Keju Sun

Carbon Chain Growth by Formyl Insertion on Rhodium and Cobalt Catalysts in Syngas Conversion

Carbon Chain Growth by Formyl Insertion on Rhodium and Cobalt Catalysts in Syngas Conversion

From Molecular Fragments to Crystals: A UV Raman Spectroscopic Study on the Mechanism of Fe-ZSM-5 Synthesis

The nucleation process of iron-exchanged zeolite Fe-ZSM-5, from the assembly of distorted tetrahedrally coordinated iron species and silicate rings in the precursor to the final Fe-ZSM-5 crystals, as well as variations in the coordination environment of iron, were studied by UV resonance Raman spectroscopy and complementary techniques.The entire sequence of crystallization events of Fe-ZSM-5 was monitored by UV Raman spectroscopy in combination with HRTEM, UV/Vis spectroscopy, X-ray diffraction patterns, and periodic DFT calculations. Fe-ZSM-5 was synthesized by an organic-free method to avoid signal interference from the organic template in Raman spectra. Framework iron atoms with resonance Raman bands at 516, 1115, and 1165 cm(-1), and a Raman band at 1016 cm(-1) are detected for Fe-ZSM-5. In the early stage of Fe-ZSM-5 synthesis, the precursor contains iron atoms in distorted tetrahedral coordination and five- and six-membered silicate rings. Nucleation by aggregation of the precursor species was monitored by UV Raman spectroscopy based on the resonance Raman effect, and confirmed by periodic DFT calculations. Evolution of iron species on the surface and in the bulk phase was monitored by UV Raman spectroscopy with excitation at 244 and 325 nm, as well as HRTEM. Nucleation takes place first in the core of the amorphous particles, and crystalline nuclei with Fe-ZSM-5 structure are formed in the core by consuming the amorphous shell. Finally the amorphous particles are completely transformed into Fe-ZSM-5 crystals.

In situ UV Raman spectroscopic study on the synthesis mechanism of AlPO-5.

Despite these efforts, a thorough understandingof the template effect, particularly the significance of the roleof templating in the channel formation of aluminophosphatemolecular sieves, is still lacking because of the complexity ofthe synthesis process.Avery powerful but relatively unexplored way of probingthis process is to perform in situ characterization underworking conditions.

A Thorough Investigation of the Active Titanium Species in TS‐1 Zeolite by In Situ UV Resonance Raman Spectroscopy

A thorough investigation of the active titanium species in TS-1 zeolite was conducted by in situ UV resonance Raman spectroscopy combined with UV/Vis diffuse reflectance spectroscopy, DFT calculations, and epoxidation experiments. A new titanium species was identified with a characteristic Raman band at 695 cm(-1) when excited at the 266 nm laser line. It is shown that the newly found titanium species is active in the epoxidation reactions in addition to the tetrahedrally coordinated titanium species. However, the acidity of the new titanium species could catalyze the ring-opening reactions of the epoxy products. It results in a lower selectivity toward the epoxy products relative to that of the tetrahedrally coordinated titanium species. The side reaction can be suppressed by the addition of a weak basic reagent.

In Situ UV Raman Spectroscopic Studies on the Synthesis Mechanism of Zeolite X

have been used to studythe formation mechanism of zeo-lites. Most studies were performed byusing ex situ tech-niques,namelybyfrequentlyremovingaliquotsofthereac-tion mixture andanalyzing thesamplesafter quenching thereaction. However, microporous zeolite-type materials areusually synthesized under hydrothermal conditions, and theneedforsamplequenchingandworkupmaycausedramaticand undeterminable structural changes.

Force reversed method for locating transition states

To identify the transition state accurately and efficiently on a high-dimensional potential energy surface is one of the most important topics in kinetic studies on chemical reactions. We present here an algorithm to search the transition state by so-called force reversed method, which only requires a rough reaction direction instead of knowing the initial state and final state. Compared to the nudged elastic band method and the dimer method that require multiple images, the present algorithm with only single image required saves significantly the computational cost. The algorithm was implemented in the first-principle periodic total energy calculation package and applied successfully to several prototype surface processes such as the adsorbate diffusion and dissociation on metal surfaces. The results indicate that the force reversed method is efficient, robust to identify the transition state of various surface processes.

