Reaction of Halogens with Oxide Surfaces BY JOHN F. J. KIBBLEWHITE AND ANTHONY J. TENCH* Chemistry Division, A.E.R.E., Harwell Received 30th May, 1973 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. The adsorption of oxygen has been studied on a wide variety of oxides. Both insulating and non-stoichiometric semi-conducting oxides have been investigated using a variety of techniques; in particular, the paramagnetic oxygen species that can be formed on such oxides have been studied in some detail using electron spin resonance indicating that 0; is formed on many 0~ides.l'~ By analogy we might expect that the halogens would react to form similar molecular ions, and could be used to investigate the charge transfer properties of oxides.Ions such as C1, have been well characterised in the alkali halides 5-8 and are formed by low temperature irradia- tion; they have also been identified on SiOa and zeolite lo surfaces where the adsorbed halogens have been y- or photo-irradiated at low temperature. Very little work has been reported on the adsorption of halogens on oxides; the adsorption of chlorine on ZnO has been studied recently and a new e.s.r.signal with a g-factor of 2.015 has been reported, there appears to be no evidence for the formation of the molecular ion.ll The adsorption of iodine on ZnO is reported to show a signal with a g-factor of 2.0068 which has been ascribed to These results are difficult to interpret because the e.s.r. evidence is contradictory and does not give a clear indica- tion of the species involved. In this work the adsorption of halogens has been studied on MgO. Although there is no indication of the formation of stable molecular ions, the results indicate that a number of changes occur on the surface of the oxide, suggesting that charge transfer reactions occur which involve oxide ions of the lattice.' EXPERIMENTAL The samples of magnesium oxide used were prepared by thermal decomposition of the carbonate in silica tubes as described previously,14 and finally heated at 1250 K for 15 h at a pressure of less than 1.33 x N m-2.Specpure chlorine and oxygen, and A.R. broming and iodine were used; the halogens were purified prior to use by several freeze-pumpine cycles and by distillation in uacuo. The amount of halide ion formed during the adsorption 72J . F. J . KIBBLEWHITE AND A . J . TEkCH 73 was determined spectrophotometrically using Hg(CNS)2 and ferric alum following the method of Swain.15 The Cary 14 spectrophotometer was calibrated using standard solutions of sodium chloride, bromide and iodide. Blanks carried out on magnesium oxide prepared in vacuo indicated that the chloride content was < 10l8 ion g-l before the addition of chlorine.The specific surface areas of the samples were obtained using a nitrogen sorptometer. The adsorption of gases on to the MgO samples was conducted using a standard gas handling line. Quantitative measurements of the amount of oxygen evolved were obtained by, first, measuring the pressure-volume characteristics and, second, passing the gas products to a gas chromatograph. The products were pumped through a liquid nitrogen trap, to remove excess chlorine, and into a Toepler pump which was then used to transfer the samples to a carrier-gas loop of a Perkin-Elmer F11 chromatograph, which incorporated a 2 metre molecular sieve 5A column. The e.s.r. spectra were obtained at 77 K on a Varian 4502 spectrometer operating at 9.2 GHz with 100 kHz modulation.In all the e.s.r. spectra shown in the figures the magnetic field increases from left to right and first derivative spectra were recorded. Spin concentra- tions were calculated by direct double integration using an on-line computer (LABCOM). The g factors were measured relative to diphenylpicrylhydrazyl taken as 2.0036. Diffuse reflectance spectra were recorded on a Cary 14 spectrophotometer using MgC03 as a reference ; the method has been described previously.16 The analogue output from the e.s.r. spectrometer and reflectance spectrophotometer was fed directly into the LABCOM l7 " on-line " computing system. This system accepts input from several instruments at the same time, the data is digitised using CAMAC l8 modular units and fed into a PDP8/I computer ; the data in the computer are displayed on an oscillo- scope.The data can be stored and