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本文以研究碳氧化物氫化反應的觸媒添加劑效應及擔體效應為主。觸媒主要使用鐵觸
媒，添加劑則為錳及鉀。擔體效應研究二氧化鈦對鎳觸媒的擔體強作用（SMSI）下所
造成的CO/H2 反應的活性，選擇性變化。研究方法因鐵錳觸媒在活化及反應過程中可
形成多種不同的二元氧化物，觸媒的內部晶相對F.T.合成反應影響至鉅，故研究以梅
斯堡光譜及x-射線粉末繞射分析測其晶相組成，以程溫規劃還原觀察觸媒還原情形，
以與F.T.合成活性測試的結果相關連。對於CH/H2 及CO2/H2反應的反應機構，本文則
利用吸附CO及CO2 的程溫規劃表面反應（Temperature Programmed Surface Reactio
n, TRSP ）測出高低溫吸附CO,CO2後氫化的TPSR圖譜，與實驗數據作參數搜尋適配，
証實所提出模式正確，並求出反應模式中的活化能等反應參數。
研究結果顯示，錳加入鐵觸媒可藉由形成一些鐵錳的二元氧化物，而穩定了觸媒的中
間氧化態，阻止了觸媒的進一步還原。鐵錳觸媒的晶相與F.T.合成的關連性非常大，
歸納我們的結果顯示對CO/H2 反應能產生最高烯類選擇性的相態為Ge1-2Mn2O,Mn3-yF
eyO4的鐵錳二元氧化物及ε′-Fe2.2C 碳化鐵°Fe10觸媒先以co, 再以H2在270℃ 活
化後,於320℃，150psig 的反應狀況下，能在81％的CO轉化率下產生41％的2－4碳低
烯類。這是本實驗室所作出最好的烯類選擇性。進一步由鐵錳觸媒吸附CO後的TPSR顯
示，錳因對碳的親合力不高，故鐵錳觸媒須預先以CO碳化後才能有高烯類產率。錳因
其有降低鐵觸媒氫化作用的能力，在F.T.合成中初期產物的烯烴可不被再次氫化故有
高烯類產率，而因錳可防止鐵錳觸媒積碳，故產物碳鏈都不長。產物中超過六碳以上
烴類都不多。鐵錳系觸媒在F.T.合成中，及事先活化過程中觸媒的氧媒氧化及碳化，
相同重要。
鐵錳系觸媒對一氧化碳氫化雖具有良好的助觸媒效果。但使用鐵錳觸媒於二氧化碳氫
化，則無法達到高烯類產率之目的，錳一但加入鐵觸媒中，觸媒二氧化碳氫化的產物
即偏向甲烷。由於二氧化碳氫化產物中含有大量竹旳水，水會造成觸媒的氧化，故推
測鐵錳觸媒在CO2/H2反應中嚴重的觸媒氧化，使觸媒表面並未形成Fe1-2Mn2O，Mn3-y
FeyO4 等烯類產生相同時，碳化鐵亦由ε′-Fe2.2C 轉為較穩定的X-Fe5C2 ，故烯類
產率不高。錳因會降低鐵觸媒對碳的親合力，而二氧化碳氫化中因大量的水蒸氣會除
去觸媒表面的碳積，故錳對鐵觸媒在二氧化碳氫化上並非一良好的添加劑。相對而言
，能增加鐵觸媒上對碳親合力的鉀，是二氧化碳氫化觸媒良好的添加劑。
對一氧化碳氫化，已証實實反應在熱力學上沒有限制，CO/H2 反應是在動力控制下進
行。但對二氧化碳氫化反應，本文進行了反應熱力學分析的計算。計算結果顯示，熱
力學平衡對二氧化碳氫化確有影響，在高溫及低壓時，反應平衡並不利於烴類的生成
而趨向於進行逆水氣轉換反應（Reverse Water Gas shift Reaction, RWGS）由二氧
化碳及一氧化碳及水。但在低溫及高壓時，反應平衡才會趨向生成烴類。
雖然在熱力學理論分析上以及在我們選定的反應條件下（350℃，10-20大氣壓）確實
可將二氧化碳及氫完全轉化成烴類。但實驗結果顯示反應器出山二氧化碳並未完全轉
化，倒是RWGS反應已趨於平衡。這乃是由於CO2 氫化步驟為先由CO2 轉化成CO，再由
CO經由F.T.合成生成烴類。CO氫化成烴類的反應速率較慢但CO2 反應生成CO的反應速
率則較快。故對我們所有的二氧化碳氫化實驗，總反應皆受制於第一步RWGS反應的化
學平衡而轉化率皆不高。RWGS趨近反應平衡的最大原因為反應器中存在有大量的水氣
。由二氧化碳氫化成烴類亦會生成多個分子的水，故對由二氧化碳氫化成烴類的研究
，如何設計一反應器移除反器中的水以提高總轉化率，將與製備送擇性優良的觸媒同
等重要。
對一氧化碳氫化，前期研究曾以一F.T.合作觸媒混合一HZSM-5沸石，在一反應器中直
接將合成氣轉化為芳香烴產物。本文中亦對二氧化碳氫化作了類似的研究。以融熔鐵
觸媒與HZSM-5沸石混合在，在350 ℃，21atm ，GHSV=60 （對鐵觸媒）的反應條件下
，CO2 的轉化率可達33％，而所生成的烴類中共21％為芳香烴產物。
鉀為CO/H反應中鐵觸媒最常用的電子促進劑，對沉澱鐵或融熔鐵觸媒，鉀之促進劑作
用已有多人研究。但對擔體分散鐵觸媒的鉀促進劑效應則較少有文獻報導。故本文中
