Monitoring Cr(Ⅵ)photoreduction at different depths by operando low-field NMR relaxometry

Chromium(VI)(Cr(VI)),a toxic metal ion,is widely present in industrial wastewater.To reduce the contamination of Cr(VI),many technologies for the photocatalytic reduction of Cr(VI)to Cr(III)have been developed in the past decades.However,the practical application of photocatalysts for the reduction of Cr(VI)inwastewater treatment is often hindered by the complicated photoreduction processes due to the sedimentation and stratification of catalyst particles that present during the treatment of the wastewater.Probing and understanding the influences of the sedimentation and stratification of the catalyst particles on the photoreduction processes are long-term challenges in the field.Herein,we demonstrate that this issue can be solved by using layer location integrated low-field time-domain nuclear magnetic resonance(LF-NMR)relaxometry.With paramagnetic Cr(III)cation as the molecular probe,we successfully monitored the Cr(VI)photoreduction processes by operando probing the 1 H T2 relaxation time of the photoreduction systems.The influences of catalyst sedimentation and the light wavelength on photocatalysis were studied and discussed.The results showed the great potential of LF-NMR relaxometry in the study of Cr(VI)photoreduction processes during industrial wastewater treatments.

A machine learning framework for low-field NMR data processing

Low-field(nuclear magnetic resonance)NMR has been widely used in petroleum industry,such as well logging and laboratory rock core analysis.However,the signal-to-noise ratio is low due to the low magnetic field strength of NMR tools and the complex petrophysical properties of detected samples.Suppressing the noise and highlighting the available NMR signals is very important for subsequent data processing.Most denoising methods are normally based on fixed mathematical transformation or handdesign feature selectors to suppress noise characteristics,which may not perform well because of their non-adaptive performance to different noisy signals.In this paper,we proposed a“data processing framework”to improve the quality of low field NMR echo data based on dictionary learning.Dictionary learning is a machine learning method based on redundancy and sparse representation theory.Available information in noisy NMR echo data can be adaptively extracted and reconstructed by dictionary learning.The advantages and application effectiveness of the proposed method were verified with a number of numerical simulations,NMR core data analyses,and NMR logging data processing.The results show that dictionary learning can significantly improve the quality of NMR echo data with high noise level and effectively improve the accuracy and reliability of inversion results.

Low-field NMR inversion based on low-rank and sparsity restraint of relaxation spectra

In this paper,we proposed a novel method for low-field nuclear magnetic resonance(NMR)inversion based on low-rank and sparsity restraint(LRSR)of relaxation spectra,with which high quality construction is made possible for one-and two-dimensional low-field and low signal to noise ratio NMR data.In this method,the low-rank and sparsity restraints are introduced into the objective function instead of the smoothing term.The low-rank features in relaxation spectra are extracted to ensure the local characteristics and morphology of spectra.The sparsity and residual term are contributed to the resolution and precision of spectra,with the elimination of the redundant relaxation components.Optimization process of the objective function is designed with alternating direction method of multiples,in which the objective function is decomposed into three subproblems to be independently solved.The optimum solution can be obtained by alternating iteration and updating process.At first,numerical simulations are conducted on synthetic echo data with different signal-to-noise ratios,to optimize the desirable regularization parameters and verify the feasibility and effectiveness of proposed method.Then,NMR experiments on solutions and artificial sandstone samples are conducted and analyzed,which validates the robustness and reliability of the proposed method.The results from simulations and experiments have demonstrated that the suggested method has unique advantages for improving the resolution of relaxation spectra and enhancing the ability of fluid quantitative identification.

Probing Development of Mierostrueture of Early Cement Paste using1H Low-field NMR

Development of microstructure of early cement paste （0-6 h） was investigated with 1H low- field NMR. It was found that T2 （transverse relaxation time） distributions of fresh cement paste were bimodal and two peaks were ＇long component＇ and ＇short component＇. Separation degree of two peaks was a sign of exchange of water within flocculation and outside flocculation. Factors such as water cement ratio, specific surface area and dosage of superplasticizer had influences on the separation degree： the separation degree increased with the water cement ratio; the separation degree of cement paste prepared with cement with a high specific surface area was zero; dosage of superplasticizer will decrease separation degree. Results also suggested that T2 distribution gradually moved to the left and T2 of long component and initial fluidity were linearly correlated.