processed, using a normal teletype mode of interaction with the system. The e.s.r. data were processed using a regression analysis to determine the base line and the spectrum integrated between chosen points. The integrated spectrum was displayed, and, if acceptable, a second integration was carried out to determine the spin concentration in comparison with the standard. The great advantage of this method is that any number of integrations can be carried out (each taking only a few seconds) on the same data to cover different ranges of magnetic field and this gives a much better assessment of the contribution from the wings of the line.For the reflectance work the spectrometer output was converted to give a scale linear in energy and processed using the Kubelka-Munk l9 function to give an absorbance scale, and the data were then analysed using a Du Pont Model 310 curve resolver. RESULTS REACTION WITH CHLORINE ADSORPTION AND DESORPTION Chlorine was found to be adsorbed on the surface of the MgO samples at 300 K in a fast process which was only partially reversible after 15 min evacuation at this temperature ; analysis of the desorbed gases showed that only chlorine was present and no oxygen was evolved. The samples turned a beige colour which remained after evacuation at 300 K. Analysis of the evolved gases after heating in vacuo at successively higher temperatures indicated that oxygen was the only component present apart from chlorine.Separate experiments on several samples showed that the desorbed oxygen approached a limiting value at 673 K and only minor quantities were evolved above this temperature (fig. 1). A similar heating experiment on a MgO sample that had no adsorbed chlorine gave a negligible desorption of oxygen con- firming that no adsorbed oxygen is present on the surface of the sample following preparation at 1273 K and subsequent sealing off (table 1). Analysis of the MgO samples subsequent to the heating experiments showed that a high level of chloride ion was present in comparison to an untreated sample. For a series of samples ranging in specific surface area (SSA) from 70 to 90 m2 g-' the74 HALOGEN REACTIONS ON MgO surface chloride coverage, expressed as chloride ions/surface MgO pair, varied be- tween 0.16-0.21 (table 1).No oxygen was evolved if the samples were kept at 300 K but colorimetric analysis showed a high chloride content corresponding to a surface coverage of 0.16. Heating the samples with adsorbed chlorine to 673 K produced a slight decrease in the specific surface area of the sample (table l), suggesting that sintering of the oxide is promoted by the presence of chloride ions. 0 temperature/K FIG. 1.-Oxygen liberated as a function of temperature after adsorption of chlorine on magnesium oxide at 300 K. The effect of excess chlorine on the analytical procedure was investigated using an aliquot of chlorine gas, identical to the amount of chlorine evolved from the sample during heating.The result showed that the chlorine contribution to the final chloride concentration was < 3 %, which is regarded as insignificant. TABLE 1 .-EVOLUTION OF OXYGEN FROM HALOGEN TREATED MgO total oxygen evolved I sample %:'i molecule g-1 M958/C12 72 4 . 6 ~ 1019 M961/C12 79 4 . 7 ~ 1019 M1008/C12 92 4 . 4 ~ 1019 M1007-f 104 < 1 . 0 ~ 1 0 ~ ~ M936/CI2 70 <4.0x lo1' maximum temperature / K 673 673 673 673 300 halide ions coveragelion ratio (surface MgO pair)-1 coverage/ion g-1 halide/@ 0.21 1 . 7 ~ 10'' 3.7 0.20 1 . 8 ~ lo2' 3.8 0.16 1 . 7 ~ 10'' 3.9 0.16 1 . 3 ~ lozo - < 1 . 7 ~ 10" - - M500/Br2 54 1.6 x 1019 873 0.1 1 0 . 6 9 ~ 1020 4.2 M5Ol /I2 67 0 . 0 4 ~ lOI9 1073 0.01 0 . 0 9 ~ lozo 22.0 * measured after thermal treatment ; 7 pure MgO which was prepared simultaneously with M1008 ; no chlorine adsorbed.REFLECTANCE MEASUREMENTS The beige colour of the sample was investigated more closely by recording the diffuse reflectance spectrum of the sample before and after chlorine treatment (fig. 2) over the range 200-800 nm. The MgO prepared in vacuo has a significant absorptionJ. F. J. KIBBLEWHITE AND A . J . TENCH 75 at 250nm and at shorter wavelengths there is some evidence of a fluorescence in agreement with previous w0rk.l The adsorption of chlorine destroys this fluoresc- ence and leads to new absorption bands at 210, 260 and 430nm and a shoulder at 350 nm ; the broad band at 260 nm is easily removed on pumping and is presumably associated with weakly adsorbed chlorine. Gaseous chlorine shows bands at 330-340 and 215 nm in CC1, 2 0 , 21 and 330 nm in the gas phase,22 which compare favourably with the bands at 350 and 210 nm observed when adsorbing chlorine on MgO.The band at 430nm is probably responsible for the beige colour because of the tail extending to long wavelengths. Comparison with the reflectance spectrum of de- hydrated MgC12 (fig. 2(e)) shows no evidence of the band at 430 nm and so it is not associated with a chloride lattice. 