Activity For Toluene Oxidation Research Articles

Ozone catalytic oxidation of toluene over triple perovskite-type catalysts modified with KMnO4.

A unique triple perovskite-type catalyst was successfully synthesized using the simple sol-gel approach, and surface acid modification was added to improve the ozone catalytic oxidation (OZCO) process ability to remove toluene more effectively. Our study indicates that La3MnCuNiO9 catalyst treated with KMnO4 shows the best toluene oxidation activity. At 250 °C, the rates of conversion and mineralization were 100% and 83%, respectively, under thermal catalytic system when C7H8 concentration = 500 ppm. During the OZCO system ([C7H8] = 20 ppm, O3/C7H8=8; room temperature), for 6 h, the conversion rate remained at 100%. The high ratios of Mn4+/(Mn4++Mn3+), Cu2+, and abundant surface oxygen species, high specific surface area, and pore volume lead to remarkable catalytic performance of this catalyst. Meanwhile, the catalyst contributes to superior stability and water resistance. The catalytic mechanism of La3MnCuNiO9 after KMnO4 treatment in the context of OZCO was further discussed. Overall, after KMnO4 treatment, the La3MnCuNiO9 catalyst reveals extraordinary catalytic activity and excellent stability combination of this catalyst with ozone exhibits high toluene removal efficiency in the OZCO system and has a good potential for industrial applications.

Regulating oxygen vacancies and hydroxyl groups of α-MnO2 nanorods for enhancing post-plasma catalytic removal of toluene

Although nonthermal plasma (NTP) technology has high removal efficiency for volatile organic compounds (VOCs), it has limited carbon dioxide (CO2) selectivity, which hinders its practical application. In this study, α-MnO2 nanorods with tunable oxygen vacancies and hydroxyl groups were synthesized by two-step hydrothermal process to enhance their activity for deep oxidation of toluene. Hydrochloric acid (HCl) was used to assist in synthesis of α-MnO2 nanorods with tunable oxygen vacancies, furtherly, more hydroxyl groups were introduced to HCl-assisted synthesized α-MnO2 by K+ supplement. The results showed that the as-synthesized nanorods exhibited superior activity, improved by nearly 30% removal efficiency of toluene compared to pristine MnO2 at SIE = 339 J/L, and reaching high COx selectivity of 72% at SIE = 483 J/L, successfully promoting the deep oxidation of toluene. It was affirmed that oxygen vacancies played an important role in toluene conversion, improving the conversion of ozone (O3) and resulting in higher mobility of surface lattice oxygen species. Besides, the enhancement of deep oxidation performance was caused by the increase of hydroxyl groups concentration. In-situ DRIFTS experiments revealed that the adsorbed toluene on catalyst surface was oxidized to benzyl alcohol by surface lattice oxygen, and hydroxyl groups were also found participating in toluene adsorption. Overall, this study provides a new approach to designing catalysts for deep oxidation of VOCs.

Insights into non-crystalline structure of solid solution Ce-Mn co-oxide nanofibers for efficient low-temperature toluene oxidation.

The controllable preparation of efficient non-crystalline solid solution catalysts is a great challenge in the catalytic oxidation of volatile organic compounds. In this work, series non-crystalline solid solution structured Ce-Mn co-oxide nanofibers were creatively prepared by adjusting Ce/Mn molar ratios using electrospinning. 0.20CeMnOx (the ratio of Ce to Mn was 0.2) displayed an outstanding low-temperature toluene oxidation activity (T90 = 233 °C). The formation of the amorphous solid solution and the unique nanofiber structure both contributed to a large specific surface area (S = 173 m2 g-1) and high adsorbed oxygen content (Oads/O = 41.3%), which enhanced the number of active oxygen vacancies. The synergies between non-crystalline structure and active oxygen species markedly improved oxygen migration rate as well as redox ability of the catalysts. Additionally, in situ diffuse reflectance infrared Fourier transform spectra showed that the absorbed toluene could be completely oxidized to CO2 and H2O with benzyl alcohol, benzaldehyde, benzoic acid, and maleic anhydride as intermediates. In summary, this study provided an alternative route for the synthesis of non-crystalline metal co-oxide nanofibers.

