Abstract
本研究是以初期潤濕法(incipient wetness method)製得鈀,鉑及銠金屬觸媒,再利用玻璃容積吸附係統求得不同溫度下,氫氣在金屬觸媒上的等溫吸附曲線.再由等溫吸附曲線求得等吸附量時氫氣壓力和溫度的關係,然後根據Clausius-Clapeyron方程式計算出吸附熱值(-qad). 從-qad與吸附量(NH/NMe, Me=Pd, Pt, Rh)的關係,發現氫氣在Pd,Pt及Rh上之強吸附為解離吸附,且其-qad會隨著氫在表面上NH/NMe的增加而下降,其值從>80kJ/mol降至 40 kJ/mol或更小.然而, 弱吸附的吸附熱值會隨金屬的性質呈不同的變化:於Pt金屬上,-qad維持在38 kJ/mol不在隨NH/NPt的增加而減少;於Pd金屬上,則出現42和21 kJ/mol兩種-qad,且不隨NH/NRh而改變;於Pd金屬上,弱吸附的-q則隨NH/NPd的增加,呈現先減少然後增加,最後達平穩的波動變化.因此從吸附熱值隨吸附量的變化,可推論出氫氣於此三種金屬上的吸附過程:低吸附量時皆為氫原子態的吸附,而在高吸附量時, 則小顆粒的Pt有雙原子態的多重吸附(multiple adsorption);Rh在低溫下有分子態的吸附,其吸附熱值為22 kJ/mol; Pd因有吸收氫氣形成氫化鈀的特性,所以吸附的氫在表面覆滿後會滲入內部, 其測得的吸收熱為35 kJ/mol. 為了探討IB族金屬對Rh金屬表面吸附氫氣特性的影響,故再將幾乎不具吸氫能力之IB族金屬二次含浸至已製成的 Rh/Al2O3上,形成雙金屬觸媒,並以2H NMR光譜研究雙金屬觸媒上氘的吸附行為. 隨著不同的Cu,Ag及Au添加量製得不 同比例(XM,M=Cu, Ag, Au)的雙金屬觸媒,於室溫下所測得的氫氣不可逆吸附量隨XM而變,其中Cu-Rh及Ag-Rh雙金屬的 NirH/NRh隨著XM的增加,呈先增加而後減少的趨勢,而Au-Rh雙金屬的NirH/NRh卻呈現隨XAu 的增加而緩緩減少. 由前者之特殊的現象推測是Cu與Ag會和Rh形成多銠[(Rh)]和多銅[(Cu)]以及(Rh)和多銀[(Ag)] 二合金相,且隨著XM的不同,此二合金在棵粒表面的比例亦不相同. 因(Rh)擁有比Rh更高的吸氫係數, 使得表面為(Rh)合金相時[XM,<0.1],氫吸附 於(Rh)的NH比Rh/Al2O3上多.然後隨著Xm的增加,(Cu)與(Ag)都會形成並覆蓋在(Rh)表面上,因而導致NH減少,直到表面 完全被(Cu)或(Ag)覆蓋後才不再減少. 然而後者因Au-Rh觸媒的氫吸附量呈現隨XAu 的增加而緩緩減少,表示Au不與Rh形成合金,且與Rh的作用力較Cu及Ag小了許多,所以聚集成較大的顆粒附著在Rh及Al2O3上. 同時也利用2H NMR光譜觀察氘吸附在雙金屬觸媒上的吸附行為,發現氘吸附在Cu-Rh及Ag-Rh雙金屬合金上的NMR光譜.其化學位移和譜寬都會隨二金屬比例的不同而變,此現象可以氘吸附在二合金上並進行快速的交換來解釋. 最後,從譜寬的變化可求得其交換活化能在Cu-Rh/Al2O3及Ag-Rh/Al2O3分別為8.6+2 kJ/mol和5+1.3 kJ/mol,表示氘吸附於(Cu)表面的吸附力比(Ag)大,使得交換時所須越過的活化能較高.因Au-Rh雙金屬觸媒上無合金的形成,故2H NMR光譜沒 有明顯的變化,也沒有交換行為.Alumina supported Pd, pt and Rh samples were prepared by anincipent wetnesstechnique. The isosteric heats of hydrogenchemisorption (qad) on these metals were determined from aseries of uptake isotherm, measured at temperatures in the range273-673 K, using the Clausius-Clapeyron equation. The heat ofhydrogen adsorption varies with the hydrogen coverage (NH/NMe,Me=Pd,Pt or Rh) and supportedmetals. At low hydrogen uptake,the hydrogen was dissociated chemisorbed on metal surfaces,the-qad decreased from >80 kJ/mol to 40 kJ/mol on increasing thehydrogen uptake.At high uptake, the -qad of hydrogen showedfollowing phenomena, i.e., 1. The -qad of the diatomicadsorption remained at a constant value of 38 kJ/molincreasingNH/NPt for Pt/Al2O3 sample. 2. Atomic and molecular hydrogenwere co-adsorbed on Rh/Al2O3 sample with -qad valueof 42 kJ/moland 21 kJ/mol, respectively. 3. The -q on Pd/Al2O3 sampleexhibits a small hump before falling to a stable10 kJ/mol whichis the heat of hydrogen absorption for the formation ofpalladium hydride. Supported bimetallic M-Rh/Al2O3 (M=Cu, Agor，u) samples of different XM [XM=NM/(NM+NRh) atomic ratio)were prepared by impregnating a Rh/Al2O3 sample with M andcharacterized withthchniques of the hydrogen chemisorption and2H NMR spectroscopy. The irreversible hydrogenuptake of the Cu-Rh and Ag-Rh bimetallic samples, measured from thechemisorption, increasesupon impregnating a small amount of Cuor Ag but decreases as the XM becomes larger than 0.1.Deuteriumatoms adsorbed on the bimetallic samples exhibit only a single2H NMR peak. However, Both the line width and the chemicalshift of these peaks vary significantly with the XM ratio.Phenomena observed from the chemisorption and NMR spectroscopyon Cu-Rh/Al2O3 andAg-Rh/Al2O3 samples may be explainedsatisfactorily by a formation of two alloy phases, i.e.,arhodium rich phases [(Rh), XCu~0.05]& a copper rich phases [(Cu), XCu~0.8], and (Rh) [XAg~0.09] & a silver rich phases [(Ag),XAg~0.9], respectively, on the surface of bimetalliccrystallites. Detected variation in the line width and thechemical shift suggest a fast chemical exchange of the deuteriumatoms adsorbed on the surface of the both alloy phases.Theactivated energy of this exchange process on Cu-Rh and Ag-Rhbimetallic samples is estimated from the temperature dependentof line width to be 8.4 +2 kJ/mol and 5 + 1.3 kJ/mol,respectively.