Abstract
Ultrathin layers of oxides deposited on atomically flat metal surfaces have been shown to significantly influence the electronic structure of the underlying metal, which in turn alters the catalytic performance. Upscaling of the specifically designed architectures as required for technical utilization of the effect has yet not been achieved. Here, we apply liquid crystalline phases of fluorohectorite nanosheets to fabricate such architectures in bulk. Synthetic sodium fluorohectorite, a layered silicate, when immersed into water spontaneously and repulsively swells to produce nematic suspensions of individual negatively charged nanosheets separated to more than 60 nm, while retaining parallel orientation. Into these galleries oppositely charged palladium nanoparticles were intercalated whereupon the galleries collapse. Individual and separated Pd nanoparticles were thus captured and sandwiched between nanosheets. As suggested by the model systems, the resulting catalyst performed better in the oxidation of carbon monoxide than the same Pd nanoparticles supported on external surfaces of hectorite or on a conventional Al2O3 support. XPS confirmed a shift of Pd 3d electrons to higher energies upon coverage of Pd nanoparticles with nanosheets to which we attribute the improved catalytic performance. DFT calculations showed increasing positive charge on Pd weakened CO adsorption and this way damped CO poisoning.
Highlights
Many nanoparticulate catalysts are prepared by wet impregnation on an oxidic support
The synthetic clay sodium fluorohectorite, (NaHec, [Na0.5]inter[Mg2.5Li0.5]oct[Si4]tetO10F2) which belongs to a handful of layered compounds that show the long-known[14] but rare phenomenon of osmotic swelling, became available.[15]
It was found that an ultrathin layer of SiO2 deposited on Mo(112) increased the metal work function by 0.5–1 eV due to dipole effects arising from charge transfer from the metal to the oxide.[6b]. To probe such a possible electronic interaction between the hectorite nanosheets and the Pd nanoparticles leading to an additional charge transfer from Pd to silicate support, electron energy loss spectra (EELS) at the Si L2,3 edge were measured (Figure 6)
Summary
Many nanoparticulate catalysts are prepared by wet impregnation on an oxidic support. Charged layered materials such as clays have been explored as supports for nanoparticles and their catalytic performance has been tested to some extent.[10] Taking advantage of the cation exchange capacity, desired cations have been introduced on and/or between the silicate layers followed by reduction (e.g. Pd, Cu, Ru)[11] or precipitation (e.g. CdS)[12] to obtain the final nanoparticulate catalysts.
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