thickness of about 32.3 nm was synthesized by a simple solvothermal method. The composite has good sensing performance and selectivity toward H2S. When the operating temperature is as low as 92 °C, the response to 100 ppm H2S reaches 84.18, and the response time is 2 s, which is extremely fast due to the open system of the two-dimensional nanosheet. A combination of gas chromatography-mass spectrometry (GC-MS) and X-ray photoelectron spectroscopy (XPS) is used to analyze the changes of H2S and the surface chemistry of WO3/Bi2W2O9 composite materials; the sensing mechanism of H2S was studied by a Kelvin probe and UV diffuse reflection. Compared with the pure phase WO3 and Bi2W2O9, good gas sensing properties of the WO3/Bi2W2O9 composite may be due to its unique heterostructure. This is the first application of WO3/Bi2W2O9 in the field of gas sensing and is of great significance for the rapid detection of H2S at low temperatures for multicomponent metal oxides.

Synthesis of W-Nb-O solid acid for catalytic combustion of low-concentration monochlorobenzene

WO3-Nb2O5 mixed oxides were prepared by citric acid based sol-gel method, and were tested for catalytic combustion of 1000 ppmv monochlorobenzene (MCB), as a typical chlorinated aromatic hydrocarbon in air pollution. Research results for the structure-function relationship revealed that the physicochemical properties of WO3-Nb2O5 were notably promoted than single component metal oxide, due to the strong electron interaction between W, Nb and O elements, which were confirmed by the techniques of H2-TPR, positron lifetime spectra and XPS characterization. WO3-Nb2O5 were amorphous, in which W and Nb elements were homogeneous dispersed. The functionalized materials contributed to the improvement of adsorption, activation, cracking and deep oxidation of MCB on the catalyst surface, and only non-negligible production of CO byproduct was detected. Both the acid property and oxidative property played important roles in the catalytic degradation process, and their cooperative catalytic effect could be optimized by the catalyst composition and preparation process. 2W1Nb (W/Nb = 2:1, molar ratio) displayed the largest acid amount and the best redox property, and thus it displayed the highest catalytic degradation performance, and 50% conversion of MCB was achieved at 280 °C with the GHSV of 9000 mL gcat−1 h−1. Besides, in the 120 h continuous reaction, only slight decrease (from 91% to 86%) for the catalytic activity of 2W1Nb was observed in initial 7 h, showing that it was desired towards the demanding conditions (such as resistance to water adsorption, chlorine poison and carbon deposition). Moreover, the WO3-Nb2O5 materials also satisfied the environment-friendly and low-cost needs for environmental catalysis, which could be a promising alternative for traditional V2O5-WO3/TiO2 catalyst system.

Synthesis of Single-Component Metal Oxides with Controllable Multi-Shelled Structure and their Morphology-Related Applications.

Multi-shelled hollow spheres metal oxides, namely materials with more than three shells, have attracted increasing attention due to their unique structure. The preparation methods of typical metal oxides including NiO, Co3 O4 and ZnO etc. have been summarized in this review. Simultaneously, the parameters that influence the ultimate morphologies, shell number as well as the compositions have also been discussed. The potential application fields in energy conversion and storage, electromagnetic wave absorption, photocatalysis that related to the unique structure are also highlighted. Finally, the future researches of multi-shelled hollow spheres metal oxides are further discussed.

Novel Synthesis of Mn-Ni-Co Oxide(MNCO)@g-C3N4 Ternary Ribbon-like Hetero Nanocomposite for Psuedocapacitor Applications

