Surface Adsorbed Oxygen Species Research Articles

Tuning the crystallinity of the MnOx catalysts to promote toluene catalytic oxidation

ABSTRACT In this paper, the MnOx catalysts with excellent toluene oxidation performance were prepared by a simple precipitation method. The physicochemical properties of the prepared MnOx catalysts were investigated by XRD, BET, H2-TPR, O2-TPD and XPS. The obtained results revealed that the crystallinity of the prepared MnOx catalysts could be effectively regulated by changing the (NH4)2CO3/Mn(NO3)2 molar ratio, and thus affecting the oxygen vacancy concentration of the prepared MnOx catalysts. The prepared MnOx-4 catalyst with the (NH4)2CO3/Mn(NO3)2 molar ratio of 4.0 had the poor crystallinity and small grain size, which effectively promoted the oxygen defects in the MnOx catalyst to be formed. At the same time, the MnOx-4 catalyst had a large specific surface area, the highest low temperature reducibility and the largest number of oxygen vacancies and surface adsorbed oxygen species, which allowed more surface oxygen species to participate in the redox reaction, and promoted the toluene deep oxidation. Therefore, when the (NH4)2CO3/Mn(NO3)2 molar ratio was 4.0, the prepared MnOx-4 catalyst exhibited an excellent toluene catalytic oxidation performance and robust catalytic stability. What’s more, the toluene oxidation conversion on the MnOx-4 catalyst reached 99% at 230°C, and the MnOx-4 catalyst showed excellent resistance to water vapour.

High coking resistance of Ni/A2Ti2O7 catalysts via modulating the A site of A2Ti2O7 (A = La, Gd) for dry reforming of methane

Ni based catalysts are currently the most investigated for dry reforming of methane (DRM) due to the advantages in intrinsic activity and cost. However, due to carbon deposition, the questionable durability greatly limits its practical application in the industry. Herein, Gd-doped Ni-La2Ti2O7 and La-doped Ni-Gd2Ti2O7 catalysts were prepared by modulating the A site of the A2Ti2O7 structure. Moderate A site doping tunes the electronic properties of the A2Ti2O7, which would not only significantly enhance the activation ability of C–H bonds of CH4, but also enhance the adsorption capacity of CO2, ultimately affecting the catalytic performance of Ni/A2Ti2O7 for the DRM. Among the modified catalysts, 2 % Gd doped Ni-La2Ti2O7 exhibited excellent stability and maintained CH4 and CO2 conversion at 70.8 % and 86.6 %, respectively, for 200 h at 800 °C with negligible carbon deposition. The fundamental reason for A site doping is that it can make the catalyst produce more surface adsorbed oxygen species, which could accelerate the elimination of deposited carbon.

Experimental and mechanistic studies on the thermo-electric synergistic catalytic oxidation of acetone over MnO2/TiO2-Ni foam catalyst

Volatile organic compounds (VOCs) cause serious environmental and human health risks and must be efficiently removed. In this work, a metal-structured catalyst of Ni foam loaded Mn/Ti oxides was used for the oxidation of acetone, a typical VOC compound. The catalyst was in-situ joule heated when applying an electric current and produced the thermo-electric synergistic oxidation (TEO) effect. Compared with conventional thermal oxidation (CTO), the oxidation rate of acetone significantly increased by 124.19 % and the energy consumption of per unit complete oxidation rate was reduced by 66.67 % in TEO reaction with a 12 A electric current at 200 °C. The efficiency gradually improved (from 16.12 % to 36.14 %) and the energy consumption of per unit complete oxidation rate decreased (from 2.85 W/% to 0.95 W/%) with the increase of electric current. Besides, the thermal (including the catalyst thermal effect and reaction gas thermal effect) and electric effects in TEO reaction were analyzed by the statistical analysis method. With the temperature increasing from 200 °C to 400 °C, the proportion of electric effect slightly decreased while that of reaction gas thermal effect increased from 5.2 % to 18.1 %. It proved that the TEO method has the advantage of improving low-temperature catalytic activity. Furthermore, the mechanism of electric effect was investigated, in which it enhances the conversion of lattice oxygen in TiO2 to surface-adsorbed oxygen species and promotes the oxidation of Mn oxides, sequentially improving the oxidation of acetone.

