Showing papers on "Temperature-programmed reduction published in 2007"

Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres

Abstract: 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 2 O 3 , goethite α-FeOOH, ferrihydrite Fe 5 HO 8 ·4H 2 O, magnetite Fe 3 O 4 and wustite 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 2 O 3 → 2 Fe 3 O 4 → 6 Fe, three-stage mechanism 3Fe 2 O 3 → 2Fe 3 O 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 3 O 4 /Fe or stoichiometrically equivalent and thermally induced, above 250 °C, phase transformation—wustite disproportionation to magnetite and metallic iron, 4FeO ⇌ Fe 3 O 4 + Fe Above 570 °C, the appearance of wustite 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 3 O 4 → Fe up to 570 °C or even the entire sequence of three-step process FeO → Fe 3 O 4 → FeO → Fe up to 880 °C should be reconsidered as a result of the accompanying FeO disproportionation wustite ⇌ 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 wustite 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 wustite → magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe 3 O 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

Steam reforming of tar from a biomass gasification process over Ni/olivine catalyst using toluene as a model compound

Abstract: A Ni/olivine catalyst, previously developed for biomass gasification and tar removal during fluidized bed steam gasification of biomass, was tested in a fixed bed reactor in toluene steam reforming as a tar destruction model reaction. The influence of the catalyst preparation parameters (nickel precursor, calcination temperature and nickel content) and operating parameters (reaction temperature, steam to carbon S/C ratio and space-time) on activity and selectivity was examined showing a high toluene conversion and a low carbon formation compared to olivine alone. The steam reforming of toluene was found to be of zero order for water and first order for toluene. Activation energy required for Ni/olivine was determined to be about 196 kJ mol−1 in accordance with literature. Catalyst activity and stability and its resistance against carbon formation were discussed on the basis of X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature programmed oxidation (TPO) results. Characterization before test (XRD, temperature programmed reduction (TPR), Mossbauer spectroscopy) have shown the presence of NiO–MgO solid solution, formed on the surface of olivine support, which explains the efficiency of the catalyst calcined at 1100 °C. After test, Ni–Fe alloys were observed (TEM, Mossbauer spectroscopy). It was suggested that magnesium oxide enhanced steam adsorption, facilitating the gasification of surface carbon and that Ni–Fe alloys prevented carbon deposition by dilution effect.

Performance and characterization of supported metal catalysts for complete oxidation of formaldehyde at low temperatures

Abstract: Activities of a series of metals (Pt, Pd, Rh, Cu, Mn) supported on TiO2 were investigated for the catalytic oxidation of formaldehyde. Among them, Pt/TiO2 was found to be the most promising catalyst. Nitrogen adsorption, hydrogen chemisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature programmed reduction (TPR) by H2 were used to characterize the platinum catalysts. Using Ce0.8Zr0.2O2, Ce0.2Zr0.8O2, SiO2 as supports instead of TiO2, the activity sequence of 0.6 wt.% platinum with respect to the supports is TiO2 > SiO2 > Ce0.8Zr0.2O2 > Ce0.2Zr0.8O2, and this appears to be correlated with the dispersion of platinum on supports rather than the specific surface areas of the catalysts. Platinum loading on TiO2 has a great effect on the catalytic activity, and 0.6 wt.% Pt/TiO2 catalyst was observed to be the most active, which could be attributed to the well-dispersed platinum surface phase. The reduction temperature greatly affects the particle size and, consequently, the catalytic activity. The smaller particle size of platinum, due to its high dispersion on support, has a positive effect on catalytic performance. Increasing formaldehyde concentration and space velocity exhibits an inhibiting effect on the catalytic activity.

