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Cobalt oxide (CoO) nanoparticles (NPs) were modified with different silanes (tetraethoxysilane – TEOS, triphenyl ethoxysilane – TPES, or trimethyl chlorosilane - TMCS) by dispersing the as-synthesized CoO NPs in n-hexane solutions containing the silanes. Fourier-transform infrared spectroscopy (FTIR) confirmed the presence of these silanes on the cobalt surfaces even after reduction in hydrogen (H 2) at 573 K. Modifying CoO-NPs with TEOS resulted in face-centred cubic (fcc)-cobalt phase upon reduction whereas TPES or TMCS modification induced a mixture of hexagonal close-packed (hcp)- and fcc-cobalt phases. The modification of cobalt surfaces with the silanes alters the adsorption properties of carbon monoxide (CO) on the catalytically active sites. Furthermore, temperature programmed desorption of CO (CO-TPD) showed that the amount of dissociatively adsorbed CO relative to amount of associatively adsorbed CO increases on silane-modified cobalt surfaces, which correlates with an improved rate of CO conversion in the Fischer-Tropsch CO hydrogenation. [Display omitted] • Successful modification of CoO NPs using tetraethoxysilane (TEOS), triphenyl ethoxysilane (TPES), and trimethyl chlorosilane (TMCS). • Alteration of cobalt crystalline phases upon silane modification, with TEOS inducing a face-centred cubic (fcc)-cobalt phase and TPES/TMCS leading to mixtures of hexagonal close-packed (hcp)- and fcc-cobalt phases. • Enhanced adsorption properties of carbon monoxide (CO) on silane-modified cobalt surfaces. • Improved rate of CO conversion in Fischer-Tropsch CO hydrogenation on silane-modified cobalt oxide nanoparticles. [ABSTRACT FROM AUTHOR]

The development of CO 2 hydrogenation catalysts for hydrocarbon production is a challenging and relevant task as it can contribute to solving of the CO 2 emission problem. In the present work, we describe our journey in the development of catalysts for operating in a slurry reactor. The screening of the preparation methods and catalyst composition yielded an active and highly selective CO 2 hydrogenation catalyst with catalytic performances comparable to the best ones reported in the literature. The most perspective catalysts were successfully evaluated in CO 2 hydrogenation using a slurry reactor. The supported catalysts were found to have a lower ability to form higher hydrocarbons compared to bulk catalysts. This could be correlated with the particle sizes of the active component of the respective catalysts. This insight may be useful in preparing tailor-made Fe-based catalysts with high chain growth probability. The important issue of mechanical strength for slurry operation was highlighted. [Display omitted] • A screening strategy focusing on chain growth and less excess-methane is presented. • The obtained catalysts show good performance comparable to best literature data. • C 2+ hydrocarbon selectivity increases with the crystallite size of the active component. • Catalysts developed in fixed-bed were successfully evaluated in a slurry reactor. [ABSTRACT FROM AUTHOR]

While hydrocarbons production by Fischer-Tropsch synthesis (FTS) is widely recognized, commercial interest in FTS products is mainly constrained by oil prices. The FTS-derived fuels, although higher quality and greener, cannot currently compete with cheaper fossil fuels, motivating to shift the FTS products distribution to higher value-added chemicals, such as olefins-C 5+. The high olefin selectivity in FTS over Co-based catalysts is usually ascribed to Co 2 C on the catalyst surface with its inherent deficiencies. This study demonstrates a novel approach to alter low-temperature FTS selectivity by applying off-gas recycling. At slow gas recycling rates, C 19+ selectivity increases due to re-adsorption and re-incorporation into the growing chain of gaseous olefins from the off-gas. High-intensity gas recycling facilitates C 5+ olefin vapors removal from the reactor by enhanced gas flow with their subsequent cut-off in the cold trap, thus reducing residence time of condensed olefins and probability of their secondary reactions. [Display omitted] • Tail gas recycling is an apt tool to tune selectivity of Fischer-Tropsch synthesis. • C 19+ selectivity rises at low gas recycle ratios due to light olefins re-adsorption. • At high gas recycle ratios, selectivity to olefins rises at the cost of heavy waxes. • Gas recycle mode speeds up the reduction of residual CoO in the catalyst by syngas. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • In situ deposition method is applied to investigate the Fe phase evolution behavior. • Fe(CO) x in syngas decomposed on SiO 2 leads to a high χ-Fe 5 C 2 content under reaction conditions. • The impregnated Fe/SiO 2 catalyst results in the Fe oxides reduction into FeO but not metallic Fe. • The FeO is mainly oxidized to Fe 3 O 4 with a low χ-Fe 5 C 2 content under reaction conditions. • The selectivity to CO 2 in iron-based FTS is systematically discussed. Abstract A new process, in situ deposition method, was developed to unveil Fe phase evolution behavior in iron-based Fischer-Tropsch synthesis (FTS). In detail, Fe(CO) x -containing syngas (CO/H 2) was fed into a fixed-bed reactor where Fe(CO) x directly decomposes to metallic Fe with subsequent favorable carburization to form active χ-Fe 5 C 2 on SiO 2 under FTS reaction conditions. This facile process gives a high χ-Fe 5 C 2 content (80–90%) without Fe 3 O 4 phase in spent catalyst. In contrast, the 5.3Fe/SiO 2 prepared by the conventional impregnation method results in very low χ-Fe 5 C 2 content (14.5%) and high Fe 3 O 4 content (47%) in spent catalyst, which is ascribed to the strong Fe oxide-SiO 2 interaction hindering its deep reduction into metallic Fe but terminating as FeO. The reduced FeO could not be efficiently carburized into χ-Fe 5 C 2 and tends to oxidize into Fe 3 O 4 during the FTS reaction. Furthermore, the factors determining CO 2 selectivity are systematically investigated based on abundant experiments. Results show that CO 2 selectivity is closely related to CO conversion, Fe phase composition, reaction conditions, and the presence of K promoter. In brief, when the factor enhances H*/C* ratio on χ-Fe 5 C 2 , the reaction of dissociative O* from CO shifts towards hydrogenation to H 2 O despite that its reaction with CO* to form CO 2 via Boudouard mechanism is kinetically favorable. In addition, CO 2 selectivity could be reduced via reverse water-gas shift (RWGS) reaction on Fe 3 O 4 with H 2 O generation. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Operando magnetic resonance measurements of water-wax behaviour in porous silica pellets. • Reactor operated at 195 °C with water pressure in the range 3–13.6 bar. • Capillary-condensation of water occurs within the initially wax-saturated pore space. • Evidence that water displaces wax from the initially wax-saturated pore space. • As much as 40% of the pore surface is estimated to be covered by water at the highest water pressures studied. Abstract Water is a major product of Fischer-Tropsch synthesis, and hence the behaviour of water within Fischer-Tropsch synthesis catalysts and its potential influence on catalyst rate and selectivity are questions of long-standing interest. The present work applies three different magnetic resonance techniques to study how water interacts with a model wax, n -octacosane, within the pore space of a porous silica of mean pore size ∼18 nm. 1H magnetic resonance spectroscopy, spin-lattice relaxation time and pulsed-field gradient measurements were performed at 195 °C, and for water pressure in the range 3–13.6 bar, conditions relevant to low temperature Fischer-Tropsch synthesis. The uptake of water within this system is shown to be very similar to that observed for capillary condensation of water within the empty pore space of the same porous silica under the same experimental conditions; suggesting that capillary condensation of water within the wax-saturated pores is occurring. The behaviour of water is characterised by two regimes. At low water relative pressures of ∼0.3 ≤ P / P 0 ≤ ∼0.8 water moves into the pore space, displacing wax from the pore surface and existing as a water-rich layer between the pore surface and an oil-rich phase in the centre of the pore; the strong interaction with the pore surface is evidenced by the short nuclear spin relaxation time values of water at the lowest pressures which then increase slightly as multi-layer adsorption at the pore surface occurs with increase in pressure. In the water relative pressure range ∼0.8 ≤ P / P 0 ≤ ∼0.97, condensation of water within the pores is observed, characterised by increases in both spin-lattice relaxation time and molecular diffusivity. Analysis of the data suggests that as much as ∼40% of the pore surface is occupied by condensed water after condensation has occurred. It is suggested that these two regimes of water behaviour inside initially wax-filled pores might explain previously reported aspects of the behaviour of Fischer-Tropsch catalyst performance as a function of pore size and amount of co-fed water. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Nitrogen-doped carbon on CNTs enhances the activation of iron particles for Fischer-Tropsch synthesis of C 2 –C 4 olefins. • Addition of sodium and sulfur leads to tandem promotion and highly active and selective iron-based Fischer-Tropsch catalysts. • Iron time yields are as high as 94∙10−5mol CO g Fe -1 s-1 (340 °C, 10 bar, H 2 /CO = 2). Abstract Carbon nanotubes with various surface modifications are utilized as supports for iron species and the resulting catalysts are used for the Fischer-Tropsch synthesis of C 2 –C 4 olefins with and without the addition of Na and S promoters. A layer of nitrogen-doped carbon on the carbon nanotube surface serves as an efficient promoter independent of the presence of Na/S. It leads to significantly faster catalyst activation as compared to the tubes without such a layer resulting from an electronic junction effect at the iron-support interface. The tandem of Na/S- plus N-doped carbon promoted catalysts provide a record high iron-time-yield of up to 94∙10−5mol CO g Fe -1 s-1 under industrially relevant Fischer-Tropsch conditions (340 °C, 10 bar, H 2 /CO = 2) and slow deactivation over more than 185 h of time on stream. Furthermore, this tandem promotion leads to enhanced C 2 –C 4 olefin and C 5+ selectivity along with reduced methanization as compared to the solely Na/S-promoted catalysts. [ABSTRACT FROM AUTHOR]

