Zhijun Sui

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.

Carbon mediated catalysis：A review on oxidative dehydrogenation

Abstract Carbon mediated catalysis has gained an increasing attention in both areas of nanocatalysis and nanomaterials. The progress in carbon nanomaterials provides many new opportunities to manipulate the types and properties of active sites of catalysts through manipulating structures, functionalities and properties of carbon surfaces. The present review focuses on progresses in carbon mediated oxidative dehydrogenation reactions of ethylbenzene, propane, and butane. The state-of-the-art of the developments of carbon mediated catalysis is discussed in terms of fundamental studies on adsorption of oxygen and hydrocarbons, reaction mechanism as well as effects of carbon nanomaterial structures and surface functional groups on the catalytic performance. We highlight the importance and challenges in tuning of the electron density of carbon and oxygen on carbon surfaces for improving selectivity in oxidative dehydrogenation reactions.

Hydrogenolysis of sorbitol to glycols over carbon nanofibers-supported ruthenium catalyst: The role of base promoter

Abstract Sorbitol hydrogenolysis over carbon nanofibers-supported Ru (Ru/CNFs) was carried out with different bases (NaOH, KOH, Mg(OH)2, Ba(OH)2, and CaO) to investigate the role of base promoter. The results indicated that all the bases used significantly enhanced the sorbitol conversion while the glycol selectivities varied with the base type and amount. CaO was the best base in terms of glycol selectivity for two reasons. CaO provided OH− for the base-promoted cleavage of C–C bonds, while it also supplied Ca2+ for complexation with the intermediate aldehydes, thus affecting the reaction pathways. We identified an optimum ratio among sorbitol concentration, Ru/CNFs catalyst, and CaO to achieve favorable glycol selectivities in sorbitol hydrogenolysis. Reaction pathways for sorbitol hydrogenolysis into glycols in aqueous solution in the presence of CaO have been proposed based on the mechanistic study.

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.

Modeling and Simulation of Coke Combustion Regeneration for Coked Cr2O3/Al2O3 Propane Dehydrogenation Catalyst

Abstract A heterogeneous model is developed for the regeneration of the Cr 2 O 3 /Al 2 O 3 catalyst for the propane dehydrogenation process by considering the internal mass transfer and external mass/heat transfer during the coke combustion. Simulation shows that under practical operating conditions, multi-steady states exist for the catalyst pellets and the catalyst temperature is sensitive to gas temperature. However, at increased mass flow rate or lowered oxygen concentration, multi-steady states will not appear. Under the strong influences of film diffusion, the coke in the packed bed reactor will first be exhausted at the inlet, while if the film diffusion resistance is decreased, the position of first coke exhaustion moves toward the outlet of the reactor.

Palladium Catalysts Supported on Fishbone Carbon Nanofibers from Different Carbon Sources

Abstract Pd catalysts (0.5% Pd) supported on fishbone carbon nanofibers (FCNFs) from different carbon sources for terephthalic acid (TA) hydro-purification were prepared and investigated. CNFs from various carbon sources, CO (FCNF-CO), CH 4 (FCNF-C1), and C 2 H 4 (FCNF-C2), have been synthesized and characterized by N 2 adsorption, XRD, and TPD-MS. The results indicated that though they all possess similar fishbone structure, the FCNF from CO has the largest specific surface area, highest graphitization degree, and a large amount of surface groups, while the FCNF from C 2 H 4 has the least specific surface area, lowest graphitization degree, and moderate surface groups. Pd/CNFs were synthesized and characterized by TEM, N 2 adsorption, XRD, and CO chemisorption. The catalytic activity for TA purification follows the sequence Pd/FCNF-CO > Pd/FCNF-C1 > Pd/FCNF-C2, which is in accordance with the sequence of Pd dispersion. It is believed that rather than the graphene arrangement, the synergic support effect of pore structure, crystallinity, and the surface chemistry dictates the Pd dispersion on FCNF and the consequent catalytic activity.

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.

Evaluation of approximations for concentration-dependent micropore diffusion in sorbent with bidisperse pore structure

Average diffusivity linear driving force (AD-LDF) and concentration-dependent diffusivity linear driving force (CDD-LDF) approximations are introduced to simplify the precise model describing the concentration-dependent micropore diffusion in bidisperse sorbents, and are compared with the precise model in predicting the dynamics of a sorption process under two different perturbations (i.e., step change perturbations and sinusoidal wave perturbation) with different concentrations imposed at the exterior surface of the bidisperse sorbent. The performance of the two approximations is validated by the precise model and experiments. The AD-LDF performs better in step adsorption and CDD-LDF does better in step desorption. Under sinusoidal wave perturbation, the CDD-LDF performs better. The different levels of consistency of the two approximations with the precise model are attributed to the different definitions of the diffusivities.

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.