Catalytic dehydrogenation of ethylbenzene with carbon dioxide: promotional effect of antimony in supported vanadium–antimony oxide catalyst

Alumina-supported vanadium oxide, VO_x/Al₂O₃, and binary vanadium–antimony oxides, VSbO_x/Al₂O₃, have been tested in the ethylbenzene dehydrogenation with carbon dioxide and characterized by S_BET, X-ray diffraction, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction and CO₂ pulse methods. VSbO_x/Al₂O₃ exhibited enhanced catalytic activity and especially on-stream stability compared to VO_x/Al₂O₃ catalyst. Incorporation of antimony into VO_x/Al₂O₃ increased dispersion of active VO_x species, enhanced redox properties of the systems and formed a new mixed vanadium–antimony oxide phase in the most catalytically efficient V_0.43Sb_0.57O_x/Al₂O₃ system.     

Engineering the electronic and geometric structure of VOx/BN@TiO2 heterostructure for efficient aerobic oxidative desulfurization

Particle size governs the electronic and geometric structure of metal nanoparticles (NPs), shaping their catalytic performances in heterogeneous catalysis. However, precisely controlling the size of active metal NPs and thereafter their catalytic activities remain an affordable challenge in ultra-deep oxidative desulfurization (ODS) field. Herein, a series of highly-efficient VO_x/boron nitride nanosheets (BNNS)@TiO₂ heterostructures, therein, cetyltrimethylammonium bromide cationic surfactants serving as intercalation agent, BNNS and MXene as precursors, with various VO_x NP sizes were designed and controllably constructed by a facile intercalation confinement strategy. The properties and structures of the prepared catalysts were systematically characterized by different technical methods, and their catalytic activities were investigated for aerobic ODS of dibenzothiophene (DBT). The results show that the size of VO_x NPs and V⁵⁺/V⁴⁺ play decisive roles in the catalytic aerobic ODS of VO_x/BNNS@TiO₂ catalysts and that VO_x/BNNS@TiO₂-2 exhibits the highest ODS activity with 93.7% DBT conversion within 60 min under the reaction temperature of 130 °C and oxygen flow rate of 200 mL·min^–1, which is due to its optimal VO_x dispersion, excellent reducibility and abundant active species. Therefore, the finding here may contribute to the fundamental understanding of structure-activity in ultra-deep ODS and inspire the advancement of highly-efficient catalyst.     

Selective oxidation of propylene to acrolein over supported V2O5/Nb2O5 catalysts: An in situ Raman, IR, TPSR and kinetic study

The vapor-phase selective oxidation of propylene (H₂CCHCH₃) to acrolein (H₂CCHCHO) was investigated over supported V₂O₅/Nb₂O₅ catalysts. The catalysts were synthesized by incipient wetness impregnation of V-isopropoxide/isopropanol solutions and calcination at 450 °C. The catalytic active vanadia component was shown by in situ Raman spectroscopy to be 100% dispersed as surface VO_x species on the Nb₂O₅ support in the sub-monolayer region (<8.4 V/nm²). Surface allyl species (H₂CCHCH_2*) were observed with in situ FT-IR to be the most abundant reaction intermediates. The acrolein formation kinetics and selectivity were strongly dependent on the surface VO_x coverage. Two surface VO_x sites were found to participate in the selective oxidation of propylene to acrolein. The reaction kinetics followed a Langmuir–Hinshelwood mechanism with first-order in propylene and half-order in O₂ partial pressures. C₃H₆-TPSR spectroscopy studies also revealed that the lattice oxygen from the catalyst was not capable of selectively oxidizing propylene to acrolein and that the presence of gas phase molecular O₂ was critical for maintaining the surface VO_x species in the fully oxidized state. The catalytic active site for this selective oxidation reaction involves the bridging VONb support bond.     

