Nanostructured transition metal silicides

Abstract

Applicability of different single-source precursors for the formation of transition metal silicide nanoparticles in an amorphous matrix and their application in hydrodesulphurisation of dibenzothiophene are investigated.<br />Addition of excess hexamethylphosphoric triamide (HMPA) to cis-Fe(CO)4(SiCl3)2 leads to the formation of the new complex [SiCl3(HMPA)3]+[Fe(CO)4SiCl3]-. Thermolysis of cis-Fe(CO)4(SiCl3)2 results in the formation of the new compound Fe2(CO)6(µ2-SiCl2)3.<br />Density functional theory (DFT) calculations are carried out for both reactions to elucidate possible reaction steps. 29Si NMR chemical shifts of 41 silicon-containing compounds (14 tetra-, 10 penta- and 17 hexacoordinated) are calculated employing DFT, using the gauge including atomic orbital (GIAO) formalism, and compared to experimental values. The hybrid B3LYP functional, the generalised gradient approximation (GGA) functional BPW91 and the meta-GGA functional HCTH407 are used in conjunction with the 6-31G(d,p) and the 6-311+G(2d) basis set and a locally dense basis set (6-311G+(2d) on Si, 6-31G(d,p)on all other atoms). The HCTH407/6-311+G(2d) level of theory consistently performs best in this study. Evidence is provided that thermal decomposition of Co(CO)4SiCl3 adsorbed on silica in a hydrogen atmosphere results in the formation of metallic cobalt nanoparticles covered with a Co2SiO4/CoO shell instead of cobalt silicide nanoparticles. The application of other supporting materials such as graphite or charcoal leads to cobalt nanoparticles. The attempt to synthesise cobalt nanoparticles in silica by a sol-gel process results in Co2SiO4 and CoO. Thermolysis of linear poly(ferrocenylsilane) at 1000°C and poly[dimethylsilyl)ferrocenyldiacetylene] at 850°C results in soft magnetic nanoparticles, well distributed in a SiC/C matrix. Both the average size of the formed nanoparticles (between 10 and 100 nm)and the size distribution depend on the choice of polymer and the thermolysis conditions. The magnetic properties of the nanoparticles are analysed as a function of their size by SQUID-measurements. The smallest Fe3Si nanoparticles (10-30 nm) are single magnetic domain particles. Further thermolysis experiments with poly[(dimethylsilylene)diacetylene] to which iron and cobalt carbonyl moieties are coordinated, and a mixture of Mn2(CO)10 and poly[(dimethylsilylene)diacetylene] result in Fe3Si, Co2Si/Co and Mn5Si3 nanoparticles, respectively, which are also well-distributed in a SiC/C matrix. SQUID measurements of Co2Si/Co nanocomposite reveal ferromagnetism in a temperature range of 3-300 K. Hydrodesulphurisation of dibenzothiophene using the synthesised nanocomposites as catalysts are performed. Kinetic studies show that the sol-gel process derived Co2SiO4/CoO performs best and all catalysts favour a direct desulphurisation route. Tetraethylsilane treatment of different metals supported on alumina (cobalt, nickel,manganese or molybdenum) at 400°C is performed. This preparation route turns the inactive molybdenum-containing material into an active catalyst towards ethylene hydrogenation. Further hydrodesulphurisation tests with dibenzothiophene reveal an improvement of the conversion by factors of 1.2-2 as compared to the untreated materials.