| 摘要: | 本研究以共沉澱法將不等量MnOx引入CeO2製備具良好redox特性的共溶性氧化物Ce1-xMnxO2作為擔體，並以含浸法製備Ag/Ce1-xMnxO2觸媒，進行揮發性有機物苯的全氧化反應，探討煅燒溫度、Ce/Mn比例、Zr的引入及Ag負載量對反應的影響。反應測試是利用空氣進入飽和蒸氣瓶內帶出定量(500~1500 ppm)的苯蒸氣於F/W = 6000~24000 ml gcat-1 h-1條件下進行苯的全氧化反應，並採BET、XRD、Raman、H2-TPR及XPS等分析方法探討觸媒之物理特性與表面性質。CeO2中引入Mn能增加其表面積，一部分Mnx+進入CeO2晶格形成良好固溶的Ce1-xMnxO2共氧化物，使晶格氧容易釋出提升redox特性，一部分MnOx則均勻分散於擔體表面，增加表面的Mn3+及缺陷氧比例；當Mn引入量x > 0.5，擔體表面的MnOx明顯聚集。以共沉澱法引入0.1的Zr製得Ce0.9-xZr0.1MnxO2擔體能使表面積再略增加，並提升redox能力。 7%Ag/Ce1-xMnxO2觸媒的苯全氧化活性優於單獨的7%Ag/CeO2及7%Ag/MnOx觸媒，活性隨Mn引入增加遞增，以7%Ag/Ce0.6Mn0.4O2有最佳活性表現(T100為185 °C)。Zr的引入對Ag/Ce0.9-xZr0.1MnxO2觸媒活性無促進效果。Ag負載於不具redox特性的SiO2與γ-Al2O3即有相當好的苯全氧化活性，觸媒表面銀價態主要為Ag0，Ag2O次之，Ag2O活性略優於Ag0。Ag/CeO2觸媒活性較Ag/SiO2觸媒佳，擔體redox特性是銀觸媒好活性的必要條件。Ce0.9-xZr0.1MnxO2比Ce1-xMnxO2更具redox特性，兩擔體本身對苯即有不錯的反應活性，前者優於後者，負載Ag後反應活性大幅提升，且活性相當。Ag為苯全氧化反應重要的活性中心，擔體redox強弱並非影響觸媒活性的主要關鍵，均勻分散於觸媒表面的MnOx亦扮演重要的角色。Ag/Ce1-xMnxO2觸媒於苯的全氧化反應經由四路徑進行：Ce1-xMnxO2擔體表面能提供活性點(均勻分散的MnOx與缺陷氧)吸附苯進行氧化反應，將苯氧化成CO2及H2O。Ag本身會吸附苯，Ce1-xMnxO2擔體可直接提供晶格氧進行氧化反應。Ag2O會吸附苯，並釋出氧進行氧化反應，Ce1-xMnxO2擔體提供晶格氧使Ag2O復原。此外，Ag會吸附氧，與苯直接進行氧化反應。 Ag/Ce1-xMnxO2觸媒進行苯的全氧化反應有相當好的活性表現，優於Ce1-xMnxO2擔體負載的一般金屬CuO與貴金屬Pd觸媒，能處理的苯濃度範圍相當廣，於適當的反應條件下有相當高的反應穩定性，應用於揮發性有機物苯的去除是一相當不錯的觸媒選擇。In this research, a series of Ce1-xMnxO2 mixed oxide supports with different compositions were prepared by co-precipitation method for enhancing the redox properties of the CeO2. Ag/Ce1-xMnxO2 catalysts were prepared by impregnation method and their performance in the oxidation of volatile organic compound benzene was evaluated. In addition to the effect of calcination temperature, the ratio of Ce/Mn, the incorporation of Zr and Ag loading were investigated, the roles of Ce1-xMnxO2 mixed oxides and Ag were also discussed. The benzene vapor feed was diluted with air into the reactor at the flow rate of 100 ml/min (F/W = 6000~24000 ml h-1gcat-1). The physical and surface properties of the prepared catalysts were characterized by BET, XRD, Raman, H2-TPR and XPS. Incorporating maganese into CeO2 increased the specific surface area of supports. Part of manganese species which entered into the ceria lattice to form Ce1-xMnxO2 solid solutions improved the redox properties, and the increase of Mn3+ and defect oxygen were caused by the part of MnOx dispersed on the surface of Ce1-xMnxO2. The aggregation of MnOx on surface occurred when the fraction of Mn in Ce1-xMnxO2 exceeded 0.5. The specific surface area and redox properties of supports could be enhanced through the incorporation of Zr into Ce1-xMnxO2 to form Ce0.9-xZr0.1MnxO2.The reactivities of the Ag catalysts supported on Ce1-xMnxO2 were higher than those supported on CeO2 and MnOx catalysts. The best performance in total oxidation of bezene was 7%Ag/Ce0.6Mn0.4O2 catalyst (T100 = 185 °C). It had no promoting effect on Ag/Ce0.9-xZr0.1MnxO2 catalysts with the introduction of Zr. The Ag catalysts supported on SiO2 and γ-Al2O3 supports without redox properties showed good activity for benzene oxidation. There were two oxidation states of silver, Ag0 and Ag+, on the surface of catalyst. The dominant species of silver was Ag0, but Ag+ was the more active of the two. The activity of Ag/CeO2 catalyst was higher than Ag/SiO2, therefore, the redox properties of supports that enhanced the catalytic activity were necessary. The redox properties of Ce0.9-xZr0.1MnxO2 were more than that of Ce1-xMnxO2, and the catalytic activity of former was also better than that of latter over benzene oxidation. However, the reactivity of Ag/Ce0.9-xZr0.1MnxO2 was equivalent to that of Ag/Ce1-xMnxO2. Ag was an important active species for total oxidation of benzene, and the part of the maganese that was in the form of well dispersed phase of MnOx also played a key role of catalytic oxidation of benzene. Nevertheless, the degree of redox behavior of supports was not a key to the reactivities. Oxidation of benzene over Ag/Ce1-xMnxO2 was along four paths. Benzene that had been adsorbed on Ce1-xMnxO2 was oxidized by the release of oxygen from Ce1-xMnxO2. The benzene could be adsorbed on Ag and oxidized by oxygen that was released from Ce1-xMnxO2. The benzene could be adsorbed on Ag2O as well, and oxidized by oxygen that was released from Ag2O. Moreover, oxygen was also adsorbed on Ag and then reacted with adsorbed benzene. Ag/Ce1-xMnxO2 catalysts whose catalytic performance in the oxidation of benzene were better than based metal CuO and noble metal Pd catalysts exhibited high reaction stability in the suitable condition, and that could eliminate a wide range of concentration of benzene. It is a very ideal and practical catalyst for complete oxidation of benzene. |
