ESR of O−2 species adsorbed on thermally activated MgO powders

Volume

14, nuinber 4

ESR OF 0;

1.5 June 1972

CHE?ACAL PHYSICS LETTERS

SPECIES

ADSGRBED

ON THERMALLY

ACTIVATED

MgO POWDERS

E.G. DEROUANE *

and V. INDOVINA Istihtto

di Chimica

Geiw-ale c Itlorganica,

Urliversir~ di Rorna, Rorna. Iral_\’

Received 2. March 1972

hIolccular adsorption OP osygen as 0; ions occurs on XlgO polvders cvncuated at 1ZOO- 1300aK with no need for prior irradiation. Several 02 species we obscrvcd and rclntcd to the prexnce of vnriuus adsorprion sites.

Numerous previous studies report the observation by ESR of oxygen chemisorbed species on irradiated [I] or slightly reduced [2,3] oxides. Theoretical approximations for those have been reviewed quite recently [l]_On MgO powders,07 [4-G] and O- [T-9] species have been detected after y or UV irradiation. We report here the observation of 0: chemisorbed species on MgO powders which have been thermall) activated in vacua at 1300-1300”K, with no need for prior irradiation. The preparation and activation of our high surface area and cstreme purity magnesium oxide polycrystalline samples are described elsewhere [lo]. Two different batches have been used. One was spectrographically standardized MgO from Johnson, kiatthey, and Co. (MgO-JM). The second was obtained by controlled decomposition in vacua of 3 MgC03 . M&OH)? . 3H20, prepared from triply distillated Mg metal (Dow Chemical Co.) and it is referred to hereafter as MgO-EP. No traces of transition metal ions were detected in this latter batch of which the purity is about lo- 100 times higher than the one of MgOJM. O? species are found to be present after G-, ad* Y~zI&

de Rechcrches” of F.N.R.S. (Belgiiuri~) to whom all should be addressed.

correspondence

sorption at room temperature on samples eventually showing an S-type (Fz) center resonance [ 14, 151 but with no need for prior irradiation. Our results are summarized and compared 10 previous ones in table 1. In all cases, a typical spectrum of 0, on MgO has been observed (see for example ref. [4] ), consisting of 3 distinct g-values of which one is close to gr (2.0023). Kanzig and Cohen I! 1] admit that the crystal field resulting from an orthorhombic environment completely removes the degeneracy of the 02 3p ?i orbitals as shown in fig. 1. Their original equations have been simplified by keeping only first and second order terms and supposing E > G > X, where A and E are energies as shown in fig. 1 while X is the spin-orbit coupling constant for O2 (0.011 eV, ref. 1161). Hence, theoreticalg-values for the Or molecule ion are given by: &-&=A&=-Q?+ab, .gY -gc

= Agdv = 7-b -a2

g,-g,=&,=Ill.

(1) -ob

1

(2) (3)

In these equations, a = x/A and b = X/E; the z axis corresponds to the internuclear axis’of 02 while the s axis is directed along an atomic 2p orbital;ge is the

Volume

14, number

4

15 June 1972

CHEMICAL PHYSICS LE’ITERS Table 1 0; species on hfg0 a)

Snmple

Pretreatments of

References

KY = 2,0018 Kv = 2.0089 & = 2.0717

[14,151

n-hlg0

irradiation with fast neutrons 02 with S, S’, or SH centers

W-hfg0

evacuation of hi& at 1 lOO’_Kand UV irrzdiation. Reaction of O2 with S-type CeIlierS

gx = 2.0011 gy = 2.0073 & = 2.0170

141

MyO-N-1

evacunticn at 12OO’K for 16 hrs.; adsorption of 112 and then Oa; evacuation of excess O2 (trxes of Hz0 present)

gx = 2.0019 g,, = 2.0070 gz = 2.G740

this work

4t 12QO’K for 16 hrs.; adsorption adsorption of 02 ; evacuation of excess 02

gx = 2.0008 s_v = 2.0074 gz = 2.0780

&&iswork

1. gx = (2.0018)“) -sy = (2.0105) gz = 2.0733 11. g, = (2.0009) gv L- (2.0085) gz = 2.0’179

