Mechanism of deactivation of iron-molybdate catalysts prepared by coprecipitation and sol-gel techniques in methanol to formaldehyde oxidation

Stoichiometric iron-molybdate and an industrial like catalyst, with the usual Mo excess, were prepared by the normal coprecipitation technique. Additionally, a Mo rich catalyst was prepared by sol-gel like technique. The catalytic stability was tested in the presence and in the absence of water in the reactor feed. Only the stoichiometric catalyst deactivates appreciably in the tested conditions. Water in the reactor feed accelerates this process and exhibits a marked reaction inhibition effect. Water seems to hamper the catalyst reoxidation during reaction. The results of the applied characterisation techniques evidenced that the deactivation mechanism involves an increase of surface reduction (higher Fe²⁺/Fe³⁺ atomic ratio) and surface Mo loss by MoO₃ sublimation and formation of Mo-methanol and Mo-water volatile compounds. Mo species migrate from the bulk to the catalyst surface to compensate the Mo loss favoured by water. Mo excess is in fact required to have a stable, active and selective iron-molybdate catalyst for the methanol to formaldehyde oxidation.     

甲醇氨氧化催化剂的制备及催化性能

铁钼催化剂用于甲醇氨氧化制氰化氢具有能耗低、活性高、工艺简单及副产物少等特点。研究催化剂制备工艺,并对甲醇氨氧化制氰化氢用铁钼催化剂进行活性评价。结果表明,甲醇氨氧化制氰化氢用铁钼催化剂的制备条件为:铁钼原子比为1.5,p H=2,750℃焙烧5 h,在此最佳条件下制备的催化剂上氰化氢收率最高。采用合适的催化剂成型方法可以有效提高甲醇氨氧化制氰化氢用铁钼催化剂利用率和催化剂活性。     

On the Synergy Effect in MoO3–Fe2(MoO4)3 Catalysts for Methanol Oxidation to Formaldehyde

Methanol oxidation to formaldehyde was studied over a series of Fe–Mo–O catalysts with various Mo/Fe atomic ratio and the end compositions Fe₂O₃ and MoO₃. The activity data show that the specific activity passes through a maximum with increase of the Mo content and is the highest for Fe₂(MoO₄)₃. The selectivity to formaldehyde, on the other hand, increases with the Mo content in the catalyst. A synergy effect is observed in that a catalyst with the Mo/Fe ratio 2.2 is almost as active as Fe₂(MoO₄)₃ and as selective as MoO₃. Imaging of a MoO₃/Fe₂(MoO₄)₃ catalyst by SEM and TEM shows that the two phases form separate crystals, and HRTEM reveals the presence of an amorphous overlayer on the Fe₂(MoO₄)₃ crystals. EDS line-scan analysis in STEM mode demonstrates that the Mo/Fe ratio in the amorphous layer is ~2.1 in the fresh catalyst and ~1.7 in the aged catalyst. The enrichment of Mo at the catalyst surface is confirmed by XPS data. Raman spectra give evidence for the Mo in the amorphous material being in octahedral coordination, which is in contrast to the crystalline Fe₂(MoO₄)₃ bulk structure where Mo has tetrahedral coordination. X-ray diffraction (XRD) analysis gives no support for the formation of a defective molybdate bulk structure. The results presented give strong support for the Mo rich amorphous structure being observed on the Fe₂(MoO₄)₃ crystal surfaces being the active phase for methanol oxidation to formaldehyde.     

Investigation of Ternary Catalysts for Methanol Electrooxidation

The electrochemical oxidation of methanol was investigated on Pt–Ru–X ternary metallic catalysts (with X=Au, Co, Cu, Fe, Mo, Ni, Sn or W). The catalysts were prepared by electrochemical deposition and dispersed in a conductive three-dimensional matrix, an electronic conducting polymer, polyaniline (PAni). A comparative study of the behaviour of several ternary catalysts towards the electro-oxidation of methanol shows that PAni/Pt–Ru–Mo is the most efficient anode at potentials up to 500mV vs RHE. This latter ternary electrocatalyst leads to current densities up to 10 times higher than those measured with PAni/Pt–Ru in this potential range. Moreover, the catalyst appears to be stable for potentials lower than 550mV vs RHE. According to EDX analysis, the good behaviour of the Pt–Ru–Mo ternary catalyst seems to result from the presence of a small amount of the third metal, at an atomic ratio close to 5%. This set of encouraging results has also been confirmed by preliminary measurements in a single cell direct methanol fuel cell (DMFC) containing a home made PAni/Pt–Ru–Mo anode. The ternary catalyst leads to higher power densities than the PAni/Pt–Ru binary catalyst under the same experimental conditions.     

