The hydrogen reduction behavior of Fe
2O
3, Fe
3O
4 and FeO is strongly influenced by time–pressure dependent process. The reduction of hematite takes place according to scheme: 3Fe
2O
3
→
2 Fe
3O
4
...→
6 FeO
→
6 Fe. The same pathway of hydrogen reduction for magnetite and wüstite is postulated. First, reduction of magnetite to wüstite Fe
3O
4
→
FeO and second step of wüstite disproponation 4FeO
→
Fe
3O
4
+
Fe. Only after the disappearance of Fe
2O
3 phase, the reduction of Fe
3O
4 to Fe can be observed. The appearance of FeO crystal phase as an intermediate compound of iron(III) oxide reduction was experimentally confirmed by XRD method above 560
°C. The complete reduction of hematite into metallic iron phase can be accomplished even at low temperature of up to 380
°C.
▪
The reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPR
H2 and TPR
CO), thermo-gravimetric and differential temperature analysis (TG-DTA-MS), and conventional and “
in situ” XRD methods. Five different compounds of iron oxides were characterized: hematite α-Fe
2O
3, goethite α-FeOOH, ferrihydrite Fe
5HO
8·4H
2O, magnetite Fe
3O
4 and wüstite FeO. In the case of iron oxide-hydroxides, goethite and ferrihydrite, the reduction process takes place after accompanying dehydration below 300
°C. Instead of the commonly accepted two-stage reduction of hematite, 3 α-Fe
2O
3
→
2 Fe
3O
4
→
6 Fe, three-stage mechanism 3Fe
2O
3
→
2Fe
3O
4
→
6FeO
→
6Fe is postulated especially when temperature of reduction overlaps 570
°C. Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe
2+
⇌
2Fe
3+
+
Fe, occurring at both the atomic scale on two-dimensional interface border Fe
3O
4/Fe or stoichiometrically equivalent and thermally induced, above 250
°C, phase transformation—wüstite disproportionation to magnetite and metallic iron, 4FeO
⇌
Fe
3O
4
+
Fe. Above 570
°C, the appearance of wüstite phase, as an intermediate of hematite reduction in hydrogen, was experimentally confirmed by “
in situ” XRD method. In the case of FeO–H
2 system, instead of one-step simple reduction FeO
→
Fe, a much more complex two-step pathway FeO
→
Fe
3O
4
→
Fe up to 570
°C or even the entire sequence of three-step process FeO
→
Fe
3O
4
→
FeO
→
Fe up to 880
°C should be reconsidered as a result of the accompanying FeO disproportionation wüstite
⇌
magnetite
+
iron manifesting its role above 150
°C and occurring independently on the kind of atmosphere—inert argon or reductive hydrogen or carbon monoxide. The disproportionation reaction of FeO does not consume hydrogen and occurs above 200
°C much easier than FeO reduction in hydrogen above 350
°C. The main reason seems to result from different mechanistic pathways of disproportionation and reduction reactions. The disproportionation reaction wustite
⇌
magnetite
+
iron makes simple wüstite reduction FeO
→
Fe a much more complicated process. In the case of thermodynamically forced FeO disproportionation, the oxygen sub-lattice, a closely packed cubic network, does not change during wüstite
→
magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe
3O
4. Depending on TPR
H2 conditions (heating rate, velocity and hydrogen concentration), the complete reduction of hematite into metallic iron phase can be accomplished at a relatively low temperature, below 380
°C. Although the reduction behavior is analogical for all examined iron oxides, it is strongly influenced by their size, crystallinity and the conditions of reduction.
For the first time, it was demonstrated that the catalytic properties of semiconductors can be activated using an external electric field only. More particularly, the TiO2 and Fe2O3 were found to ...produce noticeable amounts of carbon monoxide from carbon dioxide – water vapor gas mixture under the electric field with the strength in the 104 V/cm range. The reported process, i.e. the activation of semiconductors using the electric field is more efficient than the utilization of UV–vis radiation in terms of energy efficiency and amount of products formed.
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•Catalytic properties of semiconductors was activated using the external electric field only.•Wet CO2 was converted into CO over TiO2 and Fe2O3 in the presence of field in the 104 V/cm range.•The activity of investigated semiconductors depended on both surface and electronical properties.
Herein, we reported the method of synthesis of sodium titanates using metal-ammonia solution and their photocatalytic activity for the gas-phase reduction of carbon dioxide under visible light. The ...results of physicochemical measurements showed that the obtained samples resemble multiphase systems containing Na2Ti4O9, Na4Ti5O12, and Na0.8Ti4O8 of cylindrical forms. They demonstrated perfect adsorption of visible light and improved activity in the investigated process.
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•Intercalation of sodium into titanium dioxide was performed using a solution of sodium metal in liquid ammonia.•Multiphase systems containing sodium titanates of different stoichiometry (Na2Ti4O9, Na4Ti5O12, and Na0.8Ti4O8) were formed.•Localization of sodium ions in titanium dioxide depended on its initial concentration in liquid ammonia.•Absorption of visible light and photocatalytic activity of titanium dioxide were greatly improved.