Unveiling the crucial active sites responsible for CO, n-heptane, and toluene oxidation over Pt/ZrO2 catalyst

The surface Pt species and adsorbed oxygen on the Pt/ZrO2 catalyst were successfully modulated by altering the preparation methods, resulting in distinct catalytic behaviors for CO, n-heptane, and toluene oxidation. The Pt/ZrO2 catalyst was initially prepared via wetness impregnation and subsequently reduced in an H2 atmosphere to obtain the Pt/ZrO2-R catalyst. In contrast, the Pt/ZrO2-E catalyst was synthesized using ethanol solvent (also serving as a reducing agent) instead of water during wetness impregnation. Moreover, the pre-prepared Pt nanoparticles were loaded onto the ZrO2 surface using polyvinylpyrrolidine (as a protective and structure directing agent) dissolved in ethanol solvent to fabricate the Pt/ZrO2-EP catalyst. Among these catalysts, the Pt/ZrO2-R exhibits superior catalytic activity towards CO oxidation due to its abundant metallic Pt species. It is found that activity of Pt/ZrO2 catalyst for n-heptane oxidation is positively correlated with its surface adsorbed oxygen concentration, thus the highest activity is achieved over the Pt/ZrO2-EP catalyst possessing a higher concentration of surface adsorbed oxygen species. Unlike CO and n-heptane oxidation, toluene oxidation activity is affected by multiple factors, it is primarily promoted by synergistic catalysis between metallic Pt species (Pt0) and surface adsorbed oxygen species over the Pt/ZrO2-EP catalyst. Our preliminary findings underscore that precise modulation of sensitive active sites on surfaces plays a critical role in specific catalytic reactions.

The reduction of copper in LaFeO3, La2CuO4 and CuO three-phase system improves the efficiency of NO removal: Experimental and density functional theory calculation

Hollow spherical LaCu0.5Fe0.5O3 (LCF) and La0.8Ce0.1Cu0.5Fe0.5O3 (LCCF) catalysts are synthesized by solvothermal method, and CuO is reduced in two samples, so that the two corresponding multiphase catalysts (R-LCF and R-LCCF) were prepared and used for the selective catalytic reduction of NO with CO (CO-SCR). The results show that the balance of Ce3++Cu2+↔Cu++Ce4+ between Ce and Cu ions promotes the reduction of CuO in the Ce-doped LCCF catalyst more obviously. In addition, the reduction of Ce and Cu in R-LCCF generates more oxygen vacancies, which leads to more surface adsorbed oxygen species, thus facilitating the conversion of more NO to NOx species. Besides, the presence of reduced Cu is highly active and can inhibit the production of carbonates occupying the active site. The ability to activate both CO and NO after the evolution of Cu0 to the key species Cu+ leads to a rapid reaction between Cu+-CO and NOx species. These are the key factors for the significant increase in catalytic activity at low temperatures. It is worth noting that the CO-SCR reaction on the R-LCCF catalyst corresponds to the Langmuir-Hinshlwood (L-H) and Eley-Rideal (E-R) mechanisms at different temperature windows, respectively.