Highly active and stable plate-like ZSM‑5 supported MnOx catalysts for toluene Oxidation: Effect of preparation methods

Catalysts consisting of MnOx supported on plate-like ZSM-5 with the Si/Al ratio of 70 (MnOx/PZ5) were synthesized using four different methods: in-situ growth (MnOx/PZ5-IG), ammonia precipitation (MnOx/PZ5-AP), citric acid complexation (MnOx/PZ5-CA), and traditional impregnation (MnOx/PZ5-TI). The catalytic activity of these catalysts for toluene oxidation was evaluated, and their physicochemical properties were characterized by several technologies. The results showed the catalytic activity for toluene oxidation was significantly influenced by the preparation methods. Notably, MnOx/PZ5-IG demonstrated the highest catalytic activity for oxidation of toluene with the specific activity per unit mass of Mn with 2.12 × 10-6 at 250 °C, 3.8 times higher than that of MnOx/PZ5-CA, offering new ideas for the design of low-cost materials for VOCs elimination in the future. The excellent performance of MnOx/PZ5-IG could be attributed to the higher Mn4+ ratio with more reducible, active surface adsorbed oxygen and dispersion of MnOx, which also showed high stability. Finally, the reaction mechanism analyzed by in-situ DRIFTS found that MnOx/PZ5-IG promoted the activity of surface lattice oxygen, conducive to increasing the reaction rate and facilitating the ring opening of benzene, thus, deepening the oxidation of toluene to CO2 and H2O.

Synthesis of xCe-MnO2 with three-dimensional ultra-thin nanosheet structure and its excellent low-temperature reducibility for toluene catalysis.

A series of xCe-MnO2 (x = 0-1) catalysts were synthesized using ammonium oxalate as a precipitator via the redox precipitation method and hydrothermal synthesis method. The results indicate that 0.25Ce-MnO2 exhibited the highest catalytic activity for toluene oxidation, with the T99 of 240°C. Characterization results from XRD, Raman, SEM, TEM, EDS-mapping, BET, and other techniques reveal that the 0.25Ce-MnO2 catalyst exhibited a three-dimensional multistage ultrathin nanosheet structure by adjusting the introduction amount of Ce, with abundant active sites, and effectively formed Ce-Mn homogeneous dispersion. The larger pore size and volume of 0.25Ce-MnO2 catalyst lead to it excellent toluene transfer ability. Furthermore, compared with MnO2, the crystal pattern of 0.25Ce-MnO2 shifted to the tetragonal cryptomelane type α-MnO2 phase and exposed more crystal planes which are beneficial to catalyze toluene. H2-TPR, O2-TPD, and XPS characterization further confirmed the strong interaction between Ce and Mn oxides, which exhibited better low-temperature reducibility and oxygen migration, along with abundant Ce3+ and Mn3+ species, where lattice oxygen played a major role. Moreover, in situ DRIFTS revealed that the 0.25Ce-MnO2 catalyst showed higher adsorption and desorption capacity for toluene than the MnO2 catalyst, and benzoate species were the key intermediates for catalytic oxidation. Additionally, benzoate and surface phenolic species were the key intermediates for catalytic oxidation of MnO2. Because 0.25Ce-MnO2 possesses better ability of converting toluene to benzoate species, it exhibits better activity.