Probing Development of Microstructure of Early Cement Paste using ~1H Low-field NMR

Development of microstructure of early cement paste (0–6 h) was investigated with 1H low-field NMR. It was found that T 2 (transverse relaxation time) distributions of fresh cement paste were bimodal and two peaks were ‘long component’ and ‘short component’. Separation degree of two peaks was a sign of exchange of water within flocculation and outside flocculation. Factors such as water cement ratio, specific surface area and dosage of superplasticizer had influences on the separation degree: the separation degree increased with the water cement ratio; the separation degree of cement paste prepared with cement with a high specific surface area was zero; dosage of superplasticizer will decrease separation degree. Results also suggested that T 2 distribution gradually moved to the left and T 2 of long component and initial fluidity were linearly correlated.

In-situ Monitoring of Hydration Kinetics of Cement Pastes by Low-field NMR

Low field NMR technique was applied to investigate the hydration of cement pastes with different water to cement ratios or addition of superplasticizer. As a nondestructive method, this technique can be used to monitor the hydration kinetics process by following the changes of longitudinal relaxation time （T1） of water constrained in the pastes. The experimental results indicate that the T1 distributions of water in the fresh paste normally exhibite bimodal distribution, where the large peak is corresponding to the free water while the small one is contributed by the water stored in the flocculations. Time dependence of the weighted average T1 has a good agreement with the hydration process and could be divided into four stages, i e, initial period, dormant period, accelerated period and steady period. The hydration mechanism of each stage was described based on the theory of cement chemistry. In addition, the total signal intensity, which is proportional to the content of the physically bound water in the samples, decrease successively during the hydration reflecting the consumption of physically bound water by hydration reactions.

Analysis of Internal Moisture Change of Camphor Seeds during Drying by Low-Field NMR

In this study,low-field nuclear magnetic resonance(LF-NMR)was used to collect the transverse relaxation signals of camphor seeds that had been dried at different temperatures and for different durations.The transverse relaxation time of the hydrogen nucleus was obtained by SIRT(Simultaneous Iterative Reconstruction Technique)inversion,and the correlation between the moisture content on dry basis and the amplitude of NMR signal of camphor seeds sampled at different drying stages was analyzed.The results showed that water existed in three main forms strongly bound water,weakly bound water and free water in camphor seeds.During the drying process,the peak position of strongly bound water remained unchanged,but the peak positions of weakly bound water and free water fluctuated.Drying rate increased with drying temperature increasing.In addition,there was a very significant linear relationship between the total NMR signal amplitude and moisture content on dry basis,and the correlation coefficient reached 0.984 4.

Time-Domain Frequency Correction Method for Averaging Low-Field NMR Signals Acquired in Urban Laboratory Environment

Using a second-order helium-cooled superconducting quantum interference device gradiometer as the detector,ultra-low-field nuclear magnetic resonance(ULF-NMR)signals of protons are recorded in an urban environment without magnetic shielding.The homogeneity and stability of the measurement field are investigated.NMR signals of protons are studied at night and during working hours.The Larmor frequency variation caused by the fluctuation of the external magnetic field during daytime reaches around 5 Hz when performing multiple measurements for about 10 min,which seriously affects the results of averaging.In order to improve the performance of the averaged data,we suggest the use of a data processor,i.e.the so-called time-domain frequency correction(TFC).For a 50-times averaged signal spectrum,the signal-to-noise ratio is enhanced from 30 to 120 when applying TFC while preserving the NMR spectrum linewidth.The TFC is also applied successfully to the measurement data of the hetero-nuclear J-coupling in 2,2,2-trifluoroethanol.