Controlled assembly of novel Fe2O3/MgFe2O4 nanotubular array structures for rapid detection of dibutylamine

The development of an efficient, fast, and real-time sensor for detecting dibutylamine (C8H19N) gas remains challenging. In this study, a simple controllable and environmentally friendly solvothermal method was used to construct novel Fe2O3/MgFe2O4 nanotube array structures with heterointerfaces. The array structure exhibits a relatively large specific surface area and high-activity heterointerfaces. These features greatly promote the diffusion and adsorption of gas molecules onto the sensitive layer, effectively increasing the surface-adsorbed oxygen species and active sites of the sensing material. The sensing performance of the Fe2O3/MgFe2O4 array structure for C8H19N gas is greatly improved due to the synergistic effect of these multiple factors. The sensor showed a high response of 92.6 to 100 ppm C8H19N at 170 °C, with a short response time of 3.6 s and an ultra-low detection limit of 10 ppb. The sensitization mechanism of Fe2O3/MgFe2O4 to C8H19N was investigated by combining theoretical calculations and experimental techniques. This study broadens the application of gas sensors for the detection of volatile organic compounds (VOCs).

Catalytic Oxidation of Chlorobenzene over HSiW/CeO2 as a Co-Benefit of NOx Reduction: Remarkable Inhibition of Chlorobenzene Oxidation by NH3.

There is an urgent need to develop novel and high-performance catalysts for chlorinated volatile organic compound oxidation as a co-benefit of NOx. In this work, HSiW/CeO2 was used for chlorobenzene (CB) oxidation as a co-benefit of NOx reduction and the inhibition mechanism of NH3 was explored. CB oxidation over HSiW/CeO2 primarily followed the Mars-van-Krevelen mechanism and the Eley-Rideal mechanism, and the CB oxidation rate was influenced by the concentrations of surface adsorbed CB, Ce4+ ions, lattice oxygen species, gaseous CB, and surface adsorbed oxygen species. NH3 not only strongly inhibited CB adsorption onto HSiW/CeO2, but also noticeably decreased the amount of lattice oxygen species; hence, NH3 had a detrimental effect on the Mars-van-Krevelen mechanism. Meanwhile, NH3 caused a decrease in the amount of oxygen species adsorbed on HSiW/CeO2, which hindered the Eley-Rideal mechanism of CB oxidation. Hence, NH3 significantly hindered CB oxidation over HSiW/CeO2. This suggests that the removal of NOx and CB over this catalyst operated more like a two-stage process rather than a synergistic one. Therefore, to achieve simultaneous NOx and CB removal, it would be more meaningful to focus on improving the performances of HSiW/CeO2 for NOx reduction and CB oxidation separately.

Hierarchical ZSM-5 zeolite supported RuOx catalysts via organic mesoporous porogens for catalytic oxidation of vinyl chloride

Hierarchical ZSM-5 zeolites supported RuOx catalysts were prepared via the addition of various organic mesoporous porogens (namely starch, cellulose and sucrose) for catalytic oxidation of vinyl chloride (VC). The usage of mesoporous porogens during catalysts preparation was favorable for boosting the catalytic activity of the resultant catalysts, and 1%RuOx/HZ5-10%sucrose exhibited the optimum catalytic activity, impressive catalytic stability and long-term durability. The structure-activity dependence from catalysts characterizations revealed that the higher specific surface area, more abundant strong acid sites and surface adsorbed oxygen species were regarded as the critical parameters for the noticeable improvement of catalytic activity. Based on in situ infrared spectroscopy analysis, 1%RuOx/HZ5-10%sucrose exhibited higher adsorption capacity and more efficient conversion of VC molecules with fewer formation of organic intermediates because of its superior catalytic activity in comparison with the single 1%RuOx/HZ5. Additionally, enol and carboxylate species were demonstrated to be the key organic intermediates in catalytic VC oxidation.