10 600 800 200 400 wavelength /nm FIG. 2.-Diffuse reflectance spectra of (a) MgO in U ~ C U O ; (b) MgO with excess dorine at 300 K ; (c) MgO with excess chlorine pumped off; (d) MgO/C12 after heating to 673 K; (e) magnesium chloride ; (f) (Oi)s on MgO. In a separate series of experiments a sample was heated and the diffuse reflectance spectra recorded at various temperatures. The output was recorded directly in digital form in the computer as described in the Experimental section.The reflectance data were then analysed using the Kubelka-Munk equation l9 : where R is the reflectance, k the absorption coefficient and S the scattering coefficient, and the wavelength scale was transformed to a scale linear in energy. The spectrum of chlorine adsorbed on MgO after pumping off the excess chlorine was analysed using Gaussian profiles (fig. 3) and bands at 430,345,290,245 and 220 identified. The band (1 - R)2/2R = kS76 HALOGEN REACTIONS ON MgO at 345 nm is rapidly removed on pumping and therefore associated with physisorbed chlorine. The band at 245 nm also decreases on pumping and a similar effect has been observed when physisorbed oxygen is removed from the surface; in this latter case it has been attributed by the author to interaction of the adsorbed species with surface states of the Mg0.23 The band at 220 nm is not significantly affected by heating (fig.2(b), (c) and (d)) but it is difficult to resolve this band precisely because it was recorded at the far end of the wavelength range of the instrument. The origin of the band at 290 nm is uncertain, but it could originate from an oxychloride species of the oxide, or more probably it is associated with surface states modified by the presence of the chloride ion. The curve resolved spectrum of pure MgO showed only a band at 250 nm. wavelength/nm energy/eV FIG.3.-Curve resolved reflectance spectrum of chlorine treated MgO with excess chlorine pumped Off. 300 400 500 600 (4 temperature/K reflectance peak at 430 nm ; (b) e.s.r. spin concentration. FIG. 4.-Normalised isochronal annealing curves of the chlorine treated MgO: (a) area underJ . F . J . KIBBLEWHITE AND A . J . TENCH 77 The remaining band at 430 nm is of particular interest since it seems to be associa- ted with the interaction of chlorine with the surface to form a strongly chemisorbed species. Heating at elevated temperatures causes a reduction of intensity of the band and the isochronal annealing curve (fig. 4(a)) shows the same type of pattern as the evolution of oxygen from the surface. The reflectance spectrum of (Ol)s on MgO (prepared by reaction of oxygen with the F:(H) centre 14* 24 was also recorded for reference purposes (fig.2cf)). E.S.R. MEASUREMENTS Magnesium oxide prepared in vacuo shows no significant e.s.r. signals before the addition of halogens, apart from a trace of Cr3+ indicating an impurity level of < 1 pg g-l. A new signal (fig. 5) with g-factors of 2.0099 and 2.0020 appears immed- iately when the chlorine is adsorbed and increases in intensity at room temperature to reach a concentration of 2 x 1017 spin g-1 after about 20 min. Identification of the species giving rise to the signal is difficult because it shows no hyperfine structure which indicates the absence of chlorine containing species such as C1;.l0* 2 5 The intensity of the signal is increased by the addition of small pressures of oxygen but no broadening of the lines is observed when 1.33 kN m-2 (10 Torr) of oxygen is added. U 2.0099 2.0020 FIG.5.