Improvement of activity, selectivity, and stability of the catalyst used in Fischer-Tropsch synthesis (FTS) to produce targeted hydrocarbon products has been a major challenge. In this work, the potassium-doped iron-carbide/alumina (K-Fe 5 C 2 /Al 2 O 3 ), as a durable nanocatalyst containing small iron-carbide particles (∼ 10 nm), was applied to high-temperature Fischer-Tropsch synthesis (HT-FTS) to optimize the production of linear alpha olefins. The catalyst, suitable under high space velocity reaction conditions (14–36 N L g cat −1  h −1 ) based on the well-dispersed potassium as an efficient base promoter on the active iron-carbide surface, shows very high CO conversion (up to ∼90%) with extremely high activity (1.41 mmol CO  g Fe −1  s −1 ) and selectivity for C 5 –C 13 linear alpha olefins. [ABSTRACT FROM AUTHOR]

The explanation for CH 4 selectivity for iron based Fischer-Tropsch catalysts in the low conversion region ( i.e ., <50%) remains elusive. In this contribution, the CO conversion effect was carefully examined over four K promoted Fe catalysts (100 Fe/5.1Si/2Cu/ x K, where x = 1–5) over a wide range of CO conversion ( i.e ., 4–85%). Moreover, the effect of CO conversion on oxygenate selectivity of the Fe-K catalysts was carefully studied as well. The change in CH 4 selectivity with CO conversion was found to resemble asymmetric “V” shaped curves, with the minimum values occurring at approximately 50% CO conversion. Adding greater than x  = 2 K significantly alleviated the CO conversion effect which was attributed to the high K loading greatly decreasing the surface H coverage while improving CO adsorption. The unique CH 4 selectivity trend suggests a complicated CH 4 formation process that results from different aspects of the catalyst ( i.e ., chain growth and hydrogenation rates), and process conditions. Oxygenate selectivity was in the range of 0.7–2.8% and varied with the CO conversion and K loading. The addition of K up to x  = 3 was found to promote oxygenate formation and chain growth. The overall oxygenate distribution up to C 17 follows an Anderson-Schulz Flory (ASF) distribution, with ethanol being the dominant oxygenate. Mechanisms of oxygenate formation different from that of hydrocarbon formation ( e.g ., CO insertion versus CO dissociation) were proposed to explain the experimental results. [ABSTRACT FROM AUTHOR]

Foregoing calcination and utilizing direct reduction of cobalt nitrate led to the formation of smaller cobalt oxide nanoclusters in stronger interaction with silica support as intermediates of the activation process to Co° nanoparticles; this was demonstrated by TPR, TPR-MS, TPR-XANES, and TPR-EXAFS experiments using hydrogen. These intermediate cobalt oxides included a spinel (e.g., Co 3 O 4 ) formed from oxidation of Co 2+ species by NO 2 , which in turn converted to CoO prior to formation of the metal. To improve the reducibility, metal promoters such as Pt, Re, Ru, and Ag were added. Hydrogen chemisorption and EXAFS experiments revealed smaller nanoparticles; Co-Co metal coordination numbers were significantly lower for the H 2 -activated Co metal nanoparticles when direct reduction of the nitrate was used relative to H 2 -activated air calcined catalysts. Comparing at the same space velocity, the best catalysts were Re and Pt promoted 12%Co/SiO 2 catalysts utilizing direct reduction of the nitrate, where initial conversions in a CSTR were up to 3.8 times higher and 71% higher than unpromoted and Pt promoted air calcined catalysts, respectively. At these conditions, methane production was lower (6.8 and 8.0% for Re and Pt promoted catalysts, respectively, by direct reduction versus 12.5 and 10.1% for unpromoted and Pt promoted air calcined catalysts) and C 5+ selectivity was higher (81.2% and 81.5% for Re and Pt promoted catalysts, respectively, by direct reduction versus 73.4 and 78.8% for unpromoted and Pt promoted air calcined catalysts). The uncalcined catalysts were slightly less stable than the calcined samples, with the only exception being the rhenium promoted sample, where no visible deactivation was observed; this catalyst also had the highest catalytic activity on a per gram catalyst basis. [ABSTRACT FROM AUTHOR]

A series of silica doped alumina (SDA) [Si-Al 2 O 3 ] having high surface area and narrow pore size distribution are synthesized with varying amount of silica and used as a support material to facilitate incorporation of Co, Mg and Zr into it. These supported Co-Mg-Zr/SDA materials are used as catalysts for selective synthesis of middle distillate through Fischer-Tropsch synthesis (FTS) reaction. Trace quantity (∼0.05 wt %) of ruthenium (Ru) has also been used as a promoter for Co -Mg-Zr/SDA catalyst to improve the reducibility, selectivity towards middle distillate and increasing the catalyst life. The detail characterizations of catalysts indicate that Ru promotion and narrow pore size distribution facilitate CO conversion and middle distillate selectivity. Synthetic syngas of composition (H 2  = 28.63%, CO = 14.27%, CO 2  = 9.25%, CH 4  = 1.0%, and N 2  = 46.85%), which may be obtained after coal gasification and H 2 enrichment process by water gas shift (WGS) reaction has been used as a reactant for middle distillate synthesis. The role of Ru and silica proportion on Co-Mg-Zr/SDA is investigated through Low Temperature Fischer-Tropsch (LTFT) synthesis at 220 °C temperature and 30 bar pressure. The surface acidity of the Ru/Co-Mg-Zr/SDA catalyst plays an important role for middle distillate selectivity and CO conversion. To determine the durability of catalysts, 300 h continuous experimental run has been conducted at Gas Hourly Space Velocity (GHSV) of 500 h −1 . Among the catalysts in the present study, Ru/Co-Mg-Zr/SDA(7.5) catalyst shows highest CO conversion (94.2%) with 85% selectivity of C 6 -C 19 hydrocarbon. [ABSTRACT FROM AUTHOR]