Positive effects of the Sr doping on LaCoO3 perovskites for simultaneous catalytic oxidation performances of NO and Hg0

LaCoO3 perovskite, with a general formula of ABO3, have received significant attention as environmental functional catalysts for NO and Hg0 oxidation owing to their compositional adjustability and chemical stability. Structural modifications of the perovskite have been performed through the substitution of La atoms at the A-site with atoms of different ion radii to enhance the oxidation ability. In this study, herein, a series of La1–xSrxCoO3 (x = 0.1–0.5) perovskites were synthesized, resulting in a transition from the R–3C rhombohedral phase structure to the Pm-3m cubic phase. Sr doping was effective in improving the oxidation of NO and Hg0. The improved catalytic activity was attributable to the generation of more electronegative Co3+ and the higher content of surface-adsorbed oxygen species. Moreover, Sr doping enhanced the redox performance and reduced the reaction temperature. Notably, SrCO3 species were the main factors responsible for the reducing the activation of La1–xSrxCoO3 (x = 0.1–0.5) perovskites, which could be detected when the Sr doping content exceeded 0.4. This study provides a screening strategic idea for the designing highly active perovskite catalysts.

Natural catkins-derived single-crystal WO3 tubes rich in oxygen vacancies for high response and rapid detection of hydrazine at near room temperature

It is urgent to improve the comprehensive detection ability of chemoresistive gas sensors for highly toxic and explosive N2H4 due to less report. To this aim, we simply immersed discarded catkins in ammonium metatungstate aqueous solution to obtain a biotemplate precursor, which was then calcinated in air to controllably replicate two biomorphic WO3 materials by cross-linked nanoparticles. Amongst, single crystal WO3-6 tubes calcinated at 600 °C show good overall sensing performances towards trace N2H4 for the first time. At near room temperature (50 °C), WO3-6 sensor presents large response value (S = 120) and short response time (12 s) to 10 ppm N2H4, along with low practical detection limit (5 ppb). The above four important indexes are significantly better than those of currently reported metal oxide-based sensors. Meanwhile, it also exhibits good selectivity, satisfactory reproducibility, stability and moisture resistance. Such enhanced N2H4 sensing mechanism mainly roots from the synergy of its unique tubular structure (e.g. broad pores, oxygen vacancy defects), catalysis of W6+ ions with dangling bonds, as well as surface adsorbed oxygen species.

Micro-heater embedded Ni-SnO2 ordered nanoporous films: On-chip fabrication for fast, sensitive, and selective gas sensing towards multiple VOCs for air quality monitoring

The ability to simultaneously detect multiple volatile organic compounds (VOCs) is crucial for assessing indoor air quality and addressing significant health hazards. In this study, we present a sacrificial template approach for on-chip fabrication of two-dimensional (2D) Ni-doped SnO2 nanoporous sensing films on micro-heater substrates. The technical analysis shows that the resulting honeycomb-like ultrathin Ni-SnO2 sensing layer on a suspended substrate maintains perfect structural integrity and consistency. The Ni-SnO2 sensors exhibit exceptional characteristics, including a ppb-level detection limit, high sensitivity, fast response/recovery time, and signal repeatability for typical VOCs, while operating at a low power consumption of 25 mW. Additionally, the Ni-doping enhances the sensors' cross-sensitivity to various forms of VOCs, while causing a discernible response inhibition towards flammable or exhaust gases. Furthermore, X-ray photoelectron spectroscopy (XPS) and in-situ Raman spectra reveal a Ni2+-substitution induced surface adsorbed oxygen species enrichment mechanism for sensing enhancement. Thus, the developed Ni-SnO2 sensors hold promise as future candidates for reliable air quality assessment by real-time monitoring of overall concentrations of multiple VOCs.

Magnesium-enhanced redox property and surface acidity-basicity of LaMnO3 perovskites for efficient methane purification

Catalytic combustion has been applied to purify methane emissions from natural gas vehicle exhaust, yet it still requires the development of catalysts with both high activity and robust stability. Herein, non-equivalent promoter-magnesium ions (Mg2+) were introduced into LaMnO3 perovskites via a facile sol–gel method to partially substitute Mn sites and boost the catalytic performance. The incorporation of Mg induced the transition of Mn3+ to Mn4+ and generated oxygen vacancies, which improved the reactivity of lattice oxygen and facilitated the formation of adsorbed oxygen species. Turning the coordination state of Mn and O led to an augmentation of surface acid-basic sites on LaMn1-xMgxO3, which was beneficial for the activation of methane and adsorption of dissociated intermediates. The presence of abundant highly-reducible Mn4+ and surface-adsorbed oxygen species accelerated the complete oxidation of methane. Consequently, LaMn0.7Mg0.3O3 with the optimized surface acidity-basicity and redox property exhibited greatly enhanced catalytic activity, selectivity, water-/sulfur-resistance and thermal stability compared to LaMnO3. This study emphasizes the importance of synergistically adjusting surface properties to enhance catalytic purification performance.

