*2.2. Characterization*

X-ray diffraction (XRD) patterns of the samples were recorded using a Rigaku RINT 2100 (Tokyo, Japan) diffractometer with CuKα radiation. The operating parameters were 40 kV and 40 mA, with a 2θ step size of 0.02◦. Si (NIST SRM 640c) served as internal standard for cell parameter determination. Crystallite sizes were refined from diffraction peak broadening using a whole profile fitting procedure, as implemented in the Jade software package (version 6.11, 2010, Materials Data Inc., Livermore, CA, USA).

Sample composition was determined by atomic absorption spectroscopy (AAS) using a Varian SpectrAA 240FS (currently Agilent Technologies, Santa-Clara, CA, USA).

Surface area was measured using the Brunauer–Emmett–Teller (BET) theory [29] using a Micrometrics ASAP 2020 (Micrometrics, Norcross, GA, USA) instrument. Fifteen-point adsorption isotherms of nitrogen were collected in the P/P0 relative pressure range 0.05–0.30, where P0 is the saturation pressure at −196 ◦C. Prior to analysis, each sample was degassed under vacuum at 700 ◦C for 4 h.

X-ray photoelectron spectroscopy (XPS) data was collected using an X-ray photoelectron spectrometer ESCALAB 250 (Thermo Fisher Scientific, Waltham, MA, USA) ultrahigh vacuum (10−<sup>9</sup> bar) apparatus with an Al*K*α X-ray source and a monochromator. The X-ray beam spot size was 500 μm, and survey spectra were recorded with pass energy (PE) of 150 eV. High-energy resolution spectra were recorded using 20 eV PE. To correct for charging effects, all spectra were calibrated relative to a carbon C1s peak positioned at 284.8 eV. Processing of the XPS results was carried out using the Thermo Scientific AVANTAGE program. For accurate surface characterization by XPS, a glove box was mounted on the XPS enter lock chamber to avoid adsorption of any species from the air on the samples. To ensure that all the samples were investigated under the same experimental conditions, all samples were equilibrated for 12 h in the entry lock chamber of the XPS prior to making the measurements.

IR spectra were recorded at room temperature using a Nicolet 6700 (Thermo Scientific, Madison, WI, USA) FT-IR spectrometer with a KBr-DTGS detector in the range spanning 400–4000 cm<sup>−</sup>1. Mixtures containing 100 mg KBr and 1 mg spinel were compressed at 1 ton to generate thin plates. For each material, 64 scans of the spectrum were recorded and averaged. The spectrometer settings were at aperture of 150 and spectral resolution of 4 cm<sup>−</sup>1. Peak positions and intensities were determined by OPUS software (Billerica, MA, USA) using the second derivative and standard methods. The averaged spectrum was used to calculate the inversion parameters of the samples, employing the method of Erukhimovitch et al. [10], which uses the intensity ratios of the γ1 and γ5 modes (FTIR peaks located at ~690 and ~830 cm<sup>−</sup>1).

#### *2.3. Water Adsorption Calorimetry*

The heat of adsorption of the water–spinel surface interactions was measured using a custom-made apparatus, composed of a volumetric sorption system (ASAP2020, Micromeritics, Norcross, GA, USA) and a differential scanning calorimeter (Sensys Calvet, Setaram, Lion, France) [30]. The instrumental design and its operation have been discussed elsewhere. Here, approximately 100 mg of sample powder was put inside the sample tube, providing a total surface area of 1.0–5.6 m2. The tube is than placed inside the calorimeter, it is also connected to the sorption system via a conceive tube. Prior to measurement, a degas procedure was performed. One was cooled to 25 ◦C at the end of the degassing process, whereas the second was exposed to an oxygen atmosphere prior to cooling down (i.e., oxidized/clean). All the samples were exposed to controlled, and incremental H2O vapor doses until the partial pressure reached P/P0 = 0.3. The incremental dose was set to provide 1 μmol of H2O vapor per m<sup>2</sup> of sample surface. The heat of adsorption (ΔHads) for each dose was measured. The measurements were repeated 3–4 times for each sample to ensure reproducibility. A baseline run was performed to eliminate environmental and instrumental contributions to the signal.