-E.s.r. spectrum of chlorine adsorbed on magnesium oxide at 300 K. The g-factors are similar to those reported for 0, but no line corresponding to a gzz value is observed at high resolution. The g,, values for are a sensitive function of crystal field and could be smeared out by surface heterogeneities. No evidence for this was obtained either from inspection of the integrated spectrum (displayed using LABCOM) or by successive double integration over different field ranges. Area measurements of the first derivative signal using a planimeter show that the area of the positive and negative signal segments are equal. This confirms that the complete signal has been observed and no peaks corresponding to broadened gzz factors have been ignored.The signal was also unaltered when treated with 1.46 kN m-2 (1 1 Torr)78 HALOGEN REACTIONS ON MgO of hydrogen at 300 K, thus indicating that the signal does not arise from a species such as 0- which is known to react with hydrogen.26* 27 The absence of exchange when the sample was left in contact with oxygen enriched to 58 atom percent 1 7 0 indicates that the species is not (O;)s on the oxide surface, since this is known to exchange readily with gas phase 1702.28 The intensity of the e.s.r. signal decreases with increasing temperature (fig. 4(b)) but does not show a correlation with the de- crease of intensity of the reflectance peak at 440 nm (fig.4(a)). REACTION WITH BROMINE AND IODINE \TJl?on the bromine treated samples were heated oxygen was evolved and subsequent analysis showed that bromide ions were present. Heating samples with adsorbed iodine produced no significant oxygen release, even at temperatures as high as 1100 K ; the iodide ion concentration was also low (table 1). The reflectance spectra of the bromine and iodine treated samples were less resolved than those of the chlorine samples due to the difficulty of removing the excess halogens which have high optical absorption. However, curve resolution of the computer treated spectra suggests that after heating the bromine treated samples to 473 K bands are present at 360, 280, 235 and 215 nm. No bands could be resolved in the iodine system at 300 K or higher temperatures.E.s.r. measurements on samples with adsorbed bromine and iodine produced spectra similar to the chlorine induced signal shown in fig. 5 ; with g-factors of 2.0104 and 2.0023 for the bromine sample, and 2.0091 and 2.0020 for the iodine treated sample. Bromine containing radicals such as BrF are known to exist under certain conditions on Mg0,29 but are unstable at room temperature. The possible formation of these species was investigated by freezing bromine at the top of the sample tube with liquid nitrogen and allowing it to adsorb onto the sample in the microwave cavity. The sample became a creamy yellow but no evidence for molecular ions was observed and an e.s.r. signal similar to that shown in fig. 5 was formed. Iodine behaved similarly and the signal intensity increased slowly on standing at room temperature and could be completely destroyed at 400 K.TABLE 2.-HEATS Ot I'OKMATION AH OF SOLID OXIDE AND HALlDES chloride bromide iodide - -- oxitie AH! AH! AH! AH1 metal kcal rnol-1 AH/eV kcal rnol-1 AH/eV kcd rnol-1 AH/eV kcal moI-1 AHIeV in3gnesium (2f) 143 6.20 153 6.63 125 5.42 87 3.77 aluminium (3+) 391-401 17.20 168 7.30 126 5.46 75 3.25 silicon (4+) 216-218 9.41 liq - liq - 45 1.96 zinc (2+) 83 3.60 99 4.30 79 3.40 50 2.15 COMPETITIVE CHARGE TRANSFER When oxygen is added to a MgO sample containing F:(H) centres 14* 24 the blue colour disappears and (O;)s is formed.' Similarly, the reaction of chlorine with F2(H) centres might be expected to form (Cly)s. However, adsorption of chlorine was found to destroy the characteristic signal and blue colour, form an e.s.r.signal similar to that shown in fig. 5 and produce the beige colour also observed from unirradiated MgO treated with chlorine. The reaction of chlorine and iodine with (O;)s on MgO was also studied. The e.s.r. signal from (O;)s (fig. 6(a)) has a number of lines close to a gzz factor of 2.07, these corresponding to various different sites on the surface. The adsorption of aJ . F . J . KIBBLEWHITE A N D A . J . TENCH 79 halogen destroys these signals and gives a signal identical to that obtained by halogen adsorption on unirradiated MgO (fig. 6(b)). Quantitative adsorption/desorption measurements showed that when 6.9 x lo1' molecules oxygen per gram were adsorbed by the sample to form (OY)~, 6.9 x lo1' molecules oxygen per gram were evolved on reaction with chlorine at room temperature.This shows that (O;)s is destroyed completely by reaction with chlorine. No Cl; formation was observed and further reaction of the sample with oxygen did not reform 0; ; adsorption of iodine gave the same result. I I 1 2.0726 2.0078 2.0012 U 2.0099 2.0020 FIG. 6.