Synthesis and characterization of cyclotriphosphazenes containing silicon as single solid-state precursors for the formation of silicon/phosphorus nanostructured materials

The synthesis and characterization of new organosilicon derivatives of N3P3Cl6, N3P3[NH(CH2)3Si(OEt)3]6 (1), N3P3[NH(CH2)3Si(OEt)3]3[NCH3(CH2)3CN]3 (2), and N3P3[NH(CH2)3Si(OEt)3]3[HOC6H4(CH2)CN]3 (3) are reported. Pyrolysis of 1, 2, and 3 in air and at several temperatures results in nanostructured materials whose composition and morphology depend on the temperature of pyrolysis and the substituents of the phosphazenes ring. The products stem from the reaction of SiO2 with P2O5, leading to either crystalline Si5(PO4)6O, SiP2O7 or an amorphous phase as the glass Si5(PO4)6O/3SiO2·2P2O5, depending on the temperature and nature of the trimer precursors. From 1 at 800 °C, core−shell microspheres of SiO2 coated with Si5(PO4)6O are obtained, while in other cases, mesoporous or dense structures are observed. Atomic force microscopy examination after deposition of the materials on monocrystalline silicon wafers evidences morphology strongly dependent on the precursors. Isolated islands of size ∼9 nm are observed from 1, whereas dense nanostructures with a mean height of 13 nm are formed from 3. Brunauer−Emmett−Teller measurements show mesoporous materials with low surface areas. The proposed growth mechanism involves the formation of cross-linking structures and of vacancies by carbonization of the organic matter, where the silicon compounds nucleate. Thus, for the first time, unique silicon nanostructured materials are obtained from cyclic phosphazenes containing silicon.

Dissociative adsorption of methane on the Cu and Zn doped (111) surface of CeO2

The development of economical heterogeneous catalysts for the activation of methane is a major challenge for the chemical industry. Screening potential candidates becomes more feasible using rational catalyst design to understand the activity of potential catalysts for CH4 activation. The focus of the present paper is the use of density functional theory to examine and elucidate the properties of doped CeO2. We dope with Cu and Zn transition metals having variable oxidation state (Cu), and a single oxidation state (Zn), and study the activation of methane. Zn is a divalent dopant and Cu can have a +1 or +2 oxidation state. Both Cu and Zn dopants have an oxidation state of +2 after incorporation into the CeO2 (111) surface; however a Hubbard +U correction (+U = 7) on the Cu 3d states is required to maintain this oxidation state when the surface interacts with adsorbed species. Dissociation of methane is found to occur locally at the dopant cations, and is thermodynamically and kinetically more favorable on Zn-doped CeO2 than Cu-doped CeO2. The origins of this lie with the Zn(II) dopant moving towards a square pyramidal geometry in the sub surface layer which facilitates the formation of two-coordinated surface oxygen atoms, that are more beneficial for methane activation on a reducible oxide surface. These findings can aid in rational experimental catalyst design for further exploration in methane activation processes.