#### **3. Results and Discussion**

This study focused on the effects of surface composition and state on water–surface interactions for four different MgO•*n*Al2O3 spinel powders. The samples investigated comprised a series of nano-sized (10–15 nm) metastable spinels, with composition (*n*) ranging between 0.95 and 2.45 (Table 1). Their lattice parameters, crystallite sizes and surface areas are summarized in Table 1. The lattice parameter increased inversely with the n ratio, in keeping with literature results [31,32]. The surface areas measured by the BET method differed significantly from those calculated theoretically from XRD crystallite size (assuming spherical approximation), indicating that the samples had undergone extensive sintering during the final stages of their synthesis.


**Table 1.** As synthesized characteristics: lattice parameter, crystallite size, surface and interface areas.

Four types of surface states/conditions were addressed in this work: for simplicity, they will be referred to as "as synthesized" (AS), reduced (RD), clean (CL) and hydrated (HD). The AS condition refers to a sample after calcination. The RD condition refers to samples after the degassing procedure (Figure 2). It can be seen that the powders lost their original white color and became greyish-dark after the degassing procedure, with the samples that were richer in Al2O3 becoming markedly darker in the RD state, indicating that they were more easily reduced. Oxidation of the RD samples by exposure to 1 atm of oxygen at 700 ◦C and cooling to room temperature in a 1 atm oxygen environment resulted in the restoration of the original white color of the samples. It can be concluded that the dark color of RD samples was a result of the formation of oxygen vacancies during the degassing stage, which was reversed in the oxidization step by "refilling" the oxygen vacancies formed by the initial degas, while simultaneously keeping the surface clean. The reoxidized samples were designated CL. Finally, either RD or CL samples were exposed to H2O vapor in the water adsorption experiments to generate the HD states.

**Figure 2.** Different sample colors after degassing procedure showing different reduction level vs n: (**A**) n = 1.07; (**B**) n = 1.15; (**C**) n = 2.45.

#### *3.1. Surface State Analysis*

Typical XPS spectra in the Al2p and Mg2p regions for the four types of surface state (AS, RD, CL and HD) of MgO•*1.07*Al2O3 are shown Figures 3–6. Three types of bonds, specifically M–M, M–O and M–OH bonds (M = Al, Mg), were considered for each spectrum. In the Al2p spectra, these are assigned at ~73.0, 74.1 and 75.2 eV, respectively [33–35]. In the Mg2p spectra, their assignments are ~49.3, 50.9 and 51.8, respectively [36]. The dominant bond in AS samples (Figure 3) is the M–O bond, with less prominent, though not negligible, M–OH bonds. The RD samples (Figure 4) displayed different spectra due to the prolonged degassing procedure. Here, the amount of the hydroxyl surface species was reduced, and the presence of M–M bonds was detected. These M–M Bonds were formed due to oxygen deficiencies in the structure, as reflected in the color changes of the powders (Figure 2). In the CL samples (Figure 5), the M–M bonds disappeared and were replaced by M–O species, with the quantity of M–OH bonds being lower than in the AS samples. As expected, the HD samples exhibited the highest quantity of M–OH bonds, and no M–M bonds were identified (Figure 6). This behavior was found to be typical for all the samples in this study. The data for all samples are given in the Supplementary Materials in Tables S1 and S2.

**Figure 3.** XPS spectra, of an AS, n = 1.07 sample: Al2p de-convoluted to Al–O and Al–OH peaks (**a**), and Mg2p, de-convoluted to Mg–O and Mg–OH peaks (**b**).

**Figure 4.** XPS spectra of an RD, n = 1.07 sample, after degassing and oxidation: Al2p de-convoluted to Al–O, Al–OH and Al–Al peaks (**a**), and Mg2p, de-convoluted to Mg–O, Mg–OH and Mg–Mg peaks (**b**).

