Langmuir 1987, 3, 291-291
291
IR and UV-Vis-Near-IR Spectra of O2 Adsorbed at Low Temperature on NiO E. Escalona Platero,? G. Spoto, S. Coluccia, and A. Zecchina* Istituto di Chimica Fisica dell'llniversitci di Torino, Corso M. D'Azeglio 48, 10125 Torino, Italy Received August 11, 1986. I n Final Form: November 25, 1986 The IR spectra of O2 adsorbed reversibly at =lo0 K on NiO samples of decreasing specific surface area and increasing perfection of the microcrystals show the presence of neutral (side-on)and charged superoxo (end-on) oxygen structures. The neutral structures are formed on the (100)faces; the charged superoxo species are formed preferentially on more defective surfaces. The presence of dissociated oxygen species 02-(0-)is revealed on the UV-vis-near-IR spectra: an intense and broad band with a maximum in the 20 000-10 000-cm-' range and a tail extending in the IR is assigned to electronic transitions with chargetransfer character associated with 02-Ni3+pairs. by repulsive CO-CO interactions. (iii) the Ni2+ions of the Introduction (100) faces form (upon NO contact) reversible nitrosylic Oxygen adsorption on NiO a t room temperature is adducts stable at room temperature; also in this case the prevalently dissociative (ref 1-6 and references therein): maximum coverage is =0.5 and the NO groups form an the coverage is generally small (0 C 0 C 0.08), the maxiarray of parallel nitrosyls strongly chemically and dynammum values being found on the highest surface area ically interacting. Moreover, as in the CO case, further ~ p e c i m e n s .Due ~ to the cubic habit of the NiO microsubstantial increases of coverage are obstructed by the crystals (characterized by a prevalent exposure of (100) repulsive NO-NO interactions. faces, as illustrated by several micrographic s t ~ d i e s ) , ' ~ ~ ~ ' * ~ By analogy with the CO and NO cases, in this investithe previous observations could be interpreted as follows: gation the activity of Ni2+ions located on (100)facelets and (i) The dissociative oxygen chemisorption probably occurs other less common situations in the formation of molecular on defective surfaces (edges, steps, corners, high index reversible dioxygen complexes are investigated by IR and faces, etc.) characterized by Ni2+ions in low-coordination UV-vis spectroscopies. number situations (4-fold and 3-fold coordination) and As the percentage of Ni2+ions located on flat (100)faces with high reactivity. In fact, the fraction of ions located and on defects is strongly influenced by the dimension and in such reactive situations is estimated to be in the +lo% perfection of the microcrystals, the vibrational assignment range, the maximum value being expected on the highest of the molecular oxygen species has been made through surface area samples.s (ii) The Ni2+ions located on the the comparison of the IR spectra of O2 adsorbed at 77 K (100)faces (5-fold coordinated) are inactive toward dissoon samples with specific surface area gradually decreasing ciative chemisorption. from 100-150 m2 g-' to = 10-3 m2 8-l. In the meantime A more extensive O2chemisorption can be induced by the shape of the microcrystals after each sintering treatboth increasing or lowering the adsorption temperature. ment was monitored by electron microscopy. For instance, by increasing the temperature to 700 K, an irreversible uptake of oxygen is usually favored. On samExperimental Section ples obtained by decomposition a t low temperature in The NiO samples were the same used in ref 7, 8, and 10. vacuo of Ni(OH)2or of nickel salts, this uptake is slow and Dependingupon the sintering treatments, the specific