立方Nd2O3上过氧物种光诱导生成的18O同位素示踪考察


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	景孝廉

	佘雯瑜

	翁维正

	李建梅

	夏文生

	万惠霖

¹⁸O isotopic study of photo-induced formation of peroxide species on cubic Nd₂O₃

Xiaolian Jing, Wenyu She, Weizheng Weng , Jianmei Li, Wensheng Xia, Huilin Wan

State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China

Received: 2014-3-28. Accepted: 2014-5-14. Published: 20 August 2014.

* Corresponding author. Tel/Fax: +86-592-2185192; E-mail: wzweng@xmu.edu.cn
Tel: +86-592-2186569; Fax: +86-592-2185192; E-mail: hlwan@xmu.edu.cn

Foundation Item:This work was supported by the National Basic Research Program of China (973 Program, 2010CB732303), the National Natural Science Foundation of China (21173173, 21033006, 21373169), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1036).

Abstract: Photo-induced formation of peroxide species on cubic Nd₂O₃ was studied by in situ microprobe Raman spectroscopy using ¹⁸O as a tracer and a 325-nm laser as an excitation source. The results confirmed that the peroxide ions were formed through photooxidation of the lattice oxygen species in neodymium sesquioxide by molecular oxygen species. Under UV excitation (λ = 325 nm), the reaction between O₂ and O^2- could take place at room temperature. A fast oxygen exchange between the peroxide ions and the lattice oxygen species in Nd₂O₃ took place under the experimental conditions studied. Also, bulk lattice oxygen species in Nd₂O₃ could migrate to the surface layer and participate in the formation of peroxide ions. The migration of lattice oxygen species and the oxygen exchange between lattice oxygen and peroxide ions were promoted by UV laser irradiation.

Key words: Oxygen activation Peroxide Neodymium sesquioxide Photo-induced reaction ¹⁸O isotopic study Raman spectroscopy

1. Introduction

Owing to their excellent electronic characteristics and chemical and thermal stabilities, lanthanide oxides have been widely used as catalysts in the catalytic oxidation of light alkanes such as oxidative coupling of methane and oxidative dehydrogenation of ethane ^{[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]}. Understanding the role of activated forms of oxygen species and pathways of O₂ activation on the lanthanide oxides is therefore of fundamental importance to the above-mentioned reactions ^{[13, 14, 15, 16, 17, 18, 19]}. Our study of Ln₂O₃ (Ln = La, Nd, Sm, Gd) under O₂ by microprobe laser Raman spectroscopy revealed that laser excitation could induce the formation of peroxide species on the Ln₂O₃ surface ^{[20, 21]}. Thus, a new pathway towards molecular oxygen activation under mild conditions was discovered that presents potential application in photocatalytic reactions. Additionally, the study provided new insights in the mechanism of O₂ activation on the surface of metal oxides with stable cationic valence. In continuation of the previous research, here we examine the formation of peroxide on cubic Nd₂O₃ using ¹⁸O as tracer, aiming at further understanding the mechanisms of the photo-induced formation of peroxide ions on lanthanide sesquioxides with stable cationic valence.

2. Experimental

2.1. Sample preparation

Cubic Nd₂O₃ was prepared from Nd(OH)₃ according to reported procedures ^{[22, 23]}. First, commercial Nd₂O₃ (99.99%, Alfa Aesar) was treated at 100 °C for 100 h with a 21% O₂/N₂ flow containing water vapor (by bubbling 21% O₂/N₂ through H₂O at room temperature) to form Nd(OH)₃. Then, Nd(OH)₃ was heated at 650 °C in a flow of 21% O₂/N₂ (50 mL/min) for 3 h to produce Nd₂O₃.

2.2. Sample characterization

The crystalline structure of the sample was confirmed by X-ray powder diffraction (XRD) analysis to be cubic Nd₂O₃ (Fig. 1). The experiment was carried out on a PANalytical X’pert PRO diffractometer using Cu K_α radiation, operating at 40 kV and 30 mA. The Brunauer-Emmett-Teller (BET) surface area of Nd₂O₃ is 6.9 m²/g as measured by N₂ adsorption-desorption analysis at -196 °C on a Micromeritics TriStar II 3020. Before the measurement, the sample was degassed at 200 °C for 3 h. The scanning electron microscopy (SEM) image of the sample (Fig. 2) was obtained on a Hitachi S-4800 scanning electron microscope operating at an accelerating voltage of 15 kV. Sample preparation consists of depositing a drop of Nd₂O₃/ethanol suspension on a clean silicon substrate.

Fig. 1. XRD pattern of the prepared cubic Nd₂O₃.

Fig. 2. SEM image of the prepared cubic Nd₂O₃.

