Promoter effect of niobia on Pt/Al 2O 3 catalysts Part I. Methanol—deuterium exchange on samples containing 5% Nb 2O 5

Abstract

5% Nb 2O 5-modified Pt/Al 2O 3 with two metal loadings (0.55 and 10 wt.% Pt) have been investigated in the methane/D 2, methanol/D 2 reactions and in the formation of dimethyl ether as a function of various oxidation and hydrogenation heat treatments. As was established earlier [26], the Pt dispersion changes according to the heat treatment, being higher after high temperature oxidation and lower after high temperature reduction in hydrogen. The turnover frequency of methane exchange is not affected by the various treatments, whereas methanol exchange and dimethyl ether formation are influenced to a large extent. The data can be interpreted by the partial reduction of Nb 2O 5 during hydrogenation, followed by NbO x migration onto the platinum particles. Thus, the methyl groups in the methoxide species attached to the Lewis sites on the alumina can interact with the platinum surface for multiple exchange. The decrease in the activity after successive reactions may be attributed to the further spread of the NbO χ species, thereby creating more acidic sites, which increases the rate of DME formation.