A spillover approach to oxidation catalysis over copper and manganese mixed oxides

The activity of a hopcalite-type catalyst for H₂ and CO oxidation is compared with that of single-phase oxides CuO, Mn₂O₃ and CuMn₂O₄ (spinel) and a mixture of 1:1 CuO and Mn₂O₃ in order to elucidate the effects that are responsible for the high catalytic activity of the former. The reaction rates over the hopcalite catalyst calcined at 550°C (a mixture of CuO, Mn₂O₃ and CuMn₂O₄) are very close to those of the physical mixture of CuO and Mn₂O₃, being much greater than the rates over the single-phase oxides. CuO, Mn₂O₃ and CuMn₂O₄ show a kinetic compensation behavior both in H₂ and CO oxidation. By taking into account the activation energies and the reducibility measured by TPR it was concluded that the oxidation reactions over CuO follow a redox mechanism using lattice oxygen, while over Mn₂O₃ the mechanism is associative involving adsorbed oxygen species. Based on the TPR and kinetic results, the synergy between copper and manganese oxides in hopcalite and in the CuO–Mn₂O₃ mixture is assigned to a spillover effect.     

Oxidative dehydrogenation of propane over differently structured vanadia-based catalysts in the presence of O2 and N2O

The effect of the nature and distribution of VO_x species over amorphous and well-ordered (MCM-41) SiO₂ as well as over γ-Al₂O₃ on their performance in the oxidative dehydrogenation of propane with O₂ and N₂O was studied using in situ UV–vis, ex situ XRD and H₂-TPR analysis in combination with steady-state catalytic tests. As compared to the alumina support, differently structured SiO₂ supports stabilise highly dispersed surface VO_x species at higher vanadium loading. These species are more selective over the latter materials than over V/γ-Al₂O₃ catalysts. This finding was explained by the difference in acidic properties of silica- and alumina-based supports. C₃H₆ selectivity over V/γ-Al₂O₃ materials is improved by covering the support fully with well-dispersed VO_x species. Additionally, C₃H₆ selectivity over all materials studied can be tuned by using an alternative oxidising agent (N₂O). The improving effect of N₂O on C₃H₆ selectivity is related to the lower ability of N₂O for catalyst reoxidation resulting in an increase in the degree of catalyst reduction, i.e. spatial separation of active lattice oxygen in surface VO_x species. Such separation favours selective oxidation over CO_x formation.     

Physico-chemical characteristics and CO oxidation studies over Pd/(Mn2O3 + SnO2) catalyst

The mixed oxide catalyst (Mn₂O₃ + SnO₂) prepared by the coprecipitation method has been impregnated with Pd metal and it's catalytic behaviour for CO oxidation reaction has been investigated. In the coprecipitated material, Mn₂O₃ and SnO₂ were found to crystallise at 875 K and 1175 K, respectively, which are significantly higher than the crystallisation temperatures of individual oxides prepared under similar conditions. Results of catalytic oxidation of CO, carried out using the pulse method for the mixed oxide system and the individual oxides, suggest significant synergistic effects between these two oxides. The impregnation of palladium metal facilitated CO oxidation and the catalyst Pd/(Mn₂O₃ + SnO₂) was found to be quite effective for CO oxidation even at room temperature. Further, the CO disproportionation has been observed on palladium sites in the temperature range 350 to 400 K for the individual oxide systems.     

Structure–reactivity relationships in VOx/TiO2 catalysts for the oxyhydrative scission of 1-butene and n-butane to acetic acid as examined by in situ-spectroscopic methods and catalytic tests

Different VO_x/TiO₂ catalyst have been catalytically tested and studied by in situ-spectroscopic methods (FT-IR, UV/vis, EPR) in the oxyhydrative scission (OHS) of 1-butene and n-butane to acetic acid (AcOH). While 1-butene OHS follows the sequence butene → butoxide → ketone → AcOH/acetate with a multitude of side products also formed, n-butane OHS leads to AcOH, CO_x and H₂O only. Water vapour in the feed improves AcOH selectivity by blocking adsorption sites for acetate. The admixture of Sb₂O₃ was found to improve AcOH selectivity which is due to deeper V reduction under steady state conditions and lowering of surface acidity. VO_x/TiO₂ catalysts with sulfate-containing anatase are the most effective ones. Covalently bonded sulfate at the catalyst surface causes specific bonding of VO_x, stabilizes active V species and ensures their high dispersity.     