surface area, continuous; in contrast, on samples pretreated at higher as measured following the BET method with a Carlo Erba temperature, this phenomenon is absent.6 Sorptomatic 1800, gradually decreased from 150-100 m2 8-l (unsintered a samples) to 20-30 m2 g-' (b samples) and to 10-3 By lowering the temperature to 77 K the adsorption of m2 g-' (highly sintered c samples). Before O2 adsorption, all oxygen in molecular form is favored. samples were outgassed under high vacuum at the same temIndeed Tsyganenko et al.9 have observed some IR perature of the sintering treatment. manifestation of various molecular species at low temThe IR transmission spectra were recorded with a Perkin-Elmer perature. 580B spectrometer interfaced with a data station. The samples Similarly, IR and gravimetric investigations concerning were in form of compressed self-supporting pellets of 10 mg per the interaction of C0738and N08Jo with NiO have shown cm2. The IR cell for low-temperaturemeasurements (optical path that (i) CO and NO are adsorbed irreversibly only on -8 cm) was permanently connected to a vacuum manifold and surface defective sites (steps, edges, and corners of the O2was dosed in situ. cubic microcrystals) and (ii) the Ni2+ions of the (100)faces The temperature of the sample under the effect of the IR beam (which represent the major fraction of exposed ions) are inactive toward the adsorption of CO at room temperature; (1) Bielauski, A.; Dyrek, K.; Kluz, Z. Bull. Acad. Pol. Sci., Ser. Sci. only at 77 K is the formation of stable monocarbonylic Chim. 1966,14,795. (2) Deren, J.; Stoch, J. J. Catal. 1970,18, 249. adduds observed. At e ~ 0 . (corresponding 5 to half of the (3) Gravelle, P. C.: Teichner, S. J. Adu. Catal. 1969,20, 167. available Ni2+sites occupied) the linear carbonylic groups (4)Klissurski, D.G. 8th International Congress on Catalvsis: Berlin, adsorbed perpendicular to the surface form an array of 1984;Vol. 3, p 165. 15) Roberts. M. W.: St Smart. R. J. Chem. Soc.. Faraday Trans. 1 parallel oscillators where adsorbate-adsorbate static 1984;80, 2957: (chemical) and dynamic (dipole-dipole type) interactions (6)Larkins, F. P.; Fenshman, P. J. Tram. Faraday SOC.1970,66,1748. take place. Coverages higher than 0.5 can be obtained only (7)Escalona Platero, E.; Coluccia, S.; Zecchina, A.Surf. Sci. 1986,171, 465. by greatly increasing the CO equilibrium pressure because (8) Escalona Platero, E.; Coluccia, S.; Zecchina, A.Langmuir 1985,1, they correspond to a surface phase strongly destabilized 407. 'Present address: Departamento de Quimica Inorganica, Facultad de Quimica, Universidad de Oviedo, 33071 Oviedo, Spain.
0743-746318712403-0291$01.50/0
(9)Tsyganenko, A. A.; Rodionova, T. A.; Filimonov, V. N. React. Kinet. Catal. Lett. 1979,11, 113. (10)Escalona Platero, E.;Fubini, B.; Zecchina, A., submitted for publication in Surf. Sci.
0 1987 American Chemical Society
292 Langmuir, Vol. 3, No. 2, 1987
Escalona Platero e t al.
77K
I
3595
: Figure 1. Adsorption isotherms of O2on NiO (a sample, 120 m2 g;l) at 77 and 298 K. Open and full circles correspond to two different experiments. is estimated =lo0 K (about 20 K higher than the temperature used for the gravimetric experiments, where the liquid nitrogen temperature is effectively achieved). The UV-vis-near-IR diffuse reflectance spectra were recorded with a Varian 2390 spectrometer equipped with a reflectance attachment. The gravimetric isotherms at room temperature and at 77 K were performed with a Sartorius microbalance. The electron micrographsof the samples sintered in different ways have been obtained on a Philips 300
2.5
3.3
5
microscope.
