DFT Study on the Descriptor investigation in Direct Synthesis of Hydrogen Peroxide for Pd-Base Alloy Catalysts

Autor:	Mawan Nugraha
Rok vydání:	2019
Druh dokumentu:	學位論文 ; thesis
Popis:	107 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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Externí odkaz:	http://ndltd.ncl.edu.tw/handle/7v9u2h Zobrazit plný text záznamu