The effectiveness of the addition of diethylene glycol (DEG) to the impregnating solution in synthesizing highly dispersed Co-based catalysts supported on stabilized alumina (Al 2 O 3 (s) ) is investigated. Both the properties and catalytic performance in the Fischer-Tropsch synthesis (FTS) of the material obtained using DEG (CoDEG/Al 2 O 3 (s) ) are compared with those of a catalyst with the same formulation but prepared without the addition of DEG in the impregnating solution (Co/Al 2 O 3 (s) ). When using Co-nitrate as Co-precursor, the addition of DEG leads to a very fast and exothermic decomposition of Co-nitrate into Co oxides during the calcination step. This prevents the agglomeration of Co 3 O 4 particles, thus generating highly dispersed Co 3 O 4 crystallites on the support. In line with the decrease of the Co 3 O 4 crystallites size, the CoDEG/Al 2 O 3 (s) catalyst is more difficult to reduce than the Co/Al 2 O 3 (s) catalyst. As a result, the metallic surface of the two catalysts is very similar. Nonetheless, when tested in the FTS, the CoDEG/Al 2 O 3 (s) catalyst shows CO conversion higher than the Co/Al 2 O 3 (s) sample. This is attributed to the high intrinsic activity (i.e. high Turnover Frequency, TOF) of the small Co 0 crystallites of the CoDEG/Al 2 O 3 (s) catalyst. We explain this result assuming that the combustion phenomenon occurring during the fast calcination induced by the presence of DEG may generate structural defects on the catalyst surface that are beneficial for the FTS. Notably, the product distribution of CoDEG/Al 2 O 3 (s) catalyst is only slightly affected. [ABSTRACT FROM AUTHOR]

This paper focuses on the use of in situ and ex situ characterisation techniques to provide evidences of carbon species on a commercial iron-based Fischer-Tropsch synthesis catalyst as well as other indices of potential deactivation mechanisms. In situ XANES measurements demonstrate that re-oxidation or transformation of the active iron phase, i.e. the Hägg carbide phase, was not a significant deactivation mechanism at the studied conditions. Sintering of Hägg carbide nanoparticles is significant with increasing temperatures and time on stream. The sintering mechanism is proposed to be a hydrothermally-assisted process. In situ DRIFTS indicates the presence of different carbon species on the catalyst surface such as aliphatic hydrocarbons from wax products and oxygenate compounds such as alcohols, aldehydes/ketones and carboxylate species. Carboxylate species are resistant towards hydrogenation at 280 °C. The presence of different carbon species on the surface after wax product extraction is evident from TPH-MS measurements. GC-MS analysis shows that the strongly adsorbed carbon species remaining on the catalyst surface from wax products are mainly α-olefins and branched carboxylic species. The interaction of oxygenate compounds, especially carboxylate species with iron oxide, may form stable complexes limiting further iron catalyst carburization. STEM-EDX analysis shows that carbon is preferentially located on iron particles. [ABSTRACT FROM AUTHOR]

One of the pathways responsible for the deactivation of Fischer-Tropsch catalysts is the loss of active metal surface area due to nanoparticle agglomeration. To combat this effect efforts have been made to increase the interaction between the metal nanoparticles and the support using materials like silica. In this study, the supported metal particles were covered with a highly porous layer of silica to stabilize the Co nanoparticles on a titania support both during reduction and under reaction conditions. Co 3 O 4 nanoparticles (size range: 8–12 nm) supported on titania were stabilized by coating them with a thin layer of mesoporous silica ( ∼ 4 nm) to make Fischer-Tropsch catalysts that are less prone to sintering (Co/TiO 2 @mSiO 2 ). To mitigate the strong metal support interactions brought about by the titania and silica a Ru promoter was loaded together with the cobalt nanoparticles onto the titania (CoRu/TiO 2 @mSiO 2 ). Temperature programmed XRD studies on the evolution of the Co metal nanoparticles showed that there was no significant particle size growth under reduction conditions in the temperature range from 30 to 600 °C. Chemisorption studies following reduction under hydrogen at 350 °C and 450 °C gave results consistent with the in situ XRD data when compared to the Co/TiO 2 . Fischer-Tropsch synthesis on the Co/TiO 2 @mSiO 2 and CoRu/TiO 2 @mSiO 2 catalysts encapsulated inside the mesoporous silica shell exhibited good catalytic performance without any display of significant mass transport limitations that might arise due to a silica shell coating of the active sites. For these two catalysts the Fischer-Tropsch activity increased with reduction temperature without any significant negative changes in their selectivity due to sintering, while the activity on Co/TiO 2 decreased due to Co nanoparticle sintering. [ABSTRACT FROM AUTHOR]

Co-based catalysts are highly active for synthesizing long-chain paraffins through Fischer–Tropsch synthesis. However, their exothermic reactions can cause overheating and catalyst deactivation. To address this, thermally conductive SiC-Al 2 O 3 pellets (HeatPath™) were coated with a Co/Re/Al 2 O 3 shell, significantly boosting the CO conversion rate and the yield of C 2–4 and C 5+ hydrocarbons over a wide range of temperatures by dissipating the reaction heat. The core@shell HeatPath@Co/Re/Al 2 O 3 catalyst demonstrated high productivity (up to 19.7 g CO g cat − 1 h−1), exceptional long-term stability over 660 h time-on-stream, with high-temperature operation up to 260 °C, a low CH 4 selectivity (7.1 %) and a chain growth probability (α) > 0.8. In contrast, the irreversible deactivation of a powder Co/Re/Al 2 O 3 catalyst started at 195 °C, resulting in >60 % CH 4 selectivity and ∼100 % CO conversion due to excessive local heat. The CO conversion rate on the core@shell catalyst could be further improved by increasing the H 2 /CO ratio and temperature, with commercially viable CH 4 selectivity. [Display omitted] • core@shell SiC-Al 2 O 3 @Co-Re showed high performance in Fischer-Tropsch synthesis. • core@shell catalyst improved CO conversion rate and yield of C5 + hydrocarbons. • Heat dissipation property of SiC-Al 2 O 3 pellets enhanced Fischer-Tropsch performance. • Low CH 4 and CO 2 selectivities over a wide temperature range. • High productivity (∼19.7 g CO g cat − 1 h−1) and good stability >660 h time-on-stream. [ABSTRACT FROM AUTHOR]

Noble metals (Ru, Re, Pt, Ir) and transition metals (Mn, Y) with their atomic ratio of 0.01 to cobalt were used as promoters on the cobalt catalysts supported on mesoporous alumina. The cobalt loading was 15 wt% and all catalysts were tested for Fischer-Tropsch synthesis in a fixed-bed reactor. The process conditions for activity test were: 230 ° C and 400 psi, GHSV = 900 h −1 , and H 2 C O = 2.0 . The selectivity test for the promoted and unpromoted catalysts was performed at CO conversion ≈50% by adjusting the syngas flowrate. Addition of promoters increased the cobalt dispersion on mesoporous alumina support. The highest increase in dispersion from 4.7% to 7.6% resulted from addition of Ru. Promoters positively impacted the cobalt dispersion in the decreasing order of: Ru < Re < Pt < Ir < Mn < Y. The reduction temperature for all promoted catalysts were shifted to lower temperature. The maximum decrease in temperature happened with platinum by Δ T ∼ − 150° C for the 1st reduction temperature and Δ T ∼ − 270° C for the 2nd reduction temperature. The reducibility of the CoO x species on mesoporous alumina was extensively improved by noble metals addition and modestly enhanced by Mn and Y. Significant increase in CO conversion and C 5+ of the promoted catalysts were observed. CO conversion was increased from ∼60% to ∼86, 84, 82, 80% as a result of Ru, Re, Ir, Pt addition. Addition of Ru and Re promoters favored the heavy molecular weight hydrocarbons’ (C 5+ ) selectivity and improved it from 81.7% to 84.2 and 83.1%, respectively. [ABSTRACT FROM AUTHOR]

Ga- and In-modified ceria (Ce 0.8 Ga 0.2 O 2 , Ce 0.8 In 0.2 O 2 ) materials were used as supports for cobalt-catalyzed Fischer-Tropsch synthesis (FTS). The addition of Ga to ceria was found to improve CO conversion for cobalt-catalyzed FTS, while the addition of In tended to decrease it. A similar trend was observed with the Ag-promoted cobalt/ceria catalysts. Doping of ceria with Ga or In decreased methane and increased the selectivity to olefins and alcohols for Ag-promoted cobalt/ceria. The sum of the products of olefins and alcohols for various catalysts exhibited a decreasing trend as follows: Ag-Co/Ce-Ga > Ag-Co/Ce-In > Ag-Co/Ce. Results of H 2 -TPR-XANES showed that adding of Ga or In to ceria increases the fraction of Ce 3+ in the surface shell for both unpromoted and Ag-promoted catalysts in the range of temperature typical of catalyst activation. This partially reduced ceria plays an important role in controlling the product selectivity of cobalt-catalyzed FT synthesis. [ABSTRACT FROM AUTHOR]

