Wei-Kang Yuan

Biphasic Pd−Au Alloy Catalyst for Low-Temperature CO Oxidation

Low-temperature CO oxidation over a compositional series of Pd-Au nanoalloy catalysts supported on silica fume was studied. Except for the pure metals, these materials invariably showed biphasic separation into palladium- and gold-rich components. Performance was optimal for a catalyst of bulk composition Pd(4)Au(1), a mixture of Pd(90)Au(10) (72.5 at. %) and Pd(31)Au(69) (27.5 at. %), that was remarkably active at 300 K and more stable than a pure Au catalyst. For bulk materials dominated by Pd (Pd:Au = 16:1; 8:1; 4:1), the palladium-rich alloy fraction frequently adopted hollow sphere or annular morphology, while the gold-rich crystals were often multiply twinned. Quantitative powder X-ray diffraction (XRD) showed that under the synthesis conditions used, the Au solubility limit in Pd crystals was approximately 12 at. %, while Pd was more soluble in Au (approximately 31 at. %). This was consistent with X-ray photoelectron spectroscopy (XPS), which revealed that the surfaces of Pd-rich alloys were enriched in gold relative to the bulk composition. In situ Fourier transform infrared spectra collected during CO oxidation contained a new band at 2114 cm(-1) (attributed to linear CO-Au/Au-Pd bonds) and reduced intensity of a band at 2090 cm(-1) (arising from a linear CO-Pd bond) with escalating Au content, indicating that the Pd sites became increasingly obscured by Au. High-resolution electron micrographs (HRTEM) of the Pd-rich alloys revealed atomic scale surface defects consistent with this interpretation. These results demonstrate that gold-containing biphasic Pd nanoalloys may be highly durable alternatives for a range of catalytic reactions.

Mechanistic insight into size-dependent activity and durability in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane

We report a size-dependent activity in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane. Kinetic study and model calculations revealed that Pt(111) facet is the dominating catalytically active surface. There is an optimized Pt particle size of ca. 1.8 nm. Meanwhile, the catalyst durability was found to be highly sensitive to the Pt particle size. The smaller Pt particles appear to have lower durability, which could be related to more significant adsorption of B-containing species on Pt surfaces as well as easier changes in Pt particle size and shape. The insights reported here may pave the way for the rational design of highly active and durable Pt catalysts for hydrogen generation.

Statistical properties of visibility graph of energy dissipation rates in three-dimensional fully developed turbulence

We study the statistical properties of complex networks constructed from time series of energy dissipation rates in three-dimensional fully developed turbulence using the visibility algorithm. The degree distribution is found to have a power-law tail with the tail exponent α=3.0. The exponential relation between the number of the boxes NB and the box size lB based on the edge-covering box-counting method illustrates that the network is not self-similar, which is also confirmed by the hub-hub attraction according to the visibility algorithm. In addition, it is found that the skeleton of the visibility network exhibits excellent allometric scaling with the scaling exponent η=1.163±0.005.

Pulsed high-voltage discharge plasma for degradation of phenol in aqueous solution

Abstract Pulsed high-voltage discharge is shown in the present investigation to be effective of phenol degradation in the aqueous solution in an isothermal batch reactor with continuous gas bubbling. Removal of phenol and effects of various parameters on the removal efficiency in the aqueous solution with pulsed high-voltage corona discharge plasma are studied. It is found that phenol degradation can be raised considerably by increasing the peak voltage of the pulsed discharge and the repetition rate of pulse, or by increasing pH of the aqueous solution. Presence of a buffer such as sodium carbonate and/or a hydroxide radical scavenger such as n -butanol in the aqueous solution decreases the removal efficiency of phenol. It is also found that decreasing the diameter of discharge electrode and electric conductivity of the aqueous solution can increase the degradation efficiency. The addition of gas, especially oxygen through the hollow needle electrode, and of FeSO 4 to the aqueous solution of phenol is found to significantly enhance phenol degradation. The intermediates formed in the phenol degradation process are pyrocatechol, hydroquinone, resorcinol, 1,4-benzoquinone, and some other unidentified products. The maximum phenol degradation percentage is 62.7% when its residence time is 180 min, and the TOC of aqueous solution decreases by 83.8% after having been discharged for 420 min. The phenol degradation with pulsed high-voltage discharge in aqueous solution shows a first-order kinetics: C=C 0 e −kt .

Kinetics of Hydrogenolysis of Glycerol to Propylene Glycol over Cu-ZnO-Al2O3 Catalysts

Abstract A series of Cu-ZnO-Al 2 O 3 catalysts with various metal compositions of Cu/Zn/Al were prepared by the co-precipitation method, and screened for glycerol hydrogenolysis to propylene glycol. The catalyst with a Cu/Zn/Al molar ratio of 1 : 1 : 0.5 exhibited the best performance for glycerol hydrogenolysis, and thus selected for kinetic investigation. Under elimination of external and internal diffusion limitation, kinetic experiments were performed in an isothermal fixed-bed reactor at a hydrogen pressure range of 3.0-5.0 MPa and a temperature range of 493-513 K. Based on a dehydration-hydrogenation two-step hydrogenolysis mechanism, a two-site Langmuir-Hinshelwood kinetic model taking into account competitive adsorption of glycerol, acetol and propylene glycol was proposed and successfully fitted to the experimental data. The average relative errors between observed and predicted outlet concentrations of glycerol and propylene glycol were 6.3% and 7.6%, respectively. The kinetic and adsorption parameters were estimated by using the fourth-order Runge-Kutta method together with the Rosenbrock algorithm. The activation energies for dehydration and hydrogenation reactions were 86.56 and 57.80 kJ·mol −1 , respectively.

