Surface Defect Sites Formed on Partially and Fully Dehydrated MgO: An EPR/ENDOR Study Damien M. Murphy,* Robert D. Farley, Ian J. Purnell, Christopher C. Rowlands, and Abdul R. Yacob † National EPSRC ENDOR Centre, Department of Chemistry, UniVersity of Wales Cardiff, Cardiff CF1 3TB, United Kingdom Maria Cristina Paganini and Elio Giamello Dipartimento di Chimica IFM, UniVersita di Torino, Via P. Giuria 9, 10125 Torino, Italy ReceiVed: October 22, 1998; In Final Form: January 12, 1999 EPR and ENDOR spectroscopy have been used to investigate a variety of trapped electron centers on the surface of polycrystalline MgO. The oxide was dehydrated under vacuum at different temperatures (673- 1123 K) and UV irradiated under H 2 . The dehydration process results in the formation of surface anion vacancies, which subsequently act as excess electron traps (forming color centers). A variety of such color centers have been identified. At high activation temperatures (1123 K), surface F S + (H) color centers (type I) are formed, which have been assigned to an electron trapped by a specific anion vacancy. At slightly lower activation temperatures, a second F S + (H) color center (type II) predominates; this center has been assigned to an electron trapped in a higher coordinated surface vacancy. However, at low activation temperatures such that the oxide surface remains partially hydrated, different types of color centers are present. It is proposed that these centers arise from electron trapping at a surface cation-anion vacancy pair (tentatively assigned as P S - centers). The mechanism by which the latter center is effectively reduced by a single electron is unclear. The distribution and abundance of these different trapped electron centers varies as a function of the dehydration temperature. The results show that the surface of polycrystalline MgO containing a variety of point defects changes dramatically depending on the pretreatment conditions. Introduction Pure and doped polycrystalline MgO powders are good catalysts for many chemical reactions. 1 Nevertheless, knowledge about the actual catalytic sites or surface defects responsible for the reactivity is often imprecise and poorly understood. Although the surface properties of magnesium oxide have been studied in great depth both experimentally and theoretically, a great many questions remain unanswered. One such question concerns the collective problem associated with the nature, location, and abundance of the surface anion vacancies and the processes by which they are formed during surface dehydration. Significant theoretical advances using atomistic simulations, quantum chemical calculations, and ab initio calculations have recently been made in an attempt to understand the nature of the irregular and defective faces of oxides and the molecular processes occurring at these defects. 2-13 Anion and cation vacancies on fully dehydrated MgO surfaces have received considerable attention of late. In particular, a variety of point defects such as oxygen vacancies (F centers), magnesium vacancies (V centers), simple aggregates of defects (the diva- cancy or P center), and in some cases migration of these defects have been simulated both in the bulk 5 and at the surface of MgO. 6-11 While many of these studies have provided valuable insights into the geometric structure and electronic properties of the vacancies themselves on the dehydrated surfaces, the dynamics by which these defects form under real conditions has not been treated in depth. Many studies have been performed on the adsorption of water on oxides and on the nature of the hydrated MgO surface, 12-15 and the results clearly show a strong preference for hydroxylation at the low coordination sites. It has been shown from experiments that thermal dehydration is not a simple reversal of ambient temperature hydration, particularly during the last stages of dehydration where some surface reconstruction occurs. 16 Nevertheless, to the best of our knowledge, there is no clear theoretical study on the relationship between the state of surface dehydroxylation and the creation of surface defects. From an experimental viewpoint, it is well-known that the pretreatment conditions, such as the temperature of calcination and the extent of surface dehydration, significantly affect the surface morphology and defectivity of the oxide. 17-19 Significant attention has therefore been given to the role of the hydroxyl groups in the mechanism of surface hydration and dehydration, since these processes are ultimately responsible for the creation of surface sites active in the adsorption and catalytic behavior of MgO. 19-21 Dehydroxylation of the polycrystalline MgO surface was followed by TPD and specific peaks ascribed to energetically different forms of adsorbed water and OH groups which were removed only at elevated temperatures. 16 Coluccia et al. 22-24 used IR and photoluminesence spectroscopy to follow * Author for correspondence. † Permanent address: Universiti Teknologi Malaysia, Karung Berkunci 791, 80990 Johor Bahru, Negeri Johor Darul Ta’zim, Malaysa. 1944 J. Phys. Chem. B 1999, 103, 1944-1953 10.1021/jp984132e CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999