Composites derived from exfoliated Laponite and Mn-Al hydrotalcite prepared in inverse microemulsion: A new strategy for design of robust VOCs combustion catalysts

A novel strategy of designing combustion catalysts is proposed, in which catalytically active oxide nanoparticles, obtained by inverse microemulsion method, are trapped between randomly oriented clay layers. MnAl hydrotalcite-like compound was used as the precursor of the active phase, while synthetic smectite Laponite RD, either in sodium form (Na-L) or as an organoclay, exchanged with cetyltrimethylammonium cations (CTA-L), served as the clay component. The composite catalysts were characterized with XRF, XRD, SEM, TEM/HRTEM, TG/DSC, H2 TPR, XPS, and N2 adsorption/desorption at −196 °C. It was found that the degree of clay exfoliation was higher in materials obtained from CTA-L than in the catalysts prepared from Na-L. In organo-Laponite based composites the MnOx particles were smaller, more uniform, and better dispersed. Upon calcination they evolved towards well ordered, slightly oxygen-rich Mn3O4 nanocrystals, while in Na-L derived catalysts the MnOx grains were structurally less ordered and size-wise more diversified, of average oxidation state significantly higher than that characteristic of Mn3O4. All synthesized materials were highly active in combustion of toluene. Performance of composites based on Na-L was more susceptible to thermal degradation, while the organoclay-based catalysts improved their performance on the increase of the calcination temperature, showing eventually superior activity. The observed dependencies are discussed in terms of structure-composition-performance relationship. It is argued that, on one hand, the use of organoclay facilitates dispersion of the precursor micelles between the clay layers and prevents the occurrence of active phase coalescence, on the other, provides interlayer fuel for the additional heat input during calcination, enabling formation of well ordered, uniform, Mn3O4 nanocrystals with oxygen-rich surface of unique redox and catalytic properties. The obtained results demonstrate the potential of using organoclays as supports for the active oxide phase prepared from inverse micellar precursors.