使用拉曼光谱法测定邻-对-氢比——在燃料电池中的应用

氢是用于燃料电池的一种有吸引力的能源。氢分子有两种不同的核自旋异构体：邻位氢（同向对）和对位氢（反向对）。邻位和对位氢都表现出相同的化学性质，而它们的物理性质，如比热，彼此不同，并且由于邻位和对位异构体之间的转化需要非常长的时间，因此每个分子的处理往往与另一个不同。在环境温度下，氢气中邻位与对位的丰度比稳定在约3:1，但当它冷却成液体时，大多数氢分子转化为对位氢。然而，一部分邻位氢也变成液体，并且在液态氢中邻位到对位异构体的转化在延长的时间后可能会产生热量，这会影响储存效率。因此，当氢气被液化用于工业目的时，邻位到对位转化催化剂是必要的。在这个实验中，我们将描述，作为一个例子，通过使用拉曼光谱对邻位-对位丰度比进行简单评估UAS.COApplication Note 2/3Determination of ortho and para hydrogen ratio by using Raman spectroscopy Determination of ortho and para hydrogen ratio by using Raman spectroscopy - Application to fuel cel l Hydrogen is an attractive energy source for use with fuel cells. The hydrogen molecule has two di f ferent n u c lear spin isomers： ortho -hydrogen (same direction pair) and para-hydrogen (opposite direction pair ). Both ortho- and para-hydrogen exhibit the same chemical proper t ies ， whilst their physical proper t ies， such as the specific heat are different f rom each other， and since the conversion between the ortho- and para- isomers requires an extremely l ong time， each molecule tends to be handled as different from th e other. The abundance rat i o of ortho-to para- in hydrogen ga s at ambient temperature is stable at approx. 3：1， but when it is cooled to a liquid， most hydrogen molecules conver t to para-hydrogen. However， a portion of the ortho -hydrogen also becomes a l iquid， and the ortho- to para - isomer conversion i n liquid hydrogen after an extended t ime may generate heat， which can affect the storag e efficiency . Therefore， when hydrogen gas is l i quefied for industrial purposes， an ortho- to para- conversion catalyst is necessary. In this experiment， we will describe， as an example， a s i mple evaluat i on of the o r tho-para abundance ratio by using Raman spect r oscopy. JASCO INC. 28600 Ma r y's Court， Easton， MD 21601 USA E xper imen t a l An NRS-3100 (or NRS-5100) Laser Raman spectrophotometer was used for the measurement. A 10cm FT -IR gas cell with NaCl windows was evacuated using a vacuum pump， and then hydrogen gas with a pressure lower than one atmosphere was f illed directly into the gas cell from an H， gas cylinder. The gas cell i s mounted as shown in Figure 1， and the measurement was perfor m ed using a long working distance obj e ctive l e ns by focusing the beam i nside the cel l (Excitation wavelength； 532 nm，laser power at sample point is approximately 10 mW). Results and D iscuss i ons Figure 2 illustrates the Raman spectrum obtained for the hydrogen gas . The peaks due to the rotational transitions were seen i n the spectrum i n the range f rom 350 to 1100 cm. The system temper a ture wa s estimated f rom the Stokes/anti-Stokes ratio using equation (1) to b e approximately 300K. The ortho-para abundance ratio i s calculated f r om each par ti tion function equation (2) of the ortho- and para-hydrogen isomers by adding the peak height of the ortho- and para-hydrogen peaks in the rotation spectrum and us i ng the est i mated temperatur e of 300K i n equation (2)； the ratio of 7：3was obtained from the calculations .The hydrogen molecular rotation constan t B i n equation (2) was referred to B =59 cm- which was calculated from the equal distance of the rotation line 4B. Vibration-rotation spectrum Fig. 2： Raman spectrum of hydrogen gas B： Rotation constant， J： Rotation quantum number Pj=gj(2/+1) gj ： Each abundance ratio of ortho- and para-hydrogen Applicatio n Library： http：//www.j ascoinc .com/appl i c a t i ons Compared to the standard ortho-para ratio of hydrogen at an ambient temperature of 3：1， the ratio of para hydrogen i s higher in the experimental example. From this result ， i t is noted that some of th e ortho-hydrogen was already converted to para-hydrogen when hydrog e n i n the cylinder was liquefi e d and the cylinder filled. In t his example， th e ortho-para rat i o was obtained f rom the rotation spectrum， but ， whe n the Raman spectrum in the vicin i ty of the peaks of the rotation spec t rum is affected by extraneous light ， e.g.，fluorescence generated from t he window， the abundance ratio can also be calculated f rom the v i bration-rotation spectrum which i s separated f r om the rotat i onal spectrum peaks by using a high resolut i on measurement of the vibration-rotation spec t rum around 4160 cm1. Comments The reason why the difference in the direct i on of nuclear spin for molecules such as ortho- and para-hyd r ogen is evaluated by Raman spectroscopy (vibration spectroscopy)can be explained by quantum-mechanics. The wave fun c tion of the whole h ydrog e n molecule can be expressed as the product of the separate wave f u nct i ons for the electronic， vibrational， rotational energies and the nuclear spin as shown in equat i on (3). Para-hydrogen Ortho-hydrogen F i gure 3. Energy l evel of ortho-para hydrogen Equations (4) and (5) show the energy of the molecular vibration and rotation， respectively， calculated f rom the characteristic values of the Schrodinger equation. Note， v=(1/2元)(k /p)1/2.(v ： vibrational quantum number， k： force constant， p. reduced mass) 3) (B： rotational constant， J： rotational quantum number， I： inertia moment， for diatomic molecule，I=uR²(u：reduced mass， R： distance between atomic nuclei) Figure 3. shows the energy level of the hydrogen molecule using a quantum number of rot a tion J .Generally， i n the electronic ground state， the wave function of the electronic and vibrat i onal quanta is symmetric to the displacement of th e nucleus . On the other h a n d ， the wave f unc t ion of the nuclear spin has both symmetric and asymmet r ic cases to the displ a cement， and in each case is called ortho- and para-. The wave function o f rotation must be sy m met r ic in order to satisfy the Pauli exclusion principle.Therefore， or t ho-hydrogen exists only when the rotation level J is odd (asymmetric)， and para -hydrogen exists only when the rotation leve l J is even (symmetric). The nuclear spin i nformation such as that for ortho- and para -hydrogen can be obtained from the rotation spectrum (or vibration-rotation spectrum)of the hydrogen molecule. However， i n the case of a D e uter i um molecule (D2)， the Deuterium atom is a Bose particle ， so that the relationship between odd - even and ortho - para is contrary to the hydrog e n molecular case， with the res u lting interesting phenomenon t hat the shape of t he rotation spectrum dramatically changes.