A study of the ageing and deactivation phenomena occurring during operation of an iron molybdate catalyst in formaldehyde production

Catalyst from a Perstorp Formox formaldehyde plant, operating with high inlet concentration of methanol (10.2 vol.%), was discharged from single tubes of a multi-tube reactor after half the expected lifetime of the catalyst and again after termination of the load. Each tube was filled with two different layers of catalyst. From the inlet of the reactor the first layer was a catalyst mixed with inert rings, which was followed by a second layer of pure catalyst extending from the middle to the outlet of the reactor. Catalyst fractions from the two layers were characterized with various techniques including BET, Fourier transform Raman (FT-Raman) spectroscopy, X-ray diffraction (XRD), elemental analysis with atomic absorption spectroscopy (AAS) and activity measurements. It was found that the surface area of the catalyst in the mixed layer increases during operation while a small decrease is noticeable for the catalyst below in the pure layer. Elemental analysis, XRD and FT-Raman show that during operation of the catalyst there is migration of Mo species from the upper part of the reactor towards the outlet. Activity measurements reveal severe deactivation of the catalyst in the mixed layer. It is concluded that the deactivation primarily is due to formation of volatile species formed by the MoO₃ surface reacting methanol, causing a decrease of the MoO₃/Fe₂(MoO₄)₃ mole ratio in the catalyst. Concerning the catalyst in the pure layer, the condensation of needle-like crystals of MoO₃ mainly occurs on the external surface of the catalyst ring.     

Surface Segregation in Iron Molybdate Catalysts

The high selectivity of iron molybdate catalysts for the production of formaldehyde from methanol is somewhat surprising in view of the very different behaviour of the individual oxides of Fe and Mo for this reaction. The former, on its own, is a complete combustor of methanol, whereas the latter is highly selective. Here we use STEM (scanning transmission electron microscopy) at high resolution to image the surface of small particles of the catalyst and to show that this high selectivity is due to the dominance of the surface region by molybdenum oxide.     

甲醇氧化制甲醛铁钼催化剂活性研究

铁钼催化剂是甲醛工业生产中的重要原材料之一,其活性直接影响到催化剂的使用寿命和装置运行时间的长短。从催化剂的晶相结构原理及工业应用等几个方面对铁钼催化剂的活性进行了详细说明,阐述了催化剂活性从机理到工业化的影响因素,对进行催化剂的活性研究和工业化应用都有较高的参考价值。     

Kinetic study of methanol oxidation on carbon-supported PtRu electrocatalyst

Methanol electrooxidation was investigated on the carbon-supported PtRu electrocatalyst (1:1 atomic ratio) in acid media. X-ray diffraction measurement indicated alloying of Pt and Ru. Cyclic voltammetry of the sample reflects the amount of Ru in the catalyst and its ability to adsorb OH radicals. Tafel plots for the oxidation of 0.02-3 M methanol in the solutions containing 0.05-1 M HClO₄ and in the temperature range 27-40 °C showed reasonably well-defined linear region with the slope of about 115 mV dec⁻¹ at the low currents, irrespective of the experimental conditions employed. Reaction order with respect to methanol was found to be 0.5. A correlation between methanol oxidation rate and pseudocapacitive current of OH adsorption on Ru sites was established. It was proposed that bifunctional mechanism is operative with the reaction between methanol residues adsorbed on Pt sites and OH radicals adsorbed on Ru sites as the rate-determining step.     

Maleic anhydride formation from 1, 3-butadiene oxidation over a vanadia molybdate catalyst

Steady-state and transient studies of the partial oxidation of butadiene to maleic anhydride were performed over a Te-promoted vanadia molybdate catalyst previously used by Fiolitakis et al. (1983) for a study of maleic anhydride production from benzene. Inhibition of the partial oxidation by butadiene was observed. Pathways for CO₂ and maleic anhydride formation are different. False-start behaviour for some products was observed and attributed to conflicting contributions of adsorption and catalyst reduction or oxidation. The reaction mechanism for butadiene differs from that reported from benzene oxidation by Fiolitakis et al. (1983).     