To investigate the influence of a supercritical fluid reaction medium on nanoparticle-catalyzed Fischer-Tropsch synthesis (FTS), Fe-based nanoparticle catalysts supported on carbon nanotubes were prepared and tested under both gas phase and supercritical FTS operating conditions. The nanoparticle catalysts displayed high activity, low CH 4 selectivity, and elevated oxygenate selectivity. Conversion and selectivity towards heavy, unhydrogenated products were elevated under supercritical operation. With K promotion, aldehydes were produced under both gas phase and supercritical conditions. This study demonstrated the necessity of K promotion for aldehyde production and demonstrated that appreciable amounts of aldehydes can be produced in gas phase, Fe-catalyzed FTS. [ABSTRACT FROM AUTHOR]

Fischer-Tropsch synthesis produces light olefins and long chain hydrocarbons from syngas generated from fossil and renewable feedstocks. Molybdenum carbide exhibits catalytic properties approaching those of metal catalysts in a number of hydrogenation reactions. The present paper focuses on the effect of promotion with potassium on the structure and performance of alumina supported carburized molybdenum catalysts in Fischer-Tropsch synthesis. Molybdenum carbide catalysts were synthesized by temperature programmed carburization in methane and hydrogen. Molybdenum was highly dispersed in both calcined and carburized alumina supported catalysts. Mixed oxides of potassium, molybdenum and aluminium were uncovered in the calcined promoted catalysts. These oxides are converted in molybdenum carbide species on their carburization. Potassium promotion strengthens interaction between molybdenum and aluminium. The potassium promoted catalysts exhibited higher selectivity to light olefins and long chain hydrocarbons but lower Fischer-Tropsch reaction rate compared to the unpromoted counterpart. The observed lower Fischer-Tropsch reaction rate on the promoted catalysts was attributed to more difficult carburization of the molybdenum species and electronic effects due to the presence of potassium. The light olefin and C 5+ selectivities decrease at higher carbon monoxide conversion probably because of secondary olefin hydrogenation and hydrocarbon hydrogenolysis. [ABSTRACT FROM AUTHOR]

Fischer-Tropsch synthesis to light olefins (FTO) using biomass-derived syngas is an economical and renewable way to produce olefins. By using KMnO 4 as precursor, a number of low cost iron based catalysts was produced and their FTO performances were tested. Firstly, a series of KMnO 4 solutions with different concentrations were used to pretreat activated carbon (AC). Support was coated uniformly by K-doped birnessite MnO 2 and substantial amount of surface oxygen-containing groups and defects were obtained simultaneously. Then, iron catalysts loaded on KMnO 4 modified AC support was prepared by incipient wetness impregnation method. Characterization of the catalyst showed that α-Fe 2 O 3 nanoparticles were uniformly loaded on the surface of KMnO 4 treated AC. The valent of manganese (IV) was then transformed into Mn (II) and Mn (III) with AC served as a reducing agent. The redox reaction during preparation leads to a lower Mn chemical valence, which makes it easier to further reduction. More defects on support after calcination procedure were also observed due to the enrichment of oxygen-containing groups, which was helpful in anchoring α-Fe 2 O 3 particles and decreasing its average particle size. High-content manganese played not only as a structural promoter to anchor Fe 2 O 3 nanoparticles, but also a H 2 adsorption competitor that decreased H 2 /CO ratio over active sites to reduce the possibility of further hydrogenation of olefins. The KMnO 4 modified iron catalysts exhibited high CO conversion and olefins selectivity (∼40%) due to the improved distribution of iron and the auxiliary functions of Mn. Residual potassium is also in favor of the formation of iron carbides to increase CO conversion. [ABSTRACT FROM AUTHOR]

The paper presents the low-temperature nitrogen adsorption, TG, XRD, IR spectroscopy, XPS, TEM and SEM data for ruthenium promoted (0.2–1 wt.%) Сo-δAl 2 O 3 catalysts and characteristics of the catalysts in Fischer-Tropsch synthesis after their activation under the conditions ensuring the reduction of comparable fractions of metallic cobalt. It was shown that cobalt in oxide precursors is a component of the spinel-like Со 3-x Al x O 4 phase containing the impurity anions СО 3 2− , NO 3 − , ОН − , and NO in promoted samples, which belong to the thermolysis product of the Ru precursor. Average sizes of Со 3-x Al x O 4 crystallites are in a range of 5–10 nm. As ruthenium content in the catalyst increases upon reduction, the temperature of metallic phase formation decreases substantially (by more than 150 °C). After the reduction, selectivity of promoted catalysts for α-olefins and high-molecular hydrocarbons was higher in comparison with unpromoted catalysts, without a noticeable decrease in catalytic activity. Therewith, in 0.5–1.0 wt.% catalysts, a part of ruthenium forms individual ultradispersed metallic particles ca. 1 nm in size that are located on the surface of oxide support and are not active in Fischer-Tropsch synthesis. The oxide layer decorating the surface of metallic cobalt particles is also strongly enriched with ruthenium. In the 0.2 wt.% catalyst, the major part of ruthenium resides in metallic cobalt particles. Although the ruthenium-cobalt alloy segregates with enrichment of the surface with cobalt, the presence of ruthenium in the metallic particles and probably in the decorating oxide layer exerts a considerable effect on selectivity of the catalysts. [ABSTRACT FROM AUTHOR]

A well-defined model catalyst constituting a compromise between high surface area, porous, industrial catalysts and a planar model catalyst has been developed. It allows for measurements of catalytic activity in micro reactors, where diffusion problems are kept at a minimum, while characterization both by surface science techniques and by bulk techniques can be applied. Monodisperse, non-porous SiO2 microspheres with diameter 875 ± 25 nm have been synthesized, serving as the large area model support. These where then impregnated with pre-formed, monodisperse, colloidal Fe and FeMn nanoparticles resulting in a three-dimensional equivalent of a flat, Fe(-Mn)/SiO2 model catalysts. Characterization with electron microscopy (SEM and TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), before and after catalytic testing was performed. It was shown that the model catalysts can be used in Fischer-Tropsch synthesis experiments under industrially relevant conditions. The FTS experiments reveal that compared to the pure Fe catalyst, FeMn shows more stable activity, higher selectivity towards olefins and lower selectivity toward CH4 and CO2. Significant amounts of hydrocarbons on the catalyst surfaces and some minor indications of sintering were detected after the reaction. Formation of FeCx was detected for the Fe catalyst while no significant amounts could be seen on the Mn-promoted catalyst. [ABSTRACT FROM AUTHOR]

The reduction activities and mechanisms of cobalt-based catalysts are of great interest to industry and researchers, due to their applications in Fischer-Tropsch synthesis. Here, we investigated the reduction of alumina-supported cobalt catalysts by hydrogen using temperature-programmed reduction. We propose a five-step reduction mechanism that incorporates both amorphous and crystalline Co3O4, and includes the interaction between CoO and the Al2O3 support. Based on our proposed mechanism, we developed a kinetic model of the reduction process. The modelling results of catalysts promoted with ruthenium and lanthanum in contrast with un-promoted catalyst clearly show that the promoter improves reducibility of the catalyst. The effect of Co3O4 crystallinity was also investigated by the reduction of fresh in comparison of pre-oxidised catalyst. We conclude that high crystallinity significantly increases the difficulty of reducing Co3O4. The interaction between CoO and Al2O3 under reduction conditions to form CoAl2O4 was quantitatively simulated. The kinetic modelling confirms that the support plays an important role in catalyst reduction via the interaction between the catalyst and the support. Those kinetic modelling results are supported by in situ X-ray diffraction studies of the reduction process. [ABSTRACT FROM AUTHOR]

