Reactivity of metal oxides: Thermal and photochemical dissolution of MO and MFe2O4 (M=Ni, Co, Zn)

Dissolution rates of NiO, CoO, ZnO, α-Fe₂O₃ and the corresponding ferrites in 0.1 mol dm⁻³ oxalic acid at pH 3.5 were measured at 70 °C. The dissolution of simple oxides proceeds through the formation of surface metal oxalate complexes, followed by the transfer of surface complexes (rate-determining step). At constant pH, oxalate concentration and temperature, the trend in the first-order rate constant for the transfer of the surface complexes (k_Me; Me=Ni, Co, Zn, Fe) parallels that of water exchange in the dissolved metal ions (k_−w). Thus, the most important factor determining the rates of dissolution of metal oxides is the lability of Me-O bonds, which is in turn defined by the electronic structure of the metal ion and its charge/radius ratio. UV (384 nm) irradiation does not increase significantly the dissolution rates of NiO, CoO and ZnO, whereas hematite is highly sensitive to UV light. For ferrites, the reactivity order is ZnFe₂O₄>CoFe₂O₄?NiFe₂O₄. Dissolution is congruent, with rates intermediate between those of the constituent oxides, Fe₂O₃ and MO (M=Co, Ni, Zn), reflecting the behavior of very thin leached layers with little Zn and Co, but appreciable amounts of Ni. The more robust Ni²⁺ labilizes less the corresponding ferrite. The correlation between log k_M and log k_−w is somewhat blurred and displaced to lower k_M values. Fe(II), either photogenerated or added as salt, enhances the rate of Fe(III) phase transfer. A simple reaction mechanism is used to interpret the data.     

Correlation between structural features and vis-NIR spectra of α-Fe2O3 hematite and AFe2O4 spinel oxides (A=Mg, Zn)

Iron (III) rich pigments MgFe₂O₄ and ZnFe₂O₄ spinel ferrites and α-Fe₂O₃ hematite were synthesized by Pechini route and precipitation process, respectively. The compounds were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy and visible-NIR spectroscopy. Diffuse reflectance spectra were interpreted in regard of structural features of the three oxides in order to correlate absorption bands positions with structural parameters. It has been demonstrated that the two main absorption edges occurring in visible range (400-800 nm) can be attributed to two 2p(O²⁻)→3d(Fe³⁺) charge transfers, the energy being directly linked to the distortion degree of the [FeO₆] octahedra.     

FT-IR、XPS和DFT研究乙酰氧肟酸钠在针铁矿或赤铁矿表面的吸附机理

采用FT-IR、X射线光电子能谱以及基于周期平面波的DFT方法分别研究了乙酰氧肟酸钠(aHA)在针铁矿或赤铁矿表面上的吸附结构,并将计算得到的光电子能谱移动和电荷转移实验得到的XPS结果进行对比.红外结果显示aHA可能以五元环螯合物形式吸附于针铁矿或赤铁矿表面,DFT计算结果表明aHA中氧肟酸基团的两个氧原子与针铁矿(101)或(100)表面上的一个铁原子形成五元环结构,同时氧肟酸基团中的两个氧还相应的与针铁矿表面邻近两个铁原子成键,但氧肟酸基团中的两个氧原子却只与赤铁矿(001)表面上的一个铁原子形成五元环结构.对于针铁矿(101)、(100)和赤铁矿(001)表面上吸附位点对应的铁原子,计算得到的光电子能谱移动与实验得到的光电子能谱移动基本相等,说明了计算结果的可靠性和所得吸附结构的合理性.     