2.3. Raman spectroscopy characterization

Raman spectra were recorded on a Renishaw R1000 microprobe Raman spectrometer equipped with a charge-coupled device (CCD) detector using a 325-nm He-Cd laser as the excitation source and an in-house-built high-temperature in situ Raman cell specifically designed for the spectrometer. A diagram of the Raman cell is available elsewhere ^[24]. The microscope attachment for the spectrometer was based on a Leica DMLM system equipped with an OFR LMU-15×-NUV objective. The spectra acquisition time was varied according to the different experiments; an acquisition time of 50 s was typically used. The laser spot on the sample was about 3 μm in diameter, and the spectral resolution was ~6 cm¹. The maximum laser power of the spectrometer measured at the analysis spots was about 3 mW. However, a laser power of 0.75 mW was used in most experiments.

For the experiments performed on cubic Nd₂¹⁶O₃, the sample was first heated under flowing ¹⁶O₂ (50 mL/min, 99.995%, Linde) at 650 °C for 180-360 min to remove carbonate species and moisture. The treated Nd₂¹⁶O₃ was then cooled in the dark to 25 °C under ¹⁶O₂ before it was exposed to a focused 325-nm laser beam from the Raman spectrometer to induce formation of peroxide ions, and the resulting spectra were recorded under either ¹⁸O₂ or ¹⁶O₂ atmosphere.

The ¹⁸O-labeled Nd₂O₃ was obtained by treating cubic Nd₂¹⁶O₃ with a flow of ¹⁸O₂ (5 mL/min, 97%, CIL) at 650 °C for 6 h. The sample was then cooled in the dark under ¹⁸O₂ to 25 °C. The treated sample (¹⁸O-labeled Nd₂O₃) was then exposed to a focused 325-nm laser beam to induce formation of peroxide ions, and the resulting spectra were recorded under ¹⁸O₂ atmosphere.

3. Results and discussion

3.1. Photo-induced formation of peroxide species on Nd₂¹⁶O₃ under ¹⁶O₂ and ¹⁸O₂ atmospheres

Fig. 3(a) shows the changes in the Raman spectra when cubic Nd₂¹⁶O₃ was irradiated at varying times under ¹⁶O₂ atmosphere with a focused 325-nm laser beam at 25 °C. As observed, the Raman band at 833 cm¹, which represents the O-O stretching mode (ν_O-O) of the ¹⁶O₂² peroxide species [20,21, 25-27], began to grow at the expense of the Nd³⁺-¹⁶O^2- band at 331 cm¹. Following irradiation with the laser at 25 °C for more than 60 min, the Raman spectrum of the sample was recorded in the frequency range of 1300-1800 cm¹ and shown in Fig. 3(b). Two bands at 1554 and 1646 cm¹ could be clearly identified. The broad band at 1646 cm¹ could be assigned to the overtone of the O-O stretching vibration of the ¹⁶O₂² peroxide species because its wavenumber was almost twice that of the band at 833 cm¹. The narrow band at 1554 cm¹ could be assigned to molecular oxygen species; the O-O stretching vibration of the gas phase ¹⁶O₂ molecule could be observed at a comparable wavenumber (Fig. 4). Considering that the wavenumber of the molecular oxygen band in Fig. 3(b) shifts by ~2 cm¹ towards lower frequencies when compared with that of the gas phase ¹⁶O₂, the band at 1554 cm¹ can be attributed to a ¹⁶O₂ species adsorbed on the surface of Nd₂O₃. This band may be due to the decomposition of the peroxide species. Further evidence of the origin of the 1554 cm¹ Raman band is presented in Section 3.2.

Fig. 3. (a) In situ Raman spectra of cubic Nd₂¹⁶O₃ irradiated at varying times with a focused 325-nm laser beam under ¹⁶O₂ at 25 °C. (b) Raman spectrum (frequency range of 1300-1800 cm¹) of cubic Nd₂¹⁶O₃ following irradiation with a 325-nm laser under ¹⁶O₂ at 25 °C for more than 60 min. The laser power used to induce formation of the peroxide species was 0.75 mW.

Fig. 4. Raman spectra of gas phase ¹⁶O₂ (1) and ¹⁸O₂ (2) recorded with a 325-nm laser at 25 °C.

