Complex Oxide Catalysts Research Articles

Preparation of rare-earth metal complex oxide catalysts for catalytic wet air oxidation

Catalytic wet air oxidation (CWAO) is one of the most promising technologies for pollution abatement. Developing catalysts with high activity and stability is crucial for the application of the CWAO process. The Mn/Ce complex oxide catalysts for CWAO of high concentration phenol-containing wastewater were prepared by coprecipitation. The catalyst preparation conditions were optimized by using an orthogonal layout method and single-factor experimental analysis. The Mn/Ce serial catalysts were characterized by Brunauer-Emmett-Teller (BET) analysis and the metal cation leaching was measured by inductively coupled plasma torch-atomic emission spectrometry (ICP-AES). The results show that the catalysts have high catalytic activities even at a low temperature (80°C) and low oxygen partial pressure (0.5 MPa) in a batch reactor. The metallic ion leaching is comparatively low (Mn<6.577 mg/L and Ce<0.6910 mg/L, respectively) in the CWAO process. The phenol, CODCr, and TOC removal efficiencies in the solution exceed 98.5% using the optimal catalyst (named CSP). The new catalyst would have a promising application in CWAO treatment of high concentration organic wastewater.

Simultaneous removal of NO x and diesel soot particulates over nanometric La 2− xK xCuO 4 complex oxide catalysts

The nanometric La 2− x K x CuO 4 oxide catalysts with K 2NiF 4-type structure were prepared by auto-combustion method using citric acid as a ligand and an adjusting agent of particle-size and morphology. The structures and physico-chemical properties of these perovskite-like oxides were examined by means of XRD, FT-IR, H 2-TPR and chemical analysis. The catalytic activities for the simultaneous removal of soot and NO x were evaluated by a technique of the temperature-programmed oxidation reaction (TPO). In the La 2− x K x CuO 4 catalysts, the partial substitution of K for La at A-site leads to the increase of the concentrations of Cu 3+ and oxygen vacancy. Thus, it enhances the catalytic activity for simultaneous removal of NO x and diesel soot, and the optimal substitution amount of potassium x is equal to 0.5 among these samples. T 10, T 50, T 90 are 376, 438, 487 °C and P N 2 is 22%, respectively, for simultaneous removal of NO x and soot particulates over the La 1.5K 0.5CuO 4 catalyst under loose contact conditions between the catalyst and soot.

TiO2에 담지된 금속 산화물 촉매상에서 TCE 산화분해반응

Oxidative TCE decomposition over <TEX>$TiO_2$</TEX>-supported single and complex metal oxide catalysts has been conducted using a continuous flow type fixed-bed reactor system. Different types of commercial <TEX>$TiO_2$</TEX> were used for obtaining the supported catalysts via an incipient wetness technique. Among a variety of titanias and metal oxides used, a DT51D <TEX>$TiO_2\;and\;CrO_x$</TEX> would be the respective promising support and active ingredient for the oxidative TCE decomposition. The <TEX>$TiO_2-based\;CrO_x$</TEX> catalyst gave a significant dependence of the catalytic activity in TCE oxidation reaction on the metal loadings. The use of high <TEX>$CrO_x$</TEX> contents for preparing <TEX>$CrO_x/TiO_2$</TEX> catalysts might produce <TEX>$Cr_2O_3$</TEX> crystallites on the surface of <TEX>$TiO_2$</TEX>, thereby decreasing catalytic performance in the oxidative decomposition at low reaction temperatures. Supported <TEX>$CrO_x$</TEX>-based bimetallic oxide systems offered a very useful approach to lower the <TEX>$CrO_x$</TEX> amounts without any loss in their catalytic activity for the catalytic TCE oxidation and to minimize the formation of Cl-containing organic products in the course of the catalytic reaction.

