【作者】 曲振平；

【导师】 包信和；

【作者基本信息】 中国科学院研究生院（大连化学物理研究所） ， 物理化学， 2003， 博士

【摘要】 聚合物电极膜燃料电池（PEMFC）相对于传统的内燃机有高的能量转换效率以及零污染排放（ZEV），因此PEMFC在电动车电源上的应用受到了广泛的关注。目前燃料电池的氢源主要采用甲醇、天然气等碳氢化合物的水汽重整、部分氧化和热重整等技术得到。由于PEMFC的操作温度较低（80℃），Pt电极很容易被重整气中微量的CO（1％）所污染，因此消除氢源中的CO对PEMFC的应用十分重要。在消除富氢中CO的各种方法中，CO催化氧化无疑是最简单、最廉价和最有效的方法。迄今为止，人们研究更多的是采用Pt、Rh、Au等贵金属催化剂来催化氧化CO。从化学工艺及降低成本考虑，降低反应温度、寻找廉价金属取代或部分取代贵金属将是今后研究CO选择氧化催化的主要目标。众所周知，相对廉价的银是一种较好的乙烯环氧化催化剂，并对CO氧化有较好的反应活性。 为此本论文选择担载银为CO选择氧化反应催化剂，系统考察了载体、银担载量和反应温度等对催化剂反应活性和选择性的影响；详细研究了银催化剂的结构和粒子尺寸与催化剂活性之间的关系，提出了一个预处理对银催化剂表面再构、粒子重分散的模型以及CO在银催化剂上可能的反应机理，得到了如下的研究结果： 1．研究表明SiO₂是CO选择氧化反应银催化剂的良好载体，一定浓度和分布的表面羟基有利于银粒子在载体上的分散。CO选择氧化活性随着分子筛载体硅铝比的提高而提高，Ag-Al强相互作用降低了银催化剂的反应活性，表明担载银催化剂的载体不需要酸性。全硅SBA-15、MCM-41分子筛也是较好的银催化剂的载体，这为中孔全硅分子筛在催化中的应用开辟了一条新的道路。 2．Ag/SiO₂催化剂对消除CO有较好的低温反应活性和稳定性，低温更有利于CO的氧化；随着银担载量的提高，CO氧化的反应速率增加，并且反应温度降低。银催化剂中的次表层氧物种的含量随着银含量的增加而增多，高担载银催化剂上一氧化碳选择氧化反应的研究量的银催化剂可为Co氧化反应提供更多的【O]物种，CO与【O]物种的反应能在更低温度下（一10oC）进行。 3.氧气低温（100一300oC）处理预先己用氢气SO0oC处理过的银催化剂，由于存在阻碍反应物吸附的强吸附表面氧物种，从而降低了催化反应活性和选择性，由此表明干净的银催化剂表面对提高CO选择氧化反应活性和选择性起到至关重要的作用。随着氧气处理温度的提高（>350oC），催化剂的低温反应活性和选择性显著提高。在氧气气氛下，银粒子的稳定性在大于350℃明显降低，且在银体相中形成更多的次表层氧物种。氧气高温处理后的银催化剂再用氢气低温处理明显提高了银催化剂的反应活性，这主要是由于氧气处理形成的再构表面经低温氢气处理后，银粒子进一步分散造成的。但高温的氢气处理却降低了催化剂反应活性。氧气重新高温处理又可重新恢复银催化剂反应活性和选择性。 4.在氧气高温处理后再用氢气低温（100一300oC）处理过的银催化剂上，用原位红外技术首次观测到CO在一750C就可以发生表面氧化反应。随着氢气处理温度的升高，在银表面的CO吸附峰向低波数移动，表明银表面的电荷密度逐渐增加。CO在银粒子上的吸附强度随着CO吸附平衡压力的降低而减少，且它的覆盖度在反应温度下低于吸附饱和值。CO和O:不存在竞争吸附，因此银催化剂相对于R催化剂有较好的低温氧化选择性，反应温度的提高增强了CO的脱附能力及氢气氧化活性，从而降低了催化剂反应选择性。 5.提出一个氧气一氢气处理引起的银表面再构、粒子重分散的模型。氢气高温处理使银形成紧密堆积结构;氧气高温处理使银催化剂发生表面再构，形成大量的次表层氧物种。CO更容易吸附在已发生再构的银表面上。随后低温氢气处理提高了载体上银粒子的分散度;但高温氢气（>4O0oC）处理消除掉更多的次表层氧物种，同时也引起了银粒子的聚集。再重新用氧气高温处理又恢复了原先的再构表面。合适的氧气一氢气处理对载体上银粒子的分散十分重要。摘要 6.对CO在银催化剂上的氧化反应机理进行了初步探索，并提出了一个可能的反应模型。CO首先快速与表面吸附的氧物种反应，从而促进了氧气在银表面的吸附平衡，同时CO与高温氧气处理形成的次表层氧物种反应，次表层氧的存在促进了氧物种向银的次表层迁移，补充反应掉的次表层氧物种，然后再与吸附的CO反应生成COZ，从而构成了银催化剂表面的氧化一还原循环。高温氧气处理后的低温氢气还原形成更多表面粗糙、球形、尺寸更小的纳米银粒子，提高了反应物在银表面的吸附，而高温氢气处理不仅使银粒子发生聚集，而且也消除了大量的次表层氧物种，阻碍了表面氧物种向次表层的迁移，从而降低了反应活性。更多还原

