Coadsorption of NO and H2 at the surface of MgO monitored by EPR spectroscopy. Towards a site specific discrimination of polycrystalline oxide surfaces

Coadsorption of hydrogen and nitric oxide at the surface of polycrystalline MgO has been studied by means of electron paramagnetic resonance spectroscopy. 14NO and 15NO have been used as probes for monitoring highly exposed 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 H2 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 NO22− species on low-coordinated anions. The irreversible form of H2 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 H2–NO ratio. For high H2–NO ratios no adsorbed monomeric NO is observed whereas when lowering the H2–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 H2 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.