|关键词||Co 氧化 Co 加氢 表面谱学 Pt(111) Zno|
在现代煤化工领域中如费托合成、合成天然气、合成氨等，CO催化转化对于缓解油品的短缺，合理化利用煤炭资源具有重要的意义。此外，CO催化转化除了在燃料电池中富氢气氛下CO的去除、汽车尾气处理等方面有着广泛的应用，也常常作为基本模型反应用于研究催化反应的微观机理。CO催化转化主要分为CO氧化和CO加氢两种方式。一方面，在过渡金属催化剂表面，多数研究认为CO氧化反应机理符合Langmuir-Hinshelwood机理，但是仍有研究对此结论提出了异议，认为CO氧化反应符合Eley-Rideal或Mars-van Krevelen机理。究其原因是对于反应的中间物并不明确。目前虽然对中间物种的研究已有进展，但是对室温下Pt(111)表面CO氧化反应中间的研究还是非常少。另一方面，ZnO作为一种优良的催化剂可以催化CO加氢合成CH3OH；并且，CO2也能够通过ZnO催化加氢生成CH3OH。但是，在ZnO催化CO加氢的过程是否有部分CO发生歧化反应生成CO2后加氢却鲜有文献报道。基于以上问题，本论文利用表面谱学的方法对Pt(111)表面发生的CO氧化反应和ZnO表面发生的CO加氢反应的微观过程分别进行了研究。主要内容如下：（1） 利用原位RAIRS和LEED对Pt(111)表面O2的吸附行为进行了研究。研究发现，Pt(111)表面暴露O2后形成的原子氧以（2 × 2）结构吸附在Pt(111)表面。真空腔体中残留的CO吸附在表面后会助于O2的解离，而且会扩散到原子氧附近，与之结合后形成CO氧化反应的中间物，在红外谱图上会有1718 cm-1处的吸收谱峰出现。（2） 利用原位RAIRS和TPSR对Pt(111)表面发生的CO氧化反应进行了研究。无论是在预先吸附O2的Pt(111)表面暴露CO还是在预先吸附CO的Pt(111)表面暴露O2，在1452 cm-1处均发现了Pt(111)表面CO氧化反应的中间物C-O…O-O的吸收谱峰。只有CO和O2同时存在于表面时，中间物C-O…O-O的吸收谱峰才会存在，CO或者O2的量任一增加都会使该吸收谱峰消失。结合同位素置换实验，对中间物种的振动进行了归属。CO既能与表面分子氧反应生成CO2，又能与亚表面氧反应生CO2。500 K以下CO与表面分子氧反应生成CO2，O2活化是反应速控步骤；500 K以上CO与亚表面氧反应的生成CO2，CO吸附是反应速控步骤。CO和O2在表面的吸附能力不同，使得CO氧化生成CO2最佳配比是1:5，而不是化学计量的2:1。（3） 利用PM-IRRAS对近常压下Pt(111)表面发生的CO氧化反应进行了研究。研究发现，CO在Pt(111)表面的实际覆盖度会随着CO暴露压力的增加而增加。此外，在近常压环境下，如果O2的含量比较高，在常温下都可能使Pt(111)表面被氧化，降低CO在Pt(111)表面的吸附能力。（4） 向CO/O2混合气中引入H2，利用TPSR对H2作用下Pt(111)表面发生的CO氧化反应进行了研究。研究发现，在合适的CO与O2的比例中，H2的引入会抑制CO与亚表面氧的反应，增强氧气的解离形成原子氧，使CO与表面氧反应生成的CO2增加。但是CO/O2混合气中O2的含量过高时，H2的引入会同时抑制CO和表面氧与亚表面氧的反应，使CO2的生成量降低。将ZnO表面经CO高压处理后，进行TPD实验。研究发现，CO除了会与晶格氧发生作用生成CO2之外，还会在常温发生歧化反应。CO歧化反应的产物是CO2和C。红外结果表明CO2在常温会以碳酸盐的形式吸附在表面，碳酸盐在600 K以前即可分解完全。C在高温时会与晶格氧反应被氧化生成CO2。结合同位素置换实验对此结果进行了论证。将ZnO经CO/H2高压处理后，进行TPD实验。研究发现，CO发生了加氢反应生成了碳氢化合物，在TPD实验中分解为CO2和H2O。
The catalytic conversion of CO is of great significance for alleviating the shortage of oil products and rationalizing the utilization of coal resources in modern coal chemical such as Fischer-Tropsch synthesis, synthetic natural gas, ammonia synthesis and so on. In addition, it also has a wide range of applications not only in removal of CO at hydrogen-rich atmosphere in fuel cells and exhaust gas treatment, but also as a model reaction to study the microscopic mechanism of catalytic reactions. Catalytic conversion of CO is mainly divided into two ways including CO oxidation and CO hydrogenation. On the one hand, most studies suggested that the mechanism of CO oxidation reaction conformed to Langmuir-Hinshelwood mechanism on transition metal surface. However, there are still some debates on this conclusion. Some groups thought that CO oxidation reaction conformed to the Eley-Rideal or Mars-van Krevelen mechanism. The reason is that the intermediate for the reaction is still not clear. Although there have been some advances in the study of intermediate species, there has been little research on CO oxidation reaction on Pt(111) surface at room temperature. On the other hand, as an excellent catalyst for the synthesis of CH3OH, ZnO could catalyze CO hydrogenation to CH3OH and also catalyze CO2 hydrogenation to CH3OH. It is rarely reported that whether existence of portion of CO disproportionation to produce CO2 and subsequently CO2 hydrogenation to CH3OH. Based on the above issues, surface spectroscopic methods have been used to study the microscopic processes of CO oxidation reaction on Pt(111) surface and CO hydrogenation reaction on ZnO surface in this dissertation. Major contents were described as follows:(1) The adsorption behavior of O2 on Pt(111) surface was studied by in situ RAIRS and LEED. It was found that O2 dissociated to be atomic oxygen after exposing Pt(111) surface to O2, which formed (2 × 2) structure on the Pt(111) surface. After the residual CO in the vacuum chamber was adsorbed on the surface, it would contribute to the dissociation of O2. The residual CO could also diffuse to nearby atomic oxygen and combine with atomic oxygen to form an intermediate of CO oxidation reaction, which would be shown an absorption peak at 1718 cm-1 in the IR spectrum.