α-Fe2O3形貌对Au/α-Fe2O3催化一氧化碳氧化反应性能的影响

负载型纳米金催化剂由于其独特的化学性质在一系列氧化反应中受到广泛关注.其中,一氧化碳氧化不仅在实际应用领域(如汽车尾气处理)发挥重要作用,而且作为一种理想的模型反应用以深入研究和理解催化剂的构效关系.为了获得高效的纳米金催化剂,我们需要把金负载到载体上,载体不仅为金的分散提供必要的表面,而且还会和金产生相互作用,这种金属-载体相互作用对金的氧化态,金颗粒大小及其热稳定性均有重要影响.金属氧化物是负载金最常用的载体.为了提高纳米金催化剂的性能,需要调变金属氧化物的性质.常用的策略是调控金属氧化物的组成、晶相以及晶粒大小.此外,对金属氧化物的形貌进行精细调控也是一种重要的方法,因为具有不同形貌的氧化物可能会暴露出不同的晶面,而且可能具有不同的缺陷位点.α-Fe₂O₃是一种热稳定性强而且对环境友好的载体,可是有关其形貌对负载金催化剂在一氧化碳氧化反应中性能影响的研究尚不充分.因此,本文采用水热法合成了具有纳米球和纳米棒两种形貌的氧化铁,并采用沉积-沉淀的方法将金纳米颗粒负载于其表面.高分辨透射电镜照片显示,和氧化铁纳米球(α-Fe₂O₃(S))相比,氧化铁纳米棒(α-Fe₂O₃(R))的表面更为粗糙,具有更多的缺陷位点.Au和α-Fe₂O₃(R)之间有更强的金属载体相互作用,导致纳米棒氧化铁上的金纳米颗粒更小而且多呈半球形.相比之下,纳米球氧化铁上的金纳米颗粒较大,多呈球形,且分布不均匀.反应结果表明,Au/α-Fe₂O₃(R)具有更高的一氧化碳氧化活性.对反应后的催化剂进行表征发现,Au/α-Fe₂O₃(R)上金颗粒烧结程度较低,平均粒径从1.5增至2.4 nm,而Au/α-Fe₂O₃(S)上金颗粒烧结较为严重,平均粒径从2.0 nm增加到4.0 nm.氢气程序升温还原结果表明,Au/α-Fe₂O₃(R)具有更强的还原性,这也促进了其催化活性的提高.     

Cobalt oxide and carbon modified hematite nanorod arrays for improved photoelectrochemical water splitting

Given the proper band gap, low cost and good stability, hematite(α-Fe_2O_3) has been considered as a promising candidate for photoelectrochemical(PEC) water splitting, however suffers from the sluggish surface water oxidation reaction kinetics. In this study, a simple dip-coating process was used to modify the surface of α-Fe_2O_3 nanorod arrays with cobalt oxide(CoO_x) and carbon(C) for the improved PEC performance, with a photocurrent density at 1.6 V(vs. reversible hydrogen electrode, RHE) increased from 0.10 mA/cm~2 for the pristine α-Fe_2O_3 to 0.37 mA/cm~2 for the CoO_x/C modified α-Fe_2O_3 nanorods. As revealed by electrochemical analysis, thanks to the synergistic effect of CoO_x and C, the PEC enhancement could be attributed to the enhanced charge transfer ability, decreased surface charge recombination, and accelerated water oxidation reaction kinetics. This study serves as a good example for improving PEC water splitting performance via a simple method.     

Single-crystalline α-Fe2O3 with hierarchical structures: Controllable synthesis, formation mechanism and photocatalytic properties

A dual iron precursors system in a hydrothermal process was developed for controllable fabrication of α-Fe₂O₃ hierarchical structures with different morphologies. Micro-pines, snowflakes and bundles were successfully synthesized simply by tuning the total concentration of the two iron precursors K₄[Fe(CN)₆] and K₃[Fe(CN)₆] and their molar ratio. The obtained α-Fe₂O₃ hierarchical structures were characterized using field-emission scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis. The effect of experimental conditions on the morphologies of the α-Fe₂O₃ crystals was systematically investigated. A possible formation mechanism of different α-Fe₂O₃ hierarchical structures was proposed. Good photocatalytic properties were observed for all the hierarchical structures.     

