J . Chem. SOC., Faraday Trans. 1, 1982, 78, 1705-1715 Electron-donor Sites of Oxides as Investigated on Oxygen- 17 Exchanged CaO Surfaces BY VALERIO INDOVINA* AND DANTE CORDISCHI Centro di Studio su 'Struttura ed Attivita Catalitica di Sistemi di Ossidi', Istituto Chimica Generale ed Inorganica, Universita di Roma, Rome, Italy Received 22nd April, 198 1 The electron-donor properties of various oxides (BaO, SrO, CaO, MgO, y-Al,O, and Si0,-Al,O,), activated under vacuum in the temperature range 1000-1273 K, have been investigated by adsorption of 9,IO-dimethylanthracene (DMAN), chlorine and oxygen (90% "0-enriched or non-enriched). Upon adsorption of DMAN, radical anions are formed on CaO, whereas radical cations are formed on Si0,-AI,O,. On oxides of intermediate base strength (MgO and y-Al,O,) very low radical concentration was obtained.The data allow for a further specification of the correlation between electron acceptordonor properties of oxides and their acid-base strength. Upon adsorption of Cl,, a two g-value e x . signal (8, = 2.002, g, = 2.013) was observed on alkaline-earth oxides. A nearly identical signal was also observed on adsorption of 0, on the same oxides. Upon adsorption of 0, (non-enriched) on CaO samples, previously exchanged with 1702, the above signal showed some relevant modifications, suggesting that the paramagnetic species contains oxygen atoms. The signal is tentatively assigned to a 0;- surface species. This species is thought to be formed on the surface of alkaline-earth oxides in the electron-donation process from low-coordination oxidic sites (Oiks) toward acceptor molecules.The 0$ks sites are therefore identified as the electron-donor sites. The work also includes a volumetric study of 0, adsorption on the various oxides. By comparing volumetric adsorption data with the concentration of radicals (0; and, O;), it is possible to provide a description of the 0, adsorption mechanism on alkaline-earth oxides. The formation of anion radicals upon adsorption of various acceptor molecules (A) on the surface of alkaline-earth oxides, previously activated under vacuum at high temperature, is a well documented phenomenon. Some typical examples can be found in ref. (1)-(6). The nature of the electron-donor sites has attracted much attention. Most authors1? 4 7 5 7 appear to agree on two main points: (a) the donor centres are surface 0,- ions in low-coordination sites and (b) the mechanism of the electron ., donation is O&+A -+ O-+A'-. A point of some concern with respect to the above mechanism is that whereas the A'- radicals have been detected in several cases, there is as yet no experimental evidence for the simultaneous formation of 0- species. The arguments put forward to explain this result are not completely convincing since 0- species are known to be formed on the surface of alkaline-earth oxides and to possess a stability which allows their detection by e.s.r.* Moreover, Garrone et al. have recently proposed a mechanism for anion-radical formation which does not require electron transfer from the s01id.~ Further experimental work is therefore needed to support these suggestions as to the nature of the electron-donor sites and the electron-donation process.In the present work, we have studied the adsorption of 9,1O-dimethylanthracene, C1, and 0, (90% "0-enriched or non-enriched) on alkaline-earth oxides, Al,O, and Si0,-Al,O,. Relying also on 0, adsorption experiments on l'o-exchanged CaO 17051706 ELECTRON-DONOR SITES OF OXIDES surfaces, we have been able to show that: (a) surface oxygen ions of the oxide participate directly in the electron-donation step and (b) 0- species are formed in this step. EXPERIMENTAL MATERIALS High surface area MgO, y-Al,O, and SO,-Al,O, (alumina content 25%) were prepared as previously described.'? lo BaO, SrO and CaO were prepared by decomposition of the carbonates (Erba, RP) at 1173 K (CaO) or at 1273 K (BaO and SrO) under vacuum.B.E.T. surface area values (m2 g-l), determined by Kr adsorption at 77 K, were: MgO (200), CaO (120), SrO (3.6), BaO (0.8), Al,O, (120) and Si0,-Al,O, (120). 