Coadsorption of NO and H 2 at the surface of MgO monitored by EPR spectroscopy. Towards a site specific discrimination of polycrystalline oxide surfaces Paola Martino, Mario Chiesa, Maria Cristina Paganini, Elio Giamello * Dipartimento di Chimica IFM, Universit  a di Torino, e Unit  a INFM di Torino, Via Giuria, 7-10125 Torino, Italy Received 19 April 2002; accepted for publication 17 December 2002 Abstract Coadsorption of hydrogen and nitric oxide at the surface of polycrystalline MgO has been studied by means of electron paramagnetic resonance spectroscopy. 14 NO and 15 NO have been used as probes for monitoring highly ex- posed sites on the surface in particular as it concerns the role played by the same sites in the heterolytic dissociation of the hydrogen molecule. Two forms of dissociated hydrogen on MgO are known, the former one irreversibly bound at the surface in a wide range of temperatures, the latter desorbing H 2 upon lowering the pressure. NO is physisorbed as paramagnetic NO monomers on three distinct types of low-coordinated surface cations while a small fraction (about 2%) of NO forms NO 2 2 species on low-coordinated anions. The irreversible form of H 2 does not alter the NO chemistry as monitored by EPR, whereas, coadsorption of the two gases affects the formation of physisorbed NO monomers in a way which depends on the H 2 –NO ratio. For high H 2 –NO ratios no adsorbed monomeric NO is observed whereas when lowering the H 2 –NO ratio down to 1/10 the two NO sites with higher binding energy are still depressed while the weakest one becomes available. These results allow to identify the sites responsible for NO adsorption in monomeric form as connected to those responsible for the reversible dissociation of the hydrogen molecule. On the other hand, the sites involved in the H 2 irreversible splitting and those responsible for the adsorption of NO in either chemisorbed or physisorbed form are different one from the other. The present results provide new experimental evidence for modeling the surface active sites on MgO. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnesium oxides; Polycrystalline surfaces; Chemisorption; Physical adsorption; Hydrogen molecule; Nitrogen oxides; Surface chemical reaction 1.Introduction A detailed description, at the atomic level, of the morphology and topology of oxide surfaces is crucial in order to understand the structure and reactivity of these materials which are finding con- tinuously new applications in advanced technolo- gies [1]. * Corresponding author. Tel.: +39-11-670-7574; fax: +39-11- 670-7855. E-mail address: elio.giamello@unito.it (E. Giamello). 0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00009-8 Surface Science 527 (2003) 80–88 www.elsevier.com/locate/susc