钼酸根离子钝化层对氧化铁薄膜光电催化氧化水的性能影响研究

本文研究了氧化铁(α-Fe₂O₃)薄膜光阳极的合成及其光电催化分解水产氧的性能. 在合成氧化铁过程中,采用原位和非原位方式引入钼酸根离子,调控氧化铁薄膜的生长模式和表面特性. 实验发现原位引入钼酸根离子会显著影响氧化铁薄膜的形貌以及厚度. 而非原位表面修饰钼酸根离子则会保持氧化铁的纳米棒形貌,并有效提高其光电催化分解水的性能. 文章通过紫外-可见吸收光谱,透射电子显微镜(TEM),扫描电子显微镜(SEM),莫特-肖特基测试(M-S),电化学阻抗(EIS)以及光电催化性能测试等手段对材料的结构和性能进行了研究.     

漫反射光谱在“老挝石”颜色成因上的应用

近来,一种产自老挝的新型印章石(俗称“老挝石”)涌进国内市场,对我国印章石市场造成一定影响,“老挝石”的研究尚处于起步阶段,对其颜色成因的研究更为缺乏。采用漫反射光谱(DRS)结合X射线粉晶衍射(XRD)、红外光谱(FTIR)、X射线荧光光谱(EDXRF)等测试对红色“老挝石”的矿物成分和致色机理进行深入研究。结果显示,“老挝石”的主要矿物成分为地开石,并含有少量高岭石,化学成分中的Fe含量和“老挝石”红色色调呈正相关关系,即颜色越深Fe的含量越高。铁质矿物呈微晶集合体浸染分布于地开石的颗粒间,由于其含量低、粒度细小,常规的微区测试方法无法确认其种属。相比之下,漫反射光谱对微晶铁矿物的鉴定十分有效,对可见光波段漫反射光谱处理得到导数等,在土壤沉积物中已经被用来定量测定针铁矿和赤铁矿。该研究中“老挝石”基体与土壤沉积物均为粘土矿物,可以用漫反射光谱来判定“老挝石”中铁矿物种属。漫反射光谱一阶导数法显示,其谱峰位于565～570 nm,由此确认铁矿物的种属为赤铁矿。微晶赤铁矿分布于“老挝石”矿物颗粒间,使样品产生红色,赤铁矿含量越高,红色调越深。     

Hematite template route to hollow-type silica spheres

Hollow-type silica spheres with controlled cavity size were prepared from Fe₂O₃-SiO₂ core-shell composite particles by selective leaching of the iron oxide core materials using acidic solution. The spherical Fe₂O₃ core particles with a diameter range of 20-400 nm were first prepared by the hydrolysis reaction of iron salts. Next, the Fe₂O₃-SiO₂ core-shell particles were prepared by the deposition of a SiO₂ layer onto the surface of Fe₂O₃ particles using a two-step coating process, consisting of a primary coating with sodium silicate solution and a subsequent coating by controlled hydrolysis of tetraethoxysilicate (TEOS). The Fe₂O₃ core was then removed by dissolving with acidic solution, giving rise to hollow-type silica particles. Scanning electron microscopy clearly revealed that the cavity size was closely related to the initial size of the core Fe₂O₃ particle. According to the cross-sectional view obtained by transmission electron microscopy, the silica shell thickness was about 10 nm. The porous texture of the hollow-type silica particles was further characterized by nitrogen adsorption-desorption isotherm measurements.     

古代陶衣的微区拉曼光谱与电子探针线扫描分析

利用微区拉曼光谱与扫描电镜配置的电子探针等技术,分析了两块陶器样品的陶衣.拉曼光谱的分析结果表明,黑色陶衣的主要显色物质为炭黑,红色陶衣的显色物质为赤铁矿.电子探针线扫描分析的结果表明,红色陶衣中铁的含量明显高于内部坯体.陶器样品剖面的微区拉曼光谱分析可有效地避免样品表面其他掺杂原子的影响.