Selective Oxidation of Methanol to Formaldehyde Using Modified Iron-Molybdate Catalysts

Methanol selective oxidation to formaldehyde over a modified Fe-Mo catalyst with two different stoichiometric (Mo/Fe atomic ratio = 1.5 and 3.0) was studied experimentally in a fixed bed reactor over a wide range of reaction conditions. The physicochemical characterization of the prepared catalysts provides evidence that Fe₂(MoO₄)₃ is in fact the active phase of the catalyst. The experimental results of conversion of methanol and selectivity towards formaldehyde for various residence times were studied. The results showed that as the residence time increases the yield of formaldehyde decreases. Selectivity of formaldehyde decreases with increase in residence time. This result is attributable to subsequent oxidation of formaldehyde to carbon monoxide due to longer residence time.     

Bi_2W_xMo_(1-x)O_6固溶体制备及光催化降解有机废水

光催化剂可用于催化降解有机污染废水,在废水处理方面具有良好的发展前景。分别以乙二醇、去离子水为溶剂,采用水热法在较低温度下合成Bi_2W_xMo_(1-x)O_6固溶体,并用XRD、SEM等对产物的形貌和结构进行表征,通过对罗丹明B的降解测试固溶体光催化降解活性。结果表明,随着W与Mo原子比(R_((W/Mo)))的增加,Mo~(6+)离子逐步被W~(6+)替代,生成了无限互溶的固溶体,光催化性能也随之发生变化,其中,以去离子水为溶剂合成的固溶体在R_((W/Mo))=1.0时降解率最高,达到98%。     

States of carbon nanotube supported Mo-based HDS catalysts

As HDS catalysts, the supported catalysts including oxide state Mo, Co–Mo and sulfide state Mo on carbon nanotube (CNT) were prepared, while the corresponding supported catalysts on γ-Al₂O₃ were prepared as comparison. Firstly, the dispersion of the active phase and loading capacity of Mo species on CNT was studied by XRD and the reducibility properties of Co–Mo catalysts in oxide state over CNTs were investigated by TPR while the sulfide Co–Mo/CNT catalysts were characterized by XRD and LRS techniques. Secondly, the activity and selectivity of hydrodesulfurization (HDS) of dibenzothiophene with Co–Mo/CNT and Co–Mo/γ-Al₂O₃ were studied. It has been found that the main active molybdenum species in the oxide state MoO₃/CNT catalysts were MoO₂, rather than MoO₃ as generally expected. The maximum loading before formation of the bulk phase was lower than 6%m (calculated in MoO₃). The TPR studies revealed that that active species in oxide state Co–Mo/CNT catalysts were more easily reduced at relatively lower temperatures in comparison to those in Co–Mo/γ-Al₂O₃, indicating that the CNT support promoted the reduction of active species. Among 0–1.0 Co/Mo atomic ratio on Co–Mo/CNT, 0.7 has the highest reducibility. It shows that the Co/Mo atomic ratio has a great effect on the reducibility of active species on CNT and their HDS activities and that the incorporation of cobalt improved the dispersion of molybdenum species on CNT and mobilization. It was also found that re-dispersion could occur during the sulfiding process, resulting in low valence state Mo₃S₄ and Co–MoS_2.17 active phases. The HDS of DBT showed that Co–Mo/CNT catalysts were more active than Co–Mo/γ-Al₂O₃ and the hydrogenolysis/hydrogenation selectivity of Co–Mo/CNT catalyst was also much higher than Co–Mo/γ-Al₂O₃. For the Co–Mo/CNT catalysis system, the catalyst with Co/Mo atomic ratio of 0.7 showed the highest activity, whereas, the catalyst with Co/Mo atomic ratio of 0.35 was of the highest selectivity.     

