Insights into surface reactivity: formic acid oxidation on Cu(110) studied using STM and a molecular beam reactor

Using a combination of STM and molecular beam reactor data we summarise some important features of a model reaction (formic acid oxidation on Cu(110)) which is of general significance to surface reactivity and to catalysis. Three such features are highlighted here. The first concerns the role of weakly held species (possibly physisorbed) in surface reactions. These species, although of very short lifetime on the surface, can, nevertheless, diffuse over long distances to “find” a sparse distribution of active sites. Thus a very low coverage of oxygen on the surface of Cu(110) increases the sticking probability of all the formic acid molecules which strike the surface to high value (0.82), even though the clean surface is relatively unreactive. The important concept here is the “diffusion circle” or “collection zone” which represents the area of surface visited by the molecule in its short sojourn in the weakly held state. The second theme concerns the concept of the “flexible surface”. We show that the involvement of surface atoms in reactions directs the structure and reactivity for a particular reaction. For formic acid oxidation the liberation of Cu atoms during the removal of oxygen as water leads to gross restructuring of the surface and can lead to “compression” of one reactant (the oxygen in this case) into a lower area, higher local coverage, unreactive state (the c(6×2) oxygen structure). Thirdly, and finally, it is proposed that, for many surface reactions, the surface acts in an analogous way to a solvent, supporting a “dissolved” (highly mobile and fluxional) phase of intermediates at low coverage, which crystallise out above a critical coverage (the 2D “solubility limit”). This revised version was published online in July 2006 with corrections to the Cover Date.     

甲醇选择氧化一步法制甲缩醛催化体系研究进展

介绍了甲醇选择氧化一步法制甲缩醛的反应机理,研究了甲醇选择氧化一步法制甲缩醛催化体系的进展情况,详述了贵金属、杂多酸、钒基及其他催化体系.指出甲醇在催化剂作用下选择氧化一步法制甲缩醛工艺流程短,有利于降低生产成本,极具应用前景.最后对甲醇催化氧化一步法制甲缩醛提出了一些建议.     

Synthesis and characterization of nanocrystalline Mo–V–W–Fe–O mixed oxide catalyst and its performance in selective methanol oxidation

A mixed oxide catalyst containing Mo, V, W and Fe with the composition of 63, 23, 09 and 06 wt% respectively for the selective oxidation of the methanol to formaldehyde is in reported in this paper for the first time. The characterization of the catalyst was done using BET surface analysis, X-ray diffraction (XRD), Infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and Energy dispersive X-ray (EDX). The mixed oxide after calcination at 673 K in N₂ which was subjected for the thermal activation in N₂flow at 813 K was used for the methanol selective oxidation. The thermal treatment shows enhanced catalytic performance. Thermal activation of the nanocrystalline Mo_0.63V₂₃W_0.09Fe₀.₀₆O _x precursor oxide in nitrogen atmospheres induces partial crystallization of a Mo₅O₁₄-type oxide only in a narrow temperature range up to 813 K. XRD showed that the thermally activated mixed oxide consists of a mixture of a majority of crystalline Mo₅O₁₄-type oxide and of small amounts of crystalline MoO₃-type and MoO₂-type oxides. The structural analysis suggests that the improvement of the catalytic performance of the MoVWFe oxide catalyst in the selective oxidation of methanol is related to the formation of the catalytic active site such as Mo₅O₁₄-type mixed oxide.     

甲烷低温活化制甲醇研究进展

简要评述了当前国内外在甲烷直接部分氧化合成甲醇和甲烷经卤代后合成甲醇领域的研究进展。众多研究表明,甲烷多相催化氧化制甲醇是最具有工业化应用前景的技术,其中钼系催化剂的效果较好;以Pt、Pd配合物为催化剂的甲烷液相催化氧化法制甲醇方法具有较高的甲烷转化率和甲醇选择性,但是催化剂价格昂贵、反应条件苛刻、环境污染严重;甲烷气相均相氧化属于非催化反应,在高温高压下进行,且反应器须具备特殊的形式,结构复杂;虽然酶催化氧化法的选择性较高,但是生物酶的制备有一定难度,不适用于大范围应用,该路线目前仅限于实验室研究阶段;光催化氧化是一种环境友好的技术,反应条件温和、避免氧化剂应用,但是该类催化剂局限于半导体和盐类,还有待于进一步开发新型高效的催化剂。同时,甲烷经卤代后合成甲醇的方法对解决甲烷活化的难题具有独特优势,是一种比较有开发前景的路线,正在受到研究者们的密切关注。     

The difference in the active sites for CO2 and CO hydrogenations on Cu/ZnO-based methanol synthesis catalysts