Regulating catalytic stability of PtSnM/CeO2 (M = Mn, W, Nb) catalysts via the closely coupled multi-active sites to promote multicomponent VOCs oxidation

Pt-based commercial catalysts tend to be poisoned by chlorine species in the oxidation of chlorinated volatile organic compounds (CVOCs), which greatly limits their wide applications. It is vital to develop the active catalysts with good chlorine resistance. In the present work, the novel supported ternary metallic catalysts (PtSnM/CeO2; M = Mn, W, Nb) with the closely coupled multi-active sites show good catalytic performance and excellent chlorine resistance for the oxidation of toluene and chlorobenzene (CB), and can effectively inhibit formation of the toxic chlorine-containing by-products. The catalytic activity for toluene oxidation in the mixed VOCs system follows the sequence: PtSnMn/CeO2 > PtSnW/CeO2 ≈ PtSnNb/CeO2 > PtSn/CeO2 > Pt/CeO2. The change of the electron structure around Pt results in the difference in adsorption capacity of toluene and CB. The C−Cl bond is first dissociated at the Brønsted acid sites, and the non-chlorinated organic compounds are oxidized at the Pt sites that contact with the acid sites in the proximity. Additional water provides sufficient protons to react with Cl species, enhancing the desorption amount of HCl; furthermore, the water also reacts with the adsorbed oxygen to generate new oxygen species, which participates in the oxidation of VOCs, so as to accelerate the removal of Cl species on the catalyst surface and reduce the more toxic polychlorine by-products. This work is helpful for developing the supported noble metal catalysts with good chlorine-resistant performance in the oxidation of multicomponent VOCs under the actual industrial conditions.

Toluene and 2-propanol mixture oxidation over Mn2O3 catalysts: Study of inhibition/promotion effects by in-situ DRIFTS

Three Mn2O3 catalysts were synthetized by different methods, characterized and tested in toluene and 2-propanol (iPrOH) oxidation in single and binary component mixture. In single VOC media, the catalytic activity was strongly influenced by the specific surface area, low-temperature reducibility and active surface-oxygen species. In the binary VOCs mixture oxidation, light-off curves were shifted to lower temperatures, suggesting a promoting effect on the catalytic activity for toluene and iPrOH oxidation. The acetone production in binary mixture was also shifted to lower temperatures. The obtained results were rationalized based on the different polarity of the VOCs, the available adsorbed oxygen and the exothermic character of the VOCs combustion. These promoting effects were pointed out by in-situ DRIFTS measurements, in which the pre-adsorption of any of these two VOCs affected the adsorption/reaction of the other. For the first time, in-situ DRIFTS has been applied to elucidate the reaction mechanisms of the oxidation of toluene/2-propanol mixture on Mn2O3, which allowed to correlate the surface phenomena and the catalytic performance.

Weak Metal-Support Interaction over CuO/TiO2 Catalyst Governed Low-Temperature Toluene Oxidation.

Regulating the metal-support interaction is essential for obtaining highly efficient catalysts for the catalytic oxidation of volatile organic compounds (VOCs). In this work, CuO-TiO2(coll) and CuO/TiO2(imp) with different metal-support interactions were prepared via colloidal and impregnation methods, respectively. The results demonstrated that CuO/TiO2(imp) has higher low-temperature catalytic activity, with a 50% removal of toluene at 170 °C compared to CuO-TiO2(coll). Additionally, the normalized reaction rate (6.4 × 10-6 mol·g-1·s-1) at 160 °C over CuO/TiO2(imp) was almost four-fold higher than that over CuO-TiO2(coll) (1.5 × 10-6 mol·g-1·s-1), and the apparent activation energy value (27.9 ± 2.9 kJ·mol-1) was lower. Systematic structure and surface analysis results disclosed that abundant Cu2+ active species and numerous small CuO particles were presented over CuO/TiO2(imp). Owing to the weak interaction of CuO and TiO2 in this optimized catalyst, the concentration of reducible oxygen species associated with the superior redox property could be enhanced, thus significantly contributing to its low-temperature catalytic activity for toluene oxidation. This work is helpful in exploring the influence of metal-support interaction on the catalytic oxidation of VOCs and developing low-temperature catalysts for VOCs catalytic oxidation.

