The catalysis of H2—D2exchange by oxides

Abstract

H$_{2}$$\chembond{1,0} $D$_{2}$ exchange has been investigated at the surfaces of 13 oxides of the first transition period, at temperatures between - 195 and 20 degrees C and pressures between 1 and 5 cm. There appears to be a simple correlation between the catalytic activity per unit surface area and the electronic configuration of the metal ion. The condition for high activity is that the oxide should possess some but not too many unpaired d-electrons. Thus, high activity occurs just after the beginning of the period (Cr$_{2}$O$_{3}$) and towards the end (Co$_{3}$O$_{4}$ and NiO). If there are no unpaired d-electrons (TiO$_{2}$, V$_{2}$O$_{5}$, Cu$_{2}$O, ZnO, Ga$_{2}$O$_{3}$, GeO$_{2}$) or very few (V$_{2}$O$_{3}$, CuO) or very many (MnO, Fe$_{2}$O$_{3}$) the activity is low. A tentative explanation of the results is offered, mainly in terms of the influence of electronic configuration on the velocity of hydrogen chemisorption. The results suggest that hydrogen behaves similarly at metal and at oxide surfaces, giving catalysis by these solids a common motif. There is no simple correlation with oxides between exchange activity and semi-conductivity. Pressure dependences of the exchange velocity have been measured for Cr$_{2}$O$_{3}$, NiO, ZnO and Ga$_{2}$O$_{3}$; these throw some light on the question of the exchange mechanism.