MoO3/ZrO2催化剂的丙烷氧化脱氢制丙烯催化性能

以十六烷基三甲基溴化铵为表面活性剂,采用溶剂热法制备系列MoO_3/ZrO_2催化剂,采用H2-TPR、N_2吸附-脱附、X射线衍射等对其进行表征,并评价MoO_3/ZrO_2催化剂的丙烷氧化脱氢制丙烯催化性能。结果表明,MoO_3负载于ZrO_2载体上制备的催化剂催化活性增加,MoO_3负载质量分数为20%的MoO_3/ZrO_2催化剂,在反应温度为600℃时,丙烷转化率27.45%,丙烯选择性44.78%,丙稀收率12.29%。     

Description of active sites on molybdenum oxide as detected by isotope exchange between CO2 and MoO3

The oxygen exchange reaction between C¹⁸O₂ and MO¹⁶O₃ has been studied on well characterized MoO₃ crystallites. Temperature programmed desorption of adsorbed C¹⁸O₂ has shown that the extent of surface oxygen exchange is higher on partially reduced samples exposing the (010) face. This has been related with the easier reducibility of the (010) face. Moreover, experiments in a recycle reactor at moderate reaction temperature (623K) suggest that surface oxygen exchange is more rapid on the basal (010) face. On the other hand, when the experiment is run at higher reaction temperature (773K), using a higher ¹⁸O(C¹⁸O₂)/¹⁶O(MoO₃) ratio and for a longer period, a continuous oxygen exchange reaction occurs, specially with samples presenting lateral faces. In this latter conditions, two contributions to the exchange reaction can be distinguished. The first corresponds to a fast surface oxygen exchange. The second contribution which is more prominent over samples with (100) faces can be assigned to migration of oxygen atoms from lateral towards basal faces and/or to a slow migration of oxygen atoms from the bulk structure of MoO₃ towards the lateral faces. From the results of this study it is speculated on the importancë of a preferential oxygen migration from the bulk MoO₃ towards the lateral faces, which is related with the selective partial oxidation of olefins observed on these faces.     

Cluster model studies on oxygen sites at the (010) surfaces of V2O5 and MoO3

Cluster model studies have been performed to examine the electronic structure and adsorption properties near structurally different oxygen sites at the (010) surfaces of vanadium pentoxide, V₂O₅, and molybdenum trioxide, MoO₃. In addition, adsorption of hydrogen at the oxygen sites and desorption of OH groups has been studied in order to find site specific differences. The electronic properties and total energies of the clusters have been obtained from ab initio density functional theory (DFT) calculations. The surface oxygen sites are found to be ionic where bridging oxygens become more negative than terminal ones. Further, hydrogen adsorbs at all oxygen sites where binding is strongest at the bridge sites on the V₂O₅(010) surface whereas on MoO₃(010) the terminal sites are preferred. The latter difference can be understood by simple geometric arguments. Surface OH groups formed by H adsorption and involving terminal oxygens are strongly bound to the surface whereas those involving bridging oxygens are mobile and become available for subsequent reactions.     

Characterization and reactivity of MoO3/SiO2 catalysts in the selective catalytic oxidation of ammonia to N2

Silica-supported MoO₃ catalysts with MoO₃ loadings up to 21% w/w were prepared, characterized and tested in the selective catalytic oxidation (SCO) of ammonia to N₂ under dilute conditions. It is found that the catalysts are active and selective in the reaction, and that the catalytic performance increases on increasing the Mo loading. Crystalline MoO₃, detected over the silica support, is supposed to be the active species in the reaction. The reactivity of the catalysts is depressed by water addition to the feed at low temperatures and is enhanced by the presence of selected promoters, like Bi and Pb.     

Interactions of acetone with MoO3

Interactions of acetone and acetone/oxygen mixtures with MoO₃ were studied at 215–300 °C by pulse method. It has been shown that acetone is deoxygenated to propylene or oxidized to acetic acid, CO, and CO₂. Moreover a part of acetone is reversibly adsorbed and several minutes are necessary for acetone to be removed with a flow of carrier gas. As the surface is endowed with deoxygenating and oxidizing sites the desorbing acetone leaves the reactor mainly in the form of CO. These results agree with theoretical predictions based on the bond-strength model of active sites, according to which the sites of the first type (conversion of acetone) are localized on (100) face of MoO₃ and those of the second type (adsorption) on (001) and (101) planes. Both types of sites consist of surface anion vacancies and differ in the strength of bonding of oxygen. On the (100) face the bond strength is sufficiently large to break CO bond in acetone. On the other faces the interactions between acetone and surface are much weaker. In this context the application of acetone as poison of other reactions (e.g., conversion of methanol) is discussed.     

Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2O5

A series of SiO₂-supported MoO₃, V₂O₅, and Pt catalysts were prepared for the study of model soot oxidation with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation activities than the single-component SiO₂-supported MoO₃, V₂O₅ or Pt catalysts in the absence of SO₂ in the reactant gas. Based on the effects of NO₂ on carbon oxidation, a synergistic reaction mechanism was suggested to explain the effects of combining Pt with the oxides: Pt catalyzes the oxidation of NO with gas phase O₂ to NO₂, while MoO₃ and V₂O₅ catalyze the oxidation of carbon with NO₂. Finally, the effects of SO₂ on the carbon oxidation reaction were examined and discussed.     

Further on the mechanism of the synergy between MoO3 and α-Sb2O4 in the selective oxidation of isobutene to methacrolein: Reconstuction of MoO3 via spillover oxygen

This paper concerns the synergetic effects between a (010) oriented MoO₃ and α-Sb₂O₄ in the oxidation of isobutene to methacrolein at 420°C. The catalysts were prepared by mechanical mixture of the pure oxides prepared separately. Important increases were observed both for the conversion of isobutene and for the selectivity to methacrolein, suggesting that new selective sites have been created. The characterization of the samples before and after the reaction by SEM and XRD shows an important reconstruction of the (010) face of MoO₃ into steps exposing (100) faces, when the reaction is carried out in the presence of α-Sb₂O₄. It is known from literature that the (100) face of MoO₃ performs selectively partial oxidation, while the (010) face is not selective. On the other hand, α-Sb₂O₄ has shown to produce spillover oxygen, which flows onto the MoO₃ surface. It is, therefore, concluded that the increase of selectivity is intimately connected to the reconstruction under the action of spillover oxygen. Spillover oxygen would favour the “selective coordination” of Mo atoms or groups of atoms, typical of (100), at the expense of the “non-selective one”, typical of (010). This result corresponds to the theoretical prediction of the Remote Control theory.     

MoO_3-CeO-Co_3O_4催化剂CO催化氧化及耐硫性能

采用共沉淀法和浸渍法联用制备不同Mo质量分数的x MoO_3-6CeO-Co_3O_4催化剂,测试催化剂催化氧化CO效率及其耐硫性能,并对催化剂进行BET、SEM、FT-IR和H2-TPR等表征。结果表明,MoO_3的添加可以提高催化剂低温活性,5.61MoO_3-6CeO-Co_3O_4催化剂低温活性最佳,40℃时CO去除率达98%,耐硫性能达90 min。     

固体超强酸SO42-/MoO3-TiO2制备及催化活性的研究

以硫酸钛、钼酸铵等为原料,采用浸渍法制备固体超强酸SO24-/MoO3-TiO2催化合成乙酸异戊酯。着重探讨了影响制备催化剂活性的条件和催化剂的重复使用活性。结果表明SO24-/MoO3-TiO2最佳制备条件是:在pH为8时,将沉淀陈化24h,用浓度为0.5mol/L硫酸浸渍,在马弗炉中500℃灼烧3小时。此时SO24-/MoO3-TiO2的催化活性最高,催化乙酸异戊酯酯化率达90%左右,同时重复利用性能好。     

负载型MoO3/β分子筛催化剂的丙烷氧化脱氢性能

以β分子筛为载体,通过浸渍法负载钼制备MoO₃/β分子筛催化剂,对其进行XRD、FT-IR、BET和NH₃-TPD物化性能表征,考察其对丙烷氧化脱氢制丙烯的催化性能。结果表明,样品具有活性组分MoO₃存在,也有不同酸性能的活性位存在。负载量对催化剂的表面积和孔径有影响。单纯β分子筛载体有催化活性,不同钼负载量对催化性能有影响,负载钼质量分数为20%时,转化率达到极大值。     

