Unpromoted Iron Catalysts Research Articles

Investigations on the Zn/Fe ratio and activation route during CO hydrogenation over porous iron/spinel catalysts

In this article, a simplified iron/spinel catalyst system was adopted as the Fischer–Tropsch to light olefins (FTO) catalyst to rule out disturbances from efficient promoters (e.g., K or combination of S/Na). Supported by regular supports (e.g., Al2O3, carbon, etc.), unpromoted iron catalysts commonly have a maximum C2=–C4= hydrocarbon distribution below 28%. Supported by a composite oxide support (i.e., nominal composition, ZnAl4O7, calcined at 350 °C), our porous, unpromoted iron catalyst exhibits a maximum C2=–C4= hydrocarbon distribution of 40%, achieving a significant increase by ca. 42% in comparison with regular supports. Appropriate lifting of atomic Zn/Fe ratio, as well as, reducing at lower temperature plus mild carburization, both can make a supported iron catalyst more efficient in hindering C–C coupling and producing light olefins. The structure of ZnAl4O7 support remains stable in iron catalysts during CO hydrogenation.

Effect of MgAl2O4 Surface Area on the Structure of Supported Fe and Catalytic Performance in Fischer–Tropsch Synthesis

Two MgAl2O4 spinels with low and high surface area are compared as a support for iron Fischer–Tropsch synthesis catalysts. The textural properties of spinels have a great impact on the formation of the active phase and catalytic performance. A well‐crystallized hematite phase is formed on the low surface area spinel (SBET = 15 m2 g−1), whereas high surface area support (SBET = 163 m2 g−1) provides highly dispersed or amorphous Fe2O3. Reducibility of the catalysts in H2, CO, and CO/H2 is investigated by temperature‐programmed reduction (TPR), magnetometry, and diffuse reflectance Fourier‐transform infrared (FTIR) spectroscopy. Adding potassium to the catalyst formulation suppresses the reduction of hematite in magnetite. In synthesis gas atmosphere, the final product of hematite reduction is Hägg carbide, and its amount is higher for the low surface area support. Average carbide particle size correlates with pore diameter of the support. Spinel‐based catalysts possess low selectivity to CO2 contrary to common iron catalysts. The low surface area spinel affords a very active catalyst with high selectivity to C5+ hydrocarbons. Hydrogenation of CO over unpromoted iron catalysts seems to be a structure‐insensitive reaction.

Effects of the promotion with bismuth and lead on direct synthesis of light olefins from syngas over carbon nanotube supported iron catalysts

Light olefins are important platform molecules in chemical industry. Fischer-Tropsch synthesis provides an alternative technology for direct synthesis of light olefins from syngas, which can be generated from renewable feedstocks such as organic waste, used plastics and biomass. The Bi- and Pb-promoted iron catalysts supported by carbon nanotubes have been prepared for direct conversion of syngas to lower olefins. Compared to the un-promoted iron catalysts, a twice higher Fischer-Tropsch reaction rate and higher selectivity to lower olefins were obtained. A combination of characterization techniques reveals remarkable migration the promoting elements, which occurs during the catalyst activation. After the activation, the iron carbide nanoparticles are decorated with the promoting elements. The lower melting points and high mobility of these two metal promoters during the catalyst activation are crucial for the intimate contact between Fe and promoters. The promoting effects of bismuth and lead result in a better reducibility and easier carbidisation of iron nanoparticles. The higher yield of light olefins over the carbon nanotube supported catalysts in the presence of promoters is due to slowing down secondary hydrogenation of olefins and a decrease in the chain growth probability.

The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer–Tropsch synthesis: a quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study

The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H(2) at 303 K, whereas the presence of H(2) induced CO dissociation at higher temperatures leading to the formation of CH(4) and H(2)O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO(2) formation according to C(ads) + 2O(ads)→ CO(2(g)). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol(-1), whereas it increased to 110 kJ mol(-1) on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol(-1), which increased to 225 kJ mol(-1) in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites.

Iron catalyst supported on carbon nanotubes for Fischer–Tropsch synthesis: Effects of Mo promotion

Our studies on the application of carbon nanotubes (CNTs) as support have shown that iron catalysts supported on CNTs are active and selective catalysts for Fischer–Tropsch synthesis (FTS). However, these catalysts experienced deactivation as a result of active site agglomerations. In order to control the agglomeration of active site, which is an important step in developing a novel catalyst supported on carbon-based supports, the effects of Mo promotion on deactivation behavior of iron catalysts supported on CNTs were studied. In this work the properties and catalytic performance of unpromoted iron catalysts were compared with a promoted catalyst with different Mo contents (0.5, 1, 5, and 12wt%). Based on TEM and XRD analyses, promotion of the catalysts with Mo resulted in production of smaller metal particles compared to the unpromoted iron catalyst. According to XRD analysis, Mo species were deposited in their amorphous structure. TPR analyses showed that addition of Mo increased reduction temperature significantly. Based on TEM and XRD analyses, the particle size of the iron oxides in the unpromoted catalyst increased from 16 to 25nm under FT operating conditions, while the particle size of the iron oxide in the Mo promoted catalysts (∼12–14nm) did not change noticeably under the same operating conditions. Activity, selectivity and stability of the unpromoted and Mo promoted catalysts showed that addition of 0.5–1wt% Mo resulted in a more stable catalyst. Higher contents of Mo (5 and 12 wt%) decreased the activity of the catalysts due to catalytic site coverage and lower extent of reduction. Mo promotion (0.5–12wt%) increased the selectivity of the catalysts toward lighter hydrocarbons. The promotion of the iron catalyst with 0.5wt% of Mo stabilized the activity of the catalyst with minimal increase (2%) in methane selectivity.