Low-field NMR application in the characterization of CO_(2)geological storage and utilization related to shale gas reservoirs:a brief review

CO_(2)geological storage and utilization(CGSU)is considered a far-reaching technique to meet the demand of increasing energy supply and decreasing CO_(2)emissions.For CGSUs related to shale gas reservoirs,experimental investigations have attracted variable methodologies,among which low-field NMR(LF-NMR)is a promising method and is playing an increasingly key role in reservoir characterization.Herein,the application of this nondestructive,sensitive,and quick LF-NMR technique in characterizing CGSU behavior in shale gas reservoirs is reviewed.First,the basic principle of LF-NMR for 1H-fluid detection is introduced,which is the theoretical foundation of the reviewed achievements in this paper.Then,the reviewed works are related to the LF-NMR-based measurements of CH_(4)adsorption capacity and the CO_(2)-CH_(4)interaction in shale,as well as the performance on CO_(2)sequestration and simultaneous enhanced gas recovery from shale.Basically,the reviewed achievements have exhibited a large potential for LF-NMR application in CGSUs related to shale gas reservoirs,although some limitations and deficiencies still need to be improved.Accordingly,some suggestions are proposed for a more responsible development of the LF-NMR technique.Hopefully,this review is helpful in promoting the expanding application of the LF-NMR technique in CGSU implementation in shale gas reservoirs.

Laplace NMR谱图重建——从经典正则化到深度学习

拉普拉斯核磁共振(Laplace NMR)可以提供待测样品的扩散系数或弛豫时间等物理参数信息,是用于研究分子化学结构、动力学和相互作用的强大工具.Laplace NMR的适用性很大程度上取决于拉普拉斯逆变换相关的信号处理算法的性能.在本文中,我们首先讨论了Laplace NMR谱图重建问题的不适定性,接着回顾了经典的基于正则化约束的重建算法,并介绍了目前前沿的深度学习算法在处理Laplace反演问题方面的应用,最后总结了这些算法的优缺点,并对Laplace NMR信号处理方法未来改进方向进行了展望.

应用于太赫兹波驱动的DNP-NMR传输线

太赫兹波驱动的动态核极化增强核磁共振波谱系统可大幅提高信号灵敏度和缩短检测时间。该系统需采用传输线在驱动源回旋管与谱仪之间进行太赫兹波传输。本文针对400 MHz增强核磁共振系统设计了由准光模式变换器和波纹波导组成的太赫兹传输线,并进行了相关实验测试,测试结果表明准光模式变换器可将263 GHz回旋管TE 03模式转换成准高斯波束,其高斯成份为97%,整个传输线最终输出波束的高斯成份为96.3%。

基于LF-NMR研究双孢菇远红外辅助热泵干燥过程中水分迁移规律及品质变化

为研究双孢菇远红外辅助热泵干燥过程中水分分布规律及品质变化,采用低场核磁共振及成像技术分别对干燥过程的双孢菇进行弛豫特性及质子密度成像分析,并研究干燥前、后双孢菇微观结构和品质的变化。结果表明:在热泵温度分别为45,55,65℃时,双孢菇到达干燥终点的时间分别为510,420,390 min。双孢菇的T2反演谱有3个不同的弛豫峰:T21(3~23 ms)、T22(25~155 ms)和T23(155~1084 ms),分别代表双孢菇中的结合水、不易流动水和自由水。干燥初期,T23左迁幅度很大,干燥后期,左迁幅度变小,随着干燥时间的延长,自由水的流动性降低。干燥结束时,T23对应的弛豫峰下降约99.8%,只剩下部分T21和T22对应的峰,说明自由水几乎被完全去除,双孢菇内仅剩结合水和少量不易流动水。低场核磁共振图像显示,65℃条件下,干燥360 min,水分基本被脱除,而55℃和45℃处理组分别在干燥390 min和450 min才基本脱除。双孢菇外侧的水分先被脱除,内部的水分向外迁移,并且温度越高,内部水分迁移速率越快。另外,65℃条件下的双孢菇的BI值分别比45℃和55℃条件下的BI低30.55%和22.69%,说明65℃更能维持双孢菇原有的色泽。65℃条件下干燥的双孢菇的抗坏血酸、总酚、总黄酮含量分别比干燥前下降84.62%,55.80%和36.78%,与45℃和55℃相比,65℃下双孢菇营养成分的损失最小。本研究表明低场核磁共振及成像技术为双孢菇干燥过程中水分的变化提供了直观的参考依据,也可为双孢菇的远红外辅助热泵干燥工艺优化干燥参数、提高产品质量提供理论参考。

