A comprehensive microkinetic model based on density functional theory (DFT) calculations is constructed to explore the reaction mechanism for dry methane reforming on Ni catalyst. Three low-index facets, namely, Ni(111), Ni(100), and Ni(211), are utilized to represent the contributions from the flat, open, and stepped surfaces. Adsorption energies of all the possible reaction intermediates as well as activation energies for the elementary reactions involved in dry reforming of methane on the three Ni surfaces are calculated through DFT. These results are further employed to estimate the rate constants for the elementary reactions under realistic temperatures and pressures within the framework of transition state theory and statistical mechanics treatments. The dominant reaction pathway is identified as CH4 successive dissociation followed by carbon oxidation by atomic oxygen. The dependence of the rate-determining step on operating conditions is examined. At low CH4 and CO2 partial pressures, both CH4 dissociative adsorption and carbon oxidation would jointly dominate the overall reaction rate, while at high pressures carbon oxidation is suggested as the rate-determining step for the DRM reaction. Our findings provide a rational interpretation of contradictory experimental observations.

Conical carbon nanofibers (CNFs) exist primarily as graphitic ribbons that fold into a cylindrical structure with the formation of a hollow core. Structural analysis aided by molecular modeling proves useful for obtaining a full picture of how the size of the central channel varies from fiber to fiber. From a geometrical perspective, conical CNFs possibly have cone tips that are nearly closed. On the other hand, their fiber wall thickness can be reduced to a minimum possible value that is determined solely by the apex angle, regardless of the outer diameter. A formula has been developed to express the number of carbon atoms present in conical CNFs in terms of measurable structural parameters. It appears that the energetically preferred fiber wall thickness increases not only with the apex angle, but also with the number of atoms in the constituent graphitic cones. The origin of the empirical observation that conical CNFs with small apex angles tend to have a large hollow core lies in the fact that in graphene sheets that are more highly curved the curvature-induced strain energy rises more rapidly as the fiber wall thickens.

Platinum cluster size has a significant influence on the activity, selectivity, and stability as well as the reaction mechanism during propane dehydrogenation (PDH). Well-controlled platinum catalysts of different cluster sizes are prepared by a seed growth method and supported on calcined hydrotalcite. The Pt catalysts show strong structure-sensitive behavior both in the C–H bond activation of propane and in the C–C bond activation to yield ethylene, methane, and coke. The Pt clusters of small cluster sizes, with (211) dominating on the surface, have a lower dehydrogenation energy barrier and thus higher activity. However, large Pt clusters with Pt(111) dominating result in a weakened binding strength of propylene and an increased energy barrier for the activation of C–H bonds in propylene, which leads to higher selectivity toward propylene by lowering the possibility of deep dehydrogenation. Kinetic analysis illustrates that the reaction order in hydrogen decreases and activation energy increases with an increasing Pt cluster size. Combined with density functional theory calculations and isotope effect experiments, it gives strong evidence of the change in reaction mechanism with Pt cluster size. It suggests that on small Pt clusters that are mostly surrounded by undercoordinated surface sites, the first C–H bond activation is likely to be the rate-determining step, while the second C–H bond activation is kinetically relevant on large Pt particles with terrace sites dominating.Keywords: DFT simulation; kinetic isotope effect; platinum catalyst; propane dehydrogenation; reaction mechanism; size effects

Coupling of propane dehydrogenation with selective hydrogen oxidation is a practical strategy to achieve high propylene yield and low energy consumption. In this work, the detailed reaction mechanism on Pt(111) is explored using density functional theory calculations and microkinetic modeling to benefit the design of new catalysts. Calculated results indicate that the O2 dissociation pathway is dominant for hydrogen oxidation, and the dissociation of O2 is kinetically relevant. With the comparison between the energy barriers for dehydrogenation and oxidation, propyne is found to be the starting point for C3 oxidation. To obtain a high hydrogen oxidation rate and suppress the consumption of propylene, the catalytic performance of 11 M@Pt (M = Fe, Co, Ni, Cu, Ru, Rh, Os, Ir, Pd, Ag, and Au) core–shell surfaces is examined. Among all the core–shells, Ag@Pt not only has a high catalytic activity for hydrogen oxidation, but also exhibits a high selectivity toward propylene and is, therefore, the best candidate for selective hydrogen oxidation in the presence of propylene.

