•Fe NPs immobilized on carbon nanofibers are synthesized by CCVD method.•Both morphology and location of Fe NPs in CNFs are sensitive to H2/CO ratio.•The long polyhedron-shaped Fe NPs are highly active for NH3 decomposition.•The possible formation mechanism of shaped Fe nanoparticles is proposed.Ammonia decomposition over Fe-based catalysts is a typical structure sensitive reaction, and the shape-controlled synthesis of Fe nanoparticles based on catalytic chemical vapor deposition (CCVD) method appears to be an effective method toward enhanced catalytic activity. The objective of this work is to understand effects of the reaction parameters on structural and textural properties of the resultant Fe catalysts and thus catalytic ammonia decomposition performance. The as-obtained catalysts are characterized by multiple techniques (N2 physisorption, XRD, SEM, TEM, and Raman spectroscopy). Both the morphology and location of Fe nanoparticles are found to strongly depend on the partial pressure of H2 as well as the growth time of CNFs. The long polyhedron shaped Fe nanoparticles on CNFs exhibit the highest activity among the prepared catalysts, which are relatively higher than the activities of the reported Fe based catalysts. The possible explanation for the good catalytic performance was proposed.

•The partial substitution of Pt with Ru contributes to enhanced activity and durability.•A combination of kinetic and isotopic analyses with characterizations is conducted.•A volcano-shaped relationship for activity with entropy of activation is proposed.•Pt0.5Ru0.5/CNT exhibits appropriate entropy of activation and the highest activity.Increasing the utilization efficiency of noble metals by the synergy with less expensive metals is an important yet challenging issue in heterogeneous catalysis. Herein, exemplified with Pt-catalyzed hydrolytic dehydrogenation of ammonia borane, partial substitution of the Pt catalysts by over 11 times less expensive Ru is studied to obtain highly efficient and cost-effective Pt-based catalysts. It is observed that the Pt-Ru bimetallic catalysts, especially Pt0.5Ru0.5/CNT, deliver higher hydrogen generation activity and durability than the Pt/CNT and Ru/CNT catalysts, indicating a remarkable Pt-Ru synergy. The underlying nature of this synergy is elucidated by combining kinetic and isotopic analyses with multiple techniques such as HAADF-STEM, EDX, XRD, H2-TPR, HRTEM, XPS and ICP-AES. The appropriate activation energy and entropy of activation for the reaction are mainly responsible for the highly active Pt0.5Ru0.5/CNT catalyst, and a volcano-shaped relationship for the reactivity of these five catalysts as a function of entropy of activation is proposed. The insights revealed here might guide the rational design of catalysts for this reaction, and the methodology could be extended to unravel the nature of other bi- or multi-metallic catalysts.Download high-res image (65KB)Download full-size image

•2D pseudo-homogeneous membrane reactor models are developed for propene epoxidation with H2/O2.•Both isothermal and non-isothermal conditions are studied.•The modelling results are compared to those of the traditional three-stage/packed-bed reactors.•The membrane reactor as O2 distributor demonstrates the commercial potential of PO production.United mathematical models for four reactors, two microporous inert membrane reactors as O2 (MR-O) or H2 (MR-H) distributor and two traditional reactors (i.e., three-stage reactor (TSR) and packed-bed reactor (PBR)) with different feeding strategies, are developed to describe direct propylene epoxidation with H2 and O2 to produce propylene oxide (PO). Effects of the feeding strategies on reactor performances (i.e., C3H6 conversion, PO selectivity, PO yield and H2 efficiency) are studied under both isothermal and non-isothermal conditions by using gPROMS. Significantly different reactor performances along the catalyst length are observed and then explained by the reactants and products concentration profiles and/or the temperature distributions. Then, effects of the catalyst lengths of the four reactors are further investigated. It is found that the MR-O gives rise to the outlet C3H6 conversion of 11.3%, which exceeds the previously estimated value (i.e., 10%) with a commercial potential. Finally, an attempt is made to probe whether increasing the H2 and C3H6 feed concentrations in the MR-O without the explosion risk further improves the reactor performance. All the results indicate the potential of the MR-O for the commercial production of PO by direct propylene epoxidation with H2 and O2.Download high-res image (118KB)Download full-size image

Probing the product selectivity of Fischer–Tropsch catalysts is of prime scientific and industrial importance—with the aim to upgrade products and meet various end-use applications. In this work, the mechanisms for CH4 formation and C1–C1 coupling on a thermodynamically stable, terraced-like χ-Fe5C2 (510) surface were studied by DFT calculations. It was found that this surface exhibits high effective barriers of CH4 formation for the three cases (i.e., 3.66, 2.81, and 2.39 eV), indicating the unfavorable occurrence of CH4 formation under FTS conditions. The C + CH and CH + CH are the most likely coupling pathways, which follow the carbide mechanism. Subsequently, the effective barrier difference between CH4 formation and C1–C1 coupling was used as a descriptor to quantify FTS selectivity. A comparison of the selectivity between this surface and the reported FTS catalysts’ surfaces was discussed in detail. More interestingly, this surface shows unexpectedly high C2+ selectivity. This indicates that manipulating the crystal facet of χ-Fe5C2 catalyst can effectively tune the FTS selectivity, which will open a new avenue for highly selective Fe-based FTS catalysts.Keywords: carbide mechanism; CH4 formation; C−C coupling; Fischer−Tropsch synthesis selectivity; χ-Fe5C2 catalyst

