Characterization of surface functional groups by FT-near infrared spectroscopy

傅里叶变换近红外光谱(FT-NIR)是一种功能强大的分析方法，但除了在农业领域外还不为人所熟知。近红外区(4000 cm^-1&gt；~10000 cm^-lt；-1&gt；)覆盖了O-H键的伸缩振动和弯曲振动的组合。研究的目的是确定由酸性质引起的O-H振动，特别是固体酸性氧化物。由于近红外光谱研究的结果不多，所以用CCl4、C6H6和CHCl_3等稀释剂进行峰的归属。液体样品中的酸强度与相应的羟基的振动频率相关联如下：1)当酸强度(PKa)约为4.5时，在氯化镉溶液中观察到羟基的振动频率约为6890Cm^~(-1)~(-1)。2)当酸强度达到约3时，频率向6850 cm^-1；3)当酸强度在4.5~3之间时，在氯化镉溶液中，6890 cm^~(-1)~6850 cm^~(-1)~gt；在固体样品中，结构和硅铝比已确定的沸石在6500~7000 cm^~(-1)和5000 cm^~(-1)~gt；峰肩的位置与硅铝比的变化密切相关，表明峰肩可能与酸强度产生的羟基有关。由于观察到的沸石是暴露在空气中的，所以也检测到了吸附H2O的峰。为了清楚地识别与酸强度相对应的峰，必须采用原位和漫反射技术。研究正在进行中。

Fourier Transform Near Infrared Spectroscopy (FT-NIR) is a powerful analytical method but is not familiar except for in agricultural fields. The NIR region(4000cm^<-1>〜10000cm^<-1>) covers the overtones and combination of stretching and bending vibrations of O-H bonds. The aim of study is to identify O-H vibrations originated from the acid property, especially on solid acid oxides. As there are a few results on NIR studies, standard samples for the assignment of peaks have been employed using diluents such as CCl_4, C_6H_6 and CHCl_3. The acid strength in the liquid sample is associated to the vibration frequencies of the corresponding OH groups as follows : 1) When the acid strength(pKa) is about 4.5, the vibration frequency of OH groups is observed around 6890Cm^<-1> in CDCl_3 solution. 2) When the acid strength becomes ca. 3, the frequency is shifted to 6850cm^<-1>. 3) When the acid strength is between 4.5 and 3, the absorption peaks appear between 6890Cm^<-1> and 6850cm^<-1> in CDCl_3 solution.In solid samples, zeolites, where the structures and Si/Al ratios are identified already, show the characteristic peak shoulders at 6500-7000cm^<-1> and 5000cm^<-1>. The positions of peak shoulders are closely correlated with changes in the Si/Al ratio, suggesting that the peak shoulders might be attributed to the OH groups derived from the acid strength. Since the zeolites observed were exposed to air, the peaks from adsorbed H_2O also were detected. In order to identify clearly the peak corresponding to the acid strength, the in-situ and diffuse reflectance techniques are prerequisite. The study is under way.