9,lO-dimethylanthracene (DMAN) and n-hexane were of Reagent grade and were further purified by distillation (n-hexane) or by recrystallization (DMAN). 0, and C1, were dried before admission to the adsorption chamber. High-purity non-enriched 0, ('Air Liquide ' 99.95 %), 90 % 170-enriched 0, (Yeda, Israel) and C1, (Matheson) were used without further purification. APPARATUS AND PROCEDURE A weighed amount of specimen (ca. 0.1 g) was placed in a silica bulb equipped with a side e.s.r. tube. Samples were activated under vacuum for 5 h at 1173 K (MgO and CaO), at 1273 K (SrO and BaO) or at 1000 K (y-Al,O, and SO,-Al,O,) before exposure to 0,, C1, or DMAN, generally at 298 K.Volumetric determinations of oxygen adsorption were performed by contacting the activated samples with 0, at a pressure of ca. 50 Pa. Pressure readings were made with a differential pressure transducer (MKS, Baratron) capable of detecting variations of 0.1 Pa. The adsorption was considered complete when two successive readings at 5 min intervals did not differ by > 0.5 Pa. Total adsorption and irreversible adsorption (molecule m-,) were determined as described below. Solutions of DMAN in n-hexane ( lop2 mol drn-,) were evacuated at room temperature and successively contacted with the activated samples by means of a break-seal system.170-exchanged CaO samples were prepared by heating CaO samples, previously activated under vacuum, in the presence of 90% "0-enriched oxygen. The extent of isotopic exchange was monitored by gas-phase analysis on a mass spectrometer (VG, Micromass 601). The e.s.r. spectra were recorded at X-band frequencies on a Varian E-9 spectrometer. The absolute number of spins was determined from electronically integrated spectra using Varian 'strong pitch' (3 x 1017 spin m-l) as a standard. RESULTS ADSORPTION OF 9,lO-D I METHY L ANTHR A C ENE (DMAN) Upon adsorption from a solution of n-hexane, an e.s.r. signal consisting of seven main lines with a splitting of ca. 7.6 G* is observed on Si0,-Al,O,, previously activated under vacuum at 1000 K [fig.1, spectrum (a)]. Each line of the spectrum is further resolved into a multiplet with a splitting of ca. 3 G. The signal corresponds to ca. 4 x 1015 spin m-,. CaO, activated under vacuum at 1173 K, gives, upon adsorption from the same solution as above, an e.s.r. signal [fig. 1, spectrum (b)] showing a poorly resolved sequence of lines with a splitting of ca. 2 G. The signal intensity corresponds to 1.6 x 1015 spin m-,. y-Al,O,, activated at 1000 K, and MgO, activated at 1173 K, give weak and structureless e.s.r. signals. In table 1 the hyperfine proton-coupling constants of DMAN radicals chemisorbed on the surface of CaO and Si0,-Al,O, are compared with those of the radical cation and anion in solution. The comparison shows that an anion radical forms on the surface of CaO and a cation radical on the surface of SiO,-Al,O,.* 1 G = 10-4 T.V. INDOVINA AND D. CORDISCHI 1707 FIG. I.-E.s.r. spectra recorded at 77 K on (a) Si0,-Al,O, and (b) CaO after contacting the samples at 298 K with a solution of 9,lO-dimethylanthracene in n-hexane. TABLE 1 .-HYPERFINE PROTON-COUPLING CONSTANTS OF 9,l 0-DIMETHYLANTHRACENE RADICALS CHEMISORBED ON OXIDE SURFACES AND IN SOLUTIONa CH3 H H H radical 9-10 1-4 2-3 1, 2, 3, 4 ref. 11 cation in solution 8.00 2.54 1.19 - anion in solution 3.88 2.90 1.52 11 chemisorbed on Si02-A1203 7.6 3.0 this work chemisorbed on CaO (4.0) - - 2.0 this work - - - a Hyperfine constants are expressed in gauss (1 G = T). Estimated error: kO.1 G ADSORPTION OF CHLORINE Upon adsorption of C1, at 298 K on alkaline-earth oxides, previously activated under vacuum at 1173 K, a two g-value e.s.r.signal is observed (8, = 2.002 and g, = 2.010). The e.s.r. spectra recorded at 77 K are collected in fig. 2: MgO [spectrum (a)], CaO [spectrum (b)] and SrO [spectrum (c)]. A nearly identical signal has been1708 ELECTRON-DONOR SITES OF OXIDES v FIG. 2.