The effect of Zn in copper catalysts on the activities for both CO₂ and CO hydrogenations has been examined using a physical mixture of Cu/SiO₂+ZnO/SiO₂ and a Zn-containing Cu/SiO₂ catalyst or (Zn)Cu/SiO₂. Reduction of the physical mixture with H₂ at 573–723 K results in an increase in the yield of methanol produced by the CO₂ hydrogenation, while no such a promotion was observed for the CO hydrogenation, indicating that the active site is different for the CO₂ and CO hydrogenations. However, the methanol yield by CO hydrogenation is significantly increased by the oxidation treatment of the (Zn)Cu/SiO₂ catalyst. Thus it is concluded that the Cu–Zn site is active for the CO₂ hydrogenation as previously reported, while the Cu–O–Zn site is active for the CO hydrogenation.     

Effects of conductive polyaniline (PANI) preparation and platinum electrodeposition on electroactivity of methanol oxidation

A modified galvanostatic method, termed the ‘pulse galvanostatic method’ (PGM) was used to synthesize nanofibular polyaniline (PANI). In contrast to granular PANI prepared by the conventional galvanostatic method (GM), nanofibular PANI has better conductivity and higher specific surface area. The nanofibular PANI electrode modified by Pt microparticles, at the same Pt loading, exhibits a considerably higher electrocatalytic activity on the methanol oxidation than that of the granular PANI electrode modified by Pt microparticles. Furthermore, the PGM method can be used as a good method for Pt microparticle electrodeposition. The composite electrode composed of PANI and Pt microparticles has the best electrocatalytic activity in the experimental range. The effects of Pt loading and methanol concentration, on the electrocatalytic activity for methanol oxidation have also been researched.     

Enhancement in methanol oxidation by spontaneously deposited ruthenium on low-index platinum electrodes

Using Auger electron spectroscopy and electrochemistry we confirm that ruthenium is spontaneously deposited on platinum single-crystal faces from ruthenium(III) chloride solutions in perchloric acid. The deposit is catalytically active towards methanol present in electrolytic media. On the Pt(111) surface - the most active catalyst for the oxidation process - the data show that the packing density of ruthenium is 9.9±1.5% and increases with ruthenium concentration in the electrolytic bath, confirming the previous electrochemical coverage estimate. In the most favorable instances the ruthenium enhancement factor, the current ratio due to methanol oxidation with and without ruthenium on the surface, is equal to15 (at 0.490 V vs. RHE reference on Pt(111)). We believe this study is an important addition to our long-range efforts in investigating surface structure effects in platinum-ruthenium heterogeneous electrocatalysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

微波法制备的Pt/C催化剂对甲醇和CO的电催化氧化

用微波间断升温法制备了3种Pt/C催化剂,运用循环伏安和线行扫描方法测试甲醇和吸附态CO在不同方法制备的Pt/C催化剂上的电催化氧化情况。发现在酸性溶液中,对于相同Pt载量的Pt(2)和Pt(3)催化剂,Pt(3)具有较小的Pt平均粒径及较高的电催化活性;对于具有较高Pt载量的Pt(1)催化剂,具有最小的平均粒径和最高的电催化活性。     

Controlling reaction selectivity in the oxidation of methanol at Cu(110) surfaces

When deuterated methanol (CD₃OH) and dioxygen mixtures are coadsorbed at Cu(110) surfaces at room temperature and above, the main reaction product can be switched between formaldehyde and formate by changing the mixture composition. For oxygen-rich mixtures formaldehyde is the major product but when methanol is in 5 : 1 excess only formate is produced. It is proposed that the reaction mechanism is controlled by the microscopic structure of the surface: when the reactants are adsorbed sequentially, separate islands of oxygen and methoxy develop which inhibit further oxidation but when reactants are coadsorbed a homogeneous mixture of adsorbates is produced creating a facile pathway to formate. This revised version was published online in July 2006 with corrections to the Cover Date.     

CO oxidation on a model Cu/Rh(100) catalyst

The reaction between carbon monoxide and molecular oxygen on a model Cu/Rh(100) bimetallic catalyst was studied at 455 K using a CO/O₂ = 2 reactant gas mixture at a total pressure of 10.0 Torr. A maximum in the initial activity was observed at a Cu coverage of 1.3 monolayers. However, the Cu overlayers were found to be unstable at the reaction conditions employed in that the Cu films interact strongly with surface oxygen to form three-dimensional CuO_x clusters. The morphological modifications were found to influence markedly the catalytic properties of the surface. However, the initial catalytic activity and surface morphology of the Cu films could be restored by flashing the sample to > 750 K.     

