**1. Introduction**

Magnesium aluminate spinel (MgAl2O4, MAS) has been shown to be useful for humidity sensing and measurement applications, and as a catalyst or catalyst support for various organic reactions [1–7]. For all these applications, a detailed understanding of the surface properties of the spinel, and specifically the nature of spinel gas-surface interactions, is paramount. Nevertheless, to date, very limited data is available regarding water-surface interactions on MAS and their relation to spinel defect structure, and the information that is available is largely based on theoretical calculations [8]. MAS is the only stable intermediate phase in the Al2O3-MgO system, and at elevated temperatures (i.e., over 1300 ◦C), this system exhibits a large non-stoichiometric range [9], which can also exist at lower temperatures as a metastable nanomaterial [10–12].

The spinel structure has a general formula of AB2O4, with the lattice comprising an almost perfect, close-packed cubic arrangemen<sup>t</sup> containing 32 oxygen anions. In this arrangement, the A and B cations are situated inside the tetrahedral and octahedral interstitials, respectively. In MAS, eight Mg<sup>2</sup>+ cations

are located in the tetrahedral sites, and sixteen Al3+ cations occupy the octahedral sites [10,13–15]. The defect structure of spinel is comprised of intrinsic (i.e., Frenkel, Schottky and anti-site) defects and extrinsic (i.e., non-stoichiometric, dopant or impurity) defects. It has been established that the dominating intrinsic defect in MgAl2O4 spinel is the anti-site defect (AKA inversion), in which two cations switch places. Specifically, an Mg<sup>2</sup>+ occupies the Al3+ octahedral site and vice versa [16–20].

The inversion level (i.e., the number of tetrahedral sites occupied by Al3+ cations) is controlled by three major factors. The first is related to the thermal history of the material, reflecting the "*intrinsic*" defect concentration. The other two are due to extrinsic parameters, the first of which is MAS stoichiometry (i.e., the. "*stoichiometric*" factor), and the second is residual disorder resulting from thermal e ffects and stresses in the material synthesis process (i.e., the "*residual*" inversion).

The *intrinsic* component can be calculated using a thermodynamic model, such as that developed by O'Neill and Navrotsky [21], using experimental parameters [10]. The *stoichiometric* component can be quantified from the total defect concentration [13]. The residual inversion, resulting from the synthesis process, cannot be assessed accurately, but this type of inversion defect can be manipulated and subsequently reordered using external fields [10,13].

For MAS, the shift away from idealized stoichiometry can be considered to be due to the dissolution of MgO or Al2O3 in the spinel matrix. Departing from the stoichiometric ratio in either the Al2O3- or MgO-rich direction, results in di fferent structural defects. Spinel crystals with excess Al2O3 are characterized by *Al*•*Mg*,, which can be charge-compensated by *<sup>V</sup>Mg*, *V Al* or their combination [16,22–25], as demonstrated in the following equations:

$$4\text{-Al}\_2\text{O}\_3 \rightarrow 5\text{-Al}\_{\text{Al}}^{\text{\textdegree}} + 12\text{-O}\_{\text{O}}^{\text{\textdegree}} + 3\text{-Al}\_{\text{Mg}}^{\bullet} + \text{V}\_{\text{Al}}^{\text{\textdegree}} \tag{1}$$

$$2\cdot 4\cdot \text{Al}\_2\text{O}\_3 \rightarrow 6\cdot \text{Al}\_{\text{Al}}^{\times} + 12\cdot \text{O}\_{\text{O}}^{\times} + 2\cdot \text{Al}\_{\text{Mg}}^{\bullet} + \text{V}\_{\text{Mg}}^{\prime\prime} \tag{2}$$

$$1\,12\,\text{Al}\_2\text{O}\_3 \to 16\,\text{Al}\_{\text{Al}}^\times + 36\,\text{O}\_\text{O}^\times + 8\cdot\text{Al}\_{\text{Mg}}^\bullet + \text{V}\_{\text{Mg}}^{\prime\prime} + 2\cdot\text{V}\_{\text{Al}}^{\prime\prime\prime}\tag{3}$$

Alternatively, spinel crystals with excess MgO incorporate *MgAl* defects. In this case, the preferred charge compensation would be in the form of V•• O [16,22–25], as described by:

$$\text{3MgO} \rightarrow \text{2Mg}\_{Al}^{\prime} + \text{Mg}\_{Mg}^{X} + \text{3O}\_{O}^{X} + \text{V}\_{O}^{\bullet \bullet} \tag{4}$$

Using the Brouwer diagram, the defect types in MAS can be described in terms of the Al2O3 content [23] according to the following guidelines:

$$\frac{\text{Mg}\_{\text{Al}}^{\prime}/\text{V}\_{\text{O}}^{\bullet\bullet}}{\text{Low Al}\_{2}\text{O}\_{3}} \longleftarrow \frac{\text{Al}\_{\text{Mg}}^{\bullet}/\text{Mg}\_{\text{Al}}^{\prime}}{\text{Mg}\text{Al}\_{2}\text{O}\_{4}} \rightarrow \frac{\text{Al}\_{\text{Mg}}^{\bullet}/\text{V}\_{\text{Al}}^{\prime\prime\prime}}{\text{Molarite Al}\_{2}\text{O}\_{3}} \rightarrow \frac{\text{Al}\_{\text{Mg}}^{\bullet}/\text{V}\_{\text{Mg}}^{\prime}}{\text{High Al}\_{2}\text{O}\_{3}}\tag{5}$$

The type and quantity of defects both in the bulk and on the surface of the material changes as a function of stoichiometry. Changes in defect structure can be used for tuning the properties of a material [14,26,27], including its surface properties [2]. Surface properties are also a ffected by the environmental state in which the material is maintained. For example, ambient, clean, reduced, oxidized or humid environments all have di fferent e ffects on the surface state.

To the best of our knowledge, no experimental data regarding the e ffects of non-stoichiometry on surface-water interactions in the MgO•*n*Al2O3 system have been published, although some theoretical work has been performed. This paper, therefore, aims to study the e ffect of the surface composition on the interactions between a non-stoichiometric MgO•*n*Al2O3 spinel system and water vapor.

#### **2. Materials and Methods**