Characterization of hydrogenation active sites on LaCoO3 perovskite

LaCoO₃ becomes active for hydrogenation of ethene upon reduction in hydrogen at temperatures between 300 and 490 °C. Several aspects of the reacting system were studied in order to ascertain the nature of the active sites generated in this manner. Catalyst deactivation was evaluated by comparing rates between two successive experiments. An upper limit was estimated for the amount of polymeric residues formed after a single run: 1.1 ± 0.5 × 10¹⁴ molecules of C₂ per square centimeter. Reduced LaCoO₃ also catalyzed the self-hydrogenation of ethene. When a mixture of C₂H₄:D₂ = 1:1 was reacted over LaCoO₃ reduced to varying extents multiple-exchanged ethenes and ethanes were formed. The exchange patterns were almost unaffected by the extent of reduction. The effect of pretreatment temperatures was also evaluated. The solid in its reduced form was particularly sensitive to high-temperature treatments. The amounts of CO chemisorbed when plotted vs extent of reduction gave curves that were almost identical to the activity plots. The results reported here, discussed in terms of the current literature, are consistent with a model in which finely dispersed Co⁰, formed in the oxide matrix upon reduction, is the locus of hydrogenation activity.     

Catalytic properties in deNOx and SO2–SO3 reactions

SCR-deNO_x reaction and SO₂–SO₃ oxidation tests were carried out by different research groups over fresh and used EUROCAT oxide samples in order to characterize the reactivity of the catalysts and to compare data obtained in several laboratories (Politecnico of Milan, Università of Salerno, ENEL of Milan, Boreskov Insitute of Catalysis).

Data are presented which indicate that the used EUROCAT catalyst is slightly more active both in the deNO_x reaction and SO₂–SO₃ oxidation than the fresh sample.

An analyses of data collected over honeycomb catalysts by means of a 2D, single-channel model of the SCR monolith reactor has been performed to evaluate the intrinsic kinetic constant of the deNO_x reaction; a satisfactory comparison has been obtained between estimation of the intrinsic kinetic constant and estimation of the intrinsic catalyst activity from data collected over powdered catalysts. A good agreement has been found in the experimental results collected in the different labs, both for the deNO_x reaction and SO₂–SO₃ oxidation.     

Catalytic oxidation of naphthalene using a Pt/Al2O3 catalyst

Polycylic aromatic hydrocarbons (PAHs) are listed as carcinogenic and mutagenic priority pollutants, belonging to the environmental endocrine disrupters. Most PAHs in the environment stem from the atmospheric deposition and diesel emission. Consequently, the elimination of PAHs in the off-gases is one of the priority and emerging challenges. Catalytic oxidation has been widely used in the destruction of organic compounds due to its high efficiency (or conversion of reactants), its economic benefits and good applicability.

This study investigates the application of the catalytic oxidation using Pt/γ-Al₂O₃ catalysts to decompose PAHs and taking naphthalene (the simplest and least toxic PAH) as a target compound. It studies the relationships between conversion, operating parameters and relevant factors such as treatment temperatures, catalyst sizes and space velocities. Also, a related reaction kinetic expression is proposed to provide a simplified expression of the relevant kinetic parameters.

The results indicate that the Pt/γ-Al₂O₃ catalyst used accelerates the reaction rate of the decomposition of naphthalene and decreases the reaction temperature. A high conversion (over 95%) can be achieved at a moderate reaction temperature of 480 K and space velocity below 35,000 h⁻¹. Non-catalytic (thermal) oxidation achieves the same conversion at a temperature beyond 1000 K. The results also indicate that Rideal–Eley mechanism and Arrhenius equation can be reasonably applied to describe the data by using the pseudo-first-order reaction kinetic equation with activation energy of 149.97 kJ/mol and frequency factor equal to 3.26 × 10¹⁷ s⁻¹.     

