Weihao Weng

Oxidation of benzyl alcohol by using gold nanoparticles supported on ceria foam.

The efficacy of using cerium oxide foams as a support for Au nanoparticles and subsequent use as oxidation catalysts have been investigated. These were synthesized using L-asparagine to produce a cerium coordination polymer foam, which was calcined to give the oxide foam. Au nanoparticles were supported on the CeO(2) foams using a sol-immobilization method. The activity of the Au/foamCeO(2) for solvent-free benzyl alcohol oxidation was superior to standard Au/CeO(2) catalysts, and the activity was found to be dependent on the crystallization time of the precursor foam. A crystallization time of 4 h was found to produce the most active catalyst, which retained activity and a high selectivity to benzaldehyde (ca. 96 %) when re-used and this is related to the structure of the material. The high activity is attributed to the greater lability of surface oxygen in the support compared with commercial CeO(2) materials.

Niobium phosphates as new highly selective catalysts for the oxidative dehydrogenation of ethane

Several niobium phosphate phases have been prepared, fully characterized and tested as catalysts for the selective oxidation of ethane to ethylene. Three distinct niobium phosphate catalysts were prepared, and each was comprised predominantly of a different bulk phase, namely Nb(2)P(4)O(15), NbOPO(4) and Nb(1.91)P(2.82)O(12). All of the niobium phosphate catalysts showed high selectivity towards ethylene, but the best catalyst was Nb(1.91)P(2.82)O(12), which was produced from the reduction of niobium oxide phosphate (NbOPO(4)) by hydrogen. It was particularly selective for ethylene, giving ca. 95% selectivity at 5% conversion, decreasing to ca. 90% at 15% conversion, and only produced low levels of carbon oxides. It was also determined that the only primary product from ethane oxidation over this catalyst was ethylene. Catalyst activity also increased with time-on-line, and this behaviour was ascribed to an increase of the concentration of the Nb(1.91)P(2.82)O(12) phase, as partially transformed NbOPO(4), formed during preparation, was converted to Nb(1.91)P(2.82)O(12) during use. Catalysts with predominant phases of Nb(2)P(4)O(15) and NbOPO(4) also showed appreciable activity and selectivities to ethylene with values around 75% and 85% respectively at 5% ethane conversion. The presence of phosphorous is required to achieve high ethylene selectivity, as orthorhombic and monoclinic Nb(2)O(5) catalysts showed similar activity, but displayed selectivities to ethylene that were <20% under the same reaction conditions. To the best of our knowledge, this is the first time that niobium phosphates have been shown to be highly selective catalysts for the oxidation of ethane to ethylene, and demonstrates that they are worthy candidates for further study.

The significance of the order of impregnation on the activity of vanadia promoted palladium-alumina catalysts for propane total oxidation

The increased activity of alumina-supported palladium catalysts promoted with vanadium oxide has been investigated. Three different vanadium promoted Pd/Al2O3 catalysts with the same composition but synthesized employing sequential and co-impregnation were tested for the total oxidation of propane. The order of impregnation was critical to produce high activity catalysts. Vanadium and palladium co-impregnation on the Al2O3 support led to the most active catalyst, whereas the step-wise impregnated catalysts show a catalytic performance similar to or slightly better than unpromoted palladium catalysts. The high activity of the co-impregnated catalysts is related to the particle size and oxidation state of the palladium particles; and to the redox properties of vanadium species. The most active catalyst presents relatively large palladium particles in combination with increased reducibility of vanadium species and a relatively high amount of V4+ within the bulk of the catalyst and on the surface. STEM shows that, compared to catalysts containing only Pd or V, co-addition of the Pd and V species drastically altered the particle size distribution and morphology of the PdOx particles, and simultaneously caused the monolayer dispersion of the VOx species to become much patchier in nature. It also showed that the microstructure of the catalysts was similar for the different orders of impregnation, but some differences between the morphology of PdOx particles were observed.