this work

= 2.0011 gY = 2.007s a= = 2.0772

this work

gx = 2.0006 gv = 2.0075 g 2 = 2.0894

this work

MfO-N-2

and reaction

g-v&es

evacuation

of Hz, evacuation, MpO-JM-3

evacuation at 1300’K for 16 hrs.; adsorption of 02 by reaction with an S-type center; observarion of 2 types of 02 in nearly equal amounts I &sorbs below 600%; II desorbs above 600%

hIgO-EP-I

evacuation at 800% for 2 hrs; adsorption CXCC%02

hf@-EP-2

evacuation at 800% for 12 hrs and at 1200% for 4 hrs; adsorption of Hz, evacuation, adsoption of 02 and evacuation of excess 02

a) (

approximate

) indicates

for 12 hrs and at I200’K of 02 and evacuation of

gX

values.

electron g-value. Tabie 2 reports the values of A and E for the various OF species, calculated from the experimental K-values by using eqs. (2) and (3). gx has not been considered as values calculated from this term are quite inaccurate as a result of the presence of second order terms only. Kninzig and coworkers 120,211 have shown that the sr-electrons interact with their surroundings via (1) electrostatic crystal field interaction and (2) covalent bonding. While the former lowers the energy of the pxticular 0: molecular orbital, the latter raises the energy level. Hence, the E and A par,ameters will reflea intimately the surrounding of the 0, molecular ion and the various 0, species that have been observed then fall into 5 groups as shown in table 3. free

A previous study [33, using 170 enriched O,, showed that the 0, ion on TiO* was most probably adsorbed

with its internuclear axis parallel to the plane of the surface and perpendicular to the symmetry axis of the adsorption site. This situation is equally probable for 07_ on MS, as both 170 nuclei are also equivalent. Table 2 from experimental g-values, X = 0.014 eV, rrild eqs. (l), (Z), and (3)

A and E valuescalculared

Sample

A (eV)

E @VI

4.2 5.6

n-ME0

0.37

W-hfg0 higO-Jhf-1 hf@-Jh’l-2

0.37 0.39 0.37

hlg%JM-3 (I) MgO-JM-3 (II) higO-EP-I MgO-ET-2

0.39 0.37

3.4 4.5

0.37

5.4 5.4

0.32

6.1 5.6

Volume

14, number 4

CHEMICAL PHYSICS LETTERS a- *2p

Fig. 1. Occupation

of electronic enerpy levels for ground state.

0;

in the

Hence, we may conceive a pictorial, if not rigorous, approach to the significance of the A and E parameters in this particu!ar case. A high E value will be associated with a high electrostatic field in a direction perpendicular to the surface while a high 4 value will correspond to a low coordination environment of the 0: ion-These approximations are consistent with the

Classification

15 June 1972

treatment of Ktizig and coworkers [20,2 I J as 4 (rhe _x ,I*. - i-f; splitting) increases with increasing covalency and as a decrease in the coordination index of a surface atom usually parallels its increasing ability to induce covalent bonding. We wivilltentatively propose an explanation for such a distribution of 0, species based upon the nature of the various surface planes that may be exposed after different pretreatments of MgO powders in vacua. Namely, fresh MgO-lb1 particles evacuated below I 100°K shoed (11 l), (31 I), and (210) orientations in order of decreasing importance. After heat treatment in vacua at 1300°K, the dominant orientation became (I lO)with(21 I),( 1 I l),and (IOO)orientationsofsecondary importance [ 10). Similar observations were also reported for MgO powders obtained by decomposition of the basic carbonate [ 17 1. Hence, as the activation temperature is progressively increased, surface planes arc FAvored in the approximate sequence (i 11) + (110),(210),(211)-+(100). Group 1 species correspond to 0, ions possibly adsorbed at sites located in the (100) surface plane, as they are characterized by small A and E values, and observed after activation at 1300°K for at least 16 hrs. This assignment is in agreement with that of Tenth et al. [ 14, IS] for species obtained by reaction of 02 with F~centers. It is comfortable to note that such centers were also observed prior to O2 adsorption on the MgO-JM-3 sample.