甲醇氧化制甲醛铁钼催化剂研究

采用共沉淀法制备了不同Mo与Fe原子比的甲醇氧化制甲醛催化剂,在常压固定床反应器上对催化剂进行活性评价,采用BET、XRD和TEM对制备的催化剂进行表征。结果表明, Mo与Fe原子比为2.2~2.8时,催化剂具有较好的活性,(400~450) ℃焙烧的催化剂具有良好的比表面积、适宜的孔容和孔径,形成了较为稳定的MoO₃和Fe2(MoO₄)₃晶相,使催化剂具有更高的活性和选择性。对甲醇氧化制甲醛反应进行研究,结果表明,在反应温度(265~315) ℃,空速(8 500~13 000) h^-1时,甲醇转化率>98%,甲醛收率>93%, 500 h长周期考核,催化剂表现出良好的活性和稳定性。     

钼系多组分催化剂上的丁烯-2氧化脱氢动力学

用玻璃流动循环反应器研究了Bi_（1.5）Fe_（3.0）Ni_（4.5）Co_1Cr_（0.5）Mo_（12）K_（0.3）（原子比）氧化物催化剂（SiO_2载体）上的丁烯-2氧化脱氢动力学,发现丁烯转化速度对丁烯及氧的反应级数m及n与温度及反应物浓度有关,而且m+n≈1。 根据Redox稳态机理可认为: ①催化剂被丁烯还原的速度与还原态催化剂再氧化的速度达到稳态。 ②丁二烯在催化剂的氧化态表面部分上可逆吸附起了阻碍丁烯转化的作用,并导出速度方程为: 计算出催化剂还原步骤的活化能E_1=4.53kcal/g-mol,催化剂再氧化步骤的活化能E_2=21kcal/g-mol（>400℃）及60kcal/g-mol（<400℃）,E_2在400℃附近转折。当丁二烯阻碍作用不显著时（相对于丁烯,丁二烯分压较小,或反应温度较高）方程可以简化为 本文还对常见的钼系催化剂的丁烯转化的活化能发生转折的原因进行了讨论。     

Fe-Mo对二甲苯选择氧化催化剂的制备及其性能

以Fe(NO₃)₃和(NH₄)₆MoO₂₄为原料,采用溶胶-凝胶法制备了不同n(Fe)∶n(Mo)的催化剂,考察了该催化剂对对二甲苯催化氧化合成对苯二甲醛的催化性能。并应用X射线衍射、傅立叶变换红外光谱、紫外漫反射和热分析对催化剂进行了表征。结果表明,Fe对MoO₃改性能明显改善催化剂的活性,提高对苯二甲醛收率,原因是由于Fe₂(MoO₄)₃的生成。当n(Fe)∶n(Mo)=1∶3时,催化剂活性最佳,此时对二甲苯转化率为66.8%,对苯二甲醛选择性和收率分别为37.2%和24.8%。XRD、FT-IR和DRS结果表明,当n(Fe)∶n(Mo)=1∶3时,催化剂中有适量MoO₃存在,其催化活性最好,该催化剂能在高温维持较好的活性稳定性。     

钼铁催化剂上对二甲苯气相选择性氧化合成对苯二甲醛的研究

采用溶胶-凝胶法合成了钼铁催化剂,通过该催化剂,利用空气中的O₂进行对二甲苯选择性氧化,制备出对苯二甲醛。催化剂的活性评价结果表明,在n(Mo)∶n(Fe)∶n(Co)=2.4∶1∶0.02和焙烧温度500 ℃条件下,催化剂的活性最高;反应温度为500 ℃和空速5 500 h^-1时,对苯二甲醛的收率达到59.2%。FT-IR和XRD结果证实,催化剂的活性组分为Fe₂(MoO₄)₃与少量的MoO₃,二者具有协同作用。     

Selective Catalytic Reduction of NOx with Ammonia over Fe‐Mo/ZSM‐5 Catalysts

The Fe‐Mo/ZSM‐5 catalysts were prepared by an impregnation method. A comparison of the catalytic activity for the reduction of NO_x with ammonia over Fe‐Mo/ZSM‐5, Fe‐ZSM‐5 and Mo/ZSM‐5 catalysts was carried out. Also, the effects of the conditions used for calcination as well as of the Fe/Mo ratio on the catalytic performance of Fe‐Mo/ZSM‐5 catalysts were studied. It was found that Fe‐Mo/ZSM‐5 is more active than Fe‐ZSM‐5 and Mo/ZSM‐5 separately. Fe‐Mo/ZSM‐5 exhibited the best performance for SCR reaction with a Fe/Mo ratio of 1.5 and yielded the highest NO_x conversion of 96 % at a temperature of 430 °C. The results also showed that the performance of Fe‐Mo/ZSM‐5 is sensitive to the conditions used during calcination. The bulk phase and surface composition of the Fe‐Mo/ZSM‐5 catalysts were determined by XRD, BET, and XPS techniques, respectively. The results revealed that the surface Mo percentage is the largest when the Fe/Mo ratio is 1.5, which may be related to its higher activity for catalytic reduction of NO_x. XRD results indicate that the best catalytic performance of the Fe/Mo = 1.5 sample results from a strong interaction among Fe, Mo and HZSM‐5. In addition, it can be tentatively presumed that the surface nitrous species from the calcinations play an important role in SCR of NO_x over Fe‐Mo/ZSM‐5 catalysts.     

