Highly effective bromate reduction by liquid phase catalytic hydrogenation over Pd catalysts supported on core-shell structured magnetites: Impact of shell properties

Liquid phase catalytic reduction of bromate with supported noble metals as the catalysts is a promising method to remove bromate in water. Magnetic supports provide a feasible way to recover catalysts whose surface properties also strongly influence the catalytic efficiency. In this study, Pd nanoparticles supported on core-shell structured magnetites with varied shells (e.g., carbon, SiO₂, polypyrrole, polyaniline, polydopamine and chitosan) were prepared and catalytic reduction of bromate on the catalysts was investigated. The results showed that in comparison with other catalysts Pd/(Fe₃O₄@polyaniline) exhibited a higher catalytic efficiency due to its higher point of zero charge and surface hydrophilicity. In parallel, bromate reduction on Pd/(Fe₃O₄@polyaniline) followed the Langmuir-Hinshelwood model, confirming the crucial role of bromate adsorption. At pH 5.6 and a catalyst dosage of 0.05 g/L, 0.4 mM bromate could be completely reduced into bromide within 120 min. Furthermore, the magnetic catalysts could be effectively separated and recovered under an external magnetic field within 3 min. The results of catalyst reuse showed that after five consecutive catalytic reduction cycles Pd/(Fe₃O₄@polyaniline) retained 87% of its fresh catalyst activity. The present findings indicate that Pd/(Fe₃O₄@polyaniline) with polyaniline as the shell is a highly active, stable and recyclable catalyst for liquid phase catalytic hydrogenation of pollutants in water.

Room temperature stable CO x -free H2 production from methanol with magnesium oxide nanophotocatalysts

Methanol, which contains 12.6 weight percent hydrogen, is a good hydrogen storage medium because it is a liquid at room temperature. However, by releasing the hydrogen, undesirable CO and/or CO₂ byproducts are formed during catalytic fuel reforming. We show that alkaline earth metal oxides, in our case MgO nanocrystals, exhibit stable photocatalytic activity for CO/CO₂-free H₂ production from liquid methanol at room temperature. The performance of MgO nanocrystals toward methanol dehydrogenation increases with time and approaches ~320 μmol g^-1 hour^-1 after a 2-day photocatalytic reaction. The CO _x -free H₂ production is attributed to methanol photodecomposition to formaldehyde, photocatalyzed by surface electronic states of unique monodispersed, porous MgO nanocrystals, which were synthesized with a novel facile colloidal chemical strategy. An oxygen plasma treatment allows for the removal of organic surfactants, producing MgO nanocrystals that are well dispersible in methanol.

Influence of ZrO2 properties on catalytic hydrodechlorination of chlorobenzene over Pd/ZrO2 catalysts

Pd/ZrO(2) catalysts using different ZrO(2) as supports were prepared using the deposition-precipitation method and were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, N(2) adsorption, temperature programmed reduction, H(2) chemisorption and measurement of surface hydroxyl group. Catalytic hydrodechlorination (HDC) of chlorobenzene was used to evaluate the activity and stability of the catalyst. The results showed that ZrO(2) support calcined at 300 degrees C was amorphous in nature, whereas ZrO(2) supports calcined at 500 and 600 degrees C consisted of both monoclinic and tetragonal phases. In addition, increasing calcination temperature led to the decrease of specific surface area and surface hydroxyl group content of the ZrO(2) support. For temperature programmed reduction of PdO/ZrO(2) samples, two H(2) consumption peaks with varied reduction temperature were distinctly observed, implying the existence of different Pd species in Pd/ZrO(2) catalysts. In addition, Pd/ZrO(2) catalyst with ZrO(2) calcined at 500 degrees C had a relatively higher content of Pd species with strong metal-support interaction than other catalysts. For catalytic HDC of chlorobenzene, Pd/ZrO(2) catalyst with ZrO(2) support calcined at 500 degrees C exhibited a higher initial activity and stability as compared to other catalysts, indicative of a strong dependence of the catalytic behavior of the Pd/ZrO(2) catalyst on the support properties for catalytic HDC of chlorobenzene.

The importance of strong carbon-metal adhesion for catalytic nucleation of single-walled carbon nanotubes

Density functional theory is used to show that the adhesion between single-walled carbon nanotubes (SWNTs) and the catalyst particles from which they grow needs to be strong to support nanotube growth. It is found that Fe, Co, and Ni, commonly used to catalyze SWNT growth, have larger adhesion strengths to SWNTs than Cu, Pd, and Au and are therefore likely to be more efficient for supporting growth. The calculations also show that to maintain an open end of the SWNT it is necessary that the SWNT adhesion strength to the metal particle is comparable to the cap formation energy of the SWNT end. This implies that the difference between continued and discontinued SWNT growth to a large extent depends on the carbon-metal binding strength, which we demonstrate by molecular dynamics (MD) simulations. The results highlight that first principles computations are vital for the understanding of the binding strength's role in the SWNT growth mechanism and are needed to get accurate force field parameters for MD.

In Situ Studies of Methanol Decomposition and Oxidation on Pd(111) by PM-IRAS and XPS Spectroscopy

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.