Methane dry reforming on Ni loaded hydroxyapatite and fluoroapatite

Abstract: Calcium-hydroxyapatite and calcium-fluoroapatite loaded with different amounts of nickel were synthesized and characterized by several techniques including scanning electron microscopy, temperature programmed reduction (TPR), UV–visible–NIR and XPS spectroscopy. Three types of nickel species were detected in the two series of catalysts (i) Ni 2+ ions exchanged with Ca 2+ ions of the apatite framework. This Ni 2+ /Ca 2+ exchange seems to be restricted to nickel loadings inferior to 1 wt% Ni, (ii) small particles of NiO exhibiting strong interactions with the carriers (for x > 1 wt% Ni) and (iii) large particles of NiO which appear at high loadings. The distribution of the nickel between these three hosting sites depends on the nature of the apatite. For instance the amount of exchanged Ni 2+ ions in Ni(1)/CaHAp is twice more important than in Ni(1)/CaFAp. The Ni( x )/CaHAp and Ni( x )/CaFAp catalysts were tested in methane dry reforming with CO 2 . Methane conversion at 600 °C, increases with the nickel loading up to x = 4 where the activity is around the thermodynamic equilibrium (78%) and H 2 /CO ratio close to 1. These results were confirmed by the investigation of the catalysts activity versus the temperature. Carbon deposition on the catalysts was found to also increase with nickel loading but without provoking any significant decay of the activity after 4 h on stream. The encouraging results achieved were attributed to the synergy between the basic properties of the apatites, their aptitude to chemisorb CO 2 and the catalytic features of the supported nickel.

A highly active Cu-MgO-Cr2O3 catalyst for simultaneous synthesis of furfuryl alcohol and cyclohexanone by a novel coupling route—Combination of furfural hydrogenation and cyclohexanol dehydrogenation

Abstract: A novel coupling route highlighting the combination of hydrogenation of furfural and dehydrogenation of cyclohexanol in vapor phase conditions over Cu-MgO-Cr 2 O 3 catalyst is highly advantageous in terms of avoiding external pumping of H 2 and maintaining the formation of furfuryl alcohol and cyclohexanone, the desired products, respectively, in good amounts at atmospheric pressure. The catalytic activity of this catalyst is compared with a Cu-MgO coprecipitated catalyst and a copper chromite (commercial Cu-1800P supplied by M/s. Engelhard Corp., USA) catalyst. Cu-MgO-Cr 2 O 3 catalyst is prepared by coprecipitation method using tetraethyl ammonium hydroxide as precipitating agent. All the catalysts are characterized by BET surface area, temperature programmed reduction, XRD and XPS. The BET surface area of Cu-MgO-Cr 2 O 3 catalyst is higher indicating the presence of smaller Cu particles. XRD results also indicate the presence of smaller particles of Cu in Cu-MgO-Cr 2 O 3 catalyst. XPS results indicate the presence of Cu species (Cu 0 /Cu + ) at the surface of Cu-MgO-Cr 2 O 3 catalyst. The higher yields of the products over the Cu-MgO-Cr 2 O 3 catalyst in independent and combined reactions compared to the other two catalysts may be attributed to the promotional effect of Cr 2 O 3 , smaller Cu particle size and more number of Cu 0 /Cu + species at the surface.

Characterization and Reactivity of Alumina-Supported Pd Catalysts for the Room-Temperature Hydrodechlorination of Chlorobenzene

Abstract: A series of alumina-supported Pd catalysts were prepared by varying the metal loading between 0.5 and 5 wt. %, adopting the deposition precipitation (DP) method. These catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), pulse CO chemisorption, Brunauer−Emmett−Teller specific surface area measurement, and transmission electron microscopy. The catalytic properties of the catalysts were studied for the hydrodechlorination of chlorobenzene. The reaction was carried out in a continuous mode passing liquid chlorobenzene and gaseous hydrogen over a fixed bed of catalyst taken in a reactor operating at room temperature and atmospheric pressure. The catalysts with metal loading up to 2 wt. % demonstrated high dispersion, homogeneous distribution of active species with predominance of metal−support interaction and without any formation of β-PdH. Beyond 2 wt. % loading, the agglomeration of PdO took place forming bulk particles that reduc...

Structural properties and catalytic oxidation of benzene to phenol over CuO-impregnated mesoporous silica

Abstract: A series of copper catalysts supported on mesoporous molecular sieves MCM-41 with copper loading varying from 2 wt% to 10 wt% were synthesized by impregnation method using aqueous copper nitrate solution as the precursor. The samples were characterized by nitrogen adsorption–desorption, X-ray powder diffraction, FT-IR, UV-diffuse reflectance spectra and temperature programmed reduction. X-ray diffraction patterns indicate the presence of crystalline CuO phase above 4 wt% copper loading. TPR patterns at low temperatures reveal the presence of highly dispersed copper oxide on the surface of the support and the profiles at high temperatures indicates bulk copper oxide. The catalytic activity of the samples was evaluated towards benzene oxidation using H 2 O 2 as the oxidant in acetic acid solvent at room temperature. The effect of reaction parameters, i.e. time, temperature, catalyst amount, amount of H 2 O 2 , solvent and alkyl substitution on the substrate, were also investigated. Among the catalysts studied, 4 wt% copper loaded MCM-41 showed maximum conversion (21%) and selectivity (94%) towards phenol.