Fig. 5(a) shows the changes in the Raman spectra of cubic Nd₂¹⁶O₃ under ¹⁸O₂ upon continuous irradiation with a focused 325-nm laser beam at 25 °C. The spectra are very similar to those shown in Fig. 3(a). The onset of a peroxide Raman band corresponding to ¹⁶O₂² was observed at the expense of the Nd³⁺-¹⁶O² stretching band at 335 cm¹. This observation clearly indicates that lattice oxygen (¹⁶O²) species in Nd₂O₃ are involved in the formation of peroxide ions. Fig. 5(b) shows the Raman spectrum in the frequency range of 1300-1800 cm¹ of the Nd₂¹⁶O₃ sample following irradiation with a 325-nm laser beam at 25 °C under ¹⁸O₂ for more than 30 min. In addition to the O-O stretching vibration of the ¹⁶O₂ molecule at 1554 cm¹ and the overtone of the O-O stretching vibration of the ¹⁶O₂² peroxide ions at ~1640 cm¹, a new band at 1471 cm¹ corresponding to the O-O stretching vibration of gas phase ¹⁸O₂ (Fig. 4) was observed. Considering that ¹⁸O₂ is the major component (isotope purity >97%) in the gas phase, and that the Raman peak intensity of ¹⁶O₂ in Fig. 5(b) is much higher than that of the gas phase ¹⁸O₂, the peak at 1554 cm¹ can be rationally assigned to surface ¹⁶O₂ species. Because the 1554 cm¹ band could only be detected after the formation of peroxide species, its appearance was attributed to the decomposition of the ¹⁶O₂² peroxide ions, probably through a photo- decomposition reaction induced by irradiation with the 325-nm laser. Decomposition of Nd₂O₂(O₂) was reportedly observed following long exposure with a 488-nm Ar⁺ laser ^[28].

Fig. 5. (a) In situ Raman spectra of cubic Nd₂¹⁶O₃ irradiated at varying times with a focused 325-nm laser beam under ¹⁸O₂ at 25 °C. (b) Raman spectrum (frequency range of 1300-1800 cm¹) of cubic Nd₂¹⁶O₃ following irradiation with a 325-nm laser under ¹⁸O₂ at 25 °C for more than 30 min. The laser power used to induce formation of the peroxide species was 0.75 mW.

As suggested in the previous studies ^{[20, 21]}, the peroxide ions on lanthanide sesquioxides are generated by a photo-induced oxidation of lattice oxygen species by molecular oxygen because molecular oxygen species are always required in the formation of peroxide ions. Thus, for a reaction using ¹⁸O₂ and ¹⁶O² as reactants, the peroxide ions should be labeled with ¹⁸O atoms. However, in the Raman spectra shown in Fig. 5(a), only the peroxide ions containing ¹⁶O atoms were detected. This phenomenon may have resulted from a fast oxygen isotope exchange between lattice oxygen and the peroxide species under the experimental conditions studied. Because ¹⁶O² ions are the most abundant isotope oxygen species on the Nd₂O₃ surface, a fast isotope exchange between ¹⁶O² and ¹⁸O-labeled peroxide ions will lead to the formation of ¹⁶O₂² on the Nd₂O₃ surface. Owing to the fast isotope exchange between lattice oxygen and surface dioxygen (peroxide and molecular oxygen) species, the isotope oxygen atoms in the peroxide ions (833 cm¹) were identical to the isotope oxygen atoms in the surface molecular oxygen species (1554 cm¹), resulting from the decomposition of the peroxide ions. This is also evidenced by the absence of Raman bands corresponding to molecular oxygen species with mixed isotope oxygen atoms (¹⁸O¹⁶O) (Fig. 5(b)). These results suggest that the Raman signals of the isotope-labeled surface molecular oxygen species can be used to identify the isotope-labeled peroxide ions formed on the surface of Nd₂O₃ using ¹⁸O as a tracer. The advantage of using Raman peaks corresponding to surface molecular oxygen species towards the identification of isotope-labeled peroxide ions is that the Raman peaks associated with surface molecular oxygen species are significantly narrower than that associated with peroxide ions. Hence, a better resolution can be achieved.

