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Abstract: The heats of adsorption of different C1 and C2 molecules assumed to be present during the initial steps of the Fischer–Tropsch synthesis and activation energies for elementary steps envisioned to occur in the synthesis are calculated for Co by using the unity bond index-quadratic exponential potential (UBI-QEP) method. The preexponential factors for the elementary steps are calculated from transition-state theory, and the rate constants are calculated according to the Arrhenius equation. The activation barrier for hydrogenation of CO is found to be lower compared to hydrogen assisted dissociation of CO, which has a smaller activation barrier than direct dissociation of CO. The reaction steps with high activation barriers are eliminated. Based on this elimination two sets of elementary steps for formation of C1 and C2 alkenes and alkanes in the Fischer–Tropsch synthesis are established: one based on hydrogen assisted CO dissociation (carbide mechanism) and one based on CO hydrogenation (CO insertion mechanism). In addition, one mechanism of producing CO2 from the water–gas shift reaction is proposed. The resulting mechanisms are combined and used in the microkinetic model, which are fitted to experimental results at methanation conditions (T =483K or 493K, p =1.85bar and H2/CO=10) over a Co/Al2O3 Fischer–Tropsch catalyst. A good tuning is obtained by adjusting the C–Co and H–Co binding strengths. The microkinetic modelling based on these assumptions indicates that CO is mainly converted through hydrogenation of CO and that C2 compounds are mainly produced by insertion of CO into a metal–methyl bond. Thus, from the surface coverages and reaction rates predicted by the microkinetic modelling the mechanism can be further reduced to only include the CO insertion mechanism. Hydrogenation of CHO to CH2O is found to be the rate determining initiation step, and insertion of CO into a metal–methyl bond is found to be the rate determining step for chain growth. By using the UBI-QEP method for calculation of activation energies, the activation barriers for dissociation of CO and hydrogenation of surface carbon are found to be too large for the carbide mechanisms to occur. However, experimental data or another theoretical method is necessary in order to support or disprove the calculated activation energies in this work. [Copyright &y& Elsevier]

Abstract: Novel catalysts are presented for synthesizing carbon nanofibers (CNF) from ethylene decomposition. The catalysts are derived from hydrotalcite (HT) precursors, which contain high amount of active metal species. The synthesis process has been studied and optimized as a function of various parameters such as hydrogen content, gas space velocity, temperature, and catalyst composition. It was found that the yield, nanostructure, and quality of the CNFs were greatly influenced by these process parameters. The aim of the present article is to report the optimized large scale (128g per gram catalyst) synthesis process of high quality CNFs at relatively low temperatures (<650°C). [Copyright &y& Elsevier]

The surface curvature of the carbon nanofibers (CNF) and the strong interaction between the metal and the graphene edges cause local deformation of metal nanoparticles, and thereby inducing a high degree of microstrain. Such lattice expansion has been shown to increase the activity of Ni crystals. This provides new opportunities to manipulate the catalytic activity of the metal. [Copyright &y& Elsevier]

[Display omitted] Developing efficient heterogeneous catalysts for the aerobic oxidation of C–H bonds using oxygen molecule as an oxidant to produce value–added aryl ketones is a challenging task. In this study, a series of cementite–iron–carbon composites (referred to as Fe–C–T, where T represents the pyrolysis temperature) were synthesized using a metal–organic–framework–mediated sacrificial template strategy. The Fe–C–T composites consist of highly dispersed Fe/(Fe 3 C or Fe 2 O 3) nanoparticles surrounded by graphitic carbon on porous carbon supports. Under mild reaction conditions (1 atm of O 2 pressure, 353 K, 12 h), with tert–butyl hydroperoxide (0.25 equiv.) as the initiator in a green solvent water, the prepared Fe–C–973 achieved a high conversion of 98.4 % for ethylbenzene and a selectivity of 98.7 % for acetophenone. This performance was markedly superior to the comparable Fe–C–873 and Fe–C–1073, as well as the commercially available Fe–based catalysts. Kinetic studies revealed that the catalytic system followed first–order kinetics for the substrate ethylbenzene, and the activation energy over Fe–C–973 was remarkably low of 40.4 kJ/mol. The highly distributed Fe 3 C nanoparticles embedded in the porous carbon framework were identified as the most active sites for the reaction. [ABSTRACT FROM AUTHOR]

Abstract: Composite materials consisting of ceramic monoliths and carbon nanofibres (CNFs) have been synthesized by catalytic growth of CNFs on the γ-alumina washcoating layer covering the walls of a ceramic monolith. The composites possess a relatively uniform mesoporous layer of CNFs of relatively small diameter. The thin alumina washcoating (ca. 0.1μm) prevents the CNFs from being trapped inside the alumina pores and hence the CNFs grow freely throughout the washcoating layer to form a uniform layer of CNFs that completely covers the surface of the monolith walls. The growth temperature is found to control the thickness of the CNF layer (2–4μm), the growth rate of the nanofibres, and the mechanical strength of the resulting CNF–monolith composite. At ideal conditions, a complete adhesion of the CNF layer and higher mechanical strength than the original cordierite monolith can be obtained. The CNF layer has an average pore size of 17nm with absence of microporosity which renders these monoliths promising candidates for the use as catalyst supports, especially for liquid phase reactions. The CNFs have small diameters (5–30nm) due to the high dispersion of Ni particles in the growth catalyst and the CNFs exhibit an unusual branched structure. [Copyright &y& Elsevier]

