Abstract
藉由程溫分析技術中的程溫還原反應(temperature-programmedreduction, TPR)以及程溫脫附反應(temperature-programmeddesorption, TPD)等實驗, 本研究探討了氧氣在鉑族金屬上的鑑結及脫附情形. 鉑族金屬觸媒的製備是以初期潤濕法(incipient wetnessmethod),將鈀,鉑及銠支撐在氧化鋁,二氧化矽,以及矽鋁氧化物等擔體上.在TPR實驗中,根據觸媒上各金屬氧化物所表現之還原溫度及其氫氣消耗量等訊號,來探討鉑族金屬的氧化行為隨氧化溫度的變化情形.在TPD的實驗中,除了探討支撐性金屬氧化物的脫氧行為之外,亦廣泛了解文獻有關氧從鈀晶面脫附的行為差異. 從TPR的研究中發現,Rh/Al2O3會隨氧化溫度的上升而增加銠金屬與氧化鋁擔體間的作用,並逐漸有化學吸附氧(RhOc),表面氧化物(RhsO),氧化銠(RhOx),與Al2O3有作用力的氧化銠(RhiOx),以及嵌入Al2O3次表層的氧化物[Rh(AlO2)].銠金屬粒徑亦會影響上述氧化物種的生成.此外,嵌入Al2O3次表層的氧化物[Rh(AlO2)],可經由高溫還原處理使銠原子再回到擔體表層.對Pt/Al2O3而言,鉑氧化物的生成亦受氧化溫度影響;依次有PtOc,PtO,PtO2以及Pt aluminate.鉑金屬粒徑及氧化時間是影響上述氧化物種生成的因素,即使在室溫氧化環境下,減少鉑金屬粒徑或延長氧化時間均能促使PtO2形成.以TPR及ICP-MS實驗,配合XPS的Pt 4f位能圖,PtO2亦被證實存在Pt/SiO2中,但PtO2在SiO2上易於高溫氧化環境(>400 C)下昇華,且與擔體表現較弱的作用力.比較PtOx在Al2O3及SiO2上的還原溫度表現,可找出PtOx在矽鋁氧化物擔體上同時具有前兩者的還原行為,並且發現在Al2O3 grain及SiO2 grain的界面間有類似沸石孔洞的結構形成,此孔洞的數目會隨擔體中的矽鋁比而改變. 在TPD的應用研究中,本研究成功地建立圖譜分析方法(Graphic Analysis method),可由一個TPD圖譜直接而詳細地求得氣體從觸媒表面上脫附時的三個主要動力參數值--脫附級數(n, desorption order),脫附活化能(Ed, activationenergy)以及指數前因子(pre-exponential factor).本研究不僅將圖譜分析法運用在實際TPD實驗中,測量氧從Pt/Al2O3,Pd/Al2O3等樣品脫附的行為,並將此方法廣泛地運用在文獻上有關氧從鈀晶面脫附的實例. O2從Pt/Al2O3樣品脫附的行為中,脫附級數會隨氧化鉑物種而變化:PtO呈二次反應,PtO2則為一次.脫附活化能及指數前因子均約為一定範圍值.對Pd/Al2O3而言,氧的脫附級數均呈二次反應,其脫附活化能具有金屬分散度的相依性:Ed=175+174D kJ mol-1. O2從鈀晶面脫附的量測中,得知由改變微小吸附量即可能造成極大不同的脫附行為.氧在Pd(100)的吸附量極小時(<0.04),氧首先會分解而以原子態(Oad)吸附在強吸附基上;當這些Oad進行脫附前,結合為分子態吸附氧(O2,ad)成為脫附速率的決定步驟,故呈現二次反應,所需活化能約為230 kJ mol-1.隨氧吸附量的增加(~0.19),此時以O2,ad 的脫附成為速率決定步驟,故呈現一次反應,所需活化能也隨吸附氧量增加而減少.當吸附量再增加(>0.40),化學吸附氧會經由鈀表面的結構重組(reconstructions)而形成一層氧化鈀結構,氧從這些結構脫附的行為似像Pd/Al2O3一樣,又呈現為二次反應,指數前因子隨Ed值變化,呈現補償效應(compensation effect). 圖譜分析法是一直接而省時的分析技術,它將是一種分析程溫脫附動力學參數值極簡單而方便的方法.Two kinds of temperature-programmed technology, temperature-programmed reduction (TPR) and temperature-programmed desorption(TPD), were used as exploratory techniques to characterize theproperties of metallic catalysts. Samples of platinum metalcrystallites (Pd, Pt and Rh) finely dispersed on alumina, silicaand silica-aluminate were prepared by impregnating Al2O3, SiO2or a series of SiO2-Al2O3 with H2PdCl4, PtCl4 or RhCl3solutions. Reduced samples were oxidized over a wide temperaturerange to examine the variation of metal oxides with theoxidation temperatures (To). Oxide species formed arecharacterized according to their reduction temperature (Tr) andH2 consumption found in the TPR experiments. Besides, chemicalkinetics of oxygen desorption from metal-oxide species were alsostudied from experimental and literature TPD spectra. In TPRstudies, five kinds of oxidation products formed on Rh/Al2O3, i.e., oygen-chemisorbed rhodium (RhOc), surface rhodium oxide(RhsO), bulk rhodium-oxides (RhOx), interacting with support(RhiOx) and rhodium aluminate [Rh(AlO2)y]. The rhodium atoms inthe subsurface Rh(AlO2)y structure would segregate to thesurface of alumina upon reduction at Tr above 750 C. For Pt/Al2O3, four PtOx species, i.e., PtOc, PtO, PtO2 and platinumaluminate are formed as raising To. The optimized To for theformation of each PtOx species varied with dispersion of Pt andduration of oxdiation. Both PtO and PtO2 species are verified toco-exist on SiO2 support according XPS, TPR and ICP-MSexperiments. A Graphic Analysis method (GA method) is proposedto determin all the kinetic parameters, i.e., reaction order(n), activation energy (Ed) and pre-exponential factor (v). Thedesorption order of O2 from Pt/Al2O3 was found to vary with thespecies of platinum oxide : n=2 for PtO species, but n=1 forPtO2. On Pd/Al2O3 samples, the O2 desorption behaves in a secondorder kinetics. An obvious size-dependent relation of metalparticles (or dispersion of metal, D) is obtained for theactivation energy evolved upon O2 desorption from Pd/Al2O3according to :Ed=175+174D kJ mol-1. The chemical kinetics ofoxygen desorption from the surface of Pd crystallites were alsopursued. The desoprtion order was determined to be n=2 wheninitial coverage was less 0.04 monolayer but generally shiftedto n=1 on increasing the coverage to 0.19. As coverage largethan 0.40, the migration of adsorbed oxide ions into subsurfaceof palladium induced a surface reconstruction and a formation ofPd-O payer. Desorption of O2 from these oxide-layer structureappears a second order and keeps a constant Ed value around 165kJ mol-1. The variation in the v values with the Ed can beinterpreted by "compensation effect". The GA method is a directand time-saving technique, and would be a convenient method foranalyzing kinetic parameters for simple desorption processes.