(2) CO oxidation reaction on Pt(111) surface was studied by in situ RAIRS and TPSR. Whether the CO was exposed to O2-Pt(111) surface or O2 was exposed to CO-Pt(111) surface, the peak of intermediate C-O…O-O of CO oxidation on Pt(111) surface was found at 1452 cm-1. The peak of 1452 cm-1 appeared only co-existing both CO and O2 on the surface. Increment of either CO or O2 could make the peak disappear. In combination with the isotope displacement experiment, the vibration of intermediate species was assigned. It was also demonstrated that could CO not only react with surface oxygen but also with subsurface oxygen to produce CO2. CO could react with surface molecular oxygen to produce CO2 below 500 K and O2 activation was rate-determining step. CO could react with subsurface oxygen to produce CO2 above 500 K and CO adsorption was rate-determining step. The optimum ratio of CO and O2 was 1:5 to produce CO2 instead of the stoichiometric 2:1. This is caused by the difference in the adsorption capacity of CO and O2.(3) PM-IRRAS has been used to study CO oxidation on Pt(111) surface at near ambient pressure. It was found that the actual CO coverage on Pt(111) surface increased with increment of pressure. In addition, Pt(111) surface might be oxidized even at room temperature if the pressure of O2 was relatively higher (close to atmospheric pressure) which would decrease the CO adsorption capacity on Pt(111) surface.(4) TPSR has been used to study the effect of H2 on CO oxidation on Pt(111) after H2 was introduced into the CO/O2 mixture. It was found that H2 could inhibit the reaction of CO with subsurface oxygen and enhance the dissociation of oxygen that increasing the amount of CO2 when CO react with surface oxygen in the appropriate ratio of CO to O2. However, when the content of O2 in the CO/O2 mixture is too higher, the introduction of H2 would also inhibit the reaction of CO with surface oxygen and subsurface oxygen, resulting in a decrease in the production of CO2.(5) TPD experiments were performed after ZnO surface was subjected to high pressure treatment with CO. It was found that not only could CO react with the lattice oxygen to produce CO2, but also CO disproportionation occurs even at room temperature when ZnO was treated at high pressure with CO. The products of CO disproportionation reaction are CO2 and C. It was demonstrated that carbonate could be formed after CO2 adsorbed on the surface at room temperature and decomposed completely before 600 K through IR experiment. C could be oxidized with lattice oxygen at higher temperature to produce CO2. Those results were confirmed by isotope experiments. TPD experiments were also performed after ZnO surface was subjected to high pressure treatment with CO/H2. It was found that CO was hydrogenated to produce hydrocarbons, which were decomposed into CO2 and H2O in the TPD experiment.
|王璇. 一氧化碳催化转化的表面谱学研究[D]. 北京. 中国科学院大学,2018.|
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