烷基醇於Cu(110)表面吸附及熱分解反應

Abstract

超高真空的環境下，探討烷醇(alkyl alcohol, −OH)化合物在銅表面上的吸附及熱分解反應。以程溫脫附法(temperature-programmed desorption, TPD)偵測脫附產物。且以同步輻射光為光源的X-ray光電子能譜(X-ray photoelectron spectroscopy, XPS)，鑑定表面上中間產物的化學組態變化。據此提出含−OH基化合物在銅表面上的反應及分解的機制。於低表面覆蓋率的條件下，部分化學吸附的烷醇分子，傾向分解形成吸附在表面的氫及 alkoxide。高溫度，脫附產物隨吸附分子的種類及量不同而有變化。銅表面的烷醇分子斷氫氧鍵，形成吸附於表面的氫原子及alkoxide。吸附在銅表面上的 methoxide (CH3O-)與氫化合形成甲醇脫附，為一個可逆反應，或者 methoxide (CH3O-)再脫附一個β-H於Cu表面上形成甲醛脫離表面。而含β-H的ethoxide (CH3CH2O-)則傾向以β-elimination的方式形成乙醛而脫附，或是再與氫結合形成乙醇脫附，為一個可逆反應的進行。丙基醇、丁基醇主要也是經由β-H的脫離，以丙醛、丁醛的形式離開銅表面。少量的alkoxide在表面上脫氫產生碳的沉積。

Adsorption and thermal reactions of alcohol (ROH；R= (CH)n ,n=1~ 4) on the Cu(110) have been studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under ultra-high vacuum system. Desorption profiles of evolved gases from the surface of the Cu(110) were detected by quardrupole mass in the system of the TPD. Intermediate species adsorbed on surface were characterized with XPS using synchrotron radiation.

The chemistry of short chain alcohols on Cu surface has been studies extensively that as a result of their importance in catalytic chemistry. Previous studty of alcohols on clean Cu surfaces that can deprotonate the alcohol and produce alkoxide intermediate on the surface. In partpcular, on the clean Cu surface on which alcohols are adsorbed reversibly.

The molecular desportion measurements found that the heats of adsorption of the alcohols increased with increasing chain length. While decomposition of alcohols led to H2 and aldehyde on the surface. It has been proposed that these latter reactions proceed by alkylidyne intermediates led to carbon desposition on the surface.