Ethanol steam reforming was studied over Ni supported catalysts. The effects of support (Al
2
O
3
, Al
2
O
3
–ZnO, and Al
2
O
3
–CeO
2
), metal loading, catalyst activation method, and ...steam-to-ethanol molar feed ratio were investigated. The properties of catalysts were studied by N
2
physisorption, TPD-CO
2
, X-ray diffraction, and temperature programmed reduction. After activity tests, the catalysts were analyzed by TOC analysis. The catalytic activity measurements showed that the addition either of ZnO SSor CeO
2
to alumina enhances both ethanol conversion and promotes selectivity towards hydrogen formation. The same effects were observed for catalysts with higher metal loadings. High process temperature and high water-to-ethanol ratio were found to be beneficial for hydrogen production. An extended catalyst stability tests showed no loss of activity over 50 h on reaction stream. The TOC analysis of spent catalysts revealed only insignificant amounts of carbon deposit.
The catalytic activity of multi-walled carbon nanotubes (MWCNTs) in oxy-steam reforming of methanol (ASRM) was investigated for the first time. We demonstrate that CNTs are a potent catalytic ...material for hydrogen generation in oxy-steam reforming of methanol.
Monometallic copper and bimetallic palladium–copper catalysts supported on ZnO–Al
2
O
3
, CeO
2
–Al
2
O
3
and ZrO
2
–Al
2
O
3
were prepared by conventional impregnation method and tested in the ...methanol synthesis reaction in a gradient less reactor under elevated pressure (3.5 MPa) at 220°C. The physicochemical properties of prepared catalytic systems were studied using BET, TPR-H
2
, TPD-NH
3
, XRD, SEM-EDS and FT-IR techniques. The results of XRD and SEM-EDS measurements showed the formation of Pd–Cu alloy during the activation of bimetallic catalysts. It was found that the formed alloy was responsible for the improved activity and selectivity of catalysts in the studied reaction. Among investigated catalysts, the highest formation rate of methanol was observed with 2%Pd–20%Cu/ZnO–Al
2
O
3
system. Based on the results of FT-IR measurements it can be concluded that hydrogen molecules adsorb dissociatively on the metallic copper surface to form hydrogen atoms, increasing the hydrogen spillover effect on the metal-support interface. In contrast, CO
2
adsorb on the oxygen vacancies of the support to form carbonates, which can further undergo hydrogenation to methanol.
This review describes features of the structure of carbon nanotubes, their principal characteristics, methods of synthesis, and regions of potential application. Most attention has been paid to the ...use of nanotubes in catalysis. Examples are given of the functionalization of nanotubes and also their use as catalysts, catalytic additives, and catalyst supports.
In this paper we have demonstrated the possibility of the MWCNT growth on initially amorphous Co–Zr–N–O alloy thin film which was crystallized during heating followed by formation of Co particles on ...the surface. It was found that during CVD not only the growth of usual MWCNT array but also the formation of MWCNT arrays with a top covering layer and bilevel MWCNT arrays take place. The details of MWCNT array growth process are discussed. We have found that carbon nanotubes growth strongly depends on the Co concentration in the alloy film. The presented technique of MWCNT growth can be used in the preparation of new advanced engineering materials.
Results are given for a study of the catalytic and physicochemical properties of monometallic Cu/CeO
2
-Al
2
O
3
and bimetallic Pd-Cu/CeO
2
-Al
2
O
3
catalysts for the synthesis of methanol from ...hydrogen and carbon dioxide. The catalytic activity was determined in methanol synthesis in a gradientless reactor at 200°C and 3.5 MPa. The physicochemical properties of these catalytic systems were studied by the BET method (Brunauer−Emmett− Teller method), temperature-programmed reduction of hydrogen (TPR-H
2
), temperature-programmed desorption of ammonia (TPD-NH
3
), x-ray diffraction (XRD), and Fourier-transform IR spectroscopy (FTIR). Our results showed high activity for the palladium-promoted catalyst, which is attributed to a synergistic effect between palladium and copper as well as the formation of a PdCu alloy during activation of this bimetallic catalyst in a reducing atmosphere of 5% H
2
95% Ar.
The formation of carbon nanotubes (CNTs) arrays by chemical vapor deposition on Me-Ct-N-(O) catalyst alloys, where Ct is a catalytic metal from the group Ni, Co, Fe, Pd; Me is transition metal of ...IV-VII of the Periodic Table of Elements, with low content of catalyst has been investigated. It is shown that CNTs effectively grow, if the alloy contains Ti, V, Cr, Zr, Hf, Nb and Ta. The structure of CNT were studied by transmission and scanning-electron microscopy, energy-dispersive X-ray and the Raman spectroscopy.
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