Low pressure Fischer-Tropsch synthesis was tested using iron catalysts reduced by three different reducing gases. In this regards, three reactors were set up in parallel and each of them loaded with the same quantity of the iron catalyst (FeCuKSiO 2 ). After loading, the catalysis in the first reactor was reduced with synthesis gas (a combination of carbon monoxide and hydrogen). The catalyst in the second reactor was reduced using hydrogen (H 2 ) gas, and while that in the third reactor was reduced with carbon monoxide (CO) gas. In this study only the gas-phase products leaving the reactors were analysed and compared under the same operating conditions such as flow rate, pressure and temperature for 1000 h TOS. The results obtained indicate that the initial use of different reducing agents does not have a large impact on the long term catalyst activity but does have an impact on long term selectivity. The three reactors stabilize to about the same overall CO conversion of 14.5% which corresponds to a consumption rate of 2.64 × 10 −4 mol/min.gcat (reactivity) after about 150 h while the selectivities in the three reactors are not the same. Methane selectivity showed the trend: Carbon monoxide reduced > Syngas reduced > Hydrogen reduced. Furthermore, reducing with CO gives much more olefins than reducing with syngas and H 2 . The paraffin production rates did not follow a consistent trend. [ABSTRACT FROM AUTHOR]

Modified alumina carriers were obtained by pre-coating cobalt aluminate on the surface of common alumina. Next, supported cobalt catalysts were obtained by incipient wetness impregnation, and their FTS performances were evaluated in a fixed-bed reactor. In situ XRD results suggested that HCP Co was obtained after reducing the aluminate-modified alumina-supported cobalt catalysts, as opposed to FCC Co, which was supported on common alumina. H 2 -TPR results demonstrated that the modified catalysts possessed lower reduction temperatures. CO-TPD and in situ CO-FTIR characterizations indicated that bridged rather than linear CO adsorption occurred on the modified catalysts. The evaluation results indicated that the aluminate-modified catalysts possessed higher CO conversion rates, higher C 5 + selectivity, higher cobalt specific activity, higher C 5 + space-time yields (STYs), and lower methane selectivity than conventional catalysts in the FTS process. [ABSTRACT FROM AUTHOR]

Alkalis (Li, Na, K, Rb and Cs) promoted Fe/SiO 2 catalysts were used to investigate the influences of alkalis on the alkali-FeO x interaction, reduction behavior, and catalytic performance of iron-based Fischer-Tropsch synthesis (FTS) catalysts. Laser Raman spectroscopy (LRS), Fourier transform infrared spectroscopy (FTIR), Mössbauer effect spectroscopy (MES), H 2 temperature programmed reduction (H 2 -TPR), in situ X-ray photoelectron spectroscopy (in situ XPS) and in situ X-ray diffraction (in situ XRD) were employed to characterize the catalysts. It was found that alkalis had strong interactions with iron species in catalysts. In as-prepared catalysts, the Fe O bonds are strengthened by alkalis, which consequently influenced the reduction of catalysts. Li and Na strongly restrained the reduction of all iron oxides in catalysts. K, Rb and Cs inhibited the initial reduction step (reduction of Fe 2 O 3 to Fe 3 O 4 ), while improved the reduction of iron oxides to FeO and metallic iron. Besides, alkalis facilitated the carbonization of catalysts and the improvement increased with increasing the atomic number of alkalis. In FTS reactions, Li and Na decreased the FTS activity while K, Rb and Cs largely increased the FTS activity of iron catalysts. The selectivities to methane and alkane were decreased while those to olefin and heavier hydrocarbons were increased by alkalis. [ABSTRACT FROM AUTHOR]

High thermal conductive Al 2 O 3 @Al composite synthesized via metallic Al powders corrosion method has been applied as support for preparing Co/Al 2 O 3 @Al catalyst for high exothermic Fischer-Tropsch synthesis. The prepared Al 2 O 3 @Al composite is a hierarchical macro-mesoporous structure and consist of two parts, namely the inner metallic Al and the outer Al 2 O 3 with two kinds of morphologies. Increasing the concentration of the corrosive NaOH or extending the corrosion time could increase the specific surface area of the composite. For the prepared Co/Al 2 O 3 @Al, Co 3 O 4 phase is located on the outer plate-like Al 2 O 3 layer. The Co/Al-0.03-20 possesses higher CO conversion and C 5+ selectivity due to the appropriate cobalt reducibility, dispersion and high thermal conductivity. The thermal conductive coefficient of Co/Al 2 O 3 @Al catalysts is 13–98 times of conventional Co/Al 2 O 3 . Compared to Co/Al 2 O 3 , the Co/Al 2 O 3 @Al shows higher CO conversion and C 5+ selectivity as well as lower CH 4 and CO 2 selectivity, which is attributed to the macro-mesoporous structure and high thermal conductivity. In addition, because of the high thermal conductivity, no heat disperser should be used and a homogeneous temperature gradient maintains within the catalyst bed for Co/Al 2 O 3 @Al. [ABSTRACT FROM AUTHOR]

Ru nanoparticles (NPs) have been successfully deposited in the inner pores of TiO 2 nanotubes (TNTs), denoted as Ru-in/TNT, by vacuum assisted impregnation method for Fischer-Tropsch synthesis (FTS). The composition, morphology and electronic properties have been characterized by the XRD, TEM, N 2 -sorption, H 2 -TPR/TPD and XPS, investigating the confinement effect of TNT on the entrapped Ru NPs’ FTS performance. The Ru-in/TNT exhibits high CO conversion activity and superior deactivation-resist ability over the Ru-out/TNT (with most of Ru NPs deposited on the outer surface of TNT) and Ru/P25 (using commercial P25 powder as support) catalysts. Besides above superiority, the Ru-in/TNT facilitates the formation of long chain (C19+) products as well. The enhanced activity and selectivity can be ascribed to the confinement effect of TNT: (i) strengthen charge transfer and modulate the electronic properties of Ru species resulting in more active center site; (ii) constrain the metal sintering and metal leaching to resist deactivation during FTS; (iii) facilitate the re-adsorption of α-olefin leading to long-chain product. [ABSTRACT FROM AUTHOR]

The Single-Event MicroKinetic methodology has been successfully extended from Fe to Co catalyzed Fischer-Tropsch Synthesis. A total of 82 experiments were performed in a plug flow reactor with a H 2 to CO molar inlet ratio between 3 and 10, a temperature range from 483 to 503 K, CO inlet partial pressures from 3.7 to 16.7 kPa and space time varying between 7.2 and 36.3 (kg cat s) mol −1 . Via regression, statistically significant and physicochemically meaningful estimates were obtained for the activation energies in the model and the H, C and O atomic chemisorption enthalpies as required for the UBI-QEP method. A reaction path analysis allowed relating the observed deviations from the Anderson-Schulz-Flory distribution, i.e., a high methane and low ethene selectivity, to the symmetry numbers and a higher chemisorption enthalpy of the metal methyl species compared to the other metal alkyl species. Simulations at industrially relevant conditions show that, as a catalyst descriptor, the H atomic chemisorption enthalpy crucially determines both the CO conversion and the C 5+ selectivity. The higher FTS activity of Co compared to Fe is explained via the higher oxygen atomic chemisorption enthalpy on the latter compared to the former. [ABSTRACT FROM AUTHOR]

CNT oxidation, Co/CNT preparation and transformation of both catalyst and support in Fischer-Tropsch synthesis (FTS) were thoroughly studied. CNTs functionalized by nitric acid for 1–15 h were used as supports for Co catalysts for FTS at 190 °C and atmospheric pressure. Co supported on CNTs with the highest specific surface area and oxygen content showed the highest activity and C 5+ yield. According to TEM results after 70 h of FTS this catalyst was the most stable to Co sintering due to the large number of surface defects and oxygen groups on a support surface combined with the preserved integrity of CNTs. Activity of catalysts increased and selectivity to C 5+ decreased when the particle size diminished from 26.5 to 4.3 nm. Study of the evolution of CNT support revealed that most of the carboxylic groups decomposed at calcination. Etching of support surface during the catalyst reduction and partial pore blocking after FTS were detected. [ABSTRACT FROM AUTHOR]

The effect of phosphorus addition on silica-supported cobalt catalyst was investigated for Fischer-Tropsch synthesis (FTS). As shown by STEM images and chemisorption, the addition of phosphorus to cobalt of up to 1 wt% increased the dispersion of cobalt. Further addition of phosphorus (e.g., 3 and 5 wt.%), as demonstrated by TPR, pulse reoxidation, and XANES, significantly hindered the reduction of cobalt oxides. The cobalt FTS catalysts containing 0.5 and 1.0 wt% P exhibited greater stability in comparison with undoped and 3.0 wt% P containing cobalt catalysts. Analysis of XANES spectra at the P and Co K-edges, along with DRIFTS results of H 2 -activated cobalt catalysts, suggest that cobalt particles interact with PO 4 3− ions, indicating a role played by P in anchoring Co particles to the support, thus hindering the cobalt sintering rate. The initial selectivity to methane was slightly higher for 0.5%P-20%Co/SiO 2 and 1.0%P-20%Co/SiO 2 catalysts compared to the undoped catalyst, but at longer times differences were small. At higher loadings of P (3 wt.%), FT activity and selectivity were adversely and irreversibly affected. [ABSTRACT FROM AUTHOR]