-E.s.r. spectra recorded at 77 K on (a) activated MgO, (b) CaO and (c) SrO, after exposure to chlorine at 298 K. observed by Kibblewhite and Tenchl, upon adsorption of C1, on MgO. The species will be hereafter referred to as 0;-, the assignment being discussed below. The presence of an additional set of low-intensity lines in the low-field side of the spectra, clearly visible on the CaO sample [fig. 2, spectrum (b)], suggests the formation of a second paramagnetic species which probably contains chlorine atoms.Although an identification of this species is not possible, the formation of the C1; radical can be ruled Adsorption of Cl, at a lower temperature (146 K) also fails to give Cl;. OXYGEN RADICALS ON CaO Upon adsorption of 0, (non-enriched) at 298 K on CaO activated under vacuum at 1173 K, a two g-value signal ( g , = 2.0020, g , = 2.013) was observed in the e.s.r. spectrum recorded at 77 K in the presence of 0.1 kPa of 0, in the gas phase [fig. 3, spectrum (a)]. The signal is the same as that observed after C1, adsorption on the alkaline-earth oxides and will be therefore designated as 0:-. The spectrum of this species is broadened at higher oxygen pressure. In particular, the half-height linewidth of the sharp component at g , = 2.0020 doubles when the oxygen pressure is increasedV.INDOVINA AND D. CORDISCHI 1709 f f l x 2.5 FIG. 3.-E.s.r. spectra recorded at 77 K on CaO samples, previously activated under vacuum at 1173 K, after exposure to oxygen at 298 K. Adsorption of non-enriched 0,: (a) in the presence of 0, in the gas phase and (b) after evacuation at 298 K. Adsorption of 90% "0-enriched oxygen: (c) in the presence of 0, in the gas phase and ( d ) after evacuation at 298 K. Adsorption of non-enriched 0, on "0-exchanged CaO: (e) in the presence of 0, in the gas phase and (f) after evacuation at 298 K. from 0.1 to 1 kPa. After removal of the gas-phase 0, by evacuation at 298 K, the e.s.r. signal becomes much more complex.The spectrum observed in these conditions was previously shown to arise from three different surface species: OF, 0; and O;-.14 The three g-value signal of the 0; species (gl = 2.0023, g, = 2.0095 and g, = 2.0185) is shown in fig. 3 [spectrum (b)]. As previously reported, the 0; and 0; species are not detected in the presence of oxygen in the gas phase because of the presence of well known strong broadening effects. Upon adsorption of 0, (90% 170-enriched) on a CaO sample, thermally activated as described above, a spectrum identical to that obtained after adsorption of non-enriched 0, was observed in the presence of 0, in the gas phase [fig. 3, spectrum (c)]. By contrast, after removal of 0, at 298 K, the spectrum appears to be drastically modified. In particular, in the central region of the spectrum [fig.3, spectrum (43 the three components of the 0; species, which dominated the spectrum after adsorption of non-enriched 0, [spectrum (b)], are now absent. Thus, with 170-enriched 0,, the central parts of the spectra recorded in the presence or in the absence of oxygen differ very little: namely, only the lines of the 0;- species are clearly visible [compare spectra (c) and (d)]. This result demonstrates that the species 0;- is also present after removal of oxygen at 298 K. The 0;- signal is not observed in spectrum (b), being obscured by the more intense signal of the 0; and 0; species. An important difference with respect to spectrum (b) is the presence in spectrum (d) of a set of six low-intensity lines with a splitting of ca.5 G. These lines are most probably some of the hyperfine components of the 0; species. Indeed, according to the analysis made by Tench,15 the species (1s0,-170,-170,)-, where 0, is a lattice oxygen of the surface, should give a spectrum consisting of 36 lines centred around g, with At, = 108 G and A: = 70 G.1710 ELECTRON-DONOR SITES OF OXIDES 200 G % FIG. 4.