Effect of Structure and Composition of Non-Stoichiometry Magnesium Aluminate Spinel on Water Adsorption

Abstract

MgAl₂O₄ is used in humidity sensing and measurement, and as a catalyst or catalyst support in a wide variety of applications. For such applications, a detailed understanding of the surface properties and defect structure of the spinel, and, in particular, of the gas interactions at the spinel surface is essential. However, to the best of our knowledge, very limited experimental data regarding this subject is currently available. In this work, four spinel samples with an Al₂O₃ to MgO ratio (n) between 0.95 and 2.45 were synthesized and analyzed using X-ray photoelectron spectroscopy and water adsorption micro-calorimetry. The results showed that the spinel composition and its consequent defect structure do indeed have a distinct effect on the spinel-water vapor surface interactions. The adsorption behavior at the spinel-water interface showed changes that resulted from alterations in types and energetic diversity of adsorption sites, affecting both H₂O uptake and overall energetics. Furthermore, changes in composition following appropriate thermal treatment were shown to have a major effect on the reducibility of the spinel which enabled increased water uptake at the surface. In addition to non-stoichiometry, the impact of intrinsic anti-site defects on the water-surface interaction was investigated. These defects were also shown to promote water uptake. Our results show that by composition modification and subsequent thermal treatments, the defect structure can be modified and controlled, allowing for the possibility of specifically designed spinels for water interactions.

Keywords: defect structure; magnesium aluminate spinel; reducibility; water adsorption.

Figure 1

Crucible and cell used for the electric field heat treatments.

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.

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).

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.

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.

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.

Figure 10

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

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.