Tnble 3 of 0; species adsorbed on various

XfgO

powders

a)

--A (eW 0.32

E (eV)

0.37

0.39

-

3.4

-_

4.2-4.5

IV. 51go-EP-2 (110)

5.4-5.6

I. n-XfgO hIgO-J&l-3 (100)

V. higO-Jhl3 (100)

(I)

(II)

11. UV-hlp0 hfgO-Jhf-2 hIgO-EP-1 (110)

111.MgO-Jhl-1

6.1

(210) or (211) or (111) a) Probable assignments

are indicated

in (

). 457

Volume 14, number 4

Group V, with a very low E and a higher A value could correspond, as it appears simultaneously to group I, to ;LR0, adsorbed at a (100) face site located near an edge or a corner. This is in agreement with the experimental facts, i.e., the observation that the h¶gO-J&3(i) species desorb at a lower temperature th2n the MgO-JM-3(II) species. We assign group II species, with slightly higher E values to Ozions adsorbed 01’1 Lhe (110) surface planes (first layer) as a change from a (100) suriltce plane to a (1 10) surface will mostly alter E. Note also that these species are observed after evacuation at lower temperature for shorter times. Group III 0, ions are those with the highest E and A values and are assigned to species adsorbed on higher indices planes such as (11 l), (2 11). and (3 10) faces. Indeed, these are characterized by higher surface fields and lower coordination numbers of the surface atoms. Group IV O? ions are tentatively assigcd to O? species adsorbed on (1 IO) faces (as they have the same E value as for group 11:)but located on the second layer of atoms with high coordination index. Note here that a cut perpendicula: to the st~rface of a crystal showing a (1 IO) surface will be termina:ed by a zig-zag chain of alternating hlgff and Oz.- ions. Group II species arc those adsorbed on top of thy rippled (110) face, while group IV species NC those adsorbed on sites in the pits. Quantitative support for the previ\>us assignments is obtained by correlating the y and p p:~r;lmeters of Levine et al. [ 191 with the v&es a!‘!: :;nd II rcspective!y. y is the ratio of the surface M:!dolung constant to the corresponding bulk value. A little consideration shows that the purely eiectrostatic part of stirface tension, or surface energy, is proportiWul to (1 -y)_ If our assignment is correct, one then’espccts a continuous variation of E as a funSion of (1-r). p is the nun11 her OF oppositely charged nearest neighbors of ;1 given surface ion divided by that of a bulk ion. Then p is a *measure of the coordination number of the surface atoms and A should increas: for decreasing p values. Our correlations are sho\vn in figs. 3 and 3, and it can be seen from those that our proposed assignment

seems correct, Moreover, group III species correspond most probably to 02 molecular ions adsorbed on (2 11) surface p!anes. It is interesting

to note that stoichiometry

15 June19?2

CHEMICAL PMYSICS LETTERS

is not

1

I

0.1

t4f

i:oo1

(110)

I

I

!

I

0.2

03

0.L

0.5

(210)

tt

(211)

(1-X)

(111) face

Fig. 2. Correlation between E (eV) and (I--y). Numbers in brackets correspond to various species. Oindicate most probable assignments; 8 arc other possibilities.

