Textural Promoter Research Articles

The influence of dopants on the catalytic activity of hematite in the ethylbenzene dehydrogenation

The effect of neodymium, lanthanum, aluminum and zirconium on the textural and catalytic properties of hematite was studied in this work, which aimed to develop catalysts to ethylbenzene dehydrogenation, in the presence of steam, the main commercial route to obtain styrene. Hematite and magnetite were found in fresh and spent catalysts, respectively. All dopants increased the specific surface area of the fresh and spent catalysts and made them more resistant against reduction, but only aluminum avoided sintering during reaction. The dopants increased the catalytic activity per area of hematite, except aluminum which acted only as a textural promoter. The selectivity was decreased due to zirconium and lanthanum while the other dopants did not change this parameter. The neodymium-containing catalyst showed high levels of activity and selectivity and was able to work up to 530 °C, without deactivation, being the most promising with regard to styrene production.

Decomposition of methane over Ni-SiO2 and Ni-Cu-SiO2 catalysts: Effect of catalyst preparation method

Abstract Ni catalysts supported on silica as textural promoter prepared by different methods are studied as catalysts for the thermo catalytic decomposition (TCD) of methane to produce hydrogen and deposited carbon in a fixed-bed reactor at 700 °C. The characterization of the fresh and used catalysts and the deposited carbon has been carried out by using different techniques like powder X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The effect of copper addition and the type of copper precursor used (nitrate or oxide) on Ni activity as the catalyst of the TCD process has also been investigated. Whatever the preparation method used, all catalysts showed a high and almost constant methane conversion without apparent deactivation after 8 h on stream, revealing the good catalyst performance in the TCD process. Copper as a nickel dopant increases the hydrogen production leading to hydrogen concentrations over 80 vol% in the production gas, independent of the preparation method or the copper precursor used. All catalysts studied promote the formation of carbon nanofibers some micrometers long with different diameters, especially dependent on the presence or absence of copper. Ni-Si 1 catalysts prepared by the fusion method promote the formation of small carbon fibers with a rather narrow diameter distribution, while Ni-Cu-Si catalysts lead to the formation of larger fibers with a broad diameter distribution.

Development of chromium-free iron-based catalysts for high-temperature water-gas shift reaction

Chromium-free iron-based catalysts were prepared and studied in regard to their performance in the high-temperature water-gas shift reaction (HTS). The effects of various catalyst preparation variables (i.e., Fe/promoter ratio, pH of precipitation medium, calcination and reduction temperatures) and preparation methods were investigated. Aluminum is a potential chromium replacement in HTS catalysts. Further improvement in WGS activity of Fe–Al catalysts can be achieved by the addition of small amounts of copper or cobalt. Catalysts were characterized using BET surface area measurements, temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). As a textural promoter, aluminum and chromium prevent the sintering of iron oxides and stabilize magnetite phase by retarding its further reduction to FeO and metallic Fe. The promotional effect of Cu is found to be strongly dependent on the preparation method.

Hydrogenation of sulfur dioxide to hydrogen sulfide on chromium promoted Fe/SiO2 catalysts

The hydrogenation of sulfur dioxide to hydrogen sulfide was studied over a series of Fe-Si-Cr catalysts in a flow reactor. These catalysts were prepared by co-precipitation of iron nitrate, sodium silicate and chromium nitrate, which had Cr/Fe atomic ratios of 0–0.04 and Si/Fe atomic ratios of 0–1.2. The addition of Cr and Si dramatically improved the SO2 hydrogenation activity of the iron-based catalyst. The chromia was a textural promoter of iron, increased the catalyst surface area by preventing the iron sintering, and hence resulted in the improved hydrogenation activity. The silica acted as a support for dispersing the active species. The interaction between iron cation and silica was observed by using temperature-programmed reduction (TPR) of the fresh Fe-Si-Cr catalysts. The catalyst with a Fe/Si/Cr atomic ratio of 1/0.6/0.01 had the highest activity, which exhibited much better SO2 hydrogenation activity than the best Fe/γ-Al2O3 catalyst reported before. With a given space velocity, the former catalyst achieved 100% H2S yield at a temperature 50°C lower than the latter catalyst. X-ray diffraction (XRD) measurements indicated that FeS2 was the active species for SO2 hydrogenation, and the Fe-Si-Cr catalyst had smaller FeS2 crystal size than the Fe/γ-Al2O3 catalyst. The iron–silica interaction and the smaller crystal size might be responsible for the better SO2 hydrogenation activity observed on the Fe-Si-Cr catalyst.

