P.E. de Jongh

Driving Force for Electron Transport in Porous Nanostructured Photoelectrodes

Electron transport in a photoelectrode consisting of a porous nanostructured semiconducting or insulating network interpenetrated with an electrolyte solution is considered. Electrons, photogenerated in the solid network by light incident from the electrolyte side, travel through the system and are removed at the substrate/network boundary. We calculate the driving force for electron diffusion through the network from first principles. It is found that the driving force is in the order of kT/e divided by the thickness of the network and independent of the light intensity. Photoinduced interfacial charging of the network due to electron trapping can enhance the driving force but does not change the transport characteristics.

The role of Ni in increasing the reversibility of the hydrogen release from nanoconfined LiBH4

Nanoconfinement and the use of catalysts are promising strategies to enhance the reversibility of hydrogen storage in light metal hydrides. We combined nanoconfinement of LiBH4 in nanoporous carbon with the addition of Ni. Samples were prepared by deposition of 5-6 nm Ni nanoparticles inside the porous carbon, followed by melt infiltration with LiBH4. The Ni addition has only a slight influence on the LiBH4 hydrogen desorption, but significantly enhances the subsequent uptake of hydrogen under mild conditions. Reversible, but limited, intercalation of Li is observed during hydrogen cycling. X-ray diffraction shows that the initial crystalline 5-6 nm Ni nanoparticles are not present anymore after melt infiltration with LiBH4. However, transmission electron microscopy showed Ni-containing nanoparticles in the samples. Extended X-ray absorption fine structure spectroscopy proved the presence of Ni(x)B phases with the Ni-B coordination numbers changing reversibly with dehydrogenation and rehydrogenation of the sample. Ni(x)B can act as a hydrogenation catalyst, but solid-state 11B NMR proved that the addition of Ni also enhanced the reversibility of the system by influencing the microstructure of the nanoconfined LiBH4 upon cycling.

Heterogeneities of the Nanostructure of Platinum/Zeolite Y Catalysts Revealed by Electron Tomography

To develop structure-performance relationships for important catalysts, a detailed characterization of their morphology is essential. Using electron tomography, we determined in three dimensions the structure of Pt/zeolite Y bifunctional catalysts. Optimum experimental conditions enabled for the first time high-resolution 3D imaging of Pt particles as small as 1 nm located inside zeolite micropores. Semiautomated image analysis of 3D reconstructions provided an efficient study of numbers, size distributions, and interparticle distances of thousands of Pt particles within individual zeolite crystals. Upon extending this approach to a number of zeolite crystals of one batch of Pt/zeolite Y catalyst, heterogeneities were revealed. The Pt loading, an important parameter for catalyst performance, varied between zeolite crystals up to a factor of 35. This discovery calls for re-evaluation of catalyst preparation methods and suggests potential for lowering the nominal loading with noble metals.

Nanoconfinement of Mg6Pd particles in porous carbon: size effects on structural and hydrogenation properties

Mg6Pd nanoparticles as small as 4 nm have been synthesized inside the pores of porous carbon. They are formed by infiltration of Mg on previously formed Pd nanoparticles dispersed into carbon. Their crystalline structure, as evaluated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), differs from bulk Mg6Pd since their particle size is close to the large crystal cell (∼2 nm) of this intermetallic compound. Indeed, as compared to bulk Mg6Pd, the nanoparticles exhibit a simpler crystallographic arrangement and a higher atomic disorder. Both thermodynamic and kinetic H-sorption properties of Mg6Pd nanoparticles differ from those of bulk Mg6Pd. The H-kinetics of the Mg6Pd nanoparticles are significantly faster than bulk and are stable for at least 10 sorption cycles. Thermodynamic destabilization of the hydrided state is also observed for Mg6Pd nanoparticles. Changes in the hydrogenation properties are attributed to nanosizing as well as to the modified structure of the nanoparticles as compared to bulk Mg6Pd.

Benzene adsorption and desorption in mordenite

Abstract The zeolite-catalyzed synthesis of cumene from benzene and propene is an industrially important reaction. We used small mordenite crystals to study benzene adsorption and desorption behaviour for sodium, proton and nitric acid treated mordenite. Adsorption of benzene was for all samples fast and completed within 25 seconds at a benzene partial pressure of 0.12 bar in nitrogen at 423 K. The largest benzene uptake was found for the acid treated mordenite ∼4.5 wt.% followed by the sodium mordenite ∼4.0 wt.% and the proton mordenite ∼3.5wt.%. Lower uptake for the proton mordenite could be explained by the presence of minor blockades formed during the ion-exchange and calcination process. The higher uptake for the acid treated mordenite was explained by the partial removal of these pore blockades and more efficient stacking of benzene molecules due to the absence of cations. Desorption rates were very different for the three samples; with Na-MOR 60% of the benzene desorbed in ∼24 hours, H-MOR in ∼1hour and for the acid treated mordenite within 10 minutes. Benzene adsorption isotherms were measured for proton and acid treated mordenite. A simple Langmuir model fit yielded a maximal benzene loading for proton mordenite of 3.0-3.4 wt.%, with isosteric heats of adsorption between 43 and 52 (±5 kJ/mol). For the acid treated mordenite a maximum loading of 5.5 wt.% was found at 423 K, and slightly lower heats of adsorption. The origin of the marked differences in desorption rate is not clear, as it cannot be related to large differences in adsorption strength. However, it is clear that post-synthesis modification is a strong tool to influence the desorption and diffusion behaviour in this system.