First Principles Density Functional Study of the Adsorption and Dissociation of Carbonyl Compounds on Magnesium Oxide Nanosurfaces

Abstract

The adsorption and dissociation of three carbonyl compounds, formaldehyde, acetaldehyde, and acetone, on the magnesium oxide nanosurface, consisting of four stacked (MgO)3 hexagons, is investigated by first principles density functional theory (DFT). In the case of formaldehyde, strongly chemisorbed species, with carboxylate-like structures, are initially formed. These may subsequently undergo heterolytic cleavage of an aldehyde C-H bond to form formate ions involving a surface oxide ion and a hydride ion adsorbed over the magnesium dication [(MgH+)(HCOO-)]. For acetaldehyde, besides this reaction leading to the formation of acetate, the methyl hydrogen of the adsorbed species also tends to attach itself to a surface oxide ion, yielding surface hydroxyl ions and adsorbed [CH2=C(H)OMg]+. These results are in accord with our previous experimental and theoretical results. In particular, the shift of the aldehyde C-H vibration band to higher frequency and the appearance of OH bands in the infrared spectrum are clearly accounted for. For acetone, the mechanism is found to be similar, i.e., a methyl hydrogen shift to yield surface enolate. Again, this is in agreement with experimental studies.

Structures and Stabilities of Alkaline Earth Metal Oxide Nanoclusters: A DFT Study

“…It is well known that (MgO) 3 ring structures are competitive building blocks in the growth of very small MgO clusters [29]. This result is also in agreement with our earlier calculations [22,23] for (MgO) 12 clusters, which optimized to stacked rings from initial cubic rock salt structures.…”

“…It is gratifying to note that experimental observations of mass spectra [14,15,48,55] indicate the existence and stabilities of such stacked hexagonal rings, at least for small gas-phase clusters of MgO. Other experimental and theoretical [22,23,56] studies also provide evidence for the existence and stabilities of MgO nanotubes.…”

“…Thus, experimental knowledge of abundance of masses alone cannot distinguish between the two structures, and sophisticated calculations such as the present ones can only decide the relative stabilities. Our earlier studies [22,23] on the (MgO) 12 cluster had indicated that the (MgO) 12 nanotube, consisting of four stacked hexagonal (MgO) 3 rings, is more stable than the bulk-like cubic structure by 0.48 eV. Moreover, the calculated energy barrier for the rearrangement of the cubic structure to the tube was found to be only 0.13 eV, and hence the two structures are easily interconvertible.…”

“…Table 4 indicates that in the case of MgO, the stacked hexagonal form is the most stable, and the slab structure is higher in energy by 0.21 eV. This energy difference increases to 0.57 eV for the (MgO) 12 cluster [22,23]. For the other metal oxides, the slab structure is preferred.…”

Thermochemical Considerations of Metal Enolates

“…As an example of the opposite situation where the number of magnesium atoms exceeds that of keto or enolate groups, the adsorption of acetone on solid MgO has been suggested by spectroscopic evidence to form a surface enolate 66 . Theoretical calculations suggest the carbonyl compound (at least acetone) is more stable as the enol than as the ketone when adsorbed on the MgO surface 67 . Such calculations 68 also show that the ionic charges on oxygen in MgO are more likely to be −1 than −2.…”