Since last few years, the hassle of clean, efficient, and renewable energy sources is remarkable and thereby driving intense scientific concern in the production, storage and management of this precious energy. Among variety of electrical energy storage devices, supercapacitors (SCs), also known as electrochemical capacitors, have industrial attention because of their high power density and very long cyclic stability as compared to their counterparts such as batteries, fuel cells and conventional capacitors etc. To improve the electrochemical properties of the electrode materials, Mn-Ni-Co-Oxide(MNCO)@g-C3N4 Ribbon-like Hetero-nanoarrays were synthesized by a simple two-step approach including hydrothermal method and subsequent calcination process. This ternary metal oxide composite was directly utilized for the psuedocapacitive investigation because it is believed that these three metal elements can provide synergistic effects during the redox reaction process compared to the single component metal oxides. Due to similar atom radii of the elements, Mn-Ni-Co ternary oxide (MNCO)@g-C3N4 without crystal structure change has been identified as one kind of the most attractive supercapacitive electrode materials. The chemical composition and morphology of the material was characterized by XRD, EDS and FESEM and HRTEM techniques. The electrochemical capacitance of MNCO@g-C3N4 was examined by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance measurements. Results shows that the MNCO@g-C3N4 electrode material exhibits electrochemical activity for supercapacitor applications. A maximum specific capacitance value of 5624 F g-1 could be obtained under an applied current density of 1 Ag-1 in 6 mol L-1 KOH electrolyte. It is envisaged that the novel MNCO@g-C3N4 ternary oxide composite material can be an alternate electrode material for high-performance supercapacitor application.

Hierarchical NiCo2O4 Micro-/Nanostructures with Tunable Morphologies As Anode Materials for Li-Ion and Na-Ion Batteries

Binary metal oxide NiCo2O4 is a promising anode material for future Li-ion batteries (LIBs) and Na-ion batteries (SIBs) owing to its inherited characteristics of high theoretical capacity and low cost.1,2 In particular, NiCo2O4 possesses high electrical conductivity, reversible capacity and mechanical stability than single-component metal oxides NiO and Co3O4. 3,4 However, the large volume change and sluggish kinetics during the charge/discharge process result in a short cycling life and poor rate performance, which limits its practical application.5 The electrochemical performance of an active material highly depends on its structure (both primary and secondary structures).6 Therefore, tailored architecture design and surface engineering of NiCo2O4 are highly desirable to secure both the high transport kinetics and sustain the structure integrity for high rate and long life. In this study, one-dimensional (1D) porous NiCo2O4 microrods and microspheres are successfully synthesized by a simple solvothermal method using different solvents followed by calcination. The solvothermal process and solvents employed in the synthesis play crucial role in determining the final morphologies of NiCo2O4. Compared with NiCo2O4 microspheres, the porous microrods exhibit much better Li- and Na-ion storage properties owing to the 1D geometry to facilitate fast ion transport and the porous structure to accommodate volume expansion of NiCo2O4 during ion insertion. The porous NiCo2O4 microrods display high initial charge capacity (1046.1 mAh g-1 at 100 mA g-1), long cycling stability (719 mAh g-1 after 600 cycles at 500 mA g-1), and rate capability for lithium-ion batteries. They also show better performance in sodium-ion batteries. This work may provide a facile way for fabricating novel structured metal oxide anode materials for batteries. Figure 1. (Left) SEM images of the 1D porous NiCo2O4 microrods and microspheres for LIBs and SIBs respectively. (Right) Rate performances at various current densities of LIBs and SIBs. References Zheng, F. C.; Zhu, D. Q.; Chen, Q. W. ACS Applied Materials & Interfaces 2014, 6, (12), 9256-9264.Lee, Y.; Kim, M. G.; Cho, J. Nano letters 2008, 8, (3), 957-961.Li, B.; Feng, J.; Qian, Y.; Xiong, S. Journal of Materials Chemistry A 2015, 3, (19), 10336-10344.Jadhav, H. S.; Kalubarme, R. S.; Park, C. N.; Kim, J.; Park, C. J. Nanoscale 2014, 6, (17), 10071-10076.Chen, J.; Ru, Q.; Mo, Y.; Hu, S. RSC Advances 2015, 5, (90), 73783-73792.Ye, J.; Liu, W.; Cai, J.; Chen, S.; Zhao, X.; Zhou, H.; Qi, L. Journal of the American Chemical Society 2010, 133, (4), 933-940. Figure 1

Modifying the Surface of γ-Al2 O3 with Y2 Sn2 O7 Pyrochlore: Monolayer Dispersion Behaviour of Composite Oxides.