-E.s.r. spectra of : (a) (Oi)s on magnesium oxide ; (6) after adding chlorine 'to (O& DISCUSSION Previous studies have shown that oxides can donate electrons to electron acceptor molecules adsorbed on the surface and this has led to the suggestion l 2 that halogens can react with oxide surfaces containing readily available electrons to form adsorbed molecular halides which can be observed by e.s.r. spectroscopy.Earlier work on MgO indicates that electron donation can occur to electron acceptor molecules such as nitrobenzene 30 and trinitrobenzene 31 adsorbed on the surface. However, our results on MgO show no evidence for the stabilisation of the molecular halide ions X, during the adsorption process even when readily available electrons are trapped at the surface. Although the simple analogy between the 0: and X; ions does not appear to hold, there is clear evidence that the halogens do react strongly with the surface in a more complex process. The presence of chloride ions after adsorption of chlorine indicates that a charge transfer reaction has taken place between the adsorbed chlorine and the MgO lattice.80 HALOGEN REACTIONS ON MgO Similar charge transfer reactions have been reported when methyl iodide is adsorbed on The number of chloride ions formed corresponds to some 20 % of the surface oxide ions, which indicates far reaching changes in the surface topography and is much higher than the number of adsorbed nitrobenzene ions formed 30 on a similar surface (- 1 %).The formation of a band in the reflectance spectrum and an e.s.r. signal indicate that at least one new species has been formed. The evolution of oxygen gas on thermal treatment is accompanied by a decrease in intensity of both the optical band and the e.s.r. signal. This suggests that the new species formed must contain oxygen, and therefore arise from what were originally oxide ions of the lattice. The possibility of intrinsic or extrinsic point defects acting as sources of electrons can be excluded since the high concentrations required are more than an order of magni- tude higher than the impurity content and several orders of magnitude higher than the intrinsic vacancy concentration. It is now necessary to discuss the results in more detail, and clearly the origin of the reflectance band at 430 nm is of considerable importance.Possible species are surface oxychloride compounds, such as hypochlorite and chlorates ; however, these species are expected to absorb light in the 200-300 nm region and not at 430 nm, for example, sodium hypochlorite absorbs at 295 nm. Test reactions with lead nitrate for hypochlorite and o-tolidine for chlorate 33 gave no indication of the presence of such species.These arguments suggest that the band does not arise from chlorine containing species and is therefore probably associated with lattice oxygen ions in some unknown form. This idea is supported strongly by the observation that MgO samples irradiated in the presence of oxygen give a band at 430 nm 1 3 9 29 very similar to that obtained when chlorine is adsorbed. A comparison of the number of chloride ions formed to the number of oxygen gas molecules evolved during heating to 673 K gives a ratio of 3.8 which is close to the correct stoichiometry for the reaction of the chlorine with oxide ions of the lattice 2CI2 +202-(lattice)-+4C1- + O,(gaseous). The quantity of oxygen evolved is two orders of magnitude larger than the spin concentration of the e.s.r. signal and it seems fairly clear that the e.s.r.signal relates only to a small fraction of the total process occurring on the surface. Absolute measure- ments on the concentration of species corresponding to the reflectance peak are not possible at present, since no information is available on extinction coefficients, but the thermal stability broadly corresponds to the inverse of the oxygen evolution curve. From this comparison we have assumed that the reflectance peak at 430 nm is likely to be associated with the precursor of the evolved oxygen and thus the site that