3.2. Photo-induced formation of peroxide species on ¹⁸O-labeled Nd₂O₃ under ¹⁸O₂

To gain further insights into the mechanisms of the photo-induced formation of peroxide ions on Nd₂O₃, the experiments were performed with cubic Nd₂O₃ partially labeled with ¹⁸O. The sample was prepared by treating cubic Nd₂¹⁶O₃ with a flow of ¹⁸O₂ at 650 °C for 6 h followed by cooling under ¹⁸O₂ to 25 °C. After the treatment, the characteristic metal-oxygen vibration band of neodymium sesquioxide shifted from 336 to 320 cm¹. The position of the latter band is comparable with the band position of Nd³⁺-¹⁸O² (319 cm¹), as calculated based on the wavenumber of Nd³⁺¹⁶O² at 336 cm¹ by assuming a simple harmonic oscillator model. This result also indicates that almost all of the ¹⁶O² atoms on the surface of Nd₂O₃ microcrystal (at least those within the detection depth of the Raman spectrometer) are replaced by ¹⁸O². Fig. 6(a) shows the changes in the Raman spectra of an ¹⁸O-labeled Nd₂O₃ sample upon irradiation at varying times with a focused 325-nm laser beam under ¹⁸O₂ at 25 °C. The spectrum recorded after 1 min irradiation revealed a band with maximum at 789 cm¹. With increasing photo irradiation time, a shoulder band at 811 cm¹ appeared. This band became noticeable after the sample was irradiated for 15 min. Following irradiation of the ¹⁸O-labeled Nd₂O₃ sample under ¹⁸O₂ with a 325-nm laser for more than 30 min, the Raman spectrum of the sample was recorded in the frequency range of 1300-1800 cm¹, and the result is shown in Fig. 6(b). Three sharp peaks at 1468, 1510, and 1554 cm¹ were observed. A simple calculation based on the diatomic harmonic oscillator model using the ν_O-O band of a ¹⁶O₂² peroxide ion at 833 cm¹ gave band positions at 786 and 810 cm¹ for the ¹⁸O₂² and (¹⁸O¹⁶O)² peroxide ions, respectively. Similarly, the calculation using the ν_O-O band of a ¹⁶O₂molecule at 1556 cm¹ gave band positions at 1467 and 1512 cm¹ for ¹⁸O₂ and ¹⁸O¹⁶O, respectively. As observed, the calculated band positions for the ¹⁸O-labeled peroxide ions and molecular oxygen species are in good agreement with the experimental results shown in Fig. 6. Because the experiment was performed under flowing ¹⁸O₂ with isotope purity higher than 97%, the 1510 and 1554 cm¹ peaks observed in Fig. 6(b) can be solely attributed to molecular oxygen species (¹⁸O¹⁶O and ¹⁶O₂) on the surface of the ¹⁸O-labeled Nd₂O₃ sample. Considering that the intensity ratio of the ¹⁸O₂ and ¹⁶O₂ peaks in Fig. 6(b) is much higher than that in Fig. 5(b), and that the wavenumber of the ¹⁸O₂ peak in Fig. 6(b) shifts by ~3 cm¹ towards lower frequencies when compared with that of the gas phase ¹⁸O₂ (Fig. 4), the peak at 1468 cm¹ can also be rationally assigned to surface ¹⁸O₂ species. Based on the results and the analysis given in Section 3.1, it can be concluded that three isotope-labeled peroxide ions (i.e., ¹⁸O₂², (¹⁸O¹⁶O)², and ¹⁶O₂²) are formed under the experimental conditions studied. Because the experiment was performed under ¹⁸O₂ atmosphere, the ¹⁶O atoms detected in the peroxide ions and molecular oxygen species can only originate from the ¹⁸O-labeled Nd₂O₃ prepared by treating Nd₂¹⁶O₃ with ¹⁸O₂. The observed Raman bands corresponding to ¹⁶O-labeled peroxide ions (at 811 cm¹) and surface molecular oxygen species (at 1510 and 1554 cm¹) provided further evidence of the involvement of lattice oxygen species in the formation of peroxide ions. These results also indicated that even though the Nd₂¹⁶O₃ sample was treated with ¹⁸O₂ at 650 °C for 6 h, lattice oxygen species in the bulk phase of Nd₂O₃ microcrystals were not fully replaced by ¹⁸O².

Fig. 6. (a) In situ Raman spectra of ¹⁸O-labeled cubic Nd₂O₃ irradiated at varying times with a focused 325-nm laser beam under ¹⁸O₂ at 25 °C. (b) Raman spectrum (frequency range of 1300-1800 cm¹) of the ¹⁸O-labeled cubic Nd₂O₃ sample following irradiation with a 325-nm laser under ¹⁸O₂ at 25 °C for more than 30 min. The laser power used to induce formation of the peroxide species was 0.75 mW.

Based on the changes in the Raman spectra shown in Fig. 6, it can be deduced that molecular ¹⁸O₂ first reacts with the ¹⁸O² species on the surface of the ¹⁸O-labeled Nd₂O₃ microcrystals because most of the surface lattice oxygen species have been replaced by ¹⁸O², leading to the formation of ¹⁸O₂² peroxide ions (789 cm¹). With increasing laser irradiation time, the ¹⁶O² species in the bulk phase also participated in the formation of peroxide ions, as evidenced by the presence of (¹⁸O¹⁶O)² peroxide ion (811 cm¹), and ¹⁸O¹⁶O (1510 cm¹) and ¹⁶O₂ (1554 cm¹) molecular oxygen species on the surface of the sample. The (¹⁸O¹⁶O)² peroxide ions could have resulted from either the migration of the ¹⁶O² species from the bulk phase of the ¹⁸O-labeled Nd₂O₃ microcrystals to the surface layer followed by isotope exchange with the ¹⁸O₂² peroxide ions or direct photo-induced reaction between ¹⁶O² and ¹⁸O₂. The (¹⁸O¹⁶O)² peroxide ions can then transform to ¹⁶O₂² through isotopic exchange with ¹⁶O². The migration of lattice ¹⁶O² species from bulk to surface layer and the isotope exchange between ¹⁶O² and ¹⁸O₂² could be induced (or promoted) by the UV (λ = 325 nm) laser irradiation. To obtain experimental evidence, the effect of photo irradiation power on the formation of peroxide ions was studied. The experiments were performed by irradiating an ¹⁸O-labeled cubic Nd₂O₃ sample under ¹⁸O₂ with laser powers of 0.3 and 3 mW at 25 °C. As shown in Fig. 7(a), the Raman spectra obtained with a laser power of 0.3 mW only revealed a peroxide band, corresponding to ¹⁸O₂² (790 cm¹), within 14 min of photo irradiation, indicating that only the lattice oxygen species from the surface layer of the sample (¹⁸O²) participated in the reaction with ¹⁸O₂. This observation further indicates that migration of ¹⁶O² from the bulk phase to the surface layer and the isotope exchange between ¹⁶O² and ¹⁸O₂² can be neglected under the experimental conditions studied.However, when the laser power was raised to 3 mW (Fig. 7(b)), the band width of the peroxide Raman peak increased, and both the Raman bands corresponding to ¹⁸O₂² (790 cm¹) and (¹⁸O¹⁶O)² (811 cm¹) peroxide ions could be identified after the sample was irradiated for 5-20 min. This result clearly indicates that UV laser irradiation has an impact on the migration of lattice oxygen species and the oxygen exchange between lattice oxygen and peroxide ions. The photo-induced isotopic exchange between ¹⁸O₂ and lattice oxygen on TiO₂ at room temperature was previously reported in the literature ^{[29, 30]}.