Fischer–Tropsch synthesis: Group II alkali-earth metal promoted catalysts

The effects of four Group II alkali-earth metals (barium, beryllium, calcium and magnesium) and potassium promoters on iron Fischer–Tropsch synthesis (FTS) catalysts product selectivity, syngas conversion and productivity are compared to the unpromoted iron catalyst. Iron FTS catalysts promoted with Group II alkali-earth metals have lower overall FTS activities and lower alpha values than a potassium promoted iron catalyst, but higher values than an unpromoted catalyst. All Group II metal promoters yielded similar carbon utilization as the unpromoted iron catalyst, but higher than a potassium promoted catalyst regardless of the space time. Carbon dioxide rates indicate that a potassium promoted iron catalyst possesses better water-gas shift (WGS) activity than any Group II metal promoted catalyst when both are operated at high CO conversions. Ca and Mg can even suppress the WGS activity below that of unpromoted iron catalysts. Among the Group II metals, a Ba promoted catalyst has the highest WGS activity while Mg and Ca promoted catalysts showed the lowest WGS activity. A potassium promoted catalyst generated the highest yield of CO 2 and hydrocarbon, but the lowest methane and total liquid product (C 5+) rate. All alkali promoted catalysts yielded higher total liquid rate, but lower gas fraction than unpromoted catalysts. Alkali-earth promoted catalysts produced a slightly higher C 2–C 4 olefin ratio, but a K promoted catalyst produced a notably higher C 2 olefin fraction than Group II metal promoted catalysts.

Fischer−Tropsch synthesis: activity and selectivity for Group I alkali promoted iron-based catalysts

The impact of the Group I alkali metals upon the activity of iron catalysts has been obtained at medium pressure synthesis conditions and at the same conversion levels. The relative impact of the alkali metal depends upon the conversion level with potassium being the promoter that impacts the highest activity at all conversion levels. At low conversions, Li is nearly as effective as potassium in improving the catalytic activity but is the poorest promoter at high conversion levels. In fact, three alkalis (Li, Cs and Rb) should be viewed as inhibitors since they decrease the catalytic activity for CO conversion below that of the unpromoted iron catalyst. The differences in the impact of the various Group I alkali metals at lower (≲40%) conversions are slight but become much greater at higher CO conversion levels. The major differences of the alkali metals at higher conversion levels is due to the impact of the promoter upon the water–gas-shift (WGS) reaction. At higher conversion levels, with a synthesis gas or “syngas” of H 2/CO=0.7, the WGS reaction becomes rate controlling because hydrogen production becomes the rate limiting factor in the Fischer–Tropsch synthesis (FTS). The basicity of the promoter appears to be the determining factor for the rate of catalyst deactivation and on the secondary hydrogenation of ethene.

Preparation and adsorption, chemical and catalytic properties of iron boride fischer-tropsch catalysts

Sodium-free and sodium-promoted, iron borides were prepared by reducing anhydrous iron acetate with diborane/THF or NaBH 4/diglyme solution. These new preparation techniques provide straightforward pathways to preparation of high surface area iron borides without oxidation of the catalysts. The catalysts were characterized by hydrogen and carbon monoxide adsorptions, Moessbauer Spectroscopy, and X-ray diffraction (XRD). CO hydrogenation activities and selectivities of these catalysts were determined at 493 K to 577 K and 10 or 20 atm with a H 2/CO reactant ratio of 2. An in situ Moessbauer study indicates that the sodium-free, chemically reduced, iron boride interstitial consists mainly of a boron-rich FeB phase which transforms to a mixture of Fe 0, FeB and Fe 2B phases upon treating with hydrogen at 400°C; the corresponding sodium-containing catalyst contains mainly Fe 2B. Laboratory reactor data indicate that unpromoted iron borides have activity/selectivity properties similar to unpromoted iron catalysts for the Fischer-Tropsch Synthesis (FTS). Sodium significantly enhances activity and alkene selectivity of iron boride for FTS. Unpromoted iron borides are not stable under FTS conditions over long reaction times, however their stability is enhanced by sodium promotion. The rate of carbon monoxide hydrogenation and the average molecular weight of products obtained on iron boride catalysts increase with increasing pressure.

The inhibition by nitrogen of the chemisorption of carbon monoxide on an unpromoted iron catalyst

The effect of chemisorbed nitrogen on the adsorption of carbon monoxide at −195 °C was studied on an unpromoted iron catalyst. The results indicate that the adsorption of 1 cm 3 of nitrogen at 300 °C causes a decrease of 0.5 cm 3 in the amount of carbon monoxide adsorbed at −195 °C. The inhibition appears to be greater the lower the temperature at which the nitrogen is chemisorbed, being as high as 0.75 cm 3 when the nitrogen is adsorbed at 200 ° and as low as 0.25 cm 3 when it is adsorbed at 400 °C. The inhibitive effect of chemisorbed nitrogen is greater on the pure iron sample than previously noted for a promoted iron catalyst, though for both promoted and unpromoted iron the chemisorbed nitrogen at a synthesis temperature of 450 °C appears to be primarily in an atomic rather than molecular form on the surface.

Kinetics and isotope effect of ammonia synthesis over an unpromoted iron catalyst

The kinetics and deuterium isotope effect of the ammonia synthesis reaction over unpromoted iron catalyst have been studied at 305 °C. Some runs were carried out with mixtures of hydrogen and deuterium. The kinetic data as well as the isotope effect indicate that the rate-determining step is the chemisorption of nitrogen on a surface mainly covered with nitrogen atoms, and not with NH radicals as previously concluded for the doubly promoted catalyst. This result suggests that a part of the role of potash promoter is the change of the adsorbed species which retards the chemisorption process.