Molybdenum blue nano-rings: an effective catalyst for the partial oxidation of cyclohexane

Molybdenum blue (MB), a multivalent molybdenum oxide with a nano-ring morphology is well-known in analytical chemistry but, to date it has been largely ignored in other applications. In the present work, MB has been characterized by STEM-HAADF imaging for the first time, showing the nano-ring morphology of this complex molybdenum oxide and the ordered super-molecular framework crystals that can result from the self-assembly of these MB nano-ring units. The potential of MB as an oxidation catalyst has also been investigated, where it is shown to have excellent catalytic activity and stability in the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone which are important intermediates in the production of nylon.

Structural Characterization of Vanadium Phosphate Catalysts Prepared using a Di-block Copolymer Template

In this study, we utilize a di-block copolymer, 2-poly (styrene-alt-maleic acid) (PSMA) as a template to influence the crystallinity and morphology of a vanadium phosphate catalyst precursor. V2O5 and H3PO4 were reacted in isobutanol in the presence of a small amount of PSMA. The precursors produced are denoted as P1 (PSMA: V2O5 weight ratio = 1: 28,500) and P2 (PSMA: V2O5 weight ratio = 2: 28,500). These precursors were activated in a butane/air mixture at 400C in a fixed bed micro-reactor to produce the corresponding activated catalysts denoted as C1 and C2, respectively. From XRD analysis and catalytic performance testing, the precursors produced were found to have a high degree of crystallinity and could be activated in a much shorter time (i.e. a matter of hours instead of days) when compared to precursors obtained from standard preparation routes.

Assessment of a nanocrystal 3-D morphology by the analysis of single HAADF-HRSTEM images

This work presents the morphological characterization of CeO2 nanocrystals by the analysis of single unfiltered high-angle annular dark-field (HAADF)-high-resolution scanning transmission electron microscopy (HRSTEM) images. The thickness of each individual atomic column is estimated by the classification of its HAADF integrated intensity using a Gaussian mixture model. The resulting thickness maps obtained from two example nanocrystals with distinct morphology were analyzed with aid of the symmetry from the CeO2 crystallographic structure, providing an approximation for their 3-D morphology with high spatial resolution. A confidence level of ±1 atom per atomic column along the viewing direction on the thickness estimation is indicated by the use of multislice image simulation. The described characterization procedure stands out as a simple approach for retrieving morphological parameters of individual nanocrystals, such as volume and specific surface areas for different crystalline planes. The procedure is an alternative to the tilt-series tomography technique for a number of nanocrystalline systems, since its application does not require the acquisition of multiple images from the same nanocrystal along different zone axes.

Understanding the Growth Mechanism of CeO2 Nanocrystals by Comparison of Experimental and Simulated HAADF-STEM Images

In recent years, ceria and doped-ceria anisotropic nanocrystals (NC’s) have been produced by a variety of solution-based synthetic routes. The motivation in producing ceria-based NCs with controlled morphology is to develop high surface area catalyst supports with well-defined exposed crystal planes that exhibit optimal catalytic activity. Understanding the growth mechanism of CeO2 NC’s is vital to designing nanostructured functional materials.

Structural characterization of Niobium Phosphate Catalysts used for the Oxidative Dehydrogenation of Ethane to Ethylene

Vanadium phosphorus oxides (V-P-O) represent the most well studied heterogeneous complex oxide catalyst system. Thousands of articles have been published regarding their preparation routes, the nature of the active vanadium phosphate phase and their reaction mechanisms. A related group of compounds, niobium phosphates (Nb-P-O), are now also beginning to receive some attention from academic researchers.

Evaluation and Structural Characterization of DuPont V-P-O/SiO2 Catalysts

For several decades vanadyl pyrophosphate, (VO)2P2O7, has been used commercially as a catalyst for the partial oxidation of n-butane into maleic anhydride (MA) [1]. However due to the moderate specificity to MA production, many research groups still continue to investigate the V-P-O catalyst system with the aim of further improving its overall performance. Now a specialized form of V-P-O catalyst has been developed by DuPont for use in a fluidized bed reactor, which consists of a V-P-O core encased by a mechanically protective porous silica shell. Selected research centers worldwide have received samples of this material for analysis, aiming at combining the results generated, in an effort to further understand the functionality of the V-P-O catalyst.