Molecular dynamics simulations employing the ReaxFF reactive force field have been carried out to analyze the structural evolution of fishbone-type carbon nanofiber-supported Pt nanoparticles, with particle size ranging from 5.6 to 30.7 Å. Simulated results indicate that upon adsorption the distribution of first-shell Pt–Pt coordination number and radial distribution function change significantly in Pt nanoparticles up to 2 nm in size and that the restructuring degree of the Pt nanoparticles decreases with particle size, which is attributed both to the reduced binding energy per Pt atom bonded to support and to the increased cohesive energy of the Pt nanoparticles. In the Pt10 particle, the majority of the Pt atoms are detached from the metal particle, leading to atomic adsorption of single Pt atoms on the support. As the Pt particle size is increased to ∼3 nm, however, the crystalline degree of Pt nanoparticles is even higher than that of the corresponding isolated nanoparticles because the strong metal–support interaction has a positive effect on the crystalline degree of the upper part of Pt nanoparticles. Two surface properties of the Pt nanoparticles, namely, Pt dispersion and coordination number of surface Pt atoms, are then computed and found to decrease and increase, respectively, with particle size. Thus, on-purpose control of particle size (and hence the metal–metal and metal–support interactions) is of crucial importance for tuning the superficial structures of supported active metal particles, which eventually determine the adsorption and catalytic properties of catalysts.

•The catalytic performance of Pt particles with different shapes has been examined.•Cubic particles exhibit a higher catalytic activity for propane dehydrogenation.•Octahedral particles have a better selectivity toward propylene production.Shaped metal particles are known to possess distinct catalytic properties, which is often attributed to the catalytic activities of different particle facets. In this contribution, propane dehydrogenation on Pt particles of different shapes is investigated by a combination of experiments and DFT calculations. The selectivity toward propylene and catalyst stability in propane dehydrogenation can be enhanced by shaping Pt nanoparticles under industrially relevant conditions. Octahedral particles (12.0 nm) dominated by the Pt(1 1 1) surface have higher selectivity toward propylene production and better stability than cubic particles (11.5 nm) that are dominated by the Pt(1 0 0) surface. Combined experiments and DFT calculations suggest that the weakened binding strength of propylene and increased energy barrier for the C–H bond cleavage in propylene dehydrogenation on Pt(1 1 1) compared to Pt(1 0 0) and Pt(2 1 1) contribute to the higher selectivity toward propylene by lowering the formation possibility of the deeply dehydrogenated intermediates. When coke is formed on the Pt surface, the d-band of surrounding Pt atoms is shifted farther below the Fermi level, leading to the deactivation of Pt catalyst. Our DFT calculations provide a rational interpretation of the experimental observations regarding the shape effects on propane dehydrogenation.

Molecular dynamics simulations based on a reactive force field (ReaxFF) are performed to examine the effects of the variable morphologies of fishbone-type carbon nanofibers (f-CNFs) on the microstructures of supported Pt100 clusters. Four f-CNF cone–helix models with different basal-to-edge surface area ratios and edge plane terminations are employed. Calculated results indicate upon adsorption of Pt100 clusters a fraction of Pt atoms migrates from the metal particles onto the f-CNFs either to accumulate at the metal–support interface or to attain a single atom adsorption on the supports. With decreasing apex angle or introduction of H termination, the Pt atoms are more likely to be coordinated to the basal planes and the binding energies of the Pt100 clusters to the f-CNFs are lowered, accompanied by a lower degree of the cluster reconstruction. On the contrary, if more f-CNF edge planes are exposed, a higher Pt dispersion, lower surface first-shell Pt–Pt coordination numbers, and longer Pt–Pt surface bonds are attained. Considering the interplay between the geometric and the electronic structures of transition metal surfaces, the relationship among the support morphologies, the metal–support interactions, and the catalytic properties of the active Pt clusters is eventually elucidated.

Density functional theory calculations have been performed to investigate the effect of Sn on the catalytic activity and selectivity of Pt catalyst in propane dehydrogenation. Five models with different Sn to Pt surface molar ratios are constructed to represent the PtSn surfaces. With the increase of the Sn content, the d-band of Pt is broadened, which gives rise to a downshift in the d-band center on the PtSn surfaces. Consequently, the bonding strength of propyl and propylene on the alloyed surfaces is lowered. With the decomposition of the adsorption energy, the change in the surface deformation energy is predicted to be the dominant factor that determines the variation in the adsorption energy on the surface alloys, while on the bulk alloys the change in the binding energy makes a major contribution. The introduction of Sn lowers the energy barrier for propylene desorption and simultaneously increases the activation energy for propylene dehydrogenation, which has a positive effect on the selectivity toward propylene production. Considering the compromise between the catalytic activity and selectivity, the Pt3Sn bulk alloy is the best candidate for propane dehydrogenation.Keywords: dehydrogenation; propane; propylene; PtSn; selectivity;