To investigate the dispersion behaviour of composite oxides on supports, and to obtain better supports for Pd for CO oxidation, a series of Y2 Sn2 O7 /Al2 O3 composite oxides with different Y2 Sn2 O7 loadings were prepared by a deposition-precipitation method. XRD and X-ray photoelectron spectroscopic extrapolation methods revealed that, similar to single-component metal oxides, composite oxides can also disperse spontaneously on support surfaces to form a monolayer with a certain capacity. The monolayer dispersion capacity/threshold for Y2 Sn2 O7 on the surface of γ-Al2 O3 is 0.109 mmol per 100 m2 γ-Al2 O3 , corresponding to 7.2 wt % Y2 Sn2 O7 loading. This is the first work to demonstrate monolayer dispersion of a composite oxide on a support. After combining Y2 Sn2 O7 with γ-Al2 O3 , active oxygen species can be introduced onto the catalyst surfaces. Thus, the interaction between Pd and the support is strengthened, the dispersion of Pd is improved in comparison with the single-component Y2 Sn2 O7 support, and a synergistic effect is induced between Pd and the composite support, which is beneficial to catalyst activity. By tuning the γ-Al2 O3 surface with different amounts of pyrochlore Y2 Sn2 O7 , CO oxidation activity on 1 % Pd/Y2 Sn2 O7 /Al2 O3 was improved. These findings may provide new insights into the design and preparation of effective supported noble metal catalysts with lower contents of noble metals.

Mixed layered WO3/ZrO2 films (with and without rhodium) as active supports for PtRu nanoparticles: enhancement of oxidation of ethanol

Mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form robust sub-microstructured mixed metal oxide (WO3/ZrO2) films on glassy carbon electrode. Attractive interactions between anionic tungstate units (of the inner WO3 layer) and cationic oxo-zirconium surface species (existing in acid media on ZrO2 nanostructures in the outer layer) tend to stabilize the mixed oxide system. By dispersing bimetallic platinum-ruthenium (PtRu) nanoparticles over such active WO3/ZrO2 supports, the activity of PtRu toward the oxidation of ethanol has been enhanced in terms of both increasing the electrocatalytic currents and lowering the onset potential of this reaction. The catalytic effect is more pronounced relative to the performance of PtRu nanoparticles deposited at the same loading of 100μgcm−2 onto single-component metal oxide films, WO3 or ZrO2. Immobilization of rhodium nanoparticles onto WO3 inner layer (before over-coating with outer ZrO2 layer and deposition of PtRu) has led to further remarkable increases of electrocatalytic (voltammetric, chronoamperometric) currents recorded at the room temperature (22°C) in 0.5moldm−3H2SO4 containing 0.5moldm−3 ethanol. Among important issues is the initial activation of ethanol molecule (presumably by inducing the CC bond breaking) before catalytic electrooxidation at PtRu. Furthermore, the mixed WO3/ZrO2 film (support) is characterized by high acidity and fast charge (electron, proton) propagation during the system’s redox transitions. High population of OH reactive groups and good mobility of protons (particularly at zirconia sites) is expected to increase activity of PtRu sites toward the oxidative removal of poisoning (CO) intermediates.

Mesoporous binary metal oxide nanocomposites: Synthesis, characterization and decontamination of sulfur mustard

Mesoporous MnO2–ZnO, Fe2O3–ZnO, NiO–ZnO, and CeO2–ZnO binary metal oxide nanocomposites were studied as sorbent decontaminants against sulfur mustard. They were synthesized by precipitation pyrolysis method and characterized by means of transmission electron microscopy, scanning electron microscopy coupled with energy dispersive analysis of X rays, X ray diffraction, and nitrogen adsorption techniques. The transmission electron microscopy and nitrogen adsorption data indicated the presence of pores with diameter ranging from 10 to 70 nm in the binary metal oxide nanocomposites and these materials exhibited surface area values in the range of 76–134 m2/g. These binary metal oxide nanocomposites demonstrated large decontamination efficiencies against sulfur mustard when compared to their single component metal oxide nanoparticles. The binary metal oxide nanocomposites effectively decontaminated sulfur mustard into relatively non toxic products such as chloro ethyl vinyl sulfide, divinyl sulfide, 1,4-oxathiane, etc. The promising decontamination properties of binary metal oxide nanocomposites against sulfur mustard were attributed to the basic sites, Lewis acid sites, and the presence of these sites was confirmed by CO2 and NH3 temperature programmed desorption.

Wall-like hierarchical metal oxide nanosheet arrays grown on carbon cloth for excellent supercapacitor electrodes.