尖晶石型Mn_(1-x)M_xCo_2O_4柴油车尾气处理催化剂的制备及其应用

尖晶石型复合氧化物因具有独特的结构特征而成为相对理想的柴油车尾气处理催化剂。采用溶胶-凝胶法制备尖晶石型Mn_(1-x)M_xCo_2O_4催化剂,通过X射线衍射(XRD)和程序升温氧化(TDO)等对Mn_(1-x)M_xCo_2O_4催化剂进行表征。结果表明,制备的样品Mn_(1-x)M_xCo_2O_4均为尖晶石型复合氧化物;掺杂Cu、Ce后,催化剂的氧化性能有不同程度的变化。在固定床微型反应器上对催化剂催化活性进行评价,结果表明,与纯MnCo_2O_4相比,Mn_(0.9)Ce_(0.1)Co_2O_4催化剂催化活性提高,Mn_(0.9)Cu_(0.1)Co_2O_4催化剂催化活性降低,但CO_2选择性增加。     

Palladium–vanadium interaction in binary supported catalysts

We have studied the activity and selectivity of Pd/γ-Al₂O₃, VO_x/γ-Al₂O₃ and Pd–VO_x/γ-Al₂O₃ catalysts for the decomposition of NO and the reduction of NO with CO. Pd–VO_x/γ-Al₂O₃ catalysts were prepared by anchoring Pd(AcAc)₂ on VO_x/γ-Al₂O₃. Characterization of the binary samples by hydrogen chemisorption and TPR measurements indicated that the reduction of VO_x is enhanced by a close contact with palladium and that partially reduced vanadia decorate noble metal particles. This palladium–vanadium interaction alters the catalytic properties of palladium: the activity for NO decomposition is higher for the binary sample and, for the NO–CO reaction, both the activity and the selectivity to N₂ increase when vanadium is in contact with palladium.     

Development of vanadium-based mixed oxide catalysts for selective oxidation of H2S to sulfur

Various vanadium-based binary and multi-metallic oxides were prepared and their catalytic activities for the selective oxidation of H₂S to elemental sulfur were tested. Because the deactivation of vanadium-based catalysts originated from a relatively slow rate of reoxidation of the reduced vanadium oxide [PhD thesis, Pohang University of Science and Technology, 2000], the focus was given to increase the redox ability, especially in the reoxidation step. Stable and improved activity was observed in BiVO_x, TiVO_x, and ZrV₂O₇ at 250°C, but TiVO_x was the only catalyst that could maintain its activity below 250°C. Much higher activity was observed when VO_x/TiO₂ became multi-metallic by the incorporation of Fe, Cr, and Mo. TPR–TPO, microbalance, and XPS techniques were used to explain the redox properties of VO_x/SiO₂, VO_x/TiO₂, and V-Fe-Cr-Mo-O_x/TiO₂ catalysts in the reoxidation step.     

Oxides of copper, ceria promoted copper, manganese and copper manganese on Al2O3 for the combustion of CO, ethyl acetate and ethanol

Combustion of CO, ethyl acetate and ethanol was studied over CuO_x/Al₂O₃, CuO_x–CeO₂/Al₂O₃, CuMn₂O₄/Al₂O₃ and Mn₂O₃/Al₂O₃ catalysts. It was found that modification of the alumina with ceria before subsequent copper oxide deposition increases the activity for combustion of CO substantially, but the effect of ceria was small on the combustion of ethyl acetate and ethanol. The activity increases with the CuO_x loading until crystalline CuO particles are formed, which contribute little to the total active surface. The CuO_x–CeO₂/Al₂O₃ catalyst is more active than the CuMn₂O₄/Al₂O₃ catalyst for the oxidation of CO but the CuMn₂O₄/Al₂O₃ catalyst is more active for the combustion of ethyl acetate and ethanol.

Thermal ageing and water vapour in the feed caused a modest decrease in activity and did not affect the CuO_x–CeO₂/Al₂O₃ and CuMn₂O₄/Al₂O₃ catalysts differently. In addition, no difference in intermediates formed over the two catalysts was observed.

Characterisation with XRD, FT-Raman and TPR indicates that the copper oxide is present as a copper aluminate surface phase on alumina at low loading. At high loading, bulk CuO crystallites are present as well. Modification of the alumina with ceria before the copper oxide deposition gives well dispersed copper oxide species and bulk CuO crystallites associated to the ceria, in addition to the two copper oxide species on the bare alumina. The distribution of copper species depends on the ceria and copper oxide loading. The alumina supported copper manganese oxide and manganese oxide catalysts consist mainly of crystalline CuMn₂O₄ and Mn₂O₃, respectively, on Al₂O₃.     