Proposing a plausible model for fishbone-type carbon nanofiber (f-CNF) proves to be a challenge. A practical approach to the construction of a cone-helix model is suggested on the basis of early experimental observations and from a geometric perspective. With the introduction of disclination angle and overlap angle into graphene sheets, the resultant helical cones have variable morphologies which rationalize the broad distribution of the f-CNF apex angles. A fraction of overlap angles can produce energetically favorable cone-helix models with high densities of coincident lattice points. Once the nearest coincident points to the apex is identified, both the overlap angle and the degree of graphitic alignment can be obtained. Periodic boundary conditions are imposed on the cone-helix models to depict the f-CNF morphologies along the principal axes. The lattice strain induced by the multi-layered model is found to have a negative effect on the structural stability. After the central parts of f-CNFs are removed, the lattice strain around the cone tips is eliminated, and the cone-helix model of more graphite layers is energetically more favorable. X-ray diffraction simulations are finally conducted to evaluate the reliability of the proposed models and to reveal the identity of the reflections at the diffraction angle of 44.5°.

First-principles calculations based on density functional theory have been performed to elucidate the reaction mechanism for ethylbenzene dehydrogenation and the role of CO2 in H removal. On the basis of the experimental information and theoretical prediction, three model surfaces with Fe-, ferryl- and O-termination are constructed to represent the active Fe2O3(0 0 0 1) surface. The calculated results indicate that on all of the three surfaces the C–H activation in the methylene group followed by the dehydrogenation of the methyl group is kinetically more favorable. The energy barriers for ethylbenzene dehydrogenation are lowest on the O-terminated surface, but the generated styrene is adsorbed too strongly to be released. As CO2 decomposition and the formation of HCOO are hindered by the relatively high activation energies, CO2 cannot serve as the oxidant to recover the O- and ferryl-terminated surfaces to keep the redox cycle. At the steady state of the reaction the coupling mechanism dominates on the Fe-terminated surface, with the synergistic effect between ethylbenzene dehydrogenation and the reverse water–gas shift reaction. Since the energy barrier for the formation of COOH is comparable to that for H2 formation, both the one-step and two-step pathways are predicted to contribute to the coupling mechanism, although the former is more probable.Graphical abstractThe comprehensive mechanism for ethylbenzene dehydrogenation in the presence of CO2 on Fe2O3(0 0 0 1) with three terminations have been investigated by DFT calculations. The coupling and redox cycle mechanisms have been considered. On the basis of the calculated results, the most likely reaction pathway and the role of CO2 have been elucidated.Highlights► The mechanism for ethylbenzene dehydrogenation in the presence of CO2 is explored. ► Styrene is hard to escape from the most active O-terminated Fe2O3(0 0 0 1). ► The Fe-terminated surface dominates the reaction, with the coupling mechanism. ► Both the one-step and two-step pathways are probable while the former is dominant.

Ab initio plane wave density functional theory calculations are performed to investigate the carbon diffusion in bulk nickel during the growth of fishbone-type carbon nanofibers (CNFs). Results indicate that the octahedral interstitial sites are preferred for C dissolution relative to the tetrahedral sites. And the heat of solution of C in paramagnetic (PM) Ni is larger than that in ferromagnetic (FM) Ni because the induced C atom quenches the magnetic moments of neighboring Ni atoms. The bulk diffusion has been successfully described under two different C concentrations. At the initial CNF growth stage, the C concentration in bulk Ni is low and the calculated energy barriers for the diffusion of an isolated C atom are 1.641 eV and 1.678 eV in the Ni FM and PM state, respectively. When the C content is increased to 20 at.%, two models are established. In one case, it is assumed that all C atoms hop in the same direction at the same time, and the calculated activation energies are 1.137 eV and 1.126 eV. In the other case, only one C atom is permitted to move with the neighboring C atoms fixed at the octahedral sites and the corresponding barriers are decreased to 0.972 eV in the Ni FM state. Through these calculations, it is concluded that the magnetic state has a minor effect on the diffusion energy barrier which can be substantially lowered by the increase of C concentration in bulk Ni. Comparing the activation energy for bulk diffusion with the surface diffusion results, the reason for the formation of different CNF morphologies has been revealed.

Conical carbon nanofibers (CNFs) exist primarily as graphitic ribbons that fold into a cylindrical structure with the formation of a hollow core. Structural analysis aided by molecular modeling proves useful for obtaining a full picture of how the size of the central channel varies from fiber to fiber. From a geometrical perspective, conical CNFs possibly have cone tips that are nearly closed. On the other hand, their fiber wall thickness can be reduced to a minimum possible value that is determined solely by the apex angle, regardless of the outer diameter. A formula has been developed to express the number of carbon atoms present in conical CNFs in terms of measurable structural parameters. It appears that the energetically preferred fiber wall thickness increases not only with the apex angle, but also with the number of atoms in the constituent graphitic cones. The origin of the empirical observation that conical CNFs with small apex angles tend to have a large hollow core lies in the fact that in graphene sheets that are more highly curved the curvature-induced strain energy rises more rapidly as the fiber wall thickens.