計算水煤氣轉移反應在金與鉑(100)、(110)、(111)、(211)表面上的反應機構

Abstract

水煤氣轉移反應最佳活性的催化劑金、鉑系統性的檢驗在(111)、(100)、(110)、(211)表面結構效應對於反應的影響。首先計算水煤氣轉移反應重要的中間產物(CHO、CO、CO2、COOH、H、H2O、HCOO、O、OH)在上述表面的吸附能。從結果來看我們可以發現開放(100)、(110)、(211)的階梯型表面在大部分的例子中吸附能略強於(111)表面，而且Pt表面吸附能大於Au表面。此外，我們檢驗上述表面對於水煤氣轉移反應的三種路徑(1) 羧酸化(2)氧化還原(3)甲酸化的反應熱與活化能。Pt(111)傾向走羧酸化路徑；Pt(211)、Pt(100)、Au(100)、Au(110)、Au(211)、Au(111)傾向於氧化還原路徑；甲酸化有相對較高的反應熱，反應較不易發生。所有的Au表面的活化能都低於Pt表面，顯示Au是水煤氣轉移反應較佳的催化劑。最佳路徑的速率決定步驟沒有太大差別，顯示表面的修改可能改變反應路徑，但不影響活性。 附錄 乙醇氧化反應主要分成兩種路徑: (1)乙醇斷裂碳碳鍵發生12個電子轉移的C1路徑(2)乙醇氧化成乙醛轉移兩個電子，再氧化成乙酸轉移2個電子。催化劑活性利用循環伏安法、壽命使用計時安培法、反應產物以即時性傅立葉轉換紅外線光譜儀來鑑定並推測反應機構。本實驗以電解質濃度與反應溶液體積作為變數觀察催化劑反應機構，掃描範圍為 -0.9~0.6 V，在KOH濃度在2 M時，具有催化劑最大活性與最大穩定性，體積對於催化劑的活性沒有顯著的影響。Water gas shift reactions (WGSR) has been systematically examined on different facets of (111), (100), (110) and (211) of the most active catalysts of Pt and Au to optimize the structure effect of the catalytic reaction. Initially, the adsorption energy of key intermediates (CHO, CO, CO2, COOH, H, H2O, HCOO, O, OH) in WGSR on those facets has been computed. The energetic result finds that the opened (100) and (110) and stepped (211) facts have slightly stronger adsorption energies than (111) facet in most case and all the adspecies adsorbed stronger on Pt than Au surfaces. Furthermore, we examined the reaction energies and activation barriers for the three major pathways of WGSR, carboxyl, redox and formate, on those facets. The energetic results show that the Pt(111) prefer carboxyl pathway while the Pt(211)、Pt(100), Au(100), Au(110), Au(211) , Au(111)favor redox pathway. Formate pathway has relatively higher energetics and is less likely to occur on any surfaces. All the Au facets have lower energetics than the Pt ones, implying Au is a better catalyst for WGSR. Also, the rate determining steps of favored pathways on those surface show limited differences, indicating that the modification of facets could change the reaction pathways, but not likely alter the catalytic activity for WGSR. Appendix Ethanol oxidation reaction contains two major pathways (1)C-C cleavage involve 12 electrons transform ,called C1 pathway (2)ethanol oxidative to aldehyde including 2 electrons transform, continue to oxidative aldehyde to acetic acid including 2 electrons transform. To identify mechanism, we test activity of catalysts by CV, stability by CA, products by in situ IR. We change concentration of electrolyte and solution volume to observe mechanism .Scam range from -0.9 to 0.6 V. When electrolyte is 2 M, the catalyst had the best activity and stability and solution volume don’t had no trend to catalyst.