Particle Shape as Well as Size Affects Catalysis for Desulfurization
06 February 2007
 |
| Multi-walled MoS2 nano-octahedron. Click to enlarge. Source: A. Enyashin, TU Dresden |
German and Israeli researchers have shown for the first time that the shape, as well as the size, of molybdenum disulfide (MoS2) used as a catalyst for producing sulfur-free fuels effects the catalytic potential of the material. The results are reported in Angewandte Chemie and Nature Nanotechnology.
Very small, sulfur-rich MoS2 nanoplatelets are well known as active catalysts for the desulfurization of fuels. Recent research has shown that the catalytic potential increases dramatically with decreasing particle size.
This effect has been correlated with the specific structure along the edges of triangular nanoplatelets. In contrast to the semi-conducting bulk, the edges of the MoS2 nanoplatelets are electronically conducting and this is where sulfur-containing impurities in the fuel are decomposed.
The researchers from the Technische Universität Dresden, and the Forschungszentrum Dresden-Rossendorf, both in Germany, and at the Weizmann Institute in Rehovot, Israel, examined the properties of larger MoS2 particles, the structure of which offers many long and well-accessible edges.
They found that larger, regular three-dimensional particles offer a desulfurization potential that is similar to that of the nanoplatelets. The larger particles have an octahedral form that is similar to a bipyramid and require less effort in their production than the nanoplatelets that are synthesized directly on a gold support and cover it like a nanoconfetti.
A joint theoretical and experimental investigation correlated the particle size and shape to the structural and electronic properties that are responsible for the catalytic activity of MoS2 nanoparticles.
MoS2 nanoparticles larger than 10 nanometers are semi-conducting—like the bulk material. In contrast, within a diameter range of 3 to 7 nanometers, three-dimensional structures occur that are composed of eight equilateral triangles.
For the edges and corners of such nano-octahedra the quantum-mechanical calculations of the researchers from Dresden predict similar metallic properties to those found in the even smaller catalytically active nanoplatelets. According to the model calculations, single-walled nano-octahedra with a few hundred atoms are not stable. However, the observed multi-walled particles of nested octahedra are predicted to be more stable species which promise similar catalytic potential to the smaller nanoplatelets.
Danish researchers, reporting in the same issue of Nature Nanotechnology, have systematically mapped and classified the atomic-scale structure of triangular MoS2 nanocrystals as a function of size.
Instead of a smooth variation as expected from the bulk structure of MoS2, they observed a very strong size dependence for the cluster morphology and electronic structure driven by the tendency to optimize the sulfur excess present at the cluster edges.
Analysis of the atomic-scale structure of clusters identified the origin of the structural transitions occurring at unique cluster sizes. Their findings suggest that good size control during the synthesis of MoS2 nanostructures may be used to produce optimized catalytic MoS2 nanomaterials.
Resources:
“Nanocrystals: Catalysts on the edge”; Sibylle Gemming, Gotthard Seifert; Nature Nanotechnology 2, 21 - 22 (01 Jan 2007)
“Size-dependent structure of MoS2 nanocrystals”; Jeppe V. Lauritsen, Jakob Kibsgaard, Stig Helveg, Henrik Topsøe, Bjerne S. Clausen, Erik Lægsgaard and Flemming Besenbacher; Nature Nanotechnology 2, 53 - 58 (2007) doi:10.1038/nnano.2006.171
“Structure and Stability of Molybdenum Sulfide Fullerenes”; Andrey N. Enyashin, Dr., Sibylle Gemming, Dr., Maya Bar-Sadan, Ronit Popovitz-Biro, Dr., Sung You Hong, Yehiam Prior, Prof. Dr. 5, Reshef Tenne, Prof. Dr., Gotthard Seifert, Prof. Dr.; Angewandte Chemie International Edition Volume 46, Issue 4, Pages 623 - 627
Comments