Xiaojun Chen

Hydrogen production by methane decomposition over Ni–Cu–SiO2 catalysts: effect of temperature on catalyst deactivation

Catalytic decomposition of methane (CDM) is a simple process for the production of high-purity, COX-free (CO or CO2) hydrogen. The CDM is a moderately endothermic reaction, and high temperatures are thermodynamically favorable for achieving high methane conversion. However, Ni–Cu catalysts easily lose their activities at high temperature. To study the effect of temperature on the deactivation of Ni–Cu catalysts, a 65% Ni–15% Cu–SiO2 catalyst was prepared by the heterophase sol–gel method. A series of kinetic experiments (routes I, II, III) were designed to test the catalytic performance and generate by-product carbon structures. The effects of reaction temperature and methane dissociation rate on catalyst deactivation were studied. The phase transition temperature was estimated. Based on the kinetic experiments, TEM images, XRD data, TGA-DSC curves, and TEM-EDX data, a thoroughly deactivation study of the 65% Ni–15% Cu–SiO2 catalyst was carried out. The results of this study proved that high degree of graphitization was the key factor contributing to the deactivation of Ni–Cu catalysts. Fragmentation and phase separation at high temperature were both responsible for carbon atom enrichment and a high degree of graphitization, which in turn caused the 65% Ni–10% Cu–25% SiO2 catalyst to lose activity at high temperature.

A carboxymethyl cellulose modified magnetic bentonite composite for efficient enrichment of radionuclides

The radio-frequency (RF) plasma-induced grafting technique was employed to fabricate a carboxymethyl cellulose (CMC) grafted magnetic bentonite composite (CMC-g-MB) in Ar conditions. The grafted CMC could improve the composite dispersion ability as well as the chemical stability in strong acidic conditions. The CMC-g-MB composite exhibited fast adsorption kinetics and good adsorption capacities in the enrichment of Cs(I), Sr(II) and Co(II) from aqueous solutions, showing broad applicability for radionuclide removal. The enrichment efficiencies of radionuclides on the CMC-g-MB composite decreased in the order of Cs(I) > Sr(II) > Co(II). On the basis of the thermodynamic parameters, radionuclide adsorption on the CMC-g-MB composite was thermodynamically favorable and endothermic. Considering the non-toxicity and biodegradation of CMC, the CMC-g-MB composite presented promising potential in radioactive pollution management.

Effect of silicate on the sorption properties of kaolinite: removal of U(VI) and mechanism

The surface and sorption properties of kaolinite were analyzed as a function of silicate. Batch experiments indicate that the U(VI) sorption is promoted by the addition of silicate at low pH while is depressed at high pH. The sorption was acceptably predicted by the formation of a ternary silicate surface complex under the experimental conditions. The pseudo-second order kinetic model fit the sorption kinetics better. The sorption isotherms are more in accordance with Langmuir model and the thermodynamic parameters indicate a spontaneous and endothermic sorption process.

Analysis of hydrogen isotopes with quadrupole mass spectrometry

Hydrogen isotope separation is one of the most critical technological problems in nuclear fusion research, and, in order to assess accurately the performance of hydrogen isotope separation, quantitative analysis of hydrogen isotopes takes priority and becomes the first essential problem to be addressed. However, since hydrogen isotopes have almost identical shape, size, and chemical properties, separation and analysis of hydrogen isotopes is really not an easy task. By using the thermal-desorption spectroscopy (TDS) method, a quadrupole mass spectrometer (MS) was calibrated for the quantitative analysis of hydrogen isotopes in this paper with a methodic error less than ±3% using titanium hydride and titanium deuteride as the calibration standards. The linear response range of MS was extracted. Deviations that originated from the H+/D+/HD+ species revealing a negligible influence on real H2/D2 mixture analysis were also discussed. Due to the mass discrimination of the ion source and the isotopic fractionation effect of the molecular pump, the actual sensitivity of MS towards H2 and D2 is not the same, revealing some deviation from theoretical results.

Ultrahigh Effective H2/D2 Separation in an Ultramicroporous Metal-organic Framework Material through Quantum Sieving

An ultramicroporous MOF material, {[Fe(OH)(H2bta)](H2O)}n, was diligently selected for the experimental investigations of ultralow-temperature separation of H2/D2 through quantum sieving. With an extreme two-dimensional confinement experienced by hydrogen molecules, an extraordinary separation factor as high as 41.4 ± 0.4 at 20 K was for the first time obtained from experiment.

Screening of Metal–Organic Frameworks for Highly Effective Hydrogen Isotope Separation by Quantum Sieving

Separation of hydrogen isotopes is of great importance to produce highly pure hydrogen isotopes for numerous applications, which is however very difficult because of their almost identical thermodynamic properties. Adsorptive separation is considered as a simple, highly efficient, and cost-effective technique compared to the traditional methods, where the key is the suitable adsorbent. Herein, SIFSIX-3-Zn was screened out from the reported metal-organic frameworks (MOFs), exhibiting high selectivities for a D2/H2 mixture by quantum sieving effect. Advanced cryogenic thermal desorption spectroscopy confirms the calculation results, indicating that the selectivities for a 1:1 D2/H2 mixture at 20 K are larger than the values reported so far; especially, it shows a record value of 53.8 at 25 kPa. This demonstrates that this MOF is a promising candidate for highly effective hydrogen isotope separation.

Dissociation mechanism of H2 molecule on the Li2O/hydrogenated-Li2O (111) surface from first principles calculations

Hydrogen molecules in a purge gas are known to enhance the release of tritium from lithium ceramic materials, which has been demonstrated in numerous in-pile experiments. The static computational results suggest that the molecular adsorption of H2 on the “ideal” Li2O/hydrogenated-Li2O (111) surface encounters high dissociation barriers in various entrance channels. The surface chemical inertness of the plane can be broken by introducing vacancy defects. In the present work, a combination of static DFT calculations and ab initio molecular dynamics has been performed to investigate the H2 dissociative mechanism. Our theoretical results, that the end-on oriented H2 could dissociate on the hydrogen monomer vacancy surface with one hydrogen atom ejected into the gas phase by the abstraction channel and the parallel H2 molecule dissociates on the hydrogen dimer vacancy surface with two hydroxyls forming, suggest that hydrogen vacancy defects facilitate the adsorption and dissociation of H2 molecule. The presence of the O2− ion induced by the hydrogen vacancy provides some low energy states in which the H2 electrons can be accommodated. This is very instructive for the comprehension of phenomena that occur during the operation of a thermonuclear reactor.

Kinetic and thermodynamic studies on the interaction of europium(III) and phosphate with γ-Al2O3

The temperature effect on the co-sequestration process receives little attention. Herein, kinetic and thermodynamic studies on the co-sequestration of Eu(III) and phosphate by γ-Al2O3 were investigated by batch experiments. The standard thermodynamic parameters (∆H0, ∆S0, and ∆G0), the activation thermodynamic parameters (∆H#, ∆S#, and ∆G#), and the activation energy (EA) for the co-sequestration process were obtained. Based on EA and ∆S# values, Eu(III) and phosphate co-sequestration is controlled by chemical reactions on γ-Al2O3 and by an associative mechanism. This work highlights the temperature effect on the co-sequestration of Eu(III) and phosphate in a geological environment.