Effect of small amounts of Al on the surface silanol structure and their correlation to the improved catalytic performances of WOx/SiO2–Al2O3 in the propene self-metathesis

Surface silanol structures, acid properties, and tungsten dispersion of the sol-gel-derived 7W/SiO₂–xAl₂O₃ (x = 0.2–23 wt%) were investigated by means of ²⁹Si, ²⁷Al, and ¹H MAS NMR, NH₃-TPD, in-situ NH₃-IR spectroscopy, XRD, and Raman spectroscopy. The surface silanol structure changed upon Al and tungsten loadings; however, loading of 1 wt% Al₂O₃ appeared to be the threshold for preserving the Si(OH)Al with isolated bridge after impregnation of 7 wt% W. The 7W/SiO₂–1Al₂O₃ (1 wt% Al₂O₃) was also found to exhibit the lowest ratio of Bronsted to Lewis acid with the highest amount of Lewis acid sites and the best catalyst performances in propene self-metathesis at 550 °C in terms of both propene conversion and ethylene/butene selectivity. Despite its low tungsten dispersion, the metathesis activity was correlated well with the higher amount of tungsten carbene species, which were formed on the catalysts containing higher isolated bridge silanol and the presence of higher Lewis acid sites.     

Conductive 2D Metal-organic Framework (Co,NiCo, Ni) Nanosheets for Enhanced Non-enzymatic Detection of Urea

2D metal-organic framework (MOF) has potential applications in electrocatalysis owing to fast mass transfer, charge transfer and large specific surface area. Here, we had prepared three conductive 2D MOF based on Ni, NiCo and Co in a simple and rapid way. The 2D nanostructure of MOF was confirmed by SEM and TEM. The chemical composition was studied by XRD, Raman and XPS spectrum. The electrochemical oxidation and detection was investigated through cyclic voltammetry and current-time method. Their sensing performance for urea was determined by varying oxidation potentials and metal sites. The non-enzymatic Ni-, NiCo- and Co-MOF sensors had good catalytic activity for urea. Compared with NiCo- and Co-MOF, Ni-MOF had a wider linear range (0.5–832.5 μM), high sensitivity (1960 μA mM⁻¹ cm⁻²), low detection limit (0.471 μM), and fast response time. The sensors had well repeatability, reproducibility, and selectivity to specific interfering species. Furthermore, Ni- and NiCo-MOF modified electrode was also applied to detection of milk samples. The results showed that the recovery was satisfactory, which further confirmed the effectiveness of non-enzyme sensor. In general, the highly-sensitive 2D Ni- and NiCo-MOF modified electrode has great potential as nonenzymatic urea sensors for real samples detection in hydrogen energy, clinical diagnostics, and environmental protection, et al.     

提高金属有机骨架材料在二氧化碳热催化转化中的性能:策略及前景展望

近年来,大气中CO2的浓度不断增加,带来全球变暖等一系列严重后果,成为国际社会共同关注的环境问题.将CO2催化转化为高附加值化学品可有效降低其向大气中的排放,同时可实现其资源化利用,符合低碳社会的发展目标.目前,已有多种催化体系实现了CO2向不同化学品的转化.然而,由于CO2自身的热力学稳定性和动力学惰性,这些转化通常需要在苛刻的反应条件和较高能耗下进行.设计开发高效催化体系、实现温和条件下CO2的转化利用引起了工业界和学术界的广泛兴趣.金属有机骨架材料(MOFs)是一类由有机配体和金属中心通过配位键组装而成的有机-无机杂化材料,在很多方面展现出良好的应用性能.由于其结构的多样性、可设计性、高比表面积和多孔性等独特性质,MOFs在催化领域吸引了很多研究者的关注.其中,MOFs作为非均相催化剂在CO2热催化转化中表现出良好的应用前景,已实现多种CO2向高值化学品的转化路径.但这些催化体系也存在一些缺点,如有些MOFs材料在催化反应中稳定性差以及其微孔性对反应中的传质造成限制等.因此,设计稳定的MOFs和MOF-基材料并对其结构进行优化改性,从而在温和条件下实现高效的CO2转化具有重要意义.本文综述了提高MOFs在CO2热催化转化反应中性能的几种策略:(1)对MOFs结构中的配体进行设计,包括具有活性官能团的配体、活性配合物作为配体和引入混合配体设计多元MOF;(2)调节MOFs结构中的金属中心,设计混合金属中心和包含活性金属团簇的金属中心;(3)构筑多级孔MOFs;(4)设计MOF-基的复合材料,包括MOFs作为载体与金属纳米颗粒、活性配合物和聚合物构建复合材料;(5)利用MOFs作为前驱体制备MOF-基衍生物材料,重点阐述了如何增加MOFs作为非均相催化剂的催化活性位点以及在CO2转化反应中各位点之间的协同作用.此外,介绍了原位表征技术在MOF-基材料用于CO2固定和转化中的应用.最后,分析了MOF-基非均相催化材料在CO2热催化转化领域目前面临的问题和挑战,包括MOFs材料结构优化、催化机理研究和规模化制备等方面,并对未来的发展趋势进行了展望.     

