Study of the Formation Mechanism of Boehmite with Different Morphology upon Surface Hydroxyls and Adsorption of Chloride Ions

Abstract

Identification of the key factors of morphology control in nanomaterial synthesis is important for mechanism understanding of the reactivity–structure relationship and rational design of crystals with desired morphology. Among others, the inorganic ions in solution were found to be a key factor in controlling the final morphology. To reveal the effect of inorganic ions, the boehmite with different morphologies influenced by chloride ions is studied by a combined experimental and computational approach. The quantum mechanical calculations reveal that the chloride ions interact with crystal planes through surface hydroxyls and act as morphology-directing agent. The growth directions and exposed planes of the boehmite determined from HR-TEM images indicate an oriented-aggregation process which is consistent with the DFT calculations. It is envisaged that the defects caused by excessive hydrochloric acid are due to its dissolving property combined with the reduction of adsorption energies. Overall, all the morphologies of boehmite suggested by the calculations are confirmed by experimental results.

Phosphate alteration of chloride behavior at the boehmite–water interface: New insights from ion-probe flow adsorption microcalorimetry

“…4. Considering recent findings [40], the exothermic nature of peak A1 was attributable to Cl À interaction with "MeOHAAl 5 hydroxyls along the (0 0 1) plane. The relative intensities of peaks A1, A2, and A3 were also consistent with the relative proportion of different hydroxyls for c-AlOOH and pointed to "MeOHAAl 6 (along the (1 0 0) plane) and "MeOHAAl 4 (along the (1 0 1) plane) hydroxyls as the most likely sites accounting for peaks A2 and A3 respectively [38].…”

“…This favorability ranking is very much in-line with the relative basicity of the different hydroxyls and hence their tendency to exist in their protonated form under acidic conditions [38,40]. For example, Raybaud et al [38,40] from their DFT calculations found that the enthalpy of adsorption for NH 3 (a base) onto "MeOHAAl 5 , "MeOHAAl 6 and "MeOHAAl 4 hydroxyls was À34.3, À42.6 and À55.5 kJ mol À1 respectively-reflecting an increase in acidity (decrease in basicity) of the respective hydroxyls. Our favorability ranking also suggested that phosphate which was adsorbed to the most basic (least acidic) hydroxyls were the most susceptible to exchange by Cl À (and likely other simple anionic species).…”

“…To our knowledge, the recent work by Xia et al [40] is the only other report documenting both exothermic and endothermic sorption of Cl À with the c-AlOOH surface. Using molecular modeling calculations based on density functional theory (DFT), they found that Cl À interaction was exothermic along the (0 1 0) and (0 0 1) plane of c-AlOOH under both acidic and neutral conditions, but was endothermic along the (1 0 0) plane under neutral conditions.…”

“…4. For example, the findings of Xia and coworkers [40], suggested that peak A1 was most probably attributable to Cl À interaction with hydroxyls along the (0 1 0) and/or (0 0 1) planes. Considering the reported pK a of 6.8 [46] and point of zero charge ranging from 7.7 to 9.4 [47] for c-AlOOH, it would also be reasonable to expect Cl À to react endothermically with the (1 0 0) plane under the experimental conditions (pH 5.8) used here and thus peaks A2 and A3 would be attributable to Cl À interactions with hydroxyls on the (1 0 0) and (1 0 1) planes (in no particularly order).…”

“…Subsequently, this approach provided better insights into the nature of the Cl À /c-AlOOH interaction in the presence of phosphate. Boehmite was chosen as the model surface because of (i) its importance as a catalytic support and precursor of c-alumina (c-Al 2 O 3 ); which is one of the most common catalyst used in industrial heterogeneous catalysis [27,28], (ii) its structure and surface properties are known to be stable between pH 5-9 at room temperature [29,30], (iii) its adsorption properties [31][32][33], and (iv) its crystallography as well as hydroxyl groups that populate the exposed faces have been characterized extensively by various experimental [34][35][36][37] and mathematical modeling techniques [21,34,[38][39][40].…”