Recently, considerable efforts have been made to satisfy the future requirements of electrochemical energy storage using novel functional electrode materials. Binary transition metal oxides (BTMOs) possess multiple oxidation states that enable multiple redox reactions, showing higher supercapacitive properties than single component metal oxides. In this work, a facile hydrothermal method is provided for the synthesis of wall-like hierarchical metal oxide MMoO4 (M = Ni, Co) nanosheet arrays, which are directly grown on flexible carbon cloth for use as advanced binder-free electrodes for supercapacitors. By virtue of their intriguing structure, the resulted active material nanosheets with a high specific surface area can provide a large electroactive region, which could facilitate easy accession of electrolyte ions and fast charge transport, resulting in an enhanced electrochemical performance. Separately, the as-synthesized MMoO4 (M = Ni, Co) samples have exhibited superior specific capacitances (1483 F g(-1) of NiMoO4 and 452 F g(-1) of CoMoO4 at a current density of 2 A g(-1)), as well as excellent cycling stability (93.1% capacitance retention of NiMoO4 and 95.9% capacitance retention of CoMoO4 after 2000 cycles). The results show that the binder-free electrodes constructed by deposition of MMoO4 (M = Ni, Co) nanosheets on carbon cloth are promising candidates for the application of supercapacitors.

Single-Component TiO2 Tubular Microengines with Motion Controlled by Light-Induced Bubbles.

In this work, light-controlled bubble-propelled single-component metal oxide tubular microengines have for the first time been demonstrated. For such a simple single-component TiO2 tubular microengine in H2O2 aqueous solution under UV irradiation, when the inner diameter and length of the tube are regulated, the O2 molecules will nucleate and grow into bubbles preferentially on the inner concave surface rather than on the outer surface, resulting in a vital propulsion of the microengine. More importantly, the motion state and speed can be modulated reversibly, fast (the response time is less than 0.2 s) and wirelessly by adjusting UV irradiation. Consequently, the as-developed TiO2 tubular microengine promises potential challenged applications related to photocatalysis, such as "on-the-fly" photocatalytic degradation of organic pollutes and photocatalytic inactivation of bacteria due to the low cost, single component, and simple structure, as well as the facile fabrication in a large-scale.

Investigation of 35 Elements as Single Metal Oxides, Mixed Metal Oxides, or Dopants for Titanium Dioxide for Dye-Sensitized Solar Cells

A total of 35 elements were investigated as single component metal oxides and as dopants for TiO2 for use in dye-sensitized solar cells (DSCs). An array dispenser and scanner system was utilized for high-throughput testing of the single component metal oxides and hundreds of doped TiO2 compositions. The optimal dopant concentrations were identified and summarized according to their effect on short circuit current (ISC) and open circuit voltage (VOC). New dopant candidates were discovered, including several post-transition metals, which showed improvements in DSC performance when incorporated at compositions of 6% In TiO2, 6% Sn TiO2, and 6% Sb TiO2 (metals basis). These compositions were used to synthesize scaled-up DSC devices for detailed characterization. Incorporation of 6% In into TiO2 enhanced VOC substantially and increased ISC slightly, 6% Sn improved ISC while preserving VOC, and 6% Sb increased ISC significantly but decreased VOC. When coincorporated, Cr and Sb showed a complementary interaction, which reduced resistance to charge transport in the DSC device. The 2% Cr, 6% Sb TiO2 showed higher ISC and VOC values than 6% Sb TiO2 and considerably higher ISC than 2% Cr TiO2. The fill-factor for 2% Cr, 6% Sb TiO2 was relatively low, but the maximum power was higher than the other compositions due to the higher currents. The VOC values for DSC devices with different compositions correlated linearly with the measured flat-band potentials (EFB).

Mesoporous CuO–ZnO binary metal oxide nanocomposite for decontamination of sulfur mustard

Mesoporous CuO–ZnO binary metal oxide nanocomposites were studied as sorbent decontaminants against sulfur mustard, a well known chemical warfare agent. They were prepared by precipitation pyrolysis method and characterized by means of X-ray diffraction, transmission electron microscopy, nitrogen adsorption, Fourier transform infrared spectroscopy techniques. Obtained data indicated the presence of mesopores with diameter ranging from 2 to 80 nm and the materials exhibited relatively high surface area 86 m2 g−1 when compared to the individual metal oxide nanoparticles. Reactive sites of mesoporous CuO–ZnO binary metal oxide nanocomposites were studied by infrared spectroscopy technique using pyridine as a probe molecule. These materials demonstrated superior decontamination properties against sulfur mustard when compared to single component metal oxides and decontaminated it to divinyl sulfide, chloroethyl vinyl sulfide, hemisulfur mustard, etc.