KIT-6负载CeO2催化CO2和甲醇合成碳酸二甲酯

采用不同老化温度(80、100、120和150℃)合成了一系列KIT-6载体,并通过浸渍法制备了相应的CeO_2/KIT-6催化剂。结合X射线衍射、N_2物理吸附、NH_3程序升温脱附、CO_2程序升温脱附、透射电子显微镜、傅里叶变换红外光谱和X射线光电子能谱等表征结果,详细考察了老化温度对KIT-6结构以及CeO_2/KIT-6催化剂直接催化CO_2和甲醇合成碳酸二甲酯(DMC)反应活性的影响。结果表明,不同老化温度下制备的KIT-6均保持其独特的三维孔道结构。随着老化温度升高,KIT-6比表面积先增大后减小,当老化温度为100℃时,KIT-6比表面积达到最大(683 m~2·g~(-1))。KIT-6较高的比表面积有利于提高CeO_2分散度,进而提高暴露的活性位点数量,催化活性随催化剂表面中等碱/酸性吸附位数量和Ce~(3+)含量的增加而逐渐提高。其中,CeO_2/100-KIT-6催化剂中CeO_2颗粒尺寸最小(5.9 nm),暴露的活性位数量最高,催化活性最佳。随后,考察了反应温度和压力对CeO_2/100-KIT-6催化活性的影响。随着反应温度提高,催化活性先升高后降低,当反应温度为140℃时,催化活性最高;且催化活性随反应压力的提高而逐渐增加。在反应温度为140℃、压力为6.8 MPa条件下,催化剂经6次循环后,DMC收率由15 mmol·g_(CeO_2)~(-1)逐渐降低至2.8 mmol·g_(CeO_2)~(-1),原因归结为反应过程中CeO_2纳米颗粒发生团聚,使暴露出的活性位数量减少。     

An Aluminophosphate Molecular Sieve with 36 Crystallographically Distinct Tetrahedral Sites

The structure of the new medium‐pore aluminophosphate molecular sieve PST‐6 is determined by the combined use of rotation electron diffraction tomography, synchrotron X‐ray powder diffraction, and computer modeling. PST‐6 was prepared by calcination of another new aluminophosphate material with an unknown structure synthesized using diethylamine as a structure‐directing agent, which is thought to contain bridging hydroxy groups. PST‐6 has 36 crystallographically distinct tetrahedral sites in the asymmetric unit and is thus crystallographically the most complex zeolitic structure ever solved.     

Counting of Oxygen Defects versus Metal Surface Sites in Methanol Synthesis Catalysts by Different Probe Molecules

Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N₂O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N₂O, temperature programmed desorption of H₂, and kinetic experiments. The presence of ZnO dramatically influences the N₂O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu‐ZnO interface and can be used to easily quantify the intensity of Cu‐Zn interaction. N₂O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H₂.     

Effects of acid etching on the structure of PtNi catalyst and total exposed active sites

The integration technology of hydrogen preparation–hydrogen storage not only can utilize hydrogen energy efficiently but also can improve the selectivity of the electrode maximally. In the present work, the structure and composition of the PtNi catalyst was characterized by X-ray diffraction (XRD); and its electrochemical properties, morphology, and surface binding energy were analyzed by cyclic voltammetry (CV) and linear scanning voltammetry (LSV), scanning electron microscopy equipped with energy-dispersive spectrometry (SEM-EDS), and X-ray photoelectron spectroscopy (XPS), respectively. The effects of different acid etching treatments (e.g., etching time, etchant concentration, and etching temperature) on the structure and surface active sites were investigated by the orthogonal experiment. The experimental results reveal that after etching with 0.5 mol/L of perchloric acid for 0.5 h at 60°C, the electrode weight loss of the PtNi catalyst is mainly attributed to the large loss of Ni atoms in film layer. This results in the reduced alloy phase in film layer and the appearance of Pt characteristic diffraction peak. The relative content of Pt on the surface of the film electrode increases significantly, and the total number of active sites also increases correspondingly. The binding energy of Pt4f_7/2 decreases by 0.19 eV, and the number of active sites involved in hydrogen release decreases, indicative of the reduced promotion effect of the PtNi catalyst on hydrogen release.     