The adsorption, dissociation and desorption of CO at different coverage over two Co surfaces and the corresponding equilibrium phase diagrams under different temperatures and partial pressures have been systematically investigated; here, the results are obtained using density functional theory calculations and atomistic thermodynamics together with the periodic slab model. Our results show that the saturated adsorption on both Co(0001) and (100) surface are 7/9 ML; meanwhile, on these two surfaces, the lateral repulsive interaction has an effect on the adsorption structures and the corresponding stepwise adsorption energies of these adsorbed CO molecules, and become stronger with the increasing of CO coverage, which further leads to CO migration over Co surface; moreover, the adsorption energies decrease gradually with the increasing of CO coverage until to the saturated adsorption. According to the stepwise adsorption configurations, further calculations on CO dissociation show that only molecular adsorption CO is favored over (0001) surface; whereas on Co(100) surface, when the coverage is 1/9 ML, CO dissociation is more facile than its desorption; when the coverage is from 2/9 to 5/9 ML, only the first CO dissociation is preferred, while desorption is more favored for the left CO molecules; when the coverage is equal to or greater than 6/9 ML, CO desorption will be more favorable than its dissociation. Further, the adsorption and activation of CO under different temperatures and partial pressures from atomistic thermodynamics method well illustrate the relationship between the stable CO adsorption with the temperatures and CO partial pressure on Co(0001) and Co(100) surface, respectively. Therefore, on the basis of above results, we can obtain that the surface structures mainly affect the adsorption form of CO due to the different activity towards CO, while CO coverage will affect its adsorption energies and configurations, as well as CO dissociation on Co surfaces. The calculated stretching frequencies of CO molecule at different coverage agree with the available experimental data. This study provides a more accurate and detailed information for the process of CO adsorption and activation on Co surface. [ABSTRACT FROM AUTHOR]

The olefin (O) to paraffin (P) ratio is important in Fischer-Tropsch synthesis (FTS) and there is some disagreement as to how important diffusion, re-adsorption of olefins or other rate based phenomena are in determining this. We measured the performance of a cobalt catalyst for nearly 3000 h time on stream. During this period there was first the initial unsteady state behaviour as the catalyst settled in, then we changed the temperature, the catalyst deactivated and finally we flushed the reactor with N 2 and then resumed reaction to see if this would disturb the performance of the catalyst. We plotted the data in the standard formats typically used, but we also plotted the data in two other ways. The first way, which we refer to as the Yao plot, is a plot of P n+1 /O n+1 versus P n /O n where for us n is a carbon number from n = 2–5 as we only measure the lower hydrocarbons. We find that the data lies on two straight lines, one for n = 2 and the other for n > 2. What is important is that these straight lines hold during the settling in period, the change in temperature, the deactivation of the catalyst and even after the catalyst had been flushed with nitrogen. The olefin and paraffin distributions are thus dependent and cannot be changed independently. This result could imply that the product distribution might be determined by some sort of reaction equilibrium among some species (we refer to this as quasi-reaction equilibrium) or vapour–liquid equilibrium (VLE), or a combination of the two factors. The second plot, which we refer to as the Lu plot, is a plot of the relationship between normalised mole fractions of O n , O n+1 and P n. These plots have previously been used in distillation and reactive distillation and the behaviour that is seen for the FT system indicates that there is a stable equilibrium point and perturbations to the reactor cause the measured data to make a step change away from this point and that over time the points move back to the stable point. This behaviour is strongly suggestive of a reactive distillation map with a stable point. This is found in for example systems with reversible reactions. Our findings suggest that equilibrium processes, such as VLE and reversible reactions, may be contributing to the observed behaviour in FT and that we perhaps should not rely solely on rate based processes, such as kinetics and diffusion, in trying to understand the behaviour of FT. [ABSTRACT FROM AUTHOR]

Sequential impregnation and calcination were used to fill the pores of a commercial alumina support with alumina. In this manner, two different pore-modified alumina supports were synthesized. To account for the diminished pore size, cobalt loadings were lowered as appropriate to ensure a similar cobalt size range (e.g., clusters 14–18 nm in diameter) and comparable extents of reduction. This was confirmed by measurements from TPR, hydrogen chemisorption/pulse reoxidation, and EXAFS/XANES spectroscopies. Catalysts were tested using a slurry phase CSTR reactor at commercially relevant FTS conditions. Selectivities were compared at both high and low levels of conversion for the catalyst series. Decreasing the pore length, as measured by PSD data, appears to lessen the adverse effect that the higher relative diffusional rate of hydrogen versus carbon monoxide has on the H 2 /CO surface fugacity ratio on the catalyst surface. That is, moving the cobalt particles closer to the pore mouth may prevent the H 2 /CO ratio from being augmented at the surface of cobalt particles due to diffusion or other factors, and inhibits excessive chain termination that would lead to higher light product selectivities. Pore filling thus results in a catalyst that is similar to an egg-shell catalyst. With increased pore filling by alumina, C 1 C 4 light gas selectivities decreased, in a systematic way, by as much as 33% (relative basis), and the C 5 + selectivity improved by as much as 10.3% (relative basis) in comparison with the 22.4%Co/Al 2 O 3 (IWI) reference catalyst. This greater effectiveness offers an opportunity for more environmentally benign chemical processing. [ABSTRACT FROM AUTHOR]

The effect of acidic properties and mesoporosity created by desilication on ZSM-5 zeolites was investigated to evaluate changes in product selectivity during Fischer–Tropsch synthesis (FTS) over Fe-based bifunctional catalysts. The reaction was studied at 300 °C, 10 bar, and H 2 :CO = 2:1, using a physical mixture of the iron-based FTS catalyst and zeolites with different Si/Al ratio. Hierarchical zeolites with micro- and mesoporosity were prepared by alkaline treatment. Higher acidity (lower Si/Al ratio) in the pristine zeolite was found to be responsible for cracking of heavy hydrocarbons and the formation of heavier aromatic products, while higher Si/Al ratio induced a higher selectivity towards gasoline-range products (isoparaffins and olefins). The presence of mesopores prompted a variable behavior depending on the final acidity of the resulting Fe-based hierarchical catalyst; zeolites that retained acidity led to a slight increase in C 2 -C 4 paraffins due to overcracking of heavy molecules as a result of enhanced transport to the acidic sites through the mesopores. The products obtained with the Na + -containing alkali-treated zeolites with no acidity were similar to those obtained with the iron FTS catalyst alone, with most of the products in the paraffin range. Rather than improving the activity and selectivity to gasoline products, the better accessibility in the hierarchically-based Fe/ZSM-5 catalysts coupled to the preserved acidity of the zeolite induced the formation of aromatics and a higher amount of small paraffins (

The effects of sulfur modification (S/Fe = 0.001 ∼ 0.05, molar ratio) on the α-Al 2 O 3 -supported iron catalysts for the Fischer–Tropsch synthesis (FTS) were investigated. It was found that the presence of sulfur promoted the reduction of FeO to metallic Fe during the H 2 reduction process, but decreased the rates of reduction, carburization and carbon deposition of the catalysts when CO is used as the reducing agent. Both the sulfide (S 2− ) and sulfate species ( S O 4 2- ) are found in the S x Fe/α-Al 2 O 3 catalysts after H 2 reduction and FTS reaction. The sulfide species in the H 2 -reduced S x Fe/α-Al 2 O 3 catalysts are located on the surface of metallic Fe particles. The intensity of H 2 desorption peak around 500 K is enhanced by a small amount of sulfur, while an excess of sulfur suppresses the H 2 adsorption. The dissociative adsorption of CO on H 2 -reduced catalysts was also found to be inhibited by the presence of sulfur. The activity of the Fe/α-Al 2 O 3 catalyst can be decreased significantly by the addition of sulfur due to the inhibition of CO dissociation and thus reduce the formation of iron carbide phases under FTS reaction conditions. Resistance to sulfur poisoning of the Fe/α-Al 2 O 3 catalyst was found to improve with increasing the reaction temperature. The presence of sulfur also suppressed the formation of C 5+ hydrocarbons and shifted the products to C 2 –C 4 hydrocarbons. At the same time, the olefin to paraffin (O/P) ratio of the C 2 –C 4 hydrocarbons decreased with increasing S/Fe molar ratio. These may have resulted from the increasing of the H/C ratio on the surface of sulfur modified catalyst under FTS conditions. [ABSTRACT FROM AUTHOR]