Efeito da temperatura no desempenho catalítico de óxidos de ferro contendo cobre e alumínio

Aluminum and copper doped hematite was evaluated in the high temperature shift (HTS) reaction at several temperatures in order to find catalysts that can work in different operational conditions. It was found that the catalysts work in kinetic regime in the range of 300-400 oC. Both copper and aluminum increases the activity and selectivity. Aluminum acts as textural promoter whereas copper acts as structural one. The most promising catalyst is that with both copper and aluminum which showed higher activity and selectivity than a commercial sample. This catalyst has the advantages of being non toxic and can work at low temperatures.

Effective catalysts for direct cracking of methane to produce hydrogen and filamentous carbon: Part I. Nickel catalysts

Data obtained by studying model catalytic systems were used to develop high-loaded nickel catalysts for direct cracking of methane into hydrogen and catalytic filamentous carbon (CFC). The approach to the synthesis of these catalysts can be the basis for development of catalytic nickel systems for commercial processing of natural gas. The catalysts were synthesized by fusing nickel nitrate with zirconium nitrate, or nickel nitrate with copper-doped aluminium nitrate followed by decomposition of the mixture at 300–450°C and its additional stabilization by silica. The silica textural promoter was formed by thermal decomposition of polyethoxysilane introduced into pores of the oxide matrix. The catalysts reduced by hydrogen comprised nickel in the amount of 85–90%. Unlike preparation of co-precipitated systems, synthesis of the catalysts mentioned does not include stages of filtration and wastewater treatment. Principal characteristic of the catalysts’ effectiveness for the reaction of decomposition of methane, i.e. carbon yield per gram of nickel, of the suggested systems was close or superior to that achieved before using the best nickel or nickel–copper catalysts prepared by co-precipitation.

TPR and XRD studies of yttria-doped ceria/γ-alumina-supported copper oxide catalyst

Yttria-doped ceria (YDC) and pure ceria (CeO 2), respectively, were deposited on γ-alumina (γ-Al 2O 3) using the impregnation method; then, copper oxide was also supported on them by employing the impregnation method. For comparison, CuO/γ-Al 2O 3 catalysts were prepared in this work. The catalysts were characterized by temperature-programmed reduction (TPR) and X-ray diffraction (XRD). For CuO/γ-Al 2O 3 catalysts, two TPR peaks, namely β and γ, were observed. These have been attributed to the reduction of highly dispersed copper oxide species and bulk-like copper oxide, respectively. For CuO/CeO 2/γ-Al 2O 3 and CuO/YDC/γ-Al 2O 3 catalysts, four TPR peaks, namely α 1, α 2, β′ and γ′, could be observed. The α peaks with lower peak temperatures as compared to those of β′ and γ′ peaks have been attributed to the reduction of interface-boundary copper oxide species that contact closely and interact strongly with the supported ceria or YDC. Crystal sizes calculated from XRD measurements confirmed that yttria (Y 2O 3) addition could lead to crystal growth of ceria and correspondingly enhance the dispersion of the supported copper oxide due to the partition of YDC crystallite. Hence, this work shows that supported YDC and ceria can act bi-functionally as a textural promoter as well as a structural promoter.

New Nickel Catalysts for the Formation of Filamentous Carbon in the Reaction of Methane Decomposition

A method for intentional synthesis of metal catalysts, specifically nickel catalysts for methane decomposition, was developed. The method was based on impregnation of a porous metal oxide with the precursor of a textural promoter followed by reduction of the latter. The desired texture of NiO was provided by precalcining it at a certain temperature within the range 300 to 900°C. A family of nickel-based catalytic systems with various concentrations of the active component, metal particle sizes, and stability to deactivation in the course of synthesis of filamentous carbon was prepared. The yield of carbon and mechanical strength of carbon particles growing on the catalyst during methane decomposition were found to increase with the concentration of nickel in the catalyst, to reach their maxima at 90–96% nickel. The highest yield of carbon (375–384 g carbon per g nickel) was observed for the catalyst with particles of 10–40 nm average diameter. The effect of textural promoters (SiO2, Al2O3, MgO, TiO2, and ZrO2) on the catalyst performance was studied; the highest carbon yield was obtained with SiO2.

