小分子有机酸改性尿素的多谱学分子结构表征

尿素是我国主要的氮肥品种，但其活性高，在土壤中水解后极易通过挥发和淋洗损失，利用率低，造成养分资源浪费并污染环境。使用有机酸对尿素进行改性可以延缓尿素分解，提高尿素利用率，但有机酸与尿素的结合方式及其增效机理尚不明确。研究中选取柠檬酸和水杨酸两种小分子有机酸作为添加剂，分别加入到熔融尿素中，获得柠檬酸尿素与水杨酸尿素。利用傅里叶变换红外光谱（FTIR）和X射线光电子能谱（XPS）分析材料的化学结构，利用液相色谱-质谱联用（LC-MS）研究材料的物质组成及相对分子质量，尝试通过以上多谱学的分析方法，明晰两种有机酸与尿素的结合方式。结果表明，柠檬酸和水杨酸与尿素结合后，FTIR在3 348 cm-1处产生了加强的伯胺振动峰，推测小分子有机酸与尿素在伯胺处发生了反应。XPS C(1s)和N(1s)图谱分别出现新的碳结构（-CX）和新的氮结构（-NX），降低了柠檬酸/水杨酸中原有羧基碳结构和尿素中酰胺基氮结构的相对含量，O(1s)图谱出现C-OH化学键断裂，表明，柠檬酸/水杨酸的羧基与尿素的酰胺基相互作用生成了新的物质。LC-MS分析发现柠檬酸尿素/水杨酸尿素中的新物质可能是柠檬酸/水杨酸的羧基与尿素的酰胺基发生脱水反应，生成含有O═CNHC(O)NH₂结构的物质。因此，利用光谱分析等手段明晰了有机酸与尿素的结合方式与结合产物特征，为有机高分子与尿素反应机理的研究提供了理论依据，为后续高效肥料增效剂的筛选提供了方向。

Multispectral Structural Characterization of Low-Molecular-Weight Organic Acids Modified Urea

As the main nitrogen fertilizer in China, urea shows high activity. After hydrolysis in soil, urea is easily lost through volatilization and leaching, resulting in a low urea utilization efficiency, a waste of nutrient resources, and environmental pollution. Using organic acids to modify urea can delay urea decomposition, enhance urea use efficiency. However, the combination and enhancement mechanism is unclear. In this study, two low-molecular-weight organic acids, citric acid and salicylic acid, were selected as additives and added to molten urea to obtain urea containing citric acid (CAU) and urea containing salicylic acid (SAU). The combination of these two organic acids and urea was characterized by using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), liquid chromatography-mass spectrometry (LC-MS) and other analytical technologies. The results showed that after the combination of citric acid and salicylic acid with urea, there was an enhanced primary amine vibrational peak at 3 348 cm-1 of FTIR spectra, indicating the reaction happened on the primary amine of urea. The new carbon structure (-C_X) and nitrogen structure (-N_X) was separated from the XPS C(1s) spectra and the N(1s) spectra, respectively. These new structures led to the decrease of the carboxyl group in citric/salicylic acid and amide group of urea. In addition, the C-OH chemical bond breakage happened in the XPS O(1s) spectra. The above indicated a new substance formed through the reaction of the carboxyl group in citric/salicylic acid and the amide group of urea to form a new substance. LC-MS analysis showed that the dehydration reaction happened between the carboxyl group of citric acid/salicylic acid and the amide group of urea, and that the new substance was structured with O═CNHC(O)NH₂ will be produced in CAU or SAU. Therefore, the results from the spectral analysis and other analytical technologies used in this study clarified the combination characteristics of low-molecular-weight organic acid and urea. This founds a basis for the study on the reaction mechanism of organic polymer and urea and provides new ideas for the selection of high-efficiency fertilizer synergists.