Winning Combination of Cu and Fe Oxide Clusters with an Alumina Support for Low-Temperature Catalytic Oxidation of Volatile Organic Compounds.

A γ-alumina support functionalized with transition metals is one of the most widely used industrial catalysts for the total oxidation of volatile organic compounds (VOCs) as air pollutants at higher temperatures (280-450 °C). By rational design of a bimetal CuFe-γ-alumina catalyst, synthesized from a dawsonite alumina precursor, the activity in total oxidation of toluene as a model VOC at a lower temperature (200-380 °C) is achieved. A fundamental understanding of the catalyst and the reaction mechanism is elucidated by advanced microscopic and spectroscopic characterizations as well as by temperature-programmed surface techniques. The nature of the metal-support bonding and the optimal abundance between Cu-O-Al and Fe-O-Al species in the catalysts leads to synergistic catalytic activity promoted by small amounts of iron (Fe/Al = 0.005). The change in the metal oxide-cluster alumina interface is related to the nature of the surfaces to which the Cu atoms attach. In the most active catalyst, the CuO6 octahedra are attached to 4 Al atoms, while in the less active catalyst, they are attached to only 3 Al atoms. The oxidation of toluene occurs via the Langmuir-Hinshelwood mechanism. The presented material introduces a prospective family of low-cost and scalable oxidation catalysts with superior efficiency at lower temperatures.

Insight into the catalytic oxidation of toluene over M/ZSM-5 (M=Cu, Mn, Fe, Ce, Ti) catalysts

A series of metal oxide catalysts were prepared by impregnating Cu, Mn, Fe, Ce and Ti on ZSM-5 molecular sieve. The physicochemical properties of the catalysts were characterized by SEM, XRD, N2 adsorption/desorption, XPS, H2-TPR, and the catalytic oxidation of toluene was investigated. The results showed that Cu/ZSM-5 had rough surface, uniform distribution of metal, good pore structure, superior low-temperature reducibility and abundant adsorbed oxygen species. Cu/ZSM-5 with 5% loading exhibited excellent catalytic activity for toluene oxidation and the best sulfur resistance performance, with t90 (GHSV=24000 h−1) being 224 °C in SO2 environment. In-situ DRIFTS experiments revealed that the degradation path of toluene was as follows: toluene was first adsorbed on the surface of the catalyst to form adsorbed toluene, then it was converted into benzaldehyde and benzoic acid successively on the catalyst. And small molecule organics such as maleic acid and carboxylic acid were formed through ring opening reaction, and finally was oxidized to CO2 and H2O.

Preparation of Monolithic LaFeO3 and Catalytic Oxidation of Toluene.

Porous LaFeO3 powders were produced by high-temperature calcination of LaFeO3 precursors obtained by hydrothermal treatment of corresponding nitrates in the presence of citric acid. Four LaFeO3 powders calcinated at different temperatures were mixed with appropriate amounts of kaolinite, carboxymethyl cellulose, glycerol and active carbon for the preparation of monolithic LaFeO3 by extrusion. Porous LaFeO3 powders were characterized using powder X-ray diffraction, scanning electron microscopy, nitrogen absorption/desorption and X-ray photoelectron spectroscopy. Among the four monolithic LaFeO3 catalysts, the catalyst calcinated at 700 °C showed the best catalytic activity for the catalytic oxidation of toluene at 36,000 mL/(g∙h), and the corresponding T10%, T50% and T90% was 76 °C, 253 °C and 420 °C, respectively. The catalytic performance is attributed to the larger specific surface area (23.41 m2/g), higher surface adsorption of oxygen concentration and larger Fe2+/Fe3+ ratio associated with LaFeO3 calcined at 700 °C.

