Magnetic separations at very low magnetic field gradients (<100 tesla per meter) can now be applied to diverse problems, such as point-of-use water purification and the simultaneous separation of complex mixtures. High–surface area and monodisperse magnetite (Fe ₃ O ₄ ) nanocrystals (NCs) were shown to respond to low fields in a size-dependent fashion. The particles apparently do not act independently in the separation but rather reversibly aggregate through the resulting high-field gradients present at their surfaces. Using the high specific surface area of Fe ₃ O ₄ NCs that were 12 nanometers in diameter, we reduced the mass of waste associated with arsenic removal from water by orders of magnitude. Additionally, the size dependence of magnetic separation permitted mixtures of 4- and 12-nanometer–sized Fe ₃ O ₄ NCs to be separated by the application of different magnetic fields.

Overcoming surface defects: Dry reforming of methane with carbon dioxide creates a mixture of hydrogen and carbon monoxide—synthesis gas—which can be converted into liquid fuels. However, heterogeneous catalysts for this reaction are prone to deactivation through unwanted carbon deposition (coking) and loss of surface area of adsorbed metal nanoparticles through agglomeration (sintering). Y. Song et al. used highly crystalline fumed magnesium oxide to support molybdenumdoped nickel nanoparticle catalysts (see the Perspective by Chen and Xu). On heating, the nanoparticles migrated on the oxide surface to step edges to form larger, highly stable nanoparticles. This process also passivated sites for coking on the oxide to produce a catalyst with high activity and longevity at 800°C. Science , this issue p. 777 ; see also p. 737

Iron oxide (Fe₃O₄, magnetite) nanocrystals of 6 to 30 nm with narrow size distributions (σ = 5–10%) were prepared by the pyrolysis of iron carboxylate salts.

Post-combustion CO₂ capture and air separation are integral parts of the energy industry, although the available technologies remain inefficient, resulting in costly energy penalties. Here we report azo-bridged, nitrogen-rich, aromatic, water stable, nanoporous covalent organic polymers, which can be synthesized by catalyst-free direct coupling of aromatic nitro and amine moieties under basic conditions. Unlike other porous materials, azo-covalent organic polymers exhibit an unprecedented increase in CO₂/N₂ selectivity with increasing temperature, reaching the highest value (288 at 323 K) reported to date. Here we observe that azo groups reject N₂, thus making the framework N₂-phobic. Monte Carlo simulations suggest that the origin of the N₂ phobicity of the azo-group is the entropic loss of N₂ gas molecules upon binding, although the adsorption is enthalpically favourable. Any gas separations that require the efficient exclusion of N₂ gas would do well to employ azo units in the sorbent chemistry.