Location of Mg cations in mordenite zeolite studied by IR spectroscopy and density functional theory simulations with a CO adsorption probe.

The location of Mg cations in the channel of mordenite zeolite was studied using a combination of DFT simulations and IR spectroscopy of adsorbed CO. The calculated adsorption energies and frequencies of CO on Mg cations are in good agreement with the results from the IR spectra of adsorbed CO. It is found that the Mg cations can occupy the sites A, C, D and E in mordenite and the distribution of the Mg cations in these sites follows the priority order, site C > site A > site D and site E.

A First‐Principles Study of Carbon–Oxygen Bond Scission in Multiatomic Molecules on Flat and Stepped Metal Surfaces

Step sites over terrace sites have been suggested to be the active sites in many catalytic reactions particularly bond breaking of diatomic molecules. Aiming to provide insight into the role of step sites in multiatomic molecules bond breaking reactions and their dependence on catalysts, we present herein a systematic first‐principles study of carbon–oxygen bond scission of diatomic CO and multiatomic HCO and CH3HCO on flat and stepped Co, Rh, and Ir surfaces. We find that multiatomic molecules exhibit distinct carbon–oxygen scission activity from diatomic molecules regardless of the metal catalysts (Co, Rh, and Ir) considered: compared to the huge enhancement of step sites for CO with a barrier 0.81–1.29 eV lower than that of flat surfaces, the role of step sites for CH3CHO is substantially weakened with a barrier 0.11–0.27 eV higher than that of flat surfaces. The reason for this is the change of adsorption configurations on flat surfaces and increase of Pauli repulsion on the congested stepped sites for the dissociation of multiatomic molecules.

Stability of polar ZnO surfaces studied by pair potential method and local energy density method

The polar ZnO surfaces have received wide interests due to their higher activity than the nonpolar facets in catalysis, photo-catalysis and gas sensitivity. However, the theoretical study on the relative stability of the polar ZnO surfaces is still limited. In this work, two different methods were used to calculate the surface energy of the polar ZnO(0001)–Zn and Zn(000-1)–O surfaces. The empirical pair potential method shows that the ZnO(000-1)–O terminal is more stable than the ZnO(0001)–Zn terminal because the polarizability of surface O2− is higher than that of surface Zn2+, which is in good agreement with the experimental results. However, the classic local energy density method predicts a higher stability of the ZnO(0001)–Zn terminal. The overestimation of the stability of the ZnO(0001)–Zn terminal originates from more distribution of the transferred charge to the ZnO(0001)–Zn terminal as the electron acceptor. We propose a hybrid method to fairly redistribute the contribution of the transferred charge to electron donor and electron acceptor and make the same stability trend with the experimental studies.

First-principles study of structure sensitivity of chain growth and selectivity in Fischer–Tropsch synthesis using HCP cobalt catalysts

Structure sensitivity on chain growth and selectivity in cobalt catalyzed Fischer–Tropsch synthesis (FTS) were studied by density functional theory (DFT) calculations. It is found that at a lower CO coverage, chain growth tends to proceed via a CO insertion mechanism on close-packed Co (0001) and stepped Co, with CH4 as the main product. However, a carbide mechanism is preferable on more open Co (101) accompanied with higher selectivity to C2 hydrocarbons than CH4. The origin is identified from the structure sensitive adsorption of the key intermediates, specifically the least “saturated” C/CH species, which exhibit a relatively strong dependence on the structure evolution. With increasing CO coverage, the CO insertion mechanism becomes more favorable, and both FTS activity and C2 hydrocarbon selectivity increase on Co (0001). This work highlights the intrinsic structure and coverage effects, achieving fundamental insight that can potentially be used to design and develop improved catalysts for FTS and other important reactions in syngas conversion.