Fig. 7. Peroxide Raman bands formed upon irradiation of ¹⁸O-labeled cubic Nd₂O₃ with a focused 325-nm laser beam, operating at a power of 0.3 (a) and 3 mW (b) under ¹⁸O₂ at 25 °C.

4. Conclusions

Molecular oxygen can be transformed to peroxide ions by a photo-induced reaction with lattice oxygen species of neodymium sesquioxide. Under UV excitation (λ = 325 nm), the reaction between O₂ and O² can take place at room temperature. The experimental studies involving ¹⁸O as tracer indicated that fast oxygen exchange between peroxide ions and lattice oxygen species in Nd₂O₃ took place under the conditions studied, and that bulk lattice oxygen species in Nd₂O₃ could migrate to the surface layer and participate in the formation of peroxide species. The migration of lattice oxygen species and the oxygen exchange between lattice oxygen and peroxide ions were promoted by UV laser irradiation.

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立方Nd₂O₃上过氧物种光诱导生成的¹⁸O同位素示踪考察

景孝廉, 佘雯瑜, 翁维正 , 李建梅, 夏文生, 万惠霖

厦门大学化学化工学院化学系, 固体表面物理化学国家重点实验室, 醇醚酯化工清洁生产国家工程实验室, 福建厦门 361005

收稿日期: 2014-3-28. 接受日期: 2014-5-14. 出版日期: 2014-08-20.

基金来源：国家重点基础研究发展计划（973计划，2010CB732303）；国家自然科学基金（21173173，21033006，21373169）；教育部创新团队项目（IRT1036）.

*通讯联系人.翁维正,电话/传真: (0592)2185192; 电子信箱: wzweng@xmu.edu.cn
万惠霖,电话: (0592)2186569; 传真: (0592)2185192; 电子信箱: hlwan@xmu.edu.cn

摘要：采用原位显微Raman光谱和¹⁸O同位素示踪技术，以325 nm激光为激发光源，对立方Nd₂O₃上过氧物种的光诱导生成过程进行了详细表征，进一步证实过氧源于分子氧对晶格氧的氧化反应. 结果还表明，325 nm激光在室温下即可诱导过氧的生成，在实验条件下，生成的过氧物种可与Nd₂O₃的晶格氧发生快速的氧交换反应，位于Nd₂O₃体相的晶格氧也可迁移至样品表层进而参与过氧的生成. 325 nm激光照射有助于促进晶格氧的迁移以及晶格氧与分子氧之间的氧交换反应.

关键词：分子氧活化过氧倍半氧化钕光诱导反应 ¹⁸O同位素研究 Raman光谱

1 前言

稀土氧化物具有优良的化学和热稳定性以及独特的电子层结构. 对于甲烷氧化偶联和乙烷氧化脱氢等轻质烷烃的临氧催化转化, 稀土氧化物也具有优良的催化性能^{[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]}. 探明反应的活性氧物种以及分子氧在稀土氧化物上的活化和转化途径对于深入了解相关反应机理具有重要意义^{[13, 14, 15, 16, 17, 18, 19]}. 我们在采用Raman光谱对O₂气氛下的稀土倍半氧化物(Ln₂O₃, Ln = La, Sm, Nd, Gd)进行原位表征时发现, 激光照射可诱导Ln₂O₃表面生成过氧物种^{^{[20, 21]}}. 该发现不仅有助于深化对分子氧在阳离子价态不变的金属氧化物表面活化生成活性氧物种机理的认识, 也为分子氧在氧化物表面的活化提供了一条新途径, 因此在光催化选择氧化方面具有潜在应用价值. 在前期工作基础上, 本文采用¹⁸O同位素示踪技术对立方Nd₂O₃上过氧物种的光诱导生成过程进行了详细考察, 以期进一步深化对分子氧在阳离子价态不变的金属氧化物表面活化生成活性氧物种机理的认识.