Nickel catalysts promoted with cerium and lanthanum to reduce carbon formation in partial oxidation of methane reactions

Abstract: Mixed oxides of nickel, magnesium and aluminum were prepared by precursors hydrotalcite type and were substituted by lanthanum or cerium using the method of anion exchange. The catalysts were characterized by X-Ray Photoelectronic Spectroscopy (XPS); Energy Dispersive X-Ray Spectroscopy (EDS); Surface Area Method BET; X-Ray Diffraction (XRD); Temperature Programmed Reduction (TPR) and Catalytic Tests. Analysis of the mixed oxides suggests that the hydrotalcite type precursor after thermal treatment lead a good dispersion of nickel forming the periclase (Ni, Mg)O replaced by cations of Al 3+ and spinel structure (Ni,Mg)Al 2 O 4 . The catalysts were tested in the partial oxidation of methane and the results showed that the catalysts promoted with lanthanum and cerium led to a reduction in the velocity of the carbon formation.

Supported Pd–Cu catalysts in the water phase reduction of nitrates: Functional resin versus alumina

Abstract: Pd–Cu bimetallic catalysts supported on the cationic resin Dowex 1 × 4 (gel type poly(styrene- co -divinylbenzene) with −N(CH 3 ) 3 + Cl − groups) and basic γ-Al 2 O 3 were prepared by ion exchange and the incipient wetness impregnation method, respectively, and tested in the liquid-phase hydrogenation of nitrates in water. Various methods of the reduction of metal precursors were used. The effect of the preparation method, the temperature, the reduction conditions and the support on the properties of palladium–copper composites was investigated by means of X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and temperature programmed reduction (TPR). The Pd–Cu catalysts prepared by liquid-phase reduction of anionic chloro-complexes of palladium(II) and copper(II) immobilised in a resin, mainly isolated domains of metallic palladium and copper, if any, contained. The reduction using hydrogen joined with calcination of palladium and copper salts supported on γ-alumina led to well formed crystallites of a palladium–copper alloy. The Pd–Cu catalysts supported on γ-Al 2 O 3 exhibited higher activity of nitrate removal under given reaction conditions in a batch reactor. However, with the Pd–Cu resin-based catalysts a better final selectivity (selectivity to products other than nitrites and ammonia) at a comparable conversion of nitrates has been obtained in comparison with alumina based catalysts. The catalysts exhibited different stability with respect to the leaching of metals.

Direct synthesis of dimethyl carbonate from carbon dioxide and methanol using supported copper (Ni, V, O) catalyst with photo-assistance

Abstract: The photo-catalytic effect of a copper modified (Ni, V, O) semiconductor complex catalyst on the direct synthesis of dimethyl carbonate (DMC) from CO 2 and CH 3 OH was investigated. The synthesized catalysts were fully characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), Ultraviolet visible drift reflection spectra (UV–vis DRS) and transmission electron microscopy (TEM). The nano-scale catalyst particles were observed with TEM and light absorbance was then predicted by UV–vis DRS spectra. The pressure and temperature dependencies of the photo-catalytic activity were studied. The results demonstrated that the catalytic activity was enhanced with the assistance of ultraviolet (UV) irradiation compared with the pure thermal and surface catalytic reaction under the same reaction conditions.

Effect of the Preparation Method on the State of Nickel Ions in BEA Zeolites. A Study by Fourier Transform Infrared Spectroscopy of Adsorbed CO and NO, Temperature-Programmed Reduction, and X-Ray Diffraction

Abstract: The state of nickel ions in two BEA zeolites was studied by X-ray diffraction (XRD), temperature-programmed reduction (TPR), and Fourier transform infrared (FTIR) spectroscopy using CO and NO as probe molecules. One of the samples, NiAlBEA (0.9 wt % of Ni), was prepared by conventional ion exchange, and the other, NiSiBEA (1.0 wt % Ni), was prepared by a two-step postsynthesis method involving dealuminated SiBEA zeolite. No structural changes are observed with AlBEA after ion exchange with nickel. In contrast, the incorporation of Ni into SiBEA leads to an increase of unit cell parameters of the BEA structure and to the consumption of silanol groups in vacant T-sites of the dealuminated zeolite. In the NiAlBEA sample Ni shows three TPR peaks in the region of 600−1100 K, whereas one sharp peak at 650 K with a shoulder at 750 K is observed with the NiSiBEA sample. Adsorption of CO at 100 K on NiAlBEA results in formation of Ni2+−CO species (2200 and 2214 cm-1). The latter are partially converted, at high co...

