The activity of Cr2+ and Cr3+ ions dispersed in magnesium oxide for the hydrogen-deuterium equilibration reaction

Catalysts containing known amounts of Cr2+ and Cr3+ ions dispersed in MgO were tested to examine the role of the unusual valency state Cr2+. The catalysts were made by heating MgO impregnated with chromium nitrate in hydrogen (MC-h, Cr2+ produced) or in air (MC-a, only Cr3+ produced). Catalysts containing Cr2+ and outgassed above 870 K are active and show two distinct ranges: 78 to 200 K (Esub(a) = 4.2 kJ mol-1) and 200 to 500 K (Esub(a) = 12.6 kJ mol-1). In the low-temperature range (LTR) the activity is proportional to the Cr2+ content. Catalysts which do not contain Cr2+ are not active in the LTR. Attempts to hydrogenate ethylene on MC-h and MC-a samples failed, in both temperature ranges. It is shown that there is no special requirement for a pair of Cr2+ ions in the active centre. A molecular form of hydrogen is thought to be active in the LTR. Other aspects discussed include the role of the matrix. (author)