The hydrogen exchange reaction of surface deuteroxyl groups on MgO with H2

Abstract

The hydrogen exchange reaction of surface OD groups on MgO with H2(—OD + H2→—OH + HD) has been investigated in order to understand the relationship between the activity and the local structure of the surface hydroxyl groups by means of i.r. spectroscopy. OD groups on the Mg atom are composed of three types exhibiting different vibrations at 2771, 2765 and 2756 cm–1. These bands correspond to the following local structures on the basis of an electrostatic model: OD—Mg5C(2771 cm–1), OD—Mg4C(2765 cm–1) and OD—Mg3C(2756 cm–1), respectively. In addition to these ‘on top’ OD groups, three different kinds of multiply bridged OD groups are also observed, at 2710, 2680 and 2600 cm–1. Among the OD groups on MgO, OD—Mg3C has the highest activity, with an activation energy of 34 ± 5 kJ mol–1. OD—Mg3C—O3C has a higher activity than OD—Mg3C—O4c, where H2 dissociates at the Mg2+—O2 pairs.