Results Electron Micrographs. The electron micrographs of the high surface area samples (150-100 m2 g-', a samples) (obtained by decomposition in vacuo of Ni(OH)2a t 523 K following the method illustrated by Gravelle et aL3and successive evacuation at 573 K) show the presence of deeply fragmented hexagonal platelets (as fully illustrated in ref 8) which are aggregates of cubic NiO microcrystals with an =lo-nm edge, still preserving the shape of the original Ni(OH)2 particles. Sintering in O2 at 873 K causes the definite appearance of well-defined (100) terraces in the aggregates. In the meantime the specific surface area has decreased from 150-100 m2 g-' to 20-30 m2 g-' (b samples). Sintering at 1073 K in O2 causes a further decrease of the specific surface area to =lo m2 8-l (c samples). As already documented in ref 8 the electron micrographs now show the presence of massive particles, characterized by the overwhelming predominance of the (100)faces. Microgravimetric Isotherm. The microgravimetric isotherms carried out a t 77 and 298 K are illustrated in Figure 1 (number of O2 molecules adsorbed per 100 A2 vs. the O2equilibrium pressure; a sample). It can be seen that (i) the amount of oxygen irreversibly adsorbed at room temperature roughly corresponds to 3.2% of the monolayer capacity (expressed as the fraction of surface Ni2+ions involved in the dissociative process with respect to a total figure of Ni2+/100A2 = 11.46 as deduced on the basis of the ionic radii of Ni2 and 02-); (ii) the amount of O2 irreversibly adsorbed at 77 K corresponds to -5.6 molecules/100 A2 (i.e., about one O2 molecule to every two Ni2+ centers); (iii) on an increase in temperature to =lo0 K (i.e., nearly the same temperature of the IR cell) nearly all the adsorbed oxygen becomes reversible and can be removed by pumping (only a small fraction is left on the surface, %the monolayer); and (iv) with an corresponding to ~ 3 of increase in pressure to 200 torr (data not reported) multilayer adsorption is observed. Spectra of Adsorbed 02.The oxygen adsorption at =lo0 K causes a decrease of the transmission of NiO in
1:
F(Roo)
1.
Figure 2. (a) Upper curve: IR (difference) spectrum of O2 adsorbed at =lo0 K (50 torr) on the a sample. Lower curve: IR (difference)spectrum of O2adsorbed at 298 K. (b) Lower curve: diffuse reflectance spectrum of NiO (a sample) outgassed at 573 K. Upper curve: after dosing O2at 298 K. the whole 4000-950-cm-' range (this effect is gradually decreased on passing from a to b and c samples). The difference spectra (i.e., the spectra after O2 adsorption minus the spectrum of the clean NiO) obtained at -100 K for the a sample in two different conditions (i.e., in equilibrium with 50 torr of O2 and after successive outgassing at =lo0 K) are illustrated in Figure 2a. It can be seen that (i) in presence of O2 (50 torr) the IR spectrum consists of an extremely broad adsorption extending from 4000 to 950 cm-l (whose intensity increases with and shows a shoulder at ~ 3 2 0 0cm-l) on which
IR and UV-Vis-Near-IR Spectra of Adsorbed O2
Van dQrWaals mOlQculQs(0~), P
neutral mol. C O ~ P ~ Q X O S negatively charged mol.compl.
1085
T
0 . 0 . 0.1
1
D
B
IKX) cm-1
1400
dxl
I&
Figure 3. Effect of sintering on the IR spectrum of O2adsorbed at =lo0 K. The three curves corresponds to a, b, and c samples with specific surface area decreasing from 120 to 10 m2 g-'. The various bands are denoted by capital letters.