即針對二氧化矽擔體分散鐵觸媒作了一系列第一族鹼金屬促進劑的研究。鐵／二氧化
矽擔體觸媒之梅斯堡光譜顯示鐵金屬高度分散在SiO2擔體上，形成顆粒極小的晶粒。
加入鋰、鈉、鉀、銫等第一族鹼金屬雖因促進劑會分散在擔體上而促進作用不強，但
確可增加Fe/Sio2 觸媒的烯類及高碳產率，以及二氧化碳的產量。此研究中之特色為
作了CO/H2 反應後觸媒的恆溫一升溫脫碳分析（Isothermal and Temperature
Programmed Decarbidation， ITPDC）。分析中提出一串級反應的常溫脫碳反應模式
，經由模式計算出的理論脫碳曲線與實驗數據相適配並求出各反應步驟的碳氫化常數
。由ITPDC 的分析得知，擔體鐵觸媒上加入鹼金屬，由於擔體鐵觸媒上的鐵為高度分
散的晶粒，晶粒與晶粒間之碳並不會相連結而形成隋性碳積，故鹼金屬不會增加擔體
鐵觸媒上的積碳量。但鹼金屬卻可使碳積更緊密的聯結在擔體鐵觸媒上，各適配出來
加入鹼金屬的觸媒其氫化常數皆大幅降低，導致大部份的碳皆須在升溫狀態下才能脫
出。故推測鹼金屬對CO/H2 反應的促進劑效用，應不只於鹼金屬能加速CO的裂解，由
鹼金屬加入所造成觸媒氫化能力的減弱，應是其電子促進效用的一部份。
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This thesis is dedicated to the research of the catalyst additives and the
support effect for the hydrogenation of carbon oxides. The catalyst used
is mainly iron, the additives is manganese and alkali metals. The research
on support effect is aimed to the strong metal support interaction of
(SMSI) titanium oxide supported nickel, which cause strange activity and
selectivity changes in CO/H2 reaction. Since iron-manganese catalysts may
form various kinds of binary oxides. and the F.T. synthesis behaviou of
Fe-Mn catalysts is much related to their binary oxide structure, the
method of ressarch uses Mossbauer spectroscopy and X-ray powder
diffraction to detect the phase structure of the catalysts, and correlate
it with the activity test results. As for the CO/H2 reaction mechanisu
under support interactions,this thesis uses the temperature programmed
surface reaction (TPSR) experiment technique to measure the TPSR spectra
of pre-adsorbed co and pre-deposited carbon. To explain the TPSR spectra,
reaction models are proposed and theoretical TPSR spectra may be obtained
based on calculations using non-isothermal kinetics. By comparison between
experimental data and the theoretical TPSR curves, kinetic parameters can
be computer fitted from experimental data. THe fitted parameters are
consistent in a complete set and all TPSR spectra can be well explained by
the reaction model.