-E.s.r. spectrum recorded at 77 K on CaO after exposure to 90% "0-enriched oxygen. The details of the central part of the spectrum (out of the scale in this figure) are shown in fig. 3, spectrum ( d ) . These lines could not be detected because of their low intensity. However, as A% = A2 = 15 G and A: = A: = 10 G around g, and g,, two sets of 24 lines should be obtained with a splitting of ca.5 G. In the spectrum recorded under vacuum, the simultaneous presence of the species 0; is shown by the persistence of the line at g = 2.10 (g,-component) and by the fact that, with higher receiver gain, 11 lines centred around g, are observed (fig. 4). The &-value (75 G) is in good agreement with that reported (76 G) by Che et al. for 0; on pyridine-promoted CaO. l6 ADSORPTION OF 0, ON 170-EXCHANGED CaO SAMPLES Prior to oxygen adsorption, a CaO sample was exchanged with 90% 170-enriched oxygen. Three successive portions of enriched 0, (2.4 x mol) were contacted at 1173 K for 2 h with a CaO sample (oxygen content 4.9 x mol, expressed as 0,) previously activated under vacuum for 5 h at 1 173 K. The final extent of exchange was 26% of the total oxygen content (surface and bulk) in the CaO sample.The CaO was further activated under vacuum at 1 173 K for 1 h and then exposed to non-enriched 0, at 298 K. The spectra recorded at 77 K in the presence of 0, [spectrum (e)] and after removal of 0, [spectrum cf)] are reported in fig. 3. Spectrum (e) (species OE-), when compared with spectra (a) and (c), shows three main differences: (i) a marked broadening of the line at g, = 2.0020, (ii) the appearance of a new line at g = 2.028 and (iii) a decrease by a factor 2 of the signal intensity (from integrated spectra). The relative intensities of the three components of spectrum (e) are unaffected by varying the microwave power.* In the spectrum recorded under vacuum [spectrum (f)] the 0; signal is again visible and the component of the 0;- species at g = 2.028 is still present.* An expanded spectrum recorded with higher receiver gain does not show additional lines.V. INDOVINA A N D D . CORDISCHI 171 1 The 170-exchanged CaO sample was successively contacted with a large excess of non-enriched 0, at 1173 K for 6 h, evacuated at 1173 K for 1 h and then exposed to non-enriched 0, at 298 K. Spectra (a) and (b) were obtained. In particular, the line at g = 2.028 disappeared and the intensity of the 0;- species was restored. OXYGEN RADICAL ON OTHER OXIDES The formation of oxygen radicals on MgO, thermally activated under vacuum, has been investigated previ0us1y.l~ The main results can be summarized as follows. After 0, adsorption, MgO samples, activated at 1173 K, show a weak signal from the 0;- species.The formation of 0; and 0; observed in some cases is strongly dependent on the previous history of the sample: pre-exposure to H,17-19 and different thermal treatments in air, such as quenching or annealing of the samples after heating at high temperature. 2o On adsorption of 0, at 298 K on SrO, activated in vacuo as for the other alkaline- earth oxides, no e.s.r. signals are observed. However, if adsorption is carried out at 146 K, the signals of 0; and of 0; (the latter in trace amounts) appear. On leaving the sample for a few hours at room te.mperature the signals disappear, indicating a lower stability of these species on the surface of SrO as compared with MgO and CaO. No e.s.r. signals are observed when 0, is adsorbed on BaO, y-Al,O, and Si0,-Al,O,.VOLUMETRIC MEASUREMENTS OF 0, ADSORPTION A measurement of more strongly bound oxygen (irreversible oxygen) was taken as follows. 0, was first adsorbed at 298 K on samples activated under vacuum at 1023 K (7-Al,O, and SiO,-Al,O,), at 1173 K (MgO and CaO) or at 1273 K (BaO and SrO). TABLE 2.