Microcellular Foaming of Polypropylene/Clay Nanocomposites with Supercritical Carbon Dioxide

The solid-state foaming of two series of polypropylene (PP)/clay nanocomposites based on two different types of organoclays were studied. The wide-angle X-ray diffraction results showed that a supercritical carbon dioxide (scCO 2) saturation process could lead to the further exfoliation of clay when the initial interlayer spacing was larger enough. It was found that the incorporation of clays exerted significant impacts on cell nucleation and growth and the effect of clays was determined by their dispersion state. However, the influence of clay on the foaming of the PP/clay nanocomposites became marginal at a higher saturation pressure and a lower foaming temperature.

Multifractal analysis of the fracture surfaces of foamed polypropylene/polyethylene blends

Abstract The two-dimensional multifractal detrended fluctuation analysis is applied to reveal the multifractal properties of the fracture surfaces of foamed polypropylene/polyethylene (PP/PE) blends at different temperatures. Nice power-law scaling relationship between the detrended fluctuation function F q and the scale s is observed for different orders q and the scaling exponent h ( q ) is found to be a nonlinear function of q , confirming the presence of multifractality in the fracture surfaces. The multifractal spectra f ( α ) are obtained numerically through Legendre transform. The shape of the multifractal spectrum of singularities can be well captured by the width of spectrum Δ α and the difference of dimension Δ f . With the increase of the PE content, the fracture surface becomes more irregular and complex, as is manifested by the facts that Δ α increases and Δ f decreases from positive to negative. A qualitative interpretation is provided based on the foaming process.

Modified carbon nanotubes by KMnO4 supported iron Fischer–Tropsch catalyst for the direct conversion of syngas to lower olefins

Manganese and potassium promoters coated carbon nanotubes (i.e., MnK-CNTs) were synthesized by a redox reaction between CNTs and KMnO4, in which the CNTs act as reducing agent and as substrate for the heterogeneous nucleation of K-doped manganese oxide. The as-synthesized MnK-CNTs were employed to support Fe catalyst (i.e., Fe/MnK-CNTs, the loadings of 7.9 wt% Fe, 15.7 wt% Mn and 1.9 wt% K) for the direct conversion of syngas to lower olefins. It is revealed that Fe/MnK-CNTs catalyst is more active and stable than FeMnK/CNTs catalyst prepared by co-impregnation method using CNTs as a support. Furthermore, under similar CO conversion, the Fe/MnK-CNTs catalyst exhibits higher selectivity of hydrocarbons especially lower olefins. This could be related to the small-sized and uniform nanoparticles, the well-distributed promoters, the weak metal–support interaction and the greater defects on support, which are the consequences of the unique structural transformation of MnK-CNTs as a function of temperature and atmosphere.

Modeling of a fixed-bed reactor using the K-L expansion and neural networks

Abstract Karhunen-Loeve expansion and feedforward neural networks are combined together in modeling a wall cooled fixed-bed reactor for its on-line performance prediction. The K-L expansion is employed as a preprocessor of neural networks while the latter is used to generate the coefficients of the K-L expansion. The performance of the KL-NN model is investigated by both experimentation and simulation with benzene oxidation as a working system. It is shown that the method is effective for on-line prediction of the bed temperatures. Our conclusions are more important than just that one term can be used. Sometimes it might be two or three, but the method described in the paper is still powerful.

Ir–Re alloy as a highly active catalyst for the hydrogenolysis of glycerol to 1,3-propanediol

In this work, bimetallic Ir–Re catalysts supported on KIT-6 are prepared by tuning the thermal treatment procedures, i.e., conventional calcination and reduction (Ir–Re/KIT-6-CR) and modified direct reduction (Ir–Re/KIT-6-R) after impregnation of two metal precursors. The structure of both catalysts is intensively characterized by H2-TPR, STEM-HAADF-EDX, XPS and CO-DRIFTS. Results indicate that an Ir–Re alloy forms on the KIT-6 support when direct reduction is employed, which exhibits excellent catalytic performance in hydrogenolysis of glycerol. The formation rate of 1,3-propanediol over Ir–Re/KIT-6-R reaches 25.6 mol1,3-PD molIr−1 h−1 at 63% glycerol conversion with the addition of amberlyst-15 under 8 MPa H2, 393 K and 20 wt% glycerol aqueous solution, almost twice that over Ir–Re/KIT-6-CR. It is revealed that Re species without prior calcination treatment could be fully reduced and therefore couple with Ir to form an Ir–Re alloy structure with enhanced resistance against particle aggregation, while the calcination and subsequent reduction leads to the formation of an Ir–ReOx structure since the rhenium oxide species generated during the calcination is difficult to be reduced.