過氧化氫（H2O2）從它已被用於各種工業中，為一種重要的人體生命化學品。由於全球人口和資源需求的增加，對這種化學品的需求正在增加。因此，與該化學品相關的研究課題，特別是關於如何通過考慮成本和環境問題來生產H2O2變得相當重要。其中，如何以直接合成過氧化氫（DSHP）來取代目前常用的間接合成法相當重要。在DSHP中，催化劑的選擇起著關鍵作用。而能夠理解反應機制是提高生產率和設計催化劑的關鍵。鑑於此，本論文研究用於DHSP的合金催化劑以及評價催化機制的密度泛函理論(Density Functional Theory, DFT)，進而尋找合適的催化劑。論文由四個主題組成。所有主題均使用計算方法完成，結果通過先前文獻中的實驗結果引用或驗證。

首先，DFT研究揭示用於過氧化氫生成的鈀汞合金(Pd-Hg)催化劑的幾何和電子協同作用。DSHP面臨的主要障礙之一是如何在催化表面上保持反應中間物的完整O-O鍵結。為了應對這一挑戰，Pd-Hg合金已被展示於初步報告上，與單金屬Pd和Pd-Au合金相比，Pd-Hg具有性能優勢；然而，O2與Pd-Hg合金間的相互作用尚未得到清楚的理解。在這項研究中，密度泛函理論（DFT）計算用於研究在Pd和Pd-Hg合金表面上的O2吸附性質，表明O2吸附可以通過超氧化物或過氧化物途徑發生，並且當Hg與Pd合金化時，與單金屬Pd表面上的吸附相比，表面吸附的超氧基團更多。Pd6Hg3 / Pd（111）中的Hg導致不同於Pd（111）的電子表面結構和降低的O2吸附能。較強的O2表面相互作用，當與較弱的O-O鍵結（吸附的O2）結合時，由於在Pd-Hg表面上存在Hg，導致協同的產生幾何和電子的協同效應，使直接合成過氧化氫期間的選擇性提升。
第二，採用密度泛函理論分析的描述子(descriptor)研究。用Pd-Au和Pd-Hg合金催化劑直接合成過氧化氫。眾所周知，DSHP中使用的所選催化劑會影響DSHP的生產率。具有各種過渡金屬（TM）成分的Pd基合金催化劑通常被認為可用於直接合成H2O2。特別是PdAu和PdHg合金是已知催化活性良好的催化劑。然而，找到合適催化劑並不容易，並且通常缺乏對增強活性背後機制的基本理解。為了達到這個目的，基於密度泛函理論提出descriptors集合，以表示直接H2O2合成中Pd，PdAu和PdHg表面上的整個反應步驟。通過考慮由表面合金化引起的電子效應，由反應中間體的吸附能量如O2，O和OOH以及計算其來自基本反應步驟活化能組成的descriptors集合。另外，還考慮吸附物種的幾何因子，儘管它們的作用不太顯著。進行O2對O（Eb.O2/O）的吸附能以確定表面吸附的O2*的存在被視為形成所需產物的中間物質。通過比較OOH與O（Eb.OOH/O）的吸附能來評估選擇性。考慮到主要的熱力學和動力學特徵，結果表明當PdHg合金的表面組成原子比為6：3（即2：1）時，其選擇性最好。基於分析的結果顯示具有較少活性金屬的合金Pd表面，例如Hg和Au，可以是設計催化劑以獲得更好的催化活性和選擇性的關鍵。

第三，以密度泛函理論分析直接合成過氧化氫的Pd基核殼催化劑中之descriptor研究。本主題旨在擴展第二種方法中使用的descriptors。計算的模型M6 @ Pd32具有截角八面體（Truncated Octahedron）結構，其中M可以是Pd，Ag，Cd，Pt，Au，Hg，Ni，Cu或Zn。在這項工作中，模型的結構是具有38個原子核 - 殼模型。基於這項工作， descriptor是選擇OOH與M6 @ Pd32上各種核層M上的O（Eb.OOH/O）的吸附能量的比較。催化劑的OOH吸附越高表明選擇性越高，O吸附能越低。另考慮反應動力因素，反應速率是O2的吸附能與O（Eb.O2/O）的比較，使用反應速率來確定催化劑的選擇性。通過計算M6 @ Pd32上OOH，O2和O的吸附能，可以確定比較催化劑的選擇性。根據計算，Ni6 @ Pd32和Zn6 @ Pd32顯示出良好的DSHP選擇性催化劑。另外，亦介紹與彈性相關的催化劑選擇性，同時穩定性與表面形變有關。表面柔韌性和變形表示基於吸附的O-催化劑結構的均方根差排（root mean square dislocation, RMSD）計算的幾何descriptor。結果表明，Ni6 @ Pd32催化劑比Zn6 @ Pd32更穩定，可直接合成過氧化氫。具有38個原子核 - 殼構型的整體研究表明，Ni @ Pd優於其他催化劑。