Understanding the activation mechanism induced by NOx on the performances of VOx/TiO2 based catalysts in the total oxidation of chlorinated VOCs

A previous investigation of the chlorobenzene combustion activity of VO_x/TiO₂, VO_x–WO_x/TiO₂ and VO_x–MoO_x/TiO₂ catalysts in the presence of NO pointed out the activation effect of NO. The suggested three-step mechanism based on catalytic performances data only was: (1) chlorobenzene is oxidized on the surface of the VO_x phase (as described by Mars–van Krevelen), (2) NO gets oxidized to NO₂, mainly on WO_x and MoO_x, and (3) the in situ produced NO₂ assists O₂ in the reoxidation of the VO_x phase thus speeding up the oxidation step of the Mars–van Krevelen mechanism. The latter effect macroscopically corresponds to the observed increase of chlorobenzene conversion. This contribution aims at validating this hypothetical mechanism by pointing out the favourable occurrence of an oxidation of NO to NO₂ on the WO_x and MoO_x phases and by pointing out the higher efficiency of NO₂ than O₂ to reoxidize the reduced VO_x sites. In addition, the present contribution clearly demonstrates that, in the absence of NO, the chlorobenzene total oxidation occurred following the Mars–van Krevelen mechanism. Moreover, a thorough characterization of the oxidation state of the vanadium proving that the improvement of the catalyst activity brought by the simultaneous presence of NO and O₂ is linked to the stronger reoxidation of the VO_x active phase. Furthermore, plotting all the catalytic activity data versus the mean vanadium oxidation level clearly depicts, for the first time, the strong dependence between them. Under a mean vanadium oxidation level of 4.82 the catalyst is inactive while above 4.87 the activity is stabilized at a high level of conversion independent of the vanadium oxidation level.     

Catalyst screening for oxidative desulfurization using hydrogen peroxide

Oxidation of a mixture of thiophene, benzothiophene and dibenzothiophene with hydrogen peroxide using supported Pd, Cr₂O₃, unsupported manganese oxides and a commercial Co-Mo/Al₂O₃ as catalysts has been studied in a mixture of hexadecane and acetonitrile. Based solely on the conversion of each organic sulfur compound, the ranking of catalyst efficiency found was: supported Pd > Cr₂O₃ ≈ manganese oxides ≈ Co-Mo/Al₂O₃. The influence of the calcination temperature on synthesized manganese oxides was also investigated. Mn₃O₄, amorphous manganese compounds, Mn₂O₃ and MnO₂ showed a similar catalytic activity independent of the hydrogen peroxide concentration. According to these preliminary results, it seemed that the catalyzed decomposition of the hydrogen peroxide competes with the oxidative desulfurization, however, at short reaction time (10 min) conversions at around 60–70% of thiophene were reached.     

Vanadium-containing catalysts for the selective oxidation of H2S to elemental sulfur in the presence of excess water

Various vanadium-containing catalysts were searched for the commercial application in the selective oxidation of H₂S to elemental sulfur at low temperatures (less than 250°C) in the presence of excess (more than 35 vol.%) water. In the test of binary oxides, it was found that TiVO_x was the only catalyst that could sustain its activity without deactivation at 230°C. The best catalytic activity (85–90% sulfur yield) was obtained when VO_x/TiO₂ was incorporated with other metals such as Fe, Cr and Mo. Reaction occurred via redox mechanism and the reoxidation of reduced vanadium was the rate-limiting step. A long-term deactivation observed during the reaction was due to slower reoxidation of reduced vanadium by oxygen than the reduction by H₂S. Catalytic activities of VO_x/SiO₂, VO_x/TiO₂ and V–Fe–Cr–Mo–O_x/TiO₂ were well correlated with their redox properties that were observed by TPR/TPO and XPS measurements.     