Enhanced electrochemical reduction of CO 2 to syngas on CuZnOCNT catalyst. • Copper doping on ZnO shifts the ECR activity towards CO formation closer to the top of the volcano. • The optimal amount of copper is around 20 at% for CO formation from electrochemical reduction of CO 2. • Higher amount of Cu doping leads to metallic Cu segregation, resulting in higher H 2 evolution activity. • The 14Cu86ZnO/CNT catalyst shows best performance in terms of syngas (1:1 ratio) production at -0.9 V, better than Ag foil. Electrochemical CO 2 reduction is a promising method for the production of CO 2 neutral fuels and chemicals from renewable electricity. ZnO supported on CNT is a low-cost electro catalyst that can produce syngas from CO 2 and H 2 O. A rational catalyst design strategy for further improvement of the ZnO/CNT catalyst is to dope with copper to strengthen the binding energy of the CO intermediate, which could improve the activity. In this work, a series of CuZnO/CNT catalysts with intimate Cu and Zn contact and various Cu loadings are prepared. By varying the copper content we show that the synergy of copper and zinc improves the activity for CO formation, and the optimal copper content is 20 at%. On hydrogen evolution, the addition of copper has a two-fold effect. This reaction is enhanced by the reduced ZnO particle size obtained when copper is added, but at similar particle sizes of ZnO, a suppression is observed with increasing copper content as CO evolution is enhanced. Stability tests showed that pure ZnO phase is more stable than metallic copper on ZnO. Compared with polycrystalline silver, the CuZnO/CNT catalysts are more active for syngas formation at a useful CO: H 2 composition. This work demonstrates that the viability of DFT based rational design of electrochemical CO 2 reduction catalyst. By varying CuZnO composition as well as crystallite size, one is able to tune the electrochemical reduction activity towards CO and H 2 , and therefore achieve desired syngas ratio. [ABSTRACT FROM AUTHOR]

• The reducibility and structural stability of LaMO 3 (M = Sc - Cu) are examined. • The first four LaMO 3 perovskites have very high oxygen vacancy formation energies. • The maximum possible oxygen deficiency in LaMO 3 (M = Mn - Ni) is 0.5. • LaCuO 3 can lose one lattice oxygen atom per formula unit before it is deactivated. Periodic density functional theory calculations have been performed to examine the effect of oxygen deficiency on the structural stability of Lanthanum-based perovskites (LaMO 3), where on-site Coulomb interactions have been addressed by an additional Hubbard-type term. Calculated results indicate that with the exception of LaFeO 3 , the oxygen vacancy formation energy (Δ E f o r m a t i o n , v a c ) of LaMO 3 (M = Sc - Cu) becomes less positive when moving across the first transition metal period. The first four LaMO 3 perovskites have very high oxygen vacancy formation energies and can hardly be reduced under mild conditions, while the other five perovskites exhibit a much greater reducibility. During the formation of the first oxygen vacancy in LaMO 3 (M = Mn - Cu), the nearest neighbor transition-metal cations serve as the primary acceptors of the electrons left behind. As oxygen atoms are further removed, square-based pyramidal and tetrahedral coordination geometries appear successively, and an abrupt increase in Δ E f o r m a t i o n , v a c is observed at a specific oxygen deficiency (δ), which defines the maximum possible δ in the perovskite structures. Under this definition, the M3+ cations (M = Mn - Ni) can be possibly reduced to M2+ while LaCuO 3 may lose at most one lattice oxygen atom per formula unit before it is deactivated. [ABSTRACT FROM AUTHOR]