第四，通過DFT研究用於DSHP的核 - 殼PdNi @ Pd（111）催化劑中的高自旋Ni作用。根據前三部份的工作，O吸附能量與各種表面DSHP機制中的O2吸附能量具有相同的趨勢。通過DFT方法研究Pd（111），Pd3Ni @ Pd（111），PdNi @ Pd（111）和PdNi3 @ Pd（111）表面吸附的O，O（Eb.O）的吸附能量趨勢），根據不同的鎳組分進行探討。一旦已知O吸附能量（Eb.O）的趨勢，也可以預測O2吸附能量。與Pd（111）相比，Pd（111）上的Ni的存在降低了O吸附能（Eb.O）。較高的Ni組成導致較低的Eb.O。變化的成分Ni影響核 - 殼的幾何結構，即使當Pd：Ni的比例為1：1時，該結構也能夠（或將）從fcc變為fct。 Pd3Ni @ Pd（111）和PdNi @ Pd（111）的表面在Pd（111）上降低了14％的Eb.O。較低的Eb.O導致較低的Eb.O2。結果表明，核殼型Ni @ Pd較Pd（111）是更好的催化劑。然而，Eb.O是PdNi3 @ Pd（111）上最弱的，它可以將吸附的O2釋放到氣態。接下來，以能態密度（Density of States, DOS）來研究電子效應，同時計算了不同Ni組分的晶格變化以研究幾何效應。從不同Ni組分的d帶中心與Eb.O和晶格距離與Eb.O的比較，電子和幾何效應均顯示出對Eb.O的線性影響。然而，由DOS表示的電子效應顯示出對Eb.O變化的更敏感因素。通過這項工作，提供DSHP的設計PdNi合金的實驗基礎，實現高活性/高選擇性催化劑。

Hydrogen peroxide (H2O2) is an important chemical for human life since it has been used in various industries. The global need for this chemical is increasing due to increased population and wealth. Therefore, the research topics associated with this material, especially regarding how to produce H2O2 by considering cost and environment issues, become important. Among all, the direct synthesis of hydrogen peroxide (DSHP) is proposed to replace the indirect one. In DSHP, the chosen catalyst plays the key role. Being able to understand the reaction mechanism is the key to improve the productivity and to design a better catalyst. In view of this, the dissertation concerns the study of alloy catalyst for DHSP and the development of novel theoretical approach in assessing the catalytic mechanism, as well as finding suitable catalysts. The thesis consists of four main topics. All topics have been done using computational approaches, and the results are referred to or validated by experimental results in previous literature.

Firstly, DFT study reveals the geometric and electronic synergisms of palladium mercury alloy catalyst used for hydrogen peroxide formation. One of the main obstacles confronting the DSHP is how to maintain the unbroken O-O bonding of the intermediate species on the catalytic surface. To address this challenge Pd-Hg alloys have been used with initial reports suggesting their performance offers advantages when compared to monometallic Pd and Pd-Au alloys; however, the interactions of O2 with Pd-Hg alloys are not well characterized. In this study, density functional theory (DFT) calculations, employed to investigate O2 adsorption on the Pd and Pd-Hg alloy surfaces, suggested O2 adsorption can occur via either a superoxo or a peroxo pathway and that
when Hg is alloyed to Pd there are more adsorbed superoxo groups compared to adsorption on a monometallic Pd surface. The Hg in Pd6Hg3/Pd(111) results in an electronic surface structure different to that of Pd(111) and a reduced O2 adsorption energy. The stronger O2 surface interactions, when combined with weaker O-O bonding (of the adsorbed O2), which result from the presence of Hg on the Pd-Hg surface leads to synergistic geometric and electronic effects that result in an increased selectivity during of the synthesis of H2O2.