Molecular structure and reactivity of the group V metal oxides

The molecular structures and reactivity of the group V metal oxides (V₂O₅, Nb₂O₅ and Ta₂O₅) were compared. Their solid state structural chemistry, physical and electronic properties, number of active surface sites and their chemical reactivity properties were examined. For the bulk oxides, the solid state structural chemistry and the physical and electronic properties are well established. The number of active surface sites and the distribution of surface redox/acid sites were determined with methanol chemisorption and methanol oxidation, respectively. These studies revealed that the active surface sites present in pure V₂O₅ are primarily redox sites and the active surface sites in pure Nb₂O₅ are essentially acidic in nature. Furthermore, the surface redox sites present in pure V₂O₅ are orders of magnitude more active than the surface acid sites in pure Nb₂O₅. Consequently, the catalytic properties of bulk V₂O₅–Nb₂O₅ mixed oxides are dominated by the vanadia component. For the supported metal oxides, where the group V metal oxides are present as two-dimensional metal oxide overlayers, the structural and electronic properties are not well established in the literature. From a combination of molecular spectroscopic characterization methods (e.g., XANES, Raman, IR and UV–Vis DRS), it was possible to obtain this fundamental information. Methanol chemisorption studies demonstrated that a similar number of active surface sites are present in the supported vanadia and niobia catalyst systems. Similar to their bulk oxides, the surface vanadia species possess redox characteristics and the surface niobia species primarily possess acidic characteristics (Lewis acidity). The surface niobia species was a very sluggish redox site during oxidation reactions (e.g., methanol oxidation to formaldehyde and SO₂ oxidation to SO₃), but significantly promoted the surface vanadia redox sites for oxidation reactions that required dual surface redox and acid sites (e.g., butane oxidation to maleic anhydride and selective catalytic reduction of NO_x by NH₃ to produce N₂). These new fundamental insights are allowing for the molecular engineering of group V metal oxide catalysts (especially vanadia and niobia). In contrast, the molecular structure and reactivity properties of Ta₂O₅ catalysts are not yet established and will require significant research efforts.     

Location and size regulation of manganese oxides within mesoporous silica for enhanced antibiotic degradation

Refractory antibiotics in domestic wastewater are hard to be completely eliminated by conventional methods, and then lead to severe environmental contamination and adverse effects on public health. In present work, advanced oxidation processes (AOPs) are adopted to remove the antibiotic of sulfachloropyridazine (SCP). Nanosized Mn₂O₃ was fabricated on the SBA-15 material to catalytically activate potassium peroxydisulfate (PDS) to generate reactive oxygen radicals of ?OH and SO₄^- for SCP degradation. The effects of location and size of Mn₂O₃ were explored through choosing either the as-made or template-free SBA-15 as the precursor of substrate. Great influences from the site and size of Mn₂O₃ on the oxidation activity were discovered. It was found that Mn₂O₃ with a large size at the exterior of SBA-15 (Mn-tfSBA) was slightly easier to degrade SCP at a low manganese loading of 1.0–2.0?mmol?g^?1; however, complete SCP removal could only be achieved on the catalyst of Mn₂O₃ with a refined size at the interior of SBA-15 (Mn-asSBA). Moreover, the SO₄^- species were revealed to be the decisive radicals in the SCP degradation processes. Exploring the as-made mesoporous silica as a support provides a new idea for the further development of environmentally friendly catalysts.     

Catalytic wall reactor: Catalytic coatings of stainless steel by VOx/TiO2 and Co/SiO2 catalysts

Catalytic wall (structured) reactors and structured supports are suitable to study the catalytic properties of nanosized materials. The coating of metallic (aluminum and stainless steel) plates by thin layers of active phase is presented in two cases, VO_x/TiO₂ and Co/SiO₂, catalysts used in the oxidative dehydrogenation (ODH) of propane and in Fischer–Tropsch synthesis (FTS) of clean fuels, respectively. The preparation of coated plates and their characterisation by various methods of physicochemical analysis are described. Both chemical and physical methods were used for coating. VO_x/TiO₂ layers were obtained by grafting of Ti (on Al or stainless-steel plates) and V (on TiO₂) alkoxides and use of sol–gel media or suspension. A silica primer was deposited (on stainless-steel plate) by plasma-assisted chemical vapour deposition (PACVD) onto which Co oxide and silica were coprecipitated from sol–gel. The catalytic experiments in the respective reactions were carried out in special plate reactors and compared with those of catalytic powders. The study shows that the coating of a metallic substrate by a catalyst is not straightforward and requires specific studies dealing with both chemistry (chemical affinity between substrate and catalytic layers) and catalytic engineering (catalytic performance in taylor-made reactors).     

