低温マイクロカロリメトリーによるサイクロヘキセンの吸収とPt{111}に対する反応のエネルギー
Ole Lytken1, Wanda Lew, Jonathan J W Harris
1Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA.
Journal of the American Chemical Society
|July 12, 2008
まとめ
サイクロヘキセンは100 KでPt ((111)) に無傷に吸収するが,281 Kで脱水化し,吸収された2-サイクロヘキセニルを形成する. 吸収熱と粘着確率を測定し,表面化学に関する洞察を明らかにしました.
科学分野:
- 表面科学とは,地表科学である.
- 化学物理 化学物理
- マテリアルサイエンス 材料科学
背景:
- 金属表面における炭化水素の吸附行動を理解することは,触媒作用において極めて重要です.
- サイクロヘキセンは,C-H結合の活性化と脱水酸化を含む反応を研究するためのモデル分子です.
- プラチナ (Pt) は,多くの産業プロセスにおける重要な触媒である.
研究 の 目的:
- プラチナ (Platinum) の表面でサイクロヘキセンの吸収を調査する.
- 覆い面の関数として,吸着熱と粘着確率を決定する.
- 異なる温度でPt{111}上でサイクロヘキセンの吸収状態と反応経路を解明する.
主な方法:
- 単結晶吸附熱計は,吸附エンタルピーを測定するために使用されました.
- 初期相互作用のダイナミクスを理解するために粘着確率を測定しました.
- 実験は100〜300Kの温度範囲で行われました.
主要な成果:
- 100Kでは,サイクロヘキセンはダイシグマ結合によって無傷に吸収し,吸収熱は (130 - 47 θ - 1250 θ2) kJ/molで表されます.
- この無傷の吸収により,C-Ptシグマ結合の強度は205kJ/molになります.
- 281 Kで,サイクロヘキセンは吸収時に脱水し,吸収された2-サイクロヘキセニルとHを形成し, (174 - 700 θ + 761 θ2) kJ/molで記述される吸収熱で (174 - 700 θ + 761 θ2) kJ/mol.
- 低被覆層の粘着確率 (>0.95) は,0.10 ML以下では温度に依存しないことが判明しました.
結論:
- Pt(111) 上のサイクロヘキセンの吸収状態は温度に依存し,無傷の吸収から脱水化への移行である.
- この研究は,サイクロヘキセンとその脱水産物である Pt ((111) について,定量的な熱力学データ (形成エンタルピー,結合強度) を提供している.
- 高い初期粘着確率は,研究された条件下で,Pt{\displaystyle Pt{\displaystyle Pt{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\displaystyle Pt}{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text}}}{\text{\text{\text}}}{\text{\text{\text{\text{\text}}}{\text{\text{\text}}}{\text{\text{\text}}}{\text{\text{\text}}}{\text{\text{\text}}}{\text{\text{\text{\text{\text}{\text}{\text{\text{\text}}}}}}}{\text{
関連する概念動画
¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR
The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation
Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is...
Adsorption Isotherms I
Adsorption isotherms are mathematical models that describe how molecules in a gas or liquid phase interact with surfaces. Two of the most common isotherm models are the Langmuir and Freundlich isotherms, which relate to Type I monolayer chemisorption. The Langmuir model is based on four key assumptions:• Adsorption cannot exceed monolayer coverage.• All surface sites are equivalent.• Molecules adsorb only at vacant sites.• There are no interactions between adsorbed molecules.Consider the...
Cycloaddition Reactions: MO Requirements for Thermal Activation
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
Cycloaddition Reactions: MO Requirements for Photochemical Activation
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.