烟草果胶和纤维素含量^(13)C MultiCP/MAS NMR同时测量

果胶和纤维素是烟草细胞壁的重要组成成分,其含量对烟草品质与安全性产生重要影响。传统分析方法难以实现果胶和纤维素的同时定量。本文在优化核磁共振波谱序列条件的基础上,采用固体^(13)C多重交叉极化/魔角旋转核磁共振波谱分析技术(^(13)C MultiCP/MAS NMR)建立了烟草果胶和纤维素含量的同时测量新方法。方法以聚半乳糖醛酸和微晶纤维素为标准物质,以3-(三甲基甲硅烷基)丙酸-d4钠盐(TMSP)做为内标物质,分别建立内标法标准曲线,相关系数R2为0.999 0和0.998 2。果胶测量的检出限和定量限分别为0.38和1.28 mg·g^(–1),精密度(RSD,n=5)小于3.05%。纤维素测量的检出限和定量限为1.01和3.32 mg·g^(–1),精密度(RSD,n=5)小于2.74%。应用本方法测量烟梗、烟草薄片和烟叶等不同类型样品中的果胶和纤维素含量,并对比烟草行业标准方法的测量结果,果胶含量的相对误差在-0.95%至4.51%之间,纤维素含量的相对误差在0.77%至2.46%之间。表明^(13)C MultiCP/MAS NMR方法快速,准确,适合批量样品的分析测量,为果胶和纤维素等细胞壁类大分子的同时定量分析提供重要技术支持。

NMR、GPC和MALDI-TOF应用于聚乙二醇单甲醚的教学实验

将核磁共振(NMR)、凝胶渗透色谱(GPC)和基质辅助激光解吸/电离飞行时间质谱(MALDI-TOF)三种高级分析技术整合到本科《高分子材料研究方法》课程的实验教学中。通过分析聚乙二醇单甲醚样品,学生深入学习了高分子结构和分子量的表征方法,熟悉了这三种仪器的操作和原理。本实验显著提升了课程内容的深度和广度,激发了学生的学习兴趣,并且增强了学生的实践操作能力和创新思维。综合应用这些分析技术,使学生在高分子材料的研究中获得了更全面的理解和技能的提升。

基于^(13)C-NMR偶极去相技术的有机肥料中有机组分特征研究

【目的】在固体13C-NMR的交叉极化魔角旋转全边带抑制(cross polarization/magic angle spinning/total suppression of sidebands,CP/MAS/TOSS)技术得到的谱图中,质子碳与非质子碳的信号重叠。偶极去相(dipolar dephasing,DD)技术利用13C-1H异核耦合作用大小的差异,区分与不同质子结合的13C核基团。本研究基于DD技术探讨有机肥料中碳结构特征。【方法】供试有机肥料样品包括3种秸秆(小麦秸秆、玉米秸秆、水稻秸秆)、3种粪肥(鸡粪、牛粪、猪粪)和3种腐植酸肥(腐植酸铵、腐植酸钠、腐植酸钾)样品,利用13C-NMR谱的CP/MAS/TOSS结合DD技术,测定了不同来源的有机肥料样品图谱,比较其碳分布比例。【结果】秸秆和粪肥样品中烷氧碳组分较多,占鉴定出所有碳组分的比例分别为55%和31%~37%,非质子化的烷氧碳占比不到3%;秸秆、猪粪样品中非质子芳香碳占3%~4%,鸡粪和牛粪样品中非质子芳香碳分别占15%和11%,较秸秆和猪粪样品含有更多的非质子化芳香族化合物。粪肥和秸秆样品中甲氧基碳占比低于烷基氮。腐植酸样品以芳香碳、羧基碳和酰胺基碳为主,羧基碳和酰胺基碳占到12%~16%,总芳香族碳占48%~72%,非质子芳香碳占25%~37%,表明腐植酸芳香骨架上可能存在较多取代。【结论】基于CP/TOSS结合DD技术(偶极相移延时时间设置为40μs)分析结果发现,鸡粪和牛粪中非质子芳香碳占比高于猪粪和秸秆;腐植酸肥芳香族碳比例高于粪肥和秸秆,尤其是非质子芳香碳占比较高,表明腐植酸肥芳香化程度高于粪肥和秸秆。