Active sites of H-ZSM5 catalysts for the oxidation of nitric oxide by oxygen

The catalytic activities of H-ZSM5-18, H-ZSM5-150, Li-ZSM5-18, and H-Mag (numerical suffixes mark the Si/Al ratios of zeolites, H-Mag is the proton exchanged form of the layered sodium silicate, magadiite) were compared for the oxidation of NO by O₂ at different ratios of reactants at reaction temperatures from 25 to 600°C. H-ZSM5-18 typically has activity maxima near 25 and 400°C at most O₂/NO reactant ratios. Regardless of the partial pressures of reactants, NO±1/2O₂ NO₂ equilibria are attained at 400°C and above. The H-ZSM5-150 and Li-ZSM5-18 zeolites are only active at temperatures near 25°C. H-Mag is practically inactive at the reaction conditions used. Results indicate that Lewis acidic lattice aluminium ions and silanol hydroxyls are not active in the oxidation of NO to NO₂ over H-ZSM5 zeolites. Brønsted acidic bridging hydroxyls are probably active sites for this reaction at temperatures above 200°C.     

Effects of incorporation of Cu and Ag in Pd on electrochemical oxidation of methanol in alkaline solution

Pd fine particles were prepared by heterogeneous reaction of PdOx with dry methanol as well as by the NaBH4 reduction method. The former method was found to give Pd nanoparticles (~5nm). Similarly f.c.c. structured, single phase nanoparticles of alloy compositions Pd0.8Cu0.2, Pd0.5Cu0.5, Pd0.8Ag0.2 and Pd0.5Ag0.5 were prepared by the heterogeneous reaction of dry methanol with intimate mixtures of PdOx+CuOx and PdOx+AgNO3. The electrochemical properties of the porous unsupported electrodes, prepared from these materials, in alkaline solutions, were investigated by cyclic voltammetry and steady-state polarization measurements. Various processes taking place during potential scanning in the presence and absence of methanol in 6m KOH solution arediscussed. Steady-state polarization data indicate that the methanol oxidation reaction (MOR) activity decreases with incorporation of Cu and Ag into the Pd lattice. The extent of decrease in the MOR activity is less for Cu ad dition than for Ag addition.     

DMFC多孔Pt-Ru阳极甲醇氧化宏观动力学模型化

本文旨在建立和求解DMFC多孔阳极甲醇氧化宏观动力学理论数模。根据Pt-Ru催化剂上双位机理,得到包含CO和OH覆盖率的甲醇水解本征动力学表达式;通过对该多孔阳极微元体积中的物料和电（荷）量衡算,导出了描述电极中浓度和超电势分布的两个耦联的模型方程。经量纲1化后,获得以量纲1变量和准数表示的普遍化宏观动力学数模。该模型包括催化层厚度l、比表面积a、有效扩散系数D_e和有效液相电导率κ_e等工程参数,特别是包括了与动力学参数相关且作为厚度变量函数的CO和OH覆盖率。进而,文中还给出了DMFC多孔阳极效率因子和极化曲线的计算公式。该数模为一非线性二阶微分方程组边值问题,经解耦,可得到两个同解的微分方程。用Newman的BAND（J）程序对方程进行数值求解,并在每一节点计算中嵌入一个计算CO和OH覆盖率的子程序。将模型预测值与甲醇氧化多孔阳极的极化实验数据进行对比发现：当宏观电流密度较低时二者能很好相符;当宏观电流密度较高、CO₂形成气泡的影响变大时,实验值有规律地偏低于预测值。详细的宏观动力学分析表明：提高催化剂Pt位甲醇电分解活性以减少功率损失、优化多孔电极微观和宏观结构以削弱两相流影响,应是改善该阳极性能的重要课题。本工作也可为直接乙醇燃料电池（DEFC）、直接硼氢化物燃料电池（DBFC）阳极的理论分析提供参考。     

Comment on ``Comments on controlling reaction selectivity in the oxidation of methanol at Cu(110) surfaces'

No abstract available. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterisation of active phases of a copper catalyst for methanol oxidation under reaction conditions: an in situ X-ray absorption spectroscopy study in the soft energy range

The oxidation of methanol over copper is investigated by X-ray absorption spectroscopy in the soft X-ray range under reaction conditions. This in situ method allows the surface electronic structure of the catalyst to be correlated with its performance. The correlation reveals information about the catalytic function of various oxygen species on the surface. Oxide and metastable suboxide species affect in distinctly different ways the multiple action of copper as selective or unselective heterogeneous catalyst.     