甲醇氧化制甲醛铁钼催化剂表面结构与活性

利用共沉淀法在不同搅拌速度下制备了相同Mo/Fe原子比的甲醇氧化制甲醛催化剂,采用SEM、XRD和拉曼光谱等对催化剂进行表征,在固定床微反上评价催化剂活性和选择性。结果表明,搅拌速度增大,催化剂比表面积增大,催化活性增强,甲醛收率由600 r·min^-1时的73.8%增加到10000 r·min^-1时的95.7%（280℃）。此外,催化剂由片状的MoO₃和颗粒状的Fe₂（MoO₄）₃两部分组成,游离的片状MoO₃无明显催化活性,只有与Fe₂（MoO₄）₃结合时才具有催化活性。     

Highly active sulfided CoMo catalyst on nano-structured TiO2

High surface area (>300 m² g⁻¹) nano-structured TiO₂ oxides (ns-T) were used as CoMo hydrodesulfurization catalyst support. Cylindrical extrudates were impregnated by incipient wetness with Mo (2.8 Mo at. nm⁻²) and Co (atomic ratio Co/(Co + Mo) = 0.3). Characterization of impregnated precursors was carried out by N₂ physisorption, XRD and atomic absorption and laser-Raman spectroscopies. Sulfided catalysts (400 °C, H₂S/H₂) were studied by X-ray photoelectronic spectroscopy. As indicated by XRD and after various preparation steps (extrusion, Mo and Co impregnation and sulfiding) the nano-structured material was well preserved. XPS analyses showed that Co and Mo dispersion over the ns-T support was much higher than that on alumina. Very high surface S concentration suggested that even ns-T was partially sulfided during catalyst activation. Dibenzothiophene hydrodesulfurization activity (5.73 MPa, 320 °C, n-hexadecane as solvent) of CoMo/ns-T was two-fold to that of an alumina-supported commercial CoMo catalyst. The improvement was even more remarkable in intrinsic pseudo kinetic constant basis. No important differences in selectivity over the catalysts supported on either Al₂O₃ or ns-T were observed, where direct desulfurization to biphenyl was favored. Both Mo dispersion and sulfidability were enhanced on the ns-T support where Mo⁴⁺ fraction was notably increased (100%) as to that found on CoMo/Al₂O₃.     

重芳烃轻质化催化剂n（Ni）/n（Ni+Mo）的优化与分析

以β分子筛为载体,在保持金属总负载量不变的情况下,采用等体积浸渍法制备了4种不同n（Ni）/n（Ni+Mo）的催化剂。分别采用X射线衍射（XRD）、比表面积测试（BET）、氨程序升温脱附（NH₃-TPD）、氢程序升温还原（H₂-TPR）、氢程序升温脱附（H₂-TPD）和热重-差热分析（TG-DTG）等方法对催化剂进行了表征。结果表明,4种催化剂的酸量和酸强度相近,在n（Ni）/n（Ni+Mo）等于基准+0.2时,Mo与载体之间的相互作用最弱,其氢气吸附量最多且积炭量最少;采用某炼厂重整C₁₀⁺ 重芳烃对4种催化剂进行评价,结果表明n（Ni）/n（Ni+Mo）等于基准+0.2催化剂具有最优的催化活性和稳定性。上述结果表明,影响重芳烃轻质化催化剂活性和稳定性的关键因素是催化剂氢气吸附量的多少,氢气吸附量越多金属表面的溢流氢效应越明显,积炭前驱体被溢流氢及时消除,从而保护了催化剂的加氢活性中心不被积炭覆盖,有助于催化剂在较高活性下保持稳定。     

The catalytic activity of Ni/W bimetallic sulfide nanostructured catalysts in the hydrodesulfurization of dibenzothiophene

