Lijuan Song

Investigation on the mechanism of adsorption and desorption behavior in cerium ions modified Y-type zeolite and improved hydrocarbons conversion

Abstract Different types of Y zeolites (HY, USY and NaY) containing various amounts of cerium (Ce) were prepared with the method of liquid ion exchange and characterized by X-ray diffraction (XRD), intelligent gravimetric analyzer (IGA), and a molecular simulation technology (grand canonical Monte Carlo simulation, GCMC). A novel calculation method of desorption index (DI) was proposed and the influence of cerium cationic species on the processes of adsorption-desorption of hydrocarbon molecules (n-octane) on the cerium modified Y zeolites (CeY) was studied. The results indicated that Ce ion species played a key role in reducing desorption activation energy of n-octane on Y type zeolites, leading to an improvement of the desorption ability of the CeY and the regulation of the adsorption state of n-octane from aggregation to decentralization. These findings are essential factors for enhancing the product selectivity of light hydrocarbon component and the catalytic activity of rare earth modified Y zeolites (REY) catalysts in fluid catalytic cracking (FCC) process.

Electrical and lithium ion dynamics in Li2IrO3 from density functional theory study

Layered Li2IrO3 has been investigated using first-principles calculations within the GGA and GGA + U schemes. Within the GGA + U approach, the calculated intercalation voltage (ranging from 3.2 V to 4.1 V) is found to be in good agreement with experiments. Electronic-structure analysis shows that the band gap of pure phase Li2IrO3 is ∼1 eV, which is indicative of semiconductor properties. Moreover, further studies of lithium diffusion in bulk Li2IrO3 show that unlike the two dimensional diffusion pathways in rock salt structure layered cathode materials, lithium can diffuse in a three-dimensional pathway in Li2IrO3, with moderate lithium migration energy barriers ranging from 0.24 to 0.81 eV.

Effect of cerium ions initial distribution on the crystalline structure and catalytic performance of CeY zeolite

Abstract CeY samples with different cerium (Ce) ions initial distribution were prepared by diverse preparation processes. The correlative influence of Ce ions distribution on the intracrystalline structure and catalytic mechanism of Y-type zeolite was studied by X-ray diffraction (XRD), Rietveld refinement, X-ray fluorescence spectrometry (XRF), energy-dispersive X-ray spectroscopy (EDS), infrared (IR) spectra, NH3 temperature-programmed desorption (NH3-TPD), N2 physical adsorption-desorption and micro-activity test (MAT). The results indicated that Ce ions were concentrated on the surface (supercage) of CeY zeolite would reduce the number of acid sites and the ratio of Bronsted acid and Lewis acid (B/L). Once Ce ions concentrated in the sodalite cage, it would participate in the coordination with framework oxygen atoms under the condition of steric hindrance. Meanwhile, Ce ions would preferentially promote the B/L value of weak acid by retaining more intracrystalline framework Al atoms in sodalite cage, which was beneficial to the conversion of heavy oil in FCC process. The optimal initial-distribution of Ce ions could not only save the consumption of rare earth resources but also give the best catalytic performance of CeY zeolite catalysts, and these advantages and attributes enabled it to have the potential to be applied in industrial applications.

A DFT study of the effect of NNN Al atom on strength of Brönsted acid sites of HY zeolite

Abstract A density functional theory study has been employed to investigate the effect of next-nearest-neighbours aluminium (NNN Al) atom on the acid strength of Brönsted acid site in Y zeolite. Additionally, the distribution of NNN Al atom was also studied. The obtained results show that thermodynamically, the favourable sites for location of NNN Al atom are the diagonal sites of the four-membered ring and meta-position of hexagon I. Furthermore, the increase of NNN Al atoms causes the nonlinear reduction of the strength of Brönsted acid sites. When the number of NNN Al atoms is greater than two, the increasing extent of acid strength is not obvious with the reduction in the number of NNN Al atoms. But the acid strength will increase linearly with the further reduce of the number of NNN Al atoms. Compared with deprotonation energy (ED), ammonia adsorption energy (Eads) could give a more reasonable measuring result for the acid strength.

Impact of surface arrangement and composition on ethylene adsorption over Pd–Ag surface alloys: a computational study

The adsorption of ethylene on three low-index Pd–Ag bimetallic surfaces, which are the (111), (100), and (110) facets, is investigated using gradient-corrected periodic density functional calculations with dispersion correction. The surfaces have been modeled by 5 layers of Pd atoms and different Ag atomic concentrations, allowing us to study ethylene adsorption from 100% Pd to 25% Pd on the surfaces. The adsorption energy and the geometry have been computed for different adsorption sites (on top of a Pd or bridging two Pd atoms) for different facets and Ag atomic concentrations. For bare Pd surfaces and their surface alloys, the bridge site is always found to be more stable than any other site. For the surface alloys, the local density of states, charges of ethylene and Pd atom, and differential electron density have been investigated to illustrate the importance of ligand and ensemble effects of the guest metal Ag atoms. The adsorption is weakened because of the ensemble effect when the surface Ag atomic concentration increases. The amount of electrons transferred to the ethylene molecules from the surface increases slightly when the concentration of the surface atomic Ag increases. The ligand effect becomes more significant when the surface is closer, and its effect is not the same for different surface facets.