Methane combustion on Mg-doped LaMnO 3 perovskite catalysts

LaMn 1− x Mg x O 3 perovskite catalysts ( x=0–0.5) were synthesised by the so-called “citrates method”, characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO 3 and LaMn 0.8Mg 0.2O 3). The major results obtained are: (i) Mg substitution in the basic LaMnO 3 perovskite has a positive effect on the catalytic activity only at low x values ( x≤0.2); (ii) as opposed to the results of previous studies on the LaCr 1− x Mg x O 3 system, the role of MgO as a textural promoter is not always significant and depends strongly on the calcination temperature of the samples (800–1200°C) and on the value of x; (iii) an Eley–Rideal mechanism could satisfactorily fit the experimental kinetics results for both catalysts, even though, as opposed to LaMnO 3, the catalytic combustion over LaMn 0.8Mg 0.2O 3 seems to involve two different types of adsorbed oxygen species, depending on the operating temperature.

XRD studies of evolution of catalytic nickel nanoparticles during synthesis of filamentous carbon from methane

The XDR technique was used for studying a series of high‐loaded (90%) nickel catalysts with silica as a textural promoter. These were catalysts for direct cracking of methane at 550°C. A relation between the initial average size of active catalyst particles, carbon yield and average methane conversion was demonstrated. Genesis of these catalysts was studied including oxide precursors, reduced catalysts prior to the reaction, as well as catalysts upon their contacting the reaction medium for various periods of time from 15 min to 2 h. The active catalyst particles were shown to merge or disperse at the outset of the reaction depending on their initial size. Anyway, close average sizes ranging from 30 to 40 nm were observed by the end of the first reaction hour for all the catalytic systems providing the carbon yield of 300–385 g/g Ni. The catalytic system was shown to self‐organize in the course of direct methane cracking, i.e., the catalyst particles transform in response to the reaction conditions. If the size of nickel particles cannot vary, these catalysts are inefficient for the given process.

Cu-B2O3/SiO2, an effective catalyst for synthesis of fatty alcohol from hydrogenolysis of fatty acid esters

This work demonstrates that Cu- B2O3/SiO2 is an effective catalyst for synthesis of fatty alcohols from hydrogenolysis of fatty acid esters. Boron oxide not only acts as a textural promoter to increase the dispersion of copper, but also as a structural promoter to decrease the activation energy of hydrogenolysis by 10 kJ/mol. The optimum loading of boron oxide doped is around 6.4 wt%. This chromium-free copper catalyst is seven times as active as the commercial copper chromite (Engelhard Cu-1234E) in the hydrogenolysis of methyl esters at 240°C, 110 bar. Cu- B2O3(6.4%)/SiO2 is a promising catalyst for lower pressure (<150 bar) hydrogenolysis processes.

Coprecipitated NiAl and NiCuAl catalysts for methane decomposition and carbon deposition I. Genesis of Calcined and reduced catalysts

The structure of coprecipitated NiAl and NiCuAl catalyst with high nickel content (60–90 wt.-% Ni-alumina) has been investigated by X-ray powder diffraction, extended X-ray absorption fine structure (EXAFS) spectroscopy and Auger electron spectroscopy of calcined and reduced samples. The alumina was shown to behave as a textural promoter. The model of the reduced NiAl catalyst consisted of highly dispersed nickel and alumina particles with a NiAl spinel phase at their interface. The NiAl spinel phase had mainly an inverse structure. In the reduced sample the paracrystallinity of nickel was evidenced by precise diffraction measurements using synchrotron radiation. The spinel structure was not observed when copper was introduced. As a result, the alumina particles were more dispersed and covered the surface of the NiCu particles enriched with copper.