narrow peaks (at 3692, negative, 3595,1510,1125,and 1085 cm-l, positive) are superimposed and, (ii) after outgassing at =lo0 K, the narrow features disappear together with a consistent fraction of the broad absorption (which, however, does not totally disappear even after outgassing at room temperature). An identical broad absorption is obtained by directly contacting NiO with O2 at room temperature. In Figure 2b the reflectance spectra in the 300006000-cm-l range of the a sample before (curve 1)and after O2 adsorption at 77 K followed by desorption at 100 K (or higher temperatures) (curve 2) are illustrated. It can be seen that the dissociative oxygen chemisorption is responsible for a large increase of absorptivity in the visnear-IR range (the effect maximizes in the 20000-10000cm-l interval). It is worth noticing that the d-d transitions of Ni2+ions in octahedral coordination1'J2 3T2,(F) 3A2g(F) 8700 cm-'
-
3T1g(F) 3A2g(F)
-
3T1g(P) 3A2g(F)
'
14 900 cm-'
=24 000 cm-l
are clearly observed only on the clean sample (on the sample with adsorbed oxygen, they are obscured by the broader and stronger absorption previously mentioned). The reflectance spectrum at 77 K in the presence of 50 torr of O2has not been measured because of the experimental difficulties associated with water vapor condensation on the external walls of the cell. The IR (difference) spectra in the 1750-950-cm-' range of O2(P = 50 torr) adsorbed at ~ 1 0 K 0 on NiO samples characterized by specific surface area in the 150-100 (a), 40-30 (b), and 10-3 m2 g-' ranges (c) are compared in Figure 3 with expanded abscissa. The broken part of the spectrum of sample a (1450-1250 cm-') corresponds to a spectral range where the observation of the (difference) spectrum of the adsorbed species is not possible or subjected to serious uncertainties because of (11) Goodenough, J. B.In Progress i n Solid S t a t e Chemistry; Reiss, H., Ed.; Pergamon: Oxford, 1971; Vol. 5, p 271. (12) Figgis, B.N.Introduction to Ligand Fields; Wiley-Interscience: New York, 1967.
Langmuir, Vol. 3, No. 2, 1987 293 the presence of intense bands of unknown origin in the background spectrum of NiO. As these background bands tend to disappear upon outgassing and sintering at higher temperatures, the difference spectra of O2 adsorbed on samples b and c does not show any gap in the 14501250-cm-' interval. The relevant features of the spectra of Figure 3 are the following: (i) the complex group of bands in the 1200950-cm-' range (A bands) is greatly affected by sintering. In fact, it represents the strongest feature of the spectrum of sample a and is totally absent in the spectrum of sample c. (ii) The peak at ~ 1 5 1 cm-' 0 (B band) shows an opposite behavior. In fact, it is a minor feature on the spectrum of sample a and becomes the unique feature of the spectrum of sample c (with considerable narrowing). As in the meantime the specific surface area has decreased by a factor of =lo, it is concluded that the peak at ~ 1 5 1 cm-' 0 belongs to a surface species (B species) which is more abundant on sintered samples than on the unsintered ones. (iii) On samples with intermediate specific surface area (b samples) a broad and complex absorption is observed in the 1300-1200-cm-' range (Cbands) (this absorption is not observed on either a or c samples, so suggesting that necessary conditions for its presence are at the same time not negligible surface area and high surface dehydration). (iv) An extremely weak and broad absorption seems to be present in the 1580-1620-cm-' range (D bands) only on a and b samples. The pressure dependence of the intensity of the bands of O2 adsorbed on samples a-c is illustrated in Figure 4. The following observations can be made: (i) Absorptions A-D are reversible to outgassing at the beam temperature (~100 K). (ii) The disappearance of bands A-C is accompanied by a gradual increase of the transmission of the sample (on c samples, this effect is not observed). (iii) The transmittance of the sample, as observed before oxygen contact, is not totally recovered even after prolonged evacuation (the effect is particularly evident on a and b samples). This irreversible modification of the IR spectrum is associated with dissociatively adsorbed 02.