摘要
本文运用程序升温脱水-质谱计检测(TPD MS)和红外光谱测定(IR)法研究了12-钼磷酸(PMA)中水的存在状态及其热稳定性.解迭了PMA的TPD MS谱,发现PMA中存在有结合能量各异的7种水峰.讨论了PMA中各种水的结合强度及其对Keggin结构稳定性的影响。
The existence and thermal stability of H_2O in 12-molybdophosphoric acid (PMA)have been studied by temperature programmed desorption-mass spectrography (TPDMS) and infrared spectrography (IR). A PMA sample containing 19. 1 H_2O/P, H_3(PMo_(12)O_(40))·17. 6 H_2O, was prepared,its water content was determined according to Fig. 1. The TPD MS spectrum of H_2O(m/z=18) in PMA (solid line Fig. 3) was recorded after vacuum outgassing(0. 27Pa)at 290K (the total time of outgassing in the range 77.4-290K was 142.5min).Separated spectra (Fig.3) and their parameters simulated by spectrogram superposi-tion method and consulting table method (Tablel) show that there exist 7 kindsof H_2O in PMA. In situ IR spectra for PMA at different conditions (Fig. 4) revealed that theruptures of various oxygen bonds proceeded simultaneously at 603K (Table 2). Inte-gration of the total area of the upper temperature peaks 5-7 yielded the n value of1. 51 for water molecules desorbed per PMA unit (H_2O/P), which consists with 1.5for the protonic water. These results lead us to conclude, firstly, that the peak 1 is assigned to thewater in the secondary structure of PMA, this water is quite easy to be outgassed at290 K and its main part had been evacuated before TPD. Secondly, peaks 2-4 withmedium binding energies are assigned to three kinds of water in the hydronium ionswhich are hydrogen-bonded to bridge oxygen O_b, terminal oxygen O_t, or phosphoroxygen O_p in the primary Keggin structure. Thirdly, peaks. 5-7 with strong bindingenergies are assigned to another three kinds of water formed from protons with O_b,O_t, or O_p. Accompanying with desorption of the three kinds of protonic water, theKeggin structure is gradually destroyed till entirely collapsed at last.
出处
《物理化学学报》
SCIE
CAS
CSCD
北大核心
1990年第1期77-82,共6页
Acta Physico-Chimica Sinica
