Leon Lefferts

Drop impact upon micro- and nanostructured superhydrophobic surfaces

We experimentally investigate drop impact dynamics onto different superhydrophobic surfaces, consisting of regular polymeric micropatterns and rough carbon nanofibers, with similar static contact angles. The main control parameters are the Weber number We and the roughness of the surface. At small We, i.e., small impact velocity, the impact evolutions are similar for both types of substrates, exhibiting Fakir state, complete bouncing, partial rebouncing, trapping of an air bubble, jetting, and sticky vibrating water balls. At large We, splashing impacts emerge forming several satellite droplets, which are more pronounced for the multiscale rough carbon nanofiber jungles. The results imply that the multiscale surface roughness at nanoscale plays a minor role in the impact events for small We less than or approximately equal 120 but an important one for large We greater than or approximately equal 120. Finally, we find the effect of ambient air pressure to be negligible in the explored parameter regime We less than or approximately equal 150.

How water droplets evaporate on a superhydrophobic substrate

Evaporation of water droplets on a superhydrophobic substrate, on which the contact line is pinned, is investigated. While previous studies focused mainly on droplets with contact angles smaller than 90°, here we analyze almost the full range of possible contact angles (10°-150°). The greater contact angles and pinned contact lines can be achieved by use of superhydrophobic carbon nanofiber substrates. The time evolutions of the contact angle and the droplet mass are examined. The experimental data are in good quantitative agreement with the model presented by Popov [Phys. Rev. E 71, 036313 (2005)], demonstrating that the evaporation process is quasistatic, diffusion-driven, and that thermal effects play no role. Furthermore, we show that the experimental data for the evolution of both the contact angle and the droplet mass can be collapsed onto one respective universal curve for all droplet sizes and initial contact angles.

Selective reduction of NO to N2 in the presence of oxygen over supported silver catalysts

Selective reduction of NO (0.1%) with propylene (0.1%) in the presence of oxygen (5%) with He balance was carried out over Ag supported on Al2O3, H-ZSM5 and H-Y catalysts. Although their activities were different, all three catalysts showed high selectivity to N2 (around 95%). Through several characterizations, it was concluded that silver was present in the form of Ag2O clusters, but not in metallic form for the three catalysts. High selectivity to N2 on all the catalysts is attributed to the same active phase of silver oxide. The difference in catalytic activity is considered to be due to the amounts and the size of Ag2O clusters. Normalized activity per Ag site (TOF) was not the same (Ag/Al2O3>Ag-ZSM5>Ag-HY), which is explained by the carbon depositions and facile transformation of Ag species under redox condition over zeolite-based catalysts.

Immobilization of a layer of carbon nanofibres (CNFs) on Ni foam: A new structured catalyst support.

This work describes the preparation of new materials based on immobilizing carbon nanofibres (CNFs) on the surface of Ni foam. CNFs were catalytically synthesized by decomposition of ethene over the Ni foam. The influence of formation conditions on the morphology of the CNFs, on the mechanical stability of the CNFs?Ni-foam composite structures and on the attachment of the CNFs to the Ni foam is discussed. The surface area of the Ni?CNFs-foam composite increased with the loading of CNFs, from less than 1 m2 g?1 to 30 m2 g?1 for 50 wt.% CNFs on the foam. The layer of the CNFs was highly open with a pore volume of 1 cm3 g?1 CNFs. Stable Ni?CNFs-foam composite structures can be obtained under the conditions that the extent of corrosive metal dusting of Ni is limited, via decreasing the temperature and/or the formation time. Some metal dusting is, however, needed to form small Ni particles that allow formation of CNFs. The extent of corrosive metal dusting determines to what extent CNFs are weakly attached. The remaining CNFs, at least 80%, are remarkably strongly attached. Every single CNF is bonded to the Ni structure, probably via penetration of the CNFs into the polycrystalline Ni foam. Controlling the conditions of CNFs formation is vital in order to optimise the mechanical stability of the CNF?Ni-foam composite structures as well as the strong attachment of the CNFs to the surface of the Ni foam.

Growing a carbon nano-fiber layer on a monolith support; effect of nickel loading and growth conditions

This work describes how a new, extremely porous, hairy layer of carbon nano-fibers (CNFs) can be prepared on the surface of porous inorganic bodies, e.g. wash-coated monoliths. CNFs were prepared catalytically by methane and ethene decomposition over a Ni catalyst. The influence of the Ni particle size and growth conditions on the properties of the resulting material is reported. It turns out that the thickness of the CNF layer at the outermost surface (ca. 1 mm) as well as the diameter of the fibers increases with mean Ni particle size. The structure of this layer resembles the inverse structure of a traditional inorganic support material, combining high surface area, high porosity and low tortousity. Growing CNFs using methane leads to immediate fragmentation and doubling of the thickness of the washcoat which is independent of the amount of CNFs, forming a macro-porous composite layer of entangled alumina particles and CNFs with a typical diameter of 10?30 nm. Immediate fragmentation is due to the fact that some of the fibers are too thick for the pores in the washcoat. The total porosity decreases with the amount of CNFs whereas the surface area per gram of monolith increases. Large Ni particles are able to grow CNFs for longer times, resulting in detachment of the washcoat from the cordierite, which is caused by extensive growth of CNFs out of the washcoat. Furthermore, extended growth of CNFs inside the cordierite body causes disintegration of the monolith body when macro-pores are locally overfilled with CNFs. Methane is preferred over ethene for growing CNFs because ethene grows CNFs rapidly even on relatively large Ni particles, resulting in thick fibers up to 70 nm in the macro-porous cordierite, destroying the monolith. Controlling both the Ni particle size and Ni distribution as well as choosing the right activity of the hydrocarbon are essential to grow CNF washcoats without damaging the monolith structure.

