Nitrous oxide decomposition and low temperature parahydrogen enrichment on pure and doped rutile

Abstract

Doping with 0.1 mol % Cr₂O₃, Fe₂O₃, NiO, Nb₂O₅ or WO₃ has little effect on the catalytic properties of rutile for the decomposition of nitrous oxide in the temperature range 450–590°C. For all six catalysts, first-order kinetics with respect to initial nitrous oxide pressure were obeyed. Relatively small differences in the activation energies were observed, but an apparent compensation effect resulted in the absolute rates at 550°C and 1.33 × 10⁴ N m^–2 lying between 1.2 and 2.2 × 10¹² molecules cm^–2 s^–1.

Parahydrogen enrichment has been investigated at 77 and 90 K over the pressure range 1.33 × 10² to 2.66 × 10⁴ N m^–2 on the six catalysts used in nitrous oxide decomposition. The reaction proceeds by the paramagnetic vibrational mechanism on a small active fraction of the oxide surface. Results obtained for other samples of doped rutile, and analytical measurements on progressively dissolved material, show the existence of a surface excess concentration of dopent both at a nominal concentration of 0.1 mol % and at 100 p.p.m. Inhomogeneity in doped titanium dioxide, which has not been reported previously, is of importance in the catalytic properties of such material.