2. 实验部分

2.1. 样品制备

立方Nd₂O₃的制备方法参照文献^{^{[22, 23]}}. 将商品Nd₂O₃ (99.99%; Alfa Aesar)在100 °C下用含水蒸气的21% O₂/N₂气流(以鼓泡方式将水蒸气带入)处理100 h, 制得Nd(OH)₃. 将Nd(OH)₃在干燥的21% O₂/N₂气流中650 °C焙烧3 h制得立方Nd₂O₃样品.

2.2. 常规表征

X射线衍射(XRD)实验在PANalytical公司的X’ Pert Pro型X射线粉末衍射仪上进行. 管电流30 mA, 管电压40 kV, 使用X' Celerator超能阵列探测器, 以Cu-K_α (λ = 0.15406 nm)为辐射源, 采用石墨单色器滤光. 扫描区间为10°90°, 扫描速度为0.0167°/s, 每步时间约24 s.

样品的比表面积(A_BET = 6.9 m²/g)采用BET方法在Micromeritics TriStar II 3020型物理吸附仪上测定. 实验以N₂为吸附质, 吸附温度为-196 °C. 测试前, 样品先在200 °C下抽空处理3 h.

扫描电镜(SEM)实验在Hitachi S-4800型场发射扫描电子显微镜上进行, 加速电压为15 kV. 先将Nd₂O₃样品分散在无水乙醇中, 再将悬浮夜滴在干净的硅片上自然风干后进行SEM表征.

立方Nd₂O₃的XRD和SEM图分别示于图1和图2.

2.3. 原位Raman光谱表征

原位Raman光谱表征实验在配有Leica DMLM显微镜和OFR LMU-15×-NUV物镜以及自行研制的高温原位Raman样品池的Renishaw RT 1000激光显微拉曼光谱仪上进行. 样品池结构见文献^{^[24]}. 实验以325 nm激光(He-Cd激光器)为激发光源, 光谱仪的分辨率约为6 cm¹, 聚焦于样品表面的激光光斑直径和最大激光功率分别约为3 mm和3 mW. 摄谱所用激光功率和谱图采集时间依实验不同而不同, 多数实验的激光功率和谱图采集时间分别为0.75 mW和50 s.

合成的立方Nd₂¹⁶O₃样品首先在650 °C下通¹⁶O₂ (99.995%; Linde; 50 mL/min)处理180360 min以除去样品上的碳酸盐物种和吸附的水分. 处理后的样品随后在¹⁶O₂气氛和无光照条件下降至25 °C, 在¹⁶O₂或¹⁸O₂气氛下用聚焦后的325 nm激光束连续照射样品并用Raman光谱仪记录样品在光照过程的变化.

¹⁸O标记的Nd₂O₃样品由立方Nd₂¹⁶O₃在¹⁸O₂气流(97%; CIL; 5 mL/min)中于650 °C热处理6 h制得. 处理后的样品随后在¹⁸O₂气氛和无光照条件下降至25 °C, 在¹⁸O₂气氛下用聚焦后的325 nm激光束连续照射样品并用Raman光谱仪记录样品在光照过程的变化.

3. 结果与讨论

3.1. ¹⁶O₂和¹⁸O₂气氛下立方Nd₂¹⁶O₃上过氧物种的光诱导生成

图3(a)为¹⁶O₂气氛下的立方Nd₂¹⁶O₃在25 ºC下用325 nm激光连续照射过程的Raman谱图的变化. 由图可知, 随激光照射时间增加, 归属于Nd³⁺-¹⁶O^2-振动的Raman谱带(~331 cm^-1)强度逐渐减弱, 位于833 cm^-1处可归属为¹⁶O₂^2-过氧物种O-O键伸缩振动的Raman谱带^{^{[20, 21, 25, 26, 27]}}强度不断增强. 样品经激光连续照射60 min后, 在1300-1800 cm^-1范围内还可清晰检出两个位于1554和1646 cm^-1的谱峰(图3(b)), 其中位于1646 cm^-1的宽峰的波数略低于¹⁶O₂^2-过氧物种O-O键伸缩振动频率的2倍, 可归属为过氧物种O-O键伸缩振动的倍频峰. 1554 cm^-1谱峰的波数与分子氧(¹⁶O₂)O-O键伸缩振动的波数相近, 但较气相¹⁶O₂的谱峰(图4)低约2 cm^-1, 可指认为吸附于样品表面的分子氧物种, 该物种可能由过氧物种分解所产生. 本文3.2节将提供更多关于该谱峰本质的实验证据.