Enhanced formaldehyde oxidation over MnO2 and doped manganese-based catalysts: Experimental and theoretical Insights into mechanism and performance

Thermal catalytic degradation of formaldehyde (HCHO) over manganese-based catalysts is garnering significant attention. In this study, both theoretical simulations and experimental methods were employed to elucidate the primary reaction pathways of HCHO on the MnO2(110) surface. Specifically, the effects of doping MnO2 with elements such as Fe, Ce, Ni, Co, and Cu on the HCHO oxidation properties were evaluated. Advanced characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS), were employed to discern the physical properties and chemical states of the active components on the catalyst surface. The comprehensive oxidation pathway of HCHO on the MnO2(110) surface includes O2 adsorption and dissociation, HCHO adsorption and dehydrogenation, CO2 desorption, H2O formation and desorption, oxygen vacancy supplementation, and other elementary reactions. The pivotal rate-determining step was identified as the hydrogen migration process, characterized by an energy barrier of 234.19 kJ mol−1. Notably, HCHOO and *CHOO emerged as crucial intermediates during the reaction. Among the doped catalysts, Fe-doped MnO2 outperformed its counterparts doped with Ce, Ni, Co, and Cu. The optimal degradation rate and selectivity were achieved at a molar ratio of Fe: Mn = 0.1. The superior performance of the Fe-doped MnO2 can be ascribed to its large specific surface area, conducive pore structure for HCHO molecular transport, rich surface-adsorbed oxygen species, and a significant presence of oxygen vacancies.

Morphology and size effect of ceria on methanol oxidation in non-thermal plasma

The morphology effect and size effect of Ceria (CeO2) on methanol oxidation in non-thermal plasma (NTP) system were investigated. CeO2-Rod and CeO2-Cube with individual three sizes were synthesized and presented strong morphology-dependent and size-dependent activity, namely, CeO2-rod catalyst exhibited superior performance than CeO2-Cube, while each of them showed improved catalytic activity as the size reduced. Various characteristic methods showed that CeO2-Rod in smaller size possesses more surface adsorbed oxygen species than CeO2-Cube. Further experiments confirmed that the dependence of catalytic activity on CeO2 morphologies and CeO2 sizes was derived from their ozone utilization. To further investigate the difference of surface reaction pathways between the catalysts, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were conducted and the results showed that the methanol adsorption behavior and oxidation process on catalyst surface differ in CeO2 morphologies.

Crystal-facet-dependent activity and N2 yield of Ag/CeO2 catalysts for catalytic oxidation of N, N-Dimethylformamide

Shape-controlled CeO2 with different exposed crystal facets, CeO2 nanorods (110) and (100), nanocubes (100) and nano-octahedrons (111), were prepared as supports of Ag and then the catalytic oxidation of N, N-dimethylformamide (DMF) was investigated. Ag/CeO2 nanorod (Ag/r-Ce) catalyst exhibits better dispersion of Ag species due to the abundant oxygen vacancies of CeO2 (110) with outstanding anchoring effect, meanwhile, more Ag-O-Ce interfaces and Ag+ species are achieved because of the strong metal-support interaction and easy migration of oxygen species between Ag and CeO2 (110). Ag/r-Ce catalyst exhibits excellent catalytic activity (T90 =158 °C), 100% yield of CO2 at 240 °C, 80% yield of N2 in the temperature range of 237–400 °C, unexceptionable H2O-resistance, high stability at 240 °C for 120 h. In situ DRIFTS reveals that the dissociation of (O)C-N bond is firstly involved in DMF oxidation, and then the stepwise and repeated dehydrogenation of -N(CH3)2 and the insertion reaction of surface lattice oxygen occurs. Meanwhile the formed -HCO is completely oxidized into CO2 and H2O by surface adsorbed oxygen species and the two adjacent -NH2 species combines into N2.