**Figure 5.** XPS spectra, of a CL, n = 1.07 sample, after degassing and oxidation: Al2p de-convoluted to Al–O and Al–OH peaks (**a**): and Mg2p deconvoluted to Mg–O and Mg–OH peaks (**b**).

**Figure 6.** XPS spectra of an HD, n = 1.07 sample after degassing, oxidation, and exposure to water vapor: Al2p de-convoluted to Al–O and Al–OH peaks (**a**), and Mg2p, de-convoluted to Mg–O and Mg–OH peaks (**b**).

#### *3.2. Water Adsorption Measurments*

Heat of adsorption measurements were conducted for the reduced (RD) and fully oxidized (CL) samples. Typical heat of adsorption isotherms for MgO- and Al2O3-rich spinel samples are presented in Figure 7. It should be emphasized that in this calorimetric study, water molecules adsorbed with an enthalpy greater than −44 kJ/mol relative to vapor are referred to as "strongly bound", while those adsorbed with the enthalpy of condensation of liquid water (−44 kJ/mol) are considered "weakly bound". Furthermore, we should stress that such assignments are based solely on calorimetric data and do not reflect any structural studies of water adsorbed on the surface [37,38].

**Figure 7.** Differential enthalpies of adsorption as a function of water coverage on CL samples: an MgO-rich sample, n = 0.95 (**a**), and an Al2O3-rich sample, n = 2.45 (**b**). The blue line signifies the transition between strongly bonded and weakly bonded water, and the red line emphasizes enthalpy of −70 kJ/mol, where the site type of strongly bonded water is altered.

The heat of adsorption isotherms can be divided into segments, according to the change in their slope. Each segmen<sup>t</sup> of the isotherm may be considered as reflecting a different type of adsorption site, or site group [39,40]. The MgO-rich spinel displayed a four-step behavior (Figure 7a). In the first step, marked in purple (type A), a sharp decrease in the enthalpy of adsorption was seen, up to H2O coverage of ~2 molecules/nm2. From there, a second step was observed up to ~5 molecules/nm2, marked in violet (type B), in which the slope changed direction, and the decrease in ΔHads was moderated. At the transition point between the second and third steps ΔHads reached a value of ≈−70 kJ/mol. From this point of the curve, the blue section (type C), the ΔHads slowly decayed until the measured enthalpy corresponded to that of weakly bonded water. After reaching −44 kJ/mol (the cyan section of the curve, type D), the enthalpy fell to lower values (absolute) of about −39 kJ/mol. In general, the MgO-rich samples showed similar behavior to that of pure MgO, where surface hydroxides (i.e., Mg(OH)2) are formed [41].

In the case of the Al2O3-rich samples, the first two steps observed for the MgO-rich samples were combined into a single step (up to ~4 molecules/nm2, marked in purple, types A + B). After this point, a more moderate slope was identified, up to −44 kJ/mol (Figure 7b, blue colored, type C), with no further decreases.

The heat of the adsorption isotherms for all samples, in their CL and RD states, can be seen in Figure 8. These isotherms are a depiction of the gas adsorption amount (Figures S1 and S2) obtained in each dose with their corresponding energetic value. Table 2 summarizes the integral heat of adsorption of strongly bond water and the water coverage for all samples, as well as the amounts of Mg and Al hydroxides formed on the surface, deduced from the XPS analysis. The columns in Table 2 under the heading "Hydroxides" present differences in the surface hydroxide compositions for the initial RD and CL states and after their hydration. The measured values are in good agreemen<sup>t</sup> with the water coverage, except for the n = 0.95 sample. We believe that XPS measurements do not accurately account for the coverage in this sample, possibly due to the presence of surface contamination by adventitious species on the CL/RD samples. Such species are thought to readily adsorb due to the defective nature of MgO rich spinel [16,22–25].

