Replacement of noble metal catalysts by CuMnOx/CeOx/CH catalyst in catalytic combustion of toluene

To verify the feasibility of replacing noble metal catalysts by transition metals-based catalysts, monolithic CuMnOx/CeOx/cordierite honeycomb (CH) catalysts were prepared by conventional impregnation method and applied in microwave catalytic combustion of toluene. The research suggested that CuMnOx/CeOx/CH catalysts, demonstrated strong microwave-absorbing ability due to first dielectric loss and secondary magnetic loss, exhibited higher catalytic activities under microwave heating than electric heating. This is probably attributed to high temperature "hot spots" of microwave, the abundant pore structure of active particles of CuMnOx/CeOx/CH catalyst, and oxygen vacancies and Oads on the catalysts' surface. The research also suggested that under the conditions of an initial toluene concentration of 1500 mg m-3, airflow of 0.18 m³ h-1 and gas hourly space velocity of 7000 h-1, toluene could be completely removed at a relatively low temperature of 276 °C and mineralized at 304 °C by microwave catalytic combustion. Compared with noble metal catalysts, CuMnOx/0.03CeOx/CH catalyst had a lower light-off temperature and nearly complete combustion temperature for toluene removal and mineralization under microwave heating, which confirms the possibility of replacing noble metal catalysts. Moreover, CuMnOx/0.03CeOx/CH catalyst exhibited excellent stability at a bed temperature of 300 ℃ after six cycles of a total of 960 min and achieved a 100 % removal and 94 % mineralization of toluene. This study also identified the key mechanism behind which is the electron transfers among Cu2+/Cu+, Mn4+/Mn3+/Mn2+, and Ce4+/Ce3+ that promoted the adsorption, activation, and transformation of gaseous oxygen. Hence, toluene is firstly adsorbed by active particles and then oxidized onto the hot spots by both Oads and Olatt which follows the L-H mechanism and the MvK mechanism, respectively. Therefore, this research work provides further technological support for the development of transition metals-based catalysts and the application of microwave catalytic combustion in treating industrial VOCs waste gas.

Facet-Dependent Photocatalytic Behavior of Rutile TiO2 for the Degradation of Volatile Organic Compounds: In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy and Density Functional Theory Investigations

In this study, a custom rutile titanium dioxide (TiO2) photocatalyst with a single exposed surface was utilized to investigate the facet-dependent photocatalytic mechanism of toluene. The degradation of toluene was dynamically monitored using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technology coupled with theoretical calculations. The findings demonstrated that the photocatalytic degradation rate on the TiO2 (001) surface was nearly double that observed on the TiO2 (110) surface. This remarkable enhancement can be attributed to the heightened stability in the adsorption of toluene molecules and the concurrent reduction in the energy requirement for the ring-opening process of benzoic acid on the TiO2 (001) surface. Moreover, the TiO2 (001) surface generated a greater number of reactive oxygen species (ROS), thereby promoting the separation of photogenerated charge carriers and concurrently diminishing their recombination rates, amplifying the efficiency of photocatalysis. This research provides an innovative perspective for a more comprehensive understanding of the photocatalytic degradation mechanism of TiO2 and presents promising prospects for significant applications in environmental purification and energy fields.

Interface engineering of Mn3O4/Co3O4 S-scheme heterojunctions to enhance the photothermal catalytic degradation of toluene