Enhancing catalytic activity for toluene and acetone oxidation over ZraCo1-aOx catalysts by doping Zr to improve the oxygen activation capacity due to formation of Zr-O-Co bonds

Transition metal oxides are considered as effective catalysts for eliminating volatile organic compounds (VOCs) due to their superior characteristics of abundant reserves and low price. A series of ZraCo1-aOx composite oxides was synthesized through a facile hydrothermal method and applied for catalytic oxidation of toluene and acetone. The catalytic activity of ZraCo1-aOx is improved by adjusting the elemental content composition, especially the Zr0.4Co0.6Ox sample exhibits the superior catalytic ability for both toluene (T90 = 231 °C) and acetone (T90 = 208 °C) oxidation under WHSV of 40,000 mL·g−1·h−1. The doping of Zr in ZraCo1-aOx induces to form Zr-O-Co bonds and enlarges the specific surface area compare with pure Co3O4, which improves the redox capacity and increase active oxygen content. In situ DRIFTS tests were used to detect reaction intermediates in the toluene and acetone oxidation process on the catalyst surface, and the advantageous function of Zr doping on promoting activation of oxygen was further determined. Furthermore, the outstanding activity stability and water resistance of the Zr0.4Co0.6Ox catalyst facilitate its application in industrial environments. This work provides a theoretical basis for preparing the catalyst to eliminate VOCs in industrial applications.

Facile synthesis of β-MnO2 via ozone oxidation for enhanced performance of toluene oxidation

In this work, a series of β-MnO2 catalysts is directly synthesized via ozone oxidation using the hydrothermal method. And the effect of hydrothermal temperature on catalytic activities of toluene oxidation is investigated. The catalytic evaluation shows that O3(HT)-160 prepared at 160 °C of hydrothermal temperature exhibits the best catalytic activity of toluene oxidation compared with other catalysts. By comparison of physicochemical properties, O3(HT)-160 possesses a bigger specific surface area and larger pore volume, which are beneficial to the adsorption and diffusion of toluene molecules. Meanwhile, O3(HT)-160 performs lower reductive temperature, higher ratio of Mn4+/(Mn3++ Mn2+) and Olatt/Oads and much more oxygen vacancies, which contributes to the cycle of redox ability, the complete oxidation of intermediates and the activation of oxygen. Moreover, combined with the results of in-situ DRIFTS and TD/GC–MS, the main intermediates are benzyl alcohol, benzaldehyde, benzoic acid and propylene glycol during toluene oxidation on O3(HT)-160. Furthermore, the catalytic mechanism is proposed for toluene oxidation on O3(HT)-160. This work provides a more convenient and simpler method to prepare β-MnO2 using ozone as the oxidizer compared with potassium permanganate and potassium persulfate.

Quenching-Induced Defect-Rich Platinum/Metal Oxide Catalysts Promote Catalytic Oxidation.

Enhancing oxygen activation through defect engineering is an effective strategy for boosting catalytic oxidation performance. Herein, we demonstrate that quenching is an effective strategy for preparing defect-rich Pt/metal oxide catalysts with superior catalytic oxidation activity. As a proof of concept, quenching of α-Fe2O3 in aqueous Pt(NO3)2 solution yielded a catalyst containing Pt single atoms and clusters over defect-rich α-Fe2O3 (Pt/Fe2O3-Q), which possessed state-of-the-art activity for toluene oxidation. Structural and spectroscopic analyses established that the quenching process created abundant lattice defects and lattice dislocations in the α-Fe2O3 support, and stronger electronic interactions between Pt species and Fe2O3 promote the generation of higher oxidation Pt species to modulate the adsorption/desorption behavior of reactants. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) characterization studies and density functional theory (DFT) calculations determined that molecular oxygen and Fe2O3 lattice oxygen were both activated on the Pt/Fe2O3-Q catalyst. Pt/CoMn2O4, Pt/MnO2, and Pt/LaFeO3 catalysts synthesized by the quenching method also offered superior catalytic activity for toluene oxidation. Results encourage the wider use of quenching for the preparation of highly active oxidation catalysts.

