Electron tailoring tunes catalyst activities

Making better, more effective catalysts with predictable and reliable properties is one of the most challenging interfaces of chemical engineering and pure chemistry.

The emerging techniques of nanotechnology have great promise for designing and building catalyst particles with custom-designed surfaces, and Israel Wachs of Lehigh University in Bethlehem, Pennsylvania, is using molecular engineering to design new catalysts based on vanadium oxide.

Wachs is focusing on supported metal and metal oxide catalysts for his research, as they are, he says, ‘the workhorses of the environmental, energy and petrochemical industries. The surface of these catalysts is the crucial factor in their activity, and is particularly important at the locations where reductions and oxidations (redox reactions) can take place.

Molecular engineering provides a set of tools which can tune the electron density of surfaces, he says. In this case, Wachs chose to use titanium compounds to tailor the electron density of clusters of vanadium oxide on a silica support.

First, Wachs impregnated a porous SiO2 support material with titanium isopropoxide dissolved in isopropanol. The silica’s structure prevents the titanium complexes agglomerating to form large particles, but instead leads to the formation of nano-clusters of titania on the surface of the particle. He then soaked the particles in a solution of a vanadium salt, dried the particles off, and calcined them under a stream of air at 500°C.

The team examined the resulting particles using a variety of spectroscopic and microscopic techniques to characterise the structures on the surface, then further investigated using a technique called methanol temperature-programmed surface reaction spectroscopy.

This detects the formation of the reaction product of formaldehyde and oxygen on surface sites. Spectroscopy confirmed that ‘nano-domains’ of vanadium oxide, rather than crystalline V2O5, formed on the surface, while the TPSR showed that the more TiO2 was present at a site, the lower the temperature needed to be to desorb formaldehyde from the surface — this, Wachs says, indicates that these high-titania sites are more catalytically active.

The results showed that the molecular engineering techniques tuned both the catalytic activity and the redox selectivity of the particles, the first time that this result has been confirmed, Wachs says.

The result is a valuable step in the effort to build tailored catalysts for specific reactions, he says, particularly for reactions where the size of the active site is crucial. Further research will look at the effect of adding different ligands to the metals on the catalyst surface.