Dual Metal Active Sites in an Ir1/FeOx Single-Atom Catalyst: A Redox Mechanism for the Water-Gas Shift Reaction

Herein, we report a theoretical and experimental study of the water-gas shift (WGS) reaction on Ir₁/FeO_x single-atom catalysts. Water dissociates to OH* on the Ir₁ single atom and H* on the first-neighbour O atom bonded with a Fe site. The adsorbed CO on Ir₁ reacts with another adjacent O atom to produce CO₂, yielding an oxygen vacancy (O_vac). Then, the formation of H₂ becomes feasible due to migration of H from adsorbed OH* toward Ir₁ and its subsequent reaction with another H*. The interaction of Ir₁ and the second-neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe³⁺−O⋅⋅⋅Ir²⁺−O_vac to Fe²⁺−O_vac⋅⋅⋅Ir³⁺−O with the involvement of O_vac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir₁/FeO_x for sequential production of CO₂ and H₂.     

基于氮掺杂碳载铁复合物的锌空电池氧阴极催化剂

迫在眉睫的环境和能源问题推动人类探索可行、可靠和可再生的能源技术.锌-空气电池和氢氧燃料电池等器件显示出高能量转换效率,但是仍有许多难题有待克服,例如阴极侧上缓慢的氧还原反应(ORR),以及高昂的成本极大地限制了铂基催化剂在商业上的广泛应用.因此,开发高性能的廉价ORR催化剂具有重要意义.过渡金属碳氮化合物(M-N-C, M=Co, Fe等)成为最有希望替代铂基催化剂的一类材料, M-N-C催化剂可以通过直接热解含有过渡金属、氮和碳物种的前驱体合成.然而热解时金属原子易团聚,多孔结构不能被有效地控制,导致相对较差的催化活性.目前, MOF衍生的催化剂在能源转化和储存技术中得到了广泛的关注,其具有丰富的氮含量、高比表面积和可调的孔道结构等特点.本文报道了一种简便可靠可控的合成铁氮共掺杂碳十二面体纳米结构催化剂的方法,并作为阴极电催化剂用于锌空气电池中,测试结果证实,合成的铁氮共掺杂的纳米碳具有与铂基材料相当的活性和更加优异的稳定性.表面吸附了的邻菲罗啉铁的ZIF-8在碳化过程中,氮基团能够结合铁形成Fe Nx结构单元,因此可得到铁氮共掺杂的电催化剂.粉末X射线衍射,扫描电镜证实ZIF-8的成功合成.经过热解得到的催化剂中Fe Nx或Fe Cx衍射峰较弱,表明样品中铁含量较低,存在部分无定型铁.通过拉曼光谱分析发现,引入的邻菲罗啉在热解过程中诱导了缺陷的形成,所以Fe-NCDNA-0的ID/IG比值明显高于NC.同时ID/IG随着铁含量的增加而减少,这是因为铁可以诱导石墨化,诱导效应随着铁含量的增加而增加.分析氮气吸附-脱附等温线得出,引入邻菲罗啉之后,比表面积增加;而铁的引入因其占据了微孔结构,导致比表面积下降.同时电镜证实Fe-NCDNA-2具有较大的形貌扭曲,使得该材料具有较大的比表面积.系统的电化学研究表明,氮掺杂有利于增强ORR活性,在引入铁之后形成高效的活性中心会进一步提高催化性能.因此, Fe-NCDNA-2在碱性条件下表现出优异的ORR性能.线性扫描伏安法曲线表明,铁氮共掺杂的材料表现出与Pt/C相似的性能,其中Fe-NCDNA-2的半波电位(E1/2)为0.863 V,比商业Pt/C的电位更正(E1/2=0.841 V).同时, Fe-NCDNA-2具有更加优异的稳定性,测试30000 s后的电流保持率为80%(Pt/C:64%).在中性介质中,合成的材料也展示了较高的ORR活性.Fe-NCDNA-2的E1/2=0.715 V,催化30000 s后电流保持率77%,均优于商业Pt/C催化剂.组装的锌空气电池进一步验证其作为氧还原催化剂实际应用的可行性.相比于以Pt/C为催化剂做空气阴极的电池,以Fe-NCDNA-2组装的电池表现出更高的开路电压,更高的功率密度(184 m Wcm^-2),以及更加优异的充放电循环稳定性.该工作也有利于启发研究人员探索类似的氮掺杂过渡金属碳材料在各种催化上的应用.