Yi-An Zhu

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.

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.

Discrimination of the mechanism of CH4 formation in Fischer–Tropsch synthesis on Co catalysts: a combined approach of DFT, kinetic isotope effects and kinetic analysis

The mechanism of CH4 formation during Fischer–Tropsch synthesis on cobalt has been studied. DFT, kinetic isotope effect and kinetic analyses are combined to discriminate between the possible reaction routes of CH4 formation on Co catalysts. Nine direct reaction mechanisms were proposed from 21 elementary steps. They were first screened by DFT calculations in which the activation energies as well as the free energy profiles in each direct mechanism were compared, resulting in a reduction to six reaction mechanisms. Additional reduction was based on kinetic analysis where the reaction order was used as a descriptor. Subsequently, the kinetic isotope effect (KIE) values were calculated and compared to our previous SSITKA results. Finally, the dominating reaction route was suggested, which follows the initial elementary steps with H-assisted CO activation to form HCOH via HCO as an intermediate. It then proceeds through HCOH dissociation to CH followed by stepwise hydrogenation to CH4.

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.

Kinetics of Catalytic Dehydrogenation of Propane over Pt-Based Catalysts

Abstract Dehydrogenation of propane (DHP) is becoming an important process for increasing propylene productivity. In this review, the DHP over Pt-based catalysts is surveyed from the kinetic point of view. After a short introduction of propane dehydrogenation process in Section 1 , the DFT calculation results of the main and side reactions of the DHP over different Pt crystal planes as well as on Pt-Sn alloys are summarized in Section 2 to provide a fundamental understanding of the reaction mechanism of the DHP. In Section 3 , the macrokinetics of the DHP, coking, deactivation, and coke burning-off are summarized and then reexamined from the view of microkinetic analysis based on the DFT calculation results. Finally, the intensification of the DHP by steam dehydrogenation and selective oxidation of hydrogen are briefly reviewed.

Steam methane reforming on a Ni-based bimetallic catalyst: density functional theory and experimental studies of the catalytic consequence of surface alloying of Ni with Ag

A comprehensive understanding of the role of bimetallic alloys in surface steam reforming (SMR) reactions and carbon formation on real catalysts at an atomic level remains a challenge due to their different material properties and pressures. We report here a density functional theory (DFT) study and an experimental study of the effects of the surface composition of a Ni/Ag alloy on methane activation and steam methane reforming compared to those of a pure Ni catalyst. The Ni/Ag bimetallic catalyst demonstrates better surface alloying compared to other bimetallic catalysts, which is confirmed by X-ray photoelectron spectroscopy (XPS) and hydrogen chemisorption. The DFT calculation shows that Ag selectively substitutes for the more active Ni sites in the order Ni(211) > Ni(100) > Ni(111), so that the substitution of Ag on the Ni surface has a profound influence on the activities of the sites near Ag. These sites surrounding the Ag atom are inactive to C–H activation in the methane molecules, and Ag serves as a blocking agent to block the more active sites on the Ni nanoparticles, as suggested by both the kinetic and DFT studies. Furthermore, the study shows that the apparent activation energy increases with increasing fraction of Ag on the catalyst surface. A better understanding of the chemistry and catalytic consequence of surface alloying in SMR reactions is achieved by combining well-controlled surface alloying, detailed kinetic assessment and the DFT study. Thus, this approach provides the key to understanding the mechanism for suppressing carbon formation and is expected to have a significant implication on the general methods of SMR bimetallic catalyst design in order to have better activity and stability.

Insights into the effects of steam on propane dehydrogenation over a Pt/Al2O3 catalyst

Catalytic propane dehydrogenation over an alumina supported Pt catalyst in the presence of steam is carried out and it is found that the catalyst activity is increased and the apparent activation energy is lowered due to the presence of steam. Three possible mechanisms, i.e. co-adsorption, Langmuir–Hinshelwood and Eley–Rideal, of changes in energetics and pathways for propane dehydrogenation due to the presence of steam are explored by DFT calculation. The results show that co-adsorption of C3 species with surface oxygenated species would elevate dehydrogenation energy barriers due to repulsive interactions between them. Surface –OH is more active than surface –O in activating the C–H bond in propane and propyl species through either the Langmuir–Hinshelwood or Eley–Rideal mechanism and plays an important role in propane dehydrogenation with steam. The Langmuir–Hinshelwood mechanism is kinetically favorable, in which the activations of the first H in propane by surface −OH are the rate determining steps, but the activation energies are higher than that on a clean Pt(111) surface. The observed enhanced catalysts activity is ascribed to the lowered coking rates as well as the changes in surface coverage due to the co-adsorption of water and the surface oxygenated species.

Boosting Size‐Selective Hydrogen Combustion in the Presence of Propene Using Controllable Metal Clusters Encapsulated in Zeolite

A strategy is presented for making metal clusters encapsulated inside microporous solids selectively accessible to reactant molecules by manipulating molecular sieve size and affinity for adsorbed molecules. This expands the catalytic capabilities of these materials to reactions demanding high selectivity and stability. Selective hydrogen combustion was achieved over Pt clusters encapsulated in LTA zeolite (KA, NaA, CaA) in a propene-rich mixture obtained from propane dehydrogenation, showing pore-size dependent selectivity and coking rate. Propene tended to adsorb at channels or external surfaces of zeolite, interfering the diffusion of hydrogen and oxygen. Tailoring the surface of LTA zeolite with additional alkali or alkaline earth oxides contributed to narrowing zeolite pore size and their affinity for propene. The thus-modified Pt@KA catalyst displayed excellent hydrogen combustion selectivity (98.5 %) with high activity and superior anti-coking and anti-sintering properties.