Remote Activation of H-H Bonds by Platinum in Dilute Alloy Catalysts.

With heterogeneous catalysts, chemical promotion takes place at their surfaces. Even in the case of single-atom alloys, where small quantities of a reactive metal are dispersed within the main host, it is assumed that both elements are exposed and available to bond with the reactants. Here, we show, on the basis of in situ X-ray absorption spectroscopy data, that in alloy catalysts made from Pt highly diluted in Cu the Pt atoms are located at the inner interface between the metal nanoparticles and the silica support instead. Kinetic experiments indicated that these catalysts still display better selectivity for the hydrogenation of unsaturated aldehydes to unsaturated alcohols than the pure metals. Density functional theory calculations corroborated the stability of Pt at the metal-support interface and explained the catalytic performance as being due to a remote lowering of the activation barrier for the dissociation of H2 at Cu sites by the internal Pt atoms.

Kinetic Promotion Effect of Hydrogen and Dimethyl Disulfide Addition on Propane Dehydrogenation over the Pt-Sn-K/Al2O3 Catalyst.

The kinetic effects of co-feeding of dimethyl disulfide (DMDS) and hydrogen on propane dehydrogenation (PDH) over the Pt-Sn-K/Al2O3 catalyst were investigated by the response surface method. The 3-level Box-Behnken design for 4 factors (reaction temperature, propene, hydrogen, and DMDS flow rate) was used to design the experiment. The initial propane conversion, propene selectivity, and coking amount were chosen as responses and the results were fitted by quadratic models. The fresh and coked catalysts were characterized by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS), thermogravimetry (TG), N2 physisorption, and Fourier-transform infrared spectroscopy (FT-IR). Analysis of variance (ANOVA) results showed that the DMDS flow rate is significant for propane conversion and coking amount while hydrogen flow rate is only significant for the conversion. By using the fitted model for the response surface, it is found that DMDS can significantly reduce the coking amount at the expense of propane conversion, and hydrogen weakly affects the selectivity and coking amount. The optimal conditions to achieve maximum conversion and selectivity and minimum coking amount are not consistent. The DMDS and hydrogen flow rate should be optimized to obtain the maximum economic profit out of the propane dehydrogenation (PDH) process.

Surface phase diagrams of La-based perovskites towards the O-rich limit from first principles.

The exposed termination of transition-metal oxide surfaces plays a major role in determining the catalyst performance in redox reactions. In this contribution, the surface phase diagrams of LaMO3(001) (M = Sc-Fe) and LaMO3(110) (M = Co-Cu) are constructed by using the DFT+U method. The stabilities of six terminations derived from the stoichiometric MO2 and LaO surfaces are determined over a wide range of temperatures and oxygen partial pressures. The surface phase diagrams are calculated towards the O-rich limit in which the chemical potential of oxygen anions of perovskites equals that of gas-phase oxygen while the chemical potential of M cations is limited by thermodynamic boundary conditions. It is found that the surface phase diagrams are closely related to the reducibility of M cations, which is reflected in the oxygen adsorption energy and oxygen vacancy formation energy on the MO2- and LaO-terminated surfaces and can be measured by the third ionization energies of the M2+ cations. According to the surface phase diagrams, the most stable surface termination is predicted to be of MO2 type for LaMO3 (M = Sc-Fe) and LaO type for LaMO3 (M = Co-Cu) under solid oxide fuel cell operating conditions. Because the M cations become more readily reduced on going from left to right across the period, LaCoO3 may form an oxygen-deficient crystal structure at high temperatures and LaNiO3 and LaCuO3 would be decomposed into oxides containing the transition metals in a lower oxidation state.

Insights into Hydrogen Transport Behavior on Perovskite Surfaces: Transition from the Grotthuss Mechanism to the Vehicle Mechanism.

Hydrogen transport on transition-metal oxides is a shared process in many important physical and chemical changes of interest. In this work, DFT + U calculations have been carried out to explore the mechanism for hydrogen migration on the defect-free and oxygen-deficient LaMO3(001) (M = Cr, Mn, and Fe) surfaces. The calculated results indicate that hydrogen is preferentially adsorbed at the oxygen sites on all surfaces other than the defective LaCrO3(001), where the occupation of vacancies is energetically most favorable. The resultant O-H bonds would be weakened when oxygen vacancies are formed in their immediate vicinity because the increased electron density on the remaining ions would limit the ability of O to withdraw electrons from H. On defect-free LaMO3(001), hydrogen prefers to migrate along the [010] axis, during which the O-H bond is reoriented at the oxygen site for the hopping to proceed by the Grotthuss mechanism. In the presence of oxygen vacancies, the vehicle mechanism in which hydrogen hops together with the underlying oxygen would dominate on LaMnO3 and LaFeO3, whereas on the defective LaCrO3(001) the Grotthuss mechanism prevails. The linear scaling relations established show that the hydrogen and hydroxyl migration barriers decrease and increase, respectively, with increasing the strength of ionic bonding in perovskites, which provides a rational interpretation of the change in the preferred hydrogen migration mechanism.

BEEF-vdW+U method applied to perovskites: thermodynamic, structural, electronic, and magnetic properties.