基于小型化原子磁力计的零场NMR波谱仪搭建与测试

常规高场NMR波谱仪依托超导技术,其体积大、维护成本高,且存在样本磁化率不均匀导致的谱线展宽现象,零场或近零场NMR技术则可实现有效互补.本文自主搭建了一套基于小型原子磁力计的可移动零场NMR波谱仪,采用以多功能采集卡(National Instruments PCIe-6353)为核心的集成控制系统,仪器主磁场强度小于1nT,可实现高分辨率的J-耦合谱采集.为实现对自旋体系的有效操控,利用样品绝热初态的单脉冲激发实现对三轴脉冲线圈的精确标定.在此基础上,使用改进的组合脉冲序列实现了在^(13)C-^(1)H异核体系下的单自旋选择性操控,验证了零场NMR波谱仪的有效性.

舒肝宁注射液的^(1)H-NMR指纹图谱分析

目的对舒肝宁注射液的^(1)H-NMR指纹图谱进行分析,为其质量控制标准技术升级提供参考依据。方法使用Noesygppr1d脉冲序列[recycle delay(RD)-90°-t 1-90°-t_(m)-90°-ACQ]采集1H-NMR谱图,质子共振频率为600.13 MHz,实验温度为298 K,通过方法学考察验证分析方法可靠性,采用相关系数法和夹角余弦法评价不同批次舒肝宁注射液样品的相似度。结果对舒肝宁注射液的^(1)H-NMR指纹图谱进行分析,共鉴定出88种化学成分,且不同批次的舒肝宁注射液具有较高的相似度。结论建立了一种基于^(1)H-NMR的舒肝宁注射液质量控制方法,可为舒肝宁注射液技术升级提供参考依据。

Postmortem ^(7)Li NMR analysis for assessing the reversibility of lithium metal electrodes in lithium metal batteries

Despite the proficiency of lithium(Li)-7 NMR spectroscopy in delineating the physical and chemical states of Li metal electrodes,challenges in specimen preparation and interpretation impede its progress.In this study,we conducted a comprehensive postmortem analysis utilizing ^(7)Li NMR,employing a stan-dard magic angle spinning probe to examine protective-layer coated Li metal electrodes and LiAg alloy electrodes against bare Li metal electrodes within Li metal batteries(LMBs).Our investigation explores the effects of sample burrs,alignment with the magnetic field,the existence of liquid electrolytes,and precycling on the ^(7)Li NMR signals.Through contrasting NMR spectra before and after cycling,we identi-fied alterations in Li^(0) and Li^(+) signals attributable to the degradation of the Li metal electrode.Our NMR analyses decisively demonstrate the efficacy of the protective layer in mitigating dendrite and solid elec-trolyte interphase formation.Moreover,we noted that Li*ions near the Li metal surface exhibit magnetic susceptibility anisotropy,revealing a novel approach to studying diamagnetic species on Li metal elec-trodes in LMBs.This study provides valuable insights and practical guidelines for characterizing distinct lithium states within LMBs.

Operando NMR methods for studying electrocatalysis

The combination of electrochemical measurements with spectroscopic characterizations provides valuable insights into reaction mechanisms.Nuclear magnetic resonance(NMR)spectroscopy,as a powerful technique due to its atomic specificity and versatility in studying gas,liquid,and solid,allows the study of electrolyte solution,catalyst and catalyst-adsorbate interfaces.When applied in operando,NMR can offer molecular-level insights into various electrochemical processes.Operando NMR has been applied extensively in battery research,but relatively underexplored for electrocatalysis in the past two decades.In this mini review,we first introduce the operando electrochemical NMR setups,categorized by different probe designs.Then we review the applications of operando NMR for monitoring the electrolyte solution and the catalyst-adsorbate interface.Considering the high environmental impact of electrochemical conversion of CO_(2)into value-added products,we zoom in to the use of operando NMR in studying electrochemical CO_(2)reduction.Finally,we provide our perspective on further developing and applying operando NMR methods for understanding the complex reaction network of Cu-catalyzed electrochemical CO_(2)reduction.