-oXYGEN ADSORPTION AT 298 K irreversible oxygena oxygen radicals / 1 Owl5 molecule mP2 / 1 0-15 spin mP2 sample BaO SrO CaO MgO Si02-A1203 r-A1203 1600 170 14 0.5 0.2 0.2 0.0 7c O.Od 0.0 0.0 1 b . c a Amount of oxygen not desorbed by evacuation for 10 min at 298 K. Concentration of radicals obtained by adsorption at 146 K. No radicals were formed at 298 K. Simultaneous formation of 0; and 0; (0; = 5.5 x 1015 and 0; = 1.5 x 1015) on CaO. On SrO the species 0; is present in trace amounts.In some cases 0; and 0; are also formed on MgO (for details see the references quoted in the text). The amount of 0, determined in this way will be called total oxygen. Subsequently, the samples were evacuated for 10 min at 298 K, and a seond portion of oxygen was adsorbed, always at 298 K. The amount of oxygen adsorbed in this last experiment will be called reversible oxygen. The difference between the total adsorption and the reversible amount is the irreversible oxygen reported in table 2. Table 2 also lists the concentration of oxygen radicals as determined by e.s.r.1712 ELECTRON-DONOR SITES OF OXIDES DISCUSSION E L E C T R O N A C C E P TO R-D 0 NOR PROPERTIES OF 0 XI D E S The formation of the radical cation of DMAN on the surface of SiO,-A1,0, and that of the radical anion on the surface of CaO provide new and definitive evidence for the existence of a correlation between electron acceptor-donor properties of oxides and their acid-base properties. In fact, previous work from our group has shown that the concentration of the negative radical of nitrobenzene (NB), formed by adsorption of NB on the surface of oxides, monotonically decreases when the oxides are taken in the order of their decreasing basicity (BaO > SrO > CaO > MgO > Al,O, > Si02-A1,0,).10~ 21 Moreover, the concentration of positive radicals of hexamethylben- zene (HMB) increases when the oxides are taken in the same order of basicity given above.21 Namely, the higher the basicity of an oxide, the higher its electron-donor properties on the one hand, and the lower its electron-acceptor properties on the other.Conversely, when the concentration of the radicals of perylene (PE), anthracene (AN) and naphthalene (NA) is considered for the same oxides (again listed in the order given above), there is a minimum in the radical concentration on oxides of intermediate basicity (MgO and A120,).21 This behaviour may be explained using the assumption that radical anions of PE, AN and NA are formed on the surface of strongly basic oxides (BaO, SrO and CaO) whereas radical cations are formed on strongly acidic oxides (Si02-A1,0,). However, for alternant hydrocarbons, such as PE, AN and NA, the spectroscopic differences between their anion and cation radicals are small, both in the e.s.r. and electronic spectra.,, Therefore, in view of the poor resolution of powder spectra, an unambiguous assignment of the radicals cannot generally be made on the basis of spectroscopic evidence alone.The above considerations account for some difficulties encountered in the assignment of the e.s.r. spectrum of PE chemisorbed on the surface of A1,0,.23724 In particular it is rather difficult to decide whether the radical anion or the radical cation of PE is formed. The situation is much simpler with DMAN in view of the large difference (a factor of ca. 2) in the hyperfine coupling constants of the methyl protons for the radical anion as compared with the radical cation (table 1). Thus (i) the formation of the radical cation of DMAN on SiO,-Al,O,, (ii) the formation of the radical anion on CaO and (iii) the lack of formation of such radicals on MgO and A1,0, are in agreement with the trend observed with PE, AN and NA on the same oxide surfaces.21 The results also provide evidence for the general statement that radical anions are formed by adsorption of PE, AN and NA on basic oxides and radical cations on acidic surfaces.NATURE OF THE ELECTRON-DONOR SITE The simplest way to visualize the formation of negative radicals on the surface of oxides is to invoke a direct electron-transfer process from a surface site toward a given acceptor molecule (02, Cl,, nitrobenzene, etc.). As