(iv) The complex group of A bands is constituted by several components at 1132 (A), 1125 (A'), 1085 (A"), and 995 (A"') which disappear at different rates upon outgassing and whose relative intensity is different on passing from sample a to sample b (on samples c these bands are totally absent). In particular, the most dramatic decrease of intensity with sintering and high-temperature outgassing occurs for A' and A" species. (v) The very weak band at 1510 cm-' (and the shoulder at 1495 cm-') quickly disappears upon outgassing [a continuous upward shift (AD = 10 cm-') is observed upon lowering the O2pressure, more clearly observable on b and c samples]. In the inset in Figure 4a, the modifications induced by O2 adsorption at =lo0 K on the stretching mode of the residual hydroxyls (3692 cm-l) present at the surface of a samples (evacuated at 523 K) are illustrated. (The difference between the two spectra of the inset gives the negative and positive peaks at 3692 and 3595 cm-' illustrated in Figure 2a.) It can be noticed that, upon O2adsorption, the intensity of the narrow peak at 3692 cm-l (OH stretching) decreases, while a broader band at =3595 cm-' simultaneously increased. Similar effects were not observed for samples b and c, because the residual concentration of hydroxyls on these samples (sintered and outgassed at much higher temperature) was too low to be detected. Discussion (1) Electron Micrographs and Microcrystal Mor-
294 Langmuir, Vol. 3, No. 2, 1987
Escalona Platero e t al. Chart I
phology. A detailed discussion of the effect of the thermal treatments on the dimension and shapes of the NiO microcrystals obtained by decomposition in vacuo of Ni(OH)* has been given elsewhere: and the reader is referred to it for more detailed information. Note that (i) on passing from a to c samples, the specific surface area decreases by a factor of = l o and the dimensions and perfection of the microcrystals (with cubic habit) consequently increase; (ii) along the a, b, c series the relative concentration of ions exposed on (loo]faces (5-fold coordinated, 5c) increases (on b and c samples these ions represent 95-98% of the total); (iii) the opposite behavior is shown by the surface concentration of the ions located on edges and steps (Cfold coordinated, 4c) and of the ions located on corners (&fold coordinated, 3c) (on b and c samples the surface concentration of ions located on edges and steps is estimated to be
Ln z
c
0.1
w
0 -I
Q
u
c
8
0.1
A
Figure 4. Effect of decreasing the O2pressure and of pumping on the IR spectra of O2adsorbed at el00 K on the a, b, and c samples (parts a, b, and c, respectively). (Pressuresare in all cases 50,40,5, 5 X lo-' torr; the other curves refer to increasing times of pumping.)
mation of holes and increase of conductivity).13 This chemisorption is "cumulative" and should proceed to the mon01ayer.l~The low value of the observed coverage in-
Langmuir, Vol. 3, No. 2, 1987 295 dicates that this is not the case. So we suggest that the dissociative chemisorption occurs only on defective situations (for instance near or on edges, steps, corners, etc.), following Scheme 11. A tentative illustration of oxygen chemisorption at a surface-defective situation is illustrated in Chart I. In this illustration the dissociated oxygen is in dinegative form and occupies vacant lattice positions. Also in this case the picture is somewhat arbitrary. However, in the next paragraph some further spectroscopic considerations will reinforce this hypothesis. (3) Spectroscopic Manifestations of Oxygen Adsorbed in Dissociated Form. In order to explain the increase of conductivity which usually accompaniesthe O2 dissociation,13 it is generally assumed that the Ni3+ ions formed in the chemisorption process are not totally localized at the chemisorptionsite and can migrate as holes within the solid. In this respect it is most noticeable that (i) the transmission (in the IR) of the original sample is not totally recovered after desorption at 100 K or higher temperature (Figure 2a and 4a,b) and (ii) the oxygen adsorbed irreversibly at room temperature (or at =lo0 K) causes a darkening of the sample (spectrum 2 of Figure 2b), because of the formation of a broad and intense absorption in the 25000-5000-~m-~range [but with a tail extending its influence also in the IR (vide supra)] which masks the d-d transitions of the nickel ions. Changes of the IR transmission in the n-type semiconductors upon irreversible O2 adsorption are usually associated