图5(a)为¹⁸O₂气氛下立方Nd₂¹⁶O₃在25 °C下用325 nm激光连续照射过程的Raman谱图变化. 可以看出,该谱图的变化规律与图3(a)十分相似. 随照射时间延长, 位于833 cm^-1附近的¹⁶O₂^2-谱带逐渐增强, 同时Nd³⁺-¹⁶O^2- (331 cm^-1)谱带逐渐减弱. 这清楚地表明Nd₂¹⁶O₃上的晶格氧(¹⁶O^2-)物种参与了过氧的生成. 样品经激光连续照射30 min后, 在1300-1800 cm^-1范围内可清晰检出三个位于1471, 1554和1640 cm^-1的谱峰(图5(b)). 其中位于1640和1554 cm^-1的谱峰可分别归属为¹⁶O₂^2-过氧物种的倍频峰和吸附于样品表面的¹⁶O₂物种的O-O伸缩振动峰; 位于1471 cm^-1的谱峰与气相¹⁸O₂的峰位(图4)一致, 可归属为气相¹⁸O₂物种的O-O伸缩振动. 鉴于图5的实验是在高纯度¹⁸O₂ (同位素纯度 > 97%)气流下进行的, 而图5(b)中¹⁶O₂的谱峰(1554 cm^-1)强度显著高于气相¹⁸O₂的谱峰(1471 cm^-1), 因此位于1554 cm^-1的谱峰不是源于气相的¹⁶O₂分子, 而是来自样品表面的¹⁶O₂物种. 由于位于1554 cm^-1的表面分子氧谱峰只有在生成了过氧的Nd₂¹⁶O₃样品上才能被清晰检出, 该物种可能源于过氧物种在光照下的分解. 研究表明, 488 nm激光的照射可导致NdO₂(O₂) (Nd的过氧化合物)的分解^{^[28]}.

前期研究表明, 分子氧的存在是稀土倍半氧化物上过氧生成的必要条件之一, 过氧物种由光照下分子氧对稀土倍半氧化物的晶格氧的氧化反应所产生^{^{[20, 21]}}. 这表明在以¹⁸O₂和¹⁶O^2-为反应物的情况下, 所生成的产物中将含有¹⁸O标记过氧物种. 然而图5只检测到与¹⁶O₂^2- (833 cm^-1)对应的过氧物种. 这可能是由于生成的过氧物种与Nd₂¹⁶O₃的晶格氧之间发生了快速的氧交换反应. 由于¹⁶O^2-是Nd₂O₃表面丰度最大的氧物种, ¹⁸O标记的过氧物种与Nd₂¹⁶O₃的晶格氧之间的快速氧交换最终将导致¹⁶O₂^2-的生成. 由于晶格氧与表面过氧或分子氧之间的快速氧交换, 由过氧分解产生的表面分子氧物种(1554 cm^-1)所含的氧同位素与过氧物种中的氧同位素完全相同. 可见, 在以¹⁸O为示踪原子的实验中, 我们可借助样品上吸附态分子氧的同位素谱峰来判断Nd₂O₃表面是否生成了同位素标记的过氧物种. 与过氧物种的Raman峰相比, 表面吸附态分子氧Raman峰的峰宽要窄得多, 因此具有更高的分辨率.