Fe/Al_2O_3催化剂催化氧化兰炭废水

采用浸渍法制备Fe/Al_2O_3催化剂,采用BET、XRD和穆斯堡尔谱等进行结构和性能表征。以自制Fe/Al_2O_3为催化剂,应用催化湿式过氧化氢氧化技术处理COD为6 742 mg·L-1的兰炭废水,通过建立正交实验确定最佳实验条件,结果表明,在p H=4、过氧化氢添加量9.6 m L、反应时间150 min和反应温度80℃条件下,兰炭废水COD去除率达66.30%。对催化氧化后的废水进行GC-MS分析,确定最终氧化产物主要为乙酸。表明自制Fe/Al_2O_3催化剂具有优良的催化效果,并使大分子难降解有机污染物分解为易生化的小分子污染物,甚至被完全分解矿化。     

Catalytic reactions of C1 and C3 hydrocarbons in the presence of oxygen and oxygen-containing compounds

The reaction rates and selectivity of lower olefines formation in high temperature reactions, such as catalytic methane dehydrodimerization and catalytic pyrolysis of C₃ hydrocarbons are substantialy enhanced by oxygen and oxygen-containing compounds.     

Catalytic oxidation of hydrocarbons over Co3O4 catalyst prepared by CVD

The total oxidation of C₂H₂, C₃H₆, C₃H₈, n-C₄H₈, n-C₄H₁₀ and i-C₄H₁₀ was studied in a monolithic flow reactor under temperature-programmed mode and highly diluted conditions. The non-catalytic combustion of the investigated hydrocarbons leads to a substantial formation of CO and traces of methane at intermediate temperatures. This drawback was suppressed upon the deposition of a thin layer of Co₃O₄ on the monolith and all investigated hydrocarbons tend to light off at 250–290 °C. The apparent activation energy was found to exhibit a linear correlation with the C–H bond dissociation energy, indicating that the C–H activation is still the rate-limiting step.     

Oxidation activity and acidity of MoO3P2O5 catalysts

The oxidations of 1-butene, butadiene, and acetic acid in a large excess of air were carried out over various MoP oxide catalysts with different P₂O₅ contents and at different calcination temperatures. The oxidation activities were compared with the dehydration activity for isopropyl alcohol, which was used as a measure of the acidity of the catalysts. The acidity of the catalysts increases with an increase in the P₂O₅ content and attains a maximum at about $\text{P}\text{Mo}\text{= 0.1}$. With a further increase in P₂O₅, it decreases. P₂O₅ alone is completely inactive as a catalyst in the reactions. The acidity drops markedly with an increase in the calcination temperature. A good linear relationship is obtained between the dehydration activity and the oxidation activity for butene and butadiene, as well as the isomerization activity for butene. The activity per unit surface area for the oxidation of acetic acid is not affected by the P₂O₅ content. The results are discussed from the viewpoint of the acid-base nature of the reactants and the catalyst.     

Propene oxidation over MoO3 film deposited on an Au|YSZ|Ag system

Propene oxidation was carried out with an electrochemical reactor, MoO₃/Au|YSZ|Ag (YSZ: 8 mol.-% yttria-doped ZrO₂), at 475°C under oxygen pumping, and the catalytic activity of the thin MoO₃ film related to its crystal morphologies was discussed. The thin MoO₃ film was deposited on an Au anode by means of vacuum deposition or sputtering method at room temperature or 300°C. Each method showed a characteristic texture as well as crystal morphology of the MoO₃ film, resulting in variations in the catalytic activity. Sputtering at 300°C gave porous films composed of leaf-like crystals with preferential orientation of (010) plane parallel to the pore channel and perpendicular to the Au surface, resulting in the highest activity for acrylaldehyde production. A relatively high step density was observed on the oriented (010) plane of the leaf-like crystal. The high activity of this MoO₃ film is probably due to the high density of active sites for the partial oxidation of propene and also to their highly porous structure, which is favorable to the surface migration of oxygen to the reduced active sites.     

Catalytic dissociation of H2O by SrTiO3(100) step sites

The adsorption of H₂O on fractured and planar surfaces of SrTiO₃(100) has been studied using photoemission spectroscopy. Valence region spectra evidence molecular adsorption on the planar surface and dissociative adsorption on the fractured surface. Features arising from surface Sr (Ti) atoms on the SrO (TiO₂) terraces of SrTiO₃(100) have been resolved in core-level photoemission spectra: the surface Sr 3d (Ti 3s) features lie to lower (higher) initial energy of the bulk-derived peaks by ca. 1.0 eV (1.7 eV). These results are consistent with the expected enhancement of covalent bonding in the TiO₂-terrace surface. From the valence and core level spectra we deduce that step-sites, which connect the two types of terrace, act as catalytic centres for H₂O dissociation.