The presence of nitric oxide NO in the gas phase was shown to decrease the decomposition rate of the hydrotalcite-like Co–Al LDH phase in the catalysts prepared by co-precipitation or deposition on Al 2 O 3 under the condition of urea hydrolysis. The decrease in LDH decomposition rate is related to the formation of a more crystallized phase of the spinel-like Co–Al oxide due to the ability of nitric oxide to transfer oxygen in consecutive reactions of the oxidation to NO 2 and reduction to NO. The difference in the coherent scattering domain size of the catalyst samples precalcined in the presence of NO or in a pure inert gas is retained at all consecutive steps of the reductive heat treatment in hydrogen: after the formation of a cubic phase of the (Co, Al)O oxide and its reduction to metallic cobalt. The observed changes in the degree of crystallinity and dispersion of the active metal exert only a slight effect or even no effect on the activity of the catalysts in Fischer–Tropsch synthesis. Noteworthy are a lower selectivity for methane and a greater fraction of olefins in the products obtained on the catalysts precalcined in a flow of inert gas containing 3% NO. [ABSTRACT FROM AUTHOR]

Literature about sulfur addition over AB 2 O 4 spinel-supported cobalt CO hydrogenation catalysts is sparse. Research about the use of carbon disulfide as the sulfur carrier for the poisoning of cobalt catalysts is also sparse. In this study, effect of the addition of 0.1–1, 10, 100 and 300 ppmv of sulfur (injected as carbon disulfide) on the structure and performance of a Co/ZnAl 2 O 4 ·Al 2 O 3 CO hydrogenation catalyst was characterized (by ICP, XRD, nitrogen sorption and FESEM) and evaluated in a fixed-bed reactor. The addition of 0.1–1 ppmv of sulfur did not have a significant negative effect on the activity of Co/ZnAl 2 O 4 ·Al 2 O 3 catalyst. At S equivalent concentration of 100 ppmv, the deactivation of Co/ZnAl 2 O 4 ·Al 2 O 3 catalyst was reversible to some extent. At identical S equivalent concentration of 100 ppmv, the rate of S sorption by Co/ZnAl 2 O 4 ·Al 2 O 3 catalyst was 3.1 times higher than that by Co/γ-Al 2 O 3 catalyst, making the former a potential catalyst (or sorbent) to be loaded at the top portion (or pre-catalytic bed) of a fixed-bed FT reactor to scavenge sulfur from syngas feedstock. [ABSTRACT FROM AUTHOR]

The sulfur limit, the relationship between the sulfur added and the surface Fe atoms lost (Fe/S), and mechanism of sulfur poisoning were studied using an iron Fischer–Tropsch synthesis (FTS) catalyst (100 Fe/5.1 Si/2.0Cu/3.0K). The FTS reaction was carried out at 230–270 °C, 1.3 MPa, H 2 /CO = 0.67–0.77 and 30–70% CO conversion using a 1-L slurry phase reactor. The used Fe catalysts were characterized by XRD, Mössbauer spectroscopy and XANES spectroscopy to understand the deactivation mechanism of the Fe based catalyst after adding up to 1 ppm H 2 S in the feed. Co-feeding of 0.1 ppm H 2 S in syngas for 70 h caused a very small change in the activity of the Fe catalyst, but increasing the H 2 S level to 0.2 ppm or above resulted in measurable deactivation of the Fe catalyst over a similar time period. The limit of sulfur level in the syngas feed (sensitivity) was determined to be 50 ppb. The added sulfur improved the selectivities of the secondary reactions of olefins and the WGS reaction even though the rates for these declined. The addition of H 2 S decreased CH 4 selectivity and increased C 5+ selectivities of the Fe catalyst. The Fe/S ratio, which can be used to define the poisoning ability of sulfur for the iron catalyst, was quantified based on the deactivation data obtained. The Fe/S ratio strongly depended on temperature and decreased remarkably with increasing temperature. At 270 °C one sulfur atom was found to eliminate ∼6 surface Fe atoms, and the ratio increased to 7.2 at 260 °C and increased further to 13.5 at 230 °C. The Fe/S relationship with increasing temperature is in good agreement with sulfur sorption theory. The changes in FTS and WGS rates of the Fe catalyst by sulfur were also studied. The decreases in rates of the two reactions were nearly the same. The results of XRD and Mössbauer spectroscopy indicated that the online addition of sulfur did not greatly alter the distributions of iron carbide and magnetite. Both data sets consistently suggest an adsorption mechanism, in line with the results of reaction testing. XANES results at the S K-edge further confirmed sulfur adsorption, and some sulfide and sulfate species, likely confined to the surface zone, were detected. In this study, the sulfur tolerances of the precipitated Fe and a supported Co catalyst were compared at an identical temperature (i.e., 230 °C), and similar M/S ratios (13.5–15.0) were obtained. [ABSTRACT FROM AUTHOR]

Cobalt nanoparticles in the range of 1.7–7 nm were synthesized in water using sodium borohydride as reducing agent and subsequently immobilized on TiO 2 . Both colloidal and TiO 2 supported NPs were characterized by TEM, XRD, HRTEM, XPS, TGA, ICP techniques and their catalytic performance of the supported catalysts evaluated in the Fischer–Tropsch synthesis. As a general trend, an increase in activity and TOF was observed when the particle size decreased. These results were rationalized by the higher reducibility of the catalysts bearing the smaller nanoparticles. The unconventional stability of these relatively small CoNPs in FTS was attributed to the promoting effect of boron on the stability of the cobalt nanocrystals. [ABSTRACT FROM AUTHOR]

In this work, the performances of a spherical Co/γ-Al 2 O 3 eggshell catalyst characterized by a diameter of 600 μm and an active shell region 75 μm thick are assessed in a lab-scale fixed bed reactor operated at industrially relevant Fischer–Tropsch process conditions. Such performances are compared to those of a powder catalyst obtained by grinding and sieving a fraction of the eggshell catalyst so to obtain powders with a particle size distribution centered in the range 75–100 μm. It is shown that, thanks to the presence of modest mass transport restrictions, the eggshell catalyst outperforms the corresponding powder in terms of C 15+ specific yield. We explain these results by considering the effect of mass transport on the olefin re-adsorption probability and on the H 2 /CO ratio in proximity to the active sites. The former is increased, and results in greater selectivity to heavy hydrocarbons. The latter is also increased, due to the negative order of CO conversion kinetics with respect to CO, and results in augmented reaction rate. [ABSTRACT FROM AUTHOR]

Density functional theory is employed to investigate the Fischer–Tropsch mechanism on copper ∑5(310) tilt grain boundary together with Cu(111) surface. For the methanol formation, Cu∑5(310) can effectively reduce each energy barrier through the preferred CH 3 O intermediate route as compared with Cu(111). Furthermore, Cu∑5(310) provides a synergetic effect to modulate the energy landscape in the methanol synthesis, where the pathway associated with CH 2 OH (CH 2 O ↔ CH 2 OH ↔ CH 3 OH) presents the clear kinetic advantage than the CH 3 O route. Simultaneously, the kinetically assisted intermediate CH 2 OH enable the CH 2 production both thermodynamically and kinetically on Cu∑5(310). The formation of C2 oxygenates from CH 2 provides a critical precursor to higher alcohol synthesis. The higher activity of grain boundary is attributed to the presence of low-coordinated sites with flexible local configuration. Consequently, the adsorption strength of small species is enhanced to provide a thermodynamic driving force for C O bond scission. The adaptive character of ∑5(310) grain boundary facilitates the stabilization of the transition state, therefore lowering the activation barrier. Present work unravels the microscopic origin of the higher catalytic capacity of copper ∑5(310) grain boundary which can be helpful to guide the development of novel Cu-based catalysts for higher alcohol formation from syngas. [ABSTRACT FROM AUTHOR]

