Unravelling the Enigma of Nonoxidative Conversion of Methane on Iron Single-Atom Catalysts

The direct, nonoxidative conversion of methane on a silica-confined single-atom iron catalyst is a landmark discovery in catalysis, but the proposed gas-phase reaction mechanism is still open to discussion. Here, we report a surface reaction mechanism by computational modeling and simulations. The activation of methane occurs at the single iron site, whereas the dissociated methyl disfavors desorption into gas phase under the reactive conditions. In contrast, the dissociated methyl prefers transferring to adjacent carbon sites of the active center (Fe₁©SiC₂), followed by C−C coupling and hydrogen transfer to produce the main product (ethylene) via a key −CH−CH₂ intermediate. We find a quasi Mars–van Krevelen (quasi-MvK) surface reaction mechanism involving extracting and refilling the surface carbon atoms for the nonoxidative conversion of methane on Fe₁©SiO₂ and this surface process is identified to be more plausible than the alternative gas-phase reaction mechanism.     

Anti-Markovnikov Hydroamination of Racemic Allylic Alcohols to Access Chiral γ-Amino Alcohols

A ruthenium-catalyzed formal anti-Markovnikov hydroamination of allylic alcohols for the synthesis of chiral γ-amino alcohols is presented. Proceeding via an asymmetric hydrogen-borrowing process, the catalysis allows racemic secondary allylic alcohols to react with various amines, affording enantiomerically enriched chiral γ-amino alcohols with broad substrate scope and excellent enantioselectivities (68 examples, up to >99 % ee).     

Preparation of a novel bentonite intercalation composite by changing the chemical state of aluminum

A new environment-friendly bentonite intercalation composite was successfully obtained from alumina sol intercalation. This process used industrial grade pseudo boehmite as the aluminum source and provided with the advantage of zero emission of acidic wastewater. The bentonite intercalation composite was investigated by XRF, ICP, XRD, FT-IR, BET, and pyridine-FTIR. Results indicated that the basal spacing was enlarged from 14.72 to 15.60 Å; the specific surface area increased by 128%; and the total acid content increased from 65.32 to 245.76 μmol/g. The catalytic activity of this composite was tested by alkylation of aromatics with olefins. The results show that the weak Lewis acid generated by extra-framework aluminum and specific surface area play a decisive role, while weak Brønsted acid site is not the active site for this alkylation reaction.     

衬底温度对Ga液滴在Al_(0.4)Ga_(0.6)As表面形成纳米结构的影响

本文研究了不同衬底温度对Ga液滴在Al_(0.4)Ga_(0.6)As表面形成纳米结构的影响,当300℃≤T≤380℃时,Ga液滴演化成纳米孔(Nanohole)和盘状结构(diffusion halo),纳米结构的尺寸随温度升高而增大.当T≥385℃时,盘状结构消失,形成一定平坦的Al_xGa_(1-x)As薄膜,Ga液滴在界面处继续向下刻蚀直至耗尽,形成平均直径为75 nm,平均孔深为5.52 nm的纳米孔.本文还通过盘状结构测出平均扩散长度△R,并拟合出Ga原子在Al_(0.4)Ga_(0.6)As表面的激活能E_A=0.78(±0.01) eV和扩散前因子D_0=0.15(×4.1~(±1))10~(-2 ) cm~2s~(-1).     

Noninvasive Temperature Measurement in Dental Materials Using Nd3+, Yb3+ Doped Nanoparticles Emitting in the Near Infrared Region

In this work, the application of near infrared (NIR)-emitting NaYbF₄:1%Tm³⁺@NaLuF₄:30%Nd³⁺ core–shell nanoparticles is reported for noninvasive probing and monitoring the temperature during photopolymerization of dental materials. When excited at 808 nm, the synthesized nanoparticles emit NIR photoluminescence (PL) with two distinctive peaks at 865 and 980 nm which correspond to radiative transitions from the doped Nd³⁺ and Yb³⁺ ions, respectively. Luminescence intensity ratio between these two bands is found to vary with temperature due to temperature-dependent electronic excitation energy transfer between Nd³⁺ and Yb³⁺ ions at the core/shell interface. This finding allows luminescence ratiometric evaluation of the in situ temperature during photopolymerization of resin cement (doped with nanoparticles) in a veneer placement procedure. In addition, the NIR emission also enables PL imaging of the distribution of the adhesive under the veneer. The results highlight that rare-earth ions–doped nanoparticles with both excitation and emission in the NIR spectral range are advantageous for both PL-based nanothermometry and imaging due to the reduced attenuation of NIR light by dental ceramics.     

基于双向循环神经网络的汉语语音识别*

当前基于深度神经网络模型中,虽然其隐含层可设置多层,对复杂问题适应能力强,但每层之间的节点连接是相互独立的,这种结构特性导致了在语音序列中无法利用上下文相关信息来提高识别效果,而传统的循环神经网络虽然做出了改进,但是只能对上文信息进行利用。针对以上问题,该文采用可以同时利用语音序列中上下文相关信息的双向循环神经网络模型与深度神经网络模型相结合,并应用于语音识别。构建具有5层隐含层的模型,其中第3层为双向循环神经网络结构,其他层采用深度神经网络结构。实验结果表明:加入了双向循环神经网络结构的模型与其他模型相比,较好地提高了识别正确率;噪声对双向循环神经网络汉语识别有重要影响,尤其是训练集和测试集附加噪声类型不同时,单一的含噪声语音的训练模型无法适应不同噪声类型的语音识别;调整神经网络模型中隐含层神经元数量后,识别正确率并不是一直随着隐含层中神经元数量的增加而增加,神经元数量数目增加到一定程度后正确率出现了降低的趋势。     

Ti0.33WO3电子结构和太阳辐射屏蔽性能第一性原理研究

节能减排已成为当今社会发展的主题,对节约能源、提高太阳能的高效综合利用的新型窗用透明隔热材料的理论设计和研究尤其重要.本文采用基于密度泛函理论的计算方法,研究了六方相三氧化钨Ti掺杂前、后的晶格参数、电子能带结构、形成能和光学性质.研究结果表明,Ti掺杂后晶格体积增大,系统能量降为负值,体系具有更好的稳定性;掺杂后电子能带结构发生很大的变化,但材料仍保持n型电导率;随之,其光学性质也发生改变,掺杂前h-WO3无近红外吸收性能,掺杂后的Ti0.33WO3具有很强的近红外吸收性能.在此基础上研究了Ti掺杂h-WO3前、后的太阳辐射屏蔽性能,掺杂前无太阳辐射屏蔽性能;掺杂后的Ti0.33WO3薄膜具有可见光高透明、近红外屏蔽的性能.计算结果为Ti掺杂h-WO3在窗用透明隔热材料方面的研究提供了理论依据.     

Laser stripe extraction in additive manufacturing based on spatiotemporal noise regularization

Optical Review - The optical three-dimensional (3D) measurement technique based on the laser stripe has become increasingly important in additive manufacturing, which is necessary to extract the...     

利用新颖高效气体发生装置以氯气氧化法制备高锰酸钾实验设计及效果*

基于一种新颖高效气体发生装置,设计了以氯气为氧化剂,理论产率接近100%的将锰酸钾高效转化为高锰酸钾的实验方案。实验结果表明:使用本设计制备高纯度高锰酸钾,既可保证高锰酸钾的理论转化率,减少歧化反应副产物对纯度的影响,同时得到的副产品次氯酸钠及氯水可用做无机教学实验的基础试剂,符合“循环经济”指导思想。