Design of a novel bifunctional catalyst IrFe/Al2O3 for preferential CO oxidation

In this study, a novel bifunctional catalyst IrFe/Al₂O₃, which is very active and selective for preferential oxidation of CO under H₂-rich atmosphere, has been developed. When the molar ratio of Fe/Ir was 5/1, the IrFe/Al₂O₃ catalyst performed best, with CO conversion of 68% and oxygen selectivity towards CO₂ formation of 86.8% attained at 100 °C. It has also been found that the impregnation sequence of Ir and Fe species on the Al₂O₃ support had a remarkable effect on the catalytic performance; the activity decreased following the order of IrFe/Al₂O₃ > co-IrFe/Al₂O₃ > FeIr/Al₂O₃. The three catalysts were characterized by XRD, H₂-TPR, FT-IR and microcalorimetry. The results demonstrated that when Ir was supported on the pre-formed Fe/Al₂O₃, the resulting structure (IrFe/Al₂O₃) allowed more metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O₂ activation on the FeO_x species. Thus, a bifunctional catalytic mechanism has been proposed where CO adsorbed on Ir sites and O₂ adsorbed on FeO_x sites; the reaction may take place at the interface of Ir and FeO_x or via a spill-over process.     

Influence of metal oxides on the catalytic behavior of Au/Al2O3 for the selective reduction of NOx by hydrocarbons

The reduction of NO by propene in the presence of excess oxygen over mechanical mixtures of Au/Al₂O₃ with a bulk oxide has been investigated. The oxides studied were: Co₃O₄, Mn₂O₃, Cr₂O₃, CuO, Fe₂O₃, NiO, CeO₂, SnO₂, ZnO and V₂O₅. Under lean C₃H₆-SCR conditions, these oxides (with the exception of SnO₂) convert selectively NO to NO₂. When mechanically mixed with Au/Al₂O₃, the Mn₂O₃ and Co₃O₄ oxides and, to a much greater extent, CeO₂ act synergistically with this catalyst greatly enhancing its SCR performance. It was found that their synergistic action is not straightforwardly related to their activity for NO oxidation to NO₂. The exhibited catalytic synergy may be due to the operation of either remote control or a bifunctional mechanism. In the later case, the key intermediate must be a short-lived compound and not the NO₂ molecule in gas-phase.     

Controllable synthesis of novel nanoporous manganese oxide catalysts for the direct synthesis of imines from alcohols and amines

A novel template-free oxalate route was applied to synthesize different mesoporous manganese oxides (amorphous manganese oxide (AMO), Mn₅O₈, Mn₃O₄, MnO₂) in the narrow temperature range from 350℃ to 400℃ by controlling the calcination conditions, which were employed as the efficient catalysts for the oxidative coupling of alcohols with amines to imines. The chemical and structural properties of the manganese oxides were characterized by the methods of thermogravimetry analysis and heat flow (TG-DSC), X-ray diffraction (XRD), nitrogen sorption, scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (H₂-TPR), and inductively coupled plasma optical emission spectrometry (ICP-OES) techniques. The structures of different manganese oxides were confirmed by characterization. The M-350 (AMO) presented the maximum surface area, amorphous nature, the lowest reduction temperature, the higher (Mn³⁺ + Mn⁴⁺)/Mn²⁺ ratio, and the higher adsorbed oxygen species compared to other samples. Among the catalysts, M-350 showed the best catalytic performance using air as an oxidant, and the conversion of benzyl alcohol (BA) and the selectivity of N-benzylideneaniline (NBA) reached as high as 100% and 97.1% respectively at the lower reaction temperature (80℃) for 1 h. M-350 had also the highest TOF value (0.0100 mmol·mg^-1·h^-1) compared to the other manganese oxide catalysts. The catalyst was reusable and gave 95.8% conversion after 5 reuse tests, the XRD pattern of the reactivated M-350 did not show any obvious change. Lattice oxygen mobility and (Mn³⁺ + Mn⁴⁺)/Mn²⁺ ratio were found to play the important roles in the catalytic activity of aerobic reactions.