has donated electrons to the adsorbed chlorine. No exact correlation with the oxygen desorbed from the surface is expected since the oxygen may be held adsorbed on the surface for a while after the precursor has been destroyed.The oxygen precursor and the band at 430nm probably arise from an oxygen complex formed by abstraction of electrons from surface and subsurface 02- ions of the lattice. A number of such species are possible, for example 0-, 0 2 , O;-, and 0; ; however, some of these can be eliminated quite easily. The peak at 430 nm is consistent with 07 279 34 but not 0; ; however, the e.s.r. evidence is not consistent with the presence of either 0; or 0-. The peroxide ion 0;- is diamagnetic and would be expected to release gaseous oxygen easily on heating; however, peroxide ions usually absorb at shorter wavelengths, for example at 250 nm for BaO,. The evidence put forward in these arguments appears to be somewhat contra- dictory, and the most satisfactory model for the donor site appears to be a site in- volving two or more oxygen nuclei that have lost part of their negative charge.J .F. J . KIBBLEWHITE AND A . J . TENCH 81 Alternatively, this could be regarded as the trapping of holes on neighbouring lattice oxide ions and one possible form of this model is shown in fig. 7. Such a pair of trapped holes would form a species which is intermediate between two 0- ions and a peroxy ion OZ-. C I- C I' / / MgZt COY' Mg2' 02- Mg2+ J 02- MgZt 02- Mg2' 02- FIG. 7.--.Formation of an 0;- complex such as 0;- by electron abstraction from lattice 02- ions to form surface chloride ions. In the light of these ideas it is interesting to compare the results on the reactivity of the surface with various halogens. The data in table 1 shows that chlorine and to a lesser extent bromine are capable of replacing oxide ions of the lattice with the evolu- tion of oxygen, this is not true for iodine, although some iodide ions are formed. This is not easily understood in terms of the electron affinity values 35 from the gas phase or bond energies,36 since the energy available from the reaction : Xz +2e+2X- is 5.0 eV for C12, 5.3 eV for Br, and 4.9 eV for I2 ; which are very similar values, although we have neglected important coulombic and polarisation interactions with the charged lattice ions.However, a comparison of either the heats of formation or free energy values for the oxide and halides 37 is more useful. Since this is not an equilibrium process the heats of formation have been used (table 2) and in practice the free energy values also lead to the conclusions outlined below.From these data we can see that chlorine would be expected to liberate oxygen from MgO. The effect of bromine is somewhat surprising, and must indicate a significantly lower stabilisation of the surface oxide ions compared to the bulk. It is reassuring that iodine does not liberate oxygen and this clearly places a lower limit on the stability of the surface oxide ions. However, it is likely that some of them are able to donate one electron even to iodine and this accounts for the significant quanti- ties of iodide formed. This is a different type of electron transfer where lattice oxygen is not replaced and probably corresponds to the same situation as has been observed previously for nitrobenzene 30 and similar electron acceptors adsorbed on the surface.The concentration of the iodide ion formed is similar to that observed for the negative ions of the organic electron acceptors and is much lower than for the chloride ion. These results allow some conclusions to be drawn about the nature of the surface oxide ions. The liberation of gaseous oxygen from the surface by bromine indicates that some surface MgO ion pairs must be destabilized with respect to the normal bulk material by at least 18 kcal mol-1 (0.8 eV). The origin of this effect lies in the changed coulombic and polarisation effects present at the surface. Theoretical calculations show that the coulombic and polarisation terms act in opposite ways and the net effect is to reduce the net stabilization of the ion pairs at the surface by an amount 389 3982 HALOGEN REACTIONS ON MgO corresponding to 5.6 kcal niol-l (0.25 eV) for a (100) surface.This alone is not sufficient to account for the higher reactivity of the surface ; although the