Alkaloid Precipitant Reaction Inspired Controllable Synthesis of Mesoporous Tungsten Oxide Spheres for Biomarker Sensing.

Highly porous sensitive materials with well-defined structures and morphologies are extremely desirable for developing high-performance chemiresistive gas sensors. Herein, inspired by the classical alkaloid precipitant reaction, a robust and reliable active mesoporous nitrogen polymer sphere-directed synthesis method was demonstrated for the controllable construction of heteroatom-doped mesoporous tungsten oxide spheres. In the typical synthesis, P-doped mesoporous WO3 monodisperse spheres with radially oriented channels (P-mWO3-R) were obtained with a diameter of ∼180 nm, high specific surface area, and crystalline skeleton. The in situ-introduced P atoms could effectively adjust the coordination environment of W atoms (Wδ+-Ov), giving rise to dramatically enhanced active surface-adsorbed oxygen species and unusual metastable ε-WO3 crystallites. The P-mWO3-R spheres were applied for the sensing of 3-hydroxy-2-butanone (3H2B), a biomarker of foodborne pathogenic bacteria Listeria monocytogenes (LM). The sensor exhibited high sensitivity (Ra/Rg = 29 to 3 ppm), fast response dynamics (26/7 s), outstanding selectivity, and good long-term stability. Furthermore, the device was integrated into a wireless sensing module to realize remote real-time and precise detection of LM in practical applications, making it possible to evaluate food quality using gas sensors conveniently.

Cobalt-free double-perovskite oxide Sr2Ti0.9FeNi0.1O6 as a promising electrode for symmetric solid oxide electrolysis cells

Symmetric solid oxide electrolysis cells (SSOECs) have attracted considerable interest because of their low fabrication cost and few compatibility issues. However, the development of SSOECs is restricted by the lack of electrode materials with desirable catalytic activity and stability. Herein, novel double-perovskite Sr2Ti1−xFeNixO6 (STFNx) oxides are prepared and evaluated as electrodes for CO2 electrolysis in SSOECs. The Sr2Ti0.9FeNi0.1O6 (STFNi01) electrode shows the best performance with the minimum polarization resistance. The physicochemical characterization reveals that Ni substitution increases the concentrations of surface-adsorbed oxygen species and electrical conductivity and enhances CO2 chemical adsorption, improving the electrode surface reaction kinetics, as evidenced by the distribution of relaxation time analysis. The STFNi01 symmetric cell presents a current density of 0.91 A cm−2 at 1.6 V and 800 °C and shows no performance degradation in the durability test due to the excellent carbon deposition resistance, indicating that STFNi01 is a promising electrode material for SSOECs.

