Ex-hydroxide magnesium oxide as a model adsorbent for investigation of micropore filling mechanisms

Abstract

The thermal decomposition of magnesium hydroxide to magnesium oxide has been studied under carefully controlled conditions. Analysis of nitrogen and neopentane adsorption isotherm measurements shows that the micropore structure of the decomposition products is very well defined and consists of slit-shaped pores of width 0.93 _+ 0.03 nm from 40% to 90% decomposition. The mean pore width increases, up to ca. 1.8 nm, at higher levels of decomposition. For larger pore sizes two stages of secondary micropore filling by nitrogen (surface coverage followed by enhanced adsorption in the body of the pore) are clearly discernible. The results also show very clearly how the mechanism of micropore filling, and hence the shape of the isotherm, is dependent on the ratio of pore width to molecular diameter (dlo), and not just on d .

Preparation of MgO with High Surface Area, and Modification of Its Pore Characteristics

“…Mg(OH)2 is known to transform completely into MgO at 300 °C. 22 But, studies by others showed that some surface hydroxyls adamantly remained still at the temperature. 29,30 In situ IR study showed that hydroxylated surface was still intact at 300 °C, 30 though the amount was decreased substantially at 500 °C.…”

“…On the basis of known phenomena observed during thermal decomposition of brucite, [17][18][19][20][21][22][23][24] an explanation is proposed in Scheme 1 on how the porosity was generated. Upon being exposed to water, (100) surface of MgO is known to reconstruct into (111)-hydroxyl surface, which is topologically very close to (0001) facet of Mg(OH) 2 .…”

“…It is presumed that the decomposition follows the way others have reported on thermal decomposition of bulk brucite. [17][18][19][20][21][22][23][24] Pseudomorphic microplates of MgO would be generated from Mg(OH)2. These microplates should be facing each other, generating slit type mesopores, as are menifested by the pore analysis in this study.…”

“…[17][18][19][20][21][22] It was shown that thermally prompted decomposition of Mg(OH) 2 produced pseudomorphic microplates of MgO. The microplates were consisted of smaller nanocubes (nano-grains) of 2-3 nm big, whose (111) direction of the cube was perpendicular to the surface of the plates.…”

“…It is anticipated that the track of the decomposition must be same as that reported for the decomposition of bulk brucite. [17][18][19][20][21][22][23] The pore characteristics of modified MgO was dependent on the extent of surface hydroxylation. The case with the least amount of hydroxylation was reported in the previous report by author.…”