to changes of the free carrier population.14J5 For instance, oxygen adsorption on ZnO outgassed under vacuum, is accompanied by an increase of transparency (and a decrease of conductivity), because the formation of negatively charged surface species occurs at the expense of free electrons (carriers) populating the conduction band (which are responsible of a plasmonic-type absorption extending in the IR).14 The holes in nonstoichiometric NiO have been hypothesized to have an effective mass of the same order of magnitude of that of the free electrons:16 consequently, like in ZnO, we could hypothesize that in NiO (which is a p-type oxygen-excesssemiconductor) the formation of 02-(0-)species upon oxygen chemisorption is accompanied by the increase of the population of free or nearly free carriers (holes), with subsequent marked decrease of transparency (IR region) and increase of absorption (visnear-IR). This explanation, although attractive, is in contrast with the theory of high absorption by free carr i e r ~ .In ~~ fact, following this theory the absorption should increase proportionally to p ( p > 1) while actually the opposite is observed (the absorption increases with p ) (Figure 2a). On this basis we conclude that the large increase of absorption observed in the vis-near-IR (with a tail in the IR) is associated with high absorption by electrons in localized impurity states. In this respect it is most noticeable that by doping NiO with Li (which induces the formation of an equivalent number of Ni3+ ions) an increase of absorptivity in the IR (particularly evident at the highest P) is observed.16 This has been attributed to the excitation of impurity states (oxygen defects) situated near the absorption edge. As tentatively illustrated in Chart I, the adsorbed oxygen (14) Boccuzzi, F.; Morterra, C.; Scala, R.; Zecchina, A. J . Chem. Soc., Faraday Trans. 2 1981, 77, 2059. (15)Boccuzzi, F.; Ghiotti, G.; Chiorino, A. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1779. (16) Ksendoov, Ya. M.; Avdeenko, B. K.; Makarov, V. V. Souiet Phys.-Solid State (Engl. Transl.) 1967, 9, 828. (17) Kireev, P. S. Semiconductor Physics; Mir: Moscow 1978.
296 Langmuir, Vol. 3, No. 2, 1987 is in 3-fold coordination state and occupies a crystallographic site with two Ni3+ions and cationic vacancies in adjacent positions. This structure (associated with surface oxygen excess) is similar to that encountered in the dominant bulk (oxygen excess) nonstoichiometric NiOl+,: the cation vacancy (where the bulk oxygen excess is localized). In fact, a bulk cation vacancy traps a pair of Ni3+ions (one on each side of it) and has six oxygen ions with coordination state (5) lower than that of the bulk (6). The main difference between the two situations is represented by the lower coordination state of the 02-ions in the surface structures, which are consequently destabilized (because of the reduced Madelung potential). Electronic transition having electron-transfer character and involving adsorbed oxygen (Ni3+O2-,* N P 0 - d ) should consequently occur a t frequencies lower than the equivalent ones in the bulk (2.15 eV).ll We tentatively assign the broad absorption with maximu in the 20 0oO-10 000-cm-' range (and the tail extending in the Et)to charge-transfer transitions localized on the 2Ni3+02-,&cluster formed upon oxygen adsorption (Chart I). (4) Vibrational Manifestations of the Reversible Species. The molecules of reversibly adsorbed oxygen can be partitioned into two groups (a and b) (Figure 1): (a) molecules chemically bonded to the surface (0 < 0.5); (b) molecules physically adsorbed. From a very general point of view, the molecules of the first group, being in close contact with the asymmetric potential of the surface, are expected to be IR active, while the molecules of the second group should show the IR manifestations (if any) of the liquid. From Figures 2b, 3, and 4 it can be seen that the IR spectrum is very complex and that different types of molecular species (charged and uncharged) must be present on the surface. On the basis of the literature data, the most plausible forms of molecularly adsorbed oxygen [van der Waals molecules ( 0 J n (i), uncharged Me-.(O2) (ii), and charged Me-.0-2 (complexes) (iii)] should show IR bands in the frequency intervals represented in Figure
Escalona Platero et al. Scheme I11
$ @ jq$ + ....