Catalytic combustion properties of nanocomplex oxide for natural gas*

Abstract The catalysts of copper oxide supported on cerium dioxide were prepared by different methods for methane catalytic combustion. The effects of copper content in the catalysts and calcination temperatures of the catalysts on the catalytic activity are investigated. Results show that the complex oxide catalyst exhibits high catalytic activity for methane combustion due to the synergistic effect of CuO and CeO2. The catalyst prepared by impregnation is more active than that prepared by controlled coprecipitation even if CuO content is the same. When W (CuO)<13%, the light-off temperature and full conversion temperature for the CH4 reaction decrease with the increasing of CuO content in the catalysts. However, when the copper content is above 13%, the excess CuO has a negative effect on the catalytic activity owing to the formation of bulk CuO particles. A proper calcination temperature of 650°C can lead to a high dispersion of CuO and accordingly can enhance the catalytic activity of the composities....

Temperature Hysteresis in Oxidation of CO on Complex Oxide Catalysts

We have used thermal desorption (TD) with mass spectrometric detection of the desorbed species to study the state of the surface of oxide catalysts for oxidation of CO. Detection of weakly bound forms of water and the species H3O+ and HO 2 ⋅ in the thermal desorption spectra correlates with the existence of temperature hysteresis in these samples. The data obtained are explained by the reaction occurring in a highly active state via a heterogeneous-homogeneous mechanism, and also by the effect of local superheating.

Effect of the Composition of Mn-, Co- and Ni-Containing Perovskites on their Catalytic Properties in the Oxidative Coupling of Methane

The catalytic properties of complex oxide catalysts with ABO3 perovskite structure (A = Sr, Ba, La; B = Mn, Co, Ni) in the oxidative coupling of methane were studied with separate introduction of methane and oxygen onto the catalyst. The greatest catalytic activity was found for the nickel-containing oxides but these catalysts have much lower operational stability than Mn- and Co-containing catalysts. Modification of the oxide systems studied by partial replacement of the strontium or transition metal ions by alkali metal ions considerably improves their activity and selectivity.

Strategy in achieving propane selective oxidation over multi-functional Mo-based oxide catalysts

Based on recent scientific research and industrial approach on the selective oxidation of propane over complex metal oxide catalysts, strategical elements which are necessary for developing complicated oxide catalysts and for achieving industrial processes are summarized and discussed.

Structure dependency of Mo-V-O-based complex oxide catalysts in the oxidations of hydrocarbons

Vapor-phase oxidations of ethane, propane, and acrolein were carried out over crystalline (orthorhombic) and amorphous Mo-V-O catalysts. The Mo-V-O crystalline catalyst was prepared by hydrothermal synthesis for the first time in a new orthorhombic structure. The orthorhombic Mo-V-O catalyst showed extremely high activity for the reactions compared to the amorphous one. Te in the orthorhombic Mo-V-O catalyst increased the selectivity to acrylic acid from propane dramatically.

Selective oxidation of styrene over nanosized spinel-type Mg xFe 3− xO 4 complex oxide catalysts

Nanosized spinel-type Mg x Fe 3− x O 4 complex oxide catalysts were prepared by coprecipitation and citrate gel methods and were characterized. The crystallization temperature of spinel particles prepared by citrate gel method is about 300 °C lower than that of those prepared by coprecipitation method. Mg x Fe 3− x O 4 catalysts prepared by citrate gel method have higher dispersity and smaller particle size, leading to higher activity for styrene oxidation with H 2O 2 as oxidant. The nonstoichiometric Mg x Fe 3− x O 4 catalysts ( x<1) are more active for styrene oxidation than the pure spinel MgFe 2O 4. The major reaction (∼65–70 mol%) is oxidative CC cleavage into benzaldehyde, and the minor reaction (10 mol%) is epoxidation and its further isomerization into phenylacetaldehyde. The effects of solvent, styrene:H 2O 2 molar ratio, reaction temperature and time on the conversion and product distribution were also studied.

Combinatorial solution synthesis and characterization of complex oxide catalyst powders based on the LaMO 3 system

A polymerizable-complex synthesis method coupled with inkjet deposition was used to generate combinatorial libraries of complex oxide catalyst powders based on the LaMO 3 (M=Cr, Co) system. Strontium and calcium were substituted on the La-sites, and cobalt, chromium and combinations of Co and Cr were substituted on the B-sites of the perovskite matrix. Thermal analysis, X-ray diffraction (XRD), and catalytic activity screening were performed on the libraries. Each system was examined for its catalytic activity in the CO oxidation reaction with infrared thermography. Catalytic activity was confirmed in the La x Sr y Co z O 3− δ and La x Cr y Co z O 3− δ systems. Most of the catalytic activity was in the cobalt-rich and most perovskite-like regions. The La x Sr y Co z O 3− δ libraries exhibited variation in catalytic activity with temperature, which may be indicative of oscillatory behavior.