Characterization of unsupported copper—chromium catalysts for ethanol dehydrogenation

The dehydrogenation of ethanol to acetaldehyde has been studied over a series of unsupported copper-chromium catalysts. The results indicate that the sintering occurs in both reduction and dehydrogenation processes. The Cr 2O 3-promoted copper catalysts prevent sintering in both reduction and reaction processes. The catalyst with Cr/Cu molar ratio of 4/40 has the highest activity and stability. The temperature-programmed reduction results indicate that the good dispersion of Cr 2O 3 promoter helps to prevent sintering in the reduction process. The catalyst which contains Cu and CuCr 2O 4 has a lower initial activity and the thermal stability is as poor as the unpromoted one. Cu 0 instead of Cu 2+ is the active site. The TOFs of promoted catalysts with Cr/Cu = 1/40 to 6/40 are lower than that of unpromoted catalyst, and thus the chromic oxide is not just a textural promoter of Cu. The higher copper surface areas of promoted catalysts easily overcome the loss in TOFs.

Influence of aluminium and potassium on activity and texture of fused iron Catalysts for ammonia synthesis

BET, scanning electron microscopy and X-ray diffraction were used to investigate the texture of fused iron Catalysts which had been previously reduced and passivated, and into which potassium hydroxide was introduced by impregnation or aluminium removed by extraction. Activity tests were carried out under 10.0 MPa in the 400–520 °C temperature range. Investigations were performed on the doubly promoted (Al, Ca) and industrial, triply promoted (K, Al, Ca) Catalysts. Introduction of potassium hydroxide into the doubly promoted Catalyst increases the process rate by about 15–20 times per unit surface area. Sintering and recrystallization are observed in the Catalyst, proving that interaction takes place during the process between the potassium hydroxide introduced and the alumina present on the iron surface. Partial removal of aluminium from the surface of the doubly and triply promoted Catalysts also leads to sintering and recrystallization under ammonia synthesis conditions. This shows that the presence of alumina on the surface, and not within the iron, is responsible for textural promotion. Loss of Catalysts activity is proportional to reduction in total surface area.

Bimetallic catalysts comprised of dissimilar metals for the reduction of carbon monoxide with hydrogen

A series of bimetallic catalysts containing Co—Mo, Ru—Mo, Rh—Mo, Co—W, Ru—W and Rh—W on γ-Al 2O 3 have appreciable activity for the reduction of carbon monoxide with hydrogen under mild conditions (150, 300°C and 101 to 407 kPa). Ru-W/γ-Al 2O 3 had the highest overall activity and propensity for chain growth, Co—Mo and Co—W/γ-Al 2O 3 generated the highest yields of alkeneproducts, while Rh—Mo and (methanol and ethanol). Alkali promotion of the Ru—W/γ-Al 2O 3 catalyst increased the alkene content of the products but decreased the overall activity for carbon monoxide hydrogenation. X-ray photoelectron spectroscopy indicated that both Ru—W and Ru—Mo contain ruthenium with a slightly higher binding energy after reduction than the monometallic ruthenium catalyst. Molybdenum and tungsten are not reduced. X-ray photoelectron spectroscopy of the fresh Rh—Mo/γ-Al 2O 3 catalyst indicates the presence of Rh III 2h, the rhodium is reduced, with a binding energy close to but slightly higher than that of bulk Rh 0. Here too, the molybdenum binding energy is unaffected by reduction, and the rhodium-to-molybdenum atom ratio remains constant, suggesting little or no further sintering. High pressure (250°C, 1724–6689 kPa, hydrogen: carbon monoxide = 0.75:1.0) reduction of carbon monoxide with hydrogen by 3% Rh-2.8% Mo/γ-Al 2O 3 produced predominantly dimethyl ether and methane. This catalyst's activity and selectivity are quite stable over prolonged period (>300 h). Transmission electron microscopy of used Rh—Mo/γ-Al 2O 3 shows that there is a bimodal distribution of particles (0.5 nm and 1.0—1.5 nm). Dedicated scanning transmission electron microscopy-energy dispersive spectroscopy of the particles indicates that the larger particles are enriched in rhodium (Rh-to-Mo>1.60) and the smaller in molybdenum (Rh-to-Mo<0.9). A structural model is proposed for Rh-Mo/γ-Al 2O 3 with two separate sites leading to oxygenates and hydrocarbons. Oxygenate production is attributed to chemistry at the smaller, molybdenum-rich site. It is believed that the role of molybdenum in this site is to act as a textural promoter, providing site-isolation of the rhodium.

