利用第一原理探討CO分子吸附於Mn-modified CeO2(111)表面上之氧化反應

CO adsorption on the Mn-modified CeO2(111) surface: A First-Principles study

徐令潔(Ling-Chieh Hsu)

指導教授 ： 蔡明剛

國立臺灣師範大學/理學院/化學系/碩士(2012年)

摘要

我們使用自旋極化週期性密度泛函理論來研究CO分子吸附於Mn-doped CeO2(111)表面上之氧化反應，將Mn dope 到CeO2(111)表面，來修飾純CeO2(111)表面的活性、氧空缺的形成和分子吸附於表面的應用與氧化反應機制。在本論文中，我們在密度泛函理論之上並加入Hubbard U term (DFT+U)做計算，研究Mn dope到CeO2(111)表面上的Ce1-xMnxO2和CO吸附的結構。

將錳原子摻雜到表面中，將錳原子取代掉表面中的鈰原子，會導致強烈的結構性扭曲和較小的氧空位形成能。我們計算結果表示，Mn原子的dope有利於形成氧空缺和CO的氧化作用。當我們將Mn原子吸附於CeO2(111)表面上，則會抑制氧空缺的形成，使表面不易形成氧空缺，也不利於CO的氧化反應。將CO吸附於三種不同表面時，只有在純的CeO2(111)表面上會形成CO的物理吸附，而在Ce0.875Mn0.125O2(111)的表面上CO的吸附結會為CO的物理吸附、CO的化學吸附會形成carbonite (CO2–)。CO2的物理吸附，也計算振動頻率來確定這些穩定結構的準確性。接下來的計算加入O2分子於表面形成O原子 ，使表面恢復回Ce0.875Mn0.125O2 (111)表面，可以循環再生利用。另外，我們也計算Bader charge去分析表面上Mn原子與CeO2的氧化狀態。

關鍵字

CO吸附 ； Mn-dope CeO2(111)表面 ； 第一原理 ； DFT+U計算

並列摘要

We use spin-polarized Density Functional Theory with on-sit columbic repulsion (Hubbard U term) as DFT+U to study CO oxidation on the Mn-doped CeO2(111) surface with being denoted as Ce1-xMnxO2. The dopant – Mn is used to increase the surface activity of oxygen vacancy formation, CO absorption, and the subsequent CO oxidation.

Mn substitution in CeO2 results in significant structural distortions and oxygen vacancy formation energy reduction as compared to undoped CeO2 surface The CO oxidation is also facilitated by the presence of Mn dopant. If the Mn is only absorbed on CeO2 surface, the oxygen vacancy formation and the CO oxidation would be both hindered. Only physical absorption was found on the undoped CeO2(111) surface in comparison with CeO2(110) and CeO2(100) surfaces. CO being absroped on Ce0.875Mn0.125O2 (111) would automatically turn to the formation of carbonite (CO2–). Vibrational frequency analysis was carried out to confirm the existence of the stable geometry. The subsequent calculation takes into account the presence of O2 molecule where O2 would be decomposed into two O atoms on surface. The O atom could regenerate the Ce0.875Mn0.125O2 (111) surface in order to continue the next catalytic cycle. The Bader charge analysis is carried to characterize the oxidation state of Mn ions along the catalytic cycle.

並列關鍵字

CO adsorption ； Mn-doped CeO2 ； First-Principles study ； DFT+U

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