**Figure 8.** Heat of adsorption isotherms for all samples for RD and CL surfaces: (**a**) n = 0.95; (**b**) n = 1.07; (**c**) n = 1.15; (**d**) n = 2.45.


**Table 2.** Water adsorption data for Clean (CL) and Reduced (RD) spinel samples.

The results showed a clear relation between the Al2O3 concentration and the water adsorption in the CL samples, in terms of both water uptake and energetics. In general, as the Al2O3 concentration persisted in the reduced samples (RD), with the exception of the n = 1.07 sample. The possible origins for this apparent anomaly will be discussed below. Notably, each sample in the RD state accumulated more adsorbed water than its CL analogue (excluding n = 1.07), as its surface defect structure was altered. It is possible that some of the adsorbed water may act as an oxidizing agent, but we believe that this role is limited due to the relatively low temperature of the adsorption process.

Water uptake in the RD state was dependent on Al2O3 concentration, but this dependency is not as simple as was observed for the CL samples, even if the n = 1.07 sample is excluded. This, and the

apparently anomalous behavior of the n = 1.07 sample, are attributed to the di fferences in the level of reduction as was seen by the changes in sample coloring. Based on the color di fferences of the Al2O3-rich samples (Figure 2), we concluded that the level of reduction increased with Al2O3 concentration, but the extent of reduction was not quantitatively determined in this work. The e ffects of reduction in enhancing the extent of adsorption were evident in the increased (and similar) extent of water uptake for the n = 1.15 and n = 2.45 samples. However, the integral enthalpy of adsorption of the RD samples was lowered, relative to their CL counterparts, as the defect structure was progressively altered. These alterations, in turn, resulted in changes in the site population and its energetic diversity, the source of which lies in the newly induced material defect structure.

In spinel, water molecules are adsorbed in the vicinity of the metal ions, specifically AlxAl, MgxMg, Al•Mg, and MgAl [42]. As Al2O3 is added in excess, the Al3+ cations progressively occupy tetrahedral sites, substituting Mg<sup>2</sup>+ and disturbing the charge neutrality. To maintain charge neutrality, cation vacancies are formed, some of which are on the surface of the spinel structure [16,22–25]. Consequently, the quantity of surface cations is diminished and hence also the quantity of available adsorption sites which leads to lower quantities of adsorbed water.

An excess of Al2O3 also influences the proximity of the metal cations to the surface, as it a ffects which of the material planes have a higher tendency to be exposed. Cai et al. [43] calculated the surface stability of exposed spinel surfaces and concluded that in Al2O3-rich spinel 111\_O2(Al) plane tends to be exposed. In this plane, oxygen molecules are slightly elevated over the Al3+ cations, thus decreasing the energy of interaction at the surface. As the material becomes poorer in Al2O3, the 100\_Al(O2) plane is exposed [43]. This plane has a higher surface Gibbs free energy, thereby allowing for interactions at the surface that are more energetic.

It is important to note that the n = 0.95 samples are MgO-rich, and thus, the factors contributing to the defect structure are di fferent. Here, oxygen vacancies compensate for excess Mg, which essentially allows for better exposure of the metal cations. Moreover, MgO is highly hygroscopic, forming a surface hydroxide phase that enables additional, more energetic water uptake [41]. In the RD state, oxygen vacancies were formed, their numbers growing with Al2O3 concentration, with consequent changes in the surface defect chemistry and electronic structure. In the RD samples, a decrease in the integral enthalpy of adsorption was registered, relative to their CL counterparts. To determine the origins of this decrease, a closer analysis of the energetic diversity of the adsorption sites is required.