Ytterbium modified birnessite MnO2 for improving deep oxidation of toluene

A series of rare earth metals modified birnessite manganese dioxides (RE-Mn) was synthesized by hydrothermal method and their catalytic degradation of toluene was investigated. Yb-Mn shows the best toluene oxidation activity with the conversion of T50% and T99% at 196 and 226 °C, respectively. The physiochemical characterizations identified that the high catalytic performance and excellent stability of Yb-Mn are mainly associated with its higher molar ratios of Mn4+/Mn3+, more lattice oxygen (Olatt), and better mobility of surface adsorbed oxygen (Oads). Moreover, it is confirmed that introduction of Yb into birnessite MnO2 facilitates the production of acetic acid at lower temperature. These results can aid the design of high-efficiency manganese oxide catalysts for the catalytic combustion of VOCs.

Engineering of oxygen vacancy defect in CeO2 through Mn doping for toluene catalytic oxidation at low temperature

Catalytic oxidation is considered a highly effective method for the elimination of volatile organic compounds. Oxygen vacancy defect engineering in a catalyst is considered an effective approach for high-performance catalysts. Herein, a series of doped MnxCe1-xO2 catalysts (x = 0.05–0.2) with oxygen vacancy defects were synthesized by doping low-valent Mn in a CeO2 lattice. Different characterization techniques were utilized to inspect the effect of doping on oxygen vacancy defect generation. The characterization results revealed that the Mn0.15Ce0.85O2 catalyst has the maximum oxygen vacancy concentration, leading to increased active oxygen species and enhanced oxygen mobility. Thus, Mn0.15Ce0.85O2 catalyst showed an excellent toluene oxidation activity with 90% toluene conversion temperature (T90) of 197 °C at a weight hourly space velocity of 40,000 mL g−1 h−1 as compared to undoped CeO2 (T90 = 225 °C) and Ce based oxides in previous reports. In addition, the Mn0.15Ce0.85O2 catalyst displayed strong recyclability, water resistant ability and long-time stability. The in situ DRIFT results showed that the Mn0.15Ce0.85O2 catalyst has a robust oxidation capability as toluene is quickly adsorbed and actuated as compared to CeO2. Thus, the present work lays the foundation for designing a highly active catalyst for toluene elimination from the environment.

The influence of Ce doping on catalytic oxidation of toluene over Co3O4/iron mesh monolithic catalyst

In this paper, CeCoOx/iron mesh monolithic catalyst (CeCoOx/IM) was successfully prepared by hydrothermal method. The preparation conditions such as Ce/Co ratio, hydrothermal temperature as well as nitrate/urea ratio of the catalysts were optimized systematically. The catalytic performance results showed that when the Ce/Co ratio was 0.4, hydrothermal temperature was 160 °C and nitrate/urea ration was 1:8, the obtained catalyst possessed the best toluene oxidation activity, and the temperature at 50% and 90% toluene conversion (T50% and T90%) was 248 °C and 256 °C, respectively, which was lower than that of Co3O4/IM (260 °C and 267 °C, respectively). In addition, the active component is closely combined with the iron substrate with an exfoliation rate of 0.07%. H2-TPR results showed that the existence of rich Co3+ species and the strong interaction between Co and Ce was one of the reasons for this improvement performance. The introduction of Ce can also improve the thermal stability, water resistance and cycling stability of the catalyst. The reaction kinetics analysis showed that the Ea value was significantly reduced by Ce doping, which reached to 71.3 kJ·mol−1 for the best performance catalyst. We hope this study can provide a theoretical support for the practical application of catalyst.