Methanol synthesis by the hydrogenation of CO2 over Zn-deposited Cu(111) and Cu(110) surfaces

The hydrogenation of CO₂ over Zn-deposited Cu(111) and Cu(110) surfaces was performed at 523 K and 18 atm using a high pressure flow reactor combined with XPS apparatus. It was shown that the ZnO _x species formed on Cu(111) during reaction directly promoted the methanol synthesis. However, no such promotional effect of the Zn was observed for methanol formation on Cu(110). Thus, Zn on Cu(111) acts as a promoter, while Zn on Cu(110) acts as a poison. The activation energy and the turnover frequency are in fairly good agreement with those obtained for Cu/ZnO powder catalysts.     

Quasicatalytic and catalytic selective oxidation of methane to methanol over solid materials: a review on the roles of water

ABSTRACT

To date, although no commercial process for the selective oxidation of methane has been realized, various novel processes with effective solid materials operated at low temperature have been proposed. It is found that the addition of water in any processes not only influences the activity, selectivity, and stability of the solid materials but also affects the extraction efficiency of methanol from the product. Herein, the published results on the roles of water in the methanol production via the quasicatalytic and catalytic selective methane oxidation process using various solid materials in gas and liquid phases at low temperatures are critically reviewed.     

DMFC阳极多元合金催化剂的研究进展

介绍了直接甲醇燃料电池(DMFC)阳极Pt基三元、四元合金催化剂和非Pt基催化剂的制备方法,综述了甲醇催化氧化性能和相关反应机理的发展现状，讨论了DMFC多元合金电催化剂的发展趋势,研究结果表明,不同的合金组成和不同的催化剂载体对阳极催化剂的催化活性有着直接的影响。     

CO oxidation on a Cu(100) catalyst

The oxidation of carbon monoxide by molecular oxygen on a single crystal Cu(100) catalyst was studied at 458 K using reactant gas mixtures with CO/O₂ ratios of 2/1, 10/1 and 25/1 at a total pressure of 10 Torr. The catalytic activities were found to be strongly dependent upon the CO/O₂ ratio. Under stoichiometric reaction conditions (CO/O₂ = 2), the initial CO oxidation activity decreased sharply; with a highly reducing reaction mixture (CO/O₂ = 25), the initial activity gradually increased. These changes in catalytic activities with reactant gas mixture composition correlate with changes in surface composition, namely an increase in the surface oxygen coverage. Post-reaction TPD revealed the presence of a carbonate-like species which decomposed at ca. 630 K.     

Steam Reforming of Methanol Over Cu/CeO2 Catalysts Studied in Comparison with Cu/ZnO and Cu/Zn(Al)O Catalysts

A series of Ce_1-xCu_xO_2- mixed oxides were synthesized using a co-precipitation method and tested as catalysts for the steam reforming of methanol. XRD patterns of the Ce_1-xCu_xO_2- mixed oxides indicated that Cu²⁺ ions were dissolved in CeO₂ lattices to form a solid solution by calcination at 773K when x < 0.2. A TPR (temperature-programmed reduction) investigation showed that the CeO₂ promotes the reduction of the Cu²⁺ species. Two reduction peaks were observed in the TPR profiles, which suggested that there were two different Cu²⁺ species in the Ce_1-xCu_xO_2- mixed oxides. The TPR peak at low temperature is attributed to the bulk Cu²⁺ species which dissolved into the CeO₂ lattices, and the peak at high temperature is due to the CuO species dispersed on the surface of CeO₂. The Ce_1-xCu_xO_2- mixed oxides were reduced to form Cu/CeO₂ catalysts for steam reforming of methanol, and were compared with Cu/ZnO, Cu/Zn(Al)O and Cu/AL₂O₃ catalysts. All the Cu-containing catalysts tested in this study showed high selectivities to CO₂ (over 97%) and H₂. A 3.8wt% Cu/CeO₂ catalyst showed a conversion of 53.9% for the steam reforming of methanol at 513K (W/F = 4.9 g h mol^-1), which was higher than that over Cu/ZnO (37.9%), Cu/Zn(Al)O (32.3%) and Cu/AL₂O₃ (11.2%) with the same Cu loading under the same reaction conditions. It is likely that the high activity of the Cu/CeO₂ catalysts may be due to the highly dispersed Cu metal particles and the strong metalsupport interaction between the Cu metal and CeO₂ support. Slow deactivations were observed over the 3.8wt% Cu/CeO₂ catalyst at 493 and 513K. The activity of the deactivated catalysts can be regenerated by calcination in air at 773K followed by reduction in H₂ at 673K, which indicated that a carbonaceous deposit on the catalyst surface caused the catalyst deactivation. Using the TPO (temperature-programmed oxidation) method, the amounts of coke on the 3.8wt% Cu/CeO₂ catalyst were 0.8wt% at 493K and 1.7wt% at 513K after 24h on stream.