Steam reforming of ethanol on Ni–CeO2–ZrO2 catalysts: Effect of doping with copper, cobalt and calcium

Abstract: Steam reforming (SR) and oxidative steam reforming (OSR) of ethanol were investigated over undoped and Cu, Co and Ca doped Ni/CeO2–ZrO2 catalyst in the temperature range of 400–650 °C. The nickel loading was kept fixed at 30 wt.% and the loading of Cu and Co was varied from 2 to 10 wt% whereas the Ca loading was varied from 5 to 15 wt.%. The catalysts were characterized by various techniques, such as surface area, temperature programmed reduction, X-Ray diffraction and H2 chemisorption. For Cu and Co doped catalyst, CuO and Co3O4 phases were detected at high loading whereas for Ca doped catalyst, no separate phase of CaO was found. The reducibility and the metal support interactions were different for doped catalysts and varied with the amount and nature of dopants. The hydrogen uptake, nickel dispersion and nickel surface area was reduced with the metal loading and for the Co loaded catalysts the dispersion of Ni and nickel surface area was very low. For Cu and Ca doped catalysts, the activity was increased significantly and the main products were H2, CO, CH4 and CO2. However, the Co doped catalysts showed poor activity and a relatively large amount of C2H4, C2H6, CH3CHO and CH3COCH3 were obtained. For SR, the maximum enhancement in catalytic activity was obtained with in the order of NCu5. For Cu–Ni catalysts, CH3CHO decomposition and reforming reaction was faster than ethanol dehydrogenation reaction. Addition of Cu and Ca enhanced the water gas shift (WGS) and acetaldehyde reforming reactions, as a result the selectivity to CO2 and H2 were increased and the selectivity to CH3CHO was reduced significantly. The maximum hydrogen selectivity was obtained for Catalyst N (93.4%) at 650 °C whereas nearly the same selectivity to hydrogen (89%) was obtained for NCa10 catalyst at 550 °C. In OSR, the catalytic activity was in the order N > NCu5 > NCa15 > NCo5. In the presence of oxygen, oxidation of ethanol was appreciable together with ethanol dehydrogenation. For SR reaction, the highest hydrogen yield was obtained on the undoped catalyst at 600 °C. However, with calcium doping the hydrogen yields are higher than the undoped catalyst in the temperature range of 400–550 °C.

The influence of the preparation methods and pretreatment conditions on the properties of Ag-MCM-41 catalysts

Abstract: Silver doped mesoporous silica materials MCM-41 were prepared by the direct hydrothermal (DHT) and template ion exchange (TIE) methods. The properties of materials were investigated by the adsorption/desorption of nitrogen, X-ray diffraction (XRD), and photoacoustic infrared spectroscopy (FT-IR/PAS). Redox properties were studied by the temperature programmed reduction method and temperature programmed CO oxidation test reaction. It was found that the introduction of small amounts of silver caused structural changes of the silica materials. Silver species in the catalysts obtained by the TIE method were strongly dispersed on the silica support. XRD and TPR studies of the catalysts prepared by the DHT method indicated co-existence of the large crystallites and isolated silver ions in the silica material. Studies revealed the strong influence of the pretreatment and reaction conditions on the performance of catalysts in the oxidation of CO. Reduction of catalysts was necessary to obtain high activity at low temperatures. Complex deactivation processes were recorded above 300 °C.

Identification of cobalt species during temperature programmed reduction of Fischer-Tropsch catalysts

Abstract: Identification of cobalt species during temperature programmed reduction of Fischer-Tropsch catalysts

Surface acidic and redox properties of V-Ag-Ni-O catalysts for the selective oxidation of toluene to benzaldehyde