• A facile and scalable approach for synthesis of Fe-based MOF MIL-88A(Fe) developed. • Our approach has the advantages of mild synthesis conditions and high MOF yield. • MIL-88A(Fe) exhibited an excellent performance for oxidation of cyclohexene with O 2. • Both free radical and Fe-mediated reaction pathways involved in aerobic oxidation. Herein, we reported a facile and scalable approach for the synthesis of Fe-based MOF MIL-88A(Fe) via a simple liquid-phase route at atmosphere pressure, and its application as a heterogeneous catalyst for the efficient aerobic oxidation of cyclohexene to value-added 2-cyclohexene-1-ol. Over 130 g of spindle-shaped MIL-88A(Fe) nanocrystals with high crystallinity, monodispersed particle size, and large specific surface area were easily synthesized per batch according to our method. The resultant MIL-88A(Fe) exhibited a high catalytic activity (conversion: 81%) with 70% selectivity for 2-cyclohexene-1-ol in the aerobic oxidation of cyclohexene with O 2 as the sole oxidant under mild reaction conditions (0.5 MPa of O 2 , 353 K for 8 h), outperforming the analogues reported in the literatures. The combined experimental and theoretical approach support the reaction mechanism of the oxidation of cyclohexene via both the free radical reaction pathway and Fe-mediated reaction pathway, resulting in the selective formation of 2-cyclohexene-1-ol. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Substitution of transition metals for Cr increases the acidity of the adjacent O. • Linear scaling relations are established to identify the PDH activity descriptor. • High propylene selectivity of the M 1 -Cr 2 O 3 catalysts is achieved. • Cu 1 -Cr 2 O 3 is predicted to be the best catalyst for PDH among the 13 doped Cr 2 O 3. The catalytic behavior of M 1 -Cr 2 O 3 (M = Mn-Cu, Ru-Ag, and Os-Au) in propane dehydrogenation (PDH) has been studied by employing microkinetic modeling combined with results from periodic DFT + U calculations. Calculated results indicate that most of the single atoms concerned can be stably present on the Cr 2 O 3 surface. The adsorption energy calculations and Bader charge analysis demonstrate that the acidity of the O sites adjacent to the M sites would be enhanced upon doping, which in turn strengthens the atomic H adsorption and the co-adsorption of various PDH species. The surface H formation energy is identified as the reactivity descriptor for PDH over the M 1 -Cr 2 O 3 catalysts, and a volcano curve of the PDH activity is obtained. By calculating the difference between the propylene dehydrogenation and desorption barriers, it is found that some M 1 -Cr 2 O 3 catalysts show improved selectivity towards propylene, as compared to Cr 2 O 3. Comparison between the formation barriers of H 2 and H 2 O reveals that single-atom doping has no apparent negative effect on the catalytic stability of the Cr 2 O 3 surface. The Cu 1 -Cr 2 O 3 catalyst is finally identified as the most promising catalyst for PDH among the 13 M 1 -Cr 2 O 3 catalyst candidates, considering the catalytic activity, selectivity, stability, and cost. [ABSTRACT FROM AUTHOR]

Abstract: Carbon nanofibers (CNFs) were synthesized by CO disproportionation on iron catalyst at CO concentration between 58.3% and 75.0%, H2 concentration between 8.3% and 25.0% and reaction temperature between 833 and 913K. The time-depending rate of CNFs growth as a function of time was determined by an on-line mass spectrometer and the morphologies of all CNFs products were observed by electronic microscopy. Not only the CNFs growth rate but also the morphology of the grown CNFs were shown to vary with the three operating variables. SEM and TEM images showed that the three-dimensional morphologies of the CNFs were twist, helical or straight and an interesting relationship between the maximal growth rate and the morphology was observed. When the growth rate was between 0.8 and 0.9mmol/(sgcat), the CNFs were twist. As the growth rate increased to 1.0mmol/(sgcat), more helical nanofibers appeared. Straight nanofibers were produced when the growth rate reached the level of 1.2mmol/(sgcat). Finally when the rate of CNFs growth was high at 1.8mmol/(sgcat), the absolute majority of the solid products was amorphous carbon coexisting with some short and thick nanofibers. Under different operating conditions, the crystal faces of the catalysts had different anisotropy properties for carbon deposition, thus producing CNFs with different morphologies. [Copyright &y& Elsevier]

Abstract: Carbon nanofiber/graphite-felt composite with desirable shape and size has been synthesized by the method of catalytic chemical vapor deposition (CCVD) through the growth of carbon nanofibers (CNFs) on the surface of the graphite microfibers of the felt. Scanning electron microscopy (SEM), N2-physisorption, and a mechanical strength test have been used to characterize the morphology, the texture properties, and the mechanical strength of the CNF/graphite-felt composites, aiming at a detailed understanding of the relationship between structure and property. The physico-chemical properties, especially the mechanical strength of the composites, have been investigated as a function of the ratio (w/w) between the CNFs and the graphite felt. Both the compressive strength and the apparent density of the composites were found to vary with the CNF yield (w/w). Their maximum values are reached at a CNF yield of about 6. Very few fragments are desquamated from the composite with a CNF yield below 5 under vigorous agitation, indicating stable anchorage of CNFs on the graphite microfibers. A distinct mesoporous character, a large external surface area and high mechanical strength make CNF/graphite-felt composites attractive as a catalyst for the oxidative dehydrogenation of ethylbenzene to styrene (ODE). An optimized ratio of 4.5 between the CNF and graphite felt in the composites exists for the catalyst in the ODE reaction with respect to both the styrene yield and mechanical strength. Moreover, the composite exhibits much greater catalytic stability than activated carbon. [Copyright &y& Elsevier]