EPR Study of the Surface Basicity of Calcium Oxide. 1. The CaO-NO Chemistry M. Cristina Paganini, Mario Chiesa, Paola Martino, and Elio Giamello* Dipartimento di Chimica IFM, UniVersita ` di Torino, e Unita ` INFM di Torino, Via Giuria, 7 - 10125 Torino, Italia ReceiVed: July 8, 2002; In Final Form: September 12, 2002 The surface basic sites at the surface of calcium oxide have been explored following the interaction of nitric oxide with the solid by EPR. Various paramagnetic species are formed depending on the pressure of the gas contacted with the solid. In the initial steps of the interaction, a small amount of a NO 3 2- species is formed probably because of the presence at the surface of “anomalous” peroxy or peroxo-like groups. For higher NO pressures, four types of NO 2 2- surface species characterized by distinct values of the g and hyperfine tensors are formed by addition of the neutral molecule to basic O 2- sites of the surface. The process involves a considerable rearrangement of the spin distribution with respect to the NO molecule with a net decrease of the electron spin density on the N atom. The number of basic sites involved in this interaction is very low and amounts to a value in the range of 0.25-0.5% of the whole surface. These active sites are low coordination sites present at morphological defects of the polycrystalline solid. The concentration of the basic sites is 25 times greater than that of the corresponding sites on MgO thus confirming the higher overall basicity of CaO with respect to magnesium oxide. The surface activity shown by MgO and consisting in a low energy interaction of Lewis acid sites with the NO molecule is, on CaO, totally suppressed. 1. Introduction The surface chemistry of calcium oxide is far less investigated than the that of the isostructural magnesium oxide, the most known among the alkaline-earth oxides. MgO in fact, because of its simple structure (NaCl), its high ionicity, and its relatively simple morphology, has become in the past two decades a sort of playing ground for groups active in the surface science of oxides 1-3 (single-crystal faces or thin layers), in surface chemistry (polycrystalline materials), 4-6 and in theoretical chemistry. 7-10 The structure of real MgO surfaces, their rich defectivity, and the chemical reactivity of low coordination surface ions are progressively becoming familiar to a number of researchers. The same does not hold for CaO although this oxide finds some applications in the field of automotive exhaust treatments. 11 The only remarkable debate involving the surface chemistry of CaO that appeared in the literature in recent years concerns the discussion about its basicity in comparison to that of MgO. Tanabe and co-workers 12-14 showed in a series of experimental papers that CaO is more active than MgO in the conversion of benzaldehyde into benzyl benzoate and demonstrated that the order of base strength in the group of alkaline earth oxides is BaO > SrO > CaO > MgO. The reason for the different surface basicity between CaO and MgO has been analyzed in details by Pacchioni et al. 15 by means of ab initio cluster model calculations and rationalized on the basis of simple electrostatic arguments. The authors explain the different basicity and chemical reactivity of the two oxides considering the different Madelung potential at the two surfaces. The O 2- ion is infact unstable in the gas phase where it dissociates into O - + e - and exists in ionic solids such as alkaline-earth oxides because of the effect of the Madelung potential. Thus, a reduced Madelung potential implies a reduced stability of the oxide anion. The Madelung potential is different at different surface sites, depending on the Madelung constant of the site which in turn is a function, for a given lattice structure, of the coordination degree of the anion in question and of the lattice constant. CaO has the same cubic structure as MgO but a larger lattice constant (r CaO ) 2.399 Å, r MgO ) 2.106 Å). As a result, the electron cloud of an O 2- ion having a given coordination on CaO is more spatially diffuse compared to that of the corresponding ion in MgO and can overlap better with the orbitals of the incoming molecule explaining the higher reactivity and the increased basicity. The activity of our laboratory in recent years has been mainly devoted to understand the surface of polycrystalline MgO with particular attention to (i) the structure and properties of surface vacancies, 4,5 (ii) the interaction with metals, 16-18 and (iii) the properties of low coordination surface ions. 19 We have mainly employed in our activity the electron paramagnetic resonance (EPR) technique which revealed particularly suited to describe, by means of various paramagnetic probes, the aforementioned points (i-iii). A part from a couple of papers respectively devoted to surface color centers 20 and to H 2 -D 2 isotopic substitution on CaO 21 this oxide has not been thoroughly investigated in our laboratories till the end of the 1990s. We start with the present paper to report about the surface chemistry of CaO and about its similarities and its differences with respect to that of MgO. As it will be seen in the following, though strong analogies between the two oxides are easily observed, the higher basicity of CaO shows up in terms of a specific reactivity. In the present paper, we will describe the reactivity of the CaO surface with nitric oxide. The interest for the interaction of NO with CaO is 2-fold. NO is a component of the automotive exhaust, and thus, understanding its reactivity with CaO is important to increase the knowledge on catalytic and noncata- * To whom correspondence should be adressed. Phone: +39.011.6707574. Fax: +39.011.6707855. E-mail: elio.giamello@unito.it. 12531 J. Phys. Chem. B 2002, 106, 12531-12536 10.1021/jp0264578 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/07/2002