Manganese and potassium promoters coated carbon nanotubes (i.e., MnK-CNTs) were synthesized by a redox reaction between CNTs and KMnO4, in which the CNTs act as reducing agent and as substrate for the heterogeneous nucleation of K-doped manganese oxide. The as-synthesized MnK-CNTs were employed to support Fe catalyst (i.e., Fe/MnK-CNTs, the loadings of 7.9 wt% Fe, 15.7 wt% Mn and 1.9 wt% K) for the direct conversion of syngas to lower olefins. It is revealed that Fe/MnK-CNTs catalyst is more active and stable than FeMnK/CNTs catalyst prepared by co-impregnation method using CNTs as a support. Furthermore, under similar CO conversion, the Fe/MnK-CNTs catalyst exhibits higher selectivity of hydrocarbons especially lower olefins. This could be related to the small-sized and uniform nanoparticles, the well-distributed promoters, the weak metal–support interaction and the greater defects on support, which are the consequences of the unique structural transformation of MnK-CNTs as a function of temperature and atmosphere.

Hierarchical structured α-Al2O3 is shown to be able to effectively disperse and immobilize iron species, in comparison with commercial α-Al2O3. After promotion using an appropriate amount of sulfur, iron catalysts exhibit not only enhanced Fischer–Tropsch synthesis activity and selectivity toward lower olefins, but also increased resistance against carbon deposits.

We report a size-dependent activity in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane. Kinetic study and model calculations revealed that Pt(111) facet is the dominating catalytically active surface. There is an optimized Pt particle size of ca. 1.8 nm. Meanwhile, the catalyst durability was found to be highly sensitive to the Pt particle size. The smaller Pt particles appear to have lower durability, which could be related to more significant adsorption of B-containing species on Pt surfaces as well as easier changes in Pt particle size and shape. The insights reported here may pave the way for the rational design of highly active and durable Pt catalysts for hydrogen generation.

We demonstrate an unprecedented H2 generation activity in the hydrolytic dehydrogenation of ammonia borane over acid oxidation- and subsequent high temperature-treated CNT immobilized Pt nanocatalysts to combine the merits of defect-rich and oxygen group-deficient surfaces and unique textural properties of supports as well as optimum particle size of Pt.

•Ni nanoparticles at the tips of carbon nanofibers are synthesized by CCVD method.•The morphology of Ni catalysts is sensitive to the ratio of CH4 to H2.•The shaped Ni catalysts are highly dispersed and spatially isolated by CNFs.•The shaped Ni catalysts show high activity and stability for NH3 decomposition.•The possible formation mechanism of shaped Ni catalysts is proposed.Ni nanoparticles at the tips of carbon nanofibers (Ni-CNFs) were synthesized by catalytic chemical vapor deposition (CCVD) method using CH4 as the carbon source, and then employed as catalysts for the generation of H2 from ammonia decomposition. The morphology of Ni catalysts is highly sensitive to the ratio of CH4 to H2. Especially for the CH4/H2 ratio of 4, the as-obtained Ni-CNFs catalyst shows higher H2 formation rate, which could be due to more accessible facets to the reactants and unique shape effect. Meanwhile, this catalyst also shows good thermal stability, possibly owing to the highly dispersed and spatially isolated Ni nanoparticles by CNFs. Moreover, effect of the surface carbon coverage on the orientation of Ni crystal facets and the matching degree between graphene sheet and Ni crystal facet were investigated by DFT calculations. Finally, a possible formation mechanism of shaped Ni catalysts was discussed by combining experimental and theoretical results.

•Co–Mo bimetallic catalyst shows the synergistic effect for ammonia decomposition.•Metal amine metallate is a good active phase precursor of bimetallic catalyst.•Co–Mo nanoparticles are highly dispersed on γ-Al2O3 support.•The catalyst with monocomponent Co–Mo precursor shows higher activity and stability.Two kinds of Co-Mo bimetallic catalysts (i.e., CoMo-I/γ-Al2O3 and CoMo-II/γ-Al2O3) prepared by using different active phase precursors as well as Co and Mo monometallic catalysts were used to catalyze ammonia decomposition. The Co-Mo bimetallic catalysts show higher activity than the Co and Mo monometallic catalysts, indicating the synergistic effect between Co and Mo. More interestingly, the CoMo-I/γ-Al2O3 catalyst using monocomponent metal amine metallate (i.e., Co(en)3MoO4) as the active phase precursor exhibits higher activity and stability than the CoMo-II/γ-Al2O3 using bicomponent Co(NO3)2 and (NH4)6Mo7O24 as the active phase precursor, which could be linked to the higher content of active species Co3Mo3N for the CoMo-I/γ-Al2O3 catalyst.