THe results show: the addition of manganese to iron may stabilize the
emdium oxidation state of catalysts therefore it prohibit the full
reduction of iron. Oxidation and carbidation of the catalysts has
extremely strong effect to the F.T synthesis behavior. Our results show
that the Fe1-zMn O, Mn30yFeyO4 and small amount of ε′-Fe2.2C iron
carbide is the most powerful olefin producing phases. On our Fe10
catalyst, after pretreatment In Co and H2 at 270℃, and under the reaction
of 320℃, 150 psig, GHSV=600, the 2-4 carbon olefin selectivity may reach
41% at a total CO conversion of 86%. Further study shows that manganese
can decrease the affinity to carbon of iron catalysts, therefore high
manganese containing catalysts has to be precarbided by CO to produce the
promoter effect. Since manganese can decrease the hydrogenation ability of
iron, the olefinic primary product of F.T synthesis may not be further
hydrogenated therefore olefin production is high. For manganese has poor
carbon affinity, F.T. synthesis products on Fe-Mn catalysts are mainly
short chain hydrocarbons, and, since manganese can prevent carbidation of
iron, the carbon deposition on Fe-Mn catalysts is rather low therefore
long term activity of CO/H2 reaction may be maintained.
Although manganese has good promoter effect in CO hydrogenation, but it
behaves quite differently when Fe-Mn catalysts were used for CO2
hydrogenation. Once as manganese is added to iron, the olefin selectivity
of CO2 hydrogenation decreases drastically, and methane production vastly
increases. THis is due to the severe oxidation of the catalyst in CO2
hydrogenation. THe product in CO2 hydrogenation has a great deal of water,
and moisture in gas phase is the main cause of the iron catalysts
oxidation. Because of the destruction of the olefin production phase by
oxidation of the catalyst, Fe-Mn catalysts can not produce high olefin in
co2 hydrogenation. In Co2 hydrogenation, the water produced will react
with the carbon on catalysts therefore the promoter added should be able
to stabilize the carbon on catalysts, since maganese has no carbon
affinity, it is not a good promoter. The additive such as potassium which
can increase the carbon stability on iron catalysts should be better
additives for CO2 hydrogenation.
For CO hydrogenation, it is well known that reaction is slow and there is
no thermodynamic limitation to the syntheris. But for CO2 hydrogenation,
there is very little literature reaction thermodynamic data available.
THerefore we conduct the reaction equilibrium calculatiion to CO2
hydrogenation. The analysis shows that there is thermodynamic limitations
to CO2 hydrogenation, the conversion of CO2 to hydrocarbons may only be
favored at olw temperature and high pressure,if the temperature is high,
the reaction may shift to form CO instead of hydrocarbons.
Although under our chosen reaction conditions (350℃, 10-21 atm), the
carbon dioxide can totally convert to hydrocarbons according to reaction
thermodynamic analysis, but all our CO2 hydrogenation activity test data
show that there are only limited conversion to hydrocarbons of carbon
dioxide, instead the reverse water gas shift reaction (RWGS, CO2 + H2 --CO
+ H2O) in reactor is approaching equilibrium. This is because that there
are two steps of reaction in CO2 hydrogenation, first CO2 is convert to CO
by RWGS, then CO is further hydrogenated to hydrocarbons via F.T.
synthesis. Since the F.T. synthesis is a slow reaction, the overall
conversion of CO2 conversion is limited by the first RWGS reaction
equilibrium therefore all the conversion is not high. To produce
hydrocarbon from CO2, water must be formed and water is also the product
of RWGS reaction, therefore if the conversion is to be high in CO2
hydrogenation, practical means must be developed in reactor design to
remove the water in the reactor.