Characterization of a promoted precipitated iron catalyst for Fischer-Tropsch synthesis

A commercial precipitated catalyst for Fischer-Tropsch synthesis has been characterized by temperature-programmed techniques, carbon monoxide chemisorption, X-ray photoelectron spectroscopy, X-ray diffraction and Mössbauer spectroscopy. The catalyst consists of non-porous iron particles with a diameter close to 10 nm. Textural promotion, i.e. maintenance of the particle diameter during exposure to reaction conditions, is achieved by the coverage of the iron particles by silica islands. Upon exposure of the fresh catalyst to hydrogen at 493 K according to the manufacturer's specifications 20% of the iron is reduced to the metallic state. Only 3% of the iron atoms are exposed to the gas phase. After more than 200 h exposure to reaction conditions similar to industrial ones, the iron phase consists of more than 75% non-stoichiometric iron carbides with Fe 2.23C as the overall composition. The remaining iron atoms are either oxidized or present as carbides which show paramagnetic relaxation at 80 K. The iron atoms at the interface of the iron particles are oxidized. The pores of the catalyst pellets, i.e. the voids between the iron particles, are completely filled with liquid hydrocarbons during steady-state operation under industrial conditions.

The textural promotion of metallic iron by alumina

Textural promotion of iron catalysts, i.e., maintenance of high specific areas of reduced iron samples, is achieved by dissolved Al 2O 3 into Fe 3O 4 prior to reduction. A 10.2 wt% A1 2O 3 in Fe 3O 4 sample with a known surface composition in the reduced state was investigated to determine the mode of textural promotion of reduced iron by alumina. The amount of Fe 2+ inside the reduced iron crystallites was determined to be approx 1 wt% by Mössbauer effect spectroscopy. X-Ray line broadening indicated a 61 ± 3-nm crystal size in agreement with the BET particle size of the same sample. The paracrystallinity following Hosemann was determined to be 0.6, a value in agreement with an empirical correlation between particle size and paracrystallinity. All data presented here are compatible with Hosemann's view that molecular inclusions of FeAl 2O 4 in the iron lattice are responsible for textural promotion. Since particle size is inversely proportional to specific surface area, paracrystallinity may be a dominant cause of textural promotion in our case. Another cause may be a patchy monolayer of alumina at the surface of the iron particles, preventing sintering by a “skin” effect.

On the role of cobalt in sulfided unsupported Co-Mo hydrodesulfurization catalysts: kinetic studies and scanning electron microscopic observations

A series of unsupported sulfided Co-Mo catalysts with different Co content was investigated. The catalysts were characterized by scanning electron microscopy and tested in dibenzothiophene hydrodesulfurization. The catalytic activity, measured in a flow microreactor between 500 and 580 K from 5 to 50 × 10 5 Pa pressure, showed a pronounced maximum at Co/(Co + Mo) = 0.30. The S.E.M. revealed that the most active catalyst presented t the best homogeneous dispersion of Co and Mo and a lower deactivation. The kinetic studies at various pressures of hydrogen and hydrogen sulfide clearly showed the non-influence of these parameters on the synergetic range. Moreover, attention was paid to the hydrogenation function and the catalytic deactivation in relation to Co content. An these results showed the influence of the texture on the activity and the role of cobalt as a textural promoter in these catalysts.

Ferromagnetic resonance investigation of dispersed Ni catalysts: epitaxial and textural effects from the support

Catalysts consisting of dispersed Ni particles supported on silica and alumina, with sizes ranging from 6 to 20 nm, have been studied by ferromagnetic resonance. For the Ni on Al 2O 3 catalyst, a textural promotion effect is shown to be present and it is attributed to the possible presence of NiAl 2O 4. The FMR data confirm the epitaxial growth of Ni on SiO 2 when Ni antigorite is reduced and show that some anisotropy is still present after sintering of the catalyst at about 1200K.

Alumina as a textural promoter of iron synthetic ammonia catalysts

The location of the Al 2O 3 in a reduced singly promoted iron catalyst (3% Al 2O 3 with 250 Å iron particles) was determined by Mössbauer spectroscopy. A mildly reduced catalyst exhibits a paramagnetic phase which disappears upon further reduction. The hyperfine parameters of the reduced catalyst agree with those of pure α-Fe. These observations are best reconciled with the idea that the Al 2O 3 is present in the α-iron in the form of ca. 30 Å inclusions consisting of FeAl 2O 3 in the incompletely reduced catalyst and of Al 2O 3 itself after thorough reduction.