Iron carbides, especially χ-Fe5C2, among the active iron species in Fischer–Tropsch synthesis (FTS), are considered to be responsible for high FTS activity. CO activation pathways as the initial steps of FTS over χ-Fe5C2 were explored by spin-polarized density functional theory calculations. Surface energies of χ-Fe5C2 facets observed from the XRD patterns were first calculated, and then the corresponding equilibrium χ-Fe5C2 shape was obtained by Wulff construction. The thermodynamically stable (510) surface was predicted to have the largest percentage among the exposed crystal facets. Subsequently, the adsorption properties of CO on χ-Fe5C2 (510) were studied. Despite exhibiting lower binding energy than that at the 3F-4 site as the most stable configuration, CO adsorption at the 4F-1 site led to significant weakening of the C–O bond from both the structural and electronic properties’ points of view. Furthermore, two kinds of CO activation mechanisms (i.e., the direct and H-assisted CO dissociation) and the corresponding six kinds of CO activation pathways on χ-Fe5C2 (510) were comparatively investigated on the basis of the evolution of carbon species, in which the C–O bond cleavage and further hydrogenation of surface species were concerned. The systematic analysis of the activation properties of CO suggests the direct CO dissociation as the preferred activation pathway.

Ni-carbon nanofibers (Ni-CNFs) catalysts were synthesized in situ by the decomposition of carbon-containing gases (i.e., CH4, CO, and C2H4) over Ni/Al2O3 catalyst and directly used to catalyze ammonia decomposition. The results showed that Ni nanoparticles were found to locate at the tips of CNFs when using CH4 and CO as carbon sources, while they located at the roots of CNFs when using C2H4 as a carbon source. For ammonia decomposition, Ni catalysts at the tips of CNFs showed higher activity, which could be due to the more accessible surfaces to the reactants. Interestingly, the Ni catalyst at the tips of CNFs with CH4 as a carbon source exhibited higher activity than that with CO as a carbon source, even though the former catalyst had a larger average particle size. The possible mechanism was given by combining characterization results with our previous simulation results. Finally, when using CH4 as a carbon source, the effect of the Ni-CNFs catalysts with different growth times on the activity was further studied.

Density functional theory calculations have been performed to investigate the recombinative desorption of N on terraced Fe(100) and Fe(110) as well as stepped Fe(111) and Fe(211) surfaces. The results showed that the stepped surfaces, especially the Fe(111) surface, are more active than the terraced surfaces, which could be related to the presence of so-called C7 sites for the stepped surfaces. Carbon atoms were found to stabilize the thermodynamically unstable stepped surfaces to be the energetically favored facets and thus facilitate the preferential exposure. As these carbon atoms incorporated into the stepped surfaces in the form of surface or subsurface carbon, the activation energy for the recombinative desorption of N was lowered, which was mainly ascribed to the decrease in the d band center of surface Fe atoms. In addition, a relation between the electronic and structural properties of Fe catalysts in the absence or presence of carbon and ammonia decomposition activity was also correlated.

Hierarchical structured α-Al2O3 is shown to be able to effectively disperse and immobilize iron species, in comparison with commercial α-Al2O3. After promotion using an appropriate amount of sulfur, iron catalysts exhibit not only enhanced Fischer–Tropsch synthesis activity and selectivity toward lower olefins, but also increased resistance against carbon deposits.

We demonstrate an unprecedented H2 generation activity in the hydrolytic dehydrogenation of ammonia borane over acid oxidation- and subsequent high temperature-treated CNT immobilized Pt nanocatalysts to combine the merits of defect-rich and oxygen group-deficient surfaces and unique textural properties of supports as well as optimum particle size of Pt.

Manganese and potassium promoters coated carbon nanotubes (i.e., MnK-CNTs) were synthesized by a redox reaction between CNTs and KMnO4, in which the CNTs act as reducing agent and as substrate for the heterogeneous nucleation of K-doped manganese oxide. The as-synthesized MnK-CNTs were employed to support Fe catalyst (i.e., Fe/MnK-CNTs, the loadings of 7.9 wt% Fe, 15.7 wt% Mn and 1.9 wt% K) for the direct conversion of syngas to lower olefins. It is revealed that Fe/MnK-CNTs catalyst is more active and stable than FeMnK/CNTs catalyst prepared by co-impregnation method using CNTs as a support. Furthermore, under similar CO conversion, the Fe/MnK-CNTs catalyst exhibits higher selectivity of hydrocarbons especially lower olefins. This could be related to the small-sized and uniform nanoparticles, the well-distributed promoters, the weak metal–support interaction and the greater defects on support, which are the consequences of the unique structural transformation of MnK-CNTs as a function of temperature and atmosphere.