far as the nature of the electron-donor sites on alkaline-earth oxides is concerned, most authors agree that these consist of surface 02- ions in low-coordination sites (O&J.l9 4 7 5 7 Recently, Garrone et al.have proposed a different mechanism to explain the formation of anion radicals on the surface of alkaline-earth ~ x i d e s . ~ According to these authors, the anion radicals can be formed without any electron transfer from the solid. In fact, an XH molecule (e.g. a hydrocarbon) with a large enough electron affinity can be heterolytically chemisorbed on a surface (Me2+ 02-) site leading to species Me2+ X- and OH;. Subsequently, the carbanion can transfer an electron to a second XH molecule leading to the anion radical XH’-. Note that both mechanisms require the participation of OEts sites. These surface centres are Lewis base sites, althoughV. INDOVINA AND D. CORDISCHI 1713 they act as a source of one electron only, in the first case, and Bronsted base sites, in the second case.Accordingly, a correlation between electron-donor properties and basicity of the surface is expected in both cases. In our opinion, the mechanism proposed by Garrone et aL9 might well be operating with specific molecules and satisfactorily explain the sensitizing effect of pyridine and other molecules in promoting the formation of 0; on alkaline-earth oxides, but the participation of 065, in the electron-donor process emerges in the present study, as will be illustrated below. In particular, the species 0:- and 0; will be shown to originate from 0- species which are the first product in the oxidation of the electron-donor centre O&,. It is therefore convenient to consider first the main features of the 0:- species: 1.The centre is on the surface (or very near to it), as shown by the fact that its e.s.r. signal is substantially broadened on increasing the 0, pressure. 2. The centre contains oxygen atoms since the e.s.r. signal is different on the 'natural' CaO as compared with the 170-exchanged sample. On this latter sample, the broadening of the signal, the appearance of a new, broad component and the decrease of intensity are thought to arise from hyperfine interactions with 170. 3. The centre originates from an oxygen species already present on the CaO sample after activation. The same signal is in fact obtained whether 0, or C1, is adsorbed. An identical signal is obtained upon lS0, or 1702 chemisorption. In principle, three possible species could account for the above features: 0-, 0;- and O!-.Species 0;- with n odd and > 3, also possible in principle, do not appear likely. The 0- species can be ruled out in view of the following two main points. First, the e.s.r. spectrum of the species 0-, previously detected on alkaline-earth 25 shows features which do not agree with those of the 0;- signal. In particular, the g-values are substantially different: 2.047 and 2.0014 for 0- as compared with 2.01 and 2.0020 for 0:-. Secondly, the 0- species readily reacts with 0, leading to the species 0; and, therefore, cannot be observed in the presence of 0,. The species 0;- can also be ruled out. Two possibilities must be considered: (a) the species is the diamagnetic peroxy ion and (b) the species contains two unpaired electrons (0-. - SO-).In the first case no e.s.r. signal should be detected, whereas in the second case the molecule is in a triplet state. However, the e.s.r. spectra of the species 0- - * 0- (Vo centres), previously observed in MgO single crystals,26 are rather different from the spectrum we observe for 0:-. In particular, due to a very large D-term (> 200 G), separated e.s.r. lines are observed for the Vo centre.26 In the light of the above arguments, the 0;- species appears to be the most suitable model for the centre under discussion. 0:- centres, consisting of a triangular array of 0- species, are thought to be placed either on (1 11) surface micro-planes, which are formed during the activation of alkaline-earth oxides under vacuum at high temperature, or more simply on the corner of the oxide p a r t i ~ l e .