Oxidation of Isobutane over Complex Oxides Containing V and Mg&lt;SUB&gt;2&lt;/SUB&gt;V&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;7&lt;/SUB&gt; Catalysts Partially Substituted by Transition Metal Ions

The oxidation of isobutane was investigated over various complex vanadium oxide catalysts. Mg 2 V 2 O 7 , MgV 2 O 6 , and Mg 3 V 2 O 8 are effective catalysis for isobutene formation at >300°C. Cu 2 V 2 O 7 and CaV 2 O 6 showed intermediate activity and BiVO 4 and Ca 7 V 4 O 17 were slightly active. The active catalysts had relatively strong acidic sites, and H abstraction from the t-carbon of isobutane by acidic site was considered to be the initial and rate-determining step because a linear relationship was observed between the amount of acidic sites and the catalytic activity of the catalysts. The introduction of transition metals into the Mg sites of the Mg 2 V 2 O 7 crystal lattice formed Mg 1.8 V 2 M 0.2 O 7 (M =Mn, Cr, Fe, Co, Ni) catalysts. This substitution significantly promoted the catalytic activity without changing the selectivity for isobutene. Introduction of these transition metals increased the reactivity of lattice oxygen in Mg 1.8 V 2 M 0.2 O 7 catalysts. Decreased W/F, increased the selectivity for isobutene to approaching 100%, suggesting that the initial reaction is the oxidative dehydrogenation of isobutane without parallel CO x formation.

The Reduction of Mg–Fe–O and Mg–Fe–Al–O Complex Oxides Studied by Temperature-Programmed Reduction Combined with in Situ Mössbauer Spectroscopy

In a H2/N2 atmosphere, the reduction processes of Mg–Fe–O and Mg–Fe–Al–O complex oxide catalysts, derived from their hydrotalcite precursors, were studied by means of temperature-programmed reduction (TPR). In particular, the reduction process of the Mg–Fe–O samples with a Mg/Fe ratio of 1/1 and Mg–Fe–Al–O with a Mg/Fe/Al ratio of 2/1/1 were investigated in detail in combination with in situ Mössbauer spectroscopy and X-ray diffraction (XRD). For the Mg–Fe–O sample, two peaks were observed in the TPR profile with peak temperatures at 446 and 696°C, respectively, indicating that the reduction of the oxide catalyst proceeded via two stages. Mössbauer spectroscopy and XRD showed that the phases in the sample were MgFe2O4 and MgO before TPR. At the first TPR peak (446°C), MgFe2O4 was completely reduced to Mg1−xFexO. The Mössbauer spectrum of Mg1−xFexO exhibited a doublet with IS=0.83 mm/s and QS=0.87 mm/s. The second TPR peak (696°C) corresponded to the reduction of Mg1−xFexO to Fe0. For the Mg–Fe–Al–O sample, two peaks were also observed in the TPR profile with temperatures at 506 and 936°C, respectively. Mössbauer spectroscopy and XRD showed that the phases in the sample were α-Fe2O3, MgFeAlO4, and MgO before TPR. The Mössbauer spectrum of MgFeAlO4 was a doublet with IS=0.33 mm/s and QS=0.76 mm/s. At the first TPR peak (506°C), α-Fe2O3 was reduced to Fe2+ and Fe0, while MgFeAlO4 was transformed into Mg1−xFexO. The second TPR peak (936°C) corresponded to the reduction of Mg1−xFexO to metallic Fe0. Effects of the presence of magnesium and aluminum in the solid solutions on the reduction of iron species were discussed.