Catalytic performance of Nb2O5 modified Ru/CeO2 for dichloromethane oxidation

1 ​wt% Ru/5 ​wt% Nb2O5/CeO2 (RuNbCe) catalyst was synthesized via the incipient wetness impregnation method, and measured for the catalytic oxidation of dichloromethane (DCM). The physicochemical properties of catalysts were determined used various characterization techniques. Compared with 1 ​wt% Ru/CeO2, 5 ​wt% Nb2O5/CeO2, CeO2, 1 ​wt% Ru/xNb2O5/CeO2 (x ​= ​1 ​wt%, 2 ​wt%, and 3 ​wt%) and 1 ​wt% Ru/yMOn/CeO2 (M ​= ​Co, Cr, Fe, Mg, Cu; y ​= ​5 ​wt%), the addition of 5 ​wt% Nb2O5 obviously improved the catalytic performance of 1 ​wt% Ru/CeO2, with the T90% and apparent activation energy (Ea) being 287 oC and 43.5 ​kJ/mol, respectively, and less formation of polychlorinated byproducts at space velocity (SV) ​= ​15,000 ​mL ​g‒1 ​h‒1. The RuNbCe catalyst showed good thermal and hydrothermal stability, as well as HCl-resistance performance. We deduced that the higher oxidation performance of the RuNbCe catalyst may be attributed to the suitable acidic sites, good redox capacity, high surface adsorbed oxygen species concentration, and the strong interaction among Ru, Nb2O5 and CeO2. The RuNbCe catalyst has the potential to be applied for the elimination of chlorinated volatile organic compounds (CVOCs).

Wafer-Level Manufacturing of MEMS H2 Sensing Chips Based on Pd Nanoparticles Modified SnO2 Film Patterns.

In this manuscript, a simple method combining atomic layer deposition and magnetron sputtering is developed to fabricate high-performance Pd/SnO2 film patterns applied for micro-electro-mechanical systems (MEMS) H2 sensing chips. SnO2 film is first accurately deposited in the central areas of MEMS micro hotplate arrays by a mask-assistant method, leading the patterns with wafer-level high consistency in thickness. The grain size and density of Pd nanoparticles modified on the surface of the SnO2 film are further regulated to obtain an optimized sensing performance. The resulting MEMS H2 sensing chips show a wide detection range from 0.5 to 500ppm, high resolution, and good repeatability. Based on the experiments and density functional theory calculations, a sensing enhancement mechanism is also proposed: a certain amount of Pd nanoparticles modified on the SnO2 surface could bring stronger H2 adsorption followed by dissociation, diffusion, and reaction with surface adsorbed oxygen species. Obviously, the method provided here is quite simple and effective for the manufacturing of MEMS H2 sensing chips with high consistency and optimized performance, which may also find broad applications in other MEMS chip technologies.

Crystal Plane Effect of Co3O4 on Styrene Catalytic Oxidation: Insights into the Role of Co3+ and Oxygen Mobility at Diverse Temperatures.

In the oxidation reaction of volatile organic compounds catalyzed by metal oxides, distinguishing the role of active metal sites and oxygen mobility at specific preferentially exposed crystal planes and diverse temperatures is challenging. Herein, Co3O4 catalysts with four different preferentially exposed crystal planes [(220), (222), (311), and (422)] and oxygen vacancy formation energies were synthesized and evaluated in styrene complete oxidation. It is demonstrated that the Co3O4 sheet (Co3O4-I) presents the highest C8H8 catalytic oxidation activity (R250°C = 8.26 μmol g-1 s-1 and WHSV = 120,000 mL h-1 g-1). Density functional theory studies reveal that it is difficult for the (311) and (222) crystal planes to form oxygen vacancies, but the (222) crystal plane is the most favorable for C8H8 adsorption regardless of the presence of oxygen vacancies. The combined analysis of temperature-programmed desorption and temperature-programmed surface reaction of C8H8 proves that Co3O4-I possesses the best C8H8 oxidation ability. It is proposed that specific surface area is vital at low temperature (below 250 °C) because it is related to the amount of surface-adsorbed oxygen species and low-temperature reducibility, while the ratio of surface Co3+/Co2+ plays a decisive role at higher temperature because of facile lattice oxygen mobility. In situ diffuse reflectance infrared Fourier spectroscopy and the 18O2 isotope experiment demonstrate that C8H8 oxidation over Co3O4-I, Co3O4-S, Co3O4-C, and Co3O4-F is mainly dominated by the Mars-van Krevelen mechanism. Furthermore, Co3O4-I shows superior thermal stability (57 h) and water resistance (1, 3, and 5 vol % H2O), which has the potential to be conducted in the actual industrial application.