Promoting effect of Ce doping on catalytic performance and water resistance ability for toluene catalytic combustion over the cheap and efficient Mn8Ni2CeaOx catalysts

A series of metal oxides Mn8Ni2CeaOx (a = 0–1.5) catalysts were prepared by hydrothermal co-precipitation to investigate the effect of Ce doping on manganese-based oxides for toluene catalytic combustion reaction. Activity evaluations show that Mn8Ni2CeOx (a = 1) catalyst exhibits highly efficient toluene oxidation activity and high yield to CO2 production. Durability tests reveal that it has remarkably superior water resistance ability and promising potential for practical application. Various characterizations were conducted to explore possible structure-activity correlations. Results show that Ce introduction could construct the strong Ce-Mn interactions, generate more oxygen vacancies, facilitate lattice oxygen mobility, and increase the content of surface active oxygen. These changes efficiently utilize active oxygen species and accelerate the deep oxidation of benzyl alcohol/benzoate intermediates, therefore render activity advantages. Otherwise, Ce doping can effectively maintain the catalyst's structural stability while simultaneously inhibiting H2O adsorption on the catalyst surface under humid atmosphere, thus being conducive to the outstanding durability and water resistance ability.

Cu(Mn)MgAl mixed oxides with enhanced performance for simultaneous removal of NOx and toluene: Insight into the better collaboration of CuO and MnOx via layered double hydroxides (LDHs) precursor template

In this study, Cu(Mn)MgAl oxide catalysts prepared by layered double hydroxides (LDHs) precursor template method for simultaneous removal of NOx and toluene. Firstly, we prepared CuMgAl layered double oxides (LDO) derived from LDHs by co-precipitation method. Subsequently, Mn active species were introduced to partially replace Cu species by one-step in situ and mechanical mixing method to obtain Cu2.5Mn0.5MgAl-LDO and MnOx + Cu2.5MgAl-LDO, respectively. XRD, BET, SEM, TEM, XPS, NH3-TPD, H2-TPR and in situ DRIFTs were applied to characterize the as-prepared catalysts. The catalytic results show that the activity of Mn-doped catalysts is significantly higher than that of Cu3MgAl-LDO, and the Cu2.5Mn0.5MgAl-LDO exhibits the best catalytic activity for both NH3-SCR and toluene oxidation. The obtained Cu2.5Mn0.5MgAl-LDO shows superior redox ability and highest concentration of surface Mn4+ and Cu2+ species, which may be responsible for its excellent catalytic performance. In addition, Cu2.5Mn0.5MgAl-LDO also possesses superior reusability and good tolerance to H2O/SO2.

High-efficiency palladium/halloysite nanotubes catalyst for toluene catalytic oxidation: Characterization, performance and reaction mechanism

In this study, the halloysite treated with nitric acid was designed as a functionalized support material for favoring surface availability and dispersion of nanometric palladium (Pd) nanoparticles. The prepared supported catalysts were characterized by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), and O2 temperature-programmed desorption (O2-TPD). The catalytic performance and reaction mechanism of toluene oxidation were evaluated and proposed. Scanning and transmission electron microscopy images showed that 4–5 nm diameter Pd nanoparticles were uniformly dispersed on the external and edge surfaces of the A-Hal nanotubes and no obvious aggregations observed. The Pd nanoparticles were mainly present in the zero-valent state. The ratio of metallic Pd to Pd oxide species and adsorbed (Oads) to lattice (Olatt) presented a maximum value of 2.54 and 23.0, respectively, when the Pd loading was 0.5%. The prepared support catalysts exhibited a satisfactory catalytic performance and cycling stability for toluene conversion. Among the analyzed samples, the 0.5% Pd/A-Hal catalyst exhibited the highest catalytic activity for toluene oxidation, stability, and water resistance. Toluene conversions of 50% and 90% were obtained at 167 and 185 °C, respectively. In situ diffuse reflection Fourier transform spectroscopy measurements confirmed the intermediates generated during toluene oxidation and revealed that toluene oxidation occurred via the benzyl alcohol → benzaldehyde → benzoic acid → maleic anhydride reaction pathway over the obtained catalysts. These results were attributed to the high content of metal Pd species, abundant Oads species, and low-temperature reducibility. These results indicate that Hal support catalysts are promising materials for application in environmental protection.