【Abstract】 Polymer electrolyte membrane fuel cells (PEMFC), which have better energy efficiency than the conventional combustion engines and a zero-emission of air pollutants, have received much attention as a potential power source of electric vehicles. Restricted by the distribution and storage of hydrogen, the H2 feed gas of PEMFC is usually produced by steam reforming, partial oxidation or combination of the above techniques from methanol, natural gas. Since the PEMFC is operated at relatively low temperatures (80℃), its Pt-anode catalyst is extremely sensitive to CO contaminant (1%) in reformed gases, which will poison the catalyst and decrease the performance. Therefore, it is essential to remove the trace CO in Ha feedgas. Among the currently available methods for removing CO from H2-rich feedgas, the selective catalytic oxidation of CO with molecular oxygen is undoubtedly the most straightforward, simplest, and the most economic one. Catalysts so far proposed for this process are mainly noble metals, such as Pt, Rh, Au. However, decreasing the reaction temperatures and seeking for more economic catalysts for CO selective oxidation are the research focuses of the near future. It is well known that silver catalysts are unique for partial oxidation reactions and is not so precious as other noble metals used currently.Based on this idea, silver catalyst is a good candidate for CO selective oxidation in H2 feedgas. The effects of supports, silver loading, and reaction temperatures et al. on the activities of silver catalysts, as well as the correlation between the surface structure of silver and reaction activity, are investigated. The surface restructuring, redispersion model and the reaction mechanism for CO oxidation over silver surfaceare proposed. The following research works have been conducted:1. SiO2 is a good support of silver catalysts for CO selective oxidation. An appropriate content and distribution of silanol favors the silver particles dispersion. For zeolite support, the catalytic activity increases with the increasing of Si/Al ratio of the zeolites. The strong interaction between Ag and Al decreases the catalysts activity. Mesoporous silica materials as SB A-15, MCM-41 zeolites are also found to be better supports of silver catalysts for CO selective oxidation at low temperatures.2. Ag/SiO2 catalysts have a good performance for CO selective oxidation at low temperatures, and are quite stable. With the increasing of silver loading, the reaction rate increases and reaction temperature decreases. TPD and CO-TPSR results show that the amount of sub-surface oxygen increases with the silver loading. Thus more active [O] species will be supplied, and react with CO at much lower temperatures (-10) over silver catalyst with higher loading.3. The surface oxygen species formed by oxygen treatment at low temperatures (100-300) following H2 treatment at 500 block the adsorption of gas oxygen species, and further deactivate the silver catalysts, indicating that it is very essential to obtain a clean silver surface for CO selective oxidation. However, pretreatment with O2 at higher temperatures (>350) distinctly improves the catalytic performance at low temperatures. The stability of silver particles decreases at higher temperatures than 350, and more sub-surface oxygen species are formed. Reduction with H2 at low temperatures following the oxygen treatment at high temperatures increases the catalytic activity due to the further dispersion of silver particles; however, hydrogen reduction at higher temperatures decreases the activity and selectivity. Oxygen treatment at high temperatures will reactivate the silver catalysts. Interestingly, the changes in activity are mostly reversible.4. It is firstly observed that the reaction of CO and oxygen over silver catalysts treated by H2 at low temperatures (100-300℃) following oxygen treatment at 500℃ may occur at -75 by In-situ FTIR technique. Adsorbed CO gives a lower frequency after H2 reduction due to an increase in elec更多还原