二氧化碳分子在CaO(100)表面上的氫化反應理論計算研究

Abstract

我們利用DFT (density functional theory)-GGA 的方法，對於二氧化碳與Ni / CaO的作用方式的計算，結果包括了以下幾個部份: (1)二氧化碳在CaO (100)的吸附結構。(2) 1Ni在CaO (100)的吸附情形。 (3)二氧化碳在Ni / CaO (100)的吸附結構。(4)H2O在Ni/CaO(100)表面的解離反應探討(5)二氧化碳在Ni / CaO (100)氫化至HCOOH的可能反應機構。(6) HCOOH繼續反應至CH3OH的反應機制。並且與實驗上提出的反應機構和其他理論計算的結果作進一步的比較。 在結果中我們發現，在純的CaO(100)表面上吸附CO2的吸附能是36.07 kcal/mol，而其吸附位向CO2的O原子與表面上Ca原子dihedral angle = 0。，相較於dihedral angle = 45。的情況，吸附能較大。接著，我們又嘗試添加一顆過渡金屬Ni於CaO(100)表面上，發現Ni/CaO(100)表面對於CO2分子的吸附能達到45.231kcal/mol，比純的CaO(100)表面大了許多，我們便以此位向尋找氫化的反應途徑。 接下來，我們以水分子解離途徑，找出H原子在Ni/CaO(100)表面上的最佳吸附位向，作為CO2氫化反應的初始表面。從結果中發現，H2O分子的分解，比較有可能的路徑中，斷第一個氫原子只需要4.622 kcal/mol的能障，接著繼續跨越過20.092 kcal/mol的能障即可將水分子分解。 接著，我們將吸附於表面上O原子的氫定為H1，吸附於Ni原子上的氫定為H2，利用此氫化表面，將CO2吸附進來後氫化，使用NEB的方式尋找過渡狀態結構。We applied the periodic densityfunctional theory (DFT) to investigate the carbon dioxide adsorption sites on CaO(100) and Ni/CaO(100) surface, and we found that the largest adsorption energy was on the Ni/CaO(100) surface and calculated to be -45.20 kcal/mol. So, we used this surface as our followed hydrogenation reactions surface. We also investigated water dissociation, the first hydrogen was dissociated to oxygen on the surface with barrier 4.32 kcal/mol and the second hydrogen was with the barrier 20.09 kcal/mol. We started the hydrogenation reaction with carbon dioxide adsorbed on the hydrogenated Ni/CaO(100) surface. The hydrogenation reaction of CO2 was first forming stable formate and carboxyl structure, and then continued to form formic acid and methanol. The adsorbed formic acid was easily dissociated to adsorbed HCO and OH with the dissociation barrier¬¬ of 2.437 kcal/mol. Subsequently, the species (HCO+OH) continued to form formaldehyde (H2CO). We found that H2CO was a starting species to synthesis methanol. We also found pathways and potential energy surfaces to form methanol via CH2OH and CH3O intermediate.