Figure 9 shows normalized heat of adsorption isotherms for CL and RD samples. In these isotherms, the *x*-axis was normalized to the amount adsorbed at full coverage for each sample. This observation allows us to consider the energetic distribution of the sites. In samples n = 0.95 and 1.15, the isotherms for the RD and CL materials are almost overlapped, with the exception of the very first few sites at low relative coverage (type A), which were more energetic for the CL samples than for their RD counterparts. An inverse relationship was obtained for the n = 1.07 sample. These first few (low relative coverage) sites are very energetic and make a considerable contribution to the overall energetics. The new adsorption sites that were added after reduction are of type C, which suggests water was unable to re-oxidize the surface. Finally, the CL and RD isotherms for the n = 2.45 sample were in almost perfect alignment, suggesting that the energetic diversity of the adsorption sites was maintained regardless of the reduction process undergone by this material. Accordingly, the integral enthalpy of adsorption for this sample in the two states was similar.

At this point, it is important to address the anomalous behavior exhibited observed for the sample n = 1.07. This material has near stoichiometric composition, and, as is evident from Figure 2, was barely reduced by the degassing procedure. Nonetheless, some changes in the defect structure did occur. Jia et al. [42] showed computationally that for stoichiometric ZnGa2O4 spinel, such reduction-related defects do not always enhance water adsorption [42]. We assume that a similar explanation is appropriate here because of the proximity of the n = 1.07 sample to stoichiometry. This implies that in order for the reduction process to enhance the surface reactivity, the composition should not be near stoichiometric.

**Figure 9.** Heat of adsorption isotherms for all CL and RD samples. The coverage is normalized to the full coverage of strongly adsorbed water for each sample. (**a**) n = 0.95; (**b**) n = 1.07; (**c**) n = 1.15; (**d**) n = 2.45.

#### *3.3. E*ff*ect of Anti-Site Defects*

As discussed above, the spinel system is subject to extrinsic and intrinsic defects. The former, which can exert a considerable effect on water-surface interactions can, however, be influenced by controlling the composition of the material. The spinel system also presents intrinsic, anti-site defects that are not controllable or that are controllable only to a certain extent and require specific study and understanding. Thus, to assess the effects of anti-site defects on adsorption behavior, a MgO•*2.45*Al2O3 (*n* = 2.45) sample was heat-treated in the presence of a constant electric field (EF). FTIR spectra in the 400–1000 cm<sup>−</sup><sup>1</sup> range of the sample before and after the heat treatment are presented in Figure 10. The aim of the electric field treatment was essentially to rearrange the defects caused by the *residual* inversion without otherwise affecting the material. The thermal treatment was performed at 800 ◦C, a temperature lower than the calcination temperature of the sample (850 ◦C) so that any changes in the material as result of the heat treatment in the presence of the electric field can be attributed solely to the effect of the EF. The function of heating (to 800 ◦C) is to provide sufficient thermal energy to assist cation rearrangement, driven by the applied electric field. As a result of this treatment, highly disordered samples (*I* = 0.44) with *n* ratio of 2.45 underwent significant reordering (*I* = 0.33), as is qualitatively demonstrated in the FTIR spectrum by the decrease in the intensity of the γ5 mode (~830 cm<sup>−</sup>1) (Figure 10) [10].

**Figure 10.** FTIR spectra of MgO•2.45Al2O3 before and after application of an electric field.

After the heat treatment and subsequent inversion parameter (i) decrease, water adsorption of this re-ordered sample was measured in the same way as for all the preceding materials. The adsorption enthalpy isotherms of the two spinel samples, before and after heat treatment with the electrical field are shown in Figure 11. From the data listed in Table 3, it is apparent that following reordering, the enthalpy of adsorption decreased, as did the coverage. The extent of formation of hydroxide bonds was determined using XPS (Table 3), and the results are in agreemen<sup>t</sup> with the results of the heat of adsorption experiments. Heat treatment together with the application of an electrical field led to significantly less hydroxides being formed, with the change in Al–OH bonding being more marked than that for Mg–OH. These experimental findings can be explained by the existence of excess charge when the Al+<sup>3</sup> cations are located in the tetrahedral sites.

**Figure 11.** Differential enthalpy of adsorption as a function of water coverage of MgO•2.45Al2O3 samples before and after heat treatment in an electric field.


**Table 3.** Water adsorption by MgO•2.45Al2O3.