Building microscopic soccer balls with evaporating colloidal fakir drops

Evaporation-driven particle self-assembly can be used to generate three-dimensional microstructures. We present a unique method to create colloidal microstructures in which we can control the amount of particles and their packing fraction. To this end, we evaporate colloidal dispersion droplets on a special type of superhydrophobic microstructured surface, on which the droplet remains in Cassie–Baxter state during the entire evaporative process. The remainders of the droplet consist of a massive spherical cluster of the microspheres, with diameters ranging from a few tens up to several hundreds of microns. We present scaling arguments to show how the final particle packing fraction of these balls depends on the dynamics of the droplet evaporation, particle size, and number of particles in the system.

Review: monoclinic zirconia, its surface sites and their interaction with carbon monoxide

This review concerns monoclinic zirconia, its surface sites and their probing with carbon monoxide. The modifications of the surface sites using thermal treatments with vacuum or reactive gases are also included. In this work, we present information on the nature and manipulation of hydroxyl species and their quantities on the surface, the different types of cationic sites where CO is adsorbed linearly and their energetics, as well as the surface sites and dynamics of formate formation. We also compare the surface concentrations of the different surface species to better understand the extent and nature of the interactions. Finally, we discuss some of the remaining open questions and how to approach them.

Oxidative conversion of light alkanes to olefins over alkali promoted oxide catalysts

Alkali promoted mixed oxides were studied as catalysts for the oxidative dehydrogenation (ODH) and cracking of butane and propane. Olefin yields as high as 50% were obtained with Li/MgO-based catalysts. Magnesia-based catalysts showed higher activity for olefin production than catalysts based on zirconia and niobia. Addition of Li to magnesia increases reaction rate normalized to the specific surface area about seven times and selectivity to olefins from 40 to 70%. Li is, therefore, an essential ingredient of the catalyst in order to create the active site. Cl-containing catalysts exhibit slightly higher olefin selectivity, but chloride-free catalysts show superior stability with time on stream. Alkanes show higher conversion rates than alkenes and this surprising result explains the high selectivity to olefins. It is suggested that Li+O? defect sites are the active site for activation of the alkane via hydrogen abstraction. Production of olefins via this oxidative dehydrogenation/cracking route may be an attractive alternative for steam-cracking.

The influence of water and pH on adsorption and oxidation of CO on Pd/Al2O3—an investigation by attenuated total reflection infrared spectroscopy

Adsorption and oxidation of carbon monoxide over a Pd/Al2O3 catalyst layer was investigated both in gas phase and water. Both adsorption and oxidation of CO are significantly affected by the presence of liquid water. Water influences the potential of the metal particles as well as the dipole moment of the adsorbed CO molecule directly, which is reflected both in large red shifts and a higher infrared intensity when experiments are carried out in water. Furthermore, the rate of CO oxidation increases significantly by both the presence of water and by increasing the pH. Enhancement of the oxidation rate is attributed to a weakening of the CO bond by increasing potential of the metal particle, similar to CO oxidation over Pt/Al2O3 as recently published [S. D. Ebbesen et al., J. Catal., 2007, 246, 66]. However, on Pd/Al2O3 the oxidation of palladium is clearly promoted at increasing pH, further enhancing the oxidation of CO over Pd/Al2O3.

Presence of Lithium Ions in MgO Lattice: Surface Characterization by Infrared Spectroscopy and Reactivity towards Oxidative Conversion of Propane

The surface morphology of Li-promoted MgO catalysts prepared using the sol-gel method (sg) and wet impregnation procedure (imp), respectively, has been studied by low-temperature infrared spectroscopy of adsorbed CO molecules. The results show that step sites, as unselective catalytic centers, are the major features existing on the surface of pure MgO, and those are active toward the oxidative conversion of propane. However, the concentration of these sites is drastically reduced by the incorporation of lithium ions in the MgO lattice. In fact, the incorporated Li (+) ions tend to move into the surface region and occupy sites associated with lower coordination number (e.g., step sites). Li/MgO-sg catalysts are characterized by a higher concentration of incorporation of lithium compared to Li/MgO-imp. In the case of oxidative dehydrogenation/cracking of propane, Li/MgO-sg catalysts show higher activity and selectivity to olefins compared to materials prepared using wet impregnation. Catalytic performance differs strongly regarding (i) the amount of olefins formed, and (ii) the ratio of C(3)H(6)/C(2)H(4). It is shown that high density of active sites is essential for further oxidative dehydrogenation of propyl radicals to propylene and suppression of cracking reactions pathway.