Jarlbring Mathias, Sandström Dan E, Antzutkin Oleg N, Forsling Willis
Department of Chemical Engineering and Geoscience, Division of Chemistry, Luleå University of Technology, SE-97187 Luleå, Sweden.
Langmuir. 2006 May 9;22(10):4787-92. doi: 10.1021/la052837j.
The chemically active phosphorus surface sites defined as PO(x), PO(x)H, and PO(x)H2, where x = 1, 2, or 3, and the bulk phosphorus groups of PO4(3-) at synthetic carbonate-free fluorapatite (Ca5(PO4)3F) have been studied by means of single-pulse 1H,31P, and 31P CP MAS NMR. The changes in composition and relative amounts of each surface species are evaluated as a function of pH. By combining spectra from single-pulse 1H and 31P MAS NMR and data from 31P CP MAS NMR experiments at varying contact times in the range 0.2-3.0 ms, it has been possible to distinguish between resonance lines in the NMR spectra originating from active surface sites and bulk phosphorus groups and also to assign the peaks in the NMR spectra to the specific phosphorus species. In the 31P CP MAS NMR experiments, the spinning frequency was set to 4.2 kHz; in the single-pulse 1H MAS NMR experiments, the spinning frequency was 10 kHz. The 31P CP MAS NMR spectrum of fluorapatite at pH 5.9 showed one dominating resonance line at 2.9 ppm assigned to originate from PO4(3-) groups and two weaker shoulder peaks at 5.4 and 0.8 ppm which were assigned to the unprotonated PO(x) (PO, PO2-, and PO3(2-)) and protonated PO(x)H (PO2H and PO3H-) surface sites. At pH 12.7, the intensity of the peak representing unprotonated PO(x) surface sites has increased 1.7% relative to the bulk peak, while the intensity of the peaks of the protonated species PO(x)H have decreased 1.4% relative to the bulk peak. At pH 3.5, a resonance peak at -4.5 ppm has appeared in the 31P CP MAS NMR spectrum assigned to the surface species PO(x)H2 (PO3H2). The results from the 1H MAS and 31P CP MAS NMR measurements indicated that H+, OH-, and physisorbed H2O at the surface were released during the drying process at 200 degrees C.
已通过单脉冲1H、31P和31P CP MAS NMR研究了化学活性磷表面位点，定义为PO(x)、PO(x)H和PO(x)H2，其中x = 1、2或3，以及合成的无碳酸盐氟磷灰石（Ca5(PO4)3F）中PO4(3-)的体相磷基团。评估了每种表面物种的组成和相对含量随pH值的变化。通过结合单脉冲1H和31P MAS NMR光谱以及在0.2 - 3.0 ms范围内不同接触时间的31P CP MAS NMR实验数据，得以区分NMR光谱中源自活性表面位点和体相磷基团的共振线，还能将NMR光谱中的峰归属于特定的磷物种。在31P CP MAS NMR实验中，自旋频率设定为4.2 kHz；在单脉冲1H MAS NMR实验中，自旋频率为10 kHz。pH 5.9时氟磷灰石的31P CP MAS NMR光谱显示，在2.9 ppm处有一个主要共振线，归属于PO4(3-)基团，在5.4和0.8 ppm处有两个较弱的肩峰，归属于未质子化的PO(x)（PO、PO2-和PO3(2-)）和质子化的PO(x)H（PO2H和PO3H-）表面位点。在pH 12.7时，代表未质子化PO(x)表面位点的峰强度相对于体相峰增加了1.7%，而质子化物种PO(x)H的峰强度相对于体相峰降低了1.4%。在pH 3.5时，31P CP MAS NMR光谱中在 - 4.5 ppm处出现了一个共振峰，归属于表面物种PO(x)H2（PO3H2）。1H MAS和31P CP MAS NMR测量结果表明，表面的H+、OH-和物理吸附的H2O在200℃干燥过程中被释放。