3.2. ¹⁸O₂气氛下¹⁸O标记的立方Nd₂O₃上过氧物种的光诱导生成

为了探明Nd₂O₃上过氧物种的光诱导生成机理, 我们进一步在¹⁸O₂气氛下对¹⁸O标记的立方Nd₂O₃样品的过氧物种光诱导生成进行了考察. 实验所用¹⁸O标记的Nd₂O₃样品系通过将Nd₂¹⁶O₃在650 °C和¹⁸O₂气氛下处理6 h并在¹⁸O₂气氛下降至25 °C获得. 与Nd₂¹⁶O₃样品的Raman谱相比, 处理后的样品上氧化钕的Nd³⁺-O^2-特征振动峰由336 cm^-1红移至320 cm^-1. 后者与采用谐振子模型(假设Nd³⁺-¹⁶O^2-的振动频率为336 cm^-1)计算出的Nd³⁺-¹⁸O^2-键的振动波数(319 cm^-1)十分接近, 表明在Raman光谱的检测深度内¹⁸O标记的立方Nd₂O₃样品表面绝大多数的晶格氧物种已被¹⁸O^2-所取代. 图6(a)为¹⁸O标记的立方Nd₂O₃在25 °C和¹⁸O₂气氛下用325 nm激光连续照射过程的Raman谱图变化情况. 在照射了1 min的样品上可清晰检出位于789 cm^-1的Raman谱带, 随照射时间增加, 在811 cm^-1处还出现一个肩峰, 该峰在激光照射15 min后已非常明显. 在经325 nm激光照射30 min后样品上可清晰检出三个位于1468, 1510和1554 cm^-1的Raman峰(图6(b)). 采用谐振子模型并以¹⁶O₂^2-过氧的伸缩振动频率(833 cm^-1)为基准, 可计算出¹⁸O₂^2-和(¹⁸O¹⁶O)^2-等过氧物种氧-氧键的伸缩振动波数分别为789和810 cm^-1. 同理, 以位于1556 cm^-1的¹⁶O₂分子氧伸缩振动频率为基准, 可计算出¹⁸O₂和¹⁸O¹⁶O等分子氧物种的氧-氧键伸缩振动波数分别1467和1512 cm^-1. 可以看出, 计算所得的¹⁸O标记过氧和分子氧物种的振动频率与图6结果十分吻合, 由于实验是在同位素纯度优于97%的¹⁸O₂气流中进行的, 图6(b)中位于1510和1554 cm^-1处的谱峰只能分别来自¹⁸O标记的Nd₂O₃样品表面的¹⁸O¹⁶O和¹⁶O₂等分子氧物种. 鉴于图6(b)中¹⁸O₂和¹⁶O₂谱峰的强度比显著高于图5(b)中相同物种的强度比, 且与图4中气相¹⁸O₂的Raman峰相比, 图6(b)中¹⁸O₂的峰位向低频方向位移了约3 cm^-1, 可合理地认为, 图6(b)中位于1468 cm^-1的谱峰主要也来自样品表面的¹⁸O₂物种. 这些结果表明, 在图6的实验条件下, ¹⁸O标记的Nd₂O₃样品表面生成了¹⁸O₂^2-, (¹⁸O¹⁶O)^2-和¹⁶O₂^2-等过氧物种. 由于实验是在¹⁸O₂气流中进行的, (¹⁸O¹⁶O)^2-和¹⁶O₂^2-等过氧物种中的¹⁶O原子只能来源于¹⁸O标记的Nd₂O₃样品, 这进一步证实Nd₂O₃上的晶格氧物种参与了过氧的生成. 结果还表明, 虽然经历了650 °C通¹⁸O₂气流处理6 h, Nd₂¹⁶O₃样品上的¹⁶O并未被¹⁸O完全取代.

从图6(a)中Raman谱图的变化趋势可以看出, 由于¹⁸O标记的Nd₂O₃样品表面的绝大多数晶格氧物种已被¹⁸O^2-取代, 分子氧(¹⁸O₂)首先与样品表面的¹⁸O^2-反应生成¹⁸O₂^2-过氧物种(789 cm^-1). 随着激光照射时间的延长, 样品表面还检测到与(¹⁸O¹⁶O)^2- (811 cm^-1)对应的过氧谱峰以及与¹⁸O¹⁶O (1510 cm^-1)和¹⁶O₂ (1554 cm^-1)对应的表面分子氧谱峰(图6(b)). 图6(a)中的(¹⁸O¹⁶O)^2-过氧物种可由氧化钕体相的¹⁶O^2-迁移到样品表面再与¹⁸O₂反应直接生成, 也可通过迁移到样品表面的¹⁶O^2-与¹⁸O₂^2-之间的同位素交换反应生成. 生成的(¹⁸O¹⁶O)^2-也可进一步通过与¹⁶O^2-的同位素交换反应转化为¹⁶O₂^2-过氧物种, 后者在实验条件下可分解生成吸附于样品表面的¹⁶O₂分子氧物种(1554 cm^-1). 325 nm激光的照射对¹⁶O^2-的迁移及其与¹⁸O₂^2-的交换可能具有促进作用, 为了获得相关的实验证据, 我们进一步在25 °C和¹⁸O₂气氛下考察了激光功率(0.3和3 mW)对¹⁸O标记的Nd₂O₃上过氧物种光诱导生成的影响. 如图7所示, 在用0.3 mW激光照射的14 min过程中, 样品上仅检测到位于790 cm^-1的¹⁸O₂^2-过氧物种(图7(a)), 说明在实验条件下几乎不发生¹⁶O^2-的迁移以及¹⁶O^2-与¹⁸O₂^2-之间的同位素交换反应. 当激光功率提高到3 mW后, 过氧谱带的峰宽显著增大(图7(b)), 在照射了5-20 min的样品上可清晰检出¹⁸O₂^2- (790 cm^-1)和(¹⁸O¹⁶O)^2- (811 cm^-1)的过氧谱带, 说明激光照射的确对晶格氧迁移及其与过氧之间的氧同位素交换反应具有促进作用. 研究表明, 光照可诱导¹⁸O₂与TiO₂晶格氧之间的同位素交换^{^{[29, 30]}}.

4. 结论

分子氧可通过在光的诱导下与立方Nd₂O₃的晶格氧反应转化为过氧物种, 在325 nm激光的照射下, 上述反应在室温下即可发生. 以¹⁸O为示踪原子的实验表明, 生成的过氧物种可与Nd₂O₃的晶格氧发生快速的氧交换反应, 位于Nd₂O₃体相的晶格氧也可迁移至样品表层进而参与过氧的生成. 325 nm激光的照射有助于促进晶格氧的迁移以及晶格氧与分子氧之间的氧交换.