Transition metal oxides have high photothermal conversion capacity and excellent thermal catalytic activity, and their photothermal catalytic ability can be further improved by reasonably inducing the photoelectric effect of semiconductors. Herein, Mn₃O₄/Co₃O₄ composites with S-scheme heterojunctions were fabricated for photothermal catalytic degradation of toluene under ultraviolet-visible (UV-Vis) light irradiation. The distinct hetero-interface of Mn₃O₄/Co₃O₄ effectively increases the specific surface area and promotes the formation of oxygen vacancies, thus facilitating the generation of reactive oxygen species and migration of surface lattice oxygen. Theoretical calculations and photoelectrochemical characterization demonstrate the existence of a built-in electric field and energy band bending at the interface of Mn₃O₄/Co₃O₄, which optimizes the photogenerated carriers' transfer path and retains a higher redox potential. Under UV-Vis light irradiation, the rapid transfer of electrons between interfaces promotes the generation of more reactive radicals, and the Mn₃O₄/Co₃O₄ shows a substantial improvement in the removal efficiency of toluene (74.7%) compared to single metal oxides (53.3% and 47.5%). Moreover, the possible photothermal catalytic reaction pathways of toluene over Mn₃O₄/Co₃O₄ were also investigated by in situ DRIFTS. The present work offers valuable guidance toward the design and fabrication of efficient narrow-band semiconductor heterojunction photothermal catalysts and provides deeper insights into the mechanism of photothermal catalytic degradation of toluene.

Facile fabrication of flower-like MnO2 hollow microspheres as high-performance catalysts for toluene oxidation

A facile and robust interface reaction method for controllable synthesis of hierarchically structured flower-like MnO₂ hollow microspheres was developed at a low cost. With MnCO₃ microspheres as homologous templates, KMnO₄ was used to conduct redox reactions with the surface layer of the MnCO₃ microspheres to form porous flower-like MnO₂. Then, the internal template was removed by HCl etching to obtain flower-like MnO₂ hollow microspheres. HCl plays the dual role of removing the template and generating oxygen vacancies through acid etching. The as-prepared flower-like MnO₂ hollow microspheres exhibited excellent low-temperature catalytic activity for toluene oxidation owing to the desirable features of a high specific surface area, abundant oxygen vacancies, high content of Mn⁴⁺, a high number of acidic sites and a strong acidity. This work provides a new strategy for the facile construction of high-performance volatile organic compounds oxidation catalysts with industrial application prospects.

Insight into the effect of manganese substitution on mesoporous hollow spinel cobalt oxides for catalytic oxidation of toluene

The cobalt oxides and manganese oxides have high-activity potential for catalytic oxidation of volatile organic compounds (VOCs), while the mesoporous hollow morphology is crucial to the mass transfer of reactant and product. Therefore, it is worth investigating the effect of manganese substitution in mesoporous hollow cobalt oxides on catalytic oxidation. Herein, a partially disordered spinel structure is formed by the Mn substitution in Co₃O₄ and the mesoporous hollow microsphere is improved in morphology homogeneity with the decrease of Co/Mn ratio in the range of 1.8-28.8. The 5Co1Mn (Mn-substituted Co₃O₄ with Co/Mn at 5.4) exhibits outstanding catalytic activity for toluene oxidation with 50% CO₂ generation at 237 °C, which is 21 °C lower than Co₃O₄. Moreover, the 5Co1Mn displays satisfactory stability in reusability, lifetime, and water resistance. The small defective crystallite, mesoporous hollow morphology, and high specific surface area endow Mn-substituted Co₃O₄ with more surface chemical adsorbed oxygen, enhancing the catalytic oxidation of toluene. Theoretical calculation on (311) plane of Co₃O₄ reveals that Mn²⁺ or Mn³⁺ substitution increases the formation energy of oxygen vacancy and makes it difficult to adsorb gaseous oxygen on the defective surface. The interaction between Co and Mn impedes the improvement of toluene oxidation because the mobility of lattice oxygen, the surface distribution of Co³⁺, and the ratio of surface adsorbed oxygen to surface lattice oxygen are hindered by Mn substitution. The chemical adsorbed oxygen is more active than lattice oxygen in the oxidation of adsorbed intermediates (phenolate, benzoate species, etc.). The Langmuir-Hinshelwood mechanism dominates in the catalytic oxidation at 200-250 °C, while the catalytic oxidation follows both the Langmuir-Hinshelwood mechanism and Mars-van Krevelen mechanism above 250 °C. This work provides some enlightenment for exploring the role of surface oxygen species in VOCs oxidation and uncovering the interaction in binary spinel oxides.

