Abstract
Bearing in mind the need to develop optimal transition metal oxide-based catalysts for the combustion of volatile organic compounds (VOCs), yolk-shell materials were proposed. The constructed composites contained catalytically active Co3O4 nanoparticles, protected against aggregation and highly dispersed in a shell made of porous SiO2, forming a specific type of nanoreactor. The bottom-up synthesis started with obtaining spherical poly(styrene-co-acrylic acid) copolymer (PS30) cores, which were then covered with the SiO2 layer. The Co3O4 active phase was deposited by impregnation using the PS30@SiO2 composite as well as hollow SiO2 spheres with the removed copolymer core. Structure (XRD), morphology (SEM), chemical composition (XRF), state of the active phase (UV-Vis-DR and XPS) and reducibility (H2-TPR) of the obtained catalysts were studied. It was proven that the introduction of Co3O4 nanoparticles into the empty SiO2 spheres resulted in their loose distribution, which facilitated the access of reagents to active sites and, on the other hand, promoted the involvement of lattice oxygen in the catalytic process. As a result, the catalysts obtained in this way showed a very high activity in the combustion of toluene, which significantly exceeded that achieved over a standard silica gel supported Co3O4 catalyst.
Highlights
Considering the necessity to implement efficient methods for the elimination of volatile organic compounds (VOCs) from contaminated air, active and selective catalysts have been extensively developed, which could allow the lowering of the reaction temperature of the total oxidation of VOCs
The formed PS30 emulsion was examined by dynamic light scattering (DLS), proving the high monodispersity of the particles with diameters within the narrow range of 240–260 nm and the average ζ potential of −42.8 mV measured in deionized water at pH = 7 (Figure 1a)
The introduction of a shell made of porous SiO2 was to play a role in protecting the active phase against aggregation, which ensures a high dispersion of active sites and creates a nanospace for the reactants involved in the catalytic process
Summary
Considering the necessity to implement efficient methods for the elimination of volatile organic compounds (VOCs) from contaminated air, active and selective catalysts have been extensively developed, which could allow the lowering of the reaction temperature of the total oxidation of VOCs. The catalysts containing noble metals (e.g., Pt, Pd, Au or Ag) supported on various oxides (mainly SiO2 or γ-Al2O3) have an obvious advantage due to their extraordinary activity at low temperatures [1,2]. Co3O4, CuO, MnOx, Fe2O3 and NiO are the most common oxide systems used in the combustion of VOCs. As in the case of noble metals, the involvement of a support has a positive effect on dispersion and thermal stability of the catalytically active phase. The selection of the appropriate material forming the support depends on many factors (including its porosity, chemical activity and strength of interaction with the active phase), but a typical choice is γ-Al2O3, SiO2, TiO2, ZrO2, CeO2 and zeolites [1,5,6]. The effect of improved catalytic activity obtained in such cases is most often related to promoting the mobility of lattice oxygen and facilitating electron transfer [7]
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