Mesoporous Ta oxide reduced with bis(toluene)Ti: electronic properties and mechanistic considerations of nitrogen cleavage on the low valent surface

Abstract

Mesoporous Ta oxide with a pore size of ca. 22 Å and a BET surface area of 486 m² g⁻¹ was reduced by bis(toluene)Ti in the presence of dinitrogen and the product characterized by XPS, XRD, elemental analysis, nitrogen absorption, and variable temperature conductivity measurements. The XPS spectrum shows three different Ti species on the surface, in addition to a strong emission for a reduced nitrogen species and ammonia. There is also evidence for slight reduction of the Ta oxide mesostructure, consistent with the framework acting as an oxidizing agent. The conductivity measurements show a gradual increase in resistance with decreasing temperature, consistent with a semiconductor. In general the nitrogen content ranged from 1.5 to 3%. In order to gain insight into the nature of the cleavage of nitrogen on the reduced surface, the materials were prepared in the absence of nitrogen and then treated with nitrogen at room temperature and pressure for varying lengths of time. Surprisingly, these materials absorbed a maximum of 0.8% N and were almost insulating. This indicates that the initial Ti phase formed on the surface of the mesoporous Ta oxide is highly reactive and that it rapidly quenches itself, most likely through oxidation by the hydrated Ta oxide beneath the surface, before it has a chance to react in a second step with nitrogen. Maximum absorption of nitrogen occurred within the first 2 h, indicating that this reaction is fast. Treatment with excess moisture led to complete conversion of the nitride to ammonia in minutes, indicating that access to moisture is the rate-limiting step of the surface protonation process.