Abstract: V-Ag-O and V-Ag-Ni-O complex oxides were found to exhibit good catalytic reactivity for the selective oxidation of toluene to benzaldehyde. In order to understand the effects of each metal in the two catalysts, a series of single and complex oxides were prepared including V 2 O 5 , Ag 2 O, NiO, V-Ni-O, Ag-Ni-O, V-Ag-O and V-Ag-Ni-O with the previously optimized compositions for the binary and tertiary oxides. Data from the X-ray diffraction (XRD), infrared spectroscopy and laser Raman spectroscopy showed the formation of complex oxides with phases of silver and nickel vanadates as well as the broadening and red shift of the band of V O bonds, suggesting the formation of V O Ag and V O Ni bonds and the increased redox ability of vanadium species. The results from temperature programmed reduction (TPR) and the combined technique of TPR-XRD revealed that the addition of Ag and Ni promoted the reduction of vanadium species in the complex catalysts. Microcalorimetric adsorption of ammonia and the probe reaction of isopropanol in air demonstrated that the surface acidity was greatly decreased while the redox cycles of vanadium sites were significantly enhanced upon the addition of Ag and Ni. The reactivity for the selective oxidation of toluene was evaluated by using a fixed-bed microreactor, and was correlated with the surface acidity and redox property of the catalysts. Silver seemed to be a special promoter that not only decreased the surface acidity but also enhanced the redox cycles of vanadium sites, leading to the increased conversion of toluene and the selectivity to benzaldehyde. Such effect was even more pronounced when Ag and Ni were added simultaneously. Accordingly, the V-Ag-Ni-O catalyst studied in this work exhibited excellent performance for the selective oxidation of toluene to benzaldehyde. The conversion of toluene was found to be 8% with 95% selectivity to benzaldehyde on the V-Ag-Ni-O at 613 K.

An investigation into the effect of Cs promotion on the catalytic activity of NiO in the direct decomposition of N2O

Abstract: A series of Cs promoted NiO catalysts have been prepared and tested for direct decomposition of N2O. These catalysts are characterized by BET surface area, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of N2O (TPD-N2O) and X-ray photo electron spectroscopy (XPS). The Cs promoted NiO catalysts exhibit higher activity for the decomposition of N2O compared to bulk NiO. The catalyst with Cs/Ni ratio of 0.1 showed highest activity. The enhancement in catalytic activity of the Cs promoted catalysts is attributed to the change in the electronic properties of NiO. The characterization techniques suggest weakening of Ni–O bond thereby the desorption of oxygen becomes more facile during the reaction. The Cs promoted NiO catalyst is effective at low reaction temperature and also in the presence of oxygen and steam in the feed stream.

Promoters state and catalyst activation during ammonia synthesis over Ru/C

Abstract: Carbon-supported, promoted Ru-based catalysts for ammonia synthesis proved to be interesting substitutes for the traditional Fe-based ones. A debate recently arose on the active state of promoters, mainly Cs and Ba, and on the effect of the latter on Ru active sites. In the present work a set of Ba-, Cs- and K-promoted samples has been characterised by various techniques. Higher H 2 and O 2 uptakes have been observed during reduction and chemisorption, respectively, on Cs- and K-promoted samples supported on graphitised carbon. No evidence of this has been observed with samples supported on active carbon. This is in line with the hypothesis of alkaline promoters partial reduction under the ammonia synthesis conditions, favoured by the formation of graphite intercalation compounds. Furthermore, some suggestions are here introduced on the beneficial role of Ba, especially in increasing the support resistance to methanation. Finally, the efficacy of catalyst activation was found to depend on the nature of Ru precursor. Indeed, a prolonged activation at relatively high temperature is usually needed with chloride precursors, to remove the counterion, a poison for the catalyst, whereas less dramatic conditions are required for different precursors, such as nitrosylnitrate.

Advantages of nano-silver-carbon covered alumina catalyst prepared by electro-chemical method for drinking water purification

Abstract: The present work highlights the high efficiency of silver nano-particles deposited over carbon covered alumina (CCA), by electro-chemical deposition method over impregnation method, in controlling microorganisms in water. The anti-microbial activities of the catalysts are determined quantitatively by serial dilution followed by inoculation method. The catalytic characterization of these materials obtained by using transmission electron microscopy (TEM), X-ray diffraction (XRD) and temperature programmed reduction (TPR). TEM and X-ray line broadening technique results indicate the presence of Ag particles in nanometer size. The main advantage of Ag supported catalysts prepared by electro-chemical deposition over that made by conventional impregnation technique is that only small amount is needed and no pretreatment conditions like reduction are required for deactivation of microorganism in water. Thus, silver catalysts prepared by this method are not only efficient but also economical in restoring hydrogen economy. The combined characteristics of Al2O3 and carbon like low acidity, high mechanical strength and presence of meso pores in CCA are also helpful for getting good activity.