Secondly, descriptor study by density functional theory analysis for the direct synthesis of hydrogen peroxide using palladium–gold and palladium–mercury alloy catalysts. It is well-known that the chosen catalyst used in DSHP affects the productivity of DSHP. Pd-based catalysts with various compositions of transition metal (TM) alloys have been often considered for the direct synthesis of H2O2. In particular, PdAu and PdHg alloys are known catalysts for their good catalytic activity. However, finding a suitable catalyst with designed composition is not easy, and fundamental understanding of the mechanism behind the enhanced activity is often lacking. To facilitate the quest, descriptor sets are proposed based on Density Functional Theory to represent the whole reaction steps on Pd, PdAu and PdHg surfaces in direct H2O2 synthesis. By considering surface electronic effect caused by surface alloying compositions, descriptor sets consisting of the adsorption energy for the reaction intermediate such as O2, O and OOH and activation energy barriers are derived from elementary reaction steps. The geometric factors of adsorbed species are also considered, though they are found less prominent. The adsorption energy of O2 versus O (Eb.O2/O) is performed to determine that the presence of surface adsorbed O2* is seen as the required intermediate species to form desired product. The selectivity is assessed by comparing the adsorption energy of OOH versus O (Eb.OOH/O). Considering main thermodynamic and kinetic characteristics, the results show that PdHg alloy with the surface composition in the atomic ratio of 6:3 (namely 2:1) gives the best selectivity among others. Based on the results of the descriptor analysis, it is suggested that the alloyed Pd surface with less active metals, such as Hg and Au, can be the key to designing catalysts for better catalytic activity and selectivity.

Thirdly, descriptor study by density functional theory analysis for the direct synthesis of hydrogen peroxide using palladium–base core-shell catalyst. This study is intended to expand the descriptor use found in the second approach. The calculated model M6@Pd32 has truncated octahedron (TO) structure, where M can be Pd, Ag, Cd, Pt, Au, Hg, Ni, Cu, or Zn. In this work, the structure of the model has been proven to be the most stable of the other structures with 38 atoms core-shell model. Based on this work, a selectivity descriptor is the comparison of the adsorption energy of OOH with O (Eb.OOH/O) on the various core M on M6@Pd32. The higher catalyst selectivity indicates the higher OOH adsorption and the lower O adsorption energy. The selectivity of the catalyst is confirmed using the reaction rate which is a comparison of adsorption energy of O2 versus O (Eb.O2/O). By calculating the adsorption energy of OOH, O2, and O on M6@Pd32, the catalyst selectivity can be determined. Based on the calculation, the Ni6@Pd32 and Zn6@Pd32 showed the good selectivity catalyst for DSHP. I also introduce the catalyst selectivity related to the flexibility, while the stability connected to the surface distortion. Surface flexibility and distortion represent the geometric descriptors which calculated based on the root mean square dislocation (RMSD) of the adsorbed O-catalyst structure. The result showed that the Ni6@Pd32 catalyst is more stable than Zn6@Pd32 for direct synthesis of hydrogen peroxide. The overall study with 38 atoms core-shell configuration shows that Ni@Pd outperforms other catalysts.

Fourthly, a study of the high spin Ni role in the core-shell PdNi@Pd(111) catalyst for the DSHP by DFT. Based on my previous work, the O adsorption energy has the same trend with the O2 adsorption energy in DSHP mechanism on the various surfaces. By investigating the adsorbed O on the surface of Pd(111), Pd3Ni@Pd(111), PdNi@Pd(111), and PdNi3@Pd(111) using DFT approach, the trend of adsorption energy of O (Eb.O) has been captured based on the varied Ni composition. Once the trend of O adsorption energy (Eb.O) has been known, the O2 adsorption energy also can be predicted. The presence of Ni on the Pd(111) lowering the O adsorption energy (Eb.O) compared with that on Pd(111). The higher composition Ni leads to the lower Eb.O. The varied composition Ni affects the geometrical structure of the core-shell, even when the ratio of Pd:Ni is 1:1, the structure is able to (or will) change from fcc to fct. The surfaces of Pd3Ni@Pd(111) and PdNi@Pd(111) lowered 14% of Eb.O on Pd(111). The lower Eb.O, the lower Eb.O2. The result indicated the reason of why the core-shell Ni@Pd can be better catalyst than Pd(111) for DSHP. However, Eb.O is the weakest on the PdNi3@Pd(111) which is possible to release the adsorbed O2 to the gas state. The density of state (DOS) is investigated to study the electronic effect, while the lattice change of varied Ni composition is calculated for investigating the geometry effect. From the comparison of d-band center versus Eb.O and lattice distance versus Eb.O on varied Ni composition, both electronic and geometric effect showed the linear effect to the Eb.O. However, the electronic effect which is represented by DOS showed the more sensitive factor to the Eb.O change. By this work, the wet experiment activity is offered to realize the catalyst finding such as PdNi alloy used for DSHP.

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