Surfactant-stabilized PtRu colloidal catalysts with good control of composition and size for methanol oxidation

Well-dispersed PtRu/C catalysts were prepared by supporting surfactant-stabilized PtRu hydrosol on carbon followed by a heat-treatment at elevated temperature. The effect of the synthesis conditions and the heat-treatment on the composition and electrcatalytic properties of PtRu/C towards methanol oxidation was systematically investigated. It was found that the pH environment and the reaction temperature could greatly affect the final compositions of Pt and Ru in the PtRu catalysts. Moreover, after a post-heat-treatment process, the electrocatalytic activity of PtRu colloidal catalysts can be much improved, the enhancement of which can be largely explained by the improved alloying formation and the removal of surfactant from catalyst demonstrated by the XRD and XPS analyses, respectively.     

Electrochemical characterization of platinum–ruthenium nanoparticles prepared by water-in-oil microemulsion

The synthesis, physical characterization, decontamination and some electrocatalytic properties of PtRu nanoparticles prepared using the microemulsion method are reported. The nanoparticles are synthesized by reduction with sodium borohydride of H₂PtCl₆ and RuCl₃ in a water-in-oil microemulsion of water/polyethylenglycol-dodecylether (BRIJ^® 30)/n-heptane. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energy dispersive analysis by X-rays (EDAX) experiments were carried out to characterize the single and bimetallic nanoparticles obtained. Cyclic voltammograms (CV) of clean nanoparticles were obtained after a controlled decontamination procedure of their surfaces. CO adsorption–oxidation and methanol electrooxidation were used as test reactions to check the electrocatalytic behaviour of the bimetallic nanoparticles. Pt₈₀Ru₂₀ (nominal atomic composition) nanoparticles are the best electrocatalyst for both CO_ad and methanol oxidation. All these results show that the microemulsion method can be used to produce bimetallic nanoparticles in a very easy way. The method can be very easily scaled-up for industrial use.     

Ultra-long Pt nanolawns supported on TiO2-coated carbon fibers as 3D hybrid catalyst for methanol oxidation

In this study, TiO₂ thin film photocatalyst on carbon fibers was used to synthesize ultra-long single crystalline Pt nanowires via a simple photoreduction route (thermally activated photoreduction). It also acted as a co-catalytic material with Pt. Taking advantage of the high-aspect ratio of the Pt nanostructure as well as the excellent catalytic activity of TiO₂, this hybrid structure has the great potential as the active anode in direct methanol fuel cells. The electrochemical results indicate that TiO₂ is capable of transforming CO-like poisoning species on the Pt surface during methanol oxidation and contributes to a high CO tolerance of this Pt nanowire/TiO₂ hybrid structure.     

Partial oxidation of ethane over K2MoO4/SiO2 and potassium promoted MoO3/SiO2 catalyst

Silica supported K₂MoO₄ and potassium-promoted MoO₃ were used as catalysts for the partial oxidation of ethane in fix-bed continuous-flow reactor at 770–823 K using N₂O as oxidant. The main products of the oxidation reaction were ethylene, acetaldehyde, CO and CO₂. Addition of various compounds of potassium to the MoO₃/SiO₂ greatly enhanced the conversion of ethane and influenced the product distribution. The highest rate and selectivity for acetaldehyde formation was found on a K₂MoO₄/SiO₂ catalyst.     

Alcohol Reactivity on Zeolites and Molecular Sieves

Zeolites and molecular sieves have been extensively studied and applied in different fields. Their specific crystalline structure and composition make it possible to obtain high-activity stable catalyst systems for a wide range of chemical reactions. There are many investigations on the structural characteristics and adsorption and catalytic properties of zeolites and molecular sieves as well as on the relation between the different properties. The nature of active sites in zeolite catalysts and the mechanism of catalytic processes on their surface belong to the most important problems of the investigations.     

利用废钼催化剂生产钼酸钠的研究

叙述了利用废钼催化剂生产钼酸钠的工艺过程,并就其加碱焙烧过程主要影响因素进行了分析,得到了加碱焙烧的最佳工艺参数。结果表明该工艺稳定可行,产品达到工业级的钼酸钠标准。     

Methanol Selective Oxidation to Formaldehyde over Iron-Molybdate Catalysts

This review deals with the important industrial reaction of formaldehyde manufacture by methanol oxidation over iron molybdate catalysts. Detailed reference is made to the used catalyst, preparation techniques (coprecipitation, sol-gel like, mechanical mixing, etc.) including unsupported and supported catalysts, promoters and characterization methods. The controversial active phase assignment (stoichiometric versus Mo rich iron molybdate) is discussed. The proposed reaction mechanisms and kinetic laws for the main and side reactions are examined. The catalyst deactivation processes are reviewed and the role of Mo excess on these processes is underlined. Finally conclusions and perspectives are presented.     