Direct decomposition of nitric oxide over Ba catalysts supported on CeO2-based mixed oxides

The direct decomposition of nitric oxide (NO) over barium catalysts supported on various metal oxides was examined in the absence and presence of O2. Among the Ba catalysts supported on single-component metal oxides, Ba/Co3O4 and Ba/CeO2 showed high NO decomposition activities, while Ba/Al2O3, Ba/SiO2, and Ba/TiO2 exhibited quite low activities. The effect of an addition of second components to Co and Ce oxides was further examined, and it was found that the activities were significantly enhanced using Ce–Mn mixed oxides as support materials. XRD results indicated the formation of CeO2–MnOx solid solutions with the cubic fluorite structure. O2-TPD of the CeO2–MnOx solid solutions showed a large desorption peak in a range of relatively low temperature. The BET surface areas of the CeO2–MnOx solid solutions were larger than those of pure CeO2 and Mn2O3. These effects caused by the addition of Mn are responsible for the enhanced activities of the Ba catalysts supported on Ce–Mn mixed oxides.

Catalytic Activities of Single Component Metal Oxides for Selective Reduction of NO with Ethene.

The catalytic activities of thirteen types of single component metal oxides were measured for the selective catalytic reduction (SCR) of NO with ethene in excess oxygen and related reactions to elucidate the characteristic features of catalyst activity. The related reactions were the oxidation of NO2 with O2, the oxidation of C2H4 with O2, the reduction of NO2 with C2H4 in the absence of O2, and the reduction of NO with C2H4 in the absence of O2. Alumina exhibited the highest activity for SCR, followed by ZrO2, SNO2, MgO, CaO, Y2O3, and TiO2; WO3, Co3O4, CuO, Fe2O3, ZnO, and SiO2 showed little activity. The active catalysts including alumina had the characteristic features of high activities for both the oxidation of NO to NO2 and the reduction of NO2 with C2H4 to form N2, as well as low activity for the oxidation of C2H4 with O2. Although Co3O4, CuO, and Fe2O3 showed high activities for the oxidation of NO to NO2, they were also highly active for the oxidation of C2H4 with O2. Therefore, C2H4 was consumed without reduction of NO over these catalysts. The characteristic features for the active catalysts also support the reaction scheme that the SCR proceeds by the successive reactions of the oxidation of NO to NO2 and the reduction of NO2 with C2H4 to form N2.

Heterogeneous Basic Catalysis

Heterogeneous acid catalysis attracted much attention primarily because heterogeneous acidic catalysts act as catalysts in petroleum refinery and are known as a main catalyst in the cracking process which is the largest process among the industrial chemical processes. In contrast to these extensive studies of heterogeneous acidic catalysts, fewer efforts have been given to the study of heterogeneous basic catalysts. The types of heterogeneous basic catalysts are listed in Table 1. Except for non-oxide catalysts, the basic sites are believed to be surface O atoms. The studies of heterogeneous catalysis have been continuous and progressed steadily. They have never been reviewed in the chemical Reviews before. It is more useful and informative to describe the studies of heterogeneous basic catalysis performed for a long period. In the present article, therefore, the cited papers are not restricted to those published recently, but include those published for the last 25 years. The paper first describes the generation of basic sites before describing methods used in the characterization of basic surfaces. These are indicator methods, temperature programmed desorption (TPD) of CO{sub 2}, UV absorption and luminescence spectroscopies, TPD of H{sub 2}, XPS, IR of CO{sub 2}, IR of pyrrole, and oxygen exchange betweenmore » CO{sub 2} and the surface. The paper then discusses studies on the catalysis by heterogeneous basic catalysts. Some of these reactions are dehydration, dehydrogenation, hydrogenation, amination, alkylation, ring transformation, and reactions of organosilanes. Catalysts discussed are single component metal oxides, zeolites, non-oxide types, and superbasic catalysts. 141 refs.« less