Contemporary Problems in the Selective Catalytic Reduction of Nitrogen Oxides (NOx)

The results from investigation of the processes involved in the catalytic reduction of nitrogen oxides are reviewed: 1) with carbon monoxide and other combustible gases (H2, C n H m ) at a developed block honeycomb palladium-containing catalysts; 2) with ammonia (the SCR process) at vanadium-containing and complex oxide (Cu-Cr, Fe-Cr/γ-Al2O3) catalysts; 3) with light C1-C4 hydrocarbons at cation-exchange zeolites (pentasils, natural and synthetic mordenites). Mechanisms and kinetic models are proposed, and the regions of a positive and negative effect from oxygen and sulfur dioxide on the processes involved in the reduction of nitrogen oxides are established.

ChemInform Abstract: An Integrated Surface Science Approach Towards Metal Oxide Catalysis

The function of a metal oxide catalyst was investigated by an integrated approach, combining a variety of surface science techniques in ultrahigh vacuum with batch reactor conversion measurements at high gas pressures. Epitaxial FeO(111), Fe3O4(111) and α‐Fe2O3(0001) films with defined atomic surface structures were used as model catalysts for the dehydrogenation of ethylbenzene to styrene, a practized selective oxidation reaction performed over iron‐oxide‐based catalysts in the presence of steam. Ethylbenzene and styrene adsorb onto regular terrace sites with their phenyl rings oriented parallel to the surface, where the π‐electron systems interact with Lewis acidic iron sites exposed on Fe3O4(111) and α‐Fe2O3(0001). The reactant adsorption energies observed on these films correlate with their catalytic activities at high pressures, which indicates that the surface chemical properties do not change significantly across the pressure gap. Atomic defects were identified as catalytically active sites. Based on the surface spectroscopy results a new mechanism was proposed for the ethylbenzene dehydrogenation, where the upward tilted ethyl group of flat adsorbed ethylbenzene is dehydrogenated at Bronsted basic oxygen sites located at defects and the coupling of the phenyl ring to Fe3+ terrace sites determines the reactant adsorption–desorption kinetics. The findings are compared to kinetic measurements over polycrystalline catalyst samples, and an extrapolation of the reaction mechanism found on the model systems to technical catalysts operating under real conditions is discussed. The work demonstrates the applicability of the surface science approach also to complex oxide catalysts with implications for real catalysts, provided suitable model systems are available.

Selective oxidation of light alkanes over hydrothermally synthesized Mo-V-M-O (M=Al, Ga, Bi, Sb, and Te) oxide catalysts

Selective oxidations of ethane to ethene and acetic acid and of propane to acrylic acid were carried out over hydrothermally synthesized Mo-V-M-O (M=Al, Ga, Bi, Sb, and Te) complex metal oxide catalysts. All the synthesized solids were rod-shaped crystallites and gave a common XRD peak corresponding to 4.0 Å d-spacing. From the different XRD patterns at low angle region below 10° and from the different shape of the cross-section of the rod crystal obtained by SEM, the solids were classified into two groups: Mo-V-M-O (M=Al, possibly Ga and Bi) and Mo-V-M-O (M=Sb, and Te). The former catalyst was moderately active for the ethane oxidation to ethene and to acetic acid. On the other hand the latter was found to be extremely active for the oxidative dehydrogenation. The Mo-V-M-O (M=Sb, and Te) catalysts were also active for the propane oxidation to acrylic acid. It was found that the grinding of the catalysts after heat-treatment at 600°C in N 2 increased the conversions of propane and enhanced the selectivity to acrylic acid. Structural arrangement of the catalytic functional components on the surface of the cross-section of the rod-shaped catalysts seems to be important for the oxidation activity and selectivity.

Heterogenization of Mn and Fe complex oxidation catalysts

Homogeneous Fe and Mn oxidation catalysts can be immobilized on silica, zeolites, clays, layered double hydroxides and polymers. In addition to the well-known porphyrin catalysts, there is increasing interest in complexes with non-planar ligands. Based on a selection of examples, this paper discusses heterogenization methods, and the effects of heterogenization on the catalytic activity.