Bimetallic Pt-Co Nanoparticle Deposited on Alumina for Simultaneous CO and Toluene Oxidation in the Presence of Moisture

Carbon monoxide (CO) and hydrocarbons (HCs) generally have competitive adsorption on the active site of noble-metal nano-catalysts, thus developing an effective way to reduce the passivation of competitive reaction with each other is an urgent problem. In this study, we successfully synthesized transition metal-noble metal (Pt-M) alloys via introducing inexpensive metal elements (M = Ni, Co and Cu) into Pt particles and then deposited on alumina support to form Pt-based catalysts. Subsequently, we choose CO and toluene as polluting gases to evaluate the catalytic activities of Pt-M/Al2O3 catalysts. Introducing inexpensive metal elements (M = Ni, Co, and Cu) significantly changed the physicochemical properties and catalytic activities of these Pt-based catalysts. It can be found that the Pt-Co/Al2O3 catalyst exhibited outstanding catalytic activity for CO and toluene oxidation under mixed gas atmosphere, compared with other Pt-based catalysts, which is due to the higher dispersity, more surface adsorption oxygen, and well redox ability. Surprisingly, H2O could promote the catalytic activities for CO/toluene co-oxidation over the Pt-Co/Al2O3 catalyst. Thus, the present synthetic strategy not only opens an avenue towards the synthesis of noble metal-based catalysts, but also provides an excellent tolerance to H2O in the catalytic process.

Pt-Embedded-Co3O4 hollow structure as a highly efficient catalyst for toluene combustion

Pt embedded Co₃O₄ hollow structure nanocomposites (Pt@Co₃O₄) were facilely prepared through metal–organic frameworks (MOFs) sacrificial strategy. Compared with Pt/Co₃O₄ and bare Co₃O₄ catalyst, it shows excellent toluene combustion performance.

Efficient α-MnO2 with (2 1 0) facet exposed for catalytic oxidation of toluene at low temperature: A combined in-situ DRIFTS and theoretical investigation

The α-MnO₂ catalysts with (1 1 0), (2 1 0) and (3 1 0) crystal facets exposed were prepared via hydrothermal method and studied for the catalytic oxidation of toluene. Some characterization technologies and DFT theoretical calculation were combined to analyze the as-synthesized catalysts. The α-MnO₂ catalyst exposed with the (2 1 0) plane displayed best catalytic performance and attained complete toluene conversion at 140 °C. The O₂-TPD and XPS results exhibited the amount of surface lattice oxygen on α-MnO₂-210 catalyst was largest. Lower accumulation and faster disintegration of intermediates which was characterized by in-situ DRIFTS could be discovered on the surface of α-MnO₂-210 catalyst. The results of DFT calculation showed that the unique atomic arrangement of α-MnO₂-210 catalyst enhanced the charge separation and conversion, promoting the formation of active oxygen and the activation of toluene. The E_a of α-MnO₂-210 catalyst was 24.75 kJ mol^-1, lowest among the three catalysts. This work highlights the facet effects on catalytic property and provides new insight into the understanding of catalytic oxidation reaction mechanism of toluene.

Metal-organic framework derived Co3Se4@Nitrogen-doped porous carbon as a high-performance anode material for lithium ion batteries

In this paper, Co₃Se₄ nanoparticles embedded in nitrogen-doped porous carbon polyhedra are synthesized via a facile one-step thermal selenization, using zeolitic imidazolate framework-67 (ZIF-67) as the template. The electrochemical properties of the fabricated nanocomposite are evaluated for use as anodes for lithium ion batteries and found to exhibit a specific capacity (950 mAh g^-1 at 0.2 C) and excellent cyclic stability (899 mAh g^-1 at 1 C after 1000 cycles). Both are much higher than those of the state-of-the-art Co-Se based nanocomposites. This extraordinary lithium storage is attributed to the synergetic effect between the Co₃Se₄ nanocrystals and nitrogen-doped porous carbon framework, and is believed to offer a potential candidate anode material for next-generation lithium ion batteries.

