Reaction of halogens with oxide surfaces

Abstract

Charge transfer reactions at oxide surfaces are discussed for the halogens, chlorine, bromine and iodine as strong electron acceptors. Adsorption of chlorine and bromine on MgO prepared in vucuo is characterised by the appearance of a band at 430nm in the diffuse reflectance spectrum and an e x . signal with g-factors of 2.0099 and 2.0020. These features are destroyed on heating and oxygen is evolved. Chlorine and bromine react with the lattice oxygen ions to give halide ions corresponding to about 20 and 10 % respectively of the surface MgO ion pairs ; no evidence of the formation of molecular X i ions was obtained. These results can be understood in terms of the initial formation of charge deficient groups of oxygen ions, such as 0;-and subsequent release of oxygen. The reaction of bromine with MgO would not be expected purely on a consideration of the bulk heats of formation, and indicates an appreciable concentration of surface oxygen ions with less than 5-fold co-ordination. Iodine, as expected from the low heat of formation of the iodide, does not displace oxygen ions on the surface but some iodide ions are formed. All the halogens are able to displace oxygen adsorbed as 0, from the surface indicating that this ion is only weakly bonded to the surface.

Surface studies of magnesium oxide‐based catalysts modified with X2 or MgX2 (X = Br, I)

“…Those of MgO‐based systems whose surface has been modified with halides are much less investigated. There is some information in literature regarding the effect of modification of the MgO surface with halides on the activity of MgO as well as on the replacement of lattice oxygen of MgO by halides upon treatment of MgO with a halide‐containing compound or halogens …”

“…Moreover, the formation of MgI 2 was not achieved by treatment of nano‐MgO with 1‐iodobutane, and the surface of the nanomaterial did not diminish as much as in the case of the other halides . A study performed by Kibblewhite and Tench conducted on MgO modified with Cl 2 , Br 2 , and I 2 showed that oxygen evolution resulting from exchange of the halogen with lattice oxygen is observed only for chlorine and bromine . This means that pronounced differences should be expected when comparing the surface properties of MgO modified with bromine and iodine.…”

“…There is some information in literature regarding the effect of modification of the MgO surface with halides on the activity of MgO [7][8][9][10][11] as well as on the replacement of lattice oxygen of MgO by halides upon treatment of MgO with a halidecontaining compound or halogens. [7,10,12] It has been shown that the catalytic activity and selectivity of the halide modified MgO systems strongly depend on the initial compound introduced onto the surface. In one investigation, it was found that in comparison with unmodified MgO, the samples that were subjected to treatment with CH 4Àn Cl n (n = 1 ÷ 4) exhibited lower activity and a higher selectivity in the liquid phase of catalytic transfer hydrogenation (CTH).…”

“…[7] A study performed by Kibblewhite and Tench conducted on MgO modified with Cl 2 , Br 2 , and I 2 showed that oxygen evolution resulting from exchange of the halogen with lattice oxygen is observed only for chlorine and bromine. [12] This means that pronounced differences should be expected when comparing the surface properties of MgO modified with bromine and iodine. Therefore, the second part of this study deals with the application of both of these halogens onto the surface of MgO in order to elucidate how the type of halogen affects the surface properties, i.e.…”

The adsorption centres on polycrystalline zinc oxide induced by γ-irradiation

“…Among others this concerns the ident,ificat,ion of the ESR signal formed by bromine adsorpt,ion. Although in the light of the experiincnt'al data presented in this paper the simplest siiggest'ion would be to ascribe i t to the molecular ion Br;, there are some arguments, by analogy with the MgO-halogens system [13], that it. might be connected with oxide ions of the lattice destabilized to some extent.…”