Novel Fe‐based complex oxide catalysts for hydroxylation of phenol

Novel catalysts for the hydroxylation of phenol, Fe–Si–O, Fe–Mg–O and Fe–Mg–Si–O complex oxides, have been synthesized by a coprecipitation method. X‐ray diffraction studies show that MgFe2O4 crystallites with spinel structure are formed in Fe–Mg–Si–O and Fe–Mg–O complex oxides and the crystallite size of the metal oxide or complex oxide is reduced after addition of Si. In the hydroxylation of phenol with hydrogen peroxide, Fe‐based complex oxides exhibit high activities after a short induction period. The phenol conversion is improved when silicon is introduced into the Fe‐based complex oxides, and formation of MgFe2O4 crystals with spinel structure in the catalysts increases the diphenol selectivity. The addition of a little acetic acid to the reaction liquid can shorten the induction period effectively. Under the same reaction conditions, phenol conversion and diphenol selectivity over the Fe–Mg–Si–O catalyst are close to those over TS‐1, and furthermore, the reaction time is more than ten times shorter as compared to TS‐1. The reaction mechanism of the hydroxylation of phenol on the catalysts has been studied, and a free‐radical mechanism initiated by the formation of phenoxy free radicals is suggested.

Study of the mobility of lattice oxygen in complex oxide catalysts for partial oxidation of propylene to acrolein

The amount of oxygen in the lattice of solids that participates in the elementary stages of partial propylene oxidation is determined for two types of Co-Mo-Bi-Fe-Sb-K-O catalysts (I, II) differing in the method of introduction of antimony and potassium. Two independent methods are used: (1) on the basis of the yield of the oxygen-containing products of propylene oxidation by oxygen of the catalyst in a pulse regime and (2) with the use of Moessbauer spectroscopy. Coincidence of the results obtained by both methods indicates that the active oxygen of the catalyst lattice is formed during redox transformations of iron(III) molybdate entering the composition of the catalysts. Data on the reduction of the catalysts in a pulse regime at various temperatures, which were processed in the framework of the diffusion model, allowed the estimation of the rate constants for diffusion of the lattice oxygen. An increase in the mobility of the lattice oxygen in catalyst I, which is modified with a small amount of antimony as compared to catalyst II, results in an increase in the overall productivity of the sample and in a decrease in the selectivity of propylene oxidation to acrolein. This correlates with the increase in the total amount of the lattice oxygen participating in the process.

An integrated surface science approach towards metal oxide catalysis

The function of a metal oxide catalyst was investigated by an integrated approach, combining a variety of surface science techniques in ultrahigh vacuum with batch reactor conversion measurements at high gas pressures. Epitaxial FeO(111), Fe3O4(111) and α‐Fe2O3(0001) films with defined atomic surface structures were used as model catalysts for the dehydrogenation of ethylbenzene to styrene, a practized selective oxidation reaction performed over iron‐oxide‐based catalysts in the presence of steam. Ethylbenzene and styrene adsorb onto regular terrace sites with their phenyl rings oriented parallel to the surface, where the π‐electron systems interact with Lewis acidic iron sites exposed on Fe3O4(111) and α‐Fe2O3(0001). The reactant adsorption energies observed on these films correlate with their catalytic activities at high pressures, which indicates that the surface chemical properties do not change significantly across the pressure gap. Atomic defects were identified as catalytically active sites. Based on the surface spectroscopy results a new mechanism was proposed for the ethylbenzene dehydrogenation, where the upward tilted ethyl group of flat adsorbed ethylbenzene is dehydrogenated at Bronsted basic oxygen sites located at defects and the coupling of the phenyl ring to Fe3+ terrace sites determines the reactant adsorption–desorption kinetics. The findings are compared to kinetic measurements over polycrystalline catalyst samples, and an extrapolation of the reaction mechanism found on the model systems to technical catalysts operating under real conditions is discussed. The work demonstrates the applicability of the surface science approach also to complex oxide catalysts with implications for real catalysts, provided suitable model systems are available.

Hydrothermal synthesis of Mo–V–M–O complex metal oxide catalysts active for partial oxidation of ethane

Mo–V–M–O (M=Al, Fe, Cr and Ti) complex metal oxide catalysts have been prepared by hydrothermal synthesis for the first time and showed activity for the partial oxidation of ethane to ethene and acetic acid.