Catalytic combustion of toluene over CeO2–CoOx composite aerogels

The dispersion of active species and redox cycle of Co³⁺/Co²⁺ in cobalt based aerogels have an important influence on catalytic performance for toluene oxidation.

Facile surface improvement of LaCoO3 perovskite with high activity and water resistance towards toluene oxidation: Ca substitution and citric acid etching

We employ two facile modification methods, i.e., Ca substitution and citric acid etching, to further improve the catalytic activity of LaCoO₃ perovskite towards toluene oxidation.

Insights into the Pyrolysis Processes of Ce-MOFs for Preparing Highly Active Catalysts of Toluene Combustion

Metal organic frameworks (MOFs) have recently been used as precursors of the catalysts for the combustion of volatile organic compounds (VOCs). In the present work, three kinds of CeO2 catalysts were successfully synthesized from Ce-MOF-808, Ce-BTC, and Ce-UiO-66, with specific topological structures and coordinate environments. Catalysts with small particle size, stacking mode, and structural defects could be created by pyrolysis of Ce-MOFs, which affects the activity in the toluene combustion significantly. Raman spectra, XPS, and OSC studies were performed to reveal the formation of defect sites. The thermal redox properties were determined by H2-TPR. Catalytic activity tests were conducted on the toluene combustion, and CeO2-MOF-808 showed the best catalytic performance (T90 = 278 °C) due to its having the largest specific surface area, abundant active surface oxygen species, and low-temperature reducibility.

Fabrication of homogeneous and highly dispersed CoMn catalysts for outstanding low temperature catalytic oxidation performance

A series of homogeneous and highly dispersed CoMnO_x bimetallic oxides with different ratios were prepared through pyrolysis of CoMn-MOF-71, which was applied to the catalytic oxidation of toluene.

Highly improved acetone oxidation activity over mesoporous hollow nanospherical MnxCo3−xO4 solid solutions

Mesoporous hollow nanospherical Mn_xCo_3−xO₄ solid solutions synthesized by a facile solvothermal alcoholysis have been developed to catalyze acetone oxidation for the first time.

Integral structured Co–Mn composite oxides grown on interconnected Ni foam for catalytic toluene oxidation

Considering the three-dimensional ordered network of Ni foam-supported catalysts and the toxicity effects of volatile organic compounds (VOCs), the design of proper active materials for the highly efficient elimination of VOCs is of vital importance in the environmental field. In this study, a series of Co–Mn composite oxides with different Co/Mn molar ratios grown on interconnected Ni foam are prepared as monolithic catalysts for total toluene oxidation, in which Co_1.5Mn_1.5O₄ with a molar ratio of 1 : 1 achieves the highest catalytic activity with complete toluene oxidation at 270 °C. The Co–Mn monolithic catalysts are characterized by XRD, SEM, TEM, H₂-TPR and XPS. It is observed that a moderate ratio of Mn/Co plays significant effects on the textural properties and catalytic activities. From the XPS and H₂-TPR characterization results, the obtained Co_1.5Mn_1.5O₄ (Co/Mn = 1/1) favors the excellent low-temperature reducibility, high concentration of surface Mn³⁺ and Co³⁺ species, and rich surface oxygen vacancies, resulting in superior oxidation performance due to the formation of a solid solution between the Co and Mn species. It is deduced that the existence of the synergistic effect between Co and Mn species results in a redox reaction: Co³⁺–Mn³⁺ ↔ Co²⁺–Mn⁴⁺, and enhances the catalytic activity for total toluene oxidation.

A series of Co–Mn oxides with different Co/Mn molar ratios grown on interconnected Ni foam were prepared as monolithic catalysts for total toluene oxidation.