Previous research on CO hydrogenation using hybrid catalysts has been
reported that it can convert CO to aromatics directly. In this thesis, we
have done the same research on Co2 hydrogenation, by mixing fused iron
catalyst (ICI 35-4) with commercial HZSM-5 as the hybrid catalyst, we can
also convert CO2 to aromatics directly. At 350℃, 21 atm, GHSV=60 (to
iron), the aromatics selectivity obtained is 21% under a total Co2
conversion of 33%.
Alkali metals are the most common electronic promoters used in CO/H2
reaction catalysts. For precipitated and fused iron catalysts, the alkali
promoter effect has been well studied. But there is very literature report
on the alkali promoter effect on supported iron catalyst. THerefor in this
thesis, we conducted the research on a series alkali metal promoted
Fe-SiO2 catalysts. The Mossbauer spectra of catalysts show that iron is
very well dispersed on silica support and iron appears as very fine
crystals. The particle size of iron is very small. The addition of Li, Na
K, and Cs on 5%Fe/SiO2 has not strong promoter effect, but they do can
increase olefin, long chain hydrocarbons, and carbon dioxide production on
Fe/SiO2, we conducted the isothermal and temperature programmed
decarbidation (ITPDC) experiments after CO/H2 reaction on alkali-Fe/SiO2,
and propose a series reaction model to the hydrogenative decarbidation. By
computer data fitting with the theoretical isothermal part of the ITPDC
curve, the hydrogenation kinetic constants in the series reaction model
were obtained. The ITPDC results show that on well dispersed Fe/SiO2,
alkali metal does not increase the carbon amount on iron, but instead all
carbon are much more strongly held on Fe/SiO2 on the addition of alkali
metals. The decarbidation hydrogenation constants decreases drastically as
alkali metal was added. We suspect that the reason for not increase of
carbon is the small particle size of Fe/SiO2. The carbon on each small
ironcrystal can not link to each other therefore an inert carbon layer can
not be formed. Since the hydrogenation constants of decarbidation reaction
is drastically suppressed by alkali metals, we also suspect that the
promoter effect of alkali metal should not be only to increase CO
dissociation on iron, the selective poisoning of the hydrogenation ability
caused by alkali metal should also be taken into account.
Another part of this thesis is dedicated to CO/H2 reaction mechanism
studies by TPSR and not isothermal kinetics calculations. The first case
studied is Ni/TiO2. The TPSR spectra of pre-adsorbed CO on Ni/TiO2 shows
two distinct peaks, which should correspond CO on Ni/TiO2 shows two
distinct peaks, which should correspond to two different active reaction
sites. Comparison with the TPSR spectra on Ni/SiO2 indicates that the high
temperature peak on Ni/TiO2 has the similar behavior with Ni/SiO2,
therefore the high temperature peak on Ni/TiO2 should correspond to
reaction sites composed of pure nickel atoms, while the low temperature
peak is caused by the hydrogenation of Co adsorbed on Ni-TiOx sites which
is formed by interdiffusion of partially reduced titanium to nickel
surface during reduction. Some interrupted experiments showed that on
Ni/TiO2, a common Co/H2 reaction intermediate may be formed from both
reaction sites, this reaction intermediate is confirmed to be the active
surface carbon formed from Co dissociation by TPSR of predeposited carbon.
The CO adsorbed on Ni-tiOx sites is so easy to dissociate and the carbon
formed will migrate to the nearby nickel surface thus increases the carbon
content on nickel catalyst, therefoer the Co/H2 activity on Ni/TiO2 is
high and the selectivity will shift to long chain hydrocarbons.
The second case studied is the Co/H2 on iron catalyst, the TPSR spectra of
pre-adsorbed Co on iron catalyst also show two peaks. The first peak is
the hydrogenation of active carbon while the second peak is the
decarbidation of iron carbide. In this case, we proposed a reaction model
according to competition model, by model calculation and data fitting, we
find competition model can explain all the TPSR spectra and the CO/H2
reaction behavior on iron catalysts. This study not only proves the
competition model quantitatively, but also provide an independent
experimental evidences for the competition model.
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