Dr Jonathan Bartley - BSc (Hons) MSc PhD CChem MRSC
The methodology for preparing mixed metal oxide catalysts has changed little over the last 60 years. Typically metal nitrate solutions are co-precipitated using a base to yield precursors that are then calcined to form the oxide catalysts. Due to the crude preparation methodology, catalysts prepared in this way are a complex mixture of mixed oxide and single oxide phases. This leads to a waste of the active metals which can be present either as inactive phases or as unselective phases which reduce the activity and selectivity of the final catalyst.
We are interested in exploring new methods for synthesizing metal oxides and mixed metal oxides for use as catalysts and supports that will give improved catalyst performance and have developed a number of methodologies for preparing catalysts such as: supercritical antisolvent precipitation, the use of structure directing agents, high temperature high pressure synthesis and the use of microemulsions to prepare unsupported metal nanoparticle catalysts.
Hopcalite is a copper manganese oxide that is used for low temperature CO oxidation. The traditional co-precipitated catalyst contains the mixed metal oxide active catalyst but also copper oxide and manganese oxide. This can be seen from chemical analysis of the material using scanning transmission electron microscopy (STEM) X-ray energy-dispersive spectrometry (EDS) (Fig. 1).
Fig. 1 - Microscopy studies of the materials prepared by using the standard coprecipitation method, showing the phase segregation: a) transition electron micrograph (TEM) and b) high-resolution EDS maps from STEM
We have developed an alternative method for preparing catalysts using supercritical antisolvent precipitation. Copper and manganese acetates are dissolved in DMSO and pumped into a vessel containing supercritical (sc) CO2. The solvent and scCO2 diffuse into each other, causing the DMSO to expand, reducing its solvent power and the acetates are precipitated. This fast precipitation results in a homogeneous distribution of the components in the material (Fig. 2) that after calcination gives phase pure copper manganese oxide which has improved performance over the co-precipitated catalyst that also contains the single oxide phases.
Fig. 2 - Microscopy studies of the CuMnO materials prepared by supercritical antisolvent precipitation, showing the phase segregation: a) transition electron micrograph (TEM) and b) high-resolution EDS maps from STEM
Vanadium phosphate catalysts are used commercially for the selective oxidation of butane to maleic anhydride. The precursor (VOHPO4•0.5H2O) is prepared by reacting vanadium V oxide (V2O5) and phosphoric acid (H3PO4) in the presence of an alcohol which acts as a reducing agent and the solvent. By adding small amounts of 2-poly(styrene-alt-maleic acid) (PSMA) into the preparation the crystallinity of the precursors can be increased. This leads to very regular rhomboidal crystals, rather than the lozenge shaped crystals obtained from standard preparations (Fig. 3). This increase in crystallinity enables the activation of the precursor to the final catalyst to occur much quicker. The surface area of the catalyst is also increased as the addition of PSMA leads to thinner plates being formed.
Fig. 3 - TEM of precursors prepared with: a) no PSMA; b) PSMA to V2O5 weight ratio of 1:260; c) PSMA:V2O5 of 1:65