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.     

Effect of heat treatment on stability of gold particle modified carbon supported Pt-Ru anode catalysts for a direct methanol fuel cell

Carbon supported Au-PtRu (Au-PtRu/C) catalysts were prepared as the anodic catalysts for the direct methanol fuel cell (DMFC). The procedure involved simple deposition of Au particles on a commercial Pt-Ru/C catalyst, followed by heat treatment of the resultant composite catalyst at 125, 175 and 200 °C in a N₂ atmosphere. High-resolution transmission electron microscopy (HR-TEM) measurements indicated that the Au nanoparticles were attached to the surface of the Pt-Ru nanoparticles. We found that the electrocatalytic activity and stability of the Au-PtRu/C catalysts for methanol oxidation is better than that of the PtRu/C catalyst. An enhanced stability of the electrocatalyst is observed and attributable to the promotion of CO oxidation by the Au nanoparticles adsorbed onto the Pt-Ru particles, by weakening the adsorption of CO, which can strongly adsorb to and poison Pt catalyst. XPS results show that Au-PtRu/C catalysts with heat treatment lead to surface segregation of Pt metal and an increase in the oxidation state of Ru, which militates against the dissolution of Ru. We additionally find that Au-PtRu/C catalysts heat-treated at 175 °C exhibit the highest electrocatalytic stability among the catalysts prepared by heat treatment: this observation is explained as due to the attainment of the highest relative concentration of gold and the highest oxidation state of Ru oxides for the catalyst pretreated at this temperature.     

Partial Oxidation of Propane over Rubidium-Promoted MoO3/SiO2 Catalyst

Silica-supported Rb₂MoO₄ and rubidium-promoted MoO₃ were used as catalysts for the partial oxidation of propane in a fixed-bed continuous-flow reactor at 770–823 K using N₂O as oxidant. The main hydrocarbon products of the reaction were propylene, ethylene, propanal and methane. Addition of various compounds of rubidium to the MoO₃/SiO₂ greatly enhanced the conversion of propane and promoted the formation of propanal at the expense of ethylene and propylene. The highest yield for the production of this compound was found over Rb₂MoO₄/SiO₂ catalyst.     

Precursor-Derived Si-B-C-N Ceramics: Oxidation Kinetics

The oxidation behavior of three precursor-derived ceramics—Si_4.46BC_7.32N_4.40 (AMF2p), Si_2.72BC_4.51N_2.69 (AMF3p), and Si_3.08BC_4.39N_2.28 (T2/1p)—was investigated at 1300° and 1500°C. Scale growth at 1500°C in air can be approximated by a parabolic rate law with rate constants of 0.0599 and 0.0593 μm²/h for AMF3p and T2/1p, respectively. The third material does not oxidize according to a parabolic rate law, but has a similar scale thickness after 100 h. The results show that at least within the experimental times these ceramics develop extremely thin scales, thinner than pure SiC or Si₃N₄.     

Methanol dehydrogenation in the liquid phase with Cu-based solid catalysts

Studies of methanol dehydrogenation in the liquid phase with Cu-based solid catalysts have shown (i) the product (methylal) is totally different from that in the gas-solid phase reaction (methyl formate) even using the same catalyst, and (ii) the differences in the calcination atmosphere of the catalyst (N₂ versus dry air) have a marked effect on the product selectivity (methylal versus methyl formate). These results are in contrast to the gas-phase reaction (com-monly methyl formate). he results are discussed on the basis of surface species characterized with XPS and XRD.     

以合成气合成甲醇催化剂及其进展

本文概念了甲醇合成催化剂以及新型催化剂的研究开发状况,介绍了各类甲醇合成催化剂,锌铬催化剂,铜基催化剂,耐硫催化以及目前国际上研究最热的液相甲醇合成催化剂,并论述了常用催化剂优缺点。     

On the junction effect theory of activity in methanol synthesis catalysts

The junction effect theory which has been propounded to account for the activity in methanol synthesis of oxide supported copper catalysts has been examined in the light of published data on the mechanism of methanol synthesis on copper/zinc oxide/alumina catalysts. The absence of a formate species on the surface of the zinc oxide component of the catalyst after methanol synthesis (the formate species has been shown to be the pivotal intermediate in methanol synthesis on zinc oxide) precludes its involvement in the reaction and negates the applicability of the theory to the copper/zinc oxide/alumina system.