Two types of catalysts containing NiW bimetallic sulfide nanostructures were prepared by a chemical method employing ammonium thiotungstate and nickel nitrate as metal-sulfide precursors followed by sulfidation in H₂S/H₂ at 400 °C. The nanostructures were grown with excess of Ni, at atomic ratio R = 0.75, 0.85 (R = Ni/Ni + W). High resolution electron microscopy (HRTEM) micrographs revealed the formation of two types of nanostructures, nickel sulfide nanoparticles and long nanorods of tungsten suboxide, both coated by WS₂ layers. The Ni/W catalyst containing mostly nanorods presented twice the catalytic activity (pseudo-zero order constant rate k = 12 × 10⁻⁷ mol/s.g) of the Ni/W catalyst containing nanoparticles (k = 6.3 × 10⁻⁷ mol/s.g) with a low selectivity for tetrahydrodibenzothiophene (THDBT) and high selectivity to cyclohexylbenzene (CHB, 50 mol%). In turn the Ni/W catalyst containing nanoparticles presented a catalytic activity comparable to a Ni/Mo catalyst without inorganic fullerene (IF) nanostructures (k = 7.2 × 10⁻⁷ mol/s.g) but with higher selectivity for hydrogenation to THDBT, (14 mol%) than the sample with nanorods.     

甲醇氨氧化制氢氰酸沸腾床催化剂活性组分及其催化作用的研究

<正>铁钼氧化物不仅是甲醇氧化制甲醛的催化剂,也是甲醇氨氧化制氢氰酸的优良催化剂之一.有关前者的活性组分和催化性能的报道较多,后者报道不多,用于沸腾床上铁钼催化剂未见报道.本文用多种实验方法研究了用于沸腾床的铁钼催化剂活性组分及其催化作用.     

一种新γ相氧化铝负载的铁铝双金属催化剂用于反向水煤气转化反应(英文)

In reverse water gas shift (RWGS) reaction CO₂ is converted to CO which in turn can be used to produce beneficial chemicals such as methanol. In the present study, Mo/Al₂O₃, Fe/Al₂O₃ and Fe-Mo/Al₂O₃ catalysts were synthesised using impregnation method. The structures of catalysts were studied using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) method, inductively coupled plasma atomic emission spectrometer (ICP-AES), temperature programmed reduction (H₂-TPR), CO chemisorption, energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) techniques. Kinetic properties of all catalysts were investigated in a batch reactor for RWGS reaction. The results indicated that Mo existence in structure of Fe-Mo/Al₂O₃ catalyst enhances its activity as compared to Fe/Al₂O₃. This enhancement is probably due to better Fe dispersion and smaller particle size of Fe species. Stability test of Fe-Mo/Al₂O₃ catalyst was carried out in a fixed bed reactor and a high CO yield for 60 h of time on stream was demonstrated. Fe₂(MoO₄)₃ phase was found in the structures of fresh and used catalysts. TPR results also indicate that Fe₂(MoO₄)₃ phase has low reducibility, therefore the Fe₂(MoO₄)₃ phase signifificantly inhibits the reduction of the remaining Fe oxides in the catalyst, resulted in high stability of Fe-Mo/Al₂O₃ catalyst. Overall, this study introduces Fe-Mo/Al₂O₃ as a novel catalyst with high CO yield, almost no by-products and fairly stable for RWGS reaction.     

Preparation of MoS2 and WS2 catalysts by in situ decomposition of ammonium thiosalts

The in situ decomposition of ammonium thiometallates during the hydrodesulfurization (HDS) of dibenzothiophene (DBT), to obtain molybdenum disulfide and tungsten disulfide catalysts, was investigated. It was found that very efficient catalysts for the HDS of DBT were obtained by in situ decomposition. Mechanical uniaxial pressing of the precursors (ammonium thiometallates) affected both textural and catalytic properties of the catalysts. Surface areas of molybdenum and tungsten disulfides increased as a function of uniaxial pressing, while catalytic activities went through a maximum. For MoS₂, the hydrogenation selectivity was much higher for in situ catalysts than for ex situ ones. For WS₂ catalysts, the hydrogenation selectivity was less sensitive to the condition of decomposition (ex situ/in situ). The surface S/M (M = Mo, W) atomic ratio from the Auger signal decreased as a function of uniaxial pressing, while the C/M ratio remained almost constant at 1.6. The best catalyst showed an experimental S/Mo ratio that is slightly higher than the stoichiometric value. The effect of in situ decomposition and mechanical deformation of thiometallate precursors is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.