respected when 02. is reacted with Flcenters preexisting in our powders (MgO-JM3). Indeed, nearly 1000 0, ions appear per F: center upon 02 adsorption. This observaiiorl is in contradiction with the original results of Lunsford [4] but agrees qualitatively with the conclusion of Tenth et al. [ 13: on the adsorption of electron acceptor molecules by MgO. We must however disagree with the concluding statement of these authors, i.e., that “adsorbed molecules of 0, . .. cannot obtain electrons on the surface of MgO for negative ion formation except after y- or neutron irradiation” [ 131. To sum up briefly .our work, the novel features we want to emphasize are: (ij the fact that prior irradiation is not necessary to have electron donor characteristics ofMg0 towards low electron affinity molecules 3s 0,; thermal activation is sufficient in some cases; (ii) the availability of electrons located in traps not too far below the conduction band (but different from Fz -type centers) as no relationship seems to

Volume

14, number 4

15 June 1972

CHEMICAL PHYSICS LETTERS

References

III J. \‘?drix,

G. D;iimsi 2nd H. Imclik. Colloq. Ampere, 1963 (North-H~>ll~nd, Amstrrdam, 1369) p. 304 nnd retcrcnws rhcrcin. P. Xlerinudcau, C. Naccache and A.J. Tcnch, J. Catal. 71 (1971) 20s. 1 C. Nxcxl~c. I’. \leri3udcnu, hl. Chc and A.J. ‘Tenth. Trsns. Faraday SOC. 67 (197 1) 506. J.H. LumA‘ord and J.P. Jaync. J. Chem. Phys. 44 (1966) 1457. ISI R.L. N&on and A.J. Tenth. Trans. Faraday SK 63 (1967)

3039.

161 R.L. Nelson. !X.J. Tttnch nnd B.J. IIarmsworrh,

I L/6

516

616

(2111

(210)

(100)

(111)

(1101 top

1110) face pit

316

g

Fig. 3. Correl;ltion bctawn A (eV) 2nd p. Numbers in bmckcts correspond to various species. 0 indiate most probable x+nmcnts; e xc other possibilities.

hold between the presence or absence of Fzcenters and the fonmtion of 02 ions; (iii) the identification of at least 3, and possibly five, different adsorption sites located in various caystaliine faces. Note however that this last point was anticipated by Tenth et al. (141 and is in the line of recent work on zeolites [ 121 and TiO, [3]. The authors sre grateful to Professor M. Boudart for his encouragement and to Dr. J.C. Vedrine for his valuable suggestions.

Trans. Fnrxl~y Sot. 63 (1967) 1417. 171 .4.J. Tcnch 2nd T. La\vson. Chem. Phys. Letters 7 (1970) 459. 181\V.B. Williamson. J.H. L.unsi’ord ;md C. Nnccachc, Chem. Phys. Letters 9 (197 I) 33. 191 Ning-IImv-\I’ong and J.H. LtIilsfOid. J. Chcm. Phys. 55 (197 I) 3007. A. D~lbouille, E.G. Dcrouanti. V. lndovin:! llOl %I. BuudarrT md AU. \Ysltt‘rs. to be publishcci. 1111\I!. Kanzi: XXI !.1.51. Cohtt~~, Phys. Rev. L?tt+rs 3 (lY59) 5119. 1121 K.bl. Wang and J.H. Lunsford. J. Phys. Chwl. 74 (1970) 1512. A.J. Tcnch and R.L. Nelson, Trans. Faraday Sot. 63 I (1967) 2’54. 1141 A.J. Tenth 2nd I’. Holroyd, Chcm. Commun. 8 (1968) 471. R.L. Nelson and A.J. Tenth, J. Chem. Phys. 40 (1964) 2736. 1161 P.H. Masai, J. Chem. Phys. 43 (19653 3327. [I71 A. Lcclous, Xlem. Sot. Roy. Sci. Li&s, Ser. 6: 1 (1971) 169. [I81 AI. Tcnch and R.L. Nelson, J. Colloid Interf. Sci. 26 (1968) 364. 1191J.D. Levine and P. Mark. Phys. Rev. 144 ( 1966) 75 1. 1201 1j.R. Zcllcr ;md W. Ksnzig, Ilclv. Phys. Acta 40 (1967) 845. R.T. Shucy and H.R. Zeller. Helv. Phys. Acta 40 (1967) 873.