The recently developed BEEF-vdW exchange-correlation method provides a reasonably reliable description of both long-range van der Waals interactions and short-range covalent bonding between molecules and surfaces. However, this method still suffers from the excessive electron delocalization that is connected with the self-interaction error and, consequently, the calculated chemical and physical properties such as formation energy and band gap deviate markedly from the experimental values, especially when strongly correlated systems are under investigation. In this contribution, BEEF-vdW+U calculations have been performed to study the thermodynamic, structural, electronic, and magnetic properties of La-based perovskites. An effective interaction parameter [Formula: see text] and an energy adjustment [Formula: see text] are determined simultaneously by a mixing GGA and GGA+U method, where the enthalpy or Gibbs free energy of formation of oxides containing a transition metal in different oxidation states are fitted to available experimental data. The [Formula: see text] is found to have its origin in the fact that the GGA+U method gives rise to the offsets in the total energy that include not only the desired physical correction but also an arbitrary contribution. Calculated results indicate that the BEEF-vdW method provides a more accurate description of the bonding in the O2 molecule than the PBE method and has generally smaller [Formula: see text] values for the 3d-block transition metals, thereby giving rise to band gaps and magnetic moments that are in better agreement with the experimentally measured values.

Decoding structural complexity in conical carbon nanofibers.

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.

Origin of synergistic effect over Ni-based bimetallic surfaces: a density functional theory study.

Density functional theory calculations have been conducted to explore the physical origin of the synergistic effect over Ni-based surface alloys using methane dissociation as a probe reaction. Some late transition metal atoms (M = Cu, Ru, Rh, Pd, Ag, Pt, and Au) are substituted for surface Ni atoms to examine the variation in electronic structure and adsorption property of Ni(111). Two types of threefold hollow sites, namely, the Ni(2)M and Ni(3) sites, are taken into account. The calculated results indicate that the variation in the CH(x) adsorption energy at the Ni(2)M and Ni(3) sites is dominated by the ensemble and ligand effect, respectively, and the other factors such as surface and adsorbate distortion and electrostatic interaction affect the catalytic properties of the bimetallic surfaces to a smaller extent. Both the Brønsted-Evans-Polanyi relationship and the scaling correlation hold true on the Ni-based bimetallic surfaces. With the combination of these two linear energy relations, the corrected binding energy of atomic C is found to be a good descriptor for representing the catalytic activity of the alloyed surfaces. Considering the compromise between the catalytic activity and catalyst stability, we suggest that the Rh/Ni catalyst is a good candidate for methane dissociation.

Toward CH4 dissociation and C diffusion during Ni/Fe-catalyzed carbon nanofiber growth: a density functional theory study.

First-principles calculations have been performed to investigate CH(4) dissociation and C diffusion during the Ni∕Fe-catalyzed growth of carbon nanofibers (CNFs). Two bulk models with different Ni to Fe molar ratios (1:1 and 2:1) are constructed, and x-ray diffraction (XRD) simulations are conducted to evaluate their reliability. With the comparison between the calculated and experimental XRD patterns, these models are found to be well suited to reproduce the crystalline structures of Ni∕Fe bulk alloys. The calculations indicate the binding of the C(1) derivatives to the Ni∕Fe closest-packed surfaces is strengthened compared to that on Ni(111), arising from the upshift of the weighted d-band centers of catalyst surfaces. Then, the transition states for the four successive dehydrogenation steps in CH(4) dissociation are located using the dimer method. It is found that the energy barriers for the first three steps are rather close on the alloyed Ni∕Fe and Ni surfaces, while the activation energy for CH dissociation is substantially lowered with the introduction of Fe. The dissolution of the generated C from the surface into the bulk of the Ni∕Fe alloys is thermodynamically favorable, and the diffusion of C through catalyst particles is hindered by the Fe component. With the combination of density functional theory calculations and kinetic analysis, the C concentration in catalyst particles is predicted to increase with the Fe content. Meanwhile, other experimental conditions, such as the composition of carbon-containing gases, feedstock partial pressure, and reaction temperature, are also found to play a key role in determining the C concentration in bulk metal, and hence the microstructures of generated CNFs.

DFT study of propane dehydrogenation on Pt catalyst: effects of step sites.

Self-consistent periodic slab calculations based on gradient-corrected density functional theory (DFT-GGA) have been conducted to examine the reaction network of propane dehydrogenation over close-packed Pt(111) and stepped Pt(211) surfaces. Selective C-H or C-C bond cleaving is investigated to gain a better understanding of the catalyst site requirements for propane dehydrogenation. The energy barriers for the dehydrogenation of propane to form propylene are calculated to be in the region of 0.65-0.75 eV and 0.25-0.35 eV on flat and stepped surfaces, respectively. Likewise, the activation of the side reactions such as the deep dehydrogenation and cracking of C(3) derivatives depends strongly on the step density, arising from the much lower energy barriers on Pt(211). Taking the activation energy difference between propylene dehydrogenation and propylene desorption as the descriptor, we find that while step sites play a crucial role in the activation of propane dehydrogenation, the selectivity towards propylene is substantially lowered in the presence of the coordinatively unsaturated surface Pt atoms. As the sole C(3) derivative which prefers the cleavage of the C-C bond to the C-H bond breaking, propyne is suggested to be the starting point for the C-C bond breaking which eventually gives rise to the formation of ethane, methane and coke. These findings provide a rational interpretation of the recent experimental observations that smaller Pt particles containing more step sites are much more active but less selective than larger particles in propane dehydrogenation.

Continuous synthesis of methyl ethyl ketone peroxide in a microreaction system with concentrated hydrogen peroxide.

Methyl ethyl ketone peroxide (MEKPO) is widely used in polymer industry. It is highly sensitive to heat, friction, shock, flame or other sources of ignition, causing risks in production, storage and transportation. In this article, MEKPO is synthesized at a high throughput with concentrated hydrogen peroxide in a microreactor for on-site and on-demand production. The influences of acid concentration, residence time, feeding rate and ratio, and reaction temperature on the yield and the mass fractions of residual methyl ethyl ketone (MEK) and active oxygen of the product are systematically investigated. Under optimized condition, the reaction is completed in a few seconds, and the product contains less than 2 wt% residual MEK and has a mass active oxygen fraction higher than 22 wt%, which meets the standard for industrial application.