High quality transportation fuels for engine are certain distillates with C 5+ hydrocarbons. A promising method to produce them from other resources than petroleum is the Fischer–Tropsch synthesis (FTS) process which gives product with a wide range of boiling point. The present authors have investigated FTS by simultaneously developing a graphite supported iron based catalyst. Results indicate that a suitable amount of potassium added to the graphite-supported catalyst promoted the reduction behavior and enhanced the selectivity to liquid hydrocarbons significantly. Products were mainly composed of C 4 –C 10 α-olefins with little methane, whose distributions changed with time on stream. The α-olefins in the liquid phase reaction media promoted the selectivity of C 5+ distillate up to near 90% and while suppressing the formation of lighter hydrocarbons with higher CO conversion. A reaction network involving the secondary reactions of α-olefins was proposed. [ABSTRACT FROM AUTHOR]

During the decades of kinetic studies over iron and cobalt based Fischer–Tropsch synthesis (FTS) catalysts, various mechanisms and equations have been proposed with a wide diversity. Literature review indicates that neither general kinetic model nor the same shape for reaction rate equation has been proposed for FTS. In the present paper, a generalized mechanism was developed and verified against reported kinetic data for both iron and cobalt based FTS catalysts. Also it was shown that all various types of the proposed mechanisms can fall under the heading of one general mechanism. The proposed generalized macro kinetic model can be applied simultaneously to both iron and cobalt based FTS catalysts. [ABSTRACT FROM AUTHOR]

Deviation of C 2 (including olefin and paraffin) in the Anderson–Schulz–Flory (ASF) distribution in Fischer–Tropsch synthesis (FTS) has been observed consistently but relatively few studies have focused on this. Explanations for C 2 deviation have been proposed by previous researchers in the literature and secondary reactions of ethene are believed as the cause for the deviation. However, the ratio particularly between C 2 and C 3 olefins and other neighbouring olefins has not yet been thoroughly investigated and this might lead eventually to insights in the mechanism of Fischer–Tropsch synthesis. FTS experimental runs were conducted in a tubular fixed bed reactor on a TiO 2 -supported cobalt catalyst. The reaction conditions were varied including the H 2 /CO ratio in the feed, the reaction temperature, and the space velocity (SV) of the feed gas. The decrease of the reaction rate caused by a lack of H 2 when the H 2 /CO ratio was low is discussed. The olefin/paraffin (O/P) ratios obtained at different temperatures and SVs are presented as they relate to the three H 2 /CO ratios. We then focus on the distribution of C 2 and C 3 under varied reaction conditions, including temperature, SV, and H 2 /CO ratio, and characterize the relationship between C 2 and C 3 in terms of both the olefins and the total amounts of them. Experimental data derived from other reactor types, continuous stirred tank reactor (CSTR) and batch reactor with the same catalysts, have also been included in the investigation of C 2 and C 3 distribution. The ratio between C 2 and C 3 olefins is a linear function of CO conversion only in spite of the fact that the data is for different feed flowrates and compositions, operating temperatures and different reactors. However the ratio for the total C 2 to total C 3 was different for different H 2 /CO feed ratios, but for a fixed H 2 /CO ratio was a linear function of temperature only. [ABSTRACT FROM AUTHOR]

The effect of thermally conductive additive (Al, Cu or Zn metal powder) introduction to cobalt-based catalyst was investigated by testing in Fischer–Tropsch synthesis (FTS). Aluminum is the most promising metal for heat transfer intensification in catalytic bed since it forms an effective thermally conductive lattice, which improves the catalyst performance significantly. The introduction of copper or zinc to the catalyst composition does not result in heat transfer lattice formation; moreover, these metals demonstrate their own catalytic activity under FTS conditions. It is shown that catalyst pore volume is important: a substantial decrease in pore volume leads to formation of heat and mass transfer limitations in the catalyst pellet, which results in productivity and selectivity drop. The FTS product (C 5+ hydrocarbons) composition depends on a metal used as a thermally conductive component and its catalytic activity. [ABSTRACT FROM AUTHOR]

The paper focuses on the effect of potassium content on the structure of alumina supported molybdenum sulphide catalysts and their performance in carbon monoxide hydrogenation at the conditions favouring olefin production. Molybdenum oxide is a major phase in the unpromoted calcined catalyst. The formation of mixed K–Mo, Mo–Al and K–Mo–Al oxides occurs in the calcined catalysts in the presence of relatively small amounts of potassium. The mixed potassium molybdenum oxides readily convert into mixed molybdenum sulphides after pre-treatment with H 2 S. The presence of potassium results in a signification increase in the concentration of basic sites in the sulphided catalyst, while their strength is not much affected by the promotion. The catalytic results suggest the presence of two types of sites associated to unpromoted molybdenum sulphide and K–Mo–S species. Higher olefin (and alcohol) production rates were observed on the mixed K–Mo sulphides, while carbon monoxide hydrogenation on upromoted molybdenum sulphide leads to methane. Molybdenum oxysulphide seems not to be relevant to olefin synthesis. [ABSTRACT FROM AUTHOR]

The paper addresses the effect of sodium promotion on the structure and catalytic performance of iron catalysts supported on alumina, silica, CMK-3 and carbon nanotubes in high temperature Fischer-Tropsch synthesis giving general rules for the selection of supports for selective synthesis of olefins in the presence of promoter. Our results show that the effect of sodium promotion strongly depends on the support. In silica and CMK-3 carbon support, iron species interact with sodium with formation of inactive sodium-iron mixed oxides or carbides which results in higher olefin selectivity and chain growth probability but lower Fischer-Tropsch reaction rate. In alumina supported catalysts, sodium promotion preferentially leads to formation of sodium aluminate with a relatively small effect in Fischer–Tropsch synthesis. The effect of promotion with sodium is more pronounced on carbon nanotubes which contain iron carbide species stabilized by encapsulation in the carbon matrix. The sodium promoted catalysts supported on carbon nanotubes exhibit higher selectivity to both light and long-chain olefins. [ABSTRACT FROM AUTHOR]

The effect of ammonia on activity and selectivity of a low temperature, iron-catalyzed slurry phase Fischer–Tropsch process was studied. The ammonia content in the feed was varied systematically from 0 to 10 vol% while the total pressure was adjusted simultaneously in order to keep partial pressures constant. In addition to standard Fischer–Tropsch synthesis (FTS) products, long chained aliphatic amines, nitriles and amides were formed; the latter two product classes had not been observed by others before with iron based catalysts. The selectivities toward nitrogen containing compounds increased with increasing ammonia content while the formation rates of alcohols, aldehydes and organic acids were reduced possibly suggesting that nitrogen containing compounds are formed via oxygenates or their precursors. Common FTS descriptors such as methane selectivity, chain growth probability, olefin to paraffin ratio and double bond shift were largely unaffected at the studied levels of ammonia addition, while the activity at ammonia levels above 2 vol% decreased over and above the intrinsic deactivation rate. Although this deactivation seems reversible upon removal of ammonia from the feed gas, the selectivity is shifted toward higher water gas shift reactivity. [ABSTRACT FROM AUTHOR]

Promoter modified porous iron-based microspheres were prepared through one pot solvothermal method, and employed as catalysts during Fischer–Tropsch (FT) synthesis. Specially, a microsphere was consist of nanoparticles and pores. The effect of promoter and pore on CO conversion and on lower olefins selectivity was investigated. Fe/K and Fe/Zn displayed a high selectivity to CH 4 (>0 wt%), which was not desirable for their application in the FT process. Na doped Fe/Na catalyst displayed optimum catalytic performance. In particular, it showed an excellent selectivity toward C 5+ (59.0 wt%) while yielding a CH 4 product fraction lower than 11.3 wt%. Fe/Mn catalyst showed a C 2-4 olefins selectivity approximately 34.1 wt%, but only achieved a CO conversion of 37.4%. Furthermore, the results showed that pore size of catalysts played a key role on morphology of catalysts and on catalytic performance with time on stream. [ABSTRACT FROM AUTHOR]