values of the surface energy show considerable variation 38 it seems unlikely that they are in error by more than a factor of two and some experimental results 38 suggest a figure of about 7.4 kcal mol-l (0.33 eV). However, it is unlikely that the surface of these pow- ders can be regarded as a good (100) plane since defects such as steps, kinks, edges will be present in addition to the possibility of other crystal planes of lower symmetry than The net effect of these sites and low index surfaces is to provide surface ions of lower symmetry and therefore of greater surface energy.Clearly, these are the surface features that are most likely to be decreased on high temperature treatment but it is unlikely that they have been eliminated completely. Calculations on the ratio (7) of surface to bulk Madelung potential for NaCl type structure 41 indicate considerable variation as the surface co-ordination decreases, for a (100) surface with five-fold co-ordination y = 0.96, for a (110) surface with four-fold co-ordination y = 0.86, and for a (210) surface with three-fold co-ordination, y decreases to 0.60. The magnitude of the effects on the surface energy when both coulombic and polarisation effects are present does not appear to be known in detail.The electrostatic part of the surface energy is proportional to 1 -y, which would give an increase in the surface energy of a factor of 3.5 on going from five-fold to four-fold co-ordination. This is in good agreement with the values calculated by Shuttle- worth 42 for NaF where polarisation terms were included but are fairly small. For MgO the polarisation terms will be more important and will result in a smaller increase in surface energy ; however, it seems likely that a sufficient change in surface energy will be obtained in sites or surface planes of lower co-ordination to permit reaction of bromine with the surface with the elimination of oxygen. Chlorine on the other hand can react with a wider range of surface sites and planes and twice the coverage is obtained.From these figures it seems likely that as many as 10 % of the oxygen ions exist in sites of lower than five-fold co-ordination at the surface. This is not unreasonable since even assuming perfect cubes with (100) faces then 160 particles (specific surface area = 100 m2 g-’) will have - 3 % of surface ions in lower than five-fold co-ordination. In practice, the crystal surface will contain steps and similar defects together with a proportion of lower symmetry planes. The surface sites involved with the formation of iodide ions may be of even lower symmetry; this could correspond to situations where the oxygen ions have three-fold co-ordination on the surface (for example corner sites), and such ions could readily donate an electron. The absence of an observable e.s.r.signal from 0- can be explained in terms of a delocalisation of the electrons over several oxygen ions. The use of very powerful electron acceptors such as the halogens to explore different types of electron transfer reactions with oxide surfaces opens up a new way of exploring the surface energies of crystals and measuring the reactivity of surface ions. This approach will allow the comparison of the effects of different methods of preparation on the surface reactivity. Exploratory work on other oxides indicates that oxygen is not released when chlorine is adsorbed on N,03 and SOz, and this can be predicted from a comparison of the relevant heats of formation ; on the other hand, chlorine does react with ZnO to release oxygen,29 and this is expected from a comparison of the heats of formation of ZnO and ZnC1, (table 2).This indicates that surface studies involving the halogens must be treated with caution since in some cases reaction with the surface is of import- ance. Lastly, the desorption of (Ol)s from the surface as gaseous oxygen when halogens are adsorbed at room temperature confirms the low electron affinity of the adsororbJ . F. J . KIBBLEWHITE AND A . J . TENCH 83 oxygen on the surface. This could provide a useful way of exploring adsorbed species on a surface that has been exposed to chemical treatments and contains adsorbed ions about which little is known. A. J. Tench and P. Holroyd, Chem. Comm., 1968,471. A. J. Tench and T. 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