~ ? ~ ~ A mechanism for 0, adsorption on alkaline-earth oxides can now be considered. The following sequence of surface reactions is proposed : 2 0;- + O,(g) + 2 02,- 2 o,,, + 0;- 3 o,,, -+ 0;- (2 4 (3) 0;- + 0;- + 2 02- +O,(g). 56 FAR 11714 ELECTRON-DONOR SITES OF OXIDES The mechanism is based upon the detection by the e.s.r. technique of 0; [step (1 a)], 0; [step (2a)l and 0;- species [step (2c)l. The formation of diamagnetic species, 0;- [steps (I b) and (2b)l and 0,- [step (3)] is inferred from a comparison of the adsorption data, as determined by the volumetric method, with the concentration of radicals (table 2). The data of table 2 also suggest that reaction (1 b) is prevalent with respect to reaction (1 a) on the surface of BaO and SrO, in agreement with the more pronounced tendency of these materials to give peroxides.Accordingly, 02- surface ions taking part in reactions (1 a) and (1 b) are designated by different symbols, respectively O&,s and OE-, to underline the fact that whereas a large fraction of them can participate to step (1 b) (such as, for instance, on BaO), only those in low- coordination sites are active in step (1 a). Following step (1 a), the electron-donation step, the 0- ions formed undergo surface reactions (2a), (2b) and ( 2 c ) . The occurrence of these reactions accounts for the lack of 0- detection by e.s.r. However, the detection of 0; and 0;- species strongly supports the suggestion that 0- ions are formed first.Moreover, in the case of CaO, which may be studied in a more quantitative way, the concentrations of OF, 0; and 0;- are consistent with the stoichiometry of the adsorption scheme. In particular, from step (1 a) [O;] = [O-] and from steps (2a) to ( 2 c ) , [O-] = [O;] + 2[0:-] + 3 [ 0 ; - ] (5.5 x 1015 > 1.5 x 1015 + 3 x 0.6 x 1015). Finally, step ( 3 ) accounts for possible formation of 0,- species by surface migration of peroxy ions and 0, desorption from particular sites (near to kinks, edges or corners). TABLE 3.-cONCENTRATION OF SPECIES 0;- AFTER 0, OR c1, ADSORPTION AT 298 K C/ 1 O-I5 spin rnU2 sample 0 2 c12 10 SrO - CaO 0.6 2 MgO 0.2 0.8 A1203 0.0 0.0 Finally, we briefly consider the concentration of 0;- species formed on the surface of oxides after adsorption of C1, or 0, (table 3).As expected from the electron-affinity values, the concentrations obtained upon C1, adsorption are higher than those obtained with 0,. Furthermore, the concentration of 0;- is found to decrease mono- tonically in passing from SrO to y-A1203. This finding is relevant in view of the fact that the 0;- concentration is expected, on the basis of the mechanism proposed here, to be proportional to the concentration of O t i s donor sites. Therefore, the concentration of 0;- can be regarded as a rough measure of the concentration of the electron-donor sites. Thus, the correlation between electron-donor properties and surface basicity, previously found to rely on the concentration of anion radicals formed,l09 21 is now confirmed more directly uia the concentration of electron-donor sites. A.J. Tench and R. L. Nelson, Trans. Faraday SOC., 1967, 63, 2254. B. D. Flockhart, L. McLoughlin and R. C. Pink, J . Cafal., 1972, 25, 305. M. Che, C. Naccache and B. Imelik, J. Catal., 1972, 24, 326. D. Cordischi, V. Indovina and A. Cimino, J. Chem. SOC., Faraday Trans. 1, 1974, 70, 2189. * B. D. Flockhart, 1. R. Leith and R. C. Pink, Trans. Faraday SOC., 1969, 65, 542; 1970, 66, 469.V. INDOVINA A N D D. CORDISCHI 1715 B. D. Flockhart, P. A. F. Mollan and R. C. Pink, J. Chem. Soc., Faraday Trans. I , 1975, 71, 1192; B. D. Flockhart and R. C. Pink, J . Catal., 1980, 61, 291. S. Coluccia, A. J. Tench and R. L. Segall, J . Chem. SOC., Faraday Trans. 1, 1979, 75, 1769. A. J. Tench, T. Lawsan and J. F. J. Kibblewhite, J. Chem. Soc., Faraday Trans. 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