n
(i) van der Waals Molecules (02)n. Observable in dense phases, they can be reponsible of the very weak IR manifestations in the 1670-1460-~m-~ interval as documented in ref 18. In this respect it is most noticeable that NiO immersed in liquid O2 gives an intense peak at 1560 cm-' attributed to a surface-induced transition in liquid oxygen. (ii) Molecular Uncharged Complexes Me-.(Oz)n. Observed in cryogenic matrices (NiF2.02),19they can show IR-active modes (0-0 stretching) near to the stretching frequencies of neutral dioxygen in triplet (1555 cm-') and singlet (1385 cm-') state. (iii) Superoxo Species. Commonly found in homogeneous conditions (homogeneous oxygen carriers) ,20 they show stretching frequencies in the 1250-950-cm-' interval. Superoxo (0,) species can be divided into two subgroups: i.e., end-on and p-species absorbing in the 1250-1130- and 1130-950-cm-' intervals, respectively. By comparison of the data of the literature with the spectra of Figures 3 and 4, it is definitely concluded that the A, B, and C bands correspond to molecular species in direct contact with the surface (because they are linked to the cations), while the D bands could correspond to liquidlike physically adsorbed 02.
(18) Long, C.A.;Ewing, G. E.J. Chem. Phys. 1973,58, 4824. (19)van Leirsburg, D. A.; Dekock, C. W. J. Phys. Chem. 1974, 78, 134. (20) Jones, C.D.; Sumerville, D. A.; Basolo, F. Chem. Rev. 1979, 79, 139.
;. ' +. - . - .. ....
......
....
o=o
-
6.
+ .:
.-
t
Ni"
Ni2*
Due to the extremely low intensity of the D bands, the last assignment must be considered as highly tentative. The effect of the increase of the particle dimensions and perfection on the IR spectrum of adsorbed O2 (as illustrated in Figure 3) demonstrates the following: (i) Superoxo (0-Jspecies are originated only on defective situations (because they are present only on unsintered and partially sintered a and b samples). (ii) Uncharged complexes are originated by interaction of the oxygen molecules with the Ni2+sites of the (loo]faces (because their surface concentrations grow with sintering and they are the only surface species present on the highly sintered c samples). An illustration of the formation mechanism of the B species is represented in Scheme I11 (and Chart I) where the O2 molecule is adsorbed parallel to the surface. Following van Leirsburg and Dekocklg the bonding of O2 to Ni2+ions should be more similar to that of CO and NO (which indeed on the (100)faces of NiO form linear comp l e ~ e s ) . ~However, J~ in our case we think that, the situation is more similar to that encountered in the metalolefin Zeise's salts, because the u donation from the ir orbital of O2 to the dZ2orbital well explains the decrease in frequency (AF = -45 cm-'). The attribution of the B band to uncharged O2 molecules adsorbed parallel onto Ni2+ ions without appreciable electron transfer is strongly supported also by the low intensity of the 0-0stretching. In fact in this structure the bond stretching is not accompanied (unlike bent Me3+O-2entities) by a strong oscillation of the electronic charge from the oxygen to the metal and vice versa. It is worth mentioning that the side-on structures not only interact primarily (mainly via u donation) with single Ni2+centers but also with adjacent Ni2+ions located on the (110) directions (Chart I): this fact could ensure sufficient stability to these species. An alternative explanation of the B bands could be made in terms of an end-on structure perpendicular to the surface. However, we do not favor it because it does not explain as well the downward frequency shift observed with respect to gaseous 02.
The O2molecules on the (100)faces form a regular array of oscillators statically and d y n m i c d y interacting: indeed the presence of lateral adsorbate-adsorbate interactions is well documented by the continuous shift with 0 [from 1510 (0 = 0") to 1500 cm-' (0 O)] (Figure 4b,c), which is typical for extended assemblies of adsorbed molecules interacting via static (chemical) and dynamic (dipole-dipole) effe~ts.~JO The separation of the static (chemical) and the dynamic (dipole-dipole) contributions is not made here, because it is outside the scopes of this investigation.
-
IR and UV-Vis-Near-IR Spectra of Adsorbed O2 Chart I1
As far as the structure of A-C species (superoxospecies) is concerned, we shall discuss the A species first. As we have demonstrated in Figures 2 and 3a, the A species are more abundant on the unsintered and partially hydroxylated samples and substantially perturb the OH groups of the surface (shifting downward the OH stretching frequency from 3692 to 3595 cm-'). This implies that at least one of the A species (probably the A', because it is broader, as expected for a vibration perturbed by hydrogen bonding) is an OFzspecies formed on a Ni2+site in close proximity of an hydroxyl group as illustrated in Chart 11. The attribution of the 1125-cm-l peak to an 0-2species hydrogen bonded to an adjacent hydroxyl group is demonstrated by the nearly complete absence of this peak on b and c samples (becausethey are totally dehydroxylated). As far as the A" and A"' species are concerned, the comparison with the data of the literature20 suggests a b-superoxo structure presumably formed by interaction of oxygen with Ni2+sites in close proximity of steps (which are defective situations more abundant on unsintered materials) (see Scheme 11). The C species are present on samples which at the same time are (i) totally dehydroxylated and (ii) not too heavily sintered. These facts suggest that they are superoxo species formed on edges, steps, and corners (see, for sake of illustration, Chart I) because (i) the ions located in these positions become free from adsorbed hydroxyls only at the latest stages of the dehydration procedure and (ii) these species cannot be observed on highly sintered samples c (because the fraction of ions present on edges, steps, and corners is exceedingly low). On the basis of the experimental results presented in this paper, a further, more detailed assignment of the IR spectrum of adsorbed oxygen cannot be given without an excess of speculation. (5) Electronic Effects of Reversible 0, (Superoxo) Species. As illustrated in Figures 2b and 4a,b the for-
Langmuir, Vol. 3, No. 2, 1987 291 mation at 100 K of the species A, A', A", and C is accompanied by further loss of transparency in the IR (the effect greatly increases with 8). A reasonable explanation of this fact is as follows: the Ni3+O-2species are associated with ligand-to-metal charge-transfer transitions similar to those illustrated for irreversibly adsorbed oxygen (similar transitions have been observed at =12 000 cm-' for Co"' superoxo complexesz1which extend their influence also in the IR region). Unfortunately, the presence at 100 K of an intense and broad absorption similar to that associated with dissociated oxygen cannot be directly demonstrated by reflectance spectroscopy because of encountered experimental difficulties (the condensation of water vapor on the optical window of the reflectance cell, filled with 50 torr of 02, prevents the obtaining of reasonable spectra). However, the large increase of absorbance in the IR observed, particularly at the highest i~(Figure 2b), can be well due to the tail of an intense and broad band with maximum in the vis and/or near-IR. The previous hypothesis is well substantiated by the following facts: (i) the loss of transparency is roughly proportional to the intensity of the superoxo bands (A, A', A", and C); (ii) removal of 0-2species (obtained by outgassing at =lo0 K) causes a proportional increase of transparency; (iii) on samples where 0-2species are absent, no appreciable change of transmission is observed upon oxygen adsorption (and desorption) (c samples). The last observation is important because it confirms that the B species are neutral and do not involve appreciable electron transfer from the surface.
Conclusions Oxygen is adsorbed on NiO under dissociative and molecular forms. Sever91 molecular oxygen species are observed, which can be divided into neutral and charged species. Neutral species are formed on {loo)faces and are thought to have a side-on structure. Charged species have an end-on superoxo structure and are formed preferentially on defective situations. The irreversibly adsorbed oxygen is in 02-(0-)form and is associated with charge-transfer bands with maximum intensity in the 20 000-10 000-cm-' range with a tail extending in the infrared. Registry No. NiO, 1313-99-1; 02,7782-44-7. (21) Miskowki, V. M.; Robbins, J. L.; Treitel, I. M.; Gray, H.-B.lnOrg. Chem. 1975,14, 2318.
