Synthesis of highly dispersed cobalt oxide clusters encapsulated within LTA zeolites

Abstract

Small Co3O4 nanoparticles uniformly distributed in size were encapsulated within LTA zeolite crystals in a one-step process through hydrothermal self-assembly of crystalline frameworks around ligated Co2+ precursors. The use of bifunctional ligands containing a chelating bidentate amine functionality and an alkoxysilane moiety prevented the precipitation of Co2+ species as colloidal hydroxides in the highly alkaline synthesis gels, while also allowing the formation of linkages between precursors and the framework during the nucleation and growth of LTA crystals. Oxidative treatments of ligated compounds occluded within zeolite crystals removed ligand residues and formed small Co3O4 nanoparticles visible in transmission electron micrographs. These nanoparticles retained their small size (average diameter 1.5 nm) after oxidative treatment at 620–870 K, a reflection of their stabilization by confinement within zeolite voids. The infrared spectra of adsorbed CO on Co-LTA samples confirmed the absence of Co2+ as exchanged cations or aluminosilicates, indicating the presence of Co oxide clusters, with dynamics and stoichiometry of reduction in H2 corresponding to small Co3O4 clusters. Ethanol oxidation rates on Co-LTA samples, exchanged with K+ or Ca2+ cations to vary the diffusive properties of LTA crystals, indicated that more than 97% of the active surfaces on these Co3O4 clusters resided within zeolite crystals, where ethanol and O2 concentrations depend on the diffusive properties of the LTA framework. The Co3O4 clusters prepared by these methods, in contrast with Co2+ in exchanged or aluminosilicate forms, exhibit reactivity in CO and NO oxidation. Their turnover rates (per exposed Co atom), however, were lower than on bulk Co3O4 powders, because of the combined effects of diffusional constraints imposed by the confining framework and the small size of these clusters, which leads to lower intrinsic reactivities as a result of their more difficult reduction during catalytic redox cycles. These clusters would be attractive in catalytic applications requiring stability against sintering during reaction or regeneration, reactant or product shape selectivity, or protection from contact with large molecules that block active surfaces. Such oxide clusters cannot be formed by sequential ion exchange, detachment by reduction to Co0, and re-oxidation because the extremely high temperatures required for reduction destroy the aluminosilicate frameworks. The synthesis protocols and their mechanistic interpretations described herein